Adamantylidene revisited: flash photolysis of adamantanediazirine

Irradiation of 2-adamantane-2,3'-[3H]-diazirine (1) in isooctane at room temperature gives two primary photoproducts, 2-diazoadamantane and 2-adamantylidene (Ad:), with quantum yields of about 0.5 each. The fluorescence quantum yield of 1 is small, phi(f) approximate to 10(-4). The reactivity of Ad: was studied by nanosecond laser flash photolysis; it is best described as that of a carbene equilibrating between the singlet ground state and a low-lying triplet state. Ad: reacts with the precursor diazirine 1, pyridine, acetonitrile, tetramethylethylene, amines, water, and acetic acid with rate constants approaching the diffusion-controlled limit, as well as with molecular oxygen (k(O2) = 2.3 x 10(9) M-1 s(-t)). Reaction of Ad: with 1 (k(X) = 3.6 x 10(9) M-1 s(-1)) gives an ylide, lambda(max) = 290 nm, which forms adamantanone azine as a stable product by a reaction having activation parameters E-a = 15.5 kcal mol(-1) and A = 2 x 10(12) s(-t). At low concentrations of 1 in thoroughly dry, degassed solvents, the lifetime of Ad: reaches ca. 225 ns in isooctane, 500 ns in perfluorodecalin, and 700 ns in benzene. Diazoadamantane forms azine by a second-order reaction with a half-life of several hours at 10(-4) concentration. (C) 1998 Elsevier Science S.A. All rights reserved.