Synthesis and spectroscopic properties of novel benzochlorins derived from chlorophyll a

Methylpheophorbide-α (a chlorophyll α analog) was converted into methyl 13¹-deoxypyropheophorbide-α 2 in excellent yield. The Ni(II) complex of 2 on hydrogenation and subsequent Vilsmeier reaction with phosphorus oxychloride and 3-(N,N-dimethylamino)acrolein produced Ni(II) methyl 20-(2- formylvinyl)-13¹-deoxypyropheophorbide-α 4. In contrast to other synthetic and naturally occurring isobacteriochlorins, the isobacteriochlorin obtained by acid-catalyzed cyclization of 4 was found to be unstable and oxidized to the corresponding benzochlorin. The free base benzochlorin 7, obtained by demetalation, up on DDQ/methanol treatment produced a mixture of 13²-oxo- and 13¹-methoxy-13²-oxobenzochlorins 8,9. Surprisingly, under similar reaction conditions, an ethanolic solution of DDQ afforded 13¹,18² dioxobenzochlorin 13 as a major product. Vilsmeier reaction (POCl₃/DMF) of Ni(II) benzochlorin 6 gave unexpected 5-formyl-13²-oxobenzochlorin 15. Zn(II) complexes of the newly synthesized benzochlorins showed long wavelength absorptions in the range of 750-753 nm: Thus, compared to the respective free base analogues bathochromic shift of 4042 nm were observed. Despite extensive studies in benzochlorins, these are the first examples which exhibit such remarkable long wavelength absorptions in their electronic absorption spectra. The structures of novel benzochlorins were confirmed by extensive NMR [2D NMR (ROESY), ¹³C NMR] and X-ray crystallographic studies. On the basis of the crystal structure of oxobenzochlorin 8, the chemical reactivity of other benzochlorin analogues were examined by semiempirical molecular orbital theory. Our results are in agreement with the previous qualitative electron sextet hypothesis proposed for chlorin systems by Woodward. The fluorescence quantum yields and the singlet oxygen yields of the free base and Zn(II) benzochlorins were measured relative to tetraphenylporphyrin (TPP) in benzene.