Formation of a dihydroborole by catalytic isomerization of a divinylborane.

Diphenylamino(divinyl)borane (1a) adds two molar equivalents of Piers' borane [HB(C6F5)2] to give the expected double hydroboration product. In contrast diisopropylamino(divinyl)borane (1b) reacts cleanly already with one molar equivalent of HB(C6F5)2 to give the α-borylated tetrahydroborole derivative 10 in good yield. Subsequent treatment of 10 with benzaldehyde proceeded by retro-hydroboration to give the hydroboration product of the aldehyde plus the dihydroborole 3b. We were able to achieve the divinylborane to dihydroborole isomerization (1b to 3b) catalytically: treatment of diisopropylamino(divinyl)borane (1b) with 15 mol% of Piers' borane at elevated temperature gave (diisopropylamino)dihydroborole 3b in good yield.