Iridium complexes oxidation catalysts

A more elegant, but expensive, approach22 has been the use of soluble iridium and rhodium catalysts which contain coordinated dimethyl sulphoxide (e.g. IrHCl2(Me2SO)3) which promote the oxidation of sulphoxides in aqueous media, equation (8). The ease of oxidation depends on the substituents and this decreases in the order Me > Ph > PhCH2. This reaction is especially useful since sulphides are not oxidized under the reaction conditions due to the formation of strong complexes with the catalyst. 

Henbest and Mitchell [78] have shown that water can be used as hydrogen source with chloroiridic acid (6) as the catalyst through oxidation of phosphorous acid (59) to phosphoric acid (60) in aqueous 2-propanol. Under these conditions, no hydrogen transfer occurs from 2-propanol. However, iridium complexes with sulfoxide or phosphine ligands show the usual transfer from 2-pro-panol [79-81]. 

In contrast to the rhodium process the most abundant iridium species, the catalyst resting state, in the BP process is not the lr(l) iodide, but the product of the oxidative addition of Mel to this complex. 

In contrast to the Pt(0) and Pt(II) complexes and the corresponding Rh(I) and Rh(III) complexes, the iridium complexes have rarely been employed as hydrosilylation catalysts [1-4]. Iridium-phosphine complexes with d metal configura-tion-forexample, [Ir(CO)Cl(PPh3)2] (Vaska s complex) and [Ir(CO)H(PPh3)3]-were first tested some 40 years ago in the hydrosilylation of olefins. Although they underwent oxidative addition with hydrosilanes (simultaneously to Rh(I) com-... 

At temperatures above ca. 200°C, the decarbonylation reaction can be driven catalytically (1,4,14, 20). Scheme I illustrates the proposed catalytic reaction scheme (15,16). This catalytic reaction is slow (activity for benzaldehyde decarbonylation at 178°C is 10 turnovers hr-1) presumably because the oxidative addition of RCOX to RhCl(CO)(PPh3)2 is difficult (7, 21, 22). Consistent with this, the rate is significantly greater when IrCl(CO)(PPh3)2 is used as the catalyst (benzaldehyde, 178°C, activity is 66 turnovers hr-1) (23). Oxidative addition to iridium complexes is well known to be more facile than with their rhodium analogues. 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

The reaction of 3,4-dihydroisoquinoline A-oxide (74) and methacrylonitrile in the presence of cationic half-sandwich rhodium and iridium complexes containing a chiral diphosphine ligand was analyzed. The cycloadditions occurred with excellent regio- and diastereoselectivity and low-to-moderate enantioselectivity. Analysis of the catalytic system showed the formation of two epimeric complexes 75 containing the dipolarophile methacrylonitrile. The reaction of one of the isolated diastereopure complexes 75 with 74 afforded cycloadduct 76 with high enantioselectivity. A recycling procedure was developed in order to increase the adduct/catalyst ratio <07CEJ9746>. 

The catalytic cycle involves the same fundamental reaction steps as the rhodium system oxidative addition of Mel to Ir(I), followed by migratory CO insertion to form an Ir(III) acetyl complex, from which acetic acid is derived. However, there are significant differences in reactivity between analogous rhodium and iridium complexes which are important for the overall catalytic activity. In situ spectroscopy indicates that the dominant active iridium species present under catalytic conditions is the anionic Ir(III) methyl complex, [IrMe(CO)2l3] , by contrast to the rhodium system where the dominant complex is [Rh(CO)2l2] - PrMe(CO)2l3] and an inactive form of the catalyst, [Ir(CO)2l4] represent the resting states of the iridium catalyst in the anionic cycles for carbonylation and the WGSR respectively. At lower concentrations of water and iodide, [Ir(CO)3l] and [Ir(CO)3l3] are present due to the operation of related neutral cycles . 

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