Efficient dimeric esterification of alcohols with NBS in water using l-proline as catalyst

Article abstract of DOI:10.1080/00397911.2010.521966

The L-proline-catalyzed oxidation of aliphatic primary alcohols with N-bromosuccimide (NBS) in water at room temperature to afford the corresponding dimeric esters in good to excellent yields was described. This pathway of dimeric esterification was proved to be very simple and environmentally friendly.

Full text of DOI:10.1080/00397911.2010.521966

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Efficient Dimeric Esterification of
Alcohols with NBS in Water Using L-
Proline as Catalyst
Xiuhong Liu a , Jun Wu a & Zhicai Shang a
a Department of Chemistry , Zhejiang University , Hangzhou , China
Accepted author version posted online: 30 Jun 2011.Published
online: 14 Sep 2011.
To cite this article: Xiuhong Liu , Jun Wu & Zhicai Shang (2012) Efficient Dimeric Esterification
of Alcohols with NBS in Water Using L-Proline as Catalyst, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:1, 75-83, DOI:
10.1080/00397911.2010.521966
To link to this article: http://dx.doi.org/10.1080/00397911.2010.521966
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Synthetic Communications1, 42: 75–83, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.521966
EFFICIENT DIMERIC ESTERIFICATION OF ALCOHOLS
WITH NBS IN WATER USING L-PROLINE AS CATALYST
Xiuhong Liu, Jun Wu, and Zhicai Shang
Department of Chemistry, Zhejiang University, Hangzhou, China
GRAPHICAL ABSTRACT
Abstract The L-proline-catalyzed oxidation of aliphatic primary alcohols with N-bromosuc-
cimide (NBS) in water at room temperature to afford the corresponding dimeric esters in
good to excellent yields was described. This pathway of dimeric esterification was proved to
be very simple and environmentally friendly.
Keywords Aliphatic primary alcohols; dimeric esterification; L-proline; NBS; water
INTRODUCTION
Extensive studies on the dimeric esterification of primary alcohols have been
carried out over the past 20 years, and numerous methods have been developed.
For example, dimeric esterification could be successfully promoted by metal
complexes [Ru3(CO)12,[1] RuH2(Ph3P)4,[2,3] PdCl2,[4,5] Pd(OAc)2,[6] PhCH2Nþ
[7]
[9]
Me3Brꢀ4 MoO and [IrCl(coe)2]2[8]], chromic acids (Na2Cr2O7=H2SO4 and PCC=
Al2O3[10]), oxoammonium salt in combination with pyridine,[11] molecular iodine,[12]
and brominated reagents [Br2=KBrO3,[13] NaBrO2=CH3COOH,[14] NaBrO3=HBr,[15]
pyridinium hydrobromide perbromide (PHPB)[16] and others.[17–20]]. However, most
of these pathways still have several drawbacks from both environmental and oper-
ational points of view, such as employment of toxic and expensive metal complexes
as catalysts, use of volatile organic compounds as solvents, poor yields, and require-
ments for an anhydrous environment, strong oxidative conditions, high temperature,
long reaction times or inert gases. Thus there is still demand for simple and clean
oxidation procedures.
As the use of N-bromosuccinimide (NBS) is more convenient and safe in
organic synthesis than bromine, NBS is typically used for bromination.[21–23] In
addition, NBS=additives have received considerable attention for oxidation of
Received January 20, 2010.
Address correspondence to Zhicai Shang, Department of Chemistry, Zhejiang University, Zheda
Road, Hangzhou 310027, China. E-mail: shangzc@zju.edu.cn
75

76
X. LIU, J. WU, AND Z. SHANG
alcohols to the corresponding aldehydes or ketones, such as NBS=b-cyclodextrin,[24]
NBS=cobalt(II) acetylacetonate,[25] NBS=NH4Cl,[26] NBS=H2O2,[27] and so on.
However, NBS used for the dimeric esterification of primary alcohols has rarely been
described. Therefore, it is meaningful to develop dimeric esterification of primary
alcohols using NBS as an effective oxidizing agent, which is a simple and green
procedure.
Recently, organocatalysis using small molecules has generated great interest in
organic synthesis.[28–31] Among many organocatalysts, L-proline is very important
because it is commercially available and inexpensive and could serve as an efficient
catalyst for a range of quintessential reactions, such as the asymmetric Aldol,[32,33]
Mannich,[34,35] Michael,[36,37] a-oxidation,[38–40] Diels–Alder,[41,42] multicomponent
Biginelli,[43,44] and Knoevenagel-type reactions.[45–48] Herein, to establish more sim-
ple and safe reaction conditions, we have used NBS in the dimeric esterification. To
the best of our knowledge, this is the first successful example of efficient dimeric
esterification of aliphatic primary alcohols in water with NBS as oxidant and
L-proline as catalyst (Scheme 1).
Initially, the esterification of n-pentanol (2 mmol) was carried out with NBS
(1 equiv.) in the presence of L-proline (0.8 mmol) in water (4 ml) at room tempera-
ture, and the corresponding pentyl pentanoate was obtained in 67% yield in 2 h
(Table 1, entry 1). This reaction was used as a model to evaluate the effect of catalyst
and oxidant loading on the reaction. The results were described in Table 1. In
Table 1, it was exciting to observed that when 0.2 m mol L-proline was used as cata-
lyst and 2 equiv. NBS was used as oxidant, in 1 h the corresponding dimeric ester
could be obtained in 97% yield (Table 1, entry 10). Based on those experimental
results in Table 1, we could conclude that both the concentration of Br2, which
was generated from NBS, and the reaction time have effects on the yields of the pro-
ducts. Moreover, different organic solvents were further tested as reaction media,
and the results showed that no reaction took place in solvents such as Et2O,
dimethylsulfoxide (DMSO), and CH3CH2OH, while poor yields of dimeric ester
were obtained in CH3COCH3, CH2Cl2, and CH3CN. Obviously, water was the best
solvent for the reaction.
The efficiency of the L-proline reagent compared to various primary or second-
ary amines catalysts was also examined (Table 2). This study found that the corre-
sponding dimeric ester could be obtained in good yield in the presence of any
catalyst in Table 2. However, L-proline was a more efficient and superior catalyst
(entry 4) than other catalysts with respect to yield of the desired ester in comparison.
To show the effect of L-proline for esterification, the reaction was carried out with-
out L-proline and no reaction took place (Table 2, entry 1). L-Proline was conse-
quently ascertained to be essential for the esterification of n-pentanol with NBS in
water.
Scheme 1. Dimeric esterification of aliphatic primary alcohols with NBS using L-proline as catalyst.

L-PROLINE-CATALYZED DIMERIC ESTERIFICATION
77
Table 1. L-Proline catalyzed oxidation of n-pentanol
Entry NBS (equiv.) L-proline (mol%)
Solvent
Time (h) Yielda (%)
1
2
1.0
0.5
1.5
2.0
2.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
20
20
20
20
20
0
H2O
H2O
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
1.5
2.5
4.0
4.0
4.0
4.0
4.0
4.0
67
31
84
93
93
0
3
H2O
H2O
4
5
H2O
H2O
6
7
5
H2O
H2O
89
95
88
90
92
97
95
95
0
8
10
40
60
10
10
10
10
10
10
10
10
10
10
9
H2O
H2O
10
11
12
13
14
15
16
17
18
19
20
H2O
H2O
H2O
H2O
Et2O
CH3COCH3
CH2Cl2
CH3CN
DMSO
CH3CH2OH
7b
18b
38b
0
0
Note. Reaction conditions: n-pentanol, 2 mmol; solvent, 4 ml; at room tempera-
ture. A variety of conditions to get pentyl pentanoate were investigated including
the reaction times, solvents, and L-proline and NBS loading.
aIsolated yield.
bYield determined by GC.
Based on these optimized conditions, several aliphatic primary alcohols with
NBS utilizing L-proline in water at room temperature were examined. All reactions
were completed within 1–3 h, as indicated in Table 3. From the results in Table 3, we
could see that higher aliphatic primary alcohols underwent oxidation smoothly to
give the corresponding dimeric esters in excellent yields and high purity without
Table 2. Effect of catalysts on pentyl pentanoate yield
Entry
Catalyst
Yielda (%)
1
2
3
4
5
6
7
8
9
No catalyst
Pyrrolidine
Piperidine
L-proline
L-alanine
D-valine
0
79
88
97
83
86
90
87
86
L-lysine
L-aspartic acid
L-cystine
Note. Reaction conditions: n-pentanol, 2 mmol; NBS, 4 mmol; catalyst,
0.2 mmol; water, 4 ml; at room temperature; and 1 h.
aIsolated yield.

78
X. LIU, J. WU, AND Z. SHANG
Table 3. L-Proline-catalyzed oxidation of alcohols using NBS as oxidant
Entry
Reagent
Product
Time (h)
Yielda (%)
1
2
3.0
1.0
90
97
3
4
2.0
3.0
85
92
5
6
1.0
1.0
94
95
7
8
1.0
3.0
97
94
9
10
11
3.0
24
99
—
—
2.0
Trace
2.0
2.0
83
84
12
13
4.0
91
14
15
1.0
1.0
83
91
Note. Reaction conditions: alcohols, 2.0 mmol; NBS, 4.0 mmol; L-proline, 0.2 mmol; water, 4 ml; and at
room temperature.
aIsolated yield.
further purification by column chromatography (Table 3, entries 2 and 5–9).
1-Hexadecanol, however, could not react very well for a long time, which may be
because the 1-hexadecanol was solid and totally insoluble in water, and thus could
not be brought into contact with the reaction medium very well (Table 3, entry 10).

L-PROLINE-CATALYZED DIMERIC ESTERIFICATION
79
Moreover, branched aliphatic primary alcohols such as 3-methyl-1-butanol and
2-methyl-1-butanol were also oxidatively dimerized to the corresponding esters in
good yields accompanied by a small amount of free fatty acids (Table 3, entries 3
and 4). However, oxidation of benzyl alcohol gave a negligible extent of benzyl ben-
zoate under identical conditions; instead, the corresponding benzaldehyde and some
monobromo benzaldehydes were obtained (Table 3, entries 11).[14] On the other hand,
secondary alcohols could also participate in the reaction to afford good yields of the
corresponding carbonyl compounds, which were in accordance with previous reports
(Table 3, entries 12–15).[4,16,18]
In conclusion, we have developed a green and efficient method for the dimeric
esterification of various primary alcohols with NBS using a catalytic amount of
L-proline as catalyst in water at room temperature. The important advantages of this
procedure are as follows: (a) operational simplicity, (b) eliminating toxic organic sol-
vents as reaction medium, (c) mild reaction conditions, (d) short reaction time, (e)
good to excellent yields without further purification by column chromatography,
and (f) employment of a catalytic amount of inexpensive and easily accessible
catalyst.
EXPERIMENTAL
Most of the organic chemical substrates are commercially available and pur-
chased from Alfa Aesar. Infrared radiation (IR) spectra were recorded on an Nexus
470 Fourier transform (FT)–IR spectrophotometer, and 1H NMR data were
recorded on an Advance DMX 500-MHz spectrometer in CDCl3 solution using
tetramethylsilane (TMS) as internal standard.
General Reaction Protocol
In a typical experiment, alcohol (2 mmol), NBS (4 mmol), and water were
(4 ml) added a round-bottomed flask (25 ml). Then, L-proline (0.2 mmol) was added,
and the reaction mixture was stirred at room temperature. After the reaction was
completed, the mixture was quenched with 0.5 M aqueous Na2S2O3 (3–5 ml) and
extracted with Et2O three times (3 ꢁ 5 ml). The organic layer was washed with brine
and dried over Na2SO4. After removal of the solvent in vacuo, the product was
purified by column chromatography if necessary (n-hexane=ethyl acetate).
Spectral (IR and 1H NMR) Data of Some Representative Compounds
Butyl butyrate (Table 3, Entry 1). IR: 1473, 1724 cmꢀ1. 1H NMR (500 MHz,
CDCl3, 25 ꢂC, TMS): d ¼ 0.9 [t, J(H,H) ¼ 7.2 Hz, 6H, ꢀCH3], 1.3 (m, 2H, ꢀCH2), 1.4
(m, 2H, ꢀCH2), 1.6 (m, 2H, ꢀCH2), 2.3 [t, J(H, H) ¼ 7.3 Hz, 2H, ꢀCH2], 4.1 ppm
[t, J(H,H) ¼ 6.6 Hz, 2H, ꢀCH2].
Pentyl pentanoate (Table 3, Entry 2). IR: 1467, 1739 cmꢀ1 1H NMR
.
(500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 [t, J(H,H) ¼ 6.4 Hz, 6H, ꢀCH3], 1.3
(m, 6H, ꢀCH2), 1.6 (m, 4H, ꢀCH2), 2.3 [t, J(H, H) ¼ 7.5 Hz, 2H, ꢀCH2], 4.0 ppm
[t, J(H,H) ¼ 6.8 Hz, 2H, ꢀCH2].

80
X. LIU, J. WU, AND Z. SHANG
3-Methyl-1-butyl 3-methylbutanoate (Table 3, Entry 3). IR: 1467,
1
1738 cmꢀ1. H NMR (500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 (m, 12H, ꢀCH3),
1.1 (m, 2H, ꢀCH2), 1.5 (m, 1H, ꢀCH2), 1.6 (m, 1H, ꢀCH), 2.1 [d, 2H, J(H,
H) ¼ 3.5 Hz, ꢀCH2], 4.1 ppm [t, J(H, H) ¼ 6.9 Hz, 2H, ꢀCH2].
2-Methyl-1-butyl 2-methylbutanoate (Table 3, Entry 4). IR: 1463,
1
1737 cmꢀ1. H NMR (500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 (m, 9H, ꢀCH3), 1.2
(m, 5H, ꢀCH2 and ꢀCH3), 1.5 (m, 2H, ꢀCH2), 1.7 (m, 1H, ꢀCH2), 2.4 (m, 1H,
ꢀCH2), 3.9 (m, 1H, ꢀCH2), 4.0 ppm (m, 1H, ꢀCH2).
Hexyl hexanoate (Table 3, Entry 5). IR: 1467, 1739 cmꢀ1 1H NMR
.
(500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 [t, J(H, H) ¼ 7.9 Hz, 6H, ꢀCH3], 1.3 (m,
10H, ꢀCH2), 1.6 (m, 4H, ꢀCH2), 2.3 [t, J(H, H) ¼ 7.5 Hz, 2H, ꢀCH2], 4.1 ppm [t,
J(H, H) ¼ 6.7 Hz, 2H, ꢀCH2].
2-Ethylhexyl 2-ethylhexanoate (Table 3, Entry 6). IR: 1462, 1735 cmꢀ1
.
1H NMR (500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 (m, 12H, ꢀCH3), 1.3 (m, 12H,
ꢀCH2), 1.4 (m, 4H, ꢀCH2), 1.6 (m, 1H, ꢀCH), 2.3 (m, 1H, ꢀCH), 4.0 ppm [d,
J(H, H) ¼ 5.70 Hz, 2H, ꢀCH2].
Heptyl heptanoate (Table 3, Entry 7). IR: 1467, 1739 cmꢀ1 1H NMR
.
(500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 [t, J(H, H) ¼ 6.7 Hz, 6H, ꢀCH3], 1.3 (m,
14H, ꢀCH2), 1.6 (m, 4H, ꢀCH2), 2.3 [t, J(H, H) ¼ 7.5 Hz, 2H, ꢀCH2], 4.1 ppm [t,
J(H, H) ¼ 6.7 Hz, 2H, ꢀCH2].
Octyl octanoate (Table 3, Entry 8). IR: 1467, 1739 cmꢀ1 1H NMR
.
(500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 (t, J(H, H) ¼ 6.4 Hz, 6H, ꢀCH3), 1.3 (m,
18H, ꢀCH2), 1.6 (m, 4H, ꢀCH2), 2.3 [t, J(H, H) ¼ 7.5 Hz, 2H, ꢀCH2], 4.1 ppm [t,
J(H, H) ¼ 6.7 Hz, 2H, ꢀCH2].
1
Dodecyl dodecanoate (Table 3, Entry 9). IR: 1466, 1740 cmꢀ1. H NMR
(500 MHz, CDCl3, 25 ꢂC, TMS): d ¼ 0.9 [t, J(H, H) ¼ 6.9 Hz, 6H, ꢀCH3], 1.3 (m,
34H, ꢀCH2), 1.6 (m, 4H, ꢀCH2), 2.3 [t, J(H, H) ¼ 7.5 Hz, 2H, ꢀCH2], 4.1 ppm [t,
J(H, H) ¼ 6.7 Hz, 2H, ꢀCH2].
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