Mechanistic Investigation of Organolanthanide‐Mediated Hydroamination of Conjugated Aminodienes: A Comprehensive Computational Assessment of Various Routes for Diene Activation

The present computational mechanistic study comprehensively explores alternative scenarios for activation of the amine‐linked diene CC linkage toward CN ring closure in intramolecular hydroamination of a prototypical aminodiene by a well‐characterised lanthanocene–amido catalyst. Firstly, the non‐insertive mechanism by Scott featuring ring closure with concomitant amino proton delivery onto the diene unit has been explored and key features have been defined. This scenario has been compared with the classical stepwise insertion mechanism that involves rapid substrate association/dissociation equilibria for the 3 t ‐S1 resting state and also for azacycle intermediates 4 s , 4 a , facile and reversible exocyclic migratory diene insertion into the LaN(amido) σ‐bond, linked to turnover‐limiting LaC azacycle aminolysis. The LnN σ‐bond insertive mechanism prevails for the examined intramolecular hydroamination of (4 E ,6)‐heptadienylamine 1 t by [Cp* 2 LaCH(TMS) 2 ] starting material 2 .The following aspects are in support of this mechanism: 1) the derived rate law is consistent with the observed empirical rate law; 2) the assessed effective barrier for turnover‐limiting aminolysis does agree remarkably well with empirically determined Eyring parameters; 3) the ring‐ether double‐bond selectivity is consistently elucidated. This study reveals that the non‐insertive mechanism is not achievable for the particular lanthanocene–amido catalyst and furthermore cannot account for the observed product spectrum. Notwithstanding of these findings, the non‐insertive mechanism cannot be discarded a priori for intramolecular aminodiene hydroamination. Spatial demands around the lanthanide centre influence the two mechanisms differently. The LnN σ‐bond insertive mechanism critically relies on a sufficiently accessible lanthanide and enhanced encumbrance renders cyclisation and aminolysis steps less accessible kinetically. It contrasts with the non‐insertive mechanism, where greater lanthanide protection has a rather modest influence. The present study indicates that the non‐insertive mechanism would prevail if the lanthanide centre were to be protected effectively against CC bond approach. Notably, a different product spectrum would be expected for aminodiene hydroamination following the insertive or non‐insertive route.