Chiral salicylaldimine ligands

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... 

Enantioselection can be controlled much more effectively with the appropriate chiral copper, rhodium, and cobalt catalyst.The first major breakthrough in this area was achieved by copper complexes with chiral salicylaldimine ligands that were obtained from salicylaldehyde and amino alcohols derived from a-amino acids (Aratani catalysts ). With bulky diazo esters, both the diastereoselectivity (transicis ratio) and the enantioselectivity can be increased. These facts have been used, inter alia, for the diastereo- and enantioselective synthesis of chrysan-themic and permethrinic acids which are components of pyrethroid insecticides (Table 10). 0-Trimethylsilyl enols can also be cyclopropanated enantioselectively with alkyl diazoacetates in the presence of Aratani catalysts. In detailed studies,the influence of various parameters, such as metal ligands in the catalyst, catalyst concentration, solvent, and alkene structure, on the enantioselectivity has been recorded. Enantiomeric excesses of up to 88% were obtained with catalyst 7 (R = Bz = 2-MeOCgH4). 

Ligand substitution in salicylaldiminate [NiA2] complexes has been studied in detail.1161 Substitution in chiral [NiA2] species by chiral salen-type tetradentate H2B ligands in acetone proceeds according to Equation (7) ... 

This method of asymmetric cyclopropanation using copper catalysts which are chirally modified with salicylaldimines of optically active amines has been intensively investigated and numerous modifications of the ligands have been tested24-40-43. The use of chiral amino alcohols derived from amino acids is exceptionally successful. Thus, 2-methylpropene with ethyl diazoacetate in the presence of R-7644 (Sumitomo catalyst) gives ethyl (LS)-2,2-dimethyl-1-cyclopropanecarboxylate (2) with 92% ee, on an industrial scale24. This compound is used as a precursor of cilastm, an enzyme inhibitor. 

Intramolecular cyclopropanations with unsaturated diazo ketones have also been reported. Furthermore, enantioselective cyclopropanation with diazomethane can be achieved in up to 75% ee. In detailed mechanistic discussions, a copper(I) species, complexed with only one semicorrin ligand, and formed by reduction and decomplcxation, is suggested as the catalytical-ly active species, cisjtrans Stereoselection and discrimination of enantiotopic alkene faces should take place within a copper-carbene-alkene complex25-54"56. According to these interpretations, cisjtrans selectivity is determined solely by the substituents of the alkene and of the diazo compound (especially the ester group in diazoacetates) and is independent of the chiral ligand structure (salicylaldimine or semicorrin)25. 

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