Pyrrole-Inverted Isomer of 5,10,15,20-Tetraaryl-21-Selenaporphyrin^†

A novel 5,10,15,20-tetraaryl-21-selenaporphyrin isomer with an inverted pyrrole ring, i.e., 5,10,15,20-tetraaryl-2-aza-21-carba-22-selenaporphyrin (SeC-TArPH) has been produced by a [3 + 1] condensation of 2,5-bis(phenylhydroxymethyl)selenophene and 5,10-ditolyltripyrrin. The reaction yielded 5,20-diphenyl-10,15-bis(p-tolyl)-21-selenaporphyrin Se-DPDTPH (19%) and its isomer with an inverted pyrrole ring, i.e., 5,10-diphenyl-15,20-bis(p-tolyl)-2-aza-21-carba-22-selenaporphyrin, SeC-DPDTPH (1%). Mechanistically the synthesis of SeC-DPDTPH requires one β-condensation at the pyrrole moiety of 5,10-ditolyltripyrrin instead of the stereotypical α-condensation. The identity of inverted selenaporphyrin has been confirmed by high-resolution mass spectrometry and ¹H NMR spectroscopy. A saddle distortion mode for the inverted selenaporphyrin macrocycle SeC-DPDTPH has been determined by X-ray crystallography. NMR spectra are consistent with the existence of tautomeric equilibria that involve three tautomeric species of the neutral form of SeC-DPDTPH. The preference for the tautomer with the labile proton located at the peripheral N(2) nitrogen atom has been detected in pyridine-d₅ solution. The density functional theory (DFT) has been applied to determine the molecular and electronic structure of three tautomers of 2-aza-21-carba-22-selenaporphyrin:  {2-N, 23-N, 24-NH}, {2-N, 23-NH, 24-N}, and {2-NH, 23-N, 24-N} formally created from SeC-DPDTPH by a replacement of phenyl and tolyl groups with hydrogen. The total energies calculated using the B3LYP/6-311G//B3LYP/6-311G approach, demonstrate that relative stability of postulated tautomers decreases in the order {2-N, 23-NH, 24-N} > {2-N, 23-N, 24-NH} > {2-NH, 23-N, 24-N}. The small energy differences between tautomeric species suggests their simultaneous presence in equilibrium.