Iridium diphosphine complex

Very air-sensitive iridium diphosphine complexes carrying a peraryldiphosphine ligand, [IrCl(diphosphine)]2 (42a, 42b)(a diphosphine = BPBP b diphosphine = BINAP [47, 48]) can also activate MeOH in addition to HjO at room temperature very easily. Reaction of 42 with excess MeOH in toluene at room temperature gave ah-stable and thermally stable colorless hydrido (me thoxo) complexes, [ IrH(diphos-phine) 2( 4-OMe)2( i-Cl)]Cl (69) quantitatively (Eq. 6.21) [49]. The shucture of 69b,... 

The dinuclear iridium(l) diphosphine complexes 42 can also activate carboxylic acids easily. For example, the reaction of [IrCl(binap)]2 (42b) with an excess of acetic acid or benzoic acid in toluene at room temperature gave the corresponding (hydri-... 

We also found that iridium hydrido(hydroxo) complexes like [ lrH(diphos-phine) 2( x-OH)2( x-Cl)]Cl (43) and the precursor diphosphine complexes 42 can also catalyze the hydration of nitriles. In the presence of catalyhc amounts of these complexes, heating acetonitrile and benzonitrile with excess water at 120°C gave the corresponding amides [47, 50]. 

Since there are unresolved issues in the fine detail of reaction mechanism, it is worth recalling an earlier publication on reactive intermediates in iridium hydrogenation [61]. In general, conventional Ir diphosphine complexes turnover slowly or not at all when enantioselective hydrogenation of standard substrates is attempted, and essentially all the practical and useful recent synthetic contri-... 

The diphosphine complexes [Ir(dppe)2]+ and [Ir(dppe)(cod)]+ reportedly catalyze the decar--bonylation of aldehydes.504,505 Methanol carbonylation is catalyzed by lr(Cl)4]-H20 to yield acetic acid. The active species is thought to be an acetyl iridium(III) species, wherein the rate-determining step involves electrophilic attack of this species on methanol. The effects of added iodide ion on reaction rates have also been investigated.506... 

The sterically bulky phosphines (8) have been prepared by the Grignard method from chlorodi(t-butyl)phosphine and chlorodicyclohexylphosphine. In certain iridium(i) complexes, metallation of these phosphines occurs on the terminal olefinic carbon atom. Treatment of a, )-dialkynyl-lithium reagents with chlorodi-(t-butyl)-phosphine gives the diacetylenic diphosphines (9), which form large ring compounds when they form complexes with transition metals. ... 

The corresponding iridium enamide complexes and their alkyl hydride counterparts are much more stable, and a full NMR characterization of the alkyl hydride proved possible in the DIPAMP series. Here, as in the corresponding rhodium chemistry, the presumed dihydride precursor proved to be elusive [31]. By employing a different approach to enamide complexes in which an iridium bis-enamide complex was allowed to react with the diphosphine (Fig. 6) both major and minor enamide complexes could be prepared separately the path to one of them is shown in Fig. 6. The trick was to employ menthyl esters so that stereo-chemically homogeneous Ir complexes were formed. Some additional structural features of the intermediates were derived from detailed NMR analysis, and especially the role of the OMe group in coordinating to iridium trans to the hydride [32]. 

As recounted, these studies demonstrate that two of the three expected intermediates in asymmetric hydrogenation may be directly observed, but the expected dihydride is too fleeting. There are two further experiments which are pertinent to this issue. A related diphosphine-iridium alkene complex reacts with dihydrogen at low temperatures and a series of alkene dihydrides are observed prior to the formation of the expected alkyl hydride. Based on the H-NMR chemical shifts of the respective Ir-H species, the initial addition (or to be more correct the initially observed species) possesses H trans to alkene and H trans to phosphine only at higher temperatures does this rearrange to the expected H trans to amide and H trans to phosphine structure (Fig. 9a) [36]. A more directly relevant experiment involves para-enriched hydrogen, and in the illustrated case a transient dihydride is observed. A problem is that the spectral characteristics are not entirely in accord with expectations for the proposed structure (the supposed trans-P-Rh-H coupHng is 4 Hz rather than ca. 120 Hz), but the presence of some transient Rh dihydride is definitive based on the evi-... 

Screening of Ir diphosphine complexes. The next breakthrough was obtained when iridium was used instead of rhodium. This idea was inspired by results of Crabtree et al. [ 18] who described an extraordinarily active Ir/tricyclohexylphos-phine/pyridine catalyst that was able to hydrogenate even tetra-substituted C=C bonds. The highest ee s were observed with an Ir-bdpp catalyst in the presence of additional iodide ions (ee 84% at 0 °C), but the activity was disappointing ton s up to 10,000 and tof s of 250 h (100 bar and 25 C) with somewhat lower ee s were obtained for Ir-diop-iodide catalysts [ 17,19]. A major problem of these new Ir diphosphine catalysts was an irreversible catalyst deactivation. 

Catalytic decarbonylation of benzaldehyde using several iridium complexes has also been examined. Results of these experiments are shown in Table 8. The main points to be made here are (i) [Ir(P-P)2] catalysts have activities that are ca. twenty times lower than their Rh analogs (ii) the iridium mono-diphosphine catalysts are better than the 6w-diphosphine Ir catalysts (opposite trend noted using rhodium, see Table 5) and (iii) IrCl(CO)(PPh3)2 is a much better catalyst than RhCl(CO)(PPh3)2 and is also better than most of the iridium diphosphine catalysts. The results for the [M(P-P)2] catalysts may be explained in terms of the proposed mechanistic scheme in Figure 11.3. Since Ir-P bonds should be stronger than Rh-P bonds, the value of ki will be smaller for the Ir catalysts, thus... 

The research on catalytic decarbonylation using diphosphine complexes was supported by the National Science Foundation. The Johnson Matthey Company is acknowledged for generous loans of rhodium and iridium trichloride. 

Sablong, R. Osborn, J. A. Asymmetric hydrogenation of imines catalysed by carboxylato(diphosphine)iridium(III) complexes. Tetrahedron Asymmetry 1996, Z 3059-3062. 

Indenyl iridium(l) complexes [(77 -C9H7)lr(diphos)] 845 [diphos = 2 PPh3, dppe, rac-eypenphos, (3, 3 )-chiraphos, (R)-prophos, (R,R)-renorphos and (R)-phenphos] have been synthesized by reacting [(77 -09117) (02114)2] with the appropriate diphosphine." " ... 

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