Reduction reactions with zirconium

Of interest is the fact that the H2 required for the reduction of formyl to methoxide in reaction (73) is thought (428, 429) to come from a second zirconium moiety, ZrH2. The H2 reduction to coordinated hydroxymethyl in Eq. (72) could presumably go via a mononuclear H2M(CHO) intermediate, but such a reaction has not been demonstrated, nor has the final hydrogenolysis step via a presumed H2M(CH2OH) intermediate. Loss of water from this to give a carbene HM=CH2 (431, 432) followed by further reactions with H2 and CO, provides speculative pathways to higher hydrocarbons (417, 419a, 430, 433). 

This type of complex is derived from the mononuclear superoxo species via a further one-electron reduction of the dioxygen moiety. Cobalt is the only metal to form these complexes by reaction with dioxygen in the absence of a ligating porphyrin ring. Molybdenum and zirconium form peroxo-bridged complexes on reaction with hydrogen peroxide. In most cases the mononuclear dioxygen adducts of cobalt will react further to form the binuclear species unless specific steps are taken to prevent this. 

Facile isocyanide insertion reactions into metal-carbon, -nitrogen, -sulfur, -oxygen, - hydride, and - halide bonds have been found to readily occur. The insertion into metal-hydrides to give stable formimidines is particularly noteworthy since corresponding formyls (—CHO) are exceptionally difficult to synthesize and tend to be very unstable. There is a great deal of interest in carbon monoxide reductions, and the instability of the intermediate reduction products has made a study of the reduction process extremely difficult. Recently, however, the interaction of isocyanides with zirconium hydrides has allowed the isolation of the individual reduction steps of the isocyanide which has provided a model study for carbon monoxide reduction (39). 

In the case of the reduction of alkenes to alkanes, it does not appear that the stereo-selectivity or -specificity of this process has been studied. It is doubtful that the final steps in the reduction reactions differ from those proposed for the reduction of alkynes, and little stereo-selectivity or -specificity is to be anticipated. TTie mechanism for the reduction of alkenes with LiAlH4 in the presence zirconium(IV) chloride is proposed to occur via a metal exchange reaction followed by hydrolysis (equation 39). ... 

Both alkane reductive elimination and alkali metal reduction reactions have been used to prepare a family of bis(indenyl)zirconium sandwich complexes.104 Crystallographic characterization of the Pr1 304 and the SiMe2But variants establishes an unprecedented 7y9-hapticity of the indenyl ligand where all nine carbons of the carbocycle are engaged in bonding with the zirconium center. Interestingly, this coordination mode was computationally predicted before structural confirmation.183... 

In the course of an investigation into the syntheses and polymerization activity of bis(allyl)zirconium precatalysts, Eisen and co-workers found that the treatment of ZrCU with 2 equiv. of the lithium allyl, (tBuMe2SiGH2)2CHLi(TMEDA), in toluene afforded ( BuMezSiCH CH Zr -Cl LilTMEDA) 2 in 48% yield (Scheme 2).6 The authors concluded that reduction of MCI4 to MC13 with concomitant formation of the allyl dimer and LiCl occurs first, before reaction with further 2 equiv. of lithium allyl yields the bis(allyl)zirconium(m) complex 2. 

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