TfOH catalyzed synthesis of 9-arylfluorenes via tandem reaction under warm and efficient conditions

Article abstract of DOI:10.1039/c4ra03126a

A tandem ring-closing reaction was developed for the synthesis of 9-arylfluorenes and their derivatives. The reaction is metal-free, straightforward and efficient under warm conditions. More than 99% yield is obtained. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03126a

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TfOH catalyzed synthesis of 9-arylfluorenes via
tandem reaction under warm and efficient
conditions†
Cite this: RSC Adv., 2014, 4, 27722
*
*
Qingcui Li, Wengang Xu, Jiaxing Hu, Xiaoqing Chen, Fanglin Zhang and Hua Zheng
Received 8th April 2014
Accepted 12th June 2014
A tandem ring-closing reaction was developed for the synthesis of 9-arylfluorenes and their derivatives. The
reaction is metal-free, straightforward and efficient under warm conditions. More than 99% yield is
obtained.
DOI: 10.1039/c4ra03126a
www.rsc.org/advances
9-Aryluorenes have attracted many researchers' attention for convenient enough. In summary, an environmentally friendly
their promising potential applications in materials, including and efficient method is still desired for the synthesis of 9-aryl-
blue uorescent organic light emitting materials, thin lm uorenes. In this article, we report a method to prepare 9-aryl-
transistors, photovoltaic cells, etc.1 On the other hand, Friedel– uorenes from 2-arylbenzaldehydes in the catalyst system of
Cras reaction plays an important role in C–C bond building, TfOH and acetic anhydride via tandem Friedel–Cras reactions
especially the tandem Friedel–Cras reaction, which economi- at room temperature in acceptable to great yields.
cally constructs two C–C bonds in one step. To date, many
extramolecular tandem reactions with aromatic aldehydes have benzaldehyde
First, we attempted the tandem reaction of 2-phenyl-
with arene. Encouragingly, 2-phenyl-
been described by researchers.2 However, to our best knowl- benzaldehyde transformed into 9-mesityl-9H-uorene at 80 C
edge, intramolecular tandem reactions of aromatic aldehydes in good yield (Table 1, entry 1), but mesitylene was necessarily
are rarely reported.3 Our groups have concentrated on the solvent. Considering the acetal and acylal being active
research of the tandem reaction for its advantages in building substrates,8 1,3-propanediol was added to the reaction, but no
complicated compounds. Recently, our group reported a one- product was detected at room temperature (Table 1, entry 2).
pot three-component reaction for the synthesis of benzofuro However, to our delight, aer adding two times amount of
[2,3-c]pyridines.4 Herein, an environmentally friendly and effi- acetic anhydride, expected product was obtained in excellent
cient Tandem ring-closing Friedel–Cras reaction is developed yield in DCE at room temperature (Table 1, entry 3). If less acetic
ꢀ
for the synthesis of 9-aryluorenes.
anhydride was added to the reaction, the yield decreased
The synthetic methods of 9-aryluorenes were widely slightly to 87% (Table 1, entry 4). Notably, no product was
researched. In 2011, Chen reported a reaction of uorenes and gained, when no acetic anhydride was added at room temper-
haloarenes generates 9-aryluorenes via palladium-catalyzed ature (Table 1, entry 5).
arylation at 130 C.5 Recently, Teng's group synthesized 9-aryl-
Further, in pursuit of the catalytic version, we investigated
ꢀ
uorenes from triarylcarbinols in the presence of p-toluene- the tandem alkylation in the presence of various acids (Table 1,
sulfonic acid (TsOH), in benzene.6 In 2012, Das and his co-
workers builded a tandem ring-closing sequence catalyzed by a
Pd–Sn heterobimetallic catalyst with AgPF6 as additive,3 which
is a novel strategy about intramolecular tandem reactions. But,
according to this article, biaryl alcohols have to be synthesized
rstly from 2-arylbenzaldehydes to further prepare 9-aryl-
uorenes. Similarly, Li' and Sarkar' group developed a general
and efficient synthesis of 9-aryluorenes via Grignard addition
and Friedel–Cra cyclization reactions from 2-arylbenzalde-
hydes (Scheme 1).7 However, the strategy is not economic and
School of Chemical Engineering, Wuhan University of Technology, Wuhan, 430070,
China. E-mail: fanglinzhang0210@gmail.com; zhenghua.whut@126.com; Fax: +86
027 8774 9300
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra03126a
Scheme 1 Synthetic approaches for 9-arylfluorenes.
27722 | RSC Adv., 2014, 4, 27722–27725
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Table 1 Optimation of reaction conditionsa
Table 2 Substrate scope of tandem Friedel–Craft reactiona
Entry
Catalyst
Solvent
Time (h)
Yieldb (%)
1c
2d
3
TfOH
TfOH
TfOH
TfOH
TfOH
H2SO4
BF3$Et2O
CF3COOH
TsOH
CSA
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
Mesitylene
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
n-Hexane
Nitromethane
EtOH
24
24
0.5
24
24
22
22
24
24
24
1
1.5
1.5
24
24
24
75
NDe
98
4f
87
ND
83
5g
6
7
8
9
10
11
12
13
14
15
16
90
ND
ND
ND
96
79
87
ND
ND
ND
THF
1,4-Dioxane
a
Reactions were conducted with 0.5 mmol of 1a, 2.5 mmol of 2a and
1.0 mmol acetic anhydride in 5 ml solvent at room temperature.
b
c
Isolated yields. No acetic anhydride, mesitylene as solvent, at
d
e
80 ꢀC. No acetic anhydride, 1.0 mmol 1,3-propanediol. No product
was detected. f 0.5 mmol acetic anhydride. g No acetic anhydride.
entries 6–10). H2SO4 and BF3$Et2O could also catalyze the
reaction well (entries 6 and 7). Trace of benzylidene diacetate
was detected aer 1.5 h in the both reactions and, nally, good
yields were obtained in 83% and 90%, respectively. CF3COOH,
p-toluenesulfonic acid and camphorsulfonic acid (CSA) had no
catalytic effect (entries 7–9). Signicantly, besides in DCE, the
reaction may proceed in other non-polar solvents (Table 1,
entries 11–13). Almost identical result was obtained in DCM
(entry 11). And in nitromethane and n-hexane, good yields were
obtained (entries 12 and 13). However, in polar solvent: EtOH,
THF and 1,4-dioxane, the reaction couldn't proceed
(entries 14–16).
The activity of various groups substituted 2-phenyl-
benzaldehydes were widely investigated under the optimal
conditions (Table 1, entry 3). Methyl group substituted
2-phenylbenzaldehydes may transform into products in almost
quantitative yields (Table 2, 3ba–3da). 3ba and 3ca were
obtained in 93% and >99% yields, respectively. The substrate
bearing dimethyl group got a lower yield (84%) due to steric
hindrance (3da). Tertiary butyl substituted substrate also pro-
ceeded well to afford 3ea in >99% yield. Moreover, methoxy
group substituted substrates got different yields. 3fa was
obtained in >99% yield but 9-mesityl-2-methoxy-9H-uorene in
21% yield (3ga). The phenomenon is strange and interesting.
We try to gure it out. In contrast, 2-arylbenzaldehydes bearing
electron-withdrawing substituent transformed into products
effectively (3ia–3la). Acetoxyl, chlorine, bromine and even
a
Reactions were conducted with 0.5 mmol of 1 and 2.5 mmol of 2 under
the optimal conditions unless otherwise stated and isolated yields are
shown.9 Regioselectivity: 3ab (88 : 12); 3ac (62 : 38); stereoselectivity:
3ad (63 : 37). These ratios were determined by 1H NMR. For details
about the rotamers of 3ad, see: ref. 5.
triuoromethyl substituted 2-arylbenzaldehydes were trans-
formed almost quantitatively into desired products. Notably, to
the substrates of formyl group's the aromatic rings bearing
electron-withdrawing substituents, great yields were obtained
(3ma and 3na). We also investigated the scope of arenes (3ab–
3ag). Good to great yields were obtained with electron-rich
arenes (3ab–3ad, 3af, 3ag), while electron-decient benzene
couldn't proceed smoothly. The thing to note here is that target
products from dimethyl-substituted arenes have regioselectivity
and stereoselectivity (3ab–3ad). Unfortunately, when more
electron-decient chlorobenzene or nitrobenzene was
employed, no product was gained. Remarkably, 2-(thiophen-3-
yl)benzaldehyde cyclized into the corresponding indeno[2,1-b]-
thiophene in good yield under optimal conditions (3oa).
Furthermore, both potentially reactive C]O bonds in
substrates could convert smoothly into indeno[1,2-b]uorene
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derivatives (eqn (1)). Such products present more advantages
than monofunctional products as blue uorescent organic light
emitting materials. More importantly, indenouorene deriva-
tives are potential and vital candidates as organic semi-
conductors for use in thin-lm transistors and light-emitting
diodes.10 The methodology could also be applied to
2-phenoxybenzaldehyde to afford xanthene (7), which is a type
of important intermediate of drugs and dyes (eqn (2)).
Scheme 2 Controlled experiments.
According to our investigations, acetic anhydride has a crucial
role in the reaction. Further investigation was conducted in the
purpose of guring out the mechanism of the reaction (Scheme
˚
2). Firstly, 4 A molecular sieve was added instead of acetic
anhydride (eqn (3)) and no product was obtained, which indi-
cated acetic anhydride is not only as water absorbent. Notably,
when the acylal from 2-phenylbenzaldehyde was treated with
mesitylene in the catalysis of TfOH solely, the target compound
could be synthesized successfully in 80% yield (eqn (4)). However,
the reaction proceeds so fast that the ester intermediate can't be
obtained even though at minus 35 ꢀC with target product and the
acylal 8 being detected (eqn (5)) while no reaction with 1l
occurred (eqn (6)). We take advantage of previous research (eqn
(7))7a and a plausible mechanism is proposed (Scheme 3). With
TfOH as catalyst, acetic anhydride and the benzaldehyde afford
acylal via condensation reaction. Subsequently, arene attacks the
acylal and lead to the formation of corresponding biaryl alcohol
ester. Finally, electrophilic substitution gives 9-aryluorene.
Scheme 3 A plausible mechanism.
anhydride, The corresponding products are obtained in good to
almost quantitative yields when various 2-arylbenzaldehydes
bearing an electron-withdrawing and eletron-donating substit-
uent are treated with electron-rich arenes. The strategy can also
be applied to the synthesis of hetero indeno core and xanthene
in good yields. Studies to elucidate the detailed mechanism and
synthetic applications of this efficient and practical tandem
reaction are under way in our laboratory.
Acknowledgements
We sincerely thank National Natural Science Foundation of
China (20902030, 51273156) for nancial support.
In conclusion, we have developed a warm and efficient
tandem Friedel Cra reaction of a variety of 2-arylbenzalde-
hydes with arenes to give 9-aryluorenes and indeno[1,2-b]-
uorene derivatives in the catalyst system of TfOH and acetic
Notes and references
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