Intramolecular hydroamination/cyclisation of aminoallenes mediated by a cationic zirconocene catalyst: a computational mechanistic study.

The complete sequence of steps of a tentative catalytic cycle for intramolecular hydroamination/cyclisation (IHC) of 4,5-hexadien-1-ylamine (1) by a prototypical cationic [Cp(2)ZrCH(3)](+) zirconocene precatalyst (2) has been examined by employing a gradient-corrected DFT method. The predicted smooth overall reaction energy profile is consistent with the available experimental data, thereby providing further confidence in the proposed mechanism. Following activation of the precatalyst by protonolytic cleavage of the Zr-Me bond, the catalytically active amidoallene-Zr complex undergoes addition of an allenic C[double bond, length as m-dash]C linkage across the Zr-N sigma-bond. The alternative exo- and endocyclic pathways show similar probabilities for the sterically less encumbered reactants {1 + 2} investigated herein. However, steric factors are expected to exert control on the regioselectivity of ring closure. On the other hand, the metathesis-type transition states for subsequent protonolysis are indicated to be less sensitive to steric demands. Formation of the six-membered azacycle-Zr intermediate through intramolecular C[double bond, length as m-dash]C insertion into the Zr-N sigma-bond is predicted to be turnover limiting. The factors that govern the regioselectivity of the aminoallene IHC have been elucidated.