Transesterification of various alcohols with vinyl acetate under mild conditions catalyzed by diethylzinc using N-substituted diethanolamine as a ligand

Article abstract of DOI:10.1016/j.tetlet.2005.06.129

Commercially available, inexpensive N-phenyldiethanolamine (6) is an efficient ligand for zinc-catalyzed transesterification. The use of 6/Et ₂Zn for reactions between various alcohols and vinyl acetate at room temperature produced the desired products in good yields.

Full text of DOI:10.1016/j.tetlet.2005.06.129

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5877–5879
Transesterification of various alcohols with vinyl acetate under
mild conditions catalyzed by diethylzinc using
N-substituted diethanolamine as a ligand
a
Yoshiaki Shirae, Takashi Mino, Tae Hasegawa,
b,
b
*
a
Masami Sakamoto and Tsutomu Fujita
b
a
Graduate School of Science and Technology, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan
b
Received 6 June 2005; revised 22 June 2005; accepted 23 June 2005
Available online 14 July 2005
Abstract—Commercially available, inexpensive N-phenyldiethanolamine (6) is an efficient ligand for zinc-catalyzed transesterifica-
tion. The use of 6/Et Zn for reactions between various alcohols and vinyl acetate at room temperature produced the desired prod-
2
ucts in good yields.
Ó 2005 Elsevier Ltd. All rights reserved.
Transesterification is a convenient method for the prepa-
ration of versatile esters from inexpensive alcohols and
We investigated the effect of using 1–6 as ligands for the
zinc-catalyzed transesterification of benzyl alcohol with
vinyl acetate in toluene as a solvent at room temperature
(Table 1). Without a ligand and Et Zn, the reaction did
2
not occur (entry 1). Using Et Zn as a catalyst, the reac-
2
1
2
3
esters. Acidic or basic catalysts are commonly used
to promote this reaction in heating, but they exhibit
low chemselectivity, and labile functional groups can
be damaged. They may also lead to the formation of side
products. Although organometallic catalysts such as
tion occurred, but we obtained benzyl acetate in
low yield (entry 2). Additions of 1–3 led to lower yields
4
5
Cp* Sm(thf) , distannoxanes, and iminophosphor-
2
2
6
anes are better than simple acids, they still require long
reaction times to complete the reaction, and from an
environmental point of view, these catalysts cannot be
Table 1. Effect of ligand on the transesterification of benzyl alcohol
a
with vinyl acetate
7
recommended. N-heterocyclic carbenes are reported
Et Zn
to be quite effective, but they are difficult to prepare.
We report here an efficient method of transesterification
using a zinc catalyst with 1–6 as ligands under mild
conditions.
2
Ligand
OH
OAc + CH CHO
3
+
OAc
PhMe, r.t.
b
Entry
Ligand
Ligand:Et
2
Zn (mol %)
Yield (%)
1
2
3
4
5
6
7
8
—
—
1
—
0
30
4
1
2
1
2
3
4
5
6
: R = H, R = H
—:10
5:10
5:10
5:10
5:10
5:10
5:10
1
2
: R = H, R = Me
HO
OH
1
2
: R = H, R = Et
N
R1
R1
2
8
1
2
: R = H, R = Bn
R1
R1
R2
3
16
46
52
99
1
2
: R = Ph, R = Bn
4
1
2
: R = H, R = Ph
5
6
a
Keywords: Zinc; Transesterification; Ester; Vinyl acetate.
*
Reaction conditions: BnOH (1 mmol), vinyl acetate (5 mmol), PhMe
(2 mL), rt, 1 h.
1
Determined by H NMR.
Corresponding author. Tel.: +81 432903385; fax: +81 432903401;
e-mail: tmino@faculty.chiba-u.jp
b
0
040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.06.129

5878
Y. Shirae et al. / Tetrahedron Letters 46 (2005) 5877–5879
compared to the case without ligands (entries 3–5).
N-Benzyldiethanolamine (4) and its derivative 5 led to
moderate yields (entries 6 and 7). When N-phenyldieth-
anolamine (6) was employed, the reaction yielded the
desired product quantitatively in 1 h (entry 8). In this
case, the aldol product from acetaldehyde was not
chloroform were less effective (entries 3–6). Thus, non-
polar solvents are more suitable for this reaction.
Under optimized reaction conditions, transesterifica-
tions of vinyl acetate with benzylic alcohols bearing a
variety of functional groups were carried out at room
temperature, and the results are summarized in Table
1
detected by H NMR analysis of crude product.
3
. All substrates yielded good conversions. 4-Methoxy-
In the presence of 6 as a ligand, the effect of the solvent
was examined. As shown in Table 2, toluene and hexane
led to high yields (entries 1 and 2). By contrast, polar
solvents such as THF, acetonitrile, ethyl acetate, and
benzyl alcohol and 4-chlorobenzyl alcohol were
smoothly converted into the corresponding acetates (en-
tries 2 and 3). The reaction of the benzylic alcohol with
ester moiety in its molecule required a longer reaction
time, but no side product was identified (entry 4). This
means that the methyl ester moiety is less reactive com-
pared to the vinyl acetate and does not take part in the
transesterification. ortho-, meta-, and para-Methyl benz-
yl alcohols were also treated with vinyl acetate, which
led to high yields. No steric effect was observed, nor
did any electronic influence result from the difference
of the position of the methyl group (entries 5–7).
a
Table 2. Effect of solvent
Et Zn, 6
OH
2
OAc
+
OAc
Solvent, r.t.
b
Entry
Solvent
Yield (%)
1
2
3
4
5
6
PhMe
Hexane
THF
99
81
27
7
We investigated the transesterification of various pri-
mary alcohols (Table 4). 1-Naphthalenemethanol was
MeCN
AcOEt
16
1
CHCl
3
a
Reaction conditions: BnOH (1 mmol), vinyl acetate (5 mmol), solvent
2 mL), 6 (0.05 mmol), Et Zn (0.1 mmol), rt, 1 h.
Table 4. Transesterification of various primary alcohols with vinyl
a
acetate
(
2
b
1
Determined by H NMR.
Et Zn, 6
2
ROH
+
ROAc
OAc
OH
PhMe, r.t.
Table 3. Transesterification of various benzylic alcohols with vinyl
a
acetate
b
Entry
Alcohol
Time (h)
Yield (%)
97 (97)
OH
Et Zn, 6
OAc
2
+
OAc
PhMe, r.t.
R
R
1
2
2.5
5
b
Entry
1
Alcohol
Time (h)
Yield (%)
OH
OH
99 (95)
1
3
99 (90)
93 (78)
OH
2
OH
c
MeO
3
4
(97)
OH
3
4
2
94 (90)
96 (64)
Cl
OH
OH
4
5
4
5
(96)
(91)
OH
c
24
MeOOC
OH
Me
OH
5
6
7
3
3
3
98 (92)
93 (88)
94 (87)
OH
6
7
6
5
99 (90)
8
c
OH
O
(97)
Me
Me
OH
8
24
87 (79)
OH
O
a
a
Reaction conditions: alcohol (1 mmol), vinyl acetate (5 mmol), PhMe
2 mL), 6 (0.05 mmol), Et Zn (0.1 mmol), rt.
Determined by H NMR. Values in parentheses are isolated yields.
(0.1 mmol) and Et Zn (0.2 mmol) were used.
Reaction conditions: Alcohol (1 mmol), vinyl acetate (5 mmol),
PhMe (2 mL), 6 (0.05 mmol), Et Zn (0.1 mmol), rt.
Determined by H NMR. Values in parentheses are isolated yields.
6 (0.1 mmol) and Et Zn (0.2 mmol) were used.
(
2
2
b
c
1
b
c
1
6
2
2

Y. Shirae et al. / Tetrahedron Letters 46 (2005) 5877–5879
Et Zn (0.1 mmol)
5879
2
6
(0.05 mmol)
OH
OAc
+
OH
+
+
OAc
OAc
PhMe (0.2 M)
r.t., 1 hr
1
mmol
1 mmol
10 mmol
90
:
10
Scheme 1.
Et Zn
tion of steric hindered secondary alcohols with vinyl ace-
tate (Scheme 2). In the presence of 0.2 mmol of Et Zn
and 0.1 mmol of 6 at room temperature, benzylic
secondary alcohols, such as 1-phenylethanol and
1-(2-naphthyl)ethanol, produced moderate to good
yields.
2
2
6
OH
+
OAc
OAc
PhMe, r.t.
4 hr
2
1
mmol
5 mmol
yield : 71 %
Et Zn
In summary, we found N-phenyldiethanolamine (6)/
Et Zn to be an efficient catalyst for transesterification
2
2
6
OH +
OAc
OAc
between various alcohols and vinyl acetate. This ligand
is inexpensive and the catalyst is easy to prepare. Using
this system, a wide variety of alcohols were converted
into corresponding acetates at room temperature in
good yields.
PhMe, r.t.
2
4 hr
1
mmol
5 mmol
yield : 83 %
Scheme 2.
References and notes
easily acylated into the desired ester (entry 1). Allylic
alcohols such as cinnamyl alcohol and geraniol also
led to good yields (entries 2 and 3). Phenethyl alcohol,
1
2
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2
-phenyl-1-propanol, and 3-phenyl-1-propanol reacted
equally well, giving the corresponding acetates in good
yields (entries 4–6). Saturated liner alcohol such as dec-
anol, was effectively acylated in high yield (entry 7). The
transesterification of alcohols bearing acid-sensitive
functional groups, such as 2-hydroxymethyl-1,4-benzo-
dioxane, led to the desired products. There was no side
product but the reaction required a much longer reac-
tion time (entry 8).
1
1
3
4
6
Selective protection of a primary alcohol versus a sec-
ondary alcohol is very useful in natural product synthe-
sis. This can be achieved by using organometallic
5
systems, such as distannoxane/enol ester or Sc(OTf) /
3
8
Ac O. We attempted selective acylation by employing
2
2
benzyl alcohol and phenylethyl alcohol (Scheme 1).
The reaction was carried out in the presence of excess
vinyl acetate and was quenched after 1 h. The conver-
6
9
1
sion for each alcohol was determined by H NMR,
and the result clearly indicated that the primary benzyl
alcohol was preferentially acylated in this system.
(
2
Base on the result of the acylation of various primary
alcohols with vinyl acetate, we tested the transesterifica-
A. J. Org. Chem. 1998, 63, 2342–2347.