Ab initio study of the Wolff rearrangement of C6H4O intermediate in the gas phase

Ab initio calculations of the geometry and re-activity of 1,2-ketocarbene C 6H4O as an intermediate in organic reactions were performed using the secon d-order Moller-Plesset (MP2) perturbation theory in the 6-311G* basis set. Only the singlet state of the intermediate was considered. An oxirene-like structure (6) with a six-membered ring and a ketene-like structure (5) with a five-membered ring were localized on the potential energy surface. Attem pts to locate a quinone type structure characteristic of aliphatic ketocarb enes failed. The energy of structure 5 is similar to 70 kcal mol(-1) lower than that of structure 6. Harmonic frequencies and intensities of normal vi brations in the IR spectra of 6 and 5 were calculated. The activation energ y of the Wolff rearrangement 6 --> 5 was estimated at 12.5 kcal mol(-1). Th e geometry of the transition state of this reaction resembles the quinone-l ike structure.