Solvent-exchange with hydrogen isotopes

Reaction or exchange with stable isotopic tracers and quantitative identification of all products by mass spectrometry provides indications for molecular interactions on the surface. Reactions can be studied at steady state or by following the transient distribution of isotopic products. Langer and co-workers (25,26) presented the first steady-state mechanistic analysis for the electrocatalytic hydrogenation of ethylene on Pt in deuterated electrolytes. Proton abstraction in electroorganic synthesis has also been verified using deuterated solvents (374, 375). On-line mass spectrometry permitted indirect identification of adsorbed radicals in benzene and propylene fuel cell reactions (755,795,194). Isotopic radiotracers provided some notion on adsorption isotherms (376, 377) and surface species on electrocatalysts (208, 378, 379). 

We chose the microwave-enhanced Raney Nickel catalyzed hydrogen isotope exchange of indole and N-methylindole as our substrates and D20, CD3COCD3, CD3OD and CDC13 as the solvents. The thermal reaction had already been the subject of a recent study [44], The microwave-enhanced method was some 500-fold faster than the corresponding thermal reaction (at 40 °C). Furthermore the pattern of labeling (Scheme 13.3) varied with the choice of solvent. Thus in the case of indole it-... 

Liquid hydrogen halides do not suffer from these disadvantages and have therefore been employed by us in preference to sulphuric acid in systematic studies of hydrogen isotope exchange reactions. Parallel investigations with solvents differing in their acid-base properties facilitate the elucidation of the effect of the medium on the reactivity... 

Isotope exchange and isotope effects. The vinylic hydrogen is lost and reintroduced from the solvent during the elimination-addition process. Incorporation of deuterium from deuteriated solvents is therefore expected. Depending on the elimination mechanism, hydrogen isotope effects would be also sometimes observed when RCX=CDY is compared with RCX=CHY. 

For these exchange reactions, the catalyst (previously prereduced with hydrogen or borohydride, if required) is sealed in a preconstricted ampoule with the organic compound and isotopic water under vacuum (10 2 Torr). The tubes are shaken or stood at the required temperature, and after the reaction is completed the products are analyzed by infrared, vapor phase chromatography, NMR or mass spectrometry (if deuteration), or counted (tritiation). With some compounds, the material to be labeled may be refluxed with isotopic water in the presence of catalyst instead of using sealed tube procedures.76 Solvents such as acetic acid and ethanol have also been used 77-80 however, unless the solvent is fully labeled prior to exchange, the theoretical percentage of isotope incorporation at equilibrium under these conditions can be lowered by loss of isotope to the solvent. 

CH bonds in alpha position to carbonyl, sulfonyl, or nitro groups may undergo slow hydrogen isotope exchange with a hydroxylic solvent in the presence of a strong base. In another method of measurement of the deprotonation rate, a reagent (bromine or iodine) is added which reacts... 

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