Yan Long,Jie He,Hang Zhang,Yingkang Chen,Kang Liu,Junwei Fu,Hongmei Li,Li Zhu,Zhang Lin,Andrei Stefancu,Emiliano Cortes,Mingshan Zhu,Min Liu
Graphical Ag/Al 2 O 3 catalytic controllable synthesis of N-monomethylated amines by using CO 2 /H 2 as C 1 source has been demonstrated in a first example. The reactivity of methylated amines was reduced by the heterolytic dissociation of H 2 over the catalyst, and hence conversion of various amines to corresponding monomethylated amines was enabled with excellent selectivity. The selective synthesis of monomethylated amines with CO 2 is particularly challenging because the formation of tertiary amines is thermodynamically more favorable. Herein, a new strategy for the controllable synthesis of N-monomethylated amines from primary amines and CO 2 /H 2 is explored. First-principle calculations reveal that the dissociation of H 2 via an heterolytic route reduces the reactivity of methylated amines and thus inhibit successive methylation. In situ DRIFTS proves the process of formation and decomposition of ammonium salt by secondary amine reversible binding with H + on the Ag/Al 2 O 3 catalyst, thereby reducing its reactivity. Meanwhile, the energy barrier for the rate-determining step of monomethylation was much lower than that of overmethylation (0.34 eV vs. 0.58 eV) means amines monomethylation in preference to successive methylation. Under optimal reaction conditions, a variety of amines were converted to the corresponding monomethylated amines in good to excellent yields, and more than 90 % yield of product was obtained.
The development of transition-metal heterogeneous catalysts for economical and effective synthesis of N-methylamine, especially for the monomethylation of amines, is still challenging. Herein, two Rh-supported covalent organic framework (COF) heterogeneous catalysts Rh/MelCOF were facile-synthesized by a Schiff base reaction using melamine as a precursor, and, for the first time, it was successfully applied to the effective and high selective tandem reaction of transfer hydrogenation and monomethylation of nitroaromatic hydrocarbons, with methanol as a C1 and hydrogenation source. A series of nitroaromatic hydrocarbons, including heterocyclic or sterically hindered derivatives, can be well tolerated, and the catalyst could also be reused four times without losing significant reactivity. At the same time, the study of the Rh/MelCOF mechanism supports the hydrogen-borrowing mechanism and puts forward the reaction pathway of azobenzene as an intermediate, which is better than the hydrogen-transfer pathway from N-phenylhydroxylamine to aniline directly. This work expands the application of COF catalysts and provides an effective way to obtain mono-N-methylated amines from nitroaromatic hydrocarbons, as well as the detailed mechanism of Rh/COF-catalyzed tandem transfer hydrogenation and monomethylation of amines.
Seven new chelated cyclometalated Ir complexes of ABON,P, ABON,O, and ABON,C(carbene) based on a rigid and tunable 2-arylbenzo[d]oxazole backbone have been prepared for the N-methylation of amines. Among these three coordinated modes, ABON,C(carbene)-chelated iridium-based catalysts exhibited good performance in the monomethylation of aromatic amines with methanol (MeOH) as the green methylation reagent. The steric-modified synthesis of ABON,C(carbene) complexes was described. The most active ABON,C(carbene) complex with marginal steric hindrance as a catalyst was obtained from the benzoxazole ring without a substituent and methyl group of the benzimidazole ring on the N-heterocyclic carbene (NHC) ligand. A variety of amines including para- and meta-substituted aromatic amines, as well as heterocyclic amines, were formulated as suitable substrates. Importantly, this catalyst considerably promoted the yield of the N-methylation of ortho-substituted aromatic amines. Controlled kinetic experiments and deuterium-labeling reactions of these ortho-substituted amines were conducted under optimized conditions. On the basis of the experimental results, a plausible mechanism was proposed.
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