Base-catalyzed hydrogen isotope exchange

Base-catalyzed hydrogen isotope exchange in inactivated selenophenes occurs only in highly basic media. The relative rates of exchange of deuterium at the a-position in furan, thiophene, and selenophene in 0.4 M t-BuOK/DMSO... 

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... 

Base-catalyzed hydrogen isotopic exchange in heteroaromatic compounds 74KGS1587. 

The base-catalyzed hydrogen exchange of pyrazoles has been studied by Vaughan and conveniently summarized in <74AHC(16)l) and (B-76MI40402). The method has been used to prepare the isotopic species necessary to assign the fundamental vibration of pyrazole (Sections 4.04.1.3.7(i) and 4.04.3.3.3). 

NiFe] and [NiFeSeJ-Hydrogenasas. The hydrogen-isotope exchange reactions show that the enzyme catalyzes the heterolytic cleavage of H2 into a hydride,, and a proton, H+. To hold a hydride we need a suitable electron-deficient metal center, M, and to hold a proton we need a base, B ... 

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 mechanisms in Figures 7.8 and 7.9 are supported further by two observations (i) chiral ketones undergo both acid- and base-catalyzed race-mization, and (ii) exchange of hydrogen isotopes on the a carbon atoms is also observed. The rate constants for acid-catalyzed iodination and racemization are identical for phenyl sec-butyl ketone. The same is true for acid-catalyzed bromination and racemization of (-H )-2-(2-carboxybenzyl)indanone as... 

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