Sterically-driven methyl-halide elimination

In the presence of bulky X ligands, a facile methyl halide elimination reaction is observed (Eq. 2) [3]. In this elimination the siliconium ion complex 2, with its two N—>Si dative bonds, is converted into a neutral pentacoordinate complex 3, with only one remaining dative bond (Fig. 1, Table 1). The reaction is probably driven by partial release of steric interaction, caused by the removal of one of the A-methyl groups. This is indicated by a decrease in elimination rate in the presence of less bulky ligands, cyclohexyl and isobutyl, and the failure to observe elimination when X = methyl. The reactivity order of the halide ions follows their nucleophilicities F > Br > CF, while the less nucleophilic ttiflate ion does not react at all. 

The scope of this reaction is as yet unknown, and is under investigation. It seems likely that the elimination of methyl halide is driven by steric repulsion between the /-butyl ligand and the bulky dimethylamino donor, since it has been observed only in /-butylsiliconium salts. 

Recently, Milstein and coworkers reported an interesting CH3-I reductive elimination chemistry from Rh(III) complexes (Scheme 31) [74]. The reactions, driven by steric bulk of the pincer ligands, represent the first example of the directly observed reductive elimination from metal complexes other than group 10. It was proposed that the reactions proceed via a concerted three-centered transition state rather than the SN2-type back attack of the halide at the methyl group, as was proposed for the isoelectronic Pt(IV) complexes. To the best of our knowledge, no... 

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