Intramolecular Aminoalkene Hydroamination Mediated by a Tethered Bis(ureate)zirconium Complex: Computational Perusal of Various Pathways for Aminoalkene Activation

Abstract

The present study comprehensively explores alternative mechanistic pathways for the intramolecular hydroamination of the prototype 2,2-dimethyl-5-penten-1-amine aminoalkene (1) by bis(ureate)Zr(IV)(NMe(2))(2)(HNMe(2)) (2), which proceeds through a Zr(IV)(NHR)(2) intermediate using density functional theory (DFT) calculations. The classical stepwise σ-insertive mechanism that includes insertion of the C═C double bond into the Zr-N amido σ bond followed by Zr-C alkyl-bond aminolysis has been compared with a single-step pathway for amidoalkene → cycloamine conversion through a concerted amino proton transfer associated with N-C ring closure. Noncompetitive kinetics for reversible σ-insertive cyclization, together with the incompatibility of a turnover-limiting insertion step with observed pronounced primary kinetic isotope effects (KIEs), strongly militates against the operation of a σ-insertive mechanism. A noninsertive pathway evolving through an ordered six-center transition-state structure describing N-C bond formation at an axial Zr-N amido σ bond triggered by concurrent proton transfer from an equatorially bound substrate molecule onto the adjacent olefin-carbon center is found to prevail energetically. The proton-triggered noninsertive cyclization commencing from a catalytically relevant Zr(IV)(NHR)(2)(NH(2)R) substrate adduct is strongly downhill, followed by product expulsion via dissociative amine exchange. The assessed effective barrier compares reasonably well with the previously determined Eyring parameters, and the computationally estimated primary KIEs match the observed values pleasingly well. The present study reveals a comparable strength of substrate and product binding in relevant seven-coordinate intermediates, together with a rapid equilibrium between related primary and secondary amido species, which favors the former, as unique features of the studied catalyst. Thus, in line with experimental observations, competitive product inhibition can be discarded. On the basis of all of these findings, it is suggested that a Zr(NHR)(2)(substrate) intermediate corresponds to the catalyst resting state at high substrate concentrations, while it becomes a Zr(NHR)(2)(cycloamine) species when the product concentration is high or with the addition of excess 2-methylpiperidine, and this ambivalent behavior explains the observed operation of two distinct kinetic regimes, depending upon the extent of the reaction.