Mechanistic exploration of intramolecular aminodiene hydroamination/cyclisation mediated by constrained geometry organoactinide complexes: a DFT study.
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(NMe(2))(2)] precatalyst 2 (CGC=[Me(2)Si(eta(5)-Me(4)C(5))(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.
University of St Andrews, School of Chemistry, Purdie Building, North Haugh, St Andrews, Fife KY16 9ST, UK. st40@st-andrews.ac.uk
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