Tetrahedron Letters
Efficient acylation and transesterification catalyzed by dilithium
tetra-tert-butylzincate at low temperatures
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Miyuki Oshimura , Yuki Oda, Keita Kondoh, Tomohiro Hirano, Koichi Ute
Department of Chemical Science and Technology, Institute of Technology and Science, Tokushima University, 2-1 Minami-josanjima, Tokushima 770-8506, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The acylation and transesterification of alcohols with vinyl acetate and carboxylic esters were investi-
gated in the presence of a catalytic amount (1–10 mol %) of dilithium tetra-tert-butylzincate (TBZL) as
a catalyst. The acylation proceeded quantitatively at 0 °C within 1 h. The transesterification occurred
for a wide range of combinations of esters and alcohols at 0 to À40 °C. To the best of our knowledge, this
is the first successful transesterification at such low temperatures. The time–conversion plots for the
transesterification show that the reaction reached equilibrium within 5 min at 0 °C. The reaction pro-
ceeded quantitatively by addition of molecular sieves 4A. In addition, the transesterification proceeded
even in the presence of H2O and amines.
Received 28 January 2016
Revised 25 March 2016
Accepted 29 March 2016
Available online xxxx
Keywords:
Acylation
Transesterification
Zinc catalyst
Chemoselectivity
Ó 2016 Elsevier Ltd. All rights reserved.
Introduction
Recently, we reported that TBZL catalyzed ring-opening poly-
merization of e-caprolactone.11g In a subsequent study of the
Transesterification is a useful method for preparing various
esters.1 Acids2 and bases3 are commonly used as catalysts to pro-
mote these reactions. However, severe conditions are generally
required, that is, a high reaction temperature, long reaction time,
and large amount of catalyst. Consequently, thermally sensitive
functional groups undergo undesirable side reactions during the
transesterification.
copolymerization of e-caprolactone with 2-hydroxylethyl
methacrylate (HEMA), TBZL was found to act not only as an
anionic initiator but also as a transesterification catalyst.12 This
finding led us to investigate the acylation and transesterification
of alcohols with vinyl acetate and carboxylic esters in the presence
of a catalytic amount (1–10 mol %) of TBZL (Scheme 1).
Effective catalysts for promoting transesterification at room
temperature have been developed;4–9 for example, nitrogen–
heterocyclic carbenes catalyze transesterification of carboxylic
esters and acylation with vinyl acetate.4 The reactions are complete
within a few hours, using catalytic amounts (0.5–5.0 mol %) of the
carbenes. Et2Zn (10 mol %) catalyzes transesterification (acylation)
of benzyl alcohol (BnOH) with vinyl acetate at room temperature
in the presence of N-phenyldiethanolamine (N-PDEA, 5 mol %).7
Results and discussion
Homopolymerization of HEMA with TBZL in MeOH gave a
copolymer composed of two monomeric units, HEMA and methyl
methacrylate (Fig. S1). This result indicates that polymerization
accompanies transesterification of HEMA with MeOH at 30 °C
(Scheme 2). It is interesting to note that the reaction proceeded
at a moderate temperature.
The acylation of BnOH with vinyl acetate was examined to inves-
tigate the transesterification catalytic ability of TBZL13 (Table 1).14
Irreversible transesterification proceeded quantitatively with
A
l-oxo-tetranuclear zinc cluster catalyzes acylation of
alcohols under reflux conditions even in the presence of amine
compounds, which could impede the reaction.10
Uchiyama and coworkers11 reported that dilithium tetra-tert-
butylzincate (TBZL) catalyzed allylations of haloaromatics via
halogen–zinc exchange reactions. No protection is required for
haloaromatics containing a broad range of functional groups,
because of the unusually low basicity of TBZL.11b,c The high
chemoselectivity of TBZL enables direct polymerization of
N-isopropylacrylamide in aqueous media.11a,b
2-
tBu
2Li+
Zn
tBu
tBu
tBu
O
O
1 –10 mol% TBZL
0 to –40 °C
R3OH
R2OH
R2
R3
+
+
R1
O
R1
O
Scheme 1. Acylation and transesterification of alcohols with vinyl acetate and
carboxylic esters in the presence of TBZL.
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Corresponding author.
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.