Adj-Dicarbaporphyrinoid Systems: Synthesis, Spectroscopic Characterization, and Reactivity of 23-Carbabenziporphyrins

Article abstract of DOI:10.1021/acs.joc.9b01410

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 dimetho

Full text of DOI:10.1021/acs.joc.9b01410

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Article
adj-Dicarbaporphyrinoid Systems: Synthesis, Spectroscopic
Characterization and Reactivity of 23-Carbabenziporphyrins
Pankaj Jain, Deyaa I. AbuSalim, and Timothy D. Lash
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b01410 • Publication Date (Web): 23 Jul 2019
Downloaded from pubs.acs.org on July 23, 2019
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adj-Dicarbaporphyrinoid Systems: Synthesis, Spectroscopic Characterization
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and Reactivity of 23-Carbabenziporphyrins
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Pankaj Jain, Deyaa I. AbuSalim and Timothy D. Lash*
Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160
E-mail: tdlash@ilstu.edu
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Abstract
A new family of adj-dicarbaporphyrinoids has been prepared using the ‘2 + 2’ MacDonald
methodology. Dibutylboron triflate catalyzed condensation of 3-iodo-4-methoxy-benzaldehyde
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
o
corresponding dimethyl acetals, the iodofulvenes were metalated with Bu3MgLi at -100 C and
reacted with DMF 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 NMR 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 22 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
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connected to sp3 hybridized bridging carbons. The intriguing reactivity and unique spectroscopic
properties of benzo-23-carbabenziporphyrins makes these novel structures promising candidates
for further investigations.
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Introduction
Figure 1. Selected carbaporphyrinoid systems.
Carbaporphyrinoid systems are an important group of porphyrin analogues that have been
widely investigated over the last two decades due to their unusual reactivity and intriguing
spectroscopic properties.1 They include N-confused porphyrins 1,2,3 true carbaporphyrins such as
2 and 3,4-6 azuliporphyrins 4,7 tropiporphyrins 58 and benziporphyrins 6 (Figure 1).9 Each of
these families can generate stable organometallic derivatives10 and often undergo regioselective
oxidation reactions.1 Although 1-3 exhibit strongly aromatic characteristics, benziporphyrins are
essentially nonaromatic while azuliporphyrins have intermediary aromatic properties. However,
benziporphyrins take on a degree of diatropic character when electron-donating methoxy-groups
are present (e.g. structure 7)11,12 and this is intensified upon protonation (Figure 2). The aromatic
character of 7 can be attributed to contributions from dipolar canonical forms such as 7’ or -
delocalized structures such as 7’’ that incorporate an 18 electron anionic diaza[17]annulene
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pathway, but these interactions are limited because of the necessity for charge separation.13
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Addition of acid affords diprotonated dications 7H22+ that exhibit substantial diatropic character.
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This can be explained by the presence of resonance contributors such as 7’H2 that facilitate
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charge delocalization or by contributions from species such as 7’’H2 with cationic
[19]annulene pathways.11-13
Figure 2. Conjugation pathways in free base and protonated dimethoxybenziporphyrins.
Dicarbaporphyrinoid systems, which have two carbon atoms within the macrocyclic core, are
less well explored but a substantial number of examples have now been reported including
doubly N-confused porphyrins,14 opp- and adj-dicarbaporphyrins,15,16 dioxadicarbaporphyrins,17
dicarbachlorins,18 adj-diazuliporphyrins,19 hetero-opp-diazuliporphyrins,20 dibenziporphyrins,21
22- and 23-carbaazuliporphyrins,22,23 and 23-carbaoxybenziporphyrins.24 In addition, a
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dicarbacorrole system 8 called phenanthriporphyrin has been described (Figure 3),25,26 and
related dimeric structures have been investigated.27 Dicarbaporphyrinoids may exhibit aromatic,
nonaromatic or antiaromatic characteristics and can stabilize transition metal ions in high
oxidation states such as copper(III). Regioselective oxidation reactions have also been observed
and 22-carbaazuliporphyrins 9 were found to react with silver(I) acetate in the presence of
methanol or ethanol to give the nonaromatic dialkoxy derivatives 10 (Figure 3). Although only
one diastereomer was observed, the identity of this species (cis- or trans-) could not be
identified. Similar reactivity was recently reported for reactions of phenanthriporphyrin 8 with
copper(II) acetate in chloroform-methanol, but in this case the product 11 was generated as a
mixture of cis- and trans-isomers.26
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Figure 3. Dicarbaporphyrinoids and their oxidation products.
The interesting properties and potential applications associated with dicarbaporphyrinoids
provide the incentive to further investigate structures of this type. Given the intermediary
macrocyclic aromaticity associated with methoxybenziporphyrins, it was of interest to explore
the properties of dicarba-analogues. In particular, methoxy- and dimethoxy-23-
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carbabenziporphyrins 12 (Scheme 1) were targeted for synthesis. It was anticipated that the
desired dicarbaporphyrinoids could be constructed using a ‘2 + 2’ MacDonald condensation28
from fulvene dialdehydes 13 and 14 and the known dipyrrylmethanes 15.29 The design and
implementation of efficient syntheses of 12 are described below and the aromatic properties of
these novel macrocycles, both as the free bases and in protonated form, were assessed by proton
NMR spectroscopy and DFT calculations. Although metalation of these porphyrinoids was not
accomplished, regioselective oxidation reactions were observed.
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Scheme 1. Retrosynthetic analysis of 23-carbabenziporphyrins
Results and Discussion
Synthesis and Spectroscopic Characterization of 23-Carbabenziporphyrins
The synthesis of 23-carbabenziporphyrins 12 necessitated access to fulvene dialdehyde
intermediates (Scheme 1). In earlier work, a synthesis of fulvene aldehydes was developed using
indene enamine 16 (Scheme 2).30 Reaction of 16 with aromatic aldehydes in the presence of
dibutylboron triflate, followed by hydrolysis with aqueous sodium acetate, afforded good yields
of structurally diverse monoaldehydes 17. The main challenge at this point was the introduction
of a second formyl moiety. Attempts to generate fulvene dialdehydes by reacting
isophthalaldehydes 18 with one equivalent of 16 and Bu2BOTf were unsuccessful and only
difulvenes 19 could be isolated.31,32 Reaction of a monocyanovinyl protected isophthalaldehyde
20 with 16 gave good yields of fulvene 21 but attempts to cleave the cyanovinyl group with
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refluxing aqueous sodium hydroxide led to decomposition. Benzaldehydes 22 with vinyl or
cyano groups that could conceivably be converted into formyl units were also reacted with 16
and Bu2BOTf, but these reactions primarily led to decomposition and no fulvene products could
be isolated. All of the alternative Lewis acid catalysts that had been investigated gave poor
results and produced virtually none of the required fulvene aldehydes in any of these reactions.31
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Scheme 2. Unsuccessful routes to fulvene dialdehydes.
Given these difficulties, an alternative strategy involving metal-halogen exchange had to be
developed. Initial efforts were directed towards the synthesis of monomethoxyfulvene 13. 3-
Bromo-4-methoxybenzaldehyde (23a) was reacted with 1630 and Bu2BOTf in dichloromethane,
and hydrolyzed with aqueous sodium acetate, to give bromofulvene 24a in 62% yield (Scheme
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3). In order to install a second formyl moiety, it was necessary to protect and metalate the
structure. Reaction of 24a with trimethyl orthoformate and cerium(III) chloride afforded the
corresponding acetal 25a but reaction with n-butyllithium or tert-butyllithium failed to promote
metal-halogen exchange. In order to increase the reactivity towards metalation, the analogous
iodofulvene 24b was prepared. Reaction of 3-iodo-4-methoxybenzaldehyde (25b)33 with
enamine 16 afforded fulvene 24b in 43% yield. Protection with CH(OMe)3-CeCl3 gave the
related acetal 25b in 87% yield and this was treated with n-butyllithium in an attempt to generate
the lithiated derivative. Unfortunately, no reaction was observed under any of the conditions
investigated.
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Scheme 3. Synthesis of a methoxyfulvene dialdehyde.
As magnesium ate complexes are known to increase reactivity in metal-halogen exchange
reactions,34-40 halogen-magnesium exchange was investigated for fulvene derivatives 25a and
25b. Tributylmagnesium-ate complex n-Bu3MgLi is commonly used in these reactions and this
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can be prepared by reacting 2 equivalents of a 1.6 M solution of n-butyllithium in hexanes with a
2.0 M solution of n-butylmagnesium chloride in THF in ether at 0 °C. Metal-halogen exchange is
favored at very low temperatures and the temperature of the reactants was lowered to -100 °C.
Although protected bromofulvene 25a still failed to react, iodofulvene 25b reacted with
Bu3MgLi in anhydrous ether at -100°C to give a metalated species 26 that reacted with DMF to
afford dialdehyde 13 in 56% yield (Scheme 3).
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Scheme 4. Synthesis of 23-carbabenziporphyrins.
Fulvene dialdehyde 13 was reacted with dipyrrylmethane dicarboxylic acid 15a and TFA in
dichloromethane. Following column chromatography on silica gel and recrystallization from
chloroform-hexanes, excellent yields (65-70%) of methoxycarbabenziporphyrin 12a were
obtained (Scheme 4). This is the first example of a fully conjugated adj-dicarbaporphyrinoid
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incorporating a benzene subunit. The spectroscopic properties for dicarbaporphyrin 12a indicate
that it is a non-aromatic compound due to the presence of a cross-conjugated benzene subunit,
although the proton NMR spectrum for this compound suggests that a weak macrocyclic ring
current is present. Two tautomers of this system, 12 and 12’, are conceivable, but only one
species is observed in the proton NMR spectrum, possibly due to rapid exchange of the NH
proton. The proton NMR spectrum of the free base (Figure 4A) showed four singlets
corresponding to the meso-protons at 6.53, 7.01, 7.87 and 7.90 ppm, the inner benzene C-H
appeared at 6.02 ppm, and the inner indene showed up at 4.97 ppm. The slight upfield shifts to
the internal proton resonances, and the relatively deshielded values observed for the external
protons, suggest that the macrocycle possesses a small amount of global diatropic character that
presumably results from dipolar resonance contributors such as X or delocalized species such as
Y with a 17-atom 18 electron delocalization pathway (Scheme 4). These properties are
magnified upon addition of trace amounts of TFA to the NMR solution as the resulting
monocation 12aH+ (Scheme 5) exhibits a significant, albeit modest, aromatic ring current.
However, the precise values for these resonances depended on the concentration of TFA in the
NMR tube (Figures 4B-G). The interior indene and benzene protons shifted upfield to ca. 4.5
ppm, while the exterior meso-proton resonances gave rise to four downfield singlets between 6.8
and 8.20 ppm. Furthermore, the methyl substituents shifted downfield from 2.49 ppm in the free
base to >2.6 ppm in the monoprotonated structure. The emergence of an enhanced diatropic ring
current can be attributed to resonance contributors such as 12RH+ that possess [18]annulene
delocalization pathways (Figure 5). Contributors of this type would provide beneficial charge
delocalization in addition to aromatic characteristics.
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Figure 4. Proton NMR spectra of methoxy-23-carbabenziporphyrin 12a (ca. 1.5 mg) in CDCl3
(A, free base) and with incremental quantities of TFA (B-G, monocation 12aH+).
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Figure 5. Canonical forms of protonated 23-carbabenziporphyrins 12 possessing 18electron
delocalization pathways.
The formation of monocation 12aH+ was examined by adding small increments of TFA to an
NMR solution of ca. 1.5 mg of 12 (Figure 4). The internal proton on the benzene unit shifted
upfield from 6.02 ppm to 4.80 ppm when 0.4 µL of TFA was added, and eventually overlapped
with the internal indene proton at 4.35 ppm upon addition of 11.2 µL of TFA. At low
concentrations of TFA, the NH resonances could not be discerned but an NH peak could be
identified when 1.6 µL of TFA was added and a second NH peak became visible with 2.8 µL of
TFA. Two NH resonances gave clear singlets when 4.0 µL of TFA was added and continued to
move further upfield as the concentration of TFA was further increased. Although the changes
observed for the downfield protons were much smaller, further downfield shifts were noted as
the concentration of TFA increased (Figure 4). Profound changes to the electronic characteristics
due to the protonation of dicarbaporphyrin 12a can also be seen in the UV-vis spectra (Figure 6).
The free base gave Soret-like bands at 352 and 415 nm and a broad absorption between 500 and
750 nm (Figure 6). Titration of 12a with TFA showed the gradual transformation to a new
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species that was attributed to monocation 12aH+ (Figure 7). Isosbestic points were observed at
375 and 445 nm, indicating that no significant concentrations of any other species are present in
solution. The Soret-type band at 352 nm became more intense, while the band at 415 nm
virtually disappeared upon addition of 20 equiv of TFA (Figure 7). The longer wavelength
region between 450 and 800 nm showed Q-type bands more consistent with an aromatic
porphyrinoid. In 100% TFA, the porphyrinoid gave a completely different UV-vis spectrum due
to the formation of dication 12aH22+. This species shows a strong Soret-like band at 395 nm and
moderately strong absorptions at 482 and 674 nm (Figure 6).
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Figure 6. UV-vis spectra of 12a in 1% triethylamine-chloroform (free base, red line), 1% TFA-
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chloroform (monocation 12aH+, green line) and 100% TFA (dication 12aH2 , purple line).
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Figure 7. UV-vis spectra of 12a in chloroform with incremental quantities of TFA: 0 equiv (red
line), 1 equiv (orange) 2 equiv (light green), 3 equiv (dark green), 5 equiv (light blue), 10 equiv
(dark blue) and 20 equiv (purple).
10.0
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Figure 8. 500 MHz proton NMR spectrum of methoxy-23-carbabenziporphyrin dication 12aH2
in neat TFA.
2+
The proton NMR spectrum of 12aH2 in 100% TFA confirmed that the macrocycle had
undergone C-protonation onto the internal indene carbon atom and demonstrated that the
dication exhibited a much larger diamagnetic ring current (Figure 8). The meso-protons showed
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up as four 1H singlets at 9.04, 9.45, 9.55 and 10.31 ppm, while the internal benzene proton
moved upfield to 3.29 ppm and the internal methylene unit gave a 2H singlet at 1.11 ppm. The
methyl protons were observed at 3.43 and 3.50 ppm, shifted downfield by approximately 1 ppm
compared to the free base. Although the positive charges may play a role in the downfield shifts,
the observed differences in the chemical shifts for the internal and external sp2 hybridized CHs
( = 7.02 ppm) can only be explained by the presence of a strong aromatic ring current. The
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2+
aromatic character of 12aH2 can again be attributed to electron-donation from the methoxy
2+
substituent as this facilitates canonical forms such as 12aRH2 with 18 electron delocalization
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pathways (Figure 5). The H-1H COSY NMR spectrum for 12aH2 showed a weak correlation
between the internal CH2 and the nearby meso-protons (4J or allylic coupling), but this
interaction did not resolve in the proton NMR spectrum and only resulted in a slight broadening
of these resonances. When the proton NMR spectrum of 12a was run in 100% d-TFA, the
internal NH and indene protons, and the meso-proton at position 16, were completely exchanged.
2+
These results indicate that dication 12aH2 must be in equilibrium with a meso-protonated
dication 12a’H22+ or the related monocations 12a’H+ and 12a’’H+ (Scheme 5).
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Scheme 5. Protonation of benzo-23-carbabenziporphyrins.
Fulvene dialdehyde 13 was also reacted with dipyrrylmethanes 15b and 15c to give the related
methoxycarbabenziporphyrins 12b and 12c in 68% and 63% yields, respectively (Scheme 4).
The spectroscopic properties of these porphyrinoids were very similar to those for 12a, although
12b was insufficiently soluble in organic solvents to allow a carbon-13 NMR spectrum to be
obtained for the free base structure.
In order to magnify the aromatic characteristics of 23-carbabenziporphyrins, related structures
with two methoxy-substituents (i.e. 12d-f) were targeted for synthesis. Using the same synthetic
strategy required the availability of 3-iodo-2,4-dimethoxybenzaldehyde (27), and this precursor
was prepared by reacting 2,4-dimethoxybenzaldehyde with iodine and periodic acid in presence
of sulfuric acid and acetic acid (Scheme 6).33 However, when 27 was reacted with indene
enamine 16 in the presence of Bu2BOTf, fulvene 28 was generated instead of the required iodo-
derivative. The loss of the iodo-substituent under these reaction conditions was attributed to the
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electron-rich nature of the aryl unit, which can facilitate protonation-deiodination. Hence, a
slightly different approach was used where the iodination was carried out after the fulvene had
been formed. 2,4-Dimethoxybenzaldehyde was reacted with indene enamine 16 to afford fulvene
28, and subsequent iodination with N-iodosuccinimide in the presence of catalytic TFA gave the
required iodofulvene 29. The product was contaminated with 3-5% of a second unidentified
species that could not be removed by column chromatography. The iodo-compound was taken on
in crude form and the aldehyde unit protected as dimethyl acetal 30 with CH(OMe)3-CeCl3.
Iodine-magnesium exchange was carried out in the presence of the magnesium ate complex
Bu3MgLi at -100 °C, and subsequent addition of DMF and acid-catalyzed hydrolysis gave the
dialdehyde 14 in 70% yield (56% from monoaldehyde 29). Again, this intermediate was
contaminated with a small amount of a second species that could not be removed. Nevertheless,
this material proved to be a suitable precursor in the synthesis of the targeted
dimethoxyporphyrinoids 12d-f.
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Scheme 6. Synthesis of a dimethoxyfulvene dialdehyde.
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Reaction of dialdehyde 14 with dipyrrylmethane 15a in presence to TFA gave the
carbabenziporphyrin 12d in 51% yield (Scheme 4). As expected, the presence of two methoxy-
groups on this porphyrinoid system increased the aromatic characteristics compared to 12a-c.
The proton NMR spectrum for 12d in CDCl3 showed the meso-protons as four comparatively
downfield singlets at 6.77, 7.26, 8.09 and 8.60 ppm (Figure 9). In addition, the internal CH
protons for the indene and benzene units showed up at 3.84 and 4.88 ppm, respectively,
relatively upfield values compared to the analogous protons in 12a, which appeared at 4.97 and
6.03 ppm. Although the observed upfield and downfield shifts are still fairly weak for 12d, they
unambiguously attest to the presence of a global diamagnetic ring current (Figure 9).
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Figure 9. Partial proton NMR spectrum of dimethoxy-23-carbabenziporphyrin 12d in CDCl3
showing the internal 22-H and 23-H protons shifted to comparatively upfield values.
Addition of small amounts of TFA to solutions of 12d gave the corresponding monocationic
form 12dH+ (Scheme 5) and this species exhibited a further enhancement in macrocyclic
aromaticity. The second methoxy group provides access to additional aromatic resonance
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contributors such as 12dR’H+ that also allow for further charge delocalization (Figure 5). As was
the case for 12a, the precise values for the observed proton resonances was dependent on the
amount of TFA that was present. A typical proton NMR spectrum of 12dH+ in CDCl3 with one
drop of TFA showed the meso-protons at 6.85, 7.96, 8.45 and 8.83 ppm, while the internal
protons on the indene and benzene units gave 1H singlets at 2.19 and 2.34 ppm ( = 6.64 ppm).
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2+
At much higher concentrations of TFA, the C-protonated dication 12dH2 was generated.
Complete conversion of 12d to the dicationic form was observed in 25% TFA-CDCl3, while the
monomethoxy-porphyrinoids 12a-c were only completely converted into this form in 100%
2+
TFA. The proton NMR spectrum for dication 12dH2 showed that this species has a much
stronger aromatic ring current and the difference in chemical shifts between the internal and
external protons () was 11.43 ppm (Figure 10). The internal benzene proton gave a 1H singlet
at 1.11 ppm, while the internal methylene resonance shifted further upfield to -0.71 ppm.
Furthermore, the meso-protons were strongly deshielded giving rise to four 1H singlets at 9.58,
10.08, 10.57 and 10.72 ppm. The enhanced aromatic character can be attributed to the
availability of further resonance contributors with 18 electron delocalization pathways such as
2+
12dR’H2 due to the presence of the second methoxy-substituent (Figure 5). When the proton
NMR spectrum of 12d was obtained in 100% d-TFA, the peaks corresponding to the internal NH
and indene protons, and the resonance for the meso-proton at position 16, completely
disappeared. This result again demonstrates that deuterium exchange readily occurs at these sites
and in particular indicates that meso-protonated species such as 12d’H22+ are in equilibrium with
12dH22+ (Scheme 5).
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Figure 10. Partial proton NMR spectrum of dimethoxy-23-carbabenziporphyrin in 50% TFA-
CDCl3.
Fulvene dialdehyde 14 was also reacted with dipyrrylmethanes 15b and 15c to give
dimethoxycarbabenziporphyrins 12e and 12f in 60% and 52% yields, respectively. The free base
forms of 12d and 12e were insufficiently soluble to allow the carbon-13 NMR spectra to be
obtained. However, 12f had superior solubility characteristics and the carbon-13 NMR spectrum
showed the meso-carbons at 95.0 (3-CH), 97.4 (11-CH), 112.7 (21-CH) and 116.1 ppm (6-CH),
while the internal indene and benzene carbons appeared at 120.6 and 120.0 ppm, respectively.
The UV-vis spectrum of free base 12e in chloroform produced two broad peaks at 345 and 432
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nm. Addition of TFA led to the formation of the related monocation 12eH+ and this gave broad
peaks at 348 and 435 nm, followed by smaller bands between 500 and 800 nm. Titration of a
solution of 12e with TFA showed the gradual formation of 12eH+ with clear isosbestic points at
384 and 450 nm (Figure 12). The formation of the monocation was essentially complete upon
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2+
addition of 10 equivalents of TFA. At higher concentrations of TFA, a dicationic species 12eH2
was generated and this showed a strong Soret-type band at 413 nm, followed by several Q-type
bands (Figure 11).
Figure 11. UV-vis spectra of 12e in 1% triethylamine-dichloromethane (free base, red line), 1%
TFA-dichloromethane (monocation 12eH+, green line) and TFA (dication 12eH22+, purple line).
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Figure 12. UV-vis spectra of 12e in dichloromethane with incremental quantities of TFA: 0
equiv (red line), 0.5 equiv (orange) 1 equiv (light green), 2 equiv (dark green), 5 equiv (light
blue), 10 equiv (dark blue) and 20 equiv (purple).
The electron impact mass spectra for 12a-f showed strong [M + 2H]+ peaks and in some cases
high resolution data could not be obtained for the molecular ions. This phenomenon has
previously been observed for benziporphyrins 6.11 However, TOF ESI MS gave the expected [M
+ H]+ peaks in each case and the high resolution data confirmed the expected molecular formulae
for these structures.
Computational Investigations
Further insights into the relative stabilities and aromatic characteristics of tautomeric benzo-
23-carbabenziporphyrins (BCBs) and related protonated species were obtained by performing
DFT calculations. Initially, the unsubstituted BCB system was investigated (Table 1), and the
structures were optimized using B3LYP with the triple-ζ basis set 6-311+ +G(d,p).41 Eight
tautomeric forms were considered where one hydrogen is relocated to one of eight positions, and
these structures have been designated to show the positions of those hydrogens, e.g. as BCB-
24H, BCB-23H, etc. The relative stabilities of the tautomers were assessed using three different
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functionals, B3LYP, M06-2X, and B3LYP-D3,42,43 and the relative Gibbs free energy was also
determined with B3LYP. The results obtained using each of these methods were very similar. In
addition, the relative G values showed the same trends as the relative E values, indicating that
entropic factors were not significant. The lowest energy tautomers were BCB-24H and BCB-
25H and these differed in energy by only slightly over 1 kcal/mol (Table 1).
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Table 1. Calculated Relative Energies (kcal/mol) and NICS Values (ppm) for Selected
Unsubstituted Benzo-23-carbabenziporphyrin Tautomers. Bold: strongly shielded (aromatic);
red: strongly deshielded.
Molecule
G (298 K)
BCB-24H
0.00
BCB-25H
+0.96
BCB-23H
+14.59
BCB-22H
+26.07
(0.00/
0.00/
(+1.21/
+1.36/
(+15.75/
+14.05/
(+27.72/
+33.17/
B3LYP/
M06-2X/
0.00)
+1.26)
+15.94)
+27.85)
B3LYP-D3)
NICS(0)/NICS(1)zz
NICS(a)/NICS(1a)zz
NICS(b)/NICS(1b)zz
NICS(c)/NICS(1c)zz
NICS(d)/NICS(1d)zz
NICS(e)/NICS(1e)zz
-0.12/+1.43
-6.53/-21.34
+2.95/+5.24
-4.09/-9.86
-0.77/-7.85
-6.96/-23.69
-0.27/+0.56
-7.45/-21.78
+3.56/+3.74
-0.68/-7.46
-3.70/-8.09
-6.81/-22.45
+0.49/ +4.46
-6.42/-21.88
+1.51/ -1.01
+0.99/ -4.74
-1.07/ -8.56
-6.79/-23.61
-12.70/-30.96
-16.09/-47.37
+11.44/+23.52
+0.54/-7.07
+0.73/-6.76
-3.94/-15.96
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Table 2. Dihedral angles between individual subunits in selected benzo-23-carbabenziporphyrins
and related protonated species.
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9
Molecule
BCB-24H
ab
bc
cd
da
Average
10.06
10.16
20.75
12.88
9.20
-17.73 -10.93
1.43
-1.31
14.02
-10.16
13.34
-22.00
BCB-25H
-18.15
27.04
10.83
-15.54
-15.90
24.54
8.48
7.83
-19.92
-6.36
10.22
6.85
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22
23
24
25
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27
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BCB-24,25H+
BCB-23,24,25H2+
MeOBCB-24H
MeOBCB-25H
MeOBCB-24,25-H+
MeOBCB-23,24,25-H2+
MeO2BCB-24H
MeO2BCB-25H
MeO2BCB-24,25H+
MeO2BCB-23,24,25H2+
-15.36 -18.97
-1.73
-1.54
14.33
15.34
-1.90
-1.55
14.01
15.12
-9.30
12.88
-21.90
-18.02
9.54
9.29
-18.92
-6.53
11.00
8.11
19.92
12.09
9.93
-17.28
-17.44
25.53
7.91
12.27
-20.72
-16.41
9.84
-19.50
-7.08
19.94
11.63
The individual rings within these macrocycles are twisted relative to one another by about 10o
(Table 2) due to crowding within the porphyrinoid cavity. The favored BCB tautomers have
three hydrogens within the macrocycle and are cross-conjugated due to the presence of the arene
ring. BCB-23H, which possesses an internal methylene unit, is ca. 15 kcal/mol higher in energy,
while the aromatic tautomer BCB-22H, which sacrifices the 6 electron character of the benzene
unit, is between 26.07 and 33.17 kcal/mol higher in energy. In fact, tautomers with interrupted
conjugation due to the presence of CH2 bridges are lower in energy than BCB-22H despite the
presence of an 18 electron delocalization pathway in the latter structure (Table S1). Nucleus-
independent chemical shift (NICS) calculations were performed,44 and these confirmed that only
tautomer BCB-22H possesses global diatropic character. Large negative NICS values indicate
that a structure is aromatic, while antiaromatic compounds give large positive values and
nonaromatic species afford results that are close to zero. Standard NICS calculations are
considered to be less accurate as the effects due to σ and π electrons are included and these
results may not always reliably assess aromatic properties. For this reason, NICS calculations
zz
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were also conducted because these primarily assess the effects caused by the π system. These
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calculations were performed 1 Å above the ring (NICS(1) ) and give numerical values that are
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substantially larger than those given by NICS. As the same trends were observed using both
methods, the discussion focuses on the standard NICS results in order to avoid confusion. The m-
phenylene (ring a) and benzo-units (ring e) NICS values for the nonaromatic tautomers both
gave significantly negative values in the range of -6.17 to -7.58 ppm, demonstrating that these
units retained strong localized aromatic characteristics. The aromatic ring currents within a
macrocycle due to an applied magnetic field can also be assessed by anisotropy of induced
current density (AICD)45 and these calculations were performed on all of the tautomers. Figure
13 shows the AICD plots for the low energy tautomer BCB-24H and the high energy aromatic
form BCB-22H. No global delocalization pathway is evident for BCB-24H, but a clear 18
electron delocalization pathway is evident for BCB-22H.
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Figure 13. AICD plots for BCB-24H and BCB-22H. The latter shows a clear 18 electron
delocalization pathway.
Five monoprotonated species were considered (Table 3). Although the expected tautomer
BCB-24,25H+ was favored, tautomers BCB-23,24H+ and BCB-23,25H+ with internal CH2’s on
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the indene unit were only 1.06-3.53 kcal/mol higher in energy. The increased crowding within
the cavity leads to further distortions to the macrocycle (Table 2). Protonation onto the internal
benzene carbon is not favorable as tautomers BCB-22,25H+ and BCB-22,24H+ are 12.24-20.48
kcal/mol higher in energy. However, NICS calculations indicate that these high energy tautomers
are the only ones that possess significant global aromatic character. The instability of the arene-
protonated structures can be attributed to the magnitude of the macrocyclic aromaticity being
insufficient to offset the loss of the thermodynamically favored benzene subunit. The formation
of dications requires C-protonation and the expected indene-protonated form BCB-23,24,25H2+
was shown to be 11.70-16.14 kcal.mol lower in energy than the benzene protonated form BCB-
23,24,25H2+ (Table 4). NICS calculations show that BCB-23,24,25H2+ exhibits weakly aromatic
properties (NICS(0) = -3.01 ppm) but the high energy tautomer exhibits a much larger
diamagnetic ring current (NICS(0) = -13.33 ppm).
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35
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37
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41
42
43
Table 3. Calculated Relative Energies (kcal/mol) and NICS Values (ppm) for Tautomers of
Monoprotonated Benzo-23-carbabenziporphyrin. Bold: strongly shielded (aromatic); red:
strongly deshielded.
BCB-24,25H+
0.00
BCB-23,24H+
+1.06
BCB-23,25H+
+1.89
BCB-22,25H+
+13.21
BCB-22,24H+
+12.24
44
45
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Molecule
G (298 K)
B3LYP/
M06-2X/
(0.00/
0.00/
0.00)
(+1.43/
+2.50/
+1.91)
(+2.51/
+3.53/
+2.87)
(+14.33/
+20.48/
+14.83)
(+13.26/
+19.36/
+13.77)
B3LYP-D3)
NICS(0)/NICS(1)zz
NICS(a)/NICS(1a)zz
NICS(b)/NICS(1b)zz
NICS(c)/NICS(1c)zz
NICS(d)/NICS(1d)zz
NICS(e)/NICS(1e)zz
-1.03/-0.03
-5.97/-16.55
+3.43/+5.63
-5.76/-12.27
-4.94/-16.80
-7.00/-23.54
-1.33/-0.16
-4.80/-17.79
-0.64/-5.94
-4.87/-13.09
-2.62/-11.69
-6.70/-23.73
-1.44/-0.67
-5.72/-18.77
-0.40/-5.92
+3.71/+1.60
-5.74/-15.23
-6.89/-24.46
-13.32/-32.49
-16.78/-49.13
+15.43/+33.71
+0.13/-7.91
-11.26/-30.06
-2.62/-12.35
-13.49/-32.66
-16.73/-48.87
+13.62/+32.45
-11.25/-31.40
+0.35/-6.33
-3.23/-13.19
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Table 4. Calculated Relative Energies (kcal/mol) and NICS Values (ppm) for Benzo-23-
carbabenziporphyrin Dications. Bold: strongly shielded (aromatic); red: strongly deshielded.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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29
30
31
32
33
34
35
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37
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43
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47
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49
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51
52
53
54
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59
60
BCB-23,24,25H2+ BCB-22,24,25H2+
Molecule
G (298 K)
B3LYP/
M06-2X/
0.00
+11.70
(0.00/
(+12.28/
0.00/
+16.14/
0.00)
+12.36)
B3LYP-D3)
NICS(0)/NICS(1)zz
NICS(a)/NICS(1a)zz
NICS(b)/NICS(1b)zz
NICS(c)/NICS(1c)zz
NICS(d)/NICS(1d)zz
NICS(e)/NICS(1e)zz
-3.01/-4.25
-3.60/-11.91
-3.04/-12.17
-4.63/-9.21
-7.80/-23.78
-6.53/-24.69
-13.33/-29.99
-15.70/-46.95
+18.33/+47.59
-19.20/-48.20
-12.21/-22.90
-0.38/-5.48
2-Methoxybenzocarbabenziporphyrins (MeOBMBs) were also investigated and for the most
part the free base tautomers showed similar trends (Table 5). Again the 24H tautomer was
slightly more stable than the 25H form, while the fully aromatic tautomer MeOBCB-22H was
approx. 30 kcal/mol higher in energy. However, the NICS calculations suggest that MeOBCB-
24H and MeOBCB-25H possess weak diatropic character, although the NICS(0) values were
only -1.35 and -1.42 ppm, respectively. Monoprotonation was still favored on the nitrogens to
give MeOBCB-24,25H+, but protonation onto the inner indene carbon gave tautomers that were
only 1.27-3.10 kcal/mol higher in energy (Table 6). All three of these tautomers have higher
negative NICS values ranging from -3.00 to -3.30 ppm, and these results are in general
agreement with experimental observations. Nevertheless, the AICD plot for MeOBCB-24,25H+
does not show clear global delocalization pathways (Figure 14). Protonation onto the m-
phenylene unit can give tautomers MeOBCB-22,25H+ and MeOBCB-22,24H+, both of which
have strong diamagnetic ring currents, but these are 12.78-19.46 kcal/mol higher in energy. The
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most stable monoprotonated form, which is consistent with the observed monocations derived
from 12a-c, gives a positive NICS value for ring b and negative values for rings c and d, and
these results suggest that resonance contributors such as 12RH+ are responsible for the increased
diatropic character. Diprotonation onto the interior of the macrocycle might give rise to C-
protonation onto the indene or benzene units. However, the latter species, MeOBCB-
22,24,25H2+, is much higher in energy (Table 7). The indene-protonated structure MeOBCB-
23,24,25H2+ shows a significant increase in diatropic character (NICS(0) = -5.81 ppm) and rings
b, c and d all gave large negative NICS values. This indicates that the macrocyclic ring current
goes around the outside of these rings and indicates that conjugation pathways such as the 22
electron system found in 31 (Figure 15) are responsible for the diatropic ring currents observed
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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33
34
35
36
37
38
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40
41
42
43
2+
for 12a-cH2 . The AICD plot for MeOBCB-23,24,25H2+ confirms the presence of global
delocalization pathways and also suggests that the 22 electron circuit is favored (Figure 14).
Table 5. Calculated Relative Energies (kcal/mol) and NICS Values (ppm) for 2-Methoxy-benzo-
23-carbabenziporphyrin Tautomers. Bold: strongly shielded (aromatic); red: strongly deshielded.
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Molecule
G (298 K)
B3LYP/
M06-2X/
MeOBCB-24H
0.00
MeOBCB-25H
+0.98
MeOBCB-23H
+14.93
MeOBCB-22H
+26.44
(0.00/
0.00/
0.00)
(+1.23/
+1.36/
+1.28)
(+16.19/
+14.26/
+16.37)
0.23/+2.84
6.08/-17.52
+0.79/-2.84
+0.81/-5.16
-1.37/-9.27
-7.04/-24.00
(+28.16/
+32.55/
+28.29)
B3LYP-D3)
NICS(0)/NICS(1)zz
NICS(a)/NICS(1a)zz
NICS(b)/NICS(1b)zz
NICS(c)/NICS(1c)zz
NICS(d)/NICS(1d)zz
NICS(e)/NICS(1e)zz
-1.35/-1.67
-5.67/-15.23
+2.95/+3.34
-4.89/-12.64
-1.14/-8.13
6.84/-22.39
-1.42/-2.16
-6.90/-17.35
+3.61/+4.22
-0.95/-8.38
-4.41/-9.75
-6.78/-22.35
-11.52/-27.54
-15.13/-41.34
+9.27/+18.24
-0.50/-9.01
-0.22/-8.66
-4.74/-17.67
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Table 6. Calculated Relative Energies (kcal/mol) and NICS Values (ppm) for Tautomers of 2-
Methoxy-benzo-23-carbabenziporphyrin Monocations. Bold: strongly shielded (aromatic); red:
strongly deshielded.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
MeOBCB-24,25H+ MeOBCB-23,24H+ MeOBCB-23,25H+ MeOBCB-22,25H+ MeOBCB-22,24H+
Molecule
G (298 K)
B3LYP/
M06-2X/
0.00
+1.27
+1.78
+13.69
+12.78
(0.00/
(+1.67/
(+2.74/
(+14.88/
(+13.93/
0.00/
+2.54/
+3.45/
+19.46/
+18.61/
0.00)
+2.13)
+3.10)
+15.37
+14.43)
B3LYP-D3)
NICS(0)/NICS(1)zz
NICS(a)/NICS(1a)zz
NICS(b)/NICS(1b)zz
NICS(c)/NICS(1c)zz
NICS(d)/NICS(1d)zz
NICS(e)/NICS(1e)zz
-3.30/-5.86
-4.51/-8.86
+3.75/+5.81
-7.14/-13.85
-6.45/-22.10
-6.87/-23.14
-3.00/-4.43
-2.89/-10.01
-2.63/-11.11
-5.98/-15.84
-3.17/-13.16
-7.48/-25.33
-3.00/-5.10
-3.84/-12.24
-2.25/-11.25
+3.04/-0.47
-7.01/-17.74
-7.52/-25.62
-11.35/-27.19
-14.69/-40.63
+12.63/+26.77
-0.74/-9.36
-12.12/-31.53
-3.81/-15.13
-11.81/-28.12
-14.86/-41.24
+10.86/+25.16
-11.73/-31.61
-1.09/-9.55
-4.32/-15.72
Table 7. Calculated Relative Energies (kcal/mol) and NICS Values (ppm) for Unsubstituted 2-
Methoxy-benzo-23-carbabenziporphyrin Dications. Bold: strongly shielded (aromatic); red:
strongly deshielded.
MeOBCB-23,24,25H2+ MeOBCB-22,24,25H2+
Molecule
G (298 K)
B3LYP/
M06-2X/
0.00
+12.50
(0.00/
(+13.00/
0.00/
+14.85/
0.00)
+13.05)
B3LYP-D3)
NICS(0)/NICS(1)zz
NICS(a)/NICS(1a)zz
NICS(b)/NICS(1b)zz
NICS(c)/NICS(1c)zz
NICS(d)/NICS(1d)zz
NICS(e)/NICS(1e)zz
-5.81/-11.97
-0.22/-4.28
-6.23/-21.88
-6.68/-24.01
-9.99/-20.52
-7.97/-27.25
-10.14/-22.84
-12.03/-35.12
+15.08/+37.64
-11.11/-36.23
-12.93/-24.60
-1.97/-9.48
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Figure 14. AICD plots of MeOBCB-24,25H+ and MeOBCB-23,24,25-H2+.
Figure 15. Potential delocalization pathways in methoxy- and dimethoxy-23-
carbabenziporphyrins and related protonated species.
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