Oxirenes, stability

Now we turn from benzene to another cyclically delocalized molecule, oxirene or oxacyclopropene [172]. Is oxirene stabilized or destabilized by its 7t-electron system We can answer this question using an isodesmic equation, with B3LYP/ 6-31G energies/geometries as usual. Here we try to cancel out the strain in oxirene by having on each side of the equation about the same amount of ring strain (on each side two sp2C-0 bonds, etc) ... 

For a recent theoretical investigation of oxirene stability by Schaefer s group, see Vacek et al. (1991). 

We calculated the ASE as product energies minus reactant energies, as we did for benzene, and it came out negative, which means that the aromatic stabilization energy here is really destabilizing oxirene is antiaromatic [173]. 

The analogous thiirenium ion derived from l,5-dithia-cyclonon-7-yne 32 could not be isolated but instead the spirocyclic compound 33 was obtained (Scheme 3). The reactions were proposed to proceed through an intermediate a-thiocarbonyl-stabilized carbene analogous to the ring opening of oxirenes (Scheme 4) <2002JA8316>. 

Energetically, oxirene is found to be less stable than the oxiranylidene isomer (with 4-31G). Its low stability is also indicated by the very negative bond separation energy (Table 1) which reflects the unfavorable (antiaromatic) r-structure as well as the ff-bond strain. The total energy of oxirene is predicted at the 6-31G level to lie 89.3 kcal/mol above that of ketene.5 ) Clark, 80,38) in an earlier study of oxirene with a smaller basis also obtained a large value for this exothermicity (102.6 kcal/mol). Our calculated dipole moment (2.96D with 6-31G ) is slightly higher than the value (2.SOD) obtained by Clark.oo)... 

The bond separation energy is quite negative (—37.9 kcal/mol with 6-31G ), but less so than other antiaromatic molecules such as oxirene. This may partly be due to the distortion of the triangle noted above and also to a stabilizing interaction between the polar bonds. The total energy of oxazirine is predicted (by 6-31G ) to lie 97.3 kcal/mol above that of the most stable HCNO isomer, isocyanic acid. )... 

Bachmann, C., N Guessan, T. Y., Debu, F., Monnier, M., Pourcin, J., Aycard, J. P., Bodot, H. Oxirenes and ketocarbenes from a-diazoketone photolysis experiments in rare gas matrices. Relative stabilities and isomerization barriers from MNDOC-BWEN calculations. 

Sung, K. Substituent effects on stability of oxiranes, oxirenes, and dioxiranes. Can. J. Chem. 2000, 78, 562. 

Apart from substitution with discrete groups, oxirenes might potentially be stabilized by fusion to a benzenoid ring, as in benzooxirene delocalization of the TT-electrons of benzene should stabihze the oxirene moiety by diminishing the double bond character in that ring (cf. the rationale for stabilization of push-pull oxirenes, above) ... 

Fig. 14.5 Isodesmic reactions used to calculate strain energy and the aromatic stabilization energy of oxirene. See text...

CF3) and perfluoromethylethyloxirene (52 R = CFj, R = CFjCFj) in Ar matrices were reported, following photolysis of the corresponding diazoketones. The species identified as oxirenes could be converted into the corresponding ketenes by irradiation at 360 nm and, in accord with predictions of very low stabilization energies for oxirenes, their spectra decayed substantially when the matrices were warmed to 35 K. In a later investigation of the matrix photolysis of the diazoketones 51 (R = R = CH, CD,), the ketenes were always found to be more abundant, but species identified as oxirenes were detected as minor but well-defined products. These were stable at temperatures below 25 K and isomerized to the ketenes when irradiated with light of A > 230 nm. 

Fast-reaction techniques, employing laser flash excitation followed by UV monitoring of the reaction, show that photolysis of diazo-oxides derived from benzene and naphthalene" follows the course illustrated in Scheme 6 for the benzene series. In aqueous solution, only the ketene and the ketene hydrate are observed as short-lived intermediates. Rates of the intermediate reactions have been followed over a large pH range and show the expected profiles due to acid-base catalysis. Photol) is within a film of Novolak-resin shows evidence for the short-lived carbene intermediate, and it has been proposed that this species is stabilized as the oxirene form. However, photolysis of the C-labeled naphthalene derivatives cannot involve an oxirene intermediate 7 as no scrambling of the C label is observed in the products. ... 

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