R. Hoffmann
Mar 1, 1968
Citations
2
Influential Citations
338
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Journal
Journal of the American Chemical Society
Abstract
Extended Hiickel calculations on a distorted cyclopropane indicate the presence of a singlet trimethylene intermediate with a CCC angle of 125”, trigonal terminal methylene groups coplanar with the carbon skeleton. This molecule has a high barrier to internal rotation and a low barrier to conrotatory reclosure to cyclopropane. The first excited configuration of trimethylene and cyclopropane is a floppy molecule with no rotational barriers. The electronic structure of trimethylene is unusual with a symmetric x-type level above an antisymmetric combination. Similar level orderings, implying conrotatory closing and concerted 1,2 addition, are found in other “1,3 dipoles.” The potential surface for the addition of methylene to ethylene is explored in detail. The most symmetrical approach is symmetry forbidden, and the reaction path is unsymmetrical. It begins as a x approach and terminates as U. Because of the electronic structure of trimethylene it is possible for this unsymmetrical approach to be stereospecific. The specificity of singlet and triplet methylene additions is attributed not to the difference in spin, but to the difference in the spatial part of the wave function. The ring-opened form of cyclopropanone has an electronic structure different from that of trimethylene and other 1,3 dipoles, It is consistent with the valence-bond formulation of an oxy anion of allyl cation. A consequence of this electronic structure is a disrotatory closure back to the cyclopropanone and propensity to concerted 1,4 addition. The extended Hiickel calculations make cyclopropanone and allene oxide unstable with respect to oxyallyl (the ring-opened form). In fact they give no stability for cyclopropanone with respect to conversion to oxyallyl. A x-electron SCF-CI calculation has the ground state of oxyallyl, a triplet, with a singlet only 0.1 eV above. n this paper two aspects of cyclopropane chemistry I are discussed: the question of the existence and electronic structure of a trimethylene intermediate CH2CH2CH2, and the detailed transition-state geometry and specificities observed in the addition of methylenes to ethylenes. An important and connected problem, the cis-trans thermal isomerization of substituted cyclopropanes and the competing rearrangement to propylenes, has not yet been considered in detail.