Metal-methyl bonds

Fig. 5. The Arrhenius plot of kl/k2i. e, dimethyl zinc O, trimethyl gallium , trimethyl indium. % figures are in terms of the theoretical yield of methyl radicals that would be obtained if all methyl-metal bonds were broken. All points are corrected to 13 torr pressure (toluene+0.3-2.5 % alkyl).

Tracer studies with 13C showed that the carbon of methoxy species formed in CO hydrogenation appeared as the central C atom of isobutylene.531 Methoxy was shown to be reduced to surface methyl species that was transformed to surface acetyl group via insertion of CO into the methyl-metal bond. Aldol condensation of C2 oxygenates with formaldehyde affords iso-C4 products.532 Earlier results of a different approach for isosynthesis using metal-zeolite catalyst systems are summarized in a 1998 review.533... 

Methyl-metal bond cleavage also occurred in the reactions of gold(I), gold(III), and platinum(II) compounds with thiols and selenols (207-209). The reactions of gold(I) and platinum(II) complexes were much faster than those of gold(III), due to the operation of a radical chain mechanism in the former cases. For the reactions with diphenylphosphine, however, the following order of reactivity was found (209) ... 

Total decomposition to Ga atoms has been inferred from IR measurements (117, 120, 121). but no direct observations have been made of Ga atoms during the thermal gas-phase pyrolysis of Ga(CH3)3. GaCH3 is considered to be the most stable species, (114, 123), whereas In atoms may be formed during the decomposition of In(CH3)3 (116). The difference between the decompositions of In(CH3)3 and Ga(CH3)3 correlates well with the stronger methyl-metal bond in Ga(CH3)n (116). 

Chlorine, bromine and iodine generally cleave germole and stannole rings850,147-150. From 1,1-dimethyltetraphenylgermole or -stannole, [(Z,Z)-butadienyl]metal halides are formed in a quantitative yield (equation 49). The methyl-metal bonds remain intact. In the tin series, further chlorination or bromination gives rise to dimethyltin dihalide and dihalobutadiene (equation 50). 

Organometallic Niobium ) and Tantalum ) Compounds. Reports this year have concerned two main areas of interest the preparation of alkyl-niobium and tantalum halides and their reactions with donor ligands, and insertion reactions into methyl-metal bonds. The first examples of monomethyl niobium and tantalum compounds have been prepared by the low-temperature methylation of the appropriate halides... 

The chemistry of acetyl-CoA synthesis is thought to resemble the Monsanto process for acetate synthesis in that a metal center binds a methyl group and CO and the CO undergoes a carbonyl insertion into the methyl-metal bond. Elimination of the acetyl group is catalyzed by a strong nucleophile, iodide in the industrial process and CoA in the biochemical one. Currently, there are two views of the catalytic mechanism. 

The key elements of these carbonylation processes is the ability of a metal complex to undergo facile oxidative addition with methyl halide (especially iodide), carbon monoxide (CO) insertion into the methyl-metal bond, and reductive elimination of the acetyl group as the acetyl halide [3]. 

X 10 s ) and the phenyl (ki = 1 x lO s ) derivatives to be made at 30 C in 2,2 -diethoxydiethyl ether the cleavage to form the coordinatively unsaturated intermediate occurs more readily for the methyl-manganese bond than for the phenyl derivative. In general, phenyl-metal bonds are stronger than methyl-metal bonds... 

A. Van Rumohr and W. Sundermeyer. Chemical reactions in molten salts. XVII Electrochemical formation of methyl-metal bonds. Z. anorg. Chem., 1978,443, 37. 

We discount the likelihood of reaction R5 because two metal-bonded methyl groups have no molecular orbitals readily available for reaction. These groups are sp3 hybridized with all 4 orbitals fully occupied. The lowest lying unoccupied orbital is so much higher in energy that it is inaccessible, and there arc no obvious interactions that might enable a rehybridization to lower this energy. 

All synthetic methods described up to now (ligand displacement, acac or halo-methyl precursors, metal-bonded carbene + nucleophile, metal-bonded nucleophile + carbene) result in a metal-bonded ylide through the Ca atom. The reactivity of ylides toward metallic systems is, however, greater than anticipated and other reaction pathways could compete with simple C-bonding. 

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