Synthesis of nickel(II) isobacteriochlorins from nickel(II) complexes of chlorophyll derivatives

Abstract

Treatment of nickel(II) methyl δ-meso-[(methylthio)methyl]mesopyropheophorbide a (1c) with Raney Ni gives the expected nickel(II) δ-meso-methyl compound 5 as well as byproducts, which are shown to be the nickel(II) isobacteriochlorin 2b, nickel(II) hexahydroporphyrin 4, and methyl nickel(II) δ-meso-methyldeoxomesopyropheophorbide a 3c. The nickel(II) isobacteriochlorin is shown to consist substantially of one isomer (2b, “tcc”) by using HPLC and high-field NMR spectroscopy. When the same reaction is carried out with the δ-meso-unsubstituted nickel(II) compound 9b, similar isobacteriochlorin 11a, hexahydroporphyrin 12, and deoxo compound 10b are obtained. In this case, the isobacteriochlorin is separated by HPLC into approximately equal amounts of two isomers. On the basis of extensive NMR experiments, structures for the two isomeric nickel (II) isobacteriochlorins are proposed. Variation of the solvent used in the Raney Ni reduction allows preferential formation of either the isobacteriochlorins 11a or the deoxo compound 10b, but under no circumstances are materials formed in which both the 9-ketocarbonyl and the ring A pyrrole subunit have been reduced. The presence of the δ-meso-methyl substituent (e.g., in 1c) also has an effect on the Raney Ni reaction such that significantly higher yields of nickel(II) isobacteriochlorin 2b are obtained (compared with reduction of 9b to give 11a). © 1985, American Chemical Society. All rights reserved.