Iron-Catalyzed Esterification of Benzyl C-H Bonds to Form α-Keto Benzyl Esters

Article abstract of DOI:10.1002/adsc.201500252

An atom-economic and efficient non-precious metal-catalyzed esterification of benzyl C-H bonds has been developed. A variety of α-keto benzyl esters have been accessed in good yields through the reactions between benzyl derivatives and benzoylformic acids using iron trifluoride as a catalyst in the presence of di-tert-butyl peroxide under an inert atmosphere. This strategy provides a straightforward access to linearly expanded α-keto benzyl esters. A plausible reaction mechanism is proposed.

Full text of DOI:10.1002/adsc.201500252

UPDATES
DOI: 10.1002/adsc.201500252
À
Iron-Catalyzed Esterification of Benzyl C H Bonds to Form
a-Keto Benzyl Esters
Yue He,a Jincheng Mao,a,b,* Guangwei Rong,a Hong Yan,a and Guoqi Zhangc,*
a
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials
Science, Soochow University, Suzhou 215123, Peopleꢀs Republic of China
Fax: (+86)-512-6588-0403; e-mail: jcmao@suda.edu.cn
State Key Laboratory of Oli and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu
b
610500, Peopleꢀs Republic of China
Department of Sciences, John Jay College and The Graduate Center, The City University of New York, New York, NY
c
10019, USA
E-mail: guzhang@jjay.cuny.edu
Received: March 12, 2015; Revised: April 8, 2015; Published online: June 10, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500252.
Abstract: An atom-economic and efficient non-pre-
cious metal-catalyzed esterification of benzyl C H
concerning side reactions. It would be highly desired
to replace the existing methodologies with more effi-
cient and environmentally sustainable chemical
routes, in regard to large-scale industrial applications.
À
bonds has been developed. A variety of a-keto
benzyl esters have been accessed in good yields
through the reactions between benzyl derivatives
and benzoylformic acids using iron trifluoride as
a catalyst in the presence of di-tert-butyl peroxide
under an inert atmosphere. This strategy provides
a straightforward access to linearly expanded a-
keto benzyl esters. A plausible reaction mechanism
is proposed.
À
During the past decades, the direct C H activation
has gained considerable interest and remarkable prog-
ress has been made in this field.[7] Presently, C H ac-
À
tivation is also considered as one of the increasingly
important methods for organic synthesis due to the
direct formation of various chemical bonds including
carbon-carbon[8] and carbon-heteroatom[9] bonds. Sev-
À
eral methods towards direct C H esterification have
also been built up. For example, alkylbenzenes have
been exploited to generate a range of benzyl esters
through direct oxidative coupling reactions with car-
boxylic acids.[10] However, these known methods fea-
À
Keywords: benzyl esters; catalysis; C H activation;
iron
À
turing benzyl C H esterification usually required pre-
It is well known that benzyl esters constitute a class
of essential functional groups as well as important
structural units that are found in numerous biological-
ly active compounds.[1] Benzyl esters are also critical
precursors for a tremendous number of functional
group transformations, playing a major role in the
synthesis of medicines and natural products.[2] In addi-
tion, benzyl esters are widely used as protecting
groups for carboxylic acids and their derivatives.[3]
Therefore, the synthesis of benzyl esters has attracted
considerable attention. To date, some viable methods
have been reported for the preparation of benzyl
esters,[4] and the most common way is to use benzyl
alcohols or benzyl halides as precursors to react with
carboxylic acids[5,6] [Figure 1, Eq. (1)]. However, some
major drawbacks remain, including the use of expen-
sive or toxic starting materials (benzyl alcohols or
benzyl halides), often low efficiency and the issues Figure 1. Known strategies for benzyl ester synthesis.
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Figure 2. Examples regarding practical applications of a-keto esters.
cious metals such as palladium[11] and rhodium[12] as mediates[17] and skeletons found in bioactive com-
catalysts. In 2003, Khan and co-workers[13] reported pounds,[18] as well as useful precursors for a variety of
the use of an inorganic NaBrO3/NaHSO3 system in functional group transformations[19] (Figure 2). How-
a stoichiometric amount for the reactions of toluene ever, the synthesis of a-keto esters[20] was scarcely re-
À
with aromatic carboxylic acids to generate benzyl ported through the direct C H functionalization of al-
esters at room temperature [Eq. (2)]. Subsequently, kylbenzenes with benzoylformic acid.
Yuꢀs group[14] demonstrated a catalytic oxidative
Herein, we wish to report our recent finding of
À
À
esterification of benzyl C H bonds mediated by tetra- a facile, iron(III) trifluoride-catalyzed oxidative C H
butylammonium iodide while using tert-butyl hydro- esterification of alkylbenzenes with benzoylformic
peroxide as a co-oxidant [Eq. (3)]. Very recently, acids by di-tert-butyl peroxide (DTBP) to prepare
Zhang and co-workers[15] developed a Pd(II)-cata- a range of a-keto benzyl esters (Figure 3), which were
lyzed benzylation of carboxylic acids with toluene inaccessible through the reported catalytic methods in
under 1 atm. oxygen [Eq. (4)]. Although this protocol the literature.[11–15]
utilizes molecular oxygen as the terminal oxidant,
Initially, we examined the reaction between ben-
which can be regarded as a green synthesis for benzyl zoylformic acid (1a) and ethylenzene (2a) under the
esters, the high loading of the precious metal palladiu- metal-free conditions developed by the Yu group, but
m(II) has greatly limited its practical applications.
no desired product was observed. Hence, we moved
À
Nevertheless, the metal-catalyzed C H bond func-
tionalization of unactivated alkylbenene has proven
to be a promising strategy for the synthesis of benzyl
esters. To improve the existing methodologies of pre-
À
cious metal-catalyzed benzyl C H esterification, our
goal was to introduce inexpensive, less toxic and
earth-abundant metal alternatives such as Cu or Fe to
replace the palladium catalysts. Albeit challenging,
the replacement of precious metal catalysts with
À
earth-abundant metals for the direct C H functionali-
zation has been extremely successful in many types of
reactions in recent years.[16]
a-Keto esters, the derivatives of esters bearing an
extra functional keto group, are key chemical inter- kylbenzenes to prepare a-keto benzyl esters.
À
Figure 3. Our strategy for the C H functionalization of al-
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Iron-Catalyzed Esterification of Benzyl C H Bonds
À
Table 1. Optimization of reaction conditions for benzyl C H
the screening of various reaction conditions are sum-
marized in Table 1. To our delight, the desired a-keto
benzyl ester, 1-phenylethyl 2-oxo-2-phenylacetate (3a)
was harvested in 64% yield when CuI (20 mol%) was
utilized as a catalyst and the reaction was conducted
in the presence of DTBP at 1108C for 18 h (Table 1,
entry 1). Slightly better result (70% yield of 3a) was
obtained when CuO was used to replace CuI under
the same conditions (entry 2), whereas other copper
salts were found to be less active catalysts for the
same reaction (entries 3–6). In order to further im-
prove the efficiency for the conversion of ethylben-
zene, we then tested several iron(III) and iron(II)
salts as catalysts. Although iron(III) salts or oxide be-
haved poorly for the catalytic reaction (entries 7–10),
iron(II) dichloride promoted the reaction with a yield
comparable to those resulting from copper(II)-cata-
lyzed reactions (entry 11). Further optimization of
catalysts revealed that the corresponding ester could
be obtained in 75% yield when FeF3 was employed
(entry 12). Other organic peroxides such as tert-butyl
hydroperoxide (TBHP), tert-butyl perbenzoate
(TBPB) and dicumyl peroxide (DCP) were also eval-
uated as oxidants, but none of these improved the re-
sults (entries 13–15).
functionalization.[a]
Entry
Catalyst
Oxidant
Yield[b] [%]
1
2
3
4
5
6
7
8
CuI
CuO
CuBr
Cu(OAc)2·H2O
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBHP
DCP
64
70
60
48
43
51
30
28
33
38
58
75
55
45
33
Cu(acac)
2
CuSO4
Fe2(SO4)3
Fe2O3
9
FeCl3
10
11
12
13
14
15
Fe(acac)3
FeCl2·4H2O
FeF3
FeF3
FeF3
FeF3
TBPB
With the optimized reaction conditions in hand, we
have applied the new protocol to various alkylben-
zenes and the results are summarized in Table 2.
While the introduction of toluene gave the desired
product 3b in moderate yield, substrates bearing elec-
tron-donating groups at the 2-, 3-, or 4-position of tol-
uene coupled with benzoylformic acid 1a to afford
[a]
Reaction conditions: benzoylformic acid (0.3 mmol), eth-
ylenzene (2 mL), catalyst (20 mol%), oxidant (0.6 mmol),
1108C, 18 h, Ar.
[b]
Isolated yields.
our focus to non-precious metal-catalyzed reactions the corresponding products 3c–f in good yields. It is
between 1a and 2a in the presence of an oxidant noteworthy that propylbenzene also underwent the
À
under an argon atmosphere. The results concerning C H esterification to give product 3g in 77% yield.
Table 2. Fe(III)-catalyzed synthesis of a-keto benzyl esters from various alkylbenzenes.[a]
[a]
Reaction conditions: benzoylformic acid (0.3 mmol), solvent (2 mL), FeF3 (20 mol%), DTBP (0.6 mmol), 1108C,
18 h, Ar; all yields are for isolated products.
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Table 3. Fe(III)-catalyzed esterification of ethylbenzene with various benzoylformic acids.[a]
[a]
Reaction conditions: benzoylformic acid (0.3 mmol), ethylenzene (2 mL), FeF3 (20 mol%), DTBP (0.6 mmol),
1108C, 18 h, Ar; all yields are for isolated products.
However, other toluene derivatives containing elec-
During the search to extend the scope of substrates
tron-withdrawing halo groups were not well tolerated with the current methodology, we attempted to re-
and the corresponding esters were obtained in rela- place the benzoylformic acids with other carboxylic
tively low yields (3h–3j).
acids (Scheme 1). The aliphatic analogue, 3,3-dimeth-
The substituent effect of the other reactant, ben- yl-2-oxobutyric acid was first evaluated under the
zoylformic acid was also studied under optimal reac- standard conditions described above, however, only
tion conditions, and the relevant results are summar-
ized in Table 3. It was revealed that benzoylformic
acids with both electron-donating and electron-with-
drawing groups were smoothly transformed to the de-
sired products. Specifically, a methyl substituent at
different positions of the aryl ring (ortho-, meta- or
para-) did not affect the reactions and the corre-
sponding esters were all obtained in high yields (4a,
4b and 4c). In contrast, benzoylformic acids with halo
substituents (F, Cl or Br) on the ortho- or para-posi-
tion showed slightly poorer reactivity than that with
a Br on the meta-position (products 4d–i vs. 4j), and
the latter was converted to the ester product 4j in
70% yield. To our surprise, even in the presence of
a
strongly electron-withdrawing trifluoromethyl
group, the expected product 4k was isolated in 77%
yield. In addition, reactions involving 4-methoxyben-
zoylformic or 2-naphthoylformic acid also proceeded
well affording the desired products in 60% and 67%
yields, respectively (4l and 4m). It is worth noting that
substrates with heteroaryl substituents such as thio-
phenebenzoylformic acid showed good reactivity
À
toward benzyl C H functionalization, giving the de-
sired product 4n in 52% yield.
Scheme 1. Fe(III)-catalyzed esterification of ethylenzene
with different carboxylic acids.
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Iron-Catalyzed Esterification of Benzyl C H Bonds
Scheme 2. Reactivity test using a radical scavenger.
trace amount of the desired product 5a was detected.
Nevertheless, we were able to isolate 5a in 12% yield
after 36 h while elevating the reaction temperature to
1308C. Next, we examined simple carboxylic acids in-
cluding cinnamic acid and benzoic acid. Unexpected-
ly, the former acid reacted with ethylbenzene under
the standard conditions to give the decarboxylative
product (E)-but-1-ene-1,3-diyldibenzene 5b in moder-
ate yield, instead of any desired esters, which was,
however, reminscent of our previous findings for re-
lated coupling reactions.[21a,b] In contrast, when benzo-
ic acid was used, a trace amount of ester product 5c
was observed, which could not be improved with ele-
vated temperature.
Scheme 3. A plausible mechanism for Fe(III)-catalyzed syn-
thesis of a-keto benzyl ester through C H functionalization.
À
In order to gain insights into the reaction details of
the Fe(III)-catalyzed reactions, additional reactivity
tests were performed (Scheme 2) by introducing
2,2,6,6-tetramethylpiperidine N-oxide (TEMPO), another molecule of tert-butyl alcohol as a by-prod-
a common radical scavenger into the reaction mixture uct.
of benzoylformic acid and ethylenzene under the
In summary, we have successfully developed
standard conditions. After 18 h at 1108C, only a trace a novel, effective and direct esterification of a benzyl
À
amount of product 3a was detected, indicating that C H bond catalyzed by iron trifluoride in the pres-
the reactions were almost completely inhibited by ence of di-tert-butyl peroxide as an oxidant. The pres-
TEMPO (4 equiv.). This result provides indirect evi- ent iron catalysis system is suitable for the coupling
dence that the reactions could have proceeded reactions between a range of benzoylformic acids and
through a radical pathway.
alkybenzene substrates, accessing a variety of a-keto
Based upon the previous literature reports[14,21] in- benzyl esters in moderate to high yields. The non-pre-
À
cluding our own work on related reactions and the cious metal iron-catalyzed C H functionalization of
above results, a proposed reaction process for the alkylbenzenes provides an alternative pathway to a-
present catalytic transformation is depicted in keto benzyl esters, which could provide significant im-
Scheme 3. Initially, the di-tert-butyl peroxide readily plications for the future catalyst design. Further stud-
experienced homolysis upon heating to generate the ies on this type of reaction and the exploration of its
tert-butoxyl radical, which subsequently attacks the applications are underway in our laboratory.
À
benzyl C H bond of ethylenzene to produce the key
benzyl radical A, with the release of tert-butyl alco-
hol. The radical A is liable to be oxidized through
a one-electron transfer process assisted by the Fe(III)
Experimental Section
catalyst to form the intermediate B which contains an
active carbocation, while reducing Fe(III) to Fe(II).
The nucleophilic attack of benzoylformic acid to the
carbocation of B would eventually generate the de-
sired product C and deliver a proton. On the other
hand, once formed, Fe(II) could be oxidized to recy-
cle the Fe(III) catalyst by another tert-butyl radical,
affording a tert-butoxide anion D which is likely to
General Experimental
All the reactions were carried out under argon. Solvents
were dried and degassed by standard methods and benzoyl-
formic acid, iron trifluoride, and DTBP were commercially
available. Other benzoylformic acids were synthesized ac-
cording to the literature procedures. Flash column chroma-
tography was performed using silica gel (300–400 mesh).
Analytical thin-layer chromatography was performed using
accept the proton from the benzoylformic acid to give glass plates pre-coated with 200–400 mesh silica gel impreg-
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were measured in CDCl3 on 400 MHz or 300 MHz NMR
spectrometers with TMS as an internal reference. Products
1
were characterized by H NMR and 13C NMR spectroscop-
ies and LC-MS.
General Procedure for the Reaction between
Benzoylformic Acids and Alkylbenzenes
À
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To a Schlenk tube equipped with a magnetic stir bar was
added benzoylformic acid (0.3 mmol), alkylbenzene (2 mL),
iron trifluoride (20 mol%) and DTBP (0.6 mmol) under
argon. The resulting reaction mixture was kept stirring at
1108C for 18 h. At the end of the reaction, the reaction mix-
ture was cooled to room temperature. After removal of the
solvent, the residue was subjected to column chromatogra-
phy on silica gel using ethyl acetate and petroleum ether
mixtures to afford the desired product in high purity.
À
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Acknowledgements
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We are grateful to the grants from the Scientific Research
Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry, the Priority Academic Program
Development of Jiangsu Higher Education Institutions, and
the Key Laboratory of Organic Synthesis of Jiangsu Prov-
ince. G.Z. acknowledges the American Chemical Society Pe-
troleum Research Fund (#54247-UNI3) and the PSC-CUNY
award from the City University of New York (#67312–0045)
for support.
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