Rhodium(III)-catalyzed oxidative C-H functionalization of azomethine ylides

Article abstract of DOI:10.1002/anie.201207204

Put it on a ring: A rhodium(III) complex can catalyze the oxidative coupling of azomethine imines with olefins, leading to the synthesis of 1,2-dihydrophthalazines, olefinated aldehydes, or fused pyridines, depending on the conditions used. Copyright

Full text of DOI:10.1002/anie.201207204

Angewandte
Chemie
DOI: 10.1002/anie.201207204
À
C H Activation
À
Rhodium(III)-Catalyzed Oxidative C H Functionalization of
Azomethine Ylides**
Wencui Zhen, Fen Wang, Miao Zhao, Zhengyin Du, and Xingwei Li*
À
Metal-catalyzed oxidative C H olefination of arenes has
emerged as a powerful alternative to the traditional Heck
[1]
À
coupling for new C C bond formation. The significance of
this reaction has been elegantly demonstrated in many new
synthetic methods as well as in the total synthesis of natural
products.[2] Whereas oxidative olefination is predominantly
catalyzed by palladium complexes, rhodium(III)-catalyzed
À
À
olefination of C H bonds has increasingly received attention
owing to high efficiency, broad scope, and good functional
group tolerance.[3] RhIII-catalyzed olefination of arenes bear-
ing protic directing groups has allowed the synthesis of
lactams[4] and oxygen-containing heterocycles.[5] Moreover,
oxidative coupling of arenes with alkynes can lead to the
construction of a broad spectrum of heterocycles such as
isocoumarins,[5] indoles or pyrroles,[6] pyridines,[7] isoquino-
lines,[8] isoquinolones,[9] quinolones,[10] and pyridones.[11] Some
of these strategies have been successfully applied to the
synthesis of natural products.[12] Despite these successes, it is
Figure 1. Versatile reactivity of azomethine imines under C H activa-
tion conditions.
tadienyl; 4 mol%). Cu(OAc)2 proved to be a less effective
oxidant, and two functionalized 1,2-dihydrophthalazines, 3aa
and 4aa, were isolated in low yields. In contrast, AgOAc
proved to be effective as an oxidant and monoolefination/
cyclization product 3aa was isolated in 88% yield when the
reaction was optimized by using a slight excess of the olefin
and the oxidant [conditions A, Eq. (1); Bn = benzyl]. Fur-
À
necessary to explore the C H activation of arenes facilitated
by directing groups that not only coordinate to metals, but
also participate in subsequent reactions to provide novel
tandem transformations. This serves to both broaden the
À
scope of rhodium-catalyzed C H functionalization reactions
and to achieve molecular complexity.
We reasoned that azomethine ylides of (hetero)arylalde-
À
hydes such as 1a are suitable substrates for C H activation
À
under chelation assistance although C H activation of these
substrates has not been reported. Furthermore, the possible
=
À
À
cleavage of C N, C N, and C C bonds in the pyrazolidinone
À
ring may be coupled with C H activation, leading to versatile
and unique products (Figure 1). We now report the rich
chemistry of Rh -catalyzed oxidative C H olefination of
azomethine imines, where cleavage of all of these bonds are
involved under different conditions.
thermore, the yield of isolated 4aa was optimized to 65% by
using the olefin and AgOAc in greater excess (conditions B).
In contrast, only [3+2] dipolar addition occurred when the
rhodium catalyst was omitted (see the Supporting Informa-
tion).
III
À
We initiated our studies with a screen of the conditions for
the coupling of 1a with benzyl acrylate (2a) using [RhCp*-
(MeCN)3](SbF6)2 as a catalyst (Cp* = pentamethylcyclopen-
With the optimized conditions in hand, we next explored
the scope of this reaction. Acrylates smoothly coupled with
azomethine 1a under conditions A (Scheme 1), and products
3aa–3ae were isolated as the major products in 68-88% yield,
whereas the the corresponding diolefination product were
isolated in yields of 56–79% under conditions B. Acrylonitrile
is also a viable coupling partner, but only for monoolefina-
tion/cyclization (3af). The scope of the azomethine substrate
was next explored, using n-butyl or benzyl acrylate. Electron-
donating, electron-withdrawing, and halogen (3ha and 3ia)
groups at the para position of the benzene ring are all well
tolerated under both sets of conditions. When the ortho
position was blocked by a hologen group or fused with an
extra ring, selective monoolefination/cyclization was
obtained. Azomethines with substituents in the meta position
[*] W. Zhen,[+] F. Wang,[+] M. Zhao, Prof. Dr. X. Li
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian 116023 (China)
E-mail: xwli@dicp.ac.cn
W. Zhen,[+] Prof. Dr. Z. Du
College of Chemistry and Chemical Engineering, Northwest Normal
University, Lanzhou 730070 (China)
[+] These authors contributed equally to this work.
[**] This work was supported by the Dalian Institute of Chemical Physics
and the Chinese Academy of Sciences.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207204.
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complex 7, which was further characterized by X-ray crys-
tallography.[15] Only complex 7 successfully catalyzed the
coupling of 1a with ethyl acrylate (2c) under monoolefina-
tion/cyclization conditions, and 3ac (68%) and 4ac (12%)
were isolated in essentially the same yields as those obtained
using [RhCp*(MeCN)3](SbF6)2, which suggests that a vacant
coordination site is necessary. Although RhIII catalysts have
À
been widely applied in C H activation, reports on isolated
stable organorhodium(III) intermediates are still limited.[16]
À
Secondly, the sequencing of the ortho C H olefination
and scission of the pyrazolidinone ring was probed. Subjecting
hydrazone[17] 8 to conditions A with ethyl acrylate only
À
afforded slight decomposition, which indicates that the C H
olefination should occur prior to any opening of the
pyrazolidinone ring (Scheme 3a). To study the sequencing
Scheme 1. Coupling of azomethines with olefins. Conditions A: azome-
thine (0.50 mmol), olefin (0.65 mmol), [RhCp*(MeCN)3](SbF6)2
(4 mol%), AgOAc (1.05 mmol), DCE (4 mL), 1008C, 16 h. Condi-
tions B: azomethine (0.5 mmol), olefin (1.5 mmol), [RhCp*(MeCN)3]-
(SbF6)2 (4 mol%), AgOAc (2.25 mmol), 1,4-dioxane (4 mL), 1158C,
16 h. Yields shown are of isolated products. DCE=1,2-dichloroethane,
EWG=electron withdrawing group.
Scheme 3. Mechanistic studies. Conditions A: azomethine
(0.50 mmol), olefin (0.65 mmol), [RhCp*(MeCN)3](SbF6)2 (4 mol%),
AgOAc (1.05 mmol), DCE (4 mL), 1008C, 16 h.
À
are well-tolerated (3ld and 3md), and the C H functional-
ization preferentially occurred at the less hindered site.
Moreover, a gem-diethyl analogue reacted smoothly with
comparable reactivity and selectivity (3nd and 4nd). In
contrast to the smooth reactions of these activated olefins,
N,N-dimethylacrylamide and styrene were less effective and
reacted with different selectivity (see the Supporting Infor-
mation). Phthalazines and dihydrophthalazines are known to
be potentially bioactive.[13] Dihydrophthalazines are less
accessible, and they are typically synthesized by nucleophilic
addition to phthalazines.[14]
of the 2nd olefination and the scission of the pyrazolidinone
ring, an isolated sample of 3aa was subjected to conditions B
(benzyl acrylate), but no reaction occurred. This suggests that
the 2nd olefination should occur prior to cyclization, and that
it is also chelation assisted. Based on these results, a sequence
of 1) mono- or diolefination of azomethine, 2) opening of
pyrazolidinone ring, and 3) 6-exo-trig cyclization was pro-
posed (Scheme 4).
We next focused on the mechanistic aspects of this
chemistry. Azomethine 1a, [RhCp*Cl2]2, and NaOAc reacted
smoothly to give rhodacycle 6 in 68% yield (Scheme 2). Both
1H and 13C NMR analysis of 6 pointed to cyclometalation.
Chloride abstraction (by AgSbF6) from complex 6 afforded
The proposed ortho-olefinated azomethine intermediate
was next probed. Although we failed to prepare an azome-
=
thine bearing an ortho (E)-CH CHCN group as a starting
material, the rhodium-catalyzed one-pot reaction of aldehyde
9 with 5,5-dimethylpyrazolidinone afforded 1,2-dihydroph-
thalazine 3af in high yield (Scheme 3b), which indicates the
intermediacy of an ortho olefinated azomethine. The rhodium
catalyst is necessary in this reaction, as omission of the
catalyst afforded a mixture of unidentifiable products, which
also suggests that the rhodium catalyst promotes the scission
of the pyrazolidinone ring and/or the subsequent Michael
cyclization. The intermediacy of an olefinated hydrazone was
next explored. Hydrazone 10 cyclized smoothly to give 1,2-
Scheme 2. Rhodium(III) intermediates in catalysis.
2
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hydrazone F yields the final product, and deuterium incor-
poration into the a-methylene group is expected when
N-deuterated F cyclizes.
To better define the scope of the azomethine substrate, an
azomethine functionalized with a 2-furanyl group was used
(Scheme 5). Significant substrate effects were observed and
Scheme 4. Proposed catalytic cycle.
dihydrophthalazine 11 (68% yield) even under uncatalyzed
conditions (Scheme 3c). As well as supporting the interme-
diacy of 10, this result also indicates that the scission of the
pyrazolidinone ring is rhodium-mediated.
Additional information on the catalytic cycle was
obtained from deuterium-labeling studies using [D5]1a and
n-butyl acrylate (Scheme 3d). 1H NMR analysis of the
isolated product revealed decreased deuteration at the
position ortho to the imine group, and deuterium scrambled
Scheme 5. Formation of pyridine-fused heterocycles. Conditions: azo-
methine (0.50 mmol), olefin (0.65 mmol), [RhCp*(MeCN)3](SbF6)2
(4 mol%), AgOAc (1.50 mmol), DCE (4 mL), 908C, 16 h. Yields shown
are of isolated products.
a furo[2,3-c]pyridine was isolated and identified as a result of
À
À
À
À
to the imine C H (28%), the olefinic C H (26%), and the
a methylene positions (30%).[18] The observed loss of deute-
rium in the ortho position of the product agrees with
reversible cyclometalation, and this metalation/proto-
(deutero)demetalation should be faster than any subsequent
C H olefination and N N cleavage. Azomethines with alkyl-
and aryl-substituted furans coupled smoothly with acrylates
(68–81%). The azomethine is not limited to C2-substitution
in the furan ring. Thus, under the same conditions a C3-
À
substituted substrate undergoes C2 H bond cleavage with
reactions.[6a] Therefore, C H cleavage cannot be turnover-
high selectivity and only a single isomeric product (12sd) was
observed. Furthermore, indole-functionalized azomethine
smoothly coupled with n-butyl acrylate to afford an indole-
fused pyridine (12td) in good yield. These fused heterocycles
are widely present as the cores of natural products.[20] The
À
limiting.[19]
A proposed mechanism for the formation of product 3 is
given in Scheme 4. Reversible cyclometalation affords
a rhodium(III) intermediate A and an acid (HX, or DX for
[D5]1a). Insertion of an olefin gives metallacycle B, which
may undergo off-loop nitrogen dissociation followed by
reversible cyclometalation to give C, which can account for
the H/D exchange on the imine, because both HX and DX
can cleave the Rh-Cimine bond in intermediate C when [D5]1a
is used. Subsequent b-hydrogen elimination from B generates
ortho-olefinated azomethine D and a rhodium(III) hydride,
which reductively eliminates an HX followed by AgI oxida-
tion to regenerate the active RhIII catalyst. Intermediate D is
proposed to undergo RhIII-catalyzed reversible scission of the
pyrazolidinone ring to afford protic hydrazone F. With the
À
synthesis of these less accessible heterocycles through C H
activation is unprecedented.
A simplified pathway for this transformation, starting
from putative heteroaryl hydrazone intermediate G is pro-
posed in Scheme 6. Instead of undergoing any intramolecular
Michael reaction, G undergoes 6-e cyclization[21] to give H as
a result of stereoelectronic effects of the heterocyclic scaffold.
The final product was produced upon the elimination of a
À
reversible C N scission, deuterium is incorporated into the
pyrazolidinone ring and eventually into the olefinic position
of the product when the N-deuterated hydrazone cyclizes
back to a pyrazolidinone. Uncatalyzed cyclization of protic
Scheme 6. Proposed formation of pyridine-fused furan.
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3-methylbutenamide. In this system, the azomethine func-
tions as a oxidizing directing group, and this system is an
oxidative olefination process that uses an unprecedented
combination of external and internal oxidants. Although
there are reports of RhIII- and RuII-catalyzed couplings of
azomethines of heteroaldehydes (such as furaldehydes)
À
afforded heteroarene-fused pyridines as a result of C H
À
and N N cleavage. In addition, using Cu(OAc)2 as the
oxidant, the olefination of azomethines of these heteroarlde-
hydes gave ortho-olefinated aldehydes. Given the versatility
of this reaction system and the diversity of the products, this
method is likely to find applications in the synthesis of
complex structures.
À
À
arene C H bonds with alkenes and alkynes using N OR
oxidizing directing groups,[22] the strategy of using oxidizing
À
À
N N bonds, has not been reported for C H activation, likely
owing to limited bond polarization.[22b]
Received: September 5, 2012
Published online: && &&, &&&&
The azomethine group may also act as a potential
removable directing group[23] to produce a sequence of C H
À
=
olefination and subsequent C N bond hydrolysis. Indeed,
Keywords: azomethines · C-H activation · olefination ·
when the oxidant is simply switched to Cu(OAc)2·H2O, the
coupling of 2-furanyl functionalized azomethine with tert-
butyl acrylates proceeded with complimentary selectivity and
olefinated aldehyde 13oe was isolated in 81% yield
(Scheme 7). Moreover, the olefin substrate can be extended
to various styrenes (13og–13ol). The heterocycle is not
.
oxidative coupling · rhodium
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Scheme 7. Formation of ortho-olefinated heteroaryladehydes. Condi-
tions: azomethine (0.50 mmol), olefin (0.7 mmol), [RhCp*(MeCN)3]-
(SbF6)2 (4 mol%), Cu(OAc)2·H2O (1.05 mmol), DCE (4 mL), 1008C,
16 h. Yields shown are of isolated products.
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the azomethine limited to the C2 position (13td). In most
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À
Aldehyde is often a poor directing group for oxidative C H
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the same conditions.
In summary, we have achieved the RhIII-catalyzed oxida-
tive coupling of azomethine ylides with olefins. With AgOAc
as an oxidant, azomethines of benzaldehydes undergo selec-
tive
C H and C N cleavage to produce 1,2-dihydrophthalazines;
several intermediates in this reaction were studied to better
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À
À
4
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À
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À
[18] The scrambling of deuterium to the imine C H, the a-
methylene, and the olefinic positions of the product cannot be
ascribed to pre-scrambling of deuterium to the starting [D5]1a
because H/D exchange only occurred at the ortho positions of
the recovered starting material.
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Communications
À
C H Activation
Put it on a ring: A rhodium(III) complex
can catalyze the oxidative coupling of
azomethine imines with olefins, leading
to the synthesis of 1,2-dihydrophthala-
zines, olefinated aldehydes, or fused
pyridines, depending on the conditions
used.
W. Zhen, F. Wang, M. Zhao, Z. Du,
X. Li*
&&&& — &&&&
À
Rhodium(III)-Catalyzed Oxidative C H
Functionalization of Azomethine Ylides
6
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