A Pyrrolophanediene‐Porphycene Redox System

Abstract

DDQ) it can also be converted into the corresponding tetraketone (ceric ammonium nitrate), into the monosulfone (peracids) as well as into a still unidentified red ionic species (electrochemically). By way of contrast, tctraoxaporphyrinogen 6 has thus far always afforded the dication 1 with all reagents which effect oxidation with retention of the ring skeleton. The electrochemical oxidation of 6 also gave 1 (private communication from Prof. E. Sfeckhan, Universitlt Bonn (FRG)).[I41 C,,H,,CI,O,S,, decomp. at 240 "C, crystals from HCIO,; triclinic, space group P i , 2 = 1; a = 5.850(1), 6 = 8.994(1), c = 10.547(2) A, u = 78.90(2), p = 81.03(2), y = 82.19(2)"; e..,.,, = 1.800 measuring conditions as for 7; refinement (C,CI, 0,s anisotropic, H isotropic) taking into account 1791 reflections with F, > 4u(F,); R = 0.045, R , = 0.053 1201.[15] In the case of 2 a there are no indications of bonding between sulfur atoms that lie opposite to each other in the porphyrin core-as has been assumed and is theoretically SubStanlidled in the case of tetraphenyl-21,23-dithiaporphyrin on the basis of the extremely short S . . . S distances of 3.05 8, [4c, d] Isophlorin 1, a still elusive dihydroporphyrin first referred to by Woodwardsome 30 years ago in connection with the synthesis of chlorophyll,"' is a molecule equally interesting from the standpoint of porphyrin and annulene chemistry. It seems justified to regard 1 as a "cyclobutadiene homologue" since it constitutes an essentially planar [20]annulene associated with a degenerate, presumably very fast x-bond shift. As a consequence of its x-electron structure, 1 is expected to exhibit a pronounced tendency to undergo a two-electron oxidation with formation of resonance-stabilized diprotonated porphyrin, and hence to be a dihydroporphyrin of only fleeting existence.The anticipated ready oxidizability of 1 would explain why N,N-dihydroporphycene 2, the analogue of 1 in the detection. Other than in the case of 1, there is considerable steric hindrance of the NH-hydrogen atoms in 2. For this reason 2 is in all probability not planar but has the pyrrolophanediene structure 2 a.f3, 41 We have now found that N,N'-dihydroporphycenes are in certain cases isolable, are in fact pyrrolophanedienes, and, what was unforeseen, form redox systems with the corresponding porphycenes.Examination of the stereochemistry of porphycene 3 reveals that peripheral alkyl groups, particularly in the 9, 10, 19, 20 as well as in the 3, 6, 13, 16 positions are capable of modulating the geometry of the tetrapyrrolic macrocycle relatively strongly through non-bonding interactions. A representative example of this is the 9,10,19,20-tetrapropylporphycene 6, which is obtainable analogously to the synthesis of 3 by reductive coupling of the bipyrrole derivative 4 with titanium tetrachloride and zinc.f5] According to an X-ray structure analysis, 6, like 3, has a planar ring skeleton, but due to the repulsion of the propyl groups there is a convergence of the oppositely disposed nitrogen atoms, so that the N, cor...

Porphycenes: synthesis and derivatives

“…As a consequence, the metal cation coordination properties of this tetrapyrrolic ligand can be modified by changing the substituents on the periphery of the macrocycle. 12 With a view to mapping out the effect of such modifications on the complexation properties of porphycene, derivatives such as 56 and 57 were prepared that bear different substituents in positions 9, 10, 19 and 20 (meso positions), [52][53][54] as well as positions 3, 6, 13 and 16 (b-pyrrolic positions); the latter species bear a structural resemblance to the etioporphyrins and are referred to generically as etioporphycenes (Fig. 4).…”

Tetracyclohexyloxaquaterene

“…Inspection of the appropriate bond lengths and angles in the crystallographically unique part of the molecule shows that they are typical. The C3--C4 bond length is greater than that of C2--C3, which is a characteristic feature of non-annulene forms of such compounds; the same is observed for the thiophene (Vogel et al, 1989) and pyrrole derivatives (Dolphin, 1978).…”

Air-Stable N,N′-Dihydroporphycene: A Quinoxaline-Fused Tetrapyrrolic Macrocycle That Detects Fluoride Anion via Deprotonation

“…N , N ′-Dihydroporphycene (Figure ) is the cannonical two-electron reduced form of porphycene. N , N ′-Dihydroporphycenes resemble isophlorins, their analogues in the porphyrin series, and are typically unstable being oxidized spontaneously to porphycene upon exposure to air . Generally, N , N ′-dihydroporphycenes serve as nonisolated, direct precursors to porphycenes in the classic McMurry coupling-based synthesis introduced by Vogel .…”

“…N,N′-Dihydroporphycenes resemble isophlorins, their analogues in the porphyrin series, and are typically unstable being oxidized spontaneously to porphycene upon exposure to air. 8 Generally, N,N′dihydroporphycenes serve as nonisolated, direct precursors to porphycenes in the classic McMurry coupling-based synthesis introduced by Vogel. 2 The lack of stability associated with these intermediates is readily understandable, not only because the corresponding porphycenes are energetically favored aromatic species, but because oxidation will reduce the repulsion among the inner hydrogen atoms in the reduced form.…”

“…Vogel and co-workers employed catalytic hydrogenation to produce a meso-substituted N,N′-dihydroporphycene 2 and characterized the resulting structure by means of NMR and UV−vis absorption spectroscopy, as well as via single-crystal X-ray structural analysis (Figure 2). 8 Hayashi et al also reported a N,N′-dihydroporphycene 3 bearing CF 3 groups at the β-pyrrolic positions. 9 In the presence of an oxidant (air or 2,3-dichloro-5,6-dicyanoquinone (DDQ)), these N,N′-dihydroporphycenes were found to convert to the corresponding porphycenes.…”