Radical allylic halogenation

4 real reactions from the literature, on α,β-unsaturated esters, α,β-unsaturated acids and trans-1,2-disubstituted alkenes. Each scheme shows starting materials, conditions and products, with the yield and reference where the source reports them.

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1. Allylic halogenation: crotonic acid (an α,β-unsaturated acid) with NBS, (PhCO2)2 gives 4-bromocrotonic acid (the allylic bromide). 93% yield.

   Allylic halogenationBioorg. Med. Chem. 2014, 22, 2366

2. Allylic halogenation: (E)-hept-2-ene (a trans-1,2-disubstituted alkene) with NBS; (PhCO)2O, CCl4, Δ gives (2E)-4-bromo-2-heptene (the allylic bromide).

   Allylic halogenationOrg. Synth. Coll. Vol. 4, p. 108

3. Allylic halogenation: an α,β-unsaturated ester with AIBN gives the allylic bromide. 85% yield.

   Allylic halogenationOrg. Lett. 2013, 15, 3054

4. Allylic halogenation: 5,6-dihydro-2H-pyran-2-one (an α,β-unsaturated ester) with excess NBS, (PhCO2)2 gives the allylic bromide. yield not provided yield.

   Allylic halogenationCurr. Med. Chem. 2013, 20, 3797