σ-Insertive Mechanism versus Concerted Non-insertive Mechanism in the Intramolecular Hydroamination of Aminoalkenes Catalyzed by Phenoxyamine Magnesium Complexes: A Synthetic and Computational Study.

Abstract

The phenoxyamine magnesium complexes [{ONN}MgCH2 Ph] (4 a: {ONN}=2,4-tBu2 -6-(CH2 NMeCH2 CH2 NMe2 )C6 H2 O(-) ; 4 b: {ONN}=4-tBu-2-(CH2 NMeCH2 CH2 NMe2 )-6-(SiPh3 )C6 H2 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 N-H 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 N-C 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 N-C/C-H bond-forming pathway is energetically prohibitive in comparison to the kinetically less demanding σ-insertive pathway (ΔΔG(≠) =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 magnesium-alkyl bond aminolysis matches the experimentally determined Eyring parameter (ΔG(≠) =24.1(±0.6) kcal mol(-1) (298 K)) gratifyingly well.