Superacid deuterium exchange

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-... 

In solution chemistry too attempts have been made to protonate xenon to XeH+ in the superacid media.150 1038 Evidence for the protonation comes from suppression of proton-deuterium exchange rates of deuterium gas in the presence of xenon in strong acid medium.342... 

The nonexistence of alkane-alkene equilibrium in superacid medium has been elegantly demonstrated by the behavior of isobutane in deuterated superacid medium (DSOsF-SbFs or DF-SbFs). Isobutane at low temperature undergoes hydrogen-deuterium exchange only at the methine position through the involvement of a three-center bound pentacoordinate carbonium ion (Scheme 17). 

The isoelectronic boron analog BHC cation has a similar symmetry (31) According to the latest calculations by Olah and Rasul, protonation of BH3 to BIT is strongly exothermic by 154.4kcalmol T This indicates that hydrogen-deuterium exchange in diborane in superacids can also involve monomeric BH3 [Eq. (5.10)] ... 

Superacid DF-SbFs induces protium-deuterium exchange in isobutane [54b]. Strong acids (BF3, BFj-HjO, HF-BFj, CF3SO3H)catalyze carbonylation of alkanes including methane by carbon monoxide [54c], whereas sulfuric acid can induce carbonylation of iso- and cycloalkanes [54e,d]. In both cases, carboxylic acids are obtained. The elimination of molecular hydrogen from alkyl can occur (see a recent theoretical study of the Hz elimination from CzHs [54f]). 

G. A. Olah, Y. Halpem, J. Shen, Y. K. Mo, Electrophilic reactions at single bonds. XII. Hydrogen-deuterium exchange, protolysis (deuterolysis), and oligocondensation of alkanes with superacids, J. Am. Chem. Soc., 1973, 95, 4960-4969. 

Hydrogen (deuterium) exchange in the acid systems HF-SbFg, FSO3H-SbFs, and H2S04-SbF5 has been studied semiquantitatively. The suggested mechanism involves such species as H3+ and D3+, and the mixed cations H2D+ and HD2+, at least as transition states if not real intermediates, and provides another illustration of the similarity between gas-phase and superacid chemistry. 

Further evidence for the pentacoordinate carbonium ion mechanism of alkane protolysis was obtained in the H-D exchange reaction observed with isobutane. When isobutane is treated with deuterated superacids (DS03F SbF5 or DFiSbFs) at low temperature (—78°C) and atmospheric pressure, the initial hydrogen-deuterium exchange is observed only at the tertiary carbon. Ionization yields only deuterium-free t-butyl cation and Recovered isobutane from the reaction mixture shows at low... 

Based on the demonstration of H-D exchange of molecular hydrogen (and deuterium) in superacid solutions, Olah et al.26 suggested that these reactions go through trigonal isotopic H3+ ions (8, 9) in accordance with theoretical calculations and IR studies.27... 

Scheme 1. Superacid-promoted deuterium-hydrogen exchange on the 9-methyl-9-fluo-renyl cation (25)...

The superacidic properties of trichlorogermane are clearly manifested in the properties of its etherates (vide infra) and the ability of DGeCl3 to participate in a deuterium-hydrogen exchange reaction with methylbenzenes32 (Section VIII.E). 

Based on the demonstration of H-D exchange of molecular hydrogen and deuterium in superacid solutions,- it was suggested - that this reaction also goes through trigonal isotopic H3 ions. The structure of If, and its isoto-pomers (Fig. 6.4) is reinforced by theoretical calculations and infrared (IR) studies.- ... 

As mentioned earlier in Chapter 5 (see Section 5.4.I.6.), the H-D exchange in isobutane in superacid media is fundamentally different from the H-D exchange observed by Otvos et al. in weaker acids such as D2SO4, who found the eventual exchange of all the nine methyl hydrogens with deuterium but no exchange of the methine hydrogen was observed (Scheme 6.4). 

---