B. Barati et al. / Catalysis Communications 29 (2012) 122–126
123
Scheme 1. Alkylation of 1,3-dicarbonyl compounds catalyzed by[Ru(salophen)OTf].
3. Results and discussion
3.2. Reaction of alcohols with dicarbonyl compounds catalyzed by
[Ru(salophen)OTf]
3.1. Optimization of reaction conditions
Under these optimized conditions, addition of different alcohols to
diverse 1,3-dicarbonyls were investigated in the presence of
[Ru(salophen)OTf]. The results are summarized in Table 2. The reaction
of 2,4-pentanedione with substituted benzyl alcohols was carried out
and the corresponding products were produced in good to excellent
yields (entries 1–6). The catalyst was not applicable for conversion of
isopropanol (aliphatic secondary alcohol) to its corresponding product
(entry 7). The reaction of 1,3-diphenyl-1,3-propanedione with benzyl
alcohols afforded the desired products in high yield (entries 8 and 9).
First, the catalytic activity of the catalyst was investigated in the re-
action of acetylacetone (2,4-pentanedione) and 1-phenylethanol under
various conditions. Table 1 shows the effect of solvent, temperature and
axial ligands in the ruthenium complex on the model reaction. As
shown in Table 1, the parent [Ru(salophen)Cl] catalyst was less efficient
in this reaction both under solvent-free conditions and in solvent (en-
tries 1–5). Therefore, [Ru(salophen)OTf] was used as catalyst under
solvent-free conditions at 100 °C and the higher yield was observed in
the presence of 0.013 mmole of catalyst (entry 6). When the same reac-
tion was carried out at 90 °C, the yield reduced (entry 7). By decreasing
the catalyst amount to 0.01 mmol, lower yield was obtained (entry 8).
Next, the ability of transition metal Schiff base complexes of manga-
nese, chromium and iron (0.013 mmol) was also investigated in the
model reaction (entries 9–14). The results indicated that these catalysts
are not as efficient as [Ru(salophen)OTf].
3.3. Reaction of olefins with dicarbonyl compounds catalyzed by
[Ru(salophen)OTf]
Under the same reaction conditions used for the reaction of
dicarbonyl compounds with alcohols, we investigated the reaction of
1,3-dicarbonyl compounds with olefins (Table 3). The 2,4-pentanedione
reacted with substituted styrens in the presence of [Ru(salophen)OTf]
and gave the desired products in moderate yields (entries 1–3). The reac-
tion of indene with 2,4-pentanedione gave the corresponding alkylated
product in 50% yield (entry 4). When 1,3-diphenyl-1,3-propanedione
was used as diketone in the reaction with styrene and 4-methyl styrene,
the alkylated products were produced in 56% and 52%, respectively (en-
tries 5 and 6).
Table 1
Alkylation of acetylacetone and 1-phenylethanol under various conditionsa.
Comparison of this catalyst with previously reported alkylation cat-
alysts such as HOTf [11], [Ru(6,6'-Cl2bipy)2(H2O)2](ClO4)2[20] and
HOTf-SiO2 [22] showed that the results obtained by [Ru(salophen)
OTf] are comparable to HOTf, and both of them are slightly more effi-
cient than HOTf-SiO2 (Tables 2 and 3). While in the case of
[Ru(6,6'-Cl2bipy)2(H2O)2](ClO4)2, the reaction times are not mentioned
and therefore we could not determine the TOFs. But in the reaction of
styrenes with 1,3-dicarbonyl compounds, the [Ru(salophen)OTf] is
more efficient than the others.
The reaction mechanism can be inferred as shown in Scheme 2. Ini-
tially, 2,4-pentanedione reacts with [Ru(salophen)OTf] to generate the
intermediate 1, and affords an equivalent proton in which protonates
the alcohol to generate the stable carbocation 2(or reacts with olefin
and generates the stable carbocation 2). The nucleophilic attack of 1
to the α-carbon of the carbocation 2 gives the final product and releases
the catalyst for the next catalytic cycle. Such mechanism has been
reported previously in the alkylation of 1,3-dicarbonyl compounds cat-
alyzed by triflic acid and Ru perchlorate complex [11,20,22].
Entry
M
X
Solvent
Temperature (°C)
Yield (%)b
1
2
3
4
5
6
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Mn
Mn
Cr
Cl
Cl
Cl
Cl
neat
DCE
CH2Cl2
CHCl3
THF
neat
neat
neat
neat
neat
neat
neat
neat
neat
100
78
38
57
61
60
45
30
45
10
95
80
85
61
24
43
33
76
38
Cl
OTf
OTf
OTf
OTf
Cl
OTf
Cl
OTf
Cl
100
90
7
8c
9
100
100
100
100
100
100
100
10
11
12
13
14
Cr
Fe
Fe
Also, the reaction of acetylacetone with 1-(4-methoxyphenyl)eth-
anol or styrene was carried out in the presence of catalytic amounts of
HOTf under the same conditions as with [Ru(salophen)OTf] (Table 4).
The results showed that the yields are almost identical to those
obtained by [Ru(salophen)OTf]. These observations can be considered
a
Reaction conditions: 2,4-pentanedione (1.5 mmol),1-phenylethanol (1.0 mmol),
catalyst (0.013 mmol), solvent (2 ml).
b
Isolated yields after column chromatography.
The catalyst loading is 0.01 mmol.
c