Addition of bromine to cinnamic ester

Phenylpropynoic acid (13) can be prepared from cinnamic acid esters by the addition of bromine to the double bond followed by dehy-drobromination with alcoholic potassium hydroxide (Scheme 5.14). 

Table III shows the results of hydro-bromination and hydrochlorination of the ethyl trans-cinnamate inclusion complexes. No hydrochlorination was observed at various temperatures after long exposure of hydrogen chloride gas similar to the reaction of trans-cinnamic acid [14]. Addition of hydrogen bromide to the ester gave an optically active and regioselective product, ethyl R-(+)-3-bromo-3-phenyl-propanoate in 46 % e.e. from the a-cyclodextrin, or ethyl S-(-)-3-bromo-3-phenylpropanoate in 31 % e.e. from the 3-cyclodextrin inclusion complex.

Hydrobromination of the same substrate in the a-cyclodextrin complex gave the monobromide with the opposite configuration at 46 % e.e.. This clearly shows that ethyl trans-cinnamate forms complexes with a- and 3-cyclodextrins such that the anti-addition of hydrogen bromide occurs with high but different enantioselectivities in the two cases to yield monobromide derivatives of opposite chiralities. A detailed mechanism, however, could not be described at the present time for the observed asymmetric induction in the hydrobromination and bromination of the a-cyclodextrin complex, because no crystalline or molecular structures were determined for the ester included in a-cyclodextrin. 

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