Acid-catalyzed rearrangement of α-hydroxytrialkylsilanes

Cationic rearrangement of several alpha-hydroxysilanes is described. Treatment of both(1R,1'R,2'S)-alpha-hydroxycyclopropylsilane syn-9 and (1S,1'R,2'S)-anti-9 under aqueous H2SO4 underwent rearrangement via a common alpha-silyl cation intermediate A to give a mixture of the ring-opened (R)-vinylsilane 13, the tandem [1,2]-CC bond migration product (1R,2S,1'R)-14, and its 1'S isomer 15. On the other hand, the acidic treatment of (R,E)-(alpha-hydroxyalkenylsilane 8 or (R,Z)-8 was each accompanied with partial racemization to give an enantiomeric isomer of allylic alcohol 23 via a preferential syn-facial S(N)2' reaction, respectively. Both alpha-hydroxyalkynylsilane 6 and alpha-hydroxyalkylsilane 12 were inert to the acidic conditions; however, treatment of (R)-alpha-mesyloxyalkynylsilane 26 under aqueous H2SO4 gave a mixture of the optically active rearranged allene 27, alpha,beta-unsaturated ketone 28, and (S)-alpha-hydroxyalkynylsilane 6 with partial racermization. Comparisons of the reactivities of these alpha-hydroxysilanes under acidic conditions are also disclosed. (C) 2003 Elsevier Ltd. All rights reserved.