γ‐, Diastereo‐, and Enantioselective Addition of MEMO‐Substituted Allylboron Compounds to Aldimines Catalyzed by Organoboron–Ammonium Complexes

The first catalytic, broadly applicable, efficient, γ‐, diastereo‐, and enantioselective method for addition of O‐substituted allyl‐B(pin) compounds to phosphinoylimines (MEM=methoxyethoxymethyl, pin=pinacolato) is presented. The identity of the most effective catalyst and the optimal protecting group for the organoboron reagent were determined by consideration of the steric and electronic requirements at different stages of the catalytic cycle, namely, the generation of the chiral allylboronate, the subsequent 1,3‐borotropic shift, and the addition step. Aryl‐, heteroaryl‐, alkenyl‐ and alkyl‐substituted vicinal phosphinoylamido MEM‐ethers were thus accessed in 57–92 % yield, 89:11 to >98:2 γ:α selectivity, 76:24–97:3 diastereomeric ratio, and 90:10–99:1 enantiomeric ratio. The method is scalable, and the phosphinoyl and MEM groups may be removed selectively or simultaneously. Utility is highlighted by enantioselective synthesis of an NK‐1 receptor antagonist. With the right combination of substituents for a small‐molecule aminophenol‐based organoboron catalyst and an O‐substituted allyl boronate reagent, additions to a notable assortment of phosphinoylimines proceed efficiently and show high γ‐, diastereo‐ and enantioselectivity. The broadly applicable method has been used to prepare vicinal amino alcohols of high stereoisomeric purity for the first time.