Reactivity of the 4,5‐Didehydroisoquinolinium Cation

Abstract

The chemical properties of a 1,8-didehydronaphthalene derivative, the 4,5-didehydroisoquinolinium cation, were examined in the gas phase in a dual-cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. This is an interesting biradical because it has two radical sites in close proximity, yet their coupling is very weak. In fact, the biradical is calculated to have approximately degenerate singlet and triplet states. This biradical was found to exclusively undergo radical reactions, as opposed to other related biradicals with nearby radical sites. The first bond formation occurs at the radical site in the 4-position, followed by that in the 5-position. The proximity of the radical sites leads to reactions that have not been observed for related mono- or biradicals. Interestingly, some ortho-benzynes have been found to yield similar products. Since ortho-benzynes do not react via radical mechanisms, these products must be especially favorable thermodynamically.

Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

“…253 72 (S-T splitting = −0.6 kcal/mol) is a positively charged analog of 1,8-didehydronapthalene, which has shown interesting chemical properties 254–257 attributed to its nearly degenerate singlet and triplet states (S-T splitting = −0.9 kcal/mol). 258 Examination of the gas-phase reactions of 72 suggested that they mostly occur via radical mechanisms, as opposed to the meta -benzyne analogs discussed above.…”

“…Hence, the C4 radical site was experimentally proven to abstract the first hydrogen atom from cyclohexane, followed by the C5 radical site. 253 Analogous to the reactions with tetrahydrofuran and cyclohexane, the reactions of 72 with dimethyl disulfide and allyl iodide are likely to be initiated by atom or group abstraction by the C4 radical site.…”

Reactivity Controlling Factors for an Aromatic Carbon‐Centered σ,σ,σ‐Triradical: The 4,5,8‐Tridehydroisoquinolinium Ion

“…These results are in agreement with previouss tudies on relatedb iradicals. [3,4,6,21] However,c ompound 4 is less reactivet han monoradicals 1 and 3 toward almost all of the reagents studied. Only the least reactive monoradical 2 shows similar reaction efficiencies to that of 4.T he low reactivity of singlet (ground state) biradicals has been noted previously, [4-6, 23, 25] and is typically due to stabilizing interactions (i.e.,c oupling) between the two nonbonding electrons.…”

“…Previouss tudies have revealed that the C4 radical site is more reactive than that of C5 in biradical 5 due to its greater EA. [21] To identify which radicals itei nt riradical 7 reacts first, the first hydrogen atom abstraction product of 7 (formed upon reaction with cyclohexane) was isolateda nd allowed to react with cyclohexane( Scheme2). The measured reaction efficiency is 3%,w hichi sc omparable to that for the reaction of biradical 4 with cyclohexane (2 %), and quite different from that of 5 (18 %) or 6 (0.002 %a sd etermined in al inear quadrupole ion trap mass spectrometer [28] ).…”

“…An excess of HNO 3 was added at À3 8Ct ot he brown solution. The solution was stirred for 15 min at this temperature, allowed to warm to room temperature, and then stirred for an additional 2h.T he solution was poured into ice and made basic with aqueous NH 3 .A fter filtration and chromatography on silica gel (20:80 ethyl acetate/hexanes), 4-iodo-5-nitroisoquinoline [21] (60 mg) and 4-iodo-8-nitroisoquinoline (10 mg) were obtained. 4-Iodo-8-nitroisoquinoline: 1 The precursor for triradical 7,4 ,5,8-triiodoisoquinoline, was synthesized in the following manner.4 ,5,8-Tribromoisoquinoline [31] (0.6 g, 1.64 mmol) in THF (3 mL) was added slowly to as olution of THF (40 mL) and nBuLi (2.5 m in hexanes;4 .3 mL, 10.8 mmol) at À78 8C under nitrogen at ar ate that kept the solution temperature at or below À60 8C.…”