Hydride transfer processes from metal complexes

Reaction (64) demonstrates the production of a metal formyl complex by intermolecular hydride transfer from a metal hydride which is expected to be regenerable from H2 under catalytic conditions. Further, it provides a plausible model for the interaction of [HRu(CO)4] with Ru(CO)4I2 during catalysis, and suggests a possible role for the second equivalent of [HRu(CO)4]- which the kinetics indicate to be involved in the process (see Fig. 23). Since the Ru(CO)4 fragment which would remain after hydride transfer (perhaps reversible) from [HRu(CO)4] is eventually converted to [HRu3(CO)),] [as in (64)] by reaction with further [HRu(CO)4], the second [HRu(CO)4]- ion may be involved in a kinetically significant trapping reaction. 

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

Several transition metal mediated processes rely on the participation of Lewis acids, among these, the Nicholas reaction figures prominently. After treatment with BF3-Et20, Co2(CO)6 complexed propargylic alcohols provide the carbocation, which can be trapped intramolecularly with various nucleophiles including epoxides (eq 55), or via hydride transfer from an appropriately poised benzyl moiety (eq 56) or aromatic rings. In the latter case, access to the relatively small [7] metacyclophane derivatives was possible. 

The preceding sections have dealt with polymerization by either insertion or GTP mechanisms. Of course, vinyl monomers are also polymerizable by radical, anionic, or cationic mechanisms. In this short section, we summarize the processes which are reasonably well understood from a mechanistic viewpoint, and which involve the intervention of transition metal alkyls (or hydrides), either during initiation, propagation, or chain transfer/termination. A much larger class of polymerization reactions where redox-active transition metal complexes are used to mediate radical polymerizations by reversible atom transfer (ATRP) or other means has been extensively and recently reviewed from a mechanistic perspective and will only be briefly mentioned here. 

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