The reactivity and stability studies of benzoquinone methides by ab initiocalculations

The conformations of symmetrically hindered benzoquinone methides (=BMs), u nsymmetrically hindered BMs, and simple BM have been optimized using densit y functional theories. It is shown that the optimized geometries of symmetr ically hindered BMs are fully in accord with those expected for the plannar conformations, in which the effective hyperconjugation of symmetrically su bstituted dialky groups with ring can occur. Relative stabilization energie s calculated at the B3LYP/6-31G*//B3LYP/6-31* level by means of isodesmic e quation are 2.8-5.3 kcal/mol enhanced for symmetrically hindered BMs. This finding provides a rationalization for the previous experimental results th at the BM formation is depend upon the substituents.