CCSD(T) INVESTIGATION OF THE BERGMAN CYCLIZATION OF ENEDIYNE - RELATIVE STABILITY OF O-DIDEHYDROBENZENE, M-DIDEHYDROBENZENE, AND P-DIDEHYDROBENZENE

The energetics of the Bergman cyclization of (Z)-hexa-1,5-diyn-3-ene ( 1) to 1,4-didehydrobenzene (2) have been investigated at the CCSD(T)/6 -31G(d,p) level of theory, optimizing various structures along the rea ction path. The calculated reaction energy and classical barrier are 5 .5 and 29.5 kcal/mol, respectively. Vibrational and temperature correc tions lead to a reaction enthalpy, Delta(R)H(298), of 8.0 kcal/mol and an activation enthalpy, Delta H-double dagger(298), of 28.5 kcal/mol, indicating that previous determinations of these energies are serious ly in error. Although the transition state of the reaction is located in the exit channel, it shows no biradical character, which suggests t hat the electronic structure of 1 is retained as long as possible. Ene rgy changes in the entrance channel of the reaction are influenced by changes in strain caused by trans bending of the two triple bonds of 1 . If the terminal C atoms are pulled together to a distance of 3.0 Ang strom, the reaction becomes spontaneous (Delta H-double dagger (298) = 19.3 kcal/mol) and slightly exothermic (Delta(R)H(298) = -0.8 kcal/mo l) due to an increase of bending strain by 8.8 kcal/mol. For 1,2-dideh ydrobenzene (11), 1,3-didehydrobenzene (12), and 1,4-didehydrobenzene (2), relative energies are calculated to be 0, 13.6, and 25.4 kcal/mol , respectively, while the corresponding Delta H-f degrees(298) values are 108.7, 122.8, and 134.4 kcal/mol, respectively, at the CCSD(T)/6-3 1G(d,p) level of theory.