Enantiopure Chiral Concave 1,10‐Phenanthrolines
Chiral information has been introduced into concave 1,10-phenanthrolines of different ring sizes by using a 2,7-disubstituted naphthalene bridgehead, which causes axial chirality. A tetraphenolic 2-(dihydroxynaphthyl)-9-(dihydroxyphenyl)-1,10-phenanthroline was synthesized as a key intermediate. Two strategies were followed to obtain the bimacrocyclic chiral concave 1,10-phenanthrolines: quadruple Williamson ether synthesis or alkenylation of the OH groups and subsequent ring-closing metathesis followed by hydrogenation. The overall yields of bimacrocyles 19 were 10 to 17 % starting from the respective Suzuki coupling of the substituted arenes 11 and 13 to 2,9-dichloro-1,10-phenanthroline (5). Racemic mixtures of the three concave 1,10-phenanthrolines 19 were separated by using chiral high-performance liquid chromatography (HPLC) techniques, and their absolute stereochemistry was assigned by comparison of simulated and experimental circular dichroism (CD) spectra. The enantiopure concave 1,10-phenanthrolines were used as ligands in a copper-catalysed cyclopropanation, and their selectivity was determined by chiral gas chromatography (GC).