Angewandte
Chemie
DOI: 10.1002/anie.201405212
Polyoxometalates
A Basic Germanodecatungstate with a À7 Charge: Efficient
Chemoselective Acylation of Primary Alcohols**
Kosei Sugahara, Naoto Satake, Keigo Kamata, Takahito Nakajima, and Noritaka Mizuno*
Abstract: The synthesis of highly negatively charged polyoxo-
metalates with electrically and structurally controlled uniform
basic sites can lead to the unique base catalysis. In this work,
a g-Keggin germanodecatungstate, [g-HGeW10O36]7À (A),
having a À7 charge was, for the first time, successfully
synthesized by the reaction of [g-H2GeW10O36]6À with one
equivalent of [(n-C4H9)4N]OH under non-aqueous conditions.
The activities of germanodecatungstates for base-catalyzed
reactions dramatically increased with increase in the number
negative charges from À6 to À7. In the presence of A, various
combinations of acylating agents and primary alcohols includ-
ing those with acid-sensitive functional groups chemoselec-
tively gave the desired acylated products in high yields even
under the stoichiometric conditions.
work as structurally well-defined active sites. Monomeric
fully-occupied and lacunary Keggin-type POMs have exten-
sively been investigated (see Table S1 in the Supporting
Information), and highly negatively charged ones have
typically been synthesized as alkali-metal salts. They are
intrinsically insoluble in common organic solvents, and their
basic oxygen atoms are coordinated to alkali metal cations. In
contrast, organic-solvent-soluble alkylammonium salts of
monomeric Keggin-type POMs with less than a À7 charge
have not been isolated.[4,5] In this context, strict control of
highly negatively charged (ꢀ À7) organic-solvent-soluble
POMs has not yet been accomplished.
Acylation of alcohols is an important and frequently used
transformation in organic synthesis because esters are val-
uable chemicals for solvents, plasticizing agents, perfumes,
medicines, and lubricants. The synthesis of esters from
carboxylic acids and alcohols usually requires harsh reaction
conditions. In contrast, the acid- and base-catalyzed acylation
of alcohols with acid anhydrides, acyl chlorides, and esters as
acylating agents is promising for the synthesis of esters under
mild reaction conditions. While efficient catalytic systems for
transesterification with enol esters have been reported, the
development of catalytic systems with high activity, high
selectivity, and a wide substrate scope under stoichiometric
conditions (alcohol/enol ester= 1:1) are still desirable (see
Table S2).[6–8] Herein, we report: 1) the stepwise synthesis of
a tetra-n-butylammonium (TBA) salt of germanodecatung-
state, [g-HGeW10O36]7À (A, Figure 1), by the reaction of
isolated [g-H2GeW10O36]6À with one equivalent of TBAOH
Polyoxometalates (POMs), which are a class of anionic
metal–oxygen clusters of early transition metals, have stimu-
lated many current research activities in many fields including
catalysis, medicine, and materials science.[1,2] While various
POM catalysts have been developed for acid-catalyzed,
photocatalytic, and oxidation reactions, base catalysis has
scarcely been investigated. We have, for the first time,
discovered a unique POM for base catalysis by focusing on
the following properties:[3] 1) Construction of uniform electri-
cally and structurally controlled basic sites, which is in
contrast to that of solid bases, 2) their thermal and oxidative
stabilities, which are higher than those of organic bases (e.g.,
trialkyl phosphine), and 3) the metal-oxo moiety, which
specifically activates nucleophilic substrates. The basicities
of POMs can be increased by increasing the number of
negative charges and the deprotonated oxygen atoms can
[*] K. Sugahara, N. Satake, Dr. K. Kamata, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
Dr. T. Nakajima
RIKEN Advanced Institute for Computational Science
Computational Molecular Science Research Team
7-1-26 Minatojima-minami-machi
Figure 1. Increase in the number of negative charges and construction
of active site(s) by the deprotonation of lacunary germanodecatung-
states.
Chuo-ku, Kobe, Hyogo 650-0047 (Japan)
[**] This work was supported in part by the Japan Society for the
Promotion of Science (JSPS) through its “Funding Program for
World-Leading Innovative R&D on Science and Technology (FIRST
Program)” and a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Science, Sports, and Technology of
Japan.
under non-aqueous conditions, 2) the significant increase in
the catalytic activity for base-catalyzed reactions with an
increase in the negative charge (from À6 to À7), and
3) efficient and chemoselective acylation of various primary
alcohols, having acid-sensitive functional groups, even under
stoichiometric conditions.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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