Parent, Unsubstituted Hemiporphycene: Synthesis and Properties

Abstract

Among seven possible nitrogen-in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene (HPc), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner-cavity dimensions, orbital-energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH-stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol . Tautomerization from the higher- to the lower-energy form is detected in the lowest-excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H-bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N⋅⋅⋅N distance and the N-H-N angle.

Two Macrocycles in One Shot: Synthesis, Spectroscopy, Photophysics, and Tautomerism of 23‐Oxahemiporphycene and 21‐Oxacorrole‐5‐carbaldehyde

“…We recently reported the synthesis of parenth emiporphycene. [21] The synthesis of 23-oxahemiporhycene 10,a ccompanied by formation of 21-oxacorrole-5-carbaldehyde 11, (Scheme 1) utilizes as imilars trategy;n amely,f ormationo fa linear a,w-dialdehyde 8 containing three pyrrole and one furan subunits,a nd its reductive cyclization under McMurryr eaction conditions, followed by oxidation to the aromatic forms 10 and 11. a,w-Dialdehyde 8 was obtained by alkylation of 5-(pyrrol-2-yl)furan-2-carbaldehyde 5 by 5-formyl-5'-hydroxymethyl-N,N'-di-tert-butoxycarbonyl-2,2'-bipyrrylmethane (6), giving N,N'-di-tert-butoxycarbonyl-a,w-dialdehyde 7,a nd by furtherN Boc deprotection by thermolysis. The alkylation step with BF 3 ·Et 2 Ow as carried out in 10 %t etrahydrofuran in dichloromethane.…”

“…This value lies betweent he frequencies of two NH stretches observed in parenth emiporphycene, 3040 and 3140 cm À1 . [21] The calculated NH stretching frequencies indicatet hat the strongesti ntramolecular hydrogen bond is formed in thec ase of the 22-H tautomer,w hereas the weakesto ne may be found in the tautomer 24-H. Lack of correlation betweent automer energies and NH stretching frequencies may suggest that intramolecular H-bond energy is not the key factor governing the relative stability of the tautomers.…”

“…We plan to extend our studies by the synthesis of three additional monooxahemiporphycenes. This would provide a unique set of compounds for studies of intramolecular hydrogen bonds and tautomerism.I tw as postulated [21] that the usu- ally assumed correlation between the hydrogen-bond strength and the NÀH···N distance mayn ot hold in parent hemiporphycene, because of strongd eviations from H-bond linearity.Studies of oxahemiporphycenes should be helpful in testing this hypothesis.…”

“…This reaction provides au seful tool for obtaining macroheterocycles containingt wo-carbon bridges. It has been successfully appliedt ot he synthesis of porphyrin isomers: porphycene, [11] hemiporphycene, [12,21] and corrphycene, [15] as well as to the contracted [22] and expanded porphyrins, [23] including those containing heteroatoms other than nitrogen. [24][25][26][27][28] Formation of products rearranged under McMurry conditions, containing one-carbon bridged motifsi sp recedential,a ccording to the literature reports; [29][30][31] however, it was observed only in the case of intermolecular condensations.…”

“…The observeds mall redshift of the absorption and emission spectra with respect to hemiporphycene, as well as the small Stokes shift (25 cm À1 )a re comparable to those reported for porphyrin heteroanalogues. [1] Parent hemiporphycene, [21] as well as its octaethyl derivative [12,42] exist in the form of two interconverting trans tautomers, which are easily detectable by electronic absorption and fluorescencem easurements in nonpolars olvents. Photophysical and spectroscopiccharacterization of 10 and 11 wascarried out in polar (acetonitrile) and nonpolar (toluene)s olvents.…”