Allenylic Carbonates in Enantioselective Iridium-Catalyzed Alkylations

An
enantioconvergent C­(sp3)–C­(sp3)
coupling between racemic allenylic electrophiles and alkylzinc reagents
has been developed. An Ir/(phosphoramidite,olefin) catalyst provides
access to highly enantioenriched allenylic substitution products (93–99%
ee) with complete regiocontrol (>50:1 rr in all cases) over the
corresponding
1,3-diene isomers which are obtained predominantly when other metal
catalysts are emplyed. The synthetic utility of the products obtained
was highlighted in a variety of stereoselective transition metal-catalyzed
difunctionalization reactions. Furthermore, a combination of experimental
and theoretical studies provide support for a putative reaction mechanism
wherein enantiodetermining C–C coupling occurs via nucleophilic
attack on a highly planarized aryl butadienylium π-system that
is coordinated to the Ir center in an η2-fashion.