Propargylic alcohols hydroboration

Extension of this methodology to an enantioselective variant soon followed. In 1979 Midland showed that by using the chiral reagent derived from hydroboration of a-pinene by 9-BBN (3), deuterium labeled benzaldehyde (6b) could be reduced to enantiomerically enriched alcohol 7b in 98% ee Subsequent studies found that 3 was also useful for the enantioselective reduction of acetylinic ketones (8) to propargylic alcohols (9).4... 

The synthesis of (S)-enantiomer of propargyl alcohol is achieved by Alpine-Bo-rane obtained from (-)-a-pinene. However, (-)-a-pinene is an expensive reagent. Consequently, structurally related nopol, 6,6-dimethylbicyclo[3.1.1]hept-2-en-2-ethanol, which is a low-cost and commercial alternative to (-)-a-pinene is used. Nopol, after conversion to benzyl ether is hydroborated [1,2] with 9-BBN on reflux in THF overnight to give the borane NB-Enantrane [2] (Scheme 26.3). 

Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-l-butene with the NHC-derived borane and catalytic HNTf2 (Tf=trifluoromethanesulfonyl, CF3SO2) affects hydroboration at room temperature. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations." Different NHC-CuCl complexes catalyse hydroborations of propargylic alcohols and ethers with opposite regioselectivity a 6-NHC-CuCl (23) complex provides a-selectivity, whereas / -selectivity is achieved using a 5-NHC-CuCl (24) complex." ... 

Regioselectivity of the CuCl-catalysed hydroboration of propargylic alcohols and ethers R C=CCH(OR )R with Pin2B2 can be controlled (from 98 2 to 2 98) by the steric bulk of the OR group and the NHC ligand to copper. A rationale has been provided by assuming different approach of the reactive species to the individual substrates (87)-(89).S9... 

The convenient synthesis of the a-chloro-substituted pinacol-derived boro-nate 46 commences with the hydroboration of propargyl alcohol 57 (Scheme 5.10) [57]. Following protection of the secondary alcohol, hydroboration of the alkyne and transesterification furnishes boronate 59. Treatment of 59 with thionyl chloride stereospecifically installs the a-chloro substituent of 46. 

Haloalken-l-yl)dialkylboranes, obtained by hydroboration of propargyl halides with dialkylboranes, smoothly rearranged to ( )-allyboranes, which can be trapped with aldehydes. This one-pot three-component sequence provided linear homoallyl alcohols (Equation (145)) or //-homoallyl alcohols (Equation (146)).610-612... 

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