Unmodified rhodium-catalyzed hydroformylation of alkenes using tetrarhodium dodecacarbonyl. The infrared characterization of 15 acyl rhodium tetracarbornyl intermediates

The homogeneous catalytic hydroformylation of 20 alkenes was studied, start ing with Rh-4(CO)(12) as catalyst precursor in n-hexane as solvent, using h igh-pressure in-situ infrared spectroscopy as the analytical tool. Five cat egories of alkenes were studied, namely, cycloalkenes (cyclopentene, cycloh eptene, cyclooctene, and norbornene), symmetric internal linear alkenes (3- hexene, 4-octene, and 5-decene), terminal alkenes (1-hexene, 1-octene, 1-de cene, 1-dodecene, and 1-tekadecene), methylene cycloalkanes (methylene cycl opropane, methylene cyclobutane, methylene cyclopentane, and methylene cycl ohexane), and branched alkenes (2-methyl-2-butene, 2-methyl-2-pentene, 2-me thyl-2-heptene, and 2,3-dimethyl-2-butene). The typical reaction conditions were T = 293 K, P-H2 = 2.0 MPa (0.018 mol fraction), P-CO = 2.0 MPa (0.033 mol fraction), [alkene](0) = 0.1-10.02 mol fraction, and [Rh-4(CO)(12)](0) = 6.6 x 10(-5) mol fraction. In each experiment, with the exception of tho se involving methylene cyclopropane and the branched alkenes, the precursor Rh-4(CO)(12) was converted in good yield to the corresponding observable m ononuclear acyl rhodium tetracarbonyl intermediate RCORh(CO)(4). Due to the spectral characteristics, the intermediate RCORh(CO)(4) is assigned a trig onal bipyrimidal geometry in all cases with C-s symmetry, with the acyl gro up taking an axial position. Under the present conditions, the cycloalkenes result in one acyl complex, the symmetric internal linear alkenes result i n two acyl stereoisomers, the terminal alkenes result in three acyl complex es (two are stereoisomers), and the methylene cycloalkanes result in two ac yl complexes. The first four categories of alkenes gave rise to slightly di fferent spectral wavenumbers and relative intensities far the complexes, na mely, 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 methyle ne cycloalkanes {2110 (0.3), 2064 (0.46), 2038 (0.72), 2020 (1.0), 1704 cm( -1) (0.24)} Finally, the approximate turnover frequencies (TOF) for each sy stem were also calculated. It was found that the TOFs vary from 0.04 to 0.1 1 min(-1) between alkene categories. Thus, to a first approximation, the pr imary differences in rates of hydroformylation are due to the conversion of Rb-4(CO)(12) and not TOFs. This answers a long-standing question concernin g hydroformylation rates.