copper - enantioselective vinylation - vinylsiloxanes
Significance
<P>Fast, effective and reliable methods for obtaining chiral allylic alcohols generally
employ enantioselective resolution or alkenylzinc addition to aldehydes. These methods
either waste half of the available reagent, or use pyrophoric, potentially dangerous
reagents. Herein is presented a catalytic enantioselective method to synthesize chiral
allylic alcohols and diarylmethanols from addition of air-stable vinyl- or phenylsiloxanes
to aldehydes. Various aromatic, heteroaromatic, and easily enolizable aliphatic aldehydes
can be coupled with simple and highly substituted vinylsiloxanes with excellent enantioselectivity.</P>
Comment
<P>The CuF
2-(DBTM-SEGPHOS) catalyst is proposed to transmetallate the vinylsiloxane moiety to
generate a fluorosiloxane and a chiral vinylcopper species. This species shows opposite
reactivity to traditional organocopper reagents, displaying complete 1,2-selectivity
in one of the examples. Ligand acceleration was observed, and suggested to be due
to steric rather than electronic factors (for examples see: A. F. Littke, L. Schwarz,
G. C. Fu
J. Am. Chem. Soc. 2002,
124, 6343-6348; E. R. Strieter, D. G. Blackmond, S. L. Buchwald
J. Am. Chem. Soc. 2003,
125, 13978-13980; S. Yamasaki, M. Kanai, M. Shibasaki
J. Am. Chem. Soc. 2001,
123, 1256-1257). Finally, easily accessible 1,1′-disubstituted vinylsiloxanes were used
with modest selectivity, whereas the alkenylzinc analogue is not readily accessible.</P>
