Oxirene transition states

More definitive evidence for the formation of an oxirene intermediate or transition state was presented recently by Cormier 80TL2021), in an extension of his earlier work on diazo ketones 77TL2231). This approach was based on the realization that, in principle, the oxirene (87) could be generated from the diazo ketones (88) or (89) via the oxocarbenes 90 or 91) or from the alkyne (92 Scheme 91). If the carbenes (90) (from 88) and (91) (from 89) equilibrate through the oxirene (87), and if (87) is also the initial product of epoxidation of (92), then essentially the same mixture of products (hexenones and ketene-derived products) should be formed on decomposition of the diazo ketones and on oxidation of the alkyne this was the case. 

As far as the oxirene problem100 is concerned, we concluded in 198295 that Scheme 1 would illustrate the interconversions between ketene 74, formylcar-bene (75), and oxirene (76). The basic statement was that it might be difficult to decide whether oxirene (76) has to be regarded as an intermediate or a transition state on the reaction coordinate. 

The main advantage of MP2/6-31G optimizations over HF/3-21 ( > or HF/ 6-31G ones is not that the geometries are much better, but rather that for a stationary point, MP2 optimizations followed by frequency calculations are more likely to give the correct curvature of the potential energy surface (Chapter 2) for the species than are HF optimizations/frequencies. In other words, the correlated calculation tells us more reliably whether the species is a relative minimum or merely a transition state (or even a higher-order saddle point see Chapter 2). Thus fluorodiazomethane [91] and several oxirenes [53] are (apparently correctly) predicted by MP2 optimizations to be merely transition states, while HF optimizations... 

Fig. 9.2 A species with the symmetry of an oxirene scrambles the label in an oxo carbene. But this does not tell us whether the oxirene is an intermediate or merely a transition state...

Fig. 3.12 The best results calculated for the species in the Wolff rearrangement by Scott et al. [70]. The numbers are relative energies in kJ mol . The accuracy of the calculations does not rule out the possibility that oxirene is only a transition state connecting equivalent-energy carbenes and scrambling the label, or that the carbene is not really a stationary point (neither a minimum nor a transition state) on the potential energy sur ce...

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