Investigating the microwave-accelerated Claisen rearrangement of allyl aryl ethers: Scope of the catalysts, solvents, temperatures, and substrates

Article abstract of DOI:10.1016/j.tetlet.2020.151995

The microwave-accelerated Claisen rearrangement of allyl aryl ethers was investigated, in order to gain insight into the scope of the catalysts, solvents, temperatures, and substrates. Among the catalysts examined, phosphomolybdic acid (PMA) was found to greatly accelerate the reaction in NMP, at temperatures ranging from 220 to 300 °C. This method was found to be useful for preparing several intermediates previously reported in the literature using precious metal catalysts such as Au(I), Ag(I), and Pt(II). Additionally, substrates bearing bromo and nitro groups on the aryl portion required careful tailoring of the reaction conditions to avoid complex product profiles.

Full text of DOI:10.1016/j.tetlet.2020.151995

Tetrahedron Letters 61 (2020) 151995
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Investigating the microwave-accelerated Claisen rearrangement of allyl
aryl ethers: Scope of the catalysts, solvents, temperatures, and substrates
4
Zi Hui 1,2, Songwei Jiang 1,3, Xiang Qi , Xiang-Yang Ye ,6, Tian Xie 5,
⇑
⇑
Key Laboratory of Elemene Class Anti-Cancer Chinese Medicine of Zhejiang Province, Engineering Laboratory of Development and Application of Traditional Chinese Medicine
from Zhejiang Province, Collaborative Innovation Center of Chinese Medicines from Zhejiang Province, Holistic Integrative Pharmacy Institutes (HIPI), School of Medicine,
Hangzhou Normal University, Hangzhou, Zhejiang 311121, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The microwave-accelerated Claisen rearrangement of allyl aryl ethers was investigated, in order to gain
insight into the scope of the catalysts, solvents, temperatures, and substrates. Among the catalysts exam-
ined, phosphomolybdic acid (PMA) was found to greatly accelerate the reaction in NMP, at temperatures
ranging from 220 to 300 °C. This method was found to be useful for preparing several intermediates pre-
viously reported in the literature using precious metal catalysts such as Au(I), Ag(I), and Pt(II).
Additionally, substrates bearing bromo and nitro groups on the aryl portion required careful tailoring
of the reaction conditions to avoid complex product profiles.
Received 24 January 2020
Revised 25 April 2020
Accepted 29 April 2020
Available online 4 May 2020
Keywords:
Allyl aryl ether
Claisen rearrangement
phosphomolybdic acid (PMA), microwave
Ó 2020 Elsevier Ltd. All rights reserved.
Substituted phenols or polyphenols (and their corresponding
ethers) are a class of important compounds widely existing in nat-
ural products, pharmaceuticals, and synthetic intermediates [1].
One of the main methods to construct substituted phenols or
polyphenols is the Claisen rearrangement [2]. Since its discovery
in 1912, the Claisen rearrangement has been well studied and
reviewed [3]. Typically, long reaction times and high temperatures
are required when using conventional heating. Subsequent studies
have found that a wide range of catalysts, such as protic acids
(H2SO4, TFA, H3PO4), Lewis acids (Bi(OTf)3, AlCl3, SnCl4, BCl3), het-
eropoly acids, metal complexes, molecular sieves, and ionic liquids,
could all accelerate the reaction [4].
The microwave-accelerated Claisen rearrangement has been
reported by several research groups [5]. However, the reaction
conditions vary from one research group to the other, and are often
not reproducible using different substrates. For instance, Gupta
used Zn dust to catalyze the reaction at temperatures as low as
55 °C, but we only observed trace amounts of product formation
in the presence of Zn dust under microwave heating up to 160 °C
[6]. Furthermore, each substrate’s nature greatly affects the ease
of rearrangement, and the optimal conditions often require careful
tailoring.
In this work, we have investigated the Claisen rearrangement of
a set of allyl aryl ethers, in order to identify the optimum catalyst
and reaction conditions (solvent, temperature, reaction time, and
the substrate effects). Among the catalysts examined, phospho-
molybdic acid (PMA) was found to greatly accelerate the reaction
in NMP, at temperatures ranging from 220 to 300 °C. These condi-
tions were then applied to the synthesis of several key intermedi-
ates previously reported in the literature using precious metal
catalysts.
Initially, we chose compound 1 [7] as a model substrate for the
Claisen rearrangement studies. Under microwave irradiation in a
capped vial, compound 1 was heated to a certain temperature,
with or without catalyst, in commonly used organic solvents. The
results are presented in Table 1. The reaction in dichloromethane
at 130 °C did not produce any product after 60 min (Entry 1), while
the reaction in ethanol gave 2 in 19% yield at 150 °C (no catalyst,
60 min, Entry 2). Water has been reported to accelerate the Claisen
rearrangement by Wipf and co-workers [8]. However, when com-
pound 1 was heated in water at 180 °C [9], only trace amounts
of 2 was produced after 60 min, as suggested by TLC analysis.
Switching from ethanol or water to other high thermal resistant
solvents such as THF [6] (microwave 200 °C, 60 min, Entry 4),
DMSO [5a] (240 °C, 60 min, Entry 5), and 1,4-dioxane (240 °C,
60 min, Entry 6) was not effective in accelerating the reaction or
⇑
Corresponding authors at: Holistic Integrative Pharmacy Institutes (HIPI),
School of Medicine, Hangzhou Normal University, Hangzhou, Zhejiang 311121,
China.
E-mail addresses: xyye@hznu.edu.cn (X.-Y. Ye), xbs@hznu.edu.cn (T. Xie).
These two authors contributed equally.
0000-0002-3211-8025
0000-0003-2698-7125
0000-0002-6408-2696
0000-0001-7066-1443
0000-0003-3739-0930
1
2
3
4
5
6
https://doi.org/10.1016/j.tetlet.2020.151995
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.

2
Z. Hui et al. / Tetrahedron Letters 61 (2020) 151995
Table 1
Effects of the solvents, temperatures, reaction time, and catalysts on the microwave-assisted Claisen rearrangement of 1.a
Entry
Solvent
Temperature (℃)
Time (min)
Catalyst (amount)
Yield 2 (%)a
c
1
2
3
4
5
6
7
8
CH2Cl2
EtOH
H2O
130
150
180
200
240
240
250
250
250
250
250
250
300
300
300
160
300
300
300
300
300
300
300
250
200
150
60
60
60
60
60
60
60
60
60
30
10
30
10
5
10
30
2
2
2
2
2
2
1
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0b,
19b
tracec,
d
THF
7b
DMSO
1,4-dioxane
xylenes
N,N-diethylaniline
DMF
DMF
DMF
NMP
NMP
NMP
HMPT
NMP
NMP
NMP
NMP
NMP
NMP
NMP
21
23
80b
87b
98
90
71
90
92
77
72
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
d
A-45 ion exchange resin (0.15 equiv.)
CeCl3Á7H2O (0.059 equiv.)
Yb(OTf)3 (0.022 equiv.)
AlCl3 (0.01 equiv.)
BF3ÁEt2O (0.004 equiv.)
PMA (0.012 equiv.)
PMA (0.006 equiv.)
PMA (0.012 equiv.)
PMA (0.012 equiv.)
PMA (0.012 equiv.)
PMA (0.012 equiv.)
tracec,
46
88
79
77
91
85
81
57b
NMP
NMP
NMP
NMP
2
2
2
e
5b,
e
4b,
a
Isolated yields unless otherwise indicated. bYields were measured using HPLC. c95% of starting material was recovered. dTLC analysis. e89% of starting material was
recovered.
increasing the yield. Other higher boiling point solvents such as
(Entry 16). Other catalysts such as CeCl3Á7H2O (0.059 equiv.), Yb
(OTf)3 (0.022 equiv.) [13], AlCl3 (0.01 equiv.) [14] and BF3ÁEt2O
(0.004 equiv.) [15] were found to accelerate the reaction within
2 min, with moderate yields varying from 46 to 88% (Entries 17–
20). To our delight, the use of PMA significantly accelerated the
reaction. At 300 °C, with as low as 0.012 equiv. of PMA, the reaction
gave an excellent yield after 2 min (Entry 21), and a good yield
after 1 min (Entry 23). The yield of 2 decreased slightly when the
amount of PMA was reduced from 0.012 equiv. to 0.006 equiv.
(Entries 22 vs. 21). Furthermore, when the reaction time (2 min)
and catalyst amount (0.012 equiv. PMA) was kept constant, the
yields decreased along with the reaction temperature (Entries
24–26). It is worth noting that the yield decreased dramatically
when the reaction was performed at temperatures below 250 °C
(Entries 25–26).
Based on the results from Table 1, PMA appeared to be the best
catalyst among those examined. Next, we decided to synthesize a
set of allyl aryl ethers to probe the substrate scope, using the opti-
mised conditions described in Entry 21. Allyl aryl ethers 5 bearing
different ‘‘A” motifs were synthesized via a standard alkylation
reaction between the corresponding phenol 3 and allyl halide 4
in moderate to good yields. The Claisen rearrangement of 5 was
carried out under the conditions described in Entry 21 (Table 1),
unless otherwise indicated (Table 2).
xylenes [10], N,N-diethylaniline [11], and DMF [12] were also
examined. Reaction temperatures higher than 250 °C were avoided
to prevent the decomposition of DMF. In poor microwave absorb-
ing solvents such as xylenes, it was difficult to maintain the tem-
perature higher than 250 °C, as the microwave magnetron is not
set up to operate at full power for extended periods. Among these
three solvents, DMF appears to be the best (Entry 9 vs Entries 7 and
8). Therefore, we further investigated the model reaction using
DMF as the solvent. When the reaction time was reduced from
60 min to 30 min, then to 10 min, the yield decreased (Entries
10 and 11). Switching the solvent from DMF to NMP, gave a com-
parable yield (ca. 90%) at 250 °C for 30 min (Entry 12 vs Entry 10).
Due to the decomposition of DMF at temperatures above 250 °C,
we selected NMP as an alternative solvent to examine the effect
of higher temperatures. The reaction in NMP at 300 °C produced
an excellent yield after only 10 min (Entry 13). However, a further
decrease of the reaction time to 5 min resulted in a significant
decrease of the yield (77%, Entry 14). HPLC analysis indicates that
the starting material 1 remained. Finally, HMPT was examined at
300 °C for 10 min, and the yield was moderate and comparable
to that of the 5 min-reaction in NMP (Entry 15 vs Entry 14). It is
clear that NMP stands out as the best solvent among those exam-
ined in Table 1.
Initially, substrates bearing a single substituent on the aryl por-
tion were examined (Entries 1–7). The reaction of ortho-substi-
tuted phenyl ether 5a gave a separable mixture of 6a-1 and 6a-2,
in which the allyl group migration favored the ortho-position over
the para-position to the phenol group (Entry 1, Table 2). We
obtained a slightly higher ratio of 6a-1 to 6a-2 and a similar overall
Next, we examined the effect of various catalysts for this model
reaction. AmberlystTM 45 Resin (A-45) is a macroporous sulfonic
acid polymer catalyst designed for use in high-temperature (up
to 160 °C without decomposition) heterogenous catalysis. How-
ever, the reaction using AmberlystTM 45 at 160 °C in NMP gave only
trace amounts of product 2, and the starting material 1 remained

Z. Hui et al. / Tetrahedron Letters 61 (2020) 151995
3
Table 2
Substrate scope for the microwave-assisted Claisen rearrangement of 5.a
Entry
1
3
5
Yield 5 (%)b
92%
6
Yield 6 (%)b
55% (6a-1), 15% (6a-2) (70%, 2.4:1
ref. 5e)
2
3
57%
81%
(6b-1:6b-2 = 1:1)c (74%, 1.2:1 ref.
5e)
81% (77%, ref.5e)
43%
71%
(77%, ref. 5e)
4
96% (73%, ref. 5e)
5
6
75%
67%
88% (82%, ref. 16)
79% (77%, ref. 16)
7
87%
90% (75%, ref. 11b)
8
9
88%
67%
76% (78%, ref. 6)
69% (83%, ref. 6)
(continued on next page)

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Z. Hui et al. / Tetrahedron Letters 61 (2020) 151995
Table 2 (continued)
Entry
10
3
5
Yield 5 (%)b
6
Yield 6 (%)b
29%
78% (77%, ref. 17)
11
12
63%
54% (6k-1) (86%, ref. 18) 8% (6k-2)
50%
94%
21% (6l-1) 44% (6l-2:3l = 5:1)c
13
14
15
79%d
41%
88%
69%
73% (71%, ref. 19)
16
53% (6o-1) 16% (6o-2) Trace (6o-3)
17
89%
70% (60%, ref. 20)
18
19
38%
53%
83% (85%, ref. 21)
39% (6r) 15% (3r)

Z. Hui et al. / Tetrahedron Letters 61 (2020) 151995
5
Table 2 (continued)
Entry
3
5
Yield 5 (%)b
6
Yield 6 (%)b
28%
67%
20
42% (6s) 23% (3s)
a
Reagents and conditions: Step 1:3 (5 mmol), 4 (6 mmol), NaH (6 mmol), DMF (5 mL), r.t. Step 2: 5 (0.5 mmol), PMA (0.006 mmol), NMP (1 mL), microwave 300 °C, 2 min.
bIsolated yields unless otherwise indicated. cRatio was determined by intergration of 1H NMR spectra. dReaction without catalyst, microwave 230 °C, 20 min. eReaction
without catalyst, microwave 220 °C, 20 min. fReaction without catalyst, microwave 250 °C, 10 min. gCompound 5r-1 was prepared from 3r and allyl chloride in 53% yield
using standard alkylation conditions. Compound 5r-2 was isolated as a by-product in 28% yield.
yield compared to the conditions reported by Chu and co-workers
[5e]. When meta-substituted phenyl ether 5b was used, the
reaction produced an inseperable mixture of two products 6b-1
and 6b-2 in a 1:1 ratio (slightly higher overall yield than Chu
and co-workers [5e]), with the migration proceeding to both of
the ortho-positions (Entry 2). For substrates bearing para-
substituents such as para-methyl (5c), para-methoxy (5d), para-
fluoro (5e), para-trifluoromethyl (5f), and para-methylbenzoate
(5g), the reaction proceeded smoothly to give ortho-
rearrangement products (6c-g, Entries 3–7). It should be noted
that our method produced better yields for selected substrates
than what was reported in the literature. However, their bromo
and nitro substituted arenes (5h and 5i) gave mixtures of
products under the above conditions (Entries 11–12). Subsequent
investigation revealed that the substrates bearing bromo and
nitro groups required lower reaction temperatures and the
absence of PMA. Thus, compounds 5h and 5i underwent
rearrangement at lower temperatures (230 °C or 220 °C
respectively) to give 6h and 6i, respectively, in reasonable yields
after 20 min (Entries 8 and 9). The ortho-nitro substrate 5j gave a
single rearrangement product where the allyl group migrated to
the position next to phenol group (Entry 10). Other allyl aryl
ethers bearing either bromo or nitro groups all required lower
temperatures (220 °C) and the absence of PMA (Entries 11–13). It
Fig. 1. Possible rearrangement mechanism for substrate 5l.

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Z. Hui et al. / Tetrahedron Letters 61 (2020) 151995
Scheme 1. Applications of the microwave-accelerated Claisen rearrangement in the synthesis of useful intermediates.
is worth noting that the effect of PMA is profound with nitro-
bearing substrates such as compound 5m. The reaction
conditions with or without PMA produced different products
(6b-1 vs 6m, Entry 14 vs Entry 13). Apparently, PMA causes the
loss of the nitro group during the rearrangement. Thus,
compound 5n underwent rearrangement in the absence of PMA
to give 6n in good yield (Entry 15). Similarly, ortho-bromo
bearing substrate 5l underwent rearrangement under modified
conditions (220 °C, 20 min, in the absence of PMA) to give two
separable rearrangement products, 6l-1 and 6l-2, in a ratio of
approximately 1:1.7 (Entry 12). Compound 6l-1 was formed from
double migration of the both bromo group and the allyl group. In
addition, approximately 7% of the de-allyl product 3l (starting
material for 5l) was detected. This result suggests that the
reaction might involve multiple stepwise rearrangements. The
allyl group of 5l presumably first migrates to positions ortho- to
the phenolic hydroxyl group. The high energy intermediates then
proceed to
a lower energy aromatic system via different
pathways to furnish 6l-1, 6l-2, and 3l, respectively (Fig. 1).

Z. Hui et al. / Tetrahedron Letters 61 (2020) 151995
7
Other substrates with available ortho- and/or para- positions
generally follow the same rule. Thus, 5o underwent rearrangement
to give 6o-1 and 6o-2 in a ratio of approximately 3.4:1 with trace
amounts of 6l-3 detected (Entry 16). The complexity of the product
profile is generally due to more than one available migration posi-
tion leading to competion. For substrates with only one position
available for migration, the reaction tends to be cleaner and pro-
ceeds in higher yields, i.e. Entries 17–18.
expanded to include allyl thiophenol ether, allyl aniline, propynyl
phenol ether, and a substituted propenyl phenol ether. Finally,
the method was employed to access several intermediates previ-
ously synthesized using precious metal catalysts such as Au(I),
Ag(I), and Pt(II).
Declaration of Competing Interest
When all the ortho- and para- positions of the aryl phenol group
were occupied, the rearrangement occured differently. For
instance, 5r-1 underwent the rearrangement to give 6r and 3r
(Entry 19). The allyl group might first migrate to the ortho- position
to form a high energy intermediate, followed by the expulsion of
isobutene to produce 6r. In addition, the phenol ally ether 5r-1
might decompose to form 3r, which is the starting material for
preparation of 5r-1. Similarly, 5s underwent the rearrangement
to form 6s and 3s a ratio of approximately 2:1 (Entry 20).
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work is supported by The National Natural Science Founda-
tion of China (grant IDs: 81730108 and 81973635) and Hangzhou
Normal
University
(China)
startup
fund
(grant
ID:
The developed method was then used to prepare several key
intermediates for organic synthesis and pharmaceuticals
(Scheme 1). Compound 5t underwent double rearrangement to
give 6t in 60% yield under the standard conditions (0.012 equiv.
PMA, NMP, 300 °C, 2 min). However, the reaction of 5u, a substrate
also bearing two allyl ether groups, proceeded under modified con-
ditions (220 °C, 20 min, NMP, in the absence of PMA) to give 6u-1
(46%) and 6u-2 (14%). The results were consistent with the litera-
ture, but the reaction was complete in a much shorter time (20 min
vs 585 min) [22]. Compound 6u-1 was a key intermediate for the
preparation of a PI3K inhibitor [23]. Interestingly, 2-naphthalenyl
ether 5v underwent rearrangement and cyclization within 2 min
to afford 6v in 71% yield. Previously, several groups have reported
the synthesis of 6v involving precious metal catalysts such as
AuClPPh3, utilizing long reaction times and several synthetic steps
[24]. 1-Naphthalenyl ether 5w proceeds smoothly to give the rear-
rangement product 6w in 80% yield. Unlike the reaction of 5v,
there was no cyclization product detected for 5w. The synthesis
of 6w reported in the literature involved long reaction times at
high temperatures [25]. Furthermore, the substrate scope for this
method was further expanded. For example, allylic thio ether 5x
underwent rearrangement and cyclization in a single step to afford
two products 6x-1 and 6x-2, in a ratio of 7.3:1. Previously, com-
pound 5x was used by Dai and Yadav to synthesize 6x-2 [26]. Allyl
aniline 5y [27] underwent the rearrangement to afford ortho- pro-
duct 6y in 23% yield. Finally, 2-methylpropenyl phenyl ether 5z
(bearing a substituent on the allyl portion) rearranged to afford
6z in good yield.
In summary, the Claisen rearrangement reaction under micro-
wave irradiation was investigated in order to gain a better under-
standing of the scope of the catalysts, solvents, temperatures, and
substrates. Catalytic PMA (as low as 0.012 equiv.) was found to be
superior to the other catalysts examined, and to significantly accel-
erate the reaction greatly. NMP also appears to be the best solvent.
In general, the reaction is complete within 2–10 min at 300 °C. The
substrate scope was also investigated. In general, when ortho- and
para- positions are both available, the allyl group can migrate to
both positions, with preference for the ortho- position over the
para- position. When both ortho-positions are blocked by sub-
stituents, allyl migration occurs to the available para- position. If
all possible ortho- and para-positions are occupied, the substrates
will either decompose to the phenol or swap one of the occupied
ortho-groups with the allyl group. Bromo (but not fluoro) and nitro
groups are sensitive substituents in this reaction. For substrates
bearing these two groups, the use of PMA complicates the reaction
profiles, resulting in either the loss, or the rearrangement, of the
sensitive groups. In the absence of PMA and at a lower tempera-
ture, the sensitive groups (bromo and nitro) can be tolerated, but
the reaction requires longer times. The substrate scope was further
4125C5021920419). We thank Ms. Kefang Yang and Mr. Canyu
Chen for performing NMR studies for us.
Appendix A. Supplementary data
Supplementary data (experimental procedures, NMR spectrum)
can be found online at https://doi.org/10.1016/j.tetlet.2020.
151995.
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