AB INITIO STUDY OF CONCERTED CYCLOADDITIONS OF ALLENE, MONOFLUOROALLENE, AND 1,1-DIFLUOROALLENE WITH DIAZOMETHANE, FORMONITRILE OXIDE, CYCLOPENTADIENE, AND FURANE

摘要

Structures and energetics of reactants and transition structures of cycloadditions of allene (A), monofluoroallene (MFA), and 1,1-difluoroallene (DFA) with diazomethane, formonitrile oxide, cyclopentadiene, and furane have been investigated with the use of ab initio molecular orbital calculations. Structure optimizations were performed with both RHF/3-21G and RHF/6-31G* calculations but were limited to RHF/3-21G for the reactions of cyclopentadiene and furane. For these reactions as well as for the reactions involving DFA electronic energies were computed to the MP3/6-31G* level, whereas the full MP4/6-31G* level was also explored for all the other reactions. Kinetic contributions to activation enthalpies and entropies and solvent effects were also considered. Electronic activation energies are found to be very sensitive to the treatment of electron correlation and fail to converge to values unaffected by further theoretical improvements; indeed, the inclusion of full fourth order correlation (MP4) decreases the activation energies by 5-8 kcal/mol with respect to the preceding level of correlation (MP3). In contrast, the reactivity sequences of A, MFA, and DFA as well as regio and diastereoselectivities attain, in general, stable predictions even at rather low levels of calculation and agree well with experimental trends. The increasing reactivities of A, MFA, and DFA appear to be bound to the decreasing energies requested for the geometry deformation toward their transition structures. As for diastereofacial selectivity in the cycloaddition of MFA, the syn-attack has been found to be strongly favored by the more facile anti- than syn-deformability of the allene moiety, foreshown by a small anti-bending or syn-pyramidalization of the pi-atomic orbital of the C-2 carbon atom in the structure of the free MFA. Both syn-pyramidalization and the difference of syn/anti-deformability are traceable to the role of vicinal delocalizations between the C-2 C-3 pi-bond (in the free MFA) or the new-forming bonds (in the transition structures) and the allylic C-H and C-F bonds at C-1. Differences in vicinal delocalizations are also found to be significant in affecting the regiochemistry of fluoroallenes, even though electrostatic interactions appear to play a major role in this type of selectivity. [References: 45]