E2 elimination from vinyl halides: how to make alkynes
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص398
2025-05-29
842
An anti-periplanar arrangement of C–Br and C–H is attainable with a vinylic bromide too, provided the Br and H are trans to one another. E2 elimination from the Z isomer of a vinyl bromide gives an alkyne rather faster than elimination from the E isomer because in the E isomer the C–H and C–Br bonds are syn-periplanar.
Vinyl bromides can themselves be made by elimination reactions of 1,2-dibromoalkanes. Watch what happens when 1,2-dibromopropane is treated with three equivalents of R2NLi: fi rst, elimination to the vinyl halide, then elimination of the vinyl halide to the alkyne. The terminal alkyne is amply acidic enough to be deprotonated by R2NLi, and this is the role of the third equivalent. Overall, the reaction makes a lithiated alkyne (ready for fur ther reactions) from a fully saturated starting material. This may well be the first reaction you have met that makes an alkyne from a starting material that doesn’t already contain a triple bond.
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