Hydroboration hyperconjugative effects

The C-4 Me in an alkene (Chart 4.1) increases the placement of boron at the C-2 position and also decreases the rate of hydroboration (Chart 4.2). Apparently, the steric effect and hyperconjugation by the methyl group, which would tend to direct hydroboration to the C-3 position, are outweighed due to steric effects of the alkyl group. 

However, 1-phenylpropene undergoes hydroboration at the C-1 position. This is not what one would expect based on the steric effect of a phenyl group. This is because of the combined effects of phenyl conjugation (-K) and methyl hyperconjugation (-tK), which act to decrease the amount of electron density at the C-2 position and to increase it at the C-1 position (Fig. 4.1) [6]. Evidently, these mesomeric effects are strong enough to override the steric effects of the phenyl group. 

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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