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Pher G Andersson

Publications and source records attributed to Pher G Andersson.

16 recordsLinked to original sources
PubMed2006

Ir-catalysed asymmetric hydrogenation: Ligands, substrates and mechanism.

Relatively air and moisture tolerant cationic iridium complexes, with chiral non-racemic N,P-ligands and weakly coordinating counter ions, are efficient catalysts in the asymmetric hydrogenation of olefins. Unfunctionalised olefins are particularly difficult substrates because, in general, a polar group adjacent to the alkene bond is required for high catalytic activity and enantioselectivity. The applicability towards a variety of substrates and choice of ligands reported thus far in the literature, as well as the mechanism, selectivity issues and the importance of the anion are also discussed.

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PubMed2006

Asymmetric hydrogenation of trisubstituted olefins with iridium-phosphine thiazole complexes: a further investigation of the ligand structure.

New chiral bidentate phoshine thiazoles have been prepared and successfully applied as ligands in the homogeneous iridium-catalyzed asymmetric hydrogenation of aryl alkenes and aryl alkene esters. The ligands are designed to be highly modular and have one common chiral intermediate, from which diversity can be introduced at a late stage in the synthetic pathway. It was found that a six-member-ring backbone of the rigid ligand structure was preferred over seven- or five-member rings. In this study it is shown that the substituent pattern of the ligands has a major influence on the stereochemical outcome of the products. By applying the selectivity model proposed in this study, it is possible to match different substrates against different catalysts. In this way, good to excellent enantioselectivity can be obtained for typically difficult substrates. Geometrically different derivatives of alpha- and beta-methyl cinnamic acid ethyl esters were hydrogenated, to demonstrate the validity of the selectivity model and to verify the importance of steric and electronic matching of the catalyst and the substrate.

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PubMed2006

Mechanistic aspects of transition metal-catalyzed hydrogen transfer reactions.

In this tutorial review recent mechanistic studies on transition metal-catalyzed hydrogen transfer reactions are discussed. A common feature of these reactions is that they involve metal hydrides, which may be monohydrides or dihydrides. An important question is whether the substrate coordinates to the metal (inner-sphere hydrogen transfer) or if there is a direct concerted transfer of hydrogen from the metal to substrate (outer-sphere hydrogen transfer). Both experimental and theoretical studies are reviewed.

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PubMed2005

Mechanistic insights into the phosphine-free RuCp*-diamine-catalyzed hydrogenation of aryl ketones: experimental and theoretical evidence for an alcohol-mediated dihydrogen activation.

The commercially available chiral diamine quincorine-amine, originally derived from quinine, was found to be a highly active catalyst for ruthenium-catalyzed hydrogenation of ketones. The complex formed between the quincorine-amine, containing both a primary and a quinuclidine amino function, and RuCp*Cl catalyzes the hydrogenation of aromatic and aliphatic ketones in up to 90% ee approximately 24 times faster than previously reported Ru-diamine complexes. The pseudo-enantiomer of the quincorine-amine, i.e., quincoridine-amine, also showed high activity; however, the enantioselectivities obtained with this catalyst were lower. The reason for the lower, but opposite stereoselectivity seen with the quincoridine-amine, as compared to the quincorine-amine, was rationalized by a kinetic and computational study of the mechanism of the reaction. The theoretical calculations also revealed a significantly lower activation barrier for the alcohol-mediated split of dihydrogen, as compared to the nonalcohol-mediated process, a finding of utmost implication also for the diphosphine/diamine-mediated enantioselective hydrogenation of ketones.

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PubMed2004

Rationally designed ligands for asymmetric iridium-catalyzed hydrogenation of olefins.

A new class of chiral N,P-ligands for the Ir-catalyzed asymmetric hydrogenation of aryl alkenes has been developed. These new ligands proved to be highly efficient and tolerates a broad range of substrates. The enantiomeric excesses are in the range of the best ever reported. The results can be rationalized with the proposed selectivity model.

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PubMed2004

Origin of enantioselectivity in the Ru(arene)(amino alcohol)-catalyzed transfer hydrogenation of ketones.

The origin of the enantioselectivity in the ruthenium-catalyzed transfer hydrogenation has been studied by means of experiment and density functional theory calculations. The results clearly show that electrostatic effects are of importance, not only in the T-shaped arene-aryl interaction in the favored transition state but also between the aryl of the substrate and the amine ligand in the disfavored TS. In addition, the electrostatic interaction between the alkyl substituent of the substrate and the catalyst is of importance to the enantioselectivity. The major cause of enantioselection is found to be of nonelectrostatic origin. This inherent property of the catalytic system is discussed in terms of dispersion forces and solvent effects. Finally, a minor but well-characterized steric effect was identified. The success of this class of catalysts in the reduction of alkyl aryl ketones is based on the fact that all factors work in the same direction.

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PubMed2003

Cinchona alkaloid derived ligands in catalytic asymmetric transfer hydrogenation.

A collection of chiral quinuclidine ligands, derived from the Cinchona alkaloids quinine and quinidine, has been evaluated in the catalytic asymmetric transfer hydrogenation of aromatic ketones. It was fond that [IrCl(COD)]2 complexes of the diamines QCI-Amine and QCD-Amine gave the most active catalysts, capable of reducing a range of aromatic ketones with excellent conversions and good enantioselectivities (up to 95% ee). These are the best selectivities reported for ligands based on the quinuclidine core in an asymmetric transformation, and advocate that these ligands, commercially available in both pseudo-enantiomeric forms, will find practical use in this and other catalytic processes.

Catalysis↗
PubMed2003

New mechanistic insights into the iridium-phosphanooxazoline-catalyzed hydrogenation of unfunctionalized olefins: a DFT and kinetic study.

The reaction mechanism of the iridium-phosphanooxazoline-catalyzed hydrogenation of unfunctionalized olefins has been studied by means of density functional theory calculations (B3LYP) and kinetic experiments. The calculations suggest that the reaction involves an unexpected Ir(III)-Ir(V) catalytic cycle facilitated by coordination of a second equivalent of dihydrogen. Thus, in the rate-determining migratory insertion of the substrate alkene into an iridium-hydride bond, simultaneous oxidative addition of the bound dihydrogen occurs. The kinetic data shows that the reaction is first order with respect to hydrogen pressure. This is interpreted in terms of an endergonic coordination of this second equivalent of dihydrogen, although a rate-determining step, in which coordinated solvent is replaced by dihydrogen, could not be ruled out. Furthermore, the reaction was found to be zeroth order with respect to the alkene concentration. This correlates well with the calculated exothermicity of substrate coordination, and the catalyst is thus believed to coordinate an alkene in the resting state. On the basis of the proposed catalytic cycle, calculations were performed on a full-sized system with 88 atoms to assess the appropriateness of the model calculations. These calculations were also used to explain the enantioselectivity exerted by the catalyst.

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PubMed2002

Novel catalytic kinetic resolution of racemic epoxides to allylic alcohols.

[formula: see text] The kinetic resolution of racemic epoxides via catalytic enantioselective rearrangement to allylic alcohols was investigated. Using the Li-salt of (1S,3R,4R)-3-(pyrrolidinyl)methyl-2-azabicyclo [2.2.1] heptane 1 as catalyst allowed both epoxides and allylic alcohols to be obtained in an enantioenriched form.

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PubMed2002

New catalysts for the base-promoted isomerization of epoxides to allylic alcohols. Broadened scope and near-perfect asymmetric induction.

Optically active (1S,3R,4R)-3-[N-(trans-2,5-dialkyl)pyrrolidinyl]methyl-2-azabicyclo-[2.2.1]heptanes were evaluated as catalysts for the enantioselective beta-elimination of meso-epoxides. The (2R,5R)-dimethylpyrrolidinyl-substituted catalyst 4 exhibited exceptionally high enantioselectivity and reactivity, and several substrates were rearranged with enantioselectivities of 98-99% ee. In addition, the use of 4 allowed the first successful, true catalytic rearrangement of the difficult substrates cyclopentene oxide (81%, 96% ee) and (Z)-4-octene oxide (80%, 91% ee).

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PubMed2002

Asymmetric base-mediated epoxide isomerisation.

The base-mediated rearrangement of epoxides into allylic alcohols is a well-known synthetic transformation. The first enantioselective version of the reaction using a chiral base was reported in 1980. Since then, the reaction has received a lot of attention mostly due to the great usefulness of chiral allylic alcohols in organic synthesis. Major breakthroughs in the area were the first report on using a sub-stoichiometric amount of chiral base, and the development of chiral bases for a true catalytic reaction protocol. The present review covers the time from when the first asymmetric epoxide isomerisation reaction was reported (1980) up to now, focusing on the period 1997-2001.

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