This Feature Article intends to provide an overview of a variety of catalytic isomerization reactions that have been performed using well-defined transition metal hydride precatalysts. A particular emphasis is placed on the underlying mechanistic features of the transformations discussed. These have been categorized depending upon the nature of the substrate and in most cases discussed following a chronological order.

We investigate the distinctly different interaction of thiolate-protected cluster Au 38 (SC 2 H 4 Ph) 24 with two diverse support materials Al 2 O 3 and CeO 2 . The catalytic surfaces have been heated in different atmospheres, and the removal of the thiolate ligands has been studied. Thermogravimetry (TG), temperature-programmed process coupled with mass spectrometer (TPRDO-MS), and X-ray absorption spectroscopy (XAFS) studies were performed to understand the desorption of thiol ligands depending on conditions and support material. Depending on the atmosphere and the support material the fate of the thiol ligands is different upon heating, leading to metallic Au in the case of Al 2 O 3 and to cationic Au with CeO 2 . The thiolate removal seems to be a two-step procedure. The catalytic activity of these Au 38 -supported clusters was studied for the aerobic oxidation of cyclohexane. Conversion was higher for the gold clusters supported on CeO 2 . Surprisingly, a significant amount of cyclohexanethiol was found, revealing the active participation of the thiolate ligand in catalytic reactions. The observation also indicates that breaking and formation of C−S bonds can be catalyzed by the gold clusters. ■ INTRODUCTIONHeterogeneous catalytic processes by supported thiolateprotected clusters (Au n (SR) m ) represent an emerging field stimulated by reports showing higher activity and selectivity in several oxidation reactions 1−4 in comparison with metal nanoparticle catalysts. These clusters consist of a symmetric metal core protected by multiple gold−thiolate staples −SR− (Au−SR−) n (n = 1,2) that present size-dependent physical chemical properties, related with their quantized electronic structure and unique geometrical structure. In comparison to bulk gold, which has a face-centered cubic (fcc) structure, clusters of sizes <2 nm often show icosahedral structure. 5

The reaction of a [(PHOX)Ir(COD)](+) complex (COD = 1,5-cyclooctadiene) with dihydrogen was studied by NMR spectroscopy (PHOX = chiral phosphinooxazoline ligand). A single [(PHOX)Ir(H)(2)(COD)](+) isomer was formed as the primary product at -40 degrees C in THF. Subsequent reaction with H(2) at -40 to 0 degrees C led to a mixture of two diastereomeric [(PHOX)Ir(H)(2)(solvent)(2)](+) complexes with concomitant loss of cyclooctane. The stereochemistry of the three hydride complexes could be assigned from the NMR data. The structures and energies of the observed hydride complexes and the possible stereoisomers were calculated using density functional theory. The substantial energy differences (up to 39 kcal/mol) between the various stereoisomers demonstrate the strong influence of the chiral ligand. The observed stereoselective formation of dihydride complexes can be explained by steric effects of the PHOX ligand combined with a strong electronic influence of the coordinating N and P atoms, favoring addition of a hydride trans to the Ir-N bond.

The long-range deconjugative isomerization of a broad range of α,β-unsaturated amides, esters, and ketones by an in situ generated palladium hydride catalyst is described. This redox-economical process is triggered by a hydrometalation event and is thermodynamically driven by the refunctionalization of a primary or a secondary alcohol into an aldehyde or a ketone. Di-, tri-, and tetrasubstituted carbon-carbon double bonds react with similar efficiency; the system is tolerant toward a variety of functional groups, and olefin migration can be sustained over 30 carbon atoms. The refunctionalized products are usually isolated in good to excellent yield. Mechanistic investigations are in support of a chain-walking process consisting of repeated migratory insertions and β-H eliminations. The bidirectionality of the isomerization reaction was established by isotopic labeling experiments using a substrate with a double bond isolated from both terminal functions. The palladium hydride was also found to be directly involved in the product-forming tautomerization step. The ambiphilic character of the in situ generated [Pd-H] was demonstrated using isomeric trisubstituted α,β-unsaturated esters. Finally, the high levels of enantioselectivity obtained in the isomerization of a small set of α-substituted α,β-unsaturated ketones augur well for the successful development of an enantioselective version of this unconventional isomerization.

One-pot reactions based on catalytic isomerization of alkenes not only offer the inherent advantages of atom-, step- and redox-economy but also enable the preparation of value-added products that would be difficult to access by conventional methods.