Iridium-catalysed reactions, hydrogenation

Ramies, O., Net, G., Ruiz, A. and Claver, C. (2000) Chiral diphosphites as ligands for the rhodium- and iridium-catalysed asymmetric hydrogenation Precatalyst complexes, intermediates and kinetics of the reaction. Eur. J. Inorg. Chem., 1287-1294. 

Over all the metals studied, except cobalt, nickel and copper, the selectivity and stereoselectivity decreased slightly as the reaction proceeded. In addition to the products shown in Table 20, in the rhodium- and iridium-catalysed reactions small yields (2—3%) of buta-1 2-diene were also observed. For all the catalysts, except rhodium, iridium and platinum, which were not investigated, the initial rate kinetic orders were unity in hydrogen and zero or slightly negative (Ni) in but-2-yne. 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

Finally, the use of S/P ligands derived from (i )-binaphthol has been considered by Gladiali et al. in the asymmetric rhodium-catalysed hydrogen-transfer reduction of acetophenone performed in the presence of i-PrOH as the hydrogen donor.It was noted that racemisation occurred when the reaction time increased and consequently the corresponding alcohol was obtained in only low enantioselectivities (< 5% ee) as shown in Scheme 9.21. Similar results were more recently reported by these authors by using iridium combined with the same ligands. ... 

The metal-catalysed hydrogenation of cyclopropane has been extensively studied. Although the reaction was first reported in 1907 [242], it was not until some 50 years later that the first kinetic studies were reported by Bond et al. [26,243—245] who used pumice-supported nickel, rhodium, palladium, iridium and platinum, by Hayes and Taylor [246] who used K20-promoted iron catalysts, and by Benson and Kwan [247] who used nickel on silica—alumina. From these studies, it was concluded that the behaviour of cyclopropane was intermediate between that of alkenes and alkanes. With iron and nickel catalysts, the initial rate law is... 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

Some examples of heterogeneous hydrogenation of alkenes catalysed by T12-H2 complexes in either solid-gas or solid-liquid systems have been reported. The first of such reactions regards the hydrogenation of ethylene catalysed by crystals of the iridium(III) complex [(triphos)Ir(H)2(C2H4)]BPh4 in a tubular flow reactor... 

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