RELATIVE REACTIVITY AND MECHANISTIC STUDIES OF THE HYDRIDE-TRANSFER REAGENTS HM(CO)4L- (M = CR, W - L = CO, PR3)
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A series of anionic transition-metal hydrides have been compared according to their ability to reduce halocarbons (primarily alkyl bromides) to hydrocarbons. The 6B hydrides HM(CO)4L-(M= Cr, W; L = CO, PR3) are highly efficient hydride-transfer reagents, displacing X from a wide variety of C-X bonds, including tertiary centers, and tolerant of functionalities such as nitro groups, ketones, and aldehydes. A facile H/D exchange process with CH3OD or D20 readily converts the MH-reagents into deuterium delivery reagents. There is little difference between the reactivity of HM(CO)5-with primary vs. secondary vs. tertiary alkyl bromides; however, the cis-HM(CO)4PR3-anions are far more reactive with the less hindered primary than branched alkyl halides. A comparison of the second-order rate constants for bromide displacement from n-BuBr established an order of reactivity for simple monomeric carbonyl hydrides cis-HW(CO)4P-> cis-HCr(CO)4P-> HW(CO)6-> CpV(CO)3H-> HCr(CO)5-> HRu(CO)4-> trans-HFe(CO)3P- HFe(CO)4 (no reaction). The relative reactivity was shown to correlate with the extent of electron density localized at the M-H bond as indicated by hydride site specific contact ion pairing with Na+ in THF solution. Various mechanistic probes suggested the reagents most prone to contact ion pairing at the M-H-site to react with RX via SN2 processes whereas the complexes with the anionic charge delocalized were more prone to electron-transfer mechanisms. 1985, American Chemical Society. All rights reserved.