Acidic Devinylation of Nickel Chlorophyll Derivatives

A nickel(II) complex of methyl pyropheophorbide-a bearing a vinyl group at the 3-position was treated with methanesulfonic acid in refluxing benzene to give smoothly the corresponding 3-devinylated product. The 7- and 8-vinyl groups in the related chlorophyll-a/b derivatives were readily removed under the same conditions, while the 12-vinyl group neighboring the electron-withdrawing 13-keto carbonyl moiety was hardly defunctionalized. The devinylations blue-shifted all the visible absorption bands in dichloromethane. The deethylation at the 3-position bathochromically moved the redmost Qy maximum, and the dealkylations at the 7-, 8-, and 12-positions hypsochromically moved the Qy(0,0) maxima, which were reproduced by time-dependent density functional theory calculation. Since no vinyl periphery in the free base counterparts was removed under the mild conditions, nickel 3-unsubstituted deoxophylloerythroporphyrin in geological samples would be produced via the nickel metalation of chlorophylls-a/b/c from ancient phototrophs or their derivatives followed by the acidic 3-devinylation.