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

Structures and energetics of reactants and transition structures of cy cloadditions of allene (A), monofluoroallene (MFA), and 1,1-difluoroal lene (DFA) with diazomethane, formonitrile oxide, cyclopentadiene, and furane have been investigated with the use of ab initio molecular orb ital calculations. Structure optimizations were performed with both RH F/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 we ll as for the reactions involving DFA electronic energies were compute d to the MP3/6-31G level, whereas the full MP4/6-31G* level was also explored for all the other reactions. Kinetic contributions to activat ion 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 unaff ected by further theoretical improvements; indeed, the inclusion of fu ll 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 prediction s even at rather low levels of calculation and agree well with experim ental trends. The increasing reactivities of A, MFA, and DFA appear to be bound to the decreasing energies requested for the geometry deform ation toward their transition structures. As for diastereofacial selec tivity in the cycloaddition of MFA, the syn-attack has been found to b e strongly favored by the more facile anti- than syn-deformability of the allene moiety, foreshown by a small anti-bending or syn-pyramidali zation of the pi-atomic orbital of the C-2 carbon atom in the structur e of the free MFA. Both syn-pyramidalization and the difference of syn /anti-deformability are traceable to the role of vicinal delocalizatio ns between the C-2 C-3 pi-bond (in the free MFA) or the new-forming bo nds (in the transition structures) and the allylic C-H and C-F bonds a t C-1. Differences in vicinal delocalizations are also found to be sig nificant in affecting the regiochemistry of fluoroallenes, even though electrostatic interactions appear to play a major role in this type o f selectivity.