S. Imachi, M. Onaka / Tetrahedron Letters 45 (2004) 4943–4946
4945
supercage, which are surrounded by the CPD interacting
with the surface. The inner CPD molecules would be
exposed to some ‘internal pressure’ from the hydrophilic
zeolite surface, and hence would be activated to react
with a dienophile which comes into the confined cavity.
was not specific to the reactions performed in zeolite,
but rather governed by the inherent selectivity of each
reaction.
14
A typical procedure is described for the reaction of
CPD@NaY and methacrylonitrile: Powdery NaY
(1000 mg, JRC-Z-Y4.8 supplied from the Catalysis
Society of Japan, activated at 673 K under below 133 Pa
for 4 h) was placed in a flask. Vaporized cyclopentadiene
(3.0 mmol, prepared by pyrolysis of dicyclopentadiene at
In order to confirm the advantages of the reaction per-
formed in the zeolite cage, the reaction was also run in
organic media such as CH Cl and CPD (entries 4 and
2
2
6
), giving only poor results. Interestingly, even in water
we did not observe any obvious rate enhancement (entry
). The comparison indicates that the Diels–Alder
2
448 K) in dried N flow was passed over NaY at 273 K
7
for 1 h to afford CPD(3.0)@NaY (1198 mg). The
amount of adsorbed cyclopentadiene was determined by
an increment in the weight of NaY after adsorption. To
CPD(3.0)@NaY containing 3.0 mmol of CPD was
added neat methacrylonitrile (1.0 mmol) at 273 K using
a syringe. After the powdery mixture was stirred for 12 h
at 313 K, it was cooled down to 195 K. In order to
extract the products, CH Cl and CH CN were added at
reaction is promoted more effectively in the NaY cavities
where CPD and methacrylonitrile molecules are thickly
packed than in organic solvents where organic sub-
strates diffuse freely and occasionally bump into each
other.
Only mixing CPD, methacrylonitrile, and powdery NaY
in CH Cl afforded the cycloaddition products in only
2
2
3
2
2
195 K and the suspension was stirred for 5 min at 273 K.
The mixture was filtrated and the solvents were
removed. The crude products were purified by
distillation using a Kugelrohr apparatus to afford
13
9
% yield (entry 5). This fact proves that the pre-
adsorption of CPD molecules into NaY is essential to
obtain high reactivity.
2-cyano-2-methylbicyclo[2.2.1]hept-5-ene in 92% yield
CPD(3.0)@NaY also reacted with other representative
dienophiles to yield the corresponding cycloadducts in
good yields (Table 2). Diastereoselection of the reactions
(endo:exo ¼ 12:88).
In summary, it was revealed that nonpolar cyclopenta-
diene was encapsulated in the hydrophilic supercages of
ordinary zeolite NaY, and that remained intact as a
monomer form. By simply putting together with
dienophile reagents, the entrapped CPD showed the
greater reactivity for the Diels–Alder reactions in the
nanosized cages of NaY than in an organic or aqueous
solution.
a
Table 2. Diels–Alder reactions using CPD(3.0)@NaY
CPD(3.0)@NaY
R1
3
R3
R
2
R2
Temp.
2 h
R
1
R
1
References and notes
Entry
1
Dienophile
Temp (K)
273
Yield (%)
endo:exo
1
. Sen, S. E.; Smith, S. M.; Sullivan, K. A. Tetrahedron 1999,
5, 12657–12698.
. Okachi, T.; Onaka, M. J. Am. Chem. Soc. 2004, 126,
306–2307.
3. Pindur, U.; Lutz, G.; Otto, C. Chem. Rev. 1993, 93, 741–
61.
b
85
84:16
5
O
2
2
H
O
c
2
293
79
13:87
7
4
. Renslo, A. R.; Weinstein, R. D.; Tester, J. W.; Danheiser,
R. L. J. Org. Chem. 1997, 62, 4530–4533.
. Earle, M. J.; McCormac, P. B.; Seddon, K. R. Green
Chem. 1999, 1, 23–25.
H
b
3
4
5
313
293
313
81
78:22
75:25
30:70
5
O
O
b
d
6. Van Eldik, R.; Asano, T.; Le Noble, W. J. Chem. Rev.
989, 89, 549–688.
90 (14)
1
. Raj, C. P.; Dhas, N. A.; Cherkinski, M.; Gedanken, A.;
O
7
O
O
b
Braverman, S. Tetrahedron Lett. 1998, 39, 5413–
5
73
416.
. Rideout, D. C.; Breslow, R. J. Am. Chem. Soc. 1980, 102,
7816–7817.
. Grieco, P. A.; Nunes, J. J.; Gaul, M. D. J. Am. Chem. Soc.
1990, 112, 4595–4596.
8
9
b
6
7
293
313
80
54:46
12:88
CN
b
92
CN
1
0. Henson, N. J.; Eckert, J.; Hay, P. J.; Redondo, A. Chem.
Phys. 2000, 261, 111–124; Barthomeuf, D.; Ha, B. H.
J. Chem. Soc., Faraday Trans. 1973, 69, 2147–2157;
Monduzzi, M.; Monaci, R.; Solinas, V. J. Colloid Interface
Sci. 1987, 120, 8–14.
a
A dienophile (1.0 mmol) was reacted with CPD (3 mmol) included in
1
CPD(3.0)@NaY. endo/exo ratios were determined by H NMR.
Isolated yields.
b
c
NMR yield using Ph
3
CH as an internal standard.
d
The figure in parentheses shows the yield of the reaction in CH
1
2
Cl
2
11. The adsorption amount was determined by an increase in
the weight of NaY after adsorption of CPD.
3
(
10 mL) without NaY.