Quadricyclane Radical Cation Rearrangements: A Computational Study of the Transformations to 1,3,5‐Cycloheptatriene and Norbornadiene

An alternative skeletal rearrangement of the quadricyclane radical cation ( Q . + ) explains the side products formed in the one‐electron oxidation to norbornadiene. First, the bicyclo[2.2.1]hepta‐2‐ene‐5‐yl‐7‐ylium radical cation, with an activation energy of 14.9 kcal mol −1 , is formed. Second, this species can further rearrange to 1,3,5‐cycloheptatriene through two plausible paths, that is, a multistep mechanism with two shallow intermediates and a stepwise path in which the bicyclo[3.2.0]hepta‐2,6‐diene radical cation is an intermediate. The multistep rearrangement has a rate‐limiting step with an estimated activation energy of 16.5 kcal mol −1 , which is 2.8 kcal mol −1 lower in energy than the stepwise mechanism. However, the lowest activation energy is found for the Q . + cycloreversion to norbornadiene that has a transition structure, in close correspondence with earlier studies, and an activation energy of 10.1 kcal mol −1 , which agrees well with the experimental estimate of 9.3 kcal mol −1 . The computational estimates of activation energies were done using the CCSD(T)/6–311+G(d,p) method with geometries optimized on the B3LYP/6–311+G(d,p) level, combined with B3LYP/6–311+G(d,p) frequencies.