1,3-Dipolar cycloaddition of nitrile oxides to free and Pt-bound nitriles: a theoretical study of the activation effect, reactivity and mechanism

The cycloaddition of nitrile oxides to nitriles—free CH3CN and ligated to Pt(II) and Pt(IV) in complexes trans-[PtCl2(NCMe)2] (1) and trans-[PtCl4(NCMe)2] (2)—was investigated by theoretical methods at different levels of theory using quasi-relativistic pseudopotentials for the platinum atom. The calculations included the geometry optimization of the starting and final complexes, location of possible transition states for the reaction discussed and the intrinsic reaction coordinate calculations. The activation of the nitriles ligated to Pt(IV) can be interpreted in terms of both kinetic (activation parameters) and thermodynamic (reaction energies) viewpoints and the higher reactivity of the complex 2, in comparison with that of 1, is kinetically controlled. The calculations predict that the complex 1 should be less reactive than free acetonitrile, and the main factor determining the relative reactivity of 1 is the entropic factor. The cycloaddition of nitrile oxide to nitriles is mainly controlled by the HOMOnitrile oxide–LUMOnitrile type of interaction and occurs via a concerted asynchronous mechanism for both free and bound nitriles rather than a stepwise mechanism.