Metal-catalyzed oligomerization

Metal-catalyzed oligomerizations are important reactions concerning both their chemistry and practical applications.12 36 37 Since the catalysts [(Ti(IV), Zr(TV), Ni(I), and Ni(II) complexes] used in coordination polymerization can also be active in oligomerization, catalyst systems and mechanisms will be discussed in more detail in Section 13.2.4. A short discussion of oligomerization with emphasis on the most important processes of practical significance, however, is appropriate here. 

Many transition metals catalyze oligomerization reactions between dienes and olefins or alkynes. Possible reaction products are legion. But it is almost exclusively with zerovalent nickel that cyclic products are formed. Addition of olefins or alkynes to catalysts mentioned in the previous section suppresses the cyclooligomerization and instead gives cyclo-co-oligomerization products. 

Simple monoolefins generally yield open-chain dimers or oligomers by metal-catalyzed oligomerization. Only the nickelacyclopentane 1 catalytically dimerizes ethylene to a mixture of cyclobutane and 1-butene ... 

Carbometalation reactions play an important role in Ziegler-Aufbaureaktionen (growth reactions) of ethene to form oligomers when a stoichiometric amount of an organoaluminum compound is used. They are also crucial steps in transition metal catalyzed oligomerization and polymerization of unsaturated compounds, as well as in many cyclization. cycloaddition and cyclooligomcrization reactions. Ketone-forming carbonylative addition reactions also proceed via carbometalation steps. 

The halocyanoacetylenes 23-25 have been prepared by standard methodology [Eq. (7) see below] from cyanoacetylene (1) [30-32]. Although they have been investigated in detail from the structural and spectroscopic viewpoint, their chemistry remains largely unexplored. These compounds are not particularly unstable, and can thus be handled by more or less routine laboratory techniques. Of particular interest should be their behavior in high-temperature pyrolysis as well as their photochemical behavior. Metal-catalyzed oligomerization could turn out to be another interesting field of application of these functionalized cyanocarbons. 

Simple 1,3-dienes such as 1,3-butadiene, isoprene, and related compounds undergo efficient metal-catalyzed oligomerization. Under palladium catalysis, diene dimerization is the most common oligomerization reaction observed. Four modes of dimerization have been reported (Scheme 1) (i) [2 + 2] cycloaddition to afford 1,2-divinylcyclobutane (1) (ii) [4 -I- 2] cycloaddition to afford 4-vinylcyclohexene (2) (iii) [4 + 4] cycloaddition to afford 1,4-cyclooctadiene (3) and (iv) linear dimerization to afford 1,3,7-octatriene (4). 

Cuprous salts catalyze the oligomerization of acetylene to vinylacetylene and divinylacetylene (38). The former compound is the raw material for the production of chloroprene monomer and polymers derived from it. Nickel catalysts with the appropriate ligands smoothly convert acetylene to benzene (39) or 1,3,5,7-cyclooctatetraene (40—42). Polymer formation accompanies these transition-metal catalyzed syntheses. 

As early as 1990, Chauvin and his co-workers from IFP published their first results on the biphasic, Ni-catalyzed dimerization of propene in ionic liquids of the [BMIM]Cl/AlCl3/AlEtCl2 type [4]. In the following years the nickel-catalyzed oligomerization of short-chain alkenes in chloroaluminate melts became one of the most intensively investigated applications of transition metal catalysts in ionic liquids to date. 

Under the reaction conditions employed, the oligomerization is living the aluminium is removed from the chain in a separate process step [2]. Further research on the Aufbau reaction led to the discovery of transition metal catalyzed olefin polymerization (Ziegler-Natta catalysis) [3]. 

At lower temperatures (or in solution) and at high monomer concentration, a second chain termination process that could occur is direct j -hydrogen transfer to a second molecule of monomer. This kind of chain transfer step is now generally accepted for many transition-metal-catalyzed polymerizations, where direct /1-elimination would be too much uphill to explain the observed molecular weights, for olefin oligomerization at aluminium, a similar situation applies. Since insertion and j -hydrogen transfer have an identical concentration dependence, their ratio does not depend much on the reaction conditions (except temperature) and hence limits the molecular weight attainable in the Aufbau reaction. 

Although alkynes are highly reactive toward a wide range of transition metals, very few instances of metal-catalyzed reactions of nucleophiles with alkynes are known. This is, in part, because most stable alkyne-metal complexes are inert to nucleophilic attack, while most unstable alkyne-metal complexes tend to oligomerize alkynes faster than anything else. Hence synthetic methodology involving this process is quite limited. 

As discussed in connection with olefin-coupling reactions and shown in Fig. 4, the coupling of vinyl Grignard reagents is stereospecific and dependent upon the transition metal catalyst used (32, 33). The dimerization of ethylene, shown in Fig. 6, was also shown to produce primarily the terminal olefin 1-butene (35). The size of the metal has also been shown to influence the course of the catalyzed oligomerization reactions of butadiene. When bis-(ir-allyl) metal complexes are used as... 

Catalyzed oligomerization and co-oligomerization of conjugated dienes have been performed with a wide range of transition-metal complexes. Catalytic cyclodimerizations of conjugated dienes have also been performed selectively [27]. Thus, a catalytic amount of CpRuCl(diene) and Ag(0S02CF3) led to the formation of 1,5-cyclooctadiene, dimethylcyclooctadienes, and 6-methyl-... 

If the metal-catalyzed reactions of PMDS or BMDS were to be strictly analogous to the oligomerization of tetramethyldisiloxane, we would expect the idealized reaction shown in Eqs. (131) or (132) to occur [cf. Eq. (106)]. 

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