Vinylic Organoboranes. 7. Stereoselective Synthesis of (E)-(1-Substituted-1-alkenyl)boronic Esters by the Nucleophilic Substitution of (Z)-(1-Bromo-1-alkenyl)boronic Esters with Organolithium or Grignard Reagents. Isolation and Oxidation to Ketones.

Article abstract of DOI:10.1021/jo00376a042

The title compounds, (E)-R1CH=CR2B(OR)2 (4a-g), are prepared in a highly regio- and stereoselective manner by the reaction of (Z)-R1CH=CBrB(OR)2 (3a-g) with R2Li or R2MgX.The stereochemistry was established in two cases by isolating the E isomers 4 (R1, R2 = n-Bu; R1 = i-Pr, R2 = Me) in pure form and comparing the products with the corresponding Z isomers prepared by hydroboration of appropriate internal alkynes.Oxidation of 4 provides the corresponding ketones, R1CH2COR2, in high yields, confirming the carbon structures of the intermediate.As established previously, protonolysis of these (E)-boronic esters provides a synthesis of the pure (E)-alkenes.