Unmodified homogeneous rhodium-catalyzed hydroformylation of styrene. The detailed kinetics of the regioselective synthesis

Abstract

The homogeneous regioselective catalytic hydroformylation of styrene to (±)-2-phenylpropanal and 3-phenylpropanal was studied starting with Rh4(CO)12 as catalyst precursor in n-hexane as solvent. The reaction conditions were T = 298-313 K, PH2 = 0.27-1.01 MPa (0.002-0.0074 mol fraction), PCO = 3.0-6.0 MPa (0.042-0.075 mol fraction), [Rh4(CO)12]o = 3.5 × 10-5 to 1.7 × 10-4 mol fraction, and [C8H8]o = 0.118-0.349 mol fraction. Quantitative high-pressure in-situ infrared spectroscopic measurements were made under isobaric and isothermal conditions during the 5-h kinetic experiments. In all experiments, the disappearance of the precursor Rh4(CO)12 resulted in the formation of two observable acyl intermediates, namely the major isomer (±)-PhCH(CH3)CORh(CO)4 and the minor isomer PhCH2CH2CORh(CO)4. Clean, high-signal-to-noise spectra of the two intermediates (±)-PhCH(CH3)CORh(CO)4 and PhCH2CH2CORh(CO)4 were obtained. The disappearance of Rh4-(CO)12 was found to followed the rate expression rate = ko α [Rh4(CO)12]1.0[CO]1.3[H 2]0.9[C8H8]0.15 where ko α = (κT/h)exp(-62.6 kJ/(mol RT)) + 37.2 J/(mol R)). This rate expression suggests that cluster fragmentation of Rh4(CO)12 proceeds via CO addition and polyhedron opening as the first step. The hydrogenolysis of (±)-PhCH(CH3)CORh(CO)4 to yield the major product (±)-PhCH(CH3)CHO was found to follow the rate expression rate = ko β[PhCH(CH3)CORh-(CO)4] 1.0[CO]-1.0[H2]1.0[C 8H8]0.0 with ko β = (κT/h)exp(-78.8 kJ/(mole RT) + 15.2 J/(mol R), and the hydrogenolysis of PhCH2CH2CORh(CO)4 to yield the minor product PhCH2CH2-CHO was found to follow the rate expression rate = ko γ [PhCH2CH2CORh(CO)4]1.0[CO] -1.0-[H2]1.0[C8H8] 0.0 with ko γ = (κT/h)exp(-113 kJ/(mol RT)) - 102 J/(mol R). The two rate expressions for hydrogenolysis are consistent with (1) equilibrium controlled dissociation of a CO ligand from the acyl intermediates to form coordinatively unsaturated species, (2) a rate-limiting step involving hydrogen activation on the 4-coordinate species, and (3) product formation accompanied by the formation of a transient species {HRn(CO)3}.