Iridium-catalyzed dehydrogenation

Several iridium-catalyzed dehydrogenations of HCXH (X = O, N) linkages have been reported by pincer-iridium complexes. While these dehydrogenations are not generally considered as C-H bond activations or functionalizations, it is generally not known if they proceed via initial activation of the X-H or C-H bond [65, 66]. 

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). 

The iridium-catalyzed dehydrogenative coupling of terminal alkynes with HBpin is an attractive solution [107]... 

Density Functional Studies of Iridium Catalyzed Alkane Dehydrogenation Michael B. Hall and Hua-Jun Fan... 

DENSITY FUNCTIONAL STUDIES OF IRIDIUM CATALYZED ALKANE DEHYDROGENATION... 

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. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

In 2005, Yamamoto et al. reported the synthesis of polycyclic pyrrole-2-carboxylates 76 via a CuBr -catalyzed three-component coupling of A-benzylallylamine, ethyl gly-oxalate, and terminal alkynes, and subsequent ttansforma-tion of the glycine-tethered 1,6-enynes 75 thus obtained through a cycloisomerization/Diels-Alder cycloaddition/ dehydrogenation sequence under iridium-catalyzed conditions (Scheme 3.43) [108]. 

Yuan K, Jiang F, Bruneau C et al (2012) Iridium-catalyzed oxidant-ftee dehydrogenative C-H bond functionalization selective preparation of A-arylpiperidines through tandem hydrogen transfers. Angew Chem Int Ed 51(35) 8876-8880... 

Ishii and coworkers reported that the JV-heterocyclization of naphthylamines with diols can be achieved with an iridium catalyst. In a typical example, the reaction of 1-naphthylamine with 1,3-propanediol was carried out with a catalytic amount of IrClg (5 mol%), r c-2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) (10 mol%), and the corresponding 7,8-benzoquinoline was obtained in 96% yield (Scheme 11.11) [159]. The proposed reaction mechanism indicates that the imine intermediate is formed by the reaction of the amine and aldehyde by Ir-catalyzed dehydrogenation. Subsequent hydrogenation by the in situ generated Ir hydride leads to an aminoalcohol followed by cyclization to the desired quinoline products. 

---