CHAPTER 10: Question 4

 

Simple Answers

The role of the metal ion is to form a chelated complex involving the two carbonyl bonds as shown below. This prevents rotation around the N-CO bond, and thus ensures that the phenyl ring shields the top face of the a,b-unsaturated carbonyl system. The Michael addition of methyl cuprate then occurs the least hindered face of the alkene whilst it is held in an s-cis conformation by coordination of the copper to the carbonyl bond. The Michael addition generates an enolate which is subsequently brominated on its least hindered face to generate the observed stereoisomer of the product. The full reaction sequence is shown below.

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