Chiral copper Schiff base complexes

The catalytic asymmetric Henry reaction has been reviewed.42 Mild and efficient enantioselective nitroaldol reactions of nitromethane with various aldehydes have been catalysed by chiral copper Schiff-base complexes yielding the corresponding adducts with high yields and good enantiometric excess.43,44... 

In 1966, Nozaki et al. reported that the decomposition of o-diazo-esters by a copper chiral Schiff base complex in the presence of olefins gave optically active cyclopropanes (Scheme 58).220 221 Following this seminal discovery, Aratani et al. commenced an extensive study of the chiral salicylaldimine ligand and developed highly enantioselective and industrially useful cyclopropanation.222-224 Since then, various complexes have been prepared and applied to asymmetric cyclo-propanation. In this section, however, only selected examples of cyclopropanations using diazo compounds are discussed. For a more detailed discussion of asymmetric cyclopropanation and related reactions, see reviews and books.17-21,225... 

Chiral copper (I) semicorrin complex Chiral copper(II) Schiff base complex... 

The potentials of this and other similar chiral Schiff base complexes of copper were originally explored for the manufacture of synthetic pyre-throids, a class of highly active pesticides. Cyclopropane rings with asymmetric centers are an integral part of the molecular structures of synthetic pyrethroids. Synthetic pyrethroids were made because the natural product chrysanthemic acid, 7.69, and its derivatives have insecticidal properties. 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

Asymmetric ring-opening of saturated epoxides by organocuprates has been studied, but only low enantioselectivities (< 15% ee) have so far been obtained [49, 50]. Muller et al., for example, have reported that the reaction between cyclohexene oxide and MeMgBr, catalyzed by 10% of a chiral Schiff base copper complex, gave trans-2-methylcyclohexanol in 50% yield and with 10% ee [50]. 

A group at the Academy of Sciences in Moscow 197) has synthesized chiral threonine. Derivatives of cyclic imino acids form copper complexes with glacine and carbonyl compounds. Hydroxyethylation with acetaldehyde and decomposition of the resulting complexes produced threonine with an optical purity of up to 97-100% and with threo/allo ratios of up to 19 1 197). The chiral reagents could be recovered and re-used without loss of stereoselectivity. The mechanism of this asymmetric synthesis of amino acids via glacine Schiff base/metal complexes was also discussed 197). 

Chiral iminopyridines catalyse nitro aldol reactions with good ees in the pres- ence of copper(II) acetate, without the need for exclusion of air or moisture.143 A (g) phenylalanine-derived Schiff base - also complexed to copper(II) - is also effective, with the advantage that product configuration is easily reversed (by using the enantiomeric phenylalanine).144... 

Formyl-5-hydroxy[2.2]paracyclophane (153) was used as a chiral auxiliary in the synthesis of a-amino acids [98]. The reported enantiomeric excess was in the range of 90-98%. Racemic 153 was first prepared by Hopf and Barrett [99]. To separate the enantiomers, their Schiff bases with the dipeptide (S)valyl-(S)valine was prepared. The diastereomeric copper(II) complexes of this compound show different solubility in 2-propanol. Alternatively they can be separ-... 

Enantioselective copper-catalyzed desymmetrization of meso cyclic allylic bisdiethyl-phosphates has alsobeen conducted with dialkylzinc reagents. Piarulli and Gennari initially showed that the copper(l) complex of a chiral Schiff base catalyzed tlie enantioselective desymmetrization of 4-cyclopentene-l,3-bis(diethylphosphate) with diethyzinc (Equation 20.75). Piarulli, Gennari, and Feringa then improved upon the enantioselectivities... 

Copper complexes are also able to catal) e the insertion of a carbene into a silicon-hydrogen bond. Associated with C2-symmetric Schiff bases, CuOTf allows the formation of chiral... 

The ligand L48 belongs to the class of reduced Schiff base ligands that contains ferrocenealdehyde, instead of salicylaldehyde, and amino acid side arm. Several copper complexes of L48 are explored for their potential host-guest properties associated with the chiral cavity. 

Particularly the diazoacetic method has a certain appeal for this problem. The decomposition of the sterically hindered diazoacetic ester 74 is effective in the presence of 2,4-dimethyl-hexadien-2,5 75 involving transient unsymmetrical carbon-complexes with chiral copper complexes 76 of highly substituted schiff bases of saUcyhc aldehyde [116] affording the 1-R-trans ester 77 in high optical and chemical yield (Reaction scheme 45) [114]. 

The first asymmetric copper-catalyzed cyclopropanation using a homogeneous catalyst was reported by Noyori in 1966 [31], This reaction, which allowed the cycloaddition on styrene, was carried out with a chiral Schiff base copper complex and produced poor enantiomeric excess (Fig. 11). Further refinement of the chiral ligands produced later much better catalysts, such as the bis-oxazoline (Box) derivatives, able to provide enantioselectivities up to 99%. 

Schiff-base-copper complex 244 catalyzed intramolecular cyclopropanation to give cyclopropane-fused lactone 245 (Scheme 1.114) [171]. Chiral Schiff base 246 was employed for asymmetric cyclopropanation of dienes (Scheme 1.115)... 

On an alternative pathway, internal Cannizzaro reactions afford mandelic acid-type compounds from phenylglyoxal derivatives [152-156]. Copper complexes [153, 154], chromium perchlorate [154], cobalt Schiff s bases [155] and yttrium chloride [156] have been applied as catalysts. An asymmetric version [Eq. (7)] has been developed using phenylglyoxal (14) as substrate and a combination of Cu(OTf)2 and (S,S)-Ph-box 16 as the chiral catalyst [154]. After 24 h at room temperature isopropyl man-delate (15) was obtained with an enantioselectivity of 28% ee. 

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