An ab initio exploration of the Bergman cyclization
The Bergman cyclization is an important reaction in which an enediyne cyclizes to produce a highly reactive diradical species, p-benzyne. Enediyne motifs are found in natural antitumor antibiotic compounds, such as calicheammicin and dynemicin. Understanding the energetics of cyclization is required in order to better control the initiation of the cyclization, which induces cell death. The singlet and triplet potential energy surfaces for the Bergman cyclization of (Z)-hex-3-ene-1,5-diyne have been computed by the CCSD and EOM-SF-CCSD methods. The triplet enediyne and transition state were found to have C2 symmetry, which contrasts with the singlet reactant and transition state that possess C2v symmetry. The frontier orbitals of both cyclization pathways were analyzed to explain the large energetic barrier of the triplet cyclization. Reaction energies were calculated using CCSD(T)/cc-pVTZ single point calculations on structures optimized with CCSD/cc-pVDZ. The singlet reaction was found to be slightly endothermic (Hrxn = 13.76 kcal/mol) and the triplet reaction was found to be highly exothermic (Hrxn = -33.29 kcal/mol). The adiabatic singlet-triplet gap of p-benzyne, computed with EOM-SF-CCSD/cc-pVTZ, was found to be 3.56 kcal/mol, indicating a ground state singlet.