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Chemical processes mostly happen in fluid environments where reaction partners encounter via diffusion. The bimolecular encounters take place at a nanosecond time scale. The chemical environment (e.g., solvent molecules, (counter)ions) has a decisive influence on the reactivity as it determines the contact time between two molecules and affects the energetics. For understanding reactivity at an atomic level and at the appropriate dynamic time scale, it is crucial to combine matching experimental and theoretical data. Here, we have utilized all-atom molecular-dynamics simulations for accessing the key time scale (nanoseconds) using a QM/MM-Hamiltonian. Ion pairs consisting of a radical ion and its counterion are ideal systems to assess the theoretical predictions because they reflect dynamics at an appropriate time scale when studied by temperature-dependent EPR spectroscopy. We have investigated a diketone radical anion with its tetra-ethylammonium counterion. We have established a funnel-like transition path connecting two (equivalent) complexation sites. The agreement between the molecular-dynamics simulation and the experimental data presents a new paradigm for ion&ndash, ion interactions. This study exemplarily demonstrates the impact of the molecular environment on the topological states of reaction intermediates and how these states can be consistently elucidated through the combination of theory and experiment. We anticipate that our findings will contribute to the prediction of bimolecular transformations in the condensed phase with relevance to chemical synthesis, polymers, and biological activity.

Oxidation of 2-tert-butyl-3-(tert-butylamino)-2H-indazoles with lead(IV) oxide yields red, stable radicals, which can be distilled under high vacuum and recrystallized. They revert to their precursors on Pd-catalysed hydrogenation. Configuration and atomic distances were obtained by X-ray diffraction analysis. EPR hfc constants were assigned with the help of 15 N labeling and exchange of a phenyl hydrogen atom for a tert-butyl group. The X-ray and EPR results show that the novel radicals are to be considered as cyclic hydrazyl radicals, modified by delocalization of the unpaired electron involving the exocyclic imino group and the benzene ring.

Carboxylic acids react rapidly and quantitatively with iminodiaziridines to afford 1,2,4-trisubstituted 1-acylsemicarbazides in a multistep sequence. In this way, a carboxy group is readily converted into an azapeptide motif. Broad signals in high-field 1 H and 13 C NMR spectra recorded for the products are indicative of dynamic processes.

(Arylimino)diaziridines rearrange in several cascade reactions at temperatures 60-100 °C. Those that possess unoccupied ortho positions yield fluorescent 3-amino-2H-indazoles and 2-amino-1H-benzimidazoles. If both ortho positions are blocked by methyl groups, indazoles are not formed and deeply yellow 2-imino-2,3-dihydro-3aH-benzimidazoles are formed, which partly dimerize through Diels-Alder reaction or regenerate the aromatic system by formal loss of CH 2 . In addition, one of the methyl groups of 2,6-dimethylphenyl rings is involved in a [1,7] H shift affording orthoquinonoid intermediates which undergo 1,6-electrocyclization to furnish 2-amino-3,4-dihydroquinazolines. The formation of five-membered ring heterocycles is interpreted in terms of valence isomerization by [1,3] N shift to yield elusive 1-aryl-3-iminodiaziridines as first step. These immediately experience triaza-Cope rearrangement to benzimidazole derivatives or electrocyclic opening of the N-C bond to generate conjugated azomethine imines (1,5-dipoles), followed by their 1,5-electrocyclization to indazoles. First-order rate constants of the decay of (arylimino)diaziridines refer to the [1,3] N shifts as rate-determining steps. They are larger than the corresponding rate constants for alkylsubstituted iminodiaziridines.

Iminodiaziridines are synthesized by (i) 1,3-dehydrochlorination with potassium tert-butoxide of N-chloroguanidines, generated in situ from N,N′,N″-substituted guanidines with tert-butyl hypochlorite, and (ii) base-mediated 1,3-elimination of sulfuric acid from N,N′,N″-substituted hydroxyguanidine O-sulfonic acids. At elevated temperatures, (alkylimino)diaziridines undergo valence isomerization by 1,3-shift, [2+1] cycloelimination to afford isocyanides and diazenes, and ring-opening elimination to yield alkylideneguanidines. N′-Aryl-N-hydroxyguanidine O-sulfonic acids furnish (N-arylimino)diaziridines, but no 1-aryl-3-iminodiaziridines, instead giving rearranged isomers. Precursors containing perdeuterated tert-butyl groups give rearranged products that show complete scrambling. This indicates that 1-aryl-3-iminodiaziridines are intermediates that undergo very rapid degenerate valence isomerization. Provided that the ortho-aryl positions are substituted, high yields of (arylimino)diaziridines are obtained, along with 2-imino-2,3-dihydro-3aH-benzimidazoles. Otherwise, 2-amino-1H-benzimidazoles and strongly fluorescent 3-amino-2H-indazoles, originating from rearrangements of the elusive 1-aryl-3-iminodiaziridines, predominate. N′,N″-Diaryl-N-hydroxyguanidine O-sulfonic acids give only rearranged products: a 2-amino-1H-benzimidazole and a 6-amino-5H-dibenzo[d,f][1.3]diazepine if aryl = phenyl, or a 2-imino-2,3-dihydro-3aH-benzimidazole if aryl = mesityl. 3aH-Benzimidazoles slowly dimerize through Diels–Alder reactions. 15N NMR signals were assigned to the syn and anti ring nitrogen atoms of iminodiaziridines with the help of a combination of homonuclear NOE and HN-HMBC or HN-gHMBC experiments. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

CASSCF and CASPT2/6-31G(d) calculations have been performed on the low-lying electronic states of three, non-Kekule, hydrocarbon diradicals: 2-methylenedihydrophenalene-1,3-diyl (2), trimethylenemethane (3), and 1,8-naphthoquinodimethane (4). The computational results reveal how addition of ferromagnetic coupling groups (1,8-naphtho to 3 and vinylidene to 4) modulates the energy differences between the three lowest electronic states of 2-4. The most dramatic effect is the 30.4 kcal/mol change in the relative energies of the (1)A(1) and (1)B(2) states on addition of a vinylidene bridging group to 4 to form 2. The relative energies of the electronic states of 2-4 are discussed in terms of the topologies of the pair of nonbonding MOs for each state of each diradical, and a strategy for making (1)A(1) the ground state of a nitrogen analogue of 4 is proposed.

6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (lambda = 351 nm) and in Ar and Xe matrixes at 4.6 K (lambdaor = 345 nm), showed a six-line, high-field signal (Delta m(S) = +/- 1), centered at 3350 G in butyronitrile, along with a half-field signal (Delta m(S) = +/- 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm(-1) = 0.0105, |E/hc|/cm(-1) = 0.0014 in butyronitrile and |D/hc|/cm(-1) = 0.0107, |E/hc|/cm(-1) = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl ((3)2) as source of the EPR spectra. Linearity of the Curie-Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet-triplet energy difference (DeltaE(ST) approximately 8-10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical.

Monocyclic iminoaziridines and exo-endo diastereomers of spirocyclic iminoaziridines that are derived from norbornane are prepared in batches of up to 10 g to foster applications as building blocks in syntheses. N,N'-Disubstituted alpha-haloamidines, which are readily available in two steps from N-substituted alpha-halocarboxamides, are 1,3-dehydrohalogenated by strong bases such as alkali-metal hydrides or tert-butoxides to afford distillable oils or low-melting solids, which consist of slowly interconverting E-Z diastereomers of the title compounds. The scope and limitations are outlined for this reaction. The configurations E and Z that were assigned on the basis of homoallylic 1H-1H coupling and asymmetric solvent-induced shifts required that, in 13C NMR spectra, the observed gamma-effects of substitutents at the imino nitrogens were deshielding, contrary to the well-known shielding gamma-effects in all other types of C=N compounds. However, an NOE NMR study demonstrated unequivocally that the previous assignments are correct and hence the observed gamma-effects actually deshielding. The ranges of 15N NMR chemical shifts span more than 60 ppm. Neither the beta- nor the gamma-effects of substituents on both types of nitrogen follow a uniform increment pattern.

Treatment of the 3,3-dimethyl-2-iminoaziridine 7 with hydrogen bromide yields similar amounts of the α-bromo amidine 10a · HBr and the α,β-unsaturated amidinium bromide 11. Ring opening reactions of the 3-tert-butyl-2-iminoaziridines 8 by nucleophiles (potassium methoxide, Pirkle's alcohol 13, and 4-nitrophenol) are sluggish affording the α-amino imidates 12, 14, and 15, respectively. Compound 15a isomerises via the Meisenheimer-type intermediate 16 to furnish the α-amino amide 17. Methyl triflate converts (R)-8 into the 2-tert-butyl-3-amino-2H-azirinium triflates (R)-18 which do not undergo racemisation as shown after hydrolysis to yield the α-amino amides (R)-20. In a similar sequence, (R)-8a and b are converted into the α-amino amides (R)-26 by the action of hydrogen halides or 4-toluenesulfonic acid hydrate followed by hydrolysis of the intermediate imidoyl derivatives (R)-24. Retention of configuration is observed after treatment of (R)-8a with hydrogen chloride and of (R)-8b with hydrogen bromide. Some racemisation found in the other cases is supposed to involve (R)-24 but not the aminoazirinium ions (R)-8 · HX. In a side reaction, 8a and b decompose into methyl isocyanide (22) and the imines 23 at low temperatures, probably under general acid catalysis. The results are not consistent with the hypothesis of an electrocyclic ring opening of 3-amino-2H-azirinium ions to afford 2-amino-1-azaallyl cations (= α-imino carbocations). – The heat of protonation of the parent compounds (E)-27, (Z)-27, 30 and the structures and energies of the resulting cations (E)-28, (Z)-28, 29, and 31 were calculated by ab initio methods. Complete geometry optimisations were performed with the RHF/6-31 + G** basis set. Energies were calculated on the RHF, MP2, MP4SDTQ, and CCSD(T) levels. According to the geometric parameters, the amidine delocalisation of the free bases is abondened in the cations (E)-28, (Z)-28, and 31 while it is considerably strengthened in 29. In accord with these geometric changes, 29 is by far the most stable protonated species followed by (E)- and (Z)-28. Azirine 31 is much higher in energy. The calculated difference in (gas phase) basicity between the two nitrogen atoms of iminoaziridines appears to exclude the ring-protonated cations as intermediates also for reactions in solution.

Several 2,6-substituted barbaralanes, viz. 1d–g, have been synthesised from Meerwein's diketone (2). Treatment of dinitrile 3 with hydrochloric acid in boiling methanol followed by hydrolysis of the intermediate Pinner salt yields the dimethyl dicarboxylate 4 which is brominated to afford the exo,exo dibromo diester 5. Reductive cyclisation of 5 gives rise to the formation of dimethyl 2,6-barbaralanedicarboxylate (1d). – Addition of (phenylethynyl)cerium(III) chloride to 2 followed by acid-induced dehydration of the resulting endo,endo diol 6 yields the bis(phenylethynyl)diene 7, which is cyclised to the bis(phynylethynyl)barbaralane 1e by allylic bromination and subsequent treatment of the exo,exo dibromide 8 with the zinc-copper couple. Likewise, the exo,exo dibromo disulphone 11, which is obtained from 2 via the bis(vinyl sulphide) 9 and the bis(vinyl sulphone) 10, is converted into the bis(phenylsulphonyl)barbaralane 1f. – Trifluoromethylation of diketone 2 with trimethyl(trifluoromethyl)silane in the presence of a source of fluoride ions affords a mixture of the O silylated endo,endo diol 12 and the oxatwistane 13. Hydrochloric acid converts the latter into the hydroxy ketone 16, which yields 12 on treatment with trimethyl(trifluoromethyl)silane and tetrabutylammonium fluoride. Desilylation of 12 to furnish the endo,endo diol 14 requires prolonged treatment with hydrofluoric acid under phase-transfer catalysis. Vigorous conditions are also necessary for the dehydration of 14 with phosphorus oxychloride in pyridine to afford the bis(trifluoromethyl)diene 15. Cyclisation of 15 to the bis(trifluoromethyl)barbaralane 1g is performed in the usual way by allylic bromination and subsequent reduction of the dibromide 17 with the zinc-copper couple. – The configurations and conformations in the crystals have been elucidated by X-ray diffraction analyses for the bicyclo[3.3.1]nonane derivatives 5, 8, and 11 as well as the barbaralanes 1f and h. In the solid state, 1f appears to exist as a single non-rearranging valence tautomer. The atomic distances found for 1h indicate the presence of equilibrating non-equivalent valence tautomers. The results are compared to those from X-ray diffraction analyses of other degenerate barbaralanes and heterobarbaralanes.

Spirocyclic 1,3-dipolar cycloadducts 1 and 4 of azides to heterocyclic ketene N,N-acetals open their dihydro-1,2,3-triazole ring in the presence of weak Bronsted acids to afford novel 1,3-substituted triazenes 2X and 5X, respectively, which form colorless, crystallized tetrafluoroborates (X = BF4) and hexafluorophosphates (X = PF6). Ring-opening is reversed in alkaline solutions. Methyl triflate methylates the dihydro-1,2,3-triazole ring of 1a and 4 at N-3 and thus induces ring-cleavage to 1,3,3-trialkyltriazenes 3 and 6, respectively. The 1,3-substituted triazenes 2aX, 2bBF4, 2dX and 5X exist as tautomers that have the larger substituents, which include the heterocyclic rings, connected with the azo group (N-1). By contrast, triazene 2cPF6 has a bulky alkyl group at each terminal nitrogen and hence forms two rapidly equilibrating tautomers of similar stability. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Cyclic ketene N,X-acetals 1 are electron-rich dipolarophiles that undergo 1,3-dipolar cycloaddition reactions with organic azides 2 ranging from alkyl to strongly electron-deficient azides, e.g., picryl azide (2L; R1=2,4,6-(NO2)3C6H2) and sulfonyl azides 2M–O (R1=XSO2; cf. Scheme 1). Reactions of the latter with the most-nucleophilic ketene N,N-acetals 1A provided the first examples for two-step HOMO(dipolarophile)–LUMO(1,3-dipole)-controlled 1,3-dipolar cycloadditions via intermediate zwitterions 3. To set the stage for an exploration of the frontier between concerted and two-step 1,3-dipolar cycloadditions of this type, we first describe the scope and limitations of concerted cycloadditions of 2 to 1 and delineate a number of zwitterions 3. Alkyl azides 2A–C add exclusively to ketene N,N-acetals that are derived from 1H-tetrazole (see 1A) and 1H-imidazole (see 1B,C), while almost all aryl azides yield cycloadducts 4 with the ketene N,X-acetals (X=NR, O, S) employed, except for the case of extreme steric hindrance of the 1,3-dipole (see 2E; R1=2,4,6-(tBu)3C6H2). The most electron-deficient paradigm, 2L, affords zwitterions 16D,E in the reactions with 1A, while ketene N,O- and N,S-acetals furnish products of unstable intermediate cycloadducts. By tuning the electronic and steric demands of aryl azides to those of ketene N,N-acetals 1A, we discovered new borderlines between concerted and two-step 1,3-dipolar cycloadditions that involve similar pairs of dipoles and dipolarophiles: 4-Nitrophenyl azide (2G) and the 2,2-dimethylpropylidene dipolarophile 1A (R, R=H, tBu) gave a cycloadduct 13 H, while 2-nitrophenyl azide (2 H) and the same dipolarophile afforded a zwitterion 16A. Isopropylidene dipolarophile 1A (R=Me) reacted with both 2G and 2 H to afford cycloadducts 13G,J) but furnished a zwitterion 16B with 2,4-dinitrophenyl azide (2I). Likewise, 1A (R=Me) reacted with the isomeric encumbered nitrophenyl azides 2J and 2K to yield a cycloadduct 13L and a zwitterion 16C, respectively. These examples suggest that, in principle, a host of such borderlines exist which can be crossed by means of small structural variations of the reactants. Eventually, we use 15N-NMR spectroscopy for the first time to characterize spirocyclic cycloadducts 10–14 and 17 (Table 6), and zwitterions 16 (Table 7).

Irradiation of the amino-4H-1,2,3-triazoles 7 (λ ⩾ 280 nm) affords molecular nitrogen and 3-amino-2H-azirines 8 in high yields. As shown on the basis of spectroscopic evidence, the photoproduct of 7a exists exclusively as 3-amino-2H-azirine 8a rather than in an equilibrium with the hypothetical iminoaziridine 10. Ab initio quantum chemical calculations up to the Gaussian-3 level were performed for pairs of 3-substituted 2H-azirines (1a, 3a, 5)/aziridines (2, 4, 6) and 1-substituted cyclopropenes (11, 13, 15)/cyclopropanes with exocyclic double bonds (12, 14, 16). The G3 energies of acyclic pairs of tautomers 17−22 and other compounds were calculated for comparison. Ring strain was assessed with the help of two groups of homodesmotic reactions involving either hydrogen transfer (Equations 1−4) or exchange of carbon for nitrogen (Equations 5−7). In the cyclopropane series, relative ring strain and the stabilities of the functional groups both favour the same tautomer. The strain in aziridines − and in 2H-azirines in particular − is lower than in the corresponding cyclopropanes. This is the reason why the relative stabilities of the functional groups present in the N-heterocycles outweigh differences in strain that may be caused by the endo- or exocyclic location of the double bonds. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Deprotonation of the 2-cycloalkylbenzothiazolium perchlorates 1f-h with sodium hydride in the presence of methanesulfonyl azide (3) affords the spirocyclic dihydro-1,4-benzothiazines 9f-h together with small amounts of the 2-iminobenzothiazole 5. The 2-cycloalkylidenedihydrobenzimidazoles 11e-h are generated by deprotonation with potassium hydride from the corresponding 2-cycloalkylbenzimidazolium salts 10e-h, and trapped with 3 to yield the zwitterions 12e-h. Whereas 12h is thermally unstable, 12e-g are isolated and thermolysed at 20-80 °C. The cyclopropyl zwitterion 12e decomposes in an ill-defined way, only at temperatures above 80 °C. In contrast, 12f,g, and, in particular, 12h, decompose more readily to furnish the products of ring expansion (13f-h) and those of a [3 + 2] cycloreversion (6 and 14) of intermediate spirocyclic triazolines. Products that might be indicative of the intervention of hypothetical zwitterions of type 8 cannot be detected.

2-Alkyl-1-methylquinazolinium hexafluorophosphates 9 are deprotonated by sodium or potassium hydride to afford solutions of 2-alkylidenedihydroquinazolines 10, which were investigated by NMR spectroscopy. Trapping with methanesulfonyl azide (5a) of 10 in situ or subsequent treatment with trifluoromethanesulfonyl azide (5b) gives mixtures of colourless (15) and intensely yellow N-sulfonylimino-1,4-benzodiazepines 16 along with products due to cleavage of the exocyclic double bond of 10, viz. 11 and 13. The ethylidene compound 10b yields the bicyclic products 18 and 19, apparently by complex sequences of reactions that are triggered by removal of the acidic proton at C-2 of 16b and 16f. The structures of the products are based on spectroscopic evidence and X-ray diffraction analyses performed on 15b, 16d, 16e, and 19.

2-Alkyl-1-methylquinolinium hexafluorophosphates 1 are deprotonated by sodium or potassium hydride to afford solutions of 2-alkylidenedihydroquinolines 2, which are investigated by NMR spectroscopy. 1,3-Dipolar cycloaddition of phenyl azide to 2 yields the spirocyclic products 10. While, at 80-110 °C, the [3 + 2] cycloaddition that afforded (u)-10f is reversible and accompanied by epimerisation to give (l)-10f, thermolysis of the dimethyl compounds 10b and d affords the ring-expanded products 14b and d, respectively, in good yields along with molecular nitrogen. Irradiation of 10d with light of λ > 320 nm results in the formation of similar amounts of 14d and [3 + 2] cycloreversion products, viz. 2-diazopropane (5b) and the N-phenylimine 15d. - Trapping of 2 by methanesulphonyl azide (18a) gives mixtures of the products of ring expansion (21b, d-f, 10-50 %) and [3 + 2] cycloreversion (22a, d, 10-80 %) of the apparently very labile intermediate spirocyclic cycloadducts 19. The ratio of 21 vs. 22 is significantly improved when 18a is replaced by trifluoromethanesulphonyl azide (18b), which affords the iminodihydrobenzazepines 21i-k in 50-75 % yield. The structures of the products are based on NMR evidence and X-ray diffraction analyses performed with 21b, d, and (ax,E)-21e.

The flash vacuum pyrolysis of 3,3,5,5-tetramethylpyrazolin-4-one (4) with subsequent trapping of the products in argon at 10 K results in the formation of a complex product mixture with tetramethylcyclopropanone (5) as one of the minor constituents. The 193 nm photochemistry of matrix-isolated 4 is much cleaner, yielding cyclopropanone 5 and allene oxide 9 as the major products. The reaction of tetramethylallene 13 with oxygen atoms yields the same product mixture of 5 and 9 and thus provides an independent route to these species. The experimental IR spectra of 5 and 9 are in good agreement with the results from DFT calculations.

Oxidation with barium manganate of the enol of tetraacetylethane (10) affords tetraacetylethylene (7a) in good yield. Treatment of the 1,3-diketones 15a and b with iodosobenzene in the presence of boron trifluoride does not result in oxidative coupling yielding 10 and diacetyldibenzoylethane 17, respectively, as has been reported by Moriarty et al. Instead the known difluoroborylenolates 16 are formed. Oxidative coupling of the sodium enolate of 15b with iodine affords the meso-tetraketone meso-17 and diacetyldibenzoylethylene (E)-7b besides small amounts of rac-17 and the bisenol 19. In the presence of 1,4-diazabicyclo[2.2.2]octane, meso-17 and 19 tautomerise yielding identical equilibrium mixtures of meso-17, rac-17, and 19 (26:29:45). Dehydrogenation with 5,6-dichloro-2,3-dicyanobenzoquinone of meso-17 yields (E)-7b and an isomer (77:23) which was assigned structure (Z)-7b on the basis of spectroscopic evidence. – Tetraacetylethylene (7a) reacts with 1,2-dimethylenecyclohexane to afford two different [4+2]cycloadducts, viz. the spirocyclic dihydropyran 25 (30 %) and the tetraacetyloctalin 27. Whereas the latter is stable, the former isomerises to the latter on heating. Formation of both [4+2]cycloadducts and the isomerisation 25 27 are interpreted by invoking the zwitterion 26 as common intermediate. Monoolefines that cannot generate stable carbenium ions don't give [4+2]cycloadducts with 7a. In contrast, α-methylstyrene yields the oxa Diels–Alder product 30. The structures of (E)-7b, 19, 25, and 27 are established by X-ray diffraction analyses.

The deuterium-labelled 2,4,6,8-substituted barbaralanes [D5]-1a and b, and the two model barbaralanes 15 und 19 for the estimation of 13C chemical shifts in the slow-exchange limit are synthesised from bicyclo[3.3.1]nona-3,7-diene-2,6-dione (5). – The extension of Saunders′ isotopic perturbation method bridges the gap between the limiting cases “perturbation of shift equivalence in delocalised systems” and “perturbation of degeneracy” by considering the simultaneous presence of delocalised species of higher symmetry and skewed equilibria between localised molecules that are degenerate in the absence of the isotopic perturbation. An equation (Equation 7) is derived for such multi-component systems which describes the temperature dependence of the relative isotopic splittings in 13C spectra with three parameters, viz. the isotopic perturbation parameters ΔHP and ΔSP of the skewed equilibria and the enthalpy difference ΔH0 between the delocalised and localised species. – Relative isotopic splittings ΔδP/Δδ are calculated from estimated chemical shifts in the slow-exchange limit (Δδ) and isotopic splittings (ΔδP) of signals in variable-temperature 151-MHz 13C NMR spectra recorded for solutions of [D5]-1a and b in [D8]toluene and N,N′-dimethylpropylene urea. The results obtained from [D5]-1a in both solvents and from [D5]-1b in the former are compatible with either a skewed equilibrium between localised valence tautomers alone or the simultaneous presence of localised and small amounts of delocalised valence tautomers. In striking contrast, the small isotopic splittings themselves and their small temperature dependence, observed for solutions of [D5]-1b in N,N′-dimethylpropylene urea, demonstrate that one half of the solvated compound exists in the delocalised state [D5]-1b*, which is more stable by 2 kJ mol–1 than the equilibrating localised species [D5]-1b ⇌ [D5]-1b′.

Aus den UV/Vis-Spektren thermochromer Semibullvalene 1 und Barbaralane bei verschiedenen Temperaturen lassen sich die Enthalpiedifferenzen zwischen den beiden entarteten klassischen Strukturen 1 und 1′ und instabileren Spezies (1*), die bei groseren Wellenlangen absorbieren, ermitteln. Diese Spezies werden als hohere, delokalisierte Zustande interpretiert, die sich gerade oberhalb der flachen Potentialbarriere zwischen den Grundzustanden befinden (ΔH0 = 11 (R = H; in Butyronitril), 1 kJ mol−1 (R = Ph)).

The UV/Vis spectra of thermochromic semibullvalenes 1 and barbaralanes recorded at various temperatures yield enthalpy differences between the two degenerate classical structures 1 and 1' and the less stable species (1*) that absorb at long wavelengths. The latter species are interpreted in terms of higher, delocalized states that are located just above the flat potential energy barrier between the ground states (ΔH(0) =11 (R=H; in butyronitrile), 1 kJ mol(-1) (R=Ph)).

The position of the equilibrium between localized and delocalized states of thermochromic semibullvalenes and barbaralanes (see the Equation) depends strongly on the solvent. Dipolar aprotic solvents, particularly N,N'-dimethylpropylene urea, favor the delocalized, bishomoaromatic state (ΔH(0) =8 kJ mol(-1) (cyclohexane), ΔH(0)

Whereas 1,5-dimethylsemibullvalene (1b) equilibrates with 1,5-dimethylcyclooctatetraene (2b), the semibullvalene-2,6-dicarbonitriles 13 rearrange irreversibly to afford mixtures of the isomeric cyclooctatetraene-1,5-dicarbonitriles 14 and 15. Arrhenius and Eyring parameters of these thermal isomerisation reactions have been determined for the gas phase (1b 2b) and [D6]benzene solutions (1b 2b, 13 14 + 15). Furthermore, the activation parameters of the known rearrangement of octamethylcyclooctatetraene (3) to octamethylsemibullvalene (4) have been determined. – The data for these compounds, together with those for related compounds previously reported in the literature, show that substituents not only influence the relative stabilities of the semibullvalene and cyclooctatetraene systems but also the height of the energy barrier for their interconversion. Those substituents that lower the barrier toward the degenerate Cope rearrangement of semibullvalenes simultaneously accelerate their rearrangement to cyclooctatetraenes thus limiting the thermal stability of the former.

The activation parameters [ΔG# (200 K), ΔH#, and ΔS#] for the degenerate Cope rearrangements of barbaralane (1a) and several semibullvalenes (3a, 7a, 8a), and for those of a number of their 3,7- and 2,6-disubstituted derivatives, have been determined by variable-temperature carbon-13 NMR spectroscopy at 126 and 151 MHz. the disubstituted compounds studied include the barbaralanes 1c, d, e, h, j, k, and 2e, h, j, and the semibullvalenes 3d, e, h, 4e, h, i, l, m, and 5h–8h. Careful attention has been given to the measurement of temperatures. The data for these compound, together with those for related compounds previously reported in the literature, show that conjugating subsituents (e. g. aryl, CH, CO2R) in the 2,6 positions lower the barriers of the degenerate Cope rearrangement, whereas substituents that are electron-withdrawing through the inductive effect (e. g. Cl, CF3, SO2Ar) retard the reaction. Substituents in the 3,7-positons have little effect or are rate-retarding. The presence of 1,5-tri- and tetramethylene bridges in semibullvalenes accelerates the rearrangements, the effect being larger in the case of the former.

Mechanistic investigations by means of proton spectroscopy detected intermediates and uncovered the course of reactions in acetate-buffered [D4]methanol of primary cyclopropanediamines cis- and trans-2a with benzaldehyde (3a) or 2,2-dimethylpropanal (3b), of secondary cyclopropanediamines cis- and trans-2b with 3b, and of the ring-methylated cyclopropanediamine trans-14a and the aromatic aldehydes 3a and c. This study provided the basis of an expedient synthesis of pyrroles which takes place under exceptionally mild conditions. Irrespective of the configuration, primary (2a·2HBr) and secondary cyclopropanediammonium dibromides 2b and c·2HBr that are devoid of ring substituents react with aromatic aldehydes 3a, e–h, cinnamic aldehyde (3i), and 3b to afford 2-substituted (8a, b) and 1,2-disubstituted pyrroles (8c–i), respectively. The 3-substituted secondary trans-cyclopropanediammonium dibromides 24·2HBr furnish 1,2,4-trisubstituted pyrroles 25. While the primary 1-methylcyclopropanediammonium dibromide trans-14a·2HBr reacts regioselectively with 3a and c to produce only 2,3-substituted pyrroles 19a, c, the corresponding secondary dibromide trans-14c·2HBr gives rise to the formation of mixtures of 1,2,3- (22) and 1,2,5-trisubstituted pyrroles 23. The key step of pyrrole formation from 1,2-cyclopropanediamines and aldehydes is the ring expansion of intermediate monoiminium ions of type 5 via azomethine ylides (E, Z)-6 to yield dihydropyrrolium ions 7.

Deprotonation of the annulated tetrazolium salts 4, 6, 8, 10, and 12 with sodium or potassium hydride yields the alkylidenedihydrotetrazoles 5, 7, 9, 11, and 13, respectively. While 5a and b are unstable, even in solution at low temperatures, 7, 9, 11, and 13 form yellow oils that are distilled under high vacuum. − Irradiation of solutions of 7, 9, and 11 in [D8]toluene at −60°C yields, besides molecular nitrogen, annulated iminoaziridines that have an exocyclic CN double bond, i.e. 14, 16, and 18, respectively. In addition, an equal amount of the isomer 19 with the endocyclic CN double bond is formed from 11. On thermolysis, 14, 16, and 18 undergo [2 + 1] cycloreversion into methyl isocyanide and the cyclic imines 15, 17, and 20, respectively. By contrast, 19 rearranges thermally to yield 18. While the doubly bridged alkylidenedihydrotetrazole 13a affords only unidentified decomposition products on photolysis, its methyl homologue 13b is converted into the hexahydronaphthyridine 22 which is also formed on thermolysis. − Irradiation of 13b in a 2-methyltetrahydrofuran or butyronitrile matrix at 77 K yields a triplet diradical showing a four-line EPR spectrum centred at 3362 G and a half-field transition (at 1669 G) with a hyperfine structure. The zero-field splitting parameters |D-hc| = 0.031 cm−1 and |E-hc| = 0.0014 cm−1 are obtained by simulation of the EPR spectrum. The signal-carrier is assigned the diazatrimethylenemethane structure 23 on the basis of the close similarity between its EPR spectrum and those of trimethylenemethane (28) and tris(N-methylimino)methane (29). − Structural features are discussed that are responsible for the observed differences between the photochemical pathways.

1,4,5-Trialkyltetrazolium salts are convenient synthetic equivalents for hypothetical secondary ynamines in cycloaddition reactions to azides. Thus, the tetrazolium salts are converted into alkylaminotriazoles via deprotonation, cycloaddition to azides, and subsequent base-mediated cleavage of the resulting spirocyclic cycloadducts.

Treatment of the unsaturated ketone 6 with phenylhydrazine fails to afford the unsaturated phenylhydrazone 3 but gives the tricyclic phenylhydrazoketone exo-8, which is oxidised with iron(III) chloride to yield the phenylazoketone exo-9. The configuration of exo-9 is elucidated with the help of an X-ray diffraction analysis. The unsaturated phenylazo compound 14 is obtained from Meerwein's diketone 10 in two ways. Iodination of the phenylhydrazone 11 in the presence of pyridine followed by elimination of pyridinium iodide from the bispyridinium salt 13 with aqueous sodium hydroxide in dimethyl sulphoxide furnishes 14 in low yield. A somewhat better yield is more conveniently achieved when the dibromodiketones exo,exo- and exo,endo-12 are allowed to react with phenylhydrazine in the presence of pyridine. Bromination of 14 with N-bromosuccinimide in cyclohexane as solvent affords the labile dibromide 15. Both reagents employed, viz. the zinc/copper couple in refluxing tetrahydrofuran and butyllithium at low temperature, fail to convert 15 into the hoped-for bis(phenylazo)barbaralane 1 but instead give rise to the formation of the isomer 17, either by cyclisation of the intermediate anion 16 or by rearrangement of 1.

Lithiation of the annulated tetrazoles 6a, c with butyllithium yields the N-lithiotetrazoles 7a, c which are allowed to react with alkyl halides. Alkylation at the α-carbon atom is observed in the reaction with methyl iodide ( 6b, d), 1-bromo-2-chloroethane (7c14a), and 1,3-dibromopropane ( 16, 17), while 1,2-dichloro- and 1,2-dibromoethane give other products, viz. 11–13. Quaternisation of 6 with dimethyl sulphate affords mixtures of 1-methyl- (1) and 2-methyltetrazolium salts (8) (3:1–4:1) from which the hexafluorophosphates 1 PF6 are obtained by crystallisation. Methyl triflate converts the ω-azidonitriles 9 into the N-methylnitrilium triflates 10 which immediately undergo an intramolecular 1,3-dipolar cycloaddition to afford the 1-methyltetrazolium triflates 1 F3CSO3. Cyclisation of 16 by intramolecular N-alkylation furnishes the bisannulated tetrazolium bromide 3a · Br. Attempts to obtain the lower homologue 15, either from 14a or from 17, met with failure. Instead, 17 rearranges via 15b into 14b. The α-branched tetrazole 24 is synthesised from ethyl cyanoacetate and the bromide 18. Double cyclisation of 24 affords the bisannulated tetrazolium chloride 3b · Cl. The analogous scheme, envisaged for the synthesis of the lower homologue 32, failed in the last step owing to the high strain of this system.

Addition to the diphenyldiketone 11 of the reagent prepared from rigorously dried cerium(III) chloride and phenyllithium yields the tetraphenyldiol endo,endo-13 after extended periods of time. The configuration of this diol, which was previously assigned the (incorrect) configuration exo,exo-13, is established by an X-ray diffraction analysis. Dehydration of endo,endo-13 with sulphuric acid in acetic acid affords a mixture of the oxatwistane 16 and the tetraphenyldiene 17 (3:1). Only the latter is obtained from endo,endo-13 by the action of boron trifluoride–diethyl ether in dichloromethane solution. While attempts at allylic bromination of 17 with N-bromosuccinimide in conventional solvents inevitably lead to complete decomposition, use of cyclohexane as solvent allows to obtain solutions of allylic dibromides that can be cyclised with the zinc-copper couple to afford tetraphenylbarbaralane 9 in 58% yield based on 17. Thus, 9 is now available from the diphenyldiketone 11 in only three steps with an overall yield of 42%. – The autoxidation of 9, studied in various solvents, yields mixtures of products of which the epoxyendoperoxides 20 and 21, the unsaturated ketone 22, and a labile, probably oligomeric or polymeric product C are isolated in pure form. The structures 21 and 22 are elucidated by X-ray diffraction analysis and independent synthesis, respectively. On warming, 20 rearranges into 21. Traces of acid convert the unknown autoxidation product A instantaneously into the unsaturated ketone 22. – Rate studies show that the autoxidation of 9 starts after a short initiation period. 2,6-Di-tert-butyl-4-methylphenol exhibits powerful inhibitory effects. These results demonstrate the free radical nature of the autoxidation of 9. – The results are interpreted in terms of a mechanistic scheme involving initiation of the radical chain by addition of triplet oxygen to 9 to generate the diradical 28 followed by a radical chain 1:1 copolymerisation of 9 and oxygen to produce diradical 29. Eventually, endo addition of oxygen to 29 gives rise to the formation of the epoxyendoperoxides 20 and 21. Intramolecular hydrogen abstraction from the methylene group (C9) is accompanied by cleavage of a bridgehead bond (C1–C2) and the adjacent O–O bond to afford the extremely labile product A (perhaps 32 or 33) which undergoes acid-catalysed elimination to furnish ketone 22.

Copper-mediated cyclopropanation of dihydroimidazolones (I) using diazomethane (II) and methyl diazoacetate (VI) provides access to diazabicyclohexanones (IIIa), (IIIb), (VII), and (XI) and the first diaza[4.3.1]propellane (IIIc).

Cyclic ketene N,N-acetals derived from imidazolidine (4a, c) and 2,3-dihydrobenzimidazole (6a–c) add methanesulphonyl azide (2a) or picryl azide (2f) to afford the zwitterions 5 and 7, respectively. The structure of 7d is elucidated by X-ray crystallography. Reversibility of formation and thermal stability of the N-sulphonyl zwitterions depend on the substitution pattern at the carbon atom to which the triazenide moiety is attached: In the case of a pair of geminal methyl groups (5a, 7a) formation is irreversible and decomposition by cyclisation and subsequent reactions occurs above −20°C, while in presence of a single alkyl group (7c, d) these processes require heating to 80°C and are accompanied by partial reversion to 2a and ketene N,N-acetals (6b, c). Cyclisation of the zwitterions yields intermediate spirocyclic [3 + 2] cycloadducts, which may undergo [3 + 2] cycloreversion into N-sulphonylimine 13 and diazo compound 14 or extrude molecular nitrogen to furnish ring-expanded 2-(sulphonylimino)piperazine derivatives (9, 11).

Methanesulphonyl azide (2) reacts readily with isolated cyclic ketene N,X-acetals of type 1, viz. 10a, d, 15b, d, and 22a–c, or those that are generated in situ by deprotonation of the corresponding 2-alkylbenzazolium tetrafluoroborates, i.e. 14a 15a, 14b 15b, and 18 19. Ring-expanded products are formed by extrusion of molecular nitrogen from intermediate labile [3 + 2] cycloadducts 3 with concomitant 1,2-shift of N (route A1 12, 24) or X (route A2 16, 20, 21). In addition, 3 may undergo [3 + 2] cycloreversion into N-sulphonylimine 5 and diazoalkane 6 (route B 13, 17, 25). The configurations of the cyclic N-sulphonylamidines 16b and 21b, the N-sulphonylimine 24 and the N-sulphonylamine 27 are elucidated by means of X-ray diffraction analyses. The ratio of the (useful) ring-expansion reactions vs. the unwanted formation of 5 + 6 is hardly influenced by the solvent employed and temperature of the experiment but strongly by the nature of the potential migrating atom and the substituents at the α-carbon atom.

Iminoaziridines (11) are highly reactive synthetic equivalents for three of the four components in the Ugi four-component condensation. Thus, iminoaziridines react rapidly with carboxylic acids at temperatures as low as –20°C to afford α-amino isoimides (14), which are identical to the elusive α-adducts of isocyanides in the Ugi reaction. 1,4-Migration of the acyl group (O → α-N) in 14 furnishes the α-acylamino amides 15. Very little, if any, racemisation is observed when carboxylic acids react with nonracemic iminoaziridines [(R)-11 a,c], which are readily available. Mumm rearrangement by O → N-acyl 1,3-migration to afford α-amino imides (16 e, f) competes if the O → α-N 1,4-shift is slowed down by steric hindrance. The latter acyl shift is catalysed by carboxylic acids while the former is not. The iminoaziridines (R)-11 a,c react quantitatively and without racemisation with hydrazoic acid to produce the 5-aminoalkyltetrazoles (R)-21 a,c.

2-Iminoaziridine 19 and the pairs of isomers 22/23 and 32/33 can be obtained in high yields by base-induced 1,3-dehydro-halogenation of the corresponding α-halo amidines. Regio-selectivity of the ring closure reaction of 21 is achieved by treatment with potassium hydride as base in the presence of 18-crown-6 to afford almost exclusively 23. At low temperatures, the cyclisation of 21a under the influence of potassium tert-butoxide occurs diastereoselectively yielding (Z)-22 and (Z)-23 (87:13). Resolution of racemic 21a can be attained by single recrystallisation of the nicely crystallised mandelates of the like configuration, thus providing an efficient route to (R)- and (S)-23 of high enantiomeric purity. Chiroptical data are reported for 21a and 23. – (E)- and (Z)-22 equilibrate faster by more than one order of magnitude than (E)- and (Z)-23 but almost the same preference for the E diastereomer is found in both cases. – Only first-order decomposition into isocyanides 6 and imines is observed on thermolysis of 19, 22, 23, and 32 with activation parameters depending on the substitution pattern. In contrast, the thermal valence isomerisation 35 36 is fast enough to compete with the [2 + 1] cycloreversion of 35. Thermal racemisation of (R)-23 does not occur. Quantum-chemical calculations were performed on the parent iminoaziridines (E)- and (Z)-38, 3-amino-2H-azirine (39), the methylenediaziridines cis- and trans-40, and diastereomeric closed-shell and open-shell planar (41) and (open-shell) „mono-orthogonal” diazatrimethylenemethanes 42, 43. Complete geometry optimisations were appropriately performed with the RHF/6-31 + G** and the UHF/6-31 + G** basis sets. Energies of the closed-shell states were calculated on the RHF, MP2, MP4SDTQ, and CCSD(T) levels, those of open-shell states on the UHF, UMP2, UMP4SDTQ, and CCSD(T)/UHF levels. – (E)-38, being lowest in energy of the parent iminoaziridines (Erel = 0.0), equilibrates with (Z)-38 via an almost linear transition state (Erel = 113.4 kJ mol−1) and decomposes into hydrogen isocyanide and formaldimine in a one-step cheletropic process [Erel = 179.8 kJ mol-1, CCSD(T)/RHF] with a highly unsymmetrical transition state in which the N1–C2 bond is almost completely broken whereas the C2–C3 bond is still strong. The same is true for the cheletropic decomposition of (Z)-38 the transition state of which (Erel = 168.2 kJ mol−1) is even lower by 11.6 kJ mol−1. – Energy-rich zwitterionic transition states are found with the RHF method among which 1A′-(E,Z)-41 is lowest in energy [Erel = 203.0 kJ mol−1, CCSDT(T)]. Planar singlet (= transition states of CN-bond rotation) and triplet diazatri-methylenemethane diradicals possess energies in the range of Erel = 140–160 and 100–120 kJ mol−1 [CCSD(T)], respectively. Complete UHF optimisation of the singlet structures without symmetry constraint yielded five minima 42, 43 of mono-orthogonal geometry with similar or slightly lower energies than the planar UHF singlets. In the transition state of the valence isomerisation of (E)-38 [Erel = 170 kJmol−1, CCSD(T)/UHF], the N1–C3 bond is elongated while the plane of the methylene group is still orthogonal to the C(2) = NH plane. An activation energy of (185 ± 20 kJmol−1) is estimated for ring opening of 38 involving species with high diradical character.

The very useful singlet-oxygen ene reaction of reluctant alkenes, e.g. 1, 5, 7, 8, and, in particular, the mono(allyl hydroperoxides) 10 and 11, is handicapped by the instability of the sensitiser TPP. A comparative study of TPP, TPFPP, and TDCPP in the singlet-oxygen ene reaction of these alkenes shows that 1) the persistence increases in the order TPP < TPFPP < TDCPP, rendering TDCPP the sensitiser of choice for the generation of singlet-oxygen in non-polar solvents, 2) the persistence of the tetraarylporphyrins decreases in the presence of an alkene, 3) this decrease strongly depends on the nature of the alkene, being most pronounced in the case of cyclohexene. These results are interpreted in terms of unknown substrate-derived species which induce oxidative destruction of the tetraarylporphyrins. Alternatively, abstraction of allylic hydrogen atoms from the alkenes by the excited sensitisers may give rise to the observed substrate-dependent photobleaching. – Because the singlet-oxygen ene reaction is the key step of a 1,2-carbonyl transposition with concomitant dehydrogenation, the scope and usefulness of this sequence are distinctly improved.

Askani's synthesis of the 1,5-dialkylsemibullvalenes 1a and c was optimised on a gram scale. The as yet unknown trimethylenesemibullvalene 1b was obtained according to the same protocol which also greatly improved a recent synthesis of barbaralane (2). Allylic brominations of the dienes 5 + 6 with N-bromosuccinimide yielded complex mixtures of 7–10. Depending on the substitution pattern at the apical carbon atoms C-1 and C-5 of the bicyclo[3.3.0]octadiene system, the intermediate allylic radicals are attacked preferentially either from the exo (R = Me) or from the endo face [R, R = (CH2)3, (CH2)4]. Reductive cyclisations of the dibromides 9 and exo,exo-15 were carried out with a sonicated suspension of an activated zinc-copper couple in tetraglyme. Sweeping of the volatile products from the reaction mixtures with nitrogen at elevated temperatures allowed the convenient isolation of the semibullvalenes 1 and barbaralane (2) in reasonable purities (92–97%) and yields (57–82%).

A systematic experimental search for substituted semibullvalenes yields systems which have nearly thermoneutral reactants and products (ΔH0 ≈ 0.5–1 kJ mol−1) separated by moderately low barrier ( ΔE 0 ǂ ≈ 10–30 kJ mol −1 ), e.g. 2,6-dicyanoethylmethylsemibullvalene DEM-SBV (=7c⇌7c′). Theoretical model simulations suggest that their Cope rearrangement may be controlled by two infrared picosecond (IR ps) laser pulses: The first pump pulse excites the reactants to the transition state, represented by a delocalized wave packet with energy above the potential barrier. The second dump pulse stabilizes the products.

Short and efficient syntheses of the barbaralanes 2e-h are reported. Meerwein's diketone 9 is converted into the bis(vinyl sulphide) 10 by the action of 4-chlorothiophenol in the presence of titanium(IV) chloride and triethylamine. Oxidation of 10 with sodium perborate in acetic acid affords the bis(vinyl sulphone) 11 which is treated with potassium cyanide supported on aluminium oxide to yield the unsaturated dinitrile 8. Bromination of 8 with N-bromosuccinimide and subsequent reductive cyclisation of the resulting exo,exo dibromodinitrile 13 with the zinc-copper couple furnish 3,7-dicyanobarbaralane (2e) in 53% overall yield based on 9. – Treatment of dinitrile 8 with hydrochloric acid in boiling methanol followed by hydrolysis of the intermediate Pinner salt yields the dimethyl dicarboxylate 14 which is brominated to afford the exo,exo dibromo diester 15. Reductive cyclisation of 15 gives rise to the formation of dimethyl 3,7-barbaralanedicarboxylate (2f) in 49% overall yield based on 9. – Bicyclo[3.3.1]nonane-3,7-dione (7) is converted into a mixture of the bis(vinyl sulphones) C2- and Cs-17 (1:1) as described for 9 11. Bromination of this mixture yields a single exo,exo dibromo compound 18 which is cyclised by the zinccopper couple to afford 3,7-bis(phenylsulphonyl)barbaralane (2g) in 56% overall yield based on 7. – The known phenyl-2-oxaadamantanol 21a is cleaved by the action of boron trifluoride and acetic anhydride to yield the bicyclo[3.3.1]none-none 22. The difluoroboron complex 23 is obtained under more rigorous conditions. Addition of phenylcerium(III) chloride to 22 followed by dehydration of the endo alcohol 24 yields the diphenyldiene 25. Bromination of 25 and subsequent reductive cyclisation of the exo,exo dibromodiphenyldiene 26 furnish 3,7-diphenylbarbaralane (2h) which is isolated by preparative reversed-phase chromatography in 50% overall yield based on 7. – The configurations and conformations in the crystals are elucidated by X-ray diffraction analyses for the bicyclo[3.3.1]nonane derivatives 7, 13, 15, 18, 22–24, and 26 and the barbaralanes 2e-g. In the solid state, 2e and f exist as pairs of rapidly rearranging nonequivalent valence tautomers while the atomic distances observed for 2g indicate the presence of a single nonrearranging valence tautomer.

The 2-halo imidoyl chlorides 7 are obtained from the amide 5 and the 2-halo amides 6 by the action of phosphorus pentachloride and thionyl chloride, respectively. Non-racemic (S)-6a is converted into 7a which is racemic, however. The reaction of Lawesson's reagent with 6a furnishes the diaste-reomeric 1,3,2-thiazaphospholidine derivatives 15. Treatment of (S)-6a (98% ee) with methyl triflate affords 2-chloro imidate 8 (95% ee) which reacts with methanamine in the presence of methanammonium chloride to yield the 2-chloro amidine (S)-9a (90% ee). The 2-halo imidoyl halides 7a and b react with methanamine to produce the 2-halo amidines 9a and b. — Strong bases, e.g. potassium tert-butoxide or sodium hydride in the presence of catalytic amounts of tert-butyl alcohol, eliminate hydrogen chloride or bromide from the 2-halo amidines 9a and b and (S)-9ato yield mixtures of the 2-iminoaziridines (E)- and (Z)-4, and (E,R)- and (Z,R)-4 (83% ee), respectively. The 1,3-elimination of hydrogen bromide from 9b is diastereoselective at -30 to -40° [(E)-4:(Z)-4 = 90]. The diastereomers equilibrate at 36° with (kEZ + kZE) = (5.92 ± 0.08) . 10−5 s−1 (K = kEZ/kZE = 0.428 ± 0.013). - The thermolysis of (E)- and (Z)-4 in [D6]benzene solution yields the imine 16 and methyl isocyanide (17). The decomposition follows the first-order rate law. The following Arrhenius and Eyring parameters are calculated from five rate constants obtained in the temperature range of 70–110°: Ea = (115.2 ± 0.4) kJmol−1, IgA = (12.06 ± 0.28), δH‡ = (112.1 ± 0.4) kJmol−1, δS‡ = (-23.9 ± 0.7) JK−1 mol−1, δG‡373K = 121 kJmol−1. The enantiomeric excess of the surviving fraction of (E,R)- and (Z,R)-4 is unchanged after two half-lives at 80°.

Cyclopropanediamines, 4. – Synthesis and 1H-NMR Spectra of Pure Diastereomers of 1,2-Cyclopropanediamines and 1,2-Cyclopropanediammonium Dibromides The efficient preparation of pure diastereomers of N,N′-(1,2-cyclopropanediyl)diurethanes 16–18 and -diammonium dibromides 4, 19, and 20 is reported. Curtius rearrangement of cis- and trans-1,2-cyclopropandicarboxylic dihydrazides 14 yields di(O-benzyl) diurethanes 16 on a 10-100 g scale. The cis-1,2-cyclopropanedicarboxylic dihydrazides cis-14 tend to form bicyclic products. Thus, from cis-14a a small amount of 22a is obtained besides the diurethane cis-16a. Cyclisation is the main reaction in the case of cis-1,2-dimethyl-1,2-cyclopropanedicarboxylic hydrazide (cis-14e) which affords the bicyclic products 22e and 24, but no diurethane. The 1,2-dimethyl-1,2-cyclopropanedicarboxylic acid (27) reacts with diphenyl phosphorazidate to produce the monobenzyl ester 29. The diurethanes 16 are methylated ( 17) and benzylated ( 18) at both nitrogen atoms. The benzyloxycarbonyl group is removed from the diurethanes 16–18 by the action of hydrogen bromide in acetic acid to yield the 1,2-cyclopropanediammonium dibromides 4, 19, and 20. The 1H-NMR spectra are analysed and the results are employed as criteria for the configurations.

Addition of trimethylsilyl cyanide to the diphenylbicyclo[3.3.0]octanedione 8c is catalyzed by the potassium cyanide/18-crown-6 complex and produces a mixture of the diastereomeric bis[O-(trimethylsilyl)cyanohydrins] endo- and exo-10c (3:2). The hydrogen fluoride—pyridine complex in phosphorus oxychloride as solvent and, subsequently, an excess of pyridine convert the mixture of diastereomers 10c into the unsaturated y,y′-diphenyldinitrile 11. This is converted into the red semibullvalene 4 in a single step by treatment with hexachloroethane and concentrated aqueous sodium hydroxide in the presence of tetrabutylammonium hydroxide as phase-transfer catalyst — Above 30°C, 4 isomerizes in solution to a mixture of the cyclooctatetraenes 12 and 13. While the red crystals of 4 are stable in the atmosphere, in the dark affording the endoperoxide 16 and the yellow 2H-pyran derivative 17 (7:1) which result from parallel reactions. Only at temperatures as high s 110°C, 16 slowly rearranges to 17 which reacts further to yield well-defined but still unknown products. The simultaneous formation of 16 and 17 at low temperatures is interpreted in terms of endo and exo attack, respectively, of triplet oxygen at the benzylic carbon atoms of the semibullvalene 4 or the isomeric bicyclic diradical 21. — The structures of the new compounds are based on spectroscopic evidence and X-ray diffraction analyses of 4, endo-10c, 11, 16, and 17. The semibullvalene 4 exists as a pair of rapidly rearranging degenerate valence tautomers in solution and in the crystal as well. In the solid state, 4 exhibits apparent C2 symmetry and equal atomic distances C2–C8 and C4–C6 (201.9 pm). Because true degeneracy is highly unlikely in the crystal, the equal distribution of two non-equivalent valence tautomers at room termperature results from a fortuitous cancelling of the ΔH° and TΔS° terms governing the equilibrium. — The red colour of 4 in the crystal and in solution is due to a maximum at 444 nm which disappears on cooling. Thus, 4 not only belongs to the family of thermochromic semibullvalenes and barbaralanes devoid of a long wavelength chromophor like 1, 6 and 14, but also shows the most intensive maximum at the longest wavelength observed so far.

Conformation and Bromination of Bicyclo[3.3.1]nonane-2,6-dione and Dehydrobromination of the Bromo Ketones. – Improved Synthesis of Bicyclo[3.3.1]nona-3,7-diene-2,6-dione Bromination by copper(II) bromide of bicyclo[3.3.1]nonane-2,6-dione (1) affords the epimeric α-bromo diketones exo-and endo-5 and, eventually, the α, α′-dibromo diketones exo- and endo-7 (3:7) in high yield. Dehydrobromination of the mixtures of bromo diketones by calcium carbonate in dimethylacetamide yields the unsaturated diketones 6 (30%) and 2 (72%), respectively. Bromination of 1 by molecular bromine is less selective and furnishes mixtures of brominated diketones consisting of exo- and endo-7, 8 and the tetrabromo diketone 10. An excess of bromine yields pure 10. – The ethylene acetal 12 is brominated by pyridinium perbromide to produce the α,α′-dibromo acetal 13 which is very reluctant towards 1,2-dehydrobromination by potassium tert-butoxide. Thus, with an excess of potassium tert-butoxide in tetrahydrofuran, 1,2-dehydrobromination is achieved only on one side to afford 14. In contrast, potassium tert-butoxide in dimethyl sulphoxide induces a transannular dehydrobromination leading to the diacetal 15 of 3-bromonoradamantane-2,6-dione. – The conformations of 1,10, and 13 in the solid state are determined by X-ray diffraction analyses. From torsional angles, vicinal proton coupling constants are calculated with the aid of the Karplus equation and compared to those determined from solutions by high-field NMR spectroscopy. The reasonable agreement provides the basis for the assessment of the configuration and conformation for exo- and endo-7.

Gas-Phase Thermolysis of Pyrazolines, 3[1]. – Electronic Structure and Gas-Phase Pyrolysis of 4-Substituted 3,5,5-Trialkyl-3,5-dihydro-4H-1,2,3-triazoles Studied by Photoelectron Spectroscopy and Semiempirical Calculations The PE spectra of the 3,5-dihydro-4H-1,2,3-triazoles 1–3 have been recorded. The ionization potentials have been assigned to molecular orbitals on the basis of MNDO, AM1, and PM3 calculations. The most important occupied MOs are characterized as πNNN, πC=X (X = O, NCH3, CH2), nN-3, nNN+, nNN– and nX. The gas-phase thermolyses of 1–3 have been studied by PE-controlled real-time gas analysis. After extrusion of molecular nitrogen, the remaining reactive species cyclize to three-membered rings. At higher temperatures and in flash vacuum pyrolysis, subsequent reactions lead to smaller acyclic compounds.

Synthesis and Structure of Phenyl-Substituted Bicyclo[3.3.1]nonenes Addition of one mole of phenyllithium to bicyclo[3.3.1]nonane-2,6-dione (5) affords the hydroxy ketone 6 besides small amounts of the known diol 7. Dehydration of this mixture by sulfuric acid in acetic acid yields the unsaturated ketone 8 and the diene 1 which are separated by chromatography. Wolff-Kishner reduction of 8 furnished the hydrocarbon 2. In the same way, the known ketones exo- and endo-10 are converted into 4. The preferential formation of the less stable endo-10 is interpreted in terms of the relative stability of the conformations required in the intramolecular aldol reaction of diketone 9. The conformations of 4, 8, exo- and endo-10 are determined by X-ray diffraction analyses. Vicinal proton coupling constants are calculated from torsional angles and compared to those determined in solutions by high-field NMR spectroscopy.

2,4,6,8-Tetraphenylbarbaralane – an Orange-Red, Thermochromic Hydrocarbon Devoid of a Chromophore[1,2] The diphenylbicyclo[3.3.1]nonanedione 6 adds phenylcerium-(III) dichloride to afford a high yield of the triphenyl-2-oxa-twistanol 8. In contrast, phenyllithium yields a mixture of tri-phenylhydroxyketone exo-7, tetraphenylbicyclo[3.3.1]nona-nediol 9, and 8. The latter is dehydrated by sulfuric acid in acetic acid to produce the triphenylbicyclo[3.3.1]nonenone 10 in almost quantitative yield. Addition of phenylcerium(III) dichloride to 10 affords a 3:2 mixture of the tetraphenylbicy-clo[3.3.1]nonenols exo- and endo-11 which may be separated by chromatography. Dehydration of the mixture yields quantitatively the tetraphenyldiene 12. On treatment with an excess of butylpotassium in pentane, 12 is converted to the deep violet, crystalline dipotassium salt 17 which may be purified by reprecipitation from its tetrahydrofuran solution with pentane. When the solution of 17 in tetrahydrofuran is slowly added to an excess of 1,2-dibromoethane at –60°C, the dianion is immediately oxidized to produce the tetraphenylbarbara-lane 4 which is isolated in 30% yield as orange-red crystals after chromatography. – The structures of the new compounds are based on spectroscopic evidence and X-ray diffraction analyses of 4, 8, exo-11, and 12. The conformations in solution are inferred on the basis of vicinal proton coupling constants and a comparison with coupling constants calculated with the aid of the Karplus equation from torsional angles obtained by X-ray diffraction analyses. The conformation of exo-11 in solution closely resembles that present in the crystal. – While the barbaralane 4 exists as a pair of very rapidly rearranging degenerate valence tautomers in solution, the degeneracy is lifted in the crystal lattice. As a result, the crystal consists of two rapidly rearranging but non-equivalent valence tautomers in a ratio of 9:1 as estimated from the apparent atomic distance C2–C8 of 4 and the C2–C8 bond length of the model barbaralane 18. – The orange-red colour of 4 in the crystal and in solution results from a pronounced shoulder in the UV/Vis spectrum at 430 nm, the intensity of which strongly depends on the temperature. Thus, 4 is the first barbaralane which exhibits colour though it is lacking a classical long-wavelength chromophore.

Synthesis, NMR Spectra, and Photoelectron Spectra of Cyclic Ketene N, X1-Acetals (2-Alkylidene-N-heterocycles) Deprotonation by sodium hydride of the N1-methyl-2-alkyl-azolium salts 10c, 14–16 and 18 and of the perimidinium salt 20 as well affords the corresponding ketene N,X1-acetals 1c, 3–5, 7, and 8 as air-sensitive distillable oils or low-melting crystals. The α1-carbon atoms of cyclic ketene N,X1-acetals resonate at very high field with the smallest chemical shift of no more than δ39 for the 5-methylenedihydrotetrazol 2a. Geometries of 1a, b, 2a, and b were optimized on the Hartree-Fock level with polarized double-ζ basis sets, followed by IGLO calculations of the NMR chemical shifts. These are in excellent agreement with the unusual carbon-13 shifts observed. The electronic structures of 1a, b, 2a, b, 3a, b, 4b, 5b, 6a, and b are investigated by photoelectron spectroscopy and semi-empirical quantum chemical calculations. The observed ionization potentials are assigned to molecular orbitals on the basis of Koopmans' theorem. The order of reactivity in 1,3-dipolar cycloaddition reactions of cyclic ketene N,X1-acetals resembles that of the chemical shifts of their α1-carbon atoms.