Quinazoline synthesis via Rh(III)-catalyzed intermolecular C-H functionalization of benzimidates with dioxazolones

Article abstract of DOI:10.1021/acs.orglett.6b00691

An efficient double C-N bond formation sequence to prepare highly substituted quinazolines utilizing benzimidates and dioxazolones under the catalytic redox-neutral [Cp?RhCl₂]2/AgBF₄ system, where dioxazolones could work as an intern

Full text of DOI:10.1021/acs.orglett.6b00691

Letter
pubs.acs.org/OrgLett
Quinazoline Synthesis via Rh(III)-Catalyzed Intermolecular C−H
Functionalization of Benzimidates with Dioxazolones
Jie Wang, Shanke Zha, Kehao Chen, Feifei Zhang, Chao Song, and Jin Zhu*
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of
Coordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China
S
* Supporting Information
ABSTRACT: An efficient double C−N bond formation sequence to
prepare highly substituted quinazolines utilizing benzimidates and
dioxazolones under the catalytic redox-neutral [Cp*RhCl2]2/AgBF4
system, where dioxazolones could work as an internal oxidant to
maintain the catalytic cycle, is reported. N-Unsubstituted imine not
only acts as a directing group but also functions as a nucleophile in
postcoupling cyclization, and dioxazolone acts as a coupling partner
for access to heterocycle.
he quinazoline moiety, an abundant core of nitrogen-
containing heterocycles, is exemplified as a privileged
structure that exhibits diverse biological and therapeutic activities
such as antibacterial,1 antiviral,2 anticonvulsant,3 and anticancer
activities.4 For example (Scheme 1), erlotinib (Tarceva)5 and
Transition-metal-catalyzed C−H functionalization promises a
shortened synthetic sequence and has recently been explored for
building desired valuable C−C/C−hetero bonds or heterocycle
skeletons.11 However, due to the ubiquity of C−H bonds in
organic molecules, the presence of a nearby directing group
(DG) is usually required in order to direct positioning of a metal
catalyst so that specific C−H bond activation occurs. Based on
this strategy, while access to heterocycles (such as isoquino-
lones,12 isoquinoline,13 indole,14 and others15) has been
extensively explored in Pd-, Rh-, Ir-, Co-, and Ru-catalyzed C−
H activation, examples of quinazoline synthesis via a C−H
activation pathway are rarely reported. To date, only a few
examples16 of Pd-catalysis, starting from benzamidine, were
reported to synthesize quinazolines through intramolecular C−
H activation/cyclization. Moreover, there are several limitations,
including relatively harsh reaction conditions and/or the
requirement of external oxidants for unactivated precursors to
maintain the catalytic cycle, resulting in the generation of
undesired waste byproduct and off-cycle side reaction. In
addition, the substrates are limited to benzamidine derivatives
that suffered from not only regioselectivity but also substitution
pattern issues; thus, synthetic applications to complex molecules
might not be feasible.
To achieve the catalytic process for the synthesis of
quinazolines that is tolerant of a diversity of functional groups
and proceeds under milder reaction conditions, our attention has
been drawn to the potential chemical reactivity of readily
available benzimidate derivatives,17 which not only act as a
directing group but also work as a nucleophile in postcoupling
cyclization. Recently, dioxazolones as an amidation reagent have
played an important role in C−H activation for the synthesis of
amide derivatives.18 However, the generated amide further
undergoes intramolecular cyclization nucleophilic addition to
T
Scheme 1. Quinazoline and Popular Drugs Containing the
Quinazoline Unit
gefitinib (Iressa)6 are well-known drugs used for the treatment of
lung cancer. Prazosin7 is utilized as α-adrenergic blockers for the
treatment of high blood pressure, anxiety, and panic disorder,
and Rutaecarpine8 is one of the main active alkaloids which has
long been used to treat gastrointestinal disorders, anti-
inflammatory, headaches, and postpartum hemorrhage. Con-
sequently, increasing efforts have been devoted to developing
useful preparation methods for the generation of quinazolines,
which mainly involve (1) oxidative coupling of anilines bearing
an ortho-functional group with different carbon and nitrogen
sources;9 (2) 2-halobenzenes or 2-halobenzyl amination-
involved cyclization.10 Despite these contributions, many of
them require the addition of excess oxidants and the use of
special prefunctionalized reagents, which could result in
preparation difficulties. Therefore, the development of effective
new methods for the facile construction of quinazolines is highly
desirable.
Received: March 10, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00691
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ab
,
CO and then dehydration to furnish a heterocycle, which has
never been reported. Herein, we report the preliminary result of
an efficient double C−N bond formation sequence to prepare
highly substituted quinazolines utilizing benzimidates and
dioxazolones under the catalytic redox-neutral [Cp*RhCl2]2/
AgBF4 system, where dioxazolone could work as an internal
oxidant to maintain the catalytic cycle meanwhile as a coupling
partner for access to the heterocycle.
Scheme 2. Reaction Scope for Benzimidates
We initiated our studies with the screening of reaction
conditions in the coupling of methyl phenylimidate (1a) with 3-
phenyl-1,4,2-dioxazol-5-one (2a) (Table 1). Using the rhodium
a b
,
Table 1. Optimization of Reaction Conditions
a
Reaction conditions: 1a−1p (0.4 mmol), 2a (0.6 mmol),
yield
b
b
[Cp*RhCl2]2 (2 mol %), AgBF4 (8 mol %), DCE (2 mL). Isolated
entry
catalyst system
solvent
DCE
additive
(%)
yields.
1
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgOAc
[RhCp*Cl2]2/AgCO2CF3
[RhCp*Cl2]2/AgBF4
[RhCp*Cl2]2/AgNTf2
[RhCp*Cl2]2/AgPF6
AgBF4
82
54
46
35
61
66
72
95
80
83
0
2
CH3CN
THF
3
First, the impact of an alkyl in the alkoxy group was investigated.
We found that alkoxy groups with different sizes reacted to form
the desired products 3a−3c in 93−95% yields. Furthermore,
various methyl arylimidates were applied in this reaction. A range
of quinazolines were formed in good to excellent yields. It was
found that the reaction was sensitive to the steric demand of the
arenes. All m-substituted imidate (1k−1m) derivatives showed
excellent selectivity to the C−H bond with less steric hindrance.
The imidate with o-F (1n) also afforded the desired product in
74% yield. Various imidates having substituents at the para-
position with electron-donating substituents (e.g., Me, OMe)
reacted to form the desired products 3g−3h in 92%−96% yields.
Probably because of the electrophilic C−H activation process,
substrates with strong electron-withdrawing groups (e.g., CF3
and CO2Me) at the same position inhibited the reaction,
affording products 3i−3j in slightly lower respective yields of
75%−78%. It is noteworthy that the halo-substituted (e.g., F, Cl,
and Br) substrates performed well to afford the corresponding
products (3d−3f) in good yields. Besides, the methyl thiophene-
2-carbimidate (1o) and methyl 2-naphthimidate (1p) also
showed good reactivity in this reaction to give the excellent
regioselectivity products 3o and 3p in good yields (70% and
66%).
Furthermore, we investigated the scope of dioxazolone
derivatives (2b−2v) with methyl arylimidates (1a) as the
reaction partner (Scheme 3). 3-Phenyl-1,4,2-dioxazol-5-one
reagents bearing different electron-donating and -withdrawing
groups at different positions of the phenyl ring all coupled
smoothly with 1a. Furthermore, the 3-substituent in the 1,4,2-
dioxazol-5-one reagent is not limited to a phenyl ring, as 3-(furan
or thiophen-2-yl)-1,4,2-dioxazol-5-one also coupled to afford the
products 4r and 4s in 84% and 87% yields. Meanwhile, 3-alkyl
substituted reagents were also investigated and we were pleased
to observe that a number of 1,4,2-dioxazol-5-ones having alkyl
substituents at the 3-position were highly facile to afford 2-alky-
quinazoline products (4t−4v) in good yields.
4
1,4-dioxane
MeOH
DCE
5
6
7
DCE
8
DCE
9
DCE
10
11
12
13
14
15
16
17
DCE
DCE
[RhCp*Cl2]2
DCE
0
[RhCp*Cl2]2/AgBF4
[RhCp*Cl2]2/AgBF4
[RhCp*Cl2]2/AgBF4
DCE
NaOAc
CsOAc
HOAc
90
92
80
0
DCE
DCE
[(p-Cymene)RuCl2]2/AgBF4 DCE
[Cp*lrCl2]2/AgBF4
[RhCp*Cl2]2/AgBF4
[RhCp*Cl2]2/AgBF4
[RhCp*Cl2]2/AgBF4
DCE
DCE
DCE
DCE
42
75
6
c
18
d
19
20e
0
a
Reaction conditions: 1a (0.4 mmol), 2a (0.6 mmol), [RhCp*Cl2]2 (2
b
mol %), silver salt (8 mol %), solvent (2 mL). Isolated yields.
c
d
[RhCp*Cl2]2 (1 mol %), AgBF4 (4 mol %). [RhCp*Cl2]2 (0.5 mol
e
%), AgBF4 (2 mol %). [RhCp*Cl2]2 (0.1 mol %), AgBF4 (0.4 mol %).
complex, derived from [Cp*RhCl2]2 and AgSbF6 as a catalyst,
coupling occurred in DCE (entry 1) at 50 °C for 5 h to afford the
product 4-methoxy-2-phenylquinazoline 3a in 82% yield.
Screening of other solvents showed that DCE was the best
solvent in this transformation (compare entries 2−5). Afterward,
the effect of silver salt was investigated (compare entries 6−10),
and AgBF4 gave the best result, as the yield of 3a increased to
95% (entry 8). The control experiment uncovered that no target
product was obtained in the absence of [Cp*RhCl2]2 or AgBF4
(entries 11 and 12). Several additives were screened (entries 13−
15), but yields were slightly reduced compared to the yield in the
absence of an additive. Poor or no conversion was observed when
[Cp*RhCl2]2 was replaced by [(p-cymene)RuCl2]2 or
[Cp*IrCl2]2 catalysts (entries 16 and 17). Lower catalyst
loadings (entries 18−20) gave lower yields.
To demonstrate the utility of the product molecule, we
investigated the catalytic functionalization of 3a−3c, generated
from a quinazoline formation protocol. We were delighted to
find that the amidation reaction using 3-phenyl-1,4,2-dioxazol
−5-one (2a) can be performed under slightly modified reaction
conditions to provide the product 5a−5c and 5d in good yields
Under the obtained optimal reaction conditions mentioned
above, we explored the applicability of the scope of diversely
substituted arylimidates 1a−1p. 3-Phenyl-1,4,2-dioxazol-5-one
(2a) was kept as a representative reaction partner (Scheme 2).
B
DOI: 10.1021/acs.orglett.6b00691
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ab
,
Scheme 3. Reaction Scope for Dioxazolones
Scheme 5. Proposed Reaction Pathway
In summary, we have developed a novel method for the
convenient synthesis of quinazolines. By employing a
commercially available catalyst system, a series of benzimidates
were efficiently converted in combination with a different type of
dioxazolones into various desired products in good to excellent
yields upon isolation. The synthetic protocol proceeds with the
advantages of operational simplicity, high atom efficiency, and
broad substrate scope and offers an important basis for access to
quinazolines.
a
Reaction conditions: 1a (0.4 mmol), 2b−2v (0.6 mmol),
b
[Cp*RhCl2]2 (2 mol %), AgBF4(8 mol %), DCE (2 mL). Isolated
yields.
(Scheme 4, eq 1). Owing to the fact that quinazolin-4-amine and
quinazolin-4-one derivatives widely occur in natural products,
Scheme 4. Functionalization and Transformation of Product
Molecule
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00691.
Experimental procedure, characterization of the products
(PDF)
1
Copies of the H and 13C NMR spectra of selected
products (PDF)
Crystallographic data of complexes 3f (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jinz@nju.edu.cn.
Notes
and they show a wide range of useful biological and
pharmacological activities, we further investigated transforma-
tions of 3a. Selective transformation of the methoxy group was
realized in different reaction conditions in the absence of
transition metals. For example, ammoniation gave 2-phenyl-
quinazolin-4-amine (5e) in high yield under basic acidic reaction
conditions, while hydrolysis of the methoxy group generated 2-
phenylquinazolin-4(3H)-one (5f) in excellent yields under acidic
reaction conditions (Scheme 4, eq 2).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge support from the National Natural
Science Foundation of China (21425415, 21274058) and the
National Basic Research Program of China (2015CB856303).
Based on the previous mechanistic studies,19 a plausible
mechanism was proposed (Scheme 5). In the presence of AgBF4,
cationic [Cp*RhIII] (I) is generated in situ as the active catalyst,
which coordinates to imine and further undergoes C−H
activation to afford rhodacyclic complex (II). Coordination of
3-phenyl-1,4,2-dioxazol-5-one (2a) to rhodium (III) with further
intramolecular migratory insertion generates the RhIII amido
species (IV) with release of CO2. The complex may undergo
protonation to regenerate the active [Cp*RhIII] catalyst (I) for a
new catalytic cycle and simultaneously give the intermediate (V),
which undergoes cyclization/denitrosation to afford the quinazo-
line product 3a.
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DOI: 10.1021/acs.orglett.6b00691
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