Tetracoordinate carbon as a nucleophile? Interconversion of carbenium ions with carbonium ions possessing nearly square-pyramidal pentacoordinate carbons.

[structure: see text] Quantum chemical calculations (B3LYP and Moeller-Plesset second-order perturbation theory) on carbonium ions containing hypercoordinated carbon atoms in distorted square-pyramidal geometries are described. The importance of overall charge (in addition to the delocalization of a certain number of electrons) for the stability of such structures is explored through calculations on isoelectronic systems containing boron atoms. The effects of alkyl substitution as well as variations in the structure of the linker attached to the C(5) core substructure are explored systematically for a variety of structures that do not have severe geometric constraints that rigidly enforce unusual geometries. In addition, transition structures for reactions involving the intramolecular attack of tetracoordinate carbons on carbenium ion centers were located; in some cases, such carbenium-to-carbonium rearrangement processes (in which tetracoordinate carbon centers act as nucleophiles) are actually both exothermic and accompanied by low activation barriers.