An efficient one-pot oxidative esterification of aldehydes to carboxylic esters using B(C6F5)3-TBHP

Article abstract of DOI:10.1016/j.tetlet.2015.01.033

A simple and efficient protocol for oxidative esterification of diverse aldehydes with alcohols was accomplished with tert-butyl hydroperoxide and 1 mol % of tris(pentafluorophenyl)borane [B(C₆F₅)₃] to generate the corresponding esters in good to excellent yields. The present protocol represents compatibility with wide range of functional groups as well as exceptional tolerance toward acid labile protecting groups such as TBDPS, TBDMS, acetonide, and Boc.

Full text of DOI:10.1016/j.tetlet.2015.01.033

Tetrahedron Letters xxx (2015) xxx–xxx
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Tetrahedron Letters
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An efficient one-pot oxidative esterification of aldehydes
to carboxylic esters using B(C6F5)3–TBHP
⇑
Sravanthi Devi Guggilapu, Santosh Kumar Prajapti, Bathini Nagendra Babu
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research, Balanagar, Hyderabad 500037, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and efficient protocol for oxidative esterification of diverse aldehydes with alcohols was accom-
plished with tert-butyl hydroperoxide and 1 mol % of tris(pentafluorophenyl)borane [B(C6F5)3] to gener-
ate the corresponding esters in good to excellent yields. The present protocol represents compatibility
with wide range of functional groups as well as exceptional tolerance toward acid labile protecting
groups such as TBDPS, TBDMS, acetonide, and Boc.
Received 10 November 2014
Revised 2 January 2015
Accepted 5 January 2015
Available online xxxx
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Aldehyde
Oxidative esterification
Tris(pentafluorophenyl)borane
tert-Butyl hydroperoxide
The ester functional group is omnipresent in organic molecules
and is a prevalent structural motif found in an array of drugs, dyes,
polymers, agrochemicals, and natural products.1 Traditionally,
esters are prepared by the reaction of acids, acyl chlorides or anhy-
drides, and nitriles2,3 with alcohols. In addition, palladium-
catalyzed carbonylation of aryl halides to give carboxylic acid
derivatives is another strategic and reliable tool.4 However, these
methodologies have certain limitations.5 Recently, a potentially
powerful transformation which has regained attention is oxidative
esterification of aldehydes with alcohols under mild reaction con-
ditions.6 The traditional methods require stoichiometric amounts
of heavy-metal oxidants such as KMnO4,7 CrO3,8 hydrogen perox-
ide,9a ozone,9b oxone,9c N-iodosuccinimide,10a sodium hypochlo-
are engaged in exploring the potential of B(C6F5)3 in modern
organic synthesis, such as ring-opening of epoxides,28a aza-Ferrier
glycosylation,28b hydrosilylation of imines,29a reduction of alcohols
with silane,29b and hydrogenation of imines,29c Friedel–Crafts
alkylation of activated arenes and heteroarenes,30a
a-amidosulf-
ones,30a and Sakurai allylation.30b Recently, B(C6F5)3 has been
exploited as the Lewis acid component in frustrated Lewis pair
(FLP) chemistry to promote catalysis or to activate small
molecules.31,32
In continuation of our research in exploiting the efficiency of
B(C6F5)3 as Lewis acid catalyst,33 we disclose herein an oxidative
esterification of a variety of aldehydes with alcohols in the pres-
ence of a catalytic amount of B(C6F5)3 (Scheme 1).
rite,10b
silver
carbonate
on
celiteÒ,10c
2,3-dichloro-5,
Initially, the reaction of benzaldehyde (1 mmol) in methanol
(6 mL) was carried out with 1 mol % B(C6F5)3 and various oxidants.
Table 1 reveals that when 30% w/v H2O2, and m-CPBA (1 equiv)
were used as oxidants (Table 1, entries 1 and 2), the desired ester
was obtained in low yields along with benzoic acid and (dimeth-
oxymethyl) benzene as by-products. Further, we noticed that the
reaction proceeded with significant yields and negligible by-
products with 1 mol % B(C6F5)3 and TBHP in decane (5.5 M) in com-
parison to TBHP in 90% water (Table 1, entries 3 and 4). Other oxi-
dants benzoquinone and sodium hypochlorite (NaClO) were found
ineffective for oxidative esterification with 1 mol % B(C6F5)3 cata-
lyst (Table 1, entries 5 and 6).
6-dicyanobenzoquinone in the presence of amberlyst,11a sodium
metaperiodate,11b 1,2-dimethylindazolium,11c and mixtures of
methane sulfonic acid and aluminum oxide12 or transition-metal
catalysts such as vanadium,13 rhenium,14 silver,15 palladium,16
ruthenium,17 rhodium,18 copper,19 titanium,20 iridium,21 iron,22a
nickel,22b zinc.23 Also, electrochemical oxidations are utilized for
the same purpose.24 Very recently, calcium and magnesium chlo-
ride have also been used for the transformation of aldehydes to
their corresponding esters.25
Tris(pentafluorophenyl)borane [B(C6F5)3] has received promi-
nence as non-conventional, nontoxic, air-stable, water-tolerant,
and thermal abiding Lewis acid.26,27 Numerous research groups
Further optimization of the oxidative esterification utilized
benzaldehyde (1 mmol) in methanol (6 mL) using B(C6F5)3
(1 mol %) as catalyst and TBHP (1 equiv) as oxidant, which yielded
25% of the corresponding ester after 30 h (Table 2, entry 1). With
⇑
Corresponding author. Tel.: +91 40 23073740; fax: +91 40 23073751.
E-mail address: bathini@niperhyd.ac.in (B.N. Babu).
http://dx.doi.org/10.1016/j.tetlet.2015.01.033
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Guggilapu, S. D.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.033

2
S. D. Guggilapu et al. / Tetrahedron Letters xxx (2015) xxx–xxx
Table 3
O
O
B(C6F5)3 (1 mol%)
Solvent screening for oxidative esterification of benzaldehydea
R'
R
H
TBHP (3 equiv),
R'-OH, reflux
R
O
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
Methanol
Ethanol
18
24
30
30
48
48
48
86
82
78
75
20
—
R = alkyl, aryl or heteroaryl
R' = -CH3, -C2H5, -C3H7,-C4H9
1-Propanol
1-Butanol
2-Propanol
tert-Butanol
Benzyl alcohol
Scheme 1. General scheme for oxidative esterification of aldehydes with alcohols.
—
Table 1
a
Initial survey for oxidantsa
B(C6F5)3 (1 mol %), TBHP (5.5 M in decane) (3 equiv), benzaldehyde (1 mmol).
Isolated yields.
b
Entry
Oxidant (1 equiv)
Yieldb (%)
1
2
3
4
5
6
H2O2 aq (30%w/v)
m-CPBA
TBHP (5.5 M)
TBHP (90% water)
Benzoquinone
NaClO
12
16
25
20
—
Table 4
Oxidative esterification of aldehydes with simple alcoholsa
—
O
O
B(C6F5)3, TBHP
R'OH, reflux
a
R'
B(C6F5)3 (1 mol %), benzaldehyde (1 mmol), MeOH (6 mL), 30 h.
Isolated yields.
R
O
b
R
H
Entry
R
R1
Time (h) Product Yieldb (%)
further increase in concentration of TBHP led to an improved yield
(Table 2, entry 2). By using 3 equiv of oxidant, the desired product
was obtained in 86% yield in 18 h (Table 2, entry 3). No improve-
ment in relation to yield or reaction time was observed on increas-
ing the concentration of TBHP or catalyst (Table 2, entries 4 and 5).
As a consequence, B(C6F5)3 (1 mol %) and TBHP (3 equiv) were
selected as optimized reaction conditions for oxidative esterifica-
tion of aldehydes (Table 2, entry 3).
We also examined the use of different alcohols such as ethanol,
propanol, butanol, 2-propanol, and tert-butanol in the reaction
(Table 3). Under the optimized reaction conditions ethanol, propa-
nol, and butanol all gave the corresponding esters in very good
yields (Table 3, entries 2–4). 2-Propanol gave unsatisfactory yields
even after longer reaction time while tert-butanol and benzyl alco-
hol gave no products (Table 3, entries 5–7). These observations are
consistent with the decreased nucleophilicity or increased steric
barriers of the corresponding alcohols.
1
2
3
Me 18
1
2
86
92
NC
Me 18
Me 14
3a
3b
91
65
O2N
Pr
30
Me 14
Me 18
4a
4b
93
85c
4
O2N
Cl
Me 22
5a
5b
90
76
5
6
7
Et
28
Me 32
Me 38
6a
6b
82
75c
Me 32
7a
7b
84
85
O
Et
36
O
O
8
Me 38
Me 28
8
88
O
With the optimized reaction conditions in hand,34 we examined
the scope and generality of present methodology using function-
ally diverse aldehydes. In general, both electron withdrawing and
electron donating groups on the substituted aldehydes (Table 4,
entries 1–8) gave excellent yields of the corresponding esters.
However, as shown in Table 4, electron withdrawing groups gener-
ally increased the rate of reaction. The hydroxyl group substituted
aldehydes (Table 4, entries 9 and 10) were transformed into the
corresponding esters without affecting the hydroxyl groups. Fur-
ther, we applied the present protocol to complex compounds
which contains triple bond and ether linkage (Table 4, entries
9a
9b
83
61
HO
9
Bu
36
HO
10
Me 36
10
79
OH
OH
1135
Me 38
11
71
Cl
Cl
OH
O
1235
Me 34
12
75
O
O
O
O
O
Table 2
O
Optimization parameters for the oxidative esterification of benzaldehydea
O
1335
14
Me 24
Me 24
13
14
15
83
80
72
N
COOCH3
CHO
B(C6F5)3, TBHP
MeOH, reflux
H2N
Cl
Yieldb (%)
NH2
Entry
B(C6F5)3 (mol %)
TBHP (equiv)
Time (h)
15
Me 48
Me 24
1
2
3
4
5
1
1
1
1
2
1
2
3
4
3
30
24
18
18
18
25
58
86
85
85
16a
16b
81
75
Et
32
1635
OTBDPS
a
B(C6F5)3, TBHP (5.5 M in decane), benzaldehyde (1 mmol), MeOH (6 mL).
Isolated yields.
b
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S. D. Guggilapu et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
B(C6F5)3
Table 4 (continued)
OH
O
O
TBHP
Entry
R
R1
Time (h) Product Yieldb (%)
R'-OH
R'
R'
R
O
B(C6F5)3
R
O
R
H
Me 24
Et
17a
17b
78
72
OTBDMS
30
1735
Scheme 2. Plausible mechanism.
OTBDMS
oxy(aryl)methanol with the assistance of B(C6F5)3, followed by
tert-butyl hydroperoxide (TBHP) oxidation of the hydroxyl group
to generate the corresponding ester (Scheme 2).
In summary, the present protocol represents a general, facile,
and reproducible strategy for the oxidative esterification of various
aromatic, aliphatic, and heterocyclic aldehydes. It is noteworthy
that acid sensitive protecting groups such as TBDPS, TBDMS, aceto-
nide, and Boc were amenable to the optimized reaction conditions.
The environmentally benign, low catalyst loading, low toxicity, and
operationally simple procedure make protocol attractive.
Me 20
Et 28
18a
18b
92
87
18
N
19
20
21
Me 24
Me 24
19
90
N
20a
20b
72
61
Et
Me 24
Et 30
Me 24
Et 30
26
21a
21b
63
54
22a
22b
67
51
22
O
Acknowledgments
2335
Me 34
Me 24
23
68
N
H
O
The authors thank the Department of Pharmaceuticals, Ministry
of Chemicals and Fertilizers for providing funds and also acknowl-
edge Indian Institute of Chemical Technology, Hyderabad for the
scientific and instrumental support.
O
S
24a
24b
70
62
24
25
Et
30
N
Me 48
Me 24
25
40
HN
Cl
Supplementary data
26a
26b
82
75
Et
28
2635
N
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01.
033.
O
H
O
N
2735
Me 32
27
73
HN
O
Cl
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Please cite this article in press as: Guggilapu, S. D.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.033

4
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Please cite this article in press as: Guggilapu, S. D.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.033