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 catalyst for asymmetric hetero Diels-Alder (HDA) reaction of Danishefsky's diene with a wide variety of aldehydes. In contrast with this, (R,R)-(salen)-chromium(III) and -manganese(III) complexes well catalyze HDA reaction of simple aldehydes, while (R,S)-(salen)-chromium(III) and-manganese(III) complexes better catalyze HDA reaction of aldehydes bearing a precoordinating group. These features of metallosalen-catalyzed HDA reactions were rationalized by assuming that HDA reaction of aldehydes beating a precoordinating group would proceed through aldehyde-metallosalen complex which takes cis-β-structure.