Isothiocyanates: happy-go-lucky reagents in organic synthesis

Owing to their unique structural features, isothiocyanates (ITCs) are a class of highly useful and inimitable reagents as the -NCS group serves both as electrophile and nucleophile in organic synthesis. ITCs share a rich legacy in organic, medicinal, and combinatorial chemistry. Compared to their oxygen equivalents, isocyanates, ITCs are easily available, less unpleasant, and somewhat less harmful to work with (mild conditions) which makes them happy-go-lucky reagents. Functionalized ITCs can finely tune the reactivity of the -NCS group and thus can be exploited in the late-stage functionalization processes. This review's primary aim is to outline ITC chemistry in the construction and derivatization of heterocycles through the lens of sustainability. For ease and brevity, the sections are divided based on reactive centers present in functionalized ITCs and modes of cyclisation. Scrutinizing their probable unexplored directions for future research studies is also addressed.

Ts2O mediated deoxygenative C2-dithiocarbamation of quinoline N-oxides with CS2 and amines

A general, efficient and practical protocol for Ts2O promoted deoxygenative dithiocarbamation of quinoline N-oxides with in situ generated dithiocarbamic acids from CS2 and amines is reported. The reaction proceeded well under transition-metal free conditions to obtain a variety of novel quinoline-dithiocarbamate compounds with wide functional group tolerance and good to high yields.

Ts2O Promoted Deoxygenative C-H Dithiocarbonation of Quinoline N-Oxides with Potassium O-Alkyl Xanthates

A simple, efficient, and practical method for the synthesis of S-quinolyl xanthates was developed via Ts2O-promoted deoxygenative C-H dithiocarbonation of quinoline N-oxides with various potassium O-alkyl xanthates. The reaction performed well under transition-metal-free, base-free, and room-temperature conditions with wide substrate tolerance. Employing potassium O-tert-butyl xanthate (tBuOCS2K) as a nucleophile, some valuable quinoline-2-thiones were unexpectedly obtained in a one-pot reaction without any additional base.

TsCl promoted deoxygenative phosphorothiolation of quinoline N-oxides towards S-quinolyl phosphorothioates

A convenient, efficient and practical approach for the synthesis of S-quinolyl phosphorothioates via cheap TsCl promoted deoxygenative C2-H phosphorothiolation of quinoline N-oxides with readily available triethylammonium O,O-dialkylphosphorothioates was developed. The reaction performed well under transition-metal-free conditions at room temperature with a very short reaction time (10-20 min). Preliminary studies showed that the current transformation underwent a nucleophilic substitution process.

Phosphonium Salt-Promoted C2-H Functionalization of Heterocyclic N-Oxides

We report the development of a phosphonium salt as a remarkable activating agent that enables the direct conversion of C2-H bonds of a variety of heterocyclic N-oxides into C2-N, C2-O, or C2-S bonds with high efficiency. The phosphonium salt was prepared on a >150 g scale in a single step and is tolerant of multiple functionalities.

Ball milling synthesis of S-quinolyl xanthates via coupling of haloquinolines with potassium O-alkyl xanthates

An environmentally benign protocol that provides various S-quinolyl xanthates via a ball milling enabled cross coupling reaction of haloquinolines and readily available potassium O-alkyl xanthates is first reported. The reaction proceeded well under mild, transition metal- and solvent-free conditions, making it an attractive method for the introduction of xanthates into the quinoline scaffold.

Rhodium(II)-catalyzed synthesis of 2-aminoquinoline derivatives from 2-quinolones and N-sulfonyl-1,2,3-triazoles

Herein, we disclose a rhodium(II) catalyzed efficient and convenient method for the synthesis of 2-aminoquinoline derivatives from 2-quinolones and N-sulfonyl-1,2,3-triazoles. The reaction provides rapid access to a series of 2-aminoquinolines with moderate to excellent yields. The reaction proceeds via quinolone-hydroxyquinoline tautomerization/O-H insertion to a rhodium(II)-aza vinyl carbene intermediate generated by denitrogenation of triazole followed by rearrangement to deliver the desired product. Furthermore, we demonstrated the iodine-mediated dealkylation of a 2-aminoquinoline derivative.

Annulation strategies for diverse heterocycles via the reductive transformation of 2-nitrostyrenes

Reduction of the stable nitro group is a fundamental and widely used transformation for the construction of complex and functionalized heterocyclic architectures. The unfolding of the reactivity of the nitro group in the 2-nitrostyrene moiety not only triggers the formation of carbon-nitrogen bonds, but also offers the opportunity for annulation and heteroannulation, thereby providing a cascade process for the synthesis of highly conjugated natural and unnatural molecules. In this review, we comprehensively discuss the use of 2-nitrostyrene motifs in the synthesis of various N-heterocycles. We offer readers an overview of the synthetic achievements achieved to date, highlighting their important features, reactivities, and mechanisms.

C2-Selective, Functional-Group-Divergent Amination of Pyrimidines by Enthalpy-Controlled Nucleophilic Functionalization

Synthesis of heteroaryl amines has been an important topic in organic chemistry because of their importance in small-molecule discovery. In particular, 2-aminopyrimidines represent a highly privileged structural motif that is prevalent in bioactive molecules, but a general strategy to introduce the pyrimidine C2-N bonds via direct functionalization is elusive. Here we describe a synthetic platform for site-selective C-H functionalization that affords pyrimidinyl iminium salt intermediates, which then can be transformed into various amine products in situ. Mechanism-based reagent design allowed for the C2-selective amination of pyrimidines, opening the new scope of site-selective heteroaryl C-H functionalization. Our method is compatible with a broad range of pyrimidines with sensitive functional groups and can access complex aminopyrimidines with high selectivity.

Transition-Metal-Free Regioselective Hydroamination of Styrenes with Amino-Heteroarenes

Base-mediated anti-Markovnikov hydroamination of functionally varied styrenes with amino-substituted pyridine, quinoline, pyrimidine, pyrazine, and phenanthridine with excellent regioselectivity has been described. Double hydroamination was observed chemoselectively on secondary amine leaving...

Site-Selective Deoxygenative Amination of Azine N-Oxides with Carbodiimides under Catalyst-, Activator-, Base-, and Solvent-Free Conditions

An operationally simple method for synthesizing 2-amino azines via [3+2] dipolar cycloaddition of azine N-oxide with carbodiimide has been demonstrated. The reaction can proceed smoothly under simple heating conditions without any transition metal catalyst, activator, base, and solvent. This transformation demonstrates a broad substrate scope and produces CO₂ as the only co-product. The applicability of this method is highlighted by the late-stage modification of bioactive molecules, including quinine, (±)-α-tocopherol, and tryptamine modified quinoline.

Deoxygenative C2-heteroarylation of quinoline N-oxides: facile access to α-triazolylquinolines

A metal- and additive-free, highly efficient, step-economical deoxygenative C2-heteroarylation of quinolines and isoquinolines was achieved from readily available N-oxides and N-sulfonyl-1,2,3-triazoles. A variety of α-triazolylquinoline derivatives were synthesized with good regioselectivity and in excellent yields under mild reaction conditions. Further, a gram-scale and one-pot synthesis illustrated the efficacy and simplicity of the developed protocol. The current transformation was also found to be compatible for the late-stage modification of natural products.

Reagent-free aerobic oxidative synthesis of amides from aldehydes and isothiocyanates

A reagent-free autoxidative reaction of aldehydes with isothiocyanates is developed to readily access amides, involving capture of carboxylic acids in situ generated from aldehydes by isothiocyanates as both coupling mediators and amine surrogates.

Regioselective C-H Functionalization of Heteroarene N-Oxides Enabled by a Traceless Nucleophile

Although N-alkenoxyheteroarenium salts have been widely used as umpoled synthons with nucleophilic (hetero)arenes, the use of electron-poor heteroarenes has remained unexplored. To overcome the inherent electron deficiency of quinolinium salts, a traceless nucleophile-triggered strategy was designed, wherein the quinolinium segment is converted into a dearomatized intermediate, thereby allowing simultaneous C8-functionalization of quinolines at room temperature. Experimental and computational studies support the traceless operation of a nucleophile, which enables the previously inaccessible transformation of N-alkenoxyheteroarenium salts. Remarkably, the generality of this strategy has been further demonstrated by broad applications in the regioselective C-H functionalization of other electron-deficient heteroarenes such as phenanthridine, isoquinoline, and pyridine N-oxides, offering a practical tool for the late-stage functionalization of complex biorelevant molecules.

Recent advances in the synthesis of biologically and pharmaceutically active quinoline and its analogues: a review

Recently, quinoline has become an essential heterocyclic compound due to its versatile applications in the fields of industrial and synthetic organic chemistry. It is a vital scaffold for leads in drug discovery and plays a major role in the field of medicinal chemistry. Nowadays there are plenty of articles reporting syntheses of the main scaffold and its functionalization for biological and pharmaceutical activities. So far, a wide range of synthesis protocols have been reported in the literature for the construction of this scaffold. For example, Gould-Jacob, Friedländer, Pfitzinger, Skraup, Doebner-von Miller and Conrad-Limpach are well-known classical synthesis protocols used up to now for the construction of the principal quinoline scaffold. Transition metal catalysed reactions, metal-free ionic liquid mediated reactions, ultrasound irradiation reactions and green reaction protocols are also useful for the construction and functionalization of this compound. The main part of this review focuses on and highlights the above-mentioned synthesis procedures and findings to tackle the drawbacks of the syntheses and side effects on the environment. Furthermore, various selected quinolines and derivatives with potential biological and pharmaceutical activities will be presented.

Deoxygenative Amination of Azine-N-oxides with Acyl Azides via [3 + 2] Cycloaddition

A transition-metal-free deoxygenative C-H amination reaction of azine-N-oxides with acyl azides is described. The initial formation of an isocyanate from the starting acyl azide via a Curtius rearrangement can trigger a [3 + 2] dipolar cycloaddition of polar N-oxide fragments to generate the aminated azine derivative. The applicability of this method is highlighted by the late-stage and sequential amination reactions of complex bioactive compounds, including quinidine and fasudil. Moreover, the direct transformation of aminated azines into various bioactive N-heterocycles illustrates the significance of this newly developed protocol.

A mild and metal-free synthesis of 2- and 1-alkyl/aryl/dialkyl-aminoquinolines and isoquinolines

A simple synthetic strategy has been developed for the synthesis of 2- and 1-alkyl/aryl/dialkylaminoquinolines and isoquinolines from the easily available quinoline and isoquinoline-N-oxides, different amines, triflic anhydride as activating agent and acetonitrile as solvent in a one-pot reaction under metal-free conditions at 0 °C to room temperature.

Regioselective Arylation of Quinoline N-Oxides (C8), Indolines (C7) and N-tert-Butylbenzamide with Arylboronic Acids

Herein, we disclose Ru(II)-catalyzed regioselective distal C(sp2)-H arylation of quinoline N-oxide with arylboronic acids to 8-arylquinolines. In the developed method, the Ru(II)-catalyst shows dual activity, that is, distal C-H activation of quinoline N-oxides followed by in situ deoxygenation of arylated quinoline N-oxide in the same pot. The current catalytic method features use of Ru metal as the catalyst and arylboronic acids as the arylating source under mild reaction conditions. Use of the Rh(III)-catalyst in place of Ru(II) under the same conditions afforded 8-arylquinoline N-oxides with excellent regioselectivity. Furthermore, the developed Ru(II) catalytic system is also extended for the C(sp2)-H arylation of indolines, N-tert-butylbenzamide, and 6-(5H)-phenanthridinone. Formation of the quinoline N-oxide coordinated ruthenium adduct is found to be the key reaction intermediate, which has been characterized by single crystal X-ray diffraction and NMR spectroscopy.

The direct C–H alkenylation of quinoline N-oxides as a suitable strategy for the synthesis of promising antiparasitic drugs

This review highlights the direct C–H alkenylation of quinoline N-oxides covering the metal-free and transition-metal catalysed protocols, and the regioselectivity during the synthesis of antiparasitic drugs based on quinoline scaffold.

Metal-free insertion of sulfur dioxide with aryl iodides under ultraviolet irradiation: direct access to sulfonated cyclic compounds

Metal-free insertion of sulfur dioxide with aryl iodides and silyl enolates or allylic bromide under ultraviolet irradiation at room temperature is accomplished. This protocol provides a convenient route to sulfonated cyclic compounds under mild conditions.

Recent advances in the direct β-C(sp2)–H functionalization of enamides

Recent advances in the direct β-C(sp²)–H functionalization of enamides, mainly including arylation, alkenylation, alkynylation, alkylation, acylation, sulfonylation, phosphorylation, and others, are reported.

A Cu2O/TBAB-promoted approach to synthesize heteroaromatic 2-amines via one-pot cyclization of aryl isothiocyanates with ortho-substituted amines in water

An efficient approach to synthesize heteroaromatic 2-amines from one-pot desulfurization/dehydrogenative cyclization of aryl isothiocyanates with ortho-substituted amines in water was developed. This approach tolerated a wide range of functional groups on the aromatic ring, providing a practical and environment-friendly process to synthesize heteroaromatic 2-amines in moderate to excellent yields. A plausible mechanism was proposed and the role of TBAB and Cu2O in the present strategy was suggested with the help of ESI mass spectrometry.

Direct cyanation, hydrocyanation, dicyanation and cyanofunctionalization of alkynes

In this review, direct cyanation, hydrocyanation, dicyanation, cyanofunctionalization and other cyanation reactions of alkynes were highlighted. Firstly, the use of nitriles and development of cyanation was simply introduced. After presenting the natural properties of alkynes, cyanation reactions of alkynes were classified and introduced in detail. Transition metal catalysed direct cyanation and hydrocyanation of alkynes gave alkynyl cyanides and alkenyl nitriles in good yields. Dicyanation of alkynes produced 1,2-dicyano adducts. Cyanofunctionalization of alkynes afforded functional cyanated compounds. Thiocyanation and selenocyanation yielded the expected functional vinylthiocyanates and vinylselenocyanates. A plausible reaction mechanism is presented if available.

Cyanation of alkynes was highlighted. Direct cyanation and hydrocyanation gave alkynyl cyanides and alkenyl nitriles. Dicyanation produced 1,2-dicyano adducts. Cyanofunctionalization afforded functional cyano compounds.

Transition-metal- and oxidant-free three-component reaction of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates via a dual radical coupling process

A convenient and straightforward three-component transformation of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates has been developed, providing the target products in moderate to good yields. Compared with previous studies, the present methodology avoids the use of transition-metal catalysts and excess oxidants, providing a simple and practical alternative approach for the construction of 2-sulfonylquinolines. Control experiments indicate that a dual radical coupling process is responsible for this reaction.

TBAI-catalyzed selective synthesis of sulfonamides and β-aryl sulfonyl enamines: coupling of arenesulfonyl chlorides and sodium sulfinates with tert-amines

A simple, practical and metal-free method has been developed for the synthesis of sulfonamides and β-arylsulfonyl enamines via the selective cleavage of C-N and C-H bonds through the iodine-catalyzed oxidation of arenesulfonyl chlorides and sodium sulfinates with tert-amines. The method uses commercially available inexpensive catalysts and oxidants, and has a wide substrate scope and operational simplicity.

Direct methyl C(sp3)-H azolation of thioanisoles via oxidative radical coupling

A method for metal-free, 1,3-dibromo-5,5-dimethylhydantoin mediated methyl C(sp³)-H bond azolation of thioanisoles has been developed, affording a facile route for the construction of nitrogen-functionalized thioanisoles, possibly via a nitrogen-centered radical process. This reaction represents an important addition to the limited number of existing methods for the methyl C(sp³)-H bond functionalization of thioanisoles, and may find practical application in the synthesis of nitrogen-alkylated azoles.