Stereoselective Synthesis of γ-Substituted (Z)-Allylic Boranes via Kinetically Controlled Hydroboration of Allenes with 10-TMS-9-borabicyclo[3.3.2]decane

摘要

The hydroboration of allenes is a potentially useful but relatively undeveloped method for synthesis of allylic boranes. The hydroboration of monosubstituted allenes with di(isopinocampheyl)borane [(Ipc)_2BH] generally gives (E)-allylic boranes 4 with excellent selectivity. It is inferred, based on work by Wang who studied the hydroborations of 1-alkyl-1-trimethylsilylallenes with 9-BBN and dicyclohexylborane, that the hydroboration of allenes 1 with (Ipc)_2BH proceeds via thermodynamically controlled isomerization of the kinetically formed (Z)-allylic borane 2 by way of the methal-lylborane isomer 3 (Figure 1). Thus, allene hydroboration has not proven useful for synthesis of 2 owing to the facile 1,3-isomerization of 2 leading to 4 (Figure 1).