Synthesis of an amidosulfonate-tagged biphenyl phosphine and its application in the Suzuki-Miyaura reaction affording biphenyl-substituted amino acids in water
Abstract
An amidosulfonate-tagged phosphinobiphenyl, viz triethylammonium 2-(dicyclohexylphosphino)-4'-{[(sulfonatomethyl)amino]carbonyl}biphenyl (3), was prepared in two steps from 2-(dicyclohexylphosphino) biphenyl-4'-carboxylic acid and fully characterized including the crystal structure determination. As a highly polar phosphine ligand, compound 3 was employed in the Pd-catalyzed Suzuki-Miyaura cross-coupling of N-Boc protected 4-bromo and 4-chlorophenylalanine with aromatic boronic acids to afford the corresponding biphenyls in aqueous N,N-dimethylformamide or pure water.
The coupling was demonstrated to proceed very well and without the loss of the Boc protecting group when the bromo-substituted substrate is reacted in the presence of 1 mol.% of Pd/3 catalyst in water at 40 degrees C. Reactions with boronic acids bearing electron-withdrawing substituents and, mainly, those with the less reactive chlorophenylalanine substrate required slightly higher reaction temperature and more catalyst to achieve similar results.