Lewis acid catalysis of second-generation metallosalen complexes: An explanation for stereochemistry of asymmetric hetero Diels-Alder reaction

Chloro nitrosyl (R,S)-(salen)ruthenium(II) complex served as a good catalys t for asymmetric hetero Dials-Alder (HDA) reaction of Danishefsky's diene w ith a wide variety of aldehydes. In contrast with this, (RP)-(salen)chromiu m(III) and -manganese(III) complexes well catalyze HDA reaction of simple a ldehydes, while (R,S)-(salen)-chromium(III) and -manganese(III) complexes b etter catalyze HDA reaction of aldehydes bearing a precoordinating group. T hese features of metallosalen-catalyzed HDA reactions were rationalized by assuming that HDA reaction of aldehydes bearing a precoordinating group wou ld proceed through aldehyde-metallosalen complex which takes cis-beta -stru cture.