Hydroboration. 83. Synthesis and Hydroboration of 2-Ethylapopinene. Comparison of Monoisopinocampheylborane and Its 2-Ethyl Analogue for the Chiral Hydroboration of Representative Alkenes

Article abstract of DOI:10.1021/jo00258a021

Reduction of nopol tosylate gives the new chiral ligand (-)-2-ethylapopinene (90.2percent ee) with higher steric requirements than those of α-pinene.Reaction of borane with (-)-2-ethylapopinene in THF at 0 deg C, in a molar ratio of 1:2, provides an equilibrium mixture of mono- and dialkylboranes EapBH2 and Eap2BH.With an excess of (-)-2-ethylapopinene (1:10; BMS/2-ethylapopinene), the formation of bis(2-ethylapoisopinocampheyl)borane, Eap2BH, is essentially quantitative (ca. 98percent).Treatment of the equilibrium mixture of boranes (Eap2BH, EapBH2) with TMED at 34 deg C precipitates crystalline (EapBH2)2*TMED.Treatment of the crystalline product with BF3*OEt2 gives EapBH2 (mono(2-ethylapoisopinocampheyl)borane) of essentially 100percent ee, enantiomerically purer than the (-)-2-ethylapopinene (ca. 90percent ee) utilized for the preparation.Hydroboration of a series of olefins with EapBH2, followed by oxidation of the intermediate organoborane, produces the corresponding alcohols with significantly improved enantiomeric purities over those realized with IpcBH2 under the same conditions.Liberation of the olefin from purified EapBH2 provides (-)-2-ethylapopinene (<α>_D²³ -46.2 deg (neat) in high optical purity (>99percent ee).Metalation of (+)-α-pinene (<α>_D²³ +51.4 deg (neat)) with the Lochmann reagent, followed by alkylation with methyl iodide, provides in high optical purity (>99percent ee) (+)-2-ethylapopinene (<α>_D²³ +46.4 deg (neat)), unavailable from commercial (-)-nopol.