Cooperative Lewis Acid/Cp?CoIII Catalyzed C-H Bond Activation for the Synthesis of Isoquinolin-3-ones

Article abstract of DOI:10.1002/anie.201601003

A facile route toward the synthesis of isoquinolin-3-ones through a cooperative B(C₆F₅)₃- and Cp?Co^III-catalyzed C-H bond activation of imines with diazo compounds is presented. The inclusion of a catalytic amount of B(C₆F₅)₃ results in a highly efficient reaction, thus enabling unstable NH imines to serve as substrates. Co^III-operative catalysis: The first facile route toward the synthesis of isoquinolin-3-ones through cooperative C-H bond activation with B(C₆F₅)₃ (BCF in picture) and Cp?Co^III (Cp?=C₅Me₅) is presented. The newly developed catalytic system is highly reactive, thus unstable NH imines can serve as substrates.

Full text of DOI:10.1002/anie.201601003

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201601003
German Edition: DOI: 10.1002/ange.201601003
À
C H Activation
III
À
Cooperative Lewis Acid/Cp*Co Catalyzed C H Bond Activation for
the Synthesis of Isoquinolin-3-ones
Ju Hyun Kim, Steffen Greßies, and Frank Glorius*
Abstract: A facile route toward the synthesis of isoquinolin-3-
ones through a cooperative B(C6F5)3- and Cp*CoIII-catalyzed
À
C H bond activation of imines with diazo compounds is
presented. The inclusion of a catalytic amount of B(C6F5)3
results in a highly efficient reaction, thus enabling unstable NH
imines to serve as substrates.
À
T
ransition-metal-catalyzed C H bond activation strategies
have emerged as powerful tools for the assembly of complex
molecules.[1] From both an environmental and economic point
of view, catalysts consisting of the earth-abundant metal
cobalt have gained attention as alternatives to those of second
-row transition metals such as Pd, Ru, and Rh.[2] Although
À
remarkable progress in the field of cobalt-catalyzed C H
bond activation has been made in the past few years,
significant challenges still remain, such as achieving efficient
catalyst turnover and the development of novel transforma-
tions.[2a] C H bond-activation reactions using high-valent
Scheme 1. Synthetic methods for the preparation of isoquinolinones
À
À
and quinolinones based on C H activation strategies.
Cp*CoIII catalysts are generally limited in substrate scope due
to the requirement of strong directing groups.[2c] Thus,
efficient catalyst systems and new directing groups, specifi-
cally non-azacyclic compounds, need to be explored.
Isoquinolin-3-ones are prevalent structural motifs in
a wide variety of natural[3] and biologically active com-
pounds.[4] The basic framework of quinolinones has several
regioisomers: 2-quinolinones, isoquinolin-1-ones, and -3-ones.
In the last decade, many elegant methods toward the synthesis
Figure 1. Naturally occurring compounds containing isoquinolin-3-one
motifs.
À
of isoquinolinones have been developed through C H
activation reactions. However, most of these studies are
focused on the synthesis of isoquinolin-1-ones[5] and 2-
quinolinones[6] (Scheme 1a,b). Conversely, despite their
broad utility, no general methods have been reported to
access isoquinoline-3-ones, an important structural class
present in natural compounds (Figure 1),[3] cardiotonic drug
candidates,[4] as well as heteroacenes as a result of the unique
feature of equilibrium with the tautomeric hydroxyisoquino-
lines.[7] Traditionally, isoquinolin-3-ones are prepared in
multistep syntheses under harsh reaction conditions and
exhibit limited substitution patterns.[8] Therefore, the devel-
opment of an efficient one-pot method to give isoquinoline-3-
ones is highly desirable.
As part of our ongoing research on Cp*CoIII-catalyzed
C H activation[9] and considering the interest in isoquinolin-
À
3-ones as an important structural motif, we proposed that
imines may serve as competent directing groups for Cp*CoIII-
[10]
À
catalyzed C H activation reactions with diazo esters
to
provide isoquinoline-3-ones (Scheme 1c). To facilitate this
transformation, it was first necessary to develop an efficient
Cp*CoIII catalyst system to enable unstable NH imines to act
as directing groups. Transformations with NH imine directing
groups would be the most atom-economic way to access
N-unsubstituted isoquinoline-3-ones, as no prefunctionaliza-
tion or additional deprotection would be required. However,
À
NH imines are not widely used as a directing group in C H
activation reactions for the following considerations:[11]
1) Imines are mildly basic and can form iminium salts which
are easily hydrolyzed to the ketone or can undergo
nucleophilic addition to the electrophilic carbon
atom.[11,12]
[*] Dr. J. H. Kim, S. Greßies, Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
E-mail: glorius@uni-muenster.de
À
2) The relatively weak N H bond could generate a metal–
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201601003.
iminyl bond, which can lead to competing, undesired
reactions.[13,14]
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3) Primary aryl alkyl imines could undergo imine/enamine
tautomerization,[14] which may alter the chelation ability.
In this regard, few transformations that utilize primary
aryl alkyl imines as directing groups have been reported in
[15]
À
C H activation reactions.
In the last few years, cooperative catalytic systems have
À
attracted attention to enable C H bond activations that could
not previously be achieved by first-row transition-metal
catalysts.[16] Interestingly, we found that the addition of
catalytic amounts of B(C6F5)3 dramatically accelerates the
À
reaction rates and facilitates C H bond activation. Herein, we
III
À
report the first Lewis acid promoted Cp*Co -catalyzed C H
bond activation of imines with diazo compounds. Arguably,
this approach represents the first synthetic method toward
À
isoquinoline-3-ones through C H bond activation.
To examine whether NH imines are competent directing
III
À
groups for Cp*Co -catayzed C H activation with diazo
compounds, we selected 1-(p-tolyl)pentan-1-imine (1b) and
dimethyl 2-diazomalonate (2a) as model substrates. In an
initial set of experiments, silver salts and acetate bases were
investigated with a Cp*CoIII catalyst.[2,17] To our delight, the
desired product 3b and its transesterification derivative 3b’,
derived from the solvent, were formed in the presence of
CsOAc (10 mol%) and AgSbF6 (10 mol%) in 29% yield
(Table S1 in the Supporting Information). We suspected that
the reactivity could be improved by the presence of a Lewis
acid.[16] After extensive screening, we were pleased to find
that the yield of 3b/3b’ improves significantly up to 80% on
introduction of B(C6F5)3 (20 mol%), while other Lewis acids,
including Zn(OTf)2, Sc(OTf)3 and BF3·OEt2 were less effec-
tive (Table S1). Finally, it was found that the addition of silver
was not required for the reaction (Table S1). Structurally
similar substrates, including a range of ketoximes (N-OH, N-
OMe, N-OPiv) and N-PMP-substituted imines (PMP = N-p-
methoxyphenyl) did not give any desired product
(Table S1).[18]
Scheme 2. Substrate scope of primary ketimines and diazo com-
pounds. For the reaction conditions, see the Supporting Information.
TFE=2,2,2-trifluoroethanol.
With the optimized conditions in hand, the substrate
scope was investigated with various NH imines and diazo
compounds (Scheme 2). Electron-donating and -withdrawing
substituents on the aryl moiety were tolerated. The reaction
proceeded regioselectively when functional groups were
located at the meta and ortho positions. In a similar fashion,
m-naphthyl-substituted substrate 1i and heteroarene 1j were
also tolerated. Variation of the R1 substituents and alteration
of the diazo ester had little effect on the reaction efficiency.
Next, a variety of N-substituted imines were examined
under the optimized reaction conditions (Scheme 3). Pleas-
ingly, the reaction of N-methyl-1-phenylethan-1-imine (1’a)
proceeded smoothly to give 4a in 83% yield. Electronic
variation of the para- and meta-substituents on the aryl group
did not affect the reaction efficiency and afforded 4b–4 f. The
structure of 4c was confirmed by X-ray crystallographic
analysis.[19] The N-phenyl-substituted imine 1’g as well as the
aldimine 1’h were also competent substrates. Notably, product
4h is an important scaffold in natural compounds (Figure 1).
Other diazo compounds were also tolerated.
Scheme 3. Substrate scope of secondary ketimines, aldimines, and
diazo compounds. For the reaction conditions, see the Supporting
Information.
First, the stability of NH imine 1a was tested. Significant
decomposition (27%) of 1a was observed within 1 h in the
absence of a diazo compound under the optimized conditions
(see the Supporting Information). Previous conditions with
AgSbF6 gave 62% decomposition in the same time. It was,
therefore, concluded that an increased reaction rate was the
For a better understanding of the role of the reaction
additives, we conducted a series of experiments (Scheme 4).
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Additional preliminary experiments were carried out to
gain insight into the mechanism (see the Supporting Infor-
mation). Two competition experiments were performed. First,
a competition between p-CH3-substituted imine 1’b and imine
1’a with diazo ester 2a. In the second experiment, p-Cl-
substituted imine 1’d and imine 1’a were mixed with diazo
ester 2a. Both experiments suggested that this transformation
is more favorable for electron-rich imines. Next, a series of
deuterium labeling experiments were carried out. The use of
À
a deuterated cosolvent demonstrated that the C H activation
was reversible (see the Supporting Information). Kinetic
studies from parallel reactions revealed a KIE value (kH/kD)
À
of 1.1, thus suggesting that the C H bond cleavage of the
imine is probably not the rate-limiting step.
Based on the above results and previous reports,[9,10,14]
a plausible reaction mechanism is proposed (Scheme 5). First,
the reactive cationic Cp*CoIII species I is generated with the
Scheme 4. Studies on the role of B(C6F5)3.
key to achieving high yields. Hence, we investigated the initial
rate for the reaction between imine 1a and diazo compound
2a in the presence of different additives [Eq. (1) in
Scheme 4]. The experiments showed that the addition of
B(C6F5)3 clearly resulted in a significant rate enhancement
compared to AgSbF6. The origin of the high reactivity and the
role of B(C6F5)3 are ambiguous, and further comprehensive
mechanistic studies are required. However, a number of
experimental observations are noteworthy:
1) When parallel reactions of imine 1’a with different
additives were performed in deuterated co-solvent for
10 min, D1 incorporation in 1’a was 32% under the
optimized conditions, while only 11% with AgSbF6
[Eq. (2) in Scheme 4].
Scheme 5. Proposed reaction mechanism.
2) 1H NMR experiments of the diazo compound 2a with
different additives indicated that the formation of the
carbene on extrusion of nitrogen is accelerated in the
presence of B(C6F5)3 [Eq. (3) in Scheme 4].
assistance of B(C6F5)3. Subsequently, Cp*CoIII species I
coordinates to substrate 1, and a reversible C H bond
À
3) When B(C6F5)3 was treated with stoichiometric amounts
of [Cp*Co(CO)I2], the 11B NMR spectra showed that the
borate anion A is formed readily even at room temper-
ature by iodide abstraction from the cobalt catalyst
precursor [Eq. (4) in Scheme 4].
cleavage forms cobaltacycle II. The reaction with the diazo
compound might then occur to give the metal–carbene
intermediate III by extrusion of N2. After migratory insertion
to provide IV, rearrangement gives V. Nucleophilic addition
to the ester carbonyl group occurs in the presence of Lewis
acidic CoIII to give VI. Finally, elimination of the methoxy
group from VI and proton transfer gives the desired products
with regeneration of the catalyst.
Combining the above pieces of information, it is plausible
that B(C6F5)3 plays a dual role:
1) The generation of a catalytically active cationic CoIII
species might be facilitated by the strong Lewis acid
B(C6F5)3, which forms a noncoordinating borate anion
such as [I(C6F5)3B]À. The cationic CoIII species would be
stabilized by this borate anion.[20]
In summary, we have successfully developed a Cp*CoIII-
À
catalyzed C H activation of imines with diazo compounds to
provide the first example of an expedient method to access
highly substituted isoquinolin-3-ones with broad substrate
scope and good functional-group tolerance. In this reaction,
the addition of catalytic amounts of B(C6F5)3 results in high
reaction efficiency, thus unstable NH imines could serve as
the directing groups. Given the high reactivity, the newly
2) The inclusion of B(C6F5)3 may accelerates the rate of the
À
C H activation step and subsequent formation of the
CoIII–carbene intermediate III.
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Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft
(Leibniz Award) and the Alfried Krupp von Bohlen und
Halbach-Foundation is gratefully acknowledged. We thank
Prof. Dr. G. Erker for a generous donation of B(C6F5)3. We
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À
Keywords: carbenes · C H activation · cobalt ·
cooperative catalysis · isoquinolinones
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5577–5581
Angew. Chem. 2016, 128, 5667–5671
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Received: January 28, 2016
Published online: March 24, 2016
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