Pd-Catalyzed regioselective C-H halogenation of quinazolinones and benzoxazinones

Article abstract of DOI:10.1039/c7ob01534h

A Pd-catalyzed ortho-selective halogenation of benzoxazinone and quinazolinone scaffolds has been described employing N-halosuccinimide as both a halogen source and an oxidant reagent via C-H bond activation. This transformation shows high chemo- and regioselectivities and demonstrates a broad range of benzoxazinone and quinazolinone substrates with different functional groups and has been scaled up to the gram level.

Full text of DOI:10.1039/c7ob01534h

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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DOI: 10.1039/C7OB01534H
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Pd-catalyzed regioselective C-H halogenation of quinazolinones
and benzoxazianones
Minoo Dabiri,* Noushin Farajinia Lehi, Siyavash Kazemi Movahed and Hamid Reza Khavasi
aReceived 00th January 20xx,
Accepted 00th January 20xx
A Pd-catalyzed ortho-selective halogenation of benzoxazinone and quinazolinone scaffolds have been described employing
N-halosuccinimide as both halogen source and oxidant reagent via C−H bond activation. This transformation shows high
chemo- and rigoselectivitiy and demonstrates a broad range of benzoxazinone and quinazolinone substrates with different
functional groups and has been scaled up to gram level.
DOI: 10.1039/x0xx00000x
www.rsc.org/
cancer, anti-inflammatory, antihypertensive, anti-diabetic,
antimicrobial, antimalarial, chymotrypsin inhibitor, HSV-1 and
serine protease inhibitors, and inhibitors of leucocyte elastase
Introduction
Recently, the development in Pd-catalyzed selective
functionalization of C–H bonds has witnessed tremendous
progress as an efficient, straightforward, valuable, and atom-
economical process to form carbon-carbon and carbon-
heteroatom bonds.1 Synthetic endeavours have concentrated
on several C–H functionalization, such as amination,2 arylation,3
cyanation,4 halogenations,5 oxygenation,6 and sulfonylation7 by
activation of both sp2 and sp3 C–H bonds. Among them, Pd-
catalyzed selective ortho-halogenation of sp2 C–H bonds5 have
attracted much interest because aromatic halides are highly
valuable starting materials for synthetic elaboration due to their
role as precursors in the benzyne generation,8 nucleophilic
aromatic substitution,9 synthesis of organometallic reagents,10
as well as transition-metal catalyzed cross-coupling reactions.11
Considerable endeavours have been made for the selective
ortho-halogenation of sp2 C–H bonds by using directing groups
(DGs), including azoxybenzenes,5d carbamates,5c-5e cyano,5j
oxime ethers,5i pyridines,5f and various N-heterocycles.5a,5b
Although in some cases, the DG needs to be pre-built-in and so
removed afterward.5g,5f,5i Several halogen sources have been
used such as CaX2,12a CuX2,12b 2,3-dichloro-5,6-dicyano-1,4-
and analgesic activities (Figure 1).13 Consequently, the structure
of benzoxazinone and quinazolinone have been an intensive
research focus in synthetic chemistry.
Figure 1. Representative examples of biologically active benzoxazinone and
quinazolinone
To the best of our knowledge, transition-metal catalyzed
ortho-C-H halogenations have not been reported for
benzoxazinone and quinazolinone scaffolds. Herein, we report
that a simple, efficient, and general, methodology for the ortho-
selective halogenations of benzoxazinone and quinazolinone
scaffolds to give the ortho-halogenated products in good yields.
benzoquinone
(DDQ),12c
and
N-halosuccinimides
(NXSs).5a,5b,5d,5e,5f,5g Commonly, this reaction needs using an
external oxidant such as Cu(OTFA)212a and Cu(OAc)2.12b Among
halogen sources, NXSs act as both halogen source and oxidant
reagent.
Results and discussion
Benzoxazinone and quinazolinone scaffolds are ubiquitous
building block in many natural products and bioactive molecules
In order to optimize the reaction conditions series of
such as bouchardatin, cetilistat, dictyoquinazol A, luotonin A, experiments with different parameters were performed for a
AX-9657, and TEI-6344, and are as well defined as an excellent typical reaction of 2-phenylquinazolin-4(3H)-one and N-
class of pharmacologically active compounds exhibiting anti- bromosuccinimide (NBS). These parameters were an additive,
catalyst, solvent, oxidant, and temperature. In Pd(OAc)2 (5
mol%), NBS (1.2 eq.) as oxidant and bromine source at 100°C in
1,2-dichloroethane (DCE), failed to provide any product (Table
1, entry 1). Inspired by the work of Nicholas,5e the effect p-
toluenesulfonic acid (p-TsOH) as an acidic additive, was tested
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that the in situ generated of an electron-deficient Pd complex.5g
The yield was enhanced by the addition 0.5 eq. of p-TsOH in the
reaction medium (Table 1, entry 2). Thus, the acidic additives,
including trifluoromethanesulfonic acid (TfOH), trifluoroacetic
acid (TFA), AcOH were tested in DCE (Table 1, entries 2−5). The
best result was obtained by using p-TsOH as an acidic additive
in the model reaction (Table 1, entry 2). The loading of the acidic
additive was optimized (Table 1, entries 6 and 7). The addition
of 1 eq. of the additive did not change in yield; however, the
yield decreased when the amount of p-TsOH was reduced to
0.25 eq. Under these conditions, the effect of solvent was
investigated on the halogenation of quinazolinone (Table 1,
entries 8–10). It was found that the choice of solvent had a
significant influence on this transformation. A higher yield was
obtained when DCE was used as a solvent (Table 1, entry 2). The
desired product was slightly obtained in p-TsOH with commonly
metal salts, including PdCl2, Cu(OAc)2 and AgOAc (Table 1,
entries 11–13). Additionally, the different types of co-oxidants
such as Na2S2O8, K2S2O8, and Cu(OAc)2 were tested and showed
less efficient and afford in low yields (Table 1, entries 14−16).
The reactions were carried out at different amounts of Pd(OAc)2
ranging from 2.5 to 15 mol% (Table 1, entries 2, 17 -19). It was
found 10 mol% of Pd(OAc)2 is sufficient to push forward this
reaction. The increasing of the loading Pd(OAc)2 to 15 mol % did
not show any effect on the yield. Also, no significant
improvement in the product yield was observed even when the
reaction temperature was increased from 100 °C to 120 °C
(Table 1, entry 20).
Having obtained the optimal reaction conditions, we set out
to explore the scope of the reaction of different 2-
arylquinazolin-4(3H)-ones. The N-chlorosuccinimide (NCS) and
N-iodosuccinimide (NIS) was used as halogen sources for the
halogenation of 2-phenylquinazolin-4(3H)-one (Table 2, entries
2 and 3). The order of reactivity of NXSs under optimal
conditions is NBS > NIS > NCS (Table 2, entries 1-3). The
bromination with 2-arylquinazolin-4(3H)-ones bearing electron-
donating and electron-withdrawing groups on the aryl ring was
done. The results demonstrated that the electronic
characteristics of substituents on the aryl ring have the
significant influence on the reaction yields (Table 2, entries 4–
6). The electron-donating group (p-Me) was more reactive
compared with the electron-withdrawing group (p-NO2) (Table
2, entries 4 and 6), which could be an attributed to the
electrophilic halogenation C–H activation to form a C–Pd
bond.14 Additionally, the reaction was chemoselective in the
case of Me-substituted quinazolinone (1b). The halogenations
of benzylic positions by NXSs were reported.5f However, here 2-
(2-halo-4-methylphenyl)quinazolin-4(3H)-ones were obtained
in a chemospecificity manner (Table 2, entries 4 and 5).
Interestingly, in meta-substituted quinazolinone (1e), the I, Br
and Cl did not displace between quinazolinone ring and the
substituent (Cl) (Table 2, entries 8-10). These results shown the
steric hindrance in halogantation.
Table 1. Optimization of the reaction conditionsa.
View Article Online
DOI: 10.1039/C7OB01534H
Entry
Catalyst
(mol%)
Solvent
Additive
(equiv.)
Yield
(%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
PdCl2 (5)
DCE
DCE
DCE
DCE
DCE
-
Trace
55
24
15
43
31
56
36
44
Trace
38
19
Trace
10
p-TsOH (0.5)
TFA (0.5)
AcOH (0.5)
TfOH (0.5)
p-TsOH (0.25)
p-TsOH (1)
DCE
DCE
1,4-dioxane
CH3CN
PhCH3
DCE
p-TsOH (0.5)
p-TsOH (0.5)
p-TsOH (0.5)
p-TsOH (0.5)
p-TsOH (0.5)
p-TsOH (0.5)
Na2S2O8 (2),
p-TsOH (0.5)
K2S2O8 (2), p-
TsOH (0.5)
Cu(OAc)2 (1)
p-TsOH (0.5)
p-TsOH (0.5)
p-TsOH (0.5)
p-TsOH (0.5)
p-TsOH (0.5)
Cu(OAc)2 (5)
AgOAc (5)
Pd(OAc)2 (5)
DCE
DCE
DCE
15
16
Pd(OAc)2 (5)
Pd(OAc)2 (5)
DCE
DCE
Trace
31
17
18
19
20c
Pd(OAc)2 (2.5)
Pd(OAc)2 (10)
Pd(OAc)2 (15)
Pd(OAc)2 (10)
DCE
DCE
DCE
DCE
23
76
76
76
a2-phenylquinazolin-4(3H)-one (0.5 mmol), NBS (0.6 mol), catalyst (X mol %), additive (X
mmol), solvent (3 mL) Temp. = 100 °C, Time = 4 h; b Isolated yield;cTemp. = 120 °C
2 | J. Name., 2012, 00, 1-3
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a quinazolinone (1 mmol), NXS (1.2 mol), Pd(OAc)2 (10 mol %), p-TsOH (0.5 mmol),
Table 2. Ortho halogenation of quinazolinonesa
View Article Online
DCE (3 mL) Temp. = 100 °C, and Time = 4 h; b Isolated yield.
DOI: 10.1039/C7OB01534H
The structure of the product 2d was confirmed
unambiguously by single crystal X-ray analysis (Figure 2).
Entry
1
Quinazolinone
NXS
NBS
Product
Yieldb
76%
Figure 2. X-ray crystal structure of 2d.
2
3
4
1a
1a
NCS
NIS
63%
79%
83%
To further explore the potential of this protocol for the
halogenation, the benzoxazinones (3) were selected as an
activated C-H compound in the halogenation (Table 3). The
desired halogenated benzoxazinones were obtained in
moderate yields. Interestingly, in chlorination and iodination of
benzoxazinones, the dihalogenated product was the major
product (Table 3, entries 2, 3, 5, and 9). The increasing amount
NCS and NIS to 3 eq. increased the yield of dihalogenated
products. Additionally, the hydrolysis of the halogenated
benzoxazinones (5a-c) were observed, which could be a
reflection on the instability of benzoxazinones in acidic medium
(Table 3, entries 4, 7, and 10).15 We studied the reaction
conditions by the varying of the solvent, temperature and time
to increase the product 4 stability (Table S1). Unfortunately,
NBS
5
6
1b
NIS
76%
75%
these efforts don’t effect in elimination this by-product (5).
Also, the reaction was chemoselective in the case of Me-
substituted benzoxazinone (3d) (Table 3, entries 7 and 8).
NBS
7
8
NBS
NBS
51%
64%
9
1e
1e
NCS
NIS
50%
55%
10
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a benzoxazinone(1 mmol), NXS (1.2 mol), Pd(OAc)2 (10 mol %), p-TsOH (0.5 mmol),
Table 3. Ortho halogenation of benzoxazinonesa
View Article Online
DCE (3 mL) Temp. = 100 °C, and Time = 4 h; b Isolated yield; c NXS (3 mol).
DOI: 10.1039/C7OB01534H
Finally, the structure of the product 4d was confirmed
unambiguously by single crystal X-ray analysis (Figure 3).
Entry
1
benzoxazinone
NXS
NBS
Product
Yieldb
61%
Figure 3. X-ray crystal structure of 4d.
2
3a
3a
NCS
45%,
87%c
To demonstrate the efficiency and practicality of this
catalytic process, a 7 mmol scale bromination reaction was
conducted using 1a as a substrate, and the reaction proceeded
smoothly to afford the halogenated product 2a in 71% isolated
yield (Scheme 1).
49%,
88%c
3
4
NIS
NBS
41%
Scheme 1. Examining scalability of the bromination of 2-phenylquinazolin-4(3H)-one
5
3b
NCS
43%,
81%c
A plausible mechanism for the transformation, based on
previous literature,[5g,5d] is described in (Figure 4). Initially, a
five-membered cyclopalladium (II) intermediate I is generated
with the assistance of the nitrogen by chelation directed C–H
activation, which is generally considered to be a better
coordinating atom than oxygen. p-TsOH as an acidic additive
might be included in the in situ generation of an electron-
deficient Pd complex, Pd(p-TsO)2, which can promote more
challenging C–H activation through weak coordination.[5g]
Oxidative addition of NXS to the cyclopalladium leads to the
formation of Pd(IV) complex II. The final step involved carbon–
halogrn bond-forming reductive elimination to give the
halogenated product and regenerate active Pd(II) species to
continue the catalytic cycle. Alternatively, the reaction may also
proceed via a bimetallic Pd(III)−Pd(III) intermediate pathway for
carbon-halogen bond-forming reactions as identified by Ritter
et al.16
6
7
NCS
NBS
54%
46%
8
9
3d
NIS
68
72
NBS
10
3e
NCS
45
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the organic layer was further washed with brVinieew AsrtoiclleuOtinolinne.
Afterward, the organic layer was sepaDraOtIe: 1d0.1a0n3d9/Cd7rOieBd015o3v4eHr
anhydrous Na2SO4 and concentrated under reduced pressure
using a rotary evaporator. Purification was accomplished using
column chromatography using silica gel as the stationary phase
(n-hexane).
Conclusions
In summary, a practical, Pd(II) catalyzed ortho-selective direct
halogenation has been developed for the synthesis of the ortho-
halogenated benzoxazinone and quinazolinone scaffolds by N-
halosuccinimide as an oxidant and halogen source. The
different NXS can be employed as useful halogenating reagents
in this transformation. Additionally, this transformation can also
be scaled up to gram level.
Figure 4. Plausible mechanism for ortho-halogenation of benzoxazinone and
quinazolinone scaffolds
Experimental
Experimental section
Acknowledgements
General
All starting materials were obtained from Merck Millipore or
Sigma-Aldrich, and were used without further purification.
Melting points were measured on an Elecrtothermal 9100
apparatus and are uncorrected. Mass spectra were recorded on
an Agilent Technology (HP) 5973 NetworkMass Selective
Detector operating at an ionization potential of 70 eV. IR
We gratefully acknowledge financial support from the Research
Council of Shahid Beheshti University and financial support
provided by the Iran National Science Foundation (INSF).
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and 13CNMR spectra were obtained in DMSO-d6 using TMS as
internal standard. Elemental analyses were performed using a
Heraeus CHN-O Rapid analyzer.
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General procedure for the halogenation of quinazolinones and
benzoxazinones
To a 10 mL single-neck round-bottom flask equipped with a
magnetic stir bar were added the quinazolinones or
benzoxazinones (1 mmol), NXS (1.2 mmol), p-TsOH.H2O (0.5
mmol) and Pd(OAc)2 (0.1 mmol,) in turn. Subsequently, the
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