Mechanism of the Pyridine-Modified Cobalt-Catalyzed Hydromethoxycarbonylation of 1,3-Butadiene Ro ´bert Tuba, † La ´ szlo ´ T. Mika, † Andrea Bodor, † Zolta ´ n Pusztai, †,‡ Imre To ´th, § and Istva ´n T. Horva ´ th* ,† Department of Chemical Technology and Environmental Chemistry, Eo ¨ tvo ¨ s University, H-1117 Budapest, Pa ´ zma ´ ny Pe ´ ter se ´ ta ´ ny 1/A, Hungary, and DSM Research, Geleen, The Netherlands Received January 24, 2003 Summary: The pyridine-modified cobalt-catalyzed hy- dromethoxycarbonylation of 1,3-butadiene (1) starts by the disproportionation of Co 2 (CO) 8 to [CoPy 6 ][Co(CO) 4 ] 2 followed by the formation of HCo(CO) 4 (3). The addition of 3 to 1 leads to CH 3 CHdCHCH 2 Co(CO) 4 (4), which, depending on the conditions, can undergo facile CO insertion to yield CH 3 CHdCHCH 2 COCo(CO) 4 (5) or reversible decarbonylation to form η 3 -C 4 H 7 Co(CO) 3 (7). Pyridine accelerates the conversion of 7 to methyl-3- pentenoate (2) and the methanolysis of 5. The hydromethoxycarbonylation of 1,3-butadiene (1) to methyl 3-pentenoate (2) could be the first step in the green production of adipic acid or ǫ-caprolactam, which are key intermediates in nylon manufacture. 1,2 Co 2 (CO) 8 in the presence of pyridine (Py) represents one of the few known catalyst systems which is suitable for this reaction. 3 Although several different mechanisms have been proposed for this system, 4-7 no intermediates have been isolated and characterized under reaction condi- tions. One of the proposed mechanisms is based on the catalytic cycle of cobalt-catalyzed hydroesterification of olefins. 4c Thus, the reaction starts by the addition of HCo(CO) 4 (3) to 1, resulting in CH 3 CHdCHCH 2 Co(CO) 4 (4), which can undergo CO insertion to yield CH 3 CHd CHCH 2 COCo(CO) 4 (5). Imyanitov has suggested that the reaction of 5 with pyridine could lead to [CH 3 CHd CHCH 2 COPy] + [Co(CO) 4 ] - , which in turn could react with MeOH to give 2 and regenerate 3. 4 In contrast, Milstein has suggested the formation of the coordina- tively unsaturated species {MeOCOCo(CO) 3 } by the reaction of [Co(CO) 4 Py] + [Co(CO) 4 ] - and MeOH. 5 The addition of 1 to {MeOCOCo(CO) 3 } leads to the allyl complex (η 3 -CH 2 CHCHCH 2 COOMe)Co(CO) 3 (6), 6 which reacts with 3 to give 2 and the coordinatively unsatur- ated {Co 2 (CO) 7 }. These species have not been observed under catalytic conditions, however. We report here our high-pressure IR and NMR study on the pyridine- modified cobalt-catalyzed hydromethoxycarbonylation of 1,3-butadiene (1), which has led to the characterization of several key intermediates and the catalytic cycle. First we have investigated the addition of 1 to the equilibrium mixture 7 of Co 2 (CO) 8 and [Co(MeOH) 6 ][Co- (CO) 4 ] 2 under 75 bar of CO at 100 °C in MeOH. 8 The quantitative formation of η 3 -C 4 H 7 Co(CO) 3 (7) 9 was ob- served (the first spectrum in Figure 1.). Since no further reaction could be detected after several hours, the tem- perature was increased to 140 °C. During the next 6.5 h, 7 disappeared and the formation of 2 8d and the equilibrium mixture of Co 2 (CO) 8 and [Co(MeOH) 6 ][Co- (CO) 4 ] 2 - was observed (Figure 1). Compound 6, which could be readily isolated in THF is apparently absent in the reaction mixture. 5a,6 The formation of 7, instead of 6, was confirmed in a similar experiment by using high-pressure NMR. 10 When the reaction was performed in pyridine, [CoPy 6 ]- [Co(CO) 4 ] 2 was the only cobalt species detectable by IR 8e even at 100 °C under 75 bar of CO (the first spectrum † Eo ¨tvo ¨s University. ‡ This paper is dedicated to the memory of Mr. Zolta ´n Pusztai, a cherished colleague whose untimely death prematurely ended a brilliant career. § DSM Research. * Corresponding author. (1) Beller, M.; Cornils, B.; Frohning, C. D.; Kohlpainter, C. W. J. Mol. Catal. A 1995, 104, 17-85. (2) Dahlhoff, G.; Niederer, J. P. M.; Hoelderich, W. E. Catal. Rev. 2001, 43(4), 381-441. (3) Matsuda, A. Bull. Chem. Soc. Jpn. 1972, 46, 524-530. (4) (a) Imyanitov, N. S.; Bogoradovskaya, N. M.; Semenova, T. A. Kinet. Katal. 1978, 19, 573. (b) Imyanitov, N. S. Kinet. Katal. 1999, 40, 71. (c) Forster, D.; Hersman, A.; Morris, D. E. Catal. Rev. Sci. Eng. 1981, 23, 89. (5) (a) Milstein, D.; Huckaby, J. L. J. Am. Chem. Soc. 1982, 104, 6150-6152. (b) Milstein, D. Acc. Chem. Res. 1988, 21, 428-434. (6) Since no spectroscopic data were reported for 6, 5a we have characterized it by both IR and NMR. IR (ν(CO), n-pentane, cm -1 ): 2067 (vs), 1999 (vs), 1751 (w). 13 C NMR in d4-MeOH (ppm): syn isomer, 36.8 (CH2), 48.0 (CH2), 51.2 (CH3), 66.6 (CH), 83.3 (CH), 171.7 (CdO), 202.9 (CtO); anti isomer, 36.8 (CH2), 48.0 (CH2), 51.0 (CH3), 67.2 (CH), 82.2 (CH), 171.7 (CdO), 202.9 (CtO). (7) Mirbach, M. F.; Mirbach, M. J. J. Mol. Catal. 1985, 32, 59-75. (8) Characteristic IR bands (ν(CO), MeOH, cm -1 ): (a) Co2(CO)8, 2070 (s), 2041 (s), 2026 (s), 1858 (m); (b) [Co(MeOH)6][Co(CO)4]2, 1903 (vs); (c) 7, 2057 (s), 1988 (vs); (d) 2, 1739 (vw); (e) [CoPy6][Co(CO)4]2, 1889 (vs). (9) (a) Heck, R. F.; Breslow, D. S. J. Am. Chem. Soc. 1961, 83, 1097- 1102. (b) Bertrand, J. A.; Jonassen, H. B.; Moore, D. W. Inorg. Chem. 1963, 2, 601-604. Figure 1. Reaction of 18.08 mmol of η 3 -C 4 H 7 Co(CO) 3 (7) with CO (75 bar) and MeOH (55 mL) at 140 °C. 1582 Organometallics 2003, 22, 1582-1584 10.1021/om030058x CCC: $25.00 © 2003 American Chemical Society Publication on Web 03/15/2003