| Abstrakt: |
The formation of a metastable carbene−diazirine ylide (CDY), characterized by an UV absorption spectrum in the range of 270−290 nm and yielding azine by rearrangement, is shown to be a general process in the photolysis of diazirines. However, the yield of formation and the lifetime of CDY greatly depend on the system considered. The decreasing value of the rate constant of the reaction, carbene + diazirine → CDY, from ~109 M-1 s-1 for singlet dialkylcarbenes, Ad: (adamantylidene) and BCN: (bicyclo[3.3.1]non-9-ylidene), to ~108 M-1 s-1 for benzylchlorocarbene and to ~106 M-1 s-1 for cyclopropyl and phenylchlorocarbenes, reflects the decreasing reactivity of these carbenes. The lifetime of these ylides is determined by the value of the activation energy barrier for their rearrangement to azine, Ea ≈ 15.5 kcal/mol for dialkylcarbenes, ≈11.5 kcal/mol for alkylchlorocarbenes, and <10 kcal/mol for phenylchlorocarbene. This decrease of Ea is related to the strong stabilization of the azine when proceeding from CR2&dbd;N−N&dbd;CR2 to Ph−CCl&dbd;N−N&dbd;CCl−Ph. Another mechanism for the formation of azine, by a second-order reaction of the diazo isomer of the diazirine, is clearly identified in the case of photolysis of the BCN(N2) diazirine. |
