Interactions in Carbocations and Hiickel ' s 4 n + 2 Rule *

The molecular orbital model of hyperconjugation was utilized in rationalizing some long range interactions in carbocations. Conveniently such interactions can be detected by measuring secondary · kinetic deuterium isotope effects. Hilckel's 4n + 2 rule can be extended to interactions between the cationic center and pseudo-:n: orbitals of CH3, CH2 and CH groups respectively. Thus, in analogy to hyperconjugation one can envisage homohyperconjugation as the probable cause of normal y-deuterium isotope effects. Results are presented of studies of these effects in the adamantane and norbornane system. The stability of the 1-adamantyl cation can be ascribed to a favorable 4n + 2 configuration of a-electrons comprising the ~,y-carbon carbon and/or y-carbon hydrogen bonds. It is however not possible to distinguish between these alternatives since both could explain the observed y-isotope effects. In contrast, the four CH2 groups flanking C7 in the 7-norbornyl cation form an antiaromatic 4n pseudo-:n: system which could explain the exceptional instability of this cation. Interactions of the empty p-orbital with the subjacent 4B2 pseudo-:n: orbitals give rise to small normal y-deuterium isotope effects of homohyperconjugative origin. In this system, due to symmetry constraints hyperconjugative interaction with the ribbon orbitals of the a-framework is not possible.