Reduction by complex boron hydrides

A large number of methods have recently been found for the preparation of complex boron hydrides. The alkali borohydrides and sodium hydridotrimethoxyborate can be obtained commercially for preparation of the other compounds a few literature references must here suffice.100,1836 362 364 

Reduction by quaternary ammonium boron hydrides containing long-chain alkyl groups, e.g., hexadecyltrimethyl- and trioctylmethyl-ammonium boron hydride, can be carried out also in apolar solvents such as hydrocarbons.365 

The reduction of aliphatic and aromatic aldehydes by NaBH4 was thoroughly studied by Chaikin and Brown.373 The corresponding alcohols were formed in good yield and in most cases even at room temperature  

Only in exceptional cases did nitro, ester, epoxy, acid, lactone, lactam, or amide groups react, so that NaBH4 is particularly valuable as a selective reducing agent. An impressive example is the selective reduction of a hetero-cyclic-substituted 2-bromoacraldehyde 374 

In this reaction the bromine atom, the nitro group, and the double bond are unaffected. 

---