LASER FLASH-PHOTOLYSIS OF 2-ADAMANTANE-2,3(1)-[3H]-DIAZIRINE - A REINVESTIGATION

Two of us (MSP and SM) previously reported [S. Morgan, J.E. Jackson, M .S. Platz, J. Am. Chem. Sec. 113 (1991) 2782] that adamantylidene (Ad: ) reacts with thiophene to form an ylide which absorbs at 320 nm. It w as concluded that Ad: has a lifetime of 2.2 mu s in benzene and rather low absolute rate constants of reaction with thiophene and pyridine ( 1.75 x 10(6) M-1 s(-1) and 1.54 x 10(6) M-1 s(-1) respectively). The p resent study shows that the previously reported data, while reproducib le, were misinterpreted. Ad: is shown to be a more short-lived carbene in benzene and alkane solvents and to react with pyridine with k(pyr) = 2.0 x 10(9) M-1 s(-1) (benzene) or 2.6 x 10(9) M-1 s(-1) (cyclohexa ne) as per other simple carbenes. This interpretation is consistent wi th 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 with spiro[a damantane-2,3(1)-[3H] -diazirine] in argon purged benzene at ambient t emperature with k = 1.1 x 10(9) M-1 s(-1) to form a species with lambd a(max) = 290-300 nm. The lifetime of Ad: in argon purged benzene at am bient 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 temperatur e. (C) 1998 Elsevier Science S.A. All rights reserved.