Palladium-catalyzed oxidative carbonylation of benzylic C-H bonds via nondirected C(sp3)-H activation

Article abstract of DOI:10.1021/ja3036459

A new strategy for generating benzylpalladium reactive species from toluenes via nondirected C(sp³)-H activation has been developed. This led to construction of an efficient Pd-catalyzed reaction protocol for the oxidative carboxylation of benzylic C-H bonds to form substituted 2-phenylacetic acid esters and derivatives from inexpensive, commercially available starting materials.

Full text of DOI:10.1021/ja3036459

Communication
pubs.acs.org/JACS
Palladium-Catalyzed Oxidative Carbonylation of Benzylic C−H Bonds
via Nondirected C(sp3)−H Activation
Pan Xie, Yinjun Xie, Bo Qian, Han Zhou, Chungu Xia, and Hanmin Huang*
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, P.R. China
S
* Supporting Information
Scheme 1. Strategies toward Synthesis of 2-Phenylacetic
Acid Derivatives
ABSTRACT: A new strategy for generating benzylpalla-
dium reactive species from toluenes via nondirected
C(sp3)−H activation has been developed. This led to
construction of an efficient Pd-catalyzed reaction protocol
for the oxidative carboxylation of benzylic C−H bonds to
form substituted 2-phenylacetic acid esters and derivatives
from inexpensive, commercially available starting materials.
ransition-metal-catalyzed carbonylation of organic halides
T
or organic pseudohalides has long been recognized as a
powerful approach to carboxylic acids and their derivatives.1
Considerable effort has been devoted to broadening the scope
of carbonylation precursors by the development of methods for
direct carbonylation of other common functional groups. In this
context, carbonylation of simple C−H bonds, a strategy
involving transition-metal-catalyzed C−H bond activation2
followed by carbonylation with CO, has received a lot of
attention as a highly desirable and attractive approach for
installing carbonyl functional groups on account of its high
atom and step economy.3 Although great progress has been
made based on the directing-group-oriented carbonylation of
C(sp2)−H bonds pioneered by Fujiwara, Chatani, Orito, Yu,
and others,4 to the best of our knowledge only isolated
examples have been demonstrated on the transition-metal-
catalyzed activation of C(sp3)−H bonds followed by carbon-
ylation with CO.5 As such, the direct activation of a simple
C(sp3)−H bond followed by carbonylation is still a great
challenge.
As a key intermediate for the traditional Pd-catalyzed
carbonylation of ArCHXR, the benzylpalladium complex B is
generally generated by the oxidative addition of ArCHXR to a
Pd(0) complex, which is then reacted further with CO and
other nucleophiles to complete the desired carbonylation
process. Recent progress on transition-metal-catalyzed and
radical-involved benzylic C−H functionalization8 has prompted
us to envision that such as an active species (B) might be
generated via a single-electron-transfer (SET) process triggered
by a free radical, since the oxidative addition can proceed via a
radical mechanism.9 Herein, we describe a novel Pd-catalyzed
carbonylation of benzylic C−H bonds via nondirected C−H
activation, providing an efficient approach to a series of 2-
phenylacetic acid esters and their derivatives (Scheme 1, path
c).
Initially, we explored the viability of the process with toluene
(1a) and n-BuOH as substrates in the presence of 10 atm of
CO. Several commercially available oxidants, such as di-tert-
butyl peroxide (TBP), tert-butyl hydroperoxide (TBHP),
K2S2O8, and dicumyl peroxide (DCP), were screened as
terminal oxidants. Pd complexes were screened as catalysts
because of their excellent performance in traditional carbon-
ylation reactions.1 The carbonylation reaction of toluene, n-
BuOH, TBP, and CO (10 atm) was carried out in the presence
of Pd(Xantphos)Cl2 (5 mol%, based on TBP; Xantphos = 9,9-
dimethyl-4,5-bis(diphenylphosphino)xanthene) at 120 °C for
16 h. As a result, the desired product 3aa, together with a small
amount of tert-butyl 2-phenylacetate, was obtained in 70%
yield.10 The product resulting from carbonylation of the
C(sp2)−H bond on the phenyl ring was not observed. This
result indicated that the proposed oxidative C−H carbonylation
Substituted 2-phenylacetic acids and their derivatives are
important commodity chemicals, valuable synthetic building
blocks for agrochemicals, and active pharmaceutical ingre-
dients.6 Traditional synthetic methods for those compounds
mainly relied on the transformation of ArCHXR (A; X =
halides, pseudohalides, or OH) through transition-metal-
catalyzed carbonylation with CO or noncatalytic nucleophilic
substitution with toxic NaCN, followed by hydrolysis with
strong acids (Scheme 1, paths a and b); ArCHXR is usually
prepared from the corresponding ArCH2R, and harsh reaction
conditions and stoichiometric amounts of bases are required.7
In contrast, directed transformation of the simple ArCH2R into
the corresponding carboxylic acids or esters via transition-
metal-catalyzed C−H carbonylation with CO is arguably a
highly efficient and atom-economic method toward these
compounds that would be highly desirable.
Received: April 16, 2012
Published: June 4, 2012
© 2012 American Chemical Society
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Communication
proceeded exclusively at the C(sp3)−H bond. Encouraged by
this result, we surveyed other oxidants, such as K2S2O8, TBHP,
N-fluorobenzenesulfonimide (NFSI), and DCP, and the results
demonstrated that this reaction proceeded most efficiently
when TBP served as the oxidant (Table 1, entry 1). With
Table 2. Scope of Pd-Catalyzed Oxidative Benzylic
Carbonylation
a
a
ac
,
b
R1, R2
product, yield (%)
TON
Table 1. Screening of Reaction Conditions
entry
1
H, H
3ac, 74
3bc, 70
3cc, 71
3dc, 68
3ec, 67
3fc, 66
3gc, 59
3hc, 65
3ic, 59
3jc, 55
3kc, 63
3lc, 63
3mc, 69
3nc, 70
3oc, 68
3pc, 50
3qc, 41
3rc, 44
3sc, 53
3tc, 46
3uc, 26
288
275
232
280
249
250
250
236
220
190
220
239
230
205
265
200
139
145
178
170
82
2
2-Me, H
3-Me, H
4-Me, H
3,5-diMe, H
4-MeO, H
4-EtO, H
4-n-PrO, H
4-n-BuO, H
4-n-HexO, H
4-Ph, H
3
4
b
entry
Pd species
oxidant
ROH
product, yield (%)
5
1
2
3
4
5
6
7
PdCl2
TBP
K2S2O8
NFSI
Ag2O
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
TBP
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
MeOH
3aa, 70
6
PdCl2
3aa, 8
7
PdCl2
3aa, 20
8
PdCl2
3aa, NR
3aa, 9
9
Pd(OAc)2
Pd(dba)2
Pd(OTf)2
PdCl2
10
11
12
13
14
15
16
17
18
19
20
21
3aa, 30
d
3aa, 39
4-F, H
c
8
3aa, 54
2-Cl, H
d
9
PdCl2
3aa, 64
3-Cl, H
e
10
PdCl2
3aa, 47
4-Cl, H
f
11
PdCl2
3aa, 46
4-Br, H
12
13
14
15
16
17
PdCl2
3ab, 69 (66)
3ac, 81 (76)
3ad, 70 (63)
3ae, 54
2,6-Cl2, H
4-Ac, H
PdCl2
EtOH
PdCl2
n-PrOH
i-PrOH
i-BuOH
t-BuOH
EtOH
1-naphthyl, H
2-naphthyl, H
H, Me
PdCl2
PdCl2
3af, 55
e
PdCl2
3ag, 36
a
Condition A: 1 (15 mmol), 2c (1 mmol), TBP (0.5 mmol),
g
18
PdCl2
3ac, 80 (74)
3ac, 62
Pd(Xantphos)Cl2 (0.01 mol), CO (10 atm) at 120 °C for 16 h.
h
b
19
PdCl2
EtOH
Condition B: 1 (15 mmol), 2c (4 mmol), TBP (3 mmol),
a
Reaction conditions: 1a (15 mmol), 2 (1 mmol), oxidant (0.5
Pd(Xantphos)Cl2 (0.005 mmol), CO (10 atm) at 120 °C for 18 h.
c
mmol), PdX2 (5 mol% based on oxidant), Xantphos (5 mol% based on
oxidant), 16 h. Yields were determined by GC analysis relative to the
oxidant, with n-dodecane as internal standard (isolated yield in
Isolated yield based on TBP; 2−5% corresponding tert-butyl ester was
b
d
e
observed in all cases. 1k (10 mmol) in 1 mL of benzene. CO (30
atm).
c
d
e
parentheses). CO (5 atm). CO (20 atm). 100 °C instead of 120 °C.
f
g
h
140 °C instead of 120 °C. Pd(Xantphos)Cl2 (2 mol%). Pd-
methyl and alkoxyl groups on any position of the phenyl ring
favored the reaction, providing corresponding substituted ethyl
2-phenylacetate in moderate to good yields (Table 2, entries
2−10). Substrates 1b−1e, with multiple potentially reactive
benzylic methyl groups, could only produce the monocarbo-
nylation products (Table 2, entries 2−5). With benzene as
solvent, the solid substrate 4-methylbiphenyl (1k) could also be
transformed into the corresponding product 3kc in 63% yield
(Table 2, entry 11). Furthermore, substrates with electron-
withdrawing groups such as F, Cl, and acetyl group on the
ortho-, meta-, and para-positions of the phenyl ring were
effective under the standard conditions (Table 2, entries 12−15
and 18). The bromide-containing substrate 2p was also suitable
for this carbonylation, giving the desired ester 3pc in 50% yield,
although a small amount of methyl 4-methylbenzoate was
observed. A slight steric hindrance effect on the reactivity was
observed, which was demonstrated by the reactivy of 2,6-
dichlorotoluene (Table 2, entry 17, 41% yield). 1- and 2-
methylnaphthalene could also participate in the oxidative C−H
carbonylation reaction to give the corresponding products in
53% and 46% yields, respectively (Table 2, entries 19 and 20).
In addition, ethylbenzene was also successfully employed for
the reaction at higher CO pressure, and the corresponding
product was obtained in 26% yield (Table 2, entry 21).
(Xantphos)Cl2 (1 mol%).
Xantphos as ligand, other Pd species, including Pd(OAc)2,
Pd(dba)2, and Pd(OTf)2, were then screened (Table 1, entries
5−7), with PdCl2 being the most effective. Other commonly
used metal salts, such as Co2(CO)8, Ni(II) salts, and Cu(II)
salts, were ineffective as catalysts. Furthermore, in the absence
of a Pd catalyst or in oxidant-free conditions, no carbonylation
product was observed under otherwise identical conditions.
Screening of other phosphine ligands revealed that Xantphos
was the most effective ligand for delivering the desired product
(see Supporting Information). The effects of temperature and
pressure of CO on the carbonylation reaction were also
investigated, and the best yield could be obtained when the
reaction performed in the presence of 10 atm of CO at 120 °C.
Further optimization of the reaction conditions demonstrated
that the choice of alcohol is crucial for the success of the
present catalytic reaction, and EtOH was identified as the
optimum coupling partner (Table 1, entry 13).11 To our
satisfaction, when the catalyst loading was lowered from 5 to 2
mol%, the reaction still worked well and gave the same yield of
the carbonylation product (Table 1, entry 13 vs 18).
With the optimized conditions in hand, the substrate scope
was explored at 120 °C under 10 atm of CO using 2 mol% of
Pd(Xantphos)Cl2 as the catalyst and TBP as the oxidant. As
summarized in Table 2, electron-donating groups such as
To further demonstrate the usefulness of this transformation,
a modified procedure (condition B) with lower catalyst loading
was developed, which increased the practicality of the reaction
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Communication
process dramatically. In the modified reaction conditions, the
catalyst loading was lowered from 2 to 0.167 mol% (based on
the oxidant TBP, S/C = 600) and the reaction time was
prolonged to 18 h; the product 3ac was obtained in 48% yield
with a turnover number (TON) of 288 (Table 2, entry 1).
Other substrates were also subjected to this procedure, and
high TONs were observed for almost all of the substituted
toluenes (Table 2, entries 1−18). 1- and 2-methylnaphthalene
also proved to be good substrates for this transformation under
the same conditions, generating adducts 3sc and 3tc with TON
= 178 and 170, respectively (Table 2, entries 19 and 20).
Finally, for the substrate 1u, TON = 82 could also be achieved
when the reaction was conducted in the presence of 30 atm of
CO (Table 2, entry 21). Compared to other reported Pd-
catalyzed C−H carbonylation reactions (TON < 40 ), our
catalytic system is much more efficient.4,5
To gain some insight into the mechanism of the present C−
H carbonylation reaction, some control experiments were
conducted under the standard reaction conditions. Radical
scavengers, such as TEMPO and 1,1-diphenylethylene, were
employed in the standard reaction, and no desired carbon-
ylation product was detected. This result suggested that a free
radical process was involved in the present oxidative C−H
carbonylation reaction. Moreover, kinetic isotope effect experi-
ments were carried out under the standard conditions (Scheme
2, see Supporting Information). The observed significant
provides the benzylpalladium complex C.12 Due to the steric
hindrance of complex C, intermediate F is formed slowly
through CO insertion,13 which leads exclusively to the less
hindered intermediate D through an anion-exchange process.14
Subsequent CO insertion forms the intermediate E, which
undergoes reductive elimination to afford the final carbon-
ylation product along with regeneration of the active Pd species
for the next catalytic cycle. The corresponding tert-butyl ester
might also be produced as a minor product through the
intermediate F.
In summary, we have succeeded in developing an efficient
Pd-catalyzed carbonylation of benzylic C−H bonds with CO
through nondirected C(sp3)−H bond activation. This carbon-
ylation process represents a practical and efficient methodology
for the synthesis of substituted phenylacetic acid esters from
simple toluenes. The new strategy for generation of such a
benzylpalladium intermediate should pave the way to some new
classes of C−H functionalization reactions, complementary to
the classical synthetic methods with organic halides. Ongoing
work seeks to gain a detailed mechanistic understanding of this
reaction, and applications of this C−H bond activation strategy
in other C−H functionalization reactions are currently in
progress.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and spectroscopic data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
Scheme 2. Kinetic Isotope Effect Experiment
AUTHOR INFORMATION
■
Corresponding Author
hmhuang@licp.cas.cn
Notes
isotopic effects (kH/kD = 4.9) indicated that the benzylic C−
H bond cleavage step occurs before the rate-limiting step or
might be involved in the rate-limiting step of this trans-
formation.
Although the mechanistic details of this transformation are
not clear at the moment, on the basis of the results we obtained
here and previously,1 a plausible mechanism for the present
process can be proposed as shown in Scheme 3. In this
scenario, homolytic cleavage of the TBP produces two alkoxyl
radical intermediates, one of which abstracts a benzylic
hydrogen atom from the substrate to give the benzyl radical.
In the presence of ligands, sequential oxidation of Pd(0) with
the two radicals step by step through the SET reaction9
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Chinese Academy of
Sciences, the National Natural Science Foundation of China
(21172226 and 21133011).
REFERENCES
■
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and D were formed; for details, see Supporting Information.
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