The breaking and mending of porphyrins: reductive coupling of secochlorin bisaldehydes

The reaction of free base or Ni(II) complex secochlorin bisaldehydes 4H2 and 4Ni regenerates the ultimate starting material of the bisaldehydes, meso-tetraphenylporphyrin 2H2 and 2Ni, respectively. Depending on the reaction conditions employed (hydrazine hydrate in pyridine at reflux or hydrazine hydrate activated with sulfur in the presence of aqueous NaOH at ambient temperature), either porphyrin 2H2 is formed together with known dihydroxymorpholinochlorin 9H2 or known 2-hydroxychlorin 8H2. Two different reaction pathways for the hydrazine reaction can be derived, either involving the formation of a meso-tetraphenyl-1,4,5-triazepinoporphyrin that loses spontaneously N2 or a Wolff–Kishner-type pathway that also involves an intramolecular aldol-type reaction. Neither reaction is synthetically useful but both highlight in an impressive fashion the high thermodynamic stability of porphyrins. They also bring the ‘breaking and mending of porphyrin’ strategy to its ultimate conclusion by regenerating the starting porphyrin.