Mechanistic Exploration of Intramolecular Aminodiene Hydroamination/Cyclisation Mediated by Constrained Geometry Organoactinide Complexes: A DFT Study

摘要

The present computational mechanistic study explores comprehensively the organoactinide-mediated intramolecular hydroamination/cyclisation (IHC) of aminodienes by employing a reliable DFT method. All the steps of a plausible catalytic reaction course have been scrutinised for the IHC of (4E,6)-heptadienylamine 1t by [(CGC)Th(NMe2)(2)] precatalyst 2 (CGC = [Me2Si(eta(5)-Me4C5)(tBuN)](2-)). For each of the relevant elementary steps the most accessible pathway has been identified from a multitude of mechanistic possibilities. The operative mechanism involves rapid substrate association/dissociation equilibria for the 3t-S resting state and also for azacyclic intermediates 4a, 4s, easily accessible and reversible exocyclic ring closure, supposedly facile isomerisation of the azacycle's butenyl tether prior to turnover-limiting protonolysis. The following aspects are in support of this scenario: 1) the derived rate law is consistent with the experimentally obtained empirical rate law; 2) the accessed barrier for turnover-limiting protonolysis does agree remarkably well with observed performance data; 3) the ring-tether double-bond selectivity is consistently elucidated, which led to predict the product distribution correctly. This study provides a computationally substantiated rationale for observed activity and selectivity data. Steric demands at the CGC framework appear to be an efficient means for modulating both performance and ring-tether double-bond selectivity. The careful comparison of (CGC)4f-element and (CGC)5f-element catalysts revealed that aminodiene IHC mediated by organoactinides and organolanthanides proceeds through a similar mechanistic scenario. However, cyclisation and protonolysis steps, in particular, feature a markedly different reactivity pattern for the two catalyst classes, owing to enhanced bond covalency of early actinides when compared to lanthanides.