Force-Modulated Selectivity of the Rhodium-Catalyzed Hydroformylation of 1-Alkenes
Yichen Yu, Xujun Zheng, Chenghao Duan, Stephen L. Craig, Ross A. Widenhoefer
Abstract
Here we quantify the regio- chemo-, and enantioselectivity of the hydroformylation of 1-alkenes catalyzed by Rh(I) complexes containing chiral macrocyclic biaryl bis(phosphine) ligands as a function of mechanical force applied to the biaryl backbone of these ligands. Extension forces increase the regioselectivity of the hydroformylation of 1-octene from l:b = 1.2 to 1.9 and decrease the chemoselectivity for C9 aldehyde relative to 2-octene from 96% to 20% across a ∼230 pN change in applied force. Extension forces decrease the enantioselectivity of the hydroformylation of styrene from 28% to 18% ee across a similar force range. Variable temperature 1H and 31P NMR analysis of five-coordinate rhodium complexes of the form (P–P)Rh(CO)2H established structures in which the bisphosphine ligand occupies one equatorial and one apical position in the trigonal bipyramid with rapid interconversion of the equatorial and apical phosphorus atoms (ΔG‡223 K = 9.6–9.9 kcal/mol). Neither the isomeric composition nor fluxional behavior of these complexes was detectably perturbed across a ∼300 pN change in applied force.