Phenalenyl-based organozinc catalysts for intramolecular hydroamination reactions: A combined catalytic, kinetic, and mechanistic investigation of the catalytic cycle

Abstract

Herein, we report the synthesis and characterization of two organozinc complexes that contain symmetrical phenalenyl (PLY)-based N,N-ligands. The reactions of phenalenyl-based ligands with ZnMe 2 led to the formation of organozinc complexes [N(Me),N(Me)-PLY]ZnMe (1) and [N(iPr),N(iPr)-PLY]ZnMe (2) under the evolution of methane. Both complexes (1 and 2) were characterized by NMR spectroscopy and elemental analysis. The solid-state structures of complexes 1 and 2 were determined by single-crystal X-ray crystallography. Complexes 1 and 2 were used as catalysts for the intramolecular hydroamination of unactivated primary and secondary aminoalkenes. A combined approach of NMR spectroscopy and DFT calculations was utilized to obtain better insight into the mechanistic features of the zinc-catalyzed hydroamination reactions. The progress of the catalysis for primary and secondary aminoalkene substrates with catalyst 2 was investigated by detailed kinetic studies, including kinetic isotope effect measurements. These results suggested pseudo-first-order kinetics for both primary and secondary aminoalkene activation processes. Eyring and Arrhenius analyses for the cyclization of a model secondary aminoalkene substrate afforded ΔH ≠=11.3 kcal mol -1, ΔS ≠=-35.75 cal K -1 mol -1, and E a=11.68 kcal mol -1. Complex 2 exhibited much-higher catalytic activity than complex 1 under identical reaction conditions. The in situ NMR experiments supported the formation of a catalytically active zinc cation and the DFT calculations showed that more active catalyst 2 generated a more stable cation. The stability of the catalytically active zinc cation was further supported by an in situ recycling procedure, thereby confirming the retention of catalytic activity of compound 2 for successive catalytic cycles. The DFT calculations showed that the preferred pathway for the zinc-catalyzed hydroamination reactions is alkene activation rather than the alternative amine-activation pathway. A detailed investigation with DFT methods emphasized that the remarkably higher catalytic efficiency of catalyst 2 originated from its superior stability and the facile formation of its cation compared to that derived from catalyst 1. Phenalenyl-based organozinc complexes were used as catalysts for the intramolecular hydroamination reactions of unactivated primary and secondary aminoalkenes. DFT calculations predicted that the organozinc complex followed an alkene-activation pathway for the cyclization of secondary aminoalkenes. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.