NMR spectra of porphyrins. 18. A ring-current model for chlorophyll derivatives

A double dipole model of the macrocyclic ring current in the dihydroporphyrin ring of chlorophyll derivatives is presented and parameterized. The model includes a close-range approximation that has both first-order and second-order continuity at all points in space, except through the current loop itself. The model is tested by comparison of the proton chemical shifts of methyl pyropheophorbide a (1) with the corresponding porphyrin, 2-vinylphylloerythrin methyl ester (2). Aggregation effects were eliminated by use of the zinc(II) complexes bearing an apical pyrrolidine ligand, and substitution effects were isolated by use of the corresponding chlorin and porphyrin 9-ketals (3 and 4, respectively). The exocyclic ring E has no appreciable effect on the macrocyclic ring current, but the C9 keto function and the reduction in ring D in the chlorin both reduce the ring current by about 6 and 10%, respectively. The resulting model gives a good account of the chemical shift differences of all the protons in methyl pyropheophorbide a and 2-vinylphylloerythrin methyl ester. Owing to very low yields encountered in the preparation of 2-vinylphylloerythrin methyl ester from methyl pyropheophorbide a, a new route involving DDQ oxidation of the chlorin 9-ketal (3) is described.