Lewis Acid-Promoted Typical Friedel-Crafts Reactions Using DMSO as a Carbon Source

"Cation Pool" generated from DMSO and 1,2-dihaloethanes and their application in organic synthesis

Conventionally, carbenium and onium ions are prepared in the presence of nucleophiles due to their instability and transient nature. The nucleophiles that are unstable or inert to the reaction media cannot be used for reaction with the cationic species to access the desired compounds. To overcome these limitations, developing methods for generating organic cations irreversibly in the absence of nucleophiles is essential. The "cation pool" method developed by Yoshida and co-workers stands out as a reliable strategy to generate and accumulate the reactive cations in solution in the absence of nucleophiles. The cation pool method involves the electrolysis of the substrate in the absence of nucleophiles, usually at low temperature. Moreover, the generation of halogen and chalcogen cations through electrolysis needs extra care because of their low stability. This review covers our effort in generating and accumulating halogen cations as "cation pools", most importantly by simply heating a mixture of dimethyl sulfoxide (DMSO) and 1,2-dihaloethane (DXE, X = Cl, Br, I), and their use in the halogenation reactions. Furthermore, condition-dependent Pummerer-type fragmentations of DMSO-stabilized halogen cations to methyl(methylene)sulfonium ions and chlorodimethylsulfonium ions for synthetic applications are described.

An organocatalytic domino annulation approach via C(sp2)-OMe cleavage to unlock the synthesis of pyranochromenones enabled by HFIP

Fused pyranochromenone derivatives have extensive applications in medicinal chemistry. Herein, we report the first HFIP/TsOH catalyzed, one-pot domino reaction by cleavage of the C(sp2)-OMe bond. Control experiments reveal that 1,3,5-trimethoxybenzene is rapidly protonated in the presence of HFIP to yield a dearomatized cationic diene intermediate. The gram-scale reaction and late-stage functionalization of natural products justified the practicality of this protocol.

DMSO-DCE Triggered Chemodivergent C-Methylenation of Electron-Rich Arenes: An Easy Access to Diarylmethanes

The chemodivergent property of dimethyl sulfoxide (DMSO) along with 1,2-dichloroethane (DCE) was exploited for the incorporation of a methylene group to form diarylmethanes through a dearomatization/rearomatization process. Methyl(methylene)sulfonium ions (CH2=S+-Me) were generated by simple heating of commonly used solvents such as DMSO and DCE. These ions were subsequently trapped by electron-rich arenes and heteroarenes, resulting in the synthesis of both symmetrical and unsymmetrical diarylmethanes. This protocol was further extended to access N-methylenamides by reacting 2-naphthol with amides or nitriles in the presence of DMSO and DCE.

Cooperative Triple Catalysis Enables Deaminative α-Indolmethylation of Carbonyl Compounds with Gramines

α-Functionalization of carbonyl compounds is an important reaction in synthetic chemistry. However, the development of novel synthetic strategies to realize this reaction is challenging. This study describes the α-indolmethylation of carbonyl compounds using cooperative copper, amine, and hydrogen-bond catalysis. This reaction provides a novel and efficient strategy for developing indolmethylated carbonyl compounds by deaminative coupling of gramines.

Brønsted Acid-Catalyzed Synthesis of 4-Functionalized Tetrahydrocarbazol-1-ones from 1,4-Dicarbonylindole Derivatives

A p-toluenesulfonic acid-catalyzed cascade reaction is reported for the synthesis of 4-functionalized tetrahydrocarbazolones via the reaction of 4-(indol-2-yl)-4-oxobutanal derivatives with a variety of nucleophiles in acetonitrile or hexafluoroisopropanol. After the initial intramolecular Friedel-Crafts hydroxyalkylation, the 3-indolylmethanol intermediate is subsequently activated and reacted with the external nucleophile. The reaction conditions are crucial to avoid alternative reaction pathways, allowing direct substitution reaction with thiols, (hetero)arenes, alkenes, or sulfinates. The procedure features high overall yields to access a diverse family of compounds bearing the tetrahydrocarbazole core.

Sustainable Sulfonic Acid Functionalized Tubular Shape Mesoporous Silica as a Heterogeneous Catalyst for Selective Unsymmetrical Friedel-Crafts Alkylation in One Pot

The development of general and more sustainable heterogeneous catalytic processes for Friedel-Crafts (FC) alkylation reactions is a key objective of interest for the synthesis of pharmaceuticals and commodity chemicals. Sustainable heterogeneous catalysis for the typical FC alkylation of an easily accessible carbonyl electrophile and arenes or with two different arene nucleophiles in one-pot is a prime challenge. Herein, we present a resolution to these issues through the design and utilization of a mesoporous silica catalyst that has been functionalized with sulfonic acid. For the synthesis of sulfonic acid-functionalized mesoporous silica (MSN-SO3H), thiol-functionalized mesoporous silica was first synthesized by the co-condensation method, followed by oxidation of the thiol functionality to the sulfonic acid group. Sulfonation of mesoporous silica was confirmed by 13C CP MAS NMR spectroscopy. Further, the devised heterogeneous catalysis using MSN-SO3H has been successfully employed in the construction of diverse polyalkanes including various bioactive molecules, viz arundine, tatarinoid-C, and late-stage functionalization of natural products like menthol and Eugenol. Further, we have utilized this sustainable technique to facilitate the formation of unsymmetrical C-S bonds in a one-pot fashion. In addition, the catalyst was successfully recovered and recycled for eight cycles, demonstrating the high sustainability and cost-effectiveness of this protocol for both academic and industrial applications.

Metal-Free C(sp3)-H Bond Arylation of 3-Methylindole Derivatives via 3-Indole Imine Methides

Direct arylation of the benzylic C(sp3)-H bond is one of the most straightforward strategies for the construction of multi-aryl methanes, owing to the extraordinary step and atom economy. In this paper, we developed the first metal-free arylation of the C(sp3)-H bond in 3-methylindoles, thereby providing rapid access to a range of diaryl- and triarylmethanes with two indole rings. Mechanistically, 3-indole imine methide serves as the key intermediate. Water plays a crucial role in this process, likely serving as a proton shuttle to facilitate the key 1,3-proton transfer step in this reaction and, thus, enhance the reaction efficiency.

NH4OAc/DMSO-Promoted Benzylation of Pyrrolo[2,1-a]isoquinolines

Benzylation of pyrrolo[2,1-a]isoquinoline derivatives has been realized with various phenols by the use of ammonium acetate as a promoter (20 examples, up to 84% yield). DMSO served as the source of methylene and solvent. The employment of iron chloride as a catalyst can also afford the desired benzylated products in moderate to good yields (11 examples, up to a 74% yield).

Arylative Methylation of 2,3-Dihydropyrazines and Pyrazinones Using Dimethyl Sulfoxide as a C1 Source

Divergent synthesis of α-C-H methylated pyrazines and pyrazinones using dimethyl sulfoxide as a nonconventional methylating agent under metal-free conditions was reported. Dimethyl sulfoxide-coordinated bromine cation pools generated from the treatment of dimethyl sulfoxide and 1,2-dibromoethane undergo Pummerer-type fragmentation to afford an electrophilic methyl(methylene)sulfonium ion for reaction with a carbon nucleophile followed by aromatization to afford α-methylated pyrazines and pyrazinones.

Cooperative Friedel-Crafts Alkylation of Electron-Deficient Arenes via Catalyst Activation with Hexafluoroisopropanol

A Friedel-Crafts alkylation of electron-deficient arenes with aldehydes through ''catalyst activation'' is presented. Through hydrogen bonding interactions, the solvent 1,1,1,3,3,3, -hexafluoroisopropanol (HFIP) interacted with the added Brønsted acid catalyst pTSA•H₂ O, increasing its acidity. This activated catalyst enabled the Friedel-Crafts alkylation of electron-neutral as well as electron-deficient arenes. Strongly electron withdrawing arenes including arenes with multiple halogen atoms, NO₂ , CHO, CO₂ R, and CN, groups acted as efficient nucleophiles in this reaction. DFT studies reveal multiple roles of solvent HFIP viz; increasing the Brønsted acidity of the catalyst pTSA•H₂ O, and stabilization of the transition states through a concerted pathway enabling the challenging reaction.

Direct α-methylenation of triazines to terminal olefins with DMA

We report an efficient metal (Cu or Ni)-catalysed α-methylenation of triazines to terminal olefins using DMA as a one-carbon source. Various substituted triazine derivatives are suitable for this reaction.