Is an M-N sigma-Bond Insertion Route a Viable Alternative to the M=N [2+2] Cycloaddition Route in Intramolecular Aminoallene Hydroamination/Cyclisation Catalysed by Neutral Zirconium Bis(amido) Complexes? A Computational Mechanistic Study

This study examines alternative reaction channels for intramolecular hydroamination/cyclisation (IHC) of primary 4,5-hexadien-1-ylamine aminoallene (1) by a neutral [Cp2ZrMe2] zirconocene precatalyst (2) by using the density functional theory (DFT) method. The first channel proceeds through a [Cp2Zr(NHR)(2)] complex as the reactive species and relevant steps including the insertion of an allenic C=C linkage into the Zr-NHR sigma-bond and ensuing protonolysis. This is contrasted to the [2+2] cycloaddition mechanism involving a [Cp2Zr=NR] transient species. The salient features of the rival mechanisms are disclosed. The cycloaddition route entails the first transformation of the dormant [Cp2Zr-(NHR)(2)] complex 3B into the transient [Cp2Zr=NR] intermediate 3A', which is turnover limiting. This route features a highly facile ring closure together with a substantially slower protonolysis (k(cycloadd)>> k(protonolysis)) and can display inhibition by high substrate concentration. In contrast, protonolysis is the more facile step for the channel proceeding through the [Cp2Zr(NHR)(2)] complex as the catalytically active species. Here, C-C insertion into the Zr-C sigma-bond of 3B, which represents the catalyst resting state, is turnover limiting and Substrate concentration is unlikely to influence the rate. The regulation of the selectivity is elucidated for the two channels. DFT predicts that five-ring allylamine and six-ring imine are generated upon traversing the cycloaddition route, thereby comparing favourably with experiment, whereas the cycloimine should be formed solely along the sigma-bond insertion route. The mechanistic analysis is indicative of an operating [2+2] cycloaddition mechanism. The Zr--NHR sigma-bond insertion route, although appearing not to be employed for the reactants studied herein, is clearly suggested as being viable for hydroamination by charge neutral organozirconium compounds.