Photocatalyst- and Transition-Metal-Free Visible-Light-Promoted Intramolecular C(sp2)-S Formation

Iodoarene Activation: Take a Leap Forward toward Green and Sustainable Transformations

Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl-halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl-I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl-I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C-C and C-heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.

Recent Advances in the Development of Greener Methodologies for the Synthesis of Benzothiazoles

The benzothiazole ring system has been recognised with crucial pharmacophoric features being present among various approved drugs and clinical and pre-clinical candidates. The medicinal importance of this privileged scaffold stimulated the interest of synthetic medicinal/ organic chemists for the synthesis of its derivatives due to their diverse biological applications. In most of the reports in the literature, benzothiazoles were synthesized by cyclocondensation of 2- aminothiophenol with either carboxylic acid and its derivatives or aldehydes. However, many of these procedures involve reaction conditions that are not in conformity with sustainable chemistry development. The negative impact of chemicals and their manufacturing processes on the environment, human health, and biodiversity raises safety concerns. On the other hand, the utilization of non-renewable energy sources, use of rare earth metals as catalysts, involvement of costly chemicals, prolonged reaction time at high temperatures, and considerable waste generation diminish the greener impact of these reaction methodologies and make them non-sustainable. In order to avoid such drawbacks of the non-sustainable practices in the synthesis of benzothiazoles, there have been continuous efforts to develop greener methodologies for the construction of this bioactive scaffold. This review aims to delve into the literature reports on the recent advancements in the development of greener methodologies for the synthesis of bioactive benzothiazoles.

Catalyst-Free Photooxidative N-Acylation of Azoles with Aldehydes

A catalyst-free photochemical N-acylation of azoles with aldehydes has been developed using inexpensive BrCCl3 as the oxidant. This transition-metal- and photocatalyst-free amidation proceeded efficiently with a wide variety of substrates to give the corresponding N-acylazoles, including for the late modification of pharmaceutically active molecules, and on a gram-scale.

Metal- and base-free, aerobic photoredox catalysis with riboflavin to synthesize 2-substituted benzothiazoles

Sustainable approaches for the synthesis of 2-substituted benzothiazoles are sought after for their use in organic chemistry, bioorganic chemistry, and industrial applications. Here, we described a visible light-driven photoredox catalytic cyclization of thioanilides to afford 2-substituted benzothiazoles using riboflavin as a photocatalyst, where oxygen is used as a clean oxidant and ethanol as a greener solvent. These results provide a new photochemical route for environmentally benign synthesis.

Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

Iron(III)-Catalyzed Regioselective Synthesis of Electron-Rich Benzothiazoles from Aryl Isothiocyanates via C-H Functionalization

We report herein a direct synthetic route for the preparation of 2-arylbenzothiazoles using aryl isothiocyanates and electron-rich arenes. The synthetic route involves triflic acid promoted addition of the arenes to aryl isothiocyanates followed by FeCl₃-catalyzed C-S bond formation via C-H functionalization. The approach provides the advantage of synthesis of benzothiazoles without the conventional use of aryl aldehyde/carboxylic acid precursors employing the less expensive iron(III) catalyst.

Visible-Light-Driven Thioesterification of Aryl Halides with Potassium Thiocarboxylates: Transition-Metal Catalyst-Free Incorporation of Sulfur Functionalities into an Aromatic Ring

Reactions of acceptor-substituted aryl iodides and bromides with potassium thiocarboxylates under white light irradiation allow for the preparation of S-aryl thioesters including synthetically versatile S-aryl thioacetates. This transition-metal and external photocatalyst-free method features extremely mild reaction conditions compared with those used in transition-metal-catalyzed protocols. Reactions proceed via the initial formation of an electron donor-acceptor (EDA) complex in the ground state, which was supported by UV-vis spectra. Electron paramagnetic resonance (EPR) spin-trapping experiments using phenyl-N-tert-butylnitrone (PBN) have revealed the radical nature of the reaction.

Photoinduced Construction of a Benzothienopyridine-S,S-dioxide Framework Enabled by Polychloropyridyl Multifunctional Motifs

Reported herein is a visible-light-induced, catalyst-free intramolecular cyclization of 4-phenylsulfonyl-2,3,5,6-tetrachloropyridine, leading to rapid assembly of a series of unprecedented benzo[4,5]thieno[3,2-c]pyridine 5,5-dioxide scaffolds under mild conditions. The rational introduction of a perchloropyridin-4-yl module significantly facilitated this photoinduced process and offers a versatile platform for broad structural variation. Mechanistic studies revealed that a newly identified charge-transfer complex with carbonate is crucial to this photoinduced process.

Mechanistic Insights into Visible-Light-Driven Dearomative Fluoroalkylation Mediated by an Electron Donor-Acceptor Complex

Electron donor-acceptor (EDA) complex photochemistry has become a burgeoning topic in the synthetic radical chemistry mediated by visible light; however, the theoretical insights into the reaction mechanisms are limited. Herein, accurate electronic structure calculations at the CASPT2//CASSCF/PCM level of theory were performed to investigate the paradigm example of EDA complex-enabled photoreaction for visible-light-driven dearomative perfluoroalkylation of β-naphthol. The excitation energy levels of the EDA complex are controlled by noncovalent interactions because the photoinduced intermolecular charge is enhanced when the noncovalent interaction becomes weaker, leading to the broad spectra ranging from UVA (<380 nm) to visible light (>500 nm). The competitiveness of the radical-radical coupling over the radical chain pathway is also regulated due to the tunable radical concentrations varying the excitation wavelength.

KOtBu-Promoted Halogen-Bond-Assisted Intramolecular C-S Cross-Coupling of o-Iodothioanilides for the Synthesis of 2-Substituted Benzothiazoles

An efficacious and mild KO^tBu-promoted intramolecular C-S cross-coupling of ortho-iodothioanilides in conjunction with a catalytic quantity of phenanthroline as an additive has been described for the convenient synthesis of 2-substituted benzothiazoles. The methodology is suitable for attaining a wide variety of 2-alkyl- and 2-aryl-substituted benzothiazoles. Single-crystal XRD, DFT calculations, NMR, and UV studies suggest that halogen bonds between the units of ortho-iodothioanilides may assist in the electron transfer process.

Chemoselective Ullmann Reaction of α-Trisubstituted Thioamides: Synthesis of Novel 2-Iminobenzothiolanes

New classes of unexplored benzo[b]thiolanes are synthesized from trisubstituted thioamides through copper-catalyzed intramolecular S-arylation of thioamides for the first time. This method provides good to excellent yields with fully controlled chemoselectivity. Unusually, iminobenzo[b]thiolanes are very stable under mild acidic conditions. A plausible mechanism is proposed for the chemoselective S-arylation process.