C-methylation of Organic Substrates: A Comprehensive Overview; Methanol as a Methylating Agent: A Case of Catalysis Versatility (Part III)#

Abstract: The present account surveys the results of a myriad of works on the C-methylation of organic substrates with methanol as an eco-friendly methylating agent. The innumerable reports on this issue reveal the widespread use of a set of solid catalysts such as molecular sieves, zeolites, metal phosphates, metal oxides and transition metal complexes to accomplish such methylation. One related facet was the impact of the numbers of Brønstëd acid sites, Lewis acid sites, and Lewis base sites present in solid catalysts, such as zeolites, their ratios, and strengths that affect the distribution of the methylation products and their selectivities. Moreover, specific surface area and porosity of some solid catalysts, such as zeolites, play additional roles in the overall reaction. Not only do these catalyst properties influence the methylation outcome, the temperature, space velocity (WHSV, LHSV, GSHV), weight of catalyst per reactant flow rate (W/F), time of stream (TOS), and methanol/ substrate molar ratio also do. The treated substrates herein discussed were aromatic hydrocarbons (benzene, biphenyls, naphthalenes, toluene, xylenes), alkenes, phenolics (phenol, cresols, anisole), Nheteroarenes, carbonyls, alcohols, and nitriles. Methylation of benzene affords not only toluene as the main product but also polymethylated benzenes (xylenes, pseudocumene, hexamethylenebenzene, and also ethylbenzene as a side-chain product). Furthermore, toluene is sensitive to the reaction conditions, giving rise to ring methylation and to side-chain one (ethylbenzene and styrene), besides the formation of benzene as a disproportionation product. A number of results from the methylation of phenolic compounds bear witness to the interest of different investigators in this special research. With respect to these phenolics, concurrent O-methylation inevitably parallels the C-methylation, and the selectivity of the latter one remains dependent on the above-cited factors; ortho-cresol and 2,6-xylenol have been the main C-ring methylated phenols. Methylation of olefins with methanol over solid catalysts, leading to higher olefins, is of great interest. The chemistry involved in the methylation of N-heteroarenes, such as pyridines, indoles, and pyrroles, is significant. Application of the methylation protocols, using methanol as a reagent and transition of metal complexes as catalysts to ketones, esters, aldehydes, nitriles, and alcohols, ends up with some important molecules, such as acrylonitrile (a monomer) and isobutanol (a biofuel).