The use of gaseous allene as an allyl pronucleophile in enantioselective aldehyde reductive coupling is described. Notably, using the same antipode of chiral ligand, (S)-tol-BINAP, an inversion of enantioselectivity is observed for allene vs allyl acetate pronucleophiles. Experimental and computational studies corroborate intervention of diastereomeric π-allyliridium-C,O-benzoate complexes, which arise via allene hydrometalation (from a pentacoordinate iridium hydride) vs allyl acetate ionization (from a square planar iridium species).
Keywords: allylation; enantioselective; iridium; reductive coupling; transfer hydrogenation.