Oxocarbenes and Oxirenes

These results require some explanation. It has long been known that acyclic a-diazoketones and -esters exist predominantly in the s-cis form (89)104-108 although the energy barriers toward interconversion (89 90) 

Apart from the well-known Arndt-Eistert homologization of carboxylic acids and the many and varied intermolecular reactions of the ketenes formed 

Related rearrangements of naphthoquinonediazides have been carried out both thermally115 and photolytically.116 The formation of indole-3- 

Steppan, and R. Dietrich, Justus Liebigs Ann. Chem. 617, 20 (1958). 

The first stable benzothiete (97, X = S) was prepared by Wolff rearrangement of the diazoketone 96 (X = S) benzoxetes and benzazetes can be prepared analogously (Eq. 26).120 

Stanovnik, M. Tisler, and E. Voncina, Heterocycles 12, 761 (1979) B. Stanovnik, M. Tisler, J. Bradac, B. Budic, B. Koren, and B. Mozetic-Rescic, ibid., 457. 

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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. 

Other routes to oxocarbenes are the a-elimination of bromine from a,a-dibromo ketones (73JA2708, 73JA5416), and the elimination of CO2 or COS from dioxolenones (vinylene carbonates) or their sulfur analogues. The former reaction has not been investigated with regard to oxirene involvement the latter is discussed in Section 5.05.6.3.5. The formation of an oxocarbene from a chlorooxirane was mentioned in Section 5.05.6.3.2. 

The oxidation of phenylacetylene with dimethyldioxirane yields mandelic acid and/or phenylacetic acid.62 The initially formed oxirene rearranges via two a-oxocarbenes to phenylketene from which both products are derived. 

The antiaromatic oxirenes 11 are extremely elusive but their potential involvement in photochemical Wolff rearrangements by equilibration with the -oxocarbenes 12 has generated both theoretical and experimental interest and the subject has been reviewed <2004MR0291>. Experimental evidence for participation of oxirenes <2004MR0291> is supported by DFT calculations that suggest that oxirene corresponds to an energy minimum <2003JMT(629)263, CHEC-III(1.03.7)215>. 

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