Transfer hydrogenation catalysed

Hydrogan Transfer- An extensive review of asymmetric transfo hydrogenation is of interest i as is an improved version for selective ester reduction via hydrogen transfer from HSi(OEt)3 in the presence of Ti(OPr )4. Addition to either the same or opposite enantiofaces of prochiral alkenes is catalysed by RuH (S)-BINAP species from either Pr>OH or H2 depending on the alkene functionality. A number of alcohol-ketone transfer hydrogenations catalysed by... 

Transfer Hydrogenation Catalysed by Platinum Group Metals... 

The transfer hydrogenations of carbonyl compounds to alcohols catalysed by a variety of NHC complexes have been intensively studied. The strong bond... 

Scheme 2.4 Experimental evidence in support of the mechanism for the base-free transfer hydrogenation of carbonyl compounds catalysed by complex 43...

Even if chiral S/N ligands have proved their efficiency for promoting metal-catalysed asymmetric transfer hydrogenation, up to now they do not compete with the N/N ligands, at least in terms of enantioselectivity. ... 

I. S. 3/ >.4/ >j-2-AZANORBORNYLMETIIANOL, AN EFFICIENT LIGAND FORRUTHENIUM-CATALYSED ASYMMETRIC TRANSFER HYDROGENATION OF AROMATIC KETONES... 

Ruthenium-catalysed asymmetric transfer hydrogenation of acetophenone 133... 

In this chapter and in Chapters 10-12, we will review and validate some methods for asymmetric (transfer) hydrogenation of carbon-oxygen and carbon-carbon double bonds catalysed by non-metallic systems, homogeneous transition metal catalysts and biocatalysts. Reduction of carbon-nitrogen double bond systems will be reported in another volume of this series. 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

Enantiomerically enriched samples of 1-indanol, used to determine the enantiomer specificity by GC, can be prepared by hydrogen transfer ruthenium-catalysed reduction, as described by Ursini et al ... 

A nice analysis of non-linear effects in Rh-chiral diamine-catalysed transfer hydrogenation has been performed that reinforces the need to consider the kinetics of all of the steps in reaction manifolds (e.g. reversible formation of diastereomeric precursors and their subsequent interaction with achiral reactants). ... 

List was the first to explore this possibility, examining the Hantzsch ester mediated reduction of a,P-unsaturated aldehydes [209], Using 20 mol% of the binaphthyl derived phosphonate salt of morpholine (153) in dioxane at 50 °C, a series of P-aryl a,P-unsaturated aldehydes underwent transfer hydrogenation with Hantzsch ester 154 with excellent levels of absolute stereocontrol (96-98% ee) (Scheme 63). The method was also applied to the aliphatic substrates ( )-citral and famesal to give the mono-reduced products in 90% and 92% ee, respectively. Significantly, in line with many of the chiral secondary amine catalysed transformations described above the reactions follow a simple and practical procedure without the need for exclusion of moisture and air. 

Scheme 63 Asymmetric counteranion-directed catalysed transfer hydrogenation...

A new catalyst salt (20) that consists of an achiral ammonium ion and a chiral phosphate anion and which catalyses highly enantioselective transfer hydrogenations of ,/J-unsaturated aldehydes to the corresponding saturated derivatives has been developed. The underlying principle, namely asymmetric counteranion-directed catalysis, is claimed to be a new strategy for highly enantioselective synthesis.357... 

The role of alkali metal cations in the [ RuCL(p-cymene) 2l-pseudo-dipepLide-catalysed enantioselective transfer hydrogenation of ketones with propan-2-ol has been examined. Lithium salts were shown to increase the enantioselectivity of the reaction when 2-PrONa or 2-PrOK was used as the base. An alternative reaction mechanism for the pseudo-dipeptide-based systems, in which the alkali metal cation is an important player in the ligand-assisted hydrogen-transfer step, has been proposed.370... 

Asymmetric transfer hydrogenation of cyclic imines and iminiums in water was carried out in high yields and enantioselectivities with sodium formate as the hydrogen source and CTAB as an additive, catalysed by a water-soluble and recyclable ruthenium(II) complex of the ligand (23).371... 

A class of simple, modular, and highly efficient a-amino acid amides ligands for Ru-and Rh-catalysed asymmetric transfer hydrogenation of aromatic ketones in propan-2-ol has been developed. A remarkable feature with these ligands is the switch of product enantioselectivity observed when the amide functionality is replaced by the corresponding thioamide. The results obtained have significant mechanistic implications... 

Anomalous concentration dependence observed in the asymmetric transfer hydrogenation of imines with formic acid, catalysed by chiral rhodium-diamine complexes, has been attributed to the participation of both reactant and product in the formation of formate salt. The probable resting state of the catalyst is a rhodium hydride species.373... 

Asymmetric hydrogen transfer from 2-propanol to aromatic ketones such as acetophenone (99) has been achieved by using the same chiral Ru complex in 2-propanol containing KOH at room temperature, and (S)-1 -phenylethanol (100) with 98% ee was obtained [68,69]. Similarly, efficient Ru-catalysed transfer hydrogenation of aromatic ketones using the cyclic amino alcohol [(I. S, 3R,4i )-2-azanorbomylmetha-nol] (110) [70] and bis(oxazolinylmethyl) amine (111) [71] was reported. 

Chiral amines can be prepared by asymmetric hydrogenation, transfer hydrogenation and hydrosilylation of imines. The piperidine 137 with 98% ee was obtained by highly efficient asymmetric hydrogenation of the cyclic imines 136 catalysed by the Ti catalyst 61 [82]. Pyrrolidine 139 with 99% ee was obtained in 34% after 50%... 

Transfer hydrogenation of the imine bond in 146 from HCO2H, catalysed by the Ru chiral amine complex 114, afforded salsolidine (147) [86],... 

Hydrogenation of dienes with up to 20 1.0 diastereoselectivity and 99% ee is mediated by carbene complexes. The scope and limitations of these reactions were investigated.288 Asymmetric transfer hydrogenation to prochiral ketones, catalysed by a Ru(II) complex (10) or its dimer, with formic acid-triethylamine has been reported, (0 The protocol leads to high yields and enantioselectivity up to 96%. It has been suggested that 16-electron Ru(II) and the Ru-H intermediates are involved in this reaction.289... 

Enantioselective reduction of prochiral ketones by transfer hydrogenation is catalysed by amino acid derivatives. A mechanistic suggestion for the origin of enantioselective induction has been proposed.306 ,/M Jnsaluralcd nitriles are efficiently reduced by a Cu(I)-H species in the presence of bisphosphine ligands. The active Cu(I)-H species was generated by the reaction of copper(II) acetate and polymethylhydrosilane.307... 

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