Photochemische und nicht‐photochemische A/D‐Secocorrin→Corrin‐Cyclisierungen bei 19‐Carboxy‐ und 19‐Formyl‐1‐methyliden‐1,19‐secocorrinaten. Decarboxylierbarkeit und Deformylierbarkeit von Nickel (II)‐19‐carboxy‐ bzw. 19‐formyl‐corrinaten. Vorläufige Mitteilung

Photochemical and non‐photochemical A/D‐secocorrin→corrin‐cyclizations of 19‐carboxy‐ and 19‐formyl‐1‐methylidene‐1,19‐secocorrinates. Decarboxylation (and deformylation) of nickel(II)‐19‐carboxy‐(resp. 19‐formyl)‐corrinates Nickel (II) 1‐methylidene‐2,2,7,7,12,12‐hexamethyl‐15‐cyano‐19‐carboxy‐1,19‐secocorrinate can be induced to cyclize with concomitant decarboxylation to the corresponding corrinate (Scheme 9) . Experiments with deuterated derivatives (Scheme 10) indicate that in this decarboxylative A/D‐secocorrin→corrin‐cyclization the ring closure step precedes decarboxylation. In accord therewith is the finding that the corresponding intermediate nickel(II) 19‐carboxy‐corrinate (synthesized via photochemical cyclization of the corresponding cadmium complex, Schemes 6 and 9 ) decarboxylates under very mild conditions. Nickel(II) 1‐methylidene‐2,2,7,7,12,12‐hexamethyl‐15‐cyano‐19‐formyl‐1,19‐secocorrinate cyclizes smoothly to the corresponding 19‐formyl‐corrinate in the presence of acetic acid/triethylamine. The formyl group of the cyclization product can be eliminated hydrolytically in essentially quantitative yield by treatment with 2 N KOH in ethanolic solution (Scheme 11) . The non‐photochemical (A→D)‐cyclization of 19‐formyl‐1,19‐secocorrinoids represents formation of the corrin chromophore at the oxidation level of porphyrinogens and exemplifies how a C 1 ‐fragment that eventually leaves the ligand can fulfill a specific function in the (A→D)‐ring closure to a corrin.