pores, the complete extraction of organic compounds with
organic solvents from the zeolite pores as well as the quan-
titative analysis of the products, intact acrolein and polymers
would be difficult. The acrolein polymerization can be sup-
pressed by keeping the concentration of acrolein low. For this
purpose, two protocols were attempted: (1) The gradual forma-
tion of an acrolein monomer through the in situ monomer-
ization of a cyclic trimer of acrolein, and (2) the proper
selection of a specific solvent.
A cyclic trimer of acrolein, 2,4,6-trivinyl-1,3,5-trioxane, is a
1,3,5-trioxane on which three vinyl units are substituted at
the 2,4,6-carbons on the trioxane ring.22 Trioxane derivatives
are expected to be monomerized under acidic conditions to
generate the labile aldehydes,23 but there have been no reports
about the use of the trioxane derivative as a source of acrolein
as well as the generation of aldehydes at an appropriate rate.
We presumed that the gradual monomerization of the trimer
would supply acrolein to a reaction system in a preferable
concentration.
Derouane reported that in the liquid-phase reactions cata-
lyzed by zeolites, the concentration of reactants and/or
products in the zeolite pores were different from that in the
solution part due to the competitive adsorption of reactants,
products, and solvent into the zeolites.24 This selective adsorp-
tion is mainly governed by the polarity of each compound, and
thus the choice of the solvent affects the concentration of
reactants and products in the zeolite pores.25 The solvent effect
is responsible for the reaction rates, because a solvent competes
with reactants to diffuse in the zeolite pores, and to be adsorbed
on the active sites.26 That is to say, in a highly polar solvent, the
concentration of reactant is low in the zeolite pores, and the
reaction rate is decreased. Moreover, limiting the contact time
of products with the active sites in a highly polar solvent
can reduce the reaction rate of the consecutive reaction.26a By
exploiting this competitive adsorption effect of the solvent,
we expect that the solvent choice would lessen the undesired
acrolein polymerization.
and ethanol (5 mL) was added. The mixture was stirred for
10 min, and then the H-type zeolite was removed by filtering
through a 10-16 ¯m sintered glass funnel, washed with ethanol
(20 mL) and ethyl acetate (20 mL). To the combined filtrate was
then added an internal standard substance, and the mixture was
analyzed by gas chromatography (GC). The reaction yields by
GC analysis were calculated based on the amount of the added
acrolein. When a decreased amount (10 or 1.0 mmol) of a
benzene derivative was used, a benzene derivative in a certain
solvent (10 mL) was added to the activated H-Beta zeolite
(0.10 g) in a 10-mL flask. The yields of only 2g and 2h were
obtained by isolation.
2.2.2 Time-Course Measurements of the Amount of
Intact Acrolein: From a mixture of acrolein (1.0 mmol),
cyclohexane (40 ¯L), and solvent (5 mL) was collected an
aliquot (0.1 mL). This aliquot was diluted with 1-propanol
(1 mL) to analyze by GC. To the activated H-Beta (0.10 g) in a
10-mL flask was added the remaining mixture (ca. 5 mL) and
solvent (5 mL). After the flask was immersed in a water bath at
RT, the mixture was stirred. Each aliquot (0.2 mL) was peri-
odically picked up from the mixture. The aliquot was diluted
with 1-propanol (1.5 mL) and filtered through a 0.2 ¯m syringe
filter. The filtrate was analyzed by GC, and the area ratio of
cyclohexane to acrolein in each sample was compared with that
of the starting one.
3. Results and Discussion
3.1 Comparison between Acid Catalysts for the 1,4-
Addition of Anisole to Acrolein. We previously reported that
the addition of anisole (1a) to acrolein was induced by Na-Y
in a fair yield (Table 1, Entry 1),8c but that for the reaction to
proceed it required a large excess amount of anisole playing the
roles of both a reactant and a solvent, a large amount of Na-Y
(1.0 g) and the high reaction temperature of 154 °C. We then
searched for a much better catalytic system using homogeneous
and heterogeneous catalysts under much milder reaction con-
ditions for the 1,4-addition.
Though Na-Y (1.0 g) gave 2a in 66% yield under harsh con-
ditions at the reflux temperature of anisole (154 °C, Entry 1),
the catalytic use of Na-Y (0.1 g) at 60 °C did not induce 2a
(Entry 2). The catalytic use of H-Y (0.1 g) successfully gave
2a even at 60 °C in 77% yield with an 87% para-selectivity
(Entry 3). In this reaction, the turnover number (TON) is
defined as the 2a amount divided by the total amount of
exchangeable cations in the zeolite which is simply calculated
from the Si/Al ratio of the zeolite. The TON of Entries 1 and
3 were 0.2 and 7, respectively, proving that the protons in the
zeolite H-Y functioned as a real catalyst. Amorphous silica
alumina showed only the low catalytic activity of 8% yield
(Entry 4). Homogeneous catalysts of («)-10-camphorsulfonic
acid and BF3¢OEt2 showed no or poor activities in 0% and
26% yields, respectively (Entries 5 and 7). The stoichiometric
use of AlCl3 resulted in only a 17% yield of 2a (Entry 6). It
was reported that AlCl3 showed a moderate activity for the 1,4-
addition of anisole to methyl vinyl ketone (MVK, Supporting
Information Scheme S1),18a though MVK is a less electrophilic
α,β-unsaturated carbonyl compound than acrolein.4 AlCl3 was
considered to accelerate the acrolein polymerization rather than
the formation of 2a.
2. Experimental
2.1 Zeolites and Materials. Na-Y (Si/Al = 2.8, HSZ-
320NAA), H-Y (Si/Al = 15, HSZ-371HUA), and H-Mor
(Si/Al = 120, HSZ-690HOA) as powders were obtained from
the Tosoh Corporation (Tokyo, Japan). H-Beta (Si/Al = 75,
JRC-Z-HB150) and SiO2-Al2O3 (Si/Al = 5, JRC-SAL-2) as
powders were provided by the Catalysis Society of Japan.
The cation-exchange capacity of the zeolites is generally the
same as the aluminum content in the zeolite framework,27 so
we simply calculated the aluminum contents of Na-Y, H-Y, and
H-Beta from the Si/Al ratio of each zeolite to be 4.0, 1.1,
and 0.22 mmol g¹1, respectively. Each zeolite was activated at
400 °C/<26 Pa for 2 h just before use.
2.2 Procedures. 2.2.1 General Procedure for the 1,4-
Addition of Benzene Derivatives to Acrolein:
To the
activated H-type zeolite (0.10 g) in a 10-mL flask was added a
benzene derivative (10 mL or 10 g) of a reactant as well as a
solvent at the specified reaction temperature. After the mixture
was stirred for more than 10 min, acrolein (1.0 mmol) was
added over a period of one minute. After the reaction was
completed, the reaction vessel was placed in an ice-water bath,
© 2016 The Chemical Society of Japan