Transition metal complexes redistribution reactions

Until relatively recently, less attention has been given to catalysis of redistribution reactions by transition metal complexes. Redistributions have been observed during the course of platinum-catalyzed hydrosila-tion hence, the scrambling reaction can be a nuisance by decreasing the yield of desired hydrosilation products. A noteworthy example is the H/Cl exchange that occurs during the hydrosilation of allyl chloride [Eq. (10)]. 

In redistribution reactions catalyzed by transition metal complexes, di-or polysiloxanes are expected to share some characteristics of both monosilanes and di- or poly silanes. Similarities to the former are expected since the very reactive Si—Si bond is replaced by the less reactive Si—O bond, and to the latter because there are two or more exchangeable sites in the molecule. 

A major problem in postulating silylenoid metal complexes as intermediates in the redistribution reactions is simply that good model compounds are lacking, and the decomposition mechanisms of silyl transition metal complexes have not been systematically investigated. While there is evidence for transient R2Si species produced by thermal or photochemical means (80-83), there are no known monomeric silylene metal complexes. Several monomeric stannylene and germylene complexes are... 

Recent Developments in Theoretical Organometallic Chemistry. 15, I Redistribution Equilibria of Organometallic Compounds, 6, 171 Redistribution Reactions of Transition Metal Organometallic Complexes, 23, 9S Redistribution Reactions on Silicon Catalyzed by Transition Metal Complexes, 19, 213 Remarkable Features of (7] -Conjugated Diene) zirconocene and -hafnocene Complexes, 24, I Selectivity Control in Nickel-Catalyzed Olefln Oligomerization, 17, lOS Silyl. Germyl, and Stannyl Derivatives of Azenes. N H Part I. Derivatives of Diazene, N,H2, 23, 131... 

For clusters containing transition metals, there are closely spaced electron orbitals that can assist in electron-redistribution or electron-exchange reactions that involve excited states or slightly varying electron configurations. Stated differently, the LUMO orbitals in transition metal complexes are sufficiently close to HOMO orbitals that they can store electrons at low energy cost. Thus, metal centers on nanoclusters can accelerate reactions that involve electron redistribution. 

Molecular oxygen adducts of transition metal complexes arc of interest and importance to catalytic processes and commercial oxidation processes, as well as being intermediates in oxidation reactions. Vaska " has reviewed the nature of dioxygen bound to transition metal complexes. All known iridium dioxygen complexes possess the peroxo structure (140). Experimental data reveal that the formation of covalent Ir—(O2) bonds on dioxygen addition to IrL, is accompanied by extensive redistribution of electrons, and the electron transfer is from the iridium to dioxygen. SCF-X -SW calculations on [Ir(02)(Ph3)4] and [Ir(Ph3)4] " indicate peroxo -metal bonding. ... 

The hydrosilylation of alkenes and alkynes catalyzed by transition metal complexes is often accompanied by side reactions such as isomerization, polymerization, and hydrogenation of unsaturated compounds and/or redistribution and dehydrogenation of silicon hydrides as well as reactions in which both substrates take part, such as dehydrogenative silylation (4,13). The latter reaction, which in certain conditions permits direct production of unsaturated silyl compound, has been a subject of intense study over the last two decades. 

The redistribution reactions at the silicon atom, catalyzed by transition metal complexes have been reviewed by Curtis and Epstein (49), whereas those occurring under hydrosilylation conditions have been discussed by Speier (50). It is apparent from the numerous examples listed in the two reviews that the by-products formed during hydrosilylation arise from the following main kinds of redistribution reactions ... 

Curtis M.D., Epstein P.S. Redistribution reactions on silicon catalyzed by transition metal complexes. In Adyances in Organometalhc Chemistry, vol 19. F.G.A. Stone, R. West, eds. San Diego, CA Academic Press, 1981... 

A series of complexes in which the diorganothallium moiety is linked to various transition-metal complex anions have been prepared, RaTl-ML [MLn = M(CO)2LCp M = Mo or W L = CO or PPha R = Me,Et, or Ph). The compounds were obtained by a variety of methods, e.g. by protolytic reactions, reaction (43), metathesis reactions, reaction (44), or by redistribution... 

In the last decade, an immense amount of experimental material has been generated describing the preparation and the chemical and physical properties of transition metal n complexes and coordination compounds. Recently great emphasis has been placed on the study of the kinetics and the reaction mechanisms involving such compounds. Although redistribution reactions as defined earlier in this review and as exemplified specifically by the reaction of Eq. (168) (M = transition metal, L=coordinated ligand)... 

These redistribution reactions between metal-tin complexes not only provide extremely useful transition-metal-substituted organotin chlorides, but also demonstrate the chemical robustness of the M—Sn bonds. For example, whereas the W—Sn bond in [(CO)5WSnPh3] resists attack by dry HC1, the silicon and germanium analogs are cleaved to [C1W(C0)5] 49. 

An improvement of catalyst activity, especially for the oxidation of electron-poor, deactivated systems like p-toluic acid, can be reached by addition of other transition metal compounds to the Co/Mn/Br catalyst. The most prominent additive is zirconium(IV) acetate, which by itself is totally inactive. An addition of zirconi-um(IV) acetate (ca. 15 % of the amount of cobalt) can yield reaction rates which are higher than those observed using a tenfold amount of cobalt acetate. This amazing co-catalytic effect can be attributed to the common ability of zirconium to attain greater than sixfold coordination in solution, to the high stability of Zr toward reduction, and to the ability of zirconium or Hf to redistribute the dimer/ monomer equilibrium of dimerized cobalt acetates (Co 7Co, Co VCo " systems) by forming a weak complex with the catalytically more active monomeric Co species [17]. 

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