Palladium-catalyzed aerobic oxidative direct esterification of alcohols

Article abstract of DOI:10.1002/anie.201008073

One step from alcohol to ester: A palladium-catalyzed oxidative esterification of various benzylic alcohols with methanol and long-chain aliphatic alcohols was carried out in the presence of molecular oxygen as the oxidant (see scheme). By considering the effects of substitution and potential mechanistic pathways, the applicability of this method to a range of different substrates was shown. Copyright

Full text of DOI:10.1002/anie.201008073

Communications
DOI: 10.1002/anie.201008073
Oxidative Esterification
Palladium-Catalyzed Aerobic Oxidative Direct Esterification of
Alcohols**
Chao Liu, Jing Wang, Lingkui Meng, Yi Deng, Yao Li, and Aiwen Lei*
Ester groups are among the highly important and abundant
functional groups in chemistry and can be found in bulk
chemicals, fine chemicals, natural products, and polymers.[1]
Traditionally, esters are prepared by the reaction of activated
acid derivatives (acyl chlorides and anhydrides) with alcohols
(Scheme 1, path A),[2] a multistep process that often produces
large amounts of unwanted by-products. A more direct
approach, which uses developments in transition-metal-
of metal salts like KHSO5 (oxone) or MnO2 have to be
employed as oxidants (Scheme 1, path C).[6] The required
aldehydes are normally synthesized by selective oxidation of
alcohols.[7] All of the above three methods require several
reaction steps and produce unwanted by-products, therefore
it is part of the challenges of green and sustainable chemistry
to update these old processes.
Alcohols (as some of the most fundamental compounds)
are usually readily available as bulk chemicals. Traditionally,
alcohols could be converted into esters by multiple steps.
However, direct conversion of alcohols into esters in the
presence of catalysts could represent a step forward toward
green, economic, and sustainable processes (Scheme 1,
path D).[8] In recent years, palladium-catalyzed oxidation
reactions have been widely investigated;[7a,c,9] much more
attention has been paid to highly selective oxidations using O2
as the terminal oxidant.[7a,10] To the best of our knowledge, no
results regarding the palladium-catalyzed aerobic oxidative
direct esterification from two different alcohols have been
reported. Herein, we communicate our recent progress in this
field.
Scheme 1. Approaches for the synthesis of ester groups.
Benzylic alcohol and methanol were used in the model
reaction and molecular oxygen was employed as the oxidant
(Table 1). Initially, different palladium catalyst precursors
were tested in methanol with Na2CO3 as the base (Table 1,
entries 1–3). [PdCl2(CH3CN)2] showed the best result with
53% yield of the desired product 2a, as determined by GC,
catalyzed cross-coupling reactions, involves the carbonylation
of aryl halides (Scheme 1, path B);[3] however, the high
temperature and high CO pressure required as well as the
presence of halide anions make this reaction environmentally
unfavorable.[4] Further efforts have been devoted to the direct
synthesis of esters from the oxidative esterification of
aldehydes with alcohols,[5] in which stoichiometric amounts
Table 1: Impact of reaction parameters on the oxidative esterification of
benzylic alcohols with methanol.[a]
[*] C. Liu, J. Wang, L. Meng, Y. Deng, Y. Li, Prof. A. Lei
College of Chemistry and Molecular Sciences
Wuhan University (P. R. China)
Fax: (+86)27-6875-4067
E-mail: aiwenlei@whu.edu.cn
Homepage: http://www.chem.whu.edu.cn/GCI_WebSite/
main.htm
Entry
Solvent
Base
Catalyst
Yield [%][b]
2a
3a
1
2
3
4
5
6
7
8
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
DMSO
THF
Na2CO3
Na2CO3
Na2CO3
–
Cs2CO3
NEt3
[PdCl2(PPh3)2]
[Pd(OAc)2]
44
51
48
42
Prof. A. Lei
[PdCl2(CH3CN)2]
[PdCl2(CH3CN)2]
[PdCl2(CH3CN)2]
[PdCl2(CH3CN)2]
[PdCl2(CH3CN)2]
[PdCl2(CH3CN)2]
[PdCl2(CH3CN)2]
[PdCl2(CH3CN)2]
53
38
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, 730000 Lanzhou (P. R. China)
n.d.
64
n.d.
34
n.r.
12
n.r.
48
[**] This work was supported by the National Natural Science
Foundation of China (20772093, 20972118, and 20832003), the
Fundamental Research Funds for the Central Universities, and
Program for New Century Excellent Talents in University. We thank
Prof. Xinquan Hu (Zhejiang University of Technology) for providing
several benzylic alcohols as gifts. We thank Prof. Matthias Beller
(Rostock University) for sharing information on work in this area.
K3PO4
NaOtBu
NaOtBu
NaOtBu
74
n.d.
n.d.
n.d.
n.d.
n.d.
9[c]
10[c]
[a] Reaction conditions: 0.5 mmol benzylic alcohol, 5 mol% Pd catalyst,
1.0 mmol base, 2 mL solvent, 458C, oxygen balloon, overnight.
[b] Determined by GC–MS by using biphenyl as internal standard.
[c] MeOH/1a=5:1. n.d.=not determined; n.r.=no reaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201008073.
5144
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 5144 –5148

and 38% yield of the aldehyde 3a. By-products of benzylic
formate could not be detected by GC–MS; no reaction was
observed without addition of a base (Table 1, entry 4). Base
screening showed that the organic base NEt3 (Table 1,
entry 6) was ineffective while a stronger base (NaOtBu)
gave the best reactivity with a 74% yield of the ester, as
determined by GC; the aldehyde could not be observed
(Table 1, entry 8). Solvent effects were also tested: using
MeOH as solvent afforded the best result (Table 1, entries 8–
10). No benzylic formate was detected even in other solvents
(DMSO, THF; Table 1, entries 9–10).
desired ester 2c was isolated, no aldehyde was left (Scheme 2
bottom, (c)). Consistent with these results, the following
pathway was proposed (Scheme 3) and a possible mechanism
was outlined (see the Supporting Information).
The conditions of entry 8 in Table 1 were then applied to
investigate the substrate scope. Unfortunately, the reaction
was very sensitive to the substituent on the phenyl ring of the
benzylic alcohol. Unsubstituted benzyl alcohol gave the
desired product 2a smoothly and no aldehyde 3a was
observed (Scheme 2, (a)), but for strongly electron-donating
Scheme 3. Possible pathway for the oxidative esterification reaction.
Firstly, benzylic alcohol coordinates to the PdII center
followed by b-hydride elimination to generate benzaldehyde
(step I), which directly reacts with another alcohol to form the
corresponding hemiacetal. The desired ester is a result of the
palladium-catalyzed b-hydride elimination of the hemiacetal
(step II). In these two reaction steps, the nature of the
substituent R has a significant influence on the reactivity of
the intermediates. An electron-donating group such as p-
À
OMe makes the C H bonds of CH2 more electron-rich so that
the b hydride can be easily eliminated and it thus becomes
easier to form the corresponding aldehyde (Scheme 2, (b)).
However, a strongly electron-withdrawing group such as p-
À
NO2 makes the C H bonds more electron-deficient so that
the b hydride cannot be easily eliminated; thus, it is relatively
difficult to form the corresponding aldehyde (Scheme 2, (c)).
For the aldehyde intermediate product, the carbonyl group is
also strongly influenced by the electronic properties of the
substitutent on the phenyl ring. The electron-donating p-OMe
group passivates the carbonyl group and thus hinders the
formation of the corresponding hemiacetal, so that the
reaction will prefer the aldehyde step (Scheme 2 bottom,
(b)). However, the electron-withdrawing group p-NO2 acti-
vates the carbonyl group and makes it more electron-
deficient, thus the carbonyl group easily reacts with another
alcohol to form the corresponding hemiacetal followed by
palladium-catalyzed b-hydride elimination to result in the
desired product (Scheme 2 bottom, (c)).
According to this hypothesis, a more electron-deficient
palladium center is required, as it can promote the harder to
accomplish b-hydride elimination of the electron-deficient
benzylic alcohols and can coordinate to the corresponding
aldehyde of the electron-rich benzylic alcohols to ease the
formation of the hemiacetal. To make the catalyst more
electron-deficient, a silver salt (AgBF4) was added to remove
the anionic ligand ClÀ from the palladium catalyst precursor.
Good results were achieved with 61% and 75% yield of the
desired methyl 4-methoxybenzoate and methyl 4-nitroben-
zoate (Table 2, entries 1 and 7), respectively.
Scheme 2. Substitution effect on the oxidative esterification reaction.
groups (EDGs), such as the methoxy group, no 4-methox-
ybenzyl alcohol could be found while 4-methoxybenzalde-
hyde 3b was observed in 76% yield by NMR spectroscopy; in
addition, 20% of the desired methyl ester 2b was detected
(Scheme 2, (b)). However, for the strongly electron-with-
drawing groups (EWGs), such as p-NO2, conversion of the
substrate 4-nitrobenzyl alcohol 1c was incomplete (54%
unreacted); no formation of the aldehyde was observed and
36% of the desired ester 2c was detected (Scheme 2, (c)). In
addition, the esterifications of three typical aldehydes with
different electronic properties were carried out under the
same conditions. The benzaldehyde gave the corresponding
ester in high selectivity and good yield (Scheme 2 bottom,
(a)). The benzaldehyde substituted with the strongly electron-
donating group p-OMe gave the desired product 2b in only
14% yield in NMR spectroscopy, with 68% of aldehyde 3b
remaining unreacted (Scheme 2 bottom, (b)). For the benzal-
dehyde substituted with the strongly electron-withdrawing
group p-NO2, the reaction proceeded smoothly; 88% of the
The optimized conditions were then used for a variety of
substituted benzylic alcohols and it could be shown that the
substitution sensitivity had been overcome (Table 2). Various
substituted benzylic alcohols could be employed in this
aerobic oxidative esterification reaction. The desired ester
was isolated in 61% yield when the benzylic alcohol
Angew. Chem. Int. Ed. 2011, 50, 5144 –5148
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Communications
Table 2: Oxidative esterification of different benzylic alcohols with
completely converted into the corresponding ester with a
yield of 86% (Table 2, entry 2). For the methyl group, both
para and meta position led to good results (87% and 89%
yield, respectively; Table 2, entries 3 and 4). Strongly elec-
methanol.[a]
À
À
tron-withdrawing groups such as CF3 and NO2 could also
be tolerated under the standard conditions (Table 2, entries 5,
Entry Substrate
Product
2
Yield
[%][b]
À
6, and 7), but the different positions of the CF3 group on the
substituted phenyl ring have a significant influence on the
reactivity of the reaction, as the benzylic alcohol substituted
with a CF3 group in the para position only gave 55% of the
desired ester. In GC–MS, no aldehyde was observed but some
starting alcohol remained. In the case of m-CF3-substituted
benzylic alcohol, complete conversion was observed and the
desired product was isolated in up to 86% yield. Other
substituents on the phenyl ring (p-F, p-Ph, p-COOMe) gave
moderate to good yields (Table 2, entries 8, 9, and 10). It is
worth noting that heterocycles such as thiophene-2-methanol
and furan-2-methanol could also be used in this aerobic
oxidative esterification reaction and gave the corresponding
esters in moderate to good yields (Table 2, entries 11 and 12).
Even for cinnamyl alcohol as allylic alcohol substrate, the
desired ester was isolated in 63% yield (Table 2, entry 13).
For all reactions in Table 2, no benzyl formates or benzyl
benzoates were observed in GC–MS.
After successful application of the oxidative esterification
to benzylic alcohols with methanol, we tried to extend the
newly developed protocol to long-chain aliphatic alcohols
(Table 3). Isobutanol was the first aliphatic alcohol used in
this oxidative esterification reaction. The desired product
could be obtained by addition of an electron-deficient P-
olefin ligand,[11] which was previously developed in our
laboratory,[12] to the optimized catalyst system ([PdCl2-
(CH3CN)2] + Ag salt) in THF solution. Several benzylic
alcohols were introduced and moderate to good yields of
the desired esters were obtained by employing O2 as the
oxidant (Table 3, entries 1–4). However, when n-butanol was
used as substrate, aldol condensation of butyraldehyde with
benzaldehyde (derived for the oxidation of their correspond-
ing alcohols) dominated the reaction. By exchanging the THF
solvent for nonpolar hexane, no aldol condensation product
was formed. The desired cross-esterification product was
obtained in moderate yield (Table 3, entry 5). It was note-
worthy that the ratio of benzylic alcohol and aliphatic alcohol
is 1:2 and only trace amounts of another possible cross-
esterification product (benzyl butyrate) were observed. Other
long-chain aliphatic alcohols were then employed as sub-
strates. n-Propyl, n-pentyl, n-hexyl, n-heptyl, n-octyl alcohols,
and even n-dodecyl alcohol could be utilized to afford the
desired products (Table 3, entries 6–10 and 12).
1[c,d]
2b
2d
2e
2 f
61
86
87
89
55
86
75
2
3
4
5[c]
2g
2h
2c
6
7[c]
8
9
2i
2j
88
75
10
11
2k
2l
60
82
12
13
2m 63
2n
63
In conclusion, we have successfully developed the first
palladium-catalzyed direct aerobic oxidative esterification of
benzylic alcohols with methanol and various long-chain
aliphatic alcohols. By considering the effects of substitution
and potential mechanistic pathways, we have been able to
show the applicability of this method to a range of different
substrates to give their corresponding esters in moderate to
high yields. The challenging esterification reactions of long-
chain aliphatic alcohols were accomplished by using a P-olefin
ligand to control the selectivity. The direct nature of this route
[a] Reaction conditions: 0.5 mmol benzylic alcohol, 5 mol% [PdCl2-
(CH3CN)2], 10 mol% AgBF4, 1.0 mmol NaOtBu, 458C, O2 balloon, 2 mL
MeOH, overnight. [b] Yield of isolated product. [c] 10 mol% [PdCl2-
(CH3CN)2], 20 mol% AgBF4. [d] 36% of the corresponding aldehyde was
isolated.
substituted with the strongly electron-donating group p-OMe
was employed; the corresponding aldehyde was isolated in
36% yield (Table 2, entry 1). However, when the methoxy
group was situated in the meta position, the alcohol was
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Angew. Chem. Int. Ed. 2011, 50, 5144 –5148

Table 3: Oxidative esterification of different benzylic alcohols with long-
Experimental Section
chain aliphatic alcohols.[a]
General procedure for reactions in Table 2: In a glove box, [PdCl2-
(CH3CN)2] (6.5 mg, 0.025 mmol), NaOtBu (96.1 mg, 1.0 mmol), and
AgBF4 (9.7 mg, 0.05 mmol) were filled into a Schlenk tube. The
Schlenk tube was taken out of the box and put into an ice–water bath.
An O2 balloon was fixed and the Schlenk tube was purged 3 times.
Benzylic alcohol (0.5 mmol) and MeOH (2.0 mL) were consecutively
added to the tube. Then the reaction mixture was heated to 458C by
oil bath and was stirred overnight. Then the reaction was quenched
with a saturated aqueous NH4Cl solution and extracted with diethyl
ether (3 ꢀ 10 mL). The organic layers were combined and the pure
product was obtained by flash column chromatography (petroleum
ether and diethyl ether).
Entry
1[c]
Product
5
Yield[%][b]
74
5a
General procedure for reactions in Table 3: In a glove box,
[PdCl2(CH3CN)2] (13.0 mg, 0.05 mmol), P-olefin ligand (39.2 mg,
0.10 mmol), K3PO4 (477.0 mg, 2.25 mmol), and Ag2CO3 (27.6 mg,
0.10 mmol) were filled into a Schlenk tube. The Schlenk tube was
taken out of the box, an O2 balloon was fixed, and the Schlenk tube
was purged 3 times. Under stirring, hexane (1.5 mL) was injected into
the tube. Subsequently, benzylic alcohol (0.5 mmol) and aliphatic
alcohol (1.0 mmol) were consecutively added to the tube. The
reaction mixture was stirred at 608C overnight. Then the reaction
was quenched with a saturated aqueous NH4Cl solution and extracted
with diethyl ether (3 ꢀ 10 mL). The organic layers were combined and
the pure product was obtained by flash column chromatography
(petroleum ether and diethyl ether).
2[c]
3[c]
4[c]
5b
5c
5d
84
63
42
5
5e
5 f
5g
5h
5i
46
40
44
44
41
51
50
54
Received: December 21, 2010
Published online: April 7, 2011
6
Keywords: aerobic oxidation · alcohols · aldehydes ·
.
oxidative esterification · palladium
7
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