Question

Explain the regioselectivity of the hydroboration of indene which causes the reaction to form 2-indanol instead of 1-indanol.

Answer 1

Hydroboration is the electrophilic addition reaction to the olefin. Here Lewis acid BH₃ act as electrophile and get acted upon by pi electron density of olefin.

Regeoselective reaction: The reaction in which reagent attacks particular site in the substrate in presence of other similar kind of reaction site and particular product only results.

First step is the addition of BH₃ across double bond such that more stable carbocation results. Hence in indene case BH₃ adds onto double bond in 5 membered ring at 2 position giving carbocation at C1 which is well stabilized by resonance of benzene ring. (This kind of stability would not result if BH3 adds onto C1 and gave carbocation at C2.)

The hydride transfer and subsequent trialkyl borane formation results. Then alkaline hydrogen peroxide reaction gives trialkylboronate which on hydrolysis gives final product 2-Indanol

Hence the regioselectivity of hydroboration reaction.