Insertion olefin

Olefin insertion into arylpalladium bonds is a well-known process [14] which is usually terminated by p-hydrogen elimination, for example Eq. 6. 

The insertion process is promoted by acetate salts [16]. Reductive elimination, however, is possible through a bimolecular reaction with a base such as potassium phenoxide (Eq. 7) [17]. 

Besides monomeric complexes (Eq. 1) [3b,c], dimeric complexes have been isolated and characterized (Eq. 8) [3a,d]. 

Olefin insertion to 7c-allyl derivatives similarly may proceed through a (7-allyl intermediate. A good example is given by the formation of 1,4-hexadiene from ethylene and butadiene in the presence of rhodium chloride, equation (6-85). (See Zeigler-Natta catalyst in Chapter 7). 

One of the most recently studied reactions are those stimulated by y-irradiation. Irradiation techniques also find application in the preparation of novel as well as some more common metal 7r-complexes. The y-irradiation induced reactions of ferrocene in a variety of halogenated solvents have been studied. As shown in equations (6-86) and (6-87), ferrocene irradiated in the presence of carbon tetrachloride or carbon tetrabromide gives ferricinium tetrachloroferrate(III) and ferriciniumtetrabromoferrate(III), respectively. 

When TT-dicyclopentadienyl titanium dichloride is irradiated in the presence of carbon tetrachloride, 7r-cyclopentadienyl titanium trichloride is obtained in sufficient yield to be considered a new preparative method, equation (6-88). 

Dibiphenylchromium(I) cation disproportionates slowly upon ultraviolet irradiation, giving dibiphenylchromium(0) and a divalent chromium ion, equation (6-89). 

A novel reaction was observed in the conversion of one metal-arene n-complex to another by thermal neutron bombardment. In this way diben-zenemolybdenum was converted to dibenzenetechnetium(I), equation (6-90). 

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Ojima-Holton method 670 olefin insertion 569 (+)-ophiobolin C 716 f. organoselenides 397 ortlio-quinodimethane 155 f.,... 

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

The dynamic olefin insertion process has been modeled using various quantum mechanical methods. A concerted four-center mechanism involving a frontal copla-nar attack of the C=C unit on the Zr-H bond of 1 is associated with a low activation energy of 0-15 kcal mol and has been proposed for the reaction of ethylene (Scheme 8-2) [37]. 

A singlet carbene was proposed to account for this stereoselectivity. Attempts to produce triplet carbene by collisional deactivation with octafluorocyclo-butane were unsuccessful and stereospecific addition to olefin still occurred. However, nonstereospecific addition to olefins and larger amounts of olefinic (insertion) products result from irradiation of the phenyldiazomethane in a frozen m-butene matrix at — 196°C ... 

Henrici-Olive, G., and Olive, S. Olefin Insertion in Transition Metal Catalysis. 67, 107-127 (1976). 

By performing excellent model reactions [144], Grubbs and his co-workers demonstrated direct olefin insertion into an M-C bond. Thus, complex 115 was treated with AlEtCl2 to give complex 116, whose decomposition afforded methylcyclopentane. Under the same conditions, the polymerization of ethylene took place. In this way, the insertion of a-olefins into a Ti-C single bond in a model Ziegler-Natta catalyst system was directly observed (Eq. 9). 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

Reactions leading to the formation of the catalytically active nickel hydride species from organonickel precursors (Section III) can be regarded as model reactions for olefin oligomerization reactions. The reactions described by Eq. (8) and Scheme 3 (Section III) show that RNiX compounds (R = methyl orallyl, X = halide or acetylacetonate) activated by Lewis acids add to double bonds under mild reaction conditions (-40° or 0°C). It follows further from these reactions that under conditions leading to olefin dimerization a rapid nickel hydride /3-hydrogen elimination reaction occurs. The fact that products resulting from olefin insertion into the nickel-carbon bond are only observed when /3-hydride... 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

Brookhart s group has reported a related rhodium-catalyzed olefin insertion. To gain insight into the mechanism of this process, labeling studies were carried out under conditions where no coupling product was observed ( H NMR study at 80 °C). Deuterium loss occurred in both meta- and para-sites of the aromatic group, and deuterium incorporation was observed in the olefin (Equation (95)).89... 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

The catalytic cycle proposed for the cyclization-hydrosilylation with the cationic palladium catalyst is classified into the type D in Scheme 2. The reaction consists of an olefin insertion into palladium-silicon bond and the metathesis between palladium-carbon and hydrogen-silicon bond, regenerating the silylpalladium intermediate and releasing the product where migratory insertion of the pendant olefin into the alkylpalladium is involved before the metathesis (Scheme 26).83a... 

According to a widely accepted cyclopolymerization mechanism,67,75 there are two distinct stereochemical events for these cyclopolymerizations the already discussed stereoselectivity and stereospecificity of olefin insertion, which determine the tacticity of the polymer, and the stereoselectivity of the cyclization step, which determines the cis or trans configuration of the rings. 

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