Jean-Marie Brégeault

Transition-metal complexes for liquid-phase catalytic oxidation: some aspects of industrial reactions and of emerging technologies

This review presents recent developments in some important industrial processes. For homolytic systems, the similarities and specificities of the transformation of cyclohexane into adipic acid (via the formation and isolation of the cyclohexanol–cyclohexanone mixture) and of the process for the preparation of terephthalic acid (by oxidation of p-xylene) are analysed from a prospective viewpoint. Innovations in heterolytic chemistry are also presented. For example, the epoxidation of propene currently performed by the Arco–Lyondell method, which uses a molybdenum catalyst and tert-butyl hydroperoxide, is in competition with other systems involving recycling of the co-product, or the use of hydrogen peroxide as oxygen donor. Access to diphenols, from phenol and H2O2, is becoming more selective. New oxoperoxo complexes of titanium, rhenium, tungsten and molybdenum are presented with their potential for catalytic oxidation. Achievements of our group in the development of catalysts for C–C bond cleavage by dioxygen, and for epoxidations by H2O2 are also given. Biomimetic oxidations and systems without metal catalysts feature among the innovations. These novel systems could compete with transition-metal-based chemistry for fine chemicals, but industry increasingly focuses research and development on fully inorganic systems, and there is little impact on the large scale of catalytic complexes with organic ligands.

Assessment of MTO as a catalyst for the synthesis of acid sensitive epoxides. Use of the biphasic system H2O2/CH2Cl2 with and without bipyridine and influence of the substituents on the double bonds

Methyltrioxorhenium (MTO) catalyzes the selective epoxidation of alkenes in the biphasic medium CH2Cl2/H2O2–H2O. Especially sensitive epoxides, which could not be obtained by the use of other reagents, have been isolated in high yield by the addition of bipyridine to this reaction medium.

Novel Distorted Pentagonal‐Pyramidal Coordination of Anionic Oxodiperoxo Molybdenum and Tungsten Complexes

Model compounds for silica surfaces modified with metal complexes are provided by the mononuclear anionic diperoxo species 1 and the dinuclear complex anions 2 (M = Mo, W). They were obtained in fair to nearly quantitative yields by the reactions of Ph3 SiOH and [Ph2 Si(OH)]2 O, respectively, with an aqueous solution of [MO(O2 )2 (H2 O)2 ].

The oxidation of ketones with a heteropolyacid, H5[PMo10V2O40] and dioxygen

Substituted cycloalkanones, 1-phenylalkanones, and open-chain ketones are oxidatively cleaved by the title compound under very mild conditions.

Synthesis and characterization of highly dispersed molybdenum species in SBA-15 mesoporous molecular sieves

An original pathway involving H2O2 is described for the incorporation of highly dispersed molybdenum oxo species into the mesoporous network of an SBA-15 type silica. This procedure requires a mixture of templating agents: the triblock copolymer EO20PO70EO20 and the quaternary ammonium salt CTMACl, which are commonly used for SBA-15 and MCM-41 syntheses, respectively. Physicochemical techniques clearly demonstrate that the triblock surfactant/CTMACl molar ratio has to be carefully controlled in order to i) obtain a significant amount of Mo in the mesoporous silica and ii) retain the SBA-15 structure.

Oxidations involving phosphate species supported on TS-1: a novel class of grafted catalysts

Abstract Phosphate species deposited on titanium silicalite (TS-1) can interfere with epoxidation by hydrogen peroxide. Phosphates grafted on TS-1 are identified by solid-state31P MAS NMR. Signals at −5.1, −10.8 and −20 ppm are attributed to monodentate, chelating or bridging phosphate ions and polyphosphates, respectively. Phosphate-modified TS-1 can anchor tungsten oxoperoxo species; with aqueous hydrogen peroxide, this novel class of supported catalysts can oxidize bulky molecules under mild conditions.

Homogeneous oxidation of 1-octene by dioxygen catalyzed by rhodium species: comparison of the ‘rhodium-copper-lithium’ and ‘rhodium-bismuth-lithium’ systems

Abstract Complexes of the type [RhIII(H2O)6](A−)3 (A− = ClO4−, BF4−,…) have been tested with other rhodium compounds as catalyst precursors in the oxidation of terminal olefins to methyl ketones by dioxygen. The effect of a cocatalyst (Cu(II) or Bi(III) salt) is reported and new systems are devised. It is possible to obtain good activities as well as good selectivities with substrate/catalyst ratios of 120 (P(O2) = 1 atm, 40 ⩽ T(°C) ⩽ 70). Novel homogeneous systems based on rhodium(III), copper(II) complexes and lithium chloride are more effective in the oxidation of 1-olefins dissolved in ethanol than the ‘RhCl3·3H2O’/Cu(ClO4)2·4HMPA system, (the latter gives a precipitate of cuprous chloride). Together with rhodium-copper based systems, rhodium complexes in association with bismuth chloro-complexes and/or LiCl were found to be equally efficient in the oxidation of 1-alkenes in alcoholic media, but there is less solvent cooxidation in the ‘RhIII/CuII/LiI’ systems. The yields and the rate of reaction are not sensitive to phosphine concentration (e.g. P(C6F5)3 and P(n-Bu)3, within the limits Rh:P =1:1 to 1:2). Cycloocta-1,5-diene is mono-oxygenated by the ‘RhIII/ CuII/Li’ system at 60 °C, in a reaction leading to cyclooct-4-en-1-one (yield 82%). Our experimental results and those of other groups suggest that the key intermediates are a hydrido-rhodium complex: ‘[HRhIIIClxLy]’ and a hydroperoxy complex ‘[HOORhClxLy]’. The reaction of dioxygen with HRhCl2[PPh2(n-Bu)]3 in dry ethanol-oct-1-ene mixtures affords octane-one (yield 45% based on rhodium, T = 60 °C).

Cétonisation catalytique du méthyl-2-butène-3-ol-2 par des complexes du rhodium(III) ou du palladium(II)

Abstract A number of efficient catalytic procedures for the oxidation of acid-sensitive substrate such as 2-methyl-3-buten-2-ol to the corresponding ketone (2-methylbutan-2-ol-3-one) are compared. Reaction with dioxygen (or quinone) and a palladium precursor (PdCl 2 or [PdCl(NO 2 )(CH 3 CN) 2 ]) under mild conditions (⋍ 35°C), gave good conversions (⋍ 98%) and good selectivities (⋍ 90%).

Oxydation catalytique de l'octène-1 en présence de complexes de rhodium(III) ou de palladium(II) associés à des acides phosphomolybdovanadiques et au dioxygène

Abstract Chlororhodium(III) complexes with heteropolyacids HPAH 3+ n [PMo 12 −n V n O 40 ]) catalyze the oxidation by molecular oxygen of oct-1-ene to octan-2-one in alcoholic solvents with alcohol cooxidation ( T 60° C). The PdSO 4 HPA/water-THF/O 2 catalyst system shows the highest activity at room temperature for this ketonization.

Mixed crystals containing the dioxo complex [{Ph3SiO}2VO2]− and novel pentacoordinated oxoperoxo complex [{Ph3SiO}2VO(O2)]−: X-ray crystal structure and assessment as oxidation catalysts

[n-Bu4N][[Ph3SiO]2VO2] reacts with H2O2 to yield an oxoperoxo complex which crystallizes as a mixed-crystal compound, [P(C6H5)4][[(C6H5)3 SiO]2VO2]x[[(C6H5)3 SiO]2VO(O2)](1-x), 1(x = 0.57). It has been characterized by elemental analysis and spectroscopy (51V NMR, UV-visible and IR). The X-ray structure analysis reveals the presence of two interrelated anions: [[Ph3SiO]2VVO2]-, 1a, and [[Ph3SiO]2VVO(O2)]-, 1b with a cisoid geometry of the [VO(O2)]+ moiety. The two structures differ only slightly: anion 1a exhibits unusual tetrahedral coordination around the vanadium centre found in the precursor, whereas the geometry at the metal ion in 1b can be described as a trapezoidal pyramid. Steric constraints due to Ph3SiO- ligands and PPh4+ cations are responsible for this geometry. The reactivity of 1 in the C-C bond cleavage of 2-methylcyclohexanone under anaerobic conditions has been studied. The results suggest that peroxygen species are involved in the oxidative cleavage of C-C bonds of cycloalkanones.