Tetraaryl Tetradecahydroporphyrazins: Novel Porphyrin Derivatives Featuring a Cyclic Benzene-Ring Tetramer

Treatment of tetramethylsuccinonitrile 1 with aryl lithium compounds and subsequent quenching with chlorotrimethylsilane yields 5-aryl-3,3,4,4-tetramethyl-N-(trimethylsilyl)-3,4-dihydropyrrol-2-imines 2a-c in 49-71% yield. Attempts to crystallise 2a-c in the presence of wet air yielded the tetraaryl tetradecahydroporphyrazins 3a-c in yields of 4-84% as single diastereomers. X-ray diffraction studies of 3b and c showed that only the isomer with four aryl substituents pointing in the same direction was formed. The resulting four-bladed pinwheel-like structures were characterised by four intramolecular aromatic interactions, in which each phenyl ring points with its edge towards the centre of a neighbouring phenyl moiety, resembling the arrangement of benzene molecules in T-shaped dimers. Temperature-dependent NMR spectra give insight into the dynamic properties of the aryl substituents. Quantum chemical calculations that included dispersion corrections indicated the importance of aryl-aryl interactions for the diastereoselectivity of the reaction and for the structural properties of the single isomers observed.