HYDROBORATION .85. SYNTHESIS AND HYDROBORATION OF (-)-2-PHENYLAPOPINENE - COMPARISON OF MONO(2-PHENYLAPOISOPINOCAMPHEYL)BORANE WITH ITS 2-METHYL AND 2-ETHYL ANALOGS FOR THE CHIRAL HYDROBORATION OF REPRESENTATIVE ALKENES

The dehydration of (+)-2-phenylnopinol with POCl3provides a new chiral ligand, (-)-2-phenylapopinene (87% ee), with higher steric requirements than those of a-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)2-TMEDA. Liberation of the PapBH2 using BF3.OEt2provides the monoalkylborane in ≥99% 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 °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 ≥99% ee. The hydroboration of (-)-2-phenylapopinene with 9-borabicyclo[3.3.1]nonane (9-BBN) at 28 °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. © 1990, American Chemical Society. All rights reserved.