Highly Enantioselective Hydrogenation of Cyclic Enamides Catalyzed by a Rh-PennPhos Catalyst

Zhang, Zhaoguo; Zhu, Guoxin; Jiang, Qiongzhong; Xiao, Duan; Zhang, Xumu

doi:10.1021/jo9824605

Cited by 144 publications

(49 citation statements)

References 21 publications

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“…[48] The enamide substrates were prepared either by Fe/TMSCl/Ac 2 O reduction of the oximes in HOAc [49,50] or via Grignard reaction on the nitriles followed by reaction of the magnesium imine with Ac 2 O. [51] The hydrogenation of these substrates proceeds slower than the hydrogenation of the dehydroamino acid derivatives.…”

Section: Full Papersmentioning

confidence: 99%

Monodentate Phosphoramidites: A Breakthrough in Rhodium‐Catalysed Asymmetric Hydrogenation of Olefins

et al. 2003

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Monodentate phosphoramidites based on BINOL or substituted BINOL are excellent ligands for the rhodium-catalysed asymmetric hydrogenation of olefins. Very high enantioselectivities were obtained with MonoPhos (7a) the simplest member of this class, a ligand that is prepared in a single step from BINOL and HMPT. Turnover numbers up to 6000 have been obtained in the hydrogenation of dehydroamino acid derivatives. Enantioselectivities in the hydrogenation of dehydroamino acids are solvent dependent; in non-protic solvents they range from 95 ± 99%. Itaconic acid and its dimethyl ester could be hydrogenated with 96 and 94% e.e., respectively. Hydrogenation of aromatic enamides gave the corresponding acylated amines in 86 ± 94% e.e. Several analogous phosphoramidite ligands have been prepared. Surprisingly, bidentate ligands gave poorer results, both in terms of rate as well as enantioselectivity. Taddol-based phosphoramidites led to poor e.e. and slow rates. Methyl substituents at the 3,3'-position of BINOL led to a sharply reduced rate and a somewhat lower enantioselectivity. Bromo substituents at the 6,6'-position led to a slightly reduced rate but little effect was seen on enantioselectivity. Use of octahydro-MonoPhos (11) gave results that were very similar to those obtained with 7a. The rate of the reaction is dependent on the hydrogen pressure, however, the enantioselectivity is not affected. The rate of the dehydroamino acid hydrogenation also increases if the ligand to rhodium ratio is reduced from 2.2 to 1.5 or even to 1.0; yet, there is no deleterious effect on the enantioselectivity. Catalytic activity ceases with L/Rh 3 when dehydroamino acid derivatives were used as substrate. The reaction shows a positive non-linear effect, which confirms the presence of Rh-complexes with more than one ligand. Following the hydrogenation of methyl 2-acetamidocinnnamate with Rh(nbd) 2 BF 4 /7a by electrospray mass spectrometry showed the presence of several rhodium species. Notable are the presence of [Rh(7a)] 3 and [Rh(7a)] 4 . There is at present insufficient evidence to conclude if the active catalytic species carries one or two ligands. In view of the low cost of MonoPhos this invention might well lead to a broader application of asymmetric olefin hydrogenation for the production of enantiopure amino acids and amines.

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Section: Full Papersmentioning

confidence: 99%

Monodentate Phosphoramidites: A Breakthrough in Rhodium‐Catalysed Asymmetric Hydrogenation of Olefins

et al. 2003

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“…[13,20,28] The stereochemistry of 5 and therefore of 6 was determined by X-ray diffraction analysis. [29] The heteroaromatic enamides 8 and 9 were synthesised using a modified literature procedure [11] starting from the corresponding aryl nitriles (Scheme 2).…”

Section: Resultsmentioning

confidence: 99%

“…Solvents were reagent grade and dried and distilled before use following standard procedures. NMR spectra were recorded at room temperature in CDCl 3 General Procedure for the Synthesis of N-Acetyl-aarylenamides 3 ± 7 and 10 [13,20,28] Acetophenone (11.7 mL, 100 mmol) and hydroxylamine hydrochloride salt (14.8 g, 213 mmol) were dissolved in ethanol (150 mL) and pyridine (15 mL). After refluxing for 5 h the solvent was evaporated and water (150 mL) was added before the solution was cooled while stirring in an ice bath.…”

Section: Experimental Section General Remarksmentioning

confidence: 99%

“…Spectral data of 3 -7 and 10 were in agreement with those reported in literature. [13,20] General Procedure for the Synthesis of N-Acetyl-aarylenamides 8 and 9 [11] To a stirred solution of MeMgBr (6 mL, 3 M solution in Et 2 O) diluted with Et 2 O (20 mL) was added dropwise a solution of ArCN (17 mmol) in Et 2 O (10 mL). The mixture was heated at reflux overnight until all the starting material was converted (as indicated by TLC).…”

Section: Experimental Section General Remarksmentioning

confidence: 99%

“…One of the first examples of the asymmetric hydrogenation of an aarylenamide was reported by Kagan et al employing a complex of rhodium, with DIOP as a bidentate ligand, to afford ee×s up to 92%. [10] Higher enantioselectivities were obtained with chiral phosphine ligands like DuPHOS, BPE, [11,12,13] CDP, [14] BICP, [15] Binaphane, [16] , DIOP analogues, [17,18] bdpmi, [19] PennPhos, [20] aminophosphine BDPAB, [21] bisphosphinite spirOP [22] and TangPhos. [23,24] Bidentate ligands which can chelate rhodium were used in all these studies with the majority being chiral diphosphines.…”

Section: Introductionmentioning

confidence: 99%

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