On the factors that control the reactivity of meta-benzynes

The reactivities of eleven 3,5-didehydropyridinium and six 2,4-didehydropyridinium cations toward cyclohexane were examined in the gas phase by using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry as well as high-level quantum chemical calculations. The results unequivocally demonstrate that the reactivity of meta-benzyne analogs can be "tuned" from more radical-like to less radical-like by changing the type and position of substituents. For example, sigma-acceptor substituents at the 4-position and pi-donor substituents at the 2-position in 3,5-didehydropyridinium cations partially decouple the biradical electrons, which results in lower energy transition states, and faster radical reactions. In contrast, sigma-acceptors at the 2-position and pi-donors at the 4-position in 3,5-didehydropyridinium cations cause stronger coupling between the biradical electrons, which results in lower radical reactivity. Three main factors are found to control the reactivity of these biradicals: (1) the energy required to distort the minimum energy dehydrocarbon atom separation to the separation of the transition state, (2) the S-T splitting at the separation of the transition state, and (3) the electron affinity at the separation of the transition state.