Chemistry Letters Vol.37, No.3 (2008)
321
HSZ-930HOA (37 and 27, respectively) but had smaller pore
ring size were used, the yields of 2a profoundly decreased to 5
and 10%, respectively (Entries 6 and 8). This suggests suitably
large pore size is needed for the cyclization reaction. Other solid
acid catalysts such as Nafions SAC-13 and NR50 (Sigma-
Aldrich) could be used, although the yields of 2a (51 and
efficiently than conventional heating, although further study is
required to confirm such microwave local heating effect in
heterogeneous reaction systems.
In summary, combined use of appropriate solid acid
catalysts and microwave irradiation can efficiently promote the
intramolecular cyclization of 4-arylbutyric acids and related
compounds. Further study on development of efficient chemical
processes using microwave is under way.
42%, respectively, Entries 10 and 12) were not as high as that
obtained by the H–Beta zeolite HSZ-940HOA. In the reactions
ꢀ
of 1a at 190 C for 5 min the catalytic efficiency increased
in the order of HSZ-840HOA (5% yield) < HSZ-830HOA
This research was partially supported by the New Energy
and Industrial Technology Development Organization (NEDO),
Project of Development of Microspace and Nanospace Reaction
Environment Technology for Functional Materials. We are
grateful to Hiroko Kobashi for technical support in the measure-
ments of complex permittivities of liquid compounds.
(
(
10%) < NR50 (42%) < SAC-13 (51%) < HSZ-930HOA
71%) ꢁ HSZ-940HOA (71%). Microwave irradiation is also
effective in the catalysts other than HSZ-940HOA. When
oil bath heating was used in place of microwave irradiation,
the yields of 2a using HSZ-840HOA, HSZ-830HOA, NR50,
SAC-13, and HSZ-930HOA significantly decreased from 5,
10, 42, 51, and 71% to trace, trace, 9, 7, and 4%, respectively
References and Notes
(
Entries 5, 7, 9, 11, and 13).
In the present microwave reaction selection of solvent is al-
1
For instance: D. Lednicer, Strategies for Organic Drug
Synthesis and Design, Wiley-Interscience, New York,
1998, and references therein.
therein.
so important. For instance, replacement of 1,2-dichlorobenzene
by chlorobenzene caused very slow increase in temperature,
2
ꢀ
and maximum temperature was about 170 C, while 1,2-dichlo-
ꢀ
robenzene reached 190 C within 2 min. On the other hand, when
1,3-dichlorobenzene was used as solvent, the temperature could
3
4
ꢀ
ꢀ
become 190 C. However, the yield in the reaction at 190 C for
min (56%) was lower than that obtained in 1,2-dichloroben-
zene (71%). Measurements of complex permittivities at
5
1
1
2.45 GHz by a perturbation method using a cylindrical cavity
and a network analyzer showed the dielectric loss factors of
1
0.65, and 0.53, respectively. In general, compounds with larger
loss factors tend to absorb microwave energy more easily.
Accordingly, solvents with appropriately large loss factors
seem to be favorable for smooth progress of the microwave
reaction.
5
6
,2- and 1,3-dichlorobenzene and chlorobenzene were 2.30,
1
2
Derivatives of 1a also undergo the cyclization reaction.
ꢀ
7
8
43, 6250. b) Microwave Assisted Organic Synthesis, ed. by
J. P. Tierney, P. Lidstr o¨ m, Blackwell, Oxford, 2005. c)
Microwave-Assisted Chemical Process Technology, ed. by
Y. Wada, K. Takeuchi, CMC, Tokyo, 2006.
Thus, 4-p-tolylbutyric acid (1b) reacted at 200 C for 10 min
to give the corresponding 1-tetralone derivative 2b in 93% yield
(
Entry 14). In addition, 4-p-methoxypenylbutyric acid (1c) gave
a methoxy group-substituted product 2c in 65% yield (Entry 15).
The reaction of 1d containing somewhat electron-withdrawing
fluorine atom smoothly proceeded as well to give 2d in 95%
yield (Entry 16). Furthermore, 4-thienylbutyric acid (1e) could
also undergo the cyclization to provide a thiophene ring-contain-
ing product 2e in 65% yield (Entry 17). In these reactions isola-
tion of the products was easily achieved by standard methods.
For instance, in the cases of 1b and 1d, separation of the zeolite
catalyst by centrifugation followed by column chromatography
In the microwave reaction the reaction vessel was heated
from room temperature, while in the oil bath reaction the
vessel was immersed in a preheated oil bath. In both
reactions, some time (ca. 1 min) was needed to heat the
ꢀ
vessels to appropriately high temperature (ꢂca. 150 C).
Therefore, the total heating time adopted in the present
reaction was ca. 11 or ca. 6 min for the 10 or 5 min reaction,
respectively.
(
8
silica gel, hexane/ethyl acetate = 7/1) gave 2b and 2d in both
5% isolated yields. Besides butyric acid derivatives, a propion-
9
Supporting Information is available electronically on the
ic or a valeric acid derivative, 1f or 1g, reacted in the presence
of HSZ-940HOA under microwave irradiation to form a five-
membered or a seven-membered product, 1-indanone (2f) or
10 All products 2 were identified by comparison of GC retention
times and GC-MS spectral patterns with those of authentic
samples.
6
,7,8,9-tetrahydro-5H-benzocycloheptene-5-one (2g), respec-
tively (Entries 18 and 20). Although the yields (36% for 2f
and 8% for 2g) were not high enough, these values were better
than those obtained by conventional oil bath heating (15 and
11 H. Yamashita, H. Kobashi, K. Hiroki, J. Sugiyama, unpub-
lished results; For an example of the perturbation method,
H. Tanpo, H. Kawabata, Y. Kobayashi, Tech. Rep. IEICE
MW 2002-77, September, 2002.
2%, respectively, Entries 19 and 22). For the acceleration effect
of microwave in the present reaction, microwave is likely to
activate rather polar protic catalytic sites of solid acids more
Hiroki, Kazuaki
