Nucleophilic Addition to Singlet Diradicals: Heterosymmetric Diradicals

Abstract

In the preceding paper, we examined the addition of nucleophiles to homosymmetric diradicals and showed that the reaction occurs with no symmetry restrictions or other electronic impediments. In this work, we examine the addition of nucleophiles to heterosymmetric diradicals, by using the addition of chloride to m-dehydrotoluene as a case study. Using CASPT2 and density functional theory calculations, we show that the addition of chloride to m-dehydrotoluene is predicted to be barrierless at the asymptotic limit if C symmetry is broken, and the reaction is allowed to proceed through a nonplanar geometry. A nonplanar cyclic allene acts as the transitioning structure between open-shell and closed-shell species for the addition of chloride, with a continuous and smooth changing of the wave function by the evolution of orbital and configuration interaction coefficients, such that there is no abrupt switch from diradical to closed-shell species along the reaction coordinate. The overall conclusion from our analysis is that both homosymmetric and heterosymmetric diradicals can undergo reaction with closed-shell reagents without a barrier, and one cannot rule out the direct addition of nucleophiles to diradicals when considering the reaction mechanism.

Nucleophilic Addition to Diradicals Derived From Cycloaromatization of Maleimide‐Based Enediynes

“…The energy of the triplet state for the diradical structure 4A* was also calculated, and it is only 0.7 kcal/mol higher than that of singlet state, thus indicating that the triplet state would not affect the addition pathways present. Unlike the observation of the barrierless addition to α,3‐dehydrotoluene with chloride ion in Winter's work, [26] the Gibbs free energy barrier presented here may arise from the creation of charge separation along the cyclization path from the neutral system or the broken aromaticity required (discussed below). Moreover, the placement of nucleophilic and electrophilic reactivity in 5B* could be determined from the molecular orbital calculations.…”

“…Considering the nucleophilic addition to heterosymmetric σ,π‐diradicals, the configuration change from open‐shell to closed‐shell species would be allowed unless the symmetry of the system is broken [31] . As expected, the nonplanar cyclic allene geometry as the transition structure is involved along the addition reaction, in which the unimolecular addition process is similar to that of the reaction of α,3‐dehydrotoluene with an additional nucleophile [26] …”

“…Apart from the mentioned case of nucleophilic addition to diradicals, the suggested alternative pathway toward 5B* involving the cyclic‐allene‐like structure 4C* was indeed not well recognized yet [26] . Especially in this case, it was promoted by the unique intramolecular addition where the carbonyl group within the molecule behaved as the nucleophile.…”

“…[25] Similarly, they investigated the addition process to heterosymmetric diradicals as a representative of intermediates produced from MSC of enyne-allenes. [26] Unlike the former case, a nonplanar cyclic allene geometry has been illustrated during the approach of a chloride ion to the radical center because the intrinsic orbital symmetry is broken upon the addition process. Thus, these theoretical results may provide an explanation of the addition product observed in the reaction of α,3-dehydrotoluene formed from MSC with methanol.…”

“…demonstrated that the addition of nucleophiles to homosymmetric diradicals can proceed without any symmetry restrictions or reaction barrier according to the high‐level multireference computations [25] . Similarly, they investigated the addition process to heterosymmetric diradicals as a representative of intermediates produced from MSC of enyne‐allenes [26] . Unlike the former case, a nonplanar cyclic allene geometry has been illustrated during the approach of a chloride ion to the radical center because the intrinsic orbital symmetry is broken upon the addition process.…”