Hydroboration. 85. Synthesis and Hydroboration of (-)-2-Phenylapopinene. Comparison of Mono(2-phenylapoisopinocampheyl)borane with Its 2-Methyl and 2-Ethyl Analogues for the Chiral Hydroboration of Representative Alkenes

Article abstract of DOI:10.1021/jo00291a021

The dehydration of (+)-2-phenylnopinol with POCl3 provides a new chiral ligand, (-)-2-phenylapopinene (87percent ee), with higher steric requirements than those of α-pinene or its 2-ethyl analogue.Hydroboration of (-)-2-phenylapopinene with BH3*SMe2 (BMS) (1.2:1 ratio, respectively) provides an equilibrium mixture of the mono(2-phenylapoisopinocampheyl)borane (PapBH2) and the corresponding dialkylborane.Treatment of this mixture with tetramethylethylenediamine (TMEDA) precipitates crystalline (PapBH2)*TMEDA.Liberation of the PapBH2 using BF3*OEt2 provides the monoalkylborane in >99percent ee, thus providing the required reagent in significantly higher optical purity than the starting olefin.Hydroboration of a series of representative olefins using PapBH2 at -25 deg C, followed by oxidative workup, provides the corresponding chiral alcohols in unexpectedly lower enantiomeric purities than those obtained from the 2-methyl and 2-ethyl analogues under identical conditions.Liberation of the starting auxiliary from the borane reagent provides (-)-2-phenylapopinene of >99percent ee.The hydroboration of (-)-2-phenylapopinene with 9-borabicyclo<3.3.1>nonane (9-BBN) at 28 deg C in THF proceeds at an extremely retarded rate compared to its 2-methyl and 2-ethyl analogues.Fortunately, this lack of reactivity is easily circumvented by reacting PapBH2 with 1,5-cyclooctadiene at room temperature for 1 h, followed by thermal isomerization to provide the desired trialkylborane.