Base-Catalyzed Oligomerization

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

The coammonolysis of CH3SiHCl2 and CH3(Un)SiCl2 in the indicated ratios was carried out by the procedure as described for the ammonolysis of CH3SiHCl2 [8]. A mixture of cyclic oligomeric silazanes is to be expected in these reactions, [(CH3(H)SiNH)x(CH3(Un)SiNH)y]n, with more than one ring size present. In each preparation, the soluble, liquid products were isolated and used in the base-catalyzed polymerizations. When a ratio CH3SiHCl2/CH3(Un)SiCl2 of x l (x>l) was used in the ammonolysis reaction, the CH3(H)Si/CH3(Un)Si ratio in the soluble ammonolysis product usually was somewhat less than x l. 

Steynberg, J.P. et al.. Oligomeric fiavanoids. Part 4. Base-catalyzed conversions of (—)-fisetinidol-(+)-catechin profisetinidins with 2,3-trflwj-3,4-cw-flavan-3-ol constituent units, J. Chem. Soc., Perkin Trans. 1, 1331, 1988. 

Lithium naphthalene and sodium naphthalene (11) in solvents such as THF, diglyme, or aliphatic amines were found to be effective agents for the hydrodimerization of isoprene. Although this reaction is not strictly catalytic, it contributes to a better understanding of the base-catalyzed oligomerization of unsaturated conjugated hydrocarbons. 

The mechanism of the reaction has been studied in some detail by Hogberg i2,7i,72) jn contrast to the base-catalyzed oligomerization, the acid catalyzed process involves electrophilic aromatic substitutions by cations, as outlined in Fig. 8. Although formaldehyde does not react with resorcinol to produce cyclic oligomers, other aldehydes such as acetaldehyde and benzaldehyde give excellent yields of... 

Formation of the tetrahedral intermediate carbinolamine and subsequent elimination of water are amenable to acid-base catalysis and do not require a metal surface. The relative rates of adduct formation and subsequent dehydration to imine or enamine depend on the structure of alcohol and amine, and on the nature and strength of acidic and basic sites on the catalyst surface. It must be stressed that several side-reactions (e. g. dimerization and oligomerization, dehydration) are also acid or base-catalyzed, and good selectivity for the desired product requires proper tuning of the redox and acid-base properties of the catalyst. This is crucial in catalyst development when choosing a suitable support, additive, or modifier. Even traces of impurities remaining on the surface from the catalyst precursor can strongly influence product distribution [10]. 

Di Cosimo, Jl Apesteguia. CR. Study of the catalyst deactivation in the base-catalyzed oligomerization of acetone. Journal of Molecular Catalysis A Chemicaf 1998 130, 177-185. 

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