N‐Fused Pentaphyrins and Their Rhodium Complexes: Oxidation‐Induced Rhodium Rearrangement

Abstract

meso-Aryl-substituted pentaphyrins were isolated in the modified Rothemund-Lindsey porphyrin synthesis as a 22-pi-electron N-fused pentaphyrin ([22]NFP5) and a 24-pi-electron N-fused pentaphyrin ([24]NFP5), which were reversibly interconvertible by means of two-electron reduction with NaBH4 or two-electron oxidation with dichlorodicyanobenzoquinone (DDQ). Judging from 1H NMR data, [22]NFP5 is aromatic and possesses a diatropic ring current, while [24]NFP5 exhibits partial anti-aromatic character. Metalation of [22]NFP5 1 with a rhodium(I) salt led to isolation of rhodium complexes 9 and 10, whose structures were unambiguously characterized by X-ray diffraction analyses and were assigned as conjugated 24-pi and 22-pi electronic systems, respectively. In the rhodium(I) metalation of 1, the complex 9 was a major product at 20 degrees C, but the complex 10 became preferential at 55 degrees C. Upon treatment with DDQ, compound 9 was converted to 10 with an unprecedented rearrangement of the rhodium atom.

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

“…Although some of these species display moderate paratropicity consistent with their [24]annulenoid structures, in others marked diatropicity is observed. While this important observation was made in the initial report, [210] its significance was only appreciated in a later reinvestigation of published data.…”

“…[208,209] In contrast, attempts to prepare a fully meso-substituted pentaphyrin resulted in the isolation of N-fused systems 27def-H and 28def-H 3 . [120,162,210,211] The generality of this reaction was shown in subsequent work on Nconfused pentaphyrins, which yielded a number of singly and doubly fused macrocycles. [164,212] 27def-H and 28def-H 3 constitute two oxidation levels of N-fused pentaphyrin, corresponding to 22-and 24-electron CPs, which are interconvertible by chemical oxidation and reduction (Scheme 14).…”

“…[164,212] 27def-H and 28def-H 3 constitute two oxidation levels of N-fused pentaphyrin, corresponding to 22-and 24-electron CPs, which are interconvertible by chemical oxidation and reduction (Scheme 14). [162,210] Interestingly, N-fused pentaphyrin forms two dicarbonyl rhodium(I) complexes, 27def-Rh(CO) 2 and 28def- H 2 Rh(CO) 2 , which differ not only in the oxidation level of the macrocycle but also in the binding mode of the Rh center (Scheme 14). [210] 27def-H and 27def-Rh(CO) 2 , possess 22-electron conjugation pathways and exhibit sizable diatropic ring currents characteristic of aromatic systems.…”

“…[162,210] Interestingly, N-fused pentaphyrin forms two dicarbonyl rhodium(I) complexes, 27def-Rh(CO) 2 and 28def- H 2 Rh(CO) 2 , which differ not only in the oxidation level of the macrocycle but also in the binding mode of the Rh center (Scheme 14). [210] 27def-H and 27def-Rh(CO) 2 , possess 22-electron conjugation pathways and exhibit sizable diatropic ring currents characteristic of aromatic systems. The behavior of the 24-electron systems 28def-H 3 and 28def-H 2 Rh(CO) 2 is more complicated.…”

Figure‐Eight‐Strukturen, Möbius‐Bänder und mehr: Konformation und Aromatizität von Porphyrinoiden

“…[208,209] Beim Versuch, ein vollständig meso-substituiertes Pentaphyrin herzustellen, erhielt man lediglich die N-anellierten Systeme 27def-H und 28def-H 3 . [120,162,210,211] Diese Reaktion ließ sich verallgemeinern, denn später wurden auch N-invertierte Pentaphyrine zu einer Reihe von einfach und doppelt anellierten Makrocyclen umgesetzt. [164,212] 27def-H und 28def-H 3 sind zwei Oxidationsstufen des gleichen N-anellierten Pentaphyrins.…”