engleski

Ab initio Study of ­­­Alpha-Substituents on Acidity of Benzocyclopropene

The effect of substituents directly bound to the deprotonation site (C7) on acidity of cyclopropabenzene has been studied using the MP2/6-31+G* method. The studied substituents encompass a variety of π - and σ -accepting groups including highly electronegative atomic fluorine. It is shown that all substituents significantly enhance acidity of cyclopropabenzene with one notable exception being the methyl group. The largest enhancement is exerted by the hydrosulfonyl (SO2H) (33.7 kcal mol-1) group. It is further shown that the employed alpha-subsituents stabilize cyclopropabenzenyl anion more 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 substituents to the carbanionic site predominantly stabilize the cyclopropenyl anion by increasing 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 overlapping between the lone pair and the π -AO of the annelated bond. The reversed picture holds for the π -accepting substituents.

ab initio calculations; basicity cyclopropenyl anions; cyclopropabenzenyl anions; substituent effects

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