Epimerization of Chlorophyll Derivatives. V. Effects of the Central Magnesium and Ring Substituents on the Epimerization of Chlorophyll Derivatives.

Kinetic and equilibrium features of the epimerization in six C132-epimer pairs of chlorophyllous pigments, namely chlorophyll (Chl) a/a′, Chl b/b′, pheophytin (Pheo) a/a′, Pheo b/b′, C20-chlorinated Chl (Cl–Chl) a/a′, and Cl–Pheo a/a′, catalyzed by bases (triethylamine, pyrrolidine, and piperidine) and by Lewis acids (magnesium acetylacetonate and magnesium chloride), have been studied in diethyl ether and methanol. A significant difference was noted in the epimerization rate and equilibrium epimeric composition among the pigment systems. The difference in the rate constants was rationalized in terms of the ease in C132-proton detachment, affected by the ring π-electron density which in turn depends on the electron donation or withdrawal by the central metal and/or ring substituents. For Lewis acid-catalysis, a dominant factor appears to be the stabilization of the C131-enolate anion via formation of a loose complex with extraneous Mg2+. A synergistic action of a base (triethylamine) and a Lewis acid (Mg2...