Unmodified homogenous 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 x 10(-5) to 1.7 x 10(-4) mol fraction, and [C8H8](o) = 0.118-0.349 mol fraction. Qua ntitative high-pressure in-situ;infrared spectroscopic measurements were ma de under isobaric and isothermal conditions during the 5-h kinetic experime nts. In all experiments, the disappearance of the precursor Rh-4(CO)(12) re sulted in the formation of two observable acyl intermediates, namely the ma jor isomer (+/-)-PhCH(CH3)CORh(CO)(4) and the minor isomer PhCH2CH2CORh(CO) (4). Clean, high-signal-to-noise spectra of the two intermediates (+/-)-PhC H(CH3)CORh(CO)(4) and PhCH2CH2CORh(CO)(4) were obtained. The disappearance of Rh-4(CO)(12) was found to followed the rate expression rate = k(o)(alpha ) [Rh-4(CO)(12)](1.0)[CO](1.3)[H-2](0.9)[C8H8](0.15) where k(o)(alpha) = (k appa T/h)exp(-62.6 kJ/(mol RT)) + 37.2 J/(mol R)). This rate expression sug gests 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 foll ow the rate expression rate = k(o)(beta)[PhCH(CH3)CORh(CO)(4)](1.0)[CO](-1. 0)[H-2] 1.0[C8H8](0.0) with k(o)(beta) = (kappa T/h)exp(-78.8 kJ/(mole RT) + 15.2 J/(mol R), and the hydrogenolysis of PhCH2CH2CORh(CO)(4) to yield th e minor product PhCH2CH2CHO was found to follow the rate expression rate = k(o)(gamma) [PhCH2CH2CORh(CO)(4)](1.0)[CO](-1.0)[H-2](1.0)[C8H8](0.0) with k(o)(gamma) = (kappa T/h)exp(-113 kJ/(mol RT)) - 102 J/(mol R). The two rat e expressions for hydrogenolysis are consistent with (1) equilibrium contro lled dissociation of a CO ligand from the acyl intermediates to form coordi natively unsaturated species, (2) a rate-limiting step involving hydrogen a ctivation on the 4-coordinate species, and (3) product formation accompanie d by the formation of a transient species {HRh(CO)(3)}.