Iridium-catalysed reactions

Forster also reported HP IR measurements on iridium catalysed reactions [59]. It was noted that the iridium speciation is dependent on reaction conditions, with three different regimes being distinguishable. At intermediate [H2O], the dominant Ir species are [MeIr(CO)2l3] and [Ir(CO)2l4] . The anionic methyl complex is regarded as the active form of the catalyst in a cycle analogous to the Rh system, with carbonylation of [MeIr(CO)2l3] being rate determining. The Ir(III) tetraiodide... 

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. 

The two diasteromeric allyliridium-phosphoramidite complexes formed in the allylic 5 2 substitution of /-substituted allylic acetates, benzoates, or carbonates, with amine nucleophiles have been synthesized and characterized by NMR and X-ray diffractions. Kinetic and stereochemical studies using deuterated substrates indicate that the reaction, which occurs with a retention of configuration, proceeds by the mechanism in Scheme 3. This iridium-catalysed reaction is compared to molybdenum- and palladium-catalysed reactions. 

An interesting iridium-catalysed 5-CH boronation of 2,3-dimethylpyrazine was reported incidentally in a paper mainly devoted to the reaction of pyridines. The product 89 was used in a Suzuki coupling <06AG(I)489>. Selective mono coupling of 2,6-dichloropyrazine with boronic acids, followed by amine displacement of the second chlorine has been used to prepare potential anti-cancer compounds <06JMC407>. A full paper has been published on the chelation-driven selective Suzuki coupling of the pyridinium ylides 90 <06TL6457>. 

Scheme 3.1 Anionic and neutral cycles proposed by Forster for iridium catalysed methanol carbonylation and WGS reaction (adapted from Ref [59] by permission of The Royal Society of Chemistry).

Two classes of promoter have been identified for iridium catalysed carbonylation (i) transition metal carbonyls or halocarbonyls (ri) simple group 12 and 13 iodides. Increased rates of catalysis are achieved on addition of 1-10 mole equivalents (per Ir) of the promoter. An example from each class was chosen for spectroscopic study. An Inis promoter provides a relatively simple system since the main group metal does not tend to form carbonyl complexes which can interfere with the observation of iridium species by IR. In situ HP IR studies showed that an indium promoter (Inl3 Ir = 2 1) did not greatly affect the iridium speciation, with [MeIr(CO)2l3] being converted into [Ir(CO)2l4] as the batch reaction progressed, as in the absence of promoter. 

This represents one pathway to the formation of methane, a knovm by-product in iridium catalysed methanol carbonylation. The hydrogenolysis reaction was severely retarded by the presence of excess CO, indicating a mechanism involving initial dissociation of CO from [MeIr(CO)2l3] , prior to activation of H2. The mechanism therefore resembles that for hydrogenolysis of Rh acetyl complexes in hydroformylation. 

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. 

A new phosphoramidite ligand (1 Y = OMe), gives high enantioselectivities (92-99% ee) and regioselectivities (99% S 2 ) in iridium-catalysed allylic substitution reactions of carbonates and acetates with carbanion or primary amine nucleophiles.6 The new ligand also leads to a faster rate of reaction than other phosphoramidite ligands. 

The ligand was then used to form a variety of transition metal carbene complexes [207] (see Figure 3.72). Interestingly, more than one method for the formation of transition metal carbene complexes was successfully employed presence of an inorganic base (IC COj) to deprotonate the imidazolium salt and the silver(I) oxide method with subsequent carbene transfer to rhodium(I), iridium(I) and copperfi), respectively. The silver(I) and copper(I) carbene complexes were used for the cyclopropanation of styrene and indene with 1,1-ethanediol diacetate (EDA) giving very poor conversion with silver (< 5%) and qnantitative yields with copper. The diastereomeric ratio (endolexo) was more favonrable with silver than with copper giving almost a pnre diastereomer for the silver catalysed reaction of indene. 

Iridium-catalysed C-H-boronation can be carried out using either pinacol borane or pinacol diborane, both methods giving comparable results. Reaction occurs at a-positions of five-membered rings and is compatible with halogen substituents, as exemplified below. ... 

Iridium-based complexes also catalyse reaction 26.12, and the combination of [Ir(CO)2l2] with Ru2(CO)6l2(p-I)2 as a catalyst promoter provides a commercially viable system. 

An intramolecular Sj 2 iridium-catalysed allylic amidation of allyl carbonates has provided chiral tetrahydroisoquinolines in yields ranging from 78 to 97%, with 88-96% The catalyst is formed in situ from [Ir(cod)Cl]2 and a phosphoramidite 0 ligand. Other ring-sized products can be formed using this reaction, albeit in lower yields. 

Oxidation of Hg by Mn is catalysed by iridium and ruthenium sulphates of unknown composition, obtained by evaporating solutions of the complex chlorides in cone. H2S04-Na2S04. The rate of the catalysed reaction is invariably independent of [Hg ]. A first-order dependence on [catalyst] is observed up to ca. 10" mol 1", with little change in rate for increased catalyst. The dependence on [Mn ] is similar and the mechanism proposed is ... 

The substituent effects of norbornadiene derivatives in the iridium-catalysed asymmetric 2 -I- 2-cycloaddition reactions with arylacetylenes have been investigated. Results have shown that the atom forming the short bridge chain had the greatest effect on the enantioselectivity of the reaction. Heteroatoms such as oxygen and nitrogen resulted in excellent enantioselectivity while a carbon atom on the bridge chain decreased the enantioselectivity. " ... 

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