Site-selective C20-fluorinations of chlorophyll-a derivatives using N-fluorobenzenesulfonimide and their optical properties

Abstract

Site-selective fluorinations at the C20 position of methyl pyropheophorbides- a , which is derived from naturally occurring chlorophyll(Chl)- a , were achieved by using N -fluorobenzenesulfonimide (NFSI). All the synthetic Chl- a derivatives possessing a variety of the C3-substituents (vinyl, ethyl, formyl, acetyl, hydroxymethyl, and 1-hydroxyethyl groups) were mono-fluorinated to produce the corresponding 20-fluoro-chlorins as isolated products. These fluorinated products were characterized by 1D/2D NMR, visible absorption, and fluorescence emission spectroscopy. Owing to the strong electronegativity of the C20-fluorine atom, their red-most Qy absorption and fluorescence emission bands were bathochromically shifted except for the C3-acetyl substitutes. This study showed not only tolerance of the chlorin π-skeleton and C3-substituents to NFSI but also potential availability of the fluorinated Chl library. • N -Fuorobenzenesulfonimide was useful for fluorination of chlorophyll- a derivatives. • The fluorination site-selectively occurred at the C20 position of chlorin π-system. • The mild fluorination was performed without modification of the C3-substituents. • The electron-withdrawing C20-F partially affected molecular conformations. • The C20-fluorination specifically changed optical properties in solution.