Synthetic applications of rongalite: A green tool in the service of Diels–Alder chemistry and beyond

Construction of diaryl oxacyclic sulfones via Bi(OTf)3-catalyzed intramolecular cyclocondensation of 1,3-diaroylsulfones

We report the synthesis of six-membered saturated and unsaturated 1,4-oxasulfone derivatives via rongalite-mediated intermolecular double sulfination of α-bromoacetophenone. This was followed by Bi(OTf)3-catalyzed intramolecular condensation of 1,3-diaroylsulfones under open-vessel conditions. Furthermore, the chemical structures of the key products were determined through single-crystal X-ray analyses.

One-Pot Aminomethylation of Heteroarenes by Rongalite as In Situ C1 Source

Rongalite-mediated one-pot aminomethylation of heteroarenes using secondary amines/anilines has been developed. This transition-metal-free and mild reaction offers an efficient way to synthesize aminomethylated heteroaromatic compounds with high yields and broad functional group tolerance. Here, Rongalite plays a key role in generating the C1 unit source in situ, which triggers the aminomethylation process. This approach provides a library of aminomethylated imidazo[1,2-a]pyridines and imidazo[2,1-b]thiazoles.

Rongalite induced metal-free C(sp2)-H functionalization of indoles: direct access to 3-(sulfonylmethyl) indoles

A rongalite-induced C(sp2)-H functionalization reaction has been developed for the synthesis of 3-(phenylsulfonylmethyl) indole derivatives from indole and arylsulfonyl hydrazides. This regioselective C-H functionalization provides a wide range of C-3 sulfonylmethyl indoles with upto 90% yields. Here, rongalite functions as a C1 unit source and a single electron donor. The use of inexpensive rongalite (ca. $0.03 per 1 g), mild reaction conditions and gram-scale synthesis are some of the key features of this methodology.

Rongalite as a C1 Source: Sulfonylmethylation of Indoles with Aryl/Alkyl Hydrazides and Sulfinates

Herein, we present a sulfonylmethylation of indoles with aryl/alkyl hydrazides and sulfinates that provides viable access to sulfone compounds using rongalite as a C1 source. This protocol features readily available chemicals and simple operations, and the products were obtained in moderate to good yields.

Base controlled rongalite-mediated reductive aldol/cyclization and dimerization of isatylidene malononitriles/cyanoacetates

In this study, we developed a novel methodology involving a base-controlled, rongalite-mediated reductive/aldol reaction, followed by cyclization of isatylidene malononitriles/cyanoacetates, resulting in the synthesis of spiro[2,3-dihydrofuran-3,3'-oxindole]. Additionally, we have disclosed a rongalite-mediated dimerization process for isatylidene malononitriles, yielding dispiro[cyclopent-3'-ene]bisoxindole. The utilization of rongalite in this reaction serves a dual purpose, acting both as a reducing agent and a C1 synthon. The developed approach has several advantages like a simple reaction setup, a wide substrate scope, requiring less time, using water as a green solvent, no metal or catalyst is required and products can be easily isolated via filtration with excellent yields under mild reaction conditions.

Reductive N-Formylation of Nitroarenes Mediated by Rongalite

Herein, we disclose a straightforward approach to access transition-metal-free reductive N-formylation of nitroarenes. This reaction integrates the dual role of rongalite, which acts as a reductant and a C1 building block concurrently. This provides an alternative method for the synthesis of N-aryl formamides from nitroarenes, including the construction of a C-N bond. The utility of this protocol was demonstrated by scale-up synthesis and late-stage functionalizations of complex molecules.

Synthesis and characterization of ZrFe2O4@SiO2@Ade-Pd as a novel, recyclable, green, and versatile catalyst for Buchwald-Hartwig and Suzuki-Miyaura cross-coupling reactions

This work presents the simple synthesis of a green and novel Palladium based magnetic nanocatalyst with effective catalytic properties and reusability. These heterogeneous catalysts were prepared by the anchoring of Pd(0) on the surface of ZrFe2O4 MNPs coated with a di-substituted adenine (Ade) compound as a green linker. The as-synthesized ZrFe2O4@SiO2@Ade-Pd MNPs were methodically characterized over different physicochemical measures like VSM, EDX, Map, SEM, TEM, ICP, and FT-IR analysis. The catalytic activity of ZrFe2O4@SiO2@Ade-Pd was carefully examined for the room-temperature Carbon-Carbon coupling reaction in acetonitrile as a solvent. It is worth noting that the synthesized solid catalyst can be easily recovered with a bar magnet and reused for five cycles without decrease of catalytic activity.

Rongalite as C1 Synthon in the Synthesis of Divergent Pyridines and Quinolines

Rongalite has been used as a cheap and efficient carbon synthon for the synthesis of divergent N-heteroaromatics, including different pyridines and quinolines. The selective synthesis of different products can be achieved by employing enaminones or enaminones/anilines as reaction partners. In addition, compared with the reaction using conventional aldehyde synthons, rongalite displays an evident advantage in providing products with considerably higher product yields under milder conditions. The GC-MS analysis of the reaction process has been performed to probe the possible reaction mechanism.

Rongalite-Mediated Transition Metal- and Hydride-Free Chemoselective Reduction of α-Keto Esters and α-Keto Amides

A transition metal- and hydride-free protocol has been developed for the chemoselective reduction of α-keto esters and α-keto amides using rongalite as a reducing agent. Here, rongalite acts as a hydride-free reducing agent via a radical mechanism. This protocol offers the synthesis of a wide range of α-hydroxy esters and α-hydroxy amides with 85-98% yields. This chemoselective method is compatible with other reducible functionalities such as halides, alkenes, amides, and nitriles. The use of inexpensive rongalite (ca. $0.03/1 g), mild reaction conditions, and gram-scale synthesis are some of the key features of this methodology. Also, cyclandelate, a vasodilator drug, has been synthesized in gram scale with 79% yield.

Transition metal-free functionalization of 2-oxindoles via sequential aldol and reductive aldol reactions using rongalite as a C1 reagent

A sequential one-pot classical aldol, transition-metal and hydride-free reductive aldol reaction is reported here for C(sp3)- H functionalization of 2-oxindoles using the multifaceted reagent rongalite. Here, rongalite functions as a hydride-free reducing agent and double C1 unit donor. This protocol enables the synthesis of a wide range of 3-methylindoline-2-ones and 3-(hydroxymethyl)-3-methylindolin-2-ones from 2-oxindoles (65-95% yields), which are the synthetic precursors for many natural products. Some of the important aspects of this synthetic approach include one-pot methylation and hydroxymethylation, low-cost rongalite (ca. $0.03 per 1 g), mild reaction conditions and applicability to gram-scale synthesis.

Syntheses of 3-Aryl Tetrahydroisoquinolines via an Intermolecular [4 + 2] Cycloaddition of Sultines with Imines

The development of an intermolecular aza-Diels-Alder (DA) cycloaddition of sultines and imines is reported. By exploiting sultines as o-quinodimethane precursors and aryl imines as dienophiles in the presence of Cu(OTf)₂, an aza-DA reaction proceeds to provide a wide variety of 3-aryl tetrahydroisoquionlines in moderate to excellent yield (up to 89%). The synthetic utility of these products was demonstrated in the preparation of tetracyclic N-heterocycles, including a tetrahydroprotoberberine skeleton.

Rongalite-induced transition-metal and hydride-free reductive aldol reaction: a rapid access to 3,3'-disubstituted oxindoles and its mechanistic studies

A transition-metal and hydride-free reductive aldol reaction has been developed for the synthesis of biologically active 3,3'-disubstituted oxindoles from isatin derivatives using rongalite. In this protocol, rongalite plays a dual role as a hydride-free reducing agent and a C1 unit donor. This transition metal-free method enables the synthesis of a wide range of 3-hydroxy-3-hydroxymethyloxindoles and 3-amino-3-hydroxymethyloxindoles with 79-96% yields. One-pot reductive hydroxymethylation, inexpensive rongalite (ca. $0.03/1 g), mild reaction conditions and short reaction time are some of the key features of this synthetic method. This protocol is also applicable to gram scale synthesis.

Rongalite as C1 Synthon and Sulfone Source: A Practical Sulfonylmethylation Based on the Separate-Embedding Strategy

Rongalite has been used in several challenging synthetic transformations with operationally simple and effective protocols. However, the employment of multiple characteristics of rongalite in synthetic chemistry is comparatively little known. Herein we report a separate-embedding type sulfonylmethylation of sulfoxonium ylides in which rongalite concurrently acted as a sulfone source, C1 synthon, radical initiator, and potential reducing reagent for the first time. Notably, this facile and easy-handling reaction does not require a catalyst or prefunctionalized sulfonylmethylation reagents.

Silyloxymethanesulfinate as a sulfoxylate equivalent for the modular synthesis of sulfones and sulfonyl derivatives

An efficient protocol for the modular synthesis of sulfones and sulfonyl derivatives has been developed utilizing sodium tert-butyldimethylsilyloxymethanesulfinate (TBSOMS-Na) as a sulfoxylate (SO2 2-) equivalent. TBSOMS-Na, easily prepared from the commercial reagents Rongalite™ and TBSCl, serves as a potent nucleophile in S-alkylation and Cu-catalyzed S-arylation reactions with alkyl and aryl electrophiles. The sulfone products thus obtained can undergo the second bond formation at the sulfur center with various electrophiles without a separate unmasking step to afford sulfones and sulfonyl derivatives such as sulfonamides and sulfonyl fluorides.