Synthese bei Temperaturen unter −80°C

Abstract

Small differences in free enthalpies of activation (ΔΔG^‡) between competing reactions give rise to high selectivity at low enough temperatures. Special techniques are necessary, in order to exploit this effect in preparative-scale conversions. An experimental set-up is described here with which reactions can conveniently be carried out in up to five liter flasks at temperatures down to -150°C. In one apparatus, the cooling bath is liquid, in another one the reaction vessel is cooled directly by evaporating liquid nitrogen. Applications described are brominations of C,C double bonds and reactions of lithium enolates with electrophiles.