James H. Espenson

Mechanism of hydrogen evolution from hydridocobaloxime. [As a function of perchloric acid concentration]

The forward rate of the reaction 2HCo(dmgH)/sub 2/P(n-C/sub 4/H/sub 9/)/sub 3/ = H/sub 2/ + 2Co(dmgH)/sub 2/P(n-C/sub 4/H/sub 9/)/sub 3/ has been determined in methanol--water solutions as a function of perchloric acid concentration, 0.003 to 0.10 M. The reaction proceeds by two parallel pathways, with respective first-order and second-order dependences upon the concentration of hydridocobaloxime. Once allowance is made for an H/sup +/-dependent equilibrium with K/sub H/ = 1.3 x 10/sup 2/ M/sup -1/, which we interpret to be the protonation of oxime oxygens (a reaction known in related complexes), the first-order term proceeds at a rate directly proportional to (H/sup +/), and the second-order term is independent of (H/sup +/). Deuterium labeling experiments were also carried out. The reaction appears to proceed by parallel heterolytic and homolytic cleavage of the hydrogen--cobalt bond.

Methylrhenium trioxide as a catalyst for oxidations with molecular oxygen and for oxygen transfer

Abstract Methylrhenium trioxide (MTO) was found to be a good catalyst for the oxidation of tertiary phosphines by molecular oxygen at room temperature. Evidence is given that an intermediate Re(V) compound — CH3ReO2, or the adduct CH3ReO2·OPPh3 — is involved. The deoxygenation of epoxides, sulfoxides, N-oxides, triphenylarsine oxide and triphenylstibine oxide at room temperature was also catalyzed by MTO, with triphenylphosphine as the oxygen acceptor. A plausible reaction mechanism involves phosphine attack at a compound formed between MTO and the epoxide or other oxygen-donor compound.

Epoxidation reactions with urea–hydrogen peroxide catalyzed by methyltrioxorhenium(VII) on niobia

Abstract Soybean oils (oleic, linoleic, and linolenic acids and their methyl esters) are epoxidized readily with urea–hydrogen peroxide (UHP) when methyltrioxorhenium(VII) supported on niobia is used as the catalyst in chloroform. Simple alkenes are epoxidized by the same method. The epoxide and not a diol is produced.

The Problem and Perversity of Perchlorate

The title might just have well have contained additional words, such as “peculiarity” and “persistence,” for each in its way is further characteristic of this species. Perchlorate is indeed peculiar, in that its reactions in practice are usually not those predicted from reliable thermodynamic calculations; persistent, in that spontaneous reactions do not occur, leaving perchlorate in place, and perverse in that factors other than thermodynamics, kinetics in particular, govern its actual behavior. These issues may for some areas of chemistry create a problem, if the accumulation of perchlorate and its resistance poses a difficulty; for others, the lack of reactivity of perchlorate creates an opportunity, in that perchlorate salts can be used in many situations requiring an inert electrolyte.

OXYGEN TRANSFER REACTIONS: CATALYSIS BY RHENIUM COMPOUNDS

Publisher Summary This chapter deals with the transfer of an atom—usually oxygen, occasionally sulfur—from one species to another. The catalytic capabilities of rhenium compounds burst on the scene about one decade ago, featuring MeReO3 as a catalyst for reactions of hydrogen peroxide. It was quickly verified that peroxorhenium(VII) compounds were the active intermediates. With them, practical reactions and fundamental questions of mechanism could then be resolved. A new generation of oxorhenium compounds has now been prepared. They catalyze oxidation reactions of a different type, and appear to function by a different mechanism. They are oxorhenium(V) compounds that form usually metastable dioxorhenium(VII) intermediates. The mechanisms feature ReV(O)-to-ReVII(O)2 interconversions and catalyze oxygen-atom transfer reactions. The mechanisms show certain diversity as to the steps that enter in a kinetic sense.

Atom-transfer reactions catalyzed by methyltrioxorhenium(VII)—mechanisms and applications

Methyltrioxorhenium activates hydrogen peroxide by an electrophilic mechanism, transferring a single oxygen atom to many substrates without the intervention of free-radical intermediates. The reactions proceed without by-products, unlike those of most chemical (stoichiometric) oxidizing agents. In separate chemistry, the rhenium catalyst brings about the transfer of oxygen atoms between a pair of closed-shell molecules.

Selective C-H bond activation of arenes catalyzed by methylrhenium trioxide

Abstract Arenes, in glacial acetic acid, are oxidized to para -benzoquinones by hydrogen peroxide when methylrhenium trioxide (CH 3 ReO 3 or MTO) is used as a catalyst. In some cases an intermediate hydroquinone was also obtained in lower yield. Oxidation of the methyl side chains of various methylbenzenes did not occur. The active catalyst species are the previously-characterized η 2 -peroxorhenium complexes, CH 3 Re(O) 2 ( η 2 -O 2 ) and CH 3 Re(O)( η 2 -O 2 ) 2 H 2 O). Separate tests showed that hydroquinones and phenols are oxidized by H 2 O 2 -MTO more rapidly than the simple arenes; in the proposed mechanism they are intermediate products. Higher conversions were found for the more highly-substituted arches, consistent with their being the most reactive species toward the electrophillically-active peroxide bound to rhenium. High conversions of the less substituted members of the series were not achieved, reflecting concurrent deactivation of MTO-peroxide, a process of greater import for the more slowly-reacting substrates.

Kinetic study of epoxidations by urea–hydrogen peroxide catalyzed by methyltrioxorhenium(VII) on niobia

Relative rates were measured for the heterogeneous epoxidation of olefins with urea–hydrogen peroxide (UHP) in CDCl3 catalyzed by MeReO3 on Nb2O5. The rates are more selective than those in homogeneous MTO–H2O2 solutions. The reactivity orders among the alkenes are cis>trans and electron-rich > electron-poor. A molecular structure has been proposed for the active rhenium species which allows the rate differences to be explained in terms of energy gap between the alkene occupied π(CC) and unoccupied σ∗(OO) of the peroxorhenium moiety.

Photoaccelerated oxidation of chlorinated phenols.

Exposure to visible light increases the rate of oxidation of chlorinated phenols by hydrogen peroxide in aqueous solution in either the presence or the absence of iron-based catalysts, which may be explained by the aqueous photoreactions of chloroquinone intermediates.

Catalytic reactions of methylrhenium trioxide on solid oxide supports

Methylrhenium trioxide (MTO), supported on niobia, acts as an effective heterogeneous catalyst for certain chemical reactions. These include reactions of ethyl diazoacetate, which are equivalent to carbene-transfer processes, and selective oxidations that utilize hydrogen peroxide. Both groups of reactions, which are known to occur homogeneously, proceed efficiently and in good yield on the supported catalyst.