NMR-SPECTRA OF THE PORPHYRINS .42. THE SYNTHESIS AND AGGREGATION BEHAVIOR OF SOME CHLOROPHYLL ANALOGS

The synthesis of three analogues of pyrochlorophyll a with the selecti ve removal of the carbonyl functions is described; 7-despropionate-7-p ropyl-mesopyrochlorophyllide a (DPC), methyl 9-desoxo-mesopyrochloroph yllide a (DOC) and xo-7-despropionate-7-propyl-mesopyrochlorophyllide a (DODPC), were prepared from methyl mesopyropheophorbide a by the ext ension of known preparative methods. A detailed study of the aggregati on of these compounds in solution, using the NMR shifts observed upon dissociation of the aggregates formed in CDCl3 solution by the additio n of [H-2(4)]methanol, together with the application of the ring curre nt theory of the porphyrin ring is given. In DPC the observed aggregat ion shifts can be given a precise explanation in terms of a sandwich t ype dimer with an interplanar separation of ca. 4.2-4.7 angstrom. All the dimer models considered allow for coordination between the C-9 ket o group and the magnesium atom of the adjoining molecule, probably via a bridging water molecule. In DOC the calculated dimer shifts based o n the sandwich type dimers also give a reasonable explanation of the o bserved shifts, although now it is the C-7c carbonyl group which is po sitioned so as to complex with the magnesium atom of the adjacent porp hyrin 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 pi-pi attractive interaction, in complete contrast to the analogous metalloporphyrins.