| 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 orbitalcalculations. The reaction of acetonitrile oxide (MNO) with acrylonitrile has been also includedfor comparisons. Structure optimizations were performed at the RHF/6-31G(d) and densityfunctional B3LYP/6-31G(d) levels of approximation. Single-point electronic energies were computedup to the MP4SDTQ/6-31G(d) level. Kinetic contributions to activation enthalpies and entropieswere computed at the RHF/6-31G(d) level. Transition structures of ethene cycloadditions (prototypereactions) were also checked with the MP2/6-31G(d) approximation. Solvent effects were introducedboth at a semiempirical level (AMSOL) and at an ab initio level using the Pisa model (interlockingspheres) and the IPCM procedure (isodensity surface polarized continuum model). Electronicactivation energies are found to be very sensitive to the treatment of electron correlation and failedto converge to values unaffected by further theoretical improvements: indeed, the inclusion of fullfourth-order correlation (MP4) decreases the activation energies by 5−10 kcal/mol with respect tothe preceding level of correlation (MP3). Anyway, activation free enthalpies and entropies of thereactions under study appear to be close to the experimental values available for this class ofreactions. Still in agreement with experimental observations is the effect of solvent polarity onthe reaction rates. Theoretical regioselectivity is less sensitive to the level of calculation, althoughthe inclusion of electron correlation, both with the Moeller−Plesset technique and the use of thedensity functional theory, is able to reverse the regiochemical predictions obtained with RHFenergies for the reactions of nitrile oxides with acrylonitrile. This explains why the frontier orbitaltheory, which is based on uncorrelated HF-wave functions, cannot arrive at the correct predictionof 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.
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