Catalytic iridium complexes

Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes... 

Another area of high research intensity is the catalytic dehydrogenation of alkanes to yield industrially important olefin derivatives by a formally endothermic (ca. 35 kcal mol-1) loss of H2. Recent results have concentrated on pincer iridium complexes, which catalytically dehydrogenate cycloalkanes, in the presence of a hydrogen accepting (sacrificial) olefin, with turnover numbers (TONs) of >1000 (Equation (23)) (see, e.g., Ref 33,... 

The iridium complex composed of l/2[ Ir(OMe)(cod)2 ] and 4,4 -di-/ r/-butyl-2,2 -bipyridine (dtbpy) shows a high catalytic activity for aromatic G-H silylation of arenes by l,2-di-/z r/-butyl-l,l,2,2,-tetrafluorodisilane.142 The reaction of 1,2-dimethylbenzene with l,2-di-/< r/-butyl-l,l,2,2,-tetrafluorodisilane in the presence of l/2[ Ir(OMe)(cod)2 ] and dtbpy gives 4-silyl-l,2-dimethylbenzene in 99% yield (Equation (103)), which can be utilized for other functionalizations such as arylation and alkylation. 

A two-component bimetallic catalytic system has been developed for the allylic etherification of aliphatic alcohols, where an Ir(i) catalyst acts on allylic carbonates to generate electrophiles, while the aliphatic alcohols are independently activated by Zn(n) coordination to function as nucleophiles (Equation (48)).194 A cationic iridium complex, [Ir(COD)2]BF4,195 and an Ru(n)-bipyridine complex196 have also been reported to effectively catalyze the O-allylation of aliphatic alcohols, although allyl acetate and MeOH, respectively, are employed in excess in these examples. 

Although the transfer reaction can occur at 100 °C, the anthraphos iridium complex does not begin to show catalytic activity until 150 °C and continues to be stable to 250 °C. Therefore, we have made temperature corrections to 150°C (423 K) in Table VI and to 250°C (523 K) in Table VII. Compared to STP values, the free-energy barriers for this reaction increase by 5.6 kcal/mol for 423 K and 10.0 kcal/mol for 523 K. As expected, the enthalpies (AH and AH) hardly change (< 0.5 kcal/mol). One can also make corrections for the fact that the... 

A mechanism for the catalytic process is shown in Scheme 15, which comprises three sets of iridium complexes, namely Ir(I) species, Ir(III)-methyls... 

The commercialisation of an iridium-based process is the most significant new development in methanol carbonylation catalysis in recent years. Originally discovered by Monsanto, iridium catalysts were considered uncompetitive relative to rhodium on the basis of lower activity, as often found for third row transition metals. The key breakthrough for achieving high catalytic rates for an iridium catalyst was the identification of effective promoters. Recent mechanistic studies have provided detailed insight into how the promoters influence the subtle balance between neutral and anionic iridium complexes in the catalytic cycle, thereby enhancing catalytic turnover. 

The catalytic reduction of nitro groups is usually achieved using heterogeneous catalysts, although the iridium complex 28 has been shown to be effective for the reduction of p-nitroanisole 29 to the corresponding aniline 30 using isopropanol as the hydrogen donor (Scheme 8) [30]. In the reduction of some nitroarenes, azo compounds (Ar-N=N-Ar) could be formed as by-products or as the major product by variation of the reaction conditions. 

In this review, the recent developments in catalytic functionalization of C-H bonds by iridium complexes will be emphasized. For more information on previous work and their details, there are excellent reviews published recently on C-H bond functionalization by transition metal complexes generally [1-9], as well as with an emphasis on iridium [11, 16]. 

The success of derivatives of 1 and 2 as dehydrogenation catalysts has led to the investigation of numerous different pincer ligands for iridium-catalyzed alkane dehydrogenation. The Anthraphos pincer iridium complex (3-H2) was expected to afford even greater thermal stability (Eig. 1), and indeed, the catalyst can tolerate reaction temperatures up to 250°C [42]. The catalytic activity of 3-H2, however, is much less than that of I-H2 under comparable conditions. 

The phosphoramidite ligands that are the focus of the remainder of this chapter have prompted the investigation of ligands containing related structures. Iridium complexes of aspartic acid-derived P-chirogenic diaminophosphine oxides (DlAPHOXs) catalyze the amination [62] and alkylation [63] of aUyhc carbonates (Scheme 6). With BSA as base and catalytic amounts of NaPFs as additive, branched amination and alkylation products were obtained from cinnamyl carbonates in excellent yields and enantioselectivities. However, the yields and enantios-electivities were lower for the reactions of alkyl-substituted aUyhc carbonates. Added LiOAc increased the enantioselectivities of aUyhc alkylation products. 

In contrast, 1,5-cyclo-octadiene remains coordinated during the catalytic cycle of hydrogenation of phenylacetylene to styrene, catalyzed by the related iridium complex [Ir(C0D)( Pr2PCH2CH20Me)]BF4. This complex, which contains an ether-phosphine-chelated ligand, catalyzes the selective hydrogenation reaction via a dihydrido-cyclo-octadiene intermediate. The reaction is first order in each of catalyst, phenylacetylene and hydrogen [11] the proposed catalytic cycle is shown in Scheme 2.3. 

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