Singlet vs Triplet Reactivity of Photogenerated α,n-Didehydrotoluenes

Abstract

The reactivity of α,n-didehydrotoluenes (DHTs) in protic media (organic/aqueous mixtures) was explored by means of a combined computational and experimental approach. These intermediates were generated via a photoinduced double elimination process occurring in (chlorobenzyl)trimethylsilanes and led to the formation of a varied products distribution, depending on the isomer tested. Irradiation of ortho- and para-derivatives resulted, respectively, in the formation of triplet α,2- and α,4-DHTs, whose diradical reactivity led to both radical and polar products. On the other hand, irradiation of the meta-precursor led to the singlet α,3-DHT isomer. The latter showed a marked preference for the formation of polar products and this was rationalized, as supported by computational evidence, via the involvement of a zwitterionic species arising through interaction of the nucleophilic solvent with the benzylic position of the DHT.

Nucleophilic Addition to Diradicals Derived From Cycloaromatization of Maleimide‐Based Enediynes

“…As for the diradical/zwitterion dichotomy in α,3‐dehydrotoluene chemistry, there is increasing evidence that a marked preference of the zwitterionic to radical pathway under polar conditions or nucleophilic solvents, as supported by computational and experimental approaches [19,39] . But unlike the previous work, the nucleophilic addition to phenylic radical position in this work performed as a unimolecular process, shifting the diradical to closed‐shell polar product.…”

“…As for the diradical/zwitterion dichotomy in α,3-dehydrotoluene chemistry, there is increasing evidence that a marked preference of the zwitterionic to radical pathway under polar conditions or nucleophilic solvents, as supported by computational and experimental approaches. [19,39] But unlike the previous work, the nucleophilic addition to phenylic radical position in this work performed as a unimolecular process, shifting the diradical to closed-shell polar product. To explore if the diradical/zwitterion switch was sensitive to the polarity of environment, the < S 2 > values along the IRC were computed in toluene (ɛ = 2.37), THF (ɛ = 7.42), methanol (ɛ = 32.61) and water (ɛ = 78.35) (Figure 5).…”

“…Nevertheless, the general principle where the choice between the nonpolar and polar extremes exists every time when a chemical bond is broken, unsurprisingly endows the formed intermediates with the dual reactivity in the cycloaromatization, behaving either in a diradical or a zwitterion form. [3,17] Indeed, there has been emerging evidences of extending to the formation of zwitterions beyond diradicals when the cycloaromatization proceeds under suitable reaction conditions, such as in the presence of Au(I) catalysts, [18] nucleophilic solvents [19] or specific substituents. [20] Alabugin et al have summarized the unusual chemistry and paid special attention to the dichotomy between diradical and zwitterionic species.…”

Aryl–Cl vs heteroatom–Si bond cleavage on the route to the photochemical generation of σ,π-heterodiradicals

“…The photochemistry of compounds 1-6 can be summarized in Scheme 3. Each compound tested could in principle be suitable to undergo photoheterolytic cleavage of the Ar-Cl bond in 3 1-3 6, since the XSiR 3 group has a sufficient electron-donating ability to stabilize the corresponding triplet phenyl cation (path a) [36][37][38][39]. Sensitization experiments in the presence of acetone as triplet sensitizer did not affect somehow the distribution of products (see Tables 3, 4) pointing to the involvement of an excited triplet state in the reaction.…”

“…The following cleavage of the C-EG bond results in the formation of the desired α,n-DHT. The reaction is effective when starting from both the parent [36][37][38][39] and methoxy substituted [40] (chlorobenzyl) trimethylsilane isomers (LG = Cl, EG = Me 3 Si). Electrofugal groups able to ameliorate the water solubility of α,n-DHT precursors have been likewise designed and efficiently tested in chlorophenylacetic acids (EG = COOH) [41] and chlorobenzylphosphonic acids (EG = P(O)(OH) 2 )) [42].…”