Rational synthesis of A2B-type meso-triarylsubporphyrins

Article abstract of DOI:10.1021/ol300865s

Rational synthesis of A₂B-type meso-arylsubporphyrins has been accomplished by the condensation of triethylamine-tri-N-tripyrromethene-borane with acid chlorides. These subporphyrins are useful for evaluations of the intrinsic substituent effec

Full text of DOI:10.1021/ol300865s

ORGANIC
LETTERS
2012
Vol. 14, No. 11
2694–2697
Rational Synthesis of A2B-type
meso-Triarylsubporphyrins
Takayuki Tanaka,† Masaaki Kitano,† Shin-ya Hayashi,† Naoki Aratani,†,‡ and
Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku,
Kyoto 606-8502, Japan, and PRESTO, Japan Science and Technology Agency, 4-1-8,
Honcho, Kawaguchi-shi, Saitama 332-0012, Japan
osuka@kuchem.kyoto-u.ac.jp
Received April 5, 2012
ABSTRACT
Rational synthesis of A2B-type meso-arylsubporphyrins has been accomplished by the condensation of triethylamineÀtri-N-tripyrrometheneÀborane
with acid chlorides. These subporphyrins are useful for evaluations of the intrinsic substituent effects and the influences of substitution patterns, A3-type
versus A2B-type substitution.
Subporphyrin, a genuine ring-contracted porphyrin, has
emerged as a new functional pigment in light of its bowl-
shaped curved macrocycle, distinct 14π aromatic system,
strong emission, and intriguing substituent effect of meso-
aryl groups.1À3 The synthesis of subporphyrin was first
reported as a tribenzosubporphine in 2006,2a which was
followed by the synthesis of meso-aryl-substituted subpor-
phyrins by using tri-N-pyrrolylborane or pyridine-tri-N-
pyrrolylborane as templating precursors.2b,3a However,
the chemistry of subporphyrin still remains in its infant
stage mainly due to its poor synthetic accessibility, and
hence, new synthetic methods to produce various subpor-
phyrin derivatives are highly desired. In particular, there
had been no rational synthetic route to subporphyrins
bearing different meso-substituents. Quite recently, we
have developed the synthesis of a meso-free subporphyrin
by the condensation of in situ generated triethylamineÀ
tri-N-tripyrrometheneÀborane (2) with trimethyl ortho-
formate.4 This meso-free subporphyrin was smoothly
transformed to meso-bromo subporphyrin, from which
meso-alkynyl, meso-alkenyl, and meso-alkyl subporphyrins
were prepared by metal-catalyzed reactions. We thought that
the precursor 2 could be used for straightforward synthesis of
A2B-type subporphyrins by choosing an appropriate reac-
tion partner. In this paper, we disclose our results on the
direct and rational synthesis of A2B-type subporphyrins.
By following our reported method,4 2 was generated
from tripyrrane 15 with3 equiv of triethylamineÀborane in
1,2-dichlorobenzene at 150 °C for 2 h and was used after
† Kyoto University.
‡ PRESTO.
(1) (a) Inokuma, Y.; Osuka, A. Dalton Trans. 2008, 2517–2526. (b)
Torres, T. Angew. Chem., Int. Ed. 2006, 45, 2834–2837. (c) Osuka, A.;
Tsurumaki, E.; Tanaka, T. Bull. Chem. Soc. Jpn. 2011, 84, 679–697.
(2) (a) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoo, M.-C.; Kim, D.;
Osuka, A. Angew. Chem., Int. Ed. 2006, 45, 961–964. (b) Inokuma, Y.;
Yoon, Z. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2007, 129, 4747–
4761. (c) Inokuma, Y.; Easwaramoorthi, S.; Jang, S. Y.; Kim, K. S.;
Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2008, 47, 4840–4843. (d)
Inokuma, Y.; Easwaramoorthi, S.; Yoon, Z. S.; Kim, D.; Osuka, A.
J. Am. Chem. Soc. 2008, 130, 12234–12235. (e) Tsurumaki, E.; Saito, S.;
Kim, K. S.; Lim, J. M.; Inokuma, Y.; Kim, D.; Osuka, A. J. Am. Chem.
Soc. 2008, 130, 438–439. (f) Hayashi, S.; Inokuma, Y.; Easwaramoorthi,
S.; Kim, K. S.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2010, 49, 321–
324. (g) Hayashi, S.; Tsurumaki, E.; Inokuma, Y.; Kim, P.; Sung, Y. M.;
Kim, D.; Osuka, A. J. Am. Chem. Soc. 2011, 133, 4254–4256. (h)
Tsurumaki, E.; Hayashi, S.; Tham, F. S.; Reed, C. A.; Osuka, A.
J. Am. Chem. Soc. 2011, 133, 11956–11959.
(3) (a) Kobayashi, N.; Takeuchi, Y.; Matsuda, A. Angew. Chem., Int.
Ed. 2007, 46, 758–760. (b) Takeuchi, Y.; Matsuda, A.; Kobayashi, N.
J. Am. Chem. Soc. 2007, 129, 8271–8281. (c) Xu, T.; Lu, R.; Liu, X.;
Chen, P.; Qiu, X.; Zhao, Y. Eur. J. Org. Chem. 2008, 1065–1071.
(4) Kitano, M.; Hayashi, S.; Tanaka, T.; Yorimitsu, H.; Aratani, N.;
Osuka, A. Angew. Chem., Int. Ed. 2012, 51, DOI: 10.1002/ange.
201201853.
€
(5) (a) Bruckner, C.; Sternberg, E. D.; Boyle, R. W.; Dolphin, D.
Chem. Commun. 1997, 1689–1690. (b) Sessler, J. L.; Seidel, D.; Bucher,
C.; Lynch, V. Chem. Commun. 2000, 1473–1474.
r
10.1021/ol300865s
Published on Web 05/21/2012
2012 American Chemical Society

cooling without further purification. First, the condensa-
tion of 2 with 10 equiv of benzaldehyde was examined by
changing acid. With trifluoroacetic acid (TFA), the reac-
tion proceeded at 0 °C to give 3a in 4.1% yield (Table 1,
Table 2. Synthesis of A2B-Type Subporphyrins (3bÀi)
entry 1). The yield of 3a was decreased with BF3 OEt2
3
(entry 2), and both methanesulfonic acid (MSA) and
p-toluenesulfonic acid (p-TsOH) gave only acyclic byproducts
(entries 3 and 4). In due course, we found that HCl could be
used as an effective acid to produce 3a in 4.1% yield (entry 5),
and a better result was obtained at high temperature (entry 6).
It occurred to us that the use of benzoyl chloride instead of
benzaldehyde would work similarly or better since HCl would
be released from it. This was indeed the case, as 3a was
obtained in a better yield of 6.9% (entry 7), which was drasti-
cally improved to 17.7% at 150 °C for 30 min (entry 8). Since
benzoyl bromide or trimethyl orthobenzoate was not suitable
for this reaction under comparable conditions (entries 9 and
10), both the high reactivity of benzoyl chloride and HCl
catalysis might be crucial for the formation of subporphyrins.
The reaction mechanism is unclear at this stage but probably
involves first aroylation of 2 to form a ketone intermediate
followed by intramolecular cyclization with the aid of acid
catalysis (see the Supporting Information).6,7
On the basis of the optimized conditions, A2B-type sub-
porphyrins were prepared with various acid chlorides. The
reaction with 4-tert-butylbenzoyl chloride afforded subpor-
phyrin 3b in 6.8% yield (Table 2). In this synthesis, we
encountered a problem that the boiling point of the acid
chloride (135 °C/20 mmHg) is too high to be completely
distilled off, which made product separation quite tedious.
Thus, we invented a different workup procedure that in-
volved the addition of ethylenediamine and subsequent
rough separation through a silica gel column. The decreased
yield of 3b compared with that of 3a may be partly attri-
buted to this problem. As a trend, electron-poor acid chlo-
rides gave better yields than electron-rich ones, probably
reflecting higher reactivities toward the condensation
(Table 2). To our delight, this protocol allowed the synthesis
of meso-(2-thienyl)subporphyrin 3g and meso-(2-naphthyl)
subporphyrin 3h. Further, even meso-(pentafluorophenyl)
subporphyrin 3i that bears two ortho-substituents was pre-
pared by this reaction. Unfortunately, this protocol was not
applicable to aliphatic acid chlorides.
Table 1. Synthesis of 5,10,15-Triphenylsubporphyrin (3a)
yield
Figure 1 shows X-ray crystal structures of 3b, 3c, and
3g.8 The bowl depths were calculated to be 1.368, 1.338,
entry
reagent (equiv)
acid (equiv)
TFA (3.3)
conditions
(%)
1
2
PhCHO (10)
PhCHO (10)
0 °C, 1 h
0 °C, 1 h
4.1b
1.2b
BF3 OEt2
3
(6) For the examples of condensation reaction with acid chlorides
affording porphyrinoids and BODIPYs, see: (a) Koszarna, B.;
Voloshchuk, R.; Gryko, D. T. Synthesis 2007, 1339–1342. (b) Thoresen,
L. H.; Kim, H.; Welch, M. B.; Burghart, A.; Burgess, K. Synlett 1998,
1276–1278.
(3.3)
3
4
5
6
PhCHO (10)
PhCHO (10)
PhCHO (10)
PhCHO (10)
MSA (3.3)
p-TsOH (3.3)
HCla (10)
HCla (10)
0 °C, 1 h
0 °C, 1 h
0 °C, 1 h
150 °C
30 min
100 °C
30 min
150 °C
30 min
150 °C
30 min
150 °C
30 min
0
0
4.1b
4.6b
(7) MnO2 oxidation was conducted to oxidize subchlorins into
corresponding subporphyrins in all cases.2e
(8) Crystal data for 3b: C38H32BN3O, 0.23 (C6H14), 1.53 (CH2Cl2),
7
8
PhCOCl (10)
PhCOCl (10)
PhCOBr (10)
6.9c
17.7c
trace
0
Mw = 707.71, triclinic, space group P-1 (No. 2), a = 9.8047(3) A, b =
˚
˚
˚
11.0075(4) A, c = 16.8098(5) A, α = 99.827(2)°, β = 96.5051(19)°, γ =
3
93.876(2)°, V = 1769.06(10) A , T = 93 K, Fcalcd = 1.329 gcmÀ3, Z = 2,
˚
R1 = 0.0673 (I > 2σ(I)), Rw = 0.1998 (all data), GOF = 1.002. CCDC
874758. Crystal data for 3c: C35H26BN3O2, Mw = 531.40, monoclinic,
˚
space group P21/c (No. 14), a = 7.5185(1) A, b = 19.5104(4) A, c =
˚
9
3
˚
˚
17.6082(4) A, β = 95.3265(13)°, V = 2571.78(9) A , T = 93 K, Fcalcd
=
1.372 gcmÀ3, Z = 4, R1 = 0.0950 (I > 2σ(I)), Rw = 0.2843 (all data),
GOF = 0.996. CCDC 874759. Crystal data for 3g: C32H22BN3OS,
Mw = 507.40, monoclinic, space group P21/c (No. 14), a = 13.4058(4)
10
trimethyl
orthobenzoate (10)
˚
˚
˚
A, b = 13.7067(4) A, c = 14.2504(4) A, β = 96.5051(19)°, V =
a Et2O solution, b NMR yield after MnO2 oxidation.7 c isolated yield.
2452.45(12) A , T = 93 K, Fcalcd = 1.374 gcmÀ3, Z = 4, R1 = 0.0849
3
˚
(I > 2σ(I)), Rw = 0.2857 (all data), GOF = 1.104. CCDC 874760.
Org. Lett., Vol. 14, No. 11, 2012
2695

Figure 2. UV/vis absorption (a) and emission (b) spectra of 3bÀi
in CH2Cl2.
of 3f in CH2Cl2 is distinctly smaller than that of 4f.2b,9 These
optical properties of 3f may be ascribed to its A2B-type
substitution pattern, which may lead to intramolecular
charge-transfer type interactions. In contrast, the substitu-
ent effect of 2-thienyl group is additive and hence larger in
tris(2-thienyl) substituted subporphyrin 4g than 3g.10 This
is accounted for in terms of the intrinsic charge transfer
interaction between the subporphyrin core and 2-thienyl
substituent. meso-Pentafluorophenyl group causes a blue-
shift of Soret band of subporphyrin as observed in tris-
Figure 1. X-ray crystal structures of (a) 3b, (b) 3c, and (c) 3g (left,
top view; right, side view). Thermal ellipsoids were scaled to
25% probability. Solvent molecules and disordered parts were
omitted for clarity.
˚
and 1.318 A for 3b, 3c, and 3g, respectively, which are
similar to that of 3a (1.29 A).2b The dihedral angle of thio-
˚
(pentafluorophenyl)-substituted subporphyrin 4i (λabs
=
359, 449, and 470 nm, λem = 487 and 517 nm),11 such an
effect is smaller in 3i. It is interesting to note that single
replacement by pentafluorophenyl group causes a split
fluorescence spectrum which is similar to that of 4i.11
The electrochemical potentials of 3aÀi were measured
by cyclic voltammetry (CV) and differential-pulse voltam-
metry (DPV) experiments in CH2Cl2 containing 0.1 M
Bu4NPF6 as a supporting electrolyte (Table 4). Parent
subporphyrin 3a exhibited one reversible oxidation wave
at 0.71 V and one reversible reduction wave at À1.97 V.2c
phene ring toward subporphyrin core in 3g is 44.10°.
A2B-type meso-aryl subporphyrins prepared in this work
are useful for study on the substituent effects on the optical
properties in two ways: (1) an intrinsic substituent effect of
an aryl substituent and (2) substitution pattern effect, A3
versus A2B substitution. Large substituent effects are not
observed for 4-tert-butylphenyl (3b), 4-methoxyphenyl (3c),
4-chlorophenyl (3d), and 2-naphthyl substituents (3h) with
respect to the absorption and fluorescence spectra and
fluorescence quantum yields (Figure 2, Table 3). The small
fluorescence quantum yield of 4-bromophenyl-substituted
subporphyrin 3e is apparently due to internal heavy atom
effect, which is smaller than tris(4-bromophenyl)-substi-
tuted subporphyrin 4e (ΦF = 0.008).2b The Soret band of
4-nitrophenyl-substituted subporphyrin 3f is remarkably
broader, and interestingly the fluorescence quantum yield
(9) Easwaramoorthi, S.; Shin, J.-Y.; Cho, S.; Kim, P.; Inokuma, Y.;
Tsurumaki, E.; Osuka, A.; Kim, D. Chem.;Eur. J. 2009, 15, 12005–
12017.
(10) Hayashi, S.; Inokuma, Y.; Osuka, A. Org. Lett. 2010, 12, 4148–
4151.
(11) Saito, S.; Kim, K. S.; Yoon, Z. S.; Kim, D.; Osuka, A. Angew.
Chem., Int. Ed. 2007, 46, 5591–5593.
2696
Org. Lett., Vol. 14, No. 11, 2012

Table 3. Absorption and Fluorescence Data of 3aÀi in CH2Cl2
(Reference A3-Type Molecules 4eÀg,i Shown below)
Table 4. Electrochemical Propertiesa
oxidation
reduction
compd E1/2
E1/2
E1/2
HOMOÀLUMO gap (eV)
ox.2
ox.1
red.1
3a
3b
3c
0.71
À1.97
À2.03
À2.02
À1.97
À1.97
À1.57
À1.88
À1.91
À1.91
2.68
2.72
2.65
2.73
2.70
2.39
2.45
2.62
2.79
0.69
0.63
0.76
0.73
0.82
0.57
0.71
0.88
3d
3e
3f
0.83
0.92
0.69
0.81
1.00
3g
3h
3i
a These values were measured by cyclic voltammetry using a glassy
carbon working electrode, platinum wire counter electrode, and
Ag/AgClO4 reference electrode. The measurements were carried out in
CH2Cl2 solutions containing 0.1. M Bu4NPF6 as a supporting electrolyte.
Potentials versus ferrocene/ferrocenium ion pair. Scan rate: 0.05 V/s.
considered to be the reduction at the nitrophenyl group
(E1/2 = À1.6 V for nitrobenzene).13
In summary, we have developed the direct and rational
synthesis of A2B-type meso-arylsubporphyrins 3bÀi by the
condensation of 2 with acid chlorides in 2À11% yields.
This method is advantageous by circumventing the pre-
synthesis of the meso-free subporphyrin, meso-bromina-
tion, and metal-catalyzed subsitutiton reactions. In
addition, this method may be potentially more suited for
achieving high yields of subporphyrins, as suggested by the
yield of 3a (17.7%) which is highest among meso-arylsub-
porphyrins so far reported. Through the examinations of
the optical and electrochemical properties of 3aÀi, the
intrinsic effects of meso-aryl substituents and the substi-
tuion pattern effects have been studied. This rational
synthesis of A2B-type subporphyrins will be useful for
more elaborated molecular systems and is now actively
in progress in our laboratory.
a Excited at the Soret-like band except for 3f (λex = 462 nm).
b Absolute fluorescence quantum yield. c Fluorescence quantum yields
were determined with a reference to that of 8-amino-1-naphthalenesul-
fonic acid (ΦF = 0.37 in ethanol).
Compounds 3b, 3c, and 3e showed similar features, ex-
hibiting one oxidation potentials at 0.63À0.73 V and one
reduction potentials at À1.97À2.03 V. Curiously, the oxida-
tion waves in 3d, 3f, 3g, 3h, and 3i were somewhat split
with ΔEox (= E1/2ox.2 À E1/2ox.1) = 70À120 mV, while the
reduction waves were only shifted. The E1/2 potentials of
para-substituted subporphyrins 3aÀf are plotted as a func-
tion of Hammett’s parameter (σ) (Supporting Information).
The first oxidation potentials of 3aÀf lie on a linear slope
with F = 0.166 V, whereas a linear slope with F = 0.117 V is
obtained when the first reduction potential is plotted except
for 3f. The reaction constants F are roughly similar to those
of reported triarylsubporphyrins (F = 0.105 and 0.124 V
for the oxidation and reduction potentials, respectively)2b
and larger than those of tetraarylporphyrins (F = 0.065 and
0.073 V for the oxidation and reduction potentials, respec-
tively),12 indicating larger substituent effects of subporphyrins.
The first reduction potential of À1.57 V in 3f was
Acknowledgment. This work was supported by the Glo-
bal COE Program “International Center for Integrated Re-
search and Advanced Education in Materials Science” (No.
B-024) and Grants-in-Aid (Nos. 22245006 (A) and 20108001
“pi-Space”) for Scientific Research from MEXT. T.T. thanks
the JSPS Research Fellowship for Young Scientists.
Supporting Information Available. Experimental de-
tails and characterization of new compounds and crystal
data. This material is available free of charge via the
Internet at http://pubs.acs.org.
(12) Kadish, K. M.; Morrison, M. M. J. Am. Chem. Soc. 1976, 98,
3326–3328.
(13) (a) Bu, J.; Lilienthal; Woods, J. E.; Nohrden, C. E.; Hoang,
K. T.; Truong, D.; Smith, D. K. J. Am. Chem. Soc. 2005, 127, 6423–6429.
(b) Bento, M. F.; Medeiros, M. J.; Montenegro, M. I.; Beriot, C.;
Pletcher, D. J. Electroanal. Chem. 1993, 345, 273–286.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 11, 2012
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