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The title reactions were investigated experimentally and theoretically. The benzyne--tropone pair has been known to give a Diels–Alder [4+2] cycloadduct. However, in the present experiment using two sources of benzyne, many unexpected products were obtained. In particular, a zwitterion with a C–Cl bond is strikingly a major product, the structure of which was determined by the X-ray analysis. The bond is formed by quenching the Cl atom from the solvent CH2Cl2, which demonstrates that the reactions proceed via radical processes. DFT calculations revealed the elementary processes of the reactions. The presence of the novel zwitterion in the solid state has been interpreted in terms of the permanent dipole–dipole attractions/interactions/stabilization.

Tropone (1) reacts with ketenes 2 to yield [8+2] cycloadducts, the γ-lactones 3. The concerted [8+2] cycloaddition path is formally symmetry-allowed, but we established that it is unfavorable. Careful low-temperature NMR (1H, 13C, and 19F) spectroscopies of the reaction of diphenyl ketene (2b) or bis(trifluoromethyl) ketene (2c) with tropone (1) allowed the direct detection of a β-lactone intermediates 5b,c and novel norcaradiene species 6b,c in head-to-head configurations. The [2+2] cycloadducts 5b,c equilibrated with the norcaradienes 6b,c. The β-lactones 5b and 5c were converted to the γ-lactones 3b and 3c, respectively, in quantitative yields. The DFT calculations showed that the concerted [8+2] cycloaddition is unfavorable. The first step of the calculated reaction 1+2c is a cycloaddition which leads to a dioxetane intermediate. This initial [2+2] cycloadduct is isomerized to the β-lactone 5cvia the first zwitterionic intermediate. The β-lactone 5c is further isomerized to the product γ-lactone 3cvia the second zwitterion intermediate. Thus, 3c is not formed via the well-established two-step mechanism including zwitterionic intermediates but via a five-step mechanism composed of a [2+2] cycloaddition and subsequent isomerization (Scheme 12).

Cheletropic additions forming cyclopropane rings were studied theoretically. Ten addition paths were traced by means of density-functional-theory calculations. Two 1,4-dienes, 1,4-pentadiene, and tricyclo[5.3.1.04,9]undeca-2,5-diene were adopted as substrates. CO, SO2, C2H5PCl2, CCl2 and SiCl2 were employed as cheletropic reagents (Xs). An orbital correlation diagram of the Woodward–Hoffmann (W–H) rule and frontier molecular orbital (FMO) interactions between them were investigated in detail. The FMO interactions, HOMO (1,4-diene)→lumo (X) and homo (X)→LUMO (diene), work reasonably for the progress of the reactions. Those cause the formation of two C–X bonds and a cyclopropane ring, and alternation of double bonds to single bonds. All the additions are concerted. The easiness of the ring formation depends upon the energy gap between HOMO and lumo and that between homo and LUMO, and the spatial directions of HOMO and LUMO extensions. Symmetry conservation of the W–H rule does not hold necessarily for those addition paths. The symmetry-breaking was discussed in terms of FMO interactions.

The Woodward−Hoffmann (W−H) rule and the Fukui's frontier-orbital theory (FMO) were assessed for the title reactions. Diels−Alder reactions of the normal electron demand were investigated with density functional theory calculations. The Cs symmetry is usually conserved in [4 + 2] reactions between 1,3-dienes and olefins of symmetric geometries. However, those between very nucleophilic dienes and olefins substituted by four electron-withdrawing groups take C1-symmetry paths. This result is in contrast to the postulate of the W−H rule that the Cs symmetry should be conserved in “symmetry-allowed” paths. A [6 + 4] cycloaddition between tropone and cyclopentadiene was found not to take the Cs-symmetry path despite the symmetry-allowed reaction. Configuration analysis was carried out and gave the magnitude of various orbital interactions. The three-body orbital mixing leads to the symmetry lowering of reaction paths. FMO explains reasonably the anomaly of the cycloadditions of non-W−H rule in strong donor and ...

The fragmentation of mass spectroscopy of tropone (1) was compared with that of tropothione (2). The electron-impact mass spectroscopy of 1 afforded almost exclusively the benzene cation radical (m/z = 78). Mass spectroscopy of 2 gave similarly the m/z = 78 base peak along with an unusual (M+ − 1) peak. This hydrogen elimination was found to occur at the C(α)–H bond by the use of a 2H-labeled analogue of 2. Ab initio calculations (UB3LYP/6-31G*) showed that a π cation radical of 1 and a σ cation radical of 2 were converted to norcaradiene intermediates. Their further isomerizations led to [the benzene cation radical (m/z = 78) + CO] and [(m/z = 78) + CS], respectively. The fragmentation channel of 1 was calculated to have sufficiently small activation energies of intervening transition states to give almost exclusively the m/z = 78 peak. For a σ radical of 2, an α hydrogen moved to the sulfur atom. The resultant thiol was isomerized to a second norcaradiene and its further isomerization led to a thioketene like cation and a hydrogen atom corresponding to the unusual (M+ − 1) peak. The difference in fragmentation patterns of 1 and 2 is discussed in terms of their electronic structures.

N2 extrusion paths of the smallest monocyclic azoalkane, diazetine, its tetramethyl-substituted derivative, and their three-membered cyclic isomers were investigated by ab initio calculations. Systematic considerations of the orbital correlation diagram predicted that the routes conserving the Cs point group are symmetry allowed. CASSCF(2,2)/6-31G*, CASSCF(4,4)/6-31G*, and CASSCF(2,2)/6-311+G(2d,p) geometry optimizations were carried out and showed that the decomposition paths of the isomers followed the Woodward−Hoffmann rule. However, the paths of diazetines did not follow it. The dual orbital-mixing rule proposed here correctly explained the Jahn−Teller effect. Reaction enthalpy and activation energy were satisfactorily reproduced by the BCCD(T)/6-31G*//CASSCF(2,2)/6-31G* energies.

The mechanism of ketene−diene reactions has been studied both experimentally and theoretically. Careful experiments of the reactions of diphenylketene (1) with cyclic (s-cis) 1,3-dienes [cyclopentadiene (2) and cyclohexa-1,3-diene (3)] lead to the first direct detection of the Diels−Alder cycloadducts (10 and 11) by low-temperature NMR spectroscopy. The initially formed cycloadducts are converted to the final Staudinger products, cyclobutanones (6 and 7), by [3,3] sigmatropic (Claisen) rearrangements. In contrast, ketene 1 reacts with open-chain 1,3-dienes [2,3-dimethyl-1,3-butadiene (4) and 1-methoxy-1,3-butadiene (5)] to afford initially both the Staudinger-type (8, 9) and Diels−Alder-type cycloadducts (12, 13). The Staudinger cycloadducts (8, 9) are converted eventually to Diels−Alder products (12, 13) by the retro-Claisen rearrangement. Thus, ketene recognizes dienes in cycloadditions as ketenophiles different from olefins. [4 + 2] and [2 + 2] cycloadducts are generated and can be intermediates or pro...

Ab initio calculations on transition states and products of title reactions were performed. Possible [4+2] and [8+2] reaction paths were investigated. In the tropone ( 1 )-anhydride reaction, the [4+2] endo path was calculated to be best both kinetically and thermodynamically. The distinct selectivity of the [4+2] endo path over the [4+2] exo one was explained in terms of the secondary orbital interaction. In the tropothione ( 2 )-anhydride reaction, the [8+2] endo path, although thermodynamically the second worst, is best kinetically. The difference of the [4+2] or [8+2] selectivity is ascribed to that of HOMO shapes of 1 and 2 . The present experimental analyses support the theoretical result.

HgCl 2 complexes of tropothione derivatives and tropone are studied both experimentally and theoretically. IR spectroscopic data of these complexes are compared with theoretical ones derived from vibrational analyses. Tropone is linked with HgCl 2 in the form of a σ complex. But, tropothione and its derivatives give π complexes. Model BF 3 , BCl 3 , AlCl 3 and HgCl 2 complexes of acetone, thioacetone and tropone are all confirmed to be σ complexes. Large Lewis acids are bonded with tropothiones in π complexes. The σ−π selectivity of the Lewis-acid coordination is ascribed to the difference of HOMO shapes of tropone and tropothione.

The IR characterization of tropothione ( 1 ), the sulfur analogue of tropone ( 2 ), is described together with an argument for vibrational assignments for ν(CC) and ν(CO) in 2 . The IR spectrum of 1 is measured in five different solvents as well as in the neat state for assignments. The titled compound 1 exhibits a very strong band of the thiocarbonyl stretching vibration ( ν C  S ) at 1087 cm −1 in CCl 4 . Raman spectrum and ab initio calculated results support the assignment. With the aid of spectroscopy, we have reexamined the assignment of the tropone. The analysis gives rise to the decisive assignment of IR spectroscopies in both 1 and 2 . Two A 1 -symmetry vibrations are coupled in 2 and uncoupled in 1 , respectively.

The molecular orbital (MO) calculations predict that the the reaction of the mesylate (2) of tropone oxime with nucleophiles will lead to the ring opening of the substrate. In accord with this MO prediction we have found experimentally that the facile ring opening of 2 occurs while the oxime 3 is inert to nucleophiles. Thus, 2 reacts with nucleophiles under mild conditions and gives stereoselectively a variety of (Z,Z,Z)-trienes, as the sole products in high yields. The reaction is similar to that of the tosylate 1. The reaction has been found to be caused by the intramolecular ( HOMO → σ ∗ ) charge transfer in the Meisenheimer tetrahedral intermediate. A hyperconjugation plus E2 mechanism is proposed for the facile ring opening. The MO theory interprets why the troponoid rings of the mesylate and tosylate are cleaved easily by nucleophiles.

For the parent reacting system, HCOOOH + CH(2)=CH(2) --HCOOH + ethylene oxide, the overall potential energy surface has been determined using ab initio methods. The oxygen-addition transition state is found to be remarkably similar to that deduced experimentally. The O-H bond is retained in the transition state. Reasonable kinetic isotopic effects are obtained theoretically. The transition state leads to an unprecedented transient intermediate. That intermediate is converted to a hydrogen-bonded system between ethylene oxide and formic acid. Substituent effects on transition-state geometries are small, but the effects on activation energies are large.

The electric dipole moment of tropothione ( 1 ), a thiocarbonyl compound, and tropone ( 2 ), a carbonyl one, has been compared. The charge separation of 1 and 2 is evaluated on the basis of the dipole moment and ab initio calculations. The dipole moment (μ) of 1 is measured to be 4.42 D in carbon tetrachloride at −15°C and is larger than that ( μ = 3.71 D) of 2 . Ab initio molecular orbital calculations have reproduced these values well and have revealed the origin of this difference.

A representative azulene formation from an active troponoid precursor (2-methoxytropone) and an active methylene compound (malononitrile) has been analyzed both experimentally and theoretically. (2)H-Tracer experiments using 2-methoxy[3,5,7-(2)H(3)]tropone (2-d(3)) and malononitrile anion give 2-amino-1,3-dicyano[4,6,8-(2)H(3)]azulene (1-d(3)) in quantitative yield. New and stable (2)H-incorporated reaction intermediates have been isolated, and main intermediates have been detected by careful low-temperature NMR measurements. The detection has been guided by mechanistic considerations and B3LYP/6-31(+)G(d) calculations. The facile and quantitative one-pot formation of azulene 1 has been found to consist of a number of consecutive elementary processes: (a) The troponoid substrate, 2-methoxytropone (2), is subject to a nucleophilic substitution by the attack of malononitrile anion (HC(CN)(2)(-)) to form a Meisenheimer-type complex 3, which is rapidly converted to 2-troponylmalononitrile anion (5). (b) The anion 5 is converted to an isolable intermediate, 2-imino-2H-cyclohepta[b]furan-3-carbonitrile (6), by the first ring closure in the reaction. (c) A nucleophilic addition of the second HC(CN)(2)(-) toward the imine 6 at the C-8a position produces the second Meisenheimer-type adduct 7. (d) The second ring closure leads to 1-carbamoyl-1,3-dicyano-2-imino-2,3-dihydroazulene (11). A base attacks the imine 11, which results in generation of a conjugate base 12 of the final product, azulene 1.

Tropothione, which is valence isoelectronic with tropone, reacts concertedly with fulvenes to give 1:1 [8+ 2] cycloadducts. The product configuration is sharply different from that of the initially formed [6+4] cycloadduct between tropone and fulvene. The reaction mechanisms are studied kinetically and by molecular orbital calculations. The obtained endo form of the [8+2] product is found to arise from the secondary orbital interaction in the course of the intrinsic reaction coordinate

Ketene dimerizations to diketene and to 1,3-cyclobutanedione are studies theoretically. Their transition state (TS) structures are determined, for the first time, with correlated wavefunctions. The present calculation demonstrates that the two reactions proceed concertedly. The intrinsic reaction coordinates (IRCs) starting from the TSs are traced so as to seek the origin of the selectivity of the two reactions. At their initial stages, a one-center type charge transfer (donative orbital interaction) takes place, enhanced by polarization interactions in the charge-acceptor ketene. The back-donative orbital interactions are found to control the selectivity.

The reaction of 8,8-dicyanoheptafulvene and sodium cyclopentadienide affords under mild conditions two unusual tetracyclic cage molecules with (dicyanomethylene)tetracyclo[7.2.1.0.0]dodecadiene structures. The structures were elucitated using various modern NMR spectroscopic techniques and deuterium-labeling experiments. The easy formation of the considerably strained molecules is ascribed to a nucleophilic addition, followed by protonation and an intramolecular Diels-Alder reaction with normal electron demand. MNDO-MO calculations suggest a possible reaction mechanism

Molecular orbital (MO) calculations of two cycloadditions, ethylene to 1-thiabutadiene and to tropothione, are made so as to compare the reactivities of [4+2] and [8+2] additions. Transitionstate (TS) geometries are examined with various computational methods, followed by a search for intrinsic reaction coordinates (IRCs). Two concerted path are almost synchronous. On account of the Large lobe of frontier MOs (FMOs) on the sulfur atom, the small activation energies of the [4+2] and [8+2] additions are obtained

The reaction of 8,8-dicyanoheptafulvene and sodium cyclopentadienide affords under mild conditions two unusual tetracyclic cage molecules with (dicyanomethylene)tetracyclo[7.2.1.0.0]dodecadiene structures. The structures were elucitated using various modern NMR spectroscopic techniques and deuterium-labeling experiments. The easy formation of the considerably strained molecules is ascribed to a nucleophilic addition, followed by protonation and an intramolecular Diels-Alder reaction with normal electron demand. MNDO-MO calculations suggest a possible reaction mechanism

Tropothione S-sulfide is synthesized from tropone hydrazone with disulfur dichloride in deaerated chloroform at −78 °C. The S-sulfide is detected as an unprecedented [10π + 2π]-type cycloadduct with dimethyl acetylenedicarboxylate.

For the parent reacting system, HCOOOH + CH2CH2 → HCOOH + ethylene oxide, the overall potential energy surface has been determined using ab initio methods. The oxygen-addition transition state is found to be remarkably similar to that deduced experimentally. The O−H bond is retained in the transition state. Reasonable kinetic isotopic effects are obtained theoretically. The transition state leads to an unprecedented transient intermediate. That intermediate is converted to a hydrogen-bonded system between ethylene oxide and formic acid. Substituent effects on transition-state geometries are small, but the effects on activation energies are large.

The molecular orbital (MO) calculations predict that the the reaction of the mesylate (2) of tropone oxime with nucleophiles will lead to the ring opening of the substrate. In accord with this MO prediction we have found experimentally that the facile ring opening of 2 occurs while the oxime 3 is inert to nucleophiles. Thus, 2 reacts with nucleophiles under mild conditions and gives stereoselectively a variety of (Z,Z,Z)-trienes, as the sole products in high yields. The reaction is similar to that of the tosylate 1. The reaction has been found to be caused by the intramolecular ( HOMO → σ ∗ ) charge transfer in the Meisenheimer tetrahedral intermediate. A hyperconjugation plus E2 mechanism is proposed for the facile ring opening. The MO theory interprets why the troponoid rings of the mesylate and tosylate are cleaved easily by nucleophiles.

Ab initio calculations were performed on title reactions between butadiene and acrolein with BCl3, AlCl3, GaCl3, InCl3, ZnCl2, SnCl2, and SnCl4. A dimethyl ether molecule is explicitly considered in various reaction systems to examine solvent effects. First, the reaction path of an AlCl3-promoting reaction was examined thoroughly. This reaction has two channels. The first one involves a weak reactant-like complex (precursor) and a normal [4 + 2] addition. The second does three elementary processes, one-center addition, ring closing, and Claisen shift. The first channel is more favorable by 12.1 kcal/mol (B3LYP/6-311+G(2d,p) SCRF//B3LYP/6-31G* SCRF) than the second one. Then the first channels with other Lewis acids were traced with and without an ether molecule. The ether molecule has an appreciable effect not on geometries but on activation energies. BCl3 is desolvated and has extraordinarily strong catalytic ability. Even with the strongest catalyst, not a [2 + 4] but a normal [4 + 2] cycloaddition take...

Nucleophilic substitutions on carbonyl carbons initiated by Cl−, Ph–CO–Cl+Cl−→Cl−+Ph–CO–Cl, were studied with ab initio MO calculations. Two transition states (TS’s) were found. One is the “tetrahed-ral” TS caused by the π attack of the nucleophile. The other is the “trigonal-bipyramidal” TS initiated by the σ attack. The coexistence of two TS’s is in accord with the proposals of Bentley and Cevasko et al. based on kinetic analyses. Two independent reaction paths are interpreted in terms of the frontier-orbital theory.

A benzyne−tropothione reaction was studied experimentally and computationally. Three isomeric products were detected by a careful experiment using two benzyne sources. The three equimolar products were identified. The expected symmetry-allowed [4 + 2] or [8 + 2] cycloadduct was not detected. In order to explain the unexpected products, density functional calculations and complete active space self-consistent field (CASSCF) calculations were carried out. The benzyne is, first, added to the tropothione via one-center C−S bond formation. Then a singlet biradical intermediate is formed. In the biradical, an α hydrogen atom of the tropothione moiety is moved to the benzyne moiety. A closed-shell intermediate is generated. This allene-type intermediate is isomerized to the second intermediate. The intramolecular proton shift in the latter leads to the three products. The biradical character of the benzyne has a key role in the present reaction and was discussed in reference to other benzyne reactions.

This paper describes a new mechanistic feature for the Staudinger ketene-alkene cycloaddition reactions to give cyclobutanones. Low-temperature NMR (13C, 19F, and 1H) monitoring of a reaction between bis(trifluoromethyl)ketene (1) and ethyl vinyl ether (2) has shown that the Staudinger reaction proceeds to form initially and exclusively an alpha-methyleneoxetane (3) by [2 + 2](C=O) cycloaddition across the ketene C=O bond. The initial intermediate 3 undergoes ring cleavage to produce a 1,4-zwitterion (4), which is converted to the final [2 + 2](C=C)-type product, cyclobutanone (5). The key intermediate 3 has been isolated in its pure form and was found to be converted to the final products 5 on warming, via the 1,4-zwitterion 4. The alpha-methyleneoxetane 3 is so reactive that it reacts with methanol rapidly even at -80 degrees C via solvolysis to afford an adduct 7. The ion 4 derived from the pure isolated oxetane 3 was intercepted with acetone by a 1,4-dipolar cycloaddition to give a 1,3-dioxane 8. An open-chain alpha,beta-enone (6) has been also obtained from 3. We conclude that the (1 + 2) reaction proceeds in a new three-step mechanism; formation of an alpha-methyleneoxetane 3, a [2 + 2]-type cycloadduct across the C=O bond of ketene, followed by ring cleavage to give the zwitterion 4 and by recombination to form the final product, cyclobutanone 5. The zwitterion 4 is not equilibrating with reactants 1 and 2 but comes from the alpha-methyleneoxetane 3. Exclusive formation of another oxetane 12 has been observed in a reaction between diphenylketene (9) and methyl isopropenyl ether (11). The selectivity of initial formation of cyclobutanone or oxetane has been generalized with aid of frontier-orbital theory and ab initio calculations.

Ab initio calculations were performed on title reactions between butadiene and acrolein with BCl(3), AlCl(3), GaCl(3), InCl(3), ZnCl(2), SnCl(2), and SnCl(4). A dimethyl ether molecule is explicitly considered in various reaction systems to examine solvent effects. First, the reaction path of an AlCl(3)-promoting reaction was examined thoroughly. This reaction has two channels. The first one involves a weak reactant-like complex (precursor) and a normal [4 + 2] addition. The second does three elementary processes, one-center addition, ring closing, and Claisen shift. The first channel is more favorable by 12.1 kcal/mol (B3LYP/6-311+G(2d,p) SCRF//B3LYP/6-31G SCRF) than the second one. Then the first channels with other Lewis acids were traced with and without an ether molecule. The ether molecule has an appreciable effect not on geometries but on activation energies. BCl(3) is desolvated and has extraordinarily strong catalytic ability. Even with the strongest catalyst, not a [2 + 4] but a normal [4 + 2] cycloaddition takes place. Except for BCl(3), SnCl(4) is the strongest Lewis acid with the ether molecule. The frontier orbital, LUMO, of acrolein is distorted in the course of the reaction so that the formation of two C-C covalent bonds is possible. The precursor formation and the one-center addition were discussed also by the frontier orbital theory.

The first example of direct spectroscopic detection of transient species, 1,4-zwitterions, generated in a ketene-alkene reaction is reported. Also, a striking result of the intervention of an unprecedented "1,4-zwitterion neutral dimer" is presented in a new mechanistic pathway; the ketene-alkene reaction gives the product cyclobutanone from the initial cycloadduct alpha-methyleneoxetane.

Tropothione S-sulfide is synthesized from tropone hydrazone with disulfur dichloride in deaerated chloroform at −78 °C. The S-sulfide is detected as an unprecedented [10π + 2π]-type cycloadduct with dimethyl acetylenedicarboxylate.

Dual one-centre frontier–orbital interactions are found to control the paths of cycloadditions of ketene to ethylene, methylenimine and formaldehyde by the analysis of the intrinsic reaction coordinates calculated with the MP2/6-31G* basis set.

The photodecomposition of C3O2 into C2O and CO is studied with the ab initio MO calculation. It is found that, starting from the second excited state of C3O2 (Δu), the ground-state C2O (triplet) is yielded through the bent dissociation. The stable structure of the excited triplet state of C3O2 as an intermediate is also demonstrated.

The molecular structures of some simple three-membered ring compounds, cyclopropane, cyclopropanone, ethylene oxide and ethylenimine, were studied by means of the orbital interaction between the fragments of a molecule. The relation between the carbon-carbon distance and the bending angle of the carbon-hydrogen bonds in ethylenic part of the three-membered rings is attributed primarily to the greater extent of mixing of the unoccupied π-like MO of ethylenic fragment into the occupied MO’s of rings. The increase in the bending angle as going from ethylene oxide to cyclopropane is interpreted in terms of the interaction of an occupied π-like orbital with the unoccupied α-like orbital of ethylenic fragment through the overlap with the orbital(s) of the residual part of rings. INDO MO and localized orbital calculations support the orbital interaction scheme. The same conclusion is derived also from the electrostatic theory of molecular geometries.

Ordinary Rayleigh-Schroudinger perturbation theory with Moller-Plesset (RSMP) partitioning is used to calculate second- and third-order correlation corrections to the CHF polarizability and dipole moment of the water molecule by a finite-field procedure. Pade approximants are found to be useful in accelerating the convergence of the property perturbation expansions. Field-induced polarization functions suitable for polarizability calculations are determined. The average polarizability calculated, neglecting vibrational averaging, with Dunning's (9s5p/4s-4s2p/2s) contracted GTO basis set augmented by field-induced lslp2d/lp polarization functions is within 3 per cent of the experimental result. Correlation corrections to the dipole moment and polarizability of the water molecule calculated by the finite-field RSMP and single + double excitation CI(SDCI) methods for the same basis set are found to be in close agreement. The RSMP approach has the advantages of being size-consistent and of being capable of greater efficiency than the SCDI method. Comparative calculations carried out using Epstein-Nesbet partitioning show that through third order RSEN correlation perturbation expansions for the dipole moment and polarizability are less rapidly convergent than RSMP expansions. However, reasonable accord with RSMP results can be achieved by using Pade approximants to accelerate the convergence of RSEN energy perturbation expansions. The convergence of RSMP property correlation expansions based on the zeroth-order uncoupled-Hartree-Fock (UCHF) and coupled-Hartree-Fock (CHF) approximations are compared through third order. Whereas the CHF + RSMP expansions are for practical purposes fully converged, the UCHF + RSMP expansions are not adequately converged.

The mechanism of the conversion of vinylazides into 2H-azirines is examined by use of ab initio MO calculations. It is ascertained that vinylnitrenes have very weak bonds between the vinylazides and nitrogen molecule parts. It seems to decompose readily into the two fragments. This is one of the most characteristic features of vinylazides. The reaction is shown to proceed with N2 at the tail of the vinylnitrene part. This mechanism accounts well for experimental results of the reaction.

The state correlation diagram for the cis/trans-isomerization of azodioxymethane is presented. The direct interconversion through N—N twisting is found to be symmetry-forbidden in a thermal reaction, whereas it is symmetry-allowed photochemically. In the thermal reaction, the fragmentation of the cis form into two nitrosomethanes and their recombination to the trans form take place via the symmetry-allowed "from step to step" mechanism. The potential energy is calculated by the abinitio MO method of a 4-31G basis set and a solvent effect is found to be indispensable in reproducing the energetics of these reactions.

Trois reactions modeles sont examinees: le fluoroethane avec l'ion fluorure, le chloroethane avec F − et avec l'ion chlorure