Stereoselective synthesis of highly enantioenriched (E)-allylsilanes by palladium-catalyzed intramolecular bis-silylation:: 1,3-chirality transfer and enantioenrichment via dimer formation

Highly enantioenriched (E)-allylsilanes have been synthesized from optically active allylic alcohols on the basis of Pd-catalyzed intramolecular bis-silylation followed by highly stereospecific Si-O elimination reactions. The method involves three steps: 1) O-disilanylation of the allylic alcohols with chlorodisilanes, 2) intramolecular bis-silylation in the presence of a 1,1,3,3-tetramethylbutyl isocyanide/[Pd-(acac)(2)] (acac = acetylaceton ate) catalyst at 110° C, and 3) treatment of the reaction mixture with organolithium reagents. The overall transformation proceeds with nearly complete conservation of the enantiopurity of the starting allyl alcohols by transposition of the C=C bond. For instance, (R)-(E)-3-decen-2-ol (99.6-99.7 % ee) produced (S)-(E)-4-(organosilyl)-2-decene of 98.8-99.4% ee for a variety of silyl groups, including Me3Si, Me2PhSi, t-Bu-Me2Si, Et3Si, and iPr3Si. In the bis-silylation step, the initially formed trans-1,2-oxasiletanes immediately dimerize to stereoselectively give 1,5-dioxa-2,6-disilacyclooctanes, which are isolated in high yield by carrying out the reaction at 70°. the eight-mernDered ring compounds undergo thermal extrusion of (E)-allylsitanes in high yield at 110° C, along with formation of 1,3-dioxa-2,5-disilacyclohexane derivatives. These in turn undergo a Peterson-type elimination by treatment with nucleophiles such as BuLi and PhLi to give the (E)-allylsilanes. All of the steps involved in the sequence proceed with extremely high stereoselectivity and stereospecificity, leading to almost complete 1,3-chirality transfer through the overall transformation. The dimerization step, which forms diastereomeric intermediates, allows the synthesis of a highly enantioenriched allylsilane (99.4% ee) from an optically active allylic alcohol with lower enantiopurity (79.2% ee) by enrichment of enantiopurity. A general method for the determination of the enantiomeric excesses of (E)-allylsilanes is also described in detail.