Structures and Reaction Pathways in Rhodium(I)-Catalyzed Hydrogenation of Enamides:  A Model DFT Study

Abstract

The potential energy profile of Rh(I)-catalyzed hydrogenation of enamides has been studied for the simple model system [Rh(PH 3 ) 2 (R-acetamidoacrylonitrile)] + using a nonlocal density functional method (B3LYP). Intermediates and transition states along four isomeric pathways for dihydrogen activation have been located, and pathways for interconversion between isomeric reaction pathways have been explored. The general sequence of the catalytic cycle involves coordination of H 2 to [Rh(PH 3 ) 2 (R-acetamidoacrylonitrile)] + to form a five-coordinate molecular H 2 complex, followed by oxidative addition of the coordinated molecular hydrogen to form a dihydride complex, [RhH 2 (PH 3 ) 2 (R-acetamidoacrylonitrile)] + . This dihydride is converted into an alkyl hydride by a migratory insertion reaction. Reductive elimination of the hydrogenated acetamidoacrylonitrile completes the catalytic cycle. No computational support for alternate H 2 activation pathways, such as direct conversion of H 2 and [Rh(PH 3 ) 2 (R-acetamidoacrylonitrile)] + to an alkyl hydride, was found. Four isomeric pathways for hydrogenation are followed, corresponding to the four distinct dihydride isomers resulting from cis addition of H 2 to [Rh(PH 3 ) 2 (R-acetamidoacrylonitrile)] + . Two of these pathways are excluded from further consideration by virtue of their surprisingly high activation barriers for formation of molecular H 2 complexes. Of the two pathways with low barriers to formation of dihydride complexes, only one has a sufficiently low barrier for migratory insertion to contribute significantly to catalytic product formation. Overall, we find that formation of a dihydride is endergonic, rapid, and reversible. Migratory insertion to form an alkyl hydride constitutes the turnover-limiting step in the catalytic cycle. This conclusion is supported by comparison of computed and experimental isotope effects in catalytic enamide hydrogenation.

Asymmetric hydrogenation of α,β-unsaturated phosphonates with Rh-BisP* and Rh-MiniPHOS catalysts: Scope and mechanism of the reaction

“…5) (31). It is not necessarily a precursor of 5, because the activation barrier of the oxidative addition in an observable isomer of the molecular hydrogen complex can be higher than dissociation of dihydrogen yielding 3 (14,43,44). Our attempts to prove the presence of bound hydrogen in 10 failed, probably because the signal is broad and the concentration of 10 is relatively small.…”

“…5). Recent computational studies showed that such complexes are real minima on the potential energy surface of the asymmetric catalytic hydrogenation (14,43,44) and, therefore, their experimental observation at low temperature is quite possible. Molecular hydrogen complex 10 can form by the reaction of substrate 1 with complex 11, which is equilibrating with 4a and 4b (Fig.…”

Ligand Design for Catalytic Asymmetric Reduction

“…The stereochemistry of product is determined at the first irreversible step, 3B J 3C, although a detailed theoretical investigation suggests the possibility that the process 3B J 3C is reversible and the step 3C J 3D constitutes the turnover-limiting step. 17 The BINAP-Rh-catalyzed hydrogenation of enamides is proposed to proceed with the same Halpern-Brown mechanism. 18 CHIRAPHOS, DIPAMP, and BINAP are all chiral diphosphines with a C 2 symmetry (Figure 1.2) forming chelate complexes with transition metallic elements.…”

Combinatorial Transition‐Metal Catalysis: Mixing Monodentate Ligands to Control Enantio‐, Diastereo‐, and Regioselectivity

“…Halpern, [47b, 83] Brown, [47c, 49a] Noyori, [48] Landis, [84] and others [49] have likewise reached the same conclusion in the case of diphosphines as ligands. Moreover, the linearity of the conversion versus time plot implies strong binding of the substrate 10 (saturation kinetics).…”

“…[67] Combinatorial Transition-Metal Catalysis Angewandte Chemie et al had previously performed similar quantum mechanical (QM) studies of Rh-catalyzed olefin hydrogenations in which diphosphines serve as bidentate ligands. [84] In our reaction, as in classical hydrogenations utilizing diphosphines, the prochiral olefin first coordinates to Rh on its pro-R and pro-S side (following cod cleavage), leading to diastereomeric Rh complexes. In the case of C 2 -symmetric diphosphines, two such complexes are formed, namely the more stabile (major) and the less stabile (minor) intermediates (Halpern nomenclature in parentheses).…”