Mechanistic elucidation of the yttrium(III)-catalysed intramolecular aminoalkene hydroamination: DFT favours a stepwise σ-insertive mechanism

A comprehensive computational mechanistic study regarding intramolecular hydroamination (HA) of aminoalkenes mediated by a recently reported class of highly active cyclopentadienyl-bis(oxazolinyl)-borate {Cpo}Y-III alkyl compounds is presented. Two distinct mechanistic pathways of catalytic HA mediated by rare earth and alkaline earth compounds have emerged over the years, describing amidoalkene -> cycloamine conversion proceeding through a stepwise sigma-insertive mechanism or a concerted non-insertive N-C/C-H bond forming pathway. Notably, both mechanisms account equally for reported distinct process features. Non-competitive kinetic demands revealed for the concerted amino proton transfer associated with N-C ring closure, which commences from a {Cpo(M)}Y(NHR)center dot(NH2R) substrate adduct and evolves through a six-centre TS structure, militates against a proton-triggered non-insertive pathway to promote HA for the rare earth catalyst at hand. A stepwise s-insertive pathway, featuring rapid and reversible olefin insertion into the Y-N amido sigma-bond, linked to a less facile and irreversible intramolecular Y-C azacycle tether aminolysis, is found to prevail energetically. The assessed effective barrier for turnover-limiting aminolysis matches the empirically determined Eyring parameter well and the computationally estimated primary KIE is close to the observed values. A recent computational study revealed a similar scenario for an analogous tris(oxazolinyl) borate {To(M)}Mg system. Valuable insights into the catalytic structure-reactivity relationships have been unveiled by a comparison of {Cpo(M)}Y(NHR)- and {To(M)}Mg(NHR)-catalysed hydroaminations.