Unmodified Rhodium-Catalyzed Hydroformylation of Alkenes Using Tetrarhodium Dodecacarbonyl. The Infrared Characterization of 15 Acyl Rhodium Tetracarbonyl Intermediates

Abstract

The homogeneous catalytic hydroformylation of 20 alkenes was studied, starting with Rh4(CO)12 as catalyst precursor in n-hexane as solvent, using high-pressure in-situ infrared spectroscopy as the analytical tool. Five categories of alkenes were studied, namely, cycloalkenes (cyclopentene, cycloheptene, cyclooctene, and norbornene), symmetric internal linear alkenes (3-hexene, 4-octene, and 5-decene), terminal alkenes (1-hexene, 1-octene, 1-decene, 1-dodecene, and 1-tetradecene), methylene cycloalkanes (methylene cyclopropane, methylene cyclobutane, methylene cyclopentane, and methylene cyclohexane), and branched alkenes (2-methyl-2-butene, 2-methyl-2-pentene, 2-methyl-2-heptene, and 2,3-dimethyl-2-butene). The typical reaction conditions were T = 293 K, PH2 = 2.0 MPa (0.018 mol fraction), PCO = 2.0 MPa (0.033 mol fraction), [alkene]0 = 0.1−0.02 mol fraction, and [Rh4(CO)12]0 = 6.6 × 10-5 mol fraction. In each experiment, with the exception of those involving methylene cyclopropane and the branched alkenes, the precursor Rh4(CO)12 was converted in good yield to the corresponding observable mononuclear acyl rhodium tetracarbonyl intermediate RCORh(CO)4. Due to the spectral characteristics, the intermediate RCORh(CO)4 is assigned a trigonal bipyrimidal geometry in all cases with Cs symmetry, with the acyl group taking an axial position. Under the present conditions, the cycloalkenes result in one acyl complex, the symmetric internal linear alkenes result in two acyl stereoisomers, the terminal alkenes result in three acyl complexes (two are stereoisomers), and the methylene cycloalkanes result in two acyl complexes. The first four categories of alkenes gave rise to slightly different spectral wavenumbers and relative intensities for the complexes, namely, cycloalkenes {2109 (0.41), 2063 (0.46), 2037 (0.72), 2019 (1.0), 1699 cm-1 (0.16)}, symmetric internal linear alkenes {2108 (0.43), 2061 (0.45), 2037 (0.84), 2019 (1.0), 1693 cm-1 (0.12)}, terminal alkenes {2110 (0.35), 2064 (0.46), 2038 (0.72), 2020 (1.0), 1703 cm-1 (0.16)}, and methylene cycloalkanes {2110 (0.33), 2064 (0.46), 2038 (0.72), 2020 (1.0), 1704 cm-1 (0.24)}. Finally, the approximate turnover frequencies (TOF) for each system were also calculated. It was found that the TOFs vary from 0.04 to 0.11 min-1 between alkene categories. Thus, to a first approximation, the primary differences in rates of hydroformylation are due to the conversion of Rh4(CO)12 and not TOFs. This answers a long-standing question concerning hydroformylation rates.