Understanding the Mechanism of the Asymmetric Propargylation of Aldehydes Promoted by 1,1′-Bi-2-naphthol-Derived Catalysts

1,1'-Bi-2-naphthol (BINOL)-derived phosphoric acids catalyze the asymmetric propargylation of aldehydes. Density functional theory (DFT) calculations showed that the reaction proceeds via a six-membered transition structure (TS) in which the catalyst Brønsted acidic site interacts with the pseudoaxial cyclic boronate oxygen and the phosphoryl oxygen interacts with the formyl proton. This model accurately predicts the stereochemical outcome observed experimentally. Replacement of the phosphoric acid hydroxyl group with an N-triflyl moiety has been included in the model by calculation and a broader understanding achieved by qualitative assessment of similar reactions. We present a qualitative guide to rationalizing the experimental outcome and use this to make a prediction which was confirmed experimentally.