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

Structures and energetics of reactants and transition structures of th e cycloadditions of diazomethane (DZM) and formonitrile oxide (FNO) wi th ethene (ET), propene (PR), acrylonitrile (ACN), and methyl vinyl et her (MVE) have been investigated with the use of ab initio molecular o rbital calculations. The reaction of acetonitrile oxide (MNO) with acr ylonitrile has been also included for comparisons. Structure optimizat ions 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 Do the MP4SDTQ/6-31G(d) level. Kinetic contributions to a ctivation enthalpies and entropies were computed at the RHF/6-31G(d)le vel. Transition structures of ethene cycloadditions (prototype reactio ns) were also checked with the MP2/6-31G(d) approximation. Solvent eff ects were introduced both at a semiempirical level (AMSOL) and at an a b initio level using the Pisa model (interlocking spheres) and the IPC M procedure (isodensity surface polarized continuum model). Electronic activation energies are found to be very sensitive to the treatment o f 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 kca l/mol with respect to the preceding level of correlation (MP3). Anyway , activation free enthalpies and entropies of the reactions under stud y appear to be close to the experimental values available for this cla ss of reactions. Still in agreement with experimental observations is the effect of solvent polarity on the reaction rates. Theoretical regi oselectivity is less sensitive to the level of calculation, although t he inclusion of electron correlation, both with the Moeller-Plesset te chnique and the use of the density functional theory, is able to rever se the regiochemical predictions obtained with RHF energies for the re actions of nitrile oxides with acrylonitrile. This explains why the fr ontier orbital theory, which is based on uncorrelated KF-wave function s, cannot arrive at the correct prediction of the regiochemistry in th ese cases. Calculated solvent effects appear to influence the regioche mistry of 1,3-dipolar cycloaddition, but in general, they reinforce th e prediction obtained in vacuo.