Palladium-catalyzed cross coupling reactions of 4-bromo-6H-1,2-oxazines

Article abstract of DOI:10.3762/bjoc.5.44

A number of 4-aryl- and 4-alkynyl-substituted 6H-1,2-oxazines 8 and 9 have been prepared in good yields via cross coupling reactions of halogenated precursors 2, which in turn are easily accessible by bromination of 6H-1,2-oxazines 1. Lewis-acid promoted

Full text of DOI:10.3762/bjoc.5.44

Palladium-catalyzed cross coupling reactions of
4-bromo-6H-1,2-oxazines
Reinhold Zimmer*,1, Elmar Schmidt2, Michal Andrä1, Marcel-Antoine Duhs1,
Igor Linder1 and Hans-Ulrich Reissig*,1
Preliminary Communication
Open Access
Address:
Beilstein Journal of Organic Chemistry 2009, 5, No. 44.
1Freie Universität Berlin, Institut für Chemie und Biochemie,
Takustrasse 3, D-14195 Berlin, Germany and 2Technische Universität
Dresden, Institut für Organische Chemie, D-01061 Dresden, Germany
doi:10.3762/bjoc.5.44
Received: 04 June 2009
Accepted: 28 August 2009
Published: 16 September 2009
Email:
Reinhold Zimmer* - rzimmer@chemie.fu-berlin.de;
Hans-Ulrich Reissig* - hans.reissig@chemie.fu-berlin.de
Associate Editor: I. Marek
* Corresponding author
© 2009 Zimmer et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
alkyne; halogenation; 1,2-oxazines; palladium catalysis; pyridines
Abstract
A number of 4-aryl- and 4-alkynyl-substituted 6H-1,2-oxazines 8 and 9 have been prepared in good yields via cross coupling reac-
tions of halogenated precursors 2, which in turn are easily accessible by bromination of 6H-1,2-oxazines 1. Lewis-acid promoted
reaction of 1,2-oxazine 9c with 1-hexyne provided alkynyl-substituted pyridine derivative 12 thus demonstrating the potential of
this approach for the synthesis of pyridines.
Introduction
A broad range of synthetic applications demonstrates that 1,2- 6H-1,2-oxazines 1 and the use of the resulting products as
oxazine derivatives constitute a versatile class of N,O hetero- precursors in palladium-catalyzed cross coupling reactions.
cycles [1-13]. Considerable attention has been paid to 6H-1,2-
oxazines 1 bearing a C-4,C-5-double bond [14-18], which are Results and Discussion
useful intermediates in the synthesis of γ-lactams [19], γ-amino Not much is known about halogenated 6H-1,2-oxazines and
acids [20], amino alcohols [20], aziridines [21], pyrrolizidines only a few mostly inefficient procedures are described [28-32].
[22], and pyrrolidine derivatives [15,23,24]. In the context of This prompted us to investigate a more practical access to halo-
our ongoing exploration of the synthetic potential of these genated 6H-1,2-oxazines. Gratifyingly, the desired 4-bromo-
heterocycles we were interested to modify the substitution substituted 6H-1,2-oxazines 2a–2c could be prepared in a one-
pattern of the C-4,C-5 double bond of 6H-1,2-oxazines [25-27]. pot procedure by bromine addition to precursors 1a–1c [14] and
Herein, we describe our results dealing with the halogenation of HBr elimination by treatment with triethylamine (Scheme 1).
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Beilstein Journal of Organic Chemistry 2009, 5, No. 44.
The 4-bromo-6H-1,2-oxazines were obtained in reasonable to With the 4-halogenated 6H-1,2-oxazines 2 and 6 in hand, palla-
good yields. The bromination of 3-phenyl-substituted 6H-1,2- dium-catalyzed cross couplings offer an efficient and useful
oxazine 1a often resulted in a mixture of several brominated approach for the synthesis of novel functionalized 6H-1,2-
products which are easily separable by chromatography. oxazines. The Suzuki-coupling of the 4-bromo-substituted
Depending on the reaction scale and the amount of bromine heterocycles 2a,b with phenylboronic acid in the presence of
used (1.5 to 3 equiv) by-products such as 3a, 4 and 5 could be Pd(PPh3)4 and sodium carbonate at 80 °C in toluene gave the
isolated in varying yields. The unexpected formation of 4,5- expected 4-phenyl-substituted 6H-1,2-oxazines 8a or 8b in 82
dibromo-6H-1,2-oxazine 3a can obviously be rationalized by and 77% yield (Scheme 3).
addition of bromine to 2a and elimination of HBr during the
bromination reaction of 1a.
Scheme 3: Suzuki-couplings of 4-bromo-6H-1,2-oxazines. a)
ArB(OH)2, Pd(PPh3)4, Na2CO3, toluene, 80 °C, 3 h.
4-Bromo-6H-1,2-oxazine 2a also serves as suitable model
substrate for Sonogashira-reactions (Scheme 4). When the
coupling reaction of 2a with various terminal alkynes, such as
phenylacetylene, trimethylsilylethyne and 1-hexyne, was
Scheme 1: Brominations of 6H-1,2-oxazines. a) Br2, Et2O, −30 °C,
2 h. b) Et3N, −30 °C to r.t., overnight.
performed under typical conditions [PdCl2(PPh3)2, CuI, Et3N,
toluene], the expected 4-alkynyl-substituted heterocycles 9a–9c
were isolated in good yields. In contrast, when the same reac-
The literature describes just one related 4-chloro-substituted tion conditions were applied to the coupling of 2a and methyl
6H-1,2-oxazine which was prepared by a hetero-Diels–Alder propargyl ether, product 9d was obtained only in very low
cycloaddition–elimination sequence of 2-chloro-1-nitroso-1- yield. In addition, Sonogashira coupling of 2a and methyl
phenyl-ethene and 1-bromo-2-ethoxyethene in low yield (22%) propargyl ether performed by an alternative protocol
[32]. As demonstrated in Scheme 2, a more efficient approach (Pd(OAc)2, CuI, PPh3, NHiPr2 in DMF) afforded the expected
consists in chlorination of 6H-1,2-oxazines 1a,b by addition of product 9d and a byproduct bearing a 4-enyne moiety at 4-posi-
chlorine and subsequent base-induced dehydrochlorination. The tion. This indicates an addition of a second alkyne molecule to
expected 4-chloro-6H-1,2-oxazines 6a,b were obtained in good the primary product 9. Similar results were observed for the
yields. In analogy to the aforementioned bromination, the chlor- Sonogashira reaction of 2a with propargylic alcohol [33].
ination of 3-phenyl-6H-1,2-oxazine 1a also led to dihalogena-
tion furnishing 4,5-dichloro-substituted compound 7a as a
by-product in 13% yield.
Scheme 2: Chlorinations of 6H-1,2-oxazines. a) Cl2, Et2O, −30 °C. b)
Scheme 4: Sonogashira-couplings of 4-bromo-6H-1,2-oxazines. a)
Et3N, −30 °C to r.t.
PdCl2(PPh3)2, CuI, Et3N, toluene, r.t., 6–20 h.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 44.
Experimental
Bromination of 6H-1,2-oxazine 1a, typical
procedure
After successful simple cross couplings of mono-halogenated 2,
the 4,5-dibromo-3-phenyl-6H-1,2-oxazine 3a seemed to be an
attractive candidate for a twofold Sonogashira reaction (Scheme
5). Treatment of 3a with an excess of phenylacetylene under
conditions as described in Scheme 4 provided 5-bromo-4-
alkynyl-substituted 6H-1,2-oxazine 10a as single product in
65% yield. When the Sonogashira coupling was performed with
trimethylsilylethyne under the same reaction conditions an
inseparable 85:15-mixture of mono-alkynylated product 10b
and bis-alkynylated compound 11b was obtained in reasonable
yield. These reactions certainly deserve further optimization,
however, they already show the potential of compounds such as
3a to serve as precursors for two subsequent coupling reactions.
6H-1,2-Oxazine 1a (5.35 g, 26.3 mmol) was dissolved in
diethyl ether (200 mL) and treated with bromine (2.75 mL, 53.7
mmol) at −30 °C under argon atmosphere. After 2 h Et3N (54.0
mL, 390 mmol) was added. The reaction mixture was warmed
to r.t. overnight and quenched with water (100 mL). The
aqueous phase was extracted with CH2Cl2 (2 × 50 mL) and the
combined organic phases were dried with Na2SO4. Purification
of the crude product by column chromatography (SiO2,
hexane:EtOAc 8:1, then 4:1) gave the 4-bromo-substituted
6H-1,2-oxazine 2a (5.11 g, 69%), the 4,5-dibromo-substituted
by-product 3a (0.821 g, 9%), and starting material 1a (0.335 g,
6%).
4-Bromo-6-ethoxy-3-phenyl-6H-1,2-oxazine (2a):
yellow–brown oil. 1H NMR (CDCl3, 300 MHz): δ = 1.23 (t, J =
7.1 Hz, 3 H, CH3), AB part of ABX3 system (δA = 3.68, δB =
3.95, JAX = JBX = 7.1 Hz, JAB = 9.8 Hz, 2 H, OCH2), 5.58 (d, J
= 5.2 Hz, 1 H, 6-H), 6.70 (d, J = 5.2 Hz, 1 H, 5-H), 7.35–7.50,
7.50–7.60 (2 m, 3 H, 2 H, Ph) ppm. 13C NMR (CDCl3, 75.5
MHz): δ = 14.8 (q, CH3), 64.3 (t, OCH2), 94.7 (d, C-6), 112.9
(s, C-4), 127.8, 128.0, 128.8, 129.7, 133.1 (4 d, s, Ph, C-5),
156.2 (s, C-3) ppm. For the complete characterization, see ref.
[31].
Scheme 5: Sonogashira-couplings of 4,5-dibromo-6H-1,2-oxazines. a)
PdCl2(PPh3)2, CuI, Et3N, toluene, r.t., 4 h to overnight.
Conclusion and Perspective
In conclusion, we have successfully demonstrated that a series 4,5-Dibromo-6-ethoxy-3-phenyl-6H-1,2-oxazine (3a): brown
of 4-aryl- and 4-alkynyl-substituted 6H-1,2-oxazines 8, 9, and oil. 1H NMR (CDCl3, 300 MHz): δ = 1.25 (t, J = 7.1 Hz, 3 H,
10 are easily accessible in short reaction sequences starting CH3), AB part of ABX3 system (δA = 3.76, δB = 3.97, JAX =
from precursors 1. These 6H-1,2-oxazines should allow access JBX = 7.1 Hz, JAB = 9.7 Hz, 2 H, OCH2), 5.71 (s, 1 H, 6-H),
to many interesting five- and six-membered heterocycles. As 7.38–7.48, 7.49–7.56 (2 m, 3 H, 2 H, Ph) ppm. 13C NMR
illustrated in Scheme 6, the 4-hex-1-ynyl-3-phenyl-6H-1,2- (CDCl3, 75.5 MHz): δ = 14.7 (q, CH3), 65.0 (t, OCH2), 99.8 (d,
oxazine 9c can be converted into the trisubstituted pyridine C-6), 114.4, 124.8 (2 s, C-4, C-5), 128.1, 128.9, 129.9, 133.3 (3
derivative 12 by treatment of 9c with boron trifluoride etherate d, s, Ph), 155.9 (s, C-3) ppm. IR (neat): 3065–2900 (=C–H,
in the presence of an excess of 1-hexyne via an azapyrylium C–H), 1630 (C=N), 1600 (C=C) cm−1. HRMS (80 eV, 40 °C)
intermediate [34,35]. Additional investigations are required to m/z calcd for C12H1179Br2NO2: 358.9157; found: 358.9160.
optimize the preparation diynes of type 11. Conversion of the
Chlorination of 6H-1,2-oxazine 1b, typical
new functionalized 6H-1,2-oxazines to highly substituted
pyridine derivatives will also be reported in due course. procedure
Chlorine gas was passed into diethyl ether (28 mL) at −30 °C
until the solution became dark yellow. Then, 6H-1,2-oxazine 1b
(0.200 g, 1.00 mmol) was added and the reaction mixture was
monitored by TLC; upon complete consumption, triethylamine
(2.00 mL, 27.8 mmol) was added at −30 °C and the mixture was
slowly warmed to r.t. After addition of brine, the phases were
separated, the aqueous phase was extracted with CH2Cl2 (2 ×
20 mL) and the combined organic phases were dried with
Na2SO4. Column chromatography (SiO2, hexane,
hexane:EtOAc 9:1, then 4:1) afforded the 4-chloro-substituted
product 6b (0.182 g, 78%) as pale–yellow oil.
Scheme 6: Preparation of trisubstituted pyridine derivatives: a)
BF3·OEt2, CH2Cl2, −78 °C to r.t., overnight.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 44.
Ethyl 4-chloro-6-ethoxy-6H-1,2-oxazine-3-carboxylate (6b): 1H EtOAc 20:1, then 4:1) afforded the 4-alkynyl-substituted
NMR (CDCl3, 300 MHz): δ = 1.22 (t, J = 7.1 Hz, 3 H, CH3), 6H-1,2-oxazine 9b (0.711 g, 74%) as a colorless oil.
1.40 (t, J = 7.2 Hz, 3 H, CH3), AB part of ABX3 system (δA =
3.68, δB = 3.95, JAX = JBX = 7.1 Hz, JAB = 9.6 Hz, 2 H, 6-Ethoxy-3-phenyl-4-(trimethylsilylethynyl)-6H-1,2-oxazine
OCH2), 4.40 (q, J = 7.2 Hz, 2 H, OCH2), 5.72 (d, J = 5.0 Hz, 1 (9b): 1H NMR (CDCl3, 250 MHz): δ = 0.71 (s, 9 H, SiMe3),
H, 6-H), 6.34 (d, J = 5.0 Hz, 1 H, 5-H) ppm. 13C NMR (CDCl3, 1.21 (t, J = 7.1 Hz, 3 H, CH3), AB part of ABX3 system (δA =
75.5 MHz): δ = 13.9, 14.7 (2 q, CH3), 62.5, 64.6 (2 t, OCH2), 3.68, δB = 3.96, JAX = JBX = 7.1 Hz, JAB = 9.7 Hz, 2 H,
95.3 (d, C-6), 121.1 (s, C-4), 122.7 (d, C-5), 148.5 (s, C-3), OCH2), 5.62 (d, J = 5.1 Hz, 1 H, 6-H), 5.69 (d, J = 5.1 Hz, 1 H,
160.3 (s, C=O) ppm. IR (neat): 3105–2975 (=C–H, C–H), 1745 5-H), 7.34–7.44, 7.67–7.73 (2 m, 3 H, 2 H, Ph) ppm. 13C NMR
(C=O), 1615 (C=N) cm−1. C9H12ClNO4 (233.7): calcd. C, (CDCl3, 125.8 MHz): δ = −0.7 (q, SiMe3), 14.9 (q, CH3), 64.2
46.27; H, 5.18; N, 5.99; found: C, 46.35; H, 5.16; N, 6.08. (t, OCH2), 92.0 (d, C-6), 99.5, 101.9 (2 s, C≡C), 114.0 (s, C-4),
127.7, 128.7, 129.5, 130.1, 132.9 (4 d, s, Ph, C-5), 155.5 (s,
Suzuki-coupling of 4-bromo-substituted
6H-1,2-oxazine 2a, typical procedure
C-3) ppm. IR (neat): 3085–2900 (=C–H, C–H), 2160 (C≡C),
1620 (C=C), 1580 (C=N) cm−1. C17H21NO2Si (299.5): calcd.
6H-1,2-Oxazine 2a (0.0935 g, 0.33 mmol), phenylboronic acid C, 68.19; H, 7.07; N, 4.68; found: C, 68.17; H, 7.08; N, 4.74.
(0.122 g, 1.00 mmol) and Pd(PPh3)4 (0.016 g, 0.0138 mmol)
were dissolved in a mixture of toluene/MeOH (3 mL/0.75 mL) Acknowledgments
in a heat-gun-dried and argon-flushed flask. A 2M Na2CO3 Generous support by the Deutsche Forschungsgemeinschaft
solution (1.5 mL) was finally added and the reaction mixture (Graduiertenkolleg at the Technische Universität Dresden
was heated for 15 h at 80 °C. Then, the reaction mixture was “Struktur–Eigenschafts-Beziehungen bei Heterocyclen”), the
cooled to r.t. and washed with 2M Na2CO3 (with 1% NH3) Fonds der Chemischen Industrie and the Bayer–Schering
solution. After separation of the phases, the aqueous phase was Pharma AG is most gratefully acknowledged. We also thank
extracted with CH2Cl2 (3 × 5 mL) and the combined organic Luise Schefzig and Ute Hain for their experimental assistance.
phases were dried with Na2SO4. The crude product was puri-
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