Studies on ferrocene derivatives. Part XVIII. Microwave irradiation effect on the ligand exchange reaction between ferrocene derivatives and aromatic compound

Article abstract of DOI:10.1016/j.ica.2009.07.014

A significant accelerating effect by microwaves for the ligand exchange reaction of ferrocenes was observed. This effect is due to the absorption of microwave energy by the adduct between the ferrocene and the Lewis acid.

Full text of DOI:10.1016/j.ica.2009.07.014

Inorganica Chimica Acta 362 (2009) 4853–4856
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Note
Studies on ferrocene derivatives. Part XVIII. Microwave irradiation
effect on the ligand exchange reaction between ferrocene derivatives
and aromatic compound q
*
Yutaka Okada , Shinya Nakano
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu-shi, Shiga 5258577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A significant accelerating effect by microwaves for the ligand exchange reaction of ferrocenes was
observed. This effect is due to the absorption of microwave energy by the adduct between the ferrocene
and the Lewis acid.
Received 29 April 2009
Received in revised form 11 July 2009
Accepted 13 July 2009
Ó 2009 Elsevier B.V. All rights reserved.
Available online 19 July 2009
Keywords:
Ferrocene
Ligand exchange
Microwave irradiation
1. Introduction
Recently, the organic reaction using microwave irradiation has
been reported [7]. Some microwave irradiation effects are known,
Ferrocene has an aromatic property that allows it to undergo
many reactions. It is well known that a ligand exchange reaction
occurs when ferrocene is heated with an aromatic compound and
aluminum(III) chloride in some solutions [2]. In this reaction, the
cyclopentadienyl ring of the ferrocenes is abstracted by the alumi-
num(III) chloride. The ferrocene then decomposes into two frag-
ments; the remaining iron-containing moiety is then coordinated
by the benzene ring [3].
for example, acceleration of the reaction, increase in selectivity,
etc. There are some reports concerning the ligand exchange reac-
tion using microwave irradiation [8,9]. However, no systematic re-
search on a microwave irradiation effect for the reaction was
found.
In this study, the ligand exchanges of ferrocenes with substi-
tuted benzenes using microwave irradiation have been examined.
We describe the acceleration effect by microwaves and its origin.
With regard to the electric effect of the substituent on the reac-
tion of the 1,10-disubstituted ferrocenes with benzenes, the reac-
tivity increased by the introduction of methyl groups, e.g., the
reactivity is influenced by the I-effect of the substituent [4]. As
with the benzene ring substituent, a I-effect of the substituent
has been recognized [4]. The authors reported several steric effects
for the reactions. A significant steric effect was found for the reac-
tion of 1,10-di t-butylferrocene [5]. For the ligand exchange reac-
tions of the alkylferrocenes with substituted benzenes, the steric
effects of the substituents on the reaction were almost equal irre-
spective of the site of the substituents, either on the cyclopentadi-
enyl- or the benzene ring. However, for the benzene ring
substituents, the effect on the symmetrically-substituted benzenes
was greater than that on the unsymmetrically-substituted ben-
zenes [6].
2. Experimental
2.1. Synthesis
The ferrocene derivatives were prepared in the usual manner
[10–12].
2.2. Ligand exchange reactions under conventional heating [6]
The ligand exchange reactions were carried out under a N2
atmosphere. The ferrocenes (1.2 mmol) and benzenes (1.2 mmol)
were dissolved in 1,2-dichloroethane (12 cm3). Anhydrous alumi-
num(III) chloride (0.16 g, 1.2 mmol) was added to the solution.
The solution was refluxed for one hour. At the end of the reaction,
the solution was poured into water, and NH4PF6 was added. The
PF6 salts were identified by 1H NMR, and the chemical shifts are
summarized in Table 1. The yields were decided by the weight
q
Ref. [1].
* Corresponding author.
E-mail address: ygvictor@sk.ritsumei.ac.jp (Y. Okada).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.07.014

4854
Y. Okada, S. Nakano / Inorganica Chimica Acta 362 (2009) 4853–4856
Table 1
1H NMR chemical shifts (p.p.m.) of arene complexes of the ferrocene derivatives on acetone-d6.
1
6.25
(3H)
2
2.49
(9H)
3
5.06
(5H)
+
3
CH3
Fe
H3C
1
CH3
2
1
6.17
(3H)
2
2.50
(9H)
3
4.98
(2H)
4
4.92
(2H)
5
2.24
(2H)
6
1.13
(3H)
+
3
4
5
CH2
6
CH3
CH3
Fe
H3C
1
CH
3 2
1
6.28
(3H)
2
2.55
(9H)
3
4.99
(2H)
4
4.85
(2H)
5
1.32
(9H)
+
4
3
CH3
CH3
CH3
C
5
CH3
Fe
H3C
1
CH3
1
6.04
(3H)
2
2.55
(9H)
3
5.73
(2H)
4
5.25
(2H)
5
7.86
(2H)
6
7.53
(2H)
7
7.39
(1H)
2
3
+
4
CH3
7
Fe
5
6
H3C
1
CH3
2
1
6.02
(3H)
2
2.26
(9H)
3
5.55
(2H)
4
5.20
(2H)
5
7.74
(2H)
6
7.36
(2H)
7
2.37
(3H)
+
4
5
6
3
CH3
CH3
7
Fe
H3C
1
CH3
2
1
6.50
(6H)
2
5.25
(5H)
+
2
Fe
1
1
6.44
(6H)
2
5.18
(2H)
3
5.15
(2H)
4
2.32
(2H)
5
1.18
(3H)
+
2
3
5
CH3
4
CH2
Fe
1

Y. Okada, S. Nakano / Inorganica Chimica Acta 362 (2009) 4853–4856
4855
Table 1 (continued)
1
6.23
(6H)
2
5.06
(2H)
3
4.90
(2H)
4
1.21
(9H)
+
3
2
CH3
CH3
C
4
Fe
CH3
1
1
6.40
(5H)
2
2.55
(3H)
3
5.18
(5H)
+
3
Fe
CH3
2
1
measurement of the salts because the solubility of these salts in
water is able to ignore.
shown in Table 2. The yield between ferrocene and mesitylene
using microwaves was 31.3% for 4 min, whereas the one after con-
ventional heating was 44.3% at 1 h. This shows that the microwave
irradiation results in a shortening of the reaction time. Compared
with the yields for the substituted benzenes, the order of the yields
was found as follows:
2.3. Ligand exchange reactions under microwave irradiation
The reactions under microwave irradiation were done in the
same amounts of reagents as Section 2.2. The solution was placed
mesitylene > toluene > benzene.
The reason would be that the electron density of these aromatic
rings increases by the presence of the methyl groups in a manner
similar to the conventional heating.
For the reaction of the substituted ferrocenes, two ligand ex-
change products (A) and (B) are obtained (Scheme 1). The yield
for these reactions was significantly high by irradiation of only
4 min. Moreover, the ratio of the products (A) and (B) was not
influenced by the irradiation time. With the 4-min reaction be-
tween the ferrocene derivatives and mesitylene determined, the
in a multi-mode microwave reactor, a Shikoku Keisoku l-reactor.
The microwave irradiation was carried out by a cycle of a
1.5 min irradiation and a 0.5 min interval. The power was 300 W.
The reaction temperature was measured by a fiber-optical ther-
mometer. The solution was heated to the reflux temperature by
this irradiation conditions. The work-up of the reaction solution
was carried out in the same manner of Section 2.2.
3. Results and discussion
The yields of the ligand exchange reactions between ferrocene
and the substituted benzenes during microwave irradiation are
Table 3
Yields of ligand exchange reactions of ferrocene derivatives with mesitylene under
microwave irradiation or conventional heating.
Table 2
Yields (%)
Yields of ligand exchange reactions of ferrocene with arenes under microwave
irradiation or conventional heating.
Microwave irradiation(4 min)
Conventional heating(2 h)
t-Butylferrocene
Ethylferrocene
Ferrocene
Phenylferrocene
(4-Methylphenyl)
ferrocene
39.1
44.3
31.3
40.2
38.8
55.0
48.3
45.0
14.3
13.2
Yield (%)
Microwave irradiation/4 min
Conventional heating/1 h
Mesitylene
Toluene
Benzene
31.3
25.3
21.1
43.3
26.3
25.5
Scheme 1. Ligand exchange reaction between substituted ferrocenes and mesitylene.

4856
Y. Okada, S. Nakano / Inorganica Chimica Acta 362 (2009) 4853–4856
Table 4
between the highest and lowest value was much greater than
due to the conventional heating. These results show that the yields
for substituted ferrocenes were higher than those for the unsubsti-
tuted ferrocene. Namely, no relationship was recognized between
the yields and the substituents. This would be due to the fact that
the substituted ferrocenes possessing a dipole moment are liable to
absorb the microwaves, whereas the unsubstituted ferrocene ab-
sorbs very little. The ratio of the products (A) and (B) was similar
for the microwave irradiation and conventional heating conditions.
One or more of ferrocene, mesitylene and aluminum(III) chlo-
ride were dissolved in the solvent, and irradiated by the micro-
waves. The temperature versus irradiation time plot is shown in
Fig. 1, in which the microwave power and the amount of these sub-
stances were the same for the ligand exchange reaction. For the
substrates alone or the catalyst alone, a marked increase in the
temperature was not found. For the mixture of ferrocene or mesit-
ylene and aluminum chloride, however, the temperature increased
as well as for the exchange reaction. These results would indicate
that the microwave energy is absorbed by the complex between
the ferrocenes or mesitylene and Lewis acid. The boiling point of
1,2-dichloromethane is 83.5–84.0 °C. In this microwave irradiation
experiment, the maximum temperature was 84.7 °C, suggesting
superheating. Furthermore, as mentioned in the introduction, the
active species of this reaction is the adduct between the ferrocene
and Lewis acid. Since this species should effectively absorb micro-
wave energy, the temperature is much higher near the formed ad-
duct. The significant reaction accelerating effect would be affected
by such a high temperature.
The ratios of (A) and (B) for the ligand exchange between the ferrocene derivatives
and mesitylene under microwave irradiation or conventional heating.
Products
Microwave irradiation
Conventional heating
(A) %
(B) %
(A) %
(B) %
t-Butylferrocene
Ethylferrocene
Phenylferrocene
(4-Methylphenyl)ferrocene
33.3
69.5
80.8
75.3
66.7
30.5
19.2
24.7
35.1
57.9
67.3
80.0
64.9
42.1
32.7
20.0
Fig. 1. The temperature versus irradiation time plot when microwaves was
irradiated to one or more of ferrocene, mesitylene and aluminum(III) chloride in
1,2-dichloromethane: h: mesitylene, s: ferrocene, N: aluminum(III) chloride, ꢀ:
mesitylene + ferrocene, j: mesitylene + aluminum(III) chloride, d: ferrocene + alu-
minum(III) chloride.
References
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change in the yield was discussed. The results of the ligand ex-
change reactions during the microwave irradiation are shown to-
gether with those due to the conventional heating in Table 3. The
ratios of the products (A) and (B) are summarized at Table 4. The
highest yield by conventional heating was 55.0% for t-butylferro-
cene, and the lowest value was 13.2% for (p-methylphenyl)ferro-
cene. These results are due to the electron-donating ability of the
substituents. On the contrary, under microwave irradiation condi-
tions, the highest value was 44.3% for ethylferrocene, and the low-
est value was 31.3% for unsubstituted ferrocene. The difference