Base-catalyzed hydrogen exchange rates

Many thiophenes have been lithiated en route to other derivatives. The lithiation reaction [Eqs (6.1) and (6.2)] is related to base-catalyzed hydrogen exchange, but differs from it in the relative rates of steps (6.1) and (6.2), the latter being obviously faster and non-rate-determining for lithiation. 

Rate Data for Base-Catalyzed Hydrogen Exchange of Pyridines under Standard Conditions"... 

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... 

Two research groups have investigated the rates of the base-catalyzed hydrogen atom exchange reactions of aliphatic fluorocarbons (3,4,28,29). The results for a selected series of compounds are presented in Table 5. 

Amide hydrogen exchange rates are also dependent on temperature, with approximately three times increase in both acid- and base-catalyzed exchange rates for every increment of 10°C [9] and is described in more detail in Section 1.2.2 (see also Figure 1.6). To minimize variations in hydrogen exchange between multiple experiments, temperature should be precisely controlled ( 2°C)... 

A final, rather different example which fits in appropriately here, in that it involves hydrogen exchange, is the measurement of equilibrium and rate constants for the base-catalyzed isomerization of unsaturated sulfides, sulfoxides and sulfones193 ... 

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. 

Stadherr and Angelici459 studied the effects of various metal ions on the base-catalyzed racemization and deuterium exchange at the methine hydrogen of N,N-bis(carboxymethyl)-D-phenylgly-cine (144). The metal complexes underwent racemization at rates up to 5 x 103 times greater than for the free ligand. 

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