Enantioselective synthesis of 3,5-disubstituted thiohydantoins and hydantoins

Green metrics in mechanochemistry

The development of new green methodologies and their broader adoption for promoting sustainable development in chemistry laboratories and industry play a significant role in society, due to the economic importance of chemistry and its widespread presence in everyday life. Therefore, a sustainable approach to chemistry contributes to the well-being of the worldwide population and complies with the United Nations Sustainable Development Goals (UN SDGs) and the European Green Deal. The review highlights how batch and continuous mechanochemical methods are an eco-friendly approach for organic synthesis, with a lower environmental footprint in most cases, compared to solution-based procedures. The assessment is objectively based on the use of green metrics (e.g., atom and real atom economy, E-factor, process mass intensity, material parameter recovery, Eco-scale, stoichiometric factor, etc.) and indicators (e.g. DOZN tool and life cycle assessment, LCA, studies) applied to organic transformations such as synthesis of the amide bond, carbamates, heterocycles, active pharmaceutical ingredients (APIs), porphyrins, porous organic polymers (POPs), metal- or acid-catalysed processes, multicomponent and condensation reactions, rearrangements, etc. The generalized absence of bulk solvents, the precise control over the stoichiometry (i.e., using agents in a stoichiometrically rather than in excess), and the more selective reactions enabling simplified work-up procedures are the distinctive factors, marking the superiority of mechanochemical processes over solution-based chemistry.

Photo-on-Demand Phosgenation Reactions with Chloroform for Selective Syntheses of N-Substituted Ureas and Isocyanates

Two new reaction processes involving the in situ oxidative photochemical conversion of CHCl₃ to COCl₂ allowed selective syntheses of N-substituted ureas and isocyanates from amines. (I) A CHCl₃ solution containing an amine and an organic base under O₂ bubbling provided the urea derivative under exposure to UV light generated from a low-pressure mercury lamp at 20-40 °C. (II) A two-step reaction involving the oxidative photodecomposition of CHCl₃ at lower temperatures and subsequent sequential injections of an amine and organic base into the sample solution provided the isocyanate in high yield. The reaction processes of (I) and (II) capitalize on the solution conditions of [COCl₂] < [amine] and [COCl₂] > [amine], respectively, to result in 1:2 and 1:1 reactions. In general, isocyanates, especially aromatic and haloalkyl ones, readily undergo hydrolysis in the presence of an organic base. However, with the advantage of synthesizing the isocyanates in CHCl₃ solvent, direct addition of monoalcohols and diols to the as-prepared sample solution containing the diisocyanate allowed the one-pot syntheses of biscarbamates and polyurethanes, respectively. The reactions developed in this study are simple, safe, and inexpensive methods of synthesizing N-substituted ureas and isocyanates, and derivatives of isocyanates such as carbamates and polyurethanes. The present new methods can replace current synthetic methods using COCl₂ in both academia and industry.

Pd-Catalysed oxidative carbonylation of α-amino amides to hydantoins under mild conditions

The first example of palladium-catalysed oxidative carbonylation of unprotected α-amino amides to hydantoins is described here. The selective synthesis of the target compounds was achieved under mild conditions (1 atm of CO), without ligands and bases. The catalytic system overrode the common reaction pathway that usually leads instead to the formation of symmetrical ureas.

Synthesis of thia- and thioxo-tetraazaspiro[4.4]nonenones from nitrile imines and arylidenethiohydantoins

5-Arylidene-1-methyl-2-thiohydantoins undergo [3+2]-cycloaddition reaction with nitrile imines, generated in situ from hydrazonyl chlorides, at C=C and C=S dipolarophiles in the thiohydantoin moiety to afford thioxo-tetraazaspiro[4.4]nonenones and thia-tetraazaspiro[4.4]nonenones in moderate to good yields. The stereochemistry of these spiroheterocycles has been confirmed by X-ray diffraction studies.

Sub-stoichiometric 2D covalent organic frameworks from tri- and tetratopic linkers

Covalent organic frameworks (COFs) are typically designed by breaking down the desired network into feasible building blocks - either simple and highly symmetric, or more convoluted and thus less symmetric. The linkers are chosen complementary to each other such that an extended, fully condensed network structure can form. We show not only an exception, but a design principle that allows breaking free of such design rules. We show that tri- and tetratopic linkers can be combined to form imine-linked [4 + 3] sub-stoichiometric 2D COFs featuring an unexpected bex net topology, and with periodic uncondensed amine functionalities which enhance CO2 adsorption, can be derivatized in a subsequent reaction, and can also act as organocatalysts. We further extend this class of nets by including a ditopic linker to form [4 + 3 + 2] COFs. The results open up possibilities towards a new class of sub-valent COFs with unique structural, topological and compositional complexities for diverse applications.

Revisiting the Urech Synthesis of Hydantoins: Direct Access to Enantiopure 1,5-Substituted Hydantoins Using Cyanobenziodoxolone

A method for the synthesis of enantiopure 1,5-substituted hydantoins was developed using a hypervalent iodine cyanation reagent (cyanobenziodoxolone, CBX) as a source of electrophilic carbon. Starting from inexpensive commercially available enantiopure protected amino acids, the method allowed the synthesis of various hydantoins without epimerization. Formation of hydantoins from dipeptides was also possible, but partial epimerization was observed in this case. This synthetic strategy is user friendly as CBX is a bench-stable easy-to-handle crystalline reagent and avoids conventional multistep protocols, thus allowing the facile synthesis of a library of chiral hydantoins.

Formation of Hindered Arylcarbamates using Alkyl Aryl Carbonates under Highly Reactive Conditions

Hindered O-tert-alkyl N-arylcarbamates were conveniently prepared by treating arylamines with aryl tert-alkyl carbonates in the presence of a strong base. The new method avoids the use of sensitive and difficult-to-access dialkyl dicarbonates and isocyanates, which are most commonly used in known methods. Instead, the stable and readily accessible alkyl aryl carbonates are used. Therefore, the new method is particularly suitable for the synthesis of N-arylcarbamates that contain a complex O-alkyl moiety. Using the method, electron-rich and electron-poor, and primary and secondary arylamines can all be conveniently converted to their carbamates with acceptable yields. The method was also found equally effective for the synthesis of the less hindered O-secondary and O-primary alkyl N-arylcarbamates.