NMR spectra of the porphyrins. Part 42. The synthesis and aggregation behaviour of some chlorophyll analogues

Abstract

The synthesis of three analogues of pyrochlorophyll a with the selective removal of the carbonyl functions is described; 7-despropionate-7-propyl-mesopyrochlorophyllide a (DPC), methyl 9-desoxo-mesopyrochlorophyllide a (DOC) and 9-desoxo-7-despropionate-7-propyl-mesopyrochlorophyllide a (DODPC), were prepared from methyl mesopyropheophorbide a by the extension of known preparative methods. A detailed study of the aggregation of these compounds in solution, using the NMR shifts observed upon dissociation of the aggregates formed in CDCI₃ solution by the addition of [²H₄]methanol, together with the application of the ring current theory of the porphyrin ring is given. In DPC the observed aggregation shifts can be given a precise explanation in terms of a sandwich type dimer with an interplanar separation of ca. 4.2–4.7 A°. All the dimer models considered allow for coordination between the C–9 keto group and the magnesium atom of the adjoining molecule, probably via a bridging water molecule. In DOC the calculated dimer shifts based on the sandwich type dimers also give a reasonable explanation of the observed shifts, although now it is the C–7c carbonyl group which is positioned so as to complex with the magnesium atom of the adjacent porphyrin molecule, with a smaller interplane separation of ca. 3.7–3.9 A°, which may imply direct coordination of the carbonyl and the magnesium atom. In DODPC, in which no carbonyl function is present, there is no evidence of any aggregation in solution. Thus in this metallochlorin there is no π–π attractive interaction, in complete contrast to the analogous metalloporphyrins.