Fig. 5: The proposed mechanisms.
From: Enantioselective Cu-catalyzed double hydroboration of alkynes to access chiral gem-diborylalkanes

The reaction proceeds through 1st hydroboration cycle with HBdan and 2nd hydroboration cycle with HBpin.
From: Enantioselective Cu-catalyzed double hydroboration of alkynes to access chiral gem-diborylalkanes

The reaction proceeds through 1st hydroboration cycle with HBdan and 2nd hydroboration cycle with HBpin.