Experimental and Computational Studies on the Formation of Three para‐Benzyne Analogues in the Gas Phase

Abstract

Experimental and computational studies on the formation of three gaseous, positively-charged para-benzyne analogues in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer are reported. The structures of the cations were examined by isolating them and allowing them to react with various neutral reagents whose reactions with aromatic carbon-centered σ-type mono- and biradicals are well understood. Cleavage of two iodine-carbon bonds in N-deuterated 1,4-diiodoisoquinolinium cation by collision-activated dissociation (CAD) produced a long-lived cation that showed nonradical reactivity, which was unexpected for a para-benzyne. However, the reactivity closely resembles that of an isomeric enediyne, N-deuterated 2-ethynylbenzonitrilium cation. A theoretical study on possible rearrangement reactions occurring during CAD revealed that the cation formed upon the first iodine atom loss undergoes ring-opening before the second iodine atom loss to form an enediyne instead of a para-benzyne. Similar results were obtained for the 5,8-didehydroisoquinolinium cation and the 2,5-didehydropyridinium cation. The findings for the 5,8-didehydroisoquinolinium cation are in contradiction with an earlier report on this cation. The cation described in the literature was regenerated by using the literature method and demonstrated to be the isomeric 5,7-didehydro-isoquinolinium cation and not the expected 5,8-isomer.

Polar effects control the gas-phase reactivity of charged para-benzyne analogs

“…When the ions having m/z values of 1 and 2 were allowed to react with each of the neutral reagents, fast proton transfer was observed, indicative of the presence of the enediynes 3 and 4 , like in the earlier study [17]. The only exception was cyclohexane, which has a proton affinity too low to accept a proton from either protonated enediyne [17]. However, a second, slowly reacting isomer was also observed in each case (with the exception of cyclohexane for which only one isomer was detected).…”

“…96–98% of the ion population corresponds to the enediyne, 3 . Previous calculations have shown that during the generation of these para -benzyne analogs, ring opening occurs upon CAD of the monoradical precursors formed via cleavage of the first C-I bond from the diiodoprecursors [17]. Hence, the cause for the large difference in the amounts of the enediyne isomers is likely the difference in the ring-opening barriers of the relevant monoradicals [17].…”

“…Previous calculations have shown that during the generation of these para -benzyne analogs, ring opening occurs upon CAD of the monoradical precursors formed via cleavage of the first C-I bond from the diiodoprecursors [17]. Hence, the cause for the large difference in the amounts of the enediyne isomers is likely the difference in the ring-opening barriers of the relevant monoradicals [17]. Indeed, the calculated ring opening barrier for the monoradical iodoprecursor of 1 , 53.5 kcal mol −1 , is lower than that for 2 , 56.8 kcal mol −1 (both calculated at the (U)B3LYP/6–311G(d,p)//(U)B3LYP/6–311G(d,p) level of theory).…”

“…This approach has been successfully utilized to characterize the reactivity of many meta -benzyne analogs in Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometers [14–16]. Unfortunately, the generation of charged para -benzyne analogs in FT-ICR instruments has been unsuccessful [17]. For example, the attempted generation of the radical sites in 1 and 2 (Scheme 1) via the cleavage of two iodine atoms in 2,5-diiodopyridinium and 5,8-diiodoisoquinolinium cation precursors, respectively, involved two collision-activated dissociation (CAD) events to cleave the C-I bonds in a consecutive manner.…”

“…For example, the attempted generation of the radical sites in 1 and 2 (Scheme 1) via the cleavage of two iodine atoms in 2,5-diiodopyridinium and 5,8-diiodoisoquinolinium cation precursors, respectively, involved two collision-activated dissociation (CAD) events to cleave the C-I bonds in a consecutive manner. However, the monoradical intermediates formed upon loss of one iodine atom were found to undergo ring-opening faster than iodine atom loss upon CAD, eventually forming enediynes ( 3 and 4 ; Scheme 1) instead of the isomeric para -benzynes [17]. …”