Alkenylsilane, -Germane, -and Borane Synthesis

Treatment of trimethylsilyldibromomethane with zinc in THF affords the corresponding dizinc compound in good yield [30]. Again, the zinc dust used for these preparations was pyrometallurgy grade zinc, containing about 0.04-0.07% of lead. 

When pure, lead-free zinc is used, a catalytic amount of PbCl2 should be added. 

The silyl group may promote reduction of the C-Br bond at the a-position, as it stabilizes a radical species at this a-position. The obtained gem-dizinc species is fairly stable as it has no y8-hydrogen. In the same way, gem-dizinc species bearing germyl and boryl groups at the a-position have also been prepared in good yields [31, 32]. 

Zn in these equations denotes pyrometallurgy grade zinc, containing 0.04-0.07% lead. When pure, lead-free zinc is used, a catalytic amount of PbCl2 should be added. 

Reactions of carbonyl compounds with these gem-dizinc species gave the corresponding heteroatom-substituted alkenes. In these reactions, the addition of a titanium salt was necessary. jS-TiCls was used, except in the case of reaction of a-boryl-substituted gem-dizinc, where TiCU was used instead (Table 5.4). 

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