Cyclometalation of allylbenzene by EtAlCl₂ and Mg in the presence of Zr ansa-complexes as synthetic route to dibenzylbutane and 9,8′-neo- lignans

The aim of the research is the development of an one-pot method for the synthesis of biologically active compounds – lignans. For this purpose the ansa-zirconocenes of various structures were tested as the catalysts of the allylbenzene cyclometalation with ethylaluminum dichloride (EtAlCl₂) and Mg. The effect of organophosphorus compounds hexamethylphosphoramide (HMPA) and triphenylphosphine (PPh₃) on the chemo- and regioselectivity of the reaction was studied. It was shown that the structure of the η⁵-ligand at the Zr atom significantly affects the activity of the system, and the presence of HMPA increases the yield of cyclometalation products up to 77%. The reaction proceeds with the formation of regioisomers of 3,4-dibenzyl- and 2,4-dibenzyl-substituted alumolanes with a ratio (1-2):1, which hydrolysis provides corresponding dibenzylbutane and 9,8'-neo- lignans.