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Table 2 Hydrogenolysis–hydrogenation of different aryls and aryl alkyl ethersa
correlation was observed depending on the presence of electron
withdrawing and electron donating groups, which dictated the
formation of saturated or unsaturated alcohol or arene compounds.
In addition, the cleavage of the C–O bond of alkyl aryl ether
compounds was also achieved successfully (Table 2; entries 6–8).
For this system, instead of scCO2, different organic solvents
such as methanol, ethanol, isopropanol, tetrahydrofuran and
hexane were used along with water (see Table S1, ESI†). According to
the results, highest and lowest conversion of DPE was detected in
hexane (100%) and methanol (47%), respectively. In addition,
depending on the solvent used, product distribution was changed.
In isopropanol, the highest selectivity of cyclohexanol (92%) was
observed but the selectivity of dicyclohexyl ether was very high about
70% when hexane was used. The selectivity of cyclohexanol follows
the order of isopropanol > ethanol > methanol > tetrahydrofuran >
hexane. No definite trend of solvent polarity was detected in the
catalytic performances, but it could be stated that the reaction was
faster in non-polar solvents, which preferred hydrogenation rather
than hydrogenolysis. A strong role of the solvent was reported before
as they affected the activity and selectivity of the reaction using the
RANEYs Ni catalyst.11 It should be mentioned that although scCO2 is
considered as a non-polar solvent, it is superior to similar organic
solvents as it possesses substantially different characteristics, which
can direct the activity and selectivity of a reaction. Recycling of the
catalyst confirmed its stability (see Table 1, entries 7 and 8; ESI†).
In conclusion, we have developed a simple and highly
reactive catalytic system for the cleavage of the aryl ether bond
of DPE. Under the mild reaction conditions, it was possible to
achieve very high catalytic performance. The studied method was
also extended to different types of symmetrical and unsymmetrical
aryl and alkyl aryl ether compounds. Future work will be
directed to the mechanistic aspects to understand the actual
process for large-scale applications.
as represented in Scheme 2
Selectivity (%)
Entry Substrate
1
Conv. (%) 2(6)
3(7)
4(8)
5(9)
100
42.1 (57.9)
0.0
0.0
2
85.2
48.2 (38.6) (8.9)
46.2 (50.1) (1.2)
(7.3)
(2.5)
3
4
87.3
22.1
1.1
1.3 (55.9) (41.7)
5b
6
100
(49.9) (48.6)
0.0
0.0
60.2
(100)
—
—
—
7
52.1
55.5
(100)
(100)
—
—
—
—
—
—
8
a
Reaction conditions: catalyst : substrate = 1 : 5, PCO = 10 Mpa, PH = 0.5 Mpa,
temp. = 80 1C, water = 4 ml; reaction time: entry21 = 5 h, entry 2 = 12 h;
2
entry 3–8 = 18 h.; ‘‘—’’ = not determined. b 6 = cyclohexylmethanol.
Notes and references
1 E. Furimsky, Appl. Catal., 2000, 199, 147.
´
2 P. A. Bercedo and R. Martin, J. Am. Chem. Soc., 2010, 132, 17352;
A. Maercker, Angew. Chem., Int. Ed. Engl., 1987, 26, 972; P. Dabo,
Scheme 2
´
A. Cyr, J. Lessard, L. Brossard and H. Menard, Can. J. Chem., 1989,
77, 1225; M. Tobisu, K. Yamakawa, T. Shimasaki and N. Chatani,
Chem. Commun., 2011, 47, 2946.
cleavage product or an aromatic ring hydrogenation product
without any cleavage by simply tuning the reaction parameters.
After finding primary reaction conditions, we applied this
method to other aryl ether compounds with different substitution
patterns and the results are shown in Table 2. The results show that
the developed method was applicable for C–O bond cleavage of
unsymmetrical aryl ethers and alkyl aryl ethers. Depending on the
nature of the substrate, conversion varied from 20–100%. The
reaction of symmetrical aryl ether was faster and produced corres-
ponding hydrogenated arene and phenol compounds (Table 2;
entry 1). On the other hand, unsymmetrical aryl ether bearing
either electron withdrawing or electron donating groups or
both required a longer reaction time for the cleavage (Table 2;
3 J. Shabtai, N. K. Nag and F. E. Massoth, J. Catal., 1987, 104, 413.
4 A. G. Sergeev and J. F. Hartwig, Science, 2011, 332, 439; A. G. Sergeev,
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c
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Chem. Commun., 2013, 49, 4567--4569 4569