Non-planar porphyrins with mixed substituent pattern: bromination and formylation of ethyl-substituted tetraphenylporphyrins and tetraalkylporphyrins
A series of 5,10,15,20-tetraphenylporphyrins (TPPs) bearing different numbers and orientations of β-ethyl substituents have been used for studies on the functionalization of non-planar porphyrins. Attempts to monofunctionalize the diethyl-TPP via monobromination failed and resulted in the isolation of two different regioisomers. All ethyl-TPPs were easily converted into the perbrominated porphyrins bearing twelve peripheral substituents but with different numbers of β-bromo and -ethyl groups. These porphyrins are interesting precursors for further mixing of the substituent pattern and are novel push-pull porphyrins bearing electron donating and withdrawing groups in a defined pattern directly on the ring system. They have a similar degree of overall conformational distortion, however each with considerably altered electronic effects due to the different number of bromine atoms. In order to gain access to monofunctionalized derivatives, some of the ethyl-TPPs were converted into the monoformyl derivatives. In a similar manner, a number of 5,10,15,20-tetraalkylporphyrins were formylated, albeit in lower yields than the ethyl-TPPs. Several compounds were investigated in detail by single crystal X-ray crystallography and allowed a comparison of the influence of different substituent patterns on the conformation. The structural analyses include a comparison of different crystalline modifications for some compounds. Structural data for perbrominated ethyl-TPPs present the first examples of highly non-planar, dodecasubstituted porphyrins bearing two different types of β-substituents.