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σ‐Insertive Mechanism versus Concerted Non‐insertive Mechanism in the Intramolecular Hydroamination of Aminoalkenes Catalyzed by Phenoxyamine Magnesium Complexes: A Synthetic and Computational Study
Author(s) -
Zhang Xiaoming,
Tobisch Sven,
Hultzsch Kai C.
Publication year - 2015
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.201406468
Subject(s) - protonolysis , hydroamination , intramolecular force , chemistry , catalysis , steric effects , olefin fiber , amine gas treating , medicinal chemistry , stereochemistry , organic chemistry
The phenoxyamine magnesium complexes [{ONN}MgCH 2 Ph] ( 4 a : {ONN}=2,4‐ t Bu 2 ‐6‐(CH 2 NMeCH 2 CH 2 NMe 2 )C 6 H 2 O − ; 4 b : {ONN}=4‐ t Bu‐2‐(CH 2 NMeCH 2 CH 2 NMe 2 )‐6‐(SiPh 3 )C 6 H 2 O − ) have been prepared and investigated with respect to their catalytic activity in the intramolecular hydroamination of aminoalkenes. The sterically more shielded triphenylsilyl‐substituted complex 4 b exhibits better thermal stability and higher catalytic activity. Kinetic investigations using complex 4 b in the cyclisation of 1‐allylcyclohexyl)methylamine ( 5 b ), respectively, 2,2‐dimethylpent‐4‐en‐1‐amine ( 5 c ), reveal a first‐order rate dependence on substrate and catalyst concentration. A significant primary kinetic isotope effect of 3.9±0.2 in the cyclisation of 5 b suggests significant NH bond disruption in the rate‐determining transition state. The stoichiometric reaction of 4 b with 5 c revealed that at least two substrate molecules are required per magnesium centre to facilitate cyclisation. The reaction mechanism was further scrutinized computationally by examination of two rivalling mechanistic pathways. One scenario involves a coordinated amine molecule assisting in a concerted non‐insertive NC ring closure with concurrent amino proton transfer from the amine onto the olefin, effectively combining the insertion and protonolysis step to a single step. The alternative mechanistic scenario involves a reversible olefin insertion step followed by rate‐determining protonolysis. DFT reveals that a proton‐assisted concerted NC/CH bond‐forming pathway is energetically prohibitive in comparison to the kinetically less demanding σ‐insertive pathway (ΔΔ G ≠ =5.6 kcal mol −1 ). Thus, the σ‐insertive pathway is likely traversed exclusively. The DFT predicted total barrier of 23.1 kcal mol −1 (relative to the {ONN}Mg pyrrolide catalyst resting state) for magnesiumalkyl bond aminolysis matches the experimentally determined Eyring parameter (Δ G ≠ =24.1(±0.6) kcal mol −1 (298 K)) gratifyingly well.