The Rate‐Determining Step in the Rhodium–Xantphos‐Catalysed Hydroformylation of 1‐Octene

Abstract

The rate-determining step in the hydroformylation of 1-octene, catalysed by the rhodium-Xantphos catalyst system, was determined by using a combination of experimentally determined (1)H/(2)H and (12)C/(13)C kinetic isotope effects and a theoretical approach. From the rates of hydroformylation and deuterioformylation, a small (1)H/(2)H isotope effect of 1.2 was determined for the hydride moiety of the rhodium catalyst. (12)C/(13)C isotope effects of 1.012(1) and 1.012(3) for the alpha-carbon and beta-carbon atoms of 1-octene were determined, respectively. Both quantum mechanics/molecular mechanics (QM/MM) and full quantum mechanics calculations were carried out on the key catalytic steps, for "real-world" ligand systems, to clarify whether alkene coordination or hydride migration is the rate-determining step. Our calculations (21.4 kcal mol(-1)) quantitatively reproduce the experimental energy barrier for CO dissociation (20.1 kcal mol(-1)) starting at the (bisphosphane)RhH(CO)(2) resting state. The barrier for hydride migration lies 3.8 kcal mol(-1) higher than the barrier for CO dissociation (experimentally determined trend approximately 3 kcal mol(-1)). The computed (1)H/(2)H and (12)C/(13)C kinetic isotope effects corroborate the results of the energy analysis.

The kinetics of gas-phase propene hydroformylation over a supported ionic liquid-phase (SILP) rhodium catalyst

“…The observed reaction orders at 363 K suggest that the RDS is either alkene coordination (Reaction 2) or alkene insertion (Reaction 3) into the Rh-H bond. Support for Reaction 3 being the RDS can be drawn from the work of van Leeuwen and coworkers using homogeneous Rh-xantphos complexes [26]. While isotopic experiments ( 1 H/ 2 H and 12 C/ 13 C) were unable to determine the RDS irrefutably, their hybrid quantum mechanical/molecular mechanical (QM/MM) calculations suggested that alkene insertion is the RDS.…”

Computational aspects of hydroformylation

“…127 For the quantum mechanic part the B3LYP functional was employed along with double-z basis sets. The low level layer treated using the UFF force eld.…”

Simultaneous Orthogonal Methods for the Real-Time Analysis of Catalytic Reactions

“…Such a scenario has been suggested to be occurring in using salicylaldehyde alkene hydroacylation, 22 while the small KIE measured is similar to that proposed for hydride migration step in the hydroformylation of 1-octene. 23 Hofmann and co-workers have demonstrated increased barriers to alkyl migration in related small bite angle systems, 24 consistent with this scenario. Hydride insertion being turnover limiting in this alkyne hydroacylation is in contrast to alkene hydroacylation using aldehyde 1 and small-bite angle diphosphine Rh-catalysts in which fast and reversible hydride insertion is indicated.…”