:
The present account surveys the results of the myriad of works on 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, of Lewis acid sites, and of Lewis base sites present in solid catalysts, such as zeolites, their ratios, and
their strengths that affect the distribution of the methylation products and their selectivities. Also, specific
surface area and porosity for some solid catalysts such as zeolites play additional roles in the overall reaction.
Not only these properties of a catalyst that influence the methylation outcome but also 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. The treated substrates herein discussed were aromatic hydrocarbons (benzene,
biphenyls, naphthalenes, toluene, xylenes), alkenes, phenolics (phenol, cresols, anisole), N-heteroarenes,
carbonyls, alcohols, and nitriles. Methylation of benzene affords not only toluene as main product but also
polymethylated benzenes (xylenes, pseudocumene, hexamethylenebenzene, and also ethylbenzene as a sidechain product). Also, toluene is sensitive to the reaction conditions, giving rising to ring methylation and to sidechain one (ethylbenzene and styrene), besides the formation of benzene as a disproportionation product. Wealth
of results from the methylation of phenolic compounds bears witness to the interest of different investigators in
this special research. As to these phenolics, concurrent O-methylation inevitably parallels the C-methylation, and
the selectivity of the latter one remains depended 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 a 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 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).
Frustration across the periodic table: heterolytic cleavage of dihydrogen by metal complexes
