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

Abstract

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.

The Development of Enantioselective Rhodium-Catalysed Hydroboration of Olefins

“…The hydride and boryl ligands are trans in the reactive form of this complex. [52] Subsequent migratory insertion of the alkene into the rhodium-hydride bond produces the regioisomeric alkyl boronate esters 7 and 8. Upon reductive elimination these give the anti-Markovnikov product 9 or the Markovnikov product 10, respectively, and the catalytic species 4 is regenerated.…”

“…[53] Theoretical studies have suggested that reductive elimination is the slowest step in the overall transformation. [51,52] Among efforts to elucidate the exact mechanism, Evans has proposed on the basis of deuterium labelling studies that certain steps in the catalytic cycle, namely olefin binding to the rhodium catalyst, as well as subsequent hydride migration, are reversible. However, the level of reversibility is highly substrate-dependent.…”

“…Hydroboration has also been studied by theoretical methods. [52] In addition, the nature of the catalytic cycle has been addressed experimentally and by means of quantum chemistry methods, and this has been reviewed recently.…”

“…[51,71,72] In a competing side reaction to reductive elimination, b-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane by-product. [73] There remains however a substantial body of evidence, both experimental [74,75] and theoretical, [52,70,76] that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. [57] 3 Selectivity of Metal-Catalysed Hydroboration…”

Rhodium: Organometallic Chemistry

“…94 A related study on the hydrosilylation of alkenes (see Hydrosilation Catalysis) using RhCl(PH 3 ) 3 as a catalyst model supports the modified Chalk -Harrod mechanism, namely oxidative addition of the Si-H bond to the metal followed by alkene coordination, insertion of the alkene into the Rh-Si bond, and C-H reductive elimination.…”