Substituent effects on axial chirality in 1-aryl-3,4-dihydroisoquinolines: controlling the rate of bond rotation

A series of 1-aryl-3,4-dihydroisoquinolines (DHIQs) were synthesized and their barriers to bond rotation were determined by means of VT-NMR, dynamic HPLC or racemization studies. Although they all presented lower rotational stability than the related 1-arylisoquinolines (such as QUINAP), certain 1-aryl-DHIQ structures had a sufficiently high barrier to bond rotation to show axial chirality. These compounds included 1-(2-triflyl-1-naphthyl)-4,5-dihydroisoquinoline 4h and 1-(2-diphenylphosphanyl-1-naphthyl)-4,5-dihydroisoquinoline 4i. This discovery opens the door to the development of a new group of axially chiral N,P ligands for asymmetric synthesis and also potentially to new strategies for the synthesis of axially chiral 1-arylisoquinolines.