Diazo compounds isotopically labelled

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... 

EtOH isopentane Et20) glasses at 77 K, four products 256, 257, 258 and 259 were obtained in overall >95% yield. The major product 256 was shown by isotopic labeling studies to have been derived from the silirene 260 formed by intramolecular coupling of the bis-carbene 261, and pathways for the formation of the other products were proposed. The photolysis of the bis-diazo compound 255 was studied in detail at 405 nm and it was found that the diazirine 262 was formed this on photolysis at 305 nm also gave 260 on the pathway to 256. The chemistry is shown in Scheme 46. 

P. J. Smith and K. C. Westaway, Preparation and uses of isotopically labelled diazonium and diazo compounds , in The Chemistry of Diazonium and Diazo Compounds (Ed. S. Patai), Wiley, Chichester, 1978, pp. 709-749. 

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