Reactivity of a Substituted m-Benzyne Biradical

Fourier transform ion cyclotron resonance mass spectrometry has been employed to systematically investigate the intrinsic (solvent-free) reactivity of a 1,3-dehydrobenzene ( m -benzyne) with a pyridinium charge site in the 5-position. The m -benzyne was generated by using a combination of ion−molecule reactions and photodissociation and isolated prior to examination of its gas-phase reactions. The ionic reaction products and reaction efficiencies (second-order reaction rate constant/collision rate constant) were compared to those measured for the isomeric o -benzyne and the analogous phenyl monoradical. The m -benzyne yields some of the products formed for the o -benzyne but it also reacts via distinct radical pathways characteristic of the corresponding phenyl radical. These radical pathways are not observed for the o -benzyne. However, the reaction efficiencies measured for the m -benzyne are significantly lower than those measured for the analogous phenyl radical or the isomeric o -benzyne. These findings are partially rationalized by the relatively strong coupling (about 21 kcal mol - 1 ) between the two formally unpaired electrons in the m -benzyne that hinders radical reactions. On the other hand, the greater distance between the reactive sites in the m -benzyne makes alkyne-type addition reactions sterically and energetically less favorable than for the o -benzyne.