N-heterocyclic olefin ligand (NHO) coordinating Ir(I) complexes [Ir(cod)(κP,C,P′-NHOPPh2)]PF6 and [IrCl(cod)(κC-NHOOMe)] (cod = 1,5-cyclooctadiene, NHOPPh2 = 1,3-bis(2-(diphenylphosphanyl)ethyl)-2-methyleneimidazoline) and NHOOMe = 1,3-bis(2-(methoxyethyl)-2-methyleneimidazoline), have been tested in the transfer hydrogenation reaction; this representing the first example of the use of NHOs as ancillary ligands in catalysis. Transfer hydrogenation of aldehydes, ketones and imines was performed with the pre-catalyst [Ir(cod)(κP,C,P′-NHOPPh2)]PF6 using isopropanol as hydrogen source and it has shown excellent activities, while [IrCl(cod)(κC-NHOOMe)] decomposes during the reaction. Addition of one or two equivalents of a phosphine ligand to the latter avoids deactivation of the catalyst and improves the reaction yields for the hydrogenated product. The reaction mechanism has been investigated by means of stoichiometric studies and theoretical calculations. The formation of the active species ([Ir(κP,C,P′-NHOPPh2)(iPrO)]) has been proposed to occur via isopropoxide coordination and concomitant COD dissociation. Also, the theoretical calculation show that the NHO moiety behaves as a hemilabile ligand throughout the catalytic cycle, thus allowing the catalyst to adopt stable square planar geometries in the transition states, which reduces the overall energetic barrier of the process.

N-Heterocyclic Olefins (NHO) as ancillary ligands in catalysis and their application in transfer hydrogenation reactions

253rd National Meeting (2017)

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