Synthetic methods: Alkane H/D exchange

28 June 2008 - C-H bond activation

H/D exchange in organic chemistry is a simple tool in structure elucidation unmasking any proton (for example that of a alcohol or an amine) as an acidic proton simply by exposing it to heavy water (D2O). Extending this scope to simple alkanes is not exactly obvious but can be done with a little bit of help of organometallic chemistry.

Deuterated alkanes can be used as molecular markers in a whole range of applications. Shilov (see Shilov system) in 1969 using platinum observed deuteration in methane (no need bothering with DOI's for Soviet-Union era chemistry) opening up a new research field

In a 2008 contribution Sajiki et al. (DOI) use rhodium on carbon (think palladium on carbon), heavy water and (counter intuitively) hydrogen gas at 160°C for C-H bond activation of such compounds as dodecane (90% efficient) and cholestane (36%). Notable observations: reaction will not work without hydrogen, terminal positions are favored, the actual commercial source of Rh/C is key (Aldrich rules, Merck sucks) and organic solvents except for cyclohexane actually decrease the efficiency of this reaction. Plausible reaction mechanism: oxidative addition (activated by hydrogen) to R-H forming a R-Rh-H species, then H/D exchange to R-Rh-D followed by reductive elimination to R-D.