Acid-catalyzed hydrogen isotope

Benzannelation results in an attenuation of aromaticity effects in 4n + 2rc-systems.255 The base-catalyzed hydrogen isotope exchange and isomerization reactions of isomeric dihydrobenzazocines 135 and 136 show these compounds have moderately enhanced kinetic acidity as compared with dihydroquinoline models and is attributable to a small degree of aromatic stabilization in the incipient 1()n electron benzazocinyl anions.255... 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

The nature and position of substituents relative to the ring oxygen atom have an important effect on the acid-catalyzed isomerization of oxepins. It has been observed that, in addition to the hydrogen isotopes ( H, 2H, 3H), chloro, bromo, methyl (72E1129) and alkoxycarbonyl (79JA2470) substituents also show the migration-retention sequence found in the NIH shift. 

The results of deuterium labeling experiments shown in Scheme 37 clearly show the operation of a monohydride mechanism in the BINAP-RuOI) catalyzed hydrogenation of unsaturated carboxylic acids. However, with many olefinic substrates with a neutral, rather than anionic, secondary binding site, the products exhibit a similar degree of isotope incorporation at the two hydrogenated centers (Scheme 39). The out-... 

The specific acid-catalyzed reaction behaves in many respects like the water reaction. Thus, unusually large negative entropies of activation are found for all but those substrates which react by the acylium ion path (Schaleger and Long, 1963). At least two water molecules are involved in the transition state, the presence of at least five exchangeable hydrogens in the transition state, is deduced from solvent isotope effects in mixed water-heavy water (Salomaa et al., 1964). From the dependence of both acid-catalyzed exchange and hydrolysis in concentrated sulfuric acid on the activity of water. Lane (1964) concluded that two water molecules must be involved in the transition state. Except for the extra proton, the transition states for the acid-catalyzed and water reactions are very probably similar. 

Fig. 16. Mechanism and hydrogen isotope exchange in the Ru-BINAP-catalyzed hydrogenation of unsaturated acids and esters, reflecting the author s views...

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