Adaptation of the Rothemund Reaction for Carbaporphyrin Synthesis: Preparation of meso-Tetraphenylazuliporphyrin and Related Benzocarbaporphyrins

Electrophilic substitution of azulene has recently been shown to provide the means by which carbon–carbon bonds can be generated to form novel macrocyclic systems such as calixazulenes. These studies inspired us to develop a “one‐pot” Rothemund‐type synthesis of meso‐tetraphenylazuliporphyrin. Azuliporphyrins, a group of cross‐conjugated carbaporphyrinoids that exhibit intriguing chemistry and metallation properties, have previously only been available by multistep syntheses. In this work, azulene, pyrrole and benzaldehyde were shown to react in a 1:3:4 ratio in the presence of boron trifluoride etherate to give meso‐tetraphenylazuliporphyrin 7 a. The free base shows only a minor diatropic ring current, but addition of TFA generates the related dication which shows greatly enhanced diatropicity where the internal CH shifts from δ=+3.35 to −0.5 ppm. Addition of pyrrolidine to 7 a gave rise to a carbaporphyrin adduct which showed a porphyrin‐like UV/Vis spectrum and the internal CH shifted further upfield to give a resonance near δ=−5.7 ppm. Treatment of 7 a with tert‐butyl hydroperoxide in the presence of potassium hydroxide afforded a mixture of benzocarbaporphyrins 9 a–c. These tetraphenylcarbaporphyrins were fully aromatic by NMR spectroscopy and gave typical porphyrin‐type UV/Vis spectra with a strong Soret band near 446 nm. This new methodology makes these important porphyrin analogues readily available for further study. A remarkable three‐component reaction between azulene, benzaldehyde and pyrrole in the presence boron trifluoride etherate provides a direct route to tetraphenylazuliporphyrin and related meso‐tetraphenylcarbaporphyrins. This class of compounds shows unusual reactivity and commonly give strong absorptions above 700 nm that may lead to applications as photosensitizers.