Synthesis, stereochemical characteristics, and coordination behavior of 2,2'-binaphthyl-1,1'-biisoquinoline as a new axially chiral bidentate ligand

We describe the synthesis, stereochemical characteristics, and coordination behavior of 2,2'-binaphtyl-1,1'-biisoquinoline (BINIQ), a new axially chiral bidentate ligand. BINIQ was obtained in a racemic form by the diastereoselective Ullmann coupling of 1-(2-iodonaphthalen-1-yl)isoquinoline, which was prepared by the regioselective C-H iodination of 1-(1-naphthyl)isoquinoline. BINIQ has three chiral biaryl axes α and γ at the two naphthylisoquinoline and β at the 2,2'-binaphthyl sites and their relative configuration (αR_aβR_a , γ_a ) in solid state was confirmed by X-ray diffraction analysis. The naphthyl-isoquinoline axes α and γ were proven rigid enough in solution to allow for optical resolution by a chiral HPLC method and a solution of the optically pure BINIQ (98% ee) in chloroform-d did not result in racemization while standing at room temperature for 24 h. On the other hand, the 2,2'-binaphthyl axis is stereochemically labile and readily alternates between βR_a and βS_a at room temperature. Accordingly, (αR_{a a} γ_a )- and (αR_{a a} ,γR_a)-BINIQ are in equilibrium in solution with the former stereoisomer being dominant, though the latter is suitable as a bidentate ligand. Notably, this dynamic stereochemical behavior enabled BINIQ to readily give the relative configuration (αR_{a a} ,γR_a ) upon coordination to a copper(I) ion at room temperature.