Direct and indirect synthesis of α-chiral allylic silanes by asymmetricallylic substitution with silicon and carbon nucleophiles
Chiral copper(I)–carbene complexes are used as catalysts to displace allylic phosphates with silicon nucleophiles in highly regio- and enantioselective fashion, finally providing direct access to α-chiral allylic silanes from non-silicon-containing precursors. Distinct N-heterocyclic carbene ligands allow for either enantioconvergent or -specific displacements starting from either double bond isomer. This substantial progress in asymmetric allylic silylation complements nicely the recently developed protocols for the indirect synthesis of this motif by enantioselective allylic substitution of γ-silicon-substituted acceptors with conventional carbon nucleophiles.
The chemistry of α-chiral allylic silanes constitutes a major field of research in organic synthesis, and the ever increasing variety of their applications is accompanied by steady progress towards their efficient preparation. Among these, allylic substitution with silicon nucleophiles is a common way to access these building blocks. This approach usually relies on the catalytic generation of silicon-based copper reagents from various silicon pronucleophiles that subsequently transfer the silicon moiety onto the allylic acceptor. Over the years, this development culminated in the copper(I)-catalysed, branched-selective silylation of linear allylic chlorides (as well as phosphates) to afford racemic α-chiral allylic silanes (1, 2). Despite the significance of this fundamental transformation, an asymmetric version had remained elusive until recent independent reports by Oestreich and, shortly thereafter, Hayashi and Shintani. We wish to summarise here these seminal findings and bring these into relation with indirect asymmetric syntheses of α-chiral allylic silanes by regioselective displacement of γ-silicon-substituted allylic acceptors w ...