Laser flash photolysis of 2-adamantane-2,31-[3H]-diazirine: a reinvestigation

Two of us (MSP and SM) previously reported [S. Morgan, J.E. Jackson, M.S. Platz, J. Am. Chem. Soc. 113 (1991) 2782] that adamantylidene (Ad:) reacts with thiophene to form an ylide which absorbs at 320 nm. It was concluded that Ad: has a lifetime of 2.2 μs in benzene and rather low absolute rate constants of reaction with thiophene and pyridine (1.75×106 M−1 s−1 and 1.54×106 M−1 s−1 respectively). The present study shows that the previously reported data, while reproducible, were misinterpreted. Ad: is shown to be a more short-lived carbene in benzene and alkane solvents and to react with pyridine with kpyr = 2.0×109 M−1 s−1 (benzene) or 2.6×109 M−1 s−1 (cyclohexane) as per other simple carbenes. This interpretation is consistent with that of Bonneau, Liu, Hellrung and Wirz (accompanying paper) whose work, as well as a personal communication from the Canadian co-authors of this paper, stimulated this study. Ad: reacts rapidly swith spiro[adamantane-2,31-[3H]-diazirine] in argon purged benzene at ambient temperature with k= 1.1 × 109 M−1 s−1 to form a species with λmax = 290–300 nm. The lifetime of Ad: in argon purged benzene at ambient temperature, extrapolated to infinite diazirine dilution, is 69 ns, under these conditions. However, the lifetime of Ad: is extremely sensitive to trace amounts of water and oxygen. In rigorously purified benzene, the lifetime of Ad: is at least 553 ns at ambient temperature.