Influence of the nature of activated hydrogen isotope species on the isotope exchange efficiency, with preparation of labeled sodium 4-phenylbenzoate as example

Abstract

The influence of the nature of activated hydrogen isotope species on the isotope exchange efficiency was studied with preparation of labeled sodium 4-phenylbenzoate as example. The effect of various factors on the deuterium labeling of this compound was examined. At temperatures lower than 200°C, deuterium is mainly incorporated into the phenyl moiety, i.e., under these conditions activated hydrogen species are incorporated by the electrophilic mechanism. In the range from 260 to 300°C, the mean number of deuterium atoms incorporated into 4-phenylbenzoic acid molecules becomes approximately constant (about 8.22 deuterium atoms per molecule). At these temperatures, deuterium is efficiently incorporated both into the phenyl fragment and into the benzoic acid residue, which suggests the prevalence of the radical substitution mechanism under these conditions. At temperatures at which the isotope substitution in sodium 4-phenylbenzoate occurs by the electrophilic mechanism, 4-cyclohexylbenzoic acid is formed concurrently, i.e., the maximal yield of 4-cyclohexylbenzoic acid can be reached at temperatures that are most favorable for the isotope exchange by the electrophilic mechanism. At 200°C, the content of 4-cyclohexylbenzoic acid in the reaction mixture reaches a maximum. A sharp increase in the contribution of the radical mechanism of the process at higher temperatures led to a decrease in the yield of 4-cyclohexylbenzoic acid. It was assumed that clusters of activated hydrogen species and electrons, solvated on the support surface, undergo rearrangement with increasing temperature. Whereas the major role in labeling by the electrophilic mechanism is played by hydrogen isotope cations, at higher temperatures hydrogen isotope cations interact with electrons to form hydrogen atoms, which become active participants of the exchange process.