Hydrogen-deuterium exchange, base

Two types of hydrogen replacement are discussed here (1) the base-induced hydrogen-deuterium exchange reactions and (2) the hydrogen-metal exchange reactions. 

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... 

In pyridazine, base-catalyzed hydrogen-deuterium exchange takes place at positions 4 and 5 more easily than at positions 3 or 6. Deuteration of pyridazine 1-oxide in NaOD/DiO... 

Pyridine, 3-(dimethylamino)-amination, 2, 236 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342 Pyridine, 4-(dimethylamino)-in acylation, 2, 180 alkyl derivatives pK, 2, 171 amination, 2, 234 Arrhenius parameters, 2, 172 as base catalysts, 1, 475 hydrogen-deuterium exchange, 2, 286 ionization constants, 2, 172 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342... 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2-phenylbutane shows a similar trend. When potassium t-butoxide is used as the base, the exchange occurs with retention of configuration in r-butanol, but racemization occurs in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule coordinated to the metal ion acts as the proton donor... 

Several authors have reported that they did not succeed in isolating a number of pseudoazulenes when solutions in dilute mineral or glacial acetic acids were diluted with water or other bases.51-71,77-86 It was only possible to recover polymer-like material. In the subsequent decomposition the nucleophile attacks the conjugated acid of the pseudoazulenes at a position adjacent to the heteroatom, probably resulting in a fission. The hydrogen-deuterium exchange catalyzed by acids has been reported for systems 29,86 31,86 and 49.135... 

If a carbanion is thermodynamically accessible, but is subject to rapid quenching by internal return of C02 in the case of decarboxylation, or by a proton in carboxylation, or in a hydrogen/deuterium exchange reaction, then the carbanionic intermediate off the enzyme would give the appearance of greater basicity than its thermodynamic value would predict. The localized character of the carbanion at the 6-position of UMP requires that the proton that is removed by a base in solution initially remains closely associated, and therefore, to a great extent be transferred to the carbanion. This reduces the rate of exchange and creates a discrepancy between kinetic and thermodynamic acidities. 

This section is not complete without mentioning the hydrogen-deuterium exchange experiment by Doehring and Hoffmann (12), which indicated an enhanced acidity of the hydrogens in a tetramethylstibonium salt. In the corresponding equilibrium, stibonium ylides play the role of strong bases ... 

The a-carbon-hydrogen bonds of vinyl(phenyl)iodonium ions are relatively acidic, two examples of base catalyzed hydrogen-deuterium exchange in -oxavinyliodonium salts having been reported (equations 227 and 228)79,95. Although vinylidene-iodonium ylides... 

Rate Data for Base-Catalyzed Hydrogen-Deuterium Exchange... 

Acid-catalysed hydrogen-deuterium exchange in norcamphor has also been investigated by Werstiuk and Banerjee (1977) (DOAc—D20—DC1 medium). It was observed that exo-deuteron addition to the enol is also preferred, but with a slightly smaller selectivity (x 190). This would mean that, if torsional factors cause preferential base-catalysed exo-exchange, they also occur for acid-catalysed keto-enol tautomerism. However, the absence of important torsional strain effects on the rate constants of acid-catalysed enolisation of cyclic and bicyclic ketones contradicts this assumption. 

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