Cleavage of (octaethyl-2,5-dihydroxychlorinato)nickel(II) to give the novel 2,3-dioxo-2,3-secochlorin system

Treatment of (octaethyl-2,3-dihydroxychlorinato)nickel(II) with lead tetraacetate in benzene causes cleavage of the 2-3 bond to give the nickel(II) 2,3-secochlorin-2,3-dione system 7, a previously unknown structural type. The structure is established by elemental analysis and spectroscopic methods, and confirmed by X-ray analysis. The dione gives a bis(2,4-dinitrophenylhydrazone). Treatment with base causes an aldol-type condensation to occur to give the (2-oxo-2a-homoporphyrinato)nickel(II) 8. This substance is rather unreactive, and it has not been possible to prepare carbonyl derivatives. This lack of reactivity is rationalised in terms of charge delocalisation which reduces carbonyl double-bond character. These novel pathways are discussed in relation to the known (and different) pathways of chlorophyll catabolism which have recently been uncovered.