Chemo- and Regioselective Reduction of 5,15-Diazaporphyrins Providing Antiaromatic Azaporphyrinoids

Article abstract of DOI:10.1002/chem.201600066

Reagent-controlled chemo- and regioselective reduction of 5,15-diazaporphyrins has been developed. The selective reduction of carbon-carbon double bonds of diazaporphyrins provides 18 π aromatic isobacteriochlorin-type products, whereas the reduction of carbon-nitrogen double bonds leads to selective formation of 20 π N,N′-dihydrodiazaporphyrins in excellent yields. The distinct antiaromatic character of N,N′-dihydrodiazaporphyrins has been revealed. The free-base N,N′-dihydrodiazaporphyrin exhibits slower inner NH tautomerism than that in the corresponding 18 π porphyrins. Reagent-controlled chemo- and regioselective reduction of 5,15-diazaporphyrins has been developed. The selective reduction of carbon-carbon double bonds of diazaporphyrins provides aromatic isobacteriochlorin-type products, whereas the reduction of carbon-nitrogen double bonds leads to N,N′-dihydrodiazaporphyrins with distinct antiaromatic character.

Full text of DOI:10.1002/chem.201600066

DOI: 10.1002/chem.201600066
Communication
&
Porphyrinoids
Chemo- and Regioselective Reduction of 5,15-Diazaporphyrins
Providing Antiaromatic Azaporphyrinoids
Ayaka Yamaji,[a] Hayato Tsurugi,[b] Yoshihiro Miyake,*[a] Kazushi Mashima,[b] and
Hiroshi Shinokubo*[a]
Abstract: Reagent-controlled chemo- and regioselective
reduction of 5,15-diazaporphyrins has been developed.
The selective reduction of carbon–carbon double bonds
of diazaporphyrins provides 18p aromatic isobacterio-
chlorin-type products, whereas the reduction of carbon–
nitrogen double bonds leads to selective formation of
20p N,N’-dihydrodiazaporphyrins in excellent yields. The
distinct antiaromatic character of N,N’-dihydrodiazapor-
phyrins has been revealed. The free-base N,N’-dihydrodia-
zaporphyrin exhibits slower inner NH tautomerism than
that in the corresponding 18p porphyrins.
Porphyrinoids have attracted considerable attention, owing to
Figure 1. Reduction products of porphyrin.
their excellent optical and electronic properties. Redox pro-
cesses of porphyrinoids are a promising route for the creation
limited.[6] The synthesis of 20p porphyrinoids with both high
planarity and distinct antiaromaticity is still challenging.[7]
of novel and unique p-conjugated skeletons, because these
processes can change the structures and the number of p elec-
trons in the conjugated circuits. Reduction of regular porphyr-
ins at the peripheral double bonds of the pyrrole units pro-
vides aromatic chlorins, bacteriochlorins, and isobacteriochlor-
ins, which are the key structures of chlorophylls (Figure 1).[1]
Reduction of carbon–carbon double bonds at the meso posi-
tions of porphyrins leads to phlorins, which show nonaromatic
behavior.[2] However, isophlorins, obtained from reduction of
two imine moieties of porphyrins, are known to have nonpla-
nar 20p-electron systems.[3] In general, free-base isophlorins
adopt highly distorted conformations due to the steric repul-
sion among the four NH protons in the central cavity. Chen
and co-workers reported a 20p free-base isophlorin that
shows a nonaromatic nature because of the nonplanar saddle
conformation.[4] In 2005, Vaid and co-workers reported the syn-
thesis of an antiaromatic 20p silicon(IV) porphyrin with
a highly ruffled structure.[5] Since the isophlorin macrocycle
works as a tetravalent ligand, the choice of central metals is
5,15-Diazaporphyrins (1M, Scheme 1; M=Ni or 2H) are 18p
porphyrinoids with imine-type sp2-hybridized nitrogen atoms
at the meso positions.[8,9] Because of its relatively low-lying
LUMO, 1M is prone to undergo nucleophilic addition of alkyl-
lithium reagents.[8b] This characteristic feature of 1M prompted
us to explore the reaction of 1M with various reductants.
[a] A. Yamaji, Prof. Dr. Y. Miyake, Prof. Dr. H. Shinokubo
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University, Nagoya 464-8603 (Japan)
E-mail: miyake@apchem.nagoya-u.ac.jp
hshino@apchem.nagoya-u.ac.jp
[b] Prof. Dr. H. Tsurugi, Prof. Dr. K. Mashima
Department of Chemistry, Graduate School of Engineering Science
Osaka University, Osaka 560-8531 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600066.
Scheme 1. Chemo- and regioselective reduction of 5,15-diazaporphyrins.
Mes=2,4,6-trimethylphenyl.
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We have now succeeded in the development of reagent-
controlled chemo- and regioselective reduction of 5,15-diaza-
porphyrins 1M (Scheme 1). In this process, the selective reduc-
tion of carbon–carbon double bonds provided the 18p aro-
matic isobacteriochlorin-type product 2M, whereas the reduc-
tion of carbon–nitrogen double bonds yielded N,N’-dihydrodia-
zaporphyrins 3M selectively. The planar structure and 20p
conjugation, including lone pairs on the nitrogen atoms at the
meso positions, induce distinct antiaromatic character of 3M.
In contrast to isophlorins, free-base N,N’-dihydrodiazaporphyrin
3H2 has two NH protons in the central cavity, which is similar
to normal porphyrins.
We conducted the reaction of 1Ni with a hydride reagent
(Scheme 2). Treatment of 1Ni with 12 equivalents of NaBH4 in
the presence of Pd/C[10] in CH2Cl2/MeOH (3:1) at room temper-
ature for 2 h afforded 2Ni as a single isomer in 79% yield with
a trace amount of chlorine-type product 4Ni. In the absence
Scheme 3. a) Proposed reaction pathway for the reduction of 1Ni to 2Ni;
b) DFT-calculated molecular orbitals of 4Ni.
through coordination with the sodium cation.[8b] Protonation
of the initial adduct then affords 4Ni. The DFT calculation of
4Ni at the B3LYP/6-31G(d)+SDD level suggests that the MO
coefficient at the C17 position in the LUMO of 4Ni is signifi-
cantly larger than those at other b-positions (Scheme 3b). Con-
sequently, formation of 2Ni should be favored in reduction
with NaBH4 over the bacteriochlorin-type reduction product.
Figure 2 shows the electronic absorption spectra of 1Ni,
2Ni, and 4Ni in CH2Cl2. The characteristic changes of the ab-
sorption features of these compounds are consistent with the
general trend that is typically observed from porphyrin to
chlorine and isobacteriochlorin.[1] The Q-band of chlorin-type
Scheme 2. Reduction of 1Ni with NaBH4.
of Pd/C, 2Ni and 4Ni were obtained in 13% and 14% yields,
respectively. The X-ray diffraction analysis of 2Ni revealed the
high planarity of the macrocycle of 2Ni. Unfortunately, the as-
signment of 2Ni was not conclusive because three regioisom-
ers 2Ni, 2Ni-isomer A, and 2Ni-isomer B could not be distin-
guished by the X-ray analysis because of the highly disordered
structure (see the Supporting Information, Figure S11).[11] The
1H NMR spectrum of 2Ni displayed two doublet peaks for b-
protons in the aromatic region at d=7.5 ppm and protons on
sp3 carbons at dꢀ3.0–4.0 ppm, suggesting C2v symmetry. The
13C NMR spectrum of 2Ni also indicated that the structure of
2Ni adopted C2v symmetry. On the basis of these NMR data,
we concluded that 2Ni has the isobacteriochlorin-type struc-
ture with 18p aromatic conjugation.
Figure 2. Electronic absorption spectra of 1Ni, 2Ni, and 4Ni in CH2Cl2.
A proposed reaction pathway for the reduction of 1Ni to
2Ni is shown in Scheme 3. In this reduction process, the attack
of hydride at the pyrrole rings should be a key step for the se-
lective reduction of carbon–carbon double bonds. The initial
nucleophilic attack of hydride would occur at the C3 position
because of a directing effect of the outer nitrogen atom
4Ni was substantially redshifted. In comparison to 1Ni, the Q-
band of 2Ni was hypsochromically shifted. This shift is well
supported by the DFT calculations. The calculated HOMO–
LUMO gaps of 1Ni, 2Ni, and 4Ni were 2.73, 2.83, and 2.45 eV,
respectively. The relative intensity of the Q-band to the Soret
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band increased in the order of 1Ni, 4Ni, and 2Ni, as the
number of reduced double bonds increased.
We then found that the use of N,N’-bis(trimethylsilyl)-1,4-di-
hydropyrazine 5[12] as a reductant for 1M cleanly and selective-
ly furnished N,N’-dihydrodiazaporphyrins 3M (Scheme 4). N,N’-
Disilyl-1,4-dihydropyrazine 5 serves as a versatile and strong re-
ducing reagent for group 4–6 metal and late transition metal
chlorides.[12b,c] When we carried out the reaction of 1Ni with
1.6 equivalents of 5 in THF at room temperature, complete
conversion of 1Ni was confirmed by 1H NMR spectroscopy.
Treatment of the resulting solution with an excess amount of
MeOH afforded 3Ni in 84% yield. This protocol was also ap-
plied to the reduction of free-base diazaporphyrin 1H2 and di-
hydrodiazaporphyrin 3H2 was obtained in 78% yield. Both 3Ni
and 3H2 were easily oxidized back to diazaporphyrins 1Ni and
1H2, respectively, in solution under air.
Scheme 4. Reduction of 1M with 5.
Figure 3. X-ray crystal structures of dihydrodiazaporphyrins. Thermal ellip-
soids are scaled at 50% probability level. Mesityl groups are omitted for
clarity: a) Top and b) side views of 3Ni; c) top and d) side views of 3H2;
e) bond length alternations of 3Ni and 3H2.
[14]
The structures of 3Ni[13] and 3H2 were unambiguously re-
vealed by X-ray diffraction analysis (Figure 3). The skeletons of
3Ni and 3H2 are almost perfectly planar (Figure 3b,d) with
small mean plane deviations of 0.049 Š (3Ni) and 0.031 Š
(3H2). In addition, 3Ni and 3H2 display significant bond-length
alternations, which are diagnostic for antiaromatic porphyri-
noids (Figure 3e). These results strongly suggest that 3Ni and
3H2 exhibit antiaromaticity, owing to planar 20p conjugation.
The nucleus-independent chemical shift (NICS) values (calculat-
ed at points “a” and “b” in Figure 3a,c) are +15.5 and
+15.8 ppm, respectively, confirming the substantial paratropic
ring current effect of 3Ni and 3H2. Whereas 18p diazaporphyr-
in 1Ni exhibited b-protons in the aromatic region of d=8.8–
(Figure 4). At À508C, four doublet signals for b-protons were
detected at d=3.0–4.5 ppm. As the temperature was raised,
the four doublet signals were broadened, reaching a coales-
cence point at 258C, where the b-proton signals nearly disap-
1
9.2 ppm in its H NMR spectrum, the b-proton signals of 3Ni
were shifted significantly upfield to d=3.0–4.0 ppm. In addi-
tion, the inner two NH protons of 3H2 were observed in the
downfield region at d=24.5 ppm. These characteristic proton
chemical shifts confirm the distinct antiaromatic natures of
3Ni and 3H2. Free-base porphyrinoids with 4np conjugation
often adopt nonplanar conformations to relieve antiaromatic
destabilization. The highly planar structure of 3H2 is probably
maintained by the effective hydrogen bonding interactions in
the central cavity,[15] as indicated by the highly downfield-shift-
ed NH proton signals.
1
Unexpectedly, the H NMR spectrum of 3H2 displayed no b-
protons signals at room temperature. We then found that the
Figure 4. Variable-temperature NMR spectra of 3H2 at À50, 25, and 658C in
spectrum of 3H2 was highly dependent on temperature
[D8]THF.
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peared. Two broad signals then appeared at higher tempera-
tures. We presumed that the spectral change with temperature
resulted from the inner proton exchange of 3H2, in analogy to
NH tautomerization of porphyrins (Scheme 5a).[16] The activa-
tion energy of inner proton exchange of 3H2 was measured to
be 14.3 kcalmolÀ1 at 258C.[17] This value is substantially larger
than that of the energy barrier of NH proton exchange in typi-
cal porphyrins (ca. 12 kcalmolÀ1).[16c]
energy was calculated to be 14.6 kcalmolÀ1, which is in good
agreement with the experimental value (14.3 kcalmolÀ1).
Further evidence for our hypothesis was obtained by using
the deuterium-labeled compound [D4]3H2. The coalescence
temperature of [D4]3H2 was 458C (see the Supporting Informa-
tion, Figure S10). The activation barrier of [D4]3H2 (15.3 kcal
molÀ1 at 458C) was higher than that of 3H2 due to the kinetic
isotope effect (Scheme 5b). As shown in the X-ray structure, di-
hydrodiazaporphyrin 3H2 has rather localized p-bonds. This
considerable bond length alternation of 3H2 should be due to
Jahn–Teller distortion.[18] However, the conjugation circuit is
switched by NH tautomerism and p-bonds in the transition
state (TS) should be delocalized (Scheme 5c). The temporal de-
localization of p-electrons in the TS would enhance antiaroma-
ticity to destabilize the TS, resulting in the larger activation
barrier of NH tautomerization. In fact, the NICS value at the
ring center of the TS is +21.0 ppm, which is substantially
larger than that of 3H2 (+16.1 ppm; Figure 5).
To elucidate the reaction pathway, we monitored the reac-
1
tion of 1Ni with 5 in [D8]THF at room temperature by H NMR
spectroscopy (Scheme 6 and Figure S6 in the Supporting Infor-
mation). After 1 h, 3Ni, 3Ni-TMS, and 3Ni-TMS2 had formed in
38, 11, and 14% yields, respectively, alongside the recovery of
34% of 1Ni. In addition, we also detected the formation of
pyrazine. This result clearly shows that both the electron trans-
fer from 5 to 1Ni and the migration of trimethylsilyl (TMS)
groups are key steps in the formation of 3Ni.[12b] The initial
step would be an electronic interaction between electron-defi-
cient 1Ni and electron-rich 5, to provide the corresponding
radical ion pair. Then, the cleavage of NÀSi bonds in 5 and the
formation of NÀSi bonds in 3Ni-TMS2 would furnish 3Ni-TMS2
and pyrazine. Finally, 3Ni-TMS2 is converted into 3Ni through
desilylation with methanol.
Scheme 5. a) Inner proton exchange in 3H2; b) [D4]3H2; c) transition state in
tautomerization.
We then performed DFT calculations to obtain the transi-
tion-state structure for the inner NH migration reaction
(Figure 5). Calculations were conducted at the B3LYP/6-31+
G(d) level on model compounds in which the mesityl substitu-
ents were replaced with hydrogen atoms. The transition-state
structure was obtained by the QST2 method. The activation
The UV/Vis absorption spectra of 3Ni and 3H2 are shown in
Figure 6. The absorption spectra of 3Ni and 3H2 exhibit a sub-
stantially different shape from those of 1Ni and 1H2.[8] In par-
ticular, weak absorption bands were detected at l=700–
1200 nm (dashed lines) and are assigned to the characteristic
forbidden bands that are typical for antiaromatic porphyri-
noids.[19] The electrochemical properties of 3Ni and 3H2 were
examined by using cyclic voltammetry (see the Supporting In-
formation, Figure S13). The cyclic voltammograms of 3Ni and
Figure 5. Calculated pathway of the inner proton exchange of 3H2.
Figure 6. UV/Vis absorption spectra of 3Ni and 3H2 in CH2Cl2.
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Scheme 6. Reaction of 1Ni with 5 in [D8]THF.
3H2 showed reversible first oxidation waves at À0.717 and
À0.534 V, respectively, suggesting the donating nature of 3Ni
and 3H2.
In summary, we have demonstrated the reagent-controlled
chemo- and regioselective reduction of 5,15-diazaporphyrins
1M. The reduction of 1M with NaBH4 provides the isobacterio-
chlorin-type product 2M, whereas antiaromatic dihydrodiaza-
porphyrins 3M was obtained through the reduction with N,N’-
bis(trimethylsilyl)-1,4-dihydropyrazine 5. This is the first exam-
ple of antiaromatic porphyrinoids bearing a conjugation circuit
including the lone pairs on the nitrogen atoms at the meso po-
sitions.[20,21] The methodology described herein provides a new
route for the creation of antiaromatic compounds and will
open up the chemistry of antiaromatic heteroporphyrinoids.
Further investigation of the unique properties of 3M is cur-
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Acknowledgements
We thank Kento Kawakita and Haruka Nishiyama (Osaka Uni-
versity) for their help with the preparation of 5. This work was
supported by Grant-in-Aids for Scientific Research on Innova-
tive Areas, “p-System Figuration” (26102003) and “Element
Blocks” (25102514) and the Program for Leading Graduate
Schools “Integrative Graduate Education and Research in
Green Natural Sciences”, MEXT, Japan. H.S. acknowledges the
Asahi Glass Foundation for financial support.
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C36H34Cl6N6Ni; Mw =609.40; monoclinic; P21/c; a=8.0857(11), b=
25.718(3), c=7.1564(10) Š; b=108.2820(10)8; V=1413.1(3) Š3; Z=2;
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Keywords: antiaromaticity · chemoselectivity · porphyrinoids ·
reduction · tautomerism
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[20] Very recently, Kobayashi and co-workers have reported antiaromatic
azaporphyrinoids based on superphtalocyanines having imine sp2-hy-
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[21] After submission of our manuscript, Matano and co-workers have inde-
pendently reported the synthesis of 20p antiaromatic N,N’-diaryldiaza-
porphyrin NiII complexes. T. Satoh, M. Minoura, H. Nakano, K. Furukawa,
Y. Matano, Angew. Chem. Int. Ed. 2016, 55, DOI: 10.1002/
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Received: January 7, 2016
[17] J. Sandstrçm, Dynamic NMR Spectroscopy, Academic Press, London,
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Published online on February 3, 2016
Chem. Eur. J. 2016, 22, 3956 – 3961
www.chemeurj.org
3961
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