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Iridium complexes with a new type of N ^ N ′‐donor anionic ligand catalyze the N ‐benzylation of amines via borrowing hydrogen
Author(s) -
RuizCastañeda Margarita,
Rodríguez Ana M.,
Aboo Ahmed H.,
Manzano Blanca R.,
Espino Gustavo,
Xiao Jianliang,
Jalón Félix A.
Publication year - 2020
Publication title -
applied organometallic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.53
H-Index - 71
eISSN - 1099-0739
pISSN - 0268-2605
DOI - 10.1002/aoc.6003
Subject(s) - chemistry , heterolysis , catalysis , alkylation , amine gas treating , selectivity , ligand (biochemistry) , aniline , combinatorial chemistry , homolysis , metal , benzyl alcohol , medicinal chemistry , organic chemistry , radical , biochemistry , receptor
The development of efficient and eco‐friendly methods for the synthesis of elaborate amines is highly desired as they are valuable chemicals. The catalytic alkylation of amines using alcohols as alkylating agents, through the so‐called borrowing hydrogen process, satisfies several of the principles of green chemistry. In this paper, four neutral half‐sandwich complexes of Ru(II), Rh(III), and Ir(III) have been synthesized and tested as catalysts in the N ‐benzylation of amines with benzyl alcohol. The new derivatives contain a N ^ N ′ anionic ligand derived from 5‐(pyridin‐2‐ylmethylene)hydantoin (Hpyhy) that has never been tested in metal complexes with catalytic applications. In particular, the Ir derivatives, [(Cp*)IrX(pyhy)] (X = Cl or H), exhibit high activity along with good selectivity in the process. Indeed, the scope of the optimized protocol has been proved in the benzylation of several primary and secondary amines. The selectivity towards monoalkylated or dialkylated amines has been tuned by adjusting the amine:alcohol ratio and the reaction time. Experimental results support a mechanism consisting of three consecutive steps, two of which are Ir catalyzed, and a favorable condensation step without the assistance of the catalyst. Moreover, an unproductive competitive pathway can operate when the reaction is performed in open‐air vessels, due to the irreversible release of H 2 . This route is hampered when the reaction is carried out in close vessels, likely because the release of H 2 is reversed through metal‐based heterolytic cleavage. From our viewpoint, these results show the potential of the new catalysts in a very attractive and promising methodology for the synthesis of amines.