Synthesis and Reactivity of 23‐tert‐Butyl‐ and 23‐Phenyltetraarylazuliporphyrins: an Analysis of the Effect of Bulky Substituents on Oxidative Ring Contractions to Benzocarbaporphyrins

Abstract

Abstract6‐tert‐Butyl‐ and 6‐phenylazulene reacted with pyrrole and benzaldehyde in a molar ratio of 1:3:4 in the presence of BF3·Et2O in chloroform, followed by oxidation with DDQ, to give 23‐substituted tetraphenylazuliporphyrins in 15–20 % yield. Slightly higher yields of the related meso‐tetrakis(4‐chlorophenyl)azuliporphyrins were obtained using 4‐chlorobenzaldehyde. The presence of an electron‐donating tert‐butyl substituent increased the diatropic character of the azuliporphyrin system as determined by the proton NMR chemical shifts for the internal CH resonance, while intermediary results were noted for 23‐phenylazuliporphyrins. Addition of TFA afforded dications with increased aromatic ring currents, but electron‐donating substituents (tBu > Ph) again produced a larger upfield shift for the internal CH signal due to stabilization of the tropylium character that is required so that the system can attain carbaporphyrin‐type aromaticity. The substituted azuliporphyrins reacted with nickel(II) acetate or palladium(II) acetate to give the corresponding organometallic derivatives. In addition, oxidations with tBuOOH and KOH afforded benzocarbaporphyrin products in approximately 50 % yield. The presence of tert‐butyl or phenyl substituents did not block these oxidative ring contraction processes, and the rate of reaction was slightly increased compared to 23‐unsubstituted azuliporphyrins. The major products were 22‐tert‐butyl or phenyl‐substituted benzocarbaporphyrins and minor products with an additional formyl substituent were also isolated. These products are consistent with an initial nucleophilic addition occurring at the position adjacent to the R group on the azulene ring. Detailed mechanisms are proposed to explain these observations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)