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J Org Chem. 2012 Dec 7;77(23):10738-44. doi: 10.1021/jo302103c. Epub 2012 Nov 8.

Origin of regioselectivity in the reactions of nitronate and enolate ambident anions.

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  • 1Department of Chemistry and the Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro, Toshima-ku 171-8501, Tokyo, Japan.


The reactions of nitronates of ring-substituted phenylnitromethanes and enolates of ring-substituted 1-phenyl-2-propanones with MeOBs gave exclusively the O-methylated and C-methylated products, respectively. DFT calculations suggested that two factors, namely, intrinsic barriers and metal-cation coordination, control the C/O selectivity. The kinetic preference for O-methylation in the reactions of nitronates arises from the intrinsic barriers, which are ca. 10 kcal/mol lower for O-methylation than for C-methylation. The situation is the same for the gas-phase reaction of an enolate, in which the O-methylation is more favorable than the C-methylation. The experimentally observed C-selectivity of enolate reactions in solution is due to the metal-cation coordination, which hinders O-methylation for enolates. The effects of the enolate reactivity and the solvent on the C/O selectivity are also rationalized to arise from the two factors.

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