399037-25-3Relevant articles and documents
Ruthenium Complexes with PNN Pincer Ligands Based on (Chiral) Pyrrolidines: Synthesis, Structure, and Dynamic Stereochemistry
Bootsma, Johan,De Vries, Johannes G.,Guo, Beibei,Otten, Edwin
, (2020)
We report the synthesis of lutidine-based PNN type metal pincer complexes, using achiral (pyrrolidine) as well as chiral ((R,R)-2,5-dimethylpyrrolidine and (R)-2-methylpyrrolidine) substituents at the N side arm of the pincer ligand. With the six-coordinate saturated Ru pincers (PNN)Ru(H)(CO)(Cl), which have an aromatic pyridine ligand backbone, as the starting materials, treatment with strong base (KOtBu) generated the corresponding dearomatized pincer complexes (PNN′)Ru(H)(CO). Spectroscopic, crystallographic, and computational studies demonstrate that the C-centered chirality from the chiral pyrrolidine group exerts a small but non-negligible influence on the preferred stereochemistry at Ru (and N in the case of (R)-2-methylpyrrolidine) that is reflected in the equilibrium distribution of diastereomers of these Ru complexes in solution. Our data show that the N- and Ru-based stereogenic centers in this class of compounds are stereochemically labile and the mechanisms for epimerization are discussed. Inversion at the Ru center in the dearomatized complexes is proposed to occur via a rearomatized Ru(0) intermediate in which the Ru-bound hydride is transferred to the ligand. Support for this comes from the spectroscopic characterization of a closely related Ru(0) species that is obtained by reaction with CO. Testing these catalysts in enantioselective oxa-Michael addition or transfer hydrogenation led to racemic products, while a low ee (8%) was observed in the hydrogenation of 4-fluoroacetophenone. The lack of appreciable enantioinduction with these catalysts is ascribed to the kinetic lability of the Ru stereocenter, which results in the formation of equilibrium mixtures in which several diastereomers of the catalyst are present.
A metal-ligand cooperative pathway for intermolecular oxa-michael additions to unsaturated nitriles
Perdriau, Sébastien,Zijlstra, Douwe S.,Heeres, Hero J.,De Vries, Johannes G.,Otten, Edwin
, p. 4236 - 4240 (2015/04/14)
An unprecedented catalytic pathway for oxa-Michael addition reactions of alcohols to unsaturated nitriles has been revealed using a PNN pincer ruthenium catalyst with a dearomatized pyridine backbone. The isolation of a catalytically competent Ru-dieneamido complex from the reaction between the Ru catalyst and pentenenitrile in combination with DFT calculations supports a mechanism in which activation of the nitrile through metal-ligand cooperativity is a key step. The nitrile-derived Ru-N moiety is sufficiently Br?nsted basic to activate the alcohol and initiate conjugate addition of the alkoxide to the α,β-unsaturated fragment. This reaction proceeds in a concerted manner and involves a six-membered transition state. These features allow the reaction to proceed at ambient temperature in the absence of external base.
Nitrile biotransformations for the synthesis of highly enantioenriched β-hydroxy and β-amino acid and amide derivatives: A general and simple but powerful and efficient benzyl protection strategy to increase enantioselectivity of the amidase
Ma, Da-You,Wang, De-Xian,Pan, Jie,Huang, Zhi-Tang,Wang, Mei-Xiang
, p. 4087 - 4091 (2008/09/20)
(Chemical Equation Presented) Biotransformations of a number of racemic β-hydroxy and β-amino nitrile derivatives were studied using Rhodococcus erythropolis AJ270, the nitrile hydratase and amidase-containing microbial whole cell catalyst, under very mild conditions. The overall enantioselectivity of nitrile biotransformations was governed predominantly by the amidase whose enantioselectivity was switched on remarkably by an O- and a N-benzyl protection group of the substrates. While biotransformations of β-hydroxy and β-amino alkanenitriles gave low yields of amide and acid products of very low enantiomeric purity, introduction of a simple benzyl protection group on the β-hydroxy and β-amino of nitrile substrates led to the formation of highly enantioenriched β-benzyloxy and β-benzylamino amides and acids in almost quantitative yield. The easy protection and deprotection operations, high chemical yield, and excellent enantioselectivity render the nitrile biotransformation a useful protocol in the synthesis of enantiopure β-hydroxy and β-amino acids.
