Theoretical Study of Reactant Activation in 1,3-Dipolar Cycloadditions of Cyclic Nitrones to Free and Pt-Bound Nitriles

1,3-Dipolar cycloaddition of the cyclic nitrones (N1), (N2), (N3), and (N4) to organonitriles, RCNboth free (R = CH3, CF3) and
ligated to PtII and PtIV (in the complexes trans-[PtCl2(NCCH3)2] (1) and trans-[PtCl4(NCCH3)2] (2))was investigated extensively by theoretical methods at different levels of theory. The effectiveness of
two types of dipolarophile activation (by introducing a strong electron-acceptor group R and by coordination
to a metal center) was analyzed and compared. The influence of factors such as the nature of the cyclic
nitrone and the nature of the solvent on the reactions was also studied. The reactivity of dipoles and
dipolarophiles increases along the series N4 < N1 ≈ N3 < N2 and CH3CN < CF3CN < 1 < 2; the latter
demonstrates that the coordination of RCN to a Pt center provides an even higher activation effect upon
cycloaddition in comparison with the introduction of a strong electron-acceptor group R such as CF3. A
higher reactivity of the cyclic dipole N1 in comparison with acyclic nitrones (e.g., CH3CHN(CH3)O)
is interpreted to be a result of its exclusive existence in a more strained and hence more reactive E-
rather than Z-configuration. The activation and reaction energies have been calculated at different basis
sets and levels of theory, up to MP4(SDTQ), CCSD(T), and CBS-Q. The activation energies are weakly
sensitive to a change of the correlated methods. The consideration of the solvent effects results in the
increase of the activation barriers, and such enhancement is less pronounced for the nonpolar or low
polar solvents. The cycloadditions to CH3CN and CF3CN were found to be nearly synchronous, but
these reactions involving 1 and 2 are clearly asynchronous. Moreover, the reaction of N2 with 2 proceeds
via a very early acyclic transition state, while for all other reactions the transition states have a cyclic
nature.