E2 elimination from vinyl halides how to make alkynes

E2 elimination from vinyl halides how to make alkynes 

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. 

The base used here is LDA (lithium diisopropy lamide) made by deprotonating ePr2NH with BuLi. LDA is very basic (pKa about 35) but too hindered to be nucleophilic—ideal for promoting E2 elimination, 

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 K NLi first, 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 R2NU, and this is the role of the third equivalent. Overall, the reaction makes a lithiated alkyne (ready for further reactions) from a fully saturated starting material. This may well be the first reaction you have met that makes an alkyne from a starling material that doesn t already contain a triple bond, making an alkyne from 1,2-dibromopropane 

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