The highly enantioselective borohydride reduction of aromatic ketones or imines to the corresponding alcohols was developed in the presence of a catalytic amount of an optically active cobalt(II) complex catalyst. This enantioselective reduction is carried out using a precisely premodified borohydride with alcohols such as tetrahydrofurfuryl alcohol, ethanol and methanol. High optical yields are obtained by choosing the appropriate alcohol as modifiers and a suitable beta-ketoiminato ligand of the catalyst. The enantioselective borohydride reduction has been successfully applied to the preparation of optically active 1,3-diols, the stereoselective reduction of diacylferrocenes, and dynamic and/or kinetic resolution of 1,3-dicarbonyl compounds.
Asymmetric reduction of prochiral ketones with sodium borohydride proceeds in the presence of catalytic amounts of optically active β‐oxoaldiminatocobalt(II) complexes 1 to the corresponding optically active secondary alcohols in high chemical yields with high enantioselectivities (68–94% ee).
Lithiation of N-(1-cyanoalkyl)imines with LDA generates new N-lithiated azomethine ylide 1,3-dipoles which show enhanced reactivity toward dipolarophiles. They undergo exclusively regio- and stereoselective 3+2 cycloaddition reaction with α,β-unsaturated esters to give 1-pyrrolines after the elimination of LiCN. Metallic bases other than LDA can be also effective. Such high regio- and stereoselectivity is explained by the involvement of N-metalated azomethine ylides.
N-(Cyanomethyl)- and N-(α-cyanobenzyl)imines derived from a variety of aldehydes and ketones can tautomerize into N-protonated azomethine ylides which undergo cycloadditions with olefinic dipolarophiles. These cycloadditions are often accompanied by the elimination of HCN, mostly in a stereospecific manner, showing these imines to be synthetic equivalents of nonstabilized nitrile ylides. Stereoselectivity of the cycloadditions is discussed.
Condensation of 2-amino esters with carbonyl compounds leads to simple generation of ester-stabilized azomethine ylides which are trapped by olefinic dipolarophiles as cycloadducts. Anti ylides are exclusively involved in the cycloadditions when N-substituted 2-amino esters are employed for the ylide generation, while syn ylides from N-unsubstituted 2-amino esters. Relative stability among all possible ylide isomers has been inspected and the selective involvement of particular ylidic forms has been explained on the ground of 1,5-dipole interaction or hydrogen bonding stabilization. Endo- and exo-selectivity of the cycloadditions is also discussed.
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