Premium
Phenalenyl‐Based Organozinc Catalysts for Intramolecular Hydroamination Reactions: A Combined Catalytic, Kinetic, and Mechanistic Investigation of the Catalytic Cycle
Author(s) -
Mukherjee Arup,
Sen Tamal K.,
Ghorai Pradip Kr.,
Samuel Prinson P.,
Schulzke Carola,
Mandal Swadhin K.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201200868
Subject(s) - hydroamination , chemistry , catalysis , intramolecular force , arrhenius equation , catalytic cycle , zinc , nuclear magnetic resonance spectroscopy , photochemistry , activation energy , stereochemistry , organic chemistry
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 ( i Pr), N ( i Pr)‐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 Δ H ≠ =11.3 kcal mol −1 , Δ S ≠ =−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 .