Electronically tuned chiral ruthenium porphyrins: extremely stable and selective catalysts for asymmetric epoxidation and cyclopropanation

We report the use of three enantiomerically pure and electronically tuned ruthenium carbonyl porphyrin catalysts for the asymmetric cyclopropanation and epoxidation of a variety of olefinic substrates. The D(4)-symmetric ligands carry a methoxy, a methyl or a trifluoromethyl group at the 10-position of each of the 9-[anti-(1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracene)]-substituents at the meso-positions of the porphyrin. Introduction of a CF(3)-substituent in this remote position resulted in greatly improved catalyst stability, and turnover numbers of up to 7500 were achieved for cyclopropanation, and up to 14,200 for epoxidation, with ee values typically >90 % and approximately equal to 80 %, respectively. In one example, the axial CO ligand at the ruthenium was exchanged for PF(3), resulting in the first chiral ruthenium porphyrin with a PF(3) ligand reported to date. In cyclopropanations with ethyl diazoacetate, the latter catalyst performed exceedingly well, and gave a 95 % ee in the case of 1,1-diphenylethylene as substrate.     

Biomimetic iron-catalyzed asymmetric epoxidation of aromatic alkenes by using hydrogen peroxide

A novel and general biomimetic non-heme Fe-catalyzed asymmetric epoxidation of aromatic alkenes by using hydrogen peroxide is reported herein. The catalyst consists of ferric chloride hexahydrate (FeCl(3)6 H(2)O), pyridine-2,6-dicarboxylic acid (H(2)(pydic)), and readily accessible chiral N-arenesulfonyl-N'-benzyl-substituted ethylenediamine ligands. The asymmetric epoxidation of styrenes with this system gave high conversions but poor enantiomeric excesses (ee), whereas larger alkenes gave high conversions and ee values. For the epoxidation of trans-stilbene (1 a), the ligands (S,S)-N-(4-toluenesulfonyl)-1,2-diphenylethylenediamine ((S,S)-4 a) and its N'-benzylated derivative ((S,S)-5 a) gave opposite enantiomers of trans-stilbene oxide, that is, (S,S)-2 a and (R,R)-2 a, respectively. The enantioselectivity of alkene epoxidation is controlled by steric and electronic factors, although steric effects are more dominant. Preliminary mechanistic studies suggest the in situ formation of several chiral Fe-complexes, such as [FeCl(L*)(2)(pydic)]HCl (L*=(S,S)-4 a or (S,S)-5 a in the catalyst mixture), which were identified by ESIMS. A UV/Vis study of the catalyst mixture, which consisted of FeCl(3)6 H(2)O, H(2)(pydic), and (S,S)-4 a, suggested the formation of a new species with an absorbance peak at lambda=465 nm upon treatment with hydrogen peroxide. With the aid of two independent spin traps, we could confirm by EPR spectroscopy that the reaction proceeds via radical intermediates. Kinetic studies with deuterated styrenes showed inverse secondary kinetic isotope effects, with values of k(H)/k(D)=0.93 for the beta carbon and k(H)/k(D)=0.97 for the alpha carbon, which suggested an unsymmetrical transition state with stepwise O transfer. Competitive epoxidation of para-substituted styrenes revealed a linear dual-parameter Hammett plot with a slope of 1.00. Under standard conditions, epoxidation of 1 a in the presence of ten equivalents of H(2) (18)O resulted in an absence of the isotopic label in (S,S)-2 a. A positive nonlinear effect was observed during the epoxidation of 1 a in the presence of (S,S)-5 a and (R,R)-5 a.     

New templating strategies with salen scaffolds (Salen=N,N'-bis(salicylidene)ethylenediamine dianion)

Templated approaches towards selective organic synthesis is a common feature in nature in which nucleic acid templated synthesis plays a crucial role in various fundamental biological processes. The key feature that allows control over the amazing selectivity found in natural processes is evidently the effective molarity of the reaction partners that is mediated by the macromolecular templation event. An ongoing challenge within many chemical sciences is to exploit similar templating principles and make use of synthetic systems that are designed for specific chemical conversions. Here, we describe the recent developments that involve (metallo)salen scaffolds that are used for diverse templating events (salen=N,N'-bis(salicylidene)ethylenediamine dianion).     

Ruthenium-catalyzed asymmetric epoxidation of olefins using H2O2, part I: synthesis of new chiral N,N,N-tridentate pybox and pyboxazine ligands and their ruthenium complexes

The synthesis of chiral tridentate N,N,N-pyridine-2,6-bisoxazolines 3 (pybox ligands) and N,N,N-pyridine-2,6-bisoxazines 4 (pyboxazine ligands) is described in detail. These novel ligands constitute a useful toolbox for the application in asymmetric catalysis. Compounds 3 and 4 are conveniently prepared by cyclization of enantiomerically pure alpha- or beta-amino alcohols with dimethyl pyridine-2,6-dicarboximidate. The corresponding ruthenium complexes are efficient asymmetric epoxidation catalysts and have been prepared in good yield and fully characterized by spectroscopic means. Four of these ruthenium complexes have been characterized by X-ray crystallography. For the first time the molecular structure of a pyboxazine complex [2,6-bis-[(4S)-4-phenyl-5,6-dihydro-4H-[1,3]oxazinyl]pyridine](pyridine-2,6-dicarboxylate)ruthenium (S)-2 aa, is presented.     

Polymer-supported salen complexes for heterogeneous asymmetric synthesis: Stability and selectivity

This paper details the enantioselective performance of styrene/divinylbenzene-supported Mn- and Cr-based salen complexes for the epoxidation of olefins and the ring-opening of epoxides to azido-silyl ethers. The Mn catalyst produced the epoxides of 1,2-dihydronaphthalene, styrene, and cis-β-methylstyrene with enantiomeric excesses (ee's) of 46, 9, and 79%, respectively. For the Cr catalyst, the enantioselective ring-opening of epoxyhexane, propylene oxide, and cyclohexene oxide with trimethylsilyl azide proceeded with ee's of 34, 36, and 6%, respectively. Upon recycle of these heterogeneous catalysts, a degradation process was noted for the Mn-catalyst under the conditions for epoxidation that resulted in oxidation and decomposition of the ligand. This process also affects the homogeneous catalyst, thereby limiting the recyclability of both the homogeneous Mn catalyst and its heterogenized version for this reaction. The Cr-catalyzed reaction to ring-open epoxides employs milder conditions and allowed reuse of the heterogeneous catalyst without loss of activity or enantioselectivity through three runs with epoxyhexane. During reaction, the leaching of Cr from the heterogeneous catalyst is less than 0.1%, suggesting possible reuse of the catalyst over hundreds of cycles before reloading the polymer-supported salen ligand with metal would be necessary. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3888–3898, 1999     

非血红素铁、锰配合物催化烯烃不对称环氧化反应研究进展

手性环氧化物是重要的有机反应中间体.金属氧化酶催化的氧化反应通常具有高效、高选择性、反应条件温和和绿色的特点,模拟其中的非血红素铁加氧酶设计合成一系列手性四氮铁锰配合物催化烯烃不对称环氧化反应成为获得高产率、高对映选择性的手性环氧化物的一个重要方法.本文综述了近年来非血红素手性四氮铁锰配合物催化烯烃不对称环氧化反应的研究进展及相应的机理研究.     

Chiral model complexes for methane monooxygenase and their asymmetric catalysis of styrene epoxidation

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Ruthenium-catalyzed asymmetric epoxidation of olefins using H2O2, part II: catalytic activities and mechanism

Asymmetric epoxidation of olefins with 30 % H2O2 in the presence of [Ru(pybox)(pydic)] 1 and [Ru(pyboxazine)(pydic)] 2 has been studied in detail (pybox = pyridine-2,6-bisoxazoline, pyboxazine = pyridine-2,6-bisoxazine, pydic = 2,6-pyridinedicarboxylate). 35 Ruthenium complexes with sterically and electronically different substituents have been tested in environmentally benign epoxidation reactions. Mono-, 1,1-di-, cis- and trans-1,2-di-, tri-, and tetra-substituted aromatic olefins with versatile functional groups can be epoxidized with this type of catalyst in good to excellent yields (up to 100 %) with moderate to good enantioselectivies (up to 84 % ee). Additive and solvent effects as well as the relative rate of reaction with different catalysts have been established. It is shown that the presence of weak organic acids or an electron-withdrawing group on the catalyst increases the reactivity. New insights on the reaction intermediates and reaction pathway of the ruthenium-catalyzed epoxidation are proposed on the basis of density functional theory calculation and experiments.     

trans-[RuCl2(phosphane)2(1,2-diamine)] and Chiral trans-[RuCl2(diphosphane)(1,2-diamine)]: Shelf-Stable Precatalysts for the Rapid,Productive, and Stereoselective Hydrogenation of Ketones

A turnover number (TON) of 2 400 000 and a turnover frequency (TOF) of 63 s⁻¹ are achieved with the chiral Ru^II complex 1 (R=p-CH₃C₆H₄) in the asymmetric hydrogenation of acetophenone. Carbonyl-selective asymmetric hydrogenation of α,β-unsaturated ketones proceeds in the presence of these Ru^II catalysts, and 4-substituted cyclohexanones are selectively converted into cis alcohols.     

Spectroscopy study on the photochromism of Schiff bases N,N'-bis(salicylidene)-1,2-diaminoethane and N,N'-bis(salicylidene)-1,6-hexanediamine

The photochromism of Schiff bases N,N'-bis(salicylidene)-1,2-diaminoethane (BSE) and N,N'-bis(salicylidene)-1,6-cyclohexanediamine (BSH) was studied by steady-state and time-dependent fluorescence, UV Vis absorption spectroscopy and theoretical chemistry calculations. The experimental results show that BSH can perform the photochromism easier than BSE, may be due to the molecular topology difference.     

Catalytic asymmetric epoxidation of α,β-unsaturated carboxylic acid imidazolides and amides by lanthanide–BINOL complexes

Highly enantioselective catalytic asymmetric epoxidation of α,β-unsaturated carboxylic acid imidazolides and simple amides was developed. In the presence of 5–10 mol% of lanthanide–BINOL complexes, the reaction proceeded smoothly with high substrate generality. In particular, in the cases of α,β-unsaturated amides, there was nearly perfect enantioselectivity (>99% ee). The corresponding epoxides were successfully transformed into many types of useful chiral compounds such as α,β-epoxy esters, α,β-epoxy amides, α,β-epoxy aldehydes, α,β-epoxy β-keto ester, and α- and β-hydroxy carbonyl compounds. B3LYP density functional studies were performed to predict substrate reactivity.