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We define ΔGGA as the free energy change for the formal equilibrium: [13]G-H + 1-X-adamantane → [13]G-X + adamantane, where [13]G-H is the C13H22 fragment of all-trans graphane with 3-fold symmetry. This compares with a situation where the group X is equatorial to three cyclohexane rings with one where it is axial to three rings. ΔGGA values vary from 2.9 (CN) to 145.7 kJ mol–1 (CCl3), and this wide range means that ΔG can be calculated with confidence. ΔGGA values for Me, Et, i-Pr, and t-Bu form a regular series, 34.9, 63.3, 101.6, and 142.0, and clearly reflect the steric size of the groups. We propose a model where the six axial hydrogens surrounding X on [13]G-X provide a nearly circular constriction on the substituent close to its point of attachment but which does not extend far above this. We compare these results with A values and with calculations on 2- and 7-substituted [1(2,3)4]pentamantanes. We show that electronic effects on ΔGGA values are negligible but that they correlate well with computed cone and solid angles subtended by the substituent. more...

The synthetically versatile pinacol boronic ester group (Bpin) is generally thought of as a bulky moiety because of the two adjacent quaternary sp3 -hydribized carbon atoms in its diol backbone. However, recent diastereoselective reactions reported in the literature have cast doubt on this perception. Reported herein is a detailed experimental and computational analysis of Bpin and structurally related boronic esters which allows determination of three different steric parameters for the Bpin group: the A-value, ligand cone angle, and percent buried volume. All three parameters suggest that the Bpin moiety is remarkably small, with the planarity of the oxygen-boron-oxygen motif playing an important role in minimising steric interactions. Of the three steric parameters, percent buried volume provides the best correlation between steric size and diastereoselectivity in a Diels-Alder reaction. more...

We define Δ

Overall inversion in fused cyclohexane oligomers 2, 3, and 4 (all based on cis-decalin 1) occurs by a rolling process involving no more than two adjacent rings in twist-boat conformations at any time., Overall inversion in fused cyclohexane oligomers 2, 3, and 4 (all based on cis-decalin 1) occurs by a rolling process involving no more than two adjacent rings in twist-boat conformations at any time. These inverting rings move along the oligomer in processes that are precisely choreographed by the adjacent chairs. Actual inversion mechanisms can be stepwise [CC → TC → TT → C′T → C′C′], as for cis-decalin, but it is shown that a concerted alternative [CC → TC → C′T → C′C′] is enforced in 2. The all-cis,anti,cis-isomers of perhydrohelicenes 4 are based on the diamond lattice and have remarkably low strain energies. Helix inversion in 4 is compared with that in helicenes 5. For both, the intermediates and transition states have shapes broadly like kinked old-style telephone cables. In both cases barriers increase with the length of the system to eventually reach a plateau value of ca. 120 kJ mol–1 for 4, much lower than that for 5 (320–350 kJ mol–1). While rolling inversion only requires two adjacent rings in twist-boat conformations at any instant, inversion in propellane 6 requires all three rings be converted to twist-boats, and the S 4 symmetric hydrocarbon 7 requires all four rings to be converted to twist-boats. As a consequence, 7 probably has the highest barrier of any non-oligomeric cis-decalin derived structure (87.3 kJ mol–1 at B3LYP/6-31G*). more...

This paper uses DFT and G3(MP2) calculations to examine whether unbridged 10-membered rings can be made by (pi)6 + (pi)4 cycloadditions to (Z)- and (E)-hexatrienes, hexa-1,5-dien-3-ynes, (Z)-hexa-1,3-dien-5-ynes, hexa-1,2,3,5-tetraenes, and (Z)-hexa-3-ene-1,5-diynes. Cycloadditions to four 4pi reactants, buta-1,3-diene, butenyne, butatriene, and butadiyne, are explored. Thirty different basic cycloadditions are identified, and all are shown to be exothermic according to G3(MP2) calculations; strain energies in the products are comparable with that of cyclodecane itself, despite the presence of trans-alkene, alkyne, allene, cumulene, and s-trans diene moieties. The major obstacles to the isolation of 6 + 4 cycloaddition products are competing (pi)4 + (pi)2 cycloadditions and, especially, rapid Cope rearrangement of the products, but, in many cases, the judicious introduction of substituents can overcome these problems so that practical syntheses should be possible. Reactions between (E)-hexa-1,3,5-triene and s-trans-buta-1,3-diene are shown to have substantially lower activation energies than those involving (Z)-hexa-1,3,5-triene reacting with either s-cis- or s-trans-buta-1,3-diene. Conformationally locked derivatives of s-cis,s-cis (E)-hexa-1,3,5-trienes can lead to derivatives of (Z,Z,E)-cyclodeca-1,3,7-triene that are stable to Cope rearrangement, and reactions should proceed at close to ambient temperatures with suitable activating groups. We predict that it should be possible to prepare suitably substituted derivatives of at least 11 more highly unsaturated ring systems: (5Z,7Z)-cyclodeca-1,2,5,7-tetraene, (1Z,3Z)-cyclodeca-1,3-dien-7-yne, (2Z,7E)-cyclodeca-1,2,3,7-tetraene, (Z)-cyclodeca-1,2,3-trien-7-yne, (4Z,8E)-cyclodeca-1,2,4,8-tetraene, (Z)-cyclodeca-1,2,4,5,7-pentaene, (Z)-cyclodeca-1,2,4-trien-8-yne, (1Z,7E)-cyclodeca-1,7-dien-3-yne, (R,S,E)-cyclodeca-1,2,4,5,8-pentaene, cyclodeca-1,2,4,5,8,9-hexaene, and (R,S)-cyclodeca-1,2,4,5-tetraen-8-yne. In three other cases, we predict that cycloaddition will be followed by unusual and intriguing rearrangements. Cycloadditions can be accelerated by the presence of electron-withdrawing groups in either the 6pi or 4pi reactants. Transannular cyclizations of some products may lead to interesting stereocontrolled routes to 6,6- and/or 5,7-bicyclic structures. more...

Crisscross cycloaddition of citronellal azine (6) with 2 equiv. of TFA and powdered 3 Å molecular sieves in CH2Cl2 at reflux for 22 h afforded 37% of the desired C2-symmetric hydrazine 7 and 5%–10% of diastereomer 8 in which one of the 6–5 ring fusions is cis. Methylation of the hydrazine of 7 and reduction of the resulting salt (9) with Li in NH3 cleaved the N—N bond to give secondary tertiary amine 10 in 97% yield. Eschweiler–Clarke methylation afforded the C2-symmetric bis tertiary amine 11 in 69% yield. Racemic products were obtained in initial attempts at asymmetric catalysis using 7 or 11 as asymmetric bases, using bistertiary amine 11 as a ligand analogous to sparteine for alkyllithiums, or using the lithium amide from secondary tertiary amine 10 as an asymmetric base. Apparently, the proton is buried in the core of 11, leaving a hydrophobic surface; the free counterion is not an asymmetric catalyst. Diamine 11 may be too hindered to complex to s-BuLi. Tertiary amine 11 (pKa1 = 24.7) is more basic than DBU (pKa = 24.3) in CH3CN, in good agreement with theory.Key words: crisscross cycloaddition, azine, dipolar cycloaddition, calculation of pKa. more...

A set of C2-chiral diamines 18-21 based on 1,6-diazacyclodecane have been identified whose conjugate acids are predicted by B3LYP/6-31G calculations to have pKa values of approximately 23-6 on the water scale (pKa = 30-33 in MeCN); they are also expected to be kinetically active, but essentially nonnucleophilic. Strain relief on protonation largely determines the basicity of these compounds, and the key to the design of stronger bases is limiting conformational freedom, especially by preventing nitrogen inversion, through the introduction of additional ring fusions. 15,16-Dimethyl-15,16-diazatricyclo[9.3.1.1(4,8)]hexadecane (20) is examined in detail and shown to exist in 10 diastereomeric forms as a result of in-/out-isomerism. The predicted pKa values for these diastereomers range over 14 log units. more...

Bisher wurde kein eindeutiger Nachweis fur eine einfache, nicht katalysierte Dimerisierung von Diaminocarbenen gefunden. Es ist somit an der Zeit zu hinterfragen, welche Faktoren die Thermodynamik dieser Reaktion steuern und welche Mechanismen fur die beobachteten Dimerisierungen verantwortlich sind. In Ubereinstimmung mit qualitativen experimentellen Beobachtungen ergab die Berechnung, dass die Dimerisierung von einfachen Funf- und Sechsringdiaminocarbenen um 100 kJ mol−1 weniger begunstigt ist als die der acyclischen Pendants. Dieser grose Unterschied kann halbquantitativ durch die Anderung von Bindungs- und Torsionswinkeln um die Carbenzentren herum erklart werden. Carbene wie (Et2N)2C sind, in Einklang mit den berechneten Energiebarrieren, in THF bei 25 °C kinetisch stabil, aber sie dimerisieren schnell in Gegenwart des entsprechenden Formamidiniumions. Dieser protonenkatalysierte Prozess ist wahrscheinlich der haufigste Mechanismus der Dimerbildung. Er fuhrt zur Bildung von C-protonierten Dimeren, was in geeigneten Fallen beobachtet werden kann. Es wird auch die Moglichkeit der durch Alkalimetalle geforderten Dimerisierung diskutiert; hierfur werden einige Belege vorgestellt. more...

Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high activation energies (/=350 kJ mol(-1) from azulene) and so can only be competitive at temperatures above 1000 degrees C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a bicyclobutane intermediate is 382 kJ mol(-1). The norcaradiene-vinylidene mechanism that has been proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash thermolysis of azulene has an activation energy of 360 kJ mol(-1); subtle features of the B3LYP/6-31G(d) energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol(-1) above that of the 1-H-azulyl radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals is 7.4 kJ mol(-1) lower. Partial equilibration of the intermediates in the spiran pathway via this shift may therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer of a methylene group from one ring to the other and lies 182.3 kJ mol(-1) above that of the 1-H-azulyl radical. It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can account semiquantitatively for all the products from 1-(13)C-azulene, 9-(13)C-azulene, and 4,7-(13)C(2)-azulene, in addition to accounting for the products from methylazulenes, and the formation of naphthalene-d(0) and -d(2) from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an alternative explanation for the formation of 1-phenyl-1-buten-3-ynes. more...

The reactions of hydrazine hydrate with methyl acrylate and ethyl acrylate do not result in the formation of 1,5-diazabicyclo[3.3.0]octane-2,6-dione 1 at reaction temperatures below 200 °C; 1,2-bis(alkoxycarbonylethyl)pyrazolidin-3-one 8, 1-(alkoxycarbonylethyl)pyrazolidin-3-one 6, and 2-(alkoxycarbonylethyl)pyrazolidin-3-one 11 were isolated and identified. At 260–280 °C, however, 1 could be isolated in 20–28% yield; formation of 1 could also be observed by spectroscopic measurements when the reaction was carried out above 200 °C. It is concluded that the major pathway involves initial Michael addition to give a 1,1-disubstituted hydrazine that cannot yield 1 without undergoing unfavourable retro-Michael reactions. The apparently similar reaction of hydrazine with excess diethyl glutaconate to give 4,8-bis(ethoxycarbonylmethyl)-1,5-diazabicyclo[3.3.0]octane-2,6-dione 2 takes a quite different course. Initial Michael addition gives a 1,2-disubstituted adduct for steric reasons. Although double cyclisation of this adduct to 2 appears straightforward, retro-Michael reaction of the monocyclised adduct is very rapid and competes with the second cyclisation. The reaction of ethyl methacrylate and hydrazine hydrate at 80 °C results in a complex reaction mixture, which is transformed into 4-methylpyrazolidin-3-one 13 at 390 °C under reduced pressure. more...

Methods are described for the preparation of a range of N,N,N′,N′-tetraalkylformamidinium and N,N-dialkyliminium ions 4–20 as anhydrous salts for use as precursors to diamino- and amino-carbenes. These methods include a novel method involving nucleophilic addition to formamides followed by trapping with electrophiles such as triflic anhydride, various methods of formamide activation, exchange reactions involving orthoesters and the transamination of amidinium salts, and alkylation methods. more...

Propellane diphosphonium dications undergo addition reactions with a range of nucleophiles, and the products often have in,out-geometry with some P–P interaction, as shown by 1JPP values ranging from 46–253 Hz. X-Ray structures of 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.4]tetradecane trifluoromethanesulfonate, 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, and 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate are reported; these show that there is no simple relationship between P–P distance and 1JPP values, although the latter do correlate with the apicophilicity of the attached group. Hydride adducts, although formally protonated phosphines, can react as hydride sources, and salts of the 6-phospha-1-phosphoniabicyclo[4.4.3]tridecane ion, while stable in solution, undergo ring opening to the 1-propyl-1-phosphonia-6-phosphabicyclo[4.4.0]decane ion in the solid state. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.4.4.01,6]tetradecane ion leads, via a non-isolable hydroxide adduct, to out,in-1λ5,6-diphosphabicyclo[4.4.4]tetradecane 1-oxide. Addition to the 1,6-diphosphoniatricyclo[4.4.3.01,6]tetradecane dication leads to isolable 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, converted by excess hydroxide to the diphosphine monoxide which exists as a dynamic mixture of out,in- and out,out-isomers, the equilibrium being dependent on solvent polarity, the water content of the solvent, and the presence of metal ions. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.3.3.01,6]tetradecane dication leads to an out,in-hydroxide adduct which is rapidly converted to the out,out-monoprotonated phosphine oxide. more...

The basicity order in acetonitrile for a series of phosphines is: 1,5-diphosphabicyclo[3.3.3]undecane 1 (pKa for the protonated ion ∼ 17.9)

Barriers to rotation about the Ccarbene−N bonds in diaminocarbene (H2N)2C 1a, bis(dimethylamino)carbene ((CH3)2N)2C 1b, the related formamidinium ions (H2N)2CH+ 2a and ((CH3)2N)2CH+ 2b, and the Li+ complexes (H2N)2CLi+ 3a and ((CH3)2N)2CLi+ 3b have been calculated using density-functional theory in order to study the extent of π-bond stabilization of the carbene center. Experimental barriers from DNMR are reported for 1b and 2b and compared with those for bis(diisopropylamino)carbene 1c and the N,N,N‘,N‘-tetraisopropylformamidinium ion 2c; rotational barriers computed for 1b and 2b including thermal corrections compare well with experiment. The dimerization of 1a and 1b have been studied with (full) geometry optimization up to the levels QCISD(T)/cc-pVDZ//MP2/cc-pVDZ and B3LYP/cc-pVDZ//B3LYP/cc-pVDZ, respectively. The minimum-energy path for the dimerization of 1a has been computed using the BPW91/cc-pVDZ method. It is shown that the transition state geometries for the dimerizations of 1a and 1b have C2 a... more...

The molecular structure of 3,3-diethylpentane (tetraethylmethane) in the gas phase has been determined by electron diffraction and ab initio calculations at the MP2/6-31G* level. Five local minima on the potential energy surface were located, but only two have significant populations at room temperature. The experimental distribution of conformers was found to be 66(2)% with D2d symmetry and 34(2)% with S4 symmetry, corresponding to an energy difference ΔH° in favor of the D2d form of 3.3(2) kJ mol-1. The molecule shows significant distortion from regular tetrahedral coordination at the central carbon atom, with two CCC angles in the D2d form reduced to 106.7(8)° and two angles in the S4 form increased to 110.9(4)°. These distortions are attributed to asymmetry of the electron density distribution around the CH2 groups. more...

The structure of the monoprotonated ion, 2, of 5-methyl-1,5,9-triazabicyclo[7.3.1]tridecane, 1, as a picrate shows an NMeH+ group engaged in a transannular hydrogen bond with one of the aminal nitrogen atoms and this, in turn, induces a small but measurable structural anomeric effect in the aminal functional group. Diprotonation of 1 in CF3CO2H yields an equilibrium mixture of two isomeric bicyclic dications which are interconverted via a monocyclic ion containing a H2CN+ group and further protonation by CF3SO3H traps the iminium ion as a monocyclic trication. The structure of the monoprotonated ion, 4, of 1,4,8,11-tetraazatricyclo[9.3.1.14,8]hexadecane, 3, as a perchlorate shows the cation with effective C2 symmetry, a nearly linear, and almost symmetrical, transannular hydrogen bond, and a more pronounced structural anomeric effect in both hexahydropyrimidine units. Diprotonation of 3 gives a dication, 5, and the structure of the bis(trifluoroacetate) shows measurable shortening of the aminal C–NR2 bond in spite of the fact that the lone pair and the C–N+ bond are close to gauche. The structural effects in 2, 4, and 5 are well-reproduced by Becke–Perdew density functional theory calculations, and a natural bond orbital analysis of the parent system H3N–H+· · ·NH2CH2NH2, based on SCF and MP2 ab initio calculations is described. more...

Medium-ring diphosphines offer unique opportunities to study the interactions between phosphorus atoms. In addition, monocyclic medium-ring diphosphines may be very useful ligands in transition metal complexes. A number of useful synthetic routes to these compounds have been developed, but further efforts in this direction might be rewarding. more...

The conformational effects of the quaternary centre in (RCH2CH2)4Xq species are studied by molecular mechanics calculations on hydrocarbons, ammonium ions, and related species, and the results verified by data on quaternary ammonium ions from the Cambridge Structural Database. Prn4C and Prn4N+ have just two low-energy structures, with D2d or S4 symmetry. All other conformations suffer from g+g– non-bonding interactions and will be populated to the extent of more...

A series of 1,k + 2-diphosphabicyclo[k.l.0]alkanes 2b–e are prepared by BunLi-promoted cyclisation of 1,ω-diphosphinoalkanes, followed by alkylation and cycloalkylation with 1,ω-dihaloalkanes. These compounds appear to be exclusively cis-isomers except 1,6-diphosphabicyclo[5.4.0]undecane 2e which is a 3:1 cis/trans mixture. Mono-quaternisation of cis-1,k + 2-diphosphabicyclo[k.l.0]alkanes, followed by treatment of the mono-quaternary salts with alkyllithium or Grignard reagents produces cis-1,n-disubstituted-1,n-diphosphacycloalkanes 4 exclusively; examples containing 8-, 9- and 10-membered rings and a range of substituents on phosphorus are described. Di-quaternisation of 1,k + 2-diphosphabicyclo[k.l.0]alkanes, followed by hydrolysis, yields the trans-isomers of 1,n-disubstituted-1,n-diphosphacycloalkane monooxides 6 exclusively; reduction of these with LiAlH4 in benzene gives largely trans-1,n-disubstituted-1,n-diphosphacycloalkanes 7, but is not completely stereoselective. The structure and properties of these diphosphacycloalkanes are discussed. He(I) photoelectron spectra of 1,5-diphosphabicyclo[3.3.0]octane, 1,6-diphosphabicyclo[4.3.0]nonane and 1,6-diphosphabicyclo[4.4.0]decane show little evidence of interaction between phosphorus lone pairs, unlike the corresponding hydrazines. The medium-ring diphosphacycloalkanes, 1,5-dimethyl-1,5-diphosphacyclooctane and 1,6-dimethyl-1,6-diphosphacyclodecane also show little evidence of interaction between phosphorus lone pairs. more...

Several propellane diphosphonium salts 8 are prepared by the reaction of 1,k + 2-diphosphabicyclo[k.l.0]alkanes 6 with α,ω-alkanediol bis-triflates [bis(trifluoromethanesulfonates)]; their properties are strongly dependent on ring size. 1,6-Diphosphoniatricyclo[4.4.4.0]tetradecane bis-triflate 8d is stable in acidic aqueous solution, and reacts with nucleophiles [X = F–, MeO–, H– (from BH4–) and R– (from Grignard and alkyllithium reagents)] to give products 13–18 with partial X–P–P+ bonding, hydroxide ion gives the diphosphine monooxide 12b but even this may retain some P–P bonding. However 1,5-diphosphoniatricyclo[3.3.3.0]undecane bis-triflate 8a is hydrolysed irreversibly and much more rapidly than its [4.4.4.0] counterpart. 1,6-Diphosphoniatricyclo[4.4.3.0]tridecane bis-triflate 8c reacts with NaBH4 to give a hydride adduct 15a which is deprotonated by BunLi to give 1,6-diphosphabicyclo[4.4.3]tridecane 9c. However several attempts to prepare 1,6-diphosphabicyclo[4.4.4]tetradecane 9d led to a deep-seated rearrangement to give 1,4-bis(1-phospholan-1-yl)butane. These included reaction of hydride adduct 15b with BunLi, and debenzylations of 1-benzyl-1-phosphonia-6-phosphabicyclo[4.4.4]tetradecane trifluoromethanesulfonate 16b and 1,6-dibenzyl-1,6-diphosphoniabicyclo[4.4.4]tetradecane bromide triflate 21 with LiAlH4. Reaction of cis-1,5-dibenzyl-1,5-diphosphacyclooctane with CH2(CH2OTf)2 gives 1,5-dibenzyl-1,5-diphosphoniabicyclo[3.3.3]undecane bis-triflate which is debenzylated with LiAlH4 to 1,5-diphosphabicyclo[3.3.3]undecane 9a. Attempts to prepare 1,6-diphosphabicyclo[4.4.4]tetradecane by related methods lead to oligomerisation reactions. The structure of 1,5-diphosphabicyclo[3.3.3]undecane is reported; its He(I) photoelectron spectrum shows two well separated bands at 7.58 and 8.14 eV, and RHF/6-31G* ab initio calculations indicating that this is due to through-bond interactions. more...

Attempts to reproduce the reported preparation of 1,5-diborabicyclo [3.3.3] undecane 1 through hydroboration of triallylborane with triethylamine–borane, followed by thermolysis, have led to 1,5-dipropyl-1,5-diborocane 2 and other products, but the presence of 1 could not be detected by GC–MS. Treatment of triallylborane with monobromoborane, methanolysis and subsequent thermolysis gave 1,5-dimethoxy-1,5-diborocane 3 which could be converted to 2 by treatment with propylmagnesium bromide. more...

We make the case for benzo[c]quinolin-6-ylidene (1) as a strongly electron-donating carbene ligand. The facile synthesis of 6-trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate (2) gives straightforward access to a useful precursor for oxidative addition to low-valent metals, to yield the desired carbene complexes. This concept has been achieved in the case of [Mn(benzo[c]quinolin-6-ylidene)(CO)5](+) (15) and [Pd(benzo[c]quinolin-6-ylidene)(PPh3)2(L)](2+) L = THF (21), OTf (22) or pyridine (23). Attempts to coordinate to nickel result in coupling products from two carbene precursor fragments. The CO IR-stretching-frequency data for the manganese compound suggests benzo[c]quinolin-6-ylidene is at least as strong a donor as any heteroatom-stabilised carbene ligand reported. more...

Transketalization reactions between the diketals of 1,4-cyclohexanedione and the diketonides of pentaerythritol yield a spiropolymer which is insoluble in all solvents tried, but which can be hydrolysed to 1,4-cyclohexanedione and pentaerythritol. At short reaction times, oligomers such as 3,3,21,21-tetramethyl-1,5,10,14,19,23,26,29-octaoxapentaspiro[5.2.2.2.2.5.2.2.2.2.]untriacontane can be isolated. However, all attempts to achieve a genuine equilibrium between the monomers, oligomers and polymer were fruitless. The 13 C cross-polarization/magic angle spinning (CP/MAS) n.m.r. spectrum of the polymer shows extra lines relative to those of soluble model compounds; this is probably due to a slow ring inversion of the 1,3-dioxane rings in the solid polymer. more...

A synthetic strategy for the preparation of chains of 1,2:4,5-fused cyclohexa-1,4-diene rings, both linear and cyclic (beltenes), based on the repetitive Diets-Alder cycloaddition of p-benzoquinone with a tetramethylene-ethane synthon (3,4-dimethylenetetrahydrothiophene 1,1-dioxide) is described. This route has been developed to yield 5,1 1-dihydroxy-4,6,7,9,10,12-hexahydro-1H,3H-anthracene[2,3-c:6,7-c′]dithiophene 2,2,8,8-tetraoxide 16, a potential precursor of a tricyclic doubly-exocyclic diene, and 7,10-bis(tert-butyldimethylsilyloxy)-1,4,5,6,11,12-hexahydronaphthacene-1,4-dione 22, a potential precursor for the double dienophile 5,6,11,12-tetrahydronaphthacene-1,4,7,10-diquinone, but the generation of these Diels–Alder components and their combination to form a [9]-beltene derivative has not been accomplished. The synthesis of linear derivatives has proceeded as far as a protected heptacyclic derivative, 7,16-diacetoxy-1,4,10,13-tetrakis(tert-butyldimethylsilyloxy)5,6,8,9,14,15,17,18-octahydroheptacene 25. The 1H NMR spectrum of this compound is unexpectedly temperature dependent. more...

Hexahydropyrimidines N,N-bridged by a chain of n methylene groups (1,(n+2)-diazabicyclo[n.3.1]alkanes) adopt out,out (axial,axial) structures for n=2, 3, and 4. When n=5, the photoelectron spectrum shows evidence of the presence of some of the out, in (axial,equatorial) isomer in the gas phase, although none can be found in solution. When n=6, the compound is apparently entirely out,in in the gas phase but exists as a mixture of out,out and out,in conformers in solution. For n=1, only the diamond lattice out,in isomer can be detected in solution. These experimental data are correlated with force field (MM2) calculations; multiple minimum search methods have been used to locate all low-energy conformations more...

pK(a) values of 19.8-28.2 are reported for the conjugate acids of a large series of NHCs in water. The effects of ring size, N-substituent and C(4)-C(5) saturation on pK(a) are discussed.

1,5,9-Triazacyclododecane and related macrocyclic triamines can be conveniently constructed around a single carbon atom as template; this route permits the preparation of selectively alkylated derivatives. more...

Eschweiler-Clarke methylations of both acyclic and cyclic polyamines can lead to methylation products of fragments of the original polyamine; thus 1,5,9,13-tetra-azatridecane yields 1,1,3,3-tetramethylpropanediamine exclusively and 1,5,9-triazacyclododecane gives 45% 1,5,9-trimethyl-1,5,9-triazacyclodecane and 45% 2,6,10-trimethyl-2,6,10-triazaundecane. more...

A synthetic strategy for the preparation of chains of 1,2:4,5-fused cyclohexa-1,4-diene rings, both linear and cyclic (beltenes), based on the repetitive Diets-Alder cycloaddition of p-benzoquinone with a tetramethylene-ethane synthon (3,4-dimethylenetetrahydrothiophene 1,1-dioxide) is described. This route has been developed to yield 5,1 1-dihydroxy-4,6,7,9,10,12-hexahydro-1H,3H-anthracene[2,3-c:6,7-c′]dithiophene 2,2,8,8-tetraoxide 16, a potential precursor of a tricyclic doubly-exocyclic diene, and 7,10-bis(tert-butyldimethylsilyloxy)-1,4,5,6,11,12-hexahydronaphthacene-1,4-dione 22, a potential precursor for the double dienophile 5,6,11,12-tetrahydronaphthacene-1,4,7,10-diquinone, but the generation of these Diels–Alder components and their combination to form a [9]-beltene derivative has not been accomplished. The synthesis of linear derivatives has proceeded as far as a protected heptacyclic derivative, 7,16-diacetoxy-1,4,10,13-tetrakis(tert-butyldimethylsilyloxy)5,6,8,9,14,15,17,18-octahydroheptacene 25. The 1H NMR spectrum of this compound is unexpectedly temperature dependent. more...

The stable carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene has a pKa of 24 in (CD3)2SO and gives an elimination:substitution ratio with 2-bromopropane comparable to that of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN). more...

Attempts to reproduce the reported preparation of 1,5-diborabicyclo [3.3.3] undecane 1 through hydroboration of triallylborane with triethylamine–borane, followed by thermolysis, have led to 1,5-dipropyl-1,5-diborocane 2 and other products, but the presence of 1 could not be detected by GC–MS. Treatment of triallylborane with monobromoborane, methanolysis and subsequent thermolysis gave 1,5-dimethoxy-1,5-diborocane 3 which could be converted to 2 by treatment with propylmagnesium bromide. more...

Medium-ring diphosphines offer unique opportunities to study the interactions between phosphorus atoms. In addition, monocyclic medium-ring diphosphines may be very useful ligands in transition metal complexes. A number of useful synthetic routes to these compounds have been developed, but further efforts in this direction might be rewarding. more...

The basicity order in acetonitrile for a series of phosphines is: 1,5-diphosphabicyclo[3.3.3]undecane 1 (pKa for the protonated ion ∼ 17.9)

Propellane diphosphonium dications undergo addition reactions with a range of nucleophiles, and the products often have in,out-geometry with some P–P interaction, as shown by 1JPP values ranging from 46–253 Hz. X-Ray structures of 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.4]tetradecane trifluoromethanesulfonate, 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, and 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate are reported; these show that there is no simple relationship between P–P distance and 1JPP values, although the latter do correlate with the apicophilicity of the attached group. Hydride adducts, although formally protonated phosphines, can react as hydride sources, and salts of the 6-phospha-1-phosphoniabicyclo[4.4.3]tridecane ion, while stable in solution, undergo ring opening to the 1-propyl-1-phosphonia-6-phosphabicyclo[4.4.0]decane ion in the solid state. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.4.4.01,6]tetradecane ion leads, via a non-isolable hydroxide adduct, to out,in-1λ5,6-diphosphabicyclo[4.4.4]tetradecane 1-oxide. Addition to the 1,6-diphosphoniatricyclo[4.4.3.01,6]tetradecane dication leads to isolable 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, converted by excess hydroxide to the diphosphine monoxide which exists as a dynamic mixture of out,in- and out,out-isomers, the equilibrium being dependent on solvent polarity, the water content of the solvent, and the presence of metal ions. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.3.3.01,6]tetradecane dication leads to an out,in-hydroxide adduct which is rapidly converted to the out,out-monoprotonated phosphine oxide. more...

The reactions of hydrazine hydrate with methyl acrylate and ethyl acrylate do not result in the formation of 1,5-diazabicyclo[3.3.0]octane-2,6-dione 1 at reaction temperatures below 200 °C; 1,2-bis(alkoxycarbonylethyl)pyrazolidin-3-one 8, 1-(alkoxycarbonylethyl)pyrazolidin-3-one 6, and 2-(alkoxycarbonylethyl)pyrazolidin-3-one 11 were isolated and identified. At 260–280 °C, however, 1 could be isolated in 20–28% yield; formation of 1 could also be observed by spectroscopic measurements when the reaction was carried out above 200 °C. It is concluded that the major pathway involves initial Michael addition to give a 1,1-disubstituted hydrazine that cannot yield 1 without undergoing unfavourable retro-Michael reactions. The apparently similar reaction of hydrazine with excess diethyl glutaconate to give 4,8-bis(ethoxycarbonylmethyl)-1,5-diazabicyclo[3.3.0]octane-2,6-dione 2 takes a quite different course. Initial Michael addition gives a 1,2-disubstituted adduct for steric reasons. Although double cyclisation of this adduct to 2 appears straightforward, retro-Michael reaction of the monocyclised adduct is very rapid and competes with the second cyclisation. The reaction of ethyl methacrylate and hydrazine hydrate at 80 °C results in a complex reaction mixture, which is transformed into 4-methylpyrazolidin-3-one 13 at 390 °C under reduced pressure. more...

The 2-oxa-5-adamantyl carbocation 4 is shown to be a viable intermediate in several SN1 substitution reactions. However, attempts to observe the formation of 4 from various precursors by NMR methods (which succeed for the 1-adamantyl cation 5) failed, suggesting that 4 does not survive on longer timescales. DFT calculations suggest that the retro-Prins process (β-cleavage, Grob fragmentation) to give enantiomeric (1R,5R)- and (1S,5S)-7-methylene-2-oxoniabicyclo[3.3.1]non-2-ene cations 22 is the only realistic unimolecular escape route for 4. With the 6-31G(d) basis set, B3LYP calculation predicts that 4 is only 11 kJ mol-1 more stable than 22, and the barrier separating 4 and 22 is calculated to be only 15 kJ mol -1, so rapid equilibration of these species is likely on the laboratory time scale. The implications of this study for the mechanism of the Prins cyclisation are discussed. © 2010 The Royal Society of Chemistry. more...

The design of polymers with repeating [C(NR2)2CH2CH2] units which may simultaneously provide conformational control and contain repeating electroactive centres is discussed; (NR2)2 groups would be ideally provided by ortho-phenylenediamine derivatives, with 1,8-diaminonaphthalenes as alternatives. Oligomers containing 1,8-bis(methylamino)naphthalenes, up to the hexamer, were obtained by condensation of oligomers of CH3[COCH2CH2](n)COCH3 with 1,8-bis(methylamino)naphthalene, but attempts to prepare related oligomers from 1,2-bis(alkylamino)benzenes were unsuccessful, as only terminal ketone groups could be converted to aminals. Evidence for a strong preference for all-anti conformations of the main chain in the naphthalenediamine oligomers is provided by ring current effects on 1H NMR shifts, and by X-ray structures, which also provide evidence of intercalation in the solid state. Electrochemical studies of these oligomers show irreversible oxidation of oligomers in solution, but oxidation of longer oligomers leads to the deposition of a reddish-pink insoluble material which shows two reversible oxidation waves. Possible interpretation of these results is discussed. more...

Eschweiler-Clarke methylations of both acyclic and cyclic polyamines can lead to methylation products of fragments of the original polyamine; thus 1,5,9,13-tetra-azatridecane yields 1,1,3,3-tetramethylpropanediamine exclusively and 1,5,9-triazacyclododecane gives 45% 1,5,9-trimethyl-1,5,9-triazacyclodecane and 45% 2,6,10-trimethyl-2,6,10-triazaundecane. more...