Iridium-pincer complexes

The transfer (de)hydrogenation is fully reversible-that is, the iridium pincer complexes catalyze the alkane dehydrogenation as well as the alkene hydrogenation. 

Although neither Cjryi—H nor Cjikynyi—H bonds have been funchonalized in a catalytic process by iridium pincer complexes, a substanhal mechanishc understanding has been accumulated that may lead to catalyhc reachons in the near future. For this reason-and also due to the fact that some of these achvahon protocols are different from standard processes-a short discussion of the achievements to date is included at this point. 

Iridium pincer complexes-and to a larger extent also the corresponding rhodium complexes-have provided useful insights into the mechanism of Caryi—Ca yi bond... 

A remarkable selectivity for the formation of a-olefins has been reported by Jensen, Goldman, and co-workers [18], The iridium pincer complexes 20a and 20b were compared in the dehydrogenation of octane 17 (Scheme 5). When norbornene (18a) was used as acceptor... 

Alkane dehydrogenation took a step forward in 1996 with the report of rhodium and iridium pincer complexes that could catalyze transfer hydrogenation. While the rhodium complex was found to be active but unstable, the iridium complex was stable even after a week at 200 °C. This permitted it to efficiendy catalyze the transfer hydrogenation of cyclooctane to cyclooctene (12 t.o./min, Scheme The reaction is inhibited by high concentrations of olefin, either the... 

As mentioned above in Section 1.25.5.2, rhodium and iridium pincer complexes have been used to catalytically dehydrogenate alkanes, giving terminal olefins as the kinetic products. In a recent report by Goldman and Brookhart, the iridium Pincer complexes were combined with Schrock s alkylidene metathesis catalyst... 

In 2011, Gelman and coworkers [43] reported a PCjp P iridium pincer complex (17), which exhibited efficient catalytic activity for the conversion of benzyl alcohol to benzyl benzoate (Scheme 1.9) and was suggested to operate via a novel metal-ligand cooperation mode. Both electron-withdrawing and electron-donating substituents... 

Scheme 8.6 Suggested mechanism for the aromatization reaction in an iridium pincer complex.

Scheme 8.20 Difference in interaction of carbon dioxide and dihydrogen with rhodium and iridium pincer complexes.

Lao DB, Owens ACE, Heinekey M, Goldberg KI (2013) Partial deoxygenation of glycerol catalyzed by iridium pincer complexes. ACS Catal 3 2391-2396... 

In a series of papers, Bercaw et al. recently described a very elegant tantalum/ iridium tandem aUcane/alkene coupling ([150] and references therein). Especially exemplified with 1-hexene and heptane, this process involves the dimerization of two different aUtenes (here 1-hexene and 1-heptene) catalyzed by the hemitantallocene 108, while an iridium pincer complex 109 dehydrogenates the... 

Transfer Dehydrogenations of Alkanes and Related Reactions Using Iridium Pincer Complexes... 

A major breakthrough in transfer dehydrogenation of alkanes was achieved in 1996 by Jensen, Kaska, and coworkers [16, 17]. They reported that the iridium pincer complex ( " PCP)IrH2, la, was highly reactive and exceptionally thermally stable for transfer dehydrogenation of COA employing TBE as the acceptor [Eq. (3)]. For example, at 200°C the turnover frequency was reported to be 12/min with no noticeable catalyst decomposition over 7 days. 

Fig. 1 Examples of active PCP iridium pincer complexes for alkane dehydrogenation...

The discovery of the (de)hydrogenation steps using transition metals, in particular, the iridium pincer complexes, was essential to the success of this tandem, dualalkane metathesis reaction. As such, efforts made to improve these Ir catalytic systems will first be discussed. Subsequently, the application of these catalysts to the tandem, dual-alkane metathesis reaction will be elaborated. 

Figure 2.13 Iridium pincer complexes used for alkane dehydrogenation.

Scheme 2.15 The proposed mechanism for COA dehydrogenation using an iridium pincer complex Ref. [112],...

Dehydrogenation of the -alkane poses a regioselectivity problem for the olefinic products. Iridium pincer complexes Ir-2(H2) and Ir-3(H2) have shown kinetically high selectivity for transfer dehydrogenation of the terminal position... 

In the COA transfer dehydrogenation in the presence of the, the bisphosphi-nite iridium pincer complexes Ir-13(H2) [127] and Ir-16(H4) [126] both displayed higher activities than Ir-2(H2) (Figure 2.16). Similar results were obtained with the corresponding (p-X POCOP)IrH2 catalysts, Ir-14(H2) (X = OMe) and Ir-15(H2) (X = 3,5-(CF3)2C6H3) [127,128]. 

The synthesis and use of immobilized iridium pincer complexes on solid supports for the transfer dehydrogenation of alkanes have also been examined [131]. Three approaches are reported (i) the post-functionalization of a Mer-rifield resin by incorporating the pincer complex (ii) the covalent bonding of the catalyst to silica via a pendant alkoxysilane linker and (iii) the adsorption of the catalyst onto y-alumina via the interaction of the phenolate group on the para-position of the pincer ligand with the Lewis-acidic sites on the alumina. The last approach showed the best activity, affording thermally robust, recyclable, and active supported ( PCP)Ir (Ir-2) and ( POCOP)Ir (Ir-13) catalysts. 

Figure 2.21 Iridium pincer complexes adsorbed onto y-alumina.

Cross-Alkane Metathesis In a parallel study on cross-alkane metathesis using supported, Ta polyhydrides [34], Schrock and coworkers recently reported the dual-catalytic, homogenous, cross-alkane metathesis of n-octane, and ethylbenzene. The authors [136] employed various W-monoaryloxide pyrrolide complexes in combination with several iridium pincer complexes. For example, employing Ir-2(H2) and W-1 with a ratio of n-octane/ethylbenzene (1 1.33, in v/v) produced the best productivity toward alkylbenzenes, with good selectivity over linear alkanes (Scheme 2.20). 

Liu F, PakEB, Singh B, Jensen CM, Goldman AS. Dehydrogenation ofn-alkanes catalyzed by iridium pincer complexes regioselective formation ofa-olefins. J/lm Chem Soc. 1999 121 4086-4087. 

PCP iridium pincer complexes are a well-studied and attractive class of compounds which promote important catalytic and stoichiometric processes. Although this class of compounds are also briefly discussed in the sections describing iridium hydrides (Section 7.04.6) and iridium catalyzed C-C bond-forming processes (Section 7.04.3.5), a few notes addressing their relevance in the chemistry of hydrocarbyl iridium complexes are here necessary. Both the neutral (PCP)Ir [PCP = 3-C6H3-2,6-(CH2PBu )2] and the cationic (PNP)Ir [PNP = 2,6-bis-(di-fet -butyl phosphino... 

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