Unmodified Homogeneous Rhodium-Catalyzed Hydroformylation of Styrene. The Detailed Kinetics of the Regioselective Synthesis

The homogeneous regioselective catalytic hydroformylation of styrene to (±)-2-phenylpropanal and 3-phenylpropanal was studied starting with Rh 4 (CO) 12 as catalyst precursor in n -hexane as solvent. The reaction conditions were T = 298−313 K, P H2 = 0.27−1.01 MPa (0.002−0.0074 mol fraction), P CO = 3.0−6.0 MPa (0.042−0.075 mol fraction), [Rh 4 (CO) 12 ] o = 3.5 × 10 -5 to 1.7 × 10 -4 mol fraction, and [C 8 H 8 ] 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 Rh 4 (CO) 12 resulted in the formation of two observable acyl intermediates, namely the major isomer (±)-PhCH(CH 3 )CORh(CO) 4 and the minor isomer PhCH 2 CH 2 CORh(CO) 4 . Clean, high-signal-to-noise spectra of the two intermediates (±)-PhCH(CH 3 )CORh(CO) 4 and PhCH 2 CH 2 CORh(CO) 4 were obtained. The disappearance of Rh 4 (CO) 12 was found to followed the rate expression rate = k o α [Rh 4 (CO) 12 ] 1.0 [CO] 1.3 [H 2 ] 0.9 [C 8 H 8 ] 0.15 where k o α = (κ T / h )exp(−62.6 kJ/(mol RT )) + 37.2 J/(mol R )). This rate expression suggests that cluster fragmentation of Rh 4 (CO) 12 proceeds via CO addition and polyhedron opening as the first step. The hydrogenolysis of (±)-PhCH(CH 3 )CORh(CO) 4 to yield the major product (±)-PhCH(CH 3 )CHO was found to follow the rate expression rate = k o β [PhCH(CH 3 )CORh(CO) 4 ] 1.0 [CO] -1.0 [H 2 ] 1.0 [C 8 H 8 ] 0.0 with k o β = (κ T / h )exp(−78.8 kJ/(mole RT) + 15.2 J/(mol R ), and the hydrogenolysis of PhCH 2 CH 2 CORh(CO) 4 to yield the minor product PhCH 2 CH 2 CHO was found to follow the rate expression rate = k o γ [PhCH 2 CH 2 CORh(CO) 4 ] 1.0 [CO] -1.0 [H 2 ] 1.0 [C 8 H 8 ] 0.0 with k o γ = (κ T / 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 {HRh(CO) 3 }.