Reversal of Enantioselectivity by Catalyst Protonation: Asymmetric ­Hydrocyanation of Imines with Oxazaborolidines

Abstract

A new application of chiral oxazaborolidines in asymmetric transformation, namely the hydrocyanation of imines, is described. The effect of protonation of these catalysts on the reactivity and enantioselectivity of hydrocyanations is studied. Significant increase in reaction rate is observed upon protonation of the catalysts, along with the reversal of the sense of stereoinduction.

Key words

oxazaborolidine - asymmetric catalysis - hydrocyanation - imine - protonation

References and Notes

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• 7b

  See ref. 5f.

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8

Oxazaborolidines 1b and 1c were synthesized according to the literature procedure (see ref. 3).

9

Crystal data of 1b·HCN has been deposited at the Cambridge Crystallographic Data Centre with the allocated deposition number CCDC-281060: C₂₄H₂₃BN₂O, M = 366.25, colorless needles, 0.18 × 0.12 × 0.08 mm, monoclinic, a = 10.588 (1), b = 6.950 (1), c = 14.249 (2) Å, β = 105.07°(1), V = 1012.5 (2) ų, T = 293 (2) K, space group P2₁, Z = 2, ρ _calcd = 1.201 g/cm³, µ = 0.073 mm^-1.