THERMALLY AND PHOTOCHEMICALLY INDUCED VINYL-HYDROGEN ACTIVATION OF S-1,3,5-HEXATRIENE)](ETA-5-CYCLOPENTADIENYL)COBALT - REGIOSPECIFIC AND STEREOSPECIFIC HYDROGEN MIGRATIONS

ns-1,3,5-hexatriene)](eta5-cyclopentadienyl)cobalt (3) undergoes dimer ization to form a flyover carbene, 5, with concomitant elimination of one equivalent of trans-1,3,5-hexatriene. Structure 5 thermally rearra nges via a metal-mediated [1,5]-H shift to carbene 6: E(a) = 29.1 +/- 0.4 kcal mol-1, log A = 11.6 +/- 0.6. The structures of 5 and 6 were c onfirmed by single crystal X-ray determination. Low temperature irradi ation of 6 generates 13 which undergoes a thermally induced reversion to 6: E(a) = 19.4 +/- 0.9 kcal mol-1, log A = 10.0 +/- 1.3. Deuterium labeling studies indicate the mechanisms involved in these C-H transfo rmations are intramolecular, regio-, and stereospecific. The chemical study of this system is extended to include a variety of homologous Cp Co(triene) complexes. A comparison between the triene approach to the formation of these flyover pentadienyl carbenes and direct carbene add ition is described.