Trityl chloride (TrCl): Efficient and homogeneous organocatalyst for the solvent-free synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes by in situ formation of carbocationic system

Article abstract of DOI:10.1002/jccs.201100719

Trityl chloride (triphenylmethyl chloride, TrCl, Ph₃CCl) is utilized as an efficient and homogeneous organocatalyst for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from β-naphthol and arylaldehydes under solvent-free conditions. Moreover

Full text of DOI:10.1002/jccs.201100719

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Trityl Chloride (TrCl): Efficient and Homogeneous Organocatalyst for the Solvent-Free
Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthenes by in situ Formation of Carbocationic
System
Abdolkarim Zare,a,* Maria Merajoddin,a Fereshteh Abi,a Ahmad Reza Moosavi-Zare,b
Mohammad Mokhlesi,b Mohammad Ali Zolfigol,b,* Zhila Asgari,b Vahid Khakyzadeh,b
Alireza Hasaninejad,c Ali Khalafi-Nezhadd and Abolfath Parhamia
aDepartment of Chemistry, Payame Noor University, PO BOX 19395-3697, Tehran, Iran
bFaculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran
cDepartment of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran
dDepartment of Chemistry, College of Science, Shiraz University, Shiraz 71454, Iran
(Received: Dec. 11, 2011; Accepted: Mar. 6, 2012; Published Online: ??; DOI: 10.1002/jccs.201100719)
Trityl chloride (triphenylmethyl chloride, TrCl, Ph3CCl) is utilized as an efficient and homogeneous orga-
nocatalyst for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from b-naphthol and arylaldehydes un-
der solvent-free conditions. Moreover, a plausible mechanism is suggested based on the literature and on
in situ formation of trityl carbocation with inherent instability during the reaction.
Keywords: Trityl chloride (triphenylmethyl chloride, TrCl, Ph3CCl); Trityl carbocation;
Organocatalyst; 14-Aryl-14H-dibenzo[a,j]xanthenes; b-Naphthol; Solvent-free.
INTRODUCTION
Xanthene derivatives, especially benzoxanthenes are
the reactions are catalyzed by organic molecules in homo-
geneous media, has attracted much attention in current or-
ganic synthesis, particularly from the green chemistry point
of view.19-22 Unlike the conventional catalysis, organocata-
lysts are advantageous in many ways like high stability,
availability of the catalyst, metal-free nature, reduced tox-
icity and simple reaction conditions, and can promote a
chemical reaction through different activation modes.23
Along this line, we have recently introduced triarylmethyl
chlorides (Ar3CCl) as novel organocatalysts for the synthe-
sis of bis(indolyl)methanes,24 N-sulfonyl imines25 and 1-
amidoalkyl-2-naphtols,26 in which triarylmethyl carboca-
tion has acted as catalyst. In fact, the inherent instability of
carbocations has precluded up to now their use in catalysis
with decent turnover numbers.26 It should be mentioned
that triarylmethyl chlorides are inexpensive and can be ob-
tained commercially. These compounds have been exten-
sively applied as bulky protective groups for amino and
primary hydroxyl functional groups in multi-step organic
synthesis.27 Furthermore, triarylmethyl chlorides in combi-
nation with metal salts particularly SnCl2 have been used in
a few organic transformations.28 Nevertheless, the use of
these compounds to promote organic reactions in the ab-
sence of co-catalysts is really attractive.
of importance since they have various biological activities
including antibacterial,1 antiviral2 and anti-inflammatory3
properties.4 They have also been used in photodynamic
theraphy.5 Furthermore, some other benzoxanthenes have
found application in industries such as dyes in laser tech-
nology6 and fluorescent materials for visualization of bio-
molecules.7 The most useful protocol for the synthesis of
14-aryl-14H-dibenzo[a,j]xanthenes, as a class of benzo-
xanthenes, is the condensation of b-naphthol with alde-
hydes using Brønsted or Lewis acidic catalysts, e.g. select-
fluorTM,4 sulfamic acid,8 SiO2-Pr-SO3H,9 HBF4-SiO2,10
heteropolyacids,11 Al(HSO4)3,12 nano-TiO2,13 silica-bonded
N-propyl sulfamic acid,14 KAl(SO4)2.12H2O,15 InCl3,16
Yb(OTf)317 and ceric ammonium nitrate.18 Although some
catalysts for the synthesis of 14-aryl-14H-dibenzo[a,j]xan-
thenes are known, newer catalysts continue to attract atten-
tion for their difference with the others, high novelty and
efficacy. In addition, most of the reported methods for the
preparation of these compounds are associated with some
limitations such as long reaction times, high catalyst load-
ings, the use of toxic solvents or special apparatuses, the
use of expensive, non-available or toxic catalysts and poor
agreement with the green chemistry protocols.
In this work, we have introduced a high novelty in the
synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from b-
Development of organocatalytic processes, in which
* Corresponding author. E-mail: abdolkarimzare@yahoo.com; mzolfigol@yahoo.com
J. Chin. Chem. Soc. 2012, 59, 000-000
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Article
Zare et al.
Table 1. The solvent-free reaction of b-naphthol with 3-nitro-
naphthol and aromatic aldehydes, and used trityl chloride
(TrCl, Ph3CCl) as an efficient, attractive, inexpensive,
commercially available, homogeneous and green organo-
catalyst to promote the reaction at 120 °C in the absence of
solvent (Scheme I).
benzaldehyde in the presence of different triarylmethyl
chlorides and trityl alcohol at 120 ºC
Catalyst
Time (min) Yield[a] (%)
TOF (min-1) TON
TrCl
60
60
95
91
87
41
0.106
0.101
0.097
0.011
6.33
6.06
5.80
2.73
MMTCl
DMTCl
TrOH
60
Scheme I The solvent-free synthesis of 14-aryl-14H-
dibenzo[a,j]xanthenes by the condensation
of b-naphthol with arylaldehydes using
TrCl
240
[a] Isolated yield.
Table 2. Effect of amounts of the catalyst and temperature on the
condensation of b-naphthol with 3-nitrobenzaldehyde
Time
(min)
Yield[a]
(%)
Entry
Mol% of TrCl
Temp. (°C)
1
2
3
4
5
6
7
-
120
120
120
120
100
110
130
240
80
37
89
95
95
71
84
95
10
15
20
15
15
15
RESULTS AND DISCUSSION
60
To optimize the reaction conditions, the condensation
of b-naphthol (2 mmol) with 3-nitrobenzaldehyde (1 mmol)
was selected as a model reaction to provide 14-aryl-14H-
dibenzo[a,j]xanthene 1c, and its behavior was studied in
the presence of 15 mol% of trityl chloride (TrCl), mono-
methoxytrityl chloride [Ph2(p-MeOC6H4)CCl, MMTCl],
dimethoxytrityl chloride [Ph(p-MeOC6H4)2CCl, DMTCl]
and also trityl alcohol (TrOH) at 120 ºC under solvent-free
conditions. The results are summarized in Table 1. As it is
shown in Table 1, higher yield, TOF (turn over frequency)
and TON (turn over number) as well as shorter reaction
time were obtained when trityl chloride was utilized.
In another study, the reaction of b-naphthol (2 mmol)
with 3-nitrobenzaldehyde (1 mmol) was tested using dif-
ferent amounts of TrCl at range of 100-130 ºC in the ab-
sence solvent (Table 2). As Table 2 indicates, only 37%
yield of the product was obtained in catalyst-free condi-
tions at 120 ºC; and 15 mol% of the organocatalyst was suf-
ficient to promote the reaction efficiently at 120 ºC (Table
2, entry 3). Higher amount of TrCl and higher temperature
did not improve the reaction results (Table 2, entries 4 and
7).
60
150
100
60
[a] Isolated yield.
yields (83-95%) in relatively short reaction times (60-130
min). Thus, TrCl was efficient and general.
Based on the literature reports related to the presented
reaction shown in Scheme II, our mechanistic proposal is
that from the interaction of aldehyde and trityl chloride,
complexes of intermediates I and II could be generated via
a reversible reaction pathway.24-26
In order to validate our proposal, benzaldehyde was
reacted with trityl chloride, and then IR, 1H and 13C NMR
spectra of the resulting interacted aldehyde functional
group of proposed intermediates I and II in the reaction
mixture were compared with those in benzaldehyde as fol-
lows:26
According to the obtained FT-IR spectrum of the re-
sulting reaction mixture, stretching vibration of the car-
bonyl of the aldehyde functional group slighlty shifts to
lower frequency because of increased dipole moment char-
acter of carbonyl funcionality: IR (nujol): nmax (cm-1) of
C=O in benzaldehyde (1705) decreased to (1700) in the re-
action mixture.
Accordingly, in the 1H and 13C NMR spectra, the al-
dehyde hydrogen as well as carbonyl carbon were de-
shielded which is in agreement with the increased single
bond charachter of carbonyl functional group observed in
Next, to investigate the efficacy and the generality of
the organocatalyst, we examined the condensation of b-
naphthol with various arylaldehydes under the optimized
reaction conditions. The results are displayed in Table 3.
As it can be seen in Table 3, all aromatic aldehydes, includ-
ing benzaldehyde as well as aldehydes containing electron-
withdrawing substituents, halogens or electron-releasing
substituents on the aromatic ring, afforded the correspond-
ing 14-aryl-14H-dibenzo[a,j]xanthenes in high to excellent
2
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Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthenes
Table 3. The solvent-free preparation of 14-aryl-14H-dibenzo-
[a,j]xanthenes from â-naphthol and arylaldehydes using
TrCl
The proposed mechanism for the synthesis
of 14-aryl-14H-dibenzo[a,j]xanthenes from
b-naphthol and arylaldehydes catalyzed by
TrCl (Ph3Cl)
Scheme II
Ar
O
TrCl (15 mol%)
2
+
Ar
H
120 °C, Solvent-free
OH
O
Compound
number
Time
(min)
Yield[a]
M.p. °C
Ar
(%)
(lit.)
185-187
(182)12
1a
1b
1c
1d
1e
1f
C6H5
70
60
60
60
60
90
60
90
90
90
89
313-315
(310-311)16
4-O2NC6H4
3-O2NC6H4
2-O2NC6H4
4-ClC6H4
3-ClC6H4
2-ClC6H4
3-BrC6H4
3-HOC6H4
4-MeC6H4
92
95
93
91
96
94
91
83
96
214-216
(214)12
212-214
(214-215)13
285-287
(289-290)15
207-209
(209-211)13
210-212
(214-216)13
187-189
(190-191)13
1g
1h
1i
239-241
(242-243)16
226-228
(227)17
1j
200-202
(203-205)15
1k
1l
4-MeOC6H4
80
95
86
4-(PhCH2O)C6H4
130
163-165
[a] Isolated yield.
an intramolecular ring closure between the hydroxyl group
and the activated carbonyl to form intermediate VIII which
followed by elimination of one molecule H2O affords IX.
Subsequently, the reaction of IX with Ph3COH, produced
previously during the reaction process, gives the main
product and Ph3CCl. This proposed mechanism is in accor-
dance with the literature reports,9,12,18,24-26,29 and also by the
fact that the catalyst was completely recovered unchanged
and also triphenylmethanol couldn’t be detected after com-
pletion of the reaction as it could be observed on TLC by
comparison with the pure authentic samples. In another
study, to demonstrate that TrCl can not be converted into
TrOH and HCl by the water generated during the reaction
progress, and consequently to prove that HCl is not the ac-
tual catalyst of the process, the reaction of b-naphthol with
3-nitrobenzaldehyde was examined in the presence of the
expected amount of HCl produced by the reversible reac-
tion of TrOH with H2O which yielded the purified product
in 67% within 100 min. The reaction was also tested using
the FT-IR spectra: 1H NMR (300 MHz, CDCl3): d (ppm) of
the aldehydic hydrogen (9.78) increased to (10.04) in the
reaction mixture. 13C NMR (300 MHz, CDCl3): d (ppm) of
the carbonyl carbon (191.8) increased to (195.2) in the
reaction mixture.
These results confirm that intermediates I and II are
present in the reversible reaction media.26 Moreover, the
cationic intermediates of type I and II are known and have
been reported by Oikawa et al. for the first time.29 These
complexes, in which the carbonyl group has been activated
to accept nucleophilic attack, react with b-naphthol to pro-
vide intermediate III. This intermediate is converted into
intermediate IV by aromatization of the b-naphthol ring
followed by proton transfer. In the next step, either Ph3COH
or Ph3C+ leaves the system from intermediate IV to provide
V or VI respectively, via the reversible reactions (interme-
diate IV, V and VI are in equilibrium). Then, intermediate
V reacts with b-naphthol to produce VII which undergoes
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Article
Zare et al.
pyridine as base in which the reaction yield was 37% after
240 min. Moreover, the reaction was checked in the pres-
ence of TrCl (15 mol%) and pyridine (15 mol%) as acid
scavenger wherein the reaction well proceeded and the de-
sired product was obtained in 94% yield within 65 min (the
base didn’t affect the reaction results significantly). It is
clear that in these conditions, pyridine can abstract one pro-
ton from intermediate VII and produce pyridinium chlo-
ride; however, pyridinium chloride reacts with TrOH, pro-
duced from intermediate V to VII conversion, and forms
TrCl. To prove this, in a separate reaction, pyridinium chlo-
ride was reacted with TrOH which resulted in formation of
TrCl while also the reaction was not complete and amount
of starting materials were left unreacted. Finally, the reac-
tion was studied in the presence of pyridinium chloride (15
mol%) wherein the reaction yield was 58% after 240 min.
These evidences demonstrated that HCl is not produced
from TrCl in these conditions, and clarifies that TrCl acts as
the actual catalyst.
petroleum ether (10 mL) was added to it, refluxed and
stirred for 3 min, and filtered to separate the catalyst. The
resulting precipitate was recrystallized from EtOH (95%)
to give the pure product.
Spectral data of the products
Note: All compounds are known except 1l.
14-Phenyl-14H-dibenzo[a,j]xanthene 1a
1H NMR (400 MHz, DMSO-d6): d 6.74 (s, 1H), 6.97
(t, J = 7.6 Hz, 1H), 7.14 (t, J = 7.6 Hz, 2H), 7.45 (t, J = 7.2
Hz, 2H), 7.57 (d, J = 8.8 Hz, 2H), 7.62-7.66 (m, 4H), 7.91-
7.93 (m, 4H), 8.70 (d, J = 8.8, 2H); 13C NMR (100 MHz,
DMSO-d6): d 37.0, 117.9, 118.2, 123.9, 124.9, 126.7,
127.4, 128.4, 128.8, 129.1, 129.5, 131.1, 131.4, 146.0,
148.5.
14-(4-Nitrophenyl)-14H-dibenzo[a,j]xanthene 1b
1H NMR (300 MHz, DMSO-d6): d 6.95 (s, 1H), 7.43
(t, J = 7.2 Hz, 3H), 7.58-7.65 (m, 4H), 7.81 (d, J = 7.5 Hz,
1H), 7.90-7.95 (m, 4H), 8.14 (d, J = 7.8, 1H), 8.45 (s, 1H),
8.72 (d, J = 8.4 Hz, 2H); 13C NMR (75 MHz, DMSO-d6): d
36.4, 116.9, 118.2, 122.0, 123.6, 125.2, 127.7, 129.1,
130.0, 130.4, 131.11, 134.7, 147.9, 148.3, 148.6.
14-(3-Nitrophenyl)-14H-dibenzo[a,j]xanthene 1c
1H NMR (300 MHz, DMSO-d6): d 6.91 (s, 1H), 7.11-
7.25 (m, 1H), 7.43-7.48 (m, 2H), 7.55-7.71 (m, 4H), 7.88-
8.03 (m, 7H), 8.66 (d, J = 8.4 Hz, 2H); 13C NMR (75 MHz,
DMSO-d6): d 36.7, 116.6, 118.2, 120.1, 123.6, 124.2,
125.2, 127.6, 129.1, 129.5, 130.0, 131.1, 131.2, 134.2,
146.3, 148.4, 153.1.
In conclusion, we have developed a new method with
high novelty for the synthesis of 14-aryl-14H-dibenzo[a,j]-
xanthenes using TrCl as a green, efficient and homogenous
organocatalyst. Clean reaction, simple purification, short
reaction time, high yield, and economic availability of the
catalyst are some advantages of in this work.
EXPERIMENTAL
All chemicals were purchased from Merck or Fluka
Chemical Companies. All known compounds were identi-
fied by comparison of their melting points and spectral data
with those reported in the literature. Progress of the reac-
tions was monitored by thin layer chromatography (TLC)
using silica gel SIL G/UV 254 plates. The melting points
were recorded on a Büchi B-545 apparatus in open capil-
lary tubes. The 1H NMR (300 or 400 MHz) and 13C NMR
(75 or 100) were run on a Bruker Avance DPX, FT-NMR
spectrometers, d in ppm. Mass spectra were obtained with
Shimadzu GC-MS-QP 1100 EX model.
14-(2-Nitrophenyl)-14H-dibenzo[a,j]xanthene 1d
1H NMR (300 MHz, DMSO-d6): d 6.94 (s, 1H), 7.41-
7.46 (m, 3H), 7.56-7.65 (m, 4H), 7.79 (d, J = 2.1, 1H), 7.89-
7.94 (m, 4H), 8.13 (d, J = 7.8, 1H), 8.45 (s, 1H), 8.70 (d, J =
6, 2H); 13C NMR (75 MHz, DMSO-d6): d 36.4, 116.9,
118.1, 122.0, 122.5, 123.6, 125.2, 127.7, 129.1, 130.0,
130.4, 131.1, 134.7, 138.2, 147.9, 148.3, 148.6.
14-(4-Chlorophenyl)-14H-dibenzo[a,j]xanthene 1e
1H NMR (300 MHz, DMSO-d6): d 6.76 (s, 1H), 7.18
(d, J = 6.8 Hz, 2H), 7.46-7.64 (m, 10H), 8.92 (d, J = 7.79
Hz, 2H), 8.66 (d, J = 7.6 Hz, 2H); 13C NMR (75 MHz,
DMSO-d6): d 36.3, 117.4, 118.1, 123.4, 123.7, 125.0,
127.4, 128.8, 129.1, 129.6, 130.1, 131.1, 131.2, 144.8,
148.4.
General procedure for the synthesis of 14-aryl-14H-
dibenzo[a,j]xanthenes
A mixture of b-naphthol (0.289 g, 2 mmol) and aryl-
aldehyde (1 mmol) in a test tube was heated and stirred at
120 °C, and then TrCl (0.042 g, 0.15 mmol) was added to it.
The resulting mixture was firstly stirred magnetically, and
after solidification of the reaction mixture with a small rod,
at 120 ºC. After completion of the reaction, as monitored by
TLC, the reaction mixture was cooled to room temperature,
14-(3-Chlorophenyl)-14H-dibenzo[a,j]xanthene 1f
1H NMR (300 MHz, DMSO-d6): d 6.74 (s, 1H), 7.01
(d, J = 8.1, 1H), 7.13 (t, J = 7.8, 1H), 7.42 (t, J = 7.2, 2H),
7.52-7.66 (m, 6H), 7.91 (d, J = 8.7, 4H), 8.67 (d, J = 8.7,
4
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Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthenes
2H); 13C NMR (75 MHz, DMSO-d6): d 36.5, 117.2, 118.1,
123.7, 125.1, 126.8, 127.0, 127.5, 127.9, 129.1, 129.7,
130.7, 131.1, 131.2, 133.5, 148.2, 148.5.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge support of this
work by the Research Affairs Office of Payame Noor Uni-
versity and Bu-Ali Sina University (Grant number 32-1716
entitled development of chemical methods, reagents and
molecules), and Center of Excellence in Development of
Chemical Method (CEDCM), Hamedan, I.R. Iran.
14-(2-Chlorophenyl)-14H-dibenzo[a,j]xanthene 1g
1H NMR (300 MHz, DMSO-d6): d 6.64 (s, 1H), 6.91-
7.03 (m, 2H), 7.27 (d, J = 7.7 Hz, 2H), 7.38-7.50 (m, 5H),
7.57-7.70 (m, 2H), 7.76-7.90 (m, 4H), 8.54 (d, J = 8.4 Hz,
1H); 13C NMR (75 MHz, DMSO-d6): d 34.8, 116.9, 118.2,
123.3, 124.9, 127.4, 128.5, 128.8, 129.2, 129.8, 130.2,
130.3, 130.9, 131.4, 132.0, 143.2, 148.7.
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1H NMR (300 MHz, DMSO-d6): d 2.01 (s, 3H), 6.63
(s, 1H), 6.90 (d, J = 7.2, 2H), 7.40-7.61 (m, 8H), 7.88 (t, J =
2.7, 4H), 8.61 (d, J = 8.7, 2H); 13C NMR (75 MHz, DMSO-
d6): d 20.8, 37.4, 117.9, 118.1, 123.8, 124.9, 127.3, 128.2,
129.0, 129.3, 129.3, 131.1, 131.3, 135.8, 143.0, 148.3.
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(d, J = 8.7, 2H); 13C NMR (75 MHz, DMSO-d6): d 36.0,
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