Metal‐Free Cross‐Coupling Reactions of Aryl Sulfonates and Phosphates through Photoheterolysis of Aryl–Oxygen Bonds

Abstract

Photochemical cleavage of ArO bonds in phenyl esters substituted with electron‐donating groups offers a convenient method for the arylation of alkenes and arenes. The reaction proceeds by an SN1 mechanism via a triplet phenyl cation to form allylbenzene or biphenyl derivatives under mild conditions (see scheme).

Looking for a Paradigm for the Reactivity of Phenonium Ions

“…This approach (Scheme , a) is appealing for synthetic applications,12 whereas other methods, such as the ring‐opening of benzocyclobutene in superacids, are of spectroscopic interest 13. Some years ago we demonstrated a novel, mild access to phenonium ions by the addition of triplet phenyl cation IV , photogenerated from phenyl halides or esters,14 to olefins (Scheme , b) 15–17. The first‐formed triplet adduct cation with alkenes has a diradical character ( V ), just like the starting phenyl cation, but then intersystem crossing occurs to the more stable singlet phenethyl cation/phenonium ion system ( I / III ), which, contrary to what happens in the triplet manifold, undergoes addition of nucleophiles.…”

Leaving Groups in Metal‐Free Arylations: Make Your Choice!

“…[74] On the other hand, since the early pioneering works of Havinga [75] great attention has been given to the photochemical activation of aryl phosphates and solfonates. A decade ago, looking for new classes of photoactivatable aromatic substrates, our group discovered that electron rich aryl solfonates (triflates and mesylates) and diethylphosphates undergo photoinduced Ar-O bond heterolytic cleavage, and the resulting triplet aryl cation was exploited in metal-free arylation via triplet aryl cations, such as phenylation [76] and alkynation [34] of arenes (Scheme 15) A similar reactivity was found in the case of aryl nonaflates (Ar-OSO 2 C 4 F 9 ) [77] and trifluoroethoxy aryl sulfates. [78] Notably, these substrates can be generated in situ from the EurJOC European Journal of Organic Chemistry corresponding phenols and then irradiated under the proper arylation conditions.…”

“…Metal-free photoinduced alkynylation and arylation of aromatic sulfonates and phosphates. [34,76] Very recently, aryl triflates have been used in a synthetic protocol for the photoinduced formation of Ar-Ar, Ar-O and Ar-N bonds, by using aryl trifluoroborates, alcohols and nitriles as the nucleophiles. The reaction took place in water/acetone mixture and exhibited a discrete versatility towards different aromatic substituents (Scheme 16).…”

Aromatic Chemistry in the Excited State: Facilitating Metal‐Free Substitutions and Cross‐Couplings

“…In light of the cationic intermediate,i tw as discovered by the same group that electron-rich anilines and later phenol derivatives can both react with allyl silane to generate the Hiyama-coupling product, allyl arenes [Scheme 4A,equation (4)]. [17] Au nified mechanistic proposal to rationalize these reactions involves the formation of the phenyl cation, which is stabilized by the electron-donating group. [18] Upon absorbing light in apolar solvent (usually alcohols), 4-chloroaniline is promoted to the excited state and through intersystem crossing, it reaches the more stable triplet state and cleaves into ionic fragments.Asaresult of multiplicity conservation, at riplet aryl cation is generated.…”