Iridium diphosphine

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,... 

Cyclic imines do not have the problem of syn/anti isomerism and therefore, in principle, higher enantioselectivities can be expected (Fig. 34.8). Several cyclic model substrates 6 were hydrogenated using the Ti-ebthi catalyst, with ee-val-ues up to 99% (Table 34.5 entry 5.1), whereas enantioselectivities for acyclic imines were <90% [20, 21]. Unfortunately, these very selective catalysts operate at low SCRs and exhibit TOFs <3 h-1. In this respect, iridium-diphosphine catalysts, in the presence of various additives, seem more promising because they show higher activities. With several different ligands such as josiphos, bicp, bi-... 

Discovery of new iridium diphosphine catalysts that are more active and selective than Rh catalysts for the hydrogenation of MEA imine... 

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 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]. 

Other chiral diphosphine ligands (374), (375), (376), and (377) bind to iridium with little diastereoselectivity.603 The complexes are characterized by 1H and 31P H NMR spectroscopy. 

Micouin investigated rhodium-catalyzed hydroboration as a means of desymmetrizing meso hydrazines 59 in an important new application.36 Enantiomeric excess of up to 84% was obtained after screening diphosphines such as DIOP and BDPP (Scheme 9). Interestingly, they noted an unprecedented reversal of enantioselectivity by changing from rhodium to iridium. 

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-... 

Fig. 31.16 Stable hydrides formed from (a) amine/phosphine iridium cations and (b) fa/s-phosphine or diphosphine iridium cations.

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