One of the diastereotopic methylene protons at the double bond of A was selectively substituted by d — Physical Chemistry Chemistry Question
Stereochemistry of Bromination and Dehydrobromation
One of the diastereotopic methylene protons at the double bond of A was selectively substituted by deuterium. Bromination and subsequent dehydrobromation yields the deuteriated product B and the non-deuteriated product C.
Which configuration follows for the monodeuteriated A from the given reaction products?
The solution of this question requires the formulation of the reaction and a short argumentation why only B and C are formed.
Model Answer
The addition of bromine occurs trans (antarafacial). The elimination of HBr via an E2 mechanism also requires an anti-periplanar (= trans) arrangement of H and Br. The products given in this problem are only formed from a Z-configurated adduct.
The bromination of A and subsequent dehydrobromination yield both E,Z isomeric bromoolefins that have to be separated. Substitution of the bromine by deuterium in the Z-isomer proceeds by treatment with a metal (best: Na/t-BuOD) under retention to A.