Palladium-catalyzed β-selective direct arylation of porphyrins

Article abstract of DOI:10.1002/anie.201102318

Four birds with one stone: A palladium catalyst/potassium carbonate/pivalic acid system enables direct arylation of porphyrins by aryl bromides (see scheme; Ar=3,5-tBu₂C₆H₃). The C-H arylation shows excellent β selectivity

Full text of DOI:10.1002/anie.201102318

DOI: 10.1002/anie.201102318
Porphyrinoids
Palladium-Catalyzed b-Selective Direct Arylation of Porphyrins**
Yu Kawamata, Sumito Tokuji, Hideki Yorimitsu,* and Atsuhiro Osuka*
Porphyrins are an important class of aromatic compounds
that exhibit interesting optical and electrochemical properties
and have attracted much attention as key components of
functional materials and supramolecular architectures.[1] Cre-
ation of new porphyrins that feature intriguing properties is
hence essential in developing the chemistry and applications
of functional p-rich molecules. Peripheral functionalization of
porphyrins can effectively modify the electronic and steric
nature of the parent porphyrins, and is thus useful when
investigating new porphyrins.[2] Among the possible function-
alizations, arylation is promising since the introduced aryl
groups can induce electronic perturbation through extension
of p conjugation. Nucleophilic addition of aryllithium
reagents to meso-unsubstituted porphyrins followed by
oxidation can be employed for peripheral arylation reac-
tions.[2c,d,3] The nucleophilic arylation reaction inherently
lacks functional-group compatibility because of the high
reactivity of aryllithium reagents. Presently, the most reliable
and chemoselective functionalization is based on palladium-
catalyzed cross-coupling reactions.[4,5] However, the cross-
coupling strategy always requires preparation of halogen-
ated[5,6] or metalated[7,8] porphyrins prior to arylation. More
straightforward and efficient methods for peripheral arylation
of porphyrins are thus needed.
dimethylacetamide (DMA) at 1008C resulted in diarylation
with satisfactory efficiency (Table 1). To our surprise, the
reaction proceeded exclusively at the b positions adjacent to
the free meso position with excellent selectivity, and no meso-
arylated products were detected. Although the sequential
iridium-catalyzed borylation/palladium-catalyzed Suzuki–
Miyaura coupling had been the only efficient and practical
method for b-selective arylation,[8] this sequence often suffers
from the gradual protodeborylation of borylated porphyrins
in protic media during purification and cross-coupling pro-
cesses. Naturally, the present direct method is much more
efficient and does not suffer from such demetalation. More-
over, this method is the first palladium-catalyzed reaction that
selectively functionalizes the b positions of porphyrins.
The yields of the diarylated products 2 are generally
satisfactory, and the scope of aryl bromides is wide (Table 1).
The reactions with aryl bromides that bear no heteroatom-
containing substituents proceeded smoothly (Table 1,
entries 1–6), and the steric hindrance of 2-bromotoluene did
not retard the reaction (entry 2). Although the reaction with
4-bromoanisole was sluggish under the standard reaction
conditions, the use of potassium pivalate instead of pivalic
acid dramatically improved the yield of 2h (Table 1, entry 8).
The reactions with other functionalized aryl bromides were
slow and did not proceed to completion within 20 h (Table 1,
À
Transition-metal-catalyzed direct C H arylation of arenes
with aryl halides has been rapidly emerging as a promising
alternative to traditional cross-coupling arylations because
preparations of aryl metal reagents can be omitted.[9–11] We
envisaged that the direct arylation would be applicable to
porphyrins. After extensive screening of porphyrin substrates
and reaction conditions, we found that the pivalate-assisted
conditions developed by Fagnou and co-workers, with some
modifications, are effective for the synthesis of meso-free NiII
porphyrins.[9f,10a] Treatment of the NiII complex of 5,10,15-
tris(3,5-di-tert-butylphenyl)porphyrin (1-Ni) with aryl halides
in the presence of pivalic acid,[12] potassium carbonate, and
catalytic amounts of palladium acetate and DavePhos in N,N-
Table 1: Scope of diarylation reactions.
Entry
Ar
2
Yield[a] [%]
1
2
3
4
5
6
7
8
C6H5
2a
2b
2c
2d
2e
2 f
2g
2h
2i
76
71
81
79
80
84
72
2-MeC6H4
3-MeC6H4
4-MeC6H4
3,5-Me2C6H3
2-naphthyl
3-MeOC6H4
4-MeOC6H4
4-Me2NC6H4
4-EtO2CC6H4
4-O2NC6H4
[*] Y. Kawamata, S. Tokuji, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
E-mail: yori@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
8 (62[b])
32 (63[c])
28 (62[c])
3 (43[d])
9
10
11
[**] This work was supported by Grants-in-Aid (nos. 22245006 (A) and
20108006 “pi-Space”, and 22106523 “Integrated Organic Synthe-
sis”) from MEXT. S.T. acknowledges a JSPS Fellowship for Young
Scientists. H.Y. acknowledges financial support from Kinki Inven-
tion Center. Prof. H. Maeda and Dr. Y. Haketa (Ritsumeikan
University) are thanked for MALDI-TOF MS measurements.
2j
2k
[a] Yield of isolated product after recrystallization. [b] tBuCO2K (5 equiv)
was used instead of tBuCO2H. [c] 40 h reaction time; a solution of
Pd(OAc)2 (20 mol%) and DavePhos (40 mol%) in DMA was added
again after 20 h. [d] 60 h reaction time; a solution of Pd(OAc)2 (20
mol%) and DavePhos (40 mol%) in DMA was added every 20 h.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102318.
Angew. Chem. Int. Ed. 2011, 50, 8867 –8870
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8867

Communications
entries 9–11). We observed that the catalyst was deactivated
after 20 h, thus we simply added the catalyst repeatedly until
substrate 1-Ni disappeared, in order to considerably improve
the yields of 2i–k.
Nickel(II) proved to be the best central metal in the
substrate. The reaction of ZnII porphyrin 1-Zn (M = Zn) with
3,5-dimethylbromobenzene afforded none of the diarylated
product and the corresponding monoarylated porphyrin in
33% yield. The lower reactivity of 1-Zn, which is less
electron-deficient than 1-Ni, may indicate that the concerted
metalation/deprotonation (CMD) process[10a] would be rate-
determining (see below). No conversion was observed in the
reaction of free-base porphyrin 1-H2 (M = 2H) because a
palladium cation was captured by the substrate. Demetalation
of nickel porphyrins 4 requires highly acidic conditions, thus
reaction conditions for the direct arylation of 1-Zn and 1-H2
are now being explored to obtain the free-base porphyrin of 4.
The high efficiency of the arylation reaction culminated in
direct tetraarylation of the nickel complex of 5,15-bis(3,5-di-
tert-butylphenyl)porphyrin (3; Table 2), where four carbon–
carbon bonds were formed simultaneously with complete
b selectivity without any prefunctionalization of the porphy-
rin.[13,14]
Scheme 1. a) Plausible mechanism and b) less favorable transition
states (TS) for CMD. Ar’=3,5-tBu2C6H3.
According to Fagnou and co-workers,[9f,10a,15] the most
plausible mechanism for the arylation reaction consists of the
following elementary steps (Scheme 1a): 1) oxidative addi-
tion of an aryl bromide to Pd0 to produce an arylpalladium
bromide, 2) bromide–pivalate exchange to provide an aryl-
palladium pivalate, 3) CMD of the porphyrin with the
arylpalladium pivalate to form an aryl(porphyrinyl)palla-
dium, and 4) reductive elimination to result in arylation of the
porphyrin and liberation of the initial Pd0 complex. The more
electron-rich 1-Zn was less reactive because its b protons are
less acidic than those of 1-Ni. The low reactivity of 1-Zn also
strongly supports the hypothesis that the b palladation of the
porphyrins does not proceed by an electrophilic aromatic
substitution (SEAr)[9a,e,i] pathway but by the CMD process.
The perfect regioselectivity of the di- and tetraarylation
would be controlled by steric factors. The substituted
b protons are the most readily accessible among the periph-
eral protons, including the meso protons and the b protons
next to the meso-aryl groups (Scheme 1b). Large-scale
computational studies will be required for detailed ration-
alization of the regioselectivity.
The structures of b-arylated porphyrins 2i and 4a were
unambiguously determined by X-ray crystallographic analysis
1
(Figure 1)[16] as well as H and 13C NMR spectroscopy and
NOESY analysis (see the Supporting Information). The
introduction of the aryl groups at the b positions did not
significantly distort the structures of the parent porphyrins.
The dihedral angles between the b-aryl groups and the
porphyrin plane are 46–538, which are distinctly smaller than
those of the meso-aryl groups (64–898). In addition, the
distances between the substituted b-carbon atoms and the
adjacent benzene carbon atoms are 1.46–1.49 ꢀ, which are
shorter than those between the substituted meso-carbon
atoms and the directly connected benzene carbon atoms
(1.50–1.53 ꢀ). These structural features indicate that the b-
aryl groups are more effectively conjugated with the porphy-
rin than the meso-aryl groups.
Table 2: Attempted tetraarylation reactions.
The UV/Vis absorption spectra of the diarylated porphy-
rins 2h and 2j showed that the Soret and Q bands were
slightly but distinctly red-shifted compared to that of b-
unsubstituted 1-Ni, probably because of the extension of the
p-electronic network (Figure 2). The electronic nature of the
aryl groups likely have no significant influence on the
absorption band. On the other hand, the shifts of the first
oxidation potentials of 2 depend on the electronic nature of
the introduced b-aryl substituents. When plotted against the
2sp value (sp: Hammett substituent constant) of the substitu-
ents, the first oxidation potentials of 2 lie on a linear slope
with 1 = 0.099 (Figure 3; see the Supporting Information for
cyclic voltammograms). The 1 value is somewhat larger than
that of the nickel complexes of para-substituted tetraphenyl-
Entry
Ar
4
Yield[a] [%]
1
2
3
4
3,5-Me2C6H3
4-C8H17OC6H4
4-[Bu2NC( O)]C6H4
3,5-(MeO)2C6H3
4a
4b
4c
4d
91
38 (69[b])
(60[c])
=
13 (60[c])
[a] Yield of isolated product after recrystallization. [b] tBuCO2K (5 equiv)
was used instead of tBuCO2H; 40 h reaction time. [c] 60 h reaction time;
a solution of Pd(OAc)2 (20 mol%) and DavePhos (40 mol%) in DMA
was added every 20 h.
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Figure 3. Plot of first oxidation potentials (E1/2) of 2 against 2sp values
of the substituents (E1/2 =2sp 1).
example of b-selective functionalization. The reaction allows
the rapid synthesis of a series of new b-arylated porphyrins.
The simultaneous tetraarylation of the nickel complex of 5,15-
diarylporphyrin underscores the high efficiency and utility of
our strategy for porphyrin functionalization. The electro-
chemical characters of newly created b-arylated porphyrins
could be modulated by the substituents on the introduced aryl
groups through conjugation between the porphyrin core and
the moderately tilted b-aryl groups. Further investigations are
underway to achieve much higher catalytic activity and to
efficiently synthesize new and/or elusive porphyrin-based
functional molecules.
Figure 1. X-ray crystal structures of a) diarylated porphyrin 2i and
b) tetraarylated porphyrin 4a. Thermal ellipsoids are drawn at the 50%
probability level. Hydrogen atoms and solvent molecules are omitted
for clarity. Dihedral angles at the meso and b positions are denoted as
mi and bi , respectively.
Experimental Section
Typical procedure for the direct arylation of NiII complexes of 5,10,15-
tris(3,5-di-tert-butylphenyl)porphyrin
(1-Ni):
Porphyrin
1-Ni
(23.3 mg, 0.025 mmol), Pd(OAc)2 (1.1 mg, 0.005 mmol), DavePhos
(3.9 mg, 0.010 mmol), K2CO3 (34.5 mg, 0.25 mmol), tBuCO2H
(12.8 mg, 0.125 mmol), and 2-bromonaphthalene (51.7 mg,
0.25 mmol) were placed in a Schlenk tube. The reaction vessel was
purged with argon, and DMA (1.0 mL) was added. The resulting
mixture was then stirred at 1008C for 20 h. The reaction mixture was
diluted with CH2Cl2 (2.0 mL) and passed through a short silica gel
column. After evaporation of the solvent, the product was separated
by column chromatography on silica gel (CH2Cl2/hexane 2:1). Further
purification by recrystallization from CH2Cl2/MeOH afforded 2 f
(24.7 mg, 0.0215 mmol, 84%).
Typical procedure for the direct arylation of NiII complex of 5,15-
bis(3,5-di-tert-butylphenyl)porphyrin (3): Porphyrin
3 (22.2 mg,
0.03 mmol), Pd(OAc)2 (1.3 mg, 0.006 mmol), DavePhos (4.7 mg,
0.012 mmol), K2CO3 (41.4 mg, 0.30 mmol), and tBuCO2H (15.3 mg,
0.15 mmol) were placed in a Schlenk tube. The reaction vessel was
purged with argon, and DMA (1.0 mL) was added. 3,5-Dimethyl-
bromobenzene (81.5 mL, 0.60 mmol) was added by microsyringe. The
mixture was then heated at 1008C for 20 h. The resulting mixture was
diluted with CH2Cl2or CHCl3 (2.0 mL), passed through a pad of silica
gel, and concentrated in vacuo. The product was purified by column
chromatography on silica gel (CH2Cl2/hexane 1:5) followed by
recrystallization from CH2Cl2/MeOH to afford 4a (31.3 mg,
0.0271 mmol, 84%).
Figure 2. UV/Vis absorption spectra of 1-Ni (Ar=H, black), 2h
(Ar=4-MeOC6H4, blue), and 2j (Ar=4-EtO2CC6H4, red) in CH2Cl2
(Ar’=3,5-tBu2C6H3).
porphyrins (1 = 0.089).[17] These observations clearly reflect
the existence of small yet effective conjugation between the
porphyrin core and the peripheral b-aryl groups.
In summary, we have developed a palladium-catalyzed
direct arylation of porphyrins with aryl bromides as a rare
Received: April 4, 2011
Published online: June 3, 2011
Angew. Chem. Int. Ed. 2011, 50, 8867 –8870
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8869

Communications
À
Song, N. Aratani, H. Shinokubo, A. Osuka, J. Am. Chem. Soc.
2010, 132, 16356.
Keywords: arylation · C H activation · palladium ·
porphyrinoids
.
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[16] Crystallographic data for 2i: C81.76H97.52Cl3.76N6NiO0.37, M =
ꢀ
1362.22, triclinic, P1 (No. 2), a = 13.4668(3) ꢀ, b =
17.5454(4) ꢀ,
c = 17.8936(4) ꢀ,
a = 117.6641(12)8,
b =
90.2825(13)8, g = 99.2829(13)8, V= 3680.11(14) ꢀ3, Z = 2, T=
93 K, 1calcd = 1.229 gcmÀ3, R1 = 0.0985 (I > 2s(I)), wR2 = 0.3279
(all data), GOF = 1.052. Crystallographic data for 4a:
ꢀ
C81.56H87.55N4NiO1.16
12.8869(5) ꢀ, b = 16.4409(7) ꢀ,
107.132(2)8, b = 104.990(2)8, g = 103.723(2)8, V= 3448.9(2) ꢀ3,
Z = 2, T= 93 K, 1calcd = 1.157 gcmÀ3
R1 = 0.0983(I>2s(I)),
,
M = 1201.10, triclinic, P1 (No. 2), a =
c = 18.7479(8) ꢀ,
a =
,
wR2 = 0.3411(all data), GOF = 1.075. CCDC 819613 (2i) and
819548 (4a) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
[17] K. M. Kadish, M. M. Morrison, Inorg. Chem. 1976, 15, 980.
8870 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 8867 –8870