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Published on the web December 9, 2010
Intramolecular Condensation of 1,2-C H (CH RH) (R = O, S, and NH)
6
4
2
2
to Yield Heterocyclic Compounds over Halide-cluster Catalysts
1
2
1
1
1
Sayoko Nagashima,* Satoshi Kamiguchi, Kentaro Kudo, Tomoaki Sasaki, and Teiji Chihara
1
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570
2
The Institute of Physical and Chemical Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198
(
Received October 12, 2010; CL-100866; E-mail: nagasima@apc.saitama-u.ac.jp)
1
,2-Benzenedimethanol was reacted under a helium stream
−
H2R
RH
RH
in the presence of [(Nb6Cl12)Cl2(H2O)4]¢4H2O supported on
silica gel. When the temperature was raised above 200 °C,
catalytic activity of the cluster for cyclization appeared, yielding
R
(
R = O, S, NH)
1
,3-dihydroisobenzofuran in 91% selectivity at 350 °C. The
Scheme 1.
corresponding halide clusters of tantalum and tungsten also
catalyzed the reaction. cis-1,2-Cyclohexanedimethanol and 1,4-
butanediol exclusively produced the corresponding furans. 1,2-
Benzenedimethanamine and 1,2-benzenedimethanethiol selec-
tively afforded isoindoline and 1,3-dihydrobenzo[c]thiophene,
respectively.
100
80
100
80
60
40
20
00
6
4
2
0
0
0
0
0
Various types of halide clusters have been synthesized by
combining 19 types of Group 37, lanthanide, and actinide
transition metals with four types of halogen atoms as ligand.
1
These halide clusters have several characteristic features: metal
metal bonds that are similar to those in bulk metals, intermediate
oxidation states of the metal atoms between 1+ and 3+,
multicenter and multielectron systems, low vapor pressure, and
high melting point for metal halogenated compounds. The
clusters are expected to be thermally stable, because high
temperatures above 650 °C are required for the syntheses. Taking
all of these features into consideration, we have started to use
halide clusters as catalysts. Halide clusters have been found to
catalyze various types of reactions, the characteristics of which
can be seen in the one-step synthesis of indenes from benzal-
0
2
4
6
8
10
t / h
Figure 1. A typical reaction profile for the cyclization of 1,2-
benzenedimethanol in a helium stream over [(Nb Cl )Cl -
6
12
2
(
1
1
H O) ]¢4H O (1)/SiO at 350 °C. Following the treatment of
/SiO (10 mg) in a helium stream (600 mL h ) at 350 °C for
h, reaction was started by introduction of 1,2-benzenedi-
methanol (10% aqueous solution, 0.69 mL h , 0.50 mmol h )
to the helium stream without changing the temperature.
Conversion = products/(products + recovered 1,2-benzenedi-
methanol) © 100%, selectivity = product/(total amount of prod-
ucts) © 100%. Conversion of 1,2-benzenedimethanol ( ),
selectivity for 1,3-dihydroisobenzofuran ( ), selectivity for 2-
methylbenzyl alcohol ( ), selectivity for 2-methylbenzaldehyde
2 4 2 2
¹1
2
2
¹1
¹1
dehyde with alkyl ketones and that of 3-methylbenzofuran from
3
phenol and acetone. In these ring-closure reactions, the number
of product molecules increases, which is thermodynamically
favorable at higher temperatures under constant pressure,
according to Le Chatelier’s principle. This paper describes
another type of eliminative ring-closure reaction for compounds
containing OH, SH, or NH2 groups (Scheme 1), by taking
advantage of the thermal stabilities of the cluster catalysts.
Halide-cluster complexes were synthesized according to
published procedures. Methanol (330 mL) was added to a 1-L
flask containing [(Nb6Cl12)Cl2(H2O)4]¢4H2O (1) (1.0 g). After
dissolution of the cluster, silica gel (Nippon Aerosil, Tokyo,
(
(
), selectivity for combined amount of o-xylene and toluene
).
furnace. The catalyst sample was initially heated to 350 °C for
1 h in a helium stream (600 mL h ), followed by introduction
of 1,2-benzenedimethanol (10% aqueous solution, 0.69 mL h ,
¹
1
2
¹1
¹1
Aerosil 380 m g , 19.0 g) was added and allowed to stand for
h with occasional shaking. Then the solvent was evaporated to
¹
1
1
0.50 mmol h ) into the stream using a syringe pump at 350 °C.
The reaction was monitored every 30 min by trapping the
reaction gas followed by analysis using GLC. The reactor
effluent was collected in an ice-cold trap containing water
methanol or tetrahydrofuran for subsequent analyses with GLC
and GC/MS. Each product was identified by comparison of the
GLC retention time with that of an authentic sample.
Figure 1 shows a typical reaction profile for 1/SiO2 at
350 °C. The catalytic activity decreased with time, while the
selectivity leveled off after reaction for 3 h. Cyclization yielding
dryness under vacuum at ambient temperature. Samples of the
dried silica gel were crushed and screened to 150200 mesh. All
of the clusters were supported on the silica gel in the same way
at 5.0% by weight. The other chemicals were commercially
available and used as received. The reaction was performed in a
conventional continuous-flow microreactor operated at atmos-
pheric pressure. In a typical experiment, a weighed sample
(10 mg) of 1 supported on silica gel was packed in a borosilicate
glass tube (3 mm i.d.) and placed in the center of an electric
Chem. Lett. 2011, 40, 7880
© 2011 The Chemical Society of Japan