Ab initio molecular orbital calculations on allylic anion–olefin interactions

Abstract

Interaction of an allylic anion with an olefin has been studied by ab initio STO 3G (Gaussian 76) calculations. Approach of the two entities leads to an increase in the total energy of the system, but in all the configurations examined the HOMO energy decreases down to a separation of 2.4 Å. The calculations are used as a model to evaluate the extent of homoaromatic stabilisation in the bicyclo[3.2.1]octa-3,6-dien-2-yl anion (2). It was found that the bicyclic anion has a HOMO energy lower than that of an isolated allyl anion. Sphere-charge calculations suggest that negative charge is delocalised on to the ethylenic fragment of anion (2). These calculations provide support for the original postulate of long-range delocalisation in anion (2), and refute the recent claims by Grutzner and Jorgensen, and by Mayr, Schleyer, and their co-workers, that stabilisation of homoaromatic anions is due to an inductive effect.