Studies on Stereoregulated Reaction Using Organosilicon Reagents and Highly Selective Synteses of Heterocyclic Compounds

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Much attention has been recently focused on both syntheses of tetracoordinate allylsilanes and their applications to organic synthesis. Allylsilanes whose silicon-allyl sigma-bond is thermally stable can be highly functionalized allvl are transferred onto carbon electrophiles activated by a Lewis acid with high regio-, stereo- and chemoselectivity. One of the most interesting features in organosilicon chemistry is that stable high coordinate species can be isolated. In an extension of organosilicon compounds in organic synthesis, we have found that pentacoordinate allylsiliconates and related compounds, unknown hitherto, can be readily prepared and used as nucleophiles, some of which are reported.Topics are (1) preparations of pentacoordinate allylsiliconates, substituted by alkoxy groups, which are useful and storable reagents, regio-, chemo- and stereoselective allylations of carbonyl compounds without catalyst, and stereochemical outcomes of asymmetric allylation with optically active allylsiliconates which are interpreted by a six-membered cyclic transition state, in sharp contrast with an anti-S_<E'> mechanism for the allylation using tetracoordiante allylsilane, (2) Chemo- and stereoselective reductions of carbonyl compounds using alkoxy-substituted hydridosiliconates derived from trialkoxysilane and alkali metal alkoxides, and new catalytic asymmetric reductions of prochiral ketone using hydridosiliconates. (3) Cross-coupling reactions of alkenylsiliconates with organic halides and triflates catalyzed by a palladium complex in a stereoselective mode. (4) Syntheses of highly reactive species which were hardly obtainable and not isolable were achieved, Organosilicon reagents that acted as a synthetic equivalent of alkylideneazomethine ylids were prepared and allowed to react with alkenes, carbonyl and thicarbonyl compounds to give the corresponding [3+2] cycloadducts in high yields.