Clay based heterogeneous catalysts for carbon-nitrogen bond formation: a review

Clay and modified clay-based catalysts are widely used in organic transformation. Owing to the interlayer ions and good ion exchange capacity of clay, replacement with another ion and incorporation of different nanomaterials can be done. Due to these significant properties of clay, it can be utilized in the synthesis of various organic compounds. Carbon-nitrogen bonded compounds possess diverse applications in different fields. These compounds are prepared using different solid acid heterogeneous catalysts. This review presents a detailed discussion on clay used for the carbon-nitrogen bond formation reaction, such as the Biginelli reaction and A3 and KA2 coupling reactions. Additionally, other C-N bond formation reactions using various clay-based catalysts such as bentonite, montmorillonite, hydrotalcite and halloysite clay with various metals, metal oxides, Kegging type heteropoly acid and various nanomaterial incorporated clay heterogeneous catalysts are discussed.

Bond-Forming Processes Enabled by Silicon-Masked Aryl- and Alkyl-Substituted Diazenes

Aryl- and alkyldiimides (R-N═NH with R = aryl or alkyl) are elusive intermediates of valuable synthetic use, as they are assumed to be transient species in processes involving both carbon (with concomitant loss of N2) and nitrogen nucleophiles (with conservation of the N═N moiety). The actual compounds are fragile and as such not bench stable which is why they have not yet found the attention they deserve. Conversely, early contributions showed that the stability of the parent diimide is significantly increased by replacing the hydrogen atom by a silyl group, but the synthetic applicability of these silicon-protected aryl- and alkyldiazenes has been far less explored, in part due to the absence of general procedures for their preparation. This Perspective provides an overview of the underexplored diazene chemistry that has witnessed considerable progress in recent years and highlights the potential of this motif in a range of synthetically useful (catalytic) transformations. The rediscovered silicon-masked diazenes constitute a versatile platform possessing enhanced stability and tamed reactivity in comparison to the parent hydrogen-substituted diimides. Aryl, diazenyl, and alkyl anionic key intermediates can be selectively generated in situ under Lewis base or transition metal catalysis, giving rise to novel synthetic approaches as viable alternatives to the already existing methodologies.

Design rules for optimization of photophysical and kinetic properties of azoarene photoswitches

Azoarenes are an important class of molecular photoswitches that often undergo E → Z isomerization with ultraviolet light and have short Z-isomer lifetimes. Azobenzene has been a widely studied photoswitch for decades but can be poorly suited for photopharmacological applications due to its UV-light absorption and short-lived Z-isomer half-life (t1/2). Recently, diazo photoswitches with one or more thiophene rings in place of a phenyl ring have emerged as promising candidates, as they exhibit a stable photostationary state (98% E → Z conversion) and E-isomer absorption (λmax) in the visible light range (405 nm). In this work, we performed density functional theory calculations [PBE0-D3BJ/6-31+G(d,p)] on 26 hemi-azothiophenes, substituted with one phenyl ring and one thiophene ring on the diazo bond. We calculated the E-isomer absorption (λmax) and Z-isomer t1/2 for a set of 26 hemi-azothiophenes. We compared their properties to thiophene-based photoswitches that have been studied previously. We separated the 26 proposed photoswitches into four quadrants based on their λmax and t1/2 relative to past generations of hemi-azothiophene photoswitches. We note 8 hemi-azothiophenes with redshifted λmax and longer t1/2 than previous systems. Our top candidate has λmax and a t1/2 approaching 360 nm and 279 years, respectively. The results here present a pathway towards leveraging and optimizing two properties of photoswitches previously thought to be inversely related.

Synthesis of Non-Symmetric Azoarenes by Palladium-Catalyzed Cross-Coupling of Silicon-Masked Diazenyl Anions and (Hetero)Aryl Halides

The photoswitchable motif of azobenzenes is of great importance across the life and materials sciences. This maintains a constant demand for their efficient synthesis, especially that of non-symmetric derivatives. We disclose here a general strategy for their synthesis through an unprecedented C(sp² )-N(sp² ) cross-coupling where functionalized aryl-substituted diazenes masked with a silyl group are employed as diazenyl pronucleophiles. These equivalents of fragile diazenyl anions couple with a diverse set of (hetero)aryl bromides under palladium catalysis with no loss of dinitrogen. The competing denitrogenative biaryl formation is fully suppressed. The reaction requires only a minimal excess, that is 1.2 equivalents, of the diazenyl component. By this, a broad range of azoarenes decorated with two electron-rich/deficient aryl groups can be accessed in a predictable way with superb functional-group tolerance.

Decarbonylative/decarboxylative [4 + 2] annulation of phthalic anhydrides and cyclic iodoniums towards triphenylenes

A palladium-catalyzed decarbonylative/decarboxylative [4 + 2] annulation of phthalic anhydrides with cyclic diaryliodonium salts to synthesize triphenylenes has been developed. The reaction shows broad substrate scope with a high yield of up to 99%, and it provides an efficient and fast way to access functionalized triphenylenes in only one hour.

Microwave Assisted Rapid and Sustainable Synthesis of Unsymmetrical Azo Dyes by Coupling of Nitroarenes with Aniline Derivatives

Aromatic azo dyes are of immense commercial importance, and the development of greener routes for their synthesis is imperative due to current environmental concerns. In the present study, a microwave-assisted route has been developed for rapid and convenient synthesis of unsymmetrical azo dyes in a single step. In a metal-catalyst-free approach, an aromatic amine was used as an in situ reductant to affect its direct cross-condensation with nitroarenes to afford a variety of dispersed and water-soluble azo dyes. The electronic and substituent effects were thoroughly understood by placing suitable substituents in both nitroarenes and aniline derivatives in competitive reactions. The microwave (MW) method worked better with aniline or electron-rich aromatic amines to prepare a range of unsymmetrical azo dyes in up to 97% yields within a few minutes. The method worked well in the gram-scale synthesis of commercial dye, solvent yellow 7.

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Microwave-based green synthesis of unsymmetrical azo dyes

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Catalyst-free, rapid synthesis

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Gram-scale synthesis of commercial dyes

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Efficient synthesis of water-soluble dyes

Current state and future perspectives of cytochrome P450 enzymes for C–H and C=C oxygenation

Cytochrome P450 enzymes (CYPs) catalyze a series of C–H and C=C oxygenation reactions, including hydroxylation, epoxidation, and ketonization. They are attractive biocatalysts because of their ability to selectively introduce oxygen into inert molecules under mild conditions. This review provides a comprehensive overview of the C–H and C=C oxygenation reactions catalyzed by CYPs and the various strategies for achieving higher selectivity and enzymatic activity. Furthermore, we discuss the application of C–H and C=C oxygenation catalyzed by CYPs to obtain the desired chemicals or pharmaceutical intermediates in practical production. The rapid development of protein engineering for CYPs provides excellent biocatalysts for selective C–H and C=C oxygenation reactions, thereby promoting the development of environmentally friendly and sustainable production processes.

Cross-conjugation controls the stabilities and photophysical properties of heteroazoarene photoswitches

Azoarene photoswitches are versatile molecules that interconvert from their E-isomer to their Z-isomer with light. Azobenzene is a prototypical photoswitch but its derivatives can be poorly suited for in vivo applications such as photopharmacology due to undesired photochemical reactions promoted by ultraviolet light and the relatively short half-life (t_1/2) of the Z-isomer (2 days). Experimental and computational studies suggest that these properties (λ^max of the E isomer and t_1/2 of the Z-isomer) are inversely related. We identified isomeric azobisthiophenes and azobisfurans from a high-throughput screening study of 1540 azoarenes as photoswitch candidates with improved λ^max and t_1/2 values relative to azobenzene. We used density functional theory to predict the activation free energies and vertical excitation energies of the E- and Z-isomers of 2,2- and 3,3-substituted azobisthiophenes and azobisfurans. The half-lives depend on whether the heterocycles are π-conjugated or cross-conjugated with the diazo π-bond. The 2,2-substituted azoarenes both have t_1/2 values on the scale of 1 hour, while the 3,3-analogues have computed half-lives of 40 and 230 years (thiophene and furan, respectively). The 2,2-substituted heteroazoarenes have significantly higher λ^max absorptions than their 3,3-substituted analogues: 76 nm for azofuran and 77 nm for azothiophene.

Cu/Fe-mediated N(sp2)-arylation/alkenylation of pyridines with aryl-/alkenylboronic acids to yield versatile cationic materials

Cu/Fe-mediated N(sp2)-arylation/alkenylation of pyridines with aryl-/alkenylboronic acids to yield length-controllable and multi-responsive pyridinium salts is disclosed.