Improved syntheses of meso-tetraarylbenziporphyrins and observations of substituent effects on the diatropic characteristics of these formally nonaromatic carbaporphyrinoids

Carbaporphyrinoid systems are widely investigated but improved routes to these systems are needed. A fairly direct route to tetraphenylbenziporphyrin has been reported previously starting from isophthalaldehyde and this approach has been adapted to make use of more easily accessible isophthalic acids. Isophthalic acid and 5-tert-butylisophthalic acid were converted into the related diacyl chlorides and reacted with excess benzene or chlorobenzene to give a series of diketones. These were reduced with sodium borohydride to give the corresponding dialcohols in virtually quantitative yields and this route allows the synthesis of >10 g quantities of these key intermediates. Reaction of these dicarbinols with pyrrole and benzaldehyde or 4-chlorobenzaldehyde in the presence of BF₃·Et₂O in dichloromethane, followed by oxidation with DDQ, gave a series of eight-different tetraarylbenziporphyrins in up to 32% yield. Although benziporphyrins are generally considered to be nonaromatic, they may exhibit trace amounts of global diatropic character and this appears to be enhanced by the presence of a tert-butyl group on the benzene moiety or the introduction of 4-chlorophenyl substituents. Addition of TFA to the NMR solutions generated dications that exhibit an unambiguous ring current effect where the internal CH proton NMR resonances appear upfield in the range of 5.05-5.65 ppm, while the external pyrrolic resonances show concomitant downfield shifts to give values between 7.08 and 7.86 ppm. A tert-butyl substituent again increased the diatropic character of the macrocycle, although replacement of meso-phenyl groups with more electron-withdrawing 4-chlorophenyl units had the opposite effect in this case. These results are consistent with the aromatic characteristics deriving from canonical forms that have 18]annulene substructures. Two examples of palladium(II) benziporphyrin complexes were prepared and these results demonstrate that the presence of 4-chlorophenyl or tert-butyl groupings do not have a negative impact on the formation of organometallic derivatives. The improved yields for the reported synthetic methodology and the new information that can be obtained by systematically altering the substitution patterns around the macrocyclic periphery will facilitate further development of the chemistry of benziporphyrins.