Mechanisms for the formation of acenes from α-diketones by bisdecarbonylation

The bisdecarbonylation of bridged α-diketones has turned into an important reaction for the photochemical generation of higher acenes, in particular under matrix isolation conditions. Here, a computational mechanistic analysis of the bisdecarbonylation of dibenzobicyclo[2.2.2]octadienedione 2 to anthracene 1 is presented. The study employed the B3LYP functional in conjunction with the 6-311+G** basis set for geometry optimization on the S0, S1, and T1 potential energy surfaces as well as coupled cluster [CCSD(T)] and second-order multireference perturbation theory (MRMP2) with the cc-pVDZ basis set for evaluation of energies of intermediates and transition states. The first step of the most favorable pathway on the T1 surface has a barrier of 12 kcal mol(–1) with respect to the T1 minimum 2-3B1 and involves cleavage of the C–C bond between the bridgehead and one carbonyl atom, C(bridge)–C(O), yielding a biradical intermediate (INT1-T). On the S1 surface, the analogous step only has a barrier of less than 4 kcal mol(–1). A conical intersection of the S1 with the S0 surface exists after the transition state and provides a means for relaxation to a biradical intermediate (INT1-S) on the S0 surface. The concerted loss of two CO molecules from INT1-S has only a very small barrier. Similarly, consecutive loss of two CO molecules from the triplet state of this diradical (INT1-T) to give triplet anthracene is more favorable than ejection of triplet ethylenedione. Thus, the features identified computationally on the S0, S1, and T1 potential energy surface agree with earlier experimental observations of a fast photobisdecarbonylation within 7 ns from the triplet and singlet states of 2 and a lack of triplet ethylenedione formation.