Chiral Schiff-base salen ligands

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. 

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. 

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... 

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 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]. 

Other chiral metal catalysts such as iridium, osmium, and rhenium catalysts have also been utilized for asymmetric cyclopropanation. In particular, iridium catalyst gave exceptional enantio- and diastereoselectivities. Chiral Ir(III) salen complexes, which contain a-coordinated aryl ligands, were found to catalyze the cyclopropanation of st3rrene derivatives with TDA. Higher yields (99%), higher diastereoselectivity t/c = 1 99), and enantioselectivity (99% ee) were observed when the reactions were conducted at lower temperature (—78°C) (98). For osmium, an Schiff-based complex of [Os(fBu-salch)(OH)Cl] (H2tBu-salch... 

Reactions of A-tosyliminophenyliodinanes (PhI=NTs) as nitrene source with alkenes in the presence of chiral ligands also present a valuable method to achieve asymmetric aziridination reactions. Cinnamate esters 73 yield enantiomeric aziridinate products in good selectivities on reaction with chiral bisoxazolines 23 and Al-tosyliminophenyliodinanes in the presence of copper salts (Scheme 29) [87]. Biaryl Schiff bases 74 can also be used as ligands in the enantioselective aziridination of ciimamate esters, chromenes and st5renes [88]. Chiral C2-symmetric salen-type ligands 75 were also found to be highly effective for the... 

Enantioselective TMS cyanohydrin formation with aldehydes can be achieved using a variety of chirally-modified titanium catalytic systems chiral modifiers include Schiff bases, including salen ligands and sulphoximines. A chirally-modified yttrium complex has been employed for the same purpose (equation 8) here, the chiral modifier was a ferrocene-derived 1,3-diketone. This general area has been reviewed recently . 

The general term Salen-type is used in literature to describe the class of [O, N, N, O] tetradentate bis-Schiff base ligands. Some of the salen ligands 167,168 and 169 and their metal complexes are illustrated in Fig. 9.9. Commercial synthesis of chiral Salen complex 171, from chiral diamine salt 172 and salicylaldehyde derivative 173 is presented in Scheme 9.45 [83]. 

The base-catalysed hydrophosphonylation of aldehydes or imines (Pudovik reaction) [58] as a convenient method was widely used for the synthesis of 1-hydrox-yalkylphosphonates. Since the pioneering work of Shibuya [50] and Spilling [51] was reported, much attention has been devoted to developing enantioselective catalysts for the synthesis of chiral 1 -hydroxy alkylphosphonates. Chiral aluminium complexes were shown to be more effective chiral catalysts [59-62]. Based on the success of using A1 (salen) and A1 (salcyen) as asymmetric catalysts, Al-Schiff base complexes [63, 64] have been developed to catalyze the asymmetric addition reaction of dial-kylphosphonates and aldehydes. Tridentate Schiff base metal complexes, such as vanadium, chromium, and iron [65], have been successfully applied in many asymmetric synthetic reactions. We noticed that Al(III) complexes could catalyse the asymmetric Pudovik reaction and these ligands could be easily synthesized [66-70]. 

Due to their similarity to porphyrins and their ease of synthesis, Schiff bases have been extensively explored as ligands in oxidation catalysis [38, 44, 45]. Salen-type Hgands are the most commonly used class of Schiff bases, and are synthesized from a diamine and two equivalents of a salicylaldehyde in which chiral derivatives are readily accessed (Fig. 5.5) [46]. Additionally, Schiff base ligands coordinate and stabilize a diverse range of metals in various oxidation states making them an excellent template to explore direct 02-coupled catalytic oxidations. 

Chiral tridentate Schiff base ligands (0,N,0 binding) were recently apphed to the aerobic oxidative kinetic resolution of a-hydroxyesters using V(V) catalysts [54]. Tetradentate (0,N,N,0) salen-type ligands were incompetent in achieving asymmetric induction in these reactions, but use of tridentate Schiff base 22 lead to high values for a variety of a-hydroxy esters (Table 5.3). The catalyst was also chemoselective for alcohol oxidation with no observed epoxidation of olefin substrates (entries 4 and 5). 

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