Oxidative C-N fusion of pyridinyl-substituted porphyrins

Article abstract of DOI:10.1039/c8cc01375f

The mild (electro)chemical oxidation of pyridin-2-ylthio-meso substituted Ni(ii) porphyrins affords C-N fused cationic and dicationic pyridinium-based derivatives. These porphyrins are fully characterized and the molecular structure of one of them was confirmed by X-ray crystallography. A mechanism for the intramolecular oxidative C-N coupling is proposed based on theoretical calculations and cyclic voltammetry analyses.

Full text of DOI:10.1039/c8cc01375f

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Oxidative C-N Fusion of Pyridinyl-Substituted Porphyrins†
Mathieu Berthelot,a Guillaume Hoffmann,a Asmae Bousfiha,a Julie Echaubard,a Julien Roger,a
Hélène Cattey,a Anthony Romieu,a,b Dominique Lucas,a Paul Fleurat-Lessarda and Charles H.
Devillers*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The mild (electro)chemical oxidation of pyridin-2-ylthio-meso palladium,6c,6d,8 scandium,2,9 gold10 and organic oxidants such as
substituted Ni(II) porphyrins affords C-N fused cationic and DDQ11, PIDA,12 PIFA12-13 often under relatively harsh conditions
dicationic pyridinium-based derivatives. These porphyrins are fully (chemical oxidizers in large excess,6d high temperatures,14 acidic
characterized and the molecular structure of one of them was medium15). These synthesized π-extended compounds are
confirmed by X-ray crystallography.
A mechanism for the easier to oxidize than their starting unfused precursors due to
intramolecular oxidative C-N coupling is proposed based on the extended conjugation path. This feature could result in their
theoretical calculations and cyclic voltammetry analyses.
over-oxidation/degradation, sometimes leading to low yield
and/or poor air stability. Recently, two examples of mild
electrochemically-driven intramolecular oxidative C-C coupling
of meso-substituted-4,7-dimethoxynaphthalen-1-ylporphyrins
have been reported.16 As demonstrated by the authors, the
porphyrin cation radical was not sufficiently reactive to induce
the C-C coupling. Formation of the porphyrin dication was
necessary to promote the fusion reaction. At this applied
potential, the fused product is oxidized to its reactive dication
which may lead to further degradation. We reasoned that a
peripheral substituent which might be able to generate a
positive charge during the fusion reaction may allow to
overcome this overoxidation issue. Pyridine is known to react in
an intermolecular fashion with porphyrin cation radicals leading
to pyridinium-porphyrin derivatives.17 These latter exhibit much
higher oxidation potential (ca. from +100 to +300 mV) than the
initial porphyrins accounting for the very good selectivity of the
reaction.17a To the best of our knowledge, only two examples of
pyridine-based C-N intramolecular oxidative π-extension of an
aromatic compound are reported. The first one described the
electrochemical oxidative C-N fusion of a naphthol-substituted
pyridine but the yield was only 4%.18 In the second example the
electrochemically generated C-N benzene-based fused
compounds were non-isolated intermediates.19 To our best
knowledge, no example of C-N fused porphyrin synthesized by
the direct intramolecular oxidative C-N fusion of a peripheral
Over the last two decades π-extension of aromatic derivatives
has attracted much attention because of potential applications
in near-IR (NIR) electroluminescence displays, photovoltaic
solar cells, non-linear optical materials, photodynamic therapy
and molecular electronics.1 The additional covalent C-C bond
generated by the fusion reaction forces the aromatic core and
the aromatic substituent to be coplanar which promotes
enhanced electronic communication between both fragments.
These π-extended molecules display important changes in their
optical (bathochromic shift in their absorption/emission
spectrum, large absorption and fluorescence in the NIR range,
increase in the two-photon absorption cross section,...) and
electrochemical (drastic decrease of the HOMO/LUMO gap)
properties by comparison with the non-fused parent aromatic
core.1b,2 In particular for porphyrins, “New efficient fusion
reactions under milder conditions are highly desirable in future
developments”.3 For this purpose, “a better understanding of
the mechanism of these intramolecular oxidative couplings is
needed”.4 Currently, these additional connections are produced
by intramolecular C-C coupling using silver,5 iron,6 copper,7
a.Institut de Chimie Moléculaire de l'Université de Bourgogne UMR6302, Université
de Bourgogne Franche-Comté, 9 avenue Alain Savary, 21078 Dijon, France. E-
mail: charles.devillers@u-bourgogne.fr
heterocyclic imine with a meso and/or
 carbon from the
b.Institut Universitaire de France, 103 boulevard Saint-Michel, 75005 Paris
†Electronic Supplementary Information (ESI) available: Experimental details and
porphyrin core is reported. (2-Pyridyl)thio was selected as the
peripheral meso-substituent to assess the feasibility of this
reaction. Moreover, among the different metalloporphyrins,
the Ni(II) complex was chosen since Ni(II) insertion makes the
porphyrin more electrophilic, which should favor the C-N
characterization (1H/13C, 2D (COSY, NOESY, HSQC, HMBC) NMR, UV-Vis. spectra,
−
HRMS spectra, cyclic voltammograms) of compounds
1
-
3
and 1fus+,PF6
-
3fus+,PF6− and
−
anti-3fus2+,(PF6
)
2. Crystallographic data for
1
(CCDC 1557313),
2
(CCDC 1557314)
and 1fus+,PF6 (CCDC 1813003). Computational details: protocol to evaluate peak
potentials, detailed mechanisms, Cartesian coordinates of all compounds. See
DOI: 10.1039/x0xx00000x
−
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Journal Name
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DOI: 10.1039/C8CC01375F
Pyridine
β-pyrrolic
Phenyl
Tolyl
Scheme 1 Intramolecular Oxidative C-N Fusion Reaction of Porphyrins 1-3.
δ / ppm
9
8
7
6
Figure 1 Partial 1H NMR spectra of 1 and 1fus+,PF6− (CD3COCD3, 500 MHz, 300 K).
−
Scheme 3 Synthesis of anti-3fus2+,(PF6
)
2
Scheme 2 Synthesis of 1-3.
−
conditions were applied to
2 and 3
, providing 2fus+,PF6 and
oxidative fusion reaction. To our delight, oxidation of
3fus+,PF6 in 81 and 88 % yields respectively.† Oxidation of
−
porphyrins
1
-
3
gives for the first time, the C-N fused and
3fus+,PF6 with 1.0 eq. of PIFA leads to the doubly-fused
−
+
positively charged pyridinium-porphyrins 1fus
-
3fus+ (Scheme 1).
were
5,15-bis(p-tolyl)-10-
dicationic compound anti-3fus2+,(PF6 )2, after anion exchange
−
The synthesis of 1-3 is depicted in Scheme 2.† 7 and
8
(31% yield). Remarkably, the reaction is perfectly regioselective
as only the anti (3,13-fused) regioisomer was observed (Scheme
3). A similar regioselectivity has been previously reported when
Ni(II) 5,15-bis(4-azulenyl)porphyrin was oxidized with FeCl3
leading to the formation of two C-C bonds in an anti-
configuration.9a Theoretical study on the C-N bond formation
revealed that the anti isomer is favored both kinetically and
thermodynamically. Indeed, the SOMO of the cation radical of
-position while
-position (Figure
S102). Moreover, the anti isomer is also more stable than the
syn one by 0.5 kcal mol-1.
NMR (Figure 1 for 1fus+,PF6−) and ESI-HRMS analyses confirm
the molecular structure of the fused compounds.† In particular,
prepared
by
bromination
of
phenylporphyrin 420 and 5,15-bis(p-tolyl)porphyrin 520 with 1.2
and 0.8 eq. of NBS, respectively.21 To favor the
monobrominated porphyrin
NBS was used with . In these conditions, a mixture containing
(28%), (60%) and (12%) was obtained. Exhaustive meso-
dibromination of the porphyrin ring22 was not reached with the
free base porphyrin , even with an excess of NBS, but was
achieved with the corresponding zinc(II) complex 6 23
leading
quantitatively to the dibromoporphyrin 10. Demetalation of 10
with TFA afforded the free base porphyrin . The free base
bromoporphyrins and were then functionalized with 2-
mercaptopyridine via SNAr reactions24 providing porphyrins 11
12 and 13 in 78, 61 and 69% yield, respectively. In the last step,
8, a substoichiometric amount of
5
5
8
9
5
3fus+,PF6- presents a large coefficient on the anti
no electronic density is observed on the syn

,

9
7
,
8
9
,
the singlet integrating for one proton (two protons for anti-
nickel(II) insertion leads to the complexes
yields.
1, 2 and 3 in good
−
3fus2+,(PF6 )2) is assigned to the

-pyrrolic proton close to the
position where the fusion takes place. Besides, due to loss of
symmetry of the singly-fused molecules, the proton chemical
shifts for the pyrrolic and tolyl fragments differ from each other.
As a characteristic 1H NMR feature of these singly-fused
compounds, 1fus+,PF6− exhibits two singlets integrating each one
for 3 H at 2.73 and 2.69 ppm (in CD3COCD3) corresponding to
the methyl fragments (Figure S23).
1-3 have been characterized by NMR, HRMS, UV-vis
absorption spectroscopy, cyclic voltammetry† and, for 1‡ and 2 §
,
1
by X-ray crystallography. In the H NMR spectra (recorded in
CDCl3) of , four pyridine-based multiplets appear between
1-3
5.80 and 8.45 ppm (Figures S16, S41 and S68). Due to the
porphyrin proximity, these signals are shielded or unshielded as
compared to the 2-mercaptopyridine molecule which displays
four pyridine-based signals between 6.75 and 7.58 ppm.
The efficacy of the chemical oxidative coupling has been first
The molecular structure of 1fus
by X-ray diffraction analyses on monocrystals (Figure 2).§§
Comparing 1fus , the formation of the new Cβ-N bond
+ PF6− and
,
+ PF6− was definitively proven
,
1
evaluated on
(AgPF6, Fe(ClO4)3, DDQ/Sc(OTf)3, PIFA and PIDA), PIFA gave
satisfying results. Thus, oxidation of with 1.2 eq. of PIFA at rt
1 in CH2Cl2. Among the different tested oxidants
(1.423(5) Å) leads to a shorter Cmeso-S distance (1.744(5) vs.
1.772(3) Å) while the Cmeso-S-Cpyridine angle (105.4(2) vs.
103.02(13)°) increases. Each porphyrin interacts with two other
porphyrins by π-stacking (distance between two mean planes =
3.71 Å) leading to an infinite 1D network (Figure S99).
1
produces 1fus+,CF3CO2−. To facilitate the purification process,
CF3CO2− anion was exchanged for PF6− using an anion exchange
resin affording 1fus+,PF6− in excellent yields (98%). Similar
2 | J. Name., 2012, 00, 1-3
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O3
View Article Online
DOI: 10O.12039/C8CC01375F
O1
1 µA
O4
before electrolysis
R3
O3
R2
O2
R4
after electrolysis
O7
O8
R3
R2
−
Figure 2 Front (left, PF6 anion omitted for clarity) and side (right) Mercury views of
1fus+,PF6−. Thermal ellipsoids are scaled to the 50% probability level.
R5
R7
R6
R8
-1.5
425
-1.0
-0.5
0.0
0.5
1.0
1.5
3
E / V (vs SCE)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-
3fus+,PF6
432
-
Figure 4 Cyclic voltammetry of a 10-3 M solution of 1 in CH2Cl2 0.1 M TBAPF6 before (top)
and after (bottom) electrolysis at Eapp = 0.96 V/SCE (2.5 F),  = 100 mV.s-1, WE: Pt, Ø = 1
anti-3fus2+,(PF6 )2
439
mm.
reductions of the porphyrin ring leading to the radical anion and
dianion. Interestingly, potentials and reversibility of peaks
O2/R2 and O3/R3 remain unchanged before and after
electrolysis. These observations corroborate the formation at
the electrolysis time scale of the same product already observed
319
599
581
556
550
294
290
636
337
542
578
300 350 400 450 500 550 600 650 700 750 800
/ nm
during the CV analysis of 1. High resolution MALDI-TOF mass
−
Figure 3 UV-vis. absorption spectra of 3, 3fus+,PF6− and anti-3fus2+,(PF6
7.5×10-6 M, l = 1.00 cm).
) in DMF (C =
2
spectrometry analysis of the crude electrolyzed solution
confirms the exclusive formation of 1fus+, with an m/z peak =
730.1559 (expected m/z = 730.1570 corresponding to the loss
As compared to 1-3, the UV-Vis. absorption spectra of the of one mass unit as compared to 1 for which m/z = 731.1679
fused products† are red-shifted and a significant widening of the (expected m/z = 731.1654). The electrogenerated fused
−
Soret bands is observed in agreement with the formation of π- compound was finally purified affording 1fus+,PF6 in 71%
extended porphyrins (see as an example Figure 3 for
3
,
isolated yield. Exhaustive electrolyses of
2 and 3 at the first
−
3fus+,PF6−and anti-3fus2+,(PF6 )2).
oxidation potential in similar conditions also lead to the fused
1-3
have been analyzed by cyclic voltammetry (CV). Contrary compounds 2fus+,PF6 and 3fus+,PF6 in 52 and 72% yields
−
−
to what is commonly observed in CH2Cl2 for nickel(II) respectively. Synthesis of 3fus2+,(PF6 )2 was achieved upon
−
porphyrins,25 the first oxidation peak of 1-3 (peak O1, Figure 4 electrolysis of 3fus+,PF6− in CH3CN (23% yield).
for
1
and Figures S2 and S3 for
2
and
3
respectively) is fully
When oxidized, 2 could follow two different pathways: 1) an
irreversible which indicates that the electrogenerated intermolecular oxidative Cmeso-Cmeso coupling5,26 and/or 2) an
porphyrin cation radical is not stable at the CV time scale. intramolecular oxidative C-Npyridine coupling, leading to the
Reduction of the cation radical is only observed for scan rate fused derivative. In our conditions, we did not notice any meso-
higher than 100 V/s that gives an upper estimation of the half- meso dimer formation. The oxidation and C-N bond formation
life time of the cation radical (t1/2<5 ms). This irreversible mechanisms have been theoretically investigated.† As for
behavior indicates that 1fus+-3fus+ (with PF6− as the counter anion pyridine,17a we confirmed that the C-N fusion of
coming from the supporting electrolyte) are produced at the directly on the cation radical 1+● (Scheme 4). It proceeds with a
electrode surface during the CV analysis. For , the second and barrier of 12.8 kcal/mol corresponding to a half-life time of 0.3
1 occurs
1
third oxidation systems (peaks O2/R2 and O3/R3, Figure 4) are ms, in fair agreement with the experimental upper limit t1/2<5
fully reversible and are assigned to the oxidation of the fused ms. As expected,17a oxidation of the fused cation radical occurs
compound 1fus+,PF6− already formed at the electrode surface.
An exhaustive electrolysis at an applied potential
corresponding to peak O1 was performed in presence of 2
equivalents of K2CO3 to avoid protonation and thus inactivation
of the pyridine moiety during the electrolysis. After abstraction
of ca. 2.5 Faradays, CV of the resulting solution revealed the
disappearance of peak O1 in accordance with full consumption
of
1 and the appearance of a new irreversible reduction peak
(R6 at Epc = −0.77 V/SCE, Figure 4), which potential typically
corresponds to pyridinium reduction.17b The reversible
reduction peaks R7/O7 and R8/O8 are attributed to the
Scheme 4 Oxidation and C-N fusion of 1.
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Journal Name
푡ℎ
푡ℎ
4. J. P. Lewtak and D. T. Gryko, Chem. Commun., 2012,
View A4rti8cl,e Online
( )
푂퐴 = 0.96 V vs. 퐸 (푂1) =
푝푎 푝푎
at a lower potential than O1:
퐸
10069.
DOI: 10.1039/C8CC01375F
1.04 V. O2 and O3 correspond to the oxidation of the fused
compound. The oxidative fusion of
is oxidized, 3fus will be formed first by following the
(irreversible peak O1 in Figure S3). Then 3fus
can be easily oxidized into a cation radical that will form the
doubly fused anti-3fus molecule that is then overoxidized.
Oxidation of protonated fused radical cations are hidden as they
occur at lower potentials than O1 and O2 respectively:
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2
follows a similar route.
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When
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푡ℎ
퐸 (푂퐴)
푝푎
푡ℎ
푡ℎ
= 1.05 V (vs.
퐸
퐸 (푂1) = 1.13 V) and 퐸 (푂퐵) = 1.22 V (vs.
푝푎 푝푎
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performed in mild conditions (nearly stoichiometric amount of
PIFA oxidizer, low oxidation potential, room temperature)
leads, for the first time, to the formation of C-N fused
pyridinium-based porphyrins. These unprecedented positively
charged compounds are harder to oxidize than their precursors
which allows to reach very good selectivity and fair to good
yields for the fusion reaction. The ECEC mechanism proposed
for the intramolecular C-N coupling is supported by theoretical
calculations and voltammetric analyses. We want now to
extend the scope of this efficient reaction to other peripheral
substituents and porphyrin complexes and to explore the
different applications of these porphyrin family newcomers.
This work is supported by the CNRS, Université de
Bourgogne, Conseil Régional de Bourgogne through the "Plan
d'Actions Régional pour l'Innovation (PARI)" and the "Fonds
Européen de Développement Régional (FEDER)" programs. A.B.
acknowledges the Ministère de l’Enseignement Supérieur et de
la Recherche for a PhD grant. C. H. D. thanks the CNRS (Sept.
2015, one year “délégation CNRS”) and the Agence Nationale
de la Recherche for funding (ANR-15-CE29-0018-01). A. R.
thanks the Institut Universitaire de France (IUF) for financial
support. The authors are thankful to Dr. B. Habermeyer
(PorphyChem Company) for generous gift of 5,15-
ditolyporphyrin sample, S. Fournier for technical support, M.-J.
Penouilh for ESI-HRMS analyses. Calculations were performed
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Conflicts of interest
There are no conflicts to declare.
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§ CCDC 1557314
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§§ CCDC 1813003
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