Synthesis and physicochemical properties of (5-thiaporphyrinato)zinc and linear tetrapyrroles carrying a thioketo group

Abstract

Sulfur analogues of bilin-1-one and bilin-1,19-dione were prepared and their spectroscopic properties and reactivities were investigated. meso-Arylbilin-1,19-dione, obtained by coupled oxidation of iron meso-arylporphyrin, was converted to 19-thioxobilin-1-one via the 5-oxaporphyrin zinc complex. The 5-thiaporphyrin zinc complex was prepared by cyclization of 19-thioxobilin-1-one in 78% yield at 0 °C. At higher temperatures a mixture of (5-thiaporphyrinato)zinc and (5-oxaporphyrinato)zinc was obtained. The temperature-dependent regioselectivity was ascribed to a lower activation energy of cyclization of bilin-1-thione than that of bilin-1-one. Ring-opening of (5-thiaporphyrinato)zinc with benzenethiolate gave 19-phenylsulfanylbilin-1-thione. The rate constant of formation of (5-thiaporphyrinato)zinc from 19-phenylsulfanylbilin-1-thione (0.01 mM) and zinc acetate (0.1 mM) in CH₃CN–H₂O (1/1) at 25 °C was 10⁴ times larger than that of 19-phenylsulfanylbilin-1-one. Copper(II), cobalt(II), nickel(II), magnesium and cadmium ions also induced cyclization of 19-phenylsulfanylbilin-1-thione to afford the corresponding metal complexes of 5-thiaporphyrin. The sulfur analogues of bilin-1-one and bilin-1,19-dione showed bathochromically shifted UV-visible spectra, up to 100 nm, compared to the corresponding bilin-1-one and bilin-1,19-dione. Due to much higher reactivity and lower energy gap of bilin-1-thione than bilin-1-one, linear tetrapyrroles with a thioketo group should find applications in chemistry, biochemistry and materials science.