Vinyl groups, stereospecific coupling

Recent developments in catalysis at the atomic level are described. The use of transition metal halides which form intermediate metal n-complexes can promote both the catalytic and stoichiometric coupling of aryl Grignard reagents, the stereospecific coupling of 1,2 disubstituted vinyl groups and the double coupling of ethylene to dimers of butadiene. 

Figure 5. Stereospecific coupling of disubstituted vinyl groups...

Stereospecific coupling of 1,2-disubstituted vinyl groups is attained by reaction of a vinyl Grignard with CrClg, CoClj, PdClj, and NiBrz catalysts (50, 51). The stereospecificity of the products appears to result from the selective configuration of the transition metal halide (Fig. 20). 

Afonin, A.V, M.V. Sigalov, VK. Voronov et al. 1987. Stereospecificity of direct C- H spin-spin coupling constants in the vinyl group of N-vinylpyrroles. BAcadSci USSR Ch 6 1418-1421. 

This method was employed in the stereospecific construction of a silyl diene in a synthesis of chlorotri-cholide (Scheme 2.80). The boronic acid partner 2.233 was prepared by hydroboration and hydrolysis. The vinyl iodide was 2.235 prepared by hydroalumination-iodination. The silyl group was included in order to boost stereoselectivity in the later Diels-Alder reaction, but also served to facilitate the synthesis of the vinyl iodide coupling partner. The Suzuki coupling yielded the diene 2.236 with retention of the stereochemistry of both alkenes. Thallium hydroxide was employed as the Lewis base. Thallium-containing Lewis bases have been found to be advantageous in a number of cases, but the toxicity of thallium is a serious concern. 

Cross-Coupling Reactions. TASF(Et) activates vinyl-, alkynyl-, and allylsilanes in the Pd-mediated cross-coupling with vinyl and aryl iodides and bromides. As illustrated in eqs 8-10, the reaction is stereospecific and chemoselective. This crosscoupling protocol is remarkably tolerant towards a variety of other functional groups such as carbonyl, amino, hydroxy, and nitro. Vinylsilanes can be synthesized from hexamethyldisilane and vinyl iodides in the presence of TASF(Et) (eq 10) via cleavage of a Si-Si bond. Aryl iodides can also be synthesized by this method. TASF is superior to tetra-n-butylanunonium fluoride for these reactions. In the absence of a vinylsilane reagent, one of the methyl groups from the difluorotrimethylsilicate is substituted for the halide (eq 11). ... 

The direct coupling of two unlike alkenyl groups, for example via vinylic cuprates and vinyl iodides, has not proved useful as a stereospecific process. However, Dang and Linstrumelle have now shown that stereospecific substitution (>97%) of alkenyl iodides with a variety of Grignard reagents can occur in high yield and under mild conditions, when catalysed with tetrakis(tri-phenylphosphine)palladium (Scheme 6). Linstrumelle and his co-workers have... 

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