Insertion and elimination reactions

In homogeneous catalytic reactions, old bonds are usually broken by oxidative addition reactions and new bonds are formed by reductive elimination and insertion reactions. A few representative examples that are of relevance to catalysis are shown by Reactions 2.8-2.11. The following points deserve attention. Reactions 2.8, 2.9, and 2.10 are crucial steps in hydrogenation, polymerization, and CO-involving catalytic reactions. Reaction 2.8 is, of course, just the reverse of /8-hydride elimination. Sometimes this reaction is also called a hydride attack or hydride transfer reaction. 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

In any catalytic reaction, the catalyst undergoes a cycle that changes the state of the catalyst, effectively to make room for the reactants, release the resulting products, and start all over again as shown in Figure 16-2. The steps that a catalyst can undergo include ligand substitution, oxidative addition, reductive elimination, and insertion reactions. 

The pathways in the reaction of 1 with certain halogenoboron and -aluminum compounds are much more complicated than those described in the last chapter. Here, addition and insertion reactions take place in combination with multistep rearrangement and elimination processes. Some reactions are still not fully understood concerning the mechanistic details, but plausible reaction sequences have been suggested. 

Electrochemistry (Continued) purely organic compounds, 342 sulfide oxidation, 361 Electrode materials, 342 Electrophilic allylation, 192 attractive interaction, 196 mechanism, 192, 197 turnover-limiting step, 197 Electroreaction, asymmetric, 342 Electrostatic interaction, 328 Elimination and insertion, 3 Enamide reactions ... 

The efficiency of /-elements in catalysis originates from unconventional electrophilic pathways. In contrast to rf-elements oxidative addition/reductive elimination sequences are not accessible. Instead, substrate adduct formation, ligand exchange and insertion reactions rule the mechanistic scenarios. Therefore, the main emphasis is put on the fine-tuning of the spectator ligand of the precatalyst. 

Methylation of nickelacycle 30, obtained in the reaction between Ni(COD)(py)2 and 2-cyclopentencarboxylic acid, unexpectedly leads to the formation of cw-2-methylcyclopentanecarboxylic acid (Scheme 29) 33(0 pjijs product probably arises by methylation of isomeric nickelacycle 56, formed by 3-hydride elimination and insertion with the opposite regiochemistry. When the same reaction is carried out with a large excess of iodomethane, d.y-3-methylcyclopentanecarboxylic acid is obtained. Methylation reactions of related azanickelacycles have also been reported. ... 

For the formation of branched alkyls either from I-alkenes or internal alkenes this scheme must be modified, because now often the insertion is reversible, dependent on conditions and concentrations. A potential scheme showing the competition between the backward reaction (3-branched) and the complexation of CO reaction (4) is shown in Figure 23. At low ternperamres and sufficiently high pressures the formation of 2-alkyl species from 1-alkenes can also be irreversible. We have drawn the barriers for the backwards reaction (P-elimination) and forward reaction (CO complexation) at about the same height to indicate the competition between the two steps. 

The reaction of [PtEtaCbipy)] with methyl acrylate to give [Pt CH(Me)(C02Me) 2-(bipy)l involves an unusual reaction sequence. The first step is the displacement of the bipy by methyl acrylate. Subsequent / -elimination and insertion are finally followed by return of the bipy to the platinum. ... 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Oxidative addition of alkyl halides to Pd(0) is slow. Furthermore, alkyl-Pd complexes, formed by the oxidative addition of alkyl halides, undergo facile elimination of /3-hydrogen and the reaction stops at this stage without undergoing insertion or transmetallation. Although not many examples are available, alkynyl iodides react with Pd(0) to form alkynylpalladium complexes. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

The main example of a category I indole synthesis is the Hemetsberger procedure for preparation of indole-2-carboxylate esters from ot-azidocinna-mates[l]. The procedure involves condensation of an aromatic aldehyde with an azidoacetate ester, followed by thermolysis of the resulting a-azidocinna-mate. The conditions used for the base-catalysed condensation are critical since the azidoacetate enolate can decompose by elimination of nitrogen. Conditions developed by Moody usually give good yields[2]. This involves slow addition of the aldehyde and 3-5 equiv. of the azide to a cold solution of sodium ethoxide. While the thermolysis might be viewed as a nitrene insertion reaction, it has been demonstrated that azirine intermediates can be isolated at intermediate temperatures[3]. 

A significant effect of Lewis acids on such transamiular C-H insertion reactions has been demonstrated. Treatment of 5,6-epoxycydooctene (31) with s-BuLi/ (-)-sparteine gave allylic alcohol 32, formally the product of P-elimination, in good yield (and ee) (Scheme 5.9). In the presence of BF3-Et20, however, alcohol 33 was produced as a result of a-lithiation, in 75% yield and 71 % ee [16]. 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

C) Reactions involving ligands in two adjacent (i.e., cis) coordination positions. They include certain insertion reactions (e.g., the insertion of cyanide into the Co—C bond) and the reverse elimination reactions. 

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