Metal insertion elimination reactions

We saw in Chapter 6 that a reaction that is the reverse of 1,2-insertion— called 1,2- or (3-elimination—may also occur with metal alkyls, the product of insertion.13 Equation 8.8 details the insertion-elimination reaction that yields a vacant site for coordination in the forward direction (1,2-insertion) and requires a vacant site for the reverse ((3-elimination) pathway. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

There are several examples of the concerted mechanism. However, no report of an insertion of a carbon—carbon triple bond into a metallacyclopentadiene had appeared prior to discovery of this reaction. At low temperatures, during the reaction of zirconacyclopentadienes with DMAD, the formation of trienes (79) is observed upon hydrolysis. This clearly indicates that the benzene formation involves the insertion (addition) reaction of DMAD. As shown in Eq. 2.50, the alkenyl copper moiety adds to the carbon—carbon triple bond of DMAD and elimination of Cu metal leads to the benzene derivatives 72. Indeed, a copper mirror is observed on the wall of the reaction vessel. 

Insertion of phenyl, trimethylsilyl, and nitrile-stabilized metalated epoxides into zircona-cyclcs gives the product 160, generally in good yield (Scheme 3.37). With trimethylsilyl-substituted epoxides, the insertion/elimination has been shown to be stereospecific, whereas with nitrile-substituted epoxides it is not, presumably due to isomerization of the lithiated epoxide prior to insertion [86]. With lithiated trimethylsilyl-substituted epoxides, up to 25 % of a double insertion product, e. g. 161, is formed in the reaction with zirconacyclopentanes. Surprisingly, the ratio of mono- to bis-inserted products is little affected by the quantity of the carbenoid used. In the case of insertion of trimethylsilyl-substituted epoxides into zirconacydopentenes, no double insertion product is formed, but product 162, derived from elimination of Me3SiO , is formed to an extent of up to 26%. 

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

A. Heterolysis of the Metal-Carbon a-Bond Homolysis of the Metal-Carbon a-Bond Oxidation of Lm iM +1-R Followed by Homolysis P-Hydride Shift Reactions P-Elimination Reactions P-Elimination of Carboxylates CO Insertion/Methyl Migration... 

When the transfer reaction competes successfully with further insertion, as in the case of nickel, dimerization becomes the dominant transformation. When metal hydride elimination, in turn, is slow relative to insertion, polymeric macromolecules are formed. Ligand modification, the oxidation state of the metal, and reaction conditions affect the probability of the two reactions. Since nickel hydride, like other metal hydrides, catalyzes double-bond migration, isomeric alkenes are usually isolated. 

Theoretically, it is possible for the process of olefin coordination and insertion to continue as in Ziegler-Natta polymerization (Chapter 52) but with palladium the metal is expelled from the molecule by a p-hydride elimination reaction and the product is an alkene. For the whole process to be catalytic, a palladium(O) complex must be regenerated from the palladium(ll) product of P-hydride elimination. This occurs in the presence of base which removes HX from the palladium(II) species. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

Elimination reactions are typically found in transition metal chemistry. The conversion of allylic ethers into allylzirconocenes [18] is a well-known zirconium-mediated process of this type. In contrast, the y-elimination reactions are less frequently encountered, and only a few examples of y-elimination involving zirconocenes have been reported. When studying isonitrile insertion into zirconacycles, Whitby and coworkers observed an interesting reaction leading to a cyclopropane derivative [19]. The overall transformation is depicted in Scheme 12. Treatment of the diene 18 with preformed butene-zirconocene gave... 

This is the second chapter of a two-part review concerned with insertion reactions of transition metal-carbon a-bonded compounds. The first chapter, which appeared in Volume 11 of this series (137), provided a broad introduction to the subject of insertion reactions in general and a detailed treatment of the carbon monoxide insertion and decarbonylation. Presented herein are the insertion and elimination reactions of sulfur dioxide and of a few other unsaturated molecules. The reactions of sulfur dioxide are accorded a complete literature coverage, whereas those of the other inserting species are treated selectively. Metal-carbon a-bonded compounds of the main group elements are discussed only in the context of comparisons with their transition metal analogs. 

This review illustrates the above delineated characteristics of electron-transfer activated reactions by analyzing some representative thermal and photoinduced organometallic reactions. Kinetic studies of thermal reactions, time-resolved spectroscopic studies of photoinduced reactions, and free-energy correlations are presented to underscore the unifying role of ion-radical intermediates [29] in—at first glance—unrelated reactions such as additions, insertions, eliminations, redox reactions, etc. (Photoinduced electron-transfer reactions of metal porphyrin and polypyridine complexes are not included here since they are reviewed separately in Chapters 2.2.16 and 2.2.17, respectively.)... 

Hydride elimination reactions are characterized by the transfer of a hydrogen atom from a ligand to a metal. Eifectively, this may be considered an oxidative addition, with both the coordination number and the formal oxidation state of the metal being increased (the hydrogen transferred is formally considered as hydride, H ). The most common type is p elimination, with a proton in a p position on an alkyl ligand being transferred to the metal by way of an intermediate in which the metal, the a and P carbons, and the hydride are coplanar. An example is shown in Figure 14-11. p Elimination is the reverse of 1,2-insertion. 

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