Nonplanr Meso-Trifluoromethyl Substituted β-Octaalkylporphyrins

A trifluoromethyl (CF₃) group gives a drastic perturbation to an aromatic compound owing to not only the three-dimensional steric bulkiness but also the strong electron-withdrawing characters.  When introduced at sterically crowded meso-positions of β–alkyl substituted porphyrins, it is expected to cause major update of both the structural and electronic properties of the molecule.  However, introduction of CF₃ groups required preparation of CF₃-embedded precursors such as dipyrromethanes, resulting in tedious synthetic strategies.  This time, we performed the serendipitous but reproducible formation of meso-trifluoromethylated 2,3,7,8,12,13,17,18-octaalkylporphyrins by reactions of 5,5’-dicarboxy-3,3’,4,4’-tetraalkyl-2,2’-dipyrromethanes and large excess amount of trifluoroacetic acid without any additional sources of meso-carbons like aldehyde.  The numbers of CF₃ groups increased up to two by elongation of the reaction time and the positions of second CF₃ groups varied depending on the β-substituents.  The structural characterizations by X-ray diffraction analyses revealed their distorted conformations reflecting the steric repulsions around the CF₃ groups.  In the UV-visible absorption spectra, the lowest energy bands were red-shifted stepwise by introduction of each CF₃group.

Metallation behavior was also investigated.  In the cases of zinc complexes, the CF₃ groups gradually changed into methoxycarbonyl (CO₂Me) groups probably due to methanol in the mixture.  The influence of CO₂Me groups were smaller than CF₃groups according to the relatively flat X-ray structures and the less-shifted absorption bands.

Reference

M. Suzuki, S. Ishii, T. Hoshino, S. Neya Chem. Lett. 2014, 43, 1563–1565.