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). (C) 2000 Elsevier Science S.A. All rights reserved.