Iridium pincer catalysts

The completely saturated skeleton of carbazole 289 was used as a platform to investigate and develop selective iridium-catalyzed dehydrogenation reactions for the synthesis of net pyrroles bearing fused carbocyclic ring systems 290. Thus, complete reduction of the carbazole 288 skeleton with Pd/C at 120 °C followed by reaction with an iridium pincer catalyst underwent dehydrogenation in smooth order. These molecules were shown to have an activity window of 172-178 °C (14CC5987). 

Alkane Metathesis Using Ir-Pincer Catalysts and Olefin Metathesis In 2006, Goldman, Brookhart, and coworkers [18, 19] reported a modified, tandem w-alkane metathesis reaction employing PCP iridium pincer catalysts (dehydrogena-tion/hydrogenation) with an olefin metathesis catalyst (Figure 2.17). 

To understand the role of the y-alumina support, two experiments were carried out (i) variation ofthe molar ratio ofIr/Re and (ii) the direct addition ofy-alumina to the system. Increasing the molar amount of the Re20y catalyst (molar ratio of 1 1.8) led to a better activity. An additional amount of y-alumina provided better productivity, but parallel studies on either Ir-2(H2) or supported Ir-17(C2H4) with the olefin metathesis catalyst Mo-1 in the presence of y-alumina were found to be detrimental to the reaction selectivity [143]. All of these results strongly support that the adsorption of iridium pincer catalysts on y-alumina could prevent the... 

Determined that -20% conversion of cycloalkanes to arenas can be achieved with the iridium arsino pincer catalyst than is obtained with the analogous phosphino catalyst. 

We have found that the higher levels of conversion of cycloalkanes to arenes can be achieved with the iridium arsino pincer complex, IrH2(C5H3-2,6-(CH2AsBu 2)2) than are obtained with the analogous phosphino catalyst. However, inhibition of the arsino pincer catalyst is observed at the -20% dehydrogenation level for methylcyclohexane, decalin, and dicyclohexyl. 

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... 

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. 

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. 

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. 

Rapid hydrogen releases can be achieved under mild conditions by using transition metal complexes as catalysts. Some noble metal complexes, including Pt, Pd, Rh, Ir, and Ru, were proven to be very effective for promoting hydrogen release from AB. The Rh, Pd, and Ru complexes could catalyze AB to dehydrogenate at room temperature with suppressing the release of volatile cyclotriborazane, borazine, and poly(iminoborane) [107]. The iridium pincer... 

Pincer complexes catalyze a variety of other organic reactions [49-51]. Hence, this work is currently being extended to other metals, and other more readily accessible PCP systems. For example, as shown in Scheme 3, lO-Rfs can be converted to the iridium hydride chloride complex 15-Rfs. Closely related dihydride complexes catalyze dehydrogenations of alkanes at high temperatures [52], However, no efforts to develop recoverable catalysts have been reported to date. 

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 results of some initial experiments indicated that the catalytic activity of 1 is strongly temperature-dependent. For instance, at 100 °C, the rates are relatively low but there is an appreciable turnover [turnover frequency (TOE) = 20.5 h" j. Increasing the reaction temperature to 200 °C increases the TOE to 720 h hence, the thermal robustness of the iridium catalyst is pivotal for optimal catalyst performance. The tridentate pincer-type ligands provide a particularly stable platform for metal confinement through covalent Ir—C bonding combined with the terden-tate chelating coordination. 

Phosphinite pincer iridium systems have also been shown to have a lower tendency to oxidatively add TEE to give (vinyl)(hydride) complexes similar to 3 [18]. While this has been identified as one of the major catalyst deactivation processes in phosphine pincer iridium catalysis, apparently with complexes such as 5, only olefin coordination can occur. However, this is a considerably weaker bonding and is less detrimental to catalyst activity. Eased on steric arguments, product olefin coordination (e.g. COE) is favored over TEE coordination, and therefore at a high TON and high product concentrations the phosphinite catalysts 5 are markedly less active than the phosphine analogues 1. 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

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