Mechanistic Features of C-H Activation by ReH72

Article abstract of DOI:10.1021/ja00335a024

The synthesis and characterization of ReH7(PCy3)2 (Cy=cyclohexyl) is reported.This compound is thermally resistant to the dimerization reaction exhibited by other ReH7(PR3)2 compounds, which allows mechanistic features of C-H activation to be studied.Thus, in the temperature range 60-80 deg C, ReH7(PCy3)2 exchanges hydrogen with deuteriobenzene.Deuterium is incorporated into the complex not only as hydride ligands but also selectively at the C2 and C3 carbons of all cyclohexyl rings.Only one of the two methylene hydrogens at each of these carbons is exchangeable.It is also shown that exchange at the two adjacent carbons does not proceed by concerted dehydrogenation via a cyclohexenyl intermediate; the rate of deuterium incorporation at C3 differs from that at C2.On the basis of observation of exchange with D2, and also kinetic studies of the reaction of ReH7(PCy3)2 with phosphine and olefinic nucleophiles, all of the thermal reactions observed here are concluded to arise from the reductive elimination transient ReH5(PCy3)2.