The Unmodified Homogeneous Rhodium-Catalyzed Hydroformylation of Cyclohexene and the Search for Monometallic Catalytic Binuclear Elimination

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• 作者：Jinhai Feng and Marc Garland
• 刊名：Organometallics
• 出版年：1999
• 出版时间：April 12, 1999
• 年：1999
• 卷：18
• 期：8
• 页码：1542 - 1546
• 全文大小：98K
• 年卷期：v.18,no.8(April 12, 1999)
• ISSN：1520-6041

文摘

The homogeneous catalytic hydroformylation of cyclohexene to cyclohexanecarboxaldehydewas studied, starting with Rh₄(CO)₁₂ as catalyst precursor in n-hexane as solvent. Thereaction conditions were T = 298 K, P_H2 = 2.0 MPa (0.018 mol fraction), P_CO = 6.0 MPa(0.095 mol fraction), and [C₆H₁₀]₀ = 0.144 mol fraction. The nominal rhodium concentrationwas varied in four experiments as [Rh₄(CO)₁₂]₀ = 3.5 × 10^-5, 6.6 × 10^-5, 1.7 × 10^-4, and 3.3× 10^-4 mol fraction (where the last value represents the approximate solubility limit). Ineach of the four experiments the time-dependent concentrations of the precursor Rh₄(CO)₁₂,the organometallic intermediate C₆H₁₁CORh(CO)₄, and the organic product C₆H₁₁CHO weremeasured under isobaric and isothermal conditions using in situ high-pressure infraredspectroscopy. Complete conversion of the precursor Rh₄(CO)₁₂ to the intermediate C₆H₁₁CORh(CO)₄ was never observed during the 8 h experiments. Instead, after approximately30 min, a pseudo steady state was achieved between the species Rh₄(CO)₁₂ and C₆H₁₁CORh(CO)₄. The hydrogenolysis of C₆H₁₁CORh(CO)₄ resulted in aldehyde formation, and only 1.04-1.94% conversion of C₆H₁₀ was observed in any 8 h experiment. The turnover frequencies(TOF), based on the instantaneous concentrations of C₆H₁₁CORh(CO)₄, were calculated at15 min intervals for all four experiments, and the data were regressed according to thepolynomial TOF = (k₁ + k₂K_eq[Rh₄(CO)₁₂]^0.25). The evaluated coefficients were k₁ = (2.11 ±0.06) × 10^-3 s^-1 and k₂K_eq = (0.50 ± 0.60) × 10^-3 s^-1, where the term k₂K_eq [Rh₄(CO)₁₂]^0.25is ca. 5 × 10^-5 s^-1. These results strongly suggest that there is no statistically significantcontribution to aldehyde formation from a monometallic catalytic binuclear elimination inthe rhodium-catalyzed hydroformylation reaction under these conditions.