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

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 oxidizi...