Insight into the mechanism of oxidative kinetic resolution of racemic secondary alcohols by using manganese(III)(salen) complexes as catalysts

The oxidative kinetic resolution of various racemic secondary alcohols with PhI(OAc)(2) catalyzed by chiral [Mn(III)(salen)] complexes in the presence of KBr was studied in a water/organic solvent mixture. The dramatic, synergetic effect of additives, organic solvent, and the substituents of chiral salen ligands on the enantioselectivities of the reactions is reported. Results from UV/Vis spectroscopy and ESI-MS studies provide evidence that these reactions are induced by the formation of a high-valent manganese intermediate.     

Enantioselective epoxidation of isoflavones by Jacobsen's Mn(III)salen catalysts and dimethyldioxirane oxygen-atom source

The catalytic enantioselective epoxidation of the isoflavones 1a–f has been performed by the Mn(III)salen complexes (R,R)-3 and (S,S)-3 as catalysts and dimethyldioxirane as the oxygen-atom source to afford optically active isoflavone epoxides 2a–f. The absolute configuration of the nonracemic epoxides 2 have been determined by X-ray diffraction analysis. Our present results constitute the first examples of the preparation of optically active isoflavone epoxides.     

Enantioselective iminium-catalyzed epoxidation of hindered trisubstituted allylic alcohols

The reactivity of diastereomeric biaryl iminium cations made of a (Ra)-5,5′,6,6′,7,7′,8,8′-octahydrobinaphthyl core and exocyclic appendages derived from (S)- or (R)-3,3-dimethylbutyl-2-amine was investigated with hindered trisubstituted allylic alcohols—a class of alkenes which had not been previously studied in detail in epoxidation reactions with cyclic iminium catalysts (ee up to 98%). Surprisingly, generally strong matched/mismatched effects are observed not only in terms of reactivity but also on the enantioselectivity of the reaction (Δee up to 16%). Also, for the most hindered substrates, two sets of reaction conditions were tested in a preliminary study and little advantage was found in running reactions in MeCN/water instead of CH₂Cl₂/water/18-C-6. In any case, the presence of the hydroxyl group did not reveal any anchimeric effect.     

Studies of the catalytic activity of Fe(III)(salen) complexes as epoxidation catalysts

Two new Fe(III)(salen) complexes, FeL¹ClO₄·2H₂O (1) and FeL²ClO₄ (2) [L¹ = N,N′-ethylenebis(3-formyl-5-methylsalicylaldimine) and L² = N,N′-cyclohexenebis(3-formyl-5-methylsalicylaldimine)], have been synthesized and characterized. The catalytic activity of the complexes for epoxidation of alkenes has been investigated in the presence of two terminal oxidants PhIO and NaOCl, with two solvents CH₃CN and CH₂Cl₂. As alkenes styrene and (E)-stilbene have been chosen for investigation; styrene is a better substrate than electron-rich (E)-stilbene. The study also suggests that unlike their Mn(III) counterparts, 1 and 2 are poor epoxidation catalysts; catalysis proceeds with formation of one intermediate, rather than forming more than one intermediate depending on the terminal oxidant used. Use of exogenous neutral donor ligands such as Py, PyNO and 1-MePy is effective to improve catalytic behavior.     

A chiral Mn(III) salen complex immobilized onto ionic liquid modified mesoporous silica for oxidative kinetic resolution of secondary alcohols

A supported ionic liquid strategy has been applied for the immobilization of a chiral Mn(III) salen complex onto ionic liquid modified mesoporous silica SBA-15. The immobilized catalyst demonstrated high enantioselectivity and activity in the oxidative kinetic resolution of secondary alcohols, and could be recycled five times without obvious loss of activity.     

Dynamic kinetic resolution of allylic alcohols mediated by ruthenium- and lipase-based catalysts

An enzyme-metal combo reaction has been developed for the dynamic kinetic resolution of allylic alcohols in which racemic substrates are transformed by a lipase and a ruthenium complex in the presence of an acyl donor to allylic acetates of high optical purity in over 80% yield.     

Enantioselective aminolytic kinetic resolution (AKR) of epoxides catalyzed by recyclable polymeric Cr(III) salen complexes

Polymeric chiral Cr(III) salen complexes catalyzed regio-, diastereo-, and enantioselective aminolytic kinetic resolution (AKR) of trans-stilbene oxide, trans-β-methyl styrene oxide, and 6-CN-chromene oxide proceeded smoothly at room temperature, providing the desired anti-β-amino alcohols in high yields and enantiomeric excess (up to 100%).     

New chiral (salen)manganese(III) systems for asymmetric epoxidation of styrene

Highly enantioselelctive and repeatable epoxidation of styrene was performed by using new chiral (salen)Mn(III) catalysts, which were derived from the initial immobilization of a homogeneous (salen)Mn(III) complex on solid carriers and subsequent dispersion into ionic liquids. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Chiral Co(III)(salen)-catalysed hydrolytic kinetic resolution of racemic epoxides in ionic liquids

In the chiral Co(III)(salen)-catalysed HKR of racemic epoxides, in the presence of ionic liquids, Co(II)(salen) complex is oxidised without acetic acid to catalytically active Co(III)(salen) complex during reaction and, moreover, this oxidation state is stabilised against reduction to Co(II) complex which enables the reuse of the recovered catalyst for consecutive reactions without extra reoxidation.     

Self-assembly approach toward chiral bimetallic catalysts: bis-urea-functionalized (salen)cobalt complexes for the hydrolytic kinetic resolution of epoxides

A series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-urea hydrogen-bonding interactions. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times) in the hydrolytic kinetic resolution (HKR) of rac-epichlorohydrin in THF by facilitating cooperative activation, compared to the monomeric catalyst. In addition, one of the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions by requiring much shorter reaction time at low catalyst loading (0.03-0.05 mol %). A series of kinetic/mechanistic studies demonstrated that the self-association of two (salen)Co units through urea-urea hydrogen bonds was responsible for the observed rate acceleration. The self-assembly study with the bis-urea (salen)Co by FTIR spectroscopy and with the corresponding (salen)Ni complex by (1)H NMR spectroscopy showed that intermolecular hydrogen-bonding interactions exist between the bis-urea scaffolds in THF. This result demonstrates that self-assembly approach by using non-covalent interactions can be an alternative and useful strategy toward the efficient HKR catalysis.     

A chiral electrophilic selenium reagent to promote the kinetic resolution of racemic allylic alcohols

[reaction: see text] The first example of a kinetic resolution process promoted by electrophilic selenium reagents is reported. Racemic allylic alcohols react with half equivalents of a selenenylating agent in methanol leading to the regiospecific formation of the corresponding addition products with a very high level of facial selectivity (from 95:5 to 98:2 dr). The unreacted alcohols can be recovered in an optically enriched form (from 90 to 94% ee).     

One-pot synthesis and resolution of chiral allylic alcohols

Substituted α,β-unsaturated ketones were selectively reduced to the corresponding allylic alcohols under mild reaction conditions. The allylic alcohols thus obtained were kinetically resolved by lipase catalyzed transesterification in the same pot to afford chiral allylic alcohols in excellent enantioselectivity. Various lipases were screened for this one-pot transesterification of allylic alcohols. Effects of different solvent have also been studied under these conditions. Pseudomonas cepacia lipase immobilized on ceramic particles (PS-C) and on diatomaceous earth (PS-D) catalyzes this transesterification in diisopropyl ether in a highly efficient manner.     

Heterogeneous chiral Mn(III) salen catalysts for the epoxidation of unfunctionalized olefins immobilized on mesoporous materials with different pore sizes

Two chiral Mn(III) salen complexes were immobilized onto a series of mesoporous MCM-41 and MCM-48 materials with different pore sizes and the as-synthesized catalysts were active and enantioselective for the asymmetric epoxidation of styrene and indene. The results of XRD, FTIR, DR UV-vis, and N₂ sorption showed that the chiral Mn(III) salen complexes were anchored in the channels of mesoporous materials. The influence of organic silicane dosage on the catalytic performance was studied and the optimum dosage of organic silicane for preparing heterogeneous catalysts was determined. Furthermore, the effect of the fine-tuning of pore size on the performance of heterogeneous catalysts was discussed. In general, larger pore size of the supports could lead to higher conversions and the compatible pore size with substrate may be responsible for the improved enantiomeric excess (ee) values.     

Electronic effects in (salen)Mn-based epoxidation catalysts

Presented are density functional calculations on various Mn(salen) systems that are active catalysts in the epoxidation of olefins. Correlation of various structural properties such as Mn=O bond strengths, atomic charges, and C-O distances of evolving bonds in transition state geometries with modified Hammett constants reveal a mechanistic picture of the epoxidation reaction, supporting previous experimental results. Enantioselectivity is tied to the position of a transition state along the reaction coordinate for the first C-O bond formation step, when an olefin is approaching the epoxidation catalyst. Electronic effects exhibited by the 5,5' substituents of the salen ligand manifest themselves in a tuning of the Mn=O bond strength, which in turn influences the C-O distance of the forming bond in the transition state geometry.     

Lipase-catalyzed kinetic resolution of aryltrimethylsilyl chiral alcohols

Lipase-catalyzed kinetic resolution of aryltrimethylsilyl chiral alcohols through a transesterification reaction was studied. The optimal conditions found for the kinetic resolution of m- and p-aryltrimethylsilyl chiral alcohols, led to excellent results, high conversions (c = 50%), high enantiomeric ratios (E > 200) and enantiomeric excesses for the remaining (S)-alcohol and (R)-acetylated product (>99%). However, kinetic resolution of o-aryltrimethylsilyl chiral alcohols did not occur under the same conditions applied to the other isomers.     

Aerobic oxidative kinetic resolution of racemic alcohols with bidentate ligand-binding Ru(salen) complex as catalyst

Chiral Ru(salen)(nitrosyl) complex 1 is a useful catalyst for asymmetric aerobic oxidation of alcohols under photo-irradiation. In this study, it was found that addition of β-hydroxy ketone or 1,3-diketone had a significant influence on its asymmetric catalysis. For example, the addition of 1,3-bis(p-bromophenyl)propane-1,3-dione 9 improved the relative reaction ratio in kinetic resolution of simple racemic secondary alcohols up to 30, while the addition gave an adverse effect on desymmetrization of an acyclic meso-1,3-diol. This additive effect was considered to be attributable to the chelate formation of the β-hydroxy ketone or 1,3-diketone with a Ru(salen) complex.     

不具C2对称轴的Co（Ⅲ）（Salen)催化的末端环氧化合物的水解拆分

用（S）-3-甲基-1，2-丁二胺或（R）-1，2-丙二胺为原料与2-羟基-5-甲-3- 叔丁基苯甲醛缩合，得到不具C2对称轴的手性Salen, 与Co(Ⅱ）络各并氧化后得到 4a或4b.4a和4b成功地应用于外消旋末端环氧化合物的水解拆分，取得了较好的产 率，但产物的对映体过量值较低。     

Synthesis and activity of macrocyclized chiral Mn(III)-Schiff-base epoxidation catalysts

A series of chiral macrocyclic Mn(III)Salen complexes has been prepared with two salicylidene moieties linked in their 3 and 3′ positions by aliphatic polyether bridges of variable lengths or by a more rigid aromatic junction arm. X-ray structures of ligand precursors and of complex 8 have been performed. All complexes have been used in the asymmetric epoxidation of 1,2-dihydronaphthalene with NaOCl as oxygen atom donor and exhibited modest enantiomeric excesses. Complex 10 was selected to be tested with two cis-disubstituted olefins and several oxidants, namely NaOCl, PhIO and n-Bu₄NHSO₅. 2,2′-Dimethylchromene oxide was obtained from 2,2′-dimethylchromene with ee values of 56% and 74% when using 10 and NaOCl and PhIO, respectively.