M. Urbala / Applied Catalysis A: General 377 (2010) 27–34
33
Table 6
Thus, neat RuCl3ꢀxH2O – the stable and cheap ruthenium
compound – could be technologically attractive catalyst for highly
yielded and E-stereoselective solvent-free isomerization of 1,4-
bisallyloxybutane under air atmosphere and mild reaction
conditions. However, during the course of our research, we
observed the formation of some amount of black, flocculent
precipitate, which was insoluble in water and in popular organic
solvents (THF, Et2O, benzene, methanol, acetone, acetonitrile,
chlorinated solvents CHCl3, CH2Cl2, CCl4, CH2Cl–CH2Cl). On the
basis of the X-ray powder analysis we could raise a tentative
supposition that an amorphous polymeric organic complex type
{[RuCl2(L2)]x} is precipitated but the ruthenium black was not
detected. The resulting solid was completely inactive in the studied
isomerization, even at the high reaction temperature of 140 8C for
prolonged reaction time. According to the fact that the reusability
of a catalyst is crucial to industrial applications, the precipitation of
such complexes makes recycling of ruthenium catalyst difficult.
The effect of air and allyl hydroperoxide impurities on the model isomerization of
1,4-bisallyloxybutane catalyzed by RuCl3ꢀxH2O under solvent-free conditions.a.
Concentration
of allyl
a
(%)
y (%)
Z,Z/Z,E/E,E
%E
hydroperoxide
(ppm)
0
0c
100
100
92
100b
17/51/32
58
99.2
15/47/37
61
5
85
10/37/38
68
10
25d
87
77
6/32/39
72
48 (100)
26 (100)
0/12/14 (29/50/21)
80 (46)
a
Reaction conditions: 0.05 mol% of RuCl3, 100 8C, 1 h, argon atmosphere.
TOF = 2000/h.
b
c
Air atmosphere.
d
The reaction time: 2 h (in brackets: the reaction conditions: molar ratio S/
Ru = 1000, 140 8C, 4 h, argon atmosphere).
Our general observations reveal that the rate of the dissolution
of the precatalyst of 3 depending on the temperature is an
important factor influencing the conversion of allyl to 1-propenyl
ether. For example, when isomerization was performed with
4. Conclusion
The Ru complexes-catalyzed solvent-free isomerization of allyl
ethers can be regarded as being of industrial interest, especially
attractive for synthesis of 1-propenyl ether monomers. Practically
quantitative yield and selectivity of reaction, mild and solvent-free
reaction conditions, minimal concentration of ruthenium complex,
facility of work up and simple techniques of products separation—
e.g. distillation, possibility of the catalyst recycling and non-waste
monomer synthesis are important features of this method.
The presented results have confirmed the requirements of
highly qualified tuning of ruthenium complexes, i.e. [RuClH(-
CO)(PPh3)3], [RuCl2(PPh3)3] and RuCl3ꢀxH2O with allyl substrate
(and its impurities) and the reaction conditions to attain high yield
and selectivity of the products. In the solvent-free isomerization of
model 1,4-bisallyloxybutane, the best results with high TOF
amounting to 20,000/h were obtained with simple [RuClH(-
CO)(PPh3)3] without any catalyst promoters. However, it was
proved that the presence and concentration of allyl hydroper-
oxides (in the minimal range of 0–25 ppm) and the reaction
temperature are the most important factors in evaluating the
catalytic effectiveness and the real advantage of the soluble
ruthenium pre-catalysts for the isomerization of alkyl allyl ethers.
loading of the pre-catalyst
3 of 0.05 mol%, the complete
homogenization of pre-catalyst 3 took ca. 40–60 min at 60 8C
and only ca. 5–7 min at 100 8C.
Interestingly, the high E-stereoselectivity of isomerization has
been also observed, but the Z;Z/Z,E/E,E ratio of bis-1-propenyl ether
formed was found to be strongly dependent on the reaction
temperature and weakly on the concentration of pre-catalyst 3.
Remarkably, at 60 8C the E,E-isomer was the major product while
Z,Z-isomer was practically not detected at all (Table 5, entries 1–3).
Whereas at 80 8C the gradual decreasing of the amount of E,E
isomer down to 30% was determined with the increase in pre-
catalyst
3 loadings, at 100 8C the E–Z isomerization was
accompanied the double-bond migration from the beginning
reactions (entries 4–9, respectively).
Next, we wished to examine the effect of oxidative factors such
as oxygen in air and allyl hydroperoxides on the catalytic efficiency
of pre-catalyst 3 (Table 6). We unexpectedly found that the use of
air practically did not affect the yield of bis-1-propenyl product in
contrast to the case, when allyl hydroperoxides were used. The
presence of allyl hydroperoxides and especially their high
concentration (above 10 ppm) considerable impeded the isomeri-
zation reaction catalyzed by pre-catalyst 3.
Acknowledgements
In the search for a better catalytic system with pre-catalyst 3,
the promoters of catalyst (alcohols: ethylene glycols, methanol,
ethanol, propan-1-ol, propan-2-ol, 4-allyloxybutan-1-ol, molar
ratio alcohol/3 = 10), the bases (NaHCO3, Na2CO3, K2CO3, Cs2CO3,
KOH, Et3N, molar ratio base/3 = 10) and additional ligand PPh3
(molar ratio PPh3/3 = 20) were used under optimized reaction
conditions (100 8C, 0.05 mol% of 3, argon atmosphere) after 15, 30
and 60 min. Two series of experiments were undertaken using the
hydroperoxides—free allyl substrate and substrate containing
10 ppm of allyl hydroperoxides. The results obtained in these
investigations indicate that the tested promoters (besides organic
base) did not influence the course of isomerization—the conversion
and yield of 1-propenyl ether were the same as in the control
experiment (always 99.5–100% after 60 min). Only the use of Et3N
led to a partial decrease of the rate of migration, especially in the
case of an oxidized bisallyloxybutane (67% of 1-propenyl product
yield). Similarly, the introduction of PPh3 as additional labile ligand
contributed to a dramatic loss of the catalytic activity of 3 (under
above condition only 15% of yield was noted). The presented
results are very interesting, because neat RuCl3ꢀxH2O showed
usually low catalytic activity in organic synthesis and its utilization
as a catalyst precursor without solvent, promoters and/or labile
ligands is rather rare.
This work was supported by the Ministry of Science and Higher
Education as a research Grant (project N N209 106537).
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