Hydride transfer from transition metal alkyls

Hydride Transfer from Transition Metal Alkyls... 

Considerable mechanistic examination of hydride transfer from transition metal alkyls where M is, for example, CsH5Fe(CO)2 or C5HsRe(NO)PPh3 has been carried out. Hydride transfer may occur either from an a or -position on the alkyl framework, dependent on structural features and the identity of the transition metal. Stereoelectronic effects, kinetic factors, and electronegativity considerations have been considered in recent review articles. Because of the excellent coverage already available, no detailed discussion will be given here. 

As the second selectivity issue, the catalyst should usually favor w-aldehyde over iso-aldehyde formation. This task is mainly addressed by the right choice of ligand. The ligand influences both the electronics and sterics of the catalyst in the step of the catalytic cycle that determines regioselectivity (see -II versus iso-11 in Scheme 6.14.4). Note that the transition state leading to the linear hydroformylation product involves a linear alkyl chain attached to the metal center that requires less space compared to the branched counterpart. Moreover, the electronic properties of the ligand influence the hydride transfer from the metal complex to Cl versus C2 during formation of the metal-carbon bond. 

When represented in this way the chemistry of carbonyl complexes of transition metals becomes easier to understand. Hydroformylation reactions and other carbonylations that are catalyzed by transition-metal complexes frequently involve hydride or alkyl transfers from the metal atom to the positive carbonyl carbon (Sections 16-9G, 31-3, and 31-4) ... 

When a transition metal alkyl or a metal hydride reacts with olefin molecules to undergo successive insertions, chain growth of a polymer attached to the transition metal takes place. If -hydrogen elimination occurs from the polymer chain, a transition metal hydride coordinated with the olefin derived from the polymer chain will be produced. By displacement of the coordinated olefin from the transition metal by the other monomer olefin, the polymer with an unsaturated terminal bond is liberated with generation of a transition metal hydride coordinated with the olefin. New chain growth will follow from the hydride, with the net result of control of the molecular weight without termination of the polymerization process. The process is in fact a chain transfer process. 

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. 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

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