Iron-catalyzed direct alkenylation of sp3(C-H) bonds via decarboxylation of cinnamic acids under ligand-free conditions

Article abstract of DOI:10.1039/c3gc37131j

An example of highly efficient low-cost ferrocene-catalyzed decarboxylative C(sp²)-C(sp³) coupling reactions via C-H functionalization has been developed successfully. It is noteworthy that nanoparticles of Fe₃O₄ could be used as a reusable catalyst for 7 times in the absence of ligand.

Full text of DOI:10.1039/c3gc37131j

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Iron-catalyzed direct alkenylation of sp3(C–H) bonds via
decarboxylation of cinnamic acids under ligand-free
conditions†
Cite this: Green Chem., 2013, 15, 976
Hailong Yang, Hong Yan, Peng Sun, Yan Zhu, Linhua Lu, Defu Liu, Guangwei Rong
and Jincheng Mao*
Received 15th November 2012,
Accepted 5th February 2013
An example of highly efficient low-cost ferrocene-catalyzed decarboxylative C(sp2)–C(sp3) coupling reac-
tions via C–H functionalization has been developed successfully. It is noteworthy that nanoparticles of
Fe3O4 could be used as a reusable catalyst for 7 times in the absence of ligand.
DOI: 10.1039/c3gc37131j
www.rsc.org/greenchem
Nowadays, selective C–C bond formation has gained great Most importantly, such direct alkenylation could also be per-
interest through direct activation or functionalization of formed efficiently in the presence of a reusable Fe3O4 nano-
unreactive C–H bonds, since it avoids prefunctionalization of particle catalyst.
starting materials and can maximize atom economy.1 The use
Initially, we studied the decarboxylative coupling reaction
of unactivated sp3 C–H alkyl groups remains a major chal- of cinnamic acid (1a) and toluene (2a) using 20 mol% FeCl2 as
lenge, however, even though some progress has been made the catalyst and 2.0 equiv. of di-tert-butyl peroxide (DTBP) as
recently in this area.2 For example, a series of transition oxidant, which was stirred under an argon atmosphere at
metals, such as Ru,3 Rh,4 Ir,5 Pt,6 Pd,7 Au,8 Fe,9 Co,10 Cu,11 110 °C for 24 h. The desired product 3a was obtained in 35%
and some other metals,12 showed high efficiency for cross- yield (Table 1, entry 1). Furthermore, the yields can be slightly
dehydrogenative-coupling reactions. In addition, metal-free improved with the help of ligands (Table 1, entries 2–3). Mean-
methodologies using TBHP, TPBP, DDQ, TBAI, I2, PhI(OAc)2 or while, other commercially available iron catalysts such as FeX3
Lewis/Brønsted acids as catalysts have also been employed.13
(X = Br, Cl, F, NO3) and Fe2(SO4)3 were investigated for the reac-
The metal-catalyzed decarboxylative cross-coupling strategy tion and only lower yields were obtained (entries 4–5).
has been recently highlighted as a new synthetically useful However, Fe(acac)3 and Fe2O3 afforded good yields (entries
facility, since acids and their derivatives are low-cost and com- 6–7). Fe3O4 nanoparticles achieved a better result at 120 °C
mercially available. In 2002, Goossen and Paetzold reported (entry 8). It is surprising to find that low-cost ferrocene could
decarboxylative olefination of aryl esters for the first time.14a afford the best result (entry 9). When we varied the amount of
Since then, there have been many reports of decarboxylative catalyst, it was found that 15 mol% of catalyst can afford satis-
couplings of carboxylic acids or their salts with aryl halides, factory yields (entry 12). The use of other oxidants or radical
aryl triflates, heteroaromatic compounds and other coupling initiators such as tert-butyl hydroperoxide (TBHP), dicumyl
partners.14 However, direct decarboxylative C–H functionaliza- peroxide (DCP), (NH4)2S2O8, K2S2O8 and azodiisobutyronitrile
tion for C(sp2)–C(sp3) bond formation has been seldom (AIBN), did not afford better results (entries 13–17). However,
reported.15 Recently, we have reported an example of copper- when we raised the temperature to 120 °C, LC yield could be
catalyzed decarboxylative C(sp2)–C(sp3) coupling reactions via improved to 95%, and the desired product 3a could be isolated
a radical mechanism, but the methodology is less efficient.15a in 86% yield (entry 18).
After further investigation, here we were surprised to find that
With this optimized protocol in hand, we explored the
low-cost ferrocene could catalyze direct alkenylation of Csp3–H scope of this reaction with various cinnamic acids (Table 2). It
bonds via decarboxylative couplings with better efficiency. can be seen that electron-rich and electron-deficient cinnamic
acids as well as heteroaryl acrylic acids could afford the
desired products (3a–3o) with good yields in toluene. The
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
chloro- and fluoro-cinnamic acids could give 3b and 3c in 83%
and 85% yields, respectively. We also can see that the electron-
releasing OMe group at any position of the aromatic ring
showed good efficiency; in comparison, the ortho position
showed relatively lower efficiency (3d–3f). Di- or tri-methoxy
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
P. R. China. E-mail: jcmao@suda.edu.cn
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, 1H, 13C, HRMS spectral data and analytical data for the products. See
DOI: 10.1039/c3gc37131j
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Table 1 Screening catalytic conditions in iron-catalyzed decarboxylative C–H
functionalization reactiona
Table 2 Ferrocene-catalyzed decarboxylative C(sp2)–C(sp3) coupling of
different cinnamic acids (1) with toluene (2)a
Entry
Iron cat. (mol%)
Oxidant
Yieldb (%)
1
FeCl2 (20)
FeCl2 (20)
FeCl2 (20)
FeX3 (20) (X = Br, Cl, F, NO3)
Fe2(SO4)3 (20)
Fe(acac)3 (20)
Fe2O3 (20)
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBHP
DCP
35
44
58
Trace
20
80 (72)
62
82 (75)
83 (75)
74 (65)
77 (69)
82 (74)
25
24
NR
NR
NR
2c
3d
4
5
6
7
8e,f
9
Fe3O4 (20)
Ferrocene (20)
Ferrocene (5)
Ferrocene (10)
Ferrocene (15)
Ferrocene (15)
Ferrocene (15)
Ferrocene (15)
Ferrocene (15)
Ferrocene (15)
Ferrocene (15)
10
11
12
13
14
15
16
17
18f
(NH4)2S2O8
K2S2O8
AIBN
DTBP
95 (86)
a Catalytic conditions: cinnamic acid (0.3 mmol), toluene (2 mL), iron
catalyst (20 mol%), oxidant (2 equiv.), 110 °C, 24 h, Ar. b LC yield
determined using diphenylmethanone as an internal standard;
isolated yield based on cinnamic acid in parentheses. c Using TMEDA
(30 mol%) as the ligand. d Using 1,10-phenanthroline (30 mol%) as the
ligand. e 70–80 nm Fe3O4 nanoparticles (20 mol%). f 120 °C.
a Catalytic conditions: cinnamic acid (1) (0.3 mmol), 2a (2 mL),
ferrocene (15 mol%), DTBP (2 equiv.), 120 °C, 24 h, Ar. Isolated yield
based on cinnamic acid; trans/cis > 99 : 1. b (Z)-3-(Naphthalen-1-yl)
acrylic acid used.
cinnamic acids could also participate in the coupling success-
fully (3g–3h). Some alkyl-substituted cinnamic acids have also
been investigated, affording the desired products in yields
ranging from 81% to 83% (3i–3k). The 1-naphthylacrylic acid
gave the desired product (3l) with 76% yield. However, the decarboxylative coupling (4g–4h). Chloro-, bromo- or iodo-sub-
reaction works with low efficiency with strong electron-with- stituted toluenes also worked well in the reaction with cin-
drawing substituents such as cyano, nitro or ester groups on namic acid 1g, and the desired products (4i–4o) were acquired
the aromatic ring (3m–3o). Acetoxy-substituted cinnamic acids with moderate to good yields (48–78%). We are delighted to
also showed good tolerance and afforded the corresponding find that 4-methylbenzyl chloride could be tolerated under
products (3p–3q) with moderate yields. In addition, hetero- our catalytic condition with 68% isolated yield (4p). These
cyclic acrylic acids could also participate in the decarboxylative compounds (4i–4p) with halogen or benzyl chloride groups
coupling with toluene to produce the desired heterocyclic can be directly used for further functionalizations. Acetyl-
olefins in good yields under the standard conditions (3r–3t). substituted toluenes gave the desired products in moderate
When (Z)-3-(naphthalen-1-yl)acrylic acid was employed in yields (4q–4r).
this reaction, the corresponding product was in the trans-
The scope of the reaction about anisole and thioanisole,
configuration (3u). Therefore, it can be seen that the major where there is a heteroatom between the methyl group and
products obtained by our methodology are thermodynamically aromatic ring, is presented in Scheme 1. It can be seen that
stable.
moderate yields were obtained and generally electron-deficient
Under the optimized conditions, the scope of benzylic ethers showed better tolerance of the catalytic system (6b, 6c
hydrocarbons was also examined (Table 3). The coupling of and 6e). When 1,3-dibromopropane was applied to our cata-
different xylenes and mesitylene with 1g showed high selecti- lytic system, only the nucleophilic substitution product was
vity and only mono-coupling products (4a–4d) were obtained. obtained in 82% yield (Scheme 2).
Direct decarboxylative coupling of 1g with ethylbenzene was
Meanwhile, some experiments were carried out to study the
also successful with a slightly lower yield of 71% (4e). More- regioselectivity of the catalytic reaction. n-Propylbenzene and
over, tert-butylbenzene was also employed as a good substrate 2-bromomesitylene were employed in the decarboxylative coup-
in the coupling reaction with cinnamic acid 1g, and the ling reaction (Schemes 3 and 4). For n-propylbenzene, the
phenyl-migrating product 4f was isolated in 65% yield. Methyl- product in two isomers were acquired in a total yield up to
substituted naphthalene could also be applied in this 93% and the benzyl carbon showed higher activity for
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Table 3 Ferrocene-catalyzed decarboxylative C(sp2)–C(sp3) coupling of 1g
with different toluenes 2a
Scheme 3 Ferrocene-catalyzed oxidative decarboxylative C(sp2)–C(sp3) coup-
ling of 1g with 1-propylbenzene (9).
Scheme 4 Ferrocene-catalyzed oxidative decarboxylative C(sp2)–C(sp3) coup-
ling between 1g and 2-bromo-1,3,5-trimethylbenzene (11).
a Catalytic conditions: 3,4-dimethoxycinnamic acid (1g) (0.3 mmol),
unactivated benzylic compound (2) (2 mL), ferrocene (15 mol%), DTBP
(2 equiv.), 120 °C, 24 h, Ar. Isolated yield based on cinnamic acid.
Scheme 5 Decarboxylative coupling reaction using Fe3O4 nanoparticles.
also have good efficiency and the magnetic properties of Fe3O4
catalyst could be considered for recycling. Thus, we extended
the scope of the substrates in the reaction using Fe3O4 nano-
particles as catalyst (Scheme 5). We can see that the coupling
of 4-chlorocinnamic acid or 4-methoxycinnamic worked well
in the reaction with toluene. The reaction of cinnamic acid
with p-xylene afforded the desired product in a good yield (up
to 85%).
Scheme 1 Ferrocene-catalyzed oxidative decarboxylative Csp3–Csp2 couplings
of 1g with (thio)anisoles (5).
The recycle efficiency of Fe3O4 nanoparticle was investigated
in the coupling reaction of cinnamic acid with p-xylene. We
observed that the catalytic activity of the catalyst remained
good for eight successive runs, as presented in Table 4. After
each run, the separation of the nanoparticles of paramagnetic
Fe3O4 was highly convenient, which was achieved by using
external magnets to retain the magnetic stir bar and catalyst in
the Schlenk tube while pouring out the solution phase. The
SEM image of the Fe3O4 nanoparticle catalyst after reaction
Scheme 2 Nucleophilic substitution of 1g with 1,3-dibromopropane (7).
olefination (Scheme 3). As for 2-bromomesitylene, the desired showed that the reaction system could maintain the nano-
product was obtained in 64% yield (Scheme 4). Therefore, it meter size of the catalyst very well, which results in the high
can be seen that C–H functionalization would prefer to occur catalytic efficiency (Fig. 1b).
at the position with less steric hindrance.
To gain some understanding of the reaction, we performed
Furthermore, the ability to easily reuse the catalyst is highly some additional experiments. Radical scavengers such as
desirable for industrial application.16 In the process of screen- TEMPO, AIBN or BQ can completely inhibit the reaction,
ing the catalytic conditions, we found that Fe3O4 nanoparticles which suggested that the transformation may proceed via a
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Table 4 Recycle of the Fe3O4 nanoparticle catalyst for the decarboxylative
coupling reaction of 1a with 2ca
Run
Time (h)
Isolated yieldb (%)
0
1
2
3
4
5
6
7
24
24
24
24
24
24
24
24
85
83
82
79
75
78
76
75
a Reaction conditions: 1a (0.3 mmol), 2c (2 mL), 70–80 nm Fe3O4
nanoparticles (20 mol%), DTBP (2 equiv.), 120 °C, 24 h, Ar. b Isolated
yield based on cinnamic acid.
Scheme 7 Investigation into the reaction mechanism.
Fig. 1 (a) Images of the nanoparticle catalysts before and after the reactions;
(b) SEM image of the Fe3O4 nanoparticle catalysts after 7 reaction cycles.
Scheme 8 Proposed mechanism for the decarboxylative coupling.
cinnamic acids in the absence of any ligands. The desired pro-
ducts were obtained with moderate to good yields. The use of
unactivated benzylic compounds, which are readily available,
economical, stable, and simple to handle, could be employed
as the novel coupling partner, which could avoid the use of
some sensitive organometallic coupling partners. It is note-
worthy that our catalytic system could be effectively reused for
7 times by using Fe3O4 nanoparticle catalyst. The scope, mech-
anism, and synthetic applications of this reaction are under
further investigation.
We are grateful to the grants from International S&T
Cooperation Program of Jiangsu Province (BZ2010048), the Pri-
ority Academic Program Development of Jiangsu Higher Edu-
cation Institutions, and the Key Laboratory of Organic
Synthesis of Jiangsu Province. We thank Prof. Dr Gregory
S. Smith from the University of Cape Town, South Africa, for
helpful discussion.
Scheme 6 Effect of radical inhibitors.
radical course (Scheme 6). In the absence of toluene, only dec-
arboxylative methylation product was obtained and the decar-
boxylative self-coupling of cinnamic acid was not observed,
which can suggest that styrene radical is not formed in the
reaction system (Scheme 7a). Then we performed a kinetic
isotope effect (KIE) experiment. The KIE value of 5.0 suggests
the sp3 C–H bond cleavage is rate limiting (Scheme 7b).
Based on previous observations and literature reports,17 we
proposed a plausible catalytic cycle (Scheme 8). The catalytic
cycle starts from homolysis of DTBP to give a tert-butoxy
radical. Benzyl radical A was generated by abstraction of H
from toluenes. Reaction of the cinnamic acid with ferrocene
would produce a salt of ferricinium cinnamate B in the pres-
ence of DTBP.18 Subsequently, addition of benzyl radical A to
the α-position of the double bond of ferricinium cinnamate B
would give intermediate C. Then C proceeds via elimination of
carbon dioxide and ferrocene to generate the product.
Notes and references
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In summary, we developed a novel ferrocene-catalyzed
direct alkenylation of sp3 C–H bonds via decarboxylation of
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