Unravelling the Reaction Path of Rhodium–MonoPhos‐Catalysed Olefin Hydrogenation

Abstract

The mechanism of the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate (mac) catalysed by [Rh(MonoPhos)(2)(nbd)]SbF(6) (MonoPhos: 3,5-dioxa-4-phosphacyclohepta[2,1-a:3,4-a']dinaphthalen-4-yl)dimethylamine) was elucidated by using (1)H, (31)P and (103)Rh NMR spectroscopy and ESI-MS. The use of nbd allows one to obtain in pure form the rhodium complex that contains two units of the ligand. In contrast to the analogous complexes that contain cis,cis-1,5-cyclooctadiene (cod), this complex shows well-resolved NMR spectroscopic signals. Hydrogenation of these catalyst precursors at 1 bar total pressure gave rise to the formation of a bimetallic complex of general formula [Rh(MonoPhos)(2)](2)(SbF(6))(2); no solvate complexes were detected. In the dimeric complex both rhodium atoms are ligated to two MonoPhos ligands but, in addition, each rhodium atom also binds to one of the binaphthyl rings of a ligand that is bound to the other rhodium metal. Upon addition of mac, a mixture of diastereomeric complexes [Rh(MonoPhos)(2)(mac)]SbF(6) is formed in which the substrate is bound in a chelate fashion to the metal. Upon hydrogenation, these adducts are converted into a new complex [Rh(MonoPhos)(2){mac(H)(2)}]SbF(6) in which the methyl phenylalaninate mac(H)(2) is bound through its aromatic ring to rhodium. Addition of mac to this complex leads to displacement of the product by the substrate. No hydride intermediates could be detected and no evidence was found for the involvement at any stage of the process of complexes with only one coordinated MonoPhos. The collected data suggest that the asymmetric hydrogenation follows a Halpern-like mechanism in which the less abundant substrate-catalyst adduct is preferentially hydrogenated to phenylalanine methyl ester.

Cation‐Triggered Switchable Asymmetric Catalysis with Chiral Aza‐CrownPhos

“…The 31 P NMR spectroscopic analysis (Scheme ) showed two broad absorption peaks, which indicates that the precatalyst has two structural isomers which differ in the relative orientation of the two coordinated monodentate Aza‐CrownPhos ligands. The two isomers interconverted at room temperature and showed broad absorptions both in 1 H NMR and 31 P NMR spectra 11a…”

“…Subsequently, the precatalyst in CD 2 Cl 2 was stirred under hydrogen gas for 1 hour to remove norbornadiene 11a–d. As shown in Scheme , the 1 H NMR spectrum showed an obvious signal splitting and broadening in the region δ =2.8–4.5 ppm, and are ascribed to the protons of aza‐crown ether units.…”

Remarkably Facile Borane-Promoted, Rhodium-Catalyzed Asymmetric Hydrogenation of Tri- and Tetrasubstituted Alkenes

“…27 Scheme 5 outlines a simple unified model, an “interrupted hydroboration” pathway, 28 to account for the apparent competition between CAH and CAHB 29 under the conditions described herein. Two-point binding of the substrate ( SM ) to Rh and oxidative addition of pinBH gains access to intermediate I .…”

Ligand Self‐Sorting and Nonlinear Effects in Dinuclear Asymmetric Hydrogenation: Complexity in Catalysis

“…The structure of this complex is similar to the precatalyst synthesized from monodentate phosphorus ligands, such as the well-studied MONOPhos. [20] The final dimeric complex [Rh 2 L 4 ] quantitatively forms when 5 bars of H 2 is applied to this monomer, which effectively removes the nbd ligands.…”

“…In analogy with well-known systems, we expect solvated bis-ligated monomers to be active intermediates. [20] As they are formed from the non-self-sorted intermediate B, they are not self-sorted either, and the weakly coordinated solvent molecules make them very reactive towards H 2 and substrates.…”