Rapid intermolecular electron transfer specific to the zwitterionic form of N-(9-(10-hydroxyanthryl))piperidine (HAP)

Article abstract of DOI:10.1021/ja00005a009

The acid-catalyzed condensation of anthrone with excess piperidine affords both N-anthrylpiperidine (80-85%) and HAP (5-10%). The ¹H and ¹³C NMR spectra of N-(9-anthryl)piperidine are unexceptional and consistent with the structure assignment. However, the NMR spectra of HAP in DMSO-d₆ reveal, except for impurities, only resonances for the aliphatic protons or carbons. Treatment of the DMSO-d₆ solution with either strong acid or base results in appearance of the expected aromatic protons or carbons, with integrations and/or chemical shifts as expected for the predicted ionic forms of HAP produced under these conditions. We have interpreted the unusual NMR behavior of HAP in DMSO-d₆ in terms of intermolecular electron transfer with an impurity of the corresponding O-centered radical, observable by ESR. Of the four ionization states available to 10-N-piperidinylanthranol, only the zwitterionic form exhibits electron transfer that is rapid and reversible. The results of NMR studies on the N-methylated derivative of HAP are consistent with this interpretation.