Regio- and Stereoselective Synthesis of Multi-Alkylated Allylic Boronates through Three-Component Coupling Reactions between Allenes, Alkyl Halides, and a Diboron Reagent

Multisubstituted
allylic boronates are attractive and valuable
precursors for the rapid and stereoselective construction of densely
substituted carbon skeletons. Herein, we report the first synthetic
approach for differentially 2,3,3-trialkyl-substituted allylic boronates
that contain a stereodefined tetrasubstituted alkene structure. Copper­(I)-catalyzed
regio- and stereoselective three-component coupling reactions between gem-dialkylallenes, alkyl halides, and a diboron reagent
afforded sterically congested allylic boronates. The allylboration
of aldehydes diastereoselectively furnished the corresponding homoallylic
alcohols that bear a quaternary carbon. A computational study revealed
that the selectivity-determining mechanism was correlated to the coordination
of a boryl copper­(I) species to the allene substrate as well as the
borylcupration step.