Divergent Kinetics and Rate-Determining Steps for 1-Butene and 2-Butene Hydroformylation with a Bidentate Phosphoramidite Ligand: A Combined Experimental and Density Functional Theory Study

The hydroformylation of mixed butenes (1-butene and 2-butene) to n-valeraldehyde is an important industrial process, yet the distinct kinetic behaviors of the two isomers under identical catalytic conditions remain insufficiently understood. Herein, a comprehensive kinetic and mechanistic investigation of 1-butene and 2-butene was conducted using a robust, water- and air-stable, highly regioselective bidentate phosphoramidite ligand-modified rhodium catalyst in a homogeneous system. Systematic kinetic experiments revealed that the hydroformylation of 2-butene exhibits a strong positive dependence on H2 pressure and obvious substrate inhibition at high concentrations, whereas 1-butene shows little H2 dependence and no substrate inhibition. Using a semi-empirical hyperbolic kinetic modeling, the rate equations capture these distinct behaviors with high accuracy (R2 = 0.9746 for 2-butene, 0.9642 for 1-butene). Kinetic studies revealed distinct rate behaviors for 1-butene and 2-butene hydroformylation. Density functional theory (DFT) calculations on the n-valeraldehyde pathway further showed that hydride insertion to the internal C═C double bond has the highest barrier for 2-butene, while CO insertion to the linear rhodium n-butyl intermediate has the highest barrier for 1-butene, providing the mechanistic origin for the observed kinetic differences. The iso-valeraldehyde pathway is additionally disfavored by a 2.7 kcal·mol–1 higher RDS barrier, consistent with the good n-selectivity. In situ high-pressure FTIR spectroscopy further confirmed the five-coordinate hydride HRh(CO)2(P)2 as the catalyst resting state. These findings offer quantitative guidance for optimizing industrial hydroformylation processes using mixed butene feedstocks.