Reversible Friedel-Crafts acyl rearrangements of planar polycyclic aromatic ketones: Dibenzofluorenones

Article abstract of DOI:10.1039/c3ra43483d

Dibenzofluorenones undergo reversible Friedel-Crafts acyl rearrangements in PPA at elevated temperatures. The Friedel-Crafts acyl rearrangements of 13H-dibenzo[a,i]fluoren-13-one (DBaiF) yield both 13H-dibenzo[a,h]fluoren-13-one (DBahF) and 12H-dibenzo[b,h]fluoren-12-one (DBbhF). DBahF and DBbhF undergo reversible mutual isomerizations, and their ratio depends on the reaction conditions. The O-protonate DBahFH⁺ plays a pivotal role in the proposed mechanism of the reversible Friedel-Crafts acyl rearrangements. DBahFH⁺ may undergo proton migration to give two isomeric σ-complexes: σ-13aH-DBahF⁺ and σ-12aH-DBahF ⁺, leading, via the respective naphthyl naphthoylium ions βCOβN-βN⁺ and αCOβN-βN⁺ to O-protonates DBbhFH⁺ and DBaiFH⁺, respectively. The regioselectivity of the rearrangement is expressed by the preferred intramolecular beta-electrophilic attack in βCOβN-βN⁺ and by the preferred alpha-electrophilic attack in αCOβN-βN ⁺, which indicates a thermodynamic control. The proposed mechanism is supported by the results of the DFT calculations of the dibenzofluorenones, their O-protonates, their σ-complexes and their corresponding naphthyl naphthoylium ions at B3LYP/6-311++G(d,p). DBahF and DBaiF are the kinetically controlled products of the Friedel-Crafts acyl rearrangement, while DBbhF is the thermodynamically controlled product. The aromaticity/antiaromaticity notions in dibenzofluorenones and their O-protonates, estimated by calculated HOMA and NICS indices, are discussed. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43483d

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Reversible Friedel–Crafts acyl rearrangements of planar
polycyclic aromatic ketones: dibenzofluorenones†
Cite this: RSC Adv., 2013, 3, 21797
*
Tahani Mala'bi, Sergey Pogodin, Shmuel Cohen and Israel Agranat
Dibenzofluorenones undergo reversible Friedel–Crafts acyl rearrangements in PPA at elevated temperatures.
The Friedel–Crafts acyl rearrangements of 13H-dibenzo[a,i]fluoren-13-one (DBaiF) yield both 13H-dibenzo
[a,h]fluoren-13-one (DBahF) and 12H-dibenzo[b,h]fluoren-12-one (DBbhF). DBahF and DBbhF undergo
reversible mutual isomerizations, and their ratio depends on the reaction conditions. The O-protonate
DBahFH+ plays a pivotal role in the proposed mechanism of the reversible Friedel–Crafts acyl rearrangements.
DBahFH+ may undergo proton migration to give two isomeric s-complexes: s-13aH-DBahF+ and s-12aH-
DBahF+, leading, via the respective naphthyl naphthoylium ions bCObN-bN+ and aCObN-bN+ to O-
protonates DBbhFH+ and DBaiFH+, respectively. The regioselectivity of the rearrangement is expressed by the
preferred intramolecular beta-electrophilic attack in bCObN-bN+ and by the preferred alpha-electrophilic
attack in aCObN-bN+, which indicates a thermodynamic control. The proposed mechanism is supported by
the results of the DFT calculations of the dibenzofluorenones, their O-protonates, their s-complexes and their
corresponding naphthyl naphthoylium ions at B3LYP/6-311++G(d,p). DBahF and DBaiF are the kinetically
controlled products of the Friedel–Crafts acyl rearrangement, while DBbhF is the thermodynamically
controlled product. The aromaticity/antiaromaticity notions in dibenzofluorenones and their O-protonates,
estimated by calculated HOMA and NICS indices, are discussed.
Received 9th July 2013
Accepted 5th September 2013
DOI: 10.1039/c3ra43483d
www.rsc.org/advances
Cras acylation has been violated when the acyl group was tilted
out of the plane of the aromatic nucleus, e.g., by neighboring
Introduction
bulky substituents.4,5 Under such a scenario, deacylations and
acyl rearrangements have become feasible.6 The reversibility
studies have been focused mainly on unusual aspects of selec-
tivity, including deacylations, uni-directional rearrangements
and kinetic control versus thermodynamic control.6 The pattern
of irreversibility of Friedel–Cras acylations has been high-
lighted under classical experimental conditions, (AlCl3, a trace
of water) e.g., in the naphthalene series.6–8
Friedel–Cras acylation is considered a corner stone of organic
chemistry.1 In 1955, Gore introduced the concept of reversibility
of Friedel–Cras acylation, proposing that “The Friedel–Cras
acylation reaction of reactive aromatic hydrocarbons is a
reversible process”.2 Gore concluded that “Reversibility is an
important factor in acylation reactions”.2 In 1973, the authori-
tative monograph Friedel–Cras Chemistry stated that “acylation
differs from alkylation in being virtually irreversible”.3 For many
years it has been accepted that Friedel–Cras acylations are
usually free of rearrangements and isomerizations. The differ-
ence in behavior between Friedel–Cras acylation and Friedel–
Cras alkylation was attributed to the resonance stabilization
existing between the acyl group and the aromatic nucleus,4
which may serve as a barrier against rearrangements and
reversible processes. The pattern of irreversibility of Friedel–
The incursion of reversibility in Friedel–Cras acylation was
revealed in 1974 by Agranat et al. in the benzoylation of naph-
thalene in polyphosphoric acid (PPA) at elevated temperatures.9
The kinetically controlled 1-benzoylnaphthalene rearranged to
the thermodynamically controlled 2-benzoylnaphthalene (PPA,
ꢀ
140 C), whereas the latter underwent only deacylation to give
naphthalene. The reversibility concept was then applied to the
synthesis of linearly annelated polycyclic aromatic ketones
(PAKs) by intramolecular Friedel–Cras acyl rearrangements of
their angularly annelated constitutional isomers.10–12 Complete
reversibility of Friedel–Cras acylation was established in the
para # ortho acyl rearrangement of uorouorenones in PPA.13
Further experimental evidence in support of true reversibility of
Friedel–Cras acylations and acyl rearrangements is limited.14–23
The relative contribution of kinetic control versus thermodynamic
control in Friedel–Cras acyl rearrangements remains an open
question, in spite of the rich chemistry of Friedel–Cras
Organic Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem,
Philadelphia Bldg. #201/205, Edmond J. Safra Campus, Jerusalem 91904, Israel.
E-mail: isri.agranat@mail.huji.ac.il
† Electronic supplementary information (ESI) available: 1H- and 13C-NMR spectra
of DBaiF, DBahF, DBbhF and DBcgF; crystallographic data for compounds DBahF,
DBaiF and DBcgF; Cartesian coordinates, total and relative B3LYP/6-311++G(d,p)
energies, selected geometric parameters, HOMA and NICS indices of
dibenzouorenones, their O-protonates, s-complexes
and naphthyl
naphthoylium ions. CCDC 920378 and 920382. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3ra43483d
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acylations.1,24 We have recently reported that kinetic control wins which are non planar,27 the dibenzouorenones, except DBcgF,
out over thermodynamic control in the Friedel–Cras acyl rear- are planar (vide infra).
rangements of diacetylanthracenes in PPA.25 The conformational
variations in these PAKs, which contribute to the understanding
of the motifs of reversibility, have been described.26
Synthesis and NMR spectroscopy
We report here the results of experimental and theoretical
study of the intramolecular Friedel–Cras acyl rearrangements
of dibenzouorenones in PPA. We highlight the reversibility
characteristics and the preference of thermodynamic control in
the acyl rearrangements of these planar PAKs and the relevance
of the notions of aromaticity/antiaromaticity herein.
All the dibenzouorenones under study have been described in
the literature. DBaiF was synthesized according to a literature
procedure27 by oxidation of 13H-dibenzo[a,i]uorene, formed by
acid cyclodehydration of di-1-naphthyl methanol. DBbhF was
synthesized according to a literature procedure28 by a double
aldol condensation of benz[f]indan-1-one and o-phthaldialde-
hyde in MeOH/KOH. DBahF29,30 was synthesized in the present
study by PPA mediated acyl rearrangement of DBaiF (vide infra).
DBcgF was prepared according to a literature procedure31 by
Results and discussion
Eleven constitutional isomers of dibenzouorenone under decarboxylation of [1,10-binaphthalene]-2,20-dicarboxylic acid,
study are shown in Fig. 1. The following dibenzouorenones, obtained from 1-bromo-2-naphthoic acid.
benzologs of 9H-uoren-9-one (Fl), were considered in the
The structures of DBaiF, DBahF and DBbhF have been veri-
present study: 13H-dibenzo[a,i]uoren-13-one (DBaiF), 13H- ed by H-NMR and 13C-NMR spectroscopies (vide infra). The
dibenzo[a,h]uoren-13-one (DBahF), 12H-dibenzo[b,h]uoren- structure of DBahF has also been veried by X-ray crystallog-
12-one (DBbhF), 13H-dibenzo[a,g]uoren-13-one (DBagF), 7H- raphy (vide infra). Table 1 gives the experimental 1H-NMR
dibenzo[b,g]uoren-7-one (DBbgF), and 7H-dibenzo[c,g]uoren- chemical shis of DBaiF, DBahF and DBbhF and of their O-
7-one (DBcgF). The remaining constitutional isomers of protonates. Table 2 gives the 13C-NMR chemical shis of DBaiF,
dibenzouorenone (in Fig. 1), i.e. dibenzo[a,c]uoren-13-one DBahF and DBbhF and of their O-protonates. Complete
(DBacF, aka 13H-indeno[1,2-l]phenanthrene), 7H-benzo[hi]- assignments were made through 2-dimensional correlation
chrysen-13-one (BhiC), 7H-benzo[de]naphthacen-7-one (BdeNC), spectroscopy (DQF-COSY, HSQC, and HMBC).
1
7H-dibenzo[a,kl]anthracen-7-one (DBaklAN) and 7H-benzo[ij]-
We note the deshielding of the protons (in CDCl3) very close
pleiaden-7-one (BijP) were outside the scope of the present study. to the carbonyl group in DBaiF and DBahF: d(H1, H12) ¼ 8.95
We have focused our attention on the Friedel–Cras acyl rear- ppm (DBaiF) and d(H1) ¼ 9.13 ppm (DBahF), as compared with
rangements of DBaiF, DBahF and DBbhF. The reported study the protons peri to the carbonyl, d(H12) ¼ 8.14 ppm (DBahF) and
includes synthesis, X-ray crystallography, 1H- and 13C-NMR d(H11, H13) ¼ 8.27 ppm (DBbhF). The deshielding of the
spectroscopy, acyl rearrangements and DFT calculations.
carbonyl carbon is also noted: d(13C]O)(DBaiF)–d(13C]
The names of dibenzouorenones imply dibenzo[a,i]-, [a,h]-, O)(DBahF) ¼ 3.4 ppm and d(13C)(DBaiF)–d(13C)(DBbhF) ¼ 4.8
[b,h]-, [a,g]-, [b,g]- or [c,g]-annelation of the parent uorenone ppm. These relative downeld shis indicate contributions of
system. Dibenzouorenones may be perceived as bridged dipolar structures with partial positive charges on the carbonyl
dinaphthyl ketones. DBaiF is a bridged 1Z,1Z0-dinaphthyl carbon atoms in the order DBaiF > DBahF > DBbhF. The same
ketone (110DN); DBahF is a bridged 1Z,2Z0-dinaphthyl ketone order, but a smaller effect is indicated in the mixture of tri-
(120DN); DBbhF is a bridged 2Z,2Z0-dinaphthyl ketone (220DN); uoroacetic acid : CDCl3 (19 : 1): d(13C]O)(DBaiF)–d(13C]
DBagF is a bridged 1Z,2E0-dinaphthyl ketone; DBbgF is a O)(DBahF) ¼ 2.2 ppm and d(13C]O)(DBaiF)–d(13C]O)(DBbhF)
bridged 2Z,2E0-dinaphthyl ketone; DBcgF is a bridged 2E,2E0- ¼ 3.2 ppm. In this media dibenzouorenones exists in the form
dinaphthyl ketone. However, contrary to the dinaphthyl ketones of O-protonates: DBaiFH+, DBahFH+ and DBbhFH+ (vide infra).
Fig. 1 Isomeric C21H12O PAKs.
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Table 1 1H-NMR chemical shifts (d, ppm) of DBaiF, DBahF and DBbhF (in CDCl3) and of their O-protonates (in TFA : CDCl3 19 : 1)
d(DBaiF) d(DBaiFH+) Dd(DBaiF–DBaiFH+) d(DBahF) d(DBahFH+) Dd(DBahF–DBahFH+) d(DBbhF) d(DBbhFH+) Dd(DBbhF–DBbhFH+)
H1 8.95
H2 7.57
H3 7.41
H4 7.76
H5 7.96
H6 7.66
H7 7.66
H8 7.96
H9 7.76
H10 7.41
H11 7.57
H12 8.95
8.56
7.56
7.39
7.74
7.97
7.60
7.60
7.97
7.74
7.39
7.56
8.56
—
0.39
0.01
0.02
0.02
ꢁ0.01
0.06
0.06
ꢁ0.01
0.02
0.02
0.01
0.39
—
9.13
7.66
7.51
7.84
8.08
7.85
7.86
7.85
7.56
7.49
7.91
8.14
—
8.64
7.48
7.36
7.60
7.80
7.44
7.41
7.55
7.42–7.43
7.32
7.56
7.65
—
0.49
0.18
0.15
0.26
0.28
0.41
0.45
0.30
0.14
0.17
0.36
0.49
7.92
7.49
7.57
7.88
8.09
8.09
7.88
7.57
7.49
7.92
8.27
7.65
7.37
7.50
7.70
7.78
7.78
7.70
7.50
7.37
7.65
7.96
0.27
0.12
0.07
0.18
0.31
0.31
0.18
0.07
0.12
0.27
0.31
—
8.27
—
7.96
—
0.31
H13
—
—
In the O-protonates, the protons close to the carbonyl group structures of DBahF and DBcgF are reported here.32 PAK DBahF
exhibit considerable upeld shis relative to those in the parent crystallizes in the monoclinic space group P21/c with unit cell
ketones: Dd(1H) ¼ 0.39 ppm (H1, H12, DBaiFH+), 0.49 ppm (H1, dimensions a ¼ 13.3493, b ¼ 7.6721, c ¼ 13.0564 A, and b ¼
˚
H12, DBahFH+), 0.31 ppm (H11, H13, DBbhFH+). The carbonyl 94.888ꢀ. PAK DBcgF crystallizes in the orthorhombic space
carbons in the O-protonates are shied downeld relative to group P21/n with unit cell dimensions a ¼ 16.753, b ¼ 14.395, c
those in the parent ketones: Dd(13C) ¼ ꢁ5.9 ppm (C13, DBaiFH+), ¼ 5.793 A, and b ¼ 91.0 . Fig. 2 and 3 depict ORTEP diagrams of
ꢀ
˚
ꢁ7.1 ppm (C13, DBahFH+), ꢁ7.5 ppm (C12, DBbhFH+).
DBahF and DBcgF respectively, as determined by X-ray analysis
(anisotropic displacement parameters are drawn at the 50%
probability level). The crystallographic data of DBahF, DBcgF
and DBaiF are given in Table S1 in the ESI.†
Table 3 gives selected structural parameters derived from the
crystal structures of DBahF, DBaiF and DBcgF. The most
Molecular and crystal structures
The molecular and crystal structures of DBaiF33 and DBcgF34
have previously been reported. The molecular and crystal
Table 2 13C-NMR chemical shifts (d, ppm) of DBaiF, DBahF and DBbhF (in CDCl3) and of their O-protonates (in TFA : CDCl3 19 : 1)
Dd(DBaiF–
DBaiFH+)
Dd(DBahF–
DBahFH+)
Dd(DBbhF–
DBbhFH+)
d(DBaiF)
d(DBaiFH+)
d(DBahF)
d(DBahFH+)
d(DBbhF)
d(DBbhFH+)
C1
124.0
129.4
126.2
128.6
134.8
135.3
—
125.5
132.2
128.6
131.1
137.5
139.1
—
ꢁ1.5
ꢁ2.8
ꢁ2.5
ꢁ2.6
ꢁ2.7
ꢁ3.8
—
124.7
129.5
126.8
128.6
134.3
136.3
—
126.3
132.5
129.4
131.2
136.9
141.0
—
ꢁ1.6
ꢁ3.0
ꢁ2.6
ꢁ2.6
ꢁ2.6
ꢁ4.7
—
130.8
126.9
129.1
128.7
137.3
119.7
138.8
138.8
119.7
137.3
—
128.7
—
129.1
—
126.9
—
130.8
133.7
125.7
134.3
192.8
134.3
—
125.7
133.7
—
133.2
129.5
132.3
130.8
140.3
122.1
140.7
140.7
122.1
140.3
—
130.8
—
132.3
—
129.5
—
133.2
135.6
129.9
135.0
200.3
135.0
—
129.9
135.6
—
ꢁ2.3
ꢁ2.6
ꢁ3.2
ꢁ2.1
ꢁ3.0
ꢁ2.4
ꢁ1.8
ꢁ1.8
ꢁ2.4
ꢁ3.0
—
C2
C3
C4
C4a
C5
C5a
C5b
C6
—
—
—
—
—
—
117.9
145.4
145.4
117.9
—
135.3
134.8
128.6
—
126.2
—
129.4
—
124.0
130.2
126.8
197.6
126.8
130.2
120.3
148.8
148.8
120.3
—
139.1
137.5
131.1
—
128.6
—
132.2
—
125.5
132.6
128.9
203.5
128.9
132.6
ꢁ2.4
ꢁ3.4
ꢁ3.4
ꢁ2.4
—
ꢁ3.8
ꢁ2.7
ꢁ2.6
—
118.5
146.5
138.2
118.9
136.8
128.9
—
128.9
—
127.0
—
130.9
134.0
125.1
133.5
—
194.2
129.7
130.3
120.6
150.9
139.8
122.2
139.7
131.3
—
132.3
—
129.9
—
133.4
136.2
129.4
134.7
—
201.3
131.1
132.7
ꢁ2.1
ꢁ4.4
ꢁ1.6
ꢁ3.2
ꢁ2.9
ꢁ2.4
—
C6a
C6b
C7
ꢁ2.1
C7a
C8
—
ꢁ3.2
—
C8a
C9
ꢁ3.4
ꢁ2.6
C9a
C10
C10a
C11
C11a
C12
C12a
C12b
C13
C13a
C13b
—
—
ꢁ2.5
ꢁ2.9
—
ꢁ2.3
ꢁ1.9
ꢁ4.2
ꢁ0.7
ꢁ7.5
ꢁ0.7
—
—
ꢁ2.8
—
ꢁ2.6
ꢁ2.2
ꢁ4.3
ꢁ1.2
—
ꢁ7.1
ꢁ1.4
ꢁ2.4
ꢁ1.5
ꢁ2.4
ꢁ2.2
ꢁ5.9
ꢁ2.2
ꢁ2.4
ꢁ4.2
ꢁ1.9
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The molecules in the crystal structures of DBahF (Fig. 4) and
DBcgF (Fig. 5) are arranged in the herring-bone pattern, i.e. in
two sets of parallel planes.36 For DBahF the interplanar distance
is 341.8 pm (between x, y, z plane and 1 ꢁ x, 1 ꢁ y, 1 ꢁ z plane),
and non-parallel planes (x, y, z and 1 ꢁ x, 0.5 + y, 0.5 ꢁ z) form
dihedral angle of 68.7ꢀ. The molecules of DBahF lying in the
parallel planes demonstrate aromatic–aromatic p/p interac-
tions, with the shortest distances between the centroids of the
aromatic rings of CgA/CgD ¼ 373.2 pm, CgB/CgC ¼ 373.2 pm
(see Fig. 1 for the numbering of the aromatic rings). The
molecules lying in the non-parallel planes demonstrate C–H/p
interactions, with the shortest distances between the centroids
of one molecule (x, y, z plane) and the aromatic hydrogens of
another molecules (1 ꢁ x, 0.5 + y, 0.5 ꢁ z plane) of H5/CgD ¼
273.2 pm, H6/CgC ¼ 275.9 pm.
Fig. 2 ORTEP drawings of DBahF.
For DBcgF (Fig. 5) the interplanar distances are 379.2 pm (x,
y, z plane vs. x, y, 1 + z plane) and 357.8 pm (ꢁ0.5 + x, 0.5 ꢁ y, 0.5
ꢁ z plane vs. 1.5 ꢁ x, ꢁ0.5 + y, 1.5 ꢁ z plane), and the dihedral
angle is 68.7ꢀ (x, y, z plane vs. ꢁ0.5 + x, 0.5 ꢁ y, 0.5 ꢁ z plane).
Both p/p and C–H/p intermolecular interactions in DBcgF
are slightly weaker than in DBahF. The shortest distance
between the aromatic rings is CgD/CgB ¼ 389.2 pm, while the
shortest non-bonding C/H distance is H9/CgA ¼ 296.5 pm.
Fig. 3 ORTEP drawing of DBcgF.
Friedel–Cras acyl rearrangements
important feature of the X-ray structures of DBahF and DBaiF is
their planarity, in spite of the angular monobenzo[a]- and
dibenzo[a]annelation in DBahF and DBaiF, respectively. It
appears that despite the close proximity between the carbonyl
oxygen and the closest aromatic hydrogen, the non-bonded C]
O/H distances, which might have caused the titling of the
carbonyl group from the planes of the aromatic nuclei, are
larger than the sum of the Van-der-Waals radii of oxygen (129
pm) and hydrogen (115 pm).35 It is expected that the linearly
dibenzo[b]annelated DBbhF would also be planar. By contrast,
DBcgF is not planar due to the prohibitively short non-bonded
H1/H13 distance in the "ord" region, 214 pm, which causes a
In the present study PAKs DBaiF, DBahF and DBbhF were each
subjected to PPA at 120–160 ꢀC for 4–10 h. The results are
summarized in Table 4. The constitution of the crude products
twist around C13b–C13c bond: C13–C13a–C13b–C13c ¼ 12.2ꢀ, C13a
–
C13b–C13c–C13d ¼ 22.8ꢀ. The dihedral angle between the two
naphthalene nuclei in DBcgF, F ¼ 27.1ꢀ, is also noted. The
geometrical parameters of the molecular structures of DBahF,
DBaiF and DBcgF are in a good agreement with the respective
parameters of the B3LYP/6-311++G(d,p) calculated global
minima conformations of these ketones (vide infra).
Fig. 4 The packing diagram of DBahF, viewed down the (1, 0, 0) axis.
Table 3 Selected X-ray geometrical parameters of DBahF, DBaiF and DBcgF
Fa (deg)
cb (deg)
bc (deg)
C]O (pm)
O/Hd (pm)
C/Ce (pm)
H/He (pm)
DBaiF
DBahF
DBcgF
1.9
4.1
27.1
0.4
0.8
1.2
105.9
105.2
105.6
122.3
121.8
121.6
247.5
250.2
292.4
326.9
326.1
317.5
259.2
263.8
214.4
a
Dihedral angle between the least-square planes of two naphthyl moieties. b Pyramidalization angle at the carbonyl carbon is the improper torsion
angles C12b–C13–O1–C13a (DBaiF), C12a–C13–O1–C13a (DBahF) or C6a–C7–O1–C7a (DBcgF), respectively, minus 180ꢀ MOD 360. Bond angle C12b–C13
–
c
C13a (DBaiF), C12a–C13–C13a (DBaiF) or C6a–C7–C7a (DBcgF). The closest distance between the oxygen and an aromatic hydrogen O1/H12 (DBaiF),
O1/H1 and O1/H12 (DBahF), or O1/H6 (DBcgF). e The non-bonding distances C6/C7 and H6/H7 (DBaiF, DbahF) or C1/C13 and H1/H13 (DBcgF).
d
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(i) protonation of DBaiF to give the O-protonate DBaiFH+;
(ii) proton migration to give the s-complex s-12bH-DBaiF+;
(iii) 5-membered-ring cleavage to give 2-(20-naphthyl)-1-
naphthoylium ion aCObN-bN+;
(iv) conformational isomerization of aCObN-bN+ by rotation
0
around the C2–C2 bond;
0
(v) regioselective intramolecular acylation at C3 of the 20-
naphthyl substituent to give the s-complex s-12aH-DBahF+;
(vi) proton migration and rearomatization to give the
O-protonate DBahFH+;
(viia) proton migration to give the more stable s-complex
s-13aH-DBahF+;
(viib) alternatively, deprotonation of DBahFH+ to give the
intermediate product DBahF;
Fig. 5 The packing diagram of DBcgF, viewed down the (1,0,1) axis.
(viii) 5-membered-ring cleavage to give 3-(20-naphthyl)-2-
naphthoylium ion bCObN-bN+;
1
of the reactions was determined by H-NMR spectroscopy. The
(ix) conformational isomerization of bCObN-bN+ by rotation
constitutional isomers DBaiF, DBahF and DBbhF were distin-
guished by the low-eld 1H-NMR chemical shis of the protons
closest to the carbonyl group: H1 and H12 (doublet) in DBaiF, H1
(doublet) in DBahF and H11 and H13 (singlet) in DBbhF (Table 1)
(vide supra).
The Friedel–Cras acyl rearrangements of DBaiF in PPA yield
both DBahF and DBbhF. The ratio DBbhF/DBahF increases
signicantly (from 1 : 44 to 30 : 37) upon lengthening the
reaction time and raising the reaction temperature. Preparative
rearrangement of DBaiF in PPA (at 140 ꢀC) for three hours gave
DBahF in 25% yield. DBahF and DBbhF undergo mutual
isomerization, and their ratio depends on the reaction condi-
tions. Thus, the acyl rearrangement of the dibenzouorenones
suggests certain features of reversibility: DBaiF / DBahF #
DBbhF. Unidirectional Friedel–Cras acyl rearrangement of
11H-benzo[a]uoren-11-one to 11H-benzo[b]uoren-11-one in
PPA has briey been reported.11
0
around the C3–C2 bond;
0
(x) regioselective intramolecular acylation at C3 of the 20-
naphthyl substituent to give the s-complex s-11aH-DBbhF+;
(xi) proton migration to give the O-protonate DBbhFH+;
(xii) deprotonation of DBbhFH+ to give the nal product
DBbhF.
The O-protonate DBahFH+ plays a pivotal role in the
proposed mechanism of the consecutive rearrangements. This
O-protonate may undergo proton migration to give two isomeric
s-complexes: s-13aH-DBahF+ and s-12aH-DBahF+. The
preferred formation of s-13aH-DBahF+ allows its 5-membered
ring cleavage to give bCObN-bN+ ion. An intramolecular elec-
trophilic attack by the 2-naphthoylium ion at the beta-position
30
(C ) of the 20-naphthyl substituent gives the s-complex s-11aH-
DBbhF+, which then undergoes proton migration leading to the
linearly benzo[b]annelated O-protonate DBbhFH+. Alternatively,
the formation of the less stable s-complex s-12aH-DBahF+
would have led to aCObN-bN+ ion, which would undergo
intramolecular electrophilic attack by the 1-naphthoylium ion
A plausible mechanism of the Friedel–Cras acyl rear-
rangements of the dibenzouorenones in PPA is presented in
Scheme 1. The proposed mechanism of the consecutive rear-
rangements DBaiF / DBahF # DBbhF includes formation of
their respective O-protonates, s-complexes and naphthyl
naphthoylium ions. It involves the following steps:
10
at the alpha-position (C ) of the 20-naphthyl substituent to give
s-12bH-DBaiF+. Proton migration in the latter s-complex leads
to DBaiFH+. The regioselectivity is expressed in the preferred
alpha-carbonyl cleavage of s-13aH-DBahF+ followed by the intra-
molecular beta-electrophilic attack in bCObN-bN+ over beta-
carbonyl cleavage of s-12aH-DBahF+ followed by the alpha-elec-
trophilic attack in aCObN-bN+. The preference for beta-substitu-
tion over alpha-substitution in the Friedel–Cras acylations of
naphthalenes is indicative of a thermodynamic control (vide infra).
The preferred pathway of the rearrangements is summarized:
Table 4 Products of Friedel–Crafts acyl rearrangements of DBaiF, DBahF and
DBbhF
Products (% relative yield)
Starting
isomers
Temp
(ꢀC)
Reaction
time (h)
DBaiF
DBahF
DBbhF
DBaiF
DBaiF
DBaiF
DBaiF
DBaiF
DBahF
DBahF
DBahF
DBahF
DBbhF
DBbhF
DBbhF
140
140
140
160
160
120
140
160
160
140
140
160
3
5
8
8
10
4
6
6
10
5
8
55
54
52
33
9
0
0
0
0
44
42
34
37
43
97
76
58
60
9
1
4
14
30
48
3
23
42
40
91
66
72
DBaiF # DBaiFH+ # s-12bH-DBaiF+ # aCObN-bN #
s-12aH-DBahF+ # DBahFH+ # s-13aH-DBahF+ # bCObN-
bN+ # s-11aH-DBbhF+ # DBbhFH+ # DBbhF.
DFT study
0
0
0
DFT methods are capable of generating a variety of isolated
molecular properties quite accurately, especially via the hybrid
functionals, and in a cost-effective way.37,38 Recently, the B3LYP
34
28
6
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Scheme 1 Proposed mechanism of the Friedel–Crafts acyl rearrangements of dibenzofluorenones in PPA.
hybrid functional was successfully employed to treat dinaphthyl DDG298) are presented in Table 5; total and relative energies and
ketones, dinaphthyl thioketones and dinaphthyl diazo- selected geometrical parameters are presented in the ESI
methanes27 and overcrowded BAEs.39–41 PAKs DBaiF, DBahF, (Tables S2–S5†).
DBbhF, DBcgF, DBagF and DBbgF, their O-protonates, their s-
Dibenzouorenones (Fig. 1) adopt planar Cs or C2v confor-
complexes and the structurally related naphthyl naphthoylium mations, except for DBcgF, which is twisted around C6b–C12b
:
ions were subjected to a computational DFT study. Their rela- the dihedral angle between the naphthalene systems is F ¼ 26ꢀ;
tive B3LYP/6-311++G(d,p) Gibbs free energies (DG298 and its planar C2v conformer features prohibitively short H1/H13
Table 5 Relative B3LYP/6-311++G(d,p) free energies (DG298 and DDG298, kJ molꢁ1) of dibenzofluorenones, their respective O-protonates, s-complexes and naphthyl
naphthoylium ions
Naphthyl
DDG298 naphthoylium ions
a
b
b
b
Ketones
DG298
O-Protonates
DG298
s-Complexes
DG298
DG298
DDG298
DBbhF
DBahF
C2v
Cs
0.00
1.74
DBbhFH+
DBahFH+
DBahFH+
DBbgFH+
Cs
Cs
C1
0.00
3.99
8.81
s-11aH-DBbhF+ C1 100.05 32.51
bCObN-bN+
bCObN-bN+
aCObN-bN+
bCObN-aN+
bCObN-aN+
bCOaN-bN+
aCObN-bN+
bCOaN-bN+
aCObN-aN+
aCObN-aN+
bCOaN-aN+
bCOaN-aN+
C1 81.37
C1 79.57
C1 69.84
C1 89.13
90.22
C1 81.40
C1 67.73
C1 78.81
C1 79.05
84.64
13.64
11.84
2.11
21.40
22.49
13.67
0.00
11.08
11.32
16.91
20.05
19.40
s-13aH-DBahF+ C1
67.54 0.00
s-12aH-DBahF+ C1 100.84 33.29
DBbgF
Cs
10.10
Cs 13.03
s-6aH-DBbgF+
C1
97.60 30.06
DBbgFH+
DBaiFH+
DBagFH+
DBagFH+
Cs 15.87
C1 34.92
Cs 39.47
C1 47.70
s-7aH-DBbgF+
s-12bH-DBaiF+
C1 108.62 41.08
DBaiF
DBagF
C2v 16.14
Cs
C1
69.26
76.93
1.71
9.39
23.42
s-13aH-DBagF+ C1
s-12aH-DBagF+ C1
99.35 31.80
DBcgF
C2
55.61
DBcgFH+
C1 73.35
s-6aH-DBcgF+
C1 117.90 50.36
C1 87.78
87.13
a
Relative to the DBbhF. b Relative to DBbhFH+.
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distance (147 pm) at the ord region and serves as a transition divided into three groups on the basis of their stabilities and
state for enantiomerization of its C2 global minimum. Their HOMA indices of the benzene rings.
relative stabilities are governed by a combination of steric and
(i) The s-complexes with a benzene ring at [a]-position (s-
electronic effects. The most stable dibenzouorenones are 13aH-DBahF+, s-12bH-DBaiF+ and s-13aH-DBagF+) have the
DBbhF and DBahF which are among the least sterically strained lowest relative energies, and their single benzene rings have the
in their overcrowded regions (Table S2†), whereas the least highest HOMA indices, 0.89–0.90.
stable DBcgF has a short contact distance H1/H13 ¼ 208 pm
(ii) The s-complexes with a benzene rings at [c]-position (s-
and is non-planar. An estimate of the aromatic stabilization in 6aH-DBbgF+, s-12aH-DBagF+ and s-6aH-DBcgF+) have lower
dibenzouorenones (vide infra) shows that the most stable HOMA indices, 0.75–0.76, and high relative energies.
isomers, DBbhF and DBahF, demonstrate relatively low anti-
aromaticity of their ve-membered rings, as compared with the 12aH-DBahF+, s-11aH-DBbhF+ and s-7aH-DBbgF+) also have
least stable isomer DBcgF. high relative energies, and their benzene rings have the lowest
(iii) The s-complexes with a benzene ring at [b]-position (s-
Most of the O-protonates of dibenzouorenones are planar; HOMA indices, 0.60–0.62.
DBagFH+, DBahFH+ and DBaiFH+ with one benzene ring at [a]-
Thus, the main factor affecting the relative stabilities of the
position adopt slightly non-planar conformations with the s-complexes of dibenzouorenones is the position of the ben-
naphthalene dihedral angle F ¼ 3–7ꢀ (Table S3†), thus avoiding zannelation of the protonated ring of the uorenone moiety.
short distances between the hydroxyl and the peri-aromatic The difference between [a]- and [b]-/[c]-annelation may be
hydrogen. DBcgFH+, like DBcgF, is considerably twisted, F ¼ explained in simple resonance terms. There are seven Kekule
´
27ꢀ, with a somewhat short contact distance H1/H13 ¼ 211 pm. structures (excluding the non-protonated naphthalene system)
The order of the relative stabilities of the O-protonates is the in the case of [a]-annelation, four of them possess an aromatic
´
same as the one of the respective ketones. The most stable O- sextet. There are six Kekule structures in the case of [b]- and [c]-
protonates are planar DBbhFH+ and DBahFH+ (Table 5), while annelation, but only two of them possess an aromatic sextet.
the least stable one is highly twisted DBcgFH+. Analogously to
Naphthyl naphthoylium ions, shown in Fig. 7, are constitu-
neutral dibenzouorenones, the antiaromaticity of the central tional isomers of the O-protonates and of the s-complexes of
ve-membered ring in DBbhFH+ and DBahFH+ is lower than in dibenzouorenones. Their relative energies and selected
DBcgFH+ (vide infra), suggesting the higher stability of the geometrical parameters are presented in Table S5.† Their
formers.
designators are based on the positions of the carbonyl group
In the s-complexes of dibenzouorenones (Fig. 6), the (aCO or bCO) and of the naphthyl–naphthyl bond (aN–aN, aN–
aromatic conjugation in one naphthalene system is broken, bN, bN–aN or bN–bN). They may be formed from the s-
leaving one naphthalene system and one benzene ring. The complexes of dibenzouorenones by breaking the bond
energies and selected geometrical parameters of the s- between the carbonyl carbon and the sp3 carbon of a naphthyl
complexes of dibenzouorenones are presented in Table S4.† moiety. This process leads to rearomatization of the formerly
The HOMA indices (vide infra) of the s-complexes of dibenzo- protonated naphthalene system. Naphthyl naphthoylium ions
uorenones are presented in Fig. 6. These s-complexes may be play an important role in Friedel–Cras acyl rearrangements of
Fig.
6
HOMA indices in s-complexes of dibenzofluorenones (B3LYP/6-
Fig. 7 Naphthyl naphthoylium ions (global minima conformations only, B3LYP/
6-311++G(d,p) free energies DG298, relative to DBbhFH+, in kJ molꢁ1 are given in
parentheses).
311++G(d,p) free energies (relative to DBbhFH+) DG298 in kJ molꢁ1 are given in
parentheses; aromatic double bonds are omitted for clarity).
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dibenzouorenones (vide infra). The most stable of the naphthyl
naphthoylium ions is aCObN-bN+, which may be formed from
both s-12bH-DBaiF+ and s-12aH-DBahF+ and serves as an
intermediate for the DBaiF # DBahF rearrangement. Acylium
ion bCObN-bN+ may be obtained from both s-13aH-DBahF+
and s-11aH-DBbhF+ s-complexes and serves as an intermediate
for the DBahF # DBbhF rearrangement. Acylium ion bCOaN-
bN+ connects between s-13aH-DBagF+ and s-7aH-DBbgF+ s-
complexes and, consequently, between DBagF and DBbgF. The
remaining naphthyl naphthoylium ions, i.e. aCObN-aN+,
bCOaN-aN+ and bCObN-aN+ are formed each from a single
dibenzouorenone only (from s-12aH-DBagF+, s-6aH-DBcgF+
and s-6aH-DBbgF+, respectively, see Table S5†) and thus may
not participate in the Friedel–Cras acyl rearrangements.
The energies presented in Table 5 were calculated in the gas
phase. In order to test the potential inuence of a solvent on the
relative energies of the species under study, we have performed
solvent reaction eld calculations of DBaiF, DBahF and DBbhF,
their O-protonates, their s-complexes and the structurally
related naphthyl naphthoylium ions, using the PCM model42
(Tables S2–S5†). The relative free energies of these species
calculated in the presence of water do not differ considerably
from the respective energies calculated in gas phase, and show
the same order of stabilities: DBbhF > DBahF > DBaiF for PAKs
and their O-protonates, s-13aH-DBahF+ > s-12bH-DBaiF+ > s-
11aH-DBbhF+ > s-12aH-DBahF+ for the s-complexes.
Fig. 8 HOMA indices in selected dibenzofluorenones and their O-protonates
(B3LYP/6-311G++(d,p) free energies DG298 in kJ molꢁ1 are given in parentheses).
Aromaticity/antiaromaticity in dibenzouorenones and their
O-protonates
indices for the B3LYP/6-311G++(d,p) geometries of DBaiFH+,
DBahFH+ and DBbhFH+. The most stable O-protonates,
DBbhFH+ and DBahFH+, exhibit low aromaticity of their ve-
membered rings (HOMA5 ¼ 0.14 and 0.24). By comparison, the
ve-membered ring of the least stable DBcgFH+ is non-aromatic
The degree of aromatic/antiaromatic stabilization/destabiliza-
tion in dibenzouorenones can be estimated by various
methods, including HOMA and NICS. HOMA index43 is a
geometry-based index. The HOMA values (in the range of
+1 to ꢁ1) describe the contribution to the decrease in
aromaticity of a cyclic conjugated system due to bond elonga-
tion (EN term) and due to bond alternation (GEO term). The
HOMA indices of the central ve-membered ring and of the
21-membered perimeter of dibenzouorenones are presented
in Table S2.† Fig. 8 shows the B3LYP/6-311G++(d,p) HOMA
indices for DBaiF, DBahF and DBbhF. DBbhF and DBahF,
which have the lowest relative energies, possess lower anti-
aromaticity of their ve-membered rings (4pe) as evidenced by
their HOMA5 indices (ꢁ0.76 and ꢁ0.70, respectively). By
comparison, DBcgF has the highest antiaromaticity of its ve-
membered ring (HOMA5 ¼ ꢁ0.84). Analogously, HOMA21
indices (20pe, calculated for the whole 21-membered conju-
gated systems) of DBbhF (0.48) and DBahF (0.47) are larger
than that of DBcgF (0.41). The HOMA5 indices of dibenzo-
uorenones demonstrate a strong correlation with their rela-
tive Gibbs free energies DG298, with Pearson correlation
coefficient r ¼ ꢁ0.79. The correlation between the HOMA21
indices, which describe the overall conjugation, and DG298
values is better, r ¼ ꢁ0.92.
(HOMA5
¼ ꢁ0.04). Analogously to dibenzouorenones,
HOMA21 indices of DBbhFH+ and DBahFH+ are larger than that
of DBcgFH+. HOMA5 and HOMA21 demonstrate a very good
correlation with the relative Gibbs free energies DG298 (r ¼
ꢁ0.84 and ꢁ0.95, respectively).
NICS44 is a magnetic-based measure of aromaticity which,
like HOMA, may be employed to estimate the aromaticity/anti-
aromaticity of separate rings. NICS values reect the direction
of the ring current in a cyclic conjugated system; they are
negative for diamagnetic ring current and positive for the
paramagnetic ring current. NICS values of the ve-membered
rings of dibenzouorenones and their O-protonates are pre-
sented in Tables S2 and S3,† respectively. Fig. 9 shows the NICS
values in DBaiF, DBahF, DBbhF and in their respective O-
protonates. In all the dibenzouorenones and the O-protonates
under study, their ve-membered rings possess highly positive
NICS values, implying the paramagnetic ring current. However,
the most stable ketones, DBbhF and DBahF, and O-protonates,
DBbhFH+ and DBahFH+, have the lowest NICS values of their
ve-membered rings. The NICS values of the ve-membered
rings demonstrate a strong correlation with the respective free
energies DG298 of dibenzouorenones and the O-protonates, r
The HOMA indices of the central ve-membered ring and of
the 21-membered perimeter of the O-protonates of dibenzo-
uorenones are presented in Table S3.† Fig. 8 shows the HOMA
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highly positive NICS values of the ve-membered rings in the O-
protonates are in agreement with the notion of their anti-
aromaticity. Our NMR spectroscopy results, however, show that
the 13C chemical shis of C]O of these O-protonates in tri-
uoroacetic acid : CDCl3 (19 : 1) as compared with the corre-
sponding shis in CDCl3 do not indicate a marked effect of
antiaromaticity. Moreover, the calculated HOMA indices show,
that the ve-membered rings in the O-protonates of dibenzo-
uorenones vary from low aromatic to nonaromatic (Table S3†),
whereas the ve-membered rings in dibenzouorenones are
highly antiaromatic (Table S2†). The HOMA21 indices of the O-
protonates are also considerably higher than those of diben-
zouorenones, indicating a higher degree of aromaticity of the
O-protonates. The higher degree of aromaticity in the O-
protonates as compared to the dibenzouorenones is attributed
to the different charge distribution over the conjugated system.
Fig. 11 shows the natural charges in DBaiF, DBahF, DBbhF and
their respective O-protonates. Dibenzouorenones may be
considered as dipolar dibenzouorenylium ions. In DBaiF,
DBahF and DBbhF the partial positive charge (arising due to the
contributions of the dipolar structures) is localized mainly in
their ve-membered rings (+0.27 to +0.40), whereas the benzene
rings are nearly devoid of a positive charge (ꢁ0.16 to 0.03). In
DBaiFH+, DBahFH+ and DBbhFH+, the ve-membered rings
retain a positive charge (+0.25 to +0.41), but a signicant part of
the total positive charge is delocalized through the benzene
rings, in particular in the terminal ones (+0.11 to +0.15). This
Fig. 9 GIAO//B3LYP/6-311G++(d,p) calculated NICS values in selected diben-
zofluorenones and their O-protonates.
¼ 0.98 (excluding DBaiF, which is an outlier) and 0.90,
respectively.
It has not escaped our minds that the O-protonates of
dibenzouorenones may be considered antiaromatic. These
species may be perceived as hydroxydibenzouorenium ions
(for instance, DBaiFH+ in Fig. 10) and, accordingly, possess an
antiaromatic character due to their delocalized 4p/20p elec-
trons (4n, n ¼ 1, 5). The O-protonate of uorenone has been
shown to possess some antiaromatic character and all of its
antiaromaticity was found in its ve-membered ring.45 The
Fig. 11 Natural charges in selected dibenzofluorenones and their O-protonates
Fig. 10 Electron delocalization in DBaiFH+.
(at B3LYP/6-311G++(d,p), the summation made per each ring).
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effect is seen even in 9-hydroxyuorenylium ion (FlH+, +0.31 in undergo an acyl rearrangement to a dibenzouorenone, due to
ve-membered ring, +0.23 in six-membered rings) as its naphthyl–naphthyl aa0 bond.
compared to Fl (+0.32 in ve-membered ring, 0.02 in six-
membered rings).
In addition, the structural relationships between the diben-
zouorenones, described above, and their constitutional
isomers DBacF, BhiC, BdeNC, DBaklAN and BijP may be
considered. BhiC and BijP, like DBaiF, are structurally related to
110DN (aa0), and may be potentially obtained from the latter by
forming a bond between b- and peri-positions of the naphthyl
moieties of 110DN (aa0 + bp0, leading to BhiC) or between two
peri-positions (aa0 + pp0, leading to BijP). PAKs DBaklAN and
BdeNC, like DBahF and DBagF, are structurally related to 120DN
(ab0), and may be potentially obtained from the latter by form-
ing a bond between a- and peri-positions of the naphthyl
moieties of 120DN (ab0 + pa0, leading to DBaklAN) or between b
and para-positions (ab0 + pb0, leading to BdeNC). DBacF cannot
be formed from any of the dinaphthyl ketones dues to the
double annelation at one side of the uorenone moiety.
Dinaphthyl ketones and dibenzouorenones – structural
relationships
The interconnections of the isomers of dibenzouorenone are
presented in Scheme 2. The dibenzouorenones and their
constitutional isomers, depicted in Fig. 1, may be considered as
bridged dinaphthyl ketones. Thus, 1,10-dinaphthyl ketone
(110DN), where two naphthyl moieties are connected at their a
positions (aa0), is structurally related to DBaiF, which carries an
additional b–b bridge (aa0 + bb0). 1,20-Dinaphthyl ketone
(120DN), in which two naphthyl moieties are connected at their
a and b positions (ab0), may give rise to two dibenzo-
uorenones, DBahF with an additional b–b bridge (ab0 + bb0)
and DBagF with an additional b–a bridge (ab0 + ba0). Finally,
2,20-dinaphthyl ketone (220DN) with (bb0) connection between
its naphthyl moieties may lead to three dibenzouorenones,
Mechanism of Friedel–Cras acyl rearrangements of
dibenzouorenones
DBcgF (bb0 + aa0), DBbhF (bb0 + bb0) and DBbgF (bb0 + ba0). The mechanism of Friedel–Cras acyl rearrangements
During the Friedel–Cras acyl rearrangement the bond between proposed above is supported by the results of the DFT calcula-
the carbonyl carbon and the sp3 carbon of a naphthyl moiety is tions (see Tables 5 and S2–S5†). Recently, a computational
broken. As a result, DBaiF, DBahF and DBbhF may each model for predicting the site for electrophilic aromatic substi-
mutually undergo acyl rearrangement into other isomers by tution was reported.46 The model was based on DFT calculations
breaking the aa0, ab0 or bb0 bond, respectively, followed by of the relative stabilities of the s-complex intermediates and
rotation around the remaining bb0 bond (Scheme 2). Analo- was applied (inter alia) to Lewis acid promoted Friedel–Cras
gously, DBagF and DBbgF may interconvert by breaking the ab0 acylations. Our calculations give the following orders of relative
or bb0 bond, respectively, and rotation around the remaining ba0 free energies (DG298, kJ molꢁ1) of DBaiF, DBahF, DBbhF, DBbgF,
bond. The sixth dibenzouorenone, DBcgF, is not able to DBagF and DBcgF in each series:
Scheme 2 Structural relationships between dinaphthyl ketones and dibenzofluorenones.
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Conclusions
The dibenzouorenones under study are planar PAKs (except
DBcgF). The relative stabilities of dibenzouorenones, their O-
protonates and s-complexes are governed by a combination of
steric and electronic effects. The reversible pathway of the
Friedel–Cras acyl rearrangements (Agranat–Gore rearrange-
ment18,20,26) of dibenzouorenones established in the present
study indicates that non-planarity is not a sine qua non condi-
tion for the rearrangements. In this case, thermodynamic
control wins out over kinetic control, in contrast to the Friedel–
Cras acyl rearrangements in the diacetylanthracene series.25
The rearrangements are intramolecular. It remains to be seen
whether the thermodynamic control is dictated by the intra-
molecularity of reversible Friedel–Cras acyl rearrangements.
Fig. 12 The B3LYP/6-311++G(d,p) relative free energies (DDG298, kJ molꢁ1) of
dibenzofluorenones (K), their O-protonates (P) and s-complexes (S).
Experimental
Melting points are uncorrected. NMR spectra were recorded
with Bruker Avance II 500 and AMX 400 spectrometers; 1H-NMR
spectra were recorded at 500.2 and 400.13 MHz in CDCl3 as a
solvent and as an internal standard (d(CHCl3) ¼ 7.260 ppm).
13C-NMR spectra were recorded at 125.78 MHz using CDCl3 as
solvent and as internal standard, d(CDCl3) ¼ 77.01 ppm. 1H-
and 13C-NMR spectra of O-protonates of dibenzouorenones
were recorded using the mixture triuoroacetic acid
(TFA) : CDCl3 (19 : 1) with TFA as an internal standard (d(TFA) ¼
11.500 ppm). Complete assignments were made through 2-
dimensional correlation spectroscopy (DQF-COSY, HSQC, and
HMBC). PPA (84% weight of P2O5, dens. 1.9 g mLꢁ1) was
purchased from Acros Organics, Israel. Diethyl ether, petroleum
ether (40–60 ꢀC, PE), benzene and toluene were dried on
sodium.
ꢂ s-complexes of dibenzouorenones: DDG298 ¼ 0.0 (s-
13aH-DBahF+), 1.7 (s-12bH-DBaiF+), 9.4 (s-13aH-DBagF+), 30.1
(s-6aH-DBbgF+), 31.8 (s-12aH-DBagF+), 32.5 (s-11aH-DBbhF+),
33.3 (s-12aH-DBahF+), 41.1 (s-7aH-DBbgF+), 50.4 (s-6aH-
DBcgF+);
ꢂ O-protonated dibenzouorenones: DG298 ¼ 0.0 (DBbhFH+),
4.0 (DBahH+), 13.0 (DBbgFH+), 34.9 (DBaiFH+), 39.5 (DBagFH+),
73.4 (DbcgFH+);
ꢂ dibenzouorenones: DG298 ¼ 0.0 (DBbhF), 1.7 (DBahF),
10.1 (DBbgF), 16.1 (DBaiF), 23.4 (DBagF), 55.6 (DBcgF).
Fig. 12 depicts the relative energies of the most stable
conformations of each of the dibenzouorenone species. The
B3LYP/6-311++G(d,p) calculated order of stabilities of the s-
complexes of dibenzouorenones is s-13aH-DBahF+ z s-
12bH-DBaiF+ > s-13aH-DBagF+ > s-6aH-DBbgF+ z s-12aH-
DBagF+ > s-12aH-DBahF+ z s-11aH-DBbhF+ > s-7aH-DBbgF+ >
s-6aH-DBcgF+. According to the Hammond–Leffler postulate,47
the relative energies of the transition states leading to the
s-complexes resemble those of the s-complexes. Thus, the
DFT calculations predict that DBahF and DBaiF are the kineti-
cally controlled products of the Friedel–Cras acyl rearrange-
ment. By contrast, the calculations show that among the
dibenzouorenones, the linearly dibenzo[b]annelated DBbhF
is the most stable, and should be credited as the thermody-
namically controlled product. Likewise, among the O-proton-
ates of dibenzouorenones, the linearly dibenzo[b]annelated
DBbhFH+ is the most stable product. It should be noted,
however, that DBahF is only slightly less stable than DBbhF:
DDG298 ¼ 1.7 kJ molꢁ1 (PAKs), 4.0 kJ molꢁ1 (O-protonates).
Thus, the DFT calculations support the contention derived from
the experimental results that the Friedel–Cras acyl rearrange-
General procedure for rearrangements of dibenzouorenones
in PPA
In a 150 mL round-bottomed ask equipped with a magnetic
stirrer and under argon. PPA was heated for few minutes to the
desired temperature. A dibenzouorenone was then added. The
reaction mixture was stirred for desired time at constant
temperature. The reaction mixture was then poured into ice
water and stirred overnight. The crude products were extracted
with CH2Cl2, washed with aqueous saturated NaHCO3 and
water, and dried over MgSO4. The solvent was evaporated under
reduced pressure to give a mixture of isomers (see Table 4)
which can be detected and identied according to their 1H-NMR
spectra (Table 1).
Synthesis of 13H-dibenzo[ai]uoren-13-one (DBaiF)
DBaiF was prepared by oxidation of dibenzo[a,i]uorene with
oxygen, using 18-crown-6 ether as a catalyst, in a benzene
ments of dibenzouorenones in PPA are thermodynamically solution.48 DBaiF was obtained as red powder, mp. 269–270 ꢀC
(ref. 33, 271–272 ꢀC).
controlled and reversible. The reversible DBahF / DBaiF
1H-NMR (d, ppm) 7.406 (dt, J ¼ 8.0 Hz, J ¼ 7.1 Hz, 2H, H3,
H10), 7.573 (dt, J ¼ 8.5 Hz, J ¼ 6.7 Hz, 2H, H2, H11), 7.655 (dt, J ¼
8.0 Hz, J ¼ 7.1 Hz, 2H, H6, H7), 7.759 (d, J ¼ 8.2 Hz, 2H, H4, H9),
rearrangement was not observed experimentally, consistently
with the low population of DBaiFH+ at equilibrium (DG413
¼
34.3 kJ molꢁ1).
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7.958 (d, J ¼ 8.1 Hz, 2H, H5, H8), 8.946 (d, J ¼ 8.4 Hz, 2H,
H1, H12).
Synthesis of 7H-dibenzo[cg]uoren-7-one (DBcgF)
A mixture of [1,10-binaphthalene]-2,20-dicarboxylic acid (0.0696
g, 0.20 mmol, prepared from 1-bromo-2-naphthoic acid
according to the literature31) and acetic anhydride (2.52 mL, 20
mmol) was reuxed for 45 m. Unreacted acetic anhydride, acetic
acid and water were removed under reduced pressure, and the
solid residue heated at 280 ꢀC for 3 h under argon atmosphere.
The resulting red melt was extracted with boiling benzene (3 ꢃ
10 mL) and the solvent evaporated, yielding 0.0604 g of red
solid. The crude product was puried by sublimation at 5 mm
Hg/130 ꢀC, yielding 0.0236 g (41%) of DBcgF as dark red crystals,
13C-NMR (d, ppm): 117.88 (C6, C7), 124.02 (C1, C12), 126.17
(C3, C10), 126.76 (C13a, C12b), 128.55 (C4, C9), 129.38 (C2, C11),
130.16 (C12a, C13b), 134.82 (C4a, C8a), 135.25 (C5, C8), 145.42 (C6a,
C6b), 197.60 (C13).
DBaiFH+: 1H-NMR (d, ppm) 7.39 (t, J ¼ 7.5 Hz, J ¼ 7.7 Hz, 2H,
H3, H10), 7.56 (t, J ¼ 8.4 Hz, 2H, H2, H11), 7.60 (d, J ¼ 8.1 Hz, 2H,
H6, H7), 7.74 (d, J ¼ 8.3 Hz, 2H, H4, H9), 7.97 (d, J ¼ 8.2 Hz, 2H,
H5, H8), 8.56 (d, J ¼ 8.7 Hz, 2H, H1, H12).
13C-NMR (d, ppm): 120.31 (C6, C7), 125.48 (C1, C12), 128.63
(C3, C10), 128.94 (C13a, C12b), 131.13 (C4, C9), 132.21 (C2, C11),
132.60 (C12a, C13b), 137.47 (C4a, C8a), 139.06 (C5, C8), 148.79 (C6a,
C6b), 203.49 (C13).
ꢀ
ꢀ
mp. 223–224 C (ref. 31, 222–222.5 C, ref. 34, 225–226 ꢀC).
1H-NMR (d, ppm): 8.364 (d, J ¼ 8.2 Hz, H1), 7.919–7.888 (m,
H4), 7.844 (d, J ¼ 8.0 Hz, H5), 7.783 (d, J ¼ 8.0 Hz, H6), 7.605–
7.554 (m, H2, H3).
13C-NMR (d, ppm): 194.42 (C7]O), 146.07 (C13b), 138.84
(C4a), 132.96 (C6a), 130.12 (C5), 129.33 (C4), 128.12 (C13a), 127.92
(C1), 127.75 (C3), 126.43 (C2), 119.78 (C6).
Synthesis of 13H-dibenzo[ah]uoren-13-one (DBahF)
In a 50 mL round bottom ask equipped with magnetic stirrer
and under argon, PPA (7.3 g) was added; aer stirring for a few
minutes at 160 ꢀC, 13H-dibenzo[a,i]uorenon-13-one (DBahF)
(0.05 g) was added. The reaction mixture was stirred for 1 h at
160 ꢀC, and then poured into a mixture of ice and water (50 mL).
The crude products were extracted with CH2Cl2 (2 ꢃ 30 mL),
washed with saturated aqueous NaHCO3 solution (30 mL) and
H2O (30 mL), and dried over MgSO4. Aer ltration, the organic
solvent was evaporated in vacuum. PLC chromatography on
silica gel 60 F254 (E. Merck) of the crude products using ethyl
acetate : PE (1 : 9), gave a fraction that contained DBahF. The
compound was extracted with ethyl acetate; the solvent was
evaporated under reduced pressure to give DBahF as a yellow
X-ray diffraction study
Single crystal X-ray diffraction was carried out on a Bruker
SMART APEX CCD X-ray diffractometer equipped with a
graphite-monochromator and using MoKa radiation (l ¼
˚
0.71073 A). Single crystals were attached to glass bers, with
either epoxy glue, or mineral oil. Data were collected at 173 K.
Low temperature was maintained with a Bruker KRYOFLEX
nitrogen cryostat. The diffractometer was controlled by a Pen-
tium-based PC running the SMART soware package.49 Imme-
diately aer collection, the raw data frames were transferred to a
second PC computer for integration and reduction by the SAINT
program package.50 The structures were solved and rened
using the SHELXTL soware package51by full matrix least-
ꢀ
ꢀ
powder, 0.0127 g; yield 25%, mp. 211.0 C (ref. 29, 214 C, ref.
30, 214.0–215.0 ꢀC). Single crystal of DBahF was obtained by
crystallization from ethyl acetate.
1H-NMR (d, ppm): 9.128 (d, J ¼ 8.5 Hz, H1), 8.135 (s, H12),
8.081 (d, J ¼ 8.0 Hz, H5), 7.909 (d, J ¼ 8.0 Hz, H11), 7.86 (s, H7),
7.85 (d, J ¼ 8.5 Hz, H6), 7.85 (d, J ¼ 8.5 Hz, H8), 7.84 (d, J ¼ 8.5
Hz, H4), 7.662 (t, J ¼ 7.5 Hz, H2), 7.560 (t, J ¼ 7.5 Hz, 7.0 Hz, H9),
7.514 (t, J ¼ 7.5 Hz, 6.5 Hz, H3), 7.487 (t, J ¼ 7.0 Hz, H10).
13C-NMR (d, ppm): 194.16 (C13, C]O), 146.54 (C6a), 138.19
(C6b), 136.82 (C7a), 136.27 (C5), 134.33 (C4a), 134.04 (C11a),
2
squares on Fo with anisotropic displacement parameters for
non-hydrogen atoms.
Computational details
133.53 (C12a), 130.85 (C11), 130.26 (C13b), 129.65 (C13a), 129.51 The quantum mechanical calculations were performed with the
(C2), 128.89 (C9, C8), 128.55 (C4), 127.02 (C10), 126.80 (C3), 125.10 Gaussian09 package. Becke's three-parameter hybrid density
52
(C12), 124.68 (C1), 118.92 (C7), 118.51 (C6).
functional B3LYP,53 with the non-local correlation functional of
LC-MS: m/z ¼ 281 (M+1). IR: n ¼ 1642.9 cmꢁ1 (C]O); UV/Vis Lee, Yang, and Parr54 was used. The split valence 6-311++G(d,p)
(CHCl3, 6.7 ꢃ 10ꢁ6 M, nm): l ¼ 301, 313, 374, 349sh, 374, 392sh, basis set was employed. All structures were fully optimized
473sh.
under the symmetry constraints specied. Vibrational
DBahFH+: 1H-NMR (d, ppm): 8.64 (d, J ¼ 8.4 Hz, H1), 7.65 (s, frequencies were calculated to verify the natures of the
H12), 7.80 (d, J ¼ 8.3 Hz, H5), 7.60 (d, J ¼ 8.2 Hz, H4), 7.56 (d, J ¼ stationary points. Non-scaled thermal corrections to Gibbs' free
7.0 Hz, H11), 7.55 (d, J ¼ 7.0 Hz, H8), 7.48 (t, J ¼ 7.6 Hz, H2), 7.44 energy were used. Natural charges were computed by a full
(d, J ¼ 8.2 Hz, H6), 7.43–7.42 (m, H9), 7.41 (s, H7), 7.36 (t, J ¼ 8.3 natural orbital analysis, using NBO version 3. NMR shielding
Hz, 7.3 Hz, H3), 7.32 (t, J ¼ 8.1 Hz, 8.2 Hz, H10).
tensors were computed with the Gauge-Independent Atomic
13C-NMR (d, ppm): 201.3 (C13, C]O), 150.91 (C6a), 141.01 Orbital method. The calculations in the presence of a solvent
(C5), 139.79 (C6b), 139.71 (C7a), 136.89 (C4a), 136.23 (C11a), (H2O) were performed using Tomasi's polarized continuum
134.74 (C12a), 133.43 (C11), 132.70 (C13b), 132.51 (C2), 132.25 model (PCM),42 by placing the solute in a cavity within the
(C9), 131.30 (C8), 131.17 (C4), 131.07 (C13a), 129.88 (C10), 129.41 solvent reaction eld, with full geometry optimization of the
(C12), 129.35 (C3), 126.29 (C1), 122.15 (C7), 120.57 (C6).
structures.
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27 N. Assadi, S. Pogodin, S. Cohen and I. Agranat, Struct. Chem.,
2012, 23, 771–790.
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