A convenient formylation of 2-(Dibromomethyl)-6,11-diphenyl-5,12- naphthacenequinone using N, N -dimethylformamide or N, N -dimethylacetamide: Synthesis of 2-formyl rubrene

Article abstract of DOI:10.1055/s-0030-1258362

The main objective of this study is the synthesis of 2-formyl rubrene (5,6,11,12-tetraphenyltetracene-2-carbaldehyde). A smooth and efficient method for the formylation of 2-(dibromomethyl)-6,11-diphenyltetracene-5,12-dione is described using N,N-dimethylformamide or N,N-dimethylacetamide as both the formylating agent and solvent, without any additional base. The product, 5,12-dioxo-6,11-diphenyl-5,12-dihydrotetracene-2-carbaldehyde is obtained in quantitative yield and is converted into the target compound via three additional steps. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1258362

PAPER
287
A Convenient Formylation of 2-(Dibromomethyl)-6,11-diphenyl-5,12-naph-
thacenequinone Using N,N-Dimethylformamide or N,N-Dimethylacetamide:
Synthesis of 2-Formyl Rubrene
A
C
onvenient
F
orm
o
ylation of
A
n
ryl
D
ibromi
g
de
s
fei Wang,a Hongbin Zhao,a Zhizhi Hu,*a Masaru Kimura,*a,b Zhang Zhiqiang,a Lei Peng,a Koji Hiratsuka*c
a
Optic Photo-electronic Materials and Research Development Center, University of Science and Technology Liaoning,
Anshan 114044, P. R. of China
b
c
Department of Chemistry, Faculty of Science, Okayama University, Tsushimanaka 3-1-1, Okayama 700-8530, Japan
Chemistry Department, Faculty of Engineering, Gunma University, Tenjin-cho 1-5-1, Kiryu, Gunma 376-8515, Japan
Fax +86(412)5612902; E-mail: kimura-m@hotmail.com
Received 5 August 2010; revised 4 November 2010
mine–dimethyl sulfoxide,13 amongst others, and 2) hydrol-
ysis of the corresponding dibromobenzyl derivatives.6,14
Abstract: The main objective of this study is the synthesis of 2-
formyl rubrene (5,6,11,12-tetraphenyltetracene-2-carbaldehyde). A
smooth and efficient method for the formylation of 2-(dibromo-
methyl)-6,11-diphenyltetracene-5,12-dione is described using N,N-
dimethylformamide or N,N-dimethylacetamide as both the formy-
lating agent and solvent, without any additional base. The product,
5,12-dioxo-6,11-diphenyl-5,12-dihydrotetracene-2-carbaldehyde is
obtained in quantitative yield and is converted into the target com-
pound via three additional steps.
As direct oxidation procedures proved unsuccessful, we
investigated the second method and found this approach
provided a reliable route for the synthesis of 2-formyl ru-
brene (4), an important red-fluorescent compound, start-
ing from polycyclic compound 1. A previously reported
two-step method for conversion of the methyl group of
compound 1 into a formyl group (Scheme 1),6 involved
initial bromination to give dibromide 2a, followed by hy-
drolysis with sodium acetate–acetic acid in the presence
of a small amount of water to afford 3a. However, the hy-
drolysis step was time consuming and the use of acetic
acid was undesirable.
Key words: 2-formyl rubrene, aldehyde, formylation, N,N-dimeth-
ylformamide, N,N-dimethylacetamide
Although methods for the introduction of functional
groups to polycyclic hydrocarbons, for example, rubrene
are rare, such processes have become more important for
the synthesis of fluorescent functional dyes as organic
photo-electronic materials. In order to develop rubrenes
into useful red-fluorescent substances for electrolumines-
cence,1–3 and as fluorescent and sensitizing dyes,4,5 this
study was directed toward the challenging formylation of
rubrene at the 2-position.6 Methyl arenes are useful start-
ing materials for the regiospecific formylation of aromatic
compounds. The conversion of methyl arenes into the cor-
responding aryl aldehydes can be classified according to
the following two methods: 1) direct formylation via oxi-
dation using chromyl chloride (CrO2Cl2),7 cerium(IV)
ammonium nitrate,8 ceric trifluoroacetate,9 silver oxide,10
manganese compounds,11 selenium dioxide,12 and bro-
Ph
O
CH(OAc)2
Ph
Ph
O
O
CONMe2
Ph
O
5
6
Figure 1 Structures of intermediate 5 and by-product 6
The yield of compound 3a with respect to the use of vari-
ous solvents was investigated and the results are shown in
Ph
O
Ph
O
O
Ph
O
O
CHO
CHBr2
NBS, BPO, CCl4
reflux
NaOAc, AcOH
Ph
O
Ph
Ph
1
2a
3a
NBS = N-bromosuccinimide; BPO = benzoyl peroxide
Scheme 1 Preparation of compound 3a
SYNTHESIS 2011, No. 2, pp 0287–0291
x
x
.
x
x
.2
0
1
1
Advanced online publication: 14.12.2010
DOI: 10.1055/s-0030-1258362; Art ID: F13810SS
© Georg Thieme Verlag Stuttgart · New York

288
Y. Wang et al.
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Table 3 Formation of 3a in DMF or DMAa
Table 1. The reaction did not take place in 1,4-dioxane
and the yields were only moderate in polar aprotic sol-
vents such as N,N-dimethylformamide and dimethyl sul-
foxide. In an attempt to optimize the yield of 3a the
amount of the sodium acetate catalyst was varied and N,N-
dimethylformamide was used as the solvent. As shown in
Table 2, decreasing the amount of sodium acetate provid-
ed higher yields of compound 3a. The reaction intermedi-
ate, 2-diacetoxymethyl-5,12-dioxo-6,11-diphenyl-5,12-
dihydrotetracene (5) and the by-product, N,N-dimethyl-
5,12-dioxo-6,11-diphenyl-5,12-dihydrotetracene-2-car-
boxamide (6) were also obtained (Figure 1).
Solvent
DMF
Yield (%)b
94
92
DMA
a Reaction time = 7 h, temperature = 120 °C.
b Yield of isolated product.
the same manner. In N,N-dimethylacetamide, traces of
dimethylamine (1%) and acetic acid (0.5%) were detected
by gas chromatography. Thus, N,N-dimethylformamide
and N,N-dimethylacetamide may play an important role
other than as a solvent.
Table 1 Effect of Solvents on the Yield of 3a Starting from 2aa
Solvent
H2O
Yield (%)b
4 (3)
As shown in Schemes 2 and 3, a plausible mechanism for
the conversion of 2a into aldehyde 3a is proposed which
is similar to the Vilsmeier–Haack reaction. The formation
of 3a may be accompanied by salt 7 as a fragment partner.
This salt may undergo hydrolysis by moisture present in
the reaction mixture leading to the trace amounts of di-
methylamine and acetic acid.
THF
12 (11)
0
1,4-dioxane
DMF
60 (52)
35 (25)
DMSO
The generality of this method was studied using the sim-
ple aromatic dibromomethyl derivatives, 1-(dibromometh-
yl)benzene (2b), 1-(dibromomethyl)-3-fluorobenzene
(2c) and 1-chloro-3-(dibromomethyl)benzene (2d), and
N,N-dimethylformamide as the solvent (Table 4). The
yields of the corresponding benzaldehydes 3b–d were ex-
cellent (90–91%).
a Reaction conditions: 2a (0.5 mmol), NaOAc (1 mmol), solvent (18
mL), 100 °C, N2, 5 h.
b Yield estimated by HPLC; yield of isolated product in parentheses.
Table 2 Effect of the Amount of Sodium Acetate on the Yield of
3aa,b
Entry
2a:NaOAc
Yield (%)
O
O
3a
5
6
2
2
2
1
C
N
+
2a
C
N
R
R
1
2
3
4
1:4
53
60
83
90
18
10
8
1:2
Br
Br
1:1
O
O
1:trace
trace
C
N
N
R
C
R
a A typical procedure is included in the experimental section.
b Solvent = DMF, temperature = 100 °C.
Br
Br
Br
N
The yield of 3a demonstrated an inverse relationship to
the amount of sodium acetate catalyst used, whilst the
yield of intermediate 5 was proportional. Due to the very
high stability of diacetate 5, it could not be transformed
into 3a by addition of water or acetic acid and then heating
at reflux temperature in N,N-dimethylformamide. Treat-
ment with aqueous 10% sodium bicarbonate solution at
100 °C was required to convert 5 into the desired aldehyde
3a. When the procedure was performed in the absence of
sodium acetate in N,N-dimethylformamide at 120 °C, to
our surprise, the reaction proceeded smoothly affording
3a after seven hours in an excellent 94% yield. Using N,N-
dimethylacetamide (DMA), which has the same
CON(CH3)2 group as N,N-dimethylformamide, the yield
was 97% (Table 3). The yields were almost identical in
both solvents which presumably indicates that they act in
Br
+
3a
R
R = H or Me
7
Scheme 2 A plausible mechanism for the formation of aldehyde 3a
Br
R
OH
H2O (2 equiv)
– HBr
NH2
Br
N
Br
+
O
R
7
Scheme 3 Hydrolysis products of salt 7
The target compound, 2-formyl rubrene (4) was prepared
from 3a in three steps via protection of the formyl group
using ethylene glycol in the presence of ZrO3/SO4 at re-
flux temperature, followed by reaction with phenylmag-
Synthesis 2011, No. 2, 287–291 © Thieme Stuttgart · New York

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A Convenient Formylation of Aryl Dibromides
289
Table 4 Investigation of the Generality of the Formylation Reactiona
tively. In the presence of sodium acetate, the conversion
of reaction intermediate 5 into 3a was incomplete. The
target product, red-fluorescent 2-formyl rubrene (4) was
prepared via this modified formylation procedure.
Entry
Substrate
Product
Yield (%)b
CHO
CHBr2
1
91
All solvents and the base were commercially available (Shanghai
Chemical Reagent Co.). Column chromatography was performed
using silica gel (SiliaFlash F60, 500–800 mesh). Melting points
were measured using a Beijing Tech instrument CO, LTD X-6 melt-
ing point apparatus, and are uncorrected. IR spectra were recorded
on a WQF-200 FTIR spectrometer. 1H and 13C NMR spectra were
obtained using a Bruker Avance 500 spectrometer. Mass spectra
were recorded using Agilent Micromass UPLC/Q-Tof Micro and
Agilent 7890A/5975C spectrometers. GC analysis was performed
using an Agilent 6890N instrument. Elemental analysis were ob-
tained using a Thermo Fisher Scientific flash ea1112 analyzer.
2b
2c
2d
3b
CHO
CHBr2
2
3
90
90
F
F
3c
CHBr2
CHO
2-(Dibromomethyl)-6,11-diphenyltetracene-5,12-dione (2a)
Yield: 550 mg (62%); mp 246.2–247.1 °C
1H NMR (500 MHz, CDCl3): d = 6.62 (s, 1 H), 7.24 (s, 4 H), 7.31–
7.61 (m, 10 H), 7.88 (d, J = 7.6 Hz, 1 H), 8.11 (d, J = 7.8 Hz, 1 H),
8.24 (s, 1 H).
Cl
Cl
3d
a Temperature = 120 °C.
b Yield of isolated product.
13C NMR (125 MHz, CDCl3): d = 124.9, 127.1, 127.3, 128.0, 128.4,
128.5, 128.6, 128.9, 129.1, 129.2, 131.7, 135.1, 135.5, 140.0, 140.1,
144.4, 144.5, 146.9, 183.2, 183.3.
EI-MS: m/z = 583.3 [M + H]+.
These data are consistent with those reported.6
nesium bromide, and treatment with iron powder and
acetic acid (Scheme 4). Red-fluorescent 2-formyl rubrene
(4) has a lower-lying p–p* excited state [lmax (a) = 545
nm, e = 891, lmax (f) = 576 nm] than orange-fluorescent
rubrene [lmax (a) = 529 nm, e = 925, lmax (f) = 557 nm],
and has almost the same fluorescence quantum yield as
that of rubrene (~1) in degassed carbon tetrachloride at
room temperature.5 These properties are desirable for red-
emitters for use as electroluminescent dyes. Compound 4
has also proved amenable for assembly as an organic
light-emitting diode.1–3
5,12-Dioxo-6,11-diphenyl-5,12-dihydrotetracene-2-carbalde-
hyde (3a); Conventional Method6
A mixture of compound 2a (291 mg, 0.5 mmol) and NaOAc (4
mmol) in AcOH (35 mL) was heated at reflux temperature for 70 h.
After cooling, brine (30 mL) was added and the mixture was ex-
tracted with CH2Cl2 (60 mL). The organic layer was separated and
dried over anhyd Na2SO4. After filtration, the crude mixture was pu-
rified by flash column chromatography on silica gel (PE–EtOAc,
50:1) to give 3a in 87% yield.
Ph
O
O
O
a
b
3a
5,12-Dioxo-6,11-diphenyl-5,12-dihydrotetracene-2-carbalde-
hyde (3a); This Work
A soln of compound 2a (291 mg, 0.5 mmol) in DMF (22 mL) was
heated at 120 °C for 7 h. After cooling, brine (30 mL) was added and
the mixture was extracted with CH2Cl2 (60 mL). The organic layer
was separated and dried over anhyd Na2SO4. Evaporation of the sol-
vent and purification of the residue by flash column chromatogra-
phy on silica gel (PE–EtOAc, 50:1) gave 3a.
Ph
c
O
8
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OH
CHO
O
Yield: 215 mg (94%); yellow powder; mp 260.5–262.3 °C (Lit.6
261–263 °C).
OH
9
4
1H NMR (500 MHz, CDCl3): d = 7.31–7.33 (m, 4 H), 7.52–7.61 (m,
10 H), 8.17 (d, J = 1.6 Hz, 1 H), 8.21 (d, J = 7.9 Hz, 1 H), 8.54 (s,
1 H), 10.06 (s, 1 H).
13C NMR (125 MHz, CDCl3): d = 121.2, 127.4, 127.9, 128.5,
128.62, 128.63, 129.0, 129.2, 129.3, 129.9, 132.2, 135.6, 135.8,
138.4, 139.6, 139.8, 139.9, 144.61, 144.63, 183.3, 183.6, 190.8.
Scheme 4 Synthesis of 2-formyl rubrene (4). Reagents and condi-
tions: (a) ethylene glycol, ZrO2/SO4, 160 °C, 8 h, 82%; (b) PhMgBr,
THF, reflux, 10 h; (c) Fe powder, AcOH, reflux, 2 h, 27%.
In conclusion, during optimization studies on the conven-
tional formylation of dibromide 2a using sodium acetate,
it was gratifying to find that the reaction proceeded
smoothly to afford aldehyde 3a using only N,N-dimethyl-
formamide or N,N-dimethylacetamide as the solvent with-
out any additional base. This method was approximately
ten-times faster than that of the conventional formylation
using sodium acetate in acetic acid, and gave 3a quantita-
These data are consistent with those reported.6
Formylation (Table 2); Typical Procedure
A soln of compound 2a (291 mg, 0.5 mmol) in DMF (18 mL) was
heated at 100 °C in the presence of NaOAc (1 mmol) for 2 h. After
cooling, brine (30 mL) was added and the mixture was extracted
with CH2Cl2 (60 mL). The organic layer was separated and dried
over anhyd Na2SO4. Following evaporation of the solvent, the crude
residue was purified by flash column chromatography on silica gel
Synthesis 2011, No. 2, 287–291 © Thieme Stuttgart · New York

290
Y. Wang et al.
PAPER
(PE–EtOAc, gradient 50:1 to 30:1) to give aldehyde 3a (60%), di-
acetate 5 (10%) and dimethylamine 6 (2%).
2-(1,3)-Dioxolan-2-yl-6,11-diphenylnaphthacene-5,12-dione (8)
A soln of 3a (0.964 g, 2 mmol) in ethylene glycol (60 mL) was
stirred in the presence of ZrO3/SO4 (0.06 mmol) at 60 °C for 8 h un-
der N2. Following completion of the reaction the mixture was ex-
tracted with CH2Cl2 (60 mL), and the extract was dried over MgSO4
and evaporated. The crude residue was subjected to column chro-
matography on silica gel (PE–EtOAc, gradient 80:1 to 50:1) to give
compound 8.
2-Diacetoxymethyl-5,12-dioxo-6,11-diphenyl-5,12-dihydrotet-
racene (5)
Yield: 29.1 mg (10%); yellow powder; mp 240.1–241 °C.
IR (KBr): 3052, 2848, 1670, 1370, 1174 cm–1.
1H NMR (500 MHz, CDCl3): d = 2.13 (s, 6 H), 7.27 (s, 4 H), 7.33–
7.62 (m, 10 H), 7.70 (s, 1 H), 7.76 (d, J = 8.0 Hz, 1 H), 8.12 (d,
J = 8.0 Hz, 1 H), 8.28 (s, 1 H).
Yield: 0.87 g (82%); yellow powder; mp 269.6–270.8 °C.
IR (KBr): 3052, 1602, 1436, 1330, 1160, 972 cm–1.
13C NMR (125 MHz, CDCl3): d = 20.8, 88.7, 125.4, 127.2, 127.6,
128.5, 128.63, 128.64, 128.89, 128.90, 129.03, 129.04, 131.8,
135.6, 135.7, 140.1, 140.2, 141.0, 144.32, 144.33, 168.6, 183.6,
183.7.
1H NMR (500 MHz, CD2Cl2): d = 4.02–4.12 (m, 4 H), 5.87 (s, 1 H),
7.31 (d, J = 7.1 Hz, 4 H), 7.33–7.61 (m, 10 H), 7.79–7.81 (t, J = 8.3
Hz, 1 H), 8.05 (d, J = 8.3 Hz, 1 H), 8.14 (s, 1 H).
13C NMR (125 MHz, CDCl3): d = 29.7, 65.6, 102.7, 124.9, 126.8,
128.2, 128.69, 128.70, 128.81, 128.82, 128.84, 128.9, 131.6, 135.1,
135.4, 135.57, 135.61, 140.35, 140.38, 143.8, 144.2, 183.6, 183.7.
EI-MS: m/z = 540.8 [M]+.
Anal. Calcd for C35H24O6: C, 77.77; H, 4.48. Found: C, 77.72; H,
4.53.
EI-MS: m/z = 483.1 [M + H]+.
Anal. Calcd for C33H22O4: C, 82.14; H, 4.60. Found: C, 82.10; H,
4.64.
N,N-Dimethyl-5,12-dioxo-6,11-diphenyl-5,12-dihydrotet-
racene-2-carboxamide (6)
Yield: 5.8 mg (2%); yellow powder; mp 194.1–195.8 °C.
IR (KBr): 2920, 2852, 1672, 1650, 1257, 1011, 731 cm–1.
2-Formyl Rubrene (5,6,11,12-Tetraphenyltetracene-2-carbal-
dehyde) (4)
A soln of 8 (0.48 g, 1 mmol) in THF (15 mL) was slowly added to
a soln of PhMgBr (8 mmol) in THF, and the mixture heated at reflux
temperature for 6 h. The resulting mixture was washed with aq
NH4Cl soln (100 mL) and then extracted with CH2Cl2 (100 mL).
Concentration of the extract gave intermediate 9 (0.62 g) which was
dissolved in AcOH (30 mL). The resulting soln was heated at reflux
temperature for 2 h in the presence of Fe powder (0.7 g, 12.5 mmol).
After cooling, the reaction mixture was extracted with CH2Cl2 (100
mL), the extract was dried over MgSO4 and evaporated. The residue
was purified by column chromatography on silica gel (anhyd hex-
ane) to give compound 4.
1H NMR (500 MHz, CDCl3): d = 2.95 (s, 3 H), 3.11 (s, 3 H), 7.27
(s, 4 H), 7.32–7.61 (m, 10 H), 7.73 (d, J = 8.0 Hz, 1 H), 8.09 (s, 1
H), 8.13 (d, J = 8.0 Hz, 1 H).
13C NMR (125 MHz, CDCl3): d = 29.6, 29.7, 125.3, 127.2, 127.4,
127.5, 128.4, 128.5, 128.65, 128.68, 128.9, 129.0, 132.3, 134.8,
135.3, 135.6, 135.7, 140.0, 140.1, 141.3, 144.3, 169.7, 183.6, 183.7.
EI-MS: m/z = 482.2 [M + H]+.
Anal. Calcd for C33H23NO3: C, 82.31; H, 4.81; N, 2.91. Found: C,
82.25; H, 4.89; N, 2.82.
Yield: 0.15 g (27%); deep red solid; mp >300 °C.
IR (KBr): 3090, 2720, 1695, 1621, 1603 cm–1.
1H NMR (500 MHz, CD2Cl2): d = 6.87–6.93 (m, 8 H), 7.10–7.21
(m, 14 H), 7.43 (m, 4 H), 7.89 (s, 1 H), 9.76 (s, 1 H).
13C NMR (125 MHz, CDCl3): d = 119.5, 125.3, 125.7, 126.0, 126.1,
126.3, 126.4, 126.6, 127.3, 127.4, 127.5, 127.9, 129.3, 131.2, 132.0,
132.1, 133.4, 137.1, 140.8, 141.3, 191.8.
Benzaldehyde (3b)
Yield: 96.5 mg (91%); colorless oil; bp 203–204 °C.
IR (thin film): 3092, 2740, 2880, 1709, 1580 cm–1.
1H NMR (500 MHz, CDCl3): d = 7.61 (d, J = 7.6 Hz, 2 H), 7.71 (dd,
J = 7.3, 1.0 Hz, 1 H), 7.92–7.94 (d, J = 7.4 Hz, 2 H), 10.06 (s, 1 H).
13C NMR (125 MHz, CDCl3): d = 129.1, 129.4, 134.3, 136.9, 192.1.
EI-MS: m/z = 106.3 [M]+.
EI-MS: m/z = 560.4 [M]+.
3-Fluorobenzaldehyde (3c)
Yield: 111.6 mg (90%); colorless oil; bp 171–173 °C.
IR (thin film): 3054, 2950, 2880, 1720, 1605 cm–1.
Anal. Calcd for C43H28O: C, 92.11; H, 5.03. Found: C, 92.05; H,
5.09.
1H NMR (500 MHz, CDCl3): d = 7.47–7.51 (dd, J = 8.4, 2.3 Hz, 1
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.
H), 7.64–7.70 (m, 2 H), 7.80–7.81 (d, J = 7.2 Hz, 1 H), 10.06 (s, 1
H).
13C NMR (125 MHz, CDCl3): d = 114.7, 121.1, 125.5, 131.0, 139.0,
162.1, 191.0.
Acknowledgment
EI-MS: m/z = 124.1 [M]+.
We thank Mr. Dong Yan and Mr. Chi Haijun for mass and NMR
measurements.
3-Chlorobenzaldehyde (3d)
Yield: 126 mg (90%); colorless oil; mp 17–18 °C.
IR (thin film): 3090, 2885, 2740, 1710, 1600 cm–1.
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