The 2,3-secochlorin-2,3-dione system

Abstract

Cleavage of nickel(I1) octaethyl-2,3-dihydroxychlorin 3 with lead tetraacetate gives the novel octaethyl-2,3-secochlorin-2,3-dione system as its nickel(I1) complex 4, which on treatment with base cyclises to a novel homoporphyrinone structure 5; electronic spectra are presented: 4 and 5 are seen as representing chromophores that may arise in chlorophyll catabolism.The catabolic fate of chlorophyll has received increasing attention recently. [1][2][3] Although in the form of metalloporphyrins, a small fraction persists in sedimentary organic deposits, most of it must be destroyed during senescence. On chemical grounds the major reactive sites (shown in 1) of chlorophyll a are expected to be (a) C-132, a pseudobenzylic acidic (p-keto ester) position; ( b ) the metal-x system, readily oxidised and readily demetallated; (c) the ring A-C-3 vinyl system; ( d ) the unsubstituted meso positions (C-5,10,20-electrophilic substitution, addition); and the tertiary pseudobenzylic C-17 and C-18 positions. Adams4 postulated (Scheme 1) that radical reactions at the 17 and 18 positions could lead to cleavage of the 17-18 bond to give the secochlorindione (A). We now describe this dione in a model system, which may allow this chromophore to be recognised should it occur naturally.Osmylation of octaethylporphyrin gave the dihydroxychlorin 2 . 5 Attempted cleavage of the glycol function of this gave complex mixtures, but the nickel(I1) complex 3 reacted smoothly with lead tetraacetate in anhydrous benzene (20 "C, 15 min) to give 76% of the secochlorindione 4 as tiny green needles, C36H&&i02 (elemental analysis, accurately measured M+). The electronic spectrum (Fig. 1) showed strong broad bands at 444 nm ( E 49 000) and 648 nm ( E 13 300), while in the IR the conjugated carbonyl groups appeared as a single band at Y 1676 cm-1 (KBr). In the 1H NMR spectrum the meso signals at 6 9.03 (s) and 6 8.17 (s) showed marginal changes compared with the corresponding singlets (6 9.12, 8.25) in the spectrum of the precursor 3. The structure 4 was confirmed by X-ray crystal analysis.6 In the crystal the carbonyl groups point in opposite directions: thus the C-2 carbonyl group, for instance, lies in a plane (C-1, C-2, C-2', 0) which makes a dihedral angle of 62" to the mean plane determined by C-1, C-2, C-20 and N-21. As a consequence, the two hydrogens of each methylene group adjacent to a carbonyl are inequivalent, and in the 1H NMR spectrum appear as double quartets at 6 2.04 and 2.75.

Aza-deficient porphyrin as a ligand

“…In independent studies concerning a catabolism of chlorophyll a related porphyrinoid was detected. Thus in a two-step oxidation reaction of nickel porphyrin complex, dihydroxylation and then cleavage of ␤-␤ bond took place and yielded aromatic 2,3-secochlorin-2,3-dione nickel complex for both, octaethyl and tetraphenyl (8) derivatives [15,16].…”

Stepwise Conversion of Two Pyrrole Moieties of Octaethylporphyrin to Pyridin‐3‐ones: Synthesis, Mass Spectral, and Photophysical Properties of Mono and Bis(oxypyri)porphyrins

“…[27,28] OEP was first converted to 2,3-dihydroxychlorin 10 , followed by insertion of Ni(II) to form 10Ni and a Pb(IV)(acetate) 4 -induced diol cleavage reaction to form the Ni(II) complex of secochlorin 11Ni . Under base catalysis, this diketone underwent an intramolecular aldol condensation, resulting in the formation of the oxypyriporphyrin Ni(II) complex 12Ni .…”

Carbaporphyrinoids Containing a Pyridine Moiety: 3‐Aza‐meta‐benziporphyrin and 24‐Thia‐3‐aza‐meta‐benziporphyrin

“…[29,30] (Oxypyriporphyrin)nickel() was formed in the cyclization of (vic-dihydroxyoctaethylchlorin)-nickel() by secochlorin formation. [31,32] Reaction of dichlorocarbene with meso-octamethylcalix [4]pyrrole caused pyrrole ring expansion, affording chlorocalixpyridinopyrroles and chlorocalixpyridines, which correspond to the porphyrinogen oxidation state. [33] Paradoxically, the carbaporphyrinoid 6,11,16,21-tetraaryl-3-aza-m-benziporphyrin 2 is the very first "true" pyriporphyrin containing an unperturbed pyridine ring in the N-confused macrocyclic framework.…”