Iridium species

A recently discovered reduction procedure provides a convenient route to axial alcohols in cyclohexyl derivatives (5). The detailed mechanism of the reaction remains to be elucidated, but undoubtedly the reducing agent is an iridium species containing one or more phosphate groups as ligands. In any case, it is clear that the steric demands of the reducing agent must be extraordinary since the stereochemical outcome of the reaction is so specific. The procedure below is for the preparation of a pure axial alcohol from the ketone. 

For comparison, Battles et al. (15) determined the partial heats of sublimation of Pu02(g) and Pu0(g) above PuOi.33 over the temperature range 1937 to 2342 K by means of mass spectrometric measurements with Iridium effusion cells. The absence of Iridium oxides or Iridium species In the vapor phase Indicated that Iridium was nonreducing toward plutonia. The partial heats of sublimation calculated from the slopes of the temperature dependency data yielded values of 127.1 1.2 and 138.8 1.6 kcal/mol for Pu0(g) and Pu02(g) ... 

The coordination chemistry of iridium has continued to flourish since 1985/86. All common donor atoms can be found bound to at least one oxidation state of iridium. The most common oxidation states exhibited by iridium complexes are I and III, although examples of all oxidation states from —I to VI have been synthesized and characterized. Low-oxidation-state iridium species usually contain CO ligands or P donor atoms, whereas high-oxidation-number-containing coordination compounds are predominantly hexahalide ones. 

Actually, a similar approach was used in studying the oxidative addition of methane to an iridium complex. Hydrocarbon solvents would have reacted faster than methane with the photochemically produced unsaturated iridium species, therefore J.K. Hoyano et al chose perfluorinated hexane as being an inert solvent. The elevated pressure was necessary in order to increase the concentration of the methane in the solution sufficiently to shift equilibrium (15) to the right /20/. 

The ionic cycle is important under reaction conditions where iodide ion can exist, e.g., higher water levels (CH3OH + HI CH3I + H20) or with salt additives. However, while higher ionic iodide levels give an iridium species capable of very rapid reaction with methyl iodide, they also serve to inhibit the formation of an acyl species. The relatively slow conversion of [CH3Ir(CO)2I3] to an acyl species is almost certainly not... 

Based upon their data and upon results in the literature, the authors concluded that hydrogenations using 24 or related species as catalyst precursor proceed in solution by mechanisms involving iridium(I)/(III) formal oxidation states. During the course of their discussions, the authors made the interesting observation that the rate of gas-phase collisions between the thermalized iridium organometallic ions and D2 under their experimental conditions in the oc-topole were similar to the rate of diffusion-controlled encounters between iridium species and D2 in solution. 

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. 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

Here, it is worthy of note that dihydrogen, which is present in the medium via the WGS reaction, can also react in its own right with the iridium species... 

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. 

With a ruthenium promoter (added as [Ru(CO)4l2]), r(CO) bands due to Ru iodo-carbonyls dominated the spectrum, precluding the easy observation of iridium species. Before injection of the Ir catalyst, absorptions due to [Ru(CO)2l2(sol)2], [Ru(CO)3l2(sol)] and [Ru(CO)3l3] are present. After injection of the iridium catalyst (Ru Ir = 2 1), [Ru(CO)3l3] becomes the dominant Ru species (Figure 3.11(b)). The observations indicate that the Ru(II) promoter has a high affinity for iodide and scavenges Hl(aq) as H30 [Ru(CO)3l3] . An indium promoter is believed to behave in a similar manner to form H30 [Inl4] . These promoter species also catalyse the reaction of Hlj q) with methyl acetate (Eq. (3)), which is an important organic step in the overall process. 

Finally, in connection with the comments of Dr. Gordon, we have looked at the reduction of chlorate by hexachloroiridate. The final products, of course, are hexachloroiridate and chloride. The reaction is stoichiometric. So far, no tracer experiments have been done to try to identify any of the unstable intermediates but both of the iridium species are well known to be substitution inert, and the reaction surely involves a one-electron reduction of chlorate. 

The oxidation of trimethylene glycol and dimethyldiethylene glycol by NBS is catalysed by iridium(III) in acidic media.80 The kinetics have been investigated in the presence of mercury(II) acetate as a bromide ion scavenger, and [IrCIri H20)]2 is thought to be the reactive iridium species under the conditions employed. 

The rhodium-trimethylphosphine system is remarkable because it catalyzes the dimerization of aryl substituted acetylenes, yielding the scarce branched head-to-tail coupling product [12], The iridium species generated from [Ir(COD)Cl]2 and phosphine selectively yields linear ( ) or (Z) enynes from silylalkynes, depending on whether triaryl or tripropylphosphines, respectively, are used [16]. 

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 . 

Like the corresponding rhodium complexes, the present iridium species undergo two successive one-electron oxidations. As illustrated in Fig. 7-44 for [Fe(f -C5H4S)2]Ir(f -C5Me5)(PMe3), in these complexes loss of the second electron is also generally reversible [136]. The redox potentials of these steps are reported in Table 7-27. 

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