Ambimodal Bispericyclic [6+4]/[4+6] Transition State Competes with Diradical Pathways in the Cycloheptatriene Dimerization: Dynamics and Experimental Characterization of Thermal Dimers

The thermal dimerization of cycloheptatriene is predicted to occur by a concerted [6 + 4] cycloaddition via an ambimodal [6 + 4]/[4 + 6] transition state (TS) and a competing stepwise diradical (6 + 2) cycloaddition; both dimers subsequently undergo intramolecular [4 + 2] cycloadditions to afford thermally stable tetracyclic products. The ambimodal TS is the 10 pi-electron version of the prototype bispericyclic dimerization of cyclopentadiene discovered by Caramella et al. in 2002. Quantum mechanical studies using several common DFT functionals and post-HF methods, omega B97X-D, M06-2X, DLPNOCCSD(T), NEVPT2, and PWPB95-D3(BJ), and quasiclassical molecular dynamics simulations provide details of bond timing and bifurcation pathways. By comparing the ambimodal [6 + 4]/ [4 + 6] TS for cycloheptatriene dimerization with the ambimodal [4 + 2]/[2 + 4] TS of cyclopentadiene dimerization, we found that the high distortion energy in cycloheptatriene dimerization is the key to its relatively high energy barrier. The computational investigations were coupled with experimental studies of the cycloheptatriene dimerization, which resulted in the isolation of the two tetracyclic dimers. At lower temperature, the product from the predicted exo-[6 + 4]/[4 + 6] cycloaddition, followed by a subsequent intramolecular [4 + 2] cycloaddition, predominantly forms, while at higher temperature, the diradical (6 + 2) cycloadduct is the major product.

Automated summary

The thermal dimerization of cycloheptatriene can occur through a concerted [6 + 4] cycloaddition via an ambimodal [6 + 4]/[4 + 6] transition state or a stepwise diradical (6 + 2) cycloaddition. The resulting dimers undergo intramolecular [4 + 2] cycloadditions to form thermally stable tetracyclic products. The energy barrier in cycloheptatriene dimerization is attributed to its high distortion energy. Experimental studies confirmed the predicted reaction pathways.