New sp3 diphosphine-based rhodium catalysts for the asymmetric conjugate addition of aryl boronic acids to 3-azaarylpropenones

Abstract

Different chiral diphosphine ligands were successfully applied to the rhodium catalyzed asymmetric conjugate addition of differently substituted boronic acids to 3-azaarylpropenones containing both pyridinyl and imidazolyl cores. Atropoisomeric (S)-TetraMe-BITIANP (L1) and (S)-BITIANP (L2), together with ligands bearing mixed chirality as in (S,S,S_ax)-DIOPHEP (L3), (R,R_ax)-ISAPHOS C1 (L4) and (S,R_ax,R_ax)-ISAPHOS C2 (L5), and the ones containing a stereogenic sp³ carbon ((R,R)-ZEDPHOS (L6) and (R,R)-EPHOS (L8) and their derivatives L7 and L9) have been employed as a source of chirality in rhodium complexes. Among this last class of diphosphines, the new phosphorus-based ligand called (R,R)-EPHOS (L8) has been synthesized and employed for the first time as a chiral ligand in rhodium complexes for its catalytic activity. Computational studies suggested a cis coordination with a wide bite angle. When applied to the asymmetric conjugate addition of phenyl boronic acid to 3-azaarylpropenone 1, the catalytic system bearing L8 afforded the product 1a in a remarkable 94% e.e. in THF.