Remarkable effects of p-perfluorophenyl group on the synthesis of core-modified phosphaporphyrinoids and phosphadithiasapphyrin

Article abstract of DOI:10.1021/ol100114j

(Figure Presented) P,X,N₂-type phosphaporphyrins and phosphacalixphyrins (X = N, S) bearing a perfluorophenyl (C₆F ₅) group at the core phosphorus atom were prepared In high overall yield from 1-perfluorophenyl-2,5-di(ethoxycarbonyl)phosphole as a common starting material. In addition, P-C₆F₅ P,S ₂,N₂-type sapphyrin was successfully prepared as the first example of ring-expanded phosphorus-containing porphyrin.

Full text of DOI:10.1021/ol100114j

ORGANIC
LETTERS
2010
Vol. 12, No. 5
1112-1115
Remarkable Effects of P-Perfluorophenyl
Group on the Synthesis of
Core-Modified Phosphaporphyrinoids
and Phosphadithiasapphyrin
Takashi Nakabuchi,† Yoshihiro Matano,*,† and Hiroshi Imahori†,‡,§
Department of Molecular Engineering, Graduate School of Engineering,
Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Institute for Integrated
Cell-Material Sciences (iCeMS), Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510,
Japan, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8103, Japan
matano@scl.kyoto-u.ac.jp
Received January 15, 2010
ABSTRACT
P,X,N2-type phosphaporphyrins and phosphacalixphyrins (X ) N, S) bearing a perfluorophenyl (C6F5) group at the core phosphorus atom were
prepared in high overall yield from 1-perfluorophenyl-2,5-di(ethoxycarbonyl)phosphole as a common starting material. In addition, P-C6F5
P,S2,N2-type sapphyrin was successfully prepared as the first example of ring-expanded phosphorus-containing porphyrin.
Core modification of porphyrins, namely, replacement of
the core pyrrolic nitrogen atom by another heteroatom or
carbon, has been known as a powerful tool to alter their
optical/electrochemical properties and coordinating ability
drastically.1 Recently, we prepared the first examples of
phosphorus-containing core-modified porphyrins 1X2-4
and calixphyrins 2X5 (Figure 1) and disclosed their charac-
teristic optical and electrochemical properties, coordinating
† Department of Molecular Engineering, Graduate School of Engineering.
Figure 1. P,X,N2-Porphyrins 1X, 1XF and P,X,N2-Porphyrins 2X,
2XF (X ) N, S).
‡ Institute for Integrated Cell-Material Sciences (iCeMS).
§ Fukui Institute for Fundamental Chemistry.
(1) For reviews on core-modified porphyrins, see: (a) Latos-Graz˙yn´ski,
L. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.,
Eds.; Academic Press: San Diego, 2000; Vol 2, Chapter 14. (b) Jasat, A.;
Dolphin, D. Chem. ReV. 1997, 97, 2267–2340. (c) Furuta, H.; Maeda, H.;
Osuka, A. Chem. Commun. 2002, 1795–1804. (d) Gupta, I.; Ravikanth, M.
Coord. Chem. ReV. 2006, 250, 468–518. (e) Lash, T. D. Eur. J. Org. Chem.
2007, 5461–5481. (f) Misra, R.; Chandrashekar, T. K. Acc. Chem. Res.
2008, 41, 265–279. (g) Matano, Y.; Imahori, H. Acc. Chem. Res. 2009, 42,
1193–1204.
behavior, and reactivity. For instance, P,X,N2-porphyrins 1X
were found to possess considerably small HOMO-LUMO
gaps as compared with N4- and S,X,N2-porphyrins (X ) N,
S),2a,b,d and the 18π-systems of 1X were easily reconstructed
by complexation with zerovalent group 10 metals2c and
10.1021/ol100114j  2010 American Chemical Society
Published on Web 02/09/2010

P-oxidation with H2O2,2d affording unique 20π and/or 22π
systems. While these properties and reactivities of phos-
phaporphyrins are of interest, there were distinct drawbacks
for the synthesis due to the high reactivity of a σ3-phosphorus
atom. First, P-masking/demasking steps are necessary in the
synthesis of 1X and 2X, which increases the number of
reaction steps. Second, P-oxidation inevitably occurs in the
ring oxidation of σ3-P porphyrinogens to give significant
amounts of P-oxo side products, which severely reduces the
yield of target porphyrins 1X.2d,6 A possible solution to these
drawbacks is to improve the durability of the σ3-phosphorus
center under acidic and oxidizing conditions. In this regard,
attachment of an electron-withdrawing group onto the
phosphorus atom is a highly promising approach. Here we
report the synthesis of P,X,N2-porphyrins 1XF and P,X,N2-
calixphyrins 2XF (X ) N, S) bearing a perfluorophenyl
(C6F5) group at the core phosphorus atom. Notably, the
introduction of C6F5 group improves the chemical stability
of the σ3-phosphorus center dramatically, and both 1XF and
2XF are readily available in high overall yield from a
common starting material. Moreover, P,S2,N2-sapphyrin, the
first example of P-containing expanded porphyrin, was
successfully prepared by taking advantage of the electron-
withdrawing nature of the C6F5 group.
gen 7 in 35% yield as a mixture of three diastereomers. In
sharp contrast to the corresponding P-Ph analogues, diol 4
and phosphatripyrrane 5 are sufficiently stable against air
and acids. Therefore, it is not necessary to protect the σ3-
phosphorus center throughout the sequential BF3-promoted
dehydrative condensation reactions from 4 to 7. Finally, the
ring oxidation of the porphyrinogen 7 with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) afforded target porphyrin
1NF. In the present synthesis, only trace amounts of P-oxo
byproducts were found in the crude reaction mixture, and
the yield of 1NF from 7 (50%) was almost three times larger
than that of 1N from the P-Ph-type porphyrinogen (17%).2b,d
Alternatively, 1NF can be prepared in a one-pot procedure
from 5 and 6N in 15% yield (18% yield for stepwise
synthesis). According to a similar procedure, P,S,N2-por-
phyrin 1SF was prepared from 5 and 2,5-di[hydroxy(phe-
nyl)methyl]thiophene 6S10 in 8% yield (based on 5).
The P-C6F5-type P,X,N2-calixphyrins 2XF (X ) N, S)
were also prepared starting from 3 without P-masking
(Scheme 2). Treatment of 3 with excess MeMgBr gave
Scheme 2. Synthesis of C6F5-Type P,X,N2-Calixphyrins 2XF
The P-C6F5-type P,X,N2-porphyrins 1XF (X ) N, S)
were prepared starting from 1-perfluorophenyl-2,5-di-
(ethoxycarbonyl)phosphole 37 by a similar method used
for the synthesis of 1X2a,b,d (Scheme 1). Reaction of 3 with
Scheme 1. Synthesis of C6F5-Type P,X,N2-Porphyrins 1XF
tertiary diol 8 in 29% yield. Dehydrative condensation of 8
with excess pyrrole afforded phosphatripyrrane 9 in 83%
yield. The BF3-promoted dehydrative condensation of 9 with
6X (X ) N, S) followed by DDQ oxidation afforded P,X,N2-
calixphyrins 2NF and 2SF in 43% and 17% yields, respec-
tively.
Notably, the overall yields of P-C6F5-type P,X,N2-
porphyrins 1XF and P,X,N2-calixphyrins 2XF from 3 are
increased by 1.5-10 times as compared with those of P-Ph
analogues 1X and 2X. In addition, the number of reaction
processes is reduced by 1-2 steps.
Compounds 1XF and 2XF were isolated as purple and red
solids, respectively, and fully characterized by standard
spectroscopic techniques. In the 19F NMR spectra of 1XF
and 2XF, the ortho, meta, and para 19F signals of the P-C6F5
group were observed at δF -165.9 to -163.0, -132.7 to
-126.4, and -156.8 to -154.3, respectively. The 1H NMR
spectra of 1XF and 2XF essentially resemble those of P-Ph
counterparts 1X and 2X. For instance, the pyrrole-ꢀ protons
of 1NF appeared at the downfield region (δ 8.38-8.69)
relative to the corresponding protons of 2NF (δ 5.93-6.60).
Figure 2 summarizes differences in chemical shifts (∆δ) of
the peripheral (heterole-ꢀ) protons between 1XF and 2XF.
diisobutylaluminium hydride (DIBAH) in hexane gave 2,5-
di(hydroxymethyl)phosphole 4, which was then treated with
excess pyrrole in the presence of BF3•OEt2 to afford
phosphatripyrrane 5 in 45% yield based on 3.8 The BF3-
promoted dehydrative condensation of 5 with 2,5-bis[hy-
droxy(phenyl)methyl]pyrrole 6N9 gave σ3-P,N3-porphyrino-
Org. Lett., Vol. 12, No. 5, 2010
1113

Figure 2. Downfield shifts (∆δ) of the ꢀ protons of 1NF (black)
and 1SF (gray) from those of 2NF and 2SF.
Significant diatropic ring current effects (∆δ ) 1.82-2.75)
stemming from the 18π annulene circuit are clearly observed
for 1XF. The ring current effects in 1XF also emerged as
upfield resonances of 31P nucleus (1NF, δp -30.7 vs 2NF,
δp -15.9; 1SF, δp -22.5 vs 2SF, δp -15.8). It seems that
introduction of the electron-withdrawing C6F5 group at the
core phosphorus atom perturbs the aromaticity of P,X,N2-
porphyrin π-circuits only slightly.
Figure 3. UV-vis absorption spectra of 1S (black; data from ref
2d), 1SF (purple), and 11 (green) in CH2Cl2.
differential pulse voltammetry (DPV) (Figure S3 in Sup-
porting Information). The first oxidation potential (Eox,1) and
the first and second reduction potentials (Ered,1 and Ered,2) of
1NF and 1SF determined by DPV are shifted to the negative
side compared to the respective potentials of the P-Ph
analogues 1X (∆Eox,1 ) 0.05-0.15 V; ∆Ered,1 ) 0.01-0.06
V; ∆Ered,2 ) 0.08-0.10 V). The replacement of the P-phenyl
group with the P-perfluorophenyl group has proven to
enhance the electron-accepting ability of the phosphapor-
phyrin π-systems slightly.
The UV-vis absorption spectrum of P,N3-porphyrin 1NF
is similar to that of the P-Ph counterpart 1N (Table 1; Figure
Table 1. UV-Vis Absorption Maxima and Redox Potentials (vs
a b
,
Fc/Fc+) of 1X, 1XF, and 11 in CH2Cl2
compd λmax (Soret; Q), nm
Eox,1, V
Ered,1;Ered,2, V
1Nc
1Sc
1NF
431; 486, 522,
555, 636, 698
440; 492, 518,
547, 647, 718
433; 528, 560,
640, 704
+0.38 (ir) -1.51 (r); -1.74 (q-r)
+0.45 (ir) -1.36 (r); -1.56 (q-r)
+0.53 (ir) -1.50 (r); -1.66 (ir)
+0.50 (ir) -1.30 (ir); -1.46 (ir)
Considering rich coordination chemistry of expanded
porphyrins,12 the synthesis of phosphorus-containing ex-
panded porphyrins is a challenging subject.13 However, all
attempts to prepare expanded phosphaporphyrins from the
P-Ph-type phosphatripyrrane have been unsuccessful so far.
In this context, the successful result on the synthesis of 1XF
was quite encouraging, and we decided to use P-C6F5-type
phosphatripyrrane 5 as a key precursor for the synthesis of
expanded phosphaporphyrins. The first target, P-C6F5-type
phosphadithiasapphyrin 11,14,15 was successfully prepared
by the BF3-promoted [3 + 2] dehydrative condensation
between 5 and 5,5′- bis[hydroxy(phenyl)methyl]bithiophene
1016 followed by in situ DDQ oxidation (eq 1). It should be
emphasized again that P-masking is not involved in the
condensation/oxidation steps. The sapphyrin 11 was isolated
as an air-stable shiny green solid. The diagnostic spectral
1SF
11
441; 544, 780
500; 599, 740, 835 +0.43 (ir) -1.39 (ir); -1.59 (ir)
a Reference electrode: Ag/Ag+ [0.01 M AgNO3, 0.1 M n-Bu4NPF6
(MeCN)]. b “r”, “q-r”, and “ir” in parentheses indicate that the processes
occur reversibly, quasi-reversibly, and irreversibly. c Data from ref 2d.
S1 in Supporting Information), whereas both Soret and Q
bands of P,S,N2-porphyrin 1SF were broadened and the Q0-0
band was red-shifted by ca. 60 nm as compared to that of
1S (Table 1 and Figure 3).11 The electrochemical properties
of 1XF were examined by cyclic voltammetry (CV) and
(5) (a) Matano, Y.; Miyajima, T.; Nakabuchi, T.; Imahori, H.; Ochi,
N.; Sakaki, S. J. Am. Chem. Soc. 2006, 128, 11760–11761. (b) Matano,
Y.; Miyajima, T.; Ochi, N.; Nakabuchi, T.; Shiro, M.; Nakao, Y.; Sakaki,
S.; Imahori, H. J. Am. Chem. Soc. 2008, 130, 990–1002; Addition/
Correction: 2009, 131, 14123. (c) Matano, Y.; Fujita, M.; Miyajima, T.;
Imahori, H. Organometallics 2009, 28, 6213–6217.
(2) (a) Matano, Y.; Nakabuchi, T.; Miyajima, T.; Imahori, H.; Nakano,
H. Org. Lett. 2006, 8, 5713–5716. (b) Matano, Y.; Nakashima, M.;
Nakabuchi, T.; Imahori, H.; Fujishige, S.; Nakano, H. Org. Lett. 2008, 10,
553–556. (c) Matano, Y.; Nakabuchi, T.; Fujishige, S.; Nakano, H.; Imahori,
H. J. Am. Chem. Soc. 2008, 130, 16446–16447. (d) Nakabuchi, T.;
Nakashima, M.; Fujishige, S.; Nakano, H.; Matano, Y.; Imahori, H. J. Org.
Chem. 2010, 75, 375–389.
(6) Ring oxidation of P-masked P,X,N2-porphyrinogens (X ) N, S, O)
did not afford 18π P,X,N2-porphyrins. See ref 2b,d.
(3) In 2003, Delaere and Nguyen predicted the electronic structures
and optical properties of unsubstituted 21-phospha- and 21,23-diphos-
phaporphyrins based on the results of density functional theory (DFT)
calculations: Delaere, D.; Nguyen, M. T. Chem. Phys. Lett. 2003, 376,
329–337.
(7) Compound 3 was prepared via TiII-mediated cyclization of 1,7-
bis(ethoxycarbonyl)hepta-1,6-diyne followed by treatment with C6F5PCl2
according to the reported procedure for the synthesis of 1-phenyl-2,5-
di(ethoxycarbonyl)phosphole: Matano, Y.; Miyajima, T.; Nakabuchi, T.;
Matsutani, Y.; Imahori, H. J. Org. Chem. 2006, 71, 5792–5795.
(8) It was convenient to use 4 in a semi-purified state for the subsequent
reaction with pyrrole, although 4 is isolable as a colorless solid in 61%
yield. For details, see Supporting Information.
(4) Mathey and co-workers prepared “P-confused” carbaporphyrinoid:
Duan, Z.; Clochard, M.; Donnadieu, B.; Mathey, F.; Tham, F. S.
Organometallics 2007, 26, 3617–3620.
1114
Org. Lett., Vol. 12, No. 5, 2010

features of 11 are as follows. In the 19F and 31P NMR spectra,
11 showed three 19F signals at δF -165.2 (ortho), -128.5
(meta), and -155.2 (para), and one 31P signal at δP -28.4.
by DPV were more cathodic in comparison with the
respective values of 1SF (Table 1; Figures S3c in Supporting
Information).
1
In the H NMR spectrum of 11, peripheral (meso, pyrrole-
ꢀ, and thiophene-ꢀ) protons resonate significantly downfield
(δ 8.49-10.11), which corroborates the 22-π aromaticity of
11 (Figure 4). The relatively downfield appearance (δ
In summary, we have revealed that the P-C6F5-type
P,X,N2- porphyrins and P,X,N2-calixphyrins (X ) N, S) can
be prepared more easily than their P-Ph analogues. In
addition, the first example of phosphorus-containing core-
modified sapphyrin was successfully prepared from P-C6F5
phosphatripyrrane as the key precursor. The present results
exemplify that the functionalization at the phosphorus atom
is a highly promising strategy to bring chemical stability to
the phosphaporphyrin skeleton. Studies on the coordination
chemistry of this new series of core-modified porphyrin
ligands are now in progress.
Figure 4.
1H NMR spectrum of 11 in CDCl3. Asterisks (*) inidicate
residual solvents.
2.90-4.58) of the peripherally fused trimethylene protons
of 11 originates from the 22-π diatropic ring current effect.
It is therefore likely that the phosphole ring in 11 takes on
a noninverted conformation as represented schematically in
eq 1. This structural feature is in marked contrast to that of
previously reported X,S2,N2-sapphyrins (X ) N, O, S, Se),
where the heterocyclic rings opposite to the bithiophene unit
take on an inverted conformation.15a,b Presumably, in P,S2,N2-
sapphyrin 11, the trimethylene substituents at the phosphole
ring disturb its inversion sterically.17,18 The UV-vis absorp-
tion spectrum of 11 shows an intense Soret band at λmax 500
nm and weak Q bands at λmax 599-835 nm (Figure 3), both
of which are close to the values reported for the inverted
X,S2,N2-sapphyrins (Soret: λmax 490-510 nm, Q: λmax
600-880 nm).15a,b The redox potentials of 11 determined
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Innovative Areas (No.
21108511, “π-Space”) from MEXT, Japan. T.N. thanks a
JSPS fellowship for young scientists.
Supporting Information Available: Experimental details
and characterization data, 1H and 13C NMR spectra for new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL100114J
(15) For examples of core-modified sapphyrins, see: (a) Srinivasan, A.;
Pushpan, S. K.; Kumar, M. R.; Mahajan, S.; Chandrashekar, T. K.; Roy,
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2001, 123, 5138–5139. (d) Richter, D. T.; Lash, T. D. J. Org. Chem. 2004,
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.
(10) Ulman, A.; Manassen, J. J. Am. Chem. Soc. 1975, 97, 6540–6544
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Ed. Engl. 1997, 36, 2598–2601.
(11) The broadening and bathochromic shift of 1SF may be due to
electronic repulsion between the core sulfur atom and the P-C6F5 fluorine
atoms in 1SF.
(17) In the variable temperature 1H NMR spectra of 11 (-100 °C to
+80 °C: CD2Cl2 or CDCl2CDCl2), the signals due to the peripheral (meso,
pyrrole-ꢀ, thiophene-ꢀ, and trimethylene) protons show negligible changes
(∆δ < 0.5 ppm). This implies that the structure of π-system in 11 is not
flexible in solution.
(12) For reviews on coordination chemistry of expanded porphyrins,
see: (a) Sessler, J. L.; Seidel, D. Angew. Chem., Int. Ed. 2003, 42, 5134–
5175. (b) Shimizu, S.; Osuka, A. Eur. J. Inorg. Chem. 2006, 1319–1335,
See also ref 1b
.
(13) Mathey and co-workers prepared several phosphole-containing
macrocycles: (a) Laporte, F.; Mercier, F.; Ricard, L.; Mathey, F. J. Am.
Chem. Soc. 1994, 116, 3306–3311. (b) Deschamps, E.; Ricard, L.; Mathey,
(18) The noninverted conformation of the phosphole ring in 11 may
also be due to the absence of adjacent meso-substituents: (a) Szterenberg,
L.; Latos-Graz˙yn´ski, L. J. Mol. Struct. (Theochem) 1999, 490, 33–46. See
also: (b) Chemielewski, P. J.; Latos-Graz˙yn´ski, L.; Rachlewicz, K.
Chem.sEur. J. 1995, 1, 68–73. (c) Szterenberg, L.; Latos-Graz˙yn´ski, L. J.
Phys. Chem. A 1999, 103, 3302–3309.
F. J. Chem. Soc., Chem. Commun. 1995, 1561
(14) For a review on sapphyrins, see: Sessler, J. L.; Davis, J. M. Acc.
Chem. Res. 2001, 34, 989–997
.
.
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