Hydroborations with Acyclic Stereocontrol

Still documented a classic study involving the diastereoselective hydroboration of allylic alcohols (Equation 1) [21]. For example, when allylic alcohol 20 was allowed to react with borane, a 1 1.4 mixture of diols 21 and 22 was isolated. However, the hydroboration of 20 with the bulkier 9-BBN resulted in a pronounced preference for 21 (dr=ll 1). 

Houk has suggested that stereoelectronic effects have influence over the stereochemical course of hydroboration reactions of allylic alcohols [19]. Because borane is an electrophilic reagent, it exhibits a preference for electron-rich partners in hydroboration reactions. The more reactive conformer of an allylic alcohol is that in which the olefin avoids additional hyperconjugative interactions that would render it electron-deficient, such as jic=c ( c-x (allylic). Therefore, allylic hydroxy or alkoxy substituents tend to avoid the anti position with respect to the partially formed bonds (cf. transition structure 36). Altogether, both steric and electronic effects work in concert to support the predominance of transition structures 28 and 36, 

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