Electronic Ligand Effects on the Regioselectivity of the Rhodium-Diphosphine-Catalyzed Hydroformylation of Propene E. Zuidema, ² E. Daura-Oller, ‡ J. J. Carbo ´, ‡ C. Bo,* ,‡,§ and P. W. N. M. van Leeuwen ²,§ Van ’t Hoff Institute for Molecular Sciences, UniVersiteit Van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, Departament de Quı ´mica Fı ´sica i Inorga ` nica, UniVersitat RoVira i Virgili, Campus Sescelades, Macelli Domingo s/n, 43007 Tarragona, Spain, and Institute of Chemical Research of Catalonia (ICIQ), AVinguda Paı ¨sos Catalans 16, Campus UniVersitari, Tarragona, Spain ReceiVed October 24, 2006 Electronic effects induced by diphosphine bidentate ligands on the regioselectivity of the rhodium- catalyzed hydroformylation of propene were investigated using density functional theory based calculations (B3LYP). To this end, the key hydride migration step was evaluated for HRh(propene)(CO)L 2 (L 2 ) PF 3 , PF 3 ; PH 3 , PH 3 ; PMe 3 , PMe 3 ; PH 3 , PF 3 ; PH 3 , PMe 3 ) incorporating either two identical or two electronically distinct phosphorus moieties. The phosphorus moieties span a wide range of ligand basicities. While the electronic properties of the ligands do not influence the regioselectivity of the hydride migration reaction directly, they do govern the amount of back-donation from the metal to the alkene substrate. As a result, important differences in transition-state geometries are obtained for different ligand systems. For electron-withdrawing ligands low activation energies and trigonal-bipyramidal transition-state geometries are observed. Increasing the basicity of the diphosphine ligand leads to higher activation energies and distortion of the transition-state structures toward square-pyramidal geometries. In systems containing two electronically distinct phosphorus ligands, this geometric distortion leads to a preference for the formation of the new rhodium-alkyl σ-bond trans to the least donating phosphorus moiety, generating the most stable rhodium-alkyl isomer. In all cases, bis-equatorial coordination of the two phosphorus ligands yields considerably lower transition-state energies than equatorial-axial coordination of the same ligands. The resulting rhodium-alkyl products are stabilized relative to the reactant by electron- donating ligands. On the basis of these observations it is argued that, for electron-withdrawing and/or wide-bite-angle ligands, -hydride elimination plays an important role in determining the overall regioselectivity of the hydroformylation reaction, while for equatorial-axial coordinating ligands, the regioselectivity is determined exclusively by the relative energies of the hydride migration transition states. 1. Introduction The hydroformylation reaction is one of the most important reactions catalyzed by homogeneous transition-metal complexes in the industrial production of bulk chemicals. In order to elucidate the mechanism of the rhodium-catalyzed hydrofor- mylation (Figure 1) and ascertain the role of spectator ligands in determining the activity of the catalyst system, the reaction has been studied extensively using computational techniques. Both Morokuma and co-workers and Decker and co-workers studied the hydroformylation of ethene catalyzed by Rh(H)- (CO) 2 (PH 3 ) 2 , calculating different reaction pathways where the catalyst is ligated by either one or two phosphine moieties. 1-4 For catalysts containing a single phosphine moiety, the highest barriers were obtained for the oxidative addition of hydrogen to the catalyst, while for catalysts containing two phosphorus moieties, the highest barriers were obtained for the CO-insertion step of the catalytic cycle. More recently, Gleich et al. investigated electronic effects induced by phosphine ligands in the complete catalytic cycle of the hydroformylation of ethene, catalyzed by HRhL 3 (L ) CO, PH 3 , PF 3 , PMe 3 ). 5 These calculations suggest that the rate-determining step in the hydroformylation reaction for both the unmodified rhodium- carbonyl catalyst and catalysts modified by electron-withdrawing ligands is either the coordination of ethene to the catalyst or the insertion of ethene into the rhodium-hydride bond. This contradicts experimental studies that clearly show, for these catalyst systems, oxidative addition of hydrogen to the catalyst is rate-determining. In the hydroformylation of substituted alkenes, regioselectivity has proven to be an important parameter in the development of new catalyst systems. Experimental studies have shown that the regioselectivity of the catalyst is influenced by both the steric 6-8 and electronic 9-12 properties of the ligands surrounding the catalytically active metal center. Casey and co-workers showed that, for catalysts containing bidentate ligands, the regioselec- * To whom correspondence should be addressed. E-mail: cbo@iciq.es. ² Universiteit van Amsterdam. ‡ Universitat Rovira i Virgili. § Institute of Chemical Research of Catalonia (ICIQ). (1) Koga, N.; Jin, S. Q.; Morokuma, K. J. Am. Chem. Soc. 1988, 110, 3417. (2) Musaev, D. G.; Matsubara, T.; Mebel, A. M.; Koga, N.; Morokuma, K. Pure Appl. Chem. 1995, 67, 257. (3) Matsubara, T.; Koga, N.; Ding, Y.; Musaev, D. G.; Morokuma, K. Organometallics. 1997, 16, 1065. (4) Decker, S. A.; Cundari, T. R. J. Organomet. Chem. 2001, 635, 132. (5) Gleich, D.; Hutter, J. Chem. Eur. J. 2004, 10, 2435. (6) Freixa, Z.; Van Leeuwen, P. W. N. M. Dalton Trans. 2003, 10, 1890. (7) Reinius, H. 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