Rh- and Cu-Cocatalyzed Aerobic Oxidative Approach to Quinazolines via [4 + 2] C-H Annulation with Alkyl Azides

Article abstract of DOI:10.1021/acs.orglett.6b00774

A novel and efficient rhodium- and copper-co-catalyzed C-H bond activation and annulation for the construction of bioactively important quinazolines has been developed. This [4 + 2] annulation strategy utilizing alkyl azides as the carbon-heteroatom synthons shows high efficiency in the synthesis of six-membered benzoheterocycles containing two heteroatoms. This aerobic oxidative protocol provides a useful application of simple alkyl azides in N-heterocycle synthesis with N₂ and H₂O as byproducts.

Full text of DOI:10.1021/acs.orglett.6b00774

Letter
pubs.acs.org/OrgLett
Rh- and Cu-Cocatalyzed Aerobic Oxidative Approach to Quinazolines
via [4 + 2] C−H Annulation with Alkyl Azides
Xiaoyang Wang† and Ning Jiao*,†,‡
†State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Xue Yuan Road 38, Beijing 100191, China
‡State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A novel and efficient rhodium- and copper-co-catalyzed C−
H bond activation and annulation for the construction of bioactively
important quinazolines has been developed. This [4 + 2] annulation
strategy utilizing alkyl azides as the carbon−heteroatom synthons shows
high efficiency in the synthesis of six-membered benzoheterocycles
containing two heteroatoms. This aerobic oxidative protocol provides a
useful application of simple alkyl azides in N-heterocycle synthesis with N2
and H2O as byproducts.
ix-membered benzoheterocycles are important and common
components of cyclic compounds. Among them, quinazo-
lines are ubiquitous structural motifs found in many bioactive
compounds, natural products and pharmaceuticals.1 For
example, erlotinib and gefitinib are well-known lung cancer
drugs (Figure 1).2 Prazosin is used for curing high blood
Scheme 1. C−H Annulation for Heterocycle Construction
S
directing group (Scheme 1, path a). In contrast, the construction
of six-membered benzoheterocycles containing two heteroatoms
with carbon−heteroatom synthons is rarely achieved (Scheme 1,
path b).10
Figure 1. Quinazolines as core structures in pharmaceuticals.
pressure, anxiety, and panic disorder (Figure 1).3 Trimetrexate
has been used in the treatment of pneumocystis pneumonia
(Figure 1).4 Moreover, quinazolines are valuable precursors used
in the synthesis of functionalized materials.5 Therefore, the
construction of quinazolines has attracted considerable attention
during the past decades using preactivated substrates or
multistep reactions.6
Recently, the annulation through C−H activation has been
considered as an efficient and atom-economic strategy for the
construction of aryl and heterocyclic compounds (Scheme 1),7
which avoids the multistep preparation of preactivated starting
materials and tolerates a wide variety of functional groups.
Through this C−H annulation strategy, the [4 + 2] annulation
with alkenes and alkynes for the construction of six-membered
rings has been significantly developed (Scheme 1, path a),8,9
which provides efficient approaches to benzoheterocyclic
compounds containing one heteroatom originated from the
By using alkyl azides as substitutes for unsaturated reagents, we
developed a novel aerobic oxidative [4 + 2] C−H annulation of
imidate esters and alkyl azides (Scheme 1, path c). This
chemistry provides a practical approach for the efficient synthesis
of quinazoline derivatives. This C−H annulation strategy is
undoubtedly atom-economic and efficient (Scheme 1, path c),
but three challenging issues should be addressed to achieve this
novel quinazoline synthesis: (1) Recently, azides11 as prospective
N-sources show great advantages and high reactivity in metal-
catalyzed C−N bond formation through C−H activation even in
the absence of external oxidants.12,13 However, compared to the
well-developed amidation reactions with sulfonyl and acyl
azides,13i−m the C−H amination with alkyl azides is very
Received: March 16, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00774
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Organic Letters
Letter
challenging and limited.14 (2) More recently, the C−N bond
formation through C−H activation with azides and the
subsequent N−N bond formation coupling with the directing
group for the construction of 5-membered rings ([4 + 1]) have
been elegantly achieved by the groups of Glorius,15a Jiao,15b and
Zhu.15c Therefore, in the present design, the selectivity between
[4 + 1] for the formation of 5-membered rings and [4 + 2] for the
construction of 6-membered rings should be efficiently
controlled. To the best of our knowledge, the [4 + 2] C−H
annulation with carbon−heteroatom synthons has been
unknown until this work. (3) Four hydrogens are removed by
this synthetic design; therefore, a suitable oxidant, ideally green
and sustainable molecular oxygen,16 is desired for this oxidative
process with the generation of H2O as byproduct.
Scheme 2. [4 + 2] C−H Annulation with Different Alkyl
a,b
Azides
This project commenced with the reaction of imidate ester 1a
with benzyl azide 2a catalyzed by [Cp*RhCl2]2. Interestingly,
quinazoline product 3a was obtained in 18% yield with the
formation of a trace amount of 5-membered ring 4a when the
reaction was investigated in the presence of silver and copper
salts under air (entry 1, Table 1). The oxygen atmosphere slightly
a
Table 1. Optimization of the Reaction Conditions
b
b
t
3a
4a
entry [Rh] [Ag]
[Cu]
atmosphere (°C)
(%)
(%)
1
+
+
+
+
+
+
+
+
−
+
+
+
+
+
+
+
+
+
+
+
−
+
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(Tc)
Cu(OPiv)2
CuI
air
air
O2
O2
O2
O2
O2
Ar
120
100
100
100
100
100
90
18
54
65
41
52
67
74
trace
−
trace
19
16
34
16
6
a
b
Reaction conditions: see entry 7, Table 1. Isolated yields. Unless
2
specially noted the five-membered ring product was not obtained or
less than 5%. The reaction time is 40 h.
3
c
4
5
not affected by the steric bulk of the azides. The o-, m-, and p-
methyl-substituted benzyl azides showed similar yields (3b−d,
Scheme 2). The halogenated substrates were also tolerated in this
transformation, producing the halo-substituted quinazolines
which could be used for further coupling transformations (3g−
k). Some functional groups such as cyano, ester, and nitro groups
were compatible in the benzyl azide substrates (3l, 3m, 3p).
Notably, the simple nonyl azide containing long carbon chain
also performed well, providing the desired quinazoline 3q in 38%
yield despite the low selectivity with generation of 5-membered
ring product 4q in 36% yield.
6
7
CuI
4
8
CuI
90
−
−
−
−
9
CuI
O2
O2
O2
90
10
11
CuI
90
−
−
−
90
a
Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), [Cp*RhCl2]2
(2.5 mol %), AgSbF6 (10 mol %), 4 Å MS (50 mg), PhCl (2.0 mL), 16
h. Determined by NMR using 1,1,2,2-tetrachloroethane as internal
standard on 0.2 mmol scale.
b
improved the efficiency, but the selectivity was still unsatisfactory
(entry 3, Table 1). After a great deal of screening of different
catalysts, solvents, and additives, it is noteworthy that the yield of
the desired quinazoline 3a was improved to 74% when the
reaction was co-catalyzed by [Cp*RhCl2]2 and CuI in the
presence of AgSbF6 under oxygen atmosphere at 90 °C (entry 7,
Table 1), in which the chemoselectivity was highly selectively
controlled with only 4% 5-membered ring formation. Only trace
amount of desired product was detected when the reaction was
taken under Ar atmosphere (entry 8, Table 1), which
demonstrates the importance of O2 as an oxidant. The metal
catalysts are essential to this novel transformation (entries 9−11,
Table 1).
With the optimized conditions in hand, different kinds of
substituted benzyl azides were then tested with model imidate
ester 1a (Scheme 2). All of the reactions worked well to produce
the corresponding quinazolines in moderate to excellent yields.
The chemoselectivities were also very high, and the 5-membered
byproduct 4a was not obtained in most cases. The efficiency was
The substrate scope of aryl imidate was further investigated
(Scheme 3). Replacing ethoxy by methoxy and isopropoxy did
not decrease the reactivity, giving 5a and 5b in 70% and 68%
yield, respectively. Various aryl imidate esters containing
electron-donating groups, electron-withdrawing groups, or halo
substituents worked well under these conditions, leading to the
functionalized quinazolines with high efficiency. Naphthylimide
afforded 5i in 74% yield. It is noteworthy that the chemo-
selectivities were also very high, with no 5-membered ring
obtained in most cases (Scheme 3).
The obtained quinazoline products could be used for further
transformation or synthesis. The initial directing group contains
an alkoxy group whose conversion might further enrich the
application of the products. After being heated with hydrochloric
acid in ethanol for 1 h, the hydrolyzed product 6 was obtained
quantitatively (eq 1, Scheme 4). Further treatment with thionyl
chloride enabled the conversion of the carbonyl group to the C−
Cl bond, forming 7 in 88% yield (eq 2, Scheme 4). The
chlorinated quinazoline 7 could undergo coupling reactions for
B
DOI: 10.1021/acs.orglett.6b00774
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. [4 + 2] C−H Annulation with Different Aryl
Scheme 6. Proposed Reaction Mechanism
a
Imidates
IV regenerates the cationic Cp*Rh(III) and furnishes the
amination process with V as product. The amine V executes
the Cu-catalyzed aerobic oxidation22 to produce the correspond-
ing imine VI with the release of H2O as byproduct. Finally, the
cascade intramolecular addition and aerobic oxidative aromatiza-
tion occur, leading to the final quinazoline product.
In summary, we have developed a novel Rh- and Cu-co-
catalyzed [4 + 2] C−H annulation of imidates and alkyl azides for
the efficient synthesis of quinazolines. This chemistry provides
not only a novel approach to six-membered benzoheterocycles
containing two heteroatoms with carbon−heteroatom synthons
but also a useful application of simple alkyl azides in N-
containing compounds synthesis. With azides as N source and O2
as the terminal oxidant, this protocol gives N2 and H2O as
environmentally benign byproducts. Meanwhile, good functional
group tolerance and high atom efficiency make this protocol
useful in preparing various substituted quinazoline derivatives.
a
b
Reaction conditions: see entry 7, Table 1. Isolated yields. The yields
of five-membered ring products were 5−8% in these cases.
Scheme 4. Further Transformation of the Quinazoline
Products
ASSOCIATED CONTENT
* Supporting Information
■
S
further synthesis via the formation of C−C,17 C−N,18 C−O,19
and C−S20 bonds.
To probe the mechanism, many controlled experiments were
conducted (Scheme 5). When benzonitrile 8, benzylamine 9, and
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00774.
Experimental procedures, full characterization of products,
and copies of NMR spectra (PDF)
Scheme 5. Controlled Experiments
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jiaoning@pku.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Basic Research Program of
China (973 Program) (Grant No. 2015CB856600), the National
Natural Science Foundation of China (No. 21325206), and the
National Young Top-notch Talent Support Program is greatly
appreciated. We thank Kai Wu in this group for reproducing the
results for 3f and 5e.
benzoyl azide 10 were applied, respectively, in the system instead
of benzyl azide 2a, no desired quinazoline product was detected.
These results demonstrate that the corresponding benzonitrile,
benzylamine, or benzoyl azide intermediates were not involved in
this transformation.
According to the above results and previously reported
studies,21 the mechanism as shown in Scheme 6 was proposed.
Initially, the cationic Cp*Rh(III) catalyst I is generated with the
assistance of AgSbF6. Then the imidate group directed C−H
bond activation forms the rhodacyclic intermediate II, which
could coordinate with benzyl azide to generate species III.
Subsequently, the intermediate III undergoes migratory
insertion to afford Rh species IV. Protonation of the complex
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