NMR Spectroscopic Characterization and DFT Calculations of Zirconium(IV)-3,3′-Br2–BINOLate and Related Complexes Used in an Enantioselective Friedel–Crafts Alkylation of Indoles with α,β-Unsaturated Ketones

Experimental and theoretical studies on the structure of several complexes based on (R)-3,3′-Br2–BINOL ligand and group (IV) metals used as catalysts in an enantioselective Friedel–Crafts alkylation of indoles with α,β-unsaturated ketones have been carried out. NMR spectroscopic studies of these catalysts have been performed, which suggested that at room temperature the catalysts would form a monomeric structure in the case of TiIV and a dimeric structure in the cases of ZrIV and HfIV. Density functional theory (DFT) calculations clearly corroborate the conclusions of these experimental spectroscopic studies. The dimeric structure with a doubly bridged motif [ZrIV2(μ-(R)-3,3′-Br2–BINOL)2] where each binaphthol ligand acts as bridge between the metal centers (Novak’s model) is more stable than the dimeric structure with a doubly bridged motif [ZrIV2(μ-OtBu)2] where the tert-butoxide groups act as bridging ligands (Kobayashi’s model). The scope of the Friedel–Crafts alkylation with regard to the indole structure has been studied. Finally a plausible mechanism for the Friedel–Crafts reaction and a stereomodel for the mode of action of the catalyst that explain the observed stereochemistry of the reaction products have been proposed.