A Photochemical Route to 2‑Substituted Benzo[<i>b</i>]furans

2-Substituted benzo­[<i>b</i>]­furans were synthesized by a one-step metal-free photochemical reaction between 2-chlorophenol derivatives and terminal alkynes by tandem formation of an aryl-C and a C–O bond via an aryl cation intermediate. The mild conditions and the application to chlorophenols rather of the more expensive bromo or iodo analogues makes this procedure environmentally convenient

Wavelength Selective Generation of Aryl Radicals and Aryl Cations for Metal-Free Photoarylations

Photochemical reactions have become an important tool for organic chemists. Visible (solar) light can be conveniently adopted, however, only when using colored organic compounds or in photocatalyzed processes induced by visible light absorbing photocatalysts. Herein we demonstrate that a photolabile, colored moiety could be incorporated in a colorless organic compound with the aim of generating highly reactive intermediates upon exposure to visible (solar) light. Arylazo sulfones, colored thermally stable derivatives of aryl diazonium salts, were used as valuable substrates for the photoinduced metal-free synthesis of (hetero)­biaryls with no need of a (photo)­catalyst or of other additives to promote the reaction. Noteworthy, selective generation of aryl radicals and aryl cations can be attained at will by varying the irradiation conditions (visible light for the former and UVA light for the latter)

Aryl Imidazylates and Aryl Sulfates As Electrophiles in Metal-Free ArS_N1 Reactions

Some oxygen-bonded substituents were investigated as leaving groups in photoinduced ArS_N1 reactions. Irradiation of aryl imidazylates and of the corresponding imidazolium salts mainly caused homolysis of the ArO–S bond. However, previously unexplored trifluoroethoxy aryl sulfates were found to undergo efficient metal-free arylation. The sulfates were conveniently generated in situ by dissolving the corresponding imidazolium salts in basic 2,2,2-trifluoroethanol

Photogenerated α,<i>n</i>‑Didehydrotoluenes from Chlorophenylacetic Acids at Physiological pH

Aromatic diradicals are recognized as promising intermediates for DNA cleavage, but their formation has thus far been limited to the Bergman and Myers–Saito cycloaromatizations. We report here the phototriggered generation of all isomers of the potential DNA-cleaving α,<i>n</i>-didehydrotoluene diradicals at physiological pH, accomplished by the irradiation of chlorophenylacetic acids under mild conditions. The desired diradicals were formed upon photolysis of the chosen aromatic in aqueous phosphate buffer solution (pH = 7.3), with the consecutive elimination of biologically compatible chloride ion and carbon dioxide. Theoretical simulations reveal that the efficient decarboxylation of the primarily generated phenyl cations involves a previously not known diradical structure

From Phenyl Chlorides to α,<i>n</i>‑Didehydrotoluenes via Phenyl Cations. A CPCM–CASMP2 Investigation

Calculations with the complete active space self-consistent field (CASSCF) method were carried out for rationalizing the photochemical generation of the three isomeric didehydrotoluenes (DHTs) from the corresponding (<i>n</i>-chlorobenzyl)­trimethylsilanes. Moreover, the original CASSCF energies were corrected through the introduction of the dynamic electron correlation term (at the MP2 level) and of an appropriate solvent model (CPCM). The work demonstrated the viability of intersystem crossing (conical intersection located) leading to the lowest lying triplet state of the silanes that fragments to give the corresponding triplet phenyl cations. The <i>para</i>- and <i>ortho</i>-isomers desilylate directly from such states of radical/radical cation character and yield the corresponding DHTs in their triplet state. Different from the other isomers, the <i>meta</i>-cation has a radical/radical cation structure in both spin states and thus two potential accesses to the different spin states of the corresponding DHT

Methoxy-Substituted α,<i>n</i>‑Didehydrotoluenes. Photochemical Generation and Polar vs Diradical Reactivity

The photoreactivity of differently substituted (chloromethoxybenzyl)­trimethylsilanes in alcohols and alcohol/water mixtures has been investigated by means of a combined computational and experimental approach. Subsequent elimination of the chloride anion and the trimethylsilyl cation gives the corresponding methoxy-substituted α,<i>n</i>-didehydrotoluenes (α,<i>n</i>-MeO-DHTs). The rate of desilylation is evaluated through the competition with arylation via phenyl cation (ca. 10⁸ s^–1). α,2-MeO- and α,4-MeO-DHTs show a purely radical behavior (H abstraction from the solvent, methanol), while α,3-MeO-DHT shows mainly a ionic chemistry, as when the parent α,3-DHT is thermally generated. This is likely due to triplet–singlet surfaces crossing occurring during desilylation

Probing for a Leaving Group Effect on the Generation and Reactivity of Phenyl Cations

Phenyl cations are smoothly generated by the <i>photo</i>heterolytic cleavage of an Ar–LG bond (LG = leaving group). With the aim of evaluating the scope of the method, a series of 4-methoxy-2-(trimethylsilyl)­phenyl derivatives (sulfonic, LG = MeSO₃ and CF₃SO₃, phosphate, LG = (EtO)₂(O)­PO esters and the corresponding chloride) have been compared as probes for evaluating the leaving group ability. The photocleavage was a general reaction, with the somewhat surprising order (EtO)₂(O)­PO ∼ Cl > CF₃SO₃ > MeSO₃ (Φ = 0.50 to 0.16 in CF₃CH₂OH and lower values in MeCN–H₂O). The ensuing reactions did not depend on the LGs but only on the structure of the phenyl cation (the silyl group tuned the triplet to singlet intersystem crossing and the electrophilicity) and on the medium (formation of a complex with water slowed the electrophilic reactions)

Smooth Photocatalyzed Benzylation of Electrophilic Olefins via Decarboxylation of Arylacetic Acids

Arylacetic acids were used as sources of benzyl radicals under tetrabutylammonium decatungstate photocatalyzed conditions for the benzylation of electron-poor olefins. The reaction proceeds smoothly in a mixed aqueous medium (MeCN/H₂O 2/1) in the presence of NaHCO₃, NaClO₄, and an electron transfer agent (biphenyl). The reaction tolerates a wide variety of functional groups on the aromatic ring (whether electron donating or electron withdrawing) and can be extended to heteroaromatic analogues. The olefins have the double role of radical trap and electron acceptor. The present approach can also be extended to arylpropionic acids (including the nonsteroidal anti-inflammatory drugs ibuprofen and flurbiprofen), as well as mandelic acid derivatives

Sugar-Assisted Photogeneration of Didehydrotoluenes from Chlorobenzylphosphonic Acids

Irradiation of the three isomeric chlorobenzylphophonic acids in aqueous buffer led to a pH-dependent photochemistry. Under acidic conditions (pH = 2.5), photocleavage of the Ar–Cl bond occurred and a phenyl cation chemistry resulted. Under basic conditions (pH = 11), a photoinduced release of the chloride anion followed by the detachment of the metaphosphate anion gave α,<i>n</i>-didehydrotoluene diradicals (α,<i>n</i>-DHTs), potential DNA cleaving intermediates. At a physiological pH (pH = 7.2), both a cationic and a diradical reactivity took place depending on the phosphonic acid used. It is noteworthy that the complexation exerted by a monosaccharide (glucose or methylglucopyranoside) present in solution induced an exclusive formation of α,<i>n</i>-DHTs. The mechanistic scenario of the different photoreactivities occurring when changing the pH of the solution and the role of the various intermediates (phenyl cations, diradicals, etc.) in the process was studied by computational analysis

Decatungstate As Photoredox Catalyst: Benzylation of Electron-Poor Olefins

Excited tetrabutylammonium decatungstate (TBADT), known to activate a variety of compounds via hydrogen atom transfer (HAT), has now been applied as a photoredox catalyst for the effective oxidative cleavage of benzyl silanes and radical benzylation of reducible olefins occurring in isolated yields from poor to excellent