Fluorescent diarylindoles by palladium-catalyzed direct and decarboxylative arylations of carboxyindoles

Article abstract of DOI:10.1002/chem.200900098

A study was conducted to demonstrate the palladium-catalyzed 2,3-diarylation of carboxyindole derivatives with acryl bromides. The study also performed ester hydrolysis in the sequence to demonstrate a practical route to 2,3-diarylindoles that had differe

Full text of DOI:10.1002/chem.200900098

DOI: 10.1002/chem.200900098
Fluorescent Diarylindoles by Palladium-Catalyzed Direct and
Decarboxylative Arylations of Carboxyindoles
Mitsuru Miyasaka, Azusa Fukushima, Tetsuya Satoh, Koji Hirano, and
Masahiro Miura*[a]
The indole nucleus is found in a large number of biologi-
cally active natural and unnatural compounds, and the syn-
thesis of its derivatives is of considerable importance in or-
ganic chemistry.[1] In addition, as tryptophan is an intrinsic
fluorescent probe in proteins, indole derivatives are known
to display fascinating photophysical properties.[2] Thus, suita-
bly arylated indoles may be considered to exhibit fluores-
cence, and the wavelength and intensity may depend on the
nature of the aryl substituents. A literature search indicates
that some 2,3-diarylindoles, for example 2,3-bis(4-methoxy-
and hydroxyphenyl)indoles, show not only interesting bio-
logical activities,[3a–g] but also fluorescence in the blue re-
gion,[3e–h] whereas very little is known about their fluorescent
efficiency. Accordingly, as the development of efficient blue
emitters is currently one of the important subjects in materi-
als chemistry,[4,5] the synthesis of various related compounds
is of particular interest in terms of their physical features
and their biological behavior.
thetic strategy involves sequential ortho- and ipso-aryla-
tions,[12–14] and gratifyingly, it has been successfully utilized
in carboxyindole systems. Herein, we report the palladium-
catalyzed 2,3-diarylation of carboxyindole derivatives with
aryl bromides, as well as a practical route to 2,3-diarylin-
doles having different aryl substituents by performing ester
hydrolysis in the sequence. As a result of these studies, we
have found a highly fluorescent 2,3-diarylindole among the
products, and its photoluminescent properties are de-
scribed.[15,16]
In a typical synthesis, treatment of 1-methyl-1H-indole-2-
carboxylic acid (1a) with bromobenzene (2a) (3 equiv) in
the presence of PdACTHNUTRGNEUNG(OAc)2 (5 mol%), PCy3 (Cy=cyclohexyl,
10 mol%), and Cs2CO3 (4 equiv) in refluxing o-xylene for
4 h afforded 1-methyl-2,3-diphenyl-1H-indole (3a) in 90%
yield (Scheme 1). In the reaction of 1-methyl-1H-indole-3-
carboxylic acid (4) in place of 1a under the same conditions,
formation of the 2-monophenylated product, 1-methyl-2-
phenyl-1H-indole-3-carboxylic acid (5a) (55%) was ob-
served together with 3a (36%). Thus, the second ipso-phe-
nylation at the 3-position appears to be a relatively slower
process. The reaction of 4 at a higher temperature using me-
Transition-metal-catalyzed biaryl cross-coupling with aryl
halides and aryl metal reagents is one of the most reliable
methods for the synthesis of indoles having a variety of aryl
functions,[6] and recent advances in the metal-mediated
[7]
À
direct C H arylation reactions of heteroarenes provide an
efficient access to C2- or C3-monoarylated indoles.[8,9] How-
ever, the 2,3-diarylation reactions on an indole scaffold are
quite rare.[10] Thus, efficient methods for installing identical
and different aryl groups at the 2- and 3-positions are re-
quired.[11] During our studies on the palladium-catalyzed
direct arylation of heteroarenes with aryl halides, we ach-
ieved the 2,3-diarylation of 3-hydroxymethyl- and carboxy-
[12]
À
À
thiophenes by cleavage of C H and C C bonds. This syn-
[a] M. Miyasaka, A. Fukushima, Prof. Dr. T. Satoh, Dr. K. Hirano,
Prof. Dr. M. Miura
Department of Applied Chemistry, Faculty of Engineering
Osaka University, Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7362
Scheme 1. Phenylation of carboxyindole 1a or 4 with 2a. Reaction condi-
tions: 1a or 4 (0.5 mmol), 2a (1.5 mmol), and Cs2CO3 (2 mmol) at 170 8C
(bath temp) for 4 h. [a] Reaction of 1a in o-xylene. [b] Reaction of 4 in
o-xylene. [c] Reaction of 4 in mesitylene with addition of MS 4A
(150 mg). [d] Determined by GC as its methyl ester 7a after methylation
with MeI.
E-mail: miura@chem.eng.osaka-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900098.
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COMMUNICATION
sitylene as a solvent, however, gave 3a in an acceptable
yield (77%).
Various aryl bromides having an electron-donating or
-withdrawing group could be employed for the diarylation
reaction of 1a (entries 1–5, Table 1). The reaction using
were hydrolyzed with ethanolic KOH to quantitatively
afford the corresponding carboxylic acids. Subsequently, the
acids were subjected to the second arylation accompanied
by decarboxylation in mesitylene, and diarylindoles 8a–8 f
were obtained in good yields (Table 3).
Table 1. Reaction of 2-carboxyindoles 1 with aryl bromides 2.[a]
Table 3. Decarboxylative arylation of 5.[a]
Entry
1, R
2, Ar
3, Yield [%][b]
Entry
5, R, Ar’
2, Ar
8, Yield [%][b]
1
2
3
4
1a, Me
1a, Me
1a, Me
1a, Me
1a, Me
1b, MOM
1c, Ph
2a, Ph
3a, 90
3b, 86
3c, 93
3d, 78
3e, 79
3 f, 82
3g, 79
1
2
3
4
5
6
5a, Me, Ph
5a, Me, Ph
5a, Me, Ph
5a, Me, Ph
5b, Me, 4-CF3C6H4
5c, MOM, Ph
2b, 4-MeC6H4
2d, 4-CF3C6H4
2 f, 4-EtOCOC6H4
2g, 4-PhC6H4
2a, Ph
8a, 84
8b, 60
8c, 89
8d, 94
8e, 55
8 f, 70
2b, 4-MeC6H4
2c, 4-MeOC6H4
2d, 4-CF3C6H4
2e, 4-FC6H4
2a, Ph
5
6
2b, 4-MeC6H4
7[c]
2a, Ph
[a] Reaction
conditions:
[5]:[2]:[PdACHTUNTGRENUN(GN OAc)2]:ACHTUNRTEGG[NNNU PCy3]:ACHTUNGTRENNUNG[Cs2CO3]=
[a] Reaction
conditions:
[1]:[2]:[PdACHTUNTGRENUN(GN OAc)2]:ACHTUNRTEGG[NNNU PCy3]:CAHTNUGTREN[NUGN Cs2CO3]=
0.5:1.0:0.025:0.05:1.0 (in mmol) with MS4A (150 mg), in mesitylene at
170 8C under N2 for 6 h. [b] Yield of isolated product based on 5.
0.5:1.5:0.025:0.05:2 (in mmol), in refluxing o-xylene under N2 for 4 h.
[b] Yield of isolated product. [c] With 0.0125 mmol of Pd
0.025 mmol of P(biphenyl-2-yl)Cy2.
ACHTUNGRTEN(NUNG OAc)2 and
With the above 2,3-diarylindoles 3 and 8 in hand, a pre-
liminary survey of their solid-state photoluminescence by
using a UV lamp was carried out. It was found that 1-
methyl-2,3-bis(4-trifluoromethylphenyl)-1H-indole (3d) was
especially luminescent.
Consequently, the photoluminescence spectra of 3d and,
to examine the substituent effects on the aryl groups, those
of 3a, 3c, 8b, and 8e as well as their absorption spectra
were measured for their ethanol solutions and solid powders
(Table 4 and Figure 1). Accordingly, diarylindole 3d is
other 2-carboxyindoles was also undertaken. Under the
standard reaction conditions, N-methoxymethyl(MOM)-pro-
tection and N-phenyl-substitution were tolerated, and thus,
carboxyindoles 1b and 1c coupled with 2a to give the ex-
pected products 3 f and 3g (entries 6 and 7, Table 1).
To achieve the selective synthesis of indoles having differ-
ent aryl groups at the 2- and 3-positions, we examined a
stepwise diarylation with methyl esters of 1a and 4 as the
starting materials, as an ester function is inert under the
present conditions. The latter ester, methyl 1-methyl-1H-
indole-3-carboxylate (6a) was effectively monoarylated with
AHCTUNGTRENNUNG
2a, 2c, and 2d using P(biphenyl-2-yl)ACTHNUTRGENUG(N tBu)2 as a ligand to
Table 4. Optical properties of 1-methyl-2,3-diarylindoles.
give the corresponding 2-arylated products 7a–c with good
yields (entries 1–3, Table 2), whereas the former ester did
not react at all. The ligand PCy3 was less effective in this
case (entry 4, Table 2). MOM-protected indole 6b was also
available for use (entry 5, Table 2). Then, 7a, 7c, and 7d
[c]
[e]
Entry 3 or labsÀsl log e lemÀsl
FfÀsl
lemÀpw FfÀpw
Table 2. Reaction of 3-(methoxycarbonyl)indoles 6 with aryl bromides
2.[a]
8
[nm][a]
[nm][b]
[nm][d]
225
298
248
299
225
286
225
296
225
303
4.63
4.24
4.48
4.22
4.80
4.46
4.66
4.30
4.63
4.12
1
2
3
4
5
3a
418
0.51
0.13
0.90
0.37
0.64
419
441
422
420
426
0.65
0.44
0.97
0.56
0.76
3c
3d
8b
8e
421
436
419
438
Entry
6, R
2, Ar
7, Yield [%][b]
1
2
3
6a, Me
6a, Me
6a, Me
6a, Me
6b, MOM
2a, Ph
7a, 82
7b, 90
7c, 79
7a, 57
7d, 73
2c, 4-MeOC6H4
2d, 4-CF3C6H4
2a, Ph
4[c]
5
[a] Absorption maximum in EtOH. [b] Emission maximum in EtOH.
[c] Determined by comparison with ethanol solution of anthracene (Ff =
0.30) excited at 254 nm. [d] Emission maximum of solid powder excited
at 350 nm. [e] Absolute quantum yield determined by an integrating
sphere system.
2a, Ph
[a] Reaction conditions: [6]:[2]:[Pd
[Cs2CO3]=0.5:1.0:0.025:0.05:1.0 (in mmol), in refluxing o-xylene under
N2 for 6 h. [b] Yield of isolated product. [c] PCy3 was used as a ligand.
ACHTUNGTNERNUG(OAc)2]:[P(biphenyl-2-yl)ACHTUGNTREN(NUGN tBu)2]:-
ACHTUNGTRENNUNG
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M. Miura et al.
Experimental Section
Typical synthetic procedure (preparation of compound 3d, entry 4 in
Table 1): In a 20 mL two-necked flask were added 4-bromobenzotrifluor-
ide
ACHTUNGTREN(NUNG 2d) (1.5 mmol, 338 mg), 2-carboxyindole 1a (0.5 mmol, 87 mg), Pd-
AHCTNUGTERNNG(UN OAc)2 (0.025 mmol, 5.6 mg), PCy3 (0.05 mmol, 14 mg), Cs2CO3 (2 mmol,
652 mg), and o-xylene (2.5 mL). The resulting mixture was stirred under
N2 (balloon) at 1708C (bath temperature) for 4 h. Compound 3d
(163 mg, 78%) was isolated by filtration of the mixture through a filter
paper by using diethyl ether as eluent, followed by evaporation of the
solvents, and chromatography on silica gel using hexane–ethyl acetate
(98:2, v/v). M.p. 170–1728C; 1H NMR (400 MHz, CDCl3): d=3.69 (s,
3H), 7.23 (t, J=6.6 Hz, 1H), 7.34–7.38 (m, 3H), 7.44 (d, J=7.3 Hz, 3H),
7.53 (d, J=8.5 Hz, 2H), 7.67 (d, J=8.0 Hz, 2H), 7.75 ppm (d, J=8.0 Hz,
1H); 13C NMR (100 MHz, CDCl3): d=31.1, 109.9, 114.8, 119.5, 121.0,
123.1, 124.0 (q, J=270 Hz), 124.4 (q, J=270 Hz), 125.3 (q, J=3.8 Hz),
125.6 (q, J=3.8 Hz), 126.6, 127.8 (q, J=32 Hz), 130.4, 129.5 (q, J=
32 Hz), 131.4, 135.2, 136.6, 137.7, 138.6 ppm; elemental analysis calcd
(%) for C23H15F6N: C 65.87, H 3.61, N 3.34; found: C 65.60, H 3.59, N
3.36.
Figure 1. Photoluminescence spectra of the powders of 1-methyl-2,3-di-
ACHTUNGTRENNUNGarylindoles 3 and 8.
highly luminescent with emission maxima at 436 nm and
422 nm and with quantum yields of 0.90 and 0.97, in solution
and as a solid, respectively. The quantum efficiency of the
solid samples decreased in the following order: 3d>8e>
3a>8b>3c. This trend is the same as that in solution. In
each case, the discrepancy between the emission maxima in
solution and in the solid state is relatively small. These facts
suggest that the solid-state luminescence in each case is es-
sentially based on the intrinsic structure and electronic con-
jugation of individual molecules. This was, at least in the
case of 3d, supported by the crystal structure and packing
determined by single-crystal X-ray diffraction (Figure 2).[17]
Acknowledgements
This work was partly supported by Grants-in-Aid from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan and by coop-
erative research with Sumitomo Chemical Co., Ltd. We thank Prof. N.
Tohnai of Osaka University for the measurement of solid photolumines-
cence spectra.
À
Keywords: arylation
luminescence · palladium
·
C C coupling
·
indoles
·
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Fluorescent Diarylindoles
COMMUNICATION
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Received: January 14, 2009
Published online: February 26, 2009
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