1,7-migration of benzyl group in 2-substituted N-benzylindoles

Abstract

It has been shown that when 2-substituted N-benzylindoles are heated above 1000C in polyphosphoric acid, the benzyl group is split off and undergoes 1, 7-migration.It is known [1] that indole systems typically undergo 1,3-, 2,3-, and 3,2-rearrangements in the pyrrole ring. We reported in [2] that when a mixture of the hydrochloride of N-benzylphenylhydrazine (VI) and a ketone I-V is heated in polyphosphoric acid (PPA), the products that are formed are not the expected 2-substituted N-benzylindoles XII-XVI, but rather the corresponding N-unsubstituted indoles XVII-XXI and their 7-benzyl derivatives XXII-XXVI.We established by means of chromatographic monitoring that at relatively low temperatures (50-70"C), all that takes place is parti~/l indolization of the hydrazones VII-XI, with the formation of the N-benzyl derivatives XII-XVI and the Nunsubstituted indoles XVII-XXI. At higher temperatures (100-1200C), the processes of indolization of the hydrazones VII-XI are completed with the formation of a mixture of products of splitting off the benzyl group XVII-XXI and 1,7-migration of this group XXII-XXVI; the reaction products contain traces of the corresponding 2-substituted N-benzylindoles XII-XVI.Here we must emphasize that the electronic nature of the substituent (R = Ar, Me, COOEt) does not influence the process of rearrangement, and the corresponding 7-benzyl derivatives XXII-XXVI are formed in all cases.

Dissociative Benzyl Cation Transfer versus Proton Transfer: Loss of Benzene from Protonated N-Benzylaniline

“…The protonated N -benzylaniline first rearranges to the protonated 4-benzylaniline or 2-benzylaniline via BCT, and then loss of benzene can take place. Although the phenomenon of benzyl group rearrangement has been observed in several cases, the intrinsic features of the BCT instead of the PT resulting in gaseous fragmentations indeed attract us. In this paper the dissociative benzyl cation transfer is identified as the main driving force of benzene loss.…”