Oxidation transition metal catalysis

Keywords Carbon-fluorine, Bond activation, Transition metals, Catalysis, Oxidative addition, Fluorocarbons, CFCs, Electron transfer, Halocarbon coordination... 

Transition metal catalysis in Baeyer-Villiger oxidation of cyclic ketones with formation of lactones 98AG(E)1198. 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Figure 3.7. Some principles of homogeneous transition-metal catalysis L = PPhj c.n. = coordination number o.s. = oxidation state.

W. Liu, and M. Flytzani-Stephanopoulos, Transition metal-promoted oxidation catalysis by fluorite oxides A study of CO oxidation over Cu—Ce02, Chem. Eng. J. 64, 283—294 (1996). 

The entries into transition metal catalysis discussed so far, required the presence of a specific bond (a polar carbon-heteroatom bond for oxidative addition or a carbon-carbon multiple bond for coordination-addition processes) that was sacrificed during the process. If we were able to use selected carbon-hydrogen bonds as sacrificial bonds, then we could not only save a lot of trouble in the preparation of starting materials but we would also provide environmentally benign alternatives to several existing processes. In spite of the progress made in this field the number of such transformations is still scarce compared to the aforementioned reactions. 

Experimental observations indicate that the oxidation of cobalt (II) to cobalt (III) and the formation of ethylenediamine from N-hydroxyethylethylene-diamine occur simultaneously. This is quite the opposite to what is usually assumed in other instances of transition metal catalysis of organic reactions—for example, the catalytic effect of manganese in the oxidation of oxalic acid (7, 8), of iron in the oxidation of cysteine to cystine (22) and of thioglycolic acid to dithioglycolic acid (5, 23), of copper in the oxidation of pyrocatechol to quinone and in the oxidation of ascorbic acid (29, 30), and of cobalt in the oxidation of aldehydes and unsaturated hydrocarbons (4). In all these reactions the oxidation of the organic molecule occurs by the abstraction of an electron by the oxidized form of the metal ion. 

Biaryls have also been prepared by coupling support-bound aryl halides with aryl-zinc compounds (Figure 5.20) or with aryl(fluoro)silanes [203]. As with Suzuki or Stille couplings, these reactions also require transition metal catalysis. An additional strategy for coupling arenes on solid phase is the oxidative dimerization of phenols (Figure 5.20). 

Even though the aramatization mechanism from the putative cyclohexadienyl radical intermediate is not well understood (oxidation by the adventitious air is one possibility), the radical addition to an aromatic ring strategy does possess some advantages (a) no transition metal catalysis is required, (b) better yields are obtained than with similar procedures although the reaction is not regiospecific, (c) readily available starting materials may be used. 

G. Strukul, Transition Metal Catalysis in the Baeyer-Villiger Oxidation of Ketones , Angew. Chem. Int. Ed. Engl. 1998, 37, 1198-1209. 

This principle links the typical organometallic oxidation state-based view of transition metal catalysis reactions best with the radical reaction manifold. At the end a table that links the typical radical reaction types to the different metals catalyzing them is provided. Throughout the review, series of stable organic compounds or transition metal complexes are numbered by arabic numerals and small letters, while reactive intermediates or transition states are designated by arabic numerals followed by capital letters. 

This volume provides the reader with the most important and exiting results pertaining the use of NHC complexes in transition-metal catalysis. Following an introductory chapter, which deals with the properties of NHC compounds and discusses some insightful examples, routes to NHC complexes will be described, a prerequisite for doing catalysis. Chapters on NHC complexes in oxidation chemistry and in metathesis reactions are accompanied by a chapter on palladium-catalyzed reactions and another on catalysis by other metals. Finally, this book would be incomplete without treating applications in asymmetric catalysis, which rounds out this volume. 

G. Strukul, Transition Metal Catalysis in the Baeyer-Villiger Oxidation of Ketones, Angew. 

A widely applied strategy for the synthesis of various difunctionalized organic molecules, e.g. diols, dialdehydes, etc., relies on the oxidative cleavage of olelinic double bonds. Besides transition metal catalysis for asymmetric synthesis, periodate oxidation and ozonolysis are the standard tools for oxidative bond cleaving reactions. For economic and safety reasons, technically applicable alternatives to osmium-based chemistry and ozonolysis are of great interest. 

A few examples of the use of transition metal catalysis in conjunction with hydrogen peroxide for aldehyde oxidations have also been reported.228,229... 

A long-standing success in transition metal catalysis is the carbonylation reaction [66], in particular the synthesis of acetic acid [67]. Formally this is the insertion of CO into another bond, in particular into a carbon-halogen bond. After the oxidative addition to the transition metal (the breaking of the carbon-halogen bond), a reaction with a CO ligand takes place. This reaction is often called an insertion. Mechanistic studies have, however, shown that the actual reaction... 

The hydrophosphonylation (addition of dialkyl phosphonates), hydrophos-phinylation (addition of alkyl alkylphosphinates or alkyl phosphinates) and addition of secondary phosphine oxides to unfunctionalized alkenes and alkynes occur under free radical conditions or with transition metal catalysis.10,39 63 90... 

Three major topics have dominated research activity on thiophenes since 1996 the design and synthesis of dithienylethene molecules for application as photochromic systems (Section 3.10.2.1.3) reactions brought about under transition metal catalysis (Section 3.10.2.11) and the synthesis, characterization, and reactivity of a plethora of transition metal complexes of thiophenes (Section 3.10.6). All three had received brief mention in CHEC-11(1996) (Sections 2.10.2.2.3, 2.10.4.7.3, and 2.10.6, respectively), but together account for almost one-third of the chapter now. In addition, shorter sections have been introduced to cover the following topics one-electron oxidation of thiophenes (Section 3.10.2.2) electrochemical reactions at cathodes (Section 3.10.2.7.5) sulfur-extrusion and sulfur-transfer reactions (Section 3.10.2.10) and reactivity of silicon-linked substituents (Section 3.10.4.5). 

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