DOI: 10.1002/chem.201400240
Full Paper
&
Lewis Acids
Slow Reactant–Water Exchange and High Catalytic Performance of
Water-Tolerant Lewis Acids
[
a, b]
[a, c]
[d]
[d]
Yusuke Koito,
Kiyotaka Nakajima,
Hisayoshi Kobayashi, Ryota Hasegawa,
[e]
[a, f]
Masaaki Kitano, and Michikazu Hara*
31
Abstract: P nuclear magnetic resonance (NMR) spectro-
and [In(OTf) TMPO] complexes than other metal triflate–
3
scopic measurement with trimethylphosphine oxide (TMPO)
was applied to evaluate the Lewis acid catalysis of various
TMPO complexes. The catalytic activities of [Sc(OTf) ] and
3
[In(OTf) ] for Lewis acid-catalyzed reactions with carbonyl
3
31
metal triflates in water. The original P NMR chemical shift
and line width of TMPO is changed by the direct interaction
of TMPO molecules with the Lewis acid sites of metal tri-
compounds in water were far superior to the other metal tri-
flates, which indicates that the high stability of metal tri-
flate–carbonyl compound complexes cause high catalytic
performance for these reactions. Density functional theory
(DFT) calculation suggests that low LUMO levels of [Sc(OTf)3]
31
flates. [Sc(OTf) ] and [In(OTf) ] had larger changes in
P
3
3
chemical shift and line width by formation of the Lewis
acid–TMPO complex than other metal triflates. It originates
from the strong interaction between the Lewis acid and
and [In(OTf) ] would be responsible for the formation of
3
stable coordination intermediate with nucleophilic reactant
in water.
TMPO, which results in higher stability of [Sc(OTf) TMPO]
3
Introduction
tivity of Lewis acids has remained vague: Lewis acid catalysis,
due to both the energy levels of the highest occupied molecu-
lar orbital (HOMO) of the reactant (nucleophile) and the lowest
unoccupied molecular orbital (LUMO) of the Lewis acid (elec-
Lewis acids, represented by AlCl , BF , and SnCl , involve
3
3
4
a metal center as an electron pair acceptor that accepts the
[
1]
[1]
electron pair from a nucleophile, and are effective catalysts
for carbon–carbon bond-forming reactions in organic sol-
trophile), is more complicated than that for Brønsted acids,
which can be discussed with respect to acid strength, such as
[
2]
vents. Lewis acids are essential catalysts used for the produc-
tion of indispensable chemicals, including polymers, medicines,
and agricultural chemicals. However, the rationale for the reac-
the Hammett acidity function, H . Although most Lewis acids
0
are decomposed or deactivated in water, metal trifluorometha-
nesulfonates (triflates) ([M(OTf) ]; M: metal center, OTf: ꢀ
x
OSO CF ) are well known as the few exceptions that function
2
3
[
a] Y. Koito, Dr. K. Nakajima, Prof. M. Hara
Materials and Structures Laboratory
Tokyo Institute of Technology
in water for various reactions, including the Mukaiyama aldol
condensation of various carbonyl compounds with silyl enol
[3,4]
ethers.
[Sc(OTf) ] and scandium tris(dodecylsulfate) exhibit
3
Nagatsuta, Midori-ku Yokohama 226-8503 (Japan)
E-mail: mhara@msl.titech.ac.jp
particularly high catalytic performance for the Mukaiyama
aldol condensation, Mannich-type reaction, Friedel–Crafts alky-
lation, and allylation reactions as water-compatible Lewis acid
[
b] Y. Koito
Research Fellow of Japan Society
for the Promotion of Science (JSPS)
-3-1 Chiyoda-ku, Tokyo 102-0083 (Japan)
[5]
catalysts. These homogeneous Lewis acids can activate the
carbonyl group of a reactant even in water; electrophilic attack
of the evolved carbocation intermediate then results in the de-
5
[c] Dr. K. Nakajima
Japan Science and Technology (JST) Agency, PRESTO
[6]
4
-1-8 Honcho, Kawaguchi 332-0012 (Japan)
sired product of these reactions. Although a wide variety of
metal triflates have been recognized as active Lewis acid cata-
[d] Prof. H. Kobayashi, R. Hasegawa
Department of Chemistry and Materials Technology
Kyoto Institute of Technology
[3]
lysts in water, the catalytic performance of these metal tri-
flates is strongly dependent on the metal species. Kobayashi
et al. reported that the catalytic activities of many metal chlor-
ides, perchlorides, and triflates for the Mukaiyama-aldol reac-
Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)
[
e] Dr. M. Kitano
Materials Research Center for Element Strategy
Tokyo Institute of Technology
tion are related to their hydrolysis constants (pK ) and water
h
Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
[7]
exchange rate constants (WERC). The hydrolysis constant re-
[
f] Prof. M. Hara
Japan Science and Technology (JST) Agency, ALCA
flects the stability of metal salts in water; metal salts with
small pK are easily decomposed in water into the correspond-
4
-1-8 Honcho, Kawaguchi 332-0012 (Japan)
h
ing metal hydroxides, whereas metal salts with large pK exhib-
Supporting information for this article is available on the WWW under
h
it less interaction with electrophilic molecules, including water,
Chem. Eur. J. 2014, 20, 8068 – 8075
8068
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim