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Comparative Density Functional Study of Associative and Dissociative Mechanisms in the Rhodium(I)-Catalyzed Olefin Hydroboration Reactions

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journal contribution
posted on 28.04.2000, 00:00 by Christoph Widauer, Hansjörg Grützmacher, Tom Ziegler
The [RhCl(PH3)2]-catalyzed hydroboration reaction C2H4 + HBR2 → H3CCH2BR2 (R = OH, 2R = OCHCHO) was investigated by means of density functional theory type calculations using the Amsterdam density functional (ADF) program. In the first step, the borane adduct [RhCl(η2-HBR2)(PH3)2] (1) forms from [RhCl(PH3)2] and the borane HBR2. Subsequently, C2H4 adds to 1 to give either [RhClH(BR2)(C2H4)(PH3)2] (2) (associative pathway I) or [RhCl(η2-HBR2)(C2H4)(PH3)] (23) (dissociative pathway II). Further branching arises because on both pathways either boron migration (I.B, II.B) or hydride migration (I.H, II.H) may occur as initial product-forming steps. It is found that the associative mechanisms, I.B and I.H, have rather similar energy profiles and the formation of product complexes [RhCl(H3CCH2BR2)(PH3)2] (9, 15) by reductive elimination requires overcoming the highest activation barriers (∼9 kcal mol-1). Overall, the I.H pathway may be slightly favored over I.B for an associative mechanism. In contrast, for the dissociative mechanism migration and elimination reactions are kinetically strongly differentiated. On the II.B pathway, C−B bond formation is hindered by a high activation barrier (19.5 kcal mol-1), while reductive C−H coupling furnishing the product complex [RhCl(H3CCH2BR2)(PH3)] (31) has a low barrier (6.5 kcal mol-1). On the II.H pathway the reverse is found:  C−H formation has a low barrier (8.4 kcal mol-1), while reductive C−B formation has a high barrier (15.8 kcal mol-1). In summary, the II.B pathway may be slightly more favorable for a dissociative reaction. Since side products (i.e., vinyl boranes, alkanes) are formed on the I.B and II.B pathways, we suggest that rhodium-catalyzed hydroborations are driven into a dissociative II.H reaction channel which is easier to control kinetically by using bulky electron-withdrawing phosphines as ligands in the catalyst.