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.

مواضيع ذات صلة

Distinguishing acid derivatives by carbon NMR is difficult

E1cB eliminations in context

Anion-stabilizing groups allow another mechanism—E1cB

The regioselectivity of E2 eliminations

E2 eliminations from cyclohexanes

E2 eliminations can be stereospecific

E2 eliminations have anti-periplanar transition states

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size