N-Methylation of Amines with Methanol Catalyzed by Iridium(I) Complexes Bearing an N,O-Functionalized NHC Ligand

Abstract

A set of neutral [IrBr(L2)(κC-tBuImCH2PyCH2OMe)] and cationic [Ir(L2)(κ2C,N-tBuImCH2PyCH2OMe)]PF6(L2= cod, (CO)2) Ir(I) compounds featuring a flexible lutidine-derived polydentate ligand having NHC and -OMe as donor functions have been evaluated as catalyst precursors for the N-methylation of aniline using methanol both as a reducing agent and a C1 source. The carbonyl complexes are somewhat more active than the related diene compounds with the neutral compound [IrBr(CO)2(κC-tBuImCH2PyCH2OMe)] being the more active. A range of aromatic primary amines, including heterocyclic amines, have been selectively transformed into the corresponding N-methylamino derivatives using this catalyst at a low catalyst loading (0.1 mol %) and substoichiometric amounts of Cs2CO3(half equiv) as a base, in methanol at 423 K. For aliphatic primary amines, selective N,N-dimethylation was achieved under the same catalytic conditions. The unselective deprotonation of the methylene linkers in [IrBr(CO)2(κC-tBuImCH2PyCH2OMe)] affords two isomeric neutral complexes featuring a coordinated dearomatized pyridine core, which were converted into [Ir(OMe)(CO)2(κC-tBuImCH2PyCH2OMe)] upon addition of methanol. This compound undergoes thermal activation of a C-H bond of the tert-butyl group to give the cyclometalated iridium(I) complex [Ir(CO)2{κ2C,C-(-CH2Me2C-ImCH2PyCH2OMe)}] featuring a bidentate C,C-coordinated NHC ligand. Mechanistic investigations support a borrowing hydrogen mechanism proceeding through iridium(I) intermediates with the methoxo complex as the catalytic active species and the cyclometalated complex as the catalyst resting state. Deuterium labeling experiments have demonstrated that both species are in equilibrium under catalytic conditions, which is consistent with the exhibited catalytic activity of the cyclometalated complex.