Asymmetric Conjugate Addition of Alkynylboronates to Enones: Rationale for the Intriguing Catalysis Exerted by Binaphthols

The conjugate addition of alkynylboronates to enones catalyzed by binaphthols has been studied theoretically with DFT methods. The high reactivity of the alkynylboronate derived from binaphthol seems to arise from electronic effects since its acidic boron atom binds tightly to the enone carbonyl and lowers the activation energy of the alkynylboration step. Steric clashes between the atoms of the ligands on boron and the enone can be invoked to account for the observed facial diastereoselectivity. The competing hetero-Diels-Alder reactions are computed to be kinetically disfavored relative to alkynylborations.