Inverted N‐Confused Porphyrin Dimer

Abstract

Porphyrin (1) is a well-known tetrapyrrolic macrocycle that has four nitrogen atoms arranged inside the core that are able to co-ordinate a variety of metal ions. Normally, the porphyrin core is considered to be planar because of the resonance stabilization resulting from the 18-p electron aromatic system.[1] By contrast, porphyrin analogues display versatile structures, especially in the expanded porphyrin families where an inverted arrangement of the pyrrole rings is frequently observed.[2] The inversion of the tellurophene ring in the heteroporphyrin analogue ditelluraporphyrin (2) was demonstrated by Latos-Grażyń ski and co-workers; [3] however, such ring inversion has been rarely observed and remains unproven for the tetrapyrrolic framework (Scheme 1).Previously, we reported the formation of the N-fused porphyrin (NFP) 4 [4] from an N-confused porphyrin (NCP) 3, [5] which should proceed by ring inversion within the confused pyrrole and bond formation. In addition to this, we suggested the involvement of the inverted conformation of the confused pyrrole ring during the NH tautomerism of NCP.[6] Furthermore, density functional theory (DFT) calculations indicate that the energy difference between the inverted and normal conformations is much smaller than that of porphyrin.[7] The tilted angles were increased by the introduction of substituents at the inner carbon atom of the confused pyrrole. This observation led to the question of what would happen if a larger substituent, such as porphyrin, was introduced into the core. Would it induce pyrrole inversion and stabilize the inverted conformation of the confused pyrrole ring? To examine this possibility, we planned to synthesize an NCP dimer where the two NCP molecules are connected at both of the inner-carbon atoms. The synthetic strategy involved the initial preparation of an NFP dimmer followed by the nucleophilic, bond-cleavage reaction necessary to generate a covalently bonded NCP dimer. Both the structures have been elucidated by X-ray diffraction analysis. The confused pyrrole rings in the NCP dimer keep the inverted conformation and each NCP ring exhibits aromaticity, despite the highly distorted structure.The NFP dimer was synthesized in a similar method to that used for the meso-meso porphyrin dimer, [8] in which a Ag(I)-promoted, oxidative-coupling reaction is employed. Briefly, N-fused tetrakis-(p-tolyl)porphyrin (NFTTP, 4 a) was treated with silver(i) trifluoroacetate in CHCl 3 at reflux for 24 hours to afford the NFP dimer 5, in which the NFP units were covalently bonded to each other at the C2 carbon atoms of the fused rings, in a yield of 61 % (Scheme 2).The structure of 5 was elucidated by X-ray diffraction analysis (Figure 1).[9a] The asymmetric nature of NFP and the axial chirality of dimer 5 results in the unit cell, as determined from the crystallography data, contains an enantiomeric pair of molecules of 5. The bridging bond (1.476(9) ) was formed between C2 and C2'. Each of the NFP units was almost planar and the mean deviations from ...

Aza-deficient porphyrin as a ligand

“…The flipping of the N-confused pyrrolic moiety may be trapped by iridium(I) coordination [39], dimer formation [40] or leads to fused porphyrin 17 [41].…”

A Parallel‐Displaced Directly Linked 21‐Carba‐23‐Thiaporphyrin Dimer Incorporating a Dihydrofulvalene Motif

“…Furthermore the dimer 5 is very sensitive for the presence of even trace amount of acid in solution and easily undergoes splitting to monomeric species.Such atransformation does not have any precedence in chemistry of covalently linked dimers of porphyrinoids. [3][4][5][6] Thus the reaction of 5 with HX (X = Cl or Br,7-10 equiv) was followed by 1 HNMR spectroscopy (Scheme 3). Tw o major products were identified:t he carbathiaporphyrin monocation 4H + and 2,3-dihalo-21-carba-23-thiachlorin monocation 6H + -X, which is highly reactive and readily converted to 2-halo-21-carba-23-thiaporphyrin 7-X by HX elimination in course of column chromatography or by collidine addition.…”

A Parallel‐Displaced Directly Linked 21‐Carba‐23‐Thiaporphyrin Dimer Incorporating a Dihydrofulvalene Motif

“…[3] Thef ield of multimolecular porphyrin systems has been significantly enhanced by construction of ac ovalently linked dimers of N-confused porphyrin involving ar eactive carbon atom of coordination core (1, 2)( Scheme 1). [4,5] Thef ace-toface dimer of benzonorrole with ad irect double bond between the internal carbon atoms (3)d emonstrating strong p-p interaction between two p planes was reported. [6] Originally the inclusion of b-substituted cyclopentadiene moiety into ap orphyrin frame yielding an aromatic carbaporphyrin with b-substitution at pyrrole rings was achieved by Berlin.…”

“…[3][4][5][6] Thus the reaction of 5 with HX (X = Cl or Br,7-10 equiv) was followed by 1 HNMR spectroscopy (Scheme 3). Furthermore the dimer 5 is very sensitive for the presence of even trace amount of acid in solution and easily undergoes splitting to monomeric species.Such atransformation does not have any precedence in chemistry of covalently linked dimers of porphyrinoids.…”