Isomerization promoters

Figure 6. As expected, decreasing the substrate to catalyst molar ratio, the reaction rate increases, even though there is no difference when this molar ratio is between 600 and 300. Also, hydrogenation is preferred over isomerization reactions when this molar ratio diminishes. At 120 this selectivity is over 70% and slightly enhances with time suggesting the isomers are also hydrogenated. At 1300 the selectivity to n-octane is low and declines with time. This may mean that the hydrogenation-promoter species are inhibited or transformed to isomerization-promoter complexes during the catalytic cycle.

Of these three absorption bands, the one at 870 cm has been taken as characteristic of a cis-structure it is seen from Fig. 4 that this band is not present conspicuously in the IR spectrum of the polymer prepared in this study. Based on the observations of Kern ° who has prepared and compared poly(phenylacetylene)s from several types of chemical initiators, it can be surmised that the electioinitiated polymer is trans-rich because of cis-tram isomerization promoted by the polar solvent medium, DMF. 

Yamashita F, Kuniyasu H, Teiao J, Kambe N (2006) cis-to-trans Isomerization promoted by pyridine as a cmcial step for the selective preparation of trans-Pt(SAr)(a)(PAr 3)2. Inorg Chem 45 1399-1404... 

Fig. 10.4 Representative ene-type isomerizations promoted by TRAP—palladium complexes [19]...

BASE-PROMOTED INTERCONVERSIONS BETWEEN ACETYLENES AND CUMULENES 1. ISOMERIZATIONS WITH CATALYTIC AMOUNTS OF BASE... 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

Dual Function Catalytic Processes. Dual-function catalytic processes use an acidic oxide support, such as alumina, loaded with a metal such as Pt to isomerize the xylenes as weH as convert EB to xylenes. These catalysts promote carbonium ion-type reactions as weH as hydrogenation—dehydrogenation. In the mechanism for the conversion of EB to xylenes shown, EB is converted to xylenes... 

The discovery (116) of the base-promoted degradation of the isomeric closo-Q,. . cages provided one of the most important carborane anion systems, the isomeric nido-Q,. ][ , anions,... 

Opening by trimethylsilyl trifluoromethanesulfonate yields an adduct (54) from which trifluoromethanesulfonic acid can be eliminated to give an allylic alcohol (Scheme 47) (79JA2738) [cf. base-promoted isomerization to allylic alcohols (Section 5.05.3.2.2)]. 

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. 

Isomerization is promoted by either acids or bases. Higher alkylbenzenes are isomerized in the presence of AlCb/HCl or BF3/HF olefins with most mineral acids, acid salts and silica-alumina saturated hydrocarbons with AlCb or AlBr,3 promoted by 0.1 percent of olefins. 

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