Oxidative rearrangement and cyclisation of N-substituted amidines using iodine(III) reagents and the influence of leaving group on mode of reaction

Abstract

The products of reaction of N-substituted amidines 5 with hypervalent iodine reagents such as (diacetoxyiodo)benzene (DAIB), bis(trifluoroacetoxy)iodobenzene (BFIB) and [methoxy(tosyloxy)iodo]benzene (MTIB) are determined by the reagent, the amidine substituents and the reaction temperature. C-Alkyl-N-arylamidines 5a–d cyclise in high yield giving benzimidazoles 6 but C,N-dialkyl- and C,N-diaryl-amidines 5e–l rearrange to give products derived from an intermediate carbodiimide. Use of N²-phenylfuran-2-carboximidamide 5j leads to N-(2-furyl)acetamide 15 in good yield, illustrating a convenient route to stable derivatives of highly unstable 2-aminofuran. The rearrangement of C,N-diarylamidines on reaction with DAIB contrasts with the observed formation of benzimidazole when the same precursors are treated with lead tetraacetate (LTA). Evidence is presented to support the view that the mode of reaction is determined by the nature of the leaving group in an imide intermediate 19: very good leaving groups [e.g. PhI, N₂, AgCl and PhSO₂O^- (aq.)] appear to favour rearrangement whereas poorer leaving groups [e.g. Cl^-, Me₂S, Me₃N and PhSO₂O^- (non-aq.)] favour cyclo-α-elimination.