Lewis acid catalysts stable in water. Correlation between catalytic activity in water and hydrolysis constants and exchange rate constants for substitution of inner-sphere water ligands [22]

Full text of DOI:10.1021/ja980715q

J. Am. Chem. Soc. 1998, 120, 8287-8288
Table 1. Effect of Metal Salts in the Aldol Reactiona
8287
Lewis Acid Catalysts Stable in Water. Correlation
between Catalytic Activity in Water and Hydrolysis
Constants and Exchange Rate Constants for
Substitution of Inner-Sphere Water Ligands
Shuj Kobayashi,*,† Satoshi Nagayama, and Tsuyoshi Busujima
MXn
AlCl3
ScCl3
Sc(ClO4)3
CrCl3
MnCl2
Mn(ClO4)2
FeCl2
Fe(ClO4)2
FeCl3
Fe(ClO4)3
CoCl2
Co(ClO4)2
NiCl2
Ni(ClO4)2
CuCl2
Cu(ClO4)2
ZnCl2
Zn(ClO4)2
GaCl3
YCl3
Y(ClO4)3
RhCl3
PdCl2
AgCl
AgClO4
CdCl2
yield/%
MXn
InCl3
In(ClO4)3
SnCl2
yield/%
68c
14
4
80
81
83
78
85
trace
70 (78)b
82
Department of Applied Chemistry, Faculty of Science
Science UniVersity of Tokyo (SUT)
CREST, Japan Science and Technology Corporation (JST)
Kagurazaka, Shinjuku-ku, Tokyo 162
trace
trace
18 (40)b
39
La(OTf)3
Ce(OTf)3
Pr(OTf)3
Nd(OTf)3
Sm(OTf)3
Eu(OTf)3
Gd(OTf)3
Tb(OTf)3
Dy(OTf)3
Ho(OTf)3
Er(OTf)3
Tm(OTf)3
YbCl3
Yb(ClO4)3
Yb(OTf)3
Lu(OTf)3
IrCl3
PtCl2
AuCl
ReceiVed March 4, 1998
26 (55)b
21
Today’s environmental concerns demand clean reaction proc-
esses that do not use harmful organic solvents.1 Water is no doubt
the most environmentally friendly solvent; however, its use in
organic reaction processes is rather limited because many organic
materials do not dissolve in water, and therefore in most cases
reactions proceed sluggishly.2 In addition, many reactive inter-
mediates and catalysts are decomposed by water. This is the case
for Lewis acid catalyzed reactions, which are of great current
interest because of the unique reactivities and selectivities they
can achieve and for the mild conditions used.3 Lewis acids have
been believed to be unstable in water and therefore unusable in
aqueous solution. On the other hand, we have recently found
water-stable Lewis acids, lanthanide trifluoromethanesulfonates
(lanthanide triflates), which can be used in several carbon-carbon
bond-forming reactions in aqueous media.4 The stability and
catalytic activity of lanthanide triflates in water were ascribed to
their large ionic radii and an equilibrium between the Lewis acids
and water. We have now clarified that some metal salts other
than lanthanides are also stable Lewis acids in water and work
as catalysts.5 In addition, common characteristics, a certain range
of hydrolysis constants, and a high order of exchange rate
constants for substitution of inner-sphere water ligands (water
exchange rate constant (WERC)) have been found among these
water-stable Lewis acids.
88
90
81
85
89
86
85
7
trace
17 (7)b
trace
17 (7)b
25
47 (81)b
10
11 (92)b
84
92
84
46 (57)b
trace
5 (86)b
90
trace
trace
trace
trace
trace
15
trace
trace
trace
42 (36)b
18
HgCl2
HgCl
PbCl2
Pb(ClO4)2
BiCl3
59 (65)b
trace
Cd(ClO4)2
49 (72)b
a No adduct was obtained and the starting materials were recovered
when LiCl, NaCl, MgCl2, PCl3, KCl, CaCl2, GeCl4, RuCl3, SbCl3,
BaCl2, and OsCl3 were used. No adduct was obtained and the silyl
enol ether was decomposed when BCl3, SiCl4, PCl5, TiCl4, VCl3, ZrCl4,
NbCl5, MoCl5, SnCl4, SbCl5, HfCl4, TaCl5, WCl6, ReCl6, and TlCl3
were used. b H2O:EtOH:toluene ) 1:7:3. c Cf. ref 9.
We screened group 1-15 metal chlorides in a model reaction
of benzaldehyde with (Z)-1-phenyl-1-(trimethylsiloxy)propene (the
Mukaiyama aldol reaction) (Table 1).6 The reaction is suitable
for testing catalytic ability of the metal chlorides as Lewis acid
catalysts in aqueous media, because the silyl enol ether is water-
sensitive (especially under acidic conditions) and if the Lewis
acids hydrolyze in water, the enol ether decomposes rapidly and
the desired reaction proceeds no further. In the first screening,
the chloride salts of Fe(II), Cu(II), Zn(II), Cd(II), In(III), and Pb(II)
as well as the rare earths (Sc(III), Y(III), Ln(III)) gave promising
yields. When the chloride salts of B(III), Si(IV), P(III), P(IV),
Ti(IV), V(III), Ge(IV), Zr(IV), Nb(V), Mo(V), Sn(IV), Sb(V),
Hf(IV), Ta(V), W(VI), Re(V), and Tl(III) were used, decomposi-
tion of the silyl enol ether occurred rapidly and no aldol adduct
was obtained. This is because hydrolysis of such metal chlorides
is very fast and the silyl enol ethers were protonated then
hydrolyzed to afford the corresponding ketone. On the other hand,
no product or only a trace amount of the product was detected
using the metal chloride salts of Li(I), Na(I), Mg(II), Al(III), K(I),
Ca(II), Cr(III), Mn(II), Co(II), Ni(II), Ga(III), Ru(III), Rh(III),
Pd(II), Ag(I), Ba(II), Os(III), Ir(III), Pt(II), Au(I), Hg(II), and
Bi(III). Some of these salts are stable in water, but have low
catalytic ability. After the first screening, a second test was
performed for the more promising metals. This test was carried
out using the same aldol reaction and the corresponding metal
perchlorates or trifluoromethanesulfonates (triflates) (Table 1).7
It was found that Lewis acids based on Fe(II), Cu(II), Zn(II),
Cd(II), and Pb(II) as well as the rare earths (Sc(III), Y(III), Ln(III))
were both stable and active in water.8,9 Mn(II) and Ag(I)
perchlorates gave moderate yields of the aldol adduct.
†Present address: Graduate School of Pharmaceutical Sciences, The
University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
(1) For example, see: Anastas, P. T., Williamson, T. C., Eds.; Green
Chemistry; ACS Symposium Series 626; American Chemical Society:
Washington: DC, 1996, and references therein.
(2) (a) Li, C.-J.; Chan, T.-H. Organic Reactions in Aqueous Media;
Wiley: New York, 1997. (b) Reissig, H.-U. In Organic Synthesis Highlights;
Waldmann, H., Ed.; VCH: Weinheim, 1991; p 71. (c) Einhorn, C.; Einhorn,
J.; Luche, J. Synthesis 1989, 787.
(3) (a) Santelli, M.; Pons, J.-M. Lewis Acids and SelectiVity in Organic
Synthesis; CRC Press: Boca Raton, FL, 1995. (b) Schinzer, D., Ed.;
SelectiVities in Lewis Acid Promoted Reactions; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1989.
(4) (a) Kobayashi, S. Synlett 1994, 689. (b) Kobayashi, S. Chem. Lett. 1991,
2187. (c) Kobayashi, S.; Hachiya, I. J. Org. Chem. 1994, 59, 3590. (d)
Kobayashi, S.; Hachiya, I.; Yamanoi, Y. Bull. Chem. Soc. Jpn. 1994, 67, 2342.
(e) Kobayashi, S.; Ishitani, H. J. Chem. Soc., Chem. Commun. 1995, 1379.
(5) Scandium and yttrium triflates and related compounds were also found
to be stable Lewis acids in water. (a) Kobayashi, S.; Hachiya, I.; Araki, M.;
Ishitani, H. Tetrahedron Lett. 1993, 34, 3755. (b) Kobayashi, S.; Hachiya, I.;
Ishitani, H.; Araki, M. Synlett 1993, 472. (c) Hachiya, I.; Kobayashi, S. J.
Org. Chem. 1993, 58, 6958. (d) Kobayashi, S.; Wakabayashi, T.; Nagayama,
S.; Oyamada, H. Tetrahedron Lett. 1997, 38, 4559. (e) Kobayashi, S.;
Wakabayashi, T.; Oyamada, H. Chem. Lett. 1997, 831.
(7) Metal parts of metal perchlorates or triflates are more cationic than
those of metal chlorides, and thus metal perchlorates or triflates are more
Lewis acidic than metal chlorides. See also ref 4c.
(8) Some copper salts were reported to be stable Lewis acids in water
solution. (a) Otto, S.; Bertoncin, F.; Engberts, J. B. F. N. J. Am. Chem. Soc.
1996, 118, 7702. (b) Kobayashi, S.; Nagayama, S.; Busujima, T. Chem. Lett.
1997, 959.
(6) (a) Mukaiyama, T.; Banno, K.; Narasaka, K. J. Am. Chem. Soc. 1974,
96, 7503. See also: (b) Lubineau, A.; Meyer, E. Tetrahedron 1988, 44, 6065.
S0002-7863(98)00715-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/01/1998

8288 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Communications to the Editor
Table 2. Hydrolysis Constanta and Exchange Rate Constants for Substitution of Inner-Sphere Water Ligandsb
Although at first sight these elements seem to have little in
common, we noticed a correlation between their catalytic activity
in water and hydrolysis constants10,11 and WERC.12 pKh values
(Kh ) hydrolysis constant)13 and WERC of the cations are shown
in Table 2.10-13 Metal compounds, which gave more than 50%
yields in the aldol reaction, have pKh values from 4.3 to 10.08
and WERC greater than 3.2 × 106 M-1 s-1. There is no exception
in all the metal compounds we tested. Cations are generally
difficult to hydrolyze when their pKh values are large. In the
case that pKh values are less than 4.3, cations are easy to hydrolyze
and oxonium ions are formed. Under these conditions, silyl enol
ethers decompose rapidly. On the other hand, in the case that
pKh values are more than 10.08, the cations are too stable. The
pKh values are closely related to hydration energy and (electron)2/
(ionic radii) values. These values are correlated to the Lewis
acidity of the cations. Cations which have pKh values greater
than 10.08 have a small hydration energy as well as a small
(electron)2/(ionic radii) value, and therefore their Lewis acidity
is low. Similarly, WERC is also closely related to (electron)2/
(ionic radii) value. In general, a small (electron)2/(ionic radii)
value means a fast WERC. These requirements of pKh and
WERC values for obtaining sufficient activity as Lewis acid
catalysts in water are very strict, and if even one of them is not
fulfilled, low catalytic activity is observed. For example, several
alkali and alkaline earth elements have fast WERC, but their pKh
values are too large; consequently, almost no catalytic activity
was observed in the aldol reaction. On the other hand, Co(II)
and Ni(II) have suitable pKh values, but their WERC is smaller
than the criteria; therefore less than only 20% of the aldol adducts
were obtained even when Co(II) and Ni(II) perchlorate were used
in the aldol reaction. “Borderline” elements are Mn(II) and Ag(I).
Their pKh and WERC values are very close to the criteria limits,14
and the yields obtained in the aldol reaction are 40% and 42%,
respectively. Another interesting point is that, excluding the rare
earths, the elements fitting the criteria, Fe(II), Cu(II), Zn(II),
Cd(II), and Pb(II), are all classified as “soft acids.”15 This may
be related to interaction of these elements with water, which is a
“hard base.”15
Judging from these findings, the mechanism of Lewis acid
catalysis in water (for example, aldol reactions of aldehydes with
silyl enol ethers) can be assumed to be as follows. When metal
compounds are added to water, the metals dissociate and hydration
occur immediately. At this stage, the intramolecular and inter-
molecular exchange reactions of water molecules frequently occur.
If an aldehyde exists in the system, there is a chance for it to
coordinate to the metal cations instead of the water molecules
and the aldehyde is then activated. A silyl enol ether attacks
this activated aldehyde to produce the aldol adduct. According
to this mechanism, it is expected that many Lewis acid catalyzed
reactions should be successful in water solution.
This paper has shown the possibility of using several promising
metal compounds as Lewis acid catalysts in water. Research work
developing new aqueous reactions on the basis of these new
findings is in progress in our laboratories.
(9) Recently, indium(III) chloride-catalyzed Mukaiyama aldol reactions in
water were reported. (a) Loh, T.-P.; Pei, J.; Cao, G.-Q. J. Chem. Soc., Chem.
Commun. 1996, 1819. However, these reactions could not be reproduced in
our hands. (b) Kobayashi, S.; Busujima, T.; Nagayama, S. Tetrahedron Lett.
1998, 1579.
(10) Baes, C. F., Jr.; Mesmer, R. The Hydrolysis of Cations; Wiley: New
York, 1976.
(11) Yatsimirksii, K. B.; Vasil’ev, V. P. Instability Constants of Complex
Compounds; Pergamon: New York, 1960.
Acknowledgment. This work was partially supported by a Grant-
in-Aid for Scientific Research from the Ministry of Education, Science,
Sports, and Culture, Japan, and a SUT Special Grant for Research
Promotion.
JA980715Q
(12) Martell, A. E., Ed.; Coordination Chemistry; ACS Monograph 168;
American Chemical Society: Washington, DC, 1978; Vol. 2.
(13) Most values are -log Kxy. See ref 10 and Table 2.
(14) Actually, the hydrolysis constant of Ag(I) is 6.9 according to ref 11.
(15) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533.