Organocatalytic DYKAT of Si-Stereogenic Silanes

Abstract

Chiral organosilanes do not exist in nature and are therefore absent from the "chiral pool". As a consequence, synthetic approaches toward enantiopure silanes, stereogenic at silicon, are rather limited. While catalytic asymmetric desymmetrization reactions of symmetric organosilicon compounds have been developed, the utilization of racemic silanes in a dynamic kinetic asymmetric transformation (DYKAT) or dynamic kinetic resolution (DKR) would significantly expand the breadth of accessible Si-stereogenic compounds. We now report a DYKAT of racemic allyl silanes enabled by strong and confined imidodiphosphorimidate (IDPi) catalysts, providing access to Si-stereogenic silyl ethers. The products of this reaction are easily converted into useful enantiopure monohydrosilanes. We propose a spectroscopically and experimentally supported mechanism involving the epimerization of a catalyst-bound intermediate.

Figure 1

Mechanistic studies. (A) Time-course study with rac-1a (0.1 mmol, 1.0 equiv), 2 (0.15 mmol, 1.5 equiv), and IDPi 3d (2.5 mol %) in toluene (0.1 M) at −20 °C under Ar for 3 days. Conversions were determined by GC, and enantiomeric ratios (e.r.) were determined by HPLC. (B) Racemization studies of either (R)-1a or (S)-1a (0.1 mmol) and IDPi 3d (2.5 mol %) in toluene (0.2 M) at −20 °C for 3 days. (C) ³¹P NMR of the activation of catalyst with rac-1a (0.1 mmol, 1.0 equiv) and IDPi 3d (2.5 mol %) in toluene-d₈ (0.1 M) at −20 °C under Ar. (D) Silylation process performed under the same conditions as in (C), monitored by ³¹P NMR. (E) Reaction of (S)-1a, performed with (S)-1a (0.1 mmol, 1.0 equiv), 2 (0.15 mmol, 1.5 equiv), and IDPi 3d (2.5 mol %) in toluene-d₈ (0.1 M) at −20 °C under Ar. The variations of Cat-Si1 and Cat-H were monitored by ³¹P NMR. (F) Reaction of (R)-1a under the same conditions as in (E).

Figure 2

(A) Computational studies and (B) proposed mechanism.