Force-Modulated Selectivity of the Rhodium-Catalyzed Hydroformylation of 1-Alkenes

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 C9aldehyde 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.

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Author Stephen L Craig Chemistry

Author Ross A. Widenhoefer Chemistry