Catalytic Asymmetric Synthesis of Allenes Bearing Axial Chirality

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Optically active allenes bearing an axial chirality are important synthetic intermediates for various naturally occurring biologically active compounds. Much attention has been paid to the asymmetric synthesis of this class of compounds. Optically active allenes have been prepared by either optical resolution of racemic allenes or chirality transfer reactions of optically active precursors. In this project, some attractive methods for preparation of optically active allenes using a chiral catalyst were explored.1. Carbonylation of racemic propargylic phosphates using a palladium complex catalyst bearing an optically active bisphosphine ligand proceeds with kinetic optical resolution to give axially chiral allenecarboxylic acid esters along with recovered optically active propargylic phosphates. This is the first example of asymmetric synthesis of allenes using a catalytic amount of chiral compound. The enantioselectivity depends on CO pressure employed, and the reaction under high pressure of CO and that under atmospheric pressure of CO give allenes of oposite absolute configurations.2. Nucleophilic substitution of racemic 2,3-alkadienyl phosphates with a variety of nucleophiles using a palladium complex catalyst bearing an optically active bisphosphine ligand gives optically active substituted allenes. Mechanistis studies reveal the intermediacy of 1,3-alkadienylpalladium as a key intermediate for asymmetric induction.