Efficient synthesis of chiral 1,1'-binaphthalenes by the asymmetric suzuki-miyaura reaction: dramatic synthetic improvement by simple purification of naphthylboronic acids.
Naphthylboronic acids prepared as reported in the literature are contaminated with HCl. A very simple purification prior to their use in Suzuki-Miyaura couplings has been found to be crucial, rendering efficient some reactions that had been reported in the literature either to fail or to give extremely poor yields. With this improvement, parent boronic acids can be used instead of esters at moderate temperatures, and bromo derivatives can be used instead of iodo derivatives. Convenient access to chiral sterically hindered binaphthalene derivatives has been achieved through the use of boronic acids, bromonaphthalenes, and ferrocenylphosphane ligands. The products were obtained in good yields (95-55 %) and with good enantioselectivities (90-50 %). Bulkier ligands are less efficient in the coupling of hindered partners.