Chemoselective reduction with borane
4-(Ethoxycarbonyl)benzoic acid, the monoethyl ester of terephthalic acid, has a carboxylic acid and an ester on the same ring. LiAlH4 would reduce both groups to the alcohol.
The borane–THF complex reacts with the acid at 0 °C, evolves hydrogen and forms a triacyloxyborane, which is rapidly reduced to the alcohol; the less electron-rich ester hardly reacts. After destroying the excess borane with methanol, ethyl 4-(hydroxymethyl)benzoate is obtained. The reverse chemoselectivity (reducing the ester and keeping the acid) is achieved with the carboxylate, which does not react with LiBH4.