Oxirene, formation

An extensive series of low-temperature matrix isolation experiments has failed to provide any evidence of oxirene formation, either by diazo ketone photolysis (82CB2192) or by attempted photo-retro-cycloaddition (82CB2202). 

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. 

The molybdenum-catalyzed oxidation of alkynes by /-butyl hydroperoxide has been investigated 73JCS(P1)2851) (the epoxidation of alkenes by this system has become an important reaction Section 5.05.4.2.2(i)) but the formation of oxirenes was excluded. 

After the initial claim of the synthesis of an oxirene (by the oxidation of propyne Section 5.05.6.3.1) this system reappeared with the claim 31LA(490)20l) that 2-chloro-l,2-diphenyl-ethanone (110) reacted with sodium methoxide to give diphenyloxirene (111), but it was later shown (52JA2082) that the product was the prosaic methoxy ketone (112 Scheme 97) (the formation of 111 from 110 would be an a-elimination carbene-type reaction). Even with strong, nonnucleophilic bases, (110) failed to provide evidence of diphenyloxirene formation (64JA4866). 

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. 

A related issue is whether the carbene, when it is involved, is in equilibrium with a ring-closed isomer, an oxirene.233 This aspect of the reaction has been probed using isotopic labeling. If a symmetrical oxirene is formed, the label should be distributed to both the carbonyl and a-carbon. A concerted reaction or a carbene intermediate that did not equilibrate with the oxirene should have label only in the carbonyl carbon. The extent to which the oxirene is formed depends on the stmcture of the diazo compound. For diazoacetaldehyde, photolysis leads to only 8% migration of label, which would correspond to formation of 16% of the product through the oxirene.234... 

Formation of 2-azirine and oxirene derivatives as intermediates is assumed in a number of reactions, such as (60) and (61). As for the saturated... 

Most monoheterocycles with one cyclic double bond have been prepared by C-Z bond formation in which the Z atom acts as the nucleophile. However, for six-membered rings of this type, Diels-Alder reactions are especially important. Three-membered rings are also atypical azirines are often made by C-N bond formation from precursors in which N is electrophilic or has nitrene character, while oxirenes are but fleeting intermediates (CHEC 5.05.6.3). 

Direct dynamics calculations were carried out with quasiclassical normalmode sampling from a canonical ensemble at 923 K (the experimental reaction temperature). Simulations initiated at the vicinity of TS for rearrangement of carbene 13 to 14 via oxirene 12, and 300 trajectories were obtained at DFT methods. The preliminary results reported in the manuscript showed that preferred formation of 15a over 15b by the ratio of 1.8 7.6 depends on the method used. The results were qualitatively consistent with the value of 2.5 deduced from the experiment. The non-unity ratio likely arises from the situation that two methyl groups in 14 are dynamically unequal on the carbene formation process. 

The oxidation of phenylethyne with isolated dimethyldioxirane gives mandelic acid (61) and phenylacetic acid (60) as the main products.115 The formation of (60) and (61) is explained as involving the initial formation of phenyloxirene and therefore dimethyldioxirane-mediated oxidation of alkynes is potentially suited to study the chemistry of the elusive oxirenes. 

The photolysis of l-diazo-3-methyl-2-oxocyclododecane 122 appears to involve oxirene participation as indicated by the formation of regioisomeric enones 123 and 124 (Scheme 60) <1978CB1549, 2004RMC291>. 

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