Readily switchable one-pot 5-exo-dig cyclization using a palladium catalyst

Article abstract of DOI:10.1039/c6ra25857c

A convenient, ligand-free, Pd(OAc)₂-catalyzed one-pot reaction has been developed for the synthesis of oxazolines and oxazoles from easily available acid chlorides and propargylamine. The reaction pathways could be easily modulated to selectively obtain oxazolines or oxazoles by merely changing the additives. This method proceeds via in situ sequential nucleophilic addition/elimination reactions followed by an intramolecular 5-exo-dig cycloisomerization reaction. An interesting observation in this case is the effect of an additive: a basic additive such as triethylamine resulted in the formation of 5-methylene oxazolines, while an acidic additive such as acetic acid resulted in the formation of 5-methyloxazoles. With the current protocol we are able to obtain good to moderate yields of the desired product without the need for the isolation of intermediates.

Full text of DOI:10.1039/c6ra25857c

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Readily switchable one-pot 5-exo-dig cyclization
using a palladium catalyst†
Cite this: RSC Adv., 2017, 7, 2231
*
Jaishree K. Mali, Balaram S. Takale and Vikas. N. Telvekar
A convenient, ligand-free, Pd(OAc)2-catalyzed one-pot reaction has been developed for the synthesis of
oxazolines and oxazoles from easily available acid chlorides and propargylamine. The reaction pathways
could be easily modulated to selectively obtain oxazolines or oxazoles by merely changing the additives.
This method proceeds via in situ sequential nucleophilic addition/elimination reactions followed by an
intramolecular 5-exo-dig cycloisomerization reaction. An interesting observation in this case is the effect
of an additive: a basic additive such as triethylamine resulted in the formation of 5-methylene oxazolines,
while an acidic additive such as acetic acid resulted in the formation of 5-methyloxazoles. With the
current protocol we are able to obtain good to moderate yields of the desired product without the need
for the isolation of intermediates.
Received 26th October 2016
Accepted 15th December 2016
DOI: 10.1039/c6ra25857c
www.rsc.org/advances
drawbacks such as low yield, harsh reaction conditions, use of
expensive ligands, and use of excess amount of additive or non-
Introduction
commercially available starting materials. Nevertheless, cyclo-
isomerization of propargylamides to corresponding oxazole and
oxazoline require synthesis, isolation and purication of prop-
argylamides. Therefore, the development of single step protocol
is highly desirable to minimize solvent waste and to maximize
the yield of product. Taking this into consideration, Tran-
Oxazoles and oxazolines are ubiquitous heterocycles present in
a variety of natural products, pharmaceuticals, agrochemicals,
foods and polymers.1–4 Additionally, 2-oxazolines serve as
synthetic intermediates,5 protecting groups in organic synthesis,
and catalysts, ligands or auxiliaries in asymmetric synthesis.6,7
Several synthetic methods reported in the literature for
substituted oxazole use Robinson–Gabriel synthesis cyclo-
dehydration of 2-acylamino carbonyl compounds,8 isomer-
isation of the oxazolines,9 base promoted reactions of primary
aromatic amides and 2,3-dibromopropene,10 catalytic decom-
position of diazocarbonyl compounds in nitriles,11 reductive
desulfuration of 5-methyl-4-methylthio-1,3-oxazoles,12 func-
tionalisation (via metallation) of readily available oxazole13 and
various condensation reactions.14
DubeM et al.29 explored one-pot synthesis of oxazoles from acid
´
chlorides and propargylamines using gold-catalyst. Our curi-
osity was increased to replace expensive gold catalyst with well
known palladium catalyst.30
Gratifyingly, we found an interesting result while performing
optimization studies, use of basic additive selectively resulted in
oxazoline, while use of acidic additive selectively resulted in
oxazole. We believe, such kind of readily switchable protocol
Other straightforward methods for the synthesis of methy-
lene oxazoline and oxazole include cycloisomerization of
propargylamides.15 However, in this context cyclization of
propargylamides to the corresponding oxazoles was achieved by
strong base or strong acid using harsh conditions.16
On the other hand, Lewis acids involving not only gold17 but
also ZnI2 and FeCl3,18 cerium chloride,19 ferric bromide,20 zinc
triate,21 Bronsted acid22 catalysts have emerged to afford oxa-
zoles or oxazolines from corresponding propargylic amides.
Besides these, metals such as silver,23 copper,24 palladium,25
mercury,26 ruthenium,27 tungsten and molybdenum28 have been
explored. In fact, all of these strategies possess respective
Department of Pharmaceutical Sciences and Technology, Institute of Chemical
Technology, Mumbai 400 019, India. E-mail: vikastelvekar@rediffmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra25857c
Scheme
oxazoles.
1
Approach for the synthesis of dihydrooxazoles and
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which has wide applicability has not been explored previously
(Scheme 1).
Surprisingly, we were able to obtain oxazole 4a exclusively
when TEA was not used, although in very low amount (Table 1,
entry 14). This strongly suggests that acidity is important for the
formation of oxazole over oxazoline. To this end, a variety of
acids including organic as well as inorganic solid acid catalysts
Results and discussion
We initiated the present study with benzoyl chloride and were screened. Use of Amberlite IR 120-H, Amberlyst 15 (wet)
propargylamine as model substrates. Accordingly, propargyl- and Montmorillonite led to the formation of the desired
amine (2a, 1.82 mmol), benzoyl chloride (1a, 1.81 mmol), trie- product in 15%, 30% and 50% yields respectively. Use of
thylamine (TEA) (2.17 mmol) and 1 mol% of Pd(OAc)2 were organic acids such as pivalic acid, triuoroacetic acid and acetic
heated in toluene as a solvent, in a sealed tube at 100 ꢀC for 24 h, acid resulted in 35%, 65% and 88% yield of the desired product,
oxazoline 3a was formed in 20% yield (Table 1, entry 1). This respectively. According to these results, an additive, TEA was
results gave us a hint that Pd(OAc)2 is compatible with reaction replaced with 1.81 mmol of acetic acid to give 88% yield of the
conditions of one-pot synthesis. Further, we screened for the desired product when the reaction was carried out with 5 mol%
ꢀ
loading of Pd(OAc)2 from 2 to 10 mol% (Table 1, entries 2–13). of Pd(OAc)2 in toluene as a solvent at 100 C for 24 h (Table 1,
Remarkably, the desired product was obtained in 85% yield entry 15). Shortening of the time from 24 h to 12 h resulted in
with 5 mol% of the catalyst (Table 1, entry 3). We also tried to lower yield of the product (Table 1, entry 16). It was observed
replace TEA with inorganic or organic bases such as K2CO3 and that the reaction did not proceed towards the completion when
diisopropylethylamine but resulted in 20% and 10% yield, less than 1 equivalents of TEA and acetic acid were used
respectively. In the absence of Pd(OAc)2 and TEA, no product respectively, while their excess amount did not affect the yield of
was formed (Table 1, entry 4). Only Pd(OAc)2 without any the desired product.
additive resulted in the trace amount of product 4a (conrmed
Finally, we had optimized conditions in hands, that is for
1
by H NMR) (Table 1, entry 5). These blank reactions suggest oxazoline 3a; 1a (1.82 mmol), 2a (1.81 mmol), 5 moꢀl% of
that Pd(OAC)2 and TEA combination is crucial for selective Pd(OAc)2, TEA (2.17 mmol) in toluene as solvent at 100 C for
formation of oxazoline 3a.
5 h, and for oxazole 4a; 1a (1.82 mmol), 2a (1.81 mmol), 5 mol%
A variety of solvents were tested for this reaction, from which of Pd(OAc)2,acetic acid (1.81 mmol) in toluene as solvent at
no product was observed in DMF and DMSO. However, toluene 100 ꢀC for 24 h. Under these optimized reaction conditions,
gave the best result (Table 1, entries 3, 6–10). Remarkably, short- rstly we examined the scope of various acid chlorides for the
ening of the reaction time from 24 h to 5 h resulted in the same synthesis of oxazolines (Table 2). Aroyl chlorides with either
yield of the desired product (Table 1, entries 11 and 12). Further, electron donating group on ortho-, meta- or para-position or
putting 10 mol% of the catalyst with reaction time of 5 h did not electron withdrawing groups gave higher yields of the desired
show signicant increment in the yield (Table 1, entry 13).
product (Table 2, entries 2–6).
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst (mol%)
Additive
Solvent
Temp (ꢀC)/time (h)
3ac yield (%)
4ac yield (%)
1
2
3
4
5
6
7
8
1
2
5
—
1
5
5
5
5
5
5
5
10
5
5
TEA
TEA
TEA
—
—
TEA
TEA
TEA
TEA
TEA
TEA
TEA
TEA
—
Acetic acid
Acetic acid
Toluene
Toluene
Toluene
Toluene
Toluene
CH2Cl2
1,2-DCE
CH3CN
THF
1,4-Dioxane
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
100/24
100/24
100/24
100/24
100/24
40/24
80/24
80/24
65/24
20
45
85
0
0
0
0
0
0
<2%b
0
30
45
35
32
35
85
85
88
0
0
0
0
0
0
0
0
25
88
65
9
10
11
12
13
14
15
16
100/24
100/12
100/5
100/5
100/24
100/24
100/12
0
0
5
a
Reaction conditions: acid chloride (1.82 mmol), propargylamine (1.81 mmol), catalyst Pd(OAc)2 (mol%), additive TEA (triethylamine) 2.17 mmol or
acetic acid 1.81 mmol in 2 mL of dry solvent in a sealed tube. Detected by 1H NMR. c Isolated yield.
b
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Table 2 One pot synthesis of oxazolinesa
Table 3 One pot synthesis of oxazolesa
Entry
1
Acid chlorides; 1
Product; 4
Yield of 4b (%)
Entry
1
Acid chlorides; 1
Product; 3
Yield of 3b (%)
88
85
2
3
90
70
2
3
4
85
70
92
4
71
5
6
84
70
5
6
84
70
7
82
7
8
9
82
82
65
8
9
72
70
10
45
10
40
a
Reaction conditions: acid chloride (1.82 mmol), propargylamine (1.81
mmol), catalyst Pd(OAc)2 (5 mol%), TEA (triethylamine) 2.17 mmol in 2
a
Reaction conditions: acid chloride (1.82 mmol), propargylamine (1.81
mmol), catalyst Pd(OAc)2 (5 mol%), acetic acid 1.81 mmol in 2 mL of dry
b
mL of dry toluene in a sealed tube at 100 ꢀC for 5 h. Isolated yield.
b
toluene in a sealed tube at 100 ꢀC for 24 h. Isolated yield.
acid chlorides also gave the desired products in good to
moderate yields (Table 3, entries 8 and 9).
Aliphatic derivatives also furnished the desired product in
moderate to good yield (Table 2, entries 8 and 9). Heterocyclic
derivatives gave slightly lower yield (Table 2, entry 10). Note-
worthy to mention, multiple substituent on the ring were also
tolerated (Table 2, entry 7).
Secondly, using optimized reaction conditions available for
oxazoles, we investigated the scope for variety of substrate
(Table 3). Mono- and disubstituted aroyl chlorides were easily
transformed to give good yields of the desired products (Table 3,
entries 2–7). Both electron donating and electron withdrawing
substrates were readily converted into the desired products
(Table 3, entries 2–7). Furthermore, substituents at different
positions on the aryl group (para-, meta-, ortho-positions), did
not affect the reaction efficiency (Table 3, entries 2–6). Hetero-
Aer this successful study, we performed one control
experiment to check effect of atmospheric oxygen on the reac-
tion (Scheme 2). It is reported in the literature25b that metal-
vinylidene formed as an intermediate could be easily con-
verted to aldehyde 5a. However, in our reaction condition we
aryl derivatives gave lower yield (Table 3, entry 10). Aliphatic Scheme 2 Control experiments.
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via sequential nucleophilic addition/elimination reactions fol-
lowed by intramolecular cycloisomerization reaction via 5-exo-
dig mode. Moreover, the reaction pathway could be easily tuned
by merely changing additives such as triethylamine or acetic
acid to exclusively furnish 2-oxazolines or 2-oxazoles respec-
tively. These protocols exhibit tolerance towards wide range of
substrates to afford the desired product in moderate to good
yield, except heteroaromatics, which furnished both the
product oxazolines and oxazoles in slightly lower yield.
Aliphatic acid chlorides also offered exclusively oxazolines and
oxazoles in good yield without formation of complex mixture of
the isomerised product. Being one-pot these reactions offer
high step economy and does not require solvent exchanges or
isolation of intermediates. In future, manipulation at the 5-
methylene position of oxazolines could provide an avenue to
wide variety of functionalized oxazoles. This protocol circum-
vents the use of expensive ligands. Synthetic application of the
present work is underway in our laboratory.
Acknowledgements
Scheme 3 Plausible reaction mechanism.
The author JKM is greatly thankful to the UGC (University Grant
Commissions), New Delhi, India for providing a Basic Science
Research (BSR) fellowship and VNT thanks the Science and
Engineering Research Board (SERB), New Delhi, India, for
providing nancial support.
were not able to obtain such oxidized product 5a, and rather
mixture of 3a and 4a could be obtained. It was noted that in the
case of synthesis of oxazolines, small quantities of inactive
palladium black were observed in the solution aer the
completion of reaction. On the other hand, in case of oxazoles,
no formation of palladium black was observed. It could be
hypothesized that acetic acid signicantly helped in the
regeneration of palladium acetate. In addition, to extend the
scope of our methodology, we carried out reaction using
secondary amine such as N-methylpropargylamine, but
unidentied mixture of products was isolated.
Based on the literature and experimental details, we
proposed a plausible reaction pathway (Scheme 3). The reaction
proceeds via in situ formation of propargylamide by nucleo-
philic addition/elimination reactions of acid chlorides and
propargylamine. Palladium acetate coordinates to the carbon–
carbon triple bond of in situ formed propargylamide A to form
organo-palladium species B followed by triethylamine assisted
intramolecular nucleophilic attack of enolate on alkyne to
produce vinyl palladium C in 5-exo-dig manner. Intermediate C
can either undergo proto-demetallation to give oxazoline 3 or it
can undergo aromatization in the presence of excess of acetic
acid to give oxazole 4. The path towards the formation of
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