Stereoselective Synthesis of Highly Enantioenriched (E)‐Allylsilanes by Palladium‐Catalyzed Intramolecular Bis‐Silylation: 1,3‐Chirality Transfer and Enantioenrichment via Dimer Formation

Abstract

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 = acetylacetonate) catalyst at 110 degrees 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, tBuMe2Si, 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 degrees C. The eight-membered ring compounds undergo thermal extrusion of (E)-allylsilanes in high yield at 110 degrees 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.