Multiplicity and selectivity of cyclopent[a]acenaphthylenylidene

Cyclopent[a]acenaphthylenylidene (1) under bar has been generated by photol ysis from its precursor, the diazocompound <(1a)under bar>. Carbene (1) und er bar could be trapped in olefinic substrates to afford the spirocycloprop anes (8) under bar-<(12)under bar> In addition triplet products of (1) unde r bar such as <(13)under bar> and dimer <(14)under bar> were isolated. With trans-beta-methyistyrene (1) under bar undergoes a stereospecific addition . 2,3-Dimethylbutadiene and (1) under bar reacted to the vinylspirocyclopro pane <(16)under bar> and <(17)under bar>. Competition experiments of (1) un der bar with styrenes give a linear Hammett correlation with a rho=0.38. Th erefore, (1) under bar is regarded as a nucleophilic singlet carbene in its additions to styrenes. (C) 2000 Elsevier Science S.A. All rights reserved.