Competing Nucleophilic Displacement and Radical Chain Reduction in Reactions of Transition-Metal Hydride Anions with Alkyl Bromides

Article abstract of DOI:10.1021/ja00245a021

The reactions of group 6 metal hydrides, PPN⁺HM(CO)4L^- (M=Cr, L=CO, M=W, L=CO, P(OMe)3), with two mechanistic probes, 6-bromo-1-hexene (1) and 4-bromo-3,3-dimethyl-1-butene (3), in THF at 25 deg C were studied.Neopentyl-like probe 3 was reduced (presumably) exclusively by a radical chain process, and the second-order rate constants (k_H) for hydrogen atom abstraction from HM(CO)4L^- by the intermediate radical, 2,2-dimethyl-3-butenyl, were determinated.Unhindered probe 1 was reduced by both an S_N2 pathway and a radical chain process.The second-order rate constants for hydrogen atom abstraction from HM(CO)4L^- by 5-hexenyl were estimated, and the percentages of reduction of 1 by the S_N2 pathway and the radical chain process were calculated; the percentage of reduction by the S_N2 pathway increased in the order HCr(CO)5^- < HW(CO)5^- < HW(CO)4P(OMe)3^-.The combination of a hindered and an unhindered probe as used in this study has expanded the utility of mechanistic probes by permitting quantitation of competing pathways.