Iridium catalysts phenols

Partial reduction of phenols affords mixtures of allylic and vinylic alcohols. From the generality derived for aliphatic systems, the most hydrogenolysis of this mixture is expected with platinum, palladium, and iridium catalysts, and much less with rhodium and ruthenium, an expectation substantiated in practice. For example, hydrogenation of resorcinol in neutral medium affords 20, 19, and 70% cyclohexanediol over palladium-, platinum-, and rhodium-on-carbon, respectively (29). Many examples attest to the value of rhodium and ruthenium at elevated pressure in avoiding hydrogenolysis. 

Hydrogenation of phenol and alkylphenols over iridium catalysts is often accompanied by rather extensive hydrogenolysis even in a neutral solvent, in a similar manner as with platinum in acidic medium. Rylander and Steele observed that the hydrogenation of 3,5-dimethylphenol over 5% Ir-C in isopropyl alcohol at 100°C and 5.2 MPa H2 afforded 1,3-dimethylcyclohexane in 76% yield, compared to 25% yield over 5% Pt-C in acetic acid-hydrochloric acid.89 Similarly, 2,5-dimethylphenol caused 60% of hydrogenolysis to give 1,4-dimethylcyclohexane over the iridium catalyst in isopropyl alcohol, compared to 67% over 5% Pt-C in acetic acid-hydrochloric acid.119 Table 11.8 compares the extents of accompanying hydrogenolysis when phe-... 

Even oxygen nucleophiles have been introduced with good enantioselectivity using both palladium- and iridium-based catalysts. The conditions of the reaction need to be sufficiently mild that the product does not become a substrate for the allylic substitution, since this will ultimately lead to racemisation. Pivalate ( BuC02 ) and phenols have been used as nucleophiles, in the presence of palladium catalysts, with good results, while linear allylic carbonates are converted into chiral branched products with high ee using phenolates, aUcoxides and also hydroxylamines with iridium complexes. Sulfur nucleophiles have also been used in enantioselective allylic substitution reactions. ... 

Arenes (benzenes, phenols, and anilines etc.) and heteroarenes (indoles, pyrroles, furans, thiophenes etc.) are very important commodity chemicals. Therefore, the addition of aromatic and heteroaromatic C—H bonds to unsaturated compounds would provide a clean and economic method for adding extra value to such compounds. However, the use of an iridium complex as the catalyst in this type of asymmetric C—C bond-forming reactions was not reported until 2000. 

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. 

Recently, Shinohara et al. expanded this type of transformation to the application of iridium or iron catalysts for rearrangement of the primary cycloaddition products 37 into the phenol derivatives 38 (Scheme 13.19) [20]. 

---