502
J. Am. Chem. Soc. 2001, 123, 502-503
Enolization and Aldol Reactions of Ketone with a
La3+-Immobilized Organic Solid in Water. A
Microporous Enolase Mimic
Takehisa Dewa, Toshiyuki Saiki, and Yasuhiro Aoyama*
CREST, Japan Science and Technology Corporation (JST)
and Institute for Fundamental Research of Organic
Chemistry, Kyushu UniVersity, Hakozaki
Higashi-ku, Fukuoka 812-8581, Japan
ReceiVed March 31, 2000
ReVised Manuscript ReceiVed NoVember 7, 2000
There has recently been a renewed interest in recoverable/
reusable solid catalysts.1 Organic zeolites may be defined as
catalytically active microporous materials made up mostly of
organic components. A prototype of such materials with im-
mobilized Lewis acid centers catalyzes the Diels-Alder reactions
in a remarkable manner.2 A La3+ analogue (La host), on the other
hand, catalyzes typical base-promoted reactions such as Michael
and aldol in benzene.3 As for the media, the “green” or ecological
one is of course water.4 The present work is concerned about the
catalytic behavior of the La host in water. We report here that it
catalyzes the enolization of the ketone by the mechanism, which
suggests that these types of materials can be used as microporous
enzyme mimics.
Figure 1. (A) Binding isotherm for N2 as a guest at 77 K with the La
host and BET plot (inset), where V (mL) refers to the standard state and
PS ) 760 Torr. (B) Binding isotherm (O) for cyclohexanone as a guest
in water (50 mL) with the La host (0.02 mmol) at 298 K and initial rates
(b) of H/D exchange in cyclohexanone in D2O (2 mL) as catalyzed by
the La host (0.02 mmol) at 298 K.
As reported, treatment of anthracenebisresorcinol 1 (Chart 1)
with La(OiPr)3 in THF affords a 1:2 polycondensate,3 which, upon
aqueous workup, gives rise to an adduct 14-‚2(LaOH)2+‚6(H2O)
(La host). This formulation is consistent with the network structure
2 in which the hydrogen bond (b‚‚‚b ) O-H‚‚‚O-H) in apohost
1 is replaced by a metal coordination (b‚‚‚b ) O-‚‚‚La3+‚‚‚O-)
with a hydroxide ion and 3 water molecules on each metal center,
which is eventually hexacoordinated. The La host is microporous.
The binding isotherm for N2 at 77 K at lower guest pressures is
of the Langmuir type (Figure 1A). A BET analysis5 of the
monolayer-coverage region gives a specific surface area of ABET
) 230 m2/g.
Chart 1
The La host is completely insoluble in water but readily binds
∼30 mol of H2O with no leak of the metallic (by inductively
1
coupled plasma (ICP) analysis) or organic component (by H
NMR) into the aqueous phase, the latter being kept neutral at pH
7. The aquo complex separated from the aqueous phase easily
gets rid of extra water molecules at <150 °C (by thermogravim-
etry) to regenerate the original La host, 14-‚2(LaOH)2+‚6(H2O),
which can be identified by all the analytical6 and spectroscopic
means. Thus, the La host, while readily hydrated, is stable against
hydrolytic decomposition, in marked contrast to simple La3+
alkoxides and aryloxides.
When immersed in an aqueous solution of cyclohexanone, the
La host binds the ketone as a guest. The binding isotherm of a
saturation behavior (Figure 1B, open circles) is consistent with a
reversible Langmuir-type host-guest complexation and is ex-
pressed as guest/host ) (guest/host)sat‚K[guest]0/(1 + K[guest]0),
where (guest/host)sat ) 5.7 and K ) 5.3 M-1. The ketone adduct
recovered shows a significant complexation-induced shift to lower
wavenumber in νCdO for the bound guest (∆νCdO ) 17 cm-1).
This, coupled with the saturation guest/host ratio of ∼6, suggests
that the ketone binding is actually a water/ketone exchange as
metal ligands on each La3+ center (Scheme 1).7
When treated with the La host, the ketone in D2O undergoes
facile deuterium incorporation at the R-positions of the carbonyl
group to give 2,2,6,6-tetradeuterated ketone. When the insoluble
La host is removed by centrifugation, the supernatant liquid left
shows no reaction. Thus, the La host in the solid state must
catalyze the enolization of the ketone, a compelling process for
the H/D exchange. Under typical substrate-excess (ketone/La3+
) 10) conditions ([ketone] ) 1 M in 2 mL of D2O containing
0.1 mmol (on the formula basis) of the La host), the reaction
goes to completion with a half-life of τ ) 1.8 h, following the
first-order kinetics with respect to the ketone.8 Alicyclic and
aromatic ketones such as acetone (τ = 5.9 h) and acetophenone
(τ = 2.8 h) also undergo deuteriation at the R-positions. Neither
soluble LaCl3 nor insoluble La(OH)3 as a reference is active; no
(1) (a) Kobayashi, S.; Nagayama, S. J. Am. Chem. Soc. 1998, 120, 2985-
2986. (b) Kobayashi, S.; Endo, M.; Nagayama, S. J. Am. Chem. Soc. 1999,
121, 11229-11230.
(2) (a) Sawaki, T.; Dewa, T.; Aoyama, Y. J. Am. Chem. Soc. 1998, 120,
8539-8540. (b) Sawaki, T.; Aoyama, Y. J. Am. Chem. Soc. 1999, 121, 4793-
4798.
(3) Saiki, T.; Aoyama, Y. Chem. Lett. 1999, 797-798.
(4) Sato, K.; Aoki, M.; Noyori, R. Science 1998, 281, 1646-1647.
(5) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60,
309-319.
(6) Calcd for C26H28O12La2 (14-‚2(LaOH)2+‚6(H2O)): C, 38.54; H, 3.48;
La, 34.3. Found: C, 38.49; H, 3.04; La (ICP), 34.3.
(7) Alternatively, the ketone may be hydrogen bonded to a metal-bound
water.
10.1021/ja001140b CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/30/2000