Redistribution reactions Grignard reagents

The Kocheshkov redistribution reaction between tetra-organotins and tin(rV) halides, in appropriate mole ratios, is a general method of synthesis of organotin halides (equations 59 and 60). It is a particularly useful reaction, since it is difficult to control the alkylation of SnX4 by organolithium or Grignard reagents to the desired extent. 

The complexes [Tp ]MgR may be considered to be modifications of Grignard reagents. As such, these complexes may be expected to be subject to the facile ligand redistribution reactions of the Schlenk equilibrium. Indeed, although solutions of the tris(3,5-dimethylpyrazolyl)hydroborato derivatives [Tp ]MgR are stable at room temperature, heating to 80-120°C does result in ligand redistribution and the formation of the six-coordinate sandwich complex [Tp ]2Mg [Eq. (4), 21,22]... 

The foregoing redistribution reactions are, evidently, significantly slower than those involved in the Schlenk equilibrium of Grignard reagents. Consequently, a mixture of the alkyl derivatives [Tp ]MgCH3 and [Tp -]MgC(CH3)3 does not result in the formation of crossover products after heating at 80 C for 3 days [Eq. (5)]. 

Organoindium halides can readily be prepared by many of the synthetic methods outlined for the preparation of organoindium derivatives, but with stoichiometric control to afford the organoindium halide derivative. For example, (a) reaction of InXa with less than three equivalents of a lithium or Grignard reagent (equations 12-13), (b) reaction of Rsin with HX, which proceeds by elimination of a simple organic moiety (equation 14), (c) a redistribution reaction between InXa and Rain (equations 15-16). 

Excess alkylating reagent is required if the tetraorganotin is desired as the exclusive product. In commercial practice, the stoichiometry is kept at or below 4 1, since the cmde product is usually redistributed to lower organotin chlorides in a subsequent step and an ether is used as the solvent (86). The use of diethyl ether in the Grignard reaction has been generally replaced with tetrahydrofuran. 

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