Pyrrole-bridged quinones: π-electronic systems that modulate electronic structures by tautomerism and deprotonation

Article abstract of DOI:10.1039/d1cc02691g

A new series of π-extended quinone derivatives containing a pyrrole bridge exhibited NH/OH-type tautomerization and anion binding along with deprotonation that induced near-infrared absorption and ion-pairing assemblies.

Full text of DOI:10.1039/d1cc02691g

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Pyrrole-bridged quinones: p-electronic systems
that modulate electronic structures by
tautomerism and deprotonation†
Cite this: Chem. Commun., 2021,
57, 6983
Received 22nd May 2021,
Accepted 17th June 2021
Shinya Sugiura and Hiromitsu Maeda
*
DOI: 10.1039/d1cc02691g
rsc.li/chemcomm
A new series of p-extended quinone derivatives containing a pyrrole absorption and diradical properties.7,8 In this study, pyrrole-
bridge exhibited NH/OH-type tautomerization and anion binding bridged quinones were synthesized as p-electronic systems that
along with deprotonation that induced near-infrared absorption exhibit tautomerism between quinoidal (NH) and phenol (OH)
and ion-pairing assemblies.
forms and anion-binding and deprotonation behaviours
(Fig. 1b). This study will lead to ion-pairing assemblies whose
Modulation of the electronic structures of p-electronic systems properties are modulated by external conditions.
is essential for the control of properties in dispersed and The precursors of the target pyrrole-bridged quinones,
assembled states.1 The electronic structures can be controlled which included tert-butyl groups for stabilization, were synthe-
by associating specific species (guests) via suitable noncovalent sized via multistep reactions. Firstly, 5-(4-acetoxy-3,5-di-tert-
interactions.2 Further facile modifications can be achieved by butylphenyl)pyrrol-2-yl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
changing the location of protons, that is, protonation, depro- was obtained by the IrI-catalyzed reaction9 of the corresponding
tonation and intramolecular displacement (tautomerism) pyrrole and bis(pinacolato)diboron (Fig. 2a). The boronic acid
(Fig. 1a). Acids and bases, as p-electronic systems, are converted ester was converted to 2,5-bis(4-acetoxy-3,5-di-tert-butylphenyl)
to their charged states by deprotonation and protonation, pyrrole 1a in 53% yield by Suzuki coupling with 1-acetoxy-4-
respectively, and can thus be used as molecular sensors and bromo-2,6-di-tert-butylbenzene in the presence of Pd(PPh3)4
bio-probes.3 On the other hand, tautomerism is an intra- and K2CO3.10 The b-positions of 1a exhibited reactivity, as seen
molecular proton transfer that provides isomeric structures in the bromination using N-bromosuccinimide (NBS) to
with the sites of competitive pKa values, inducing different afford b-brominated 1b in 79% yield.11 Furthermore,
electronic structures and properties.4 A p-electronic system tetraaryl-substituted 1c was synthesized in 40% yield by Suzuki
bearing a mobile proton is pyrrole, which is a five-membered coupling between 1b and phenylboronic acid. After several
heteroaromatic ring and a building unit of extended p-systems trials, pyrrole-based quinoidal derivatives 2a–c were obtained
such as porphyrins. Porphyrin has two NH and two N sites
inside the macrocycle, thus exhibiting protonation, deproto-
nation5 and tautomerism.6 The modification of pyrrole units in
conjunction with proton-accepting and electron-withdrawing
p-units, even in acyclic forms, results in p-electronic systems
exhibiting intramolecular and intermolecular NH proton
migration and corresponding electronic properties. A candidate
proton-accepting and electron-withdrawing p-unit attached to
pyrrole is p-quinone methide. p-Electronic systems substi-
tuted with two p-quinone methide moieties on both sides
exhibit unique electronic properties, including near-infrared
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University,
Fig. 1 (a) Conceptual figure of structure modulations by proton location
(e.g., 2-pyridone; left: tautomerism, right: protonation/deprotonation) and
(b) tautomerization of quinoidal pyrrole, as a parent structure, which
is observed as the quinoidal (NH) and phenol (OH) form (left) and a
deprotonated form (right).
Kusatsu 525-8577, Japan. E-mail: maedahir@ph.ritsumei.ac.jp
† Electronic supplementary information (ESI) available: Synthetic procedures,
analytical data, computational details and cif files for the single-crystal X-ray
analysis. CCDC 2076439–2076443. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d1cc02691g
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Fig. 2 (a) Synthesis of pyrrole-bridged quinones 2a–c and (b) single-
crystal X-ray structures of (i) 2a and (ii) 2c with atom numbering. Atom
colour code in (b) and following figures: brown, pink, blue and red refer to
carbon, hydrogen, nitrogen and oxygen, respectively.
in 66–74% yields by reactions of 1a–c with excess anhydrous
FeCl3 for 5 min in CH2Cl2 (Fig. 2a).12,13 Longer reaction time
(410 min) resulted in lower yields of 2a–c due to the formation
of byproducts. The characterization of the obtained compounds
was conducted using 1H and 13C NMR and ESI-TOF-MS. It is
noteworthy that the 1H NMR spectra revealed the presence of the
quinoidal form 2a–cNH and phenol form 2a–cOH, where the ratios
were dependent on the solvent polarity (vide infra).
Single-crystal X-ray analysis revealed the exact geometries
and packing structures of 2a,c in the solid state (Fig. 2b and
Fig. S16, S17, ESI†), exhibiting the quinoidal forms (2a,cNH).
Hydrogen bonding of water molecules with the pyrrole NH and
carbonyl unit of the p-quinone methide moieties was observed.
Fig. 3 (a) UV/vis absorption spectra of (i) 2a and (ii) 2b (0.010 mM) in
CH3CN (dielectric constant15 er: 35.94; red), MeOH (32.66; orange), acet-
one (20.56; blue), CH2Cl2 (8.93; green) and n-hexane (1.88; light blue) and
photographs under visible light and (b) 1H NMR spectra of (i) 2a and (ii) 2b
in CD2Cl2 at 20 1C.
respectively. On the other hand, 2b,c in CH2Cl2 showed brown
and green colours, respectively, derived from their phenol
forms at lmax of 462 and 467 nm, respectively (Fig. 3a(ii)).17
In MeOH and CH3CN, the absorption of 2b at B450 nm (purple
region) decreased, while new absorption bands were observed
at B600–850 nm (650 and 830 nm in CH3CN). Furthermore, in
acetone, 2b showed a dark green solution with a large and
sharp absorption at 830 nm, which was also observed in
CH3CN. This peak was assigned to the deprotonated species
(red region, vide infra).7a 2a,c also exhibited small absorptions
at B800 nm in acetone.
Quantitative analyses of the tautomers 2aNH and 2aOH were
conducted using 1H NMR. In CD2Cl2, pyrrole NH, quinone methide
CH, and pyrrole CH signals of 2aNH were observed at 8.25, 7.39/7.28
and 7.44 ppm, respectively, whereas quinone methide CH, phenol
CH, pyrrole CH and OH signals of 2aOH appeared at 8.43/7.54, 8.05,
7.68/7.44 and 5.79 ppm, respectively. 2aNH was a major tautomer
with an integration ratio of 89 : 11 (Fig. 3b(i)), providing an equili-
brium constant of 0.12 for NH/OH tautomerization (KT = [2aOH]/
[2aNH]) and the Gibbs energy (DG) of 1.23 kcal molꢀ1 at 20 1C
(Table 1). The observation was consistent with the theoretically
estimated stability of 1.38 kcal molꢀ1 for 2aNH compared to 2aOH
at the PCM-B3LYP/6-31+G(d,p)(CH2Cl2)//B3LYP/6-31+G(d,p) level
0
0
The bond lengths in the quinone methide moieties (C2(2 )–C3(3 )
,
0
0
C5(5 )–C6(6 )) of 2a,c were in the range of 1.34–1.36 Å, suggesting
double bond characters of quinoidal structures. The dihedral
angles between p-quinone methide moieties and the core
pyrrole unit of 2c were 10.21 and 14.81, which were slightly
larger than those of 2a (8.51 and 10.21), due to steric hindrance
by the b-phenyl rings. These values of 2c were smaller than
those of tetraarylpyrroles, such as 2,3,4,5-tetraphenylpyrrole
(25.21 and 34.31),14 due to the quinoid structure.
The tautomerism of 2a–c between the quinoidal and
phenol forms was examined by UV/vis absorption spectra
(Fig. S39–S41, ESI†). A blue solution with an absorption max-
imum (lmax) at 625 nm was detected for 2a in CH2Cl2
(Fig. 3a(i)), which was ascribed to the quinoidal form 2aNH
(observed in the green region) by electronic transition spectra
simulated by time-dependent density functional theory (TD-
DFT) (Fig. S32, ESI†).16 No notable differences were observed in
the UV/vis absorption spectra in more polar solvents (MeOH
and CH3CN). In contrast, absorption bands observed at
400–500 nm in n-hexane were correlated with the theoretical
spectrum of 2aOH. Polar and less polar solvents induced the
preferred formation of the quinoidal and phenol forms,
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Table 1 KT (KT = [2OH]/[2NH]) and DG (kcal molꢀ1) values of 2a–c in several
solvents at 20 1C
2a
2b
2c
CD2Cl2
CDCl3
KT: 0.12,
DG: 1.23
KT: 0.26,
DG: 0.78
Only 2aNH
Only 2bOH
KT: 1.24,
DG: ꢀ0.13
Only 2bOH
KT: 2.19,
DG: ꢀ0.46
Only 2cNH
a
CD3CN
—
a
A mixture of 2bNH, 2bOH and 2bꢀ.
(Fig. S20, ESI†).16,18 However, 2b existed only as 2bOH (Fig. 3b(ii)),
whereas 2c showed the tautomers with a KT value of 1.24 and DG of
ꢀ0.13 kcal molꢀ1 at 20 1C in CD2Cl2 (Table 1 and Fig. S47, ESI†).
The preferred tautomers were controlled by pyrrole b-substituents
and resulting pyrrole NH acidities, based on the rather competitive
pKa values of pyrrole and phenol (23.0 and 18.0, respectively, in
DMSO).19
In CDCl3 (er(CHCl3): 4.81), which is less polar than CD2Cl2,
the KT values of 2a,c increased (Table 1 and Fig. S43, S48, ESI†).
In more polar solvents, such as CD3CN, only quinoidal forms
(2a, cNH) were observed. On the other hand, 2b existed only as
Fig. 4 (a) UV/vis absorption spectra of 2aꢀ (solid line), prepared from 2a
(broken line) upon the addition of TBAOH, and photographs under visible
light in CH2Cl2 (0.010 mM) at 20 1C and (b) 1H NMR spectrum of 2aꢀ,
prepared from 2a (overlayed in grey) upon the addition of TBAOH
(1.5 equiv.) in CD2Cl2 at 20 1C.
1
2bOH in CDCl3 and provided complicated 1H NMR signals in for Clꢀ. In the H NMR spectral changes upon the addition of
CD3CN and acetone-d6 (Fig. S8 and S46, ESI†), suggesting TBACl in CD2Cl2, the signals of 2a corresponding to pyrrole NH
partial deprotonation in polar solvents, which was in agree- and quinone methide CH shifted downfield due to hydrogen-
ment with the UV/vis absorption spectra. The control of tauto- bonding formation (Fig. S54, ESI†). The signals of 2c, as a
merism by solvent polarity can be explained by the effective mixture of tautomers, corresponding to pyrrole NH and qui-
interactions between the pyrrole NH unit and the solvent none methide CH in 2cNH shifted downfield, while the signals
molecules in the quinoidal forms. The OH site in the phenol of 2cOH showed no shifts upon the addition of Clꢀ because of
forms is less suitable for interactions due to bulky tert-butyl the negligible interaction with Clꢀ. The signals of 2cOH
groups. Furthermore, variable-temperature (VT) 1H NMR in decreased, while those of 2cNH increased as 2cNH interacted
CD2Cl2 (1 mM) for 2a revealed decreased KT values at lower with Clꢀ, suggesting that the tautomeric equilibrium shifted
temperatures, as seen in 0.02 at ꢀ40 1C. The temperature- toward the quinoidal form by anion binding (Fig. S56, ESI†).
dependent KT values provided an enthalpy DH of 4.3 kcal molꢀ1 Furthermore, 2b, which existed as 2bOH in CD2Cl2, changed to
and entropy DS of 10.5 cal Kꢀ1 molꢀ1 via the van’t Hoff plots, 2bNH by Clꢀ binding in CD2Cl2. The signals of 2bNH corres-
indicating that 2aNH was more enthalpically stable than 2aOH ponding to pyrrole NH and quinone methide CH at ꢀ50 1C
(Fig. S50, ESI†). A similar tendency was observed for 2c, gradually emerged upon the addition of Clꢀ (Fig. S55, ESI†).
providing smaller DH and DS values of 1.8 kcal molꢀ1 and
The deprotonation of 2a, providing the anionic species 2aꢀ
6.3 cal Kꢀ1 molꢀ1, respectively, whereas 2b exhibited only 2bOH upon the addition of tetrabutylammonium hydroxide (TBAOH),
even at ꢀ50 1C (Fig. S51 and S52, ESI†). The smaller DH value in was confirmed by the UV/vis absorption spectral changes with
2c is ascribed to the more distorted geometry of 2cNH, which is the support of theoretical studies (Fig. 4a and Fig. S20, S23, S27,
seen in the X-ray and optimized structures.
S33, S57, ESI†). In CH2Cl2, the absorption maximum of 2a at
Using pyrrole NH as a hydrogen-bonding donor,20 quinoidal 625 nm disappeared, with the appearance of a sharp absorption
pyrrole derivatives displayed anion binding via the supporting band of 2aꢀ at 773 nm upon the addition of TBAOH. The near-
interactions of quinone methide CH sites. In CH2Cl2, the UV/vis infrared band was derived from the delocalization of the
absorption of 2a, existing mainly as 2aNH, at 625 nm decreased negative charge and the resulting extended p-electronic state,
upon the addition of Clꢀ as a tetrabutylammonium (TBA) salt, as seen in cyanine derivatives.22 No pyrrole NH signal was
1
with the appearance of a new red-shifted absorption at 644 nm observed in the H NMR of 2aꢀ in CD2Cl2, while the quinone
ascribed to the Clꢀ complex 2aNHꢁClꢀ (Fig. S53, ESI†). UV/vis methide CH signal at the pyrrole N side showed a downfield
absorption spectral changes of 2a in CH2Cl2 revealed a binding shift from 7.28 to 8.28 ppm due to hydrogen bonding with the
constant (Ka) of 300 Mꢀ1, which was larger than that of pyrrole deprotonated N site (Fig. 4b). The signals of the pyrrole CH and
(10 Mꢀ1),21 suggesting the electron-withdrawing effect of the neighbouring quinone methide CH exhibited downfield shifts
p-quinone methide units and hydrogen bonding by the qui- from 7.44 to 7.54 ppm and from 7.39 to 7.51 ppm, respectively
none methide CH units. On the other hand, 2b,c, existing (Fig. S58, ESI†). Similar deprotonation behaviours of 2b, c
mainly in the phenol form in CH2Cl2, exhibited anion- were observed in the UV/vis absorption and 1H NMR spectra
binding abilities with Ka values of o1 and 2 Mꢀ1, respectively, (Fig. S57, S59, S60, ESI†).
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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b-substituents, were stabilized by hydrogen bonding with polar
solvents and anions via the pyrrole NH. Deprotonated species,
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diverse fascinating ion-pairing assemblies in combination with
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This work was supported by JSPS KAKENHI Grant Numbers
JP18H01968 for Scientific Research (B) and JP20H05863 for
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.
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