Oxidative annulation of β-aminoporphyrins into pyrazine-fused diporphyrins

Article abstract of DOI:10.1002/anie.201108037

Dimers are a girl's best friend: The oxidative fusion reaction of β-aminoporphyrins with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) under high-dilution conditions furnishes pyrazine-fused diporphyrins 1 in high yields. In a further application of the

Full text of DOI:10.1002/anie.201108037

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DOI: 10.1002/anie.201108037
Porphyrin Chemistry
Oxidative Annulation of b-Aminoporphyrins into Pyrazine-Fused
Diporphyrins**
Masanari Akita, Satoru Hiroto, and Hiroshi Shinokubo*
Covalently linked porphyrin arrays have attracted much
attention as biomimetic light harvesting systems for photo-
synthesis, optoelectronic devices, and molecular wires.[1] The
construction of covalent linkages between porphyrin units
often employs palladium-catalyzed cross-coupling[2] or oxida-
tive coupling reactions.[3] In particular, the oxidative dimeri-
zation of porphyrins offers a versatile and reliable method for
the construction of multiporphyrin arrays. Treatment of meso-
unsubstituted porphyrins with strong oxidants, such as
AgPF6,[4a,b]
(DDQ)-Sc(OTf)3
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
[4c]
(Tf = trifluoromethanesulfonyl), and
phenyliodine bis(trifluoroacetate),[4d] yields directly linked
porphyrin oligomers with high regioselectivity. These trans-
formations always include bond formation at the meso
position because of its high reactivity.
Among such oxidative coupling reactions, a fusion reac-
tion allows the connection of two p-electronic systems by two
or more bonds in a single step. This strategy is highly useful
for constructing rigid and planar structures, which would be
effective for p-conjugation and suppression of energetic
decay in the photoirradiated excited state. However, such
oxidative fusion reactions with porphyrins have been limited
to the formation of meso–b doubly linked and meso–meso, b–
b, b’–b’ triply linked diporphyrins.[4c] In the course of our
research on b-functionalized porphyrins, we serendipitously
found an oxidative dimerization of b-aminoporphyrins that
efficiently provides pyrazine-fused diporphyrins. Interest-
ingly, the reaction proceeds smoothly at the b positions with
perfect regioselectivity to construct two direct linkages in one
step.[5]
Scheme 1. Synthesis and oxidation of b-aminoporphyrins. Reaction
=
conditions: a) Ph2C NH (3.0 equiv), [Pd2(dba)3]·CHCl3 (5 mol%),
Xantphos (10 mol%), Cs2CO3 (2.0 equiv), dioxane, reflux, 24 h;
b) Conc. HCl, THF, reflux, 1 h; c) DDQ (4 equiv), CHCl3, RT, 1.5 h for
4a and 1h for 4b; d) H2SO4, TFA, RT, 1.5 h; e) Zn(OAc)2·2H2O,
MeOH, CHCl3, reflux, 3 h.
We examined the oxidation of 3a, which was prepared
through the Pd-catalyzed cross-coupling of porphyrin bistri-
flate 1a[7] with benzophenone imine as the key step
(Scheme 1). The addition of 4.0 equiv of DDQ into a dilute
solution of 3a in chloroform (1.0 mmolLÀ1) at room temper-
ature afforded 4a as a single product in 63% yield. The
reaction was sensitive to the reaction media, and no reaction
occurred in dichloromethane. The high-resolution mass
spectrum of 4a contained the parent mass-ion peaks at m/
z = 1938.0031 (calcd for
C124H142N12Ni2Na: 1938.0080
[M+Na+]), which suggests the formation of a dimeric species
of 3a. The 1H NMR spectrum of 4a has four doublet
resonances and one singlet resonance for the b protons,
which indicates the presence of unsymmetrical porphyrin
units. Relative to 3a, two signals from the aryl protons were
shifted to lower field at d = 8.17 and 8.14 ppm, probably as
a result of a deshielding effect by the neighboring porphyrin
ring current. The NOESY spectrum of 4a (Figure S10 in the
Supporting Information) showed correlations between these
aryl protons and a meso proton of the porphyrin. On the basis
of these spectral data, we assigned 4a as a pyrazine-fused
diporphyrin (Scheme 1).[8,9] The oxidation of b-monoamino-
porphyrin 3b also afforded the corresponding pyrazine-fused
dimer 4b in 82% yield. The present reaction also proceeded
with free-base b-aminoporphyrin 3c to produce 4c in 11%
yield (in two steps from 2c). Furthermore, free-base 4c was
obtained in 84% yield by demetalation of 4b with H2SO4. 4b
was then converted to biszinc complex 4d in 60% yield upon
[*] M. Akita, Dr. S. Hiroto, Prof. Dr. H. Shinokubo
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University, Aichi, 464-8603 (Japan)
E-mail: hshino@apchem.nagoya-u.ac.jp
[**] This work at Nagoya University was supported by Grant-in-Aids for
Scientific Research (no. 23655033 and no. 22750036) from MEXT
(Japan) and the Global COE program in Chemistry of Nagoya
University. H.S. acknowledges the Toray Science Foundation for
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108037.
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Figure 2. UV/Vis absorption spectra of 3a (g), 3b (b), 4a
(ca), 4b (d), and tris(3,5-di-tert-butylphenyl)porphyrin NiII (8,
c) in CH2Cl2.
electrochemical analysis. Table 1 summarizes the electro-
chemical data of 3a, 3b, 4a, and 4b measured by cyclic
voltammetry (Figure S15 in the Supporting Information). The
dimers 4a and 4b have relatively smaller DE values (the gap
between the first oxidation and the first reduction potentials
Table 1: Summary of cyclic voltammetry of 3a, 3b, 4a, 4b, and tris(3,5-
di-tert-butylphenyl)porphyrin NiII (8).[a]
Compound
Eox2 [V]
Eox1 [V]
Ered [V]
DE [V]
8
–
–
–
0.524
0.148
0.307
0.270
0.488
À1.84
À1.88
À1.84
À1.61
À1.58
2.36
2.02
2.15
1.88
2.07
Figure 1. X-ray crystal structures of 4b and 7: a) Top view and b) side
view of 4b. c) Top view of 7. Hydrogen atoms and meso-aryl groups of
4b are omitted for clarity. Thermal ellipsoids are at 50% probability
level.
3a
3b
4a
4b
0.448
0.729
[a] Solvent: CH2Cl2; electrolyte: Bu4NPF6; working electrode: Pt; counter
electrode: Pt; reference electrodte: Ag/AgClO4; scan rate: 0.1 VsÀ1
potentials are recorded versus ferrocene/ferrocenium ion couple.
;
treatment with Zn(OAc)2·2H2O. The structure of 4b was
unambiguously elucidated by X-ray diffraction analysis
(Figure 1).[10] A suitable crystal was obtained by vapor
diffusion of acetonitrile into a solution of 4b in carbon
tetrachloride. The whole structure of 4b is twisted as a result
of the ruffled nickel(II) porphyrin subunits, in contrast to
a quite planar geometry around the central pyrazine ring. The
1
(Eox ÀEred)), than the monomers 8 and 3b, which indicates the
decrease in the HOMO–LUMO gaps. Furthermore, splitting
of the oxidation potential was detected for both 4a and 4b.
These results suggest effective electronic interaction between
the two porphyrin rings.[12] Notably, the difference in the
oxidation potential between 4a and 3b is smaller than that for
the reduction potential. This trend is also detected for 8 and
4b. This result implies that the formation of the pyrazine ring
mainly affects the LUMOs of porphyrin rings to reduce the
HOMO–LUMO gaps.
À
bond length of C1 C2 is 1.45 ꢀ, which is almost equivalent to
2
2
À
a standard C(sp ) C(sp ) single bond length (ca. 1.47 ꢀ).
Furthermore, the bond lengths in the central pyrazine core
À
À
À
are 1.40 ꢀ for C2 C3, 1.34 ꢀ for C3 N5, and 1.34 ꢀ for N5’
C2. On the basis of these bond lengths, the harmonic
oscillator model of aromaticity (HOMA) value of the
pyrazine moiety is calculated to be 0.977, which indicates
strong aromaticity in the pyrazine ring.
The oxidative fusion reaction is not limited to 5,10,15-
triarylporphyrins 3, which have one unsubstituted meso
position. Oxidation of 2-aminotetraarylporphyrins with the
Dess–Martin periodinane reagent is known to produce the
corresponding porphyrin a-diones in moderate yields.[13] In
contrast, we found that treatment of 2-amino-5,10,15,20-
tetraphenylporphyrin (5) with DDQ in chloroform afforded
the pyrazine-fused dimer 6 in 18% yield (Scheme 2). The
pyrazine-fused structure was assigned by its highly symmetric
1H NMR spectrum, which has only three signals for the
b protons. Furthermore, the high-resolution mass spectrum,
Figure 2 shows the UV/Vis absorption spectra of 3a and
4a, as well as tris(3,5-di-tert-butylphenyl)porphyrin nickel(II)
(8). Aminoporphyrin 3a has a typical spectrum for a mono-
meric porphyrin. The red-shifted Q-band of 3a is a result of
electron donation from the amino groups to the porphyrin
core. In contrast, 4a has split Soret bands at 399, 411, and
489 nm.[11] The Q-band has a substantial bathochromic shift
owing to effective p-conjugation between the two porphyrin
rings. Such intramolecular interactions were investigated by
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Figure 3. UV/Vis absorption spectra of 4b (d), 6 (c), and 7
(g) in CH2Cl2.
spectrum of 7 are not split, which suggests the presence of
negligible exciton-coupling interactions between the two
porphyrin chromophores. This spectral characteristic of 7 is
in line with reported b–b singly linked diporphyrins.[14] The
electrochemical properties of 6 were also investigated by
cyclic voltammetry (Figure S16 in the Supporting Informa-
tion). As for 4a and 4b, 6 has two reversible oxidation
potentials at 0.575 and 0.419 V and two reversible reduction
potentials at À1.60 and À1.72 V. These results also support
the pyrazine-fused dimeric structure of 6.
In summary, we have developed an oxidative dimerization
of b-aminoporphyrins by the formation of a fused pyrazine
ring. The dimer has effective p-conjugation, as shown by the
significantly red-shifted absorption spectra. This dimerization
protocol is promising for connecting p-conjugated molecules
to construct rigid p-systems in a one-pot operation. Further-
more, the oxidation of 2-amino-5,10,15,20-tetraphenylpor-
phyrin with DDQ provides the diamino, b–b linked dipor-
phyrin 7, which is a route to C2-symmetric diamines. This
porphyrin dimer would be applicable as a chiral ligand or an
organocatalyst. Further investigation on oxidative fusion
reactions and the application of diporphyrins is ongoing.
Scheme 2. Oxidation of b-aminotetraphenylporphyrin.
contained the parent mass-ion peaks at m/z = 1365.3172
(calcd for C88H53N10Ni2: 1365.3156 [M+H+]), which also
confirmed the formation of 6. The formation of the pyra-
zine-fused dimer 6 is rather remarkable, since the dimeriza-
tion process would be required to endure the steric repulsion
between the meso-phenyl groups on the two coupling
porphyrins.
Interestingly, we could isolate another product 7 in 14%
1
yield from the reaction mixture. The H NMR spectrum of 7
has six doublet resonances for b protons and two NH protons.
The resonances from one of the phenyl groups were detected
in the shielded region at 7.30, 6.46, 5.90, 5.05, and 4.24 ppm.
The dimeric structure was confirmed by high-resolution mass
spectrometry. On the basis of these data, we characterized 7
as a b–bꢁ singly linked diporphyrin (Scheme 2). The structure
of 7 was finally elucidated by X-ray diffraction analysis
(Figure 1c).[14] A fine crystal was obtained by vapor diffusion
of ethanol into a solution of 7 in chlorobenzene. The two
porphyrin units are connected directly at the b position
adjacent to the amino group, and the dihedral angle between
the two proximal pyrrole units is 548. The bond length of the
linkage between the two porphyrins is 1.46 ꢀ, which is normal
Received: November 15, 2011
Revised: January 5, 2011
Published online: February 8, 2012
2
2
À
for a C(sp ) C(sp ) single bond and indicates little p-
conjugation between the two porphyrins. The phenyl group
on one porphyrin moiety is overlapped with the other
porphyrin ring and the average distance between the two
moieties is 3.18 ꢀ, which suggests the existence of a p–p
interaction. Diporphyrin 7 is a C2-symmetric diamine and
probably has axial chirality as a result of restricted bond
rotation similar to that in 1,1’-binaphthyl-2,2’-diamine, which
is widely used as a chiral source in asymmetric syntheses. We
are currently attempting the optical resolution of 7.
Keywords: fused-ring systems · oxidation · porphyrinoids ·
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pyrazines
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