Spin–Spin Coupling Between Two meta‐Benzyne Moieties In a Quinolinium Tetraradical Cation Increases Their Reactivities

Abstract

The reactivity of a carbon‐centered σ,σ,σ,σ‐type singlet‐ground‐state tetraradical containing two meta‐benzyne moieties was examined in the gas phase. Surprisingly, the tetraradical showed higher reactivity than its individual meta‐benzyne counterparts. The reactivity of meta‐benzynes is controlled by their (calculated) distortion energy ΔE2.3, singlet–triplet spitting ΔES–T, and electron affinity (EA2.3) of the meta‐benzyne moiety at the transition state geometry for hydrogen‐atom abstraction reactions. The addition of a second meta‐benzyne moiety to a meta‐benzyne does not significantly change EA2.3. However, ΔE2.3 is substantially decreased for both meta‐benzyne moieties in the tetraradical, and this explains their higher reactivities. The decrease in ΔE2.3 for each meta‐benzyne moiety in the tetraradical is rationalized by stabilizing spin–spin coupling between one radical site in each meta‐benzyne moiety. Therefore, spin–spin coupling between the meta‐benzyne moieties in this tetraradical increases its reactivity, whereas spin–spin coupling within each meta‐benzyne moiety decreases its reactivity.

Spin‐Spin Coupling between the Biradical Moieties in Aromatic Tetraradicals Increases their Reactivity

“…However, 1 has a substantial ΔE S-T (À 22.2 kcal mol À 1 for the 5,7-moiety; Table 1), which was initially expected to greatly hinder its radical reactions but was found not to do so. Similar findings were made previously [10] for the isomeric tetraradical 2 that also contains two but different meta-benzyne moieties (Table 1).…”

“…A σ,σ,σ,σ-type singlet tetraradical cation containing two metabenzyne moieties, the 2,4,5,7-tetradehydroquinolinium cation, was found to be more reactive than the related meta-benzynes in the gas phase. A similar finding was made previously [10] for an isomeric tetraradical, the 2,4,6,8-tetradehydroquinolinium cation. This enhanced reactivity of both tetraradicals is rationalized based on a decrease in the distortion energies (ΔE 2.30 ) of their meta-benzyne moieties, making these moieties, and hence the tetraradicals themselves, more reactive.…”

“…For the lowest-energy structure, both the meta-benzyne moieties have a DAS of 1.4-1.5 Å. This differs drastically from the potential-energy surface calculated [10] earlier for 2: in this case, the lowest-energy (global) minimum occurs at a geometry in which the 2,4-meta-benzyne moiety is bicyclic (DAS: 1.5 Å) but the 6,8-meta-benzyne moiety almost has the DAS (2.1 Å) of the TS geometry (2.3 Å). These potential energy surfaces help rationalize the greater reactivity of 2 than 1 as the 6,8meta-benzyne moiety in 2 already has a geometry very similar to the TS for radical reactions.…”

Spin‐Spin Coupling between the Biradical Moieties in Aromatic Tetraradicals Increases their Reactivity

“…However, 1 has a substantial ΔE S-T (À 22.2 kcal mol À 1 for the 5,7-moiety; Table 1), which was initially expected to greatly hinder its radical reactions but was found not to do so. Similar findings were made previously [10] for the isomeric tetraradical 2 that also contains two but different meta-benzyne moieties (Table 1).…”

“…A σ,σ,σ,σ-type singlet tetraradical cation containing two metabenzyne moieties, the 2,4,5,7-tetradehydroquinolinium cation, was found to be more reactive than the related meta-benzynes in the gas phase. A similar finding was made previously [10] for an isomeric tetraradical, the 2,4,6,8-tetradehydroquinolinium cation. This enhanced reactivity of both tetraradicals is rationalized based on a decrease in the distortion energies (ΔE 2.30 ) of their meta-benzyne moieties, making these moieties, and hence the tetraradicals themselves, more reactive.…”

“…For the lowest-energy structure, both the meta-benzyne moieties have a DAS of 1.4-1.5 Å. This differs drastically from the potential-energy surface calculated [10] earlier for 2: in this case, the lowest-energy (global) minimum occurs at a geometry in which the 2,4-meta-benzyne moiety is bicyclic (DAS: 1.5 Å) but the 6,8-meta-benzyne moiety almost has the DAS (2.1 Å) of the TS geometry (2.3 Å). These potential energy surfaces help rationalize the greater reactivity of 2 than 1 as the 6,8meta-benzyne moiety in 2 already has a geometry very similar to the TS for radical reactions.…”

Do Diradicals Behave Like Radicals?

“…If the diradical (easily generated by chemical ionization of an iodo-substituted precursor) is charged, its reactions can be studied in great detail mass spectrometrically. Studies of Kenttämaa and co-workers have in this fashion provided us with much carefully reasoned experimental detail on m -benzyne reactivity. − These molecules behave primarily as radicals but can be induced to react as electrophiles upon heteroatomic substitution . These reactivity patterns are in line with the expectations resulting from our general analysis outlined in section .…”

Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

“…In order to examine the efficiencies of specific reactions (i.e., H atom and I atom abstractions), the H and I atom abstraction efficiencies (total efficiency × branching ratio of atom abstraction) for cyclohexane and allyl iodide were plotted as a function of the (calculated) EA v for a series of (iso)­quinolinium-based mono- and biradicals (Figures and ; note that only those biradicals that underwent major I atom abstraction were included in the I atom abstraction plot). , The monoradicals showed a linear correlation for both reactions (Figures and ; note that corresponding plots of the natural logarithm of the H or I atom abstraction efficiency vs EA v (data not shown) gave fits almost as good). While a correlation between the natural logarithm of H atom abstraction efficiency and EA v for related monoradicals has been published previously, no correlations have been reported for I atom abstraction reactions of radicals similar to those described here.…”

Spin‐Spin Coupling between the Biradical Moieties in Aromatic Tetraradicals Increases their Reactivity

“…However, 1 has a substantial ΔE S-T (À 22.2 kcal mol À 1 for the 5,7-moiety; Table 1), which was initially expected to greatly hinder its radical reactions but was found not to do so. Similar findings were made previously [10] for the isomeric tetraradical 2 that also contains two but different meta-benzyne moieties (Table 1).…”

“…A σ,σ,σ,σ-type singlet tetraradical cation containing two metabenzyne moieties, the 2,4,5,7-tetradehydroquinolinium cation, was found to be more reactive than the related meta-benzynes in the gas phase. A similar finding was made previously [10] for an isomeric tetraradical, the 2,4,6,8-tetradehydroquinolinium cation. This enhanced reactivity of both tetraradicals is rationalized based on a decrease in the distortion energies (ΔE 2.30 ) of their meta-benzyne moieties, making these moieties, and hence the tetraradicals themselves, more reactive.…”

“…For the lowest-energy structure, both the meta-benzyne moieties have a DAS of 1.4-1.5 Å. This differs drastically from the potential-energy surface calculated [10] earlier for 2: in this case, the lowest-energy (global) minimum occurs at a geometry in which the 2,4-meta-benzyne moiety is bicyclic (DAS: 1.5 Å) but the 6,8-meta-benzyne moiety almost has the DAS (2.1 Å) of the TS geometry (2.3 Å). These potential energy surfaces help rationalize the greater reactivity of 2 than 1 as the 6,8meta-benzyne moiety in 2 already has a geometry very similar to the TS for radical reactions.…”

Do Diradicals Behave Like Radicals?

“…If the diradical (easily generated by chemical ionization of an iodo-substituted precursor) is charged, its reactions can be studied in great detail mass spectrometrically. Studies of Kenttämaa and co-workers have in this fashion provided us with much carefully reasoned experimental detail on m -benzyne reactivity. − These molecules behave primarily as radicals but can be induced to react as electrophiles upon heteroatomic substitution . These reactivity patterns are in line with the expectations resulting from our general analysis outlined in section .…”

Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

“…In order to examine the efficiencies of specific reactions (i.e., H atom and I atom abstractions), the H and I atom abstraction efficiencies (total efficiency × branching ratio of atom abstraction) for cyclohexane and allyl iodide were plotted as a function of the (calculated) EA v for a series of (iso)­quinolinium-based mono- and biradicals (Figures and ; note that only those biradicals that underwent major I atom abstraction were included in the I atom abstraction plot). , The monoradicals showed a linear correlation for both reactions (Figures and ; note that corresponding plots of the natural logarithm of the H or I atom abstraction efficiency vs EA v (data not shown) gave fits almost as good). While a correlation between the natural logarithm of H atom abstraction efficiency and EA v for related monoradicals has been published previously, no correlations have been reported for I atom abstraction reactions of radicals similar to those described here.…”

Spin‐Spin Coupling between the Biradical Moieties in Aromatic Tetraradicals Increases their Reactivity

“…However, 1 has a substantial ΔE S-T (À 22.2 kcal mol À 1 for the 5,7-moiety; Table 1), which was initially expected to greatly hinder its radical reactions but was found not to do so. Similar findings were made previously [10] for the isomeric tetraradical 2 that also contains two but different meta-benzyne moieties (Table 1).…”

“…A σ,σ,σ,σ-type singlet tetraradical cation containing two metabenzyne moieties, the 2,4,5,7-tetradehydroquinolinium cation, was found to be more reactive than the related meta-benzynes in the gas phase. A similar finding was made previously [10] for an isomeric tetraradical, the 2,4,6,8-tetradehydroquinolinium cation. This enhanced reactivity of both tetraradicals is rationalized based on a decrease in the distortion energies (ΔE 2.30 ) of their meta-benzyne moieties, making these moieties, and hence the tetraradicals themselves, more reactive.…”

“…For the lowest-energy structure, both the meta-benzyne moieties have a DAS of 1.4-1.5 Å. This differs drastically from the potential-energy surface calculated [10] earlier for 2: in this case, the lowest-energy (global) minimum occurs at a geometry in which the 2,4-meta-benzyne moiety is bicyclic (DAS: 1.5 Å) but the 6,8-meta-benzyne moiety almost has the DAS (2.1 Å) of the TS geometry (2.3 Å). These potential energy surfaces help rationalize the greater reactivity of 2 than 1 as the 6,8meta-benzyne moiety in 2 already has a geometry very similar to the TS for radical reactions.…”

Do Diradicals Behave Like Radicals?

“…If the diradical (easily generated by chemical ionization of an iodo-substituted precursor) is charged, its reactions can be studied in great detail mass spectrometrically. Studies of Kenttämaa and co-workers have in this fashion provided us with much carefully reasoned experimental detail on m -benzyne reactivity. − These molecules behave primarily as radicals but can be induced to react as electrophiles upon heteroatomic substitution . These reactivity patterns are in line with the expectations resulting from our general analysis outlined in section .…”

Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

“…In order to examine the efficiencies of specific reactions (i.e., H atom and I atom abstractions), the H and I atom abstraction efficiencies (total efficiency × branching ratio of atom abstraction) for cyclohexane and allyl iodide were plotted as a function of the (calculated) EA v for a series of (iso)­quinolinium-based mono- and biradicals (Figures and ; note that only those biradicals that underwent major I atom abstraction were included in the I atom abstraction plot). , The monoradicals showed a linear correlation for both reactions (Figures and ; note that corresponding plots of the natural logarithm of the H or I atom abstraction efficiency vs EA v (data not shown) gave fits almost as good). While a correlation between the natural logarithm of H atom abstraction efficiency and EA v for related monoradicals has been published previously, no correlations have been reported for I atom abstraction reactions of radicals similar to those described here.…”