Deuterium exchangeability

Protonated methanes and their homologues and derivatives are experimentally indicated in superacidic chemistry by hydrogen-deuterium exchange experiments, as well as by core electron (ESCA) spectroscopy of their frozen matrixes. Some of their derivatives could even be isolated as crystalline compounds. In recent years, Schmidbaur has pre-... 

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... 

Each act of proton abstraction from the a carbon converts a chiral molecule to an achi ral enol or enolate ion The sp hybridized carbon that is the chirality center m the start mg ketone becomes sp hybridized m the enol or enolate Careful kinetic studies have established that the rate of loss of optical activity of sec butyl phenyl ketone is equal to Its rate of hydrogen-deuterium exchange its rate of brommation and its rate of lodma tion In each case the rate determining step is conversion of the starting ketone to the enol or enolate anion... 

Proton Abstraction. Although the exopolyhedral hydrogens of nido and arachno boranes are generally considered hydridic, the bridge hydrogens are acidic as first demonstrated by titration of and deuterium exchange (71). Some typical reactions are... 

Electrophilic Attack. A variety of boranes, heteroboranes, and metaHaboranes undergo electrophilic substitution. SusceptibiUty of boranes to electrophilic attack is often detected by deuteron—proton exchange experiments. Eor example, electrophilic hydrogen—deuterium exchange of occurs at the l-,2-,3-, and 4-positions when exposed to DCl in the presence of AlCl (81). The trend to increasing positive sites in is... 

The deuterium exchange reactions in the H2S/H2O process (the GS process) occur in the tiquid phase without the necessity for a catalyst. The dual-temperature feature of the process is illustrated in Figure la. Dual-temperature operation avoids the necessity for an expensive chemical reflux operation that is essential in a single-temperature process (11,163) (Fig. lb). 

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... 

In pyridazin-3(2//)-one only the hydrogen at position 6 is replaced at 180 °C, while in 3-hydroxypyridazine 1-oxide the deuterium exchange takes place first at position 6 and then at position 4 at 140 °C. 5-Hydroxy- and 5-methoxy-pyridazine 1-oxide exchange only the hydrogen at position 6 at 150 °C. 

The NMR spectra of pyrido[2,3-d]pyridazine Af-oxides reduced derivatives and quaternary salts have also been studied and alkaline deuterium exchange reactions investigated <77BSF919). 

Substituted tetrazoles readily exchange the 5-hydrogen for deuterium in aqueous solution. A major rate-enhancing effect is observed with copper(II) or zinc ions due to complexation with the heterocycle. The rate of base-induced proton-deuterium exchange of 1-methyltetrazole is 10 times faster than 2-methyltetrazole (77AHC(2l)323). 

The hydrogen-deuterium exchange rates for 1,2-dimethylpyrazolium cation (protons 3 and 5 exchange faster than proton 4 Section 4.04.2.1.7(iii)) have been examined theoretically within the framework of the CNDO/2 approximation (73T3469). The final conclusion is that the relative reactivities of isomeric positions in the pyrazolium series are determined essentially by inductive and hybridization effects. 

In theory two carbanions, (189) and (190), can be formed by deprotonation of 3,5-dimethylisoxazole with a strong base. On the basis of MINDO/2 calculations for these two carbanions, the heat of formation of (189) is calculated to be about 33 kJ moF smaller than that of (190), and the carbanion (189) is thermodynamically more stable than the carbanion (190). The calculation is supported by the deuterium exchange reaction of 3,5-dimethylisoxazole with sodium methoxide in deuterated methanol. The rate of deuterium exchange of the 5-methyl protons is about 280 times faster than that of the 3-methyl protons (AAF = 13.0 kJ moF at room temperature) and its activation energy is about 121 kJ moF These results indicate that the methyl groups of 3,5-dimethylisoxazole are much less reactive than the methyl group of 2-methylpyridine and 2-methylquinoline, whose activation energies under the same reaction conditions were reported to be 105 and 88 kJ moF respectively (79H(12)1343). 

Comparison of UV data for 3-aminoisothiazoles with those of reference compounds confirms that they exist in the 3-amino form. A more recent investigation of 4-aminoisothiazole (76MI41701) using deuterium exchange experiments of the type described in Section 4.01.5.2, and analysis of the symmetric and antisymmetric NH2 stretching frequencies in its IR spectrum, show that this compound also exists in the 4-amino form. 

Azaindolizine, 5-chloro-nucleophilic substitution, 4, 458 8-Azaindolizine, 7-chloro-nucleophilic substitution, 4, 458 Azaindolizines basicity, 4, 454 electronic spectra, 4, 445 electrophilic substitution, 4, 453 halogenation, 4, 457 hydrogen/deuterium exchange, 4, 458 NMR, 4, 447, 449 nucleophilic attack, 4, 458 protonation, 4, 453 reaction with isothiocyanates, 4, 513 reactions, 5, 267 reviews, 4, 444 UV spectra, 4, 446, 449 Azaindolizines, amino-tautomerism, 4, 452... 

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