Abstract
The interaction of [IrCl(COD)]2 with PPh3 in isopropanol has been investigated for various P/Ir ratios, in the absence or presence of a strong base (KOtBu), at room temperature and at reflux. At room temperature, PPh3 adds to the metal center to yield [IrCl(COD)(PPh3)] and additional PPh3 only undergoes rapid degenerative ligand exchange. Subsequent addition of KOtBu affords [IrH(COD)(PPh3)2] as the main compound, even for high P/Ir ratios, although very minor amounts of products having a “HIr(PPh3)3” core are also generated. Warming to the solvent reflux temperature results in a rapid (<1 h) and quantitative COD removal from the system as hydrogenated products (54.4% of cyclooctene plus 32.2% of cyclooctane according to a quantitative GC analysis) and in the eventual generation of [IrH3(PPh3)3]. The latter is observed as a mixture of the fac and mer isomers in solvent-dependent proportions. Other minor products, one of which is suggested to be mer-cis-[IrH2(OiPr)(PPh3)3] by the NMR characterization, are also generated. These results show that, contrary to certain previously published assumptions, systems of this kind are unlikely to function via a COD-containing active species in transfer hydrogenation catalysis conducted in hot isopropanol in the presence of a strong base.
Highlights
Transfer hydrogenation is a catalyzed process that allows unsaturated compounds such as aldehydes, ketones and imines to be reduced by a hydrogen donor DH2, which is transformed to a by-product D, typically isopropanol giving acetone or formic acid giving CO2 [1,2,3,4,5,6,7]
A strong base was added at room temperature
The results described here clearly demonstrate that the previously reported transfer hydrogenation processes catalyzed by [IrCl(COD)]2/phosphine ligand/strong base in isopropanol at high temperature likely generate active species that are devoid of the COD ligand
Summary
Transfer hydrogenation is a catalyzed process that allows unsaturated compounds such as aldehydes, ketones and imines to be reduced by a hydrogen donor DH2, which is transformed to a by-product D, typically isopropanol giving acetone or formic acid giving CO2 [1,2,3,4,5,6,7]. It should be underlined that certain Rh and Ir complexes are known to be active catalysts for the transfer hydrogenation from alcohols to olefins [66,67,68,69,70] In this contribution, we report our investigations of the reaction between [IrCl(COD)]2 and PPh3 under a variety of conditions, including those typical of catalytic transfer hydrogenation. Our investigation has retraced a few already well established transformations, but has brought to the surface a few unexpected and surprising results
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