Iridium addition initiated

In 1957 (2) platinic, ruthenium, and iridium chlorides were shown to be catalysts leading to very rapid additions, sometimes below room temperature, of many kinds of SiH compounds. These findings initiated much activity, chiefly in industrial research laboratories, in several countries, because they indicated that the manufacture of new organosilicon monomers and many new silicone polymers and copolymers would become commercially practicable for the first time. 

Much like the enol systems discussed in Sect. 6.1, enamines are predictably difficult substrates for most iridium asymmetric hydrogenation catalysts. Both substrate and product contain basic functionahties which may act as inhibitors to the catalyst. Extended aromatic enamines such as indoles may be even more difficult substrates for asymmetric hydrogenation with an additional energetic barrier to overcome. Initial reports by Andersson indicated a very difficult reaction indeed (Table 14) [75]. Higher enantioselectivities were later reported by Baeza and Pfaltz (Table 14) [76]. 

Complex 77 has also been reported to catalyze the oxidative dimerization of alcohols to esters when the reactions are performed in the presence of base [76]. The presence of base presumably encourages the reversible attack of the alcohol onto the initially formed aldehyde to give a hemiacetal, which is further oxidized to give the ester product. Alcohols 87 and 15 were converted into esters 88 and 89 with good isolated yields (Scheme 20). Alternative iridium catalysts have been used for related oxidative dimerization reactions, and the addition of base is not always a requirement for the reaction to favor ester formation over aldehyde formation [77, 78]. 

The first iridium catalysts for allylic substitution were published in 1997. Takeuchi showed that the combination of [fr(COD)Cl]2 and triphenylphosphite catalyzes the addition of malonate nucleophiles to the substituted terminus of t -allyliridium intermediates that are generated from allylic acetates. This selectivity for attack at the more substituted terminus gives rise to the branched allylic alkylation products (Fig. 4), rather than the linear products that had been formed by palladium-catalyzed allylic substitution reactions at that time [7]. The initial scope of iridium-catalyzed allylic substitution was also restricted to stabilized enolate nucleophiles, but it was quickly expanded to a wide range of other nucleophiles. 

Westcott et al. have also observed the exclusive Mnear product 3-02NC,5H4CH2CH2Bcat in the hydroboration of 3-nitrostyrene by [lrCl2(T -C5Me5)]2 and HBcat [45]. Similarly, the Hnear isomer 4-(Bcat)2NC6H4CH2CH2Bcat could be preferentially obtained in the iridium-mediated hydroboration of 4-vinylaniline as an example of a mild route to anihne derivatives containing boronate esters. Westcott et al. claimed a mechanism that may not necessarily proceed via conventional pathways that invoke initial oxidative addition of HBcat to the metal center. 

The result of theoretical investigations have suggested that cleavage of a B—H bond occurs to initiate ammonia borane dehydrogenation [38]. Alternatively, the oxidative N—H addition of ammonia to the dehydrogenated intermediate C may constitute a feasible reaction pathway due, in particular, to the fact that ammonia and aniline oxidative addition to la and related iridium-PCP systems has been reported experimentally [39]. 

By contrast, the isomerization of silyl olefins and addition of silylacetylenes =C—H bond into imines catalyzed by iridium complexes appears to serve as a suitable route for the synthesis of silylfunctionahzed organic compounds. Hence, the acquisition of experimental data on catalysis by iridium complexes in silicon chemistry may be regarded as an initial stage in the quest for catalytic processes leading to the synthesis of other p-block (e.g. B, Ge, Sn, P)-carbon bond-containing compounds. 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

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. 

All data were collected on toluene solutions (28°C) which were 5 x 10 1 Min iridium the results of a representative experiment are shown in Figure 1. It proved necessary to use the simple substrates in considerable (20X) excess in order to detect the oxidative addition reaction using the measured equilibrium constant, we then calculated the [Ir(III)]/[Ir(I)] (Rjjx) which would exist if the initial concentrations of substrate and 1 were both 5 x 10-l> M. Those calculated values are shown in Table I, together with our results for the chelating substrates. 

A different catalytic cycle for alkene hydroamination is initiated by the oxidative addition of the N-H bond to the metal, followed by insertion of the alkene into the metal-nitrogen bond and reductive elimination to form the amine. The oxidative addition of unactivated N-H bonds to platinum(O) complexes is thermodynamically unfavorable, so the catalytic cycle cannot be completed17, but the successful iridium(I)-catalyzed amination of norbornene with aniline has been reported18. 

Hydrogen addition to Ir(CO)(dppe)(X) leads first to the isomer with X trans to one phosphorus of dppe in a reversible reaction. This kinetic isomer then rearranges to the thermodynamic isomer that has CO trans to a phosphorus atom of dppe. For X = H or PPh3, no rearrangement of the initially formed product is observed . Addition of H2 to an iridium complex containing the optieally active diphosphine chiraphos [bis(S),(S)-2,3-(diphenylphosphino)butane] was described - . The H2 addition to analogues of... 

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