Aromatic pathways in mono- and bisphosphorous singly Möbius twisted [28] and [30]hexaphyrins

Magnetically induced current densities and strengths of currents passing through selected bonds have been calculated for monophosphorous [28]hexaphyrin ((PO)[28]hp) and for bisphosphorous [30]hexaphyrin ((PO)(2)[30]hp) at the density functional theory level using our gauge-including magnetically induced current (GIMIC) approach. The current-density calculations yield quantitative information about electron-delocalization pathways and aromatic properties of singly M?bius twisted hexaphyrins. The calculations confirm that (PO)[28]hp sustains a strong diatropic ring current (susceptibility) of 15 nA T(-1) and can be considered aromatic, whereas (PO)(2)[30]hp is antiaromatic as it sustains a paratropic ring current of -10 nA T(-1). Numerical integration of the current density passing through selected bonds shows that the current is generally split at the pyrroles into an outer and an inner pathway. For the pyrrole with the NH moiety pointing outwards, the diatropic ring current of (PO)[28]hp takes the outer route across the NH unit, whereas for (PO)(2)[30]hp, the paratropic ring current passes through the inner C(β)=C(β) double bond. The main diatropic ring current of (PO)[28]hp generally prefers the outer routes at the pyrroles, whereas the paratropic ring current of (PO)(2)[30]hp prefers the inner ones. In some cases, the ring current is rather equally split along the two pathways at the pyrroles. The calculated ring-current pathways do not agree with those deduced from measured (1)H NMR chemical shifts.     

The induced current strengths and aromatic pathways of heteroporphyrins and their antiaromatic derivatives

The magnetically induced current strengths as well as nucleus independent chemical shifts of aromatic heteroporphyrins and antiaromatic 22,24‐dideazaheteroporphyrins have been studied using the density functional theory method. The induced current strengths and pathways are obtained by numerical integration of the induced current densities following the specific chemical bonds. The total induced current strengths of antiaromatic 22,24‐dideazaheteroporphyrins is about 6 nA/T weaker than the one for the heteroporphyrins in absolute value. The substitution of pyrrole NH groups by O and S atoms does not change the total induced current strengths. The induced currents around the molecular macroring split at the heterocycles (pyrrole, furan, and thiophene) into the inner and outer routes. The heteroatoms (N, O, and S) have high resistance and consequently lead to a weaker induced current strength than the one passing the outer route in aromatic heteroporphyrins. For antiaromatic 22,24‐dideazaheteroporphyrins, the heteroatoms enhance the current strength and change the main current pathway into the inner route. The induced current strength following the NH moiety is stronger than the one passing the oxygen moiety of furan ring and the sulfur moiety of the thiophene ring in both heteroporphyrins and 22,24‐dideazaheteroporphyrins. © 2015 Wiley Periodicals, Inc.     

Tetrakisdehydro[16]annuleno[18]annulene

Tetrakisdehydro[16]annuleno consisting of trisdehydro[16]annulene and bisdehydro[18] annulene has been synthesized. Induction of para- and diamagnetic ring currents in 16- and 18-membered rings, respectively, was clearly recognized by the ¹H NMR spectroscopy. A marked suppresion of the diatropicity in the 18π moiety was observed in the same trend as observed in tetrakisdehydro[14]annuleno[16]annulene.     

The aromatic character of magnesium porphyrins

The aromaticity of magnesium porphyrins have been studied by calculating the nuclear magnetic shieldings in selected points outside the molecules. The strength of the induced ring currents for a given magnetic field have been obtained by using the aromatic-ring-current-shielding (ARCS) method. Nucleus-independent chemical shift (NICS) calculations provide additional information about the current routes in the multiple-ring systems. The total aromatic pathway of magnesium porphyrins must be considered as a superposition of several (4n + 2) pi-electron Huckel pathways. We found that all beta-unsaturated pyrrolic rings have local ring currents the strength of which is 70-90% of the current strength for the pyrrole molecule. The present study also shows that the 18pi-[16]annulene aromatic pathway does not exist in magnesium porphyrins until all four pyrrolic units are saturated in the beta-position.     

Synthesis, spectroscopic, and in vitro photosensitizing efficacy of ketobacteriochlorins derived from ring-B and ring-D reduced chlorins via pinacol-pinacolone rearrangement

In this report, we present a regioselective oxidation of a series bacteriochlorins, which on reacting with either ferric chloride (FeCl(3)) or 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) yielded the corresponding ring-B or ring-D reduced chlorins. The effect of the number of electron-withdrawing groups present at the peripheral position, with or without a fused isocyclic ring (ring-E), did not make any significant difference in regioselective oxidation of the pyrrole rings. However, depending on the nature of substituents, the intermediate bis-dihydroxy bacteriochlorins on subjecting to pinacol-pinacolone reaction conditions gave various ketochlorins. The introduction of the keto-group at a particular position in the molecule possibly depends on the stability of the intermediate carbocation species. The newly synthesized bacteriochlorins show strong long-wavelength absorption and produced significant in vitro (Colon26 cells) photosensitizing ability. Among the compounds tested, the bacteriochlorins containing a keto-group at position 7 of ring-B with cleaved five-member isocyclic ring showed the best efficacy.     

Annulenoannulenes

Annulenoannulenes formally consist of two annulenes fused together. Hitherto, almost all the known representatives of this class of compounds were the strongly dehydrogenated forms, containing a few triple bonds and several cumulated double bonds. Besides ortho-annelated dehydroannulenoannulenes also such compounds containing bridges made up of two or four carbon atoms (“acetylene-cumulene” annulenoannulenes) were obtained by multistep syntheses. Most dehydroannulenoannulenes are stable and form colored crystals. In addition to dehydro[4n+2]annuleno[4n+2] annulenes with 14-, 18-, and 22-membered individual rings, two dehydro[4n]annuleno[4n+2]annulenes with 16- and 14-, and 16- and 18-membered rings have also been synthesized. Planar combinations of two [4n]-rings of this type and size are not known.—The dehydrogentated[14]annuleno[14]annulene containing a bridge of two carbon atoms has been studied in detail; however, the problem of the induction of a diamagnetic ring current in compounds of this type has still not been completely clarified.     

Beiträge zur Chemie der Pyrrolpigmente, 2. Mitt.: Die Röntgenphotoelektronenspektren einiger Pyrrolpigmente

The charge densities on the nitrogen atoms of symmetrically substituted dipyrromethenes, porphins, chlorins and bacteriochlorins were determined by X-ray photoelectron spectroscopy of the N_1s-levels. Calculation by means of a π-electron- and an all-valence-electron-method (SCF-LCAO-MO-PPP and CNDO/2) reveals good correlation of both methods with experiment. Both are comparable in predicting the energy of the highest occupied orbitals which were studied by measuring the valenceband regions.     

Diepoxy[18]annulene(10.0) : (Z,E,Z,E,Z)‐Diepoxy[18]annulen(10.0) – ein hochdynamisches Annulen

Diepoxy[18]annulenes(10.0): ( Z , E , Z , E , Z )‐Diepoxy[18]annulene(10.0) – a Highly Dynamic Annulene The McMurry reaction of (all‐E)‐5,5′‐([2,2′‐bifuran]‐5,5′‐diyl)bis[penta‐2,4‐dienal] ( 13 ) only occurs intramolecularly to give a mixture of the diepoxy[18]annulenes(10.0) 6 and 7 . Tetraepoxy[36]annulene(10.0.10.0) resulting from an intermolecular McMurry reaction is not formed. According to spectroscopic data, 6 is (Z,E,Z,E,Z)‐ and 7 (Z,E,E,Z,E)‐configured. The ¹H‐NMR data confirm that in 6 the (E)‐ethene‐1,2‐diyl bonds (C(11)=C(12) and C(15)=C(16)) rotate around the adjacent σ‐bonds. Beginning at −70°, this rotation freezes, and 6 is becoming a diatropic aromatic ring system. Beside [18]annulene itself, (Z,E,Z,E,Z)‐diepoxy[18]annulene(10.0) 6 is the only hitherto known [18]annulene derivative with dynamic properties.     

Visualising aromaticity of bowl-shaped molecules

Superposition of slices (planar maps of induced current density calculated within the ipsocentric pseudo-π model of electronic response to a magnetic field) gives a simple route to visual diagnosis of ring-current aromaticity in bowl-shaped molecules. Results are presented for currents in the recently synthesized indenocorannulene precursors of [60]fullerene.     

Tetrakisdehydro[18]annuleno[20]annulene

Tetrakisdehydro[18]annuleno[20]annulene has been synthesized. The ¹H NMR spectra clearly indicate the induction of dia- and paramagnetic ring currents in 18- and 20-rings, respectively. A marked suppression of the diatropicity in the 18π moiety was observed being in the same trend in other tetrakisdehydro[4n]annuleno[4n′+2]annulenes. The ¹H NMR spectroscopic behavior of the annulenoannulenes is consistent with theoretical conclusion.     

Polycyclic antiaromatic hydrocarbons

Magnetically induced current densities have been calculated for a series of hydrocarbons consisting of hexadehydro[12]annulene rings alternatingly fused with benzenes. The calculations show that all molecular rings of the studied molecules sustain paramagnetic ring currents. The new class of molecules is therefore coined polycyclic antiaromatic hydrocarbons (PAAH).     

Palladium vacataporphyrin reveals conformational rearrangements involving Hückel and Möbius macrocyclic topologies

5,10,15,20-Tetraaryl-21-vacataporphyrin (butadieneporphyrin, an annulene-porphyrin hybrid) which contains a vacant space instead of heteroatomic bridge acts as a ligand toward palladium(II). The metal ion of square-planar coordination geometry is firmly held via three pyrrolic nitrogen atoms where the fourth coordination place is occupied by a monodentate ligand or by an annulene part of vacataporphyrin. The macrocycle reveals the unique structural flexibility triggered by coordination of palladium. The structural rearrangements engage the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment which serves as a linker between two pyrrolic rings of vacataporphyrin albeit the significant ruffling of the tripyrrolic block is also of importance. Two fundamental modes of interactions between the palladium ion and annulene moiety have been recognized. The first one resembles an eta(2)-type interaction and involves the C(2)C(3) unit of the butadiene part. Alternatively the profound conformational adjustments allowed an in-plane coordination through the deprotonated trigonally hybridized C(2) center of butadiene. The coordinated vacataporphyrin acquires Hückel or extremely rare M?bius topologies readily reflected by spectroscopic properties. The palladium vacataporphyrin complexes reveal Hückel aromaticity or M?bius antiaromaticity of [18]annulene applying the butadiene fragment of vacataporphyrin as a topology selector. The properties of specific conformers were determined using (1)H NMR and density functional theory calculations.     

Studies towards the total synthesis of di- and sesterterpenes with dicyclopenta[a,d]cyclooctane skeletons. Three-component approach to the A/B rings building block

Sesqui- and sesterterpenes of ophiobolin and fusicoccin families are important synthetic targets because of complexity of structure and potentially useful physiological activities, including anti-tumor activity. A synthesis of versatile building blocks for these terpenoids is described. Cyclopenta[8]annulene rings system with properly dislocated substituents was constructed using as key steps ring closing metathesis reaction and Wagner - Meerwein rearrangement. Ring closing metathesis reaction leading to cyclopenta[8]annulene was studied in detail.     

Aromatic and antiaromatic gold(III) hexaphyrins with multiple gold-carbon bonds

Au(III) metalation of hexakis(pentafluorophenyl) [26]hexaphyrin led to formation of aromatic mono-Au(III) hexaphyrin and bis-Au(III) hexaphyrin, in which the inner pyrrolic beta-protons are activated to form gold-carbon bonds, hence accommodating Au(III) ion with a NNCC core in a square planar manner. Two-electron reductions of these complexes with NaBH4 provided the corresponding [28]hexaphyrin complexes which exhibit distinct paratropic ring currents.     

Synthesis and Antiaromatic Properties of Highly Planar Dithiaamethyrin

2,5‐Bis(4‐propyl‐2‐pyrrolyl)thiophene was reacted with aryl aldehydes to afford large porphyrinoids; [24]dithiaamethyrin(1.0.0.1.0.0) and [36]trithianonaphyrin(1.0.0.1.0.0.1.0.0). X‐ray crystallography of the dithiaamethyrin revealed a highly planar ring structure with mean plane deviation (MPD) value of 0.053 Å. A large positive NICS(0) value (+13.9 ppm) calculated for this planar 24π system unambiguously indicates an antiaromatic character that is consistent with a remarkably low field ¹H chemical shift of the inner NH proton at 24.0 ppm. The magnitude of the paratropic ring current effect in the 24π amethyrin framework in the free base form and the diprotonated form was discussed on the basis of the MPD value and the NICS value. The present dithiaamethyrin is much more strongly antiaromatic than the dodecaalkylamethyrin in their free base forms, but the order inversed in their diprotonated forms.     

The first example of Diels-Alder cycloaddition of ortho-xylylenes to meso-tetraarylporphyrins containing electron-deficient β,β-double bonds

β-Nitro-5,10,15,20-tetraphenylporphyrin and its zinc complex, or 2,7-dinitro-5,10,15,20-tetraphenylporphyrin, react with 1,3-dihydrobenzo[c]thiophene 2,2-dioxide in refluxing 1,2,4-trichlorobenzene, giving rise to chlorins, bacteriochlorins or isobacteriochlorins. The products obtained are attractive intermediates for further functionalization of porphyrins and may be of potential use as sensitizers in photodynamic therapy.     

Reactions involving di-trans-[12]annulenes

The low temperature complete dehydrohalogenation of pentabromocyclododecene (C12H17Br5) with potassium tert-butoxide in THF followed by exposure to potassium metal leads to the formation of the anion radical of 1,5-di-trans-[12]annulene, which loses hydrogen and undergoes ring closure to form the anion radical of 11,12-dihydro-[8]annuleno-[6]annulene. This product can, in turn, be isolated as its neutral molecule via reoxidation with iodine. A [12]annulene obtained via the dimerization of 1,5-hexadiyne in the presence of 18-crown-6 and potassium tert-butoxide undergoes ring closure, with concomitant loss of hydrogen, to yield the heptalene anion radical. It follows that the heptalene anion radical precursor was the 1,7-di-trans isomer of [12]annulene.     

Mobilité conformationnelle et migration des liaisons π dans le [24]annulène

Conformational Mobility and Migration of the π Bonds of the [24]annulene. The configuration and the conformation of [24]annulene have been determined after a detailed analysis of its ¹H-NMR spectrum recorded at −95°. At this temperature, molecular dynamics is practically frozen, and the spectrum can be correctly simulated considering eight magnetic sites with the relevant couplings. The [24]annulene exhibits alternation of the double and the single bonds with the CTTTCTTTCTTT sequence (C=cis, T=trans) expressing the connectivity of the double bonds. The signal of the 9 protons pointing inside the ring is 7.72 ppm at lower field than the signal of the 15 outer protons; this indicates a marked paramagnetic ring current. Molecular dynamics is revealed by the dependence of the spectrum upon the temperature; the simulation of the line shape of these spectra indicates that the [24]annulene in solution exists as an equilibrium of two conformers A and B ( B / A ≤0.05), both having the same configuration. Each of these conformers undergoes two isodynamic processes: a migration of the π bonds on the adjacent single bonds (bond shift) described by V and a conformational mobility described by K. The two conformers interconvert extremely rapidely. Conformer A complies with C_3h symmetry, conformer B with C₃ symmetry. The enthalpy, entropy, and free energy of activation for the processes described by V and K in the major conformer A have been determined: these processes are slower than those observed in [16]annulene. From their values, we could deduce that the resonance energy in the [24]-7³annulene is negative and of the order of −9 to −10 kcal⋅mol⁻¹.     

The influence of benzene rings on aromatic pathways in the porphyrins

Magnetically induced current density for tetraazaporphyrins (H₂ATP), phthalocyanine (H₂Pc), and tetrabenzoporphyrin (H₂TBP) molecules has been computed. The calculated current strengths for H₂ATP and H₂TBP were found to be close to these of free base porphyrin (27 nA/T). The current strengths appeared to have greater value than the same ones for H₂Pc (21.7 nA/T). The joining of benzene rings to free base porphyrin and to H₂ATP causes to appear the additional weak ring current densities. The H₂Pc have a degree of aromaticity less than porphyrins according to magnetic criterion. © 2013 Wiley Periodicals, Inc.     

Vibrational Spectra and Molecular Vibrational Behaviors of All-Carboatomic Rings,cyclo[18]carbon and Its Analogues

The vibrational spectra of cyclo[18]carbon and its analogues, cyclo[2n]carbon (n=3 to 15), were carefully simulated and characterized. The in-plane C−C stretching vibrations shows strong rigidity, while out-of-plane motions seem to be extremely flexible. The solvation effect can enhance signal strengths of the vibrational spectra, but does not evidently change the shape of the spectral curves. The infrared and Raman spectra of cyclo[2n]carbons are quite sensitive to ring size in the range of n=3 to 7, while the size only modestly affects peak positions and strengths for larger rings. Molecular dynamic trajectories show that the fluctuation period of the skeleton of cyclo[18]carbon is basically constant at different temperatures, and they are all about 300 fs. With increase of simulation temperature, the ring distortion due to thermal motion is notable and becomes much stronger. However, neither ring breaking nor isomerization in cyclo[18]carbon is observed during the simulations untill 298.15 K.