Thermal Reactions of B-Alkyl-9-borabicyclononane (9-BBN). Evidence for Unusually Facile Dehydroboration with B-Pinanyl-9-BBN

Article abstract of DOI:10.1021/ja00366a026

B-Alkyl-9-BBN compounds undergo a slow olefin-alkyl group exchange when refluxed with an olefin in tetrahydrofuran.The half-life of the process for B-trans-2-methylcyclopentyl- and B-3-methyl-2-butyl-9-BBN is approximately 4 days.However, B-3-pinanyl-9-BBN undergoes an exceptionally rapid olefin-alkyl group exchange with a half-life of less than 10 h.Kinetic and competition experiments support a dehydroboration-hydroboration process.The isomerization of the B-3-pinanyl-9-BBN to B-myrtanyl-9-BBN, which one would expect to accompany such a facile dehydroboration, is not seen until 145 deg C.After 24 h at 165 deg C, the reaction reaches equilibrium with B-trans-myrtanyl-9-BBN as the major product.Treatment of this organoborane with benzaldehyde liberates the rare (+)-β-pinene.The facility with which B-3-pinanyl-9-BBN undergoes dehydroboration has important consequences for asymmetric reductions with this reagent.