Current Organic Chemistry - Volume 23, Issue 16, 2019

The present account surveys the results of the plethora of works on Nmethylation of nitrogen-containing substrates, mainly amines. The countless reports in the literature on this issue reveal the emergence of a set of methylating agents, which include: methanol, dimethyl carbonate, formaldehyde/formic acid, carbon dioxide/reductant, methyl iodide, dimethylsulfate, peroxides, dimethylsulfoxide, tetramethylammonium salts, and other unusual ones. Types of the methylating agents including, catalyst, solvent, base, ligand, reducing agent and other reaction conditions such as temperature and time would greatly affect the extent of selectivity of N-monomethylation vis-à-vis N,N-dimethylation. The degree of acidity or alkalinity of catalysts such as the solid catalysts (i.e. zeolites) showed a substantial impact on the selectivity and the course of methylation, leading to design adequate catalysts or to bring suitable modifications to the existing ones. Although this account takes into consideration all types of methylating agents, it is worthwhile to mention that the relatively recent works have been focused on the utilization of eco-friendly methylating agents, including carbon dioxide/reductant, methanol, and dimethylcarbonate. N-Methyl-containing drugs were successfully synthesized with some methylating agents under specified conditions. In some instances, unexpected products and events from the planned N-methylation of some nitrogen-containing molecules occurred. N-Formylation occurred as an intermediate or concomitant reaction when amines were subjected to catalyze methylation with methanol, formaldehyde/ formic acid, and carbon dioxide/reductant. The occasionally depicted mechanisms would elucidate the carbon and hydrogen sources of the affixing methyl group on the nitrogen site. Peculiarly, methylation involving methanol as a methylating agent and transition metal catalysis called for borrowing hydrogen process as a new mechanistic approach.

Following numerous applications of Wittig reaction now functionalized phosphonium salts are gaining attention due to their characteristic properties and diverse reactivity. This review is focused on α-alkoxyalkyl triphenylphosphonium salts: an important class of functionalized phosphonium salts. Alkoxymethyltriphenylphosphonium salts are majorly employed in the carbon homologation of carbonyl compounds and preparation of enol ethers. Their methylene insertion strategy is extensively demonstrated in the total synthesis of a wide range of natural products and other important organic molecules. Similarly enol ethers prepared thereof are important precursors for different organic transformations like Diels-Alder reaction, Claisen rearrangement, Coupling reactions, Olefin metathesis and Nazarov cyclization. Reactivity of these α-alkoxyalkylphosphonium salts have also been studied in the nucleophilic substitution reactions. A distinctive application of this class of phosphonium salts was recently reported in the phenylation of carbonyl compounds under very mild conditions. Synthesis of structurally diverse alkoxymethyltriphenylphosphonium salts with variation in alkoxy groups as well as counter anions are reported in literature. Here we present a detailed account of different synthetic methodologies for the preparation of this unique class of quaternary phosphonium salts and their applications in organic synthesis.

The interest in organophosphorus compounds with a C-P bond is due to their wide use in various fields, especially in medicine and agrochemistry. Prominent examples of anti-cancer, antibacterial, and anti-HIV agents are therapeutic candidates containing a phosphonic acid group fragment. This review provides modern synthetic methods for obtaining phosphorylated aromatic and heteroaromatic compounds with the participation of complexes and salts of various metals developed in recent years as well modern protocol - electrochemical synthesis which allows carrying out reactions at room temperature and normal pressure with no additional oxidants or bases. Herein, we demonstrate new trends and evolution of phosphorylation reactions in catalysis.

An efficient and facile synthesis of substituted pyrimidine derivatives through Biginelli type reaction was achieved in high yields via one-pot reaction of aromatic aldehydes, active methylene compounds and urea or thiourea in the presence of choline chloride- urea mixture as a deep eutectic solvent or cerium (IV) ammonium nitrate (CAN) as a catalyst at different conditions. The reaction was carried out using different ratio of CAN in different solvents to determine the optimum conditions. In addition, 2-mercapto-6-oxo- 4-(thiophen-2-yl)-1,6-dihydropyrimidine-5-carbonitrile was employed in the synthesis of pyrimidinylhydrazide and its corresponding hydrazone. The structural formula of all derivatives was confirmed and characterized by their elemental analyses and spectral data (IR, MS, 1H NMR, 13C NMR).

A simple, straightforward and efficient method has been developed for the synthesis of (E)-3-(arylimino)indolin-2-one derivatives and (E)-2-((4-methoxyphenyl)imino)- acenaphthylen-1(2H)-one. The synthesis of these biologically-significant scaffolds was achieved from the reactions of various substituted anilines and isatins or acenaphthaquinone, respectively, using commercially available, environmentally benign and naturally occurring organic acids such as mandelic acid or itaconic acid as catalyst in aqueous medium at room temperature. Mild reaction conditions, energy efficiency, good to excellent yields, environmentally benign conditions, easy isolation of products, no need of column chromatographic separation and the reusability of reaction media are some of the significant features of the present protocol.