Salicylaldimine-derived ligands

By modifying the auxiliary ligand to o-aminated salicylaldimine derivatives (Figure 15.9), dinuclear Zn complexes are obtained. These novel systems can effect hydroamination at room temperature with a variety of heteroatom-functionalized secondary aminoalkene substrates [278]. 

In general, the introduction of spatially hindered phenols into coordination compounds may produce stable free-radical forms [138b—140]. A series of metal complexes with redox ligands, containing derivatives of 2,6-di-t-butylphenols n- or a-connected, or vicinal fragments in the coordination environment of the central metal atom, were synthesized in this way 7i-aryl [141], Tt-cr-allyl [142] compounds, nitrile complexes [143], metal glioximates [144], salicylaldiminates [145,146], por-phyrines [147-149], and phthalocyanines [150,151],... 

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). 

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. 

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