Synthesis of dibenzo[c,e]oxepin-5(7H)-ones from benzyl thioethers and carboxylic acids: Rhodium-catalyzed double C-H activation controlled by different directing groups

Article abstract of DOI:10.1002/anie.201500486

A rhodium(III)-catalyzed cross-coupling of benzyl thioethers and aryl carboxylic acids through the two directing groups is reported. Useful structures with diverse substituents were efficiently synthesized in one step with the cleavage of four bonds (C-H, C-S, O-H) and the formation of two bonds (C-C, C-O). The formed structure is the privileged core in natural products and bioactive molecules. This work highlights the power of using two different directing groups to enhance the selectivity of a double C-H activation, the first of such examples in cross-oxidative coupling.

Full text of DOI:10.1002/anie.201500486

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201500486
German Edition: DOI: 10.1002/ange.201500486
À
C H Activation
Synthesis of Dibenzo[c,e]oxepin-5(7H)-ones from Benzyl Thioethers
À
and Carboxylic Acids: Rhodium-Catalyzed Double C H Activation
Controlled by Different Directing Groups**
Xi-Sha Zhang, Yun-Fei Zhang, Zhao-Wei Li, Fei-Xian Luo, and Zhang-Jie Shi*
Abstract: A rhodium(III)-catalyzed cross-coupling of benzyl
thioethers and aryl carboxylic acids through the two directing
groups is reported. Useful structures with diverse substituents
were efficiently synthesized in one step with the cleavage of
À
À
À
four bonds (C H, C S, O H) and the formation of two bonds
À
À
(C C, C O). The formed structure is the privileged core in
natural products and bioactive molecules. This work highlights
the power of using two different directing groups to enhance
À
the selectivity of a double C H activation, the first of such
examples in cross-oxidative coupling.
B
iaryl compounds are important structures in organic
synthesis, materials chemistry, and drug molecules.[1] In
À
biaryl synthesis, the use of selective double C H bond
activation[2] usually leads to a mixture of regioisomers.
However, nondirecting strategies can be utilized,[3] including
the control of electronics,[4] sterics,[5] and use of an excess[6] of
substrates (Scheme 1a, see 1a–d). To further control the
regioselectivity and enhance the reactivity, an intramolecular
strategy was designed and applied to construct fused rings[7,8]
(Scheme 1a, see 2). Another efficient and widely used
strategy is the introduction of a directing group to one of
the two partners (Scheme 1a, see 3). Having controlled the
Scheme 1. New strategy to for the direct oxidative coupling of two
different arenes based on control by two directing groups.
À
regioselectivity of C H activation in one arene, the selectivity
À
of the second C H activation was always ensured by selection
of electron-rich,[9] electron-deficient,[10] or bulky[11] substrates
(Scheme 1a, see 3a–c). All these strategies suffer from the
limitation of either substrate scope or relatively poor
regioselectivity. Our hypothesis to solve this problem was to
examples of the homocoupling of arenes with directing
groups.[12]
Herein we use a thioether and carboxylic acid as directing
À
groups in the designed double C H cross-coupling reaction.
À
control the selectivity of a double C H bond activation by
Our choice in directing group derives from the wide
application of carboxylic acids as directing groups in palla-
dium[13] and rhodium[14] catalysis, and our previous interest in
using two directing groups, thus introducing a second direct-
ing group to the second arene partner (Scheme 1a, see 4).
This novel approach is unknown, although there are a few
À
the development of thioethers as directing groups in C H
activation,[15] especially in rhodium catalysis[16] (Scheme 1b).
More interestingly, during our study we finally found that
[*] X.-S. Zhang, Y.-F. Zhang, Z.-W. Li, F.-X. Luo, Prof. Dr. Z.-J. Shi
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education
À
after the double C H coupling, in situ intramolecular cycli-
zation generated a dibenzo[c,e]oxepin-5(7H)-one. From this
aspect, this reaction was ideal because after the reaction the
thioether directing group could be removed in situ[17] and the
carboxylic acid group became part of the product.[18]
College of Chemistry and Molecular Engineering and
Green Chemistry Center, Peking University
Beijing, 100871 (China)
Dibenzooxepinones and their analogues can be found in
natural products and bioactive molecules (Scheme 1c).[19]
Antitumor activity,[19a] tyrosine kinase inhibition activity,[19b,d]
cytotoxic activity,[19c,20] and antimicrotubule activity[21] have
been investigated for some of these molecules. Traditionally,
this dibenzo[c,e]oxepin-5(7H)-one structure was constructed
by a sequence of conventional cross-coupling and C O bond-
forming cyclization. For example, the biphenyl structure was
mainly constructed from organohalides and/or organometal-
E-mail: zshi@pku.edu.cn
Prof. Dr. Z.-J. Shi
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
[**] Support of this work by the “973” Project from the MOST of China
(2009CB825300) and NSFC (Nos. 20925207, and 21002001) is
gratefully acknowledged.
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500486.
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Chemie
lic reagents.[19b,20–22] For the cyclization step, it is essential that
two functional groups be installed at the ortho posi-
tions.[19b,22,23] The multistep sequence and the prefunctional-
ization of starting materials make the synthesis complicated
and non-economical. Herein, with our direct one-step
method, the starting materials are very simple and readily
available. Notably, prefunctionalization to prepare organo-
halides and organometallic reagents was avoided.
Our initial studies indicated that the oxidative coupling
product 3aa was observed in 6% yield upon isolation from
the reaction of the thioether (1a) and benzoic acid (2a), with
[Cp*Rh(CH3CN)3](SbF6)2 as the catalyst and AgOAc as
oxidant (Scheme 2a). The structure of 3aa was unambigu-
Table 1: Screening of reaction conditions for the coupling of thioethers
with carboxylic acids.
Entry
x
y
Oxidant (z)
T [8C]
Yield [%][a]
1[c]
2[c]
3[c]
4
5
6
0.1
0.1
0.1
0.1
0.2
0.3
0.3
0.3
0.3
0.6
0.6
0.6
0.2
0.2
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.2
0.2
0.2
AgOAc (2.0)
AgNO3 (2.0)
AgNO3 (3.0)
AgNO3 (3.0)
AgNO3 (3.0)
AgNO3 (3.0)
AgNO3 (3.0)
AgNO3 (3.0)
AgNO3 (3.0)
AgNO3 (3.0)
AgNO3 (4.0)
AgNO3 (4.0)
120
120
120
120
120
120
140
160
160
160
160
160
(6)[b]
(11)[b]
19 (13)[b]
23
35
41
7
8
52
66
9[d]
10[d]
11[d]
12[e]
70 (53–68)[b]
71
71 (63)[b]
0
[a] Determined by NMR spectroscopy using BnOMe as an internal
standard. [b] Yield of isolated product. [c] tAmylOH as solvent.
[d] [{Cp*RhCl2}2] (2.5 mol%) + AgSbF6 (40 mol%) instead of [Cp*Rh-
(CH3CN)3](SbF6)2. [e] No catalyst.
Table 2: Substrate scope for aryl carboxylic acids.[a]
Scheme 2. Initial results and some control experiments. Cp*=C5Me5.
ously determined by X-ray crystallography of its single
crystal. Basically, this designed product may arise from the
À
intramolecular double C H coupling of benzyl benzoate (4),
however such an ester cannot be transformed into the desired
product under the same reaction conditions (Scheme 2b),
thus further confirming our double directing group strategy.
Control experiments with the sufoxide 5 and sulfone 6 also
resulted in no product (Scheme 2c), thus ruling out the
possibility of sulfoxide or sulfone as actual directing groups
(see the Supporting Information for the control experiments
run under standard reaction conditions).
Based on our initial result, we continued to optimize the
reaction conditions (Table 1). After systematic screening, we
found that the oxidant and solvent played a vital role and
AgNO3, and toluene provided the optimal combination
(entries 3 and 4). By changing the ratio between the two
partners, with the thioether in excess, the yield was elevated to
41% (entry 6). The yield can be further increased to 66% by
increasing the temperature to 1608C (entry 8). By changing
[a] Reaction conditions: [{Cp*RhCl2}2] (3.1 mg, 0.005 mmol), AgSbF6
(27.6 mg, 0.08 mmol), AgNO3 (136.0 mg, 0.8 mmol), carboxylic acid (2,
0.2 mmol), and thioether (1a; 83.0 mg, 81.6 mL, 0.6 mmol) were added
to a 50 mL Wattecs reaction tube before adding solvent (toluene, 2 mL).
Then the tube was sealed and heated at 160 8C for 8–12 h. [b] Some
inseparable contaminant was observed in the 13C NMR spectrum.
[c] Used 5 mol% [Cp*Rh(CH3CN)3](SbF6)2 as catalyst. [d] Run on
0.5 mmol scale at 1608C for 10 h, then 1408C for 15 h.
the cationic rhodium catalyst to
a combination of
[{Cp*RhCl2}2] and AgSbF6 and increasing AgNO3 to 4.0
equivalents, a more reproducible yield of 71% was obtained
(entry 11). The reaction cannot proceed in the absence of the
rhodium catalyst (entry 12).
With the optimal reaction conditions in hand, we first
investigated the substrate scope of the aryl carboxylic acids
(Table 2). A series of electron-neutral acids reacted smoothly,
thus giving the products in good yields (3aa, 3ab, 3ac, 3ah).
However, electron-rich (3ad) and electron-poor (3ae, 3af,
3ag) acids showed reduced reactivity (see the Supporting
Information for potential side reactions). The ortho- and
meta-substituted substrates also reacted smoothly, although
sterically encumbered ones showed lower efficiency (3ai,
3aj). It should be noted that halides like F, Cl, and Br could be
tolerated in this reaction to and resulted in moderate yields
(3af–ah, 3ak–ao, 3aq), thus revealing the possibility of
further functionalization. Apart from monosubstituted ben-
zoic acids, disubstituted benzoic acids and naphthoic acids
(3al–aq) could also be used as suitable substrates.
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We then investigated the substrate scope of the thioethers.
Firstly, we found that the substituents on the S atom played
a vital role in enhancing the efficiency. Alkyl groups gave very
good yields (1a,b) while aryl groups (1c,d) showed very low
efficiency (Table 3). With the methyl group as a substituent on
carboxylic acids might occur, thus reducing the yield.[24]
However, by either replacing the thioether with benzalde-
hyde or running the reaction in the absence of thioether, the
reaction did not occur, thus indicating the aldehyde was not
the key intermediate and the thioether was essential (Sche-
me 4a,b). The desired product was not obtained from 4 under
Table 3: The effect of substituents on sulfur atom.
S, we further investigated the reactivity of substituted
thioethers (Table 4). Different functional groups, such as F,
Cl, Br, COOMe, OMe, CF3, etc., all tolerated the reaction
conditions and gave the desired products in moderate to good
yields (3ej–gj, 3ha–ja, 3lj). Again, electron-neutral sub-
strates showed the best efficiency. In addition, substrates
having ortho and meta substituents also reacted well (3kj–lj,
3ma).
Scheme 4. Control experiments and intermediate study.
the standard reaction conditions (see Scheme 2b; also see the
Supporting Information). To further investigate the inter-
mediate, we synthesized 9. Indeed, it was transformed into the
desired product in high yield under the standard reaction
conditions (Scheme 4c), thus indicating it might be a key
intermediate in the catalytic cycle. This cyclization step can be
promoted by either the rhodium catalyst or silver salt.[25]
Further efforts showed that the kinetic isotope effect (KIE)
of the carboxylic acid and thioether were 2.0 and 4.9,
Table 4: Substrate scope for thioether derivatives.[a]
À
respectively, thus indicating that C H activation for both
arenes was possibly involved in the rate-determining step (see
the Supporting Information).
Based on these mechanistic studies, we proposed the
mechanism shown in Scheme 5. The first thioether-directed
À
rate-determining first C H bond activation forms the inter-
mediate A. Carboxylic acid coordination and ligand exchange
then forms the intermediate C, from which a relatively facile
À
second C H bond activation occurrs to form the intermediate
À
D. Reductive elimination forms the C C bond coupling
intermediate E and rhodium(I) species. With the assistance of
the rhodium catalyst or silver salt, E is cyclized to the final
product. Reoxidation of rhodium(I) into rhodium(III) by
AgNO3 completes the catalytic cycle. The intermediates A
and D were detected by ESI-HRMS (see the Supporting
[a] Reaction conditions: [{Cp*RhCl2}2] (3.1 mg, 0.005 mmol), AgSbF6
(27.6 mg, 0.08 mmol), AgNO3 (136.0 mg, 0.8 mmol), carboxylic acid (2,
0.2 mmol), and thioether 1 (0.6 mmol) were added to a 50 mL Wattecs
reaction tube before adding solvent (toluene, 2 mL). Then the tube was
sealed and heated to 1608C for 8–12 h. [b] Used 2.4 equiv of 1 f.
[c] Benzyl(ethyl)sulfane (1b).
À
Information). At this stage the carboxylic acid C H activa-
À
tion/thioether C H activation sequence cannot be ruled out.
With 1h as a substrate, we observed small amounts of both
the aldehyde 7 (derived from 1h) and ester 8 (derived from
1h and 2; Scheme 3). As a result, the relatively low efficiency
arises in part from esterification. In addition, in the presence
of the silver salt at high temperature, the decarboxylation of
In summary, we have developed a rhodium-catalyzed
double C H bond coupling reaction between benzyl thio-
À
ethers and aryl carboxylic acids to synthesize dibenzooxepi-
nones. Here, two directing groups were used and they were
either incoorporated into the product or removed in situ after
the reaction. The substrate scope is good and several func-
tional groups could be tolerated. The synthetic application of
this method in useful natural products and further investiga-
tion of the mechanism are underway.
Experimental Section
General procedures: [{Cp*RhCl2}2] (3.1 mg, 0.005 mmol), AgSbF6
(27.6 mg, 0.08 mmol), AgNO3 (136.0 mg, 0.8 mmol), and carboxylic
acid (2a; 24.4 mg, 0.2 mmol) were added to a 50 mLWattecs reaction
tube (sealed tube) under an atmosphere of air. Then the thioether
Scheme 3. Byproduct analysis.
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Scheme 5. Proposed mechanism.
(1a; 83.0 mg, 81.6 mL, 0.6 mmol) was added by using a microinjector
before adding solvent (toluene, 2 mL). After that, the tube was sealed
and the Wattecs parallel reactor heated to 1608C for 12 h. After the
reaction, the system was cooled to room temperature and the reaction
mixture concentrated under vacuum before purification by flash
chromatography on silica gel with petroleum ether/EtOAc (10:1 to
6:1) to give the product 3aa.
Supporting information for this article is available on the WWW
under http://www.angewandte.org. CCDC 1040225 (3aa) contains the
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obtained free of charge from The Cambridge Crystallographic Data
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À
Keywords: C H activation · carboxylic acids · cross-coupling ·
rhodium · synthetic methods
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Received: January 18, 2015
Published online: March 10, 2015
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