Mechanism, metal hydride methyl transfer

In agreement with this mechanism terminal groupings (methyl from polyethylene and isopropyl from polypropene) increase in concentration. When deuterium is used the end groups are CH2D [34] and with tritium two atoms per molecule have been found one from initiation by the metal hydride bond and the other on chain transfer [82]. 

On the basis of DFT study of hydrogenation of dimethyl carbonate catalysed by a PNN-ligated ruthenium complex, a direct metal hydride and ligand proton transfer mechanism with three cascade catalytic cycles for the hydrogenation of dimethyl carbonate, methyl formate, and formaldehyde to methanol has been proposed. ... 

The first peroxide oxygen atom is transferred to the alkene by a peroxymetalation pathway involving coordination of both Oz and alkene to the metal in (71a) or (71b), followed by the formation of a peroxometallacycle (72a) or (72b) which decomposes to give the methyl ketone and the rhodium-oxo (73a) or -hydroxo (73b) species. The second hydroxo oxygen atom is transferred via an internal cis hydroxymetalation mechanism (Wacker type), which produces a second mole of ketone and regenerates the initial Rh111 hydride or the Rh1 species by reductive elimination of the latter. Two plausible alternative versions of the peroxymetalation pathway can be envisaged133 and these are described below. 

Alternatively a different mechanism was proposed by several other authors, namely the direct isomerization of the olefinic starting material to isomeric olefins which then are hydroformylated in the second stage of the reaction. However, from the excellent work of P. Pino, S. Pucci and F. Pia-centi [89], who reacted optically active (- -)(S)-3-methyl-l-pentene, it can clearly be seen that under normal oxo conditions little or no isomerization of the olefinic double bond occurs prior to the hydrocarbonyl addition. Such a type of isomerization is only to be expected if very low carbon monoxide partial pressures are applied [61, 88, 89, 914]. The results of Pino etal. suggest that a multistep isomerization occurs while the olefin is attached to the metal. This could be the case if the hydride transfer, which is a necessary step in this isomerization, is faster than olefin exchange. This assumption is in line with earlier work on ironcarbonyl catalyzed isomerizations. 

The methyl hydrogens in the tri-p-phenoxo complex [ Rh(C5Mes) 2(OPh)3][(PhO)5H4] exchange moderately quickly ti/2 — 40 min at 20°C) with deuterium in [ He] acetone solution. A mechanism was proposed involving formation of the enolate complex 4, followed by transfer of a methyl hydrogen to the metal to form a fulvene-rhodium hydride species 5 from which the H can be eliminated by transfer to enolate to give coordinated acetone. 

P(OMe)3] with Ati=4.2x 10 s" (30 °C). As with the osmium complexes just discussed this rate is several orders of magnitude lower than that of methane elimination from the corresponding methyl hydride while reductive elimination of methane from [Co(Me)2 P(OMe)s 4]+ exhibits an intermediate reactivity. The latter reaction is consequently thought to proceed via a multistage mechanism in which carbene complexes (resulting from a-hydrogen transfer to the metal) play an important role. Elimination of CH4 from the methyl hydride complex is probably a concerted process according to these workers. 

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