Thermally and photochemically induced vinyl-hydrogen activation of [η-1,2:3,4-(trans-1,3,5-hexatriene)](η-cyclopentadienyl)cobalt: Regio- and stereospecific hydrogen migrations |
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Authors: | Joseph A KingJr K Peter C Vollhardt |
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Institution: | Department of Chemistry, University of California, Berkeley, and the Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720 USA |
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Abstract: | 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: Ea = 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: Ea = 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. |
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Keywords: | Cobalt Photochemistry Hydrogen migration Bond activation Thermochemistry Metallocarbene |
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