Thermally and photochemically induced vinyl-hydrogen activation of 4-1,2:3,4-(trans-1,3,5-hexatriene)>(η5-cyclopentadienyl)cobalt: regio- and stereospecific hydrogen migrations

Article abstract of DOI:10.1016/0022-328X(94)80169-X

<η⁴-1,2:3,4-(trans-1,3,5-hexatriene)>(η⁵-cyclopentadienyl)cobalt (3) undergoes dimerization to form a flyover carbene, 5, with concomitant elimination of one equivalent of trans-1,3,5-hexatriene.Structure 5 thermally rearranges 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 confirmed by single crystal X-ray determination.Low temperature irradiation 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 transformations are intramolecular, regio-, and stereospecific.The chemical study of this system is extended to include a variety of homologous CpCo(triene) complexes.A comparison between the triene approach to the formation of these flyover pentadienyl carbenes and direct carbene addition is described. Key words: Cobalt; Photochemistry; Hydrogen migration; Bond activation; Thermochemistry; Metallocarbene