Yield:-

Reaction Conditions:

Stage #1: 2-Methylbutanoic acidwith phosphorus tribromide at 20;Inert atmosphere;Sealed tube;Hell-Volhardt-Zelinski Bromination;
Stage #2: with bromine at 110; for 6 h;Inert atmosphere;Sealed tube;Hell-Volhardt-Zelinski Bromination;

Steps:

4.1. General procedure for Scheme 2

General procedure: 1 (5.0 mmol), and PBr3 (0.16 mL, 1.65 mmol) were sequentiallyadded under air to a microwave vial equipped with a stir bar, rubbercap, and aluminum cover cap under nitrogen atmosphere (purity99.95%). The resulting mixture vigorously stirred under nitrogenatmosphere overnight at room temperature. After this time, Br2(0.28 mL, 5.5 mmol) was added to the reaction mixture and thetemperature was raised up 110C. When the temperature wasreached at 110C, inside pressure was released vi a needle shown inScheme 1. After releasing the pressure, the resulting mixturevigorously stirred under nitrogen atmosphere for 3 h at 110C.Next, additional Br2 (0.14 mL, 2.8 mmol) was added to the mixtureto complete the reaction. After stirring for 3 h at 110C, the mixturewas cooled to room temperature and cyclohexene was added to themixture (1.0 mL, 20.0 mmol). The resulting crude a-bromo acidbromide 3 was used for the next step.

References:

Murata, Yumi;Takeuchi, Kentaro;Nishikata, Takashi [Tetrahedron,2019,vol. 75,# 18,p. 2726 - 2736]