Iridium complexes, catalyst

Intermolecular asymmetric aminations are at an early stage of development, and consequently much lower turnover frequencies and catalytic yields have been observed at this stage. In the example shown, a key aspect is the activation of the iridium complex catalyst by fluoride ion [111] (Scheme 38). 

Further development of iridium-complex catalysts was initiated by BP Chemicals in the 1990s, with the hope of identifying reaction conditions under which high activity and selectivity could be achieved. An additional aim was to develop a catalyst that is more robust in the presence of low water concentrations than the rhodium complex catalyst thus, some similarity to the Celanese lithium-iodide stabilized rhodium catalyst was sought. A series of patents provide detail of the discovery by BP of promoters that enhance the activity of an iridium/iodide carbonylation catalyst and, crucially, attain optimum rate at relatively low water concentrations, as illustrated in Figure 2 [116-119]. 

The original mechanistic investigations of iridium/iodide-catalyzed methanol carbonylation were conducted by Forster [6,7,19,115,132-135]. Some other studies were also reported in the late 1970s [136-138]. Since the 1990s, interest in the fundamental aspects of the reaction mechanism has been rekindled by the industrial significance of iridium-complex catalysts. 

The commercial processes for methanol carbonylation discussed above all employ homogeneous rhodium complex or iridium complex catalysts that require an iodide cocatalyst. The highly corrosive nature of acidic iodide-containing solutions and the costly product separation steps mean that catalytic process that avoid these problems are potentially attractive,... 

Water-gas shift reaction iridium complex catalysts, 1160... 

In this reaction, JV-benzylideneimine was formed initially by the reaction of aniline with the aldehyde, and the iridium complex catalyst serves as a Lewis acid. 

By a suitable choice of conditions (metal hydrides or metal/ammonia) ketones at the 1-, 2-, 4-, 6-, 7-, 11-, 12- and 20-positions in 5a-H steroids can be reduced to give each of the possible epimeric alcohols in reasonable yield. Hov/ever, the 3- and 17-ketones are normally reduced to give predominantly their -(equatorial) alcohols. Use of an iridium complex as catalyst leads to a high yield of 3a-alcohol, but the 17a-ol still remains elusive by direct reduction. 

More recently, the same type of hgand was used to form chiral iridium complexes, which were used as catalysts in the hydrogenation of ketones. The inclusion of hydrophihc substituents in the aromatic rings of the diphenylethylenediamine (Fig. 23) allowed the use of the corresponding complexes in water or water/alcohol solutions [72]. This method was optimized in order to recover and reuse the aqueous solution of the catalyst after product extraction with pentane. The combination of chiral 1,2-bis(p-methoxyphenyl)-N,M -dimethylethylenediamine and triethyleneglycol monomethyl ether in methanol/water was shown to be the best method, with up to six runs with total acetophenone conversion and 65-68% ee. Only in the seventh run did the yield and the enantioselectivity decrease slightly. 

The iridium complex 35 has been also used as catalyst for the transfer hydrogenation of substituted nitroarenes [34]. Good to very good conversions were observed (2.5 mol%, in refluxing isopropanol, 12 h). A mixture of two products was obtained, the relative ratio of which depends on the concentration of added base (KOH) and catalyst. (Scheme 2.5)... 

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

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