Organic synthesis using haloboration reaction. Part 9. A direct and selective synthesis of (Z,Z)-1-bromo-1,3-dienes and (E,Z)-1,3-dienes by the hydroboration-bromoboration sequence of two alkynes
1-Alkenyldibromoboranes readily obtainable by the hydroboration of first alkynes with dibromoborane-dimethylsulfide followed by treatment with tribromoborane, react without any difficulty with second 1-alkynes as bromoborating agents to give (1-a1keny1)(2-bromo-1-alkeny)- bromoboranes (5). The reaction of 5 with iodine in the presence of potassium acetate provides (Z,Z)-1-bromo-1,3-dienes (6), specifically