Rearrangement of N-substituted porphyrins. Preparation and structure of homoporphyrins

Abstract

The preparation of "homoporphyrin" derivatives-porphyrins possessing a two-carbon meso bridge-is reported. These new tetrapyrrolic macrocycles were obtained from N-CHzCOzEt-porphyrins via nickel(I1) salt catalyzed rearrangement, The reaction involves formation of intermediate nickel(I1) cationic complex and cyclization to an aziridine fused to a pyrrole, followed by migration of a C-C bond to the meso position and electrocyclic ring-opening of the resultant cyclopropanic intermediate. The stereospecificity of this rearrangement is high. These homoporphyrins derivatives show a ring inversion at 1 IO0 with a barrier of ca. 30 kcal mol-'.The aromaticity of the porphin nucleus 1 has been well established both by its chemical and physical propertie~.l-~ The macrocyclic conjugated system can thus be considered in some ways as homologous to benzene, as are [4n + 21 ann~l e n e s .~The idea of comparing porphyrins lacking a bridge carbon, namely corroles (derivatives of 2), with cyclopentadiene has been well illustrated by the ease of formation of anions from corroles or metallo~orroles.'.~~~ In order to complete the comparison, it only remained to prepare and study the chemical behavior of the "homoporphyrin" system 3. This new tetrapyrrolic macrocycle should display 1 2 3 chemical properties related, to a certain extent, to those of cycloheptatriene and the corresponding heterocyclic systems, as azepine. At the time we obtained our results on homoporphyrins chemistry, the only known compounds possessing the required skeleton was the azahomoporphyrin 4 prepared by G~-i g g .~ Although well characterized, Grigg's compound was found to be unstable, undergoing ring contraction to porphyrin 5 either upon heating (M = H2) or treatment with a divalent cation (M = Zn, Cu). This reaction is similar to the ring contraction of N-carbethoxyazepine. 8 We found that the nickel(I1) promoted rearrangement of N-substituted porphyrins led to homoporphyrin derivatives in good yield, and therefore we undertook a study of the mechanistic details of that new reaction and of the thermal behavior of homop~rphyrins.~ Callot. ResultsSynthesis of Homoporphyrins 7 and 8. The substituted porphyrin 6 was prepared according to the literature procedureI0 and treated with a mixture of Ni(I1) bis (acety1)acetonate and Ni(I1) carbonate in 1,2-dichloroethane. The resultant mixture consisted of three major compounds: homoporphyrins 7 (57%) and 8 (4%) in addition to the known tetraphenylporphinatonickel(I1) (NiTPP) (8%). Compounds 7 and 8 were very similar (infrared, mass spectra, HFfi N HN ChHi CfiK 6 7, R = H, R -CO,C,H, (endo) 8, R = CO?C,H,; R = H (exo)visible spectra) except for the 'H N M R and chromatographic data. Visible spectra of both isomers were compared with those of unsaturated tetrapyrrolic derivatives whose conjugation has been interrupted, e.g., azahomoporp h y r i n~,~ bilatrienes,' I and phlorins.'* The presence of two major absorption bands at ca. 450 and 680 nm could be related to a certain amount of deformation of the macrocycle.I3...

Metamorphosis of Tetrapyrrole Macrocycles

“…A classic example is the Ni II -catalyzed rearrangement of 18 (Scheme 5) involving first the formation of a Ni II cationic complex, then cyclization to an aziridine, CÀC bond migration to a meso position, and subsequent electrocyclic ring opening of the cyclopropanoic intermediate. [16] Conceptionally, a similar case is the transformation of corroles into hemiporphycenes (formally homocorroles), which was reported here last year by Paolesse et al [17] Reaction of 5,10,15-triphenylcorrole 20 with diiodocarbene gave the corresponding 5-iodo-10,15,20-triphenylporphyrin in traces and larger amounts of the hemiporphycene 21 (Scheme 5) through a ring-expansion reaction. Such a one-step procedure is more facile than the original total syntheses of hemiporphycenes.…”

“…[18] Hemiporphycenes are also accessible by ring contraction of homoporphyrins. [19] Likewise, a brominated porphycene has been shown to undergo a thermal ring contraction to isocorrole [porphyrin-(2.0.1.0)] in basic medium. [20] As corroles can undergo ring expansion to porphyrins, porphyrins can undergo ring contraction to corroles.…”

Isoporphycene: The Fourth Constitutional Isomer of Porphyrin with an N4 Core—Occurrence ofE/Z Isomerism

“…[3] If a further incentive for the synthesis of the isomers was needed, this was provided by the inverted porphyrins discovered simultaneously by a Japanese and a Polish group. [4] Porphycene (1), which is approximately 2 kcal mol À1 more stable than porphyrin according to ab initio calculations (BLYP/6-31G** method), has meanwhile been joined by hemiporphycene (2) [6] and corrphycene (3; each as octaalkyl derivatives).…”

Bridging the Gap between Porphyrins and Porphycenes: Substituent‐Position‐Sensitive Tautomerism and Photophysics in meso‐Diphenyloctaethylporphyrins

“…[5][6][7][8] The first constitutional isomer, porphycene (Pc, Scheme 1) was synthesized in 1986. [9] Four more isomers have since been obtained: hemiporphycene, [10,11] corrphycene, [12,13] isoporphycene, [14,15] and "inverted" ("confused") porphyrin. [16,17] Thus far, research has been dominated by studies on porphycenes.…”