Chiral Brønsted Acid-Catalyzed Asymmetric Allyl(propargyl)boration Reaction of ortho-Alkynyl Benzaldehydes: Synthetic Applications and Factors Governing the Enantioselectivity

Chiral Brønsted acid-catalyzed allyl­(propargyl)­boration of ortho-alkynyl benzaldehydes gives rise to ω-alkynyl homoallylic­(homopropargylic)­alcohols that can be further transformed to complex molecular scaffolds via subsequent hydroalkoxylation, ring-closing enyne metathesis (RCEYM), or intramolecular Pauson–Khand reaction (PKR). Optimizations of each two-step transformation is reported. A strong dependence between enantioselectivities and the nature of the substitution at the alkynyl moiety is observed, showcasing that the triple bond is not merely a spectator in this transformation. Density functional theory (DFT) calculations (M06-2X/6-311+G­(d,p)–IEFPCM//B3LYP/6-31G­(d)) show that this dependence is the result of the steric and electronic properties of the alkyne substituent.