Diborane as a reducing agent—II: The reduction of indole and pyrrole carbonyl derivatives☆

Abstract Diborane reduction of indole and pyrrole ketones readily affords the corresponding alkyl indoles and pyrroles, even when the nitrogen is substituted. Reduction of 3-formylindoles leads to a mixture of the methylindole, two types of dimeric products and traces of higher polymeric products, whereas 2-formylindole and 2-formylpyrrole give largely polymers. These results are discussed in relation to the mechanisms both of diborane reductions, and of electrophilic substitution in indoles.