Aldehydes with propargylic boranes

Table 6.20 Reactions of representative aldehydes and ketones with propargylic borane [1]...

Asymmetric reduction of a,f -acetylenic ketones. This borane can be used to reduce 1-deulerio aldehydes to chiral (S)-l-deulerio primary alcohols in 90% optical yields. It also reduces a,/ -acctylcnic ketones to (R)-propargylic alcohols with enantiomeric purity of 73-100%. The ee value is increased by an increase in the size of the group attached to the carbonyl group. The value is also higher in reductions of terminal ynones. Alcohols of the opposite configuration can be obtained with the reagent prepared from (— )-a-pinene. 

In the same way, reaction of (8) with allenyl- or propargyl-stannanes affords intermediate borane derivatives which, upon reaction with aldehydes, produce the expected adducts with high selectivities (eqs 14 and 15). ... 

R)-Propargylic alcohols have also been produced, this time in 73—100% e.e., by asymmetric reduction of a,/3-acetylenic ketones with the chiral borane prepared from (+)-a-pinene and 9-borabicyclo[3.3.1]nonane (9-BBN) (Scheme 10), a system already known to reduce aldehydes to chiral alcohols cf. 2,115). These results compare favourably with those from the LiAlH4- Darvon complex above, and the availability of both (+)-and (-)-o -pinene means that either (R)-or (5)-propargylic alcohols may be produced. 

There have been significant discoveries of methods that enable the enantioselective addition of an alkyne to an aldehyde or a ketone [182]. The resulting chiral propargyl alcohols are amenable to a wide variety of subsequent structural modifications and function as useful, versatile chemical building blocks. In 1994, Corey reported the enantioselective addition reactions of boryl acetylides such as 292, prepared from the corresponding stannyl acetylenes (e.g., 291) in the presence of the oxazaborolidine 293 as the chiral catalyst (Scheme 2.36) [183]. Both aliphatic and aromatic aldehydes were demonstrated to participate in these addition reactions, which proceeded in high yields and with impressive enantioselectivity. The proposed transition state model 295 is believed to involve dual activation both of the nucleophile (acetylide) and of the electrophile (aldehyde). The model bears a resemblance to the constructs previously proposed for alkylzinc addition reactions (Noyori, 153) and borane reductions (Corey. 188). 

---