Olefin complexes, substitution reactions

It is generally believed that the cobalt-carbon a-bond is rather unstable and reactive. However, cobaloximes, model complexes of vitamin B12, form simple alkylcobaloximes by several ways, such as reductive alkylation, addition of olefins, and substitution reaction. The cobalt-carbon bonds thus formed are surprisingly stable, and undergo a variety of cleavage reactions, such as/3-elimination, coupling, substitution with nucleophiles, and other reactions. The effect of this specific ligand is remarkable. 

At the beginning of the 1970s a convenient procedure was described for converting olefins into substituted butanedioates, namely through a Pd(II)-cata-lysed bisalkoxycarbonylation reaction. So far various catalytic systems have been applied to this process, but it took twenty years before the first examples of an enantioselective bisalkoxycarbonylation of olefins were reported. Ever since, the asymmetric bisalkoxycarbonylation of alkenes catalysed by palladium complexes bearing chiral ligands has attracted much attention. The products of these reactions are important intermediates in the syntheses of pharmaceuticals such as 2-arylpropionic acids, the most important class of... 

The cationic dinuclear oxo-gold(m) complexes of substituted bipyridyls react with olefins to give the stable coordination compounds shown in Scheme 79. The reactions are often incomplete, produce many byproducts, and give low yields of the olefin complexes/... 

In marked contrast to the results of Gassman and Schrock, major differences were noted by Casey and co-workers in a series of studies utilizing phenylcarbene-substituted W(0) complexes in reactions with olefins. The H NMR spectra of new phenylcarbene tungsten and iron (69) complexes indicate a substantial positive charge residing on the carbene carbon, and as expected, these complexes readily form ylides on reaction with phosphines ... 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

Nucleophilic substitution reactions of halide anions in aprotic solvents are often accompanied by elimination reactions. For instance, reactions of secondary alkyl halides with potassium fluoride solubilized in acetonitrile with the aid of 18-crown-6 [3] give olefins as the main reaction product (Liotta and Harris, 1974). Similarly, the dicyclohexyl-18-crown-6 complex of potassium iodide acted exclusively as a base in its reaction with 2-bromo-octane in DMF (Sam and Simmons, 1974). The strongly basic character of weakly solvated fluoride has been exploited in peptide synthesis (Klausner and Chorev, 1977 Chorev and Klausner, 1976). It was shown that potassium fluoride solubilized... 

Markovic and Hartwig isolated and characterized the first intermediate in iridium-catalyzed allylic substitution [100]. They isolated the metalacyclic iridium-phosphor-amidite fragment containing COD and the olefinic portion ofN- l -phenylallyl)aniline, the product of the allylic substitution reaction between cinnamyl carbonate and aniline (5 in Scheme 22). This complex containing the product of allylic substitution was first detected by NMR spectroscopy during catalytic reactions. It was then isolated, prepared independently, and shown to be chemically and kinetically competent to be an intermediate in allylic substitutions. 

We discussed this catalysis recently (141st National Meeting of the American Chemical Society, March 1962) in terms of an olefin insertion reaction involving a Pt(II) olefin complex (3). We found that catalysis was only accomplished by platinum compounds capable of coordinating olefins. For example, substitution by tertiary phosphines blocks coordination by olefins and greatly reduces the catalytic activity of Pt(II). The substitution by phosphines does not affect the ability of the complexes to cleave the Si—H bond, however. The hindering of a catalytic reaction by blocking coordination sites is a common occurrence and is, I think, a persuasive... 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Olefinic compounds will often insert into carbon-transition metal bonds as CO does, and this reaction is an important step in many catalytic syntheses. When this step is combined with an oxidative addition of an organic halide to a palladium(O) complex in the presence of a base, a very useful, catalytic olefinic substitution reaction results (26-29). The oxidative addition produces an organopalladium(II) halide, which then adds 1,2 to the olefinic reactant (insertion reaction). The adduct is unstable if there are hydrogens beta to the palladium group and elimination of a hydridopalladium salt occurs, forming a substituted olefinic product. The hydridopalladium salt then reforms the... 

Since the substitution reaction succeeded so well with olefins, the obvious extension to acetylenes was tried. Of course, only terminal acetylenes could be used if an acetylenic product was to be formed. This reaction has been found to occur but probably not by a mechanism analogous to the reaction of olefins (43,44). It was found that the more acidic acetylene phenylacetylene reacted with bromobenzene in the presence of triethylamine and a bisphos-phine-palladium complex to form diphenylacetylene, while the less acidic acetylene, 1-hexyne did not react appreciably under the same conditions. The reaction did occur when the more basic amine piperidine was used instead of triethylamine, however (43). Both reactions occur with sodium methoxide as the base (44). It therefore appears that the acetylide anion is reacting with the catalyst and that a reductive elimination of the disubstituted acetylene is... 

The known reactions of fluorinated olefins, arenes, and heterocycles with metal carbonyl anions, to afford fluorovinyl or fluoroaryl complexes resulting from net displacement of fluoride ion (see Section II1,G), prompted us to attempt such substitution reactions with OFCOT, in order to generate the required metal-substituted heptafluorocyclooctatetraenes. 

Collman et al. have recendy reported that threitol-strapped manganese(IH)-porphyrin complex 7 shows high enantioselectivity in the epoxidation of a wide range of olefins when the reaction is carried out in the presence of 1,5-dicyclohexylimidazole (Scheme 6B. 12) [22], The substituted imidazole in this reaction appears to play the same role as the imidazole in Inoue s reaction. Detailed understanding of the mechanism of asymmetric induction by 7 needs further investigation, the major pathway should accommodate the steric interaction between the olefinic substituent and the inner oxygen atom of the threitol strap (Figure 6B.6). Thus, the pathway a is likely to be the major pathway,... 

Electrophilic substitution reactions to olefins have been recorded on vinyl silanes116 (57). Scheme 27 reports the electrophilic substitution of the SiMe3 group by acyl halides. Probably, this reaction too occurs via a complex between the electrophilic part of the reagent and the tt system of the olefin, but other mechanisms are possible. 

Photochemical reactions have been used for the preparation of various olefin, and acetylene complexes (7). Application to the coordination of dienes as ligands has not been used extensively, so far. In this article the preparative aspects of the photochemistry of carbonyls of the group 6 and group 7 elements and some key derivatives, with the exception of technetium, with conjugated and cumulated dienes will be described. Not only carbonyl substitution reactions by the dienes, but also C—C bond formation, C—H activation, C—H cleavage, and isomerizations due to H shifts, have been observed, thereby leading to various types of complexes. 

As the olefin becomes more complex, more reaction pathways and products become possible and the dynamics are even more complicated. In the case of the reaction of F with various butenes [585, 587], both CH3 and H substitution pathways can occur via long-lived collision complexes. The relative cross sections for the two different channels agree with RRKM predictions in most cases, indicating energy randomisation and some... 

The effective atomic number (EAN) rule is useful for interpreting how ligands with more than one double bond are attached to the metal. Essentially, each double bond that is coordinated to the metal functions as an electron pair donor. Among the most interesting olefin complexes are those that also contain CO as ligands. Metal olefin complexes are frequently prepared from metal carbonyls that undergo substitution reactions. 

Olefin substitution reactions. Some olefins form more stable complexes than others. Therefore, it is possible to carry out substitution reactions in which one olefin replaces another. For several common olefins, the order of stability of complexes analogous to Zeise s salt is... 

The electrophilic activation of a C—C multiple bond as a result of coordination to an electron-deficient metal ion is fundamental to much of organometallic chemistry, both conceptually and in synthetic applications (11). The Wacker process, a classic example of an efficient catalytic oxidation, is an important industrial reaction, used for the conversion of ethylene into acetaldehyde. The catalytic reaction begins with the coordination of ethylene to a Pd(ll) center, leading to activation of the ethylene moiety. The key step is the reaction of the metal-olefin complex with a nucleophile to give substituted metal-alkyl species (12). The integration of this reaction into a productive catalytic cycle requires the eventual cleavage of the newly generated M—C bond. 

The reactions of iridium olefin complexes are not restricted to reactions with phosphines. Amines have also employed in bridge-splitting and substitution reactions with [Ir(COD)Cl]2, especially chelating diamines. The reactions proceed to yield [Ir(COD)N-N]2 compounds. A fertile chemical area involves the irw(pyrazolyl)borate (see Tris(pyrazolyl)borates) family of compounds with the monoethylene and bisethylene complexes serving as reactive entries in this field. ... 

Another unusual behavior of NHC TM complex was recently reported by Nolan et al. and involves the insertion of an NHC into a platinum-olefin bond. In the course of preparation of new NHC-containing platinum complexes by reaction of equimolar amounts of [(l,5-hexadiene)PtCl2] and free NHC, the substitution product (286) in which one coordinated double bond was substituted by an NHC was isolated in good yield (Scheme 48). A by-product was also... 

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