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

Abstract

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 σ-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)·(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 σ-insertive pathway, featuring rapid and reversible olefin insertion into the Y–N amido σ-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.