Alkenylcopper Reagents Prepared from Stereodefined Alkenylboronate Esters. Reaction with Allylic Halides as a Convenient Route to Stereodefined 1,4-Dienes

The stereo-defined synthesis of 1,4-dienes based on in situ generated stereo-defined alkenylcopper reagents is presented. The hydroboration of alkynes with dibromoborane-methyl sulfide complex followed by treatment with trimethylene glycol provides stable (E)-1-alkenylboronate esters. These boronate esters readily undergo “ate” complexes with a hindered base such as potassium-tert.-butoxide. The transmetalation of the alkenyl group from boron to copper via the “ate” complexes retains the original stereochemistry defined from the starting alkenylboronate esters. The effect of representative bases on stereodefined alkenylboronate esters and subsequent reaction of these boronate esters in the transmetalation reaction with copper(I) bromidemethyl sulfide is investigated. The resulting stereo-defined alkenyl copper species generated in situ readily couple with allylic bromide to give the corresponding 1,4-dienes with retention of stereochemistry. Since (Z)-1-alkenylboronate esters are easily accessible, both cisand trans-isomeric 1,4-dienes are synthesized.