Transition metal hydrides as reagents in organic and organometallic synthesis
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1987
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Abstract
The group 6 anionic transition metal hydrides, HM(CO)4L- (M = Cr, W; L = CO, P(OMe)3 were found to be convenient reagents in organic and organometallic synthesis. These reagents offer the advantages of high selectivity, relative ease of preparation, and the potential for catalytic activity through regeneration using inexpensive reagents. These hydrides were found to reduce carbonyl functionalities within organic substrates, such as aldehydes, as well as carbonyls attached to metal centers. The reduction of acyl and alkyl substrates was also possible. Reduction of alkyl and acyl halides was easily achieved with high selectivity. When the alkyl or acyl moiety was attached to a metal center, reaction with the group 6 hydrides eliminated the organic product through a binuclear reductive elimination pathway. The formation of a heterobimetallic bridging hydride intermediate was observed in some of these reactions. Addition of an activated olefin, such as acrylonitrile, to selected anionic transition metal hydrides and alkyls resulted in formation of the alkyl addition product. Olefin addition could also occur to the heterobimetallic hydride, HFeW(CO)9-. Addition of a metal-bound olefin, (CH2=CHCN)Fe(CO)4, to the group 6 hydrides also resulted in formation of a metal alkyl. However, in this case a heterobimetallic product resulted due to a M-M' adduct reaction concomitant to the hydride transfer. Elemental sulfur was also found to be an active substrate for hydride addition. Reaction of S8 with selected anionic hydrides and alkyls resulted in the formation of the addition product, RSM(CO)n-. Reactivity differences were noted in systems employing one metal center vs. systems employing two metal centers. A measure of the effect of the second metal center could be achieved by comparison of a similar monomeric and dimeric system. The reactivites of MeFe(CO)4- and MeFe(CO)4W(CO)5- were tested as a gauge for this effect. As well as exhibiting successful H- (two-electron) transfer behavior, the anionic metal hydrides could function as efficient one-electron transfer reagents. Addition of two equivalents of hydride to a series of metal carbonyl dimers, M2(CO)[n]L[m], resulted in dimer disruption yielding the metal carbonyl anions.
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Major chemistry