Effective N-methylation of nitroarenes with methanol catalyzed by a functionalized NHC-based iridium catalyst: a green approach to N-methyl amines†
Compound [IrBr(CO)2(κC-tBuImCH2PyCH2OMe)] featuring a flexible pyridine/OMe functionalized NHC ligand κ1C coordinated efficiently catalyzes the selective N-monomethylation of nitroarenes using methanol as both the reducing agent and the C1 source. A range of functionalized nitroarenes including heterocyclic or sterically hindered derivatives have been efficiently converted to the corresponding N-monomethyl amines in good yields at low catalyst loadings using sub-stoichiometric amounts of Cs2CO3 as a base. Mechanistic investigations support a borrowing-hydrogen mechanism in which methanol acts as the hydrogen source and methylating agent. Further, the hydrogen transfer reduction of nitrobenzene to aniline under optimized reaction conditions should proceed through a direct mechanism involving nitrosobenzene and N-phenylhydroxylamine intermediates.