Acid-catalyzed hydrogen isotope exchange

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... 

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

Hydrogen isotope exchange at the nitrogen atom of A, A -dimethylanilinium ions is catalyzed by acid " and nitrosonium ion. The latter reaction has been studied at acidities for which the subsequent chemical reaction between NO and amines is much slower. The mechanism postulates the formation of a loose complex between NO" and the anilinium ion, followed by proton loss from the nitrogen. This process is related to the direct diazotization of some anilinium ions, and to the nitrous acid-catalyzed nitration and oxidation of the A, A -di-methylanilinium ion. 

The electrophilic aromatic substitution reaction may be advantageously modeled by isotopic exchange reactions. Shatenshtein and his co-workers77-79 studied hydrogen exchange catalyzed by acids and bases in nonaqueous solution, and their studies throw considerable light on both electrophilic substitution and protophilic (base-induced) replacement of hydrogen in the system (see Section V, A). 

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. 

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