Base-Catalyzed Exchange Reactions

A completely symmetrical system in which the mechanism can be fully described is the ethanolysis of ethyl trifluoroacetate (Johnson, 1964). This exchange reaction is catalyzed by general bases. Ofthemany possible general base-catalyzed schemes listed in Table 1, only two are possible which include a tetrahedral intermediate since the reactants are equal to the products. These two paths are the general base path which involves the anionic tetrahedral intermediate (path A) and the specific... 

The reaction takes place extremely rapidly and if D2O is present in excess all the alcohol is con verted to ROD This hydrogen-deuterium exchange can be catalyzed by either acids or bases If D30 is the catalyst in acid solution and DO the catalyst in base wnte reasonable reaction mech anisms for the conversion of ROH to ROD under conditions of (a) acid catalysis and (b) base catalysis... 

Solvent for Base-Catalyzed Reactions. The abihty of hydroxide or alkoxide ions to remove protons is enhanced by DMSO instead of water or alcohols (91). The equiUbrium change is also accompanied by a rate increase of 10 or more (92). Thus, reactions in which proton removal is rate-determining are favorably accompHshed in DMSO. These include olefin isomerizations, elimination reactions to produce olefins, racemizations, and H—D exchange reactions. 

Base-catalyzed hydrogen exchange has been summarized for five-membered rings (74AHC(l6)l). In many reactions of this type the protonated azole is attacked by hydroxide... 

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. 

One commonly used procedure for studying the steric stability of the carbanion is comparison of the rate of base-catalyzed H/D exchange reaction (kel) on the chiral carbon with the rate of racemization (fcrac). By this comparison, inversion, racemization or... 

TABLE 16. Base-catalyzed H/D exchange reactions of sulfonyl compounds 70-73... 

It is obvious that the primary amines formed in this reaction will be uncontaminated by secondary or tertiary amines (unlike 10-44). The reaction is usually rather slow but can be conveniently speeded by the use of a dipolar aprotic solvent such as DMF or with a crown ether. Hydrolysis of the phthalimide, whether acid or base catalyzed (acid catalysis is used far more frequently), is also usually very slow, and better procedures are generally used. A common one is the Ing-Manske procedure,in which the phthalimide is heated with hydrazine in an exchange... 

Similar behavior was found for carbanions generated by base-catalyzed hydrogen exchange (Reaction 12-1) ... 

Some representative data on base-catalyzed H/D exchange reactions of several optically active sulfonyl compounds 70-73 are listed in Table 16. In all these reactions, 10. 

N-Nitroso compounds have been found to exist as syn and anti rotamers [30, 31] due to restricted rotation of the N-N bond resulting from nitrogen lone-pair delocalization (Fig. 3.2). This delocalization causes the hydrogens at the a-carbons to become acidic as evident by their base-catalyzed reactions, such as exchange with deuterium... 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

Fig. 9.2. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrites. Pathway a Base-catalyzed hydrolysis with liberation of nitrite. Pathway b Reversible nitro-syl exchange between organic nitrites and alcohols. Pathway c General acid catalysis with concerted mechanism in the acid hydrolysis of organic nitrites.

Another example of a 1,3-exoannular rearrangement, involving a 1,3-carbon exchange, is the base-catalyzed conversion of 2-amino-3-formylthiophenes into 3-cyanothiophenes [77JCR(S)294]. The reaction in-... 

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