title: Computational mechanistic elucidation of the intramolecular aminoalkene hydroamination catalysed by iminoanilide alkaline-earth compounds.
abstract: A comprehensive computational exploration of plausible alternative mechanistic pathways for the intramolecular hydroamination (HA) of aminoalkenes by a recently reported class of kinetically stabilised iminoanilide alkaline-earth silylamido compounds [{N^N}Ae{N(SiMe3)2}⋅(thf)n] ({N^N} = iminoanilide; Ae = Ca, Sr, Ba) is presented. On the one hand, a proton-assisted concerted N-C/C-H bond-forming pathway to afford the cycloamine in a single step can be invoked and on the other hand, a stepwise σ-insertive pathway that involves a fast, reversible migratory olefin 1,2-insertion step linked to a less rapid, irreversible metal-C azacycle tether σ-bond aminolysis. Notably, these alternative mechanistic avenues are equally consistent with reported key experimental features. The present study, which employs a thoroughly benchmarked and reliable DFT methodology, supports the prevailing mechanism to be a stepwise σ-insertive pathway that sees an initial conversion of the {N^N}Ae silylamido into the catalytically competent {N^N}Ae amidoalkene compound and involves thereafter facile and reversible insertive N-C bond-forming ring closure, linked to irreversible intramolecular Ae-C tether σ-bond aminolysis at the transient {N^N}Ae alkyl intermediate. Turnover-limiting protonolysis accounts for the substantial primary kinetic isotope effect observed; its DFT-derived barrier satisfactorily matches the empirically determined Eyring parameter and predicts the decrease in rate observed across the series Ca>Sr>Ba correctly. Non-competitive kinetic demands militate against the operation of the concerted proton-assisted pathway, which describes N-C bond-forming ring closure triggered by concomitant amino proton delivery at the C=C linkage evolving through a multi-centre TS structure. Valuable insights into the catalytic structure-activity relationships are unveiled by a detailed comparison of [{N^N}Ae(NHR)] catalysts. Moreover, the intriguingly opposite trends in reactivity observed in intramolecular (Ca>Sr>Ba) and intermolecular (Ca<Sr<Ba) HA catalysis for the studied family of iminoanilide alkaline-earth amido catalysts are rationalised.
authors: University of St Andrews, School of Chemistry, Purdie Building, North Haugh, St Andrews, Fife KY16 9ST (United Kingdom), Fax:(+44) 1797-383-652). st40@st-andrews.ac.uk.
authors: Sven
authors: S
authors: Tobisch
mesh_terms: D000475:Alkenes / Q000737:chemistry*; D000586:Amination; D000588:Amines / Q000737:chemistry*; D000813:Anilides / Q000737:chemistry*; D002384:Catalysis; D003500:Cyclization; D007097:Imines / Q000737:chemistry*; D008673:Metals, Alkaline Earth / Q000737:chemistry*; D008962:Models, Theoretical; D009942:Organometallic Compounds / Q000737:chemistry*
chemical_list: D000475:Alkenes; D000588:Amines; D000813:Anilides; D007097:Imines; D008673:Metals, Alkaline Earth; D009942:Organometallic Compounds
keywords: alkaline-earth metals; density functional calculations; homogeneous catalysis; hydroamination; reaction mechanisms