meta-Benzyne Reacts as an Electrophile

Abstract

Examination of gas-phase reactions of various nucleophiles with meta-benzyne analogues that carry a positively charged substituent has revealed that the meta-benzyne moiety is susceptible to nucleophilic attack. Addition of nucleophiles to the meta-benzyne moiety occurs rapidly without energy-demanding uncoupling of the singlet biradicals' formally unpaired electrons. The resulting zwitterionic intermediate may undergo fragmentation either by a homolytic bond cleavage to yield net-radical type products or by a heterolytic bond cleavage that generally leads to replacement of the original charged group with the incoming nucleophile. The intermediate was characterized experimentally and computationally and found to be a low-energy species. The nonradical reactivity reported here is common for meta-benzynes with positively charged substituents, and is predicted also to dominate the chemistry of anionic and neutral meta-benzynes.

Reactivity of the 3,4,5‐Tridehydropyridinium Cation—An Aromatic σ,σ,σ‐Triradical

“…After isolation, 6 and 7 were allowed to react with reagents for varying periods of time as described previously. [9,16,21,22,27] The structures of all radical species were confirmed by using structurally diagnostic reactions. [16,21] All molecular orbital calculations were carried out with the Gaussian 98 [28] electronic structure program suite.…”

“…The gas-phase reactions of this triradical are compared to those of triradical 4, as well as to those of two related s,s-biradicals-the previously unreported 3,4-didehydropyridinium cation (7) and the previously reported 3,5-didehydropyridinium cation (8) [9] -and two previously reported s-monoradicals-3-dehydropyridinium cation (9) [10] and 4-dehydropyridinium cation (10; Table 1). [8] Upon collisional activation, the protonated triradical precursor, 3,4,5-triiodopyridinium cation, loses first the I atom at C4.…”

Chemical Properties of a para-Benzyne

“…3,[9][10][11] The isolated mono-and biradicals were allowed to react with reagents for variable periods of time. The second-order reaction rate constants (k exp ) and the reaction efficiencies (k exp /k coll ) 12 were determined as described 3,[9][10][11] previously.There is a significant amount of evidence in the literature suggesting that the mono-and biradicals studied here should be stable with respect to isomerization. 3,[9][10][11] Indeed, all of the reactions of 1-4 follow pseudo-first-order kinetics, which suggests that the radical populations are isomerically pure.…”

“…The second-order reaction rate constants (k exp ) and the reaction efficiencies (k exp /k coll ) 12 were determined as described 3,[9][10][11] previously.…”

“…These techniques can be extended to reactive neutral molecules via ions that contain the reactive group of interest and a chemically inert charged group for mass spectrometric manipulation ("distonic ion approach"). 3 One benefit associated with such experiments is that intrinsic (solventfree) properties can be determined which provides information that is crucial for the understanding of reactivity in any environment.…”