Recovering from hydroborations

The hydroboration described in reaction A of Fig. 5 mainly occurs on the (R)-alkene and leaves the unreacted (,S)-oIefin having a 65% e.e. [29]. A similar level of stereoselection (70% e.e.) was estimated for the unreacted (R)-allenic sulfone recovered from the conjugate addition of half equivalent of (R)-a-methylbenzylamine to the racemic compound (reaction B) [30]. 

Reactions n>itb alknes. Caserio et at. have reported that partial hydroboration of racemic 1,3-dimethylallene and 1,3-diphenylallene with ( + )-Pn4B2H2 [from (-)- -pincnc] gave recovered (—)-allcne of moderate activity. The method has been studied in greater detail by Moore et al. and they report that in every case examined the recovered alicne is enriched in the R enantiomer. Brown s reagent is thus useful for preparation of optically active allenes however, only moderate activities result and. of course, some of the alIcne is irrevocably lost in the process. 

The saturate and aromatic hydrocarbons were separated from the alcohols by elution chromatography on alumina deactivated with 3.8% water in a ratio of 5 g of alumina to 1 g of oil, by successive elutions with (1) 30 mL of cyclohexane and (2) 50 mL of 2% benzene in cyclohexane. The alcohols were recovered for future analysis by washing the column with 100 mL of ethyl ether. The solvent was removed from the hydrocarbon fraction and the yield of saturates plus aromatics calculated back to the neutral oil. The percentage of olefins was determined by difference. The percentage of aromatic material was determined as the difference between the saturates and aromatics from the hydroboration and the saturates from acid absorption. 

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