Investigation on Claisen rearrangement of allyl phenyl ethers in near-critical water

Article abstract of DOI:10.1007/s11164-013-1433-4

Catalyst-free Clasien rearrangement of allyl phenyl ethers were investigated in near-critical water. The effects on the reaction in near-critical water and conventional conditions were compared. The results demonstrate that near-critical water could greatly accelerate the Claisen rearrangement of allyl phenyl ethers. This process is simple, fast, efficient and environmentally benign.

Full text of DOI:10.1007/s11164-013-1433-4

Res Chem Intermed
DOI 10.1007/s11164-013-1433-4
Investigation on Claisen rearrangement of allyl phenyl
ethers in near-critical water
•
Shangyou Xiao Yi He Guang Xu Qi Liu
•
•
Received: 16 July 2013 / Accepted: 1 October 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Catalyst-free Clasien rearrangement of allyl phenyl ethers were inves-
tigated in near-critical water. The effects on the reaction in near-critical water and
conventional conditions were compared. The results demonstrate that near-critical
water could greatly accelerate the Claisen rearrangement of allyl phenyl ethers. This
process is simple, fast, efficient and environmentally benign.
Keywords Allyl phenyl ethers Á Near-critical water Á Clasien rearrangement
Introduction
Water is an abundant and cheap solvent, which has emerged in recent years as a
versatile solvent for organic chemistry [1–3]. The types of organic reactions in
water are broad, including hydrolysis [4], hydration [5], reduction [6], alkylation
[7], and Heck [8] and Suzuki reactions [9]. However, the reactions of many organics
in water are restricted by its high polarity and consequent poor solubility of
organics. Near-critical water, which has a lower dielectric constant, weaker
hydrogen bonds, and a higher isothermal compressibility than ambient liquid water,
can overcome these limitations to facilitate organic reactions. It is interesting that
the products usually crystallize from solution directly after cooling in many cases
because of the poor solubility of organics in water, without further purification.
The Claisen rearrangement is a fundamental organic reaction for the production
of a large variety of important intermediates [10–13]. Claisen rearrangement under
conventional conditions usually suffers from the disadvantages of long reaction time
and the generation of a large amount of waste solvent. Some successful applications
of Lewis acid-catalyzed Claisen rearrangements have been reported to reduce the
S. Xiao (&) Á Y. He Á G. Xu Á Q. Liu
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, China
e-mail: xsy6508@163.com
123

S. Xiao et al.
reaction temperature [14, 15]. However, the catalysts used, such as SnCl2 and BCl3,
are toxic and moisture-sensitive. Therefore, an efficient and environmentally
friendly way should be developed to conquer these drawbacks. To the best of our
knowledge, only a few investigations on Claisen rearrangements of allyl phenyl
ethers in near-critical water have been reported.
In this study, we with to show a versatile method for the catalyst-free Clasien
rearrangement of allyl phenyl ethers from various phenols by the following strategy
(Scheme 1): allyl phenyl ethers (2a–2e) prepared from various phenols (1a–1e) with
alkyl bromide underwent [3] sigmatropic (Claisen) rearrangement in near-critical
water to obtain O-allylphenols (3a–3e), respectively.
Experimental
Materials and instruments
Staring materials were purchased from commercial sources and used without further
purification. Melting points were measured using a micro-melting point X-4
apparatus and were uncorrected. IR spectra were recorded as KBr discs on a
Shimadzu IR Affinity-1 spectrometer. All 1H NMR spectra were recorded on Bruker
500 MHz spectrometer in DMSO-d6 using TMS as an internal standard.
General procedure for the preparation of allyl phenyl ethers (2a–2e)
In presence of anhydrous potassium carbonate, a mixture of different substituted
phenols, allyl bromide, were placed into a 250-mL flask in a ratio of 1:1.2:1 at
70 °C, using ethanol as solvent. The reaction mixture was refluxed for 5 h and
monitored by TLC. After completion of the reaction, it was filtered and
concentrated in vacuum. The residue was dissolved in ethyl acetate, washed with
saturated NaCl solution, and then dried with MgSO4. After filtration, the solvent
OH
O
OH
R1
+
Near-critical water or
conventional heating
R1
R1
K2CO3
EtOH
Br
°C
R2
R2
R2
1a-1e
2a-2e
3a-3e
1a, 2a, 3a: R1 = NO2 , R2 = H; 1b, 2b, 3b: R1 = H, R2= NO2;
1c, 2c, 3c: R1 = H, R2= Cl; 1d, 2d, 3d: R1= H, R2 = C(CH3)3;
1e, 2e, 3e: R1 = H, R2 = H
Scheme. 1 Synthesis of compounds 3a–3e
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Investigation on Claisen rearrangement of allyl phenyl ethers
Table 1 Yields of the allyl
Allyl phenyl ethers
phenyl ethers
R1
R2
Yield (%)
2a
2b
2c
2d
2e
NO2
H
H
84
79
71
82
63
NO2
Cl
H
H
C(CH3)3
H
H
was removed in vacuum to give crude products, 2a–2e (Table 1), which were
purified by silica gel chromatographic column with ethyl acetate:petroleum
ether = 1:8–1:6 as eluent.
Comparison of Claisen rearrangement in near-critical water and conventional
method
Two equal portions of allyl phenyl ethers were placed into a 25-mL stainless
hydrothermal synthesis reactor and a 5-mL round flask, respectively. The
hydrothermal synthesis reactor was filled with 15 mL deionized water.
In a systematic study, the neat allyl phenyl ethers were heated in water at 220 °C.
To make a comparison with conventional methods, parallel experiments were
conducted with a traditional solvent-free oil bath heating in the round flask with
magnetic stirring at 220 °C. The neat ether was refluxed for several hours. After
completion of the reaction, the products were further purified by column
chromatography to get O-allylphenols .
Spectroscopic data for the products
2a
1
Yellow liquid. H NMR (CDCl3, 500 MHz) d: 4.69 (dd, J = 3.50, 1.50 Hz, 2H),
5.34 (dd, J = 10.50, 1.00 Hz, 1H), 5.50 (dd, J = 17.00, 1.00 Hz, 1H), 6.00–6.08 (m,
1H), 7.03 (t, J = 8.00 Hz, 1H), 7.08 (d, J = 8.50 Hz, 1H), 7.52 (t, J = 16.00 Hz, 1H),
7.84 (dd, J = 8.00, 1.50 Hz, 1H); IR (KBr) m: 3,084, 1,608, 1,523, 993 cm-1.
3a
Yellow liquid. 1H NMR (CDCl3, 500 MHz) d: 3.41 (d, J = 6.50 Hz, 2H), 5.03–5.07
(m, 2H), 5.87–5.96 (m, 1H), 6.85 (t, J = 7.50 Hz, 1H), 7.39 (d, J = 7.00 Hz, 1H),
7.92 (d, J = 8.50 Hz, 1H), 10.87 (s, 1H); IR (KBr) m: 3,290, 3,082, 1,639, 1,541,
1,450 cm-1
.
2b
Yellow liquid. 1H NMR (CDCl3, 500 MHz) d: 4.64 (d, J = 5.00 Hz, 2H), 5.35 (dd,
J = 10.50, 1.00 Hz, 1H), 5.44 (dd, J = 17.50, 1.00 Hz, 1H), 6.01–6.09 (m, 1H), 6.97
123

S. Xiao et al.
(d, J = 9.00 Hz, 2H), 8.19 (d, J = 9.00 Hz, 2H); IR(KBr) m: 3,086, 1,647, 1,591,
1,500, 1111 cm-1
.
3b
1
Pale yellow solid. m.p. 77–78 °C. H NMR (CDCl3, 500 MHz) d: 3.58 (d, J =
6.50 Hz, 2H), 5.32–5.35 (m, 2H), 5.85 (s, 1H), 6.08–6.16 (m, 1H), 7.00 (d, J =
9.50 Hz, 1H), 8.17 (t, J = 7.00 Hz, 2H); IR (KBr) m: 3,356, 3,080, 1,639, 1,587,
1,492 cm-1.
2c
1
Yellow liquid. H NMR (CDCl3, 500 MHz) d: 4.49(d, J = 5.00 Hz, 2H), 5.29 (dd,
J = 10.50, 1.00 Hz, 1H), 5.40 (dd, J = 17.50, 1.50 Hz, 1H), 6.00–6.06 (m, 1H), 6.83
(d, J = 9.00 Hz, 2H), 7.22 (dd, J = 16.50, 8.00 Hz, 2H); IR (KBr) m: 3,084, 1,649,
1,595, 1,091, 684 cm-1
.
3c
1
Yellow liquid. H NMR (CDCl3, 500 MHz) d: 3.48 (d, J = 6.50 Hz, 2H), 5.00 (s,
1H), 5.26–5.31 (m, 2H), 6.05–6.12 (m, 1H), 6.85 (d, J = 8.50 Hz, 1H), 7.19 (t, J =
7.00 Hz, 2H); IR(KBr) m: 3,539, 3,080, 1,637, 655, 1490 cm-1
.
2d
1
Yellow liquid. H NMR (CDCl3, 500 MHz) d: 1.29 (s, 9H), 4.51 (d, J = 5.00 Hz,
2H), 5.27 (dd, J = 10.50, 1.00 Hz, 1H), 5.4 (dd, J = 17.50, 1.50 Hz, 1H), 6.02–6.10
(m, 1H), 6.85 (d, J = 9.00 Hz, 2H), 7.19 (d, J=9.00 Hz, 2H); IR (KBr) m: 3,039,
2,962, 1,647, 1,581, 1,118 cm-1.
3d
1
Yellow liquid. H NMR (CDCl3, 500 MHz) d: 1.28 (s, 9H), 3.40 (d, J = 6.00 Hz,
2H), 4.96 (s, 1H), 5.13–5.18(m, 2H), 5.99–6.07 (m, 1H), 6.73 (d, J = 8.00 Hz, 1H),
7.10–7.15 (m, 2H); IR (KBr) m: 3,510, 3,078, 1,639, 2,964, 1,506 cm-1
.
2e
1
Yellow liquid. H NMR (CDCl3, 500 MHz) d: 4.52 (dd, J = 3.50, 1.00 Hz, 2H),
5.27 (dd, J = 10.50, 1.00 Hz, 1H), 5.41 (dd, J = 17.50, 1.50 Hz, 1H), 6.02–6.10 (m,
1H), 6.93 (t, J = 7.50 Hz, 3H), 7.25 (d, J = 7.50 Hz, 2H); IR (KBr) m: 3,072, 3,030,
1,649, 1,598, 1,078 cm-1
.
123

Investigation on Claisen rearrangement of allyl phenyl ethers
3e
1
Yellow liquid. H NMR (CDCl3, 500 MHz) d: 3.41(d, J = 6.50 Hz, 2H), 4.98 (s,
1H), 5.14–5.18 (m, 1H), 5.98–6.05 (m, 1H), 6.80 (d, J = 8.00 Hz, 1H), 6.87–6.90(m,
1H), 7.10–7.14 (m, 2H); IR (KBr) m: 3,500, 3,076, 1,637, 3,035, 1,492 cm-1
.
Results and discussion
Initially, 2a was used as model substrate for the optimization of Claisen rearrangement
in the stainless hydrothermal synthesis reactor. The effect of temperature (120, 150,
190, 210, 220 °C) was studied (Fig.1). It barely reacted at 120 °C after heating for
90 min. When the temperature was raised to 150 °C, it was found that a small amount
of 3a was generated while the majority was the starting material. Dramatically, up to
190 °C, ortho-Claisen rearrangement occurred almost exclusively; the yield was 75 %
when heated for 90 min. We found that, at 210 °C, the yield of 3a increased to 90 %
after 40 min. Longer residence time led to a small increase in yield (e.g., 92 % for
60 min). This might indicate that the temperature is a key factor for an efficient Claisen
rearrangement reaction. Therefore, we further enhanced the reaction temperature. At
220 °C, even higher yields were obtained. When the reaction time was 20 min,
compound 3a was generated in 85 % yield, and the yield could reach as high as 93 %
after only 40 min. Both 1H NMR and HPLC analysis showed that there were only two
components in the crude product : the starting material and the target product, which
indicated the high selectivity of the reaction.
Subsequently, the influence of reaction time for the rearrangement was investigated
at 220 °C. When the reaction time was 20 min, 2a was converted to 3a in 85 % yield in
near-critical water, while the conventional solvent-free Claisen rearrangement
reaction gave only 15 % yield (Fig.2a), and even after 5 h, the yield was only 82 %.
We continued our study with a number of other substituted substrates. Since there are
100
220°C
90
80
70
60
50
40
30
20
10
0
210°C
190°C
150°C
120°C
0
20
40
60
80
100
Time/min
Fig. 1 The product yields (%) versus time (min) of 2a in near-critical water at different temperatures
123

S. Xiao et al.
100
90
80
70
60
50
40
30
20
10
0
a
b
80
70
60
50
40
30
20
10
0
0
50
100 150 200 250 300
Time/min
0
100
200
300
400
500
Time/min
c 100
90
d
100
90
80
70
60
50
40
30
20
10
80
70
60
50
40
30
20
10
0
0
0
100 200 300 400 500
Time/min
0
100 200 300 400 500
Time/min
e
70
60
50
40
30
20
10
0
In near-critical water
Conventional heating
0
100 200 300 400 500
Time/min
Fig. 2 The product yields (%) versus time (min) of 3a–3e. a o-Nitro-phenyl allyl ether (2a); b p-nitro-
phenyl allyl ether (2b); c p-chloro-phenyl allyl ether (2c); d p-tert-butyl-phenyl allyl ether (2d); e allyl
phenyl ether (2e). The reaction temperature was 220 °C
some differences in the chemical properties among the substrates, diverse reaction
times were tested for these reactions. For instance, 2b was heated in near-critical water
for 60 min to give 3b in 75 % yield, while the yields of conventional reactions for the
same time were no more than 16 % (Fig.2b). When heated for as long as 7 h, the yield
reached 47 %. 2c was converted to 3c in 92 % yield when reacted for 60 min in near-
critical water, while it only gave 59 % with a conventional heating method for the
same time (Fig.2c). From Fig.2, we can infer that the near-critical water can markedly
accelerate the Claisen rearrangement to afford the corresponding O-allylphenols in
high yields. This may confirm the point that near-critical water can act as a convenient
solvent, catalyst, and reagent for typically acid- or base-catalyzed reactions [16].
123

Investigation on Claisen rearrangement of allyl phenyl ethers
Table 2 Yields at different time points in near-critical water
Reactant
R1
R2
Yield (%)
10 min
20 min
30 min
2a
2b
2c
2d
2e
NO2
H
H
60
37
47
45
25
82
49
59
59
34
90
53
78
76
42
NO2
Cl
H
H
C(CH3)3
H
H
Furthermore, the effect of various substituted groups were also investigated. The
results are summarized in Table 2, clearly showing that allyl phenyl ethers bearing
both electron–donating and electron-withdrawing substituents gave the desired O-
allylphenols in better yields than those of unsubstituted substrates.
Conclusion
This work has successfully performed the aromatic Claisen rearrangement in near-
critical water in the absence of a catalyst. This procedure is simple, fast, efficient,
and environmentally benign. Although the convenience of near-critical water as a
solvent and/or medium for preparative organic chemistry has been demonstrated,
potential opportunities will only be fully realized through high conversions and
yields for a diverse range of reactions, as well as the introduction of environmen-
tally benign isolation techniques. Our work is progressing on these fronts.
Acknowledgment We thank the Ministry of Science and Technology of China (No. 2010DFA32680)
for their generous financial support.
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