Alkylated intermediate

In principle, polymers equivalent to those obtained from vinyl and divinyl monomers may be synthesized by this method. The product in the above example possesses the same chain structure as polyethylene. The polymerization process, notwithstanding the likelihood of a metal alkyl intermediate, should conform satisfactorily to stepwise condensation. However, the product, and those obtained by Friedel-Crafts condensation as well, lack the recurrent functional groups which generally characterize condensation polymers. 

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

A plausible mechanism proposed for this reaction involves migratory insertion of an olefin into the Pd-Si bond of a paUadium-silyl intermediate I followed by migratory insertion of the pendant olefin into the resulting Pd-C bond of II forming palladium-alkyl intermediate III. Reaction of Iff with hydrosilane releases the carbocy-cle to regenerate the palladium-silyl complex I (Scheme 3-21) [61]. 

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

Because the cis-decalin molecule extends its two methine carbon-hydrogen bonds on the same side in contrast to frans-decalin, the carbon-hydrogen bond dissociation of adsorbed decalin would be advantageous to the cis-isomer on the catalyst surface (Figure 13.17). A possible reaction path by octalin to naphthalene in dehydrogeno-aromatization of decalin will be favored to the cis-isomer, since its alkyl intermediate provides the second hydrogen atom from the methine group to the surface active site easily. 

Sites for growth (on-top sites) and the alkyl intermediate (SpN) bound on them... 

Chain propagation is started from a methylene group and terminated by desorption of 1-alkenes or alkanes. Propeller-type mobility of the olefin ligand renders possible CH3 branching of the growing chain, as demonstrated by the scheme. The growth probability is determined by the ratio of rates of formation of the alkyl intermediate and of the desorption of 1-alkenes, and to a minor extent of alkanes. 

One of the main consequences of this change concerns the branching toward the formation of the alcoholate or the release of aldehyde and hydrogenation toward the alkyl intermediate formation. Coordination of the alkali cation weakens the O-C bond, favoring the hydrogenolysis of this bond. Correspondingly, it reduces the chance of alcoholate formation and aldehyde release. This assumption may explain why a2 is only marginally affected by the nature of A (A = Li, Na, K, Cs). 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Yields of EUPb are therefore constricted by adsorption and formation of passivating coatings, and rates are affected by side reactions of partially alkylated intermediates ... 

Since S03/H2S04 is clearly not the most desirable system for industrial applications, a formidable challenge is to find an oxidant that oxidizes Pt(II) much faster than S03 does, operates in an environmentally friendly solvent, and can be (like SVI/SIV) reoxidized by oxygen from air. Ideally, the reduced oxidant would get reoxidized in a continuous process, such that the oxidant acts as a redox mediator. In addition, the redox behavior has to be tuned such that the platinum(II) alkyl intermediate would be oxidized but the platinum(II) catalyst would not be completely oxidized. Such a system that efficiently transfers oxidation equivalents from oxygen to Pt(II) would be highly desirable. A redox mediator system based on heteropolyacids has been reported for the Pt-catalyzed oxidation of C-H bonds by 02, using Na8HPMo6V6O40... 

Monohydride (MH) catalysts, such as [RhH(CO)(PPh3)3], react with substrates such as alkenes, according to Scheme 1.1, yielding rhodium-alkyl intermediates which, by subsequent reaction with hydrogen, regenerate the initial monohydride catalyst. This mechanism is usually adopted by hydrogenation catalysts which contain an M-H bond. 

From all the above observations, it was concluded that, for diphosphine chelate complexes, the hydrogenation stage occurs after alkene association thus, the unsaturated pathway depicted in Scheme 1.21 was proposed [31 a, c, 74]. The monohydrido-alkyl complex is formed by addition of dihydrogen to the en-amide complex, followed by transfer of a single hydride. Reductive elimination of the product regenerates the active catalysts and restarts the cycle. The monohydrido-alkyl intermediate was also observed and characterized spectroscopically [31c, 75], but the catalyst-substrate-dihydrido complex was not detected. 

Alkene insertion into the Pd-H bond with formation of an alkyl intermediate (Eq. (5)). 

Kinetic studies show that insertion (the enantioselection step) is very rapid, and that the rate-determining step is the hydrogenolysis of the M-C bond. Nonetheless, under H2-starving conditions, there is evidence that fi- I elimination can be competitive with hydrogenolysis. />-H elimination of the alkyl intermediate gives back the starting alkene and, through an equilibration process, it... 

The first step is coordination of the ethylene through its n orbital. The ethylene is trans to Cl with the C=C bond in the Cl-Ru-H plane. Facile migratory insertion (AE = 7.6 kcal.mol 1) of the coordinated ethylene in the Ru-H bond leads to an alkyl intermediate 6.2 kcal.mol 1 less stable than the n ethylene complex. The alkyl intermediate has a strong P C-H agostic interaction as illustrated by the unusually long agostic C-H bond (1.221 A) which helps to stabilize the unsaturation in the formally 14-electron alkyl intermediate. 

Figure 8. Selected 14-electron alkyl intermediates and their relative energy (kcalmol 1) with respect to the corresponding tc-olefin complex...

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