Ab Initio Study of the Regiochemistry of 1,3-Dipolar Cycloadditions. Reactions of Diazomethane and Formonitrile Oxide with Ethene, Propene, Acrylonitrile, and Methyl Vinyl Ether.

Abstract

Structures and energetics of reactants and transition structures of the cycloadditions of diazomethane (DZM) and formonitrile oxide (FNO) with ethene (ET), propene (PR), acrylonitrile (ACN), and methyl vinyl ether (MVE) have been investigated with the use of ab initio molecular orbital calculations. The reaction of acetonitrile oxide (MNO) with acrylonitrile has been also included for comparisons. Structure optimizations were performed at the RHF/6-31G(d) and density functional B3LYP/6-31G(d) levels of approximation. Single-point electronic energies were computed up to the MP4SDTQ/6-31G(d) level. Kinetic contributions to activation enthalpies and entropies were computed at the RHF/6-31G(d) level. Transition structures of ethene cycloadditions (prototype reactions) were also checked with the MP2/6-31G(d) approximation. Solvent effects were introduced both at a semiempirical level (AMSOL) and at an ab initio level using the Pisa model (interlocking spheres) and the IPCM procedure (isodensity surface polarized continuum model). Electronic activation energies are found to be very sensitive to the treatment of electron correlation and failed to converge to values unaffected by further theoretical improvements: indeed, the inclusion of full fourth-order correlation (MP4) decreases the activation energies by 5-10 kcal/mol with respect to the preceding level of correlation (MP3). Anyway, activation free enthalpies and entropies of the reactions under study appear to be close to the experimental values available for this class of reactions. Still in agreement with experimental observations is the effect of solvent polarity on the reaction rates. Theoretical regioselectivity is less sensitive to the level of calculation, although the inclusion of electron correlation, both with the Moeller-Plesset technique and the use of the density functional theory, is able to reverse the regiochemical predictions obtained with RHF energies for the reactions of nitrile oxides with acrylonitrile. This explains why the frontier orbital theory, which is based on uncorrelated HF-wave functions, cannot arrive at the correct prediction of the regiochemistry in these cases. Calculated solvent effects appear to influence the regiochemistry of 1,3-dipolar cycloaddition, but in general, they reinforce the prediction obtained in vacuo.