Friedel-Crafts Acylation Reaction Research Articles

Conversion of waste polystyrene into porous adsorbents for efficient removal of hazardous pollutants: Adsorption properties and adsorption mechanism

Herein, the porous adsorbent material (PS-SAS) was prepared by introducing a large number of carboxyl groups on the benzene ring of waste polystyrene (WPS) by Friedel-Crafts acylation reaction using succinic anhydride as the modifying group, followed by further alkaline modification, and was used for the removal of hazardous dye and heavy metal ion. The chemical structure and surface morphology of the prepared adsorbent were assessed via Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), optical microscope (OM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The prepared adsorbent gave highly efficient separation performance towards malachite green (MG) and Cu2+ with corresponding maximum sorption capacities of 2231 mg/g and 156 mg/g at 298 K. The experimental data indicated that the sorption process followed the Langmuir adsorption isotherm and the pseudo-second-order kinetic model. Moreover, thermodynamic studies also demonstrated that the sorption process occurred spontaneously. The adsorption mechanisms of the adsorbent for MG include hydrogen bonding, electrostatic attraction, n-π interactions, and π-π interactions, while for Cu2+ via electrostatic attraction, ion exchange, and complexation. Furthermore, the selective adsorption, regeneration, dynamic sorption, and simulated wastewater experiment demonstrated that the adsorbent is a promising adsorbent. And phytotoxicity test proved that the prepared adsorbent can effectively reduce the toxicity of dye and metal ions. In summary, the transformation of waste polystyrene into an adsorbent represents a promising solution for water purification.

Reactions of Benzyl Phosphine Oxide/Sulfide with (COCl)2: Synthesis of Novel Acyl Chloride-Substituted Chlorophosphonium Ylides.

New reactions of benzyl phosphine oxide/sulfide with oxalyl chloride are presented. The resulting reactive intermediates, acyl chloride-substituted chlorophosphonium ylides, are capable of undergoing esterification and Friedel-Crafts acylation reactions, ultimately yielding either methyl 2-(2-bromophenyl)-2-(diphenylphosphoryl)acetate or β-carbonyl-diarylphosphine oxide derivatives. Additionally, when an alkynyl group is contained in the acyl chloride-substituted chlorophosphonium ylide, intramolecular cyclization occurs, leading to the formation of a pair of trans- and cis-dichlorophosphonyl benzofulvene isomers. The generation process of acyl chloride-substituted chlorophosphonium ylide was carefully monitored by using 31P{1H} NMR spectroscopy, and a plausible reaction mechanism was proposed.

Catalytic Advantages of SO3H-Modified UiO-66(Zr) Materials Obtained via Microwave Synthesis in Friedel-Crafts Acylation Reaction.

The catalytic activity and stability of sulfonic-based UiO-66(Zr) materials were tested in the Friedel-Crafts acylation of anisole with acetic anhydride. The materials were prepared using microwave-assisted synthesis, producing microporous materials with remarkable crystallinity and physicochemical features as acid catalysts. Different ratios between both organic ligands, terephthalic acid (H2BDC) and monosodium 2-sulfoterephthalic acid (H2BDC-SO3Na), were used for the synthesis to modulate the sulfonic content. The sulfonic-based UiO-66(Zr) material synthesized with a H2BDC/H2BDC-SO3Na molar ratio of 40/60 exhibited the best catalytic performance in the acidic-catalyzed Friedel-Crafts acylation reaction. This ratio balanced the number of sulfonic acid sites and their accessibility within the UiO-66 microporous structure. The catalytic performance of this material increased remarkably at 200 °C, outperforming reference acids and commercial heterogeneous catalysts such as Nafion-SAC-13 and Amberlyst-70. Additionally, the best sulfonic-based UiO-66(Zr) material proved to be stable in four successive reaction cycles, maintaining both its catalytic activity and its structural integrity.

The methods of synthesis of 2-aminobenzophenones — key precursors of 1,4-benzodiazepines

The review article briefly presents the pharmacological profile and mechanism of action of 1,4-dibenzodiazepines, and the structures of the most important drugs from this class of compounds. The strongest emphasis is placed on presentation of various methods for the synthesis of 2-aminobenzophenones — the most often used precursors in the synthesis of 1,4-benzodiazepines and numerous classes of azaarenes. The reactions or their sequences were grouped according to retrosynthetic strategies based on structural similarities and differences of the reagents used. There were presented not only the most classical methods like Friedel-Crafts acylation or Grignard reaction, but also more advanced transformations utilizing various catalysts and more sophisticated approaches involving the synthesis of heterocyclic systems followed by their reductive or oxidative cleavage. Selected nuances of the presented reactions were discussed, including fragments of mechanisms, limitations of the applicability of a given method, and the advantages of modern solutions over the oldest and more classical methods.

Sulfonated mesoporous TUD-1: An innovative environmentally friendly solid acid catalyst for various organic transformations

TUD-1, a novel sponge-like three-dimensional mesoporous silica material, was synthesized in one-step using a hydrothermal technique and triethanolamine as a template. To enhance its acidic properties, the TUD-1 material underwent sulfonation with varying amounts of sulfonic groups (1 and 3 wt%). Structural characteristics were determined using XRD, FTIR, Raman, BET analysis, HRTEM, SEM, and XPS. Surface acidities of the catalysts were evaluated through non-aqueous potentiometric titration and FTIR analysis of chemisorbed pyridine. The performance of catalysts was assessed in various reactions, including the Pechmann reaction, Friedel-Crafts acylation reaction, and Biginelli reaction for synthesis of 7-hydroxy-4-methyl-2H-chromen-2-one, aromatic ketones and 3, 4-dihydropyrimidin-2(1H)-one. The findings confirm that the TUD-1 mesoporous structure did not undergo any significant alteration post sulfonation. The sulfonated catalysts exhibited higher surface acidities than TUD-1. A correlation between surface acidity, particularly Brönsted acid sites, and catalytic efficiency in selected reactions was evident from both catalytic testing and characterization. Under moderate conditions, TUD-1-3SO3H exhibited outstanding catalytic performance and remarkable reusability across all selected reactions.

Alternative Reactions to Friedel-crafts Acylation on Highly Activated Substrates

Abstract: Friedel-Crafts acylation (FCAcyl) is the most widespread method used to prepare aryl ketones and aldehydes. However, depending on the type of group attached to the benzene, their derivatives influence the electronic characteristics and structural orientations of the compounds during acylation; thus, the groups are very important for the success of the reaction. The existence of strong electron-donating groups, such as polyhydroxy/ polyalkoxyphenols and anilines on the aromatic ring, makes this reaction difficult. To overcome these problems and with the aim of obtaining aromatic ketones from benzene compounds, appropriate methodologies were described. Therefore, this review consists of showing the importance and applicability of the Houben- Hoesch and Sugasawa reactions as alternatives for the Friedel-Crafts acylation of polyhydroxy/ polyalkoxyphenols and anilines, respectively. The main advances used in the original methodologies were also described. The use of these reactions as an alternative to the renowned Friedel-Crafts acylation reactions should be taken into consideration as an important synthetic tool because there is the possibility of reducing steps, with consequent improvement of yield, in addition to optimizing reaction performance.

SYNTHESIS AND BIOLOGICAL ACTIVITIES OF ACETAMINOPHEN AND IBUPROFEN METAL COMPLEXES OR DERIVATIVES: A REVIEW

We reviewed scientific literature on the synthesis of acetaminophen and ibuprofen, as well as their derivatives and biological properties. The synthesis of acetaminophen involves the acetylation of 4-aminophenol and acetic anhydride, while ibuprofen is synthesised by reacting isobutyl benzene and acetic anhydride in four continuous reaction stages, which are Friedel-Crafts acylation, carbonyl reduction, chloride substitution, and Grignard reaction. To obtain their derivatives, modifications have been made either by complexing the main structure of the drug compound with metal elements or adding certain desired moieties, such as thiourea, amide, ammonium, halogen, silicon, and 1,3,4-oxadiazole. Ibuprofen and acetaminophen have been recognised as effective painkillers and anti-inflammatories. Recently, their derivatives have been implicated in a variety of biological effects. The biological activities of acetaminophen and ibuprofen derivatives have been reported to exhibit urease inhibition and inflammatory inhibition, as well as inhibit the proliferation of breast cancer cells MCF-7. Overall, this review article describes the synthesis of acetaminophen and ibuprofen derivatives, complete with their biological activities such as antimicrobial, antifungal, anti-inflammatory, urease inhibitors, and anticancer.

Synthesis and Characterization of DOPO-Containing Poly(2,6-dimethyl-1,4-phenylene oxide)s by Oxidative Coupling Polymerization.

A set of polyphenylene oxides incorporating DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) functionality, denoted as DOPO-R-PPO, was synthesized by copolymerization of 2,6-dimethylphenol (2,6-DMP) with various DOPO-substituted tetramethyl bisphenol monomers. In the initial step, a Friedel-Crafts acylation reaction was employed to react 2,6-DMP with different acyl chlorides, leading to the formation of ketone derivatives substituted with 2,6-dimethylphenyl groups. Subsequently, the ketones, along with DOPO and 2,6-DMP, underwent a condensation reaction to yield a series of DOPO-substituted bisphenol derivatives. Finally, polymerizations of 2,6-dimethylphenol with these DOPO-substituted bisphenols were carried out in organic solvents using copper(I) bromide/N-butyldimethylamine catalysts (CuBr/DMBA) under a continuous flow of oxygen, yielding telechelic PPO oligomers with DOPO moieties incorporated into the polymer backbone. The chemical structures of the synthesized compounds were characterized using various analytical techniques, including Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H NMR), phosphorus nuclear magnetic resonance (31P NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). When compared to conventional poly(2,6-dimethyl-1,4-phenylene oxide)s with a similar molecular weight range, all DOPO-PPOs exhibited higher glass transition temperatures, enhanced thermal degradability, and increased char yield formation at 800 °C without compromising solubility in organic solvents.

Hierarchical Zeolites Prepared Using a Surfactant-Mediated Strategy: ZSM-5 vs. Y as Catalysts for Friedel-Crafts Acylation Reaction.

Hierarchical ZSM5 and Y zeolites were prepared through a surfactant-mediated strategy with NH4OH changing the duration of the treatment and the amount of CTAB surfactant and taking as reference multiples of the critical micellar concentration (CMC). The materials were characterized using powder X-ray diffraction, N2 adsorption isotherms at -196 °C, and SEM and TEM microscopy. The catalytic performance was evaluated in Friedel-Crafts acylation of furan with acetic anhydride at 80 °C. The alkaline surfactant-mediated treatment had different effects on the two zeolites. For ZSM5, the CTAB molecular aggregates can hardly diffuse inside the medium-size pores, leading mainly to intercrystalline mesoporosity and increased external surface area, with no positive catalytic impact. On the other hand, for large-pore Y zeolite, the CTAB molecular aggregates can easily diffuse and promote the rearrangement of crystal units around micelles, causing the enlargement of the pores, i.e., intracrystalline porosity. The optimized Y-based sample, treated for 12 h with a CTAB amount 32 times the CMC, shows an increase in product yield and rate constant that was not observed when a higher amount of surfactant was added. The reuse of spent catalysts upon thermal treatment at 400 °C shows a regeneration efficiency around 90%, showing good potentialities for the modified catalysts.

Friedel-Crafts reactions for biomolecular chemistry.

Chemical tools and principles have become central to biological and medical research/applications by leveraging a range of classical organic chemistry reactions. Friedel-Crafts alkylation and acylation are arguably some of the most well-known and used synthetic methods for the preparation of small molecules but their use in biological and medical fields is relatively less frequent than the other reactions, possibly owing to the notion of their plausible incompatibility with biological systems. This review demonstrates advances in Friedel-Crafts alkylation and acylation reactions in a variety of biomolecular chemistry fields. With the discoveries and applications of numerous biomolecule-catalyzed or -assisted processes, these reactions have garnered considerable interest in biochemistry, enzymology, and biocatalysis. Despite the challenges of reactivity and selectivity of biomolecular reactions, the alkylation and acylation reactions demonstrated their utility for the construction and functionalization of all the four major biomolecules (i.e., nucleosides, carbohydrates/saccharides, lipids/fatty acids, and amino acids/peptides/proteins), and their diverse applications in biological, medical, and material fields are discussed. As the alkylation and acylation reactions are often fundamental educational components of organic chemistry courses, this review is intended for both experts and nonexperts by discussing their basic reaction patterns (with the depiction of each reaction mechanism in the ESI) and relevant real-world impacts in order to enrich chemical research and education. The significant growth of biomolecular Friedel-Crafts reactions described here is a testament to their broad importance and utility, and further development and investigations of the reactions will surely be the focus in the organic biomolecular chemistry fields.

Advancements in lewis acid catalysis for friedel-crafts acylation reactions

Advancements in lewis acid catalysis for friedel-crafts acylation reactions

Biosynthesis of 4-Acyl-5-aminoimidazole Alkaloids Featuring a New Friedel-Crafts Acyltransferase.

Friedel-Crafts acylation (FCA) is a highly beneficial approach in organic chemistry for creating the important C-C bonds that are necessary for building intricate frameworks between aromatic substrates and an acyl group. However, there are few reports about enzyme catalyzed FCA reactions. In this study, 4-acyl-5-aminoimidazole alkaloids (AAIAs), streptimidazoles A-C (1-3), and the enantiopure (+)-nocarimidazole C (4) as well as their ribosides, streptimidazolesides A-D (5-8), were identified from the fermentation broth of Streptomyces sp. OUCMDZ-944 or heterologous S. coelicolor M1154 mutant. The biosynthetic gene cluster (smz) was identified, and the biosynthetic pathway of AAIAs was elucidated for the first time. In vivo and in vitro studies proved the catalytic activity of the four essential genes smzB, -C, -E, and -F for AAIAs biosynthesis and clarified the biosynthetic process of the alkaloids. The ligase SmzE activates fatty acyl groups and connects them to the acyl carrier protein (ACP) holo-SmzF. Then, the acyl group is transferred onto the key residue Cys49 of SmzB, a new Friedel-Crafts acyltransferase (FCase). Subsequently, the FCA reaction between the acyl groups and 5-aminoimidazole ribonucleotide (AIR) occurs to generate the key intermediate AAIA-nucleotides catalyzed by SmzB. Finally, the hydrolase SmzC catalyzes the N-glycosidic bond cleavage of the intermediates to form AAIAs. Structural simulation, molecular modeling, and mutational analysis of SmzB showed that Tyr26, Cys49, and Tyr93 are the key catalytic residues in the C-C bond formation of the acyl chain of AAIAs, providing mechanistic insights into the enzymatic FCA reaction.

Zn-Co bimetallic catalysts before and after calcination: Research on their different properties for cycloaddition and Friedel-Crafts acylation reactions

A series of bimetallic Zn-Co catalysts before and after calcination were prepared by simple MOFs template methods. The effects of calcination on their structures were investigated by XRD, FT-IR, TPD, XPS and EPR characterization methods, and their catalytic performances were compared in the cycloaddition reactions of CO2 and propylene oxide and acylation reactions of anisole and acetyl chloride, respectively. The results show that uncalcined ZnCo2-MOFs catalyst has excellent catalytic performance for cycloaddition reaction due to the presence of acid-base synergism, whereas the strong acidity of calcined ZnCo2OX catalyst is very favorable for acylation reaction. This is due to the fact that calcination changes the structure of ZnCo2-MOFs catalyst, causes charge migration on the surface and results in the conversion of lattice oxygen to oxygen vacancies, which brings a large difference in the acidity and basicity of catalysts.

A new strategy for facile and rapid separation of astatine-211 from nitric acid medium

Developing fast and efficient separation avenues can promote either fundamental study or practical application of astatine-211(211At). This work presents a new solid-phase extraction (SPE) separation strategy based on functional resin for recovering 211At from the irradiated bismuth (Bi) target mildly and rapidly, without switching HNO3 medium into HCl system. Four functional resins containing acyl-resin, thioether, sulfonate- and sulfonate-groups had been prepared via Friedel-Crafts acylation and alkylation reactions successfully, and subsequently characterized by FTIR, EA, SEM etc. Acyl-resin from these functional materials have shown good retention towards 211At but low binding affinity to all possible metal impurities (Zn2+, Cu2+, Ga3+and Bi3+) in simulated separation experiments. Static batch sorption experiments indicate that the pseudo-second-order kinetic model is more suitable for describing the binding behavior of 211At on Acyl-resin. This reveals that chemisorption dominates the reaction between the target radionuclide and functional material. During several practical separations in medically-used scale, over 98.5 % of 211At can be well retained on SPE column with metal impurities removed in the process of loading target solution and removing non-target radionuclides. More importantly, 60.03 %∼79.05 % of 211At on the Acyl-resin SPE column is recoverable by NH2OH-NH3·H2O-CH3OH elution process, and over 60 % of the radioactivity distributes in volatile organic solvent (CH3OH). The whole SPE separation process can be completed within 20 min, giving 211At product with high radionuclidic purity (≥ 99 %), low metal impurities (Bi3+≤ 17.37 μg and Cu2+≤ 2.91 μg) and high radiolabeling availability. This work implies that acyl-groups can act as potential binding motifs not only for developing new 211At separation methods but also designing bifunctional conjugate agents for astatinated compounds based on coordination bonds.

A Molecular Electron Density Theory Study of the Domino Reaction of N-Phenyl Iminoboranes with Benzaldehyde Yielding Fused Bicyclic Compounds.

The reaction of N-phenyl iminoborane with benzaldehyde yielding a fused aromatic compound, recently reported by Liu et al., has been studied within the Molecular Electron Density Theory (MEDT). Formation of the fused aromatic compound is a domino process that comprises three consecutive reactions: (i) formation of a weak molecular complex between the reagents; (ii) an intramolecular electrophilic attack of the activated carbonyl carbon of benzaldehyde on the ortho position of the N-phenyl substituent of iminoborane; and (iii) a formal 1,3-hydrogen shift yielding the final fused aromatic compound. The two last steps correspond to a Friedel-Crafts acylation reaction, the product of the second reaction being the tetrahedral intermediate of an electrophilic aromatic substitution reaction. However, the presence of the imino group adjacent to the aromatic ring strongly stabilizes the corresponding intermediate, being the reaction product when the ortho positions are occupied by t-butyl substituents. This domino reaction shows a great similitude with the Brønsted acid catalyzed Povarov reaction. Although N-phenyl iminoborane can experience a formal [2+2] cycloaddition reaction with benzaldehyde, its higher activation Gibbs free energy compared to the intramolecular electrophilic attack of the activated carbonyl carbon of benzaldehyde on the ortho position of the N-phenyl substituent, 6.6 kcal·mol-1, prevents the formation of the formal [2+2] cycloadduct. The present MEDT study provides a different vision of the molecular mechanism of these reactions based on the electron density.

Synthesis of amine-functionalized magnetic porous organic polymers for effective extraction of phenolic endocrine disrupting chemicals

Amine-functionalized porous organic polymers (EDA-POP-PC and EDA-POP-BT) were prepared by post-modification of POP-PC and POP-BT with ethylenediamine (EDA) for the first time. The POP-PC and POP-BT were obtained through Friedel-Crafts acylation reaction of potassium tetraphenylborate with p-phthaloyl chloride (PC) and 1,3,5-benzenetricarbonyl trichloride (BT), respectively. The EDA-POPs exhibited superior adsorption capacity for phenolic endocrine disrupting chemicals (EDCs). After magnetically functionalization, the obtained M-EDA-POP-PC was employed as a magnetic adsorbent for enrichment of phenolic EDCs from real samples prior to high performance liquid chromatography-ultraviolet detection. The current strategy showed low detection limits (S/N = 3) of 0.02–0.07, 0.04–0.08 and 0.04–0.10 ng mL−1 for river water, white peach juice and lychee juice, respectively. The method recoveries were 81.7%–115% with relative standard deviations below 8.6%. The proposed strategy showed good practicality for sensitive determination of phenolic EDCs in real samples.

Amorphous carbon nanosheets suitable for deep eutectic solvent electrolyte toward cryogenic energy storage

The emerging deep eutectic solvent (DES) electrolyte has great potential in realizing commercial-scale application of electric double-layer capacitors (EDLCs) served in low temperature environment. That goal, however, rests with how to design the interface structure of electrode materials for well-matching with DES electrolyte. Herein, porous carbon nanosheets (PCNs) were obtained from coal tar pitch through Friedel–Crafts acylation reaction and melting salt intercalation process. The morphology, specific surface area and porosity of porous carbon nanosheets were regulated by tailoring the abundance of the dangling-bonds grafted on the CTP molecules. Profiting from the large specific surface area, suitable pore structure and good two-dimensional structure to provide more active sites and enhance ion transport capacity, the PCNs-0.10 delivers a maximal specific capacitance of 504F g−1 at 0.1 A g−1, which is overmatch than most of previously reported for other carbon materials. As-assembled symmetrical EDLCs using K+ DES electrolyte, can be assembled to work at −40 °C to 75 °C and exhibit satisfactory energy density. The strategy proposed here has opened a new way for exploring the large-scale preparation of electrode materials suitable for ultra-low temperature capacitors.

Regiodivergent C-H Acylation of Arenes by Switching from Ionic to Radical Type Chemistry Using NHC-Catalysis.

The Friedel-Crafts acylation which belongs to the class of electrophilic aromatic substitutions is a highly valuable and versatile reaction in synthesis. Regioselectivity is well predictable and determined by electronic as well as steric factors of the (hetero)arene substrate. In this communication, a radical approach for the acylation of arenes and heteroarenes is presented. C-H Acylation is achieved through mild cooperative photoredox/NHC radical catalysis with the cross coupling of an arene radical cation with an NHC-bound ketyl radical as a key step. As compared to the classical Friedel-Crafts acylation, a regiodivergent outcome is observed upon switching from the ionic to the radical mode. In these divergent reactions, aroyl fluorides act as the acylation reagents in both the ionic as well as the radical process.

Carboxyl and amino acid functionalized indole-based polymer for ultrafast uranium extraction in aqueous solution

High-efficient and fast extraction of uranium is of great significance not only for environmental remediation, but also meeting the shortage of uranium storage. A novel type of porous indole-based polymer (PHTPIAAs) decorating with amino acid and carboxyl groups was fabricated by a simple Friedel-Crafts acylation reaction. The as-prepared PHTPIAAs possessed encouraging and effective elimination properties for uranium in aqueous solution by taking advantages of the cooperativity of cation-π interaction and complexation. The experimental results showed that the PHTPIAA-2 exhibited ultrafast uptake rate (50.0 min) to uranium with a high removal efficiency of 97.2%, and the maximal uptake capacity was up to 457 mg g−1 calculated by Langmuir model at 298 K. The regeneration of PHTPIAA-2 was fulfilled expediently with hydrochloric acid (HCl, 0.1 mol L−1) as eluant, and the removal efficiency of the reborn PHTPIAA-2 maintained 80.2% after 5 cycles. These findings demonstrated that this new indole-based polymer could be considered as a valuable material in the extraction and recovery of uranium from various uranium-containing aqueous solutions.

Friedel-Crafts acylation of anisole with acetic anhydride over single- to multiple-layer MWW zeolites: Catalytic behavior and kinetic mechanism

Numerous efforts have been dedicated to develop solid acid catalysts with higher activity and stability replacing the homogeneous catalysis for Friedel-Crafts (FC) acylation reaction. In this study, single- to multiple-layer MWW zeolites are employed as well-defined model catalysts to investigate the influencing mechanism of microscale structure of MWW zeolites on the kinetics for acylation of anisole with acetic anhydride. It is found that the acylation of anisole is solely carried out on the external acid sites of MWW zeolites and follows Langmuir-Hinshelwood mechanism. The single-layer MWW (SL-MWW) with more external acid sites exhibits lower catalytic activity for acylation of anisole in batch reaction, whereas the recycle stability of SL-MWW is clearly higher than that of multiple-layer MWW (ML-MWW). Kinetics results reveal that a large equilibrium constant of product adsorption together with the confinement effect within 12MR pockets on surface of SL-MWW results in difficult desorption of product and small rate constant. Therefore, the favorable mass-transfer and diffusion of SL-MWW is restrained in batch reaction and at low temperature condition. Based on these findings, the continuous flow fixed-bed process under an elevated temperature (493 K) was conducted to underline the structural advantages of SL-MWW zeolite. The lifetime of SL-MWW can reach to 300 h in the optimal reaction conditions, which is potential catalyst for industrial application in the field of FC acylation reactions.