A general and efficient palladium-catalyzed carbonylative synthesis of 2-aryloxazolines and 2-aryloxazines from aryl bromides

Article abstract of DOI:10.1002/chem.201202652

Oxazoline is OK! A general and efficient method for the synthesis of oxazolines has been developed (see scheme). This allowed the preparation of 27 five-membered-ring heterocycles and 11 six-membered-ring heterocycles in moderate to good yields. Copyright

Full text of DOI:10.1002/chem.201202652

COMMUNICATION
DOI: 10.1002/chem.201202652
A General and Efficient Palladium-Catalyzed Carbonylative Synthesis of 2-
Aryloxazolines and 2-Aryloxazines from Aryl Bromides
Xiao-Feng Wu,*[a, b] Helfried Neumann,[b] Stephan Neumann,[b] and Matthias Beller*[b]
Palladium-catalyzed coupling reactions offer a powerful
À
toolbox for the construction of C X (X=C, N, O, etc.)
bonds, which are frequently applied for the synthesis of
pharmaceuticals, agrochemicals, and advanced materials.[1,2]
With the importance of these reactions in mind, both aca-
demic and industrial chemists are continuously investigating
and developing new applications. Among the palladium-cat-
alyzed coupling reactions, palladium-catalyzed carbonylation
reactions are elegant in the preparation of carbonyl-contain-
ing compounds.[3] By applying CO as a cheap C1 source, the
carbon chain of the parent molecules can be easily in-
creased, introducing one or more carbonyl groups into the
Scheme 1. Methods for oxazoline synthesis.
molecules.
Oxazolines are an important family of chemicals that
Table 1. Palladium-catalyzed carbonylative synthesis of oxazolines: Ex-
exist in naturally occurring iron chelators, cytotoxic cyclic
amination of ligands and solvents.[a]
peptides, and antimitotic and neuroprotective agents.[4] They
also contribute to the flavor of a numbers of foods.[5] More-
over, oxazolines are valuable intermediates in various or-
ganic transformations.[6] Because of the importance of oxa-
Entry
Ligand
([mol%])
Solvent
Yield
[%][b]
zolines, a number of methodologies have been developed
for the production of these compounds (Scheme 1);[7] they
are generally prepared from the corresponding carboxylic
acids, carboxylic esters, nitriles, aldehydes, or other carbon-
yl-containing chemicals. However, it would be extremely in-
teresting if a methodology could be found that can apply
CO as the carbonyl source to react with more available aryl
bromides as the starting materials, instead of using chemi-
cals that already contain a carbonyl group. To the best of
our knowledge, no report on the carbonylative synthesis of
2-oxazolines has been published.[8] Because of the impor-
tance of 2-oxazolines, and our ongoing interest in palladi-
um-catalyzed carbonylation reactions,[9] we wish to report,
herein, a general and efficient palladium-catalyzed carbony-
lative synthesis of 2-oxazolines.
AHCTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
BuPAd2 (6)
PCy3 (6)
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
toluene
DMF
36
13
27
27
32
P
N
DPPP (3)
DPPF (3)
DPEphos (3)
BuPAd2 (6)
BuPAd2 (6)
BuPAd2 (6)
BuPAd2 (6)
BuPAd2 (6)
BuPAd2 (6)
29
51
<1
<1
<1
<1
<1
DMSO
DMAc
NMP
10
11
12
CH3CN
[a] PdACHTNURGTNE(NUG OAc)2 (2 mol%), a ligand, a solvent (2 mL), NEt3 (3 mmol), CO
(5 bar), bromobenzene (1 mmol), 2-chloroethylamine hydrochloride
(1 mmol), 1108C, 16 h. Ad=adamantyl; Cy=cyclohexyl; DPPP=1,3-
bis(diphenylphosphino)propane; DPPF=1,1’-bis(diphenylphosphino)fer-
rocene; DPEphos=bis(diphenylphosphinophenyl)ether; DMAc=dime-
thylacetamide; NMP=N-methylpyrrolidone. [b] The yields, based on the
amount of bromobenzene used, were determined by GC using hexade-
cane as the internal standard.
The first set of reactions were carried out with six differ-
ent ligands (Table 1, entries 1–6), in the presence of Pd-
[a] Dr. X.-F. Wu
Department of Chemistry
AHCTUNTGREN(GNUN OAc)2 (2 mol%), NEt3 (3 mmol), and bromobenzene
Zhejiang Sci-Tech University, Xiasha Campus
Hangzhou, Zhejiang Province, 310018 (P.R. China)
E-mail: xiao-feng.wu@catalysis.de
(1 mmol), in dioxane (2 mL), under a CO (5 bar) atmos-
phere, at 1108C for 16 h. The ligand BuPAd2 gave a 36%
yield of 2-phenyloxazoline (Table 1, entry 1). Bidentate li-
gands showed similar results (Table 1, entries 4–6). Next, we
applied BuPAd2 as the ligand to check the effect of other
solvents on the reaction (27–32% yield; Table 1, entries 7–
[b] Dr. X.-F. Wu, Dr. H. Neumann, S. Neumann, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢀt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
Chem. Eur. J. 2012, 00, 0 – 0
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12). Nevertheless, a 51% yield of oxazoline was produced
by using toluene as the solvent (Table 1, entry 7), but the
other tested solvents resulted in only traces of the desired
product. Herein, all of the reactions gave full conversion of
bromobenzene and the rest of the starting material was
transformed into the dehalogenation product (benzene) in-
stead of into oxazoline.
Next, we chose BuPAd2 in toluene to determine the effect
of using different bases (Table 2, entries 1–5), but no better
Table 2. Palladium-catalyzed carbonylative synthesis of oxazolines: Ex-
amination of bases and additives.[a]
Scheme 2. Proposed reaction mechanism.
Para-methyl- and ortho-isopropylbromobenzene gave
yields of 70 and 65%, respectively, of the corresponding ox-
azolines (Table 3, entries 2 and 3). Good yields of the de-
sired products were also produced from the corresponding
naphthyl substrates (Table 3, entries 4–6). Methoxyl-, meth-
ylthiol-, and N,N-methyldiamino-substituted aryl bromides
were transformed into oxazolines in 65–89% yield (Table 3,
entries 7–10). Not only electron-donating groups, but also
electron-withdrawing groups were tolerated in the reaction
and gave the corresponding products in good yields
(Table 3, entries 11–19; 61–85% yield). In addition, eight
different heterocycles were successfully transformed into 2-
heterocycle-substituted oxazolines in moderate to good
yields (Table 3, entries 20–27; 50–89% yield).
Not only five-membered-ring oxazolines, but also six-
membered-ring oxazines can be synthesized by using this
procedure (Table 4). Both electron-donating- and electron-
withdrawing-group-substituted aryl bromides were reacted
and gave the corresponding products in good yield (Table 4,
entries 2–7; 74–85% yield). Furthermore, four heterocycle-
containing aryl bromides were reacted with the aminochlor-
ide to give the desired products in good yields (Table 4, en-
tries 8–11; 60–89% yield).
Entry
Base
(3 mmol)
Additive
(1 mmol)
CO
[bar]
Yield
[%][b]
G
G
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
DiPEA
DBU
TMEDA
DBACO
DMAP
NEt3
NEt3
NEt3
NEt3
NEt3
–
–
–
–
–
–
5
5
5
5
5
5
5
5
40
0
33
0
0
55[c]
60[c]
44[c]
80[c]
66[d]
45[e]
MgSO4
Na2SO4
MgSO4
MgSO4
MgSO4
10
10
10
10
11
NEt3
[a] PdACHTUNGTRENNUNG(OAc)2 (2 mol%), BuPAd2 (6 mol%), toluene (2 mL), base, CO,
bromobenzene (1 mmol), 2-chloroethylamine hydrochloride (1 mmol),
1108C, 16 h. DiPEA=N,N-diisopropylethylamine; DBU=1,8-diazabicy-
cloundec-7-ene; TMEDA=tetramethylethylenediamine; DABCO=1,4-
diazabicycloACHTUNGTRENNUNG[2.2.2]octane; DMAP=4-dimethylaminopyridine. [b] The
yields, based on the amount of bromobenzene used, were determined by
GC using hexadecane as the internal standard. [c] Toluene (4 mL).
[d] PdACHTUNGTRENNUNG(OAc)2 (1 mol%), BuPAd2 (3 mol%), toluene (4 mL). [e] 1008C,
toluene (4 mL).
yield was observed. The use of 4 mL of toluene can increase
the yield of the desired product to 55% (Table 2, entry 6).
MgSO4 and Na2SO4 were tested as additives; MgSO4 im-
proved the yield of oxazoline to 60% (Table 2, entry 6), but
Na2SO4 decreased the yield of oxazoline to 44%. To our de-
light, the yield was improved to 80% by running the reac-
tion under 10 bar of CO (Table 2, entry 9). A 66% yield of
oxazoline could still be formed in the presence of only
In conclusion, a general and efficient methodology has
been developed for the synthesis of oxazolines. This allowed
the preparation of 27 five-membered-ring heterocycles and
11 six-membered-ring heterocycles in moderate to good
yields.
1 mol% PdACHTUNGTRENNUNG(OAc)2 (Table 2, entry 10).
Based on our experiments, the most probable reaction
mechanism is proposed in Scheme 2. The first step is the ox-
idative addition of bromobenzene to the Pd0 species, fol-
lowed by the coordination and insertion of CO to form the
acylpalladium intermediate. After nucleophilic attack of 2-
chloroethylamine and reductive elimination, N-(2-chloroe-
thyl)arylamide is formed as the key intermediate. Under the
assistance of a base and high temperatures, 2-aryloxazoline
is produced as the final product.
With the best reaction conditions in hand, we tested dif-
ferent substrates in the reaction (Table 3). The generality
and variability of this methodology was proven by the syn-
thesis of 27 different oxazolines.
Experimental Section
General information: All reactions were performed by using standard
Schlenk techniques (argon). Gas chromatography was performed on a
Hewlett–Packard HP 6890N chromatograph with an HP5 column. Chem-
icals were purchased from Fluka, Aldrich, and Strem and used as re-
ceived. The cataCXium A ligand is available from Strem or directly from
Solvias. Toluene was distilled from CaH2.
General procedure:
A vial (12 mL) was charged with PdACHTUNGTRENNUNG(OAc)2
(2 mol%), BuPAd2 (6 mol%), MgSO4 (1 mmol), 2-chloroethylamine hy-
drochloride (1 mmol), and a stirring bar. Then, toluene (4 mL), an aryl
bromide (1 mmol), and NEt3 (3 mmol) were injected by syringe. The vial
(or several vials) was (were) placed in an alloy plate, which was transfer-
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Synthesis of 2-Aryloxazolines from Aryl Bromides
COMMUNICATION
Table 3. Palladium-catalyzed carbonylative synthesis of 2-aryl-4,5-dihydrooxazoles.[a]
Entry
Aryl bromide
Oxazoline
Yield
[%][b]
Entry
15
Aryl bromide
Oxazoline
Yield
[%][b]
1
2
78[c]
70
61
70
16
3
4
65
85
17
18
82
71
5
6
7
80
83
72
19
20
21
74
57
52
8
89
22
80
9
70
65
23
24
50
70
10
11
85
25
89
12
13
81
81
78
26
27
74
68
14
[a] PdACHTUNGTRENNUNG(OAc)2 (2 mol%), BuPAd2 (6 mol%), toluene (4 mL), NEt3 (3 mmol), CO (10 bar), aryl bromide (1 mmol), 2-chloroethylamine hydrochloride
(1 mmol), 1108C, 16 h. [b] The yields, based on the amount of aryl bromide used, were determined by NMR spectroscopy. [c] Yield of the isolated prod-
uct.
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X.-F. Wu, M. Beller et al.
Table 4. Palladium-catalyzed carbonylative synthesis of 2-aryl-5,6-dihy-
moving the solvent under vacuum, 1,4-dimethoxybenzene (20 mg) was
added as an internal standard. A part of the mixture was removed for
NMR analysis to determine the yield. All of the products are known
compounds.
dro-4H-1,3-oxazines.[a]
Representative purification procedure: The solution was extracted 3–5
times with ethyl acetate (2–3 mL). After evaporation of the organic sol-
vent the residue was adsorbed on silica gel and the crude product was pu-
rified by column chromatography using n-heptane/AcOEt (4:1) as the
eluent.
2-Phenyl-4,5-dihydrooxazole: 1H NMR (300 MHz, CDCl3): d=7.87–7.97
(m, 2H), 7.28–7.45 (m, 3H), 4.32 (t, 2H, J=9.3 Hz), 3.97 ppm (t, 2H, J=
9.4 Hz); 13C NMR (75 MHz, CDCl3): d=164.5, 131.2, 128.3, 128.1, 127.8,
67.5, 54.9 ppm; GC-MS (EI, 70 eV): m/z (%): 147 [M]+ (80), 117 (100),
77 (20), 51 (10).
Entry
1
Aryl bromide
Oxazine
Yield
[%][b]
77[c]
2-Phenyl-5,6-dihydro-4H-1,3-oxazine: 1H NMR (300 MHz, CDCl3): d=
7.93–8.02 (m, 2H), 7.33–7.47 (m, 3H), 4.42 (t, 2H, J=4.9 Hz), 3.49–3.59
(m, 2H), 2.03 ppm (t, 2H, J=6.4 Hz); 13C NMR (75 MHz, CDCl3): d=
166.4, 132.2, 128.6, 127.6, 127.4, 61.6, 35.8, 27.9 ppm; GC-MS (EI, 70 eV):
m/z (%): 161 [M]+ (50), 160 (50), 105 (100), 77 (45), 51 (10).
2
74
3
4
80
85
Acknowledgements
The authors thank the state of Mecklenburg–Vorpommern and the Bun-
desministerium fꢀr Bildung und Forschung (BMBF) for financial support.
We also thank Mrs. S. Leiminger, Dr. W. Baumann, and Dr. C. Fischer
(all LIKAT) for analytical support.
5
6
7
70
78
80
Keywords: aryl bromides · carbonylation · heterocycles ·
oxazolines · palladium
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Synthesis of 2-Aryloxazolines from Aryl Bromides
COMMUNICATION
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Received: July 25, 2012
Published online: && &&, 0000
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X.-F. Wu, M. Beller et al.
Heterocycle Synthesis
X.-F. Wu,* H. Neumann, S. Neumann,
M. Beller* ..................... &&&& — &&&&
Oxazoline is OK! A general and effi-
cient method for the synthesis of oxa-
zolines has been developed (see
of 27 five-membered-ring heterocycles
and 11 six-membered-ring heterocycles
in moderate to good yields.
A General and Efficient Palladium-
Catalyzed Carbonylative Synthesis of
2-Aryloxazolines and 2-Aryloxazines
from Aryl Bromides
scheme). This allowed the preparation
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ÝÝ
These are not the final page numbers!