Electrophilic substitution in indoles—V: Indolenines as intermediates in the benzylation of 3-substituted indoles

Abstract 3-Benzyl-2-p-methoxybenzylindole is formed either by treatment of the Grignard derivative of 3-benzylindole with p -methoxybenzyl chloride, or by benzylation of the Grignard derivative of 3- p -methoxybenzylindole. The same intermediate 3-benzyl-3- p -methoxybenzyl indolenine must be involved in both reactions. Cyclization of 3-( o -hydroxymethylbenzyl) indole with phosphorous pentoxide in hot benzene affords 1,4-dihydro-2,3-benzcarbazole. The intermediacy of a spirocyclic indolenine in this reaction is supported by the formation of a spirocyclic indoline in a related cyclization which occurs in the diborane reduction of 3( o -carboxybenzyl) indole. These results confirm that electrophilic substitution at the 2-position of a 3-substituted indole is an indirect process, and in the light of this work published observations on the reactions of indoles with aromatic aldehydes and ketones are re-interpreted.