Further Studies on the Synthesis of meso-Tetraarylazuliporphyrins under Lindsey−Rothemund Reaction Conditions and Their Conversion into Benzocarbaporphyrins

Azulene has been shown to react with pyrrole and a series of aromatic aldehydes in the presence of boron trifluoride etherate to give meso‐tetraarylazuliporphyrins 6. Good yields of azuliporphyrins were obtained for benzaldehyde, 4‐chlorobenzaldehyde, 4‐bromobenzaldehyde, and 4‐iodobenzaldehyde, and under dilute conditions p‐tolualdehyde gave respectable yields. In each case, substantial amounts of meso‐tetraarylporphyrins were also formed and a minor fraction of carbaporphyrin by‐products could be detected, but otherwise no other macrocyclic products could be identified. 4‐Nitrobenzaldehyde gave relatively poor yields of the corresponding azuliporphyrin, while p‐anisaldehyde only gave trace amounts of product. Pentafluorobenzaldehyde gave variable results, although in this case a large number of additional by‐products were identified including N‐fused pentaphyrin, hexaphyrin, and higher order porphyrinoids, but no expanded azulene‐containing macrocycles could be detected. Azuliporphyrins undergo reversible nucleophilic substitution on the seven‐membered ring with pyrrolidine, benzenethiol, hydrazine, or benzylamine to give carbaporphyrin adducts. This property appears to facilitate an oxidative ring contraction of azuliporphyrins 6 with tert‐butyl hydroperoxide in the presence of potassium hydroxide to produce mixtures of benzocarbaporphyrins 19 and 20. Tetraaryl‐benzocarbaporphyrins exhibit slightly reduced diatropic ring currents compared to their meso‐unsubstituted counterparts, although their UV/Vis spectra are very porphyrin‐like and exhibit strong Soret bands near 450 nm. The benzocarbaporphyrins undergo reversible protonation to give monocationic and dicationic species. The latter involves C‐protonation to generate an internal CH2 within the macrocyclic cavity. X‐ray crystallography of tetraphenylbenzocarbaporphyrin 19a confirms that the preferred tautomer has the two NHs on either side of the indene subunit, in agreement with previous theoretical and spectroscopic studies. In addition, the presence of phenyl substituents at the 5,20‐positions was found to tilt the indene moiety substantially by 27.4(1)° relative to the [18]annulene substructure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)