Step Economy Research Articles

Elemental Sulfur Promoted Cyclization of o‐Chloronitrobenzenes and Aryl Isothiocyanates to Furnish 2‐Aminobenzothiazoles

AbstractMethods to afford 2‐aminobenzothiazoles often involve the use of o‐aminothiophenols or aniline‐typed precursors. Herein we describe a method to furnish 2‐aminobenzothiazoles through the cyclization of o‐chloronitrobenzenes and aryl isothiocyanates with the assistance of elemental sulfur. From the perspective of step economy, our strategy allows for the direct use of nitroarene precursors without further reduction to anilines. The scope of the reaction with respect to both substrates was investigated, and it was found that electronic properties of the aryl isothiocyanate was important for obtaining acceptable yields.

Reductive Coupling of N-Heteroarenes and 1,2-Dicarbonyls for Direct Access to γ-Amino Acids, Esters, and Ketones Using a Heterogeneous Single-Atom Iridium Catalyst.

Despite their significant importance, the challenges in direct and diverse synthesis of N-heterocyclic γ-amino acids/esters/ketones hamper exploration of their applications. Herein, by developing a multifunctional heterogeneous iridium single-atom catalyst composed of silica-confined iridium species and a boron-doped ZrO2 support (Ir-SAs@B-ZrO2/SiO2), we describe its utility in establishing a new reductive coupling reaction of N-heteroarenes and 1,2-dicarbonyls for selective and diverse construction of the as-described compounds in a straightforward manner. The striking features, including good substrate and functionality tolerance, high step and atom economy, exceptional catalyst reusability, and diversified product post-transformations, highlight the practicality of the developed chemistry. Mechanistic studies reveal that the synergy between the active Ir sites and acidic support favors a chemoselective reduction of the more inert N-heteroarenes and affords requisite enamine intermediates. In this work, the concept on precise transformation of reductive intermediates will open a door to further develop useful tandem reactions by rational catalyst design.

Controllable Synthesis of Oxa‐/Thia‐Spirohydroquinolines by [3+(2+1)] Annulation of Aromatic Amines with Bimolecular O/S‐Heterocyclic Ketones

Abstract[3+(2+1)] Annulation represents the most straightforward and atom‐economical strategies in organic synthesis for the efficient construction of diverse cyclic structural units and complex molecules. Herein, we present a Lewis acid Sc(OTf)3‐catalyzed controlled synthesis of two different regioselective oxa/thia‐spirohydroquinolines (C=C at the α/β‐site of O/S‐atom) from aromatic amines and bimolecular O/S‐heterocyclic ketones via intermolecular [3+(2+1)] annulation. This method features mild reaction conditions, high step economy and atom utilization, and the O/S‐heterocyclic ketone can act as both two‐ and one‐carbon synthons. Furthermore, density functional theory (DFT) calculations indicate that the intrinsic cause of this unique regioselectivity is caused by the proton abstraction step.

Asymmetric Alkylative Difunctionalization of Carbon–Carbon Double Bonds by Aliphatic C─H Bond Activation

AbstractThis paper highlights the latest advancements in asymmetric alkylative difunctionalization of carbon–carbon double bonds by utilizing aliphatic C─H substrates as alkyl radical precursors. By integrating different hydrogen atom transfer (HAT) processes with various transition metal catalytic systems, various alkane substrates can be efficiently converted into corresponding alkyl radicals, which then participate in radical additions to carbon–carbon double bonds followed by stereoselective functionalizations with a third component. These strategies leverage inexpensive, abundant, and readily available alkane feedstocks to rapidly construct complex chiral molecules, demonstrating high step and atom economy. This paper focuses on recent progress and applications in this area, provides a comprehensive perspective on current developments, and suggests future directions in alkane‐involved asymmetric transformations.

Visible-Light Photoredox-Catalyzed Direct Carboxylation of Tertiary C(sp3)-H Bonds with CO2: Facile Synthesis of All-Carbon Quaternary Carboxylic Acids.

Direct carboxylation of C-H bonds with CO2 represents an attractive strategy to synthesize valuable carboxylic acids with high atom, step, and redox economy. Although great progress has been achieved in this field, catalytic carboxylation of tertiary C(sp3)-H bonds still remains challenging due to their inherent inertness and significant steric hindrance. Herein, we report a direct carboxylation of tertiary benzylic C(sp3)-H bonds with CO2 via visible-light photoredox catalysis. Various all-carbon quaternary carboxylic acids, which are of significant importance in medicinal chemistry, are successfully obtained with high yields. This direct carboxylation is characterized by good functional group tolerance, broad substrate scope, and mild operational conditions. Furthermore, our methodology enables the efficient and rapid synthesis of key drug or bioactive molecules, such as carbetapentane, caramiphen, and PRE-084 (σ1 receptor agonist), and facilitates various functionalizations of C(sp2)-H bonds using the directing ability of target carboxylic acids, thus highlighting its practical applications. Mechanistic studies indicate that a carbanion, which serves as the key intermediate to react with CO2, is catalytically generated via a single electron reduction of a benzylic radical through a consecutive photoinduced electron transfer process.

Synthesis of Polynuclear Aromatic Hydrocarbons by Palladium-catalyzed C-H Bond Functionalization

: Nowadays palladium-catalyzed C–H bond activation is a useful approach for the synthetic transformation of organic compounds due to step economy, the use of non-prefunctionalized substrates and reduced chemical wastes. Among the various synthetic strategies, palladium catalyzed intra and inter-molecular C–H bond activation has recently drawn a lot of interest to synthesize the decorated π-conjugated polycyclic aromatic hydrocarbon. In this review, we have focused on recent progress along with previous strategies to synthesize various polynuclear aromatic hydrocarbons (PAHs) by the use of Pd-catalyzed C–H bond activation. We have also discussed the mechanistic details of the reaction intra and inter-molecular C–H bond activation.

The Progress of Reductive Coupling Reaction by Iron Catalysis.

The transition metal catalyzed coupling reaction has revolutionized the strategies for forging the carbon-carbon bonds. In contrast to traditional cross-coupling methods using pre-prepared nucleophilic organometallic reagents, reductive coupling reactions for the C-C bonds formation provide some advantages. Because both coupling partners are reduced in the final products using a stoichiometric amount of a reductant, this approach not only avoids the need to use sensitive organometallic species, but also provides an orthogonal and complementary access to classical coupling reaction. Notably, the reductive coupling reactions feature readily available fragments, promote good step economy, exhibit high functional group tolerance and unique chemoselectivity, which have propelled their increasingly popular in the organic synthesis. In recent years, due to the low price, minimal toxicity, and environmentally benign character, iron-catalyzed carbon-carbon coupling reactions have garnered significant attention from the organic synthetic chemists and pharmacologists, especially the iron-catalyzed reductive coupling. This review aims to provide an insightful overview of recent advances in iron-catalyzed reductive coupling reactions, and to illustrate their possible reaction mechanisms.

Cost‐Effective Carbon Quaternization with Redox‐Active Esters and Olefins

AbstractQuaternary carbons are embedded in various natural products, pharmaceuticals, and organic materials. However, constructing this valuable motif is far from trivial. Conventional approaches mainly rely on classical polar disconnections and encounter bottlenecks concerning harsh conditions, functional group tolerance, regioselectivity, and step economy. In this context, Kawamata, Baran, Shenvi, and co‐workers recently demonstrated that two feedstock chemicals, alkyl carboxylic acids and olefins, could be utilized to construct tetrasubstituted carbons in the presence of an inexpensive iron porphyrin catalyst and a suitable reductant combination through quaternization of the radical intermediates. The method enables access to various sterically encumbered quaternary carbons under mild and robust conditions. Taking a complete detour from conventional approaches, the present heteroselective radical–radical coupling simplifies the synthesis of quaternary carbon‐containing molecules through an innovative and distinctive disconnection approach.

Cost-Effective Carbon Quaternization with Redox-Active Esters and Olefins.

Quaternary carbons are embedded in various natural products, pharmaceuticals, and organic materials. However, constructing this valuable motif is far from trivial. Conventional approaches mainly rely on classical polar disconnections and encounter bottlenecks concerning harsh conditions, functional group tolerance, regioselectivity, and step economy. In this context, Kawamata, Baran, Shenvi, and co-workers recently demonstrated that two feedstock chemicals, alkyl carboxylic acids and olefins, could be utilized to construct tetrasubstituted carbons in the presence of an inexpensive iron porphyrin catalyst and a suitable reductant combination through quaternization of the radical intermediates. The method enables access to various sterically encumbered quaternary carbons under mild and robust conditions. Taking a complete detour from conventional approaches, the present heteroselective radical-radical coupling simplifies the synthesis of quaternary carbon-containing molecules through an innovative and distinctive disconnection approach.

An environmentally benign process to synthesize vanillin and other substituted phenyl aldehydes using natural phenylpropenes

The limited vanillin (3a) production from plant sources requires identifying some renewable and sustainable approaches for its synthesis. This study aimed to develop an efficient, eco-friendly process for synthesizing vanillin (3a) from eugenol (1a) and eugenol-rich essential oils. The chemical methodology for vanillin (3a) synthesis involved base-mediated isomerization of eugenol (1a) to isoeugenol (2a), followed by OsO4/NaIO4 mediated oxidation of isoeugenol to vanillin (3a) using different additives such 1,4-diazabicyclo[2.2.2]octane (DABCO) and substituted pyridines in reusable environment-friendly solvents. Use of 2,6-dimethylpyridine and 2,6-dimethylpyridine N-oxide as additives in the oxidation step offered a significantly higher product yield (vanillin 3a, 70 %). The process synthesized vanillin (3a) irrespective of the cis/ trans stereochemistry of isoeugenol (2a). The peculiarity of the method relates to converting eugenol (1a) to vanillin (3a) without phenolic group protection, which offers step economy. Besides efficient vanillin (3a) synthesis, the process's general implications involve converting other naturally occurring phenylpropenes or phenylpropenes-enriched oils to the corresponding phenyl aldehydes (59–82 % yield).

Denitrative Functionalization of Nitroarenes: Recent Progress and Future Perspectives

Nitroarenes are fundamental feedstocks in the chemical industry. Their relatively inert C−NO2 bond allows for late‐stage modifications of molecules and exhibits complete orthogonality to the reactivity of C−halogen bond in cross‐coupling reactions. The denitrative functionalization of nitroarenes holds significant appeal due to it avoids the use of aryl halides, thereby simplifying reaction steps and improving atom and step economy. Recent progress in direct denitrative functionalization of nitroarenes includes palladium‐catalyzed denitrative coupling, copper‐catalyzed denitrative coupling, as well as metal‐free one‐pot C−NO2 functionalization. In this review, we provide a concise overview of these advancements, detailing the reaction features and mechanisms. This summary aims to highlight the versatility and efficiency of denitrative functionalization methodologies, offering insights into their potential applications and inspiring further research in this promising area of organic synthesis.

Synthesis of Chromone‐3‐phosphonates via Arbuzov‐Type C−P Cross Coupling from o‐Hydroxyphenyl Enaminones and Phosphites

AbstractWith an in situ C−H iodination tactic, a method for the synthesis of chromone‐3‐phosphonates was developed with trialkyl/triaryl phosphites as the reaction partners of o‐hydroxyphenyl enaminones by palladium catalysis. The product formation consists of cascade C−H iodination, chromone annulation, and Arbuzov‐type C−P cross coupling as major transformations. In addition to providing an enaminone‐based synthetic method to chromone‐3‐phosphonates, the work shows the advantage of step economy by skipping the separate operation for preparing the iodo‐functionalized chromone intermediate.

Sustainable (-)-Ambrox Production: Chemistry Meets Biocatalysis.

(-)-Ambrox, the most prominent olfactive component of ambergris, is one of the most widely used biodegradable fragrance ingredients. It is traditionally produced from the diterpene sclareol chemically modified and cyclized into (-)-ambrox. The availability of the new feedstock (E)-β-farnesene produced by fermentation opened new routes to (E,E)-homofarnesol as a precursor to (-)-ambrox. Combining the chemical transformation of (E)-β-farnesene to (E,E)-homofarnesol and its enzymatic cyclization with an engineered Squalene Hopene Cyclase provided a new sustainable route for the production of (-)-ambrox at industrial scale. Compared to the traditional synthesis from sclareol, the new and innovative route from (E)-β-farnesene improves atom and step economy, reduces waste production, solvent and energy consumption.

Photomediated One-Pot Three-Component Approach Enables the Formal Direct N-Acylation/Sulfonylation and α-C-H Functionalization of 1,2,3,4-Tetrahydroisoquinoline.

N-Acyl/sulfonyl-α-functionalized 1,2,3,4-tetrahydroisoquinolines (THIQs) are significant structural motifs in organic synthesis and drug discovery. However, the one-pot approach enabling direct difunctionalization of THIQs remains challenging. Herein we report a photomediated one-pot three-component strategy to access N-acyl/sulfonyl-α-functionalized THIQs. This method features the use of oxygen (from air) as the green oxidant, high atom and step economy, and decent structural diversity. The synthetic applicability of the method was further demonstrated via the facile construction of valuable bioactive molecules. Mechanistic studies indicated that oxidation with singlet oxygen and the acceptor-less dehydrogenation were involved in the photoredox process.

Ru(0)-catalysed synthesis of borylated polyene building blocks by cross-dimerisation toward cross-coupling.

Conjugated and non-conjugated polyenes are important substructures and are often found in biologically active compounds and natural products. Their preparation often needs multiple steps or iterative reactions and as a result, they have poor step economies. In this feature article, we show a new methodology to prepare these substructures by combinations of cross-dimerisation giving borylated polyenes and subsequent cross-coupling reactions. This divergent reaction strategy allows for the opportunity to access many bioactive compounds and natural products as well as some electronic materials.

Copper‐Catalyzed One‐Pot Three‐Component Aminophosphinoylation of 1,4‐Naphthoquinone

AbstractA copper‐catalyzed one‐pot three‐component aminophosphinoylation reaction of 1,4‐naphthoquinone with disubstituted phosphine oxides and arylamines has been developed. Various disubstituted phosphine oxides and arylamines with diverse structures and electronic properties can work smoothly with 1,4‐naphthoquinone, afforded a series of corresponding aminophosphinoylated 1,4‐naphthoquinones with up to 96 % isolated yield. The present method features easily available reagents, operational simplicity, broad substrate scope, high atom and step economy. The potential practicality of this protocol was further demonstrated by gram‐scale synthesis.

2,5-Disubstituted structures of 1,3,4-oxadiazole and 1,3,4-thiodiazole for anti-inflammatory potential: DFT approach for the structural aspects

We suggest a [3 + 2] annulation method that combines aromatic acid with acyl hydrazide using p-TSA, resulting in the formation of highly substituted 1,3,4-oxadiazole derivatives. These reactions take place efficiently under specific conditions: microwave irradiation, employing a metal-free environment, utilizing the environmentally friendly solvent PEG-400, and requiring only a short reaction time. Furthermore, employing similar conditions in the presence of Lawesson's reagent yields 1,3,4-thiadiazole derivatives. This method exhibits several notable characteristics, including atom and step economy, utilization of easily accessible raw materials, and simplicity in operation. From the DFT approach, the optimized geometrical parameters are obtained using the B3LYP/6–311++G (d,p) level of theory. There is a decrease in the energy gap and NLO property increases in nature and leads to more biological activity. Notably, there was a reduction in the energy gap, accompanied by an increase in the nonlinear optical property, and suggested the synthesized compounds exhibit heightened biological activity. Furthermore, these compounds demonstrate promising anti-inflammatory and anti-diabetic activity, as evidenced by protein denaturation assay studies.

Microwave-Assisted Synthesis of β-N-Aryl Glycoamphiphiles with Diverse Supramolecular Assemblies and Lectin Accessibility.

Glycoamphiphiles have attracted considerable interest in a broad range of application fields owing to their solution and bulk-state self-assembly abilities. Despite their importance, the straightforward synthesis of glycoamphiphiles consisting of a hydrophilic carbohydrate linked to a hydrophobic aglycone remains one of the major challenges in glycosciences. Here, a rapid, simple, and efficient synthetic access to chemically stable glycoamphiphiles at physiological pH, namely, N-(β-d-glycosyl)-2-alkylbenzamide, is reported. It leverages the nonreductive amination of unprotected carbohydrates with ortho-substituted aniline derivatives which could be readily obtained by reacting commercially available primary alkylamines with isatoic anhydride. This strategy avoids protection and deprotection of sugar hydroxyl groups and the use of reductive agents, which makes it advantageous in terms of atom and step economy. Moreover, in order to circumvent the cons of classical N-aryl glycosylation, we investigate the use of microwave as a heat source that provides fast, clean, and high-yield β-N-arylation of unprotected carbohydrates. Their self-assembly into water led to multiple morphologies of dynamic supramolecular glycoamphiphiles that were characterized to assess their ability to bind to lectins from pathogenic bacteria. Biophysical interactions probed by isothermal titration microcalorimetry revealed micromolar affinities for most of the synthesized glycoamphiphiles.

Regioselective Syntheses of 1,4- and 1,6-Dicarbonyl Compounds via Photoredox-Based Oxidative Heterocoupling of Enolsilanes with Oxygen as an Oxidant.

A photoredox-based oxidative heterocoupling of enolsilanes to the corresponding 1,4- and 1,6-dicarbonyl compounds was developed by using Mes-Acr+BF4- as the photocatalyst, and oxygen was used as the oxidant. This newly developed chemistry adheres to the principles of atom economy, step economy, and redox economy, making it a concise and efficient method.

Rh‐catalyzed three‐component synthesis of diverse N‐aryl pyrazoles via cascade pyrazole annulation and aryl C‐H addition to alkenes

A novel Rh‐catalyzed three‐component synthesis of N‐(o‐alkylaryl) pyrazoles via cascade pyrazole annulation and aryl C‐H conjugate addition to alkenes has been developed with enaminones, hydrazines, and functionalized alkenes as starting materials. The raw materials used in this reaction are simple, cheap, and readily available, and the reactions feature excellent step economy by furnishing products via the formation of a pyrazole ring and a key C‐H activation in one‐step operation. Moreover, the reaction proceeds smoothly under air atmosphere, making it easy to operate.