Ab initio study of the effect of α-substituents on the acidity of cyclopropabenzene

The effect of substituents directly bound to the deprotonation site (C-7) on the acidity of cyclopropabenzene was studied using the MP2/6-31+G* method. The studied substituents encompass a variety of pi- and a-accepting groups including the highly electronegative fluorine atom. It is shown that all substituents enhance the acidity of cyclopropabenzene with the effect being the smallest (0.02 kcal mol(-1)) for the methyl group and the largest (33.7kcal mol(-1)) for the hydrosulfonyl group. It is further shown that the alpha-subsituents employed stabilize the cyclopropabenzenyl anion less efficiently than the cyclopropenyl anion, with the effect being more pronounced for the substituents acting via inductive/field effects. This is attributed to the fact that attachment of inductively/field acting substiments to the carbanionic site predominantly stabilize the cyclopropenyl anion by increasing the s character of the lone pair, diminishing the antiaromatic character of the three-membered ring at the same time. Hence these two effects are operative in concert. The opposite occurs in related cyclopropabenzenyl anions. Here, the rehybridization at the carbanionic center does stabilize the lone pair, but decreases the anionic resonance of the whole system, because of a decrease in overlap between the lone pair and the pi-AO of the annelated bond. The reverse picture holds for the conjugatively acting substituents. Copyright (c) 2005 John Wiley & Sons, Ltd.