Ruthenium-Sulfonamide-Catalyzed Direct Dehydrative Condensation of Benzylic C-H Bonds with Aromatic Aldehydes

Article abstract of DOI:10.1021/jacs.6b08863

The first catalytic dehydrative condensation of the benzylic C-H bonds of toluene and p-xylene with aromatic aldehydes is reported herein. This protocol provides highly atom-economical access to stilbene and p-distyrylbenzene derivatives, whereby water is the sole byproduct. The reaction is based on the deprotonation-functionalization of benzylic C-H bonds through η⁶-complexation of the arenes, which is realized for the first time using a catalytic amount of a transition metal activator. The key to the success of this method is the use of a sulfonamide anion as a catalyst component, which appears to facilitate not only the deprotonation of the benzylic C-H bonds but also the formation of a C-C bonds via an electrophilic tosylimine intermediate.

Full text of DOI:10.1021/jacs.6b08863

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Communication
Ruthenium Sulfonamide-Catalyzed Direct Dehydrative
Condensation of Benzylic C–H Bonds with Aromatic Aldehydes
Shin Takemoto, Eri Shibata, Mitsuaki Nakajima, Yoshihiro
Yumoto, Mayuko Shimamoto, and Hiroyuki Matsuzaka
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08863 • Publication Date (Web): 02 Nov 2016
Downloaded from http://pubs.acs.org on November 2, 2016
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Ruthenium SulfonamideꢀCatalyzed Direct Dehydrative Condensation of Benzylic C–H Bonds with Aroꢀ
matic Aldehydes
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Shin Takemoto,* Eri Shibata, Mitsuaki Nakajima, Yoshihiro Yumoto, Mayuko Shimamoto, and
Hiroyuki Matsuzaka*
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.
Supporting Information Placeholder
oxidant.8 However, in many cases, relatively expensive
oxidants such as Ag(I) or Cu(II) salts are required in
stoichiometric amounts.
ABSTRACT: The first catalytic dehydrative condensa-
tion of the benzylic C–H bonds of toluene and p-xylene
with aromatic aldehydes is reported herein. This proto-
col provides highly atom-economical access to stilbene
and p-distyrylbenzene derivatives, whereby water is the
sole byproduct. The reaction is based on the deprotona-
tion–functionalization of benzylic C–H bonds through
η6-complexation of the arenes, which is realized for the
first time using a catalytic amount of a transition metal
activator. The key to the success of this method is the
use of a sulfonamide anion as a catalyst component,
which appears to facilitate not only the deprotonation of
the benzylic C–H bonds but also the formation of a C–C
bonds via an electrophilic tosylimine intermediate.
Scheme 1. Synthetic approaches toward Stilbenes
Stilbene and p-distyrylbenzene moieties are useful
structural motifs in pharmaceuticals and optoelectronic
materials,.1-3 and tThe development of new atom-
economical methods for their preparation is therefore of
considerable academic and industrial interest.4 Conven-
tional synthetic routes to stilbenes rely predominantly on
Wittig5 and Heck reactions,6 both of which, although
successful, present certain drawbacks. On one hand, the
Wittig reaction (Scheme 1a) requires multiple steps and
hazardous reagents to generate phosphorus ylide inter-
mediates, and it produces stoichiometric quantities of
halogen- and phosphorus-containing byproducts. The
sSyntheses via the Heck reaction (Scheme 1b), on the
other hand, produce less waste, but may present regiose-
lectivity issues when introducing the halogen (X) group
on the aromatic ring. The recently emerged catalytic
aromatic C–H olefination reactions (Scheme 1c),7 which
use directing groups (DGs) such as carboxylic acids and
carboxylic amides to ensure high regioselectivity, repre-
sent a more selective and direct approach to stilbenes.
These oxidative C–H olefin coupling reactions can be
highly atom-economical and cost-effective, provided
that molecular oxygen or air are used as the terminal
Herein, we report a new catalytic process that yields
stilbenes and p-distyrylbenzenes in a highly atom-
economical fashion via the direct dehydrative condensa-
tion of the benzylic C–H bonds of toluene and p-xylene
with aromatic aldehydes (Scheme 1d). The reaction ap-
pears to proceed through the benzylic deprotonation-
functionalization of toluene and p-xylene activated
through π-coordination to a cationic Cp*Ru complex.
Although existing examples of side-chain functionaliza-
tion of π-coordinated aromatics have needed stoichio-
metric amounts of transition metal activators,9,10 we now
demonstrate for the first time a catalytic version of such
transformation using a novel cooperative catalysis of a
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cationic Cp*Ru(η6-arene) complex and a sulfonamide
anion NHTs- (Ts = p-toluenesulfonyl).
The pronounced difference between the catalytic ac-
1
2
3
4
5
6
7
8
tivity of the primary and secondary tosylamide anions
(Table 1; cf. entries 4, 6, and 7) indicates that the role of
the NHTs- anion is not restricted to act as a base. We
envisaged that the NHTs- anion might react with the al-
dehyde to form a tosylimine intermediate and hence fa-
cilitate the C–C bond formation step of the catalytic cy-
cle, considering that tosylimines are generally more
electrophilic than the corresponding aldehydes.13 As a
support for this hypothesis, the condensation reaction
between pre-synthesized tosylimine p-ClC6H4CH=NTs
and toluene was effected by 10 mol% of catalyst 1 to
afford the p-chlorostilbene 3a in 58%yield (eq 1).
Table 1. Screening of Active Catalyst Components.
9
Entry
1
MLn
X–
Base
Yield (%)a
10
11
12
13
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Cp*Ru+
Cp*Ru+
Cp*Ru+
Cp*Ru+
Cp*Ru+
Cp*Ru+
Cp*Ru+
Cp*Ru+
Cp*Ru+
Cp*Ru+
CpRu+
NHTs–
OTf–
Cl–
Cl–
Cl–
Cl–
Cl–
Cl–
Cl–
none
50
0
2
none
3
none
0
4
KNHTs
KNHMs
KNMeTs
KNtBuTs
KHMDS
KOtBu
K3PO4
40
24
2
5
6
7
2
8
1
9
2
10
11b
12b
13b
14b,c
15b
16b
Cl–
2
Scheme 2. Proposed Catalytic Cycle
PF6–
PF6–
PF6–
OTf–
PF6–
none
KNHTs
KNHTs
KNHTs
KNHTs
KNHTs
KNHTs
8
Cp*Fe+
CpFe+
(PCP)Ru+
5
9
0
+
Mn(CO)3
Cr(CO)3
0
0
a Determined by GC. b Reaction time 19 h. c PCP = 1,5-
bis(diphenylphosphino)pent-3-yl.
Our initial discovery of the this dehydration dehydra-
tive condensation of toluene with an aromatic aldehyde
in the presence of [Cp*Ru(η6-toluene)][NHTs] (1; Cp*
= η5-C5Me5)11 and the subsequent screening tests for the
optimal catalyst composition are detailed in Table 1.
When a mixture of 1 and a 10-fold excess of p-
chlorobenzaldehyde (2a) in toluene was heated at to 130
ºC, p-chlorostilbene (3a) was produced in 50% yield
relative to 2a (entry 1), indicating that the solvent tolu-
ene used as solvent was also acted as a reactant and was
subjected to a catalytic condensation with the aldehyde.
As expected, analogous ruthenium complexes containing
less basic triflate or chloride counteranions displayed no
catalytic activity for this reaction (entries 2 and 3).
However, the catalyst generated in situ from [Cp*Ru(η6-
toluene)]Cl and KNHTs was similarly effective as the
pre-isolated NHTs- salt 1 (entries 1 and 4). Screening of
several potassium bases in combination with precatalyst
[Cp*Ru(η6-toluene)]Cl (entries 4-10) revealed that pri-
mary sulfonamide anions, in particular NHTs-, play an
essential role in this catalytic process. The η6-toluene
complexes of the [CpRu]+, [CpFe]+, and [Cp*Fe]+ frag-
ments (entries 11–13) were much less effective than that
of the [Cp*Ru]+ fragment, while the η6-toluene com-
plexes of [(PCP)Ru]+,12 [Mn(CO)3]+, and [Cr(CO)3]
were totally inactive (entries 14-16).
Scheme 2 depicts a proposed catalytic cycle for the
dehydrative condensation of toluene with the aldehyde
2a via a tosylimine intermediate. The initial interaction
between 1 and the aldehyde would occur by nucleophilic
addition of the NHTs– anion to form the intermediate A
containing a hemiaminolate anion. Benzylic deprotona-
tion14 from the [Cp*Ru(η6-toluene)]+ cation by the oxy
anion in A with and subsequent elimination of water
could
form
the
tosylimine
and
η5-
methylenecyclohexadienyl intermediate B.15 A nucleo-
philic attack of the exo-methylene carbon in B to the
tosylimine followed by a 1,2-elimination of the NHTs–
anion would form an η6-stilbene complex D, which
could subsequently undergo ligand exchange with tolu-
ene to complete the catalytic cycle.
Having identified 1 as a desired catalyst and with a
likely mechanistic scenario in hand, we next examined
optimization of reaction conditions using catalyst 1 and
the aldehyde 2a as a test substrate (Table 2). Raising the
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o
reaction temperature from 130 C to 150 C showed a
considerable increase in the reaction rate and the yield of
stilbene 3a (entries 1 and 2). Although there was a glar-
ing discrepancy between the conversion of 2a and the
yield of 3a in entries 1-3, a dramatically improved selec-
tivity for the production of 3a was obtained when the
reaction was conducted with molecular sieves 4Å (MS
4Å; entry 4). Complete conversion of 2a and 98% yield
of 3a was achieved with 5 mol% catalyst loading in 24 h
(entry 5), although further reduction of catalyst loading
(2.5 mol%) resulted in a slower reaction (entry 6).
of Ru catalyst (entries 1-5), while p-cyanobenzaldehyde,
parent benzaldehyde, and p- and o-tolualdehydes were
less reactive and required 10 mol% catalyst loading to
afford the corresponding stilbenes in acceptable yield
(entries 6-9). The aldehydes with o-chloro, p-NO2, and
p-OMe functionalities were not very compatible in this
reaction (entries 10-12).
1
2
3
4
5
6
7
8
We next extended our protocol to include substrates
other than toluene, namely, p-xylene (Table 4a). For this
purpose, the combination of [Cp*Ru(ꢀ-OEt)]2 (4)16 and
NH2Ts was found to be a convenient precatalyst that
could generate the catalyst [Cp*Ru(η6-p-xylene)]NHTs
in situ17 and facilitate the condensation of p-xylene with
2 equiv of aromatic aldehydes to afford a series of p-
distyrylbenzene derivatives (5a–5j) in good yields. The
compound 5j, bearing a carbazole functionality, is
known as a useful host material for blue organic light-
9
10
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12
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Table 2. Optimization of Reaction Conditions.
emitting diodes.18
In these experiments, the p-
distyrylbenzene products, which are generally highly
crystalline, precipitated as microcrystalline solids from
the reaction medium and could thus be easily isolated by
filtration.19 Hence, the present catalytic protocol pro-
vides an operationally simple procedure for the synthesis
of relatively simple p-distyrylbenzene derivatives. Ad-
ditionally, we also demonstrate a reaction of m-xylene
with the benzaldehyde 2a, which affords the m-
distyrylbenzene derivative 6a in 74% yield (Table 4b).20
Entry
X
10
10
5
T
time (h) additive
conv. (%)a yield (%)a
1
2
3
4
5
6
130
150
150
150
150
150
4
4
none
none
88
100
71
50
72
58
70
98
51
4
none
5
4
MS 4Å
MS 4Å
MS 4Å
70
5
24
24
100
57
2.5
a Determined by GC analysis.
Table 4. Dehydrative Condensation of p- and m-Xylene
with Aromatic Aldehydes.a,b
Table 3. Scope of Aromatic Aldehydes in the Catalytic
Dehydrative Condensation with Toluene.a
Entry
1
R
p-Cl
Ru mol% conv. (%)b product
yield (%)
5
5
100
100
100
91
3a
3b
3c
3d
3e
3f
95
90
94
85
92
60
87
88
85
5
2
p-F
Entry
ArCHO
Product
5a
Yield (%)b
3
p-CF3
p-Br
m-Cl
p-CN
H
5
1
2
3
4
5
6
7
8
9
p-ClC6H4CHO
p-FC6H4CHO
p-CF3C6H4CHO
m-ClC6H4CHO
p-NCC6H4CHO
PhCHO
81
93
71
94
73
49
80
91
70
4
5
5b
5
5
100
78
5c
6
10
10
10
10
5
5d
7
100
100
100
21
3g
3h
3i
5e
8
p-Me
o-Me
o-Cl
5f
9
p-MeC6H4CHO
o-MeC6H4CHO
p-MeOC6H4CHO
5g
10
11
12
3j
5h
p-NO2
p-MeO
5
26
3k
3l
8
5i
10
30
20
a Isolated yields are reported. b Determined by GC.
10
5j
4878
With optimized conditions in hand, we explored the
scope of aromatic aldehydes in the dehydrative conden-
sation with toluene (Table 3). Aromatic aldehydes with
electron-withdrawing halogen and -CF3 groups in para
or meta positions were smoothly transformed into the
corresponding stilbenes 3a-e in good yield with 5 mol%
a
Catalyst loading: 5 mol% 4 and 10 mol% TsNH2 rela-
tive to ArCHO. b Isolated yields are reported.
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Int. Ed. 2015, 54, 14518. (d) Srimani, D.; Leitus, G.; Ben-David, Y.;
Milstein, D. Angew. Chem. Int. Ed. 2014, 53, 11092.
In summary, we have developed a ruthenium-
sulfonamide-catalyzed direct dehydrative condensation
between the benzylic C–H bonds of toluene and p-
xylene and aromatic aldehydes. This method provides
highly atom-economical access to relatively simple stil-
bene and p-distyrylbenzene derivatives, both of which
are valuable structural motifs in pharmaceuticals and
optoelectronic materials. This catalytic process repre-
sents the first benzylic deprotonation-functionalization
of η6-coordinated arenes promoted by a catalytic quanti-
ty of a transition metal activator.
1
2
3
4
5
6
7
8
(5) For examples of Wittig reactions to synthesize stilbene and dis-
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on
the ACS Publications website.
Experimental procedures and product characterization
data (PDF).
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.T.: takemoto@c.s.osakafu-u.ac.jp
*E-mail for H.M.: matuzaka@c.s.osakafu-u.ac.jp
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This study was supported by the JSPS KAKENHI grants
JP15K05457 and JP15K05459. This work was also partial-
ly supported by the JSPS KAKENHI grants JP16H01038
(“Precisely Designed Catalysts with Customized Scaffold-
ing”) and 15H00958 (“Stimuli-responsive Chemical Spe-
cies for the Creation of Functional Molecules”). The au-
thors are also grateful to the TOYOTA Motor Corporation
for financial support.
(11) For the synthesis and X-ray structure of 1, see: Takemoto, S.;
Yumoto, Y.; Matsuzaka, H. J. Organomet. Chem. 2016, 808, 97.
(12) Takaya, J.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 5756.
(13) Appel, R.; Chelli, S.; Tokuyasu, T.; Troshin, K.; Mayr, H. J.
Am. Chem. Soc. 2013, 135, 6579.
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(18) (a) Lee, J.-H.; Woo, H.-S.; Kim, T.-W.; Park, J.-W. Opt. Ma-
ter. 2002, 21, 225. (b) Hosokawa, C.; Higashi, H.; Nakamura, H.;
Kusumoto, T. Appl. Phys. Lett. 1995, 67, 3853.
(19) The stilbene derivatives p-MeC6H4CH=CHAr, resulting from
the 1:1 condensation reaction between p-xylene and ArCHO, were
formed as minor products (for details, see Supporting Information).
(20) Other methylarene substrates such as p-chloro- and p-
methoxytoluene did not react with 2a under similar conditions.
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