Comparative density functional study of associative and dissociative mechanisms in the rhodium(I)-catalyzed olefin hydroboration reactions

The [RhCl(PH3)(2)]-catalyzed hydroboration reaction C2H4 + HBR2 --> H3CCH2B R2 (R=OH, 2R = OCH = CHO) was investigated by means of density functional t heory type calculations using the Amsterdam density functional (ADF) progra m. In the first step, the borane adduct [RhCl(eta(2)-HBR2)(PH3)(2)] (1) for ms from [RhCl(PH3)(2)] and the borane HBR2. Subsequently, C2H4 adds to 1 to give either [RhClH(BR2)(C2H4)(PH3)(2)] (2) (associative pathtoay I) or [Rh Cl-(eta(2)-HBR2)(C2H4)(PH3)] (23) (dissociative pathway II). Further branch ing arises because on both pathways either boron migration (I.B, II.B) or h ydride 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)(PH 3)(2)] (9, 15) by reductive elimination requires overcoming the highest act ivation barriers (similar to 9 kcal mol-l). Overall, the I.H pathway may be slightly favored over I.B for an associative mechanism. In contrast, for t he dissociative mechanism migration and elimination reactions are kinetical ly strongly differentiated. On the II.B pathway, C-B bond formation is hind ered 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 l ow 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 forma tion has a high barrier (15.8 kcal mol(-1)). In summary, the II.B pathway m ay be slightly more favorable for a dissociative reaction. Since side produ cts (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 dissoci ative II.H reaction channel which is easier to control kinetically by using bulky electron-withdrawing phosphines as ligands in the catalyst.