Tosyl bromide Page 18
Methyl hex-5-ynoate
AIBN (recrystallised from methanol from crude)
toluene or benzene

Add methylhex-5-ynoate (1eq, 6.96mmol) to benzene or toluene (ca. 100mL) and stir. Add AIBN (0.25eq), tosyl bromide (1.2eq) and warm to external oil bath temperature of 65-70 ^oC under nitrogen atmosphere. Monitor by tlc after a few hours or after overnight. If little reaction has occurred add more AIBN (0.25eq) and continue monitoring in this way. Typical conditions require around 0.25-0.5eq AIBN reaction time 16h to 80h. When adjudged complete, remove solvent, column by flash column chromatography. Yield: 63 %, colourless oil.

This is a reliable way to make these functionalised trans-bromo-sulfonyl alkenes in 60-80 % yield. Tolerant of amide and ester functionalities, others not tried. The variation in reaction times is not easy to explain, but once the reaction starts to go it is normally over within 16h or so. The products column very well and are stable (room temp/dessicator or fridge) for several months. The alkenyl proton is a distinctive marker, occurring here at 6.7ppm. Seems to work well on small (50mg) or large (1.5g) scale. Trans geometry of double bond confirmed by nOe studies as shown below.

For E-5-bromo-6-toluenesulphonyl-methylhex-5-enoate: 1H NMR (CDCl₃): 7.71 (2H, d, J=8.3Hz, H2 & H6) 7.30 (2H, d, J=8.1Hz), 6.71 (1H, s, TsCH=CBr-), 3.63 (3H, s, CH₃-OCOCH₂-), 3.04 (2H, t, J=7.4Hz, CH₃-OCOCH₂-), 2.38 (3H, s CH₃-Ar), 2.32 (2H, t, J=7.5Hz, TsCH=CBr-CH₂-), 1.84-1.94 (2H, app. quintet, -CH₂CH₂COCH₃-). ¹H-¹H nOe (CDCl₃): Irradiation of alkene proton at 6.71ppm showed 0.8% enhancement in intensity of aryl protons H2 & H6 at 7.71ppm.

Caddick, S.; Hamza, D.; Wadman, S.N. Tetrahedron Lett. 1999, 40, 7285-7288