Stabilization and activation of unstable propynal in the zeolite nanospace and its application to addition reactions

Article abstract of DOI:10.1039/c7cy01161j

Propynal (HC≡C-CHO), having both a C≡C triple bond and a formyl group in a molecule, is a promising building block but its labile property to easily polymerize often narrows its application for organic synthesis. In a similar way to unstable molecules, such as formaldehyde and acrolein, propynal is also stabilized and remains unchanged in supercages of Na-Y zeolite for over 30 days at ambient temperature. There, the carbonyl oxygen atoms of propynal coordinate to sodium ions in Na-Y which was proved by a ¹³C-DD/MAS-NMR analysis. In addition, propynal adsorbed in zeolite is sufficiently activated to allow unprecedented reactions; i.e., (1) a 1,3-dipolar cycloaddition with electron-deficient α-diazocarbonyl compounds, (2) a 1,4-addition with mono-, di-, and trimethoxy-substituted benzenes, and (3) a [2 + 2] cycloaddition of unactivated cycloalkenes. The nanospace of the zeolites keeps the products from dimerization during reaction (1) and from successive side-reactions in reaction (2). Quantum chemical calculations demonstrated that reaction (3) proceeds via a one-step-like non-concerted mechanism to afford the corresponding [2 + 2] cycloadducts. These three reactions can produce valuable synthetic intermediates retaining both a formyl group and a C=C double bond.

Full text of DOI:10.1039/c7cy01161j

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Stabilization and activation of unstable propynal in the zeolite
nanospace and its application to addition reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Daijiro Hayashi, Yuta Igura, Yoichi Masui, Makoto Onaka*
Propynal (HC≡C−CHO) having both a C≡C triple bond and a formyl group in a molecule is a promising building block, but its
labile property to easily polymerize often narrows its application for organic synthesis. In a similar way to unstable
molecules, such as formaldehyde and acrolein, propynal is also stabilized and remains unchanged in the supercages of the
Na-Y zeolite for over 30 days at ambient temperature, where the carbonyl oxygen atoms of the propynal coordinate to
sodium ions in Na-Y which was proved by a 13C-DD/MAS-NMR analysis. In addition, the propynal adsorbed in the zeolite is
sufficiently activated to allow unprecedented reactions; i.e., (1) The 1,3-dipolar cycloaddition with electron-deficient α-
diazocarbonyl compounds, (2) the 1,4-addition with mono-, di-, and trimethoxy-substituted benzenes, and (3) the [2 + 2]
cycloaddition of unactivated cycloalkenes. The nanospace of the zeolites keeps the products from dimerization during
reaction (1) and from successive side-reactions in reaction (2). Quantum chemical calculation demonstrated that reaction
(3) proceeds via a one-step-like non-concerted mechanism to afford the corresponding [2 + 2] cycloadducts. The three
reactions can produce valuable synthetic intermediates retaining both a formyl group and a C=C double bond.
DOI: 10.1039/x0xx00000x
www.rsc.org/catalysis
go in and out have been applied as catalysts or promoters not
only to gas-phase transformations of petrochemicals-based
materials, but also to liquid-phase organic reactions for fine
Introduction
Alkynes conjugated with a carbonyl group, such as esters or
ketones, are widely utilized for organic synthesis. Propynal
chemicals synthesis for decades.3 We have been interested
and engaged in probing the behaviors of easily polymerizable
or short-lived organic molecules such as formaldehyde,
acrolein and secondary benzylic cations confined in the zeolite
pores at ambient temperature as well as developing their
molecular transformations from the viewpoint of synthetic
organic chemistry.4,5,6 We discovered that the zeolites can
stabilize and activate the molecules adsorbed in the pores. For
example, capturing polymerizable formaldehyde and acrolein
in the supercages of the faujasite zeolite Na-Y and preserving
them in monomer forms at ambient temperature for days.4
Our quantum chemical calculation studies disclosed how such
labile molecules interacted with the zeolite pores and why
they were so stabilized:7 The calculated models showed that
three molecules of the monomeric formaldehyde and one
molecule of 1,3,5-trioxane, a cyclic trimer of formaldehyde,
encapsulated in the supercage of Na-Y were at equilibrium in
favour of the former. The reason is that the former is more
advantageous than the latter in terms of the coordination
energy of formaldehyde monomers to sodium ions in the Na-Y.
It was also discovered that the stabilized aldehydes in Na-Y
were sufficiently activated to react with some nucleophiles,
such as olefins and aromatics, to give homoallylic alcohols
through the Prins reactions of formaldehyde as well as 3-
arylpropanals via the 1,4-addition reactions to acrolein.4
(HC
propiolaldehyde, is the simplest conjugated ynal, and its
addition to the C C triple bond can lead to a product having
both formyl group and C=C double bond. As a
≡C−CHO), also known as propargyl aldehyde or
≡
a
a
consequence, propynal is considered to be one of the versatile
building blocks for the construction of valuable organic
compounds, but its use in organic synthesis has been scarce
compared with those of related acetylenic carbonyl
compounds such as alkyl propiolate and 3-butyn-2-one. One
crucial reason is the instability of propynal, which easily
polymerizes even at −20 °C without any catalyst, or even at
−78 °C in the presence of bases like pyridine.1 Therefore, a
reaction promoter, which can suppress the propynal
polymerization, is highly demanded to broaden the
applications of propynal in organic synthesis.
In order to shield labile compounds from the exterior
circumstances and to survive them, artificial organic cage
compounds, such as hemicarcerands, cavitands, and
networked molecular capsules, have been developed.2
Inorganic zeolites composed of open subnano-sized pore
systems where organic substrates and products are allowed to
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Scheme 1 Three types of propynal reaction discussed in this study.
We further discovered that in place of a stoichiometric
amount of the Na-Y zeolite, a catalytic amount of acidic
zeolites, such as H-Y and H-Beta, promoted the 1,4-addition of
benzene derivatives to acrolein at much lower temperatures.5
In order to suppress the self-polymerization of acrolein in the
study, we developed two protocols to maintain the
concentration of acrolein in the zeolite pores as low as
possible; i.e., (1) the slow in-situ generation of acrolein from its
oligomeric precursor in the zeolite pores, and (2) competitive
adsorption of a suitable solvent into the zeolite pores.
Scheme 3
α,β-unsaturated
starting materials for the syntheses of
unsaturated-

-butyrolactones,12

-lactams,14 cyclopentenes,15
,-

-lactones,13
carboxylate esters,16 and β2-amino acids,17 which are induced
by the N-heterocyclic carbene (NHC) catalysis. As Bode pointed
out, however, the cinnamaldehydes are not commercially
available, so they have to be prepared through troublesome
multiple steps or by the use of precious coupling catalysts
(Supplementary Information Scheme S1).18 For example,
cinnamaldehydes can be prepared through the reaction of
benzaldehydes with the Horner-Emmons reagent followed by
the conversion of the ester to the aldehyde (Scheme 3a),19 or
through the Heck reaction of bromobenzenes with acrolein
(Scheme 3b).20 Only one report about the 1,4-addition of
aromatics to propynal was published in 1986 using furan as a
nucleophile and formic acid as both the promoter and the
solvent (Scheme 3c).21 A few reports were published on the
1,4-addition of benzene derivatives to α,β-unsaturated
carbonyl compounds having a terminal triple bond,22 but no
example with heterogeneous catalysts.
Considering our findings described above, we presume that
1) Na-Y-encapsulated propynal remains in the monomer form,
and 2) the acid sites of Na-Y and H-Y promote three types of
reaction with propynal as shown in Scheme 1:
Pyrazoles are recognized as the core structures in a broad
variety of bioactive compounds, such as insecticides,
antibacterial agents and anticancer drugs, and they are
generally prepared through the reactions of 1,3-dicarbonyl
compounds with hydrazine derivatives (Scheme 2a).8 The 1,3-
dipolar cycloadditions of propynal to azides, the Huisgen
cycloaddition, have been reported to produce triazole
derivatives (Scheme 2b),9 but there were only three
reports9,10,11 about the reactions of propynal with diazo
compounds to form pyrazoles (Scheme 2c).
The 1,4-addition of benzene derivatives to propynal afford
cinnamaldehyde derivatives having a terminal formyl group
and an internal C=C double bond, but there have been no
precedents. The cinnamaldehydes turn out to be the key
The [2 + 2] cycloaddition of alkynes to alkenes is one of the
important methods to synthesize cyclobutenes which have
been used not only as valuable synthetic intermediates, but
also as monomers for polymerization.23 For example, the [2 +
2] cycloaddition of methyl propiolate to unactivated alkenes
catalyzed by aluminium chloride was intensively studied by
Snider,24 indicating that monosubstituted and 1,2-
disubstituted alkenes mainly gave [2 + 2]-type cycloadducts
rather than ene-type adducts. The [2 + 2] cycloadducts tend to
cause ring opening reactions with heat to give unsaturated
carbonyl compounds, so the [2 + 2] cycloaddition should be
performed as mildly as possible. Only one example of the [2 +
2] cycloaddition with propynal was reported by Hafner using
2 | J. Name., 2012, 00, 1-3
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Procedure for the 1,4-addition of methoxybenzenes with propynal
In the case of anisole or 1,3-dimethoxybenzene as
nucleophile, the procedure for each experiment is described in
Table 2 or Table 3.
a
In Scheme 5, to the activated Na-Y (1.0 g) in a 30-mL flask
was added 1,3,5-trimethoxybenzene (10 g). After the
suspension was stirred for 10 min at 100 °C, propynal (1.0
mmol) was added over a period of one minute. After the
reaction was completed in 3 h, the reaction vessel was placed
in an ice-water bath, and ethanol (5 mL) was added. The
mixture was stirred for 10 min, then the Na-Y was removed by
filtration through a 10–16 μm sintered glass funnel, and
washed with ethanol and ethyl acetate. To the combined
filtrate was added triphenylmethane as an internal standard
substance, and the mixture was analyzed by gas
chromatography (GC). The reaction yield was calculated based
on the propynal amount.
cyclopenta[cd]azulene which has a large dipole moment
(Scheme 4).25
Experimental section
Zeolites and materials
Powdery Na-Y (Si/Al=2.75, HSZ-320NAA) and H-Y (Si/Al=15,
HSZ-371HUA) were obtained from the Tosoh Corporation
(Japan). Powdery SiO2-Al2O3 (Si/Al=5.3, JRC-SAL-2) was
provided by the Catalysis Society of Japan. The cation
exchange capacity of the zeolites is generally the same as the
aluminium content in the zeolite framework,26 so we simply
calculated the aluminium contents of the Na-Y and H-Y from
the Si/Al ratio of each zeolite to be 4.0 mmol g-1 and 1.1 mmol
g-1, respectively. The Na-Y and H-Y were activated at 400 °C /
<26 Pa for 4 h and 2 h just before use, respectively.
General procedure for the [2 + 2] cycloaddition of alkenes with
propynal
To the activated Na-Y (1.0 g) in a 30-mL flask was added CH2Cl2
(10 ml) and propynal (1.0 mmol), and the suspension was
stirred for 30 min at RT prior to the reaction. To the
suspension was then added an alkene (3.0 mmol), and the
reaction mixture was continuously stirred for a specific
reaction time at RT. To the reaction mixture was then added
methanol (5 mL) and the mixture was stirred for 10 min. The
Na-Y was removed by filtration through a 10–16 μm sintered
glass funnel, and washed with methanol and ethyl acetate.
After the filtration through a Celite pad with ethyl acetate, the
combined filtrate was concentrated under reduced pressure.
This crude product was analyzed by NMR with
triphenylmethane as the internal standard. The reaction yield
was calculated based on the propynal amount.
Propynal was synthesized according to Sauer’s report.27 α-
Diazoacetophenone and 1-diazo-3,3-dimethyl-2-butanone
were synthesized as previously reported by Shioiri.28 The other
reactants were commercially available (Supplementary
Information Section I) and purified before use.
General procedure for the 1,3-dipolar cycloaddition of α-
diazocarbonyl compounds
To the activated Na-Y (1.0 g) in a 30-mL flask was added CH2Cl2
(10 ml) and propynal (1.0 mmol), and the suspension was
stirred for 30 min at room temperature (RT) prior to the
reaction. To the suspension was then added an α-
diazocarbonyl compound (1.5 mmol), and the reaction mixture
was continuously stirred for a specific reaction time at RT. To
the reaction mixture was then added methanol (5 mL) and the
mixture was stirred for 10 min. The Na-Y was removed by
filtration through a 10–16 μm sintered glass funnel, and
washed with methanol and ethyl acetate. After the filtration
through a Celite pad with ethyl acetate, the filtrate was
concentrated under reduced pressure. The recrystallization
from ethyl acetate produced a pyrazole compound as a white
solid. The residue of the product in the solution part was
purified on silica gel to afford the additional pyrazole. The
isolated yield was calculated based on the propynal amount.
Results and discussion
Capture and stabilization of propynal in Na-Y zeolite
For the effective organic synthesis using propynal, it is desired
to keep propynal from polymerizing. Labile carbonyl
compounds, such as formaldehyde and acrolein, were found to
be stabilized in the monomer form in the Na-Y pores, so
propynal is also thought to be applicable.4 Propynal vaporized
in a stream of nitrogen was adsorbed on powdery Na-Y at 0 °C
and the actual amount of the adsorbed propynal in 1.0 g of Na-
Y
was 3.8 mmol, which is hereafter referred to as
propynal(3.8)@Na-Y.
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and 5.8 ppm, respectively, than those of propynal in CDCl3
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DOI: 10.1039/C7CY01161J
13
2
(Fig.1a). On the other hand, the C chemical shift values of C
for the propynal(3.8)@Na-Y showed a slightly high-field shift
compared with that of propynal in CDCl3. Such chemical shifts
of the C1, C2, and C3 were also observed in the 13C-DD/MAS-
NMR spectrum of acrolein adsorbed on Na-Y,5 proving that the
carbonyl oxygen atoms of propynal strongly coordinate to the
Lewis acidic sodium ions in the zeolite pores. Surprisingly, after
storage for 30 days at ambient temperature, the
propynal(3.8)@Na-Y sample remained unchanged without any
formation of polymerized products (Fig.1c).
1,3-Dipolar cycloaddition of α-diazocarbonyl compounds with
propynal.
We reported the Na-Y promoted the 1,3-dipolar cycloaddition
of ethyl α-diazoacetate to alkynes having electron-withdrawing
groups such as esters as well as keto and formyl groups.11 For
example, propynal smoothly reacted with ethyl α-diazoacetate
(1a) to afford the corresponding pyrazole 2a. Pyrazoles with a
formyl group are known to be in equilibrium with their
dimer.29 Indeed, a mixture of the monomer (2a) and dimer (3a
)
was detected by 13C NMR in DMSO-d6 at RT (Fig.2a); the two
Fig.
1
13C NMR spectra of propynal: (a) Propynal in CDCl3, (b)
propynal(3.8)@Na-Y, and (c) propynal(3.8)@Na-Y after storage for 30 days
at RT.
sets of signals at 74.7 and 75.2 ppm in the 13C NMR spectrum
were assigned as the C–OH of two diastereomers of 3a. 2a was
also assigned by comparison with the NMR spectrum at 60 °C
in which only 2a was included (Fig.2b). Since the size of 3a was
too large to be accommodated in the pores, we expected that
the monomer product 2a would stay in the Na-Y pores. To
determine whether 2a or 3a was present in the Na-Y during
the 1,3-dipolar cycloaddition, we evacuated the reaction
mixture including Na-Y and analyzed it with 13C-CP/MAS-NMR
at RT (Fig.2c). The C–OH peaks of 3a at around 75 ppm were
To confirm that propynal was in the monomer form in the
Na-Y, the propynal(3.8)@Na-Y sample was analyzed by 13C-
DD/MAS-NMR spectroscopy. Propynal(3.8)@Na-Y showed
three distinctive peaks at 182.5, 88.4 and 81.2 ppm which
were assigned to the C1, C3 and C2 carbons of propynal,
respectively (Fig.1b). The 13C chemical shift values of C1 and C3
for the propynal(3.8)@Na-Y showed the low-field shift by 6.0
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Table 1 1,3-Dipolar cycloaddition of α-diazocarbonyl compounds to
Table 2 1,4-Addition of anisole to propynala
propynala
Na-Y
(g)
1.0
1.0
—
—
1.0
1.0
H-Y
(g)
—
Temp.
(°C)
80
140
80
80
80
80
Time
(h)
1
Entry
Yieldd (%)
para:ortho
1b
2b
3
<1
14
35
22
43
42
—
—
4
82:18
85:15
84:16
80:20
81:19
0.10
0.10
0.10
0.10
12
12
12
12
4c
5
6c
Entry
1: R
Additive
Na-Y
—
Na-Y
—
Time (h)
Yieldb (%)
b
a Reaction conditions: 4a (10 mL), propynal (1.0 mmol). Reaction mixture was
stirred for 0.5 h at RT prior to the reaction. c Propynal was added using a syringe
pump over 11 h. d Yield was determined by GC analysis.
1
2
3
4
5
6
1
1
1
1
24
24
83
30
93
1
90
6
1a: OEt
1b: Ph
which was proved to proceed even after the exterior surface of
the acidic zeolite was selectively poisoned by using a bulky
base.5 We then envisaged that the coexisting Na-Y would
probably work as a reservoir which could entrap 5a and keep
5a from any side-reactions induced by the acidic H-Y. Indeed,
the combined use of Na-Y (1.0 g) and a catalytic amount of H-Y
(0.1 g) increased the yield to 43% (Table 2 entry 5).32 The slow
addition of propynal to 4a in the presence of only H-Y using a
syringe pump did not improve the yield of 5a, but the slow
addition in the presence of Na-Y and H-Y similarly increased
the yield (Table 2 entries 4 and 6).
Na-Y
—
1c: t-Bu
a Reaction conditions: 1 (1.5 mmol), propynal (1.0 mmol), additive (1.0 g), CH2Cl2
b
(10 mL), RT. Isolated yield of both 2 and 3. 2 includes two prototropic
tautomers.30
lost, and the carbonyl carbon peak of 2a was observed at
around 185 ppm, indicating that 2a rather than 3a favourably
stayed in the Na-Y.
Although 2a was obtained even without Na-Y in 30% yield,
α-diazoacetophenone
(
1b
)
and 1-diazo-3,3-dimethyl-2-
We next conducted the reaction of an excess amount (10
mL) of 1,3-dimethoxybenzene (4b) having cooperative ortho-
para influences by the two methoxy groups. The 1,4-addition
of 4b to propynal (1.0 mmol) was promoted by Na-Y (1.0 g) to
give 5b in 47% yield including the exclusive 4-adduct (Table 3,
entry 1). A by-product (6b) derived from the 1,2-addition was
also obtained in 2% yield. The combined use of Na-Y and H-Y
showed almost the same catalytic activity as the use of Na-Y
(Table 3, entry 3). The slow addition of propynal improved the
yields to around 60% (Table 3, entries 2 and 4).
A more highly electron-rich benzene derivative, 1,3,5-
trimethoxybenzene (4c), also reacted with propynal to give 5c
in 43% under the same reaction conditions of Table 3, Entry 1
(Scheme 5 and Supplementary Information Table S1). These
results indicated that 4b and 4c had a sufficient nucleophilic
reactivity to add to propynal induced by a relatively weak
Lewis acid promoter of Na-Y, and that a more strongly acidic
catalyst like H-Y was required to promote the 1,4-addition of
the less nucleophilic 4a than 4b and 4c. Phenol and N,N-
dimethylaniline with more electron-donating groups, such as
OH and NMe2, than a methoxy group were not converted into
the corresponding cinnamaldehyde derivatives. The amine
group was considered to work as a base (a conjugate acid of
N,N-dimethylaniline; pKa = 5.01, 25 °C in water33) to promote
the polymerization of propynal because one drop of propynal
turned the N,N-dimethylaniline solution brown. It was
concluded that a methoxy group is inert to propynal, and that
butanone (1c), which are less reactive to the 1,3-dipolar
cycloaddition than α-diazoacetate, were not able to form
pyrazoles without Na-Y (Table 1, entries 2, 4, and 6). With Na-
Y, however, the α-diazoketones were converted into 2a, 2b,
and 2c in greater than 83% yields (Table 1, entries 1, 3, and 5).
In these Na-Y promoted 1,3-dipolar cycloadditions, propynal is
thought to be activated by the coordination of the carbonyl
oxygen atoms to the Lewis acidic sodium ions in the zeolite
pores and undergo additions to the α-diazocarbonyl
compounds.11
1,4-Addition of benzene derivatives to propynal
As we previously reported that the 1,4-additions of benzene
derivatives to acrolein were mediated by Na-type or H-type
zeolites,4b,5 we expected that the Na-Y and/or H-Y would also
promote the similar 1,4-additions of benzene derivatives to
propynal. First, we carried out the reaction of an excess
amount (10 mL) of anisole having an electron-donating
methoxy group (4a) with propynal (1.0 mmol) as shown in
Table 2. One gram of Na-Y slightly promoted the 1,4-addition
at 140 °C, but had no affect at 80 °C (Table 2 entries 1 and 2).
Surprisingly, the catalytic use of acidic H-Y (0.1 g) was found to
facilitate the 1,4-addition to afford 5a in 35% yield (Table 2
entry 3) accompanied by some successive side-reactions.22b,31
The present addition reaction is also considered to proceed
inside the H-Y pores in a similar way to the acidic zeolite-
catalyzed 1,4-addition of benzene derivatives to acrolein,
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Table 3 1,4-Addition of 1,3-dimethoxybenzene to propynala
Yield (%)
Entry
Na-Y (g)
H-Y (g)
Temp. (°C)
Time (h)
Isomer selectivity of 5bf(%)
5bd
47
58
51
59
6be
2
2
5
1b
2c
3
1.0
1.0
1.0
1.0
—
—
0.10
0.10
100
100
100
100
3
3
3
3
98
98
97
97
4c
5
a Reaction conditions: 4b (10 mL), propynal (1.0 mmol). b Reaction mixture was stirred for 0.25 h at RT prior to the reaction. c Propynal was added using a syringe pump
over 1 h. d Yield was determined by GC analysis. e Yield was calculated by the effective carbon number based on GC. The major isomer was the 4-adduct, and the
f
minor one having the same mass number was also detected in less than a few percent.
methoxy-substituted benzenes are proper carbon nucleophiles reaction,24 7a underwent the cycloaddition to propynal which
for the reactions of propynal.
was activated by Na-Y and the other acidic additives in Table 4
and Supplementary Information Table S2. Using SiO2-Al2O3 or
H-Y as a Brønsted acid catalyst, 8a was obtained in ca. 10%
yields (Table 4, entries 2 and 3). Although AlCl3 and EtAlCl2
were reported to be suitable Lewis acid catalysts for the [2 + 2]
cycloaddition to methyl propiolate24, which is more stable to
Lewis acids than propynal, they were not applicable to the
labile propynal (Table 4, entries 4 and 5). Na-Y gave the highest
yield of 8a among the five catalysts (23%, Table 4, entry 6)
although the yield was not satisfactory.34 The use of polar
[2 + 2] Cycloaddition of alkenes to propynal
The [2 + 2] cycloaddition of alkenes to propynal has not yet
been reported, but we predicted that the Lewis acid property
of Na-Y might be sufficient to induce the [2 + 2] cycloaddition.
As cyclohexene (7a) prefers the [2 + 2] cycloaddition to the ene
Table 4 [2 + 2] Cycloaddition of cyclohexene to propynal under different
reaction conditionsa
Entry
Additive
—
SiO2-Al2O3
H-Y
AlCl3
EtAlCl2
Na-Y
Na-Y
Na-Y
Na-Y
Amount
—
1.3 g
Solvent
7a
Time (h)
209
1
24
8
8
72
72
72
72
Yieldc (%)
1
2
3
4
5
6
7
8
9
0
10
5
0
0
23
0
0
b
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
AcOEt
CH3CN
7a
0.1 g
1.0 equiv.
0.5 equiv.
1.0 g
1.0 g
1.0 g
1.0 g
9
a Reaction conditions: 7a (3.0 mmol), propynal (1.0 mmol), CH2Cl2 (10 mL), and
an additive at RT. b JRC-SAL-2, Si/Al = 5.3. c Yield was determined by the 1H NMR
analysis of the reaction mixture.
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solvents, such as AcOEt and CH3CN, as well as an excess the [2 + 2] cycloaddition of alkenes to methyl propiolate,
amount of 7a in place of CH2Cl2 did not improve the yield Snider proposed the one-step concerted mechanism taking on
(Table 4, entries 7–9).
We also employed more nucleophilic cyclic vinyl ethers as hand, Huisgen concluded that the [2 + 2] cycloaddition
a large degree polar character (Scheme 7a).24 On the other
ynophiles, such as 3,4-dihydro-2H-pyran
(7b) and 2,3- proceeded through the two-step mechanism based on the
dihydrofuran (7c), yielding 8b and 8c in 31% and 27% yields, kinetic and stereochemical evidence,36 and most of reports by
respectively (Scheme 6a and 6b). On the other hand, linear other researchers adopted the zwitterionic intermediacy
vinyl ethers (7d) did not produce the [2 + 2] cycloadduct (Scheme 7b).25,37
(Scheme 6c).
To discuss the reaction mechanism for the [2 + 2]
For the [2 + 2] cycloaddition, the “one-step mechanism” cycloaddition of 7b to propynal, the transition state of this
and the “two-step mechanism” were proposed.35 In general, reaction was investigated. As a function of the length of the
the one-step reaction involves the concerted [π2a + 2s]-type two bonds, C1–C8 and C6–C7, the Gibbs free energy of each
π
orbital interaction, and the two-step reaction goes through a point was calculated at the B3LYP/6-311+G level (Fig. 3,
zwitterion intermediate in a non-concerted way. Concerning Supplementary Information Section IV). Based on the
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2
3
4
(a) D. J. Cram, M. E. Tanner and R. Thomas, Angew. Chem.
calculation, only one saddle point was found, which indicated
that the reaction went through the one-step mechanism. At
the saddle point, the length of the C1–C8 bond and the C6–C7
bond are 2.84 Å and 1.63 Å, respectively. Meanwhile, the
length of the C1–C8 bond and the C6–C7 bond of the product 8b
are 1.54 Å and 1.53 Å, respectively. It is clear that at the saddle
point, the C6–C7 bond nearly forms, while the C1–C8 bond
hardly forms. These results indicated that the C6 carbon on 7b
first attacks the C7 carbon of propynal directly leading to a
zwitterionic structure without any formation of an
intermediate (Scheme 7c).38
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Int. Ed. Engl., 1991, 30, 1024–1027; (b) T. Iwasawa, R. J.
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Hooley and J. Rebek, Science, 2007, 317, 493–496; (c) G.-H.
Ning, Y. Inokuma and M. Fujita, Chem. Asian J., 2014, 9, 466–
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(a) M. Guisnet and M. Guidotti, Catalysts for Fine Chemical
Synthesis: Microporous and Mesoporous Solid Catalysts, ed.
E. G. Derouane, John Wiley & Sons, Chichester, 2006, vol. 4,
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(a) T. Okachi and M. Onaka, J. Am. Chem. Soc., 2004, 126
,
2306–2307; (b) S. Imachi and M. Onaka, Chem. Lett., 2005,
34, 708–709.
D. Hayashi, T. Narisawa, Y. Masui and M. Onaka, Bull. Chem.
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Y. Masui, T. Hattori and M. Onaka, J. Am. Chem. Soc., 2017,
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5
6
7
8
Conclusions
M. Tomita, Y. Masui and M. Onaka, J. Phys. Chem. Lett.,
2010, 1, 652–656.
Once propynal was adsorbed into the supercages of Na-Y, the
labile aldehyde was stabilized and survived for over 30 days
even at ambient temperature without self-polymerization. The
Na-Y-adsorbed propynal showed low-field shifts of C1 and C3 in
its 13C-DD/MAS-NMR, which indicated the coordination of a
carbonyl oxygen atom of propynal to the sodium ion in the
zeolite. In addition, the Na-Y-adsorbed propynal was
sufficiently and concurrently activated to react with three
types of ynophile which have been only rarely reported so far:
(1) Three electron-deficient α-diazocarbonyl compounds
underwent the 1,3-dipolar cycloaddition to propynal to afford
the corresponding pyrazoles in good yields, (2)
cinnamaldehyde derivatives were produced by the 1,4-
addition of the mono-, di-, and trimethoxy-substituted
benzenes to propynal. Especially in the case of the reactions of
anisole with propynal, the combined use of Na-Y and a
catalytic amount of H-Y improved the yields because the
Brønsted acid sites of H-Y catalyzed the addition reaction and
the Na-Y functioned as a reservoir for the desired products and
B. Stanovnik and J. Svete, Science of Synthesis Houben-Weyl
Methods of Organic Chemistry, ed. R. Neier, Thieme,
Stuttgart/New York, 2002, Category 2, chap. 12.1.1.2.1.1,
pp.22–82.
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[2 + 2] cycloaddition of three cycloalkenes to propynal yielded
the corresponding cyclobutenes. Our quantum chemical
calculation demonstrated that the alkene first attacks propynal
in the Michael addition-type fashion via the one-step-like non-
concerted mechanism. The three types of reaction can
produce valuable products containing a formyl group and a
C=C double bond.
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Acknowledgements
The computations were performed using Research Center for
Computational Science, Okazaki, Japan. The present study was
partially supported by JSPS KAKENHI Grant Numbers
JP23105511 and JP16H04562. This paper is dedicated to
Professor Teruaki Mukaiyama in celebration of his 90th
birthday (Sotsuju).
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.
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38 If the faujasite framework with sodium ions is included in
this calculation, the corresponding zwitterionic structure
state should be quite stabilized.
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