Nonplanar distortion modes for highly substituted porphyrins

The syntheses, structures, and spectroscopic properties of some novel dodecaphenyl- and dodecaalkylporphyrins have been investigated with the aim of designing model compounds to aid in evaluating theoretical models of porphyrin nonplanarity. It was found that dodecaphenylporphyrin (3) and the dodecaalkylporphyrins 4a-c could be prepared using standard synthetic procedures and that these porphyrins adopted very distorted nonplanar structures in the crystalline state. In the crystal structure of 3 the pyrrole rings were tilted with respect to the mean plane of the porphyrin to give a saddle conformation, whereas in the crystal structure of the nickel(II) complex of 4c (Ni4c) the pyrrole rings were twisted with respect to the mean plane to give a ruffled conformation. Nonplanar conformations with the same distortion modes were obtained as minimum energy structures for Ni3 and Ni4c using a molecular mechanics force field, which showed that the observed nonplanar conformations were due to steric repulsions between the peripheral substituents. Low-temperature NMR spectra indicated that the solution conformations of 3 and Ni4c were consistent with those found in the crystal structures. Several conformations of Ni4b and Ni4c with different quasi-axial and quasi-equatorial orientations of the alkyl chains were observed in solution, and saddle and ruffled distortion modes were clearly differentiated for the dications and nickel(II) complexes of porphyrins 4b and 4c. Finally, an attempt was made to synthesize the very sterically hindered dodecaalkylporphyrin 5 using the same conditions applied to porphyrins 4a-c. This gave only the 5,15-dihydroporphyrin 6, presumably because the more spatially demanding six-membered ring prevented the oxidation of 6 to porphyrin 5 using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. 44 refs., 14 figs., 4 tabs.