Fluorinated Ligands for Selective Catalytic Reactions

The LLB type catalysts were also successfully applied in the asymmetric ni-troaldol reaction of quite unreactive a,a-difluoro aldehydes. However, catalytic asymmetric nitroaldol reaction of a broad variety of a,a-difluoro aldehydes proceeded satisfactorily when using the heterobimetallic asymmetric catalysts with modified, 6,6 -disubstituted BINOL ligands [22]. The best results were obtained with the samarium (III) complex (5 mol%) generated from 6,6 -bis[(tri-ethylsilyl)ethynyl BINOL with enantioselectivities up to 95% ee. The ( -configuration of one representative nitroaldol adduct showed that the nitronate reacted preferentially on the Si face of aldehyde in the presence of (P)-LLB (20 mol% 74% yield 55% ee). It is noteworthy that the enantiotopic face selection for a,a-difluoro aldehydes is reverse to that for nonfluorinated aldehydes. The stereoselectivity for a,a -difluoro aldehydes is identical with that of P-oxa-aldehydes, suggesting that the fluorine atoms at the a-position have a great influence on enantioface selection. 

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