Hydroboration of Chloro-, Acetate-, and Acetal-Functionalized Alkenes

Hydroboration of Chloro-, Acetate-, and Acetal-Functionalized Alkenes 

9-BBN reduces many of the functional groups at a relatively slower rate [18] than that of hydroboration. 

However, all other allylic haloalkenes with an internal double bond undergo hydroboration by placing the boron p to the halogen substituent. These products immediately undergo elimination, followed by rapid rehydroboration of the new terminal double bond (Eqs. 5.7, 5.8) [13]. 

The electronic effects of the hydroboration of allyl halides have been studied and it is found that a Cl, Br, or I substituent at the y position has a rate-reducing effect of 25, 21.7, or 14.1, respectively [11]. 

The order of rate reduction Cl Br I is in the order of increasing electron-withdrawing effect (inductive and mesomeric) of the halogen. However, in a l-halo-3-methyl-2-butene system, the rate-retarding effect of halogen substituent on hydroboration at the p position is a factor of 2, much less as explained above, where hydroboration occurs, at the y position. 

---