Rotational isomerization of 3-substituents in synthetic chlorophyll derivatives

Methyl pyropheophorbides-a possessing a (pseudo)planar substituent at the 3-position were prepared from naturally occurring chlorophyll-a. Some of the semisynthetic π-conjugates with the chlorin skeleton took two atropisomeric conformations for the sterically demanding 3-substituents, CONMe2, NHCOMe, C[CHC(CN)2]C(CN)2, and Ph(2,3,4,5-Ph4). Their rotational isomerization in a solution was analyzed by 1H NMR and HPLC. Zinc complex of the 3-aryl-chlorin gave a large energy barrier for the rotation of the C3C31 single bond (estimated ΔG‡=108 kJ mol−1 at 20 °C) and the atropisomerically pure conformers were separated at room temperature.