Regioselectivity of bromofluorination of functionalized 1-alkenes

The regiochemistry of bromofluorinations of 1-alkenes with the combination of N-bromosuccinimide and amine/HF reagents mainly depends on the character of functional groups in the neighborhood of the double bond and only weakly from the fluorinating agent. While mono-substituted terminal alkenes mainly yield Markovnikov-oriented products, electron withdrawing groups in allylic or homoallylic position to the double bond destabilize carbenium centers in 2-position. Consequently, the part of anti-Markovnikov-oriented bromofluorides increases. Besides, with dropping reactivity of alkenes, the formation of dibromides becomes a competitive side reaction of bromofluorination. The extent of this process depends on the nature of the amine/HF reagent. This reaction is most important in case of the more nucleophilic trimethylamine bishydrofluoride, while this process is neglectible with pyridinium poly(hydrogen fluoride) (Olah’s reagent).