Insertion alkyl migration promotion

Upon the addition of Lewis (i.e. All ) and protonic non-compSexing. acids (i.e. HPF5) promoters to [Ru(00)3X3] catalysts, improvements in selectivity to valuable products (acetic acid + ethyl acetate) and in reaction rate are observed. This is believed to be due to an acceleration by acids of the alkyl migration-carbonyl insertion step of the process, as discussed in detail in a recent paper (10). 

Carbonyl insertion in MeFe( -C5H5)(PPh3)(CO) is promoted by BF3, with high stereospecificity, by means of alkyl migration. For the CO insertion in MeMn(CO)5, rate enhancements up to one order of magnitude were found in the presence of protonic acids the reaction was, however, complicated by the cleavage of the Mn-Me bond to give methane,... 

Evidence can be found in the literature that CO insertion into a metal-alkyl bond (actually, an alkyl migration) is promoted by an extra-molecular... 

The CO insertion can be promoted by le oxidation of metals. A partial positive charge on the CO carbon induced by increase of electrophilicity of the metal would facilitate the nucleophilic alkyl migration. The effect of electron deficiency is to promote incorporation of the solvent molecules that may assist the CO insertion [67]. For example, rate enhancement of CO insertion into CpFeCH3(CO)2 (Cp = 7 -cyclopentadienyl) by treatment with Ce(IV) and trityl salts has been demonstrated (Eq. 7.6) [68]. 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

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