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Theoretical Study of the Asymmetric Conjugate Alkenylation of Enones Catalyzed by Binaphthols

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journal contribution
posted on 04.07.2008, 00:00 by Robert S. Paton, Jonathan M. Goodman, Silvina C. Pellegrinet
To rationalize the experimental results observed in the asymmetric conjugate addition of alkenylboronates to enones catalyzed by binaphthols and shed light into the factors controlling the rate, the selectivity, and the substituent effects of this process, a theoretical DFT study has been performed. The calculations suggest the catalytic cycle is finely balanced. Reversible exchange of methoxy ligands gives rise to the binaphthol-derived alkenylboronate, which is highly Lewis acidic and strongly coordinates to the enone carbonyl in a reversible fashion, lowering the energy barrier for the subsequent conjugate addition step. The key asymmetric step goes through a sofalike transition structure in which the boron atom is strongly bound to the carbonyl oxygen and lies in the plane of the enone moiety. A steric clash between one of the iodine atoms of the ligand and one face of the enone seems to be responsible for the facial discrimination. The alternative reaction channel in which only one methoxy ligand of the alkenylboronate is exchanged was investigated too and was computed to be disfavored. The [4 + 2] and the [4 + 3] pathways for the competitive hetero-Diels−Alder reaction were also found to be disfavored relative to the conjugate alkenylboration. In addition, the effects of substitution on the enone and the alkenylboronate have been evaluated. Calculations correctly reproduced the experimental reactivity trends and enantiomeric ratios.