Unravelling the Reaction Path of Rhodium-MonoPhos-Catalysed Olefin Hydrogenation (original) (raw)

Sulfonamido−Phosphoramidite Ligands in Cooperative Dinuclear Hydrogenation Catalysis

Journal of the American Chemical Society, 2009

Natural enzymes often contain active sites constructed from multiple metal centers that are capable of cooperative substrate activation. 1 Reports of artificial dinuclear structures for homogeneous transition-metal catalysis are still scarce, despite interesting examples showing enhanced reactivity and selectivity compared with the analogous mononuclear species. 2 Indeed, all of the rhodium-based catalysts for asymmetric hydrogenation of alkenes used to date are mononuclear, and the potential for use of bimetallic analogues in this industrially important reaction is unexplored. We now present a new class of anionic P-N-bridging ligands based on the sulfonamido-phosphoramidite scaffold that form neutral boat-shaped Rh-P-N-Rh-bridged dinuclear species, which are active in the asymmetric hydrogenation of acetamidoalkene substrates via a proposed unprecedented cooperative binuclear mechanism.

Novel Contributions to the Mechanism of the Enantioselective Hydrogenation of Dimethyl Itaconate with Rhodium Complexes

Chemistry-a European Journal, 2008

Itaconic acid and its derivatives such as dimethyl itaconate can easily be transformed into pharmaceutically interesting chiral methyl succinates via enantioselective hydrogenation. [1] In contrast to a- [2] and b-dehydroamino acid derivatives mechanistic investigations concerning itaconates are hardly available. Investigations of the system Rh-dipamp/dimethyl itaconate (dipamp = 1,2-ethandiylbis[(2-methoxyphenyl)phenylphosphine]) by Brown et al. resulted in the detection of only one species in the 31 P NMR spectroscopy at À45 8C. From the rather large P-P coupling constant of 40 Hz they concluded the coordination of the CÀC double bond and the b-carboxy group to the rhodium center. Line-shape analysis of 31 P NMR spectra of the substrate complexes of dimethyl itaconate with a Rh-bisphosphinite and an aminophosphine-phosphinite provided support for a significantly higher rate of the intramolecular interconversion between the diastereomers compared to the intermolecular conversion via the solvate complex. For the Rh-DPPP complex (DPPP = 1,3-bis(diphenylphosphino)propane) the formation of the hydrido alkyl species was shown to be reversible in H/D exchange studies. By means of the PHIP method catalyst-substrate dihydride complexes could be detected in the asymmetric hydrogenation of dimethyl itaconate with cationic rhodium(I)-bis(phosphinite) complexes. Reetz and co-workers studied the mechanism of enantioselective hydrogenation of dimethyl itaconate with Rh complexes containing monodentate binol-based phosphites. DFT calculations support the validity of the lockand-key principle which was already proven experimentally for one a-and several b-dehydroamino acid derivatives. [2b, 3] 2 ]BF 4 proceeds via the expected Michaelis-Menten kinetics. Independent of the substrate concentration average values of the pseudo-rate constant k 2 = 83 min À1 and the Michaelis constant K m = 1.66 10 À2 mol L À1 result, as well as an enantioselectivity of 91 % (see Supporting Information). Halpern et al. for the very same catalyst but with the prochiral olefin (Z)-methyl acetamido cinnamate (6.8 10 4 L mol À1 , derived from activation parameters of ref.

Activation of Molecular Hydrogen over a Binuclear Complex with Rh 2 S 2 Core: DFT Calculations and NMR Mechanistic Studies

Journal of The American Chemical Society, 2004

The dicationic complex [(triphos)Rh(µ-S)2Rh(triphos)] 2+ , 1 (modeled as 1c) [triphos ) CH3C(CH2-PPh2)3], is known to activate two dihydrogen molecules and produce the bis(µ-hydrosulfido) product [(triphos)-(H)Rh(µ-SH)2Rh(H)(triphos)] 2+ , 2 (modeled as 2b), from which 1 is reversibly obtained. The possible steps of the process have been investigated with DFT calculations. It has been found that each d 6 metal ion in 1c, with local square pyramidal geometry, is able to anchor one H2 molecule in the side-on coordination. The step is followed by heterolytic splitting of the H-H bond over one adjacent and polarized Rh-S linkage. The process may be completed before the second H2 molecule is added. Alternatively, both H2 molecules are trapped by the Rh2S2 core before being split in two distinct steps. Since the ambiguity could not be solved by calculations, 31 P and 1 H NMR experiments, including para-hydrogen techniques, have been performed to identify the actual pathway. In no case is there experimental evidence for any Rh-(η 2 -H2) adduct, probably due to its very short lifetime. Conversely, 1 H NMR analysis of the hydride region indicates only one reaction intermediate which corresponds to the monohydride-µ-hydrosulfide complex [(triphos)-Rh(H)(µ-SH)(µ-S)Rh(triphos)] 2+ (3) (model 5a). This excludes the second hypothesized pathway. From an energetic viewpoint the computational results support the feasibility of the whole process. In fact, the highest energy for H2 activation is 8.6 kcal mol -1 , while a larger but still surmountable barrier of 34.6 kcal mol -1 is in line with the reversibility of the process. Stein, M.; Brecht, M.; Ogata, H.; Higuchi, Y.; Lubitz, W. J. Am. Chem. Soc. 2003, 125, 83. (d) Stein, M.; van Lenthe, E.; Baerends, E. J.; Lubitz, W. J. Phys. Chem. A 2001, 105, 416. (e) Pavlov, M.; Siegbahn, P. E. M.; Blomberg, M. R. A.; Crabtree, R. H.

Ab Initio Studies of Rhodium(I)−N-Alkenylamide Complexes with cis- and trans-Coordinating Phosphines: Relevance for the Mechanism of Catalytic Asymmetric Hydrogenation of Prochiral Dehydroamino Acids

Organometallics, 1997

A theoretical study on cationic Rh(I)-N-alkenylamide complexes is presented, which are important intermediates in the asymmetric hydrogenation affording N-acylamino acids. The assumption of different intermediates investigated is based on the inter-and intramolecular equilibrium of diastereomeric complexes in the hydrogenation mechanism. The geometry optimizations were performed at the MP2 level of theory using relativistic pseudopotentials for rhodium, followed by QCISD(T) single-point calculations. The intermediates containing cis-or trans-coordinated phosphines have been compared structurally and energetically, and the influence of solvents is discussed. In the equilibrium consisting of uncoordinated substrate and substrate complexes, all tautomeric complexes were found in the calculation and have a minimum on the potential hypersurface. However, the occurrence of enamine/ imine and amide/imine-ol tautomers could be excluded for the catalytic reaction, because they implement irreversible steps (C-H activation, deprotonation) which finish the catalytic cycle and may therefore be responsible for turnover-limiting steps. Only the calculation of an intermediate showing hetero-π-allyl type coordination can explain satisfactorily the crucial interconversion of the diastereomeric major/minor complexes in a noncoordinating medium. This intermediate is favored for the rationalization of the intramolecular equilibrium. For trans-coordinating diphosphine ligands a T-shaped intermediate with only a coordinated nitrogen atom is proposed.

Catalytic Hydrogenation of Norbornadiene by a Rhodium Complex in a Self-Folding Cavitand

Angewandte Chemie International Edition, 2010

Reactions mediated by supramolecularly encapsulated transition metal complexes are rare, [2] as cavities large enough to encapsulate the transition structure are required. Natural proteins, and hydrogen-bonded, metal-mediated, or hydrophobically driven synthetic multimolecular assemblies provide such spaces. For example, Raymond, Bergman et al. used M 4 L 6 assemblies as vessels for cationic metal sandwich complexes and controlled the reactivity of the corresponding half-sandwich. Herein we show that deep cavitands can also perform chemistry in this regard and present their unprecedented behavior in a catalytic hydrogenation mediated by a rhodium(I) complex.