Reversible Friedel-Crafts acylations of anthracene: Rearrangements of acetylanthracenes

Article abstract of DOI:10.2174/157017809787893118

Treatment of 1-acetylanthracene (1-AcAN) and 9-acetylanthracene (9-AcAN) with PPA at 80-120 °C leads to the nearly complete conversion of these isomers into 2-acetylanthracene (2-AcAN), an illustration of the Agranat-Gore rearrangement of polycyclic aromatic ketones (PAKs). Ab initio MP2/6-31(d) calculations predict the following order of stabilities: for O-complexes: 9-AcAN>1-AcAN>2-AcAN; for ketones: 2-AcAN>1-AcAN>9-AcAN; for O-protonated ketones: 2-AcAN>1-AcAN>9-AcAN. Thus, 9-AcAN is the kinetically controlled product, whereas 2-AcAN is the thermodynamically controlled product. No reverse rearrangements of 2-AcAN to either 9-AcAN or 1-AcAN and of 1-AcAN to 9-AcAN were observed. The results strengthen the pattern of reversibility in Friedel-Crafts acylations of PAHs.

Full text of DOI:10.2174/157017809787893118

Letters in Organic Chemistry, 2009, 6, 237-241
237
Reversible Friedel-Crafts Acylations of Anthracene: Rearrangements of
Acetylanthracenes
Tahani Mala'bi, Sergey Pogodin and Israel Agranat*
Department of Organic Chemistry, The Hebrew University of Jerusalem, Philadelphia Bldg #201/205, Edmond J Safra
Campus, Jerusalem 91904, Israel
Received January 24, 2009: Revised February 02, 2009: Accepted February 09, 2009
Abstract: Treatment of 1-acetylanthracene (1-AcAN) and 9-acetylanthracene (9-AcAN) with PPA at 80-120 ºC leads to
the nearly complete conversion of these isomers into 2-acetylanthracene (2-AcAN), an illustration of the Agranat-Gore re-
arrangement of polycyclic aromatic ketones (PAKs). Ab initio MP2/6-31(d) calculations predict the following order of
stabilities: for ꢀ-complexes: 9-AcAN>1-AcAN>2-AcAN; for ketones: 2-AcAN>1-AcAN>9-AcAN; for O-protonated ke-
tones: 2-AcAN>1-AcAN>9-AcAN. Thus, 9-AcAN is the kinetically controlled product, whereas 2-AcAN is the thermo-
dynamically controlled product. No reverse rearrangements of 2-AcAN to either 9-AcAN or 1-AcAN and of 1-AcAN to
9-AcAN were observed. The results strengthen the pattern of reversibility in Friedel-Crafts acylations of PAHs.
Keywords: Friedel-Crafts acetylation, PPA, Overcrowding, Agranat-Gore rearrangement, Deacylation, Ab initio calculations.
INTRODUCTION
tion and the rearrangement of the kinetically controlled ke-
tone to the thermodynamically controlled ketone presumably
involves the corresponding ꢀ-complexes and O-protonated
ketones as intermediates.
"Acylation differs from alkylation in being virtually irre-
versible" [1], free of rearrangements and isomerizations [2,
3]. This difference in behavior between Friedel-Crafts acyla-
tion and Friedel-Crafts alkylation was attributed to the reso-
nance stabilization existing between the acyl group and the
aromatic nucleus [2], which may serve as a barrier against
rearrangements and reversible processes. However, if the
acyl group is tilted out of the plane of the aromatic nucleus,
the resonance stabilization is reduced and the picture of irre-
versibility of Friedel-Crafts acylation may be challenged [2,
4].
1
8
9
2
3
7
6
10
5
4
AN
O
H3C
CH3
O
C
C
The concept of reversibility in Friedel-Crafts acylations
[5, 6] was put forward in 1955 by Gore, who proposed that
"the Friedel-Crafts acylation reaction of reactive hydrocar-
bons is a reversible process" [5]. Gore concluded that "Re-
versibility is an important factor in acylation reactions" [5].
Their studies have been focused mainly on unusual aspects
of selectivity, including deacylations, one-way rearrange-
ments and kinetic versus thermodynamic control [7]. The
incursion of reversibility in Friedel-Crafts acylations was
revealed by Agranat, et al. in the benzoylation of naphtha-
lene in polyphosphoric acid (PPA) [8]. The reversibility con-
cept was then applied to the synthesis of linearly annelated
polycyclic aromatic ketones (PAKs) by intramolecular
Friedel-Crafts rearrangements of their angularly annelated
constitutional isomers [9, 10]. Complete reversibility in
(Z)-1-AcAN
(E)-1-AcAN
O
CH3
C
C
CH3
O
(E)-2-AcAN
(Z)-2-AcAN
CH3
O
Friedel-Crafts acylation was established in para
ortho
C
rearrangements of fluorofluorenones in PPA [11]. Features
of reversibility in Friedel-Crafts acylation of fluorene and in
the acetylphenanthrene series have recently been reported
[12, 13]. The mechanism of reversible Friedel-Crafts acyla-
9-AcAN
*Address correspondence to these authors at the Department of Organic
Chemistry, The Hebrew University of Jerusalem, Philadelphia Bldg
#201/205, Edmond J Safra Campus, Jerusalem 91904, Israel; Tel: 972-2-
6585221; Fax: 972-2-6511907; E-mail: isria@vms.huji.ac.il
We report here features of reversibility in Friedel–Crafts
acylation in the acetylanthracene series, using PPA. We
highlight the rearrangements of the kinetically controlled 9-
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

238 Letters in Organic Chemistry, 2009, Vol. 6, No. 3
Mala'bi et al.
acetylanthracene (9-AcAN) and 1-acetylanthracene (1-
AcAN) to the thermodynamically controlled 2-acetylanthr-
acene (2-AcAN), an illustration of the Agranat–Gore rear-
rangement. The proposed mechanism is supported by the
results of ab initio calculations of the isomeric acetylanthra-
cenes, their O-protonated ketones and their ꢀ-complexes.
Diacetylanthracenes are not included in the report.
petroleum ether 1:4): 0.60 (1-AcAN), 0.56 (2-AcAN), and
0.73 (9-AcAN).
The attempted rearrangements and deacetylation of 1-
AcAN, 2-AcAN and 9-AcAN in PPA [20] were carried out
at various temperatures and for various periods of time ac-
cording to the general procedure, described in the experi-
mental section. PPA was used as the solvent of the reaction,
as well as a source of protons, which is a necessary (but not
sufficient) condition for reversibility. The reactions in PPA
are also homogeneous, in contrast to some of the classical
Friedel–Crafts acylations.
RESULTS AND DISCUSSION
Anthracene (AN) is essentially a planar polycyclic aro-
matic hydrocarbon (PAH). Due to the D2h symmetry of AN,
three constitutional isomers of acetylanthracene (AcAN) are
possible: 1-acetylanthracene (1-AcAN), 2-acetylanthracene
(2-AcAN), and 9-acetylanthracene (9-AcAN). In 1-AcAN
and 2-AcAN, both the (E)- and the (Z)-diastereomers should
be considered. The E,Z-isomerizations are expected to be
facile processes (low energy barriers at room temperature,
vide infra). The three constitutional isomers of AcAN can be
categorized, depending on the degree of their overcrowding:
(i) the non-overcrowded isomer 2-AcAN, in which the acetyl
group is flanked by two ortho-hydrogens (H1, H3); (ii) the
overcrowded isomer 1-AcAN, in which the overcrowding is
due to the presence of one hydrogen atom (H9) peri to the
acetyl group; (iii) the severely overcrowded isomer 9-AcAN
(assuming the planar conformation), in which the over-
crowding is due to two peri-hydrogens (H1, H8) to the acetyl
group. The overcrowding in 1-AcAN and 9-AcAN should
result in significant deviations of the acetyl groups from the
plane of the anthracene nucleus, which is expected to en-
courage reversibility and rearrangements.
Treating 9-AcAN with PPA at 120 ºC for 1 h gave AcAN
isomers and AN in the ratio 70:30. The ratio of 2-AcAN to
9-AcAN was 40:30, while 1-AcAN was not formed at all.
After 6 h at 120 ºC, the distribution of AN, 2-AcAN, and 9-
AcAN was 58:12:30, respectively. At 80 ºC after 1.5 h, the
corresponding distribution was 88:7:5.
Treating 1-AcAN with PPA at 80 ºC for 6 h gave 1-
AcAN, 2-AcAN, and AN in the ratio 58:13:29, respectively.
At 100 ºC after 1 h, the corresponding distribution was
34:16:50, while 9-AcAN was not formed at all. At 110 ºC
after 2 h, the complete conversion of 1-AcAN into AN and
2-AcAN was achieved, with the ration of the products 80:20.
At 120 ºC after 1 h, the rearrangements of 1-AcAN resulted
also in diacetylations, giving, in addition to AN and 2-
AcAN, 1,5-diacetylanthracene (ꢁ=9.56 (s, H9, H10) ppm) [21]
and 1,8-diacetylanthracene (ꢁ=10.18 (s, H9) ppm) [22]. The
distribution of AN, 1-AcAN, 2-AcAN, 1.5-AcAN, and 1,8-
AcAN was 55:6:24:9:6, respectively.
Treating 2-AcAN with PPA at 60-155 ºC for 1 h did not
give any rearranged acetylanthracenes. Deacetylation started
at 130 ºC, after 1 h giving AN and 2-AcAN in the ratio of
14:86. After 1 h at 155 ºC, deacetylation to AN was almost
complete; the ratio of AN:2-AcAN was 97:3.
The Friedel-Crafts acetylations of anthracene have previ-
ously been studied by Gore, using the classical system acetyl
chloride/AlCl3, under a variety of conditions [7, 14-17]. The
orientation depended not only on the solvent, but also on
temperature, duration of reaction, and on the proportion of
reactants. 9-AcAN was initially formed, and often precipi-
tated from solution as an aluminium chloride complex. Upon
raising the temperature and prolonging the reaction, deacyla-
tion was promoted, and 1-AcAN accumulated [7]. Using
nitrobenzene as a solvent led under certain conditions to 2-
AcAN, which was also formed by the isomerization of 9-
AcAN [18]. Using an ionic liquid system such as 1-methyl-
3-ethylimidazolium chloride/AlCl3 at 0 ºC, the initial product
of the acetylation of AN was 9-AcAN, which subsequently
underwent a slow disproportionation to AN and 1,5- and 1,8-
diacetylanthracenes [19]. Small amounts of 1-AcAN and 2-
AcAN were also formed [19].
The crystal and molecular structures of 1-AcAN and 9-
AcAN were reported in the literature [23, 24]. The carbonyl
group of 9-AcAN is almost orthogonal to the plane of the
anthracene ring system: the twist angle C9a-C9-C11-O=88º.
1-AcAN crystallizes as the (Z)-diastereomer. Its carbonyl
group is twisted out of the plane of the anthracene ring sys-
tem by 27º (C9a-C1-C11-O). Ab initio MP2/6-31G(d) calcula-
tions (Table 1) of 9-AcAN (Fig. 1) and (Z)-1-AcAN showed
the torsion angles of the carbonyl group to be 71º and 20º,
respectively. The calculated structure of (Z)-1-AcAN is
slightly more overcrowded (the O...H9 distance is 215 pm)
than the X-ray structure (223 pm); the sum of the Wan-der-
Vaals radii of oxygen and hydrogen is 244 pm [25]. The cal-
culations predict nearly planar structures for both (E)- and
(Z)-2-AcAN (the twist angle C1-C2-C11-O=179º and 1º, re-
spectively), which are not overcrowded (the O...H9 distances
are 242-243 ppm). (Z)-1-AcAN diastereomer was more sta-
ble than its (E)-diastereomer by ꢂH298=7.4 kJ/mol, whereas
(Z)-2-AcAN diastereomer was less stable than (E)-2-AcAN
by ꢂH298=2.6 kJ/mol. The nearly orthogonal [1-AcAN]
(twist angle 92º) serves as the transition state for the easy
E,Z-diastereomerization of 1-AcAN, with the low enthalpy
barrier of 1.2 kJ/mol, relative to (E)-1-AcAN. The corre-
sponding O-protonated acetylanthracenes (all global min-
ima) have the following torsion angles: 32º (9-AcAN), 4º
In the present study, the three constitutional isomers of
AcAN were distinguished by the 1H-NMR chemical shifts of
their methyl groups, by the low-field chemical shifts of the
hydrogen atoms peri and ortho to the acetyl groups, and by
the low-field shifts of H9 and H10: 1-AcAN: ꢁ=2.81 (s, CH3),
9.48 (s, H9), 8.18 (s, H2), 8.45 (s, H10) ppm; 2-AcAN: ꢁ=2.76
(s, CH3), 8.65 (s, H1), 8.58 (s, H9), 8.44 (s, H10) ppm; 9-
AcAN: ꢁ=2.81 (s, CH3), 8.03-8.06 (H1, H8), 8.49 (s, H10)
ppm. Anthracene was characterized by ꢁ(H9, H10)=8.39 (s)
ppm. The characteristic 13C-NMR chemical shifts of the
methyl carbon atoms should also be noted: 29.7 (1-AcAN),
26.5 (2-AcAN), and 33.6 (9-AcAN) ppm. The three isomers
of AcAN differ also in their TLC Rf values (ethyl acetate–

Reversible Friedel-Crafts Acylations of Anthracene
Letters in Organic Chemistry, 2009, Vol. 6, No. 3
239
Table 1. Relative Enthalpies (MP2/6-31G(d)) and Selected Geometrical Parameters of Acetylanthracenes, and Their O-Protonated
and ꢀ-Complexes
ꢁH298
ꢁꢁH298
C9a-C1-C11-O
C1-C2-C11-O
C9a-C9-C11-O
deg
ꢂ(C1)a
ꢂ(C2)a
ꢂ(C9)a
deg
C1–C11
C2–C11
C9–C11
pm
O...Hb
kJ/mol
kJ/mol
pm
Ketones
(E)-1-AcAN
(Z)-1-AcAN
(E)-2-AcAN
(Z)-2-AcAN
9-AcAN
21.5
14.1
0.0
145.1
19.8
178.9
1.0
4.3
4.4
149.9
149.6
149.3
149.7
150.4
167.5
211.0
212.3
218.0
161.6
158.7
141.4
141.1
141.1
141.2
141.8
251.3
214.8
243.4
242.1
257.3
349.6
260.8
357.1
261.3
369.3
241.0
229.7
211.6
237.9
235.8
229.2
1.2
2.6
1.2
24.9
60.6
69.5
70.3
80.4
14.0
20.1
19.5
15.7
0.0
70.7
66.8
147.0
81.3
143.8
60.6
116.8
165.6
4.2
1.6
ꢁ-Complexes
(sc)-1-AcAN
(ac)-1-AcAN
(sc)-2-AcAN
(ac)-2-AcANd
(sc)-9-AcAN
(ac)-9-AcAN
(E)-1-AcAN
(Z)-1-AcAN
(E)-2-AcAN
(Z)-2-AcAN
9-AcAN
46.6
55.4
56.3
66.3
0.00
6.0
42.9c
16.0c
15.4c
15.3c
47.5c
50.4c
9.9
O-Protonated
ketones
6.2
179.3
1.2
1.4
0.7
1.1
27.5
31.8
2.3
aC9a–C1–C11–C2, C1–C2–C11–C3, or C9a–C9–C11–C8a improper torsion angles, respectively.
bThe shortest distance between the carbonyl oxygen and an aromatic hydrogen.
cC9a–C1–H1–C2, C1–C2–H2–C3, or C9a–C9–H9–C8a improper torsion angles, respectively.
dThe structure was optimized using loose convergence criteria (vide infra).
((Z)-1-AcAN), and 179º ((E)-2-AcAN). The O-protonated
(Z)-1-AcAN diastereomer was more stable than its (E)-
diastereomer by ꢀH298=3.8 kJ/mol, whereas the
O-protonated (Z)-2-AcAN diastereomer was less stable than
(E)-2-AcAN by ꢀH298=0.7 kJ/mol. The twist angle of the
carbonyl group in the O-protonated (Z)-1-AcAN is substan-
tially reduced as compared with the twist angle in the neutral
(Z)-1-AcAN due to the more efficient delocalization of the
positive charge into the aromatic system, which leads also to
the shortening of the C1–C11 distance (141.1 pm vs. 149.6
pm).
give the following orders of enthalpies of the three acetylan-
thracene constitutional isomers in each series (Table 1):
•
ꢁ-complexes: ꢀꢀH298=0.0 (9-AcAN), 46.6 (1-AcAN),
56.3 (2-AcAN) kJ/mol.
•
O-protonated ketone cations: ꢀꢀH298=0.0 ((E)-
2-AcAN), 15.7 ((Z)-1-AcAN), 27.5 (9-AcAN)
kJ/mol.
•
ketones:
ꢀꢀH298=0.0
((E)-2-AcAN),
14.1
((Z)-1-AcAN), 24.9 (9-AcAN) kJ/mol.
The MP2 calculated order of the stabilities of ꢁ-
complexes 9-AcAN>1-AcAN>2-AcAN is in agreement with
simple resonance considerations [26].
Ab initio MP2 calculations of the acetylanthracenes, their
O-protonates, and ꢁ-complexes suggest that the acetylation
reaction proceeds through the formation of a ꢁ-complex
from AN and acetyl cation (via a respective transition state).
The ꢁ-complex then isomerizes to an O-protonated ketone,
which gives the resulting ketone. MP2/6-31(d) calculations
According to the Hammond–Leffler principle [27], the
relative energies of the transition states leading to the
ꢁ-complexes resemble those of the ꢁ-complexes: ꢀH‡(9-

240 Letters in Organic Chemistry, 2009, Vol. 6, No. 3
Mala'bi et al.
Fig. (1). The MP2/6-31(d) optimized structures of 9-AcAN (a), 9-AcAN O-protonate (b), and 9-AcAN ꢁ-complex (c).
AcAN)<ꢀH‡(1-AcAN)<ꢀH‡(2-AcAN). Thus, the MP2 cal-
culations predict that 9-AcAN is the kinetically controlled
product, whereas 2-AcAN is the thermodynamically con-
trolled product. 1-AcAN occupies an intermediary position:
its formation is kinetically controlled relative to 2-AcAN and
thermodynamically controlled relative to 9-AcAN. The
above results give a much simpler picture of the behavior of
acetylanthracenes in PPA, as compared with acetylphenan-
threnes [12]. In the AcAN series, the kinetically controlled
products, 9-AcAN and 1-AcAN, undergo deacylation and
rearrangements to the thermodynamically controlled product
2-AcAN. No reverse rearrangement of 2-AcAN to either 9-
AcAN or 1-AcAN or 1-AcAN to 9-AcAN were observed.
Furthermore, the deacylation of 2-AcAN to give anthracene
could be affected only at high temperatures (130–155 ºC).
These results are partially attributed to the large energy dif-
ferences among the three constitutional isomers of AcAN as
the kinetically controlled ꢁ-complexes and as the thermody-
namically controlled O-protonated ketones.
tionary points (minima, transition states). Non-scaled ther-
mal corrections to enthalpy were used.
1-Acetylanthracene
1-AcAN was synthesized using AlCl3 and acetyl chloride
in CH2Cl2 solvent (1 h, 40 ºC) [21], mp. 110 ºC (lit. 109 ºC
1
[21]; TLC Rf 0.60. H NMR: ꢂ=9.48 (s, H9), 8.45 (s, H10),
8.18 (d, J=8.4 Hz, H2), 8.08 (m, H3), 8.00 (d, J=6.8 Hz, H4,
H5), 7.49 (m, H6, H7, H8), 2.81 (s, CH3). 13C NMR: ꢂ=201.41
(C=O), 134.80 (C), 132.99 (C), 131.89 (C), 131.48 (C),
129.60 (CH), 129.15 (CH), 127.72 (CH), 127.59 (C), 126.85
(CH), 126.10 (CH), 125.95 (CH), 125.75 (CH), 123.38 (CH),
29.72 (CH3).
2-Acetylanthracene
2-AcAN was synthesized using AlCl3 and acetyl chloride
in nitrobenzene solvent [19], mp. 177 ºC (lit. 174–178 ºC
1
[19]; TLC Rf 0.56. H NMR: ꢂ=8.65 (s, H1), 8.58 (s, H9),
8.44 (s, H10), 8.02 (m, H4, H5, H8), 7.53 (m, H3, H6, H7), 2.76
(s, CH3). 13C NMR: ꢂ=197.80 (C=O), 134.04 (C), 133.25
(C), 132.66 (C), 131.60 (CH), 130.38 (C), 128.97 (CH),
128.70 (CH), 128.40 (C), 128.19 (CH), 126.66 (CH), 126.22
(CH), 125.93 (CH), 122.71 (CH), 26.5 (CH3).
The results of the present study provide additional sub-
stance to the emerging picture of reversibility of Friedel-
Crafts acylations of PAKs in PPA, although complete re-
versibility in the AcAN series has not been realized.
EXPERIMENTAL SECTION
9-Acetylanthracene
Melting points are uncorrected. NMR spectra were re-
2-AcAN was synthesized using AlCl3 and acetyl chloride
in CH2Cl2 solvent (45 min, 0 ºC) [19, 21], mp. 78 ºC (lit. 75–
76 ºC [23]; TLC Rf 0.73. 1H NMR: ꢂ=8.49 (s, H10), 8.04 (m,
H1, H8), 7.85 (m, H4, H5), 7.51 (m, H2, H3, H6, H7), 2.81 (s,
CH3). 13C NMR: ꢂ=207.82 (C=O), 136.58 (C), 130.89 (C),
128.66 (CH), 128.04 (CH), 126.60 (CH), 126.44 (C), 125.29
(CH), 124.14 (CH), 33.64 (CH3). 1H and 13C-NMR spectra of
acetylanthracenes have been previously reported [22, 29-31].
1
corded with a Bruker DRX 400 spectrometer. H and 13C
NMR spectra were recorded at 400 MHz using CDCl3 as
solvent and as internal standard, ꢂ(CHCl3)=7.26 ppm. An-
thracene and polyphosphoric acid (PPA) (84% weight of
P2O5, density 2.05 g/mL) were obtained from Acros Organ-
ics. Dichloromethane, n-hexane, petroleum ether (bp. 40–60
ºC) and ethyl acetate (all AR grade) were obtained from
BioLab.
The quantum mechanical calculations were performed
using the Gaussian03 [28] package. The split valence 6-
31G(d) basis set was employed. All structures were fully
optimized using standard convergence criteria (loose con-
vergence criteria were used for the calculation of (ac)-2-
AcAN ꢁ-complex to prevent the optimization process from
leading to the more stable isomer (sc)-2-AcAN). Vibrational
frequencies were calculated to verify the nature of the sta-
General Procedure for Friedel-Crafts Rearrangements of
Acetylanthracenes in PPA
The reaction was carried out in a 250 mL round-
bottomed flask equipped with a reflux condenser protected
from moisture, a septum cork, an argon inlet and a magnetic
stirrer. The flask was heated in an oil bath to the desired
temperature. PPA (40 g) was added to the flask, held at the
reaction temperature. The acetylanthracene isomer (0.3 g, 14

Reversible Friedel-Crafts Acylations of Anthracene
Letters in Organic Chemistry, 2009, Vol. 6, No. 3
241
[15]
[16]
[17]
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Adams, C. J.; Earle, M. J.; Roberts, G.; Seddon, K. R. Chem.
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Paquette, L. A. Encyclopedia of Reagents for Organic Synthesis,
Wiley: Chichester, 1995; Vol. 6, p. 4169.
Bassilios, H. F.; Shawky, M.; Salem, A. Y. Recl. Trav. Chim. Pays-
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Sarobe, M.; Jenneskens, L. W.; J. Org. Chem., 1997, 62, 8247.
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mmol) was then added to the reaction flask under an argon
atmosphere, with magnetic stirring. The reaction mixture
was stirred for the desired period at the predetermined tem-
perature. At the end of the reaction, the reaction mixture
(usually a dark red complex) was poured into ice–water/ice
and stirred overnight. After the complex had completely dis-
integrated, the product was extracted with dichloromethane
and the organic fraction was washed successively with
aqueous sodium bicarbonate and water, dried (MgSO4) and
the solvent was evaporated in vacuum. The constitution of
[18]
[19]
[20]
[21]
[22]
[23]
1
the crude product was examined by H-NMR spectroscopy
using the chemical shifts of the methyl hydrogen and the
low-field hydrogens of the isomeric acetylanthracenes (vide
supra). Anthracene was identified by following chemical
shifts: ꢀ = 7.44 (m, 4H, H2, H3, H6, H7), 7.98 (m, 4H, H1, H4,
H5, H8), 8.39 ppm (s, 2H, H9, H10).
[24]
[25]
[26]
[27]
[28]
Muller, P. Glossary of terms used in physical organic chemistry
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