Salen Catalyst Research Articles

The first asymmetric catalytic halo aldol reaction of β-iodo allenoates with aldehydes by using chiral salen catalyst

The first asymmetric catalytic halo aldol reaction of β-iodo allenoates with aldehydes was established. The reaction was successfully achieved by using ( R, R)-SalenAlCl as the chiral catalyst and LiI as an additive at 0 °C in dichloromethane. Moderate to good yields and up to 62% ee were obtained. The new system showed a good substrate scope in which both aromatic aldehydes and aliphatic aldehydes can be employed. The reaction provided the first catalytic and enantioselective approach to chiral β-iodo Baylis–Hillman ester adducts.

Aldehyde olefination with a ruthenium(II) salen catalyst

A ruthenium salen complex of the type Ru(II)(salen)(PPh 3) 2 can be successfully applied as catalyst for the olefination of a broad variety of aldehydes. Dependent on the electron richness of the applied aldehydes, good to very good olefin yields are obtained and high E/ Z-selectivities are reached at 80 °C reaction temperature with ethyl diazo acetate being the reaction partner. The reaction rate depends on the electron-donor capabilities of the aldehydes. Electron-poor aldehydes undergo faster reactions than electron-rich aldehydes, but both electron-rich and bulky aldehydes can be transformed to olefins in very good yields as well. Very good E/ Z-selectivities and quantitative olefin yields are obtained for aromatic aldehydes when the reaction is performed in the room-temperature ionic liquid (Bmim)PF 6 and toluene.

Chiral Vanadyl Salen Complex Anchored on Supports as Recoverable Catalysts for the Enantioselective Cyanosilylation of Aldehydes. Comparison Among Silica, Single Wall Carbon Nanotube, Activated Carbon and Imidazolium Ion as Support.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Impregnation of dimeric Cr III(salen) on silica and its application in epoxide asymmetric ring opening reactions

The heterogeneous asymmetric ring opening (ARO) of epoxides with trimethylsilylazide (TMSN 3) catalysed by a dimeric Cr(salen) catalyst impregnated on a silica material combines high enantioselectivities for epoxide and ring opened product with good conversions and suffers only from low leaching in both batch and continuous flow experiments.

Mechanistic studies on the asymmetric alkylation of amino ester enolates using a copper(ii)salen catalyst

Hammett data indicate that the asymmetric alkylation of enolates catalysed by copper(II)salen complex 1, proceeds by an asynchronous S(N)2 reaction and that the role of the catalyst is to enhance the nucleophilicity of the enolate.

A dynamic supramolecular system exhibiting substrate selectivity in the catalytic epoxidation of olefins

A dynamic supramolecular system involving hydrogen bonding between a Mn(III) salen catalyst and a Zn(II) porphyrin receptor exhibits selectivity for pyridine appended cis-beta-substituted styrene derivatives over phenyl appended derivatives in a catalytic epoxidation reaction.

Asymmetric double ring-opening of a C2h-symmetric bis-epoxide: improved enantiomeric excess of the product through enantioselective desymmetrisation and ‘proof-reading’ steps

A new strategy in asymmetric synthesis is described in which the desymmetrisation of a C(2h)-symmetric molecule is followed by a subsequent enantioselective 'proof-reading' step. The double asymmetric ring-opening of the bis-epoxide (1R*,3R*,5S*,7S*)-4,8-dioxa-tricyclo[5.1.0.0(3,5)]octane with azidotrimethylsilane, catalysed by a chiral chromium Salen catalyst, was studied. The reaction involves the initial asymmetric ring-opening of the bis-epoxide to give the intermediate in moderate enantiomeric excess (ca. 50% ee); the second ring-opening step yields the required diazido diol, (1S,3S,4S,6S)-4,6-diazidocyclohexane-1,3-diol, in 72% yield and 70% ee. The origin of proof reading stems from the diversion of the minor enantiomer of the intermediate to a centrosymmetric by-product, a process which improves the enantiomeric excess of the required product. Using alternative conditions, the reaction was optimised to yield the required product in >98% ee.

Catalytic, Asymmetric Synthesis of α‐Acetoxy Amides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

CO 2 fixation using recoverable chromium salen catalysts: use of ionic liquids as cosolvent or high-surface-area silicates as supports

Two series of recoverable and reusable chromium salen catalysts for the CO 2 addition to styrene epoxide to form cyclic carbonate have been tested under supercritical conditions (100 bar and 80 °C). In one of the cases, tetra- tert-butylsalen chromium (substrate-to-catalyst ratio 875) was dissolved in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6), and after the products were extracted with diethyl ether the ionic liquid containing the complex was reused up to four consecutive runs. However, only modest conversions ( > 50 % ) and product selectivities ( > 79 % ) were achieved under these conditions, although decreasing the substrate-to-catalyst ratio showed some enhancement in the epoxide conversion. In the other system, a chromium salen catalyst was adsorbed on aminopropylsilyl-modified silica and ITQ-2. The solids were tested as heterogeneous catalysts but complete desorption of the complexes was observed after the first use. The best recoverable catalyst of the series was obtained when chromium salen was covalently anchored to silica support through a linker resulting from nucleophilic substitution between aminopropylsilyl-modified silica and chloromethyl-substituted complex. In this case, the solid was still active after the fourth reuse and exhibited 100% carbonate selectivity at about 70% epoxide conversion.

Chiral vanadyl salen complex anchored on supports as recoverable catalysts for the enantioselective cyanosilylation of aldehydes. Comparison among silica, single wall carbon nanotube, activated carbon and imidazolium ion as support

The activity and enantiomeric excess (ee) (in some cases >85%) obtained for the asymmetric addition of trimethylsilyl cyanide to aldehydes using different heterogeneous chiral catalysts are compared. A library of recoverable catalysts was developed by immobilization of a chiral vanadyl salen complex having a terminal carbon–carbon double bond onto a series of scaffolds including silica, single-wall carbon nanotubes, activated carbon and room-temperature ionic liquids. The covalent linkage has been achieved by radical initiated addition of mercapto groups to C C. The highest enantiomeric excesses, similar to those obtained in the homogeneous phase, were achieved using silica as support or with the homogeneous tetra- tert-butyl salen catalyst dissolved in an imidazolium ionic liquid. The use of silica as support permits an easier separation and reuse of the catalyst from the reaction media.

Catalytic, asymmetric synthesis of α-acetoxy amides

Non-racemic cyanohydrin acetates are readily available from aldehydes and potassium cyanide/acetic anhydride by use of bimetallic titanium (salen) catalyst 1 . Treatment of the cyanohydrin acetates with a platinum(II) phosphinito catalyst 3 under neutral conditions results in chemoselective hydrolysis to the corresponding α-acetoxy amides in a racemization free process.

Coordination of CrIII(Salen) on Functionalised Silica for Asymmetric Ring Opening Reactions of Epoxides

The heterogeneous asymmetric ring opening (ARO) ofmeso-epoxides with trimethylsilylazide (TMSN3) catalysed by a Cr(salen) catalyst coordinated on an aminopropyl functionalised silica material combines good enantioselectivities with high conversions and suffers low leaching (<1%).

New chiral cobalt salen complexes containing Lewis acid BF3; a highly reactive and enantioselective catalyst for the hydrolytic kinetic resolution of epoxides

A new type of chiral cobalt salen complexes bearing BF3 Lewis acid proved to be reactive and enantioselective in the hydrolytic resolution of terminal epoxides. The polymer type salen catalysts also showed a high enantioselectivity in the same reaction.

Synthesis of Racemic and Enantiomerically Enriched a-Oxyfunctionalized Benzocyclanones and Chromanones by Dimethyldioxirane and Dimethyldioxirane/Mn(III) salen System

Enolacetates of benzocyclanones and chromanones were synthesized and treated with dimethyldioxirane and the asymmetric oxidizing system dimethyldioxirane/chiral, non-racemic Mn(III) salen complex/axial ligand. The latter reagent resulted in the corresponding enantiomerically enriched cyclic α-hydroxy ketones and their acetates in moderate-to-good yields and modest enantioselectivity under mild and neutral conditions from tetralone and chromanone. On the contrary, flavanone provided poor yields due to the competitive C–H insertion at position 2. The use of R,R-Mn(III)salen catalyst induced an S absolute configuration at the position α in the whole series.

Zr[bis(salicylidene)ethylenediaminato]-mediated Baeyer-Villiger oxidation: stereospecific synthesis of abnormal and normal lactones.

Baeyer-Villiger oxidation of racemic bicyclic cyclobutanones with Zr[bis(salicylidene)ethylenediaminato] (salen) complex 1 as catalyst in the presence of a urea-hydrogen peroxide adduct was found to proceed enantiospecifically. The enantiotopos selection in the oxidation was governed primarily by the Zr(salen) catalyst, although migratory aptitude (methine > methylene > methyl) in Baeyer-Villiger oxidation affected the selection to a varied extent, depending on the substrate structures; one enantiomer of cyclobutanones gave exclusively a normal lactone expected from the migratory aptitude, and the other enantiomer gave an abnormal lactone preferentially, the formation of which is counter to the migratory aptitude. Furthermore, the rates of abnormal lactone formation were found to be faster than those of normal lactone formation in most of the oxidations examined. For example, the enantiomer of racemic bicyclo[3.2.0]heptan-6-one giving an abnormal lactone reacted 2.2 times faster than the other enantiomer giving a normal lactone. To our knowledge, this example of chemocatalytic Baeyer-Villiger oxidation giving an abnormal lactone in preference to a normal lactone has been previously unreported. This unusual behavior is likely to be attributable to strict control of stereoelectronic demand in Baeyer-Villiger oxidation and chiral recognition by complex 1.

Epoxidation of unfunctionalized olefins by Mn(salen) catalyst using organic peracids as oxygen source: a theoretical study.

The mechanism and origin of asymmetric induction in the Mn(III)(salen)-catalyzed epoxidation by peracetic acid have been elucidated by the density functional [Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (B3LYP)] method in two different regimes: with and without an axial ligand. The acylperoxo complexes of Mn(II,III,IV) in cisON cisNO and trans geometrical configurations cannot compete with the catalyst-free Prilezhaev epoxidation. Instead, oxo species perform epoxidation following the O-O bond cleavage in the acylperoxo complexes. The epoxidation may proceed in a concerted and/or radical-mediated stepwise manner. The actual mechanism of the epoxidation depends on the electronic and oxidation state of the oxo species and the nature of the axial ligand. The olefin can approach the reactive MnO fragment of both cis and trans-l-isomers of the plain oxo species along multiple distinct directions: native approaches. The native approaches are used to rationalize the inversion of the absolute configuration of the product epoxide due to the axial ligand.

Asymmetric synthesis of ( R)- and ( S)-2-trifluoromethylepinephrine

An asymmetric synthesis of ( R)- and ( S)-2-trifluoromethylepinephrine ( 1R and 1S ) is presented. Trifluoromethylation involves nucleophilic aromatic substitution of halobenzene 4 most likely via a copper mediated CF 3 anion equivalent generated in situ. The asymmetric step involves conversion of 3,4-dimethoxy-2-trifluoromethylbenzaldehyde ( 5 ) to silyl cyanohydrin ( 6R and 6S ) using a chiral salen catalyst in the presence of titanium. 1R and 1S are potential alternatives to currently used vasoconstrictors in local anesthetic formulations.

Enantioselectivities of Chiral Ti(IV) Salen Complexes Immobilized on MCM-41 in Asymmetric Trimethylsilylcyanation of Benzaldehyde

A new heterogeneous system for catalytic trimethylsilylcyanation of benzaldehyde has been developed by immobilizing Ti(IV) salen onto ordered mesoporous silica (MCM-41). The immobilization was performed according to different methods: (i) direct condensation of silanol on the silica surface with Ti(IV) salen and (ii) multigrafting of salicylaldehyde derivatives and diaminocyclohexane using 3-mercaptopropyl-functionalized MCM-41 as a starting material. The heterogenized salen catalysts showed a high enantioselectivity for the addition of trimethylsilyl cyanide to benzaldehyde.

Chromotropism of Cr(salen) moiety in zeolite matrix: synthesis, characterization and catalytic activity study of Cr(salen)-NaY hybrid catalyst

Abstract The [Cr(salen)] + [salenH 2 = N , N ′-bis(salicylaldehyde)-ethylenediimine] complex moiety is immobilized in NaY matrix to obtain a new hybrid catalytic system. The resulting yellow colored zeolite immobilized Cr(salen) catalyst on washing with CH 3 OH and H 2 O turns to light grey. The spectroscopic measurements showed that the interaction between axially coordinated H 2 O and Cr(III) in [Cr(salen)(H 2 O) 2 ] + moiety becomes weaker on the entrapment of the complex moiety in Y zeolite supercage. A good catalytic performance of the newly prepared hybrid catalyst was observed in epoxidation reaction of norbornene and styrene.

Development of Bifunctional Salen Catalysts: Rapid, Chemoselective Alkylations of α‐Ketoesters.

AbstractFor Abstract see ChemInform Abstract in Full Text.