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Acid-catalyzed condensation of a naphtho[2,3- f ]indane dialdehyde with a tripyrrane, followed by an oxidation step, afforded an anthro[2,3- b ]-21-carbaporphyrin. The presence of a fused anthracene unit induced minor bathochromic shifts and did not significantly affect the aromatic characteristics of the carbaporphyrin core. Protonation led to the formation of a monocation with similar diatropic properties, but the dication generated in the presence of a large excess of trifluoroacetic acid had a weakened Soret band absorption and a broad absorption at 754 nm. Nucleus-independent chemical shift (NICS) calculations indicate that the dication is only weakly aromatic and possesses a 32-atom 30π electron delocalization pathway. Alkylation with methyl iodide and potassium carbonate gave a 22-methyl derivative that reacted with palladium(II) acetate to afford an aromatic palladium(II) complex. Upon heating, the methyl group migrated from the nitrogen to the internal carbon atom and the resulting complex exhibited diminished aromatic character. A comparison with related carbaporphyrin complexes without ring fusion or with benzo- or naphtho-fused units demonstrated that the diatropic character decreased with increasing conjugation. NICS calculations and anisotropy of induced current density (AICD) plots confirmed this trend and showed that the remaining aromatic properties of the anthrocarbaporphyrin complex were due to a 30π electron circuit that extends around the entire anthracene unit.

Pyrazoloporphyrins (PzPs), which are porphyrin analogues incorporating a pyrazole subunit, are examples of carbaporphyrin-type structures with a carbon atom within the macrocyclic cavity. DFT calculations were used to assess a series of 17 PzP tautomers, nine monoprotonated species and four related diprotonated PzP dications. The geometries of the structures were optimized using M06-2X/6-311++G(d,p), and the relative stabilities computed with the cc-PVTZ functional. Nucleus independent chemical shifts, both NICS(0) and NICS(1) zz , were calculated, and the anisotropy of the induced current density (AICD) plots were generated for all of the species under investigation. The results for free base PzPs show that fully aromatic PzP tautomers are not significantly more stable than weakly aromatic cross-conjugated species. In addition, strongly aromatic structures with internal CH 2' s are much less stable, a feature that is also seen for protonated PzPs. The degree of planarity for the individual macrocycles does not significantly correlate with the stability of these structures. The results allow significant aromatic conjugation pathways to be identified in many cases, and provide insights into the aromatic properties of this poorly studied system. These investigations also complement experimental results for PzPs and emphasize the need for further studies in this area.

Reaction of 6-nitrochrysene with ethyl isocyanoacetate in the presence of a non-nucleophilic base gave a c -annulated pyrrole ethyl ester that was used to prepare chrysene-fused tripyrranes and a chrysopyrrole dialdehyde. Chrysene-fused tripyrranes were reacted with a pyrrole dialdehyde, but poor yields of chrysoporphyrins were obtained. However, condensation of the chrysopyrrole dialdehyde with a series of tripyrranes afforded excellent yields of chrysoporphyrins and an acenaphtho-chrysoporphyrin. Iron(III) chloride mediated oxidative cyclization of a dihexylchrysoporphyrin afforded a benzopyrene-fused porphyrin that exhibited a strongly red-shifted electronic absorption spectrum. DFT calculations show that both chrysoporphyrins and the benzopyrene-fused porphyrin have tautomers that possess 34π electron delocalization pathways that pass through the porphyrin nucleus and the fused polycyclic aromatic hydrocarbon (PAH) units. Protonation gave dications that favor 36-atom 34π electron circuits. c -Annulated pyrrole dialdehydes were also condensed with a carbatripyrrin to generate PAH-fused carbaporphyrins that retained fully aromatic properties.

Oxyquinoliziniporphyrins, novel carbaporphyrinoids that incorporate quinolizone units, were synthesized. These structures exhibit moderate diamagnetic ring currents that are greatly enhanced upon protonation. Addition of trifluoroacetic acid initially gave a monocation, but this was readily converted into a dicationic species. The aromatic character of the free base and protonated species was assessed by proton NMR, nucleus-independent chemical shift calculations, and anisotropy of induced ring current plots. Stable Ni(II) and Pd(II) complexes are also reported.

Isocitrate dehydrogenase 1 (IDH1) mutations that generate the oncometabolite 2-hydroxyglutarate (2-HG) from α-ketoglutarate (α-KG) have been identified in many types of tumors and are an important prognostic factor in gliomas. 2-HG production can be determined by hyperpolarized carbon-13 magnetic resonance spectroscopy (HP- 13 C-MRS) using [1- 13 C]-α-KG as a probe, but peak contamination from naturally occurring [5- 13 C]-α-KG overlaps with the [1- 13 C]-2-HG peak. Via a newly developed oxidative-Stetter reaction, [1- 13 C-5- 12 C]-α-KG was synthesized. α-KG metabolism was measured via HP- 13 C-MRS using [1- 13 C-5- 12 C]-α-KG as a probe. [1- 13 C-5- 12 C]-α-KG was synthesized in high yields, and successfully eliminated the signal from C5 of α-KG in the HP- 13 C-MRS spectra. In HCT116 IDH1 R132H cells, [1- 13 C-5- 12 C]-α-KG allowed for unimpeded detection of [1- 13 C]-2-HG. 12 C-enrichment represents a novel method to circumvent spectral overlap, and [1- 13 C-5- 12 C]-α-KG shows promise as a probe to study IDH1 mutant tumors and α-KG metabolism., (© 2021 John Wiley & Sons Ltd. This article has been contributed to by US Government employees and their work is in the public domain in the USA.)

The reaction of a carbatripyrrin with a tellurophene dicarbinol in the presence of BF 3 ·Et 2 O, followed by oxidation with DDQ, afforded the first example of a telluracarbaporphyrin. Although this system exhibits strongly aromatic characteristics, it is prone to air oxidation, giving rise to a hydroxy derivative that was characterized by X-ray crystallography. The initial telluracarbaporphyrin reacted with palladium(II) acetate to give a stable organometallic complex, and X-ray crystallography showed that the palladium cation was coordinated to all four atoms in the CNTeN core. An oxacarbatripyrrin was also reacted with a tellurophene dialcohol to give an air-stable porphyrin analogue with a CNTeO core. Nonmetalated telluracarbaporphyrins showed relatively short Te-N separations that strongly implied the involvement of hypervalent tellurium interactions. Furthermore, despite the presence of a very large tellurium atom, the tellurophene subunit is not strongly pivoted away from the mean macrocyclic plane as would be expected in the absence of these interactions. The aromatic properties of heterocarbaporphyrins were assessed by proton NMR spectroscopy, NICS calculations, and AICD plots. In addition, the relative stability of hydroxytelluraporphyrins in comparison to their tellurophene oxide tautomers was investigated and the aromatic characteristics of these oxidized structures were evaluated.

Site-selective fluorination of aliphatic C-H bonds remains synthetically challenging. While directed C-H fluorination represents the most promising approach, the limited work conducted to date has enabled just a few functional groups as the arbiters of direction. Leveraging insights gained from both computations and experimentation, we enabled the use of the ubiquitous amine functional group as a handle for the directed C-H fluorination of Csp 3 -H bonds. By converting primary amines to adamantoyl-based fluoroamides, site-selective C-H fluorination proceeds under the influence of a simple iron catalyst in 20 minutes. Computational studies revealed a unique reaction coordinate for the catalytic process and offer an explanation for the high site selectivity., Competing Interests: The authors declare no conflicts of interest., (This journal is © The Royal Society of Chemistry.)

Bis(pyrrolylmethyl)azulene dialdehydes underwent intramolecular McMurry coupling, and following oxidation with DDQ and addition of trifluoroacetic acid, gave aromatic azulitriphyrin cations. The proton NMR spectra for these contracted carbaporphyrinoids showed the internal CH upfield at ca. 2 ppm, while the bridging methine units appeared downfield between 8 and 10 ppm, demonstrating that the macrocycles possess significant 14π electron diamagnetic ring currents. Although protonated azulitriphyrins(1.2.1) proved to be very stable, the free base forms were unstable and could not be isolated. Treatment of an azulitriphyrin with KO t -Bu- t -BuOOH gave rise to oxidative ring contractions that afforded the first examples of benzocarbatriphyrins. These contracted porphyrinoids also exhibit aromatic ring currents, and the internal CH resonances shifted upfield to 0.5-0.7 ppm. Protonation afforded cations with slightly enhanced diatropic properties. Nucleus independent chemical shift and anisotropy of induced current density calculations confirmed that benzocarbatriphyrins are 14π electron delocalized aromatic systems that bypass the fused arene units.

Acid-catalyzed condensation of azulidipyrrane aldehydes with a dihydrodipyrrin carbaldehyde afforded the first examples of azulichlorins. These macrocyclic products were isolated in a monoprotonated form, and the free bases proved to be somewhat unstable. The monocations were strongly diatropic, and proton NMR spectroscopy showed the internal C-H at ca. -2 ppm. Addition of TFA gave the related dications, but these exhibited significantly reduced aromatic ring currents. Reaction of an azulichlorin with tert -butyl hydroperoxide and KOH in dichloromethane/methanol gave a benzocarbachlorin and two related aldehydes. The UV-vis spectrum for the benzocarbachlorin showed a split Soret band at 414 and 430 nm, together with a strong chlorin-like absorption at 684 nm. The proton NMR spectrum indicated that the carbachlorin is strongly aromatic and the internal C-H was observed at -4.64 ppm. Addition of TFA afforded a C-protonated dication with a significantly increased diatropic ring current. The proton NMR spectrum, NICS calculations, and AICD plots indicated that the system favors a 22π electron delocalization pathway that runs through the fused benzo unit. Addition of TFA to the benzocarbachlorin aldehydes primarily led to the formation of monocations, and the generation of C-protonated dications was no longer favored due to the presence of electron-withdrawing formyl moieties.

Modifying the electronic properties of olefins is the quintessential approach to tuning alkene reactivity. In this context, the exploration of trifluoromethyl groups as divergent electronic modifiers has not been considered. In this work, we describe a copper-mediated 1,2-(bis)trifluoromethylation of acetylenes to create E-hexafluorobutenes (E-HFBs) under blue light in a single step. The reaction proceeds with high yield and E/Z selectivity. Since the alkyne captures two trifluoromethyl groups from each molecule of bpyCu(CF 3 ) 3 , mechanistic studies were conducted to illuminate the role of the reactants. Interestingly, E-HFBs exhibit remarkable stability to standard olefin functionalization reactions in spite of the pendant trifluoromethyl groups. This finding has significant implications for medicine, agroscience, and materials., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new family of adj -dicarbaporphyrinoids has been prepared using the "2 + 2" MacDonald methodology. Dibutylboron triflate catalyzed condensation of 3-iodo-4-methoxybenzaldehyde with an indene enamine afforded an iodofulvene aldehyde, and a related dimethoxyfulvene was similarly prepared in two steps from 2,4-dimethoxybenzaldehyde. Following protection as the corresponding dimethyl acetals, the iodofulvenes were metalated with Bu 3 MgLi at -100 °C and reacted with dimethylformamide to give the required fulvene dialdehyde intermediates. Acid-catalyzed condensation with three different dipyrrylmethanes afforded a series of benzo-23-carbabenziporphyrins in 52-70% yields. The proton nuclear magnetic resonance spectra for these adj -dicarbaporphyrinoids indicate that these macrocycles are slightly diatropic. Monoprotonation afforded cationic species with slightly larger aromatic ring currents, and under strongly acidic conditions, C-protonated dications were generated with substantial diatropic properties. The aromatic character of these structures was supported by nucleus-independent chemical shifts and anisotropy of induced current density calculations. The computational results indicate that the dications favor 23-atom 22π electron delocalization pathways. The benzo-23-carbabenziporphyrins were selectively oxidized with silver(I) acetate in dichloromethane-methanol to give stable nonaromatic structures with two additional methoxy substituents connected to sp 3 hybridized bridging carbons. The intriguing reactivity and unique spectroscopic properties of benzo-23-carbabenziporphyrins make these novel structures promising candidates for further investigations.

Electrospray ionization (ESI) combined with ion mobility spectrometry (IMS) and mass spectrometry (MS) techniques is used to examine the Biginelli reaction in an ensemble of ions generated from droplets. We find evidence for rapid dihydropyrimidinone formation from condensation of ethyl acetoacetate, benzaldehyde, and urea on the very short timescales associated with the electrospray process (∼10 μs to ∼1.0 ms). Control bulk-solution reactions show no product formation even after several days. This implies that the in-droplet reaction rate is enhanced by an astonishing factor. Examination of the reaction conditions and characterization of the intermediates en route to product shows evidence for variations in the reaction mechanism. IMS separation shows that the Knoevenagel condensation intermediate from benzaldehyde and ethyl acetoacetate exists as both the cis - and trans -isomer, in a ∼5 to 1 ratio. We suggest that the dramatic acceleration arises because of increased reagent confinement as electrosprayed droplets shrink. The ability of IMS-MS to resolve intermediates (including isomers) provides a new means of understanding reaction pathways.

The direct, catalytic substitution of unactivated alcohols remains an undeveloped area of organic synthesis. Moreover, catalytic activation of this difficult electrophile with predictable stereo-outcomes presents an even more formidable challenge. Described herein is a simple iron-based catalyst system which provides the mild, direct conversion of secondary and tertiary alcohols to sulfonamides. Starting from enantioenriched alcohols, the intramolecular variant proceeds with stereoinversion to produce enantioenriched 2- and 2,2-subsituted pyrrolidines and indolines, without prior derivatization of the alcohol or solvolytic conditions., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

DFT studies have been carried out on 46 azuliporphyrin (AzP) and tropiporphyrin (TrP) tautomers, cations, and dications. The structures were minimized using DFT-B3LYP/6-311++G(d,p), and the relative stabilities of the tautomers for each series were computed with M06-2X and B3LYP-D3 functionals. Nucleus independent chemical shifts, both NICS(0) and NICS(1) zz , were calculated for both the center of the macrocycles and the individual pyrrolic and carbocyclic subunits. In addition, anisotropy of induced current density (AICD) plots were generated for all of the species under investigation. The results provided insights into the favored conjugation pathways and aromatic properties of these important carbaporphyrinoid systems. The π delocalization pathways for AzP were complex, and contrary to previous speculations, there was no evidence for tropylium characteristics in the favored free base structure. The global diatropicity of AzP was found to increase with increasing solvent polarity, in agreement with experimental observations. TrP was shown to be planar and to possess a large global diamagnetic ring current. However, NICS calculations and AICD plots indicated that the cycloheptatrienyl components of the two most favored TrP tautomers have paratropic characteristics. These results are not fully consistent with the proton NMR spectra for TrPs. A dihydroTrP and a Diels-Alder adduct were investigated, and these showed exceptionally large shielding effects with NICS(1) zz values of -36.53 and -36.92 ppm, respectively, in agreement with experimental results. A silver(III) complex of tropiporphyrin was also examined, and this showed no paratropic character for the seven-membered ring, although in this case, the system was found to be severely twisted. Overall, the results provide important information about the electronic properties of AzP and TrP tautomers, as well as for related protonated species, and complement the extensive experimental studies that have been conducted on these systems.

In an attempt to prepare quatyrin derivatives, which are hydrocarbon analogues of the porphyrins, azulene-appended fulvene carbinols were self-condensed in the presence of BF 3 ·Et 2 O. Although these investigations failed to give structures related to quatyrin, expanded porphyrin analogues were obtained instead. In dichloromethane, the major macrocyclic product consisted of three fulvene units, but when chloroform was used as the reaction solvent, a tetrafulvene macrocycle was isolated. Self-condensation of a furan-appended fulvene alcohol gave trace amounts of an opp-dioxadicarbaporphyrin. An alternative route to this novel system was developed where a dioxacarbatripyrrin was condensed with an indene dialdehyde in the presence of HBr. The heterodicarbaporphyrinoid proved to have strong globally aromatic properties as assessed by 1 H NMR spectroscopy, anisotropy of induced current density, and NICS calculations. In the presence of excess trifluoroacetic acid, an unstable aromatic cation was formed by C-protonation of an indene subunit. This species was also highly diatropic, and the 1 H NMR spectrum gave an unusually high Δδ value of 17.46 ppm.

Acid-catalyzed condensation of a benzo[ f]indane dialdehyde with a tripyrrane, followed by an oxidation step, afforded the first example of a naphtho[2,3- b]-21-carbaporphyrin. This π-extended porphyrinoid system is strongly aromatic and gave a porphyrin-like UV-vis spectrum with a Soret band at 432 nm. Protonation with TFA gave a monocation, but under highly acidic conditions a C-protonated dication was generated. Reaction of the naphthoporphyrin with ferric chloride produced a 21-chloro derivative. Alkylation with methyl iodide and potassium carbonate gave a 22-methyl derivative, and this reacted with palladium(II) acetate to afford a palladium(II) complex in which the internal methyl group had migrated from a nitrogen to a carbon atom. Treatment of the naphthocarbaporphyrin with silver(I) acetate generated the corresponding silver(III) complex. In naphtho[2,3- b]-21-carbaporphyrin and many of its derivatives, the aromatic conjugation pathways appear to bypass the naphthalene unit, and for this reason the UV-vis spectra were little affected. However, the diprotonated dication and the palladium(II) complex have aromatic pathways that pass through the naphthalene moiety, and this leads to large bathochromic shifts for these species. The results provide insights on the influence of fused aromatic units on the reactivity, spectroscopic properties, and aromatic characteristics of carbaporphyrinoid systems.

The installation of trifluoromethyl groups has become an essential step across a number of industries such as agrochemicals, drug discovery, and materials. Consequently, the rapid introduction of this critical functional group in a predictable fashion would benefit current practitioners in those fields. This communication describes a mild trifluoromethylation of benzylic C-H bonds with high selectivity for the least hindered hydrogen atom. The reaction provides monotrifluoromethylation and proceeds in an environmentally friendly acetone/water solvent system. The method can be used to install benzylic trifluoromethyl groups on highly functionalized drug molecules.

The mechanisms for new palladium-catalyzed divergent reactions of triazolopyridines were investigated by means of DFT calculations. Previously, it was observed experimentally that cross-coupling at the C7-position of triazolopyridines occurred when a strong base was used, whereas the reaction could be diverted to the C3-position if a weak base was employed. Calculations suggest that a strong base, such as tert-butoxide, can easily deprotonate C7-H, independent of the palladium metal, and deliver the preactivated substrate to palladium, which can reductively eliminate the final product. Without a strong base, the palladium(II) center reacts with the ring-opened diazo imine isomer of triazolopyridine to initially form a palladium(II)-carbene intermediate, which undergoes migratory insertion followed by β-hydride elimination to afford a 1,1-disubstituted alkene., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Connectivity-Based Hierarchy (CBH) is an effective error-cancellation scheme for the determination of chemically accurate thermochemical properties of a variety of organic and biomolecules. Neutral molecules and open-shell radicals have already been treated successfully by this approach utilizing inexpensive computational methods such as density functional theory. Herein, we present an extension of the method to a new class of molecules, specifically, organic cations. Because of the presence of structural rearrangements involving hydrogen migrations as well as unusual structures such as bridged cations, the application of the standard CBH protocol to a test set of 25 cations leads to significant errors due to ineffective bond-type matching. We propose an adjusted protocol to overcome such limitations to achieve highly effective error cancellation. The modified CBH methods, in conjunction with a wide range of density functionals, reproduce G4 energies for the test set of organic cations accurately within 1-2 kcal/mol at a reduced computational cost.

The first examples of porphyrin analogues incorporating pyrene units are reported. Acid-catalyzed condensation of a pyrene dialdehyde with a tripyrrane, followed by oxidation with DDQ, afforded a polycyclic aromatic hydrocarbon (PAH)-porphyrin hybrid in 38% yield. Pyreniporphyrin proved to be devoid of global aromatic character, but upon protonation aromatic mono- and dicationic species were generated. In the proton NMR spectrum for the dication, the internal CH was shifted upfield to approximately +3 ppm. NICS calculations and ACID plots confirmed the diatropic nature of these structures. Pyreniporphyrin reacted with palladium(II) acetate to give excellent yields of a palladium(II) complex that showed weakly diatropic properties. Treatment of the pyrene dialdehyde with phenylmagnesium bromide generated a dicarbinol that reacted with excess pyrrole in the presence of boron trifluoride etherate to give a tripyrrane analogue. Lewis acid catalyzed ring closure with a thiophene dialcohol in 2% ethanol-dichloromethane afforded a tetraphenylthiapyreniporphyrin in 31% yield. This porphyrinoid was nonaromatic in the free-base form but showed significant diatropicity upon protonation. These results demonstrate that PAH-porphyrin hybrids are easily accessible, and this strategy may allow the incorporation of even larger aromatic subunits.

This communication describes a mild, amide-directed fluorination of benzylic, allylic, and unactivated C-H bonds mediated by iron. Upon exposure to a catalytic amount of iron(II) triflate (Fe(OTf) 2 ), N-fluoro-2-methylbenzamides undergo chemoselective fluorine transfer to provide the corresponding fluorides in high yield. The reaction demonstrates broad substrate scope and functional group tolerance without the use of any noble metal additives. Mechanistic and computational experiments suggest that the reaction proceeds through short-lived radical intermediates with F-transfer mediated directly by iron.

The conformations of five doubly neo-confused porphyrin structures and fifty-four neo-confused N-confused porphyrin (neo-C-NCP) tautomers were minimized using DFT-B3LYP/6-311++G(d,p). The relative stabilities of the tautomers for each series were computed using M06-2X and B3LYP-D functionals, and the bond lengths and bond angles were calculated. Nucleus-independent chemical shifts and anisotropy of induced current density plots were used to provide further insights into the diatropic properties of each species. The results demonstrate that porphyrinoid structures with two modified rings of this type are far less stable than porphyrins, N-confused porphyrins, or neo-confused porphyrins, but the fully conjugated tautomers retain highly diatropic characteristics. Nevertheless, the lowest-energy neo-C-NCP tautomers have similar relative energies to known doubly N-confused porphyrins, and the results suggest that these types of porphyrin isomers may be synthetically accessible.

Base-catalyzed condensation of dicyclopentadienylmethane with a dipyrrylmethane dialdehyde gave a dicarbachlorin with an internal CH2 group. This unusual porphyrinoid retained highly diatropic characteristics and exhibited a porphyrin-like UV-vis spectrum.

Oxidation of tetraarylazuliporphyrins with silver(I) acetate in refluxing chloroform-acetonitrile afforded good yields of 21-oxyazuliporphyrins. Although hydroxyazuliporphyrin tautomers can be considered for this system, spectroscopic results and density functional theory calculations indicate that the keto form is favored, and this was confirmed by single-crystal X-ray diffraction. Oxyazuliporphyrins formally possess a 24π electron delocalization pathway, but the proton NMR spectra are consistent with macrocycles that have diatropic ring currents. Nucleus independent chemical shift and anisotropy of induced current density calculations also confirmed the diatropic nature of these macrocycles, although these results indicated that the seven-membered ring is antiaromatic. However, while the NMR spectra showed the azulene protons at atypically high field values, the results are consistent with a nonaromatic cycloheptatrienyl unit. Protonation gave dicationic products that exhibited enhanced diatropic character. Oxyazuliporphyrins readily form metalated derivatives with Ni(II), Pd(II), and Pt(II), and these complexes exhibited significant diatropic character even though the macrocycle is highly distorted. X-ray diffraction characterization of palladium(II) and platinum(II) complexes demonstrated that these derivatives are structurally virtually identical to a previously reported copper(II) oxyazuliporphyrin.

The conformations of a series of benziporphyrins, naphthiporphyrins, oxybenziporphyrins, and related structures were minimized using DFT-B3LYP/6-311++G(d,p). The relative stabilities of the tautomers for each series were calculated using M06-2X and B3LYP-D functionals, and bond lengths were obtained. The diatropic properties of each species were assessed by calculating nucleus independent chemical shifts (NICS) and the related NICSzz values. Although benziporphyrin and naphthiporphyrin tautomers essentially exhibit no aromatic properties, mono- and diprotonated species show weakly diatropic characteristics in agreement with experimental observations. Benziphlorins, intermediates in the MacDonald "3 + 1" synthesis of benziporphyrins, were also examined and tautomers with methylene units adjacent to the benzene ring were shown to be far more stable than tautomers with a CH2 bridge between two pyrrolic units. 2-Hydroxybenziporphyrin was shown to be significantly less stable than the aromatic tautomer oxybenziporphyrin, although diprotonation leads to a species with somewhat reduced diatropicity. Related systems were also analyzed and the results demonstrate that benziporphyrins and naphthiporphyrins range from structures with no measurable macrocyclic aromaticity to strongly aromatic systems that exhibit large diamagnetic ring currents.

Previous attempts to prepare tropone-fused carbaporphyrins by reacting peroxides with azuliporphyrins under basic conditions afforded benzocarbaporphyrins instead. In this study, a methoxyazulitripyrrane condensed with a pyrrole dialdehyde in the presence of TFA, followed by oxidation with ferric chloride, to give a tropone-fused carbaporphyrin following a spontaneous demethylation. The porphyrinoid gave a modified UV-vis spectrum showing multiple bands in the Soret region, and the proton NMR spectrum showed that it has a reduced diamagnetic ring current in comparison to other carbaporphyrins. The tropone-fused derivative failed to react with tert-butyl hydroperoxide and potassium hydroxide, demonstrating that this type of structure is not an intermediate in the formation of benzocarbaporphyrins. However, the reaction with silver(I) acetate gave the corresponding silver(III) complex. Condensation of the methoxyazulitripyrrane with 2,5-thiophenedicarbaldehyde gave a related tropone-fused thiacarbaporphyrin together with a methoxythiaazuliporphyrin. Treatment of the carbaporphyrins with DBU resulted in the formation of anionic species, while addition of acid afforded dicationic structures. DFT studies were performed on a series of tautomers, protonated species, and anionic structures related to these tropone-fused carbaporphyrins, and NICS calculations were carried out. These results allowed favorable conjugation pathways to be identified. In addition, these studies predicted that protonation initially occurs on the carbonyl moiety rather than on the expected interior pyrrolenine nitrogen atom.

An adj-dicarbaporphyrin was prepared by carrying out a base-catalyzed MacDonald reaction between bis(3-indenyl)methane and a dipyrrylmethane dialdehyde. The porphyrinoid system exhibited highly diatropic characteristics, and the proton NMR spectrum gave resonances at -5.74 and -6.24 ppm for the internal NH and CH protons, respectively. The UV-vis spectrum was also porphyrin-like, giving a Soret band at 455 nm and a series of Q bands at longer wavelengths. Addition of trifluoroacetic acid gave a C-protonated monocation, and at higher acid concentrations a dicationic species was observed. Addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) afforded a monodeprotonated porphyrinoid anion. All of these species retained highly diatropic characteristics. Density functional theory calculations showed that a nonplanar tautomer with four internal hydrogens was favored, in agreement with the spectroscopic data. Nucleus-independent chemical shift calculations also confirmed the aromatic characteristics of the free-base, cationic, and anionic structures. The dicarbaporphyrin reacted with palladium(II) acetate in refluxing acetonitrile to give an unusual tripalladium sandwich complex consisting of two dianionic palladium(II) dicarbaporphyrin units surrounding a palladium(IV) cation with unique η(5) interactions involving meso-carbon atoms.

The conformations of eight neo-confused porphyrin (neo-CP) structures and seven related benzo-neo-confused porphyrins (B-neo-CPs) were minimized using DFT-B3LYP/6-311++G(d,p). In addition, in order to provide contrasts to the B-neo-CP tautomers, a series of twelve benzoporphyrin (BP) tautomers and twelve benzocarbaporphyrins (BCPs) were also analyzed, and two N-confused porphyrin (NCP) tautomers were also considered. The relative stability of the tautomers for each series was computed, and the bond lengths and bond angles were calculated. Surprisingly, all of the neo-CP and B-neo-CP tautomers were near planar. The fully conjugated members of these series showed a significant degree of bond length alternation, unlike porphyrins or the aromatic forms of NCP, BP or BCP. The most stable tautomer of neo-CP was calculated to be 27.04 kcal mol(-1) higher in energy than porphyrin and 9.95 kcal mol(-1) higher in energy than the most favored NCP tautomer. The most stable tautomer of B-neo-CP was similarly shown to be 26.40 kcal mol(-1) higher in energy than benzoporphyrin. The favored tautomers also place the internal NH opposite to the neo-confused ring to minimize steric and lone pair-lone pair interactions, and to provide improved hydrogen bonding interactions. Nucleus independent chemical shifts (NICS) demonstrated that the fully conjugated neo-CP and B-neo-CP tautomers exhibit a significant amount of diatropic character, but these values are somewhat lower than the results obtained for porphyrins, BPs or BCPs. BCPs have three internal hydrogens, but the favored tautomer places the two pyrrolic hydrogens on each side of the indene unit to maximize hydrogen bonding interactions. Tautomers with an internal CH2 unit are also feasible, but these are energetically less favorable, have reduced diatropicity, and show significant bond length alternation. The results indicate that 18π electron delocalization pathways that pass through a fused benzene ring are less favorable than alternative delocalization pathways that bypass this unit.

Monocarbaporphyrins, dicarbaporphyrins, tricarbaporphyrins, and quatyrins can all potentially exist in a number of tautomeric forms, but little is known about these species. In addition, no examples of tricarbaporphyrins or quatyrins are known at this time. In order to get information on the relative stabilities of carbaporphyrin tautomers, a series of aromatic and nonaromatic structures were assessed using computational methods. The conformations of 41 carbaporphyrin structures, together with four tautomers of porphyrin and the related antiaromatic species didehydroquatyrin, were minimized using DFT-B3LYP/6-311++G(d,p). The relative stabilities of the tautomers for each series were computed, and the bond lengths and bond angles were calculated. Many aromatic carbaporphyrin structures have very crowded cavities with up to six hydrogens being present. However, this was not sufficient to destabilize the aromatic structures relative to the nonaromatic forms. Furthermore, nucleus-independent chemical shifts (NICS) demonstrated that all of the porphyrinoids with 18-π-electron delocalization pathways are highly diatropic. adj-Dicarbaporphyrin and tricarbaporphyrin favor tautomers with an internal methylene group, while two interior methylenes are present in the favored tautomers for quatyrin. The results provide valuable information that will be helpful in designing synthetic routes to higher carbaporphyrinoid systems.

A series of diformylbenzophenones were generated by sequentially reacting protected bromobenzaldehydes with n-butyllithium and ethyl N,N-dimethylcarbamate. The acetal protective groups were cleaved with refluxing formic acid. Vilsmeier-Haack formylation of 2,2',4,4'-tetramethoxybenzophenone also afforded a related dialdehyde. MacDonald "2 + 2" condensation of three benzophenone dialdehydes with a dipyrrylmethane gave oxophlorin analogues constructed from two benzene and two pyrrole rings. The free base oxodibenziphlorins were rather unstable in solution, and in most cases these porphyrinoids were isolated as the corresponding trifluoroacetate salts. The spectroscopic properties of 6-oxo-adj-dibenziphlorins are consistent with a nonaromatic ring system. DFT calculations indicated that the macrocycles considerably diverge from planarity, particularly when methoxy substituents are present on the arene rings.