Stereochemistry of Imine Reduction by a Hydroxycyclopentadienyl Ruthenium Hydride

Casey, Charles P.; Bikzhanova, Galina A.; Guzei, Ilia A.

doi:10.1021/ja056402u

Cited by 65 publications

(57 citation statements)

References 19 publications

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“…This notion was reinforced by the analogy to the Noyori-hydrogenation catalysts. [56] In those systems, a metal complex effects heterolytic cleavage of H 2 yielding a metal hydride and a protonated ligand. Frustrated Lewis pairs effect similar heterolytic H 2 activation without the need for a transition metal.…”

Section: Catalytic Hydrogenations By Phosphine/borane Flpsmentioning

confidence: 99%

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

2009

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Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

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Section: Catalytic Hydrogenations By Phosphine/borane Flpsmentioning

confidence: 99%

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

2009

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“…Amine nitrogen inversion is extremely rapid (DG {~7 .5 kcal mol À1 ) and had previously precluded determination of the stereochemistry of reduction of imines by any reagent or catalytic system [60,61]. Deuterium labeling experiments established that the reduction of N-aryl imines by 2-tol-d 2 is largely stereospecific [62]. Reduction of PhCH¼NPh by deuterated 2-tol-d 2 at À60 C produced a 2:1 mixture of stereoisomers of deuterated 9-d 2 :9-d 2 * (Scheme 20); the major stereoisomer 9-d 2 has D and Ru added to the same face of the imine C¼N bond.…”

Section: Scheme 17mentioning

confidence: 99%

Shvo’s Catalyst in Hydrogen Transfer Reactions

Warner

Casey

Bäckvall

2011

Bifunctional Molecular Catalysis

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This chapter reviews the use of Shvo's catalyst in various hydrogen transfer reactions and also discusses the mechanism of the hydrogen transfer. The Shvo catalyst is very mild to use since no activation by base is required in the transfer hydrogenation of ketones or imines or in the transfer dehydrogenation of alcohols and amines. The Shvo catalyst has also been used as an efficient racemization catalyst for alcohols and amines. Many applications of the racemization reaction are found in the combination with enzymatic resolution leading to a dynamic kinetic resolution (DKR). In these dynamic resolutions, the yield based on the starting material can theoretically reach 100%. The mechanism of the hydrogen transfer from the Shvo catalyst to ketones (aldehydes) and imines as well as the dehydrogenation of alcohols and amines has been studied in detail over the past decade. It has been found that for ketones (aldehydes) and alcohols, there is a concerted transfer of the two hydrogens involved, whereas for typical amines and imines, there is a stepwise transfer of the two hydrogens. One important question is whether the substrate is coordinated to the metal or not in the hydrogen transfer step(s). The pathway involving coordination to activate the substrate is called the inner-sphere mechanism, whereas transfer of hydrogen without coordination is called the outer-sphere mechanism. These mechanistic proposals together with experimental and theoretical studies are discussed.

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“…26 The transfer hydrogenation of imines to amines and the corresponding reverse reaction are expected to show significant differences to the ketone/alcohol reaction because of the differences in basicity, susceptibility to nucleophilic attack and their ability to bind to metal ions. There have been fewer investigations into the imine/amine reaction using organometallic catalysts, although kinetic and isotope labelling studies using a cyclopentadienone ruthenium catalyst 27,28 have attempted to differentiate an inner sphere mechanism, involving direct coordination of the substrate to the metal, and an outer sphere process in which the amine/imine nitrogen does not bind directly to the ruthenium. Distinguishing between stepwise and concerted hydride and proton transfer steps is also controversial 29 .…”

Section: Introductionmentioning

confidence: 99%

The kinetics and mechanism of the organo-iridium-catalysed enantioselective reduction of imines

et al. 2016

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The iridium complex of pentamethylcyclopentadiene and (S,S)-1,2-diphenyl-N ′ -tosylethane-1,2-diamine is an effective catalyst for the asymmetric transfer hydrogenation of imine substrates under acidic conditions. Using the Ir catalyst and a 5:2 ratio of formic acid: triethylamine as the hydride source for the asymmetric transfer hydrogenation of 1-methyl-3,4-dihydroisoquinoline and its 6,7-dimethoxy substituted derivative, in either acetonitrile or dichloromethane, shows unusual enantiomeric excess (ee) profiles for the product amines. The reactions initially give predominantly the (R) enantiomer of the chiral amine products with >90% ee but which then decreases significantly during the reaction. The decrease in ee is not due to racemisation of the product amine, but because the rate of formation of the (R)-enantiomer follows first-order kinetics whereas that for the (S)-enantiomer is zero-order. This difference in reaction order explains the change in selectivity as the reaction proceeds -the rate formation of the (R)-enantiomer decreases exponentially with time while that for the (S)-enantiomer remains constant. A reaction scheme is proposed which requires rate-limiting hydride transfer from the iridium hydride to the iminium ion for the first-order rate of formation of the (R)-enantiomer amine and rate-limiting dissociation of the product for the zero-order rate of formation of the (S)-enantiomer.

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Stereochemistry of Imine Reduction by a Hydroxycyclopentadienyl Ruthenium Hydride

Cited by 65 publications

References 19 publications

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

Shvo’s Catalyst in Hydrogen Transfer Reactions

The kinetics and mechanism of the organo-iridium-catalysed enantioselective reduction of imines

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