BINAP-CuH-catalysed enantioselective allylation using alkoxyallenes to access 1,2-syn-tert,sec-diols

Herein, we present an economical method for highly enantioselective and diastereoselective Cu-BINAP-catalysed reductive coupling of alkoxyallenes with a range of electronically and structurally diverse ketones to afford 1,2-syn-tert,sec-diols, using PMHS as the hydride source. This reductive coupling has also been efficiently employed in the enantioselective desymmetrization of prochiral cyclic ketones harboring quaternary centres, in high yields with exclusive diastereoselectivity. Density Functional Theory (DFT) calculations are used to elucidate that the reaction is facilitated by a kinetically favourable "open" Z-enolate copper-alkoxyallene conformer, occurring at a lower Gibbs free energy barrier (by 3.9 kcal mol^-1) than its E-enolate counterpart, dictating the stereoselectivity. Subsequently, this Z-enolate conformer synchronizes with appropriate nucleophilic faces to achieve the targeted syn-diastereoselectivity in the product through six-membered chair-like transition states during ketone addition.