A more sustainable isothiocyanate synthesis by amine catalyzed sulfurization of isocyanides with elemental sulfur

Na2CO3-promoted synthesis of thiocarbamates from isocyanates and thiols under mild conditions

Here, we developed an efficient method for the synthesis of s-alkyl or aryl thiocarbamates catalyzed by Na2CO3 under mild conditions using alkyl or aryl isocyanates and thiophenols or thiols. In situ reaction with deuterated tetrahydrofuran (d8-THF) was carried out to investigate the reaction mechanism. This method has the advantages of high yield, good functional group tolerance and high atom economy.

Visible-Light-Promoted Cascade Coupling of 2-Isocyanonaphthalenes with Elemental Sulfur and Amines to Construct Naphtho[2,1-d]thiazol-2-Amines

A visible-light-induced strategy has been explored for the synthesis of naphtho[2,1-d]thiazol-2-amines through ortho-C-H sulfuration of 2-isocyanonaphthalenes with elemental sulfur and amines under external photocatalyst-free conditions. This three-component reaction, which utilizes elemental sulfur as the odorless sulfur source, molecular oxygen as the clean oxidant, and visible light as the clean energy source, provides a mild and efficient approach to construct a series of naphtho[2,1-d]thiazol-2-amines. Preliminary mechanistic studies indicated that visible-light-promoted photoexcitation of reaction intermediates consisting of thioureas and DBU might be involved in this transformation.

Electro-oxidative three-component cascade coupling of isocyanides with elemental sulfur and amines for the synthesis of 2-aminobenzothiazoles

2-Aminobenzothiazoles are commonly encountered in various functional compounds. Herein, we disclose an electro-oxidative three-component reaction for the effective synthesis of 2-aminobenzothiazoles under mild conditions, utilizing non-toxic and abundant elemental sulfur as the sulfur source. Both aliphatic amines and aryl amines demonstrate good compatibility at room temperature, highlighting the broad functional group tolerance of this approach. Additionally, elemental selenium demonstrated reactivities comparable to those of elemental sulfur.

Recent advancement in the synthesis of isothiocyanates

Isothiocyanates exhibit various biological characteristics, including antimicrobial, anti-inflammatory, and anticancer properties. Their significance extends to synthetic chemistry, where they serve as valuable platforms for versatile transformations. Consequently, they have attracted the attention of biologists and chemists. This review summarizes recent advancements in the synthesis of isothiocyanates. Access to a variety of starting materials is important to prepare isothiocyanates with diverse structures. This review categorizes synthetic methods into three types based on the starting materials and functional groups: (i) type A, derived from primary amines; (ii) type B, derived from other nitrogen functional groups; and (iii) type C, derived from non-nitrogen groups. Recent trends in synthetic methods have revealed the prevalence of type-A reactions derived from primary amines. However, type B reactions have rarely been reported. Notably, over the past four years, there has been a notable increase in type C reactions, indicating a growing interest in non-nitrogen-derived isothiocyanates. Overall, this review not only outlines the advancements in the synthesis of isothiocyanates but also highlights trends in the methodology.

Cyrene: a very reactive bio-based chiral ketone in diastereoselective Passerini reactions

Cyrene, a green bioderived solvent from waste cellulose, was applied to the synthesis of novel α-acyloxyamide derivatives through a Passerini-3CR with carboxylic acids and isocyanides with good yields and diastereoselectivities under mild conditions. Cyrene showed exceptionally high reactivity and the degree of diastereoselection was dependent mostly on the isocyanide. DFT calculations as well as the experimental findings indicated that both kinetic and thermodynamic effects might explain the results.

Bacterial Isothiocyanate Biosynthesis by Rhodanese-Catalyzed Sulfur Transfer onto Isonitriles

Natural products bearing isothiocyanate (ITC) groups are an important group of specialized metabolites that play various roles in health, nutrition, and ecology. Whereas ITC biosynthesis via glucosinolates in plants has been studied in detail, there is a gap in understanding the bacterial route to specialized metabolites with such reactive heterocumulene groups, as in the antifungal sinapigladioside from Burkholderia gladioli. Here we propose an alternative ITC pathway by enzymatic sulfur transfer onto isonitriles catalyzed by rhodanese-like enzymes (thiosulfate:cyanide sulfurtransferases). Mining the B. gladioli genome revealed six candidate genes (rhdA-F), which were individually expressed in E. coli. By means of a synthetic probe, the gene products were evaluated for their ability to produce the key ITC intermediate in the sinapigladioside pathway. In vitro biotransformation assays identified RhdE, a prototype single-domain rhodanese, as the most potent ITC synthase. Interestingly, while RhdE also efficiently transforms cyanide into thiocyanate, it shows high specificity for the natural pathway intermediate, indicating that the sinapigladioside pathway has recruited a ubiquitous detoxification enzyme for the formation of a bioactive specialized metabolite. These findings not only elucidate an elusive step in bacterial ITC biosynthesis but also reveal a new function of rhodanese-like enzymes in specialized metabolism.

Sodium Carbonate-Promoted Formation of 5-Amino-1,2,4-thiadiazoles and 5-Amino-1,2,4-selenadiazoles with Elemental Sulfur and Selenium

Sodium carbonate-promoted facile synthesis of 5-amino-1,2,4-thiadiazoles and 5-amino-1,2,4-selenadiazoles with elemental sulfur and selenium, respectively, was developed. This method was carried out with O2 in the air as the green oxidant, and it has several advantages, including low cost, low toxicity, and stable sulfur and selenium sources, good to excellent yields with water as the sole byproduct, simple operation, and a broad substrate scope. Preliminary mechanistic studies indicate that the formation of the 1,2,4-thiadiazole ring and the 1,2,4-selenadiazole ring undergoes different processes.

Synthesis of highly substituted alkenes by sulfur-mediated olefination of N-tosylhydrazones

Tetraphenylethylenes (TPEs) are well-known for their aggregation-induced emission properties. The synthesis of TPE derivatives, as well as other highly substituted olefins, generally requires the use of hazardous reagents, such as metalorganic compounds, to overcome the high activation energies caused by the sterically congested double bond. Herein, we present an efficient and metal-free procedure for the synthesis of tetraarylethylenes via alkylidene-homocoupling of N-tosylhydrazones, derived from readily available benzophenones, in excellent yields. The method relies only on cheap and benign additives, i.e. elemental sulfur and potassium carbonate, and easily competes with other established procedures in terms of scope, yield and practicability. A mechanistic study revealed a diazo compound, a thioketone and a thiirane as key intermediates in the pathway of the reaction. Based on this, a modified method, which allows for selective alkylidene-cross-coupling, generating a broader scope of tri- and tetrasubstituted olefins in good yields, is showcased as well.

Dihydrolevoglucosenone (Cyrene™), a new possibility of an environmentally compatible solvent in synthetic organic electrochemistry

Dihydrolevoglucosenone (DLG or Cyrene™) solvent is a green dipolar solvent produced from cellulose waste. Different studies have demonstrated that it can successfully replace dipolar solvents, such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and N-methylpyrrolidinone (NMP), in a variety of chemical reactions. In this paper, the first application of DLG in organic electrosynthesis is described, with results of its use in the electrochemical reduction of benzophenone derivatives (ca. E = -1.75 V vs. Ag/AgCl), as a greener alternative to other dipolar solvents with environmental concerns. Conductivity measurements show that the solvent presents conductivity and viscosity limitations that can be overcome by using EtOH as a cosolvent. The DLG/EtOH mixture resulted in a convenient solvent to carry out galvanostatic electroreductions of starting materials that exhibit high potential value. Furthermore, the reaction pathway (1e- or 2e-) was found to be dependent on the supporting electrolyte used; TBABF4 favored 2e- reduction to the corresponding alcohol (52-85%), whereas LiClO4 promoted C-C pinacolic coupling (47-70%).

Cyrene: a bio-based solvent for the Mizoroki-Heck reaction of aryl iodides

The development of greener and more sustainable methods, as well as the adaptation of already existing protocols to more environmentally friendly procedures, has become crucial for organic synthesis. The introduction and utilization of greener solvents is a very promising alternative, especially when they can replace toxic organic solvents in the known and widely used organic reactions. Cyrene has appeared to be an excellent alternative solvent for a number of organic reactions. In this work, the development of a new, greener and more economical protocol for the Mizoroki-Heck reaction is described, using Cyrene as the green solvent and Pd/C as the palladium catalyst source. A wide substrate scope for the coupling of aryl iodides with acrylamides, acrylates, acrylic acid, acrylonitrile and styrene was demonstrated. The recyclability of Cyrene and the leaching of palladium in the final product were examined in order to enhance the industrial applicability of this protocol. Furthermore, the synthesis of the natural product piperlotine A is reported.

Cyrene: A Green Solvent for the Synthesis of Bioactive Molecules and Functional Biomaterials

In the panorama of sustainable chemistry, the use of green solvents is increasingly emerging for the optimization of more eco-friendly processes which look to a future of biocompatibility and recycling. The green solvent Cyrene, obtained from biomass via a two-step synthesis, is increasingly being introduced as the solvent of choice for the development of green synthetic transformations and for the production of biomaterials, thanks to its interesting biocompatibility, non-toxic and non-mutagenic properties. Our review offers an overview of the most important organic reactions that have been investigated to date in Cyrene as a medium, in particular focusing on those that could potentially lead to the formation of relevant chemical bonds in bioactive molecules. On the other hand, a description of the employment of Cyrene in the production of biomaterials has also been taken into consideration, providing a point-by-point overview of the use of Cyrene to date in the aforementioned fields.

Sustainable One-Pot Cellulose Dissolution and Derivatization via a Tandem Reaction in the DMSO/DBU/CO2 Switchable Solvent System

New sustainable concepts have to be developed to overcome the increasing problems of resource availability. Cellulose derivatives with tunable material properties are promising biobased alternatives to existing petroleum-derived polymeric materials. However, the chemical modification of cellulose is very challenging, often requiring harsh conditions and complex solubilization or activation steps. More sustainable procedures toward novel cellulose derivatives are therefore of great interest. Herein, we describe a novel concept combining two approaches, (i) tandem catalysis and (ii) cellulose derivatization, by applying a single catalyst for three transformations in the DMSO/DBU/CO₂ switchable solvent system. Cellulose was functionalized with four different biobased isothiocyanates, which were formed in situ via a catalytic sulfurization of isocyanides with elemental sulfur, preventing the exposure and handling of the isothiocyanates. The degree of substitution of the formed O-cellulose thiocarbamates was shown to be controllable in a range of 0.52-2.16 by varying the equivalents of the reactants. All obtained products were analyzed by ATR-IR, ¹H, ¹³C, and ³¹P NMR spectroscopy as well as size exclusion chromatography, elemental analysis, differential scanning calorimetry, and thermal gravimetric analysis. Finally, the tandem reaction approach was shown to be beneficial in terms of efficiency as well as sustainability compared to a stepwise synthesis. Recycling ratios ranging from 79.1% to 95.6% were obtained for the employed components, resulting in an E-factor of 2.95 for the overall process.

Recent Advances in the Synthesis of Isothiocyanates Using Elemental Sulfur

Isothiocyanates (ITCs) are biologically active molecules found in several natural products and pharmaceutical ingredients. Moreover, due to their high and versatile reactivity, they are widely used as intermediates in organic synthesis. This review considers the best practices for the synthesis of ITCs using elemental sulfur, highlighting recent developments. First, we summarize the in situ generation of thiocarbonyl surrogates followed by their transformation in the presence of primary amines leading to ITCs. Second, carbenes and amines afford isocyanides, and the further reaction of this species with sulfur readily generates ITCs under thermal, catalytic or basic conditions. Additionally, we also reveal that in the catalyst-free reaction of isocyanides and sulfur, two—until this time overlooked and not investigated—different mechanistic pathways exist.