Intramolecular nucleophilic SN2 substitution at the tetrahedral carbon atom:: an ab initio study

Pentacoordination of carbon atom in bicyclic organic compounds of the pentalene type was studied by the ab initio RHF/6-31G** and MP2(full)/6-31G** methods. It was shown that intramolecular S(N)2 reactions with energy barriers within the energy scale of NMR spectroscopy can occur in systems in which a linear orientation of the attacking and leaving groups is realized. The barrier to the intramolecular nucleophilic substitution reaction in 2,3-dihydro-3-formylmethylenefuran is 36.9 (RHF) and 27.7 kcal mol(-1) (MP2) and decreases to 16.4 and 19.4 kcal mol(-1), respectively, in the case of diprotonation at the O atoms in this system. For model pentalene type compounds containing electron-deficient B atoms in the ring, the ab initio calculations predict a further decrease in the barrier height (down to less than 10 kcal mol(-1)).