Porphyrin Analogues of a Trityl Cation and Anion

Article abstract of DOI:10.1002/chem.201600473

Porphyrin-stabilized meso- or β-carbocations were generated upon treatment of the corresponding bis(4-tert-butylphenyl)porphyrinylcarbinols with trifluoroacetic acid (TFA). Bis(4-tert-butylphenyl)porphyrinylcarbinols were treated with TFA to generate the corresponding carbocations stabilized by a meso- or β-porphyrinyl group. The meso-porphyrinylmethyl carbocation displayed more effective charge delocalization with decreasing aromaticity compared with the β-porphyrinylmethyl carbocation. A propeller-like porphyrin trimer, tris(β-porphyrinyl)carbinol, was also synthesized and converted to the corresponding cation that displayed a more intensified absorption reaching over the NIR region. meso-Porphyrinylmethyl carbanion was generated as a stable species upon deprotonation of bis(4-tert-butylphenyl)(meso-porphyrinyl)methane with potassium bis(trimethylsilyl)amide (KHMDS) and [18]crown-6, whereas β-porphyrinylmethyl anions were highly unstable.

Full text of DOI:10.1002/chem.201600473

DOI: 10.1002/chem.201600473
Communication
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Stereochemistry |Hot Paper|
Porphyrin Analogues of a Trityl Cation and Anion
Kenichi Kato,[a] Woojae Kim,[b] Dongho Kim,*[b] Hideki Yorimitsu,*[a] and Atsuhiro Osuka*[a]
more general synthetic methods to generate such cations. On
the other hand, porphyrin-stabilized carbanions have not been
characterized in the literature to the best of our knowledge.
Herein, we disclose the effective generation of porphyrin ana-
logues of a trityl cation by using porphyrinyllithium reagents.
We also report the generation of a porphyrin analogue of
a trityl anion.
Abstract: Porphyrin-stabilized meso- or b-carbocations
were generated upon treatment of the corresponding
bis(4-tert-butylphenyl)porphyrinylcarbinols with trifluoro-
acetic acid (TFA). Bis(4-tert-butylphenyl)porphyrinylcarbi-
nols were treated with TFA to generate the corresponding
carbocations stabilized by a meso- or b-porphyrinyl group.
The meso-porphyrinylmethyl carbocation displayed more
effective charge delocalization with decreasing aromaticity
compared with the b-porphyrinylmethyl carbocation. A
propeller-like porphyrin trimer, tris(b-porphyrinyl)carbinol,
was also synthesized and converted to the corresponding
Porphyrinyllithium reagents recently developed by us are
useful reactive species possessing high nucleophilicity.[4] In
fact, meso- and b-lithioporphyrins prepared from the
corresponding iodoporphyrins[5] furnished the corresponding
carbinols 1OH and 2OH by their reactions with 4,4’-di-tert-
butylbenzophenone, respectively (Scheme 1). In the next step,
we attempted to synthesize trisporphyrinylcarbinols by using
lithioporphyrins and soon found that tris(meso-porphyrinyl)-
carbinol was very difficult to synthesize due to serious steric
hindrance. On the other hand, tris(b-porphyrinyl)carbinol 3OH
was formed in approximately 40% yield by reaction of the b-
lithioporphyrin with dimethyl carbonate, but the reaction was
not very clean, hampering the isolation of 3OH. After further
optimization, we found that 3OH was obtained in a better
yield of 72% when b-lithioporphyrin generated upon treat-
ment of b-iodoporphyrin with 4-(N,N-dimethylamino)pheny-
lithium[6] was used for the reaction with dimethyl carbonate in
the presence of LiCl. The addition of LiCl also improved the
yield of 2OH significantly. (Scheme 1, conditions b,c). These
carbinols 1OH, 2OH, and 3OH were reduced to the corre-
sponding triarylmethanes 1H, 2H, and 3H by ionic hydrogena-
tion reaction with HBF4·OEt2 and BH3·NEt3 in excellent to quan-
titative yield (Scheme 1, conditions d). These newly synthesized
compounds have been characterized by high-resolution APCI-
or MALDI-TOF-MS and NMR experiments (see the Supporting
Information).
cation that displayed
a more intensified absorption
reaching over the NIR region. meso-Porphyrinylmethyl
carbanion was generated as a stable species upon depro-
tonation
of
bis(4-tert-butylphenyl)(meso-porphyrinyl)-
methane with potassium bis(trimethylsilyl)amide (KHMDS)
and [18]crown-6, whereas b-porphyrinylmethyl anions
were highly unstable.
Since its discovery as the first identified carbocation in 1901,[1]
the triphenylmethyl cation has played an indispensable role in
the understanding of a huge number of reaction mechanisms
as well as the development of fundamental concepts about p-
electronic delocalization. This unusually stable carbocation and
its derivatives have found widespread usage as protecting
groups, Lewis acids, and chromophores and fluorophores.[2]
With various important functions of porphyrins taken into
consideration, porphyrin-stabilized carbocations are attractive
molecules in terms of effective electronic delocalization and
resultant low-energy absorption. Despite these promises,
porphyrin-stabilized carbocations have been scarcely studied
so far.[3] In these studies, strong absorptions were observed in
NIR region for porphyrin-stabilized cations as a consequence
of effective charge delocalization. It is desirable to develop
The structures of 1OH and 3OH have been revealed by
single-crystal X-ray diffraction analysis to be diarylporphyrinyl
carbinol and trisporphyrinylcarbinol, respectively (Figure 1a–c).
In 3OH, the three porphyrin moieties are tilted with dihedral
1
angles of 40-558 in a propeller-like manner. The H NMR spec-
trum of 1OH in CDCl3 (Figure 2) was slightly broad at room
temperature but became sharp at 608C, probably due to rota-
tional hindrance at room temperature. On the other hand, that
of 2OH was sharp even at room temperature, suggesting less
[a] K. Kato, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku Kyoto, 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
1
rotational hindrance. The H NMR spectrum of 3OH was rather
broad at room temperature in CDCl3 and did not become
almost sharp until heating at 1208C in CDCl2CDCl2, which indi-
cated larger rotational hindrance (Supporting Information).
When the measurement temperature was cooled at À608C,
[b] W. Kim, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
E-mail: dongho@yonsei.ac.kr
1
the H NMR spectrum of 3OH became clear. A singlet at d=
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600473.
10.48 ppm due to the meso-protons, a broad singlet at d=
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Scheme 1. Synthesis of porphyrinylcarbinols and their conversions into the corresponding cations, hydrides, and anion. a) i) 1.5 equiv nBuLi, 3.0 equiv LiCl,
THF, À988C, 30 min, ii) 2 equiv 4,4’-di-tert-butylbenzophenone, À988C to RT, 1 h; b) i) 1.5 equiv nBuLi, THF, À988C, 30 min, ii) 2 equiv 4,4’-di-tert-butylbenzo-
phenone, À988C to RT, 1 h;. c) i) 1.00 equiv 4-(N,N-dimethylamino)phenyllithium, 3.0 equiv LiCl, THF, À988C, 30 min, ii) 0.28 equiv dimethyl carbonate, À988C
to RT, 1 h; d) i) 10 equiv HBF4·OEt2, CH2Cl2, RT, 10 miin, ii) 5.0 equiv BH3·NEt3, RT, 10 min, e) TFA, CH2Cl2, RT, f) 20 equiv [18]crown-6, 20 equiv KHMDS, THF, RT.
Ar=3,5-di-tert-butylphenyl.
Figure 1. X-ray crystal structures of 1OH and 3OH. a) Top view of 1OH.
b) Top view and c) side view of 3OH (one of two independent molecules is
shown). The thermal ellipsoids are drawn at 50% probability. tert-Butyl
groups, all hydrogen atoms, and solvent molecules are omitted for clarity.
Kohn–Sham orbital representations on the optimized structures at the
B3LYP/6-31G(d) level. LUMOs of d) 1+’ and e) 2+’. f) HOMO of 1À’.
5.63 ppm due to the hydroxyl proton, seven signals due to the
pyrrolic b-protons, nine signals due to the meso-aryl protons,
and six signals due to the protons of the tert-butyl groups indi-
cated that rotations of the porphyrinyl groups and 3,5-di-tert-
1
Figure 2. Aromatic region of the H NMR spectra of a) 1OH, b) 1+, c) 2OH,
butylphenyl groups were frozen. On the other hand, the
1H NMR spectrum of 3H at room temperature showed sharp
d) 2+, e) 3OH, f) 3+, g) 1H, and h) 1À. Peaks marked with * are due to
residual solvents and protonated water. Ar=3,5-di-tert-butylphenyl and
signals due to the pyrrolic b-protons and broad signals due to
Ar*=4-tert-butylphenyl.
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the aryl protons, which have been ascribed to less rotational
hindrance around the central carbon (Supporting Information).
Cations 1+, 2+, and 3+ were readily generated upon ad-
dition of trifluoroacetic acid (TFA) (ca. 0.5%) to their solutions
units.[7] Namely, each porphyrin may receive one-third of the
cation effect. In addition, the 1H NMR spectrum of 3+ revealed
substantial rotational hindrance of the porphyrinyl groups
(Supporting Information). Different from 1+ and 2+, 3+ dis-
played an absorption spectrum that contained a relatively
sharp Soret band at l=424 nm and a broad band around
l=958 nm.
1
in CH2Cl2 or CDCl3. In the H NMR spectrum of 1+ taken at
room temperature, the signals due to the pyrrolic protons
were observed as sharp signals in an upfield region (d=7.95–
7.21 ppm) as compared with those (d=8.68–8.40 ppm) of
1OH. Considering the positive charge at the periphery of the
porphyrin, these upfield shifts were unexpected but can be ra-
tionalized in terms of decreased diatropic ring current associat-
ed with the effective charge delocalization onto the porphyrin
p-system, which gives rise to an increased contribution of non-
aromatic resonance contributor. Cation 1+ characteristically
displayed a smeared Soret band at l=424 nm and a broad
band around l=713 nm (Figure 3). Similarly, the pyrrolic
We have attempted to generate porphyrinylmethyl carb-
anions by treating 1H, 2H, and 3H with bases under various
conditions. Treatment of 1H with potassium bis(trimethylsilyl)-
amide (KHMDS) in THF at room temperature did not cause any
change, but we found that in the presence of [18]crown-6,[8]
the solution gave rise to an immediate vivid color change from
red to green. The green solution showed a Soret band at
l=476 nm, a Q-band at l=627 nm, and a broad band at
l=847 nm. The 1H NMR spectrum of the green species in
[D8]THF indicated signals due to the pyrrolic b-protons in the
range of 6.99~6.39 ppm. We assigned the green species as
meso-porphyrinylmethyl carbanion 1À. In line with this assign-
ment, addition of acetic acid to the green solution led to the
recovery of exactly the same absorption spectrum as 1H. The
absorption spectral features indicated that more porphyrinic
character remained in 1À as compared with cations 1+ and
2+. Despite our extensive efforts, we could not find suitable
conditions to generate anions 2À and 3À from 2H and 3H.
Treatments of 2H and 3H with KHMDS and [18]crown-6 under
the same conditions caused color changes but the solution
colors changed rapidly to black in a minute, indicating rapid
decomposition.
The observations discussed above were supported by DFT
calculations at the B3LYP/6-31G(d) level[9] on 1+’, 2+’, and
1À’, in which the tert-butyl groups are replaced by hydrogen
atoms. The HOMO–LUMO gaps of 1+’, 2+’, and 1À’ have
been calculated to be 1.93, 1.67, and 1.94 eV, respectively,
which are roughly consistent with the optical HOMO–LUMO
gaps estimated from the edges of the absorption spectra. It is
conceivable that the vacant p-orbitals of the cations or the
lone pair electrons of the anion are effectively conjugated with
the porphyrinic electronic networks (Figure 1d–f, Supporting
Information). NICS(0) values[10] have also been calculated at the
B3LYP/6-31G(d) and HF/6-31G(d) level (Supporting Informa-
tion). These calculations indicated that the NICS(0) values were
substantially decreased in 1+’ (À6.22~À4.12), 2+’ (À13.28~
À9.47), and 1À’ (À10.57~À8.59), compared those of 0’
(À19.55–À19.39), supporting our interpretations of 1H NMR
spectra.
Figure 3. UV/vis/NIR absorption spectra of a) 0, 1+, 2+, and 3+ in CH2Cl2
and b) 1H and 1À in THF.
Next, we have carried out femtosecond transient absorption
(fs-TA) measurements in order to scrutinize the excited-state
dynamics of tris(b-porphyrinyl)carbinol 3OH, and its cation 3+
. As shown in Figure 4a, 3OH shows simultaneous peak shifts
in the ground-state bleaching (GSB) and excited-state absorp-
tion (ESA) band on the subpicosecond timescale after photoex-
citation. By using global analysis with singular value decompo-
sition (SVD), we could obtain decay-associated spectra (DAS) of
three transient components. The initial decay with t=0.76 ps
has been associated with the transition from the lowest-
protons of 2+ were upfield shifted (d=8.80–8.20 ppm) from
those (d=8.84–8.32 ppm) of 2OH and the absorption spec-
trum of 2+ showed a smeared Soret band at l=407 nm and
a broad band around l=816 nm. Upfield shifts of the pyrrolic
b-protons were also observed for 3+ (d=9.41–8.50 ppm) but
their extents were much smaller compared with those of 1+
and 2+. This may be explained by considering equal delocali-
zation of the single positive charge over the three porphyrin
1
energy (p,p*) state of the porphyrin core to the NiII-centered
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stituent on the sp3 central carbon atom plays no measureable
role in the excited state.
The spectral evolution of transient absorption and decay
profiles of 3+ were entirely different from those of 3OH and
3H (Figure 4b). Interestingly, we found that 3+ did not reveal
any subpicosecond decay time constant, implying the fast de-
1
3
activation from the (p,p*) state to the NiII-centered (d,d) state
did not occur in this case. In other words, the electron in the
excited-state of 3+ was not localized within the nickel of one
porphyrin unit. This observation can be a clear evidence that
3+ behaved as a single chromophore owing to its effective
delocalization of the electrons and positive charge over the
entire molecular framework both in the ground and excited
state. Thus, we can directly assign a time constant with t=
3 ps as a vibrational or structural relaxation process and t=
10 ps as an excited-state lifetime of 3+. The diminished life-
time of 3+ again indicates that the excited-state depopulation
3
did not originate from the (d,d) state and reflects the reduced
HOMO–LUMO gap due to the effective delocalization and
stabilization of the positive charge.
In summary, porphyrin analogues of the trityl cation and
anion were generated and characterized with UV/vis/NIR
absorption and NMR spectroscopies. Positive charge was
delocalized more effectively to a meso-porphyrinyl group with
decreasing aromatic characters of porphyrin than to a b-por-
phyrinyl group. The meso-porphyrinylmethyl carbanion dis-
played the absorption reflecting remaining porphyrinic nature,
in contrast to the meso-porphyrinylmethyl carbocation. The
charge-induced decreased aromaticity of porphyrin macrocy-
cles was also supported by calculated NICS(0) values. We finally
confirmed the effective delocalization and stabilization of the
positive charge in 3+ by using fs-TA measurements, which
showed entirely different excited-state dynamics in comparison
to the neutral species.
Acknowledgements
The work at Kyoto was supported by a Grants-in-Aid from
MEXT (no.: 25107002 “Science of Atomic Layers”) and from
JSPS (no.: 25220802 (Scientific Research (S)), 24685007 (Young
Scientists (A)), 26620081 (Exploratory Research)), and by ACT-C,
JST. The authors thank Prof. Dr. H. Maeda and Dr. Y. Bando (Rit-
sumeikan University) for MALDI-TOF-MS measurements. H.Y.
thanks the Kansai Research Foundation for Technology Promo-
tion and The Asahi Glass Foundation for financial support. The
research at Yonsei University was supported by the Global Re-
search Laboratory (GRL) Program (2013K1A1A2A02050183) of
the Ministry of Education, Science and Technology (MEST) of
Korea.
Figure 4. Transition absorption (top) and decay-associated spectra (bottom)
of a) 3OH, and b) 3+ in CH2Cl2 after photoexcitation at 530 nm.
3(d,d) state and is responsible for TA spectral changes on the
subpicosecond timescale. The second time constant with t=
20 ps can be assigned as a vibrational cooling process from
3
3
hot (d,d) to the relaxed (d,d) state. Following this process, the
relaxed 3(d,d) state decayed to the ground state with t=
180 ps. The observed excited-state dynamics of 3OH were
quite similar to those of NiTPP (Supporting Information),[11]
which indicated that 3OH behaved like a monomer in the ex-
cited state. This is presumably due to negligible excitonic inter-
actions between three porphyrin moieties, despite of their
proximity. However, relaxation time constants of 3OH were
slightly faster than those of NiTPP, which can be ascribed to
the large flexibility of trimeric system. Moreover, the excited-
state dynamics of triarylmethane 3H, were exactly the same as
those of 3OH (Supporting Information). This implies the sub-
Keywords: carbanions · carbocations · electronic structure ·
lithium · porphyrinoids
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Received: February 1, 2016
Published online on April 8, 2016
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