Multiple Bonds Between Main Group Elements and Transition Metals. 151.1 Trioxorhenium(VII) Alkoxide Complexes

Rhenium-catalysed reactions in chemical synthesis: selected case studies

This Perspective presents and discusses a selection of examples that reinforce the enabling and distinctive reactivity provided by homogeneous rhenium catalysis in chemical synthesis. Specifically, the ability for lower oxidation...

Deoxydehydration of glycerol in presence of rhenium compounds: reactivity and mechanistic aspects

Re compounds in different oxidation states are activated during a delay time into an active Re alkoxide precipitate catalysing the DODH of glycerol.

Regiochemical control in the metal-catalyzed transposition of allylic silyl ethers

A novel mode of regiochemical control over the allylic [1,3]-transposition of silyloxy groups catalyzed by Re2O7 has been developed. This strategy relies on a cis-oriented vinyl boronate, generated from the Alder-ene reaction of homoallylic silyl ethers and alkynyl boronates, to trap out the allylic hydroxyl group. The resulting cyclic boronic acids are excellent partners for cross-coupling reactions. High chirality transfer is observed for the rearrangement of enantioenriched allylic silyl ethers.

Rhenium(VII) Oxide Catalyzed Heteroacylative Ring-Opening Dimerization of Tetrahydrofuran

Re(2)O(7), which is known primarily as a strong oxidant, was found to be a highly selective Lewis acid catalyst that affects the heteroacylative dimerization of THF at room temperature. This multicomponent reaction, which involves THF, trifluoroacetic anhydride (TFAA), and a carboxylic acid, produces a nonsymmetrical diester, RCO(2)(CH(2))(4)O(CH(2))(4)OCOCF(3), in high yields. The reaction is quite general with respect to the carboxylic acid but is highly selective for unsubstituted THF in preference to other cyclic ethers. It is also highly selective for TFAA in preference to other anhydrides. Isotope labeling experiments indicate that two of the five oxygen atoms in the product originate from THF; one comes from rhenium oxide, and the two carbonyl oxygens originate from the carboxylic acid and from TFAA. The catalytic cycle, which is proposed on the basis of these experiments, involves a multistep sequence of nucleophilic attacks, starting with an attack of a rhenium oxo ligand on a coordinated THF, then attack of the resultant alkoxide ligand on a second coordinated THF, nucleophilic addition of the resultant alkoxide ligand to the coordinated carboxylic acid (an intramolecular metal-oxygen bond metathesis), and, finally, electrophilic cleavage of the other coordinated alkoxide by TFAA to produce the nonsymmetrical diester. This synthetically useful reaction highlights the unique, frequently avoided Lewis acidity of transition-metal oxides.

MTO Schiff-Base Complexes: Synthesis, Structures and Catalytic Applications in Olefin Epoxidation

Several Schiff-base ligands readily form complexes with methyltrioxorhenium(VII) (MTO) by undergoing a hydrogen transfer from a ligand-bound OH group to a ligand N atom. The resulting complexes are stable at room temperature and can be handled and stored in air without problems. Due to the steric demands of the ligands they display distorted trigonal-bipyramidal structures in the solid state, as shown by X-ray crystallography, with the O(-) moiety binding to the Lewis acidic Re atom and the Re-bound methyl group being located either in cis or trans position to the Schiff base. In solution, however, the steric differences seem not to be maintained, as can be deduced from (17)O NMR spectroscopy. Furthermore, the Schiff-base ligands exchange with donor ligands. Nevertheless, the catalytic behaviour is influenced significantly by the Schiff bases coordinated to the MTO moiety, which lead either to high selectivities and good activities or to catalyst decomposition. A large excess of ligand, in contrast to the observations with aromatic N-donor ligands, is detrimental to the catalytic performance as it leads to catalyst decomposition.

Heterogenization of an organorhenium(vii) oxide on a modified mesoporous molecular sieve

A novel route to heterogenize an organorhenium(VII) oxide, derived from the well examined methyltrioxorhenium(VII) (MTO), on the surface of an iodosilane-modified MCM-41 is applied. The successful grafting of the -CH(2)-ReO3 moiety on the surface was evidenced by 1H CP MAS NMR, IR spectroscopy, TG-MS, and elemental analysis. XRD and TEM analyses confirm the retaining of long-range ordering throughout the grafting process. The rhenium loading of the mesoporous material after heterogenization of MTO is found to be 1.25 wt%. Despite containing formally a derivative of the very sensitive benzyltrioxorhenium(VII) the material is stable at room temperature and applicable as a heterogeneous catalyst for aldehyde olefination reactions.