Domino ring-opening/carboxamidation reactions of N-tosyl aziridines and 2-halophenols/pyridinol: Efficient synthesis of 1,4-benzo- And pyrido-oxazepinones

Article abstract of DOI:10.1021/ol902598d

"Chemical Equation Presented" A domino process is described for the synthesis of 1,4-benzo- and pyrido-oxazepinones by one-pot sequential ring-opening/carboxamidation reactions of various N-tosylaziridines with a range of 2-halophenols/pyridinol under phase-transfer catalysis.

Full text of DOI:10.1021/ol902598d

ORGANIC
LETTERS
2010
Vol. 12, No. 1
192-195
Domino Ring-Opening/Carboxamidation
Reactions of N-Tosyl Aziridines and
2-Halophenols/Pyridinol: Efficient
Synthesis of 1,4-Benzo- and
Pyrido-oxazepinones
Gagan Chouhan and Howard Alper*
Centre for Catalysis Research and InnoVation, Department of Chemistry, UniVersity of
Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada, K1N 6N5
howard.alper@uottawa.ca
Received November 9, 2009
ABSTRACT
A domino process is described for the synthesis of 1,4-benzo- and pyrido-oxazepinones by one-pot sequential ring-opening/carboxamidation
reactions of various N-tosylaziridines with a range of 2-halophenols/pyridinol under phase-transfer catalysis.
The strained three-membered ring heterocycles, aziridines,
are considered to be versatile building blocks for the
synthesis of various valuable nitrogen-containing compounds
because of their unique ability to function as carbon
electrophiles.1 Aziridines can undergo a ring-opening reaction
with various nucleophiles to provide direct access to a range
of substituted amines.2 During the last several decades,
domino reactions have attracted significant attention by many
organic chemists to prepare complex molecules, as they can
enhance the synthetic efficiency, avoid the separation of
intermediates, and reduce the amount of waste.3 Thus, the
development of new methods for the simultaneous formation
of two or more new bonds (usually C-C and C-N(O) bonds)
under the same reaction conditions in a single step is quite
advantageous since it allows the rapid buildup of molecular
complexity from relatively simple starting materials.4 Transition-
(1) (a) Yudin, A. K. Aziridines and Epoxides in Organic Synthesis;
Wiley-VCH, Verlag GmbH & Co: Weinheim, 2006. (b) Singh, G. S.;
D’Hooghe, M.; De Kimpe, N. Chem. ReV. 2007, 107, 2080. (c) Olsen, C. A.;
Franzyk, H.; Jaroszewski, J. W. Eur. J. Org. Chem. 2007, 1717. (d)
Hodgson, D. M.; Humphreys, P. G.; Hughes, S. P. Pure Appl. Chem. 2007,
79, 269. (e) Watson, I. D. G.; Yu, L.; Yudin, A. K. Acc. Chem. Res. 2006,
39, 194. (f) Padwa, A.; Murphree, S. S. ARKIVOC 2006, 3, 6. (g) Hu, X. E.
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Grigg, R.; Kilner, C.; Dumronchai, N.; Kongkathip, B.; Kongkathip, N.;
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10.1021/ol902598d  2010 American Chemical Society
Published on Web 12/08/2009

metal-catalyzed reactions, particularly palladium-catalyzed reac-
tions, are a valuable synthetic tool for achieving this
goal.5,6
In the context of our research interest on the chemistry of
aziridines7 and application of Pd-catalyzed domino processes
for the preparation of various heterocycles,8 we report a new
domino ring-opening/carboxamidation reaction of N-tosy-
laziridines and 2-halophenols/pyridinol under phase-transfer
catalysis to give 1,4-benzo- and pyrido-oxazepinones in fine
yields (Figure 1).9-12 It is well-known that many compounds
at 300 psi of carbon monoxide in the presence of Pd(OAc)2
(0.03 mmol), triphenylphosphine (PPh3) (0.03 mmol), and
Cs2CO3 (3 equiv) in THF at 110 °C for 24 h. The reaction
afforded 60% yield of the desired 1,4-benzoxazepinone 3a
(Table 1, entry 1). No significant improvement in the yield
Table 1. Optimization of the Domino Reaction Conditions for
the Reaction of N-Tosylaziridine of Cyclohexene 1a with
2-Iodophenol 2aa
catalyst/phosphine
(mol %)
CO
(psi)
T
3a
entry
base
(°C) (%)b
1
2
3
4
5
6
Pd(OAc)2 (3)/PPh3 (3)
Pd(OAc)2 (3)/PPh3 (3)
PdCl2(PPh3)2 (3)/PPh3 (3)
PdCl2(PPh3)2 (1.5)/PPh3 (1.5) 200 K2CO3
PdCl2(PPh3)2 (1.5)/PPh3 (1.5) 200 K2CO3
PdCl2(PPh3)2 (1.5)/PPh3 (1.5) 200 K2CO3
300 Cs2CO3 110
60
63
65
66
300 Cs2CO3
200 Cs2CO3
80
80
80
Figure 1. Strategy for the domino ring-opening/carboxamidation
reaction.
80 61c
80 76d
a Reaction conditions: 1a (1 mmol), 2a (1.1 mmol), THF (5 mL), 24 h.
b Isolated yield. c Reaction was carried out in dioxane. d With phase-transfer
catalyst benzyltriethylammonium chloride (TEBA, 10 mol %).
possessing benzo- and pyrido-oxazepinone scaffolds display
biological activity of medicinal interest.13
Initially, the domino ring-opening/carboxamidation reac-
tions were examined by reacting the N-tosylaziridine of
cyclohexene 1a (1 mmol) with 2-iodophenol 2a (1.1 mmol)
of the product 3a was observed when the nature of the Pd
catalyst was changed, and the reaction was carried out at
low pressure of carbon monoxide and lower temperature
(Table 1, entries 2 and 3). The change of the base from
Cs2CO3 to K2CO3 provided 66% yield of the desired 1,4-
benzoxazepinone 3a (Table 1, entry 4). As solvent can have
a significant effect on the reaction, we have further studied
this domino reaction in the more polar solvent dioxane.
However, change of the solvent proved less efficient and
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Org. Lett., Vol. 12, No. 1, 2010
193

benzoxazepinones on domino reaction with N-tosylaziridines
1a and 2a (Table 2, products 3b, 3f, 3i, and 3m). The domino
reaction of N-tosylaziridines 1a,b with 2-iodopyridinol also
afforded the corresponding 1,4-pyridoxazepinones in good
yield (Table 2, products 3g and 3n).
Table 2. Domino Ring-Opening/Carboxamidation Reaction of
Cyclic N-Tosylaziridines 1a-d with 2-Halophenols/Pyridinol
2a-ja
It is interesting to note that the domino reaction of seven-
membered ring aziridines 1c with 2-iodophenol 2a does not
provide the desired 1,4-benzoxazepinone under the optimized
reaction conditions. However, when the reaction was carried
out at 110 °C with 400 psi carbon monoxide pressure in
DMF for 24 h, the desired product was isolated in 72% yield
(Table 2, product 3o). The domino reaction also works well
with indene N-tosylaziridine 1d (Table 2, product 3p).
Apart from the cyclic N-tosylaziridines, various acyclic
N-tosylaziridines 4a-d were also examined for this domino
ring-opening/carboxamidation reaction, and the results are
listed in Table 3. A variety of N-(p-toluenesulfonyl)-2-
benzylaziridines 4a-c were reacted under the optimized
reaction conditions with different 2-iodophenols/pyridinol to
afford the corresponding 1,4-benzo- and pyrido-oxazepinones
in 64-91% isolated yields (Table 3). The reaction is
completely regioselective and provides only one regioisomer.
The domino reaction with N-(p-toluenesulfonyl)-2,3-diethy-
laziridine required higher temperature (110 °C), CO pressure
(400 psi), and polar solvent DMF to afford 1,4-benzoxaze-
pinone in moderate yield (Table 3, product 5j).
a Reaction conditions: 1a-d (1.0 mmol), 2a-j (1.1 mmol), PdCl2(PPh3)2
(1.5 mol %), triphenylphosphine (1.5 mol %), TEBA (10 mol %), K2CO3
(3 equiv), THF (5 mL), 80 °C, 24 h. b Isolated yield with 2-bromophenols.
Reaction conditions: N-tosylaziridines (1.0 mmol), 2-bromophenols (1.1
mmol), PdCl2(PPh3)2 (3 mol %), dppf (6 mol %), 18-crown-6 (10 mol %),
K2CO3 (3 equiv), DMF (5 mL), 130 °C, 48 h. c Reaction was carried out
in DMF with 400 psi CO pressure at 110 °C, 24 h.
Table 3. Domino Ring-Opening/Carboxamidation Reaction of
Acyclic N-Tosylaziridines 4a-d with 2-Halophenols/Pyridinol
2a-fa
did not increase the isolated yield of the desired product
(Table 1, entry 5). The yield of 3a increased to 76% when
the phase-transfer catalyst, benzyltriethylammonium chloride
(TEBA, 10 mol %), was used (Table 1, entry 6).14
The stereochemistry of the product was determined by
measuring the coupling constant of the methine proton (Ha)
and by 1D NOE and the NOESY NMR spectra of the product
3a. Trans stereochemistry was determined by the coupling
constant of the proton Ha at 3.71 ppm (ddd, 1H, J ) 11.53,
11.50, and,3.35 Hz) and further confirmed by 1D NOE and
NOESY spectra as no NOE effects were detected between
Ha and Hb (see Supporting Information).
Under the optimized reaction conditions, the scope of this
reaction was further examined by treating different N-
tosylaziridines with a variety of 2-halophenols/pyridinol. The
results are summarized in Table 2. The reaction of the
N-tosylaziridine of cyclohexene 1a with 2-iodophenol bearing
electron-donating (Table 2, products 3c and 3d) and electron-
withdrawing (Table 2, product 3e) groups provides lower
yields of the desired 1,4-benzoxazepinones. Similar results
were obtained with N-tosylaziridines of cyclopentene 1b on
reacting with 2-iodophenols 2c-e (Table 2, products 3j-l).
In contrast, 2-iodophenols bearing phenyl and methyl ester
groups provide higher yields of the corresponding 1,4-
a Reaction conditions: 1a-d (1 mmol), 2a-j (1.1 mmol), PdCl2(PPh3)2
(1.5 mol %), triphenylphosphine (1.5 mol %), TEBA (10 mol %), K2CO3
(3 equiv), THF (5 mL), 80 °C, 24 h. b Isolated yield with 2-bromophenols.
Reaction conditions: N-tosylaziridines (1 mmol), 2-bromophenols (1.1
mmol), PdCl2(PPh3)2 (3 mol %), dppf (6 mol %), 18-crown-6 (10 mol %),
K2CO3 (3 equiv), DMF (5 mL), 130 °C, 48 h. c Reaction was carried out
in DMF with 400 psi CO pressure at 110 °C, 24 h.
This domino reaction was next studied with 2-bromophe-
nol. Thus, N-tosylaziridine of cyclohexene 1a was reacted
with 2-bromophenol 2h under the optimized reaction condi-
(14) (a) Albanese, D.; Landini, D.; Penso, M. Tetrahedron 1997, 53,
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194
Org. Lett., Vol. 12, No. 1, 2010

these conditions, the starting material was converted to 1,4-
benzoxazepinone 3a in 71% isolated yield (Table 2, product
3a). Several other 2-bromophenols were used for this type
of domino reaction with various N-tosylaziridines under the
above-mentioned optimized reaction conditions to give
desired 1,4-benzoxazepinones in good to excellent yields
(Table 2, products 3e and 3f, Table 3, products 5a and 5g).
A possible reaction mechanism for the formation of benzo-
and pyrido-oxazepinones 3 or 5 is shown in Figure 2. First,
the base-catalyzed ring-opening of N-tosylaziridines 1 or 4
with 2-halophenols/pyridinol 2 under phase-transfer catalysis
condition generates the amine 6. The oxidative addition of
6 to the in situ generated palladium(0) species17 leads to a
palladium complex 7. Insertion of carbon monoxide into the
aryl carbon-palladium bond of 7 affords 8, and nucleophilic
attack of the tosyl-amine on an aroylpalladium complex gives
intermediate 9. The latter undergoes reductive elimination
affording 1,4-benzo- and pyrido-oxazepinones 3 or 5 with
regeneration of palladium(0).
In summary, we have developed a new efficient synthetic
method for the synthesis of 1,4-benzo- and pyrido-oxazepi-
nones via a domino process through one-pot ring-opening/
carboxamidation reaction sequences of N-tosylaziridines with
2-halophenols/pyridinol under phase-transfer conditions. The
method works efficiently with a range of N-tosylaziridines
and 2-halophenols/pyridinol to provide facile access to a
variety of 1,4-benzo- and pyrido-oxazepinones.
Figure 2. Proposed reaction mechanism.
tions; however, none of the desired product was obtained.
Increasing the reaction temperature (110 °C) or CO pressure
(300 psi) or using a different phosphine ligand (dppp, PtBu3)
with toluene as the solvent was also unsuccessful. After some
experimentation, we found that this type of domino reactions
works well by employing PdCl2(PPh3)2 (3 mol %) as the Pd
catalyst, bisdiphenylphosphino ferrocene (dppf, 6 mol %),
K2CO3 (3 equiv) as the base, and 18-crown-6 as the PTC at
400 psi CO pressure in DMF at 130 °C for 48 h.15,16 Under
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada for providing
financial support for this research.
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copies of 1H NMR and 13C NMR spectra for substrates and
products. This material is available free of charge via the
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