Chiral Schiff-base salen

Very reeently Kureshy et al. [98] further reported non-salen chiral Schiff base derived Ti complexes as eatalysts 70, 71 (Figure 23) in the KR of meso-siiXheae oxide, cyclohexene oxide, cyelooetene oxide and cA-butene oxide with anilines. The study deliberated upon the role of several ehiral and achiral additives with these catalysts to give chiral y9-amino alcohols with high enantioselectivity ee, >99%) in excellent yield (>99%) at 0 °C in lOh. Unlike the monomerie version 72 the chiral catalyst 70 used in this study was recoverable and recyclable several times with retention of its performance (Table 10)... 

The need in new inexpensive, safe and effective processes for asymmetric sulfide oxidations is determined by pharmaceutical industry requirements [38], Recently, inexpensive and active. systems based on hydrogen peroxide as oxidant and non-toxic chiral iron(III) complexes as catalysts have been reported [39-41 ]. Different mctal-salen complexes have also been previously employed as catalysts for oxidation of sulfides with PhIO Mn "(salen) [42-44], salen) 45], salen) [46], The mechanism proposed in [46] involves intermediate formation of 0x0 iron(lV)-salcn cation radical, that seems doubtful based on the experimental results obtained. In this Chapter we present asymmcinc version of the latter system [(salen )Fc ClJ/PhlO (where salcn stands for the corresponding chiral Schiff base ligands. Scheme 5) and an NMR investigation of the active intermediates. 

Pd complexes of the Schiff base salen N,N -bis-(sahcyHdene)-ethylenediamine) and its chiral derivative salen (R,R-W,N -bis-(3,5-di-terr-butylsaHcyhdene)-l,2-cyclohexanediamine) have been immobilized in the large cages of zeolites FAU and EMT, where the intracrystalline complex formation has been proved by UV-VIS spectroscopy [179]. These host/guest compounds are active catalysts for... 

A more recent alternative approach, developed by Jacobsen and co-workers, concerns the catalytic asymmetric epoxidation of unfunctionalized olefins using cheap NaOCl as oxidant in the presence of Mn complexes of chiral Schiff bases as catalysts, the so-called salene (Fig. 3-4). Values of 97% e.e. have been achieved using cis-disubstituted or trisubstituted alkenes. Equation 3-15 describes the Jacobsen epoxidation of olefins schematically. 

Epoxidation of alkenes with iodosylbenzene can be effectively catalyzed by the analogous salen or chiral Schiff base complexes of manganese(in), ruthenium(II), or ruthenium(III). For example, the oxidation of indene with iodosylbenzene in the presence of (/ ,5)-Mn-salen complexes as catalysts affords the respective (15,2/ )-epoxyindane in good yield with 91-96% ee [704]. Additional examples include epoxidation of alkenes with iodosylbenzene catalyzed by various metalloporphyrins [705-709], corrole metal complexes, ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazoUnyl)pyridine [710,711]. 

Titanium-catalyzed oxidations with 35% aqueous H2O2 using Schiff-base (salen) titanium 0x0 complexes as catalysts showed very high activity [28]. The oxidation of methyl phenyl sulfide required only 0.1mol% of catalyst. The use of chiral salen complexes gave low enantioselectivity (<20% ee). 

Structure 2.49 is a complex with a chiral Schiff base ligand and is probably the first example of an asymmetric homogeneous catalyst used in catalytic cyclopropanation reaction (see Section 7.5). The ligand in structure 2.50 is similar to SALEN but has two chiral centers. It has been used effectively as a chiral catalyst in some epoxidation reactions. 

The power of this methodology lies in the ability to prepare unnatural amino acid derivatives by asymmetric alkylation of prochiral enolates. Several asymmetric alkylations of the alanine derivative 7, catalysed by the C2-symmetrical quaternary ammonium salt 6d, have been reported these reactions yield unnatural amino acids such as 8 in high enantiomeric excess (Scheme 2) [7]. The chiral salen complex 9 has also been shown to be an effective catalyst for the preparation of a,a-dialkyl a-amino acids [8, 9]. For example, benzylation of the Schiff base 10 gave the a-methyl phenylalanine derivative 11 in 92% ee (Scheme 3) [8]. Similar reactions have been catalysed by the TADDOL 12, and also give a,a-dialkyl a-amino acids in good enantiomeric excess [10]. 

Sigman and Jacobsen reported the first example of a metal-catalyzed enantioselective Strecker-type reaction using a chiral Alnl-salen complex (salen = N,N -bis(salicyhdene)-ethylenediamine dianion) [4]. A variety of N-allylimines 4 were evaluated in the reaction catalyzed by complex 5 to give products 6, which were isolated as trifluoroacetamides in good yields and moderate-to-excellent enantioselectivities (Scheme 3). Substituted arylimines 4 were the best substrates, while alkyl-substituted imines afforded products with considerably lower ee values. Jacobsen and co-workers also reported that non-metal Schiff base catalysts 8 and 9 proved to be effective in the Strecker reaction of imines 7 with hydrogen cyanide to afford trifluoroacetamides 10 after reaction with trifluoroacetic anhydride, since the free amines were not stable to chromatography (Scheme 4) [5]. 

Metallo-(salen)-catalyzed oxidation. In 1986, Pasini and co-workers62 developed chiral oxotitanium(IV)-Schiff base complex as catalyst for the oxidation of methyl phenyl sulfide. While the catalytic activity of the Pasini system was excellent (catalyst substrate ratio, 1 1000 to 1 1500), the enantioselection was unfortunately low (< 20% ee) for catalyst 31 (Fig. 4). 

Porphyrin complexes such as (porph)MnCl or (porph)Mn=0 have been much studied for the oxidation of alkanes, alkenes, and other organic compounds using O-transfer agents such as PhIO, H202, or 02. Schiff base complexes have also been used since chiral complexes (e.g., of the salen type) can be readily made on a ton scale and the enantiomers separated in pure forms. Thus the epoxidation... 

One of the classical tetradentate Schiff bases is the open-chain N2O2 donor ligand salen (137), which has flexibihty sufficient to permit it to form different geometric isomers on complexation. The ability of the ubiquitous salen and close analogues to bind O2 reversibly as its cobalt complex, as in [(02)Co(salen)(py)], has been central to most investigations of its coordination chemistry. Complexes of the [Co(salen)] family also mediate the electrocatalytic reduction of O2 to H2O2, whereas those incorporating chiral centers on... 

Bi, W.-Y.,Lii, X.-Q., Chai, W.-L., etal. (2008) Construction and NIR luminescent property of hetero-bimetallic Zn—Nd complexes from two chiral salen-type Schiff-base Ugands. Journal of Molecular Structure, 891, 450. 

A tridentate Schiff base Cr(III) complex derived from l-amino-2-indanol catalyzes the enantioselective ring opening of meso A-2,4-dinitrobenzyl aziridines with TMSN3 (Sch. 18) [96]. The chiral (salen)metal complexes, used in the enantioselective ring opening of epoxides, were found to be much less effective (for Cr) or inactive (for Co). 

Narasaka has reported that TADDOL-Ti dichloride catalyzes the asymmetric addition of trimethylsilylcyanide to aromatic and aliphatic aldehydes (Sch. 63) [148]. The reactions proceed only in the presence of MS 4A. In reactions with aliphatic aldehydes a chiral cyanotitanium species obtained by mixing of the TADDOL-Ti dichloride and trimethylsilylcyanide before addition of the aldehydes acts as a better chiral cyanating agent and affords higher enantiomeric excesses. Chiral titanium complexes obtained from an alcohol ligand and salicylaldehyde-type Schiff bases and a salen ligand have been reported to catalyze the asymmetric addition of hydrogen cyanide or... 

The assymetric Strecker reaction of diverse imines, including aldimines as well as ketoimines, with HCN or TMSCN provides a direct access to various unnatural and natural amino acids in high enantiomeric excesses, using soluble or resin-linked non-metal Schiff bases the corresponding chiral catalysts are obtained and optimized by parallel combinatorial library synthesis [93]. A rather general asymmetric Strecker-type synthesis of various imines and a, 9-unsaturated derivatives is catalyzed by chiral bifunctional Lewis acid-Lewis base aluminum-containing complexes [94]. When chiral (salen)Al(III) complexes are employed for the hydrocyanation of aromatic substituted imines, excellent yields and enatio-selectivities are obtained [94]. 

Previously, Pasini [27] and Colonna [28] had described the use chiral titani-um-Schiff base complexes in asymmetric sulfide oxidations, but only low selec-tivities were observed. Fujita then employed a related chiral salen-titanium complex and was more successful. Starting from titanium tetrachloride, reaction with the optically active C2-symmetrical salen 15 led to a (salen)titani-um(IV) dichloride complex which underwent partial hydrolysis to generate the t]-0x0-bridged bis[(salen)titanium(IV)] catalyst 16 whose structure was confirmed by X-ray analysis. Oxidation of phenyl methyl sulfide with trityl hydroperoxide in the presence of 4 mol % of 16 gave the corresponding sulfoxide with 53% ee [29]. 

A difficult challenge in developing ARO reactions with carbon nucleophiles is identifying a reagent that is sufficiently reactive to open epoxides but at the same time innocuous to chiral metal catalysts. A recent contribution by Crotti clearly illustrates this dehcate reactivity balance. The lithium enolate of acetophenone added in the presence of 20 mol % of the chiral Cr(salen) complex 1 to cyclohexene oxide in very low yield but in 84% ee (Scheme 10) [23]. That less than one turnover of the catalyst was observed strongly suggests that the lithium enolate and the Schiff base catalyst are not compatible under the reaction conditions. 

Alkylation of alanine methyl ester Schiff base by chiral salen-metal catalysts,... 

ALKYLATION OF ALANINE METHYL ESTER SCHIFF BASE BY CHIRAL SALEN-METAL CATALYSTS, a-BENZYL-ALANINE METHYL ESTER... 

The ship-in-a-bottle technique is perhaps the most common method for encapsulation of transition metal complexes. In this way the tetradentate Schiff base ligand SALEN (bis-salicylidene) ethylenediamine can diffuse through the 12 MR windows of faujasite. Then, when complexed with a previously exchanged metal ion, nearly square planar coordination geometry is formed inside the a-cages [97-100], Mn complexes with a chiral ligand, prepared by the ship-in-a-bottle technique inside Y and EMT zeolites, have enantioselectively carried at the epoxidation of olefins [101,102]. 

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