MOLECULAR-ORBITAL THEORY OF THE ELECTRONIC-STRUCTURE OF MOLECULES .39. HIGHLY UNUSUAL STRUCTURES OF ELECTRON-DEFICIENT CARBON-COMPOUNDS - REVERSAL OF VANT-HOFF STEREOCHEMISTRY IN BBC-RING SYSTEMS

It may be possible to violate all of van't Hoffs stereochemical rules! When two geminal hydrogens of methane, of ethylene, of allene, or of butatriene are replaced by a three-membered ring comprised of two BH groups, ab initio molecular orbital calculations indicate preference for anti-van't Hoff geometries: planar (HB)iCH7 (6b), perpendicular (HB)iC=CH2(7b), planar (HB)2C=C=CH2 (8b), and perpendicular (HB)2C=C=C=CH2 (9b). These forms are estimated to be 15-20 kcal/mol more stable than the van't Hoff alternatives, 6a-9a.The van't Hoff forms (6a-9a) exhibit classical Lewis twocenter-two-electron bonding, with six a electrons for the three diboracyclopropane ring bonds. In the anti-van't Hoff forms (6b-9b),the same rings have only four a electrons; the two remaining electrons occupy an aromatic, cyclopropenium-ion-like 7r orbital. The geometrical consequences are shortening of the BB bonds and widening of the HBB angles in 6b-9b over 6a-9a, and, most particularly, retention of the exocyclic C1=C2 double bonds in the anti-van't Hoff forms(7b-9b).Both the perpendicular ethylene 7b and its less stable planar isomer 7a are found to be local minima, with a rotation barrier between them. Triplet forms of 6-9 do not appear to be competitive in energy with the singlets. Prospects for the experimental verification of these predictions are analyzed. Isolobal transition metal analogues may be best suited for this purpose. © 1979, American Chemical Society. All rights reserved.