Catalytic cyclometallation of allylbenzenes by EtAlCl2 and Mg as new route to synthesis of dibenzyl butane lignans

The cyclometallation of allylbenzenes (Ar = Ph, 4-MeO-Ph, 3,4-(MeO) 2 -Ph) with EtAlCl 2 (Et 2 AlCl) and Mg in the presence of Zr and Ti catalysts (Dzhemilev reaction) has been studied. The reaction run with high diastereoselectivity and gives cyclic organoaluminum compounds, which deuterolysis or hydrolysis gave 2 R ( S ), 3 R ( S )-dibenzyl butanes with yield of 48–69%. The study of catalyst structure effect on the substrate conversion, reaction chemo- and stereoselectivity showed that the best results were obtained in the case of Cp 2 ZrCl 2 among the tested complexes. The enantioselectivity of neomenthylindenyl or neomenthyltetrahydroindenyl zirconium catalysts in the reaction did not exceed 20%ee. The trans -configuration of the substitutes in the metallacycles formed in the reaction has been proved by X-ray analysis of the hydrolysis product – 2 R ( S ), 3 R ( S )-dimethyl-1,4-bis[(4′- methoxyphenyl)methyl]-butane. The proposed method could be used for the one-pot diastereoselective synthesis of dibenzyl butane lignans from readily available allylbenzenes.