A valence bond study of the Bergman cyclization: Geometric features, resonance energy, and Nucleus-Independent Chemical Shift (NICS) values

The Bergman cyclization of (Z)-hex-3-ene-1,5-diynes (1, enediynes). which produces pharmacologically important DNA-cleaving biradicals (1,4-benzyne, 2), was studied by using Hartree-Fock (HF) and density-functional theory (DFT) based valence bond (VB) methods (VB-HF and VB-DFT, respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space {CASSCF(6 × 6)} computations, while the quality of VB-DTF utilizing the same three configurations improves upon CASSCF(6 × 6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2, while the most important biradical configuration in 2 plays a negligible role in 1. The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state (TS). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate are non-synchronous; the TS is geometrically ≈80% productlike and electronically ≈70% reactantlike. While the π resonance in the TS is very small, it is large (64.4 kcal mol^-1) for 2 (cf. benzene = 61.5 kcal mol^-1). As a consequence, suhstituents operating on the σ electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional σ resonance in 2 results in unusually high values for the nucleusindependent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to σ resonance to which the NICS procedure is relatively sensitive.