Kinetic and spectroscopic properties of carbene-diazirine ylides

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 dia zirines. However, the yield of formation and the lifetime of CDY greatly de pend on the system considered. The decreasing value of the rate constant of the reaction, carbene + diazirine --> CDY, from similar to 10(9) M-1 s(-1) for singlet dialkylcarbenes, Ad: (adamantylidene) and BCN: (bicyclo[3.3.1] non-9-ylidene), to similar to 10(8) M-1 s(-1) for benzylchlorocarbene and t o similar to 10(6) M-1 s(-1) for cyclopropyl and phenylchlorocarbenes, refl ects the decreasing reactivity of these carbenes. The lifetime of these yli des is determined by the value of the activation energy barrier for their r earrangement to azine, E-a approximate to 15.5 kcal/mol for dialkylcarbenes , approximate to 11.5 kcal/mol fur alkylchlorocarbenes, and <10 kcal/mol fo r phenylchlorocarbene. This decrease of E-a is related to the strong stabil ization of the azine when proceeding from CR2=N-N=CR2 to Ph-CCl=N-N=CCl-Ph. Another mechanism for the formation of azine, by a second-order reaction o f the diazo isomer of the diazirine, is clearly identified in the case of p hotolysis of the BCN(N-2) diazirine.