Studies on the phthalidation of heteroarenes: A facile preparation of 3-(heteroaryl)phthalides via triflic acid mediated phthalidation

Article abstract of DOI:10.1055/s-0033-1340551

A triflic acid mediated heteroarylation of phthalaldehydic acid in 1,2-dichloroethane at reflux temperature leads to the formation of 3-heteroarylphthalides. This method for the phthalidation of heteroarenes can be utilized for the successful preparation of mono-, bis- and tris- heteroarylphthalides. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1340551

LETTER
▌509
lSettteur dies on the Phthalidation of Heteroarenes: A Facile Preparation of
3-(Heteroaryl)phthalides via Triflic Acid Mediated Phthalidation
Phthalidation of Heteroarenes
Meganathan Nandakumar, Elumalai Sankar, Arasambattu K. Mohanakrishnan*
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai-600 025, Tamil Nadu, India
Fax +91(44)22352494; E-mail: mohanakrishnan@unom.ac.in; E-mail: mohan_67@hotmail.com
Received: 19.10.2013; Accepted after revision: 09.12.2013
synthesis of complex heteroarylphthalides. The require-
ment for di- as well as tri-phthalide units on heteroarene
scaffolds has prompted us to explore an alternative meth-
od to provide simultaneous incorporation of phthalide
functions.
Abstract: A triflic acid mediated heteroarylation of phthalaldehy-
dic acid in 1,2-dichloroethane at reflux temperature leads to the for-
mation of 3-heteroarylphthalides. This method for the phthalidation
of heteroarenes can be utilized for the successful preparation of
mono-, bis- and tris-heteroarylphthalides.
Key words: triflic acid, heteroarylation, 3-heteroarylphthalides,
Brønsted acid, Lewis acid
The Lewis acid mediated arylation of benzylic bromides
has been exploited in our group for the synthesis of a va-
riety of π-conjugated heterocycles.13 In continuation of
this work, we have undertaken a systematic study on the
Lewis acid mediated arylation of 3-substituted phthalides
1a–c and phthalaldehydic acid (1d).
3-Arylphthalides are routinely prepared by Friedel–Crafts
phthaloylation of arenes followed by reductive cycliza-
tion, and we have adopted this method for the synthesis of
a range of heteroarylphthalides.1 Alternatively, halogen–
metal exchange reactions of 2-halobenzoic acids,2 as well
as 2-halobenzoates3 have been employed widely for the
synthesis of 3-arylphthalides. The preparation of 3-ar-
ylphthalides has also been achieved by nickel-mediated
coupling of 2-bromobenzoates with aryl aldehydes.4 Oth-
er methods described for the formation of 3-ar-
ylphthalides involve transition metal mediated cascade
reactions of phthalaldehydes with arylboronic acids.5 A
straightforward synthesis of several 3-arylphthalides has
been achieved by the arylation of phthalaldehydic acid
with arenes using aqueous sulfuric acid,6 perchloric acid7
and methanesulfonic acid.8 The reaction of phthalaldehy-
dic acid with aryl Grignards has also been employed for
the synthesis of a wide variety of 3-arylphthalides.9 More-
over, the preparation of 3-arylphthalides via palladium-
mediated arylation of phthalaldehydic acid with an aryl-
boronic acid has been disclosed.10 An efficient synthesis
of 3-indolyl substituted phthalides via Friedel–Crafts re-
actions of indoles with 2-formylbenzoic acid in aqueous
medium has also been reported.11
The known bromo compound 1a, upon heating at reflux
temperature with 1-hexylcarbazole using one equivalent
of zinc bromide (ZnBr2) in 1,2-dichloroethane (DCE) for
10 hours led to the isolation of the desired lactone 3a in
N
O
O
C6H13
O
2a
O
ZnBr2 (1 equiv), 1,2-DCE
reflux, 10 h
N
Br
21%
C6H13
1a
3a
O
2a
ZnBr2(1 equiv), 1,2-DCE
O
3a
reflux, 10 h
10%
OAc
1b
O
2a
ZnBr2 (1 equiv), 1,2-DCE
Me
Me
Me
O
The most common method for the preparation of 3-ar-
ylphthalides, involving Friedel–Crafts phthaloylation fol-
lowed by reductive cyclization, suffers from the
disadvantage of requiring excess Lewis acid as well as a
reduction step. It should be noted that during the forma-
tion of the mono-ketoacid in the Friedel–Crafts step, the
substrate is deactivated and hence the possibility for fur-
ther phthaloylation is difficult. Moreover, in the case of
electron-rich heteroarenes such as ethylenedioxythio-
phene (EDOT), the use of a Lewis acid led to polymeriza-
tion.12 Other reported methods are not amenable to the
3a
reflux, 10 h
15%
O
O
1c
O
2a
ZnBr2 (1 equiv), 1,2-DCE
H
3a
OH
reflux, 5 h
51%
O
1d
2a
ZnBr2, MgSO4 (1 equiv)
1d
3a
1,2-DCE, reflux, 5 h
62%
SYNLETT 2014, 25, 0509–0514
Advanced online publication: 31.01.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340551; Art ID: ST-2013-D0982-L
© Georg Thieme Verlag Stuttgart · New York
Scheme 1 Carbazolyation of phthalides 1a–d mediated by ZnBr2

510
M. Nandakumar et al.
LETTER
carbonyl unit would seem to be responsible for reducing
Table 1 Effect of the Catalyst on the Carbazolylation of 1d
significantly the yield of the phthalide 3a.14
Entry
Catalyst
Conditionsa
Yield (%)
Accordingly, the carbazolylation of 1d was carried out
with different Lewis and Brønsted acid catalysts and the
obtained results are presented in Table 1.
1
2
3
4
5
6
7
8
9
ZnBr2
reflux, 5 h
reflux, 4 h
r.t., 5 h
62c
74c
21c
46c
51c
80
BF3·OEt2
AlCl3
Among the Lewis acids employed for the carbazolylation
of 1d, boron trifluoride–diethyl ether complex (BF3·OEt2)
gave the highest yield (Table 1, entry 2). The maximum
yield of the lactone 3a was obtained with triflic acid (Ta-
ble 1, entry 6). It should be noted that, with the exception
of triflic acid, in all the other cases, some unreacted 2a
was always recovered.
ZrOCl2·8H2O
PTSA
reflux, 12 h
reflux, 9 h
reflux, 3 h
r.t., 3 h
CF3SO3Hb
TMSOTf
Amberlyst®
40c
43c
72c
To generalize the phthalidation reaction, 5,6-dimethoxy-
3-hydroxyphthalide (6) was prepared via benzylic bromi-
nation followed by hydrolysis (Scheme 2). 1H NMR spec-
troscopic analysis of 6 confirmed its existence mainly as
the hydroxyphthalide along with a minor amount of the
corresponding phthalaldehydic acid.
reflux, 12 h
Montmorillonite K10 reflux, 4 h
a Reactions were carried out using 2a (1 equiv), 1d (1.2 equiv), Brøn-
sted/Lewis acid catalyst (1 equiv), anhydrous MgSO4 (1 equiv).
b Triflic acid (0.2 equiv) was used.
c Calculated yield based on recovered 2a.
MeO
MeO
OMe
OMe
OMe
MeO
K2CO3
THF, H2O
AIBN, NBS
CCl4
21% yield. Similarly, the zinc bromide mediated carbazo-
lylation of acetate 1b and pivalate 1c produced lactone 3a
in low yields. However, carbazolylation of phthalaldehy-
dic acid (1d) using zinc bromide led to the formation of
lactone 3a in 51% yield (Scheme 1). The yield of the lac-
tone 3a could be enhanced further by performing the zinc
bromide mediated carbazolylation in the presence of an-
hydrous magnesium sulfate. Thus, among substrates 1a–
d, the zinc bromide mediated carbazolylation was best
with phthalaldehydic acid (1d). With substrates 1a–c, the
preferential coordination of zinc bromide with the lactone
8 h
61%
reflux, 2 h
79%
O
O
Br
O
HO
O
5
O
4
O
6
Scheme 2 Preparation of 5,6-dimethoxy-3-hydroxyphthalide (6)
The phthalidation of carbazole using triflic acid prompted
us to explore the reaction with various heteroarenes (Ta-
ble 2).
Table 2 Phthalidation of Heteroarenes 2a–f
Entry
Substrate
Product
Conditionsa
Yield (%)
R2
R1
reflux, 3 h
reflux, 4 h
80
70
1
2a
O
O
N
C6H13
3a R1,R2 = H
7a R1,R2 = OMe
R2
R1
O
O
O
O
r.t., 4 h
r.t., 4 h
65
66
2
S
O
S
O
2b
3b R1,R2 = H
7b R1,R2 = OMe
Synlett 2014, 25, 509–514
© Georg Thieme Verlag Stuttgart · New York

LETTER
Phthalidation of Heteroarenes
511
Table 2 Phthalidation of Heteroarenes 2a–f (continued)
Entry
Substrate
Product
Conditionsa
Yield (%)
R2
R1
S
S
r.t., 5 h
r.t., 2 h
73
68
S
3
S
O
O
2c
3c R1,R2 = H
7c R1,R2 = OMe
R2
R1
reflux, 5 h
reflux, 4 h
80
78
4
S
S
O
O
2d
3d R1,R2 = H
7d R1,R2 = OMe
R2
R1
reflux, 7 h
reflux, 8 h
70
69
5
S
O
O
S
2e
3e R1,R2 = H
7e R1,R2 = OMe
R2
R1
N
N
reflux, 6 h
reflux, 3 h
76
75
6
O
O
3f R1,R2 = H
2f
7f R1,R2 = OMe
a Reactions were carried out using substrate 2 (1 equiv), 1d or 6 (1.2 equiv), triflic acid (0.2 equiv), anhydrous MgSO4 (1 equiv), 1,2-DCE.
Carbazole 2a, upon heating at reflux temperature with phthalaldehydic acid (1d) or 5,6-dimethoxy-3-hy-
phthalaldehydic acid (1d) or 5,6-dimethoxy-3-hy- droxyphthalide (6), using triflic acid in 1,2-dichloroeth-
droxyphthalide (6) using triflic acid (20 mol%) in 1,2-di- ane at reflux temperature, furnished the respective
chloroethane, furnished phthalides 3a or 7a in 80% and lactones 3f and 7f in comparable yields. It should be noted
70% yields, respectively (Table 2, entry 1). However, the that the presence of methoxy groups on the hy-
expected phthalidation of highly electron-rich 3,4-ethyl- droxyphthalide 6 did not have any significant influence on
enedioxythiophene (2b) could be performed at room tem- the yields of the corresponding heteroarylphthalides. The
perature affording the lactones 3b and 7b in very similar obtained yields of 5,6-dimethoxyphthalides 7a–f were
yields (Table 2, entry 2). In the case of 2,2′-bithiophene comparable with those of phthalides 3a–f.
(2c), the phthalidation could also be performed at room
temperature (Table 2, entry 3).
The triflic acid mediated phthalidation reaction was then
successfully applied to the synthesis of symmetrical bis-
Benzo[b]thiophene (2d), upon heating at reflux tempera- lactones. The thienyl lactones 8 and 3c, upon reaction
ture with 1d or 6 using triflic acid in 1,2-dichloroethane with phthalaldehydic acid (1d) using triflic acid (40
led to the isolation of 2-benzo[b]thienylphthalides 3d or mol%) in 1,2-dichloroethane at reflux temperature, fur-
7d in high yields (Table 2, entry 4). The phthalidation of nished the respective diphthalides 9 and 10. Under similar
dibenzothiophene (2e) could be smoothly performed with conditions, 3,4-ethylenedioxythiophenyl lactone 3b was
1d and 6 to afford the respective phthalides 3e and 7e (Ta- converted into the bis-lactone 11 in 58% yield (Scheme
ble 2, entry 5). The reaction of triphenylamine (2f) with 3).
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 509–514

512
M. Nandakumar et al.
LETTER
O
1d
O
1d
S
MgSO4, CF3SO3H
MgSO4, CF3SO3H
S
S
2c
15
S
O
O
S
n
1,2-DCE, reflux, 8 h
O
O
1,2-DCE, reflux
12 h, 89%
n
O
O
O
O
10
9 n = 1 (60%)
10 n = 2 (62%)
8 n = 1
3c n = 2
O
O
1d
O
1d
O
MgSO4, CF3SO3H
MgSO4, CF3SO3H
S
O
O
3b
S
1,2-DCE, reflux
12 h, 79%
S
1,2-DCE, reflux
6 h, 58%
O
O
O
16
O
11
Scheme 3 Preparation of symmetrical diphthalides
O
O
1d
MgSO4, CF3SO3H
O
O
1-Hexylcarbazol-3-ylphthalide 3a, upon reaction with
5,6-dimethoxy-3-hydroxyphthalide (6) in the presence of
triflic acid in 1,2-dichloroethane at reflux temperature, led
to the isolation of unsymmetrical diphthalide 12 in 62%
yield (Scheme 4). Under identical conditions, the hetero-
arylphthalides 3c and 3f were converted into unsymmetri-
cal diphthalides 13 and 14 in 58% and 61% yields,
respectively.
2a
1,2-DCE, reflux
24 h, 86%
N
C6H13
17
O
O
1d
O
O
MgSO4, CF3SO3H
2f
OMe
MeO
1,2-DCE, reflux
24 h, 80%
N
O
O
O
O
18
6
MgSO4, CF3SO3H
3a
Scheme 5 One-pot bis-phthalidations of heteroarenes
1,2-DCE, reflux
24 h, 62%
N
C6H13
Similarly, the reaction of triphenylamine (2f) with phthal-
aldehydic acid (1d) (3.3 equiv) led to the isolation of
triphthalide 19 in 87% yield (Scheme 6). Under identical
conditions, 1,3,5-tri(thiophen-2-yl)benzene (20) was
smoothly converted into the respective triphthalide 21.
12
OMe
6
OMe
MgSO4, CF3SO3H
S
S
3c
1,2-DCE, reflux
24 h, 58%
O
In summary, a straightforward and general method for the
preparation of 3-heteroarylphthalides has been achieved
via triflic acid mediated Friedel–Crafts phthalidations of
heteroarenes. This phthalidation reaction has been suc-
cessfully applied to the synthesis of a variety of heteroaryl
diphthalides and triphthalides. The heteroaryl diph-
thalides and triphthalides reported herein cannot be ob-
tained via conventional Friedel–Crafts phthaloylation
followed by reductive cyclization, thereby opening up the
possibility to explore these readily available hetero-
arylphthalides as intermediates for the syntheses of ben-
zo[c]thiophene analogues.1,15,16 In particular, the
heteroaryl diphthalides and triphthalides reported herein
may find applications in the field of polycyclic aromatic
heterocycles.17
O
O
O
13
O
O
O
O
OMe
6
N
OMe
MgSO4, CF3SO3H
3f
1,2-DCE, reflux
12 h, 61%
14
Scheme 4 Preparation of unsymmetrical diphthalides
Finally, one-pot bis-phthalidations of bithiophene 2c, 1,3-
dithienylbenzo[c]thiophene (15), N-hexylcarbazole (2a)
and triphenylamine (2f) with phthalaldehydic acid (1d)
(2.2 equiv) using triflic acid (40 mol%) in 1,2-dichloro-
ethane at reflux temperature afforded the corresponding
diphthalides 10, 16, 17 and 18 in good yields (Scheme 5).
Acknowledgment
We thank the Department of Science and Technology (DST), New
Delhi for financial support. M.N. and E.S. thank the Council of Sci-
entific and Industrial Research (CSIR), New Delhi for JRF Fellow-
Synlett 2014, 25, 509–514
© Georg Thieme Verlag Stuttgart · New York

LETTER
Phthalidation of Heteroarenes
513
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O
O
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O
O
1d
N
MgSO4, CF3SO3H
2f
1,2-DCE, reflux
12 h, 87%
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O
O
19
O
O
S
1d
S
MgSO4, CF3SO3H
(13) (a) Sivasakthikumaran, R.; Nandakumar, M.;
Mohanakrishnan, A. K. J. Org. Chem. 2012, 77, 9053.
(b) Sureshbabu, R.; Saravanan, V.; Dhayalan, V.;
Mohanakrishnan, A. K. Eur. J. Org. Chem. 2011, 922.
(c) Dhayalan, V.; Arul Clement, J.; Jagan, R.;
1,2-DCE, reflux
24 h, 89%
S
S
S
O
O
S
20
Mohanakrishnan, A. K. Eur. J. Org. Chem. 2009, 531.
(14) General Procedure: Phthalidation of N-Hexylcarbazole
(2a): To a stirred suspension of N-hexylcarbazole 2a (0.30
g, 1.20 mmol), 2-carboxybenzaldehyde (0.22 g, 1.43 mmol)
and anhyd MgSO4 (0.29 g, 1.20 mmol) in anhyd DCE (15
mL), 3 drops of triflic acid (0.04 g, 0.024 mmol) were added,
and the mixture was refluxed for 3 h. It was then poured into
sat. NaHCO3 solution (50 mL), and the resulting mixture
was extracted with CH2Cl2 (3 × 20 mL). The combined
organic layers were washed with H2O (2 × 30 mL) and dried
(Na2SO4). Removal of the solvent followed by column
chromatographic purification (silica gel, 10% EtOAc in
hexane) afforded lactone 3a as a thick pale-yellow liquid
(0.37 g, 80% yield). 1H NMR (300 MHz, CDCl3):  = 8.06–
8.00 (m, 3 H, ArH), 7.64–7.55 (m, 2 H, ArH), 7.52–7.47 (m,
1 H, ArH), 7.42–7.32 (m, 3 H, ArH), 7.29–7.22 (m, 2 H,
ArH), 6.58 (s, 1 H, CH), 4.24 (t, J = 7.2 Hz, 2 H, CH2), 1.87–
1.78 (m, 2 H, CH2), 1.34–1.27 (m, 6 H, CH2), 0.90–0.86 (m,
3 H, CH3). 13C NMR (75 MHz, CDCl3):  = 170.7, 150.2,
140.7, 134.1, 129.0, 126.2, 126.0, 125.8, 125.3, 124.7,
123.0, 122.8, 122.3, 120.3, 119.6, 119.0, 109.0, 108.8, 83.8,
43.0, 31.4, 28.7, 26.7, 22.4, 13.9.
O
O
21
Scheme 6 One-pot tri-phthalidations of heteroarenes
ships. The authors thank the Department of Science and
Technology Funds for the Improvement of Science and Technology
(DST-FIST) for NMR facilities. The authors also thank SAIF, IIT
Madras for HRMS data.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SuppInigfor
m
o
ti
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© Georg Thieme Verlag Stuttgart · New York
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