Chemical and electrochemical oxidation of N-alkyl cyclo[n]pyrroles

Abstract

Cyclo[n]pyrroles ( n = 6, 7, and 8), are expanded porphyrins that lack bridging meso carbon atoms. They adopt conformations that are near-planar and show features that are consistent with global aromaticity. In previous work, involving studies of cyclo[8]pyrroles, it was found that, in contrast to what is true for porphyrins, reductive per- N -alkylation led to the formation of non-planar products that displayed few signs of extended conjugation. These octa- N -alkylated species could be reoxidized electrochemically to produce dicationic, octa- N -substituted analogues of the starting aromatic cyclo[8]pyrroles. We have now extended this study to cyclo[6]pyrrole, and have found that reductive per- N -methylation can be readily effected using methyl iodide. However, in contrast to what is true in the case of cyclo[8]pyrroles, efforts to obtain the fully N -ethylated or N -benzylated products proved unsuccessful. Moreover, in the case of the hexa- N -methylated cyclo[6]-pyrrole product no viable chemical means of oxidizing it to the putative aromatic dication could be found. However, evidence for the formation of such species was obtained by electrochemical means. The differences between cyclo[6]- and cyclo[8]pyrrole highlight the effect of ring size in mediating the redox and reactivity chemistry within what is otherwise a closely matched pair of porphyrin analogues.