Enantioselective catalysis with complexes of asymmetric P,N-chelate ligands

Helmchen, Günter; Kudis, Steffen; Sennhenn, Peter; Steinhagen, Henning

doi:10.1351/pac199769030513

Cited by 122 publications

(49 citation statements)

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“…Since no observable complexation of malonate occurs when sodium dimethyl malonate is added to the -allyl complex in the absence of a CO source, we believe addition of CO to form the tricarbonyl complex 14 must occur before formation of the malonate complex 15. Finally, reductive elimination occurs to afford the enantio-enriched product and the anionic Mo(CO) 4 L complex (16), which is one of the observed resting states of the catalyst along with the -allyl complex (54).…”

Section: Resultsmentioning

confidence: 99%

“…Early work in this area focused on Pd complexes as catalysts, but their utility in organic synthesis was limited because, in general, unsymmetrical substrates afforded the achiral linear product (1)(2)(3)(4)(5). More recently, a limited number of ligands have been designed such that the branched product is now accessible with high enantiomeric excess (ee) by using Pd catalysis (6)(7)(8)(9).…”

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confidence: 99%

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Mechanistic studies of the molybdenum-catalyzed asymmetric alkylation reaction

Hughes

Lloyd‐Jones

Krska

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

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M etal-catalyzed asymmetric allylic alkylations are widely used in organic synthesis as efficient and practical methods to form carbon-carbon bonds with high enantio-and regioselectivity. Early work in this area focused on Pd complexes as catalysts, but their utility in organic synthesis was limited because, in general, unsymmetrical substrates afforded the achiral linear product (1-5). More recently, a limited number of ligands have been designed such that the branched product is now accessible with high enantiomeric excess (ee) by using Pd catalysis (6-9). Additionally, several other transition metals and ligands have been developed to regioselectively provide the branched product. These reactions can be grouped into two categories: (i) those that retain the stereochemistry of the branched substrate, and (ii) those in which the stereochemistry is lost. When starting with branched substrates, Ru (10-13), Rh (14,15), Fe (20,21), and W (22) complexes generally provide product wherein the stereochemistry of the substrate is retained, although recent exceptions have now been reported (23,24). These reactions may proceed by means of intermediates in which isomerization (K eq in Scheme 1) is slow relative to nucleophilic reaction. In contrast, Mo-based catalysis (25-39) proceeds with loss of substrate stereochemistry and provides the opportunity in reactions with asymmetric ligands to obtain high product ee, starting with either the linear substrate or racemic branched product (Scheme 1). These reactions proceed by a dynamic kinetic asymmetric transformation (40) with rapid equilibration of the two diastereomeric -allyl complexes before nucleophilic attack (Scheme 1).The stereochemistry of the two steps shown in Scheme 1 is arbitrarily shown as retention for oxidative addition and retention for the nucleophilic displacement. Extensive work on the Pd-catalyzed allylic alkylation has shown that the overall stereochemical outcome is generally retention, with the two steps of the process both proceeding with inversion (41-46). More recently, the Ir-catalyzed alkylation has also been shown to proceed by inversion-inversion in constrained systems (19). For the Mo-catalyzed reaction, Trost and coworkers (47, 48) have determined that the reaction proceeds with overall retention, but the stereochemistry of each step in the reaction has not been elucidated.A few studies have been carried out to determine the stereochemistry of the oxidative addition reaction in the Mocatalyzed reaction. Faller and Linebarrier (49), Rubio and Liebeskind (50), and Kuhl et al. (51) have shown that oxidative addition in stoichiometric reactions of Mo(CO) 3 (MeCN) 3 with cyclic and acyclic allylic acetates proceeds with retention of configuration. On the other hand, Ward et al. (52) discovered the stereochemistry of the oxidative addition is influenced by steric factors and experimental conditions, and oxidative addition of a single substrate can follow either an inversion or retention pathway, depending on the solvent and concentration, suggesti...

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Mechanistic studies of the molybdenum-catalyzed asymmetric alkylation reaction

Hughes

Lloyd‐Jones

Krska

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

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“…Apart from a constant shift of the resonances (Dd = 0.6 ppm in 13 C and Dd = 1 ppm in 1 H), these are essentially identical. Furthermore we compared long-range coupling constants determined in the isotropic phase with the corresponding coupling constants obtained in the anisotropic phase under magic angle sample spinning (MAS) conditions (Tables SI 5 and SI 6 in the Supporting Information).…”

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confidence: 87%

Determination of the Conformation of the Key Intermediate in an Enantioselective Palladium‐Catalyzed Allylic Substitution from Residual Dipolar Couplings

et al. 2009

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“…[1] Three models for the design of highly selective catalysts for allylic substitutions have proven most successful: [2] "side arm guidance" of nucleophiles, [3] "chiral pockets" generated by Trosts C 2 -symmetric diphosphanes based on 2-(diphenylphosphino)benzoic acid (dppba), [4] and "electronic differentiation" by different donor atoms, such as in P,N ligands (e.g., phosphinooxazolines; PHOX). [5] Phosphinooxazoline catalysts enantioselectivities arise from electronic differentiation between coordinating P and N atoms, leading to preferential attack of nucleophiles trans to phosphorus. [6] In addition, enantioselectivities are also a function of steric differentiation-that is, more stable exo versus endo Pd-h 3 -allylic intermediates [7] -which has been studied by X-ray analyses, NMR investigation, and quantum-chemical computations (Scheme 1).…”

Section: Introductionmentioning

confidence: 99%

Ligand Bite Governs Enantioselectivity: Electronic and Steric Control in Pd‐Catalyzed Allylic Alkylations by Modular Fenchyl Phosphinites (FENOPs)

Goldfuß

Löschmann²,

Röminger

2004

Chemistry A European J

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Modular fenchyl phosphinites (FENOPs) containing different aryl units-phenyl (1), 2-anisyl (2), or 2-pyridyl (3)-are efficiently accessible from (-)-fenchone. For comparison of the influence of the different aryl units on enantioselectivities and reactivities, these FENOPs were employed in Pd-catalyzed allylic alkylations. The strongly chelating character of P,N-bidentate 3 is apparent from X-ray structures with PdCl2 ([Pd3Cl2]), and with allyl-Pd units in ([Pd3(eta1-allyl)] and [Pd3(eta3-allyl)]). FENOP3 gives rise to a PdL* catalyst of moderate enantioselectivity (42 % ee, R product). Surprisingly, higher enantioselectivities are found for the hemilabile, monodentate FENOPs 1 (83 % ee, S enantiomer) and 2 (69 % ee, S enantiomer). Only small amounts of 1 or 2 generate selective PdL* catalysts, while complete abolition of enantioselectivity appears with unselective PdL*2 species with higher FENOP concentrations in the cases of 1 or 2. Computational transition structure analyses reveal steric and electronic origins of enantioselectivities. The nucleophile is electronically guided trans to phosphorus. endo-Allyl arrangements are favored over exo-allyl orientations for 1 and 2 due to Pd-pi-pyridyl interactions with short "side-on" Pd-aryl interactions. More remote "edge-on" Pd-pi-aryl interactions in 3 with Pd-N(lp) coordination favor endo-allyl units slightly more and explain the switch of enantioselectivity from 1 (S) and 2 (S) to 3 (R).

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Enantioselective catalysis with complexes of asymmetric P,N-chelate ligands

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Cited by 122 publications

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Mechanistic studies of the molybdenum-catalyzed asymmetric alkylation reaction

Mechanistic studies of the molybdenum-catalyzed asymmetric alkylation reaction

Determination of the Conformation of the Key Intermediate in an Enantioselective Palladium‐Catalyzed Allylic Substitution from Residual Dipolar Couplings

Ligand Bite Governs Enantioselectivity: Electronic and Steric Control in Pd‐Catalyzed Allylic Alkylations by Modular Fenchyl Phosphinites (FENOPs)

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