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

Abstract

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.

Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation

“…An investigation of the vibrational normal modes of N • + shows that it has four low frequencies (377, 385, 395 and 460 cm −1 ). The Table 2 Relative electronic energies and Gibbs energies (in kcal mol −1 ) for all stationary points involved in the three isomerization pathways from N normal mode of 385 cm −1 , for example, deforms the structure towards the pseudo-Jahn-Teller distorted transition structure between N • + and Q • + described previously 9, 16 ; and the 395 cm −1 mode describes the C 2v symmetric motion towards the conical intersection. 9 The normal modes responsible for cleavage of the bridgehead-methylene bonds, i.e.…”

“…As has been noted before for radical cation systems (see, e.g., ref. 16), the SCF reference wavefunctions for most of the stationary points optimized with MP2/6-311+G(d,p) are found to be heavily spin-contaminated: 0.763 (N • + ) ≤ S 2 ≤ 1.023 (CHT • + ) for the minima and 0.905 (TS2) ≤ S 2 ≤ 1.147 (TS1) for the transition structures. In contrast, for B3LYP the S 2 values are 0.753-0.765 for the minima and 0.754-0.773 for the transition structures.…”

“…15 Also, starting from BHE • + , a multistep isomerization pathway leading to the formation of CHT • + was identified. At the above mentioned level of theory, the ratelimiting step on this path has a barrier of 16.5 kcal mol −1 above Q • + , whereas the barrier was found 16 to be higher (19.3 kcal mol −1 ) when BHE • + rearranges to CHT • + via BHD • + . The novel distonic BHE • + intermediate is characterized, moreover, by a homoaromatic stabilization in a three-centre two-electron bond.…”

“…Recently, we have undertaken computational studies 15 for the isomerization reactions of Q • + to the bicyclo[3.2.0]hepta-2,6-diene radical cation (BHD • + ), as well as for the further rearrangement 16 to CHT • + (see Scheme 1). Using quantum chemical B3LYP, MP2 and CCSD(T) methods, 17 a mechanism was proposed for the skeletal rearrangement of Q • + to BHD • + , which was found to occur via a stable intermediate, the bicyclo[2.2.1]hepta-2ene-5-yl-7-ylium radical cation (BHE • + ).…”

Resonance theory of catalytic action of transition‐metal complexes: Isomerization of quadricyclane to norbornadiene catalyzed by metal porphyrins

“…In this case, the estimated barrier is only 4.8 kcal/mol . The isomerization of the QC cation to the NB cation is well studied in the literature . The reaction path maintaining the C 2 v symmetry implies that the highest point on the potential energy profile is a conical intersection.…”