Nonplanr Meso-Trifluoromethyl Substituted β-Octaalkylporphyrins

Monday, 25 May 2015: 15:20
Lake Michigan (Hilton Chicago)
M. Suzuki, T. Hoshino, and S. Neya (Chiba University)
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.


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