Photolysis oxirene

Neither ground-state ethynol (hydroxyacetylene) (80) nor carbenaoxirane (81) appears to be a viable point of ingress to the oxirene-methanoylcarbene system, as both can isomerize to ketene by lower-energy pathways. The limited experimental information available on carbenaoxirane (Section 5.05.6.3.4(f/)) indicates that it is indeed largely isolated from the oxirene-methanoylcarbene manifold (but note the photolysis of ketene in Section 5.5.6.3.4(ff)) appropriate labelling experiments with (the unknown) ethynol have not been performed. 

The isomerization of carbenaoxirane (81) to oxirene seems unlikely, as ab initio calculations predict a more facile conversion to ketene (Figure 3). However, oxirene involvement has been demonstrated for ketene photolysis (Scheme 102) (80PAC1623) and this may proceed through vibrationally excited (81) cold carbenaoxiranes would appear to be even... 

Of much greater promise than thermolysis for the detection of oxirene is low-temperature photolysis. Strausz and coworkers (80JOC2271) investigated the photolysis, in an argon matrix at low temperatures, of 1,3-dioxolenone (vinylene carbonate 125). However, the only C2H2O species detected was ketene. The sulfur derivatives of (125), compounds (126) and... 

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). 

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... 

In recent work (2008) ultrafast photolysis of a potential diazo ketone precursor of p-biphenylmethyloxirene failed to detect the oxirene, the UV absorption of which could, however, have been hidden by another band [8], and in a combined experimental/computational (ab initio and molecular dynamics with DFT) study, flash thermolysis of a formal Diels-Alder adduct was interpreted as affording acetylmethyloxirene and benzene [9]. 

The photochemical reaction between ozone and dimethylacetylene has been examined using both matrix isolation and ab initio methods <1999PSA7280>. The photolysis of hexafluoro-2-butyne and ozone forming oxirene... 

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>. 

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. 

Photolysis of (204) in methanol gives rise to (208) and (209), products from Wolff rearrangement and hydrogen-abstraction, respectively. No traces of (212) were detected, implying that a symmetrical intermediate such as (210) could not have intervened (Scheme 45). It appears that fused-ring oxirenes are still elusive species and further research in this area is warranted. 

The oxirene participation is up to 100% in gas-phase photolysis, and less in solution. Oxirenes can also be involved in thermolysis usually to a smaller extent than in photolysis, their significance increasing with the temperature. Matlin and... 

Even when oxirenes are intermediates, their lifetimes may be very short. Photolysis of diazoacetaldehyde in an argon matric at 8 K with concomitant IR-observation showed fomation of ketene, but no absorption due to oxirene could be detected 26). 

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