The design and development of new chiral ligands to enable precise stereocontrol in a wide variety of reactions is one of the most important branches of organic synthesis. To date, the development of hybrid ligands containing both σ-donating and π-donating groups has attracted considerable attention, with unprecedented reactivities and stereoselectivities being observed. Therefore to develop efficient hybrid chiral ligands with novel structural motifs, we envisage that helicene would be a suitable π-donor efficiently to construct a helical environment around a metal center. In this context, we herein describe our recent efforts to develop a series of novel chiral [5]helicene-derived phosphine ligands (L1, with a 7,8-dihydro[5]helicene core structure, and L2, with a fully aromatic [5]helicene core structure). The prepared ligands, and in particular L1, were found highly effective in the asymmetric allylation of 1,3-diphenylallyl acetate with indoles and etherification with alcohols. Furthermore, in the asymmetric Suzuki-Miyaura coupling reaction, L2 exhibited excellent enantioselectivities. Finally, density functional theory studies were employed to propose a model that accounts for the origin of such high enantioselectivity in these reactions.
Keywords: Suzuki-Miyaura cross-coupling; asymmetric allylation; helicene; helix; phosphine ligand.