Alkene elimination polymerization

The formation of alkenes and alkene-related polymerization products can seriously reduce the yields of desired alkane products from secondary alcohols, which can undergo elimination reactions. For example, reduction of 2-octanol at 0° with boron trifluoride gas in dichloromethane containing 1.2 equivalents of tri-ethylsilane gives only a 58% yield of n-octane after 75 minutes (Eq. II).129 The remainder of the hydrocarbon mass comprises nonvolatile polymeric material.126... 

The alkene elimination product usually dissociates readily but remains coordinated in some cases, as in a compound of the type (21-XXII).106 The /3-H elimination is the predominant termination step in catalytic alkene oligomerizations and polymerizations. 

The reaction of alkenes and other unsaturated substances with transition metal hydrido or alkyl complexes is of prime importance in catalytic reactions such as hydrogenation, hydroformylation, and polymerization (see Chapter 22). It is one of the major methods for synthesizing metal-to-carbon bonds. The reverse reaction, the /3-hydride or /3-alkyl transfer-alkene elimination reaction has already been discussed (Section 21-3). 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Mechanistic studies of the rearrangement activity of the ring-opening metathesis polymerization catalyst [Ru(H20)6]2+ were reported for unfunctionalized alkenes (112). The mechanism was found to be intermolecular, the alkene isomerization proceeding through an addition-elimination mechanism with a metal hydride catalytic species. This interpretation was... 

A new mechanism to interpret alkene formation in Fischer-Tropsch synthesis has been presented 499-501 There is a general agreement that hydrocarbon formation proceeds according to the modified carbene mechanism. Specifically, CO decomposes to form surface carbide and then undergoes hydrogenation to form surface methine (=CH), methylene (=CH2), methyl and, finally, methane. Linear hydrocarbons are formed in a stepwise polymerization of methylene species. When chain growth is terminated by p-hydride elimination [Eq. (3.61)], 1-alkenes may be formed,502 which is also called the alkyl mechanism ... 

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. 

Migratory insertion is the principal way of building up the chain of a ligand before elimination. The group to be inserted must be unsaturated in order to accommodate the additional bonds and common examples include carbon monoxide, alkenes, and alkynes producing metal-acyl, metal-alkyl, and metal-alkenyl complexes, respectively. In each case the insertion is driven by additional external ligands, which may be an increased pressure of carbon monoxide in the case of carbonylation or simply excess phosphine for alkene and alkyne insertions. In principle, the chain extension process can be repeated indefinitely to produce polymers by Ziegler-Natta polymerization, which is described in Chapter 52. 

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. 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

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. 

Polymerization reactions follow an insertion mechanism, that is, alkene coordination to a vacant site on the active metal species, followed by a migratory alkyl transfer step. The addition of donor molecules which can compete with the alkene for coordination sites is therefore a means of reducing the rate of propagation and allows /3-H elimination to take place, so that a polymerization reaction might be converted to oligomerization or dimerization. On the other hand, metals which... 

Because the elementary reactions of cationic alkene polymerizations are directly related to the organic chemistry of carbocations, Chapter 2 will investigate electrophilic additions to double bonds, nucleophilic substitution, electrophilic aromatic substitution, and elimination reactions. 

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