Friedel-Crafts alkylation of ferrocene with Z-cyclooctene and cyclohexene

Article abstract of DOI:10.1016/S0022-328X(99)00313-7

Cycloalkylated ferrocene derivatives, potentially useful as liquid burning rate modifiers for the HTPB/AP-based composite rocket propellants, were synthesized by the Friedel-Crafts alkylation of ferrocene with Z-cyclooctene or cyclohexene in the presence of AlCl₃ as a Lewis acid catalyst. The reaction with Z-cyclooctene yields a mixture of main products containing up to four C₈H₁₅ substituents, each of which exists in several isomeric forms, as recognized by MS and GC-MS techniques. Two isomeric monosubstituted products, cyclooctylferrocene and (1-methylcycloheptyl)ferrocene, isolated by means of preparative GC, were identified by ¹³C-NMR spectroscopy. In the case of cyclohexene, cyclohexylferrocene and a 2:1 mixture of 1,1′-dicyclohexylferrocene and 1,3-dicyclohexylferrocene were isolated and identified by MS and ¹³C-NMR spectroscopy.

Full text of DOI:10.1016/S0022-328X(99)00313-7

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 587 (1999) 122–126
Friedel–Crafts alkylation of ferrocene with Z-cyclooctene and
cyclohexene
a
b
c,
c
Friedrich-Wilhelm Grevels , Ayla Kuran , Saim O
8
zkar *, Metin Zora
a
Max-Planck-Institut f u¨ r Strahlenchemie, Postfach 101365, D-45413 M u¨ lheim an der Ruhr, Germany
Defense Industry Research and De6elopment Institute, TUBITAK-SAGE, P.K. 16 Mamak, 06261 Ankara, Turkey
b
c
Department of Chemistry, Middle East Technical Uni6ersity, 06531 Ankara, Turkey
Received 9 March 1999; received in revised form 24 May 1999
Abstract
Cycloalkylated ferrocene derivatives, potentially useful as liquid burning rate modifiers for the HTPB/AP-based composite
rocket propellants, were synthesized by the Friedel–Crafts alkylation of ferrocene with Z-cyclooctene or cyclohexene in the
presence of AlCl as a Lewis acid catalyst. The reaction with Z-cyclooctene yields a mixture of main products containing up to
3
four C H substituents, each of which exists in several isomeric forms, as recognized by MS and GC–MS techniques. Two
8
15
isomeric monosubstituted products, cyclooctylferrocene and (1-methylcycloheptyl)ferrocene, isolated by means of preparative GC,
13
were identified by C-NMR spectroscopy. In the case of cyclohexene, cyclohexylferrocene and a 2:1 mixture of 1,1%-dicyclohexyl-
13
ferrocene and 1,3-dicyclohexylferrocene were isolated and identified by MS and C-NMR spectroscopy. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: Ferrocene; Alkylation; Friedel–Crafts; Cyclooctene; Cyclohexene
1. Introduction
meric binder or is a part of its polyurethane backbone
4]. For this purpose, Butacene (SNPE, France) has
[
Ferrocenes are known as highly efficient burning-rate
been developed, which contains the ferrocene unit
chemically bound to the HTPB polymer [1,5]. A large
number of polymeric ferrocenes, produced from differ-
ent precursor monomers, are also found in the litera-
ture [4,5]. However, the cost of ferrocene derivatives of
this type is quite high due to the laborious syntheses.
The high price of Catocene and Butacene, for example,
is the main impediment for their use as burning-rate
modifier in the rocket applications.
catalysts for composite rocket propellants [1,2]. Al-
though ferrocene derivatives are superior to other tran-
sition metal compounds for this purpose, their use has
some drawbacks arising from their tendency of migra-
tion in the bulk of the material and their sensitivity
toward oxidation by air [3]. In this way, they are
detrimental to the aging behavior and reliability of the
propellant. A binuclear ferrocene drivative, 2,2%-bis-
(
ethylferrocenyl)propane (Catocene) has been devel-
Therefore, the synthesis of new, more easily accessi-
ble ferrocene derivatives as effective burning-rate
modifiers would be of great importance. Bulky sub-
stituents on the cyclopentadienyl rings of ferrocene,
such as cyclohexyl or cyclooctyl groups, are anticipated
to hinder the migration in the HTPB-based composite
propellants without affecting the processing and me-
chanical properties, the aging behavior and the lifetime
of the propellant. Herein we report on the synthesis of
cyclohexyl or cyclooctyl substituted ferrocenes by
Friedel–Crafts alkylation as potentially useful additives
for composite rocket propellants [6,7].
oped to reduce the migration tendency [1]. In hydroxyl
terminated polybutadiene (HTPB)-based propellants,
this additive has a migration rate lower than that of
mononuclear ferrocene derivatives such as ethyl- or
n-butylferrocene. In principle, migration can be com-
pletely prevented if the ferrocene is bound to the poly-
*
Corresponding author. Fax: +90-312-2101280.
E-mail address: sozkar@.metu.edu.tr (S. Ozkar)
8
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00313-7

F.-W. Gre6els et al. / Journal of Organometallic Chemistry 587 (1999) 122–126
123
2
. Results and discussion
The crude product obtained in this way is a mixture
5
5
of substituted ferrocenes, Fe(h -C H
R )(h -
5
5−m
m
The reaction of ferrocene (1) with Z-cyclooctene has
C H
R ) (R=C H ), as recognized by means of
5
5−n n 8 15
been explored under various conditions [8]. Among a
variety of strong Lewis acids used as catalysts, best
results were obtained with anhydrous aluminum chlo-
ride. Aluminum chloride has already been shown to
catalyze the Friedel–Crafts alkylation of olefins, which
involves the carbocation formation by nucleophilic at-
tack of olefin on aluminum [9]. Although in general the
reaction was found to be accelerated upon raising the
temperature, thermally mild conditions are favorable
with respect to minimizing side reactions such as olefin
polymerization. Moreover, in the absence of a solvent
the reaction has totally failed to yield the desired prod-
ucts, in contrast to the findings of Neuse and Trifan
using a related system [9]. Optimum conditions involve
heating a 1:4:1 molar ratio of ferrocene (1), Z-cy-
clooctene, and aluminum chloride in n-octane at 125°C,
the lowest temperature at which the system is
homogeneous.
mass spectrometry with temperature-controlled evapo-
ration of the sample. The series of spectra displayed in
Fig. 1 illustrates the successive appearance of ferrocene
derivatives with increasing number of C H sub-
stituents, each of which exhibits the molecular ion as
the most intense peak. Compounds with (m+n)=1
(ꢀ15%), 2 (ꢀ40%), 3 (ꢀ35%), and 4 (ꢀ10%) are the
main components of the product mixture, as estimated
from the integrated intensities of the respective molecu-
lar ions in the spectra recorded throughout the course
of the evaporation.
8
15
More detailed information is obtained from gas chro-
matography in combination with MS detection, which
reveals the presence of several isomers associated with
each of the above molecular ion peaks. Even with only
+
one C H15 substituent (M =296), three individual
8
species are detected in ca. 10:3:2 ratio. The first two
isomers could be isolated by means of preparative GC
13
13
1
and, on the basis of their C- and C{ H}-NMR
spectra, were identified as cyclooctylferrocene (2) and
(
1-methylcycloheptyl)ferrocene (3).
In either case, the cyclopentadienyl carbon atoms
give rise to four signals, one singlet associated with the
carbon atom carrying a substituent, two doublets for
the remaining two pairs of carbon atoms in the substi-
tuted ring, and one doublet of higher intensity for the
unsubstituted ring. Consistent with the pattern expected
for the cyclooctyl substituent, the main product (2)
exhibits five signals in the aliphatic region, one of which
is a doublet, while the other four appear as triplets
(
seven CH , six of which are pairwise equivalent).
2
The same number of five aliphatic carbon resonances
is also found in the spectrum of compound 3. However,
in this case one singlet and one quartet are observed in
addition to three triplets of comparable intensities (six
CH , pairwise equivalent) indicative of cyclooctyl ring
2
contraction with formation of the 1-methylcycloheptyl
group. The ring contraction leading to the compound 3
might be attributed to the formation of a more stable
carbocation during the course of reaction, which, in
fact, requires further investigation. The identity of the
Fig. 1. Mass spectra recorded during the temperature-controlled
5
5
evaporation of the crude product mixture, Fe(h -C H
R )(h -
5
5−m
m
C H
R ) (R=C H15), obtained from the Friedel–Crafts alkyla-
5
5−n
n 8
tion of ferrocene with Z-cyclooctene. The selected spectra refer to
+
third isomer with M =296 remains a matter of specu-
those temperatures at which the respective molecular ions exhibit
lation. Further ring contraction of the C H sub-
+
+
8
15
maximum intensity: (A) 65°C (M 296, m+n=1); (B) 90°C (M
+
+
stituent with conversion into an ethyl- or
dimethyl-cyclohexyl group seems possible [10].
4
6
06, m+n=2); (C) 150°C (M 516, m+n=3); (D) 190°C (M
26, m+n=4).

124
F.-W. Gre6els et al. / Journal of Organometallic Chemistry 587 (1999) 122–126
Due to the thermal decomposition under the conditions
The signal pattern associated with 5 clearly reflects
the presence of two equivalent, monosubstituted
cyclopentadienyl rings. It exhibits three signals for the
cyclopentadienyl carbons, viz., two doublets for the
two CH pairs and one singlet for the unsubstituted car-
bon in each ring. Four resonances are observed in the
aliphatic region: one doublet for the CH carbon and
three triplets for the single CH group and the two CH
of preparative gas chromatography, none of the numer-
+
ous polyalkylated ferrocenes with M =406 (m+n=2),
5
16 (m+n=3), and 626 (m+n=4), recognized by
means of GC–MS combination, could be isolated from
the reaction mixture. Thus, no information concerning
the occurrence of homoannular versus heteroannular
multiple substitution is available from this system.
Fortunately, the analogous Friedel–Crafts alkylation
of ferrocene with cyclohexene gives a clue in this respect.
GC analysis of the crude product shows two prominent
peaks, eachaccountingforaboutonethirdofthematerial.
These two components, separated from the reaction
mixture by means of preparative gas chromatography
with 98 and 89% purity (by GC analysis), are recognized
as mono- and dialkylated ferrocenes on the basis of the
2
2
pairs.
With this assignment, a total of four C carbon signals
p
is left over for the second component of the sample,
consistent with a homoannularly disubstituted ferrocene.
In detail, this part of the spectrum includes, in addition
to one intense doublet for an unsubstituted C ligand, one
p
singlet and two more doublets, attributable to either a
+
1,2- or 1,3-disubstituted C ring. The distinction between
p
molecular ions, M =268 and 350, respectively, ob-
these two possibilities, in favor of structure 6, rests on
the relative intensities and positions of the latter two
doublets, one of which represents two equivalent CH
carbons and, thus, is of higher intensity than the other
one. The weaker signal appears at higher field and, hence,
the corresponding CH unit is concluded to be located
between the two substituted carbon atoms, as indicated
in formula 6. In the aliphatic region, compound 6 exhibits
five signals, i.e. one more than 5. This is due to the fact
that the cyclohexyl rings of 6, unlike those of 5, lack an
internal plane of symmetry. Hence, all six carbon atoms
are chemically different, but accidental overlap of two
served in the mass spectra of the two samples. In addition,
the GC of the crude reaction mixture shows some
unreacted ferrocene (less than 10%) and a group of peaks
(
altogether less than 10%) which, considering the reten-
tion time, are attributable to trialkylated ferrocenes.
The first one of the two isolated samples proved to be
13
CH
(a doublet for one CH and four triplets for the five CH2).
,1%-Dicyclohexylferrocene (5) has already been ob-
2
resonances reduces the number of resonances to five
pure cyclohexylferrocene (4), as concluded from the C-
1
3
1
and C{ H}-NMR spectra. In addition to a total number
of four signals associated with the carbon atoms of the
unsubstituted and the monosubstituted cyclopentadienyl
rings, one doublet and three triplets are observed in the
1
tained from the reaction of cyclopentadienylidenecyclo-
hexane with lithium aluminum hydride or potassium
hydroxide followed by iron(II) chloride and characterized
on the basis of sole elemental analysis [13]. The alkylation
of ferrocene with cyclohexene in the presence of boron
trifluoride as a Lewis acid catalyst has been reported to
give also the compound 5, in addition to cyclohexylfer-
rocene 4, both characterized on the basis of elemental
analysis and IR spectral data [14].
aliphatic region, indicative of the CH and CH units of
2
a cyclohexyl substituent. The compound 4 has been
obtained from the catalytic hydrogenation of phenylfer-
rocene [11] or cyclohexenylferrocene [12], and character-
1
ized on the basis of H-NMR chemical shifts that are in
agreement with our data, though obviously not enough
for a thorough structural assignment.
The second sample contains two products in ca. 2:1
ratio, as recognized from a more careful analysis by gas
As previously reported for many other olefins [8],
multiple substitution occurs to a large extent in the
Friedel–Crafts alkylation of ferrocene with either cy-
clooctene or cyclohexene. Disubstitution takes place
preferentially in the heteroannular fashion, as explicitly
proven for the case of cyclohexene, probably because of
the steric effect of the bulky cycloalkyl groups [8].
13
13
1
chromatography. Accordingly, the C- and C{ H}-
NMR spectra exhibit two sets of resonances with relative
intensities that are consistent with the above ratio of the
two components and, altogether, identify the more
abundant compound as 1,1%-dicyclohexylferrocene (5)
and the other one as 1,3-dicyclohexylferrocene (6).
5
5
The effect of the Fe(h -C H
R )(h -C H5−nRn)
5
5−m
m
5
product mixtures, with both R=cyclooctyl and cyclo-
hexyl, on the thermal decomposition of ammonium
perchlorate and their performance as burning rate
modifier in the HTPB/AP-based composite rocket propel-
lantsarecurrentlyunderinvestigation. Theresultsofthese
studies will be reported elsewhere.

F.-W. Gre6els et al. / Journal of Organometallic Chemistry 587 (1999) 122–126
125
3. Experimental
124 Hz; 2 CH ), 26.36 (t, 127 Hz; 1 CH ), 25.89 (t, 124
2 2
1
Hz; 2 CH ). H: l 4.12 (s, 5 H), 4.05 (broad s, 4 H),
2
3.1. General
2.49 (m, 1 H), 1.92 (m, 2 H), 1.74–1.42 (m, 14 H). MS:
+
2
96 (M , 100), 234 (13), 212 (18), 199 (8), 186 (9), 121
Ferrocene (Merck), cyclohexene, Z-cyclooctene, and
(15), 56 (7).
1
3
aluminum chloride (Aldrich) were used as received,
while n-octane (Aldrich) was distilled from
sodiumꢀbenzophenone ketyl prior to use. All reactions
were performed under nitrogen atmosphere. NMR
spectra were recorded on a Bruker AM 400 instrument
NMR data of 3 (CDCl3). C: l 103.25 (s; 1 C at Cp),
68.12 (d, 174 Hz; 5 CH at C ), 66.82 (d, JCH=175 Hz;
p
2 CH at C ), 65.19 (d, 169 Hz; 2 CH at C ), 42.31 (t,
p
p
126 Hz; 2 CH ), 36.12 (s; 1 C), 30.80 (t, 126 Hz; 2
2
CH ), 28.75 (q, 125 Hz; 1 CH ), 23.53 (t, 124 Hz; 2
2
3
1
13
1
(
400.1 MHz for H and 100.6 MHz for C).
CH ). H: l 4.03 (s, 5 H), 3.96 (m, 4 H), 1.57 (m, 2 H),
2
+
1
.50–1.30 (m, 10 H), 1.11 (s, 3 H). MS: 296 (M , 100),
3.2. Friedel–Crafts alkylation of ferrocene with
239 (15), 226 (11), 199 (8), 186 (19), 121 (13), 56 (7).
Z-cyclooctene
A solution of ferrocene (1) (10.00 g, 0.054 mol),
3.3. Friedel–Crafts alkylation of ferrocene with
cyclohexene
3
Z-cyclooctene (28 cm , 0.20 mol), and anhydrous alu-
3
minum chloride (7.20 g, 0.054 mol) in n-octane (50 cm )
was refluxed (125°C) for a period of 8 h. The gradual
disappearance of ferrocene was followed by TLC, using
A solution of ferrocene (1) (10.00 g, 0.054 mol),
3
cyclohexene (28 cm , 0.20 mol), and anhydrous alu-
3
2
:1 methanolꢀdichloromethane as eluent. After cooling
minum chloride (7.20 g, 0.054 mol) in n-octane (50 cm )
down, the mixture was quenched with ice-water and
then extracted with methylene chloride (4×150 cm ).
was reacted under similar conditions as described
above, yielding 16.0 g crude product as a brown viscous
oil.—GC analysis (Siechromat instrument; capillary
column with 0.5 mm OV1 film, Ø=0.25 mm and l=10
3
The combined extracts were dried over anhydrous
sodium sulfate and then filtered through a layer of
celite and silica gel. Removal of the volatiles in vacuo
yielded a brown viscous oil (17.20 g).—By means of
MS analysis (Finnigan MAT 8200 instrument, EI ion-
ization), with temperature-controlled evaporation of the
sample (−20°C to +380°C), the crude product mix-
m; 1.2 bar H carrier gas; temperature 70–280°C, 8°C
2
−
1
min , FID) of the crude product shows, apart from
some unreacted ferrocene, two major product peaks
(each accounting for about one third of the material)
and an additional group of product peaks (less than
10%) at longer retention time.—Preparative GC (simi-
lar conditions as described above) was employed to
separate cyclohexylferrocene (4) (first main fraction,
98% purity by GC) and a mixture of 1,1%-dicyclohexyl-
ferrocene (5) and 1,3-dicyclohexylferrocene (6) (second
main fraction, 58% of 5 and 32% of 6, by GC) from the
crude product.
ture was shown to contain about 15% mono-, 40% di-,
+
3
5% tri-, and 10% tetrasubstituted ferrocenes (M =
296, 406, 516, and 626, respectively; representative spec-
tra are displayed in Fig. 1). GC (HP 5890/2 instrument;
capillary column with 0.5 mm OV1 film, Ø=0.25 mm
and l=10 m; 1.2 bar H carrier gas; temperature
2
−
1
70–300°C, 10°C min ) in combination with MS de-
13
tection (Finnigan MAT SSQ7000, EI ionization) re-
vealed the presence of several isomers associated with
each molecular ion.—Preparative GC (Gerstel AMPG-
NMR data of 4 (CDCl3). C: l 95.94 (s; 1 C at Cp),
68.32 (d, 175 Hz; 5 CH at C ), 66.72 (d, 175 Hz; 2 CH
p
at C ), 66.09 (d, 173 Hz; 2 CH at C ), 37.55 (d, 126 Hz;
p
p
6
6
0 apparatus; packed column with Silicone SE-30 on
1 CH), 34.44 (t, 127 Hz; 2 CH ), 26.73 (t, 125 Hz; 2
2
1
0–80 mesh Chromosorb P, Ø=20 mm and l=1.5 m;
CH ), 26.45 (t, 125 Hz; 1 CH ). H: l 4.12 (s, 5 H), 4.06
2
2
3
−1
N2 carrier gas, 565 cm min ; 350°C and 230°C
injection and column temperature, respectively; 0.5 cm
(s, 4 H), 2.25 (m, 1 H), 1.98 (m, 2 H), 1.76 (m, 2 H),
3
+
1.45–1.10 (m, 6 H). MS: 268 (M , 100), 225 (9), 199
of a sample solution in toluene injected per cycle, with
back flushing of higher boiling components in each
cycle) was employed to separate cyclooctylferrocene (2)
(13), 198 (14), 186 (10), 134 (8), 121 (22), 56 (12).
13
C-NMR data of 5 and 6 (CDCl ). 5: l 95.95 (s; 2 C
3
at C ), 67.61 (d, 174 Hz; 4 CH at C ), 66.60 (d, 172 Hz;
p
p
(
91% purity by GC) and (1-methylcycloheptyl)ferrocene
4 CH at C ), 37.54 (d, ꢀ127 Hz; 2 CH), 34.52 (t, 127
p
(
3) (84% purity by GC; 95% in a second run with
Hz; 4 CH ), 26.75 (t, ꢀ127 Hz; 4 CH ), 26.45 (t, ꢀ127
2
2
pre-enrichment from of the crude product by means of
HPLC on Nucleosil N-7-C18 with methanol as the
Hz; 2 CH ). 6: l 94.79 (s; 2 C at C ), 68.98 (d, 175 Hz;
2
p
®
5 CH at C ), 64.70 (d, ꢀ174 Hz; 2 CH at C ), 64.59 (d,
p
p
mobile phase).
NMR data of 2 (CDCl3). C: l 98.01 (s; 1 C at Cp),
ꢀ174 Hz; 1 CH at C ), 37.60 (d, ꢀ127 Hz; 2 CH),
p
1
3
34.37 (t, ꢀ127 Hz; 2 CH ), 34.24 (t, ꢀ127 Hz; 2 CH ),
2
2
6
2
8.43 (d, ꢀ170 Hz; 5 CH at C ), 66.86 (d, ꢀ 170 Hz;
26.73 (t, ꢀ127 Hz; 4 CH ), 26.50 (t, ꢀ127 Hz; 2 CH ).
MS of 5/6: 350 (M , 100), 268 (4), 266 (4), 200 (4), 199
p
2
2
+
CH at C ), 66.65 (d, ꢀ 170 Hz; 2 CH at C ), 37.40
p
p
(
d, 126 Hz; 1 CH), 34.07 (t, 124 Hz; 2 CH ), 27.11 (t,
(5), 198 (6), 186 (5), 134 (5), 121 (6), 56 (4).
2

1
26
F.-W. Gre6els et al. / Journal of Organometallic Chemistry 587 (1999) 122–126
burning rates solid propellants, in: Proceedings of 22nd Joint
Acknowledgements
Propulsion Conference of AIAA, Huntsville, AL, 1986.
6] For Friedel–Crafts reactions, see (a) G.A. Olah, R. Krishna-
murti, S. Prakash, in: B.M. Trost, I. Fleming, G. Pattenden
[
Financial support from the Turkish State Planning
Organization, TUBITAK-SAGE, the Turkish Academy
of Science, and the Deutsche Forschungsgemeinschaft
is gratefully acknowledged. Thanks are due to the
technical staff of the GC and MS service laboratories
for their excellent work.
(
Eds.), Comprehensive Organic Synthesis, vol. 3, Pergamon
Press, Oxford, 1991. (b) R.M. Roberts, A.A. Khalaf, Friedel–
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science, New York, 1963–1965.
[
7] For synthesis of some substituted ferrocenes, see: (a) A.N.
Nesmeyanov, N.S. Kochetkova, Doklady Chem. 114 (1957) 583.
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[
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