A Comparative Investigation: Group 9 Cp?M(III)-Catalyzed Formal [4 + 2] Cycloaddition as an Atom-Economic Approach to Quinazolines

Article abstract of DOI:10.1021/acs.orglett.6b00716

A comparative study on the catalytic activity of different group 9 [Cp?M(III)] complexes in the formal [4 + 2] cycloaddition of arenes with rarely explored free imines and dioxazolones for the construction of multisubstituted quinazolines is reported herein. This investigation revealed that the cobalt catalyst is uniquely suited to this transformation due to its strong Lewis acidity and high sensitivity to steric hindrance.

Full text of DOI:10.1021/acs.orglett.6b00716

Letter
pubs.acs.org/OrgLett
A Comparative Investigation: Group 9 Cp*M(III)-Catalyzed Formal
[4 + 2] Cycloaddition as an Atom-Economic Approach to
Quinazolines
Xiaoming Wang, Andreas Lerchen, and Frank Glorius*
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstrasse 40, 48149 Munster, Germany
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* Supporting Information
ABSTRACT: A comparative study on the catalytic activity of different group
9 [Cp*M(III)] complexes in the formal [4 + 2] cycloaddition of arenes with
rarely explored free imines and dioxazolones for the construction of
multisubstituted quinazolines is reported herein. This investigation revealed
that the cobalt catalyst is uniquely suited to this transformation due to its
strong Lewis acidity and high sensitivity to steric hindrance.
he quinazoline motif is a key core structure commonly
found in many natural products, pharmaceutical com-
Over the past few decades, the development of the first-row
transition-metal-catalyzed reactions has attracted great attention
because of the use of cheap, earth-abundant, and less toxic base
metals.6 Among them, high-valent Cp*Co(III) catalysts have
been developed as robust, powerful, and cheap metal catalysts for
the C−H activation of arenes, as demonstrated in several reports
by the groups of Kanai and Matsunaga,7 Ellman,8 Chang,9
Ackermann,10 Glorius,11 and others.12,13 Notably, most
applications of [Cp*Co(III)] complexes, however, are limited
to reactions similar to those developed with [Cp*Rh(III)]
complexes.14 Since metals of the same group have different
physicochemical properties (e.g., ionic radius and electro-
negativity), it is highly rewarding to investigate factors
controlling their catalytic activities between Cp*Co(III) and
Cp*Rh(III)/Ir(III) to further expand the utility of the
Cp*Co(III) catalysts.7f−h,9a,d,11b,12g We report herein a Cp*M-
(III)-catalyzed formal [4 + 2] cycloaddition of arenes with a
rarely explored free imine unit and readily available dioxazolones.
This reaction represents a new complementary process to
synthesize multisubstituted quinazolines, which features a broad
scope and functional group tolerance (Figure 1c). This
comparative study on the catalytic activity of different Group 9
[Cp*M(III)] complexes revealed that the Cp*Co(III) complex
is uniquely capable of catalyzing this C−H amidation and
cyclization process affording diversely substituted quinazolines
directly from simple starting materials due to its strong Lewis
acidity and high sensitivity to steric hindrance.
T
pounds, pesticides, and functional organic materials (Figure
1a).1,2 For example, 4-oxy-substituted quinazolines have recently
Figure 1. Examples of biologically active molecules and comparison
between traditional methods and this work.
attracted interest due to their anticancer and anti-HIV biological
properties.3 Although there are various methods to construct
these privileged structures, most of them employ comparatively
less available starting materials such as ortho-functionalized
anilines, suffer from a limited scope of substituents at the 2- or 4-
positions, or involve multistep procedures or harsh reaction
conditions.4,5 Thus, it is highly desirable to develop new
convenient and efficient approaches to synthesize multi-
substituted quinazolines from simple starting materials. From
the viewpoint of synthesis analysis, formal [4 + 2] cycloaddition
of arenes with a free imine and a formal “cyano-unit” would be a
highly effective and atom-economic approach to directly
construct multisubstituted quinazolines (Figure 1b).
Recently, the groups of Chang, Li, Jiao and Ackermann
developed several attractive Cp*Rh(III) and/or Cp*Co(III)-
catalyzed C−H amidation strategies of arenes containing stable
and strongly coordinating directing groups and employing
dioxazolones as user-friendly amidating reagents.15 Inspired by
these results, we designed a novel tandem approach to synthesize
quinazolines where a Cp*M(III) catalyst mediates both an initial
Received: March 11, 2016
© XXXX American Chemical Society
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Letter
C−H amidation and a subsequent cyclization. For this purpose,
we selected the rarely explored free imine unit as atom-economic
directing group and chose readily available dioxazolones as
amidating reagents (Figure 1c). As is known, tandem C−H
amidation (or amination)/cyclization approach is a challenging
and less developed approach for the construction of hetero-
cycles.16 Indeed, the successful realization of this desirable
transformation would require overcoming several intricate
challenges, including the risk of decomposition of the CNH
unit in the substrates and amidated intermediates because of the
Lewis acidity of [Cp*M(III)] complexes, as well as the
compatibility of the reaction conditions for the C−H amidation
and intramolecular cyclization process. Furthermore, both the
CNH unit in the substrate and the pyrimidine core in the
product could act as directing group that can lead to competing
C−H amidation, and overamidation could feasibly occur.
isolated in 29% yield in the end of the reaction. In the ESI-MS
spectra of the reaction mixture with Cp*Ir(III) as catalyst, the
intermediate 3aa could be obviously detected, indicating that the
cyclization process may be more difficult using this catalyst. Only
the reaction with the Cp*Co(III) catalyst afforded the clean
formation of 4aa. These results clearly demonstrate that, among
the group 9 triad metals, Cp*Co(III) is the optimum catalyst for
this tandem process, mediating the cyclization step more
efficiently than Cp*Ir(III) and avoiding the overamidation
encountered with Cp*Rh(III).
Under the optimized conditions using the Cp*Co(III)
catalyst, the scope with respect to the amidating reagent 2 was
first examined (Scheme 1). Thus, aryl-substituted dioxazolones
Scheme 1. Synthesis of Quinazolines Using 1a and Various
Dioxazolones
In order to examine and compare the catalytic performance of
different group 9 triad [Cp*M(III)] catalysts, we selected ethyl
benzimidate 1a as a model substrate and reacted it with 2a (see
the Supporting Information for details of the reaction
conditions). To our delight, all of the Cp*M(III) complexes
led to the formation of the desired product 4aa in the presence of
NaOAc (Figure 2a). The reaction employing Cp*Co(III) as the
bearing various electron-donating and electron-withdrawing
groups at different positions of the aryl ring reacted smoothly
with 1a, affording the corresponding products 4aa−ai in good to
excellent yields. Furthermore, substituents on the amidating
reagents were not limited to aryl groups. The heterocycle-bearing
dioxazolone also reacted with 1a to afford the desired product 4aj
in 97% yield. Moreover, alkyl substituents could be readily
installed at the 2-position of the corresponding quinazoline
products 4ak and 4al in synthetically useful yields using methyl-
and cyclohexyl-substituted amidating reagents.
Subsequently, we tested the versatility of the Cp*Co(III)-
catalyzed C−H amidation/cyclization with arylimidates. As
shown in Scheme 2, various para-substituted arylimidates
smoothly afforded the desired products 4ba−ha in high yields
irrespective of the substrate electronics. For unsymmetrical
substrates, such as meta-substituted and meta,para-disubstituted
arylimidates, the reactions showed excellent selectivity (>20:1)
for the C−H bond with less steric hindrance, selectively
Figure 2. Comparison of the reaction performance of Cp*M(III) in the
synthesis of 4aa. (a) 1a (0.1 mmol), 2a (0.15 mmol), NaOAc (50 mol
%), DCE (1.0 mL), 60 °C, 5 h. GC yields are shown and isolated yield is
given in parentheses. (b) Reaction profiles until 3.0 h. Cp*Rh =
[Cp*RhCl2]2 (5 mol %), Cp*Ir = [Cp*IrCl2]2 (5 mol %) and Cp*Co =
[Cp*Co(CO)I2] (10 mol %).
Scheme 2. Synthesis of Quinazolines Using Various
Substrates 1 and 2a
catalyst led to the highest yield of 4aa (98%), while much lower
yields were observed with Cp*Rh(III) and Cp*Ir(III) (55% and
39%, respectively). No reaction was observed in the absence of
the Cp*M(III) complexes, indicating that the reaction is indeed a
Cp*M(III)-catalyzed process. In addition to assessing the
reaction outcome, the rate of formation of 4aa in each case
was also compared. As shown in Figure 2b, the reaction with
Cp*Ir(III) as catalyst was very slow, whereas 4aa was produced
fastest with Cp*Co(III). However, the reaction catalyzed by
Cp*Rh(III) exhibited a more complicated kinetic profile where
the yield of 4aa appeared to decrease over time. This observation
implies that the desired product 4aa is further transformed into
byproducts under the reaction conditions. Indeed, analysis of
each of the reaction mixtures at 1 and 5 h revealed that the
overamidation product 5aa was formed when Cp*Rh(III) was
used as catalyst. Moreover, this compound (5aa) could be
a
For reaction conditions, see the Supporting Information.
B
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Letter
delivering products 4ia−ka in good yields. Moreover, an ortho-
substituted imidate also afforded the desired product 4la in 77%
yield. It should be noted that the catalytic system tolerated
various kinds of functional groups on the arene such as EtO,
PhO, NMe2, CO2Et, F, and Cl, which can be transformed to
other functionalities. Additionally, 2-naphthyl imidate and ethyl
thiophene-3-carbimidate also showed good reactivity in these
reactions, giving the corresponding products 4ma and 4na.
Finally, the effect of modifying the ethylimidate directing group
was examined. Switching to 2-methoxyethylimidate did not
decrease the reactivity, and the corresponding product 4oa was
isolated in 94% yield. Moreover, N-tert-butylbenzimidamide,
diphenylmethanimine, and 1-phenylpentan-1-imine, which all
feature different imine-based directing groups, also led smoothly
to the cyclized products 4pa−ra in moderate to good yields.
Overall, this methodology is a highly efficient and convenient
strategy to synthesize multisubstituted quinazolines, with a broad
scope of substituents at the benzene position as well as in the 2-
and 4-positions from simple starting materials.
product 4aa. By careful analysis of the reaction outcomes with
different metals, the factors controlling their catalytic activities
between Cp*Co(III) and Cp*Rh(III)/Ir(III) were investigated
(Scheme 4). Compared with its Cp*Rh(III) and Cp*Ir(III)
Scheme 4. Analysis of the Reaction Outcomes with Different
Cp*M(III) as the Catalysts
It should be noted that the standard product 4aa, further
entitled as HMJ-53A, can inhibit arachidonic acid-induced
platelet aggregation (Scheme 3).3c,d Additionally, the hydrolysis
congeners, Cp*Co(III) is the most Lewis acidic catalyst and is
expected to promote the cyclization of intermediate 3aa more
efficiently.11b Indeed, the amidated intermediate 3aa was not
detected during the reaction using Cp*Co(III) as catalyst
whereas this compound was observed in the ESI-MS of the
reaction mixture when Cp*Ir(III) was employed. The addition
of Lewis acid [Sc(OTf)3 (20 mol %)] in the reaction with
Cp*Ir(III) as catalyst could improve the yield of 4aa (78%),
showing that the cyclization may be promoted by a Lewis acid.
Moreover, using 4aa as the starting material under the standard
conditions, the reaction with the Cp*Co(III) catalyst did not
lead to further amidation to 5aa and 4aa was recovered (86%),
while Cp*Rh(III) showed high reactivity, affording 5aa in 81%
yield and Cp*Ir(III) also showed reactivity (15% yield of 5aa).
This is proposed to be due to the smallest ionic radius of cobalt
among the group 9 metals, meaning that Cp*Co(III) is more
sensitive to steric hindrance than Cp*Rh(III).7h Thus, steric
repulsion between the Cp*-ligand and the substituent groups in
the pyrimidine-core of 4 (e.g., the ethoxy, and benzo-part in 4aa)
is more significant with Cp*Co(III) than with Cp*Rh(III)
(Scheme 4, G). Therefore, the coordination of the cobalt catalyst
to 4aa is less favorable and the overamidation process observed
with Cp*Rh(III) is prevented. These combined effects of high
Lewis acidity and small ionic radius are crucial in making
Cp*Co(III) such an efficient and selective catalyst for this
transformation.
In summary, we have demonstrated the excellent efficiency
and selectivity of the Cp*Co(III) catalyst in a novel synthesis of
quinazolines involving a tandem direct C−H amidation and
cyclization.18 The cobalt complex exhibited significantly higher
reactivity than Cp*Ir(III), while the overamidation observed
with Cp*Rh(III) was suppressed using Cp*Co(III). This unique
reactivity can be attributed to the strong Lewis acidity and the
high sensitivity to steric hindrance of Cp*Co(III) catalyst. The
highly efficient methodology developed herein represents a
convenient strategy to synthesize multisubstituted quinazolines
of interest for drug discovery and photophysical applications.
Scheme 3. Synthesis of Biologically Active Molecules
of 4 opens an alternative access to quinazolinone derivatives17
that are also known for their biological and medicinal activities,
such as 6ak, which is a natural product from Bacillus cereus17b and
6ik, which could be utilized in the synthesis of antimetabolite
drug raltitrexed and ICI 198583.17c,d Compared to the
traditional syntheses starting from multifunctionalized sub-
strates, the approach presented herein offers the desired
quinazoline products in an efficient, one-step procedure under
mild conditions from easy and readily available starting materials.
Moreover, using this procedure, a convenient and simple
synthesis for such derivatives could be helpful for drug discovery,
especially for the compiling of libraries. Additionally, several
compounds synthesized herein bearing electron-donating
substituents such as 4ma, 4ea, and 4ad showed potential
applications as colorimetric and luminescent pH sensors (for
details, see the Supporting Information).2a
For analysis of a possible reaction mechanism, the kinetic
isotope effect (KIE) of 2.36 was measured for parallel
experiments, indicating that C−H activation is plausibly involved
in the rate-determining step. Moreover, a competition experi-
ment between electronically different arylimidates showed that
the electron-rich substrate is more favored. Based on these
experiments and previously reported studies,15 a plausible
catalytic cycle is proposed in Scheme S1 (see the Supporting
Information). The reaction likely involves the directed Cp*Co-
(III)-catalyzed C−H amidation to afford intermediate 3aa. This
amide intermediate is proposed to undergo Lewis acid-promoted
nucleophilic addition and dehydration, affording the desired
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00716.
C
DOI: 10.1021/acs.orglett.6b00716
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Organic Letters
Experimental procedures, spectroscopic data, and mech-
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: glorius@uni-muenster.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
́ ́
(11) (a) Yu, D.-G.; Gensch, T.; de Azambuja, F.; Vasquez-Cespedes,
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■
This work was supported by the European Research Council
under the European Community’s Seventh Framework Program
(FP7 2007-2013)/ERC Grant Agreement No. 25936 and the
Alexander von Humboldt Foundation (X.W.). We also thank Dr.
Matthew N. Hopkinson and Dr. Eric A. Standley (both
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Q.; Vasquez-Cespedes, S.; Gensch, T.; Glorius, F. ACS Catal. 2016, 6,
́
quez-Ces
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pedes, S.; Yu, D.-G.;
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quez-
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Westfalische Wilhelms-Universitat Munster) for helpful dis-
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2352. (f) Kim, J. H.; Greßies, S.; Glorius, F. Angew. Chem., Int. Ed. 2016,
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