Silanes transition-metal halides

Biaryls have also been prepared by coupling support-bound aryl halides with aryl-zinc compounds (Figure 5.20) or with aryl(fluoro)silanes [203]. As with Suzuki or Stille couplings, these reactions also require transition metal catalysis. An additional strategy for coupling arenes on solid phase is the oxidative dimerization of phenols (Figure 5.20). 

This is a reaction that seems veiy attractive for synthesis but, in the absence of a transition met catalyst, the yields are very low. We showed in the last chapter how vinyl silanes can be made wit control over stereochemistry and converted into lithium derivatives with retention. Neither of thes vinyl metals couple with vinyl halides alone. But in the presence of a transition metal—Cu (I) for I and Pd(0) for Sn—coupling occurs stereo specifically and in good yield. 

If neither the halides nor their metal derivatives have a stable structure, how can we expect to control their reactions Things are not quite as bad as that. The bromides and chlorides are stable at low temperatures in the absence of Lewis acid catalysts or radical generators. The Grignard reagents often react cleanly to give the less substituted double bond isomer, presumably by the type of process shown in 12. We shall be seeing methods to control transition metal allyls and allyl silanes which make these intermediates valuable reagents in synthesis, and you should finish this chapter in a confident mood without fear of fluxional allyl compounds. 

Cross-coupling Reactions. Trimethylsilylmethylmagnesium chloride reacts with organic halides (or pseudohalides), especially aryl and alkenyl halides, in the presence of transition metal catalysts. The reactions directly provide allylic or benzylic trimethyl-silanes of significant synthetic use. 

In common with the synthesis of trifluoroisopropenyl-boronic acid, trifluoro-isopropenylsilane can be also prepared in 80% yield by treatment of 2-BrTFP with 1.2 equivalents of magnesium in the presence of 2.0 equivalents of dimethylphenylsilyl chloride in THF at — 10°C to room temperature (Scheme 26.45). This reagent can employ as CF2=C -CH2 synthon, but there have been no reports on the transition metal-catalyzed cross-coupling reaction of tri-fluorinated silane with halides. 

The Rochow Process. Rochow found that alkyl and aryl halides react directly with silicon when their vapors contacted silicon at elevated temperatures to produce complex mixtures of organosilicon halides. The reaction is promoted by a wide variety of metals from both the main group and the transition series, but the most efficient catalyst is copper. The most studied reaction of this type is the reaction between methyl chloride and silicon to give dimethyldichlorosilane and methyltrichlorosilane. Dimethyldichloro-silane is major feedstock silane for methylsilicon polymers. 

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