Lifetimes of Simple Ketocarbenes

摘要

Decomposition of a-diazoketones (1, Scheme 1) readily produces ketocarbenes (2) and ketenes (3, Scheme 1) which can be intercepted with typical traps. Oxirene species have also been implicated as intemediaries on the basis of labeling studies, and evidence for dimethyloxirene has been obtained in cryogenic matrices. Kaplan, Meloy, and Mitchell have reported convincing evidence that the photochemistry of a-diazoketones is highly dependent on their conformation. Photolysis of the syn isomer is believed to lead directly to ketene and nitrogen, whereas photolysis of the anti isomer releases carbene and nitrogen which can subsequently isomerize to ketene (3) or oxirene (4) or react with external trapping agents. Increasing steric bulk of the substituent R (Scheme 1) leads to a decrease in the extent of the anti conformer at equilibrium and a decrease in the yield of carbene. These rules are widely used, but other interpretations have been advanced.