Naphthologs of overcrowded bistricyclic aromatic enes: (E)-bisbenzo[a] fluorenylidene

Article abstract of DOI:10.1007/s11224-012-0146-4

(E)-11H-Bisbenzo[a]fluorenylidene (E-6) was synthesized by Barton's double extrusion diazo-thione coupling method from 11H-benzo[a]fluoren-11-thione (11) and 11-diazo-11H-benzo[a]fluorene (13). The reaction is probably thermodynamically controlled; in the event that the less stable Z -6 is also formed, it would rapidly undergo Z → E diastereomerization to give E -6. The B3LYP/6-311G(d,p) calculated diastereomerization barrier for Z -6 → E -6 is ΔG ₂₉₈ = 57.0 kJ/mol (13.6 kcal/mol). The calculated equilibrium constant K _eq(E -6 → Z -6) = 92:8 (at 298 K) is indicative of a marked diastereoselectivity of the reaction leading to E -6. The structure of E-6 was established by ¹H-NMR and ¹³C-NMR spectroscopies and by X-ray analysis. PAE E-6 crystallizes in the monoclinic space group C2/c. The unit cell of the crystal structure E -6 contains eight molecules, arranged as four pairs of enantiomers. PAE E -6 adopts a twisted conformation with the pure twist of the central C¹¹=C ^11′ bond ω = 39. The dihedral angle ν in E -6 is 60.6, which is significantly higher than the respective dihedral angle in PAEs Z -6, 2, E -7, Z -7, 14, and 15. The large syn-pyramidalization angles at C ¹¹ and C^11′ (χ = 12.6 and 14.8) of E-6 indicates the enhanced strain in the fjord regions of the molecule. The enhanced twist is primarily attributed to the double benzo[a]annelation of the bifluorenylidene moiety at the fjord regions. The B3LYP/6-311G(d,p) calculated structure of E -6 is in a very good agreement with the experimental X-ray structure. PAE E -6 adopts a twisted conformation in solution, with the downfield chemical shift of H¹/H^1′ (8.31 ppm); H¹⁰/H ^10′ (δ = 7.20 ppm) and H⁹/H^9′ (δ = 6.86 ppm) in E -6 are positioned above the planes of the opposing naphthalene rings. PAEs E -6 and Z -6 are significantly higher in energy than their corresponding benzo[b]annelated isomers E -7 and Z -7.

Full text of DOI:10.1007/s11224-012-0146-4

Struct Chem (2013) 24:1229–1240
DOI 10.1007/s11224-012-0146-4
ORIGINAL RESEARCH
Naphthologs of overcrowded bistricyclic aromatic enes:
(E)-bisbenzo[a]fluorenylidene
•
•
•
Naela Assadi Sergey Pogodin Shmuel Cohen
Israel Agranat
Received: 10 September 2012 / Accepted: 28 September 2012 / Published online: 26 October 2012
Ó Springer Science+Business Media New York 2012
Abstract (E)-11H-Bisbenzo[a]fluorenylidene (E-6) was
synthesized by Barton’s double extrusion diazo-thione
coupling method from 11H-benzo[a]fluoren-11-thione (11)
and 11-diazo-11H-benzo[a]fluorene (13). The reaction is
probably thermodynamically controlled; in the event that
the less stable Z-6 is also formed, it would rapidly undergo
Z ? E diastereomerization to give E-6. The B3LYP/6-
311G(d,p) calculated diastereomerization barrier for
Z-6 ? E-6 is DG298 = 57.0 kJ/mol (13.6 kcal/mol). The
calculated equilibrium constant Keq(E-6 ? Z-6) = 92:8
(at 298 K) is indicative of a marked diastereoselectivity of
the reaction leading to E-6. The structure of E-6 was
molecule. The enhanced twist is primarily attributed to the
double benzo[a]annelation of the bifluorenylidene moiety
at the fjord regions. The B3LYP/6-311G(d,p) calculated
structure of E-6 is in a very good agreement with the
experimental X-ray structure. PAE E-6 adopts a twisted
conformation in solution, with the downfield chemical shift
0
0
0
of H1/H1 (8.31 ppm); H10/H10 (d = 7.20ppm) and H9/H9
(d = 6.86 ppm) in E-6 are positioned above the planes of
the opposing naphthalene rings. PAEs E-6 and Z-6 are
significantly higher in energy than their corresponding
benzo[b]annelated isomers E-7 and Z-7.
1
established by H-NMR and 13C-NMR spectroscopies and
Keywords Crystal structure ꢀ NMR ꢀ DFT calculations ꢀ
Diastereomerization ꢀ Diazo-thione coupling ꢀ
Pyramidalization ꢀ Steric strain
by X-ray analysis. PAE E-6 crystallizes in the monoclinic
space group C2/c. The unit cell of the crystal structure E-6
contains eight molecules, arranged as four pairs of enan-
tiomers. PAE E-6 adopts a twisted 0conformation with the
pure twist of the central C11=C11 bond x = 39°. The
dihedral angle m in E-6 is 60.6°, which is significantly
higher than the respective dihedral angle in PAEs Z-6, 2,
Introduction
The bistricyclic aromatic enes [1, 2] (1, BAEs) (Fig. 1)
have fascinated chemists since bifluorenylidene (2) was
synthesized in 1875 [3], dixanthylene (3) was synthesized
in 1895 [4], and thermochromism and piezochromism were
revealed in bianthrone (4) [5, 6]. They can be classified into
homomerous BAEs (1, X = Y) and heteromerous BAEs
(1, X = Y) [7]. They may be viewed as bridged tetra-
arylethenes or as tetrabenzofulvalenes. These systems are
attractive substrates for the study of the ground state con-
formations and dynamic stereochemistry of overcrowded
polycyclic aromatic enes (PAEs) [1, 2, 8, 9]. Bifluoreny-
lidene (2) and the PAE bis-4H-cyclopenta[def]phenan-
thren-4-ylidene (5) [10] with their central five-membered
rings are fullerene fragments and potential starting mate-
rials for the preparation of buckyballs [11]. The topic of
BAEs has previously been reviewed [1, 2, 9, 12–14].
E-7, Z-7, 14, and 15. The large syn-pyramidalization
angles at C11 and C11 (v = 12.6° and 14.8°) of E-6 indi-
cates the enhanced strain in the fjord regions of the
0
N. Assadi ꢀ S. Pogodin ꢀ S. Cohen ꢀ I. Agranat (&)
Organic Chemistry, Institute of Chemistry,
The Hebrew University of Jerusalem, Philadelphia Building,
#201/205, Edmond J. Safra Campus, 91904 Jerusalem, Israel
e-mail: isri.agranat@mail.huji.ac.il
N. Assadi
e-mail: naela_as@yahoo.com
S. Pogodin
e-mail: s.pogodin@mail.huji.ac.il
S. Cohen
e-mail: shmuel.cohen2@mail.huji.ac.il
123

1230
Struct Chem (2013) 24:1229–1240
Fig. 1 Bistricyclic aromatic
enes
10
X
central 5-membered rings, which increases the nonbonding
coplanar distances 1ꢀꢀꢀ10 and 8ꢀꢀꢀ80, compared to the cor-
responding distances in their central six-membered ring
analogs (e.g., dixanthylene (3) and bianthrone (4)). This
topology introduces a propensity towards planarity of the
tricyclic moieties in 2, while concomitantly rendering them
sterically less demanding, especially in the twisted con-
formation [2]. Bifluorenylidenes and the putative E- and
Z-biphenalenylidenes are the only known homomerous
BAEs with a twisted global minimum conformations
[20–23]. In the parent 2, the pure twist is 32°–33° [22].
Recently, the inherent potential of a new generation of
electron acceptors in organic electronics, based on the bi-
fluorenylidene backbone has been reported [24]. The ver-
satility of the bifluorenylidene core as a function of
substitution and the extension of the p-electron conjugated
4
5
10a
8a
4a
6
7
3
2
A
B
E
8
1
9
9a
fjord
fjord
8a'
9a'
2'
3'
9'
7'
6'
1'
C
8'
F
Y
D
4a'
10a'
5'
4'
10'
BAE (1)
X = Y: homomerous
X = Y: heteromerous
Intramolecular Overcrowding has been defined as a steric
effect shown by aromatic structures in which certain intra-
molecular distances between nonbonded atoms are smaller
than the sum of the van der Waals radii of the atoms involved
[15, 16]. BAEs (1) are overcrowded in the fjord regions. This
intramolecular overcrowding requires out-of-plane defor-
mations to alleviate prohibitively close contacts of non-
bonded atoms in the fjord regions on both sides of0the central
system has been highlighted [24]. The addition of one
0
electron across the C9=C9 bond of 2 is highly favorable,
due to the relief of steric strain/overcrowding resulting in a
reduced twist and the gain in aromaticity in one tricyclic
moiety to a 14p-electron (4n ? 2, n = 3) system [24].
Such electron acceptors are potentially important com-
pounds for photoinduced charge transfer from an electron-
donating material to an electron-accepting component,
used as an active layer for organic photovolcanic devices
(OPVs). Benzannelation of a BAE leads to a PAEs.
We report here the synthesis, spectroscopic properties and
molecular and crystal structure of the PAE E-11H-bis-
benzo[a]fluorenylidene (E-6), a violet, doubly benzannelat-
ed, highlytwistedbifluorenylidenederivative. Theresultsofa
0
0
0
0
double 0bond C9=C9 0(C1ꢀꢀꢀC1 , C8ꢀꢀꢀC8 , H1ꢀꢀꢀH1 , H8ꢀꢀꢀH8 ,
0
0
C1ꢀꢀꢀH1 , C8ꢀꢀꢀH8 , C1 ꢀꢀꢀH1, C8 ꢀꢀꢀH8). A variety of confor-
mations of BAEs has been identified. The unique molecular
architecture of BAEs gives rise to three basic conformations:
anti-folded, syn-folded, and twisted [14, 17]. Each confor-
mation represents a compromise between p-delocalization
and steric strain. The bridges X and Y play an important role
in the relative stabilities of the three conformations, leading
in favorable cases to thermochromism [17, 18].
O
6
7
4
5
5a
6
7
8
3
5
4a
4b
6a6b
G
8
4a
A
A
B
B
11a
4
9
E
2
E
9a
9
11
11
3
G11b
11a
10a
10a
10
10
1
2'
1
1'
2
3'
O
3
1'
10'
11a'
10a'
10'
10a'
11b'
11a'
H
11'
11'
9'
2
9a'
2'
3'
9'
4'
F
F
4a'
5'
D
C
D
C
8'
4b'
4a'
6b'
6a'
H
8'
O
5a'
6'
4'
5'
7'
7'
6'
(E)-7
(E)-6
O
4
5
(Z )-7
(
Z
)-6
Bifluorenylidene (2) (1, X,Y:–), the smallest BAE, is a
unique case among the homomerous BAEs, because of its
Fig. 2 PAEs 6 and 7
123

Struct Chem (2013) 24:1229–1240
1231
DFT study of the conformations of the diastereomers E-6 and
Z-6, and a comparison with their constitutional isomers
(E)-11H-bisbenzo[b]fluorenylidenes (E-7) and (Z)-11H-bis-
benzo[b]fluorenylidenes (Z-7) are also reported (Fig. 2).
condenser, magnetic stirrer, and glass beads, ketone 8 (4.22 g,
18.34 mmol) was dissolved in toluene (70 mL) and KOH
(pellets, 12.0 g, 214.22 mmol) was added. The reaction mix-
ture was stirred with reflux for 4 h. The color of the reaction
mixture turned brown. The reaction mixture was cooled and
poured into water. The organic layer was washed several times
with 10 % aqueous KOH, The orange aqueous fractions were
combined and acidified with HCl (32 %), to give a white solid.
It was extracted with diethyl ether; the organic layer was col-
lected, dried on Na2SO4, filtered and the solvent evaporated
under reduced pressure. The yellow solid product was re-
crystallized from CCl4 to give9 aslightyellowpowder, 1.86 g,
yield 42 %, mp 183 °C (lit. [26] 185–186 °C).
Experimental section
Melting points are uncorrected. All NMR spectra were
1
recorded with a Bruker DRX 500 spectrometer; H-NMR
spectra were recorded at 500.2 MHz using CDCl3 as sol-
vent and as internal standard, d(CHCl3) = 7.26 ppm.
13C-NMR spectra were recorded at 125.78 MHz using
CDCl3 as solvent and as internal standard, d(CDCl3) =
77.01 ppm. Complete assignments were made through
two-dimensional correlation spectroscopy (DQF-COSY,
HSQC, and HMBC). Diethyl ether, benzene, and toluene
were dried on sodium. Lawesson’s reagent and KOH were
purchased from Acros; hydrazine hydrate was purchased
from Fluka; 1-indanone was purchased from Merck KGaA.
4
4
1H-NMR (d, ppm): 7.429 (t, J = 2.0 Hz, J = 1.5 Hz,
3
1H), 7.454 (d, J = 7.5, 2H), 7.484–7.503 (m, 2H), 7.587
(td, 3J = 7.5 Hz, 4J = 1.0 Hz, 5J = 0.5 Hz, 1H), 7.792 (s,
1H), 7.796 (d, 3J = 8.0 Hz, 1H), 7.833–7.860 (m, 2H),
3
7.977 (d, J = 8.0 Hz, 1H).
13C-NMR (d, ppm): 126.02 (C–H), 126.18 (C–H),
126.98 (C–H), 127.15 (C–H), 127.31 (C–H), 127.42 (C–H),
127.71 (C–H), 128.13 (C–H), 129.29 (C), 135.82 (C–H),
131.53 (C–H), 132.16 (C–H), 132.60 (C), 133.26 (C),
138.73 (C), 143.40 (C), 172.37 (C=O).
11H-Benzo[b]fluorene-11-one (8)
Ketone 8 was prepared according to the literature proce-
dure with some modifications [25]. A solution of 1-inda-
none (6 g, 44.7 mmol) in methanol (25 mL) was added to a
solution of o-phthalaldehyde (6 g, 44.7 mmol) in methanol
(25 mL) with magnetic stirring. The mixture was stirred for
30 min; 28 % KOH in methanol (16 mL) (prepared from
KOH 5.6 g and methanol 20 mL) was then added drop-
wise. The color of the reaction mixture turned black. The
reaction mixture was refluxed with stirring for 3 h; a yel-
low precipitate was formed. The reaction mixture was
cooled to rt and filtered off, the yellow powder was washed
with methanol. 4.22 g of 8 was obtained; yield 41 %, mp
153 °C (lit. [25] 152 °C).
11H-Benzo[a]fluorene-11-one (10)
Ketone 10 was prepared according to the literature procedure
with some modifications [26]. A mixture of 2-(2-naph-
thyl)benzoic acid 9 (1.86 g, 7.50 mmol) and PPA (40 g) was
stirred at 75 °C for 4 h under Ar. The color of the reaction
mixture turned deep brown-black. The reaction mixture was
poured on ice and water and stirred for 1 h. The yellow solid
was filtered off, dissolved inCH2Cl2, the solutionwas washed
with aqueous Na2CO3 and water. The organic layer was dried
on Na2SO4, filtered and the solvent evaporated under reduced
pressure. The crude product was purified by column chro-
matography on silica gel (PE:EtAc, 95:5) to give 10 as an
orange powder, yield 50 %, mp 130 °C (lit. [26] 132 °C).
1H-NMR (d, ppm): 7.345 (td, 3J = 7.5 Hz, 3J = 7.0 Hz,
4J = 1.0 Hz, H2), 7.464 (td, 3J = 8.0 Hz, 3J = 7.5 Hz,
4J = 1.0 Hz, H8), 7.540 (td, 3J = 7.5 Hz, 4J = 1.0 Hz,
H7), 7.558 (td, 3J = 7.5 Hz, 3J = 7.0 Hz, 4J = 1.0 Hz,
3
3
1H-NMR (d, ppm): 7.259 (t, J = 8.2 Hz, J = 6.4 Hz,
H9), 7.405–7.478 (m, H3, H7, H8), 7.586 (td, J = 8.4 Hz,
3
3
3
H3), 7.705 (d, J = 7.5 Hz, H4), 7.750 (d, J = 7.5 Hz,
4J = 1.0 Hz, 5J = 0.6 Hz, H2, H6), 7.623 (d, 3J = 8.2 Hz,
3
3
3
H1), 7.819 (d, J = 8.0 Hz, H6), 7.853 (s, H5), 7.882 (d,
H10), 7.771 (d, J = 8.3 Hz, H4), 7.971 (d, J = 8.2 Hz,
3J = 8.5 Hz, H9), 8.163 (s, H10).
H5), 8.974 (d, J = 8.5 Hz, H1).
3
13C-NMR (d, ppm): 119.00 (C5), 120.93 (C4), 124.37
(C1), 124.85 (C10), 126.87 (C8), 128.72 (C6), 128.92 (C7),
192.12 (C2), 130.50 (C9), 132.73 (C9a), 133.58 (C10a),
134.93 (C3), 136.12 (C11a), 136.86 (C5a), 138.33 (C4b),
144.00 (C4a), 192.99 (C=O).
13C-NMR (d, ppm): 118.02 (C6), 119.92 (C7), 123.79
(C10), 124.26 (C1), 126.38 (C8), 126.83 (C11a), 128.49 (C4),
129.22 (C9), 129.40 (C2), 130.15 (C11b), 134.16 (C3),
134.39 (C4a), 134.55 (C10a), 135.85 (C5), 143.85 (C6b),
146.11 (C6a), 195.36 (C11).
2-(2-Naphthyl)benzoic acid (9)
11H-Benzo[a]fluorene-11-thione (11)
Acid 9 was prepared according to the literature procedure
[26]. In a round-bottomed flask equipped with a reflux
In a round-bottomed flask equipped with a reflux condenser
protected with a CaCl2 tube and a magnetic stirrer, ketone
123

1232
Struct Chem (2013) 24:1229–1240
10 (0.1 g, 0.43 mmol) was dissolved in dry benzene
(30 mL), Lawesson’s reagent (0.089 g, 0.22 mmol) was
added. The reaction mixture was stirred with reflux for 5 h.
The color of the yellow reaction mixture turned red. After
cooling to rt, the solvent was evaporated under vacuum
almost to dryness. The crude product was purified by col-
umn chromatography on dry silica gel (PE:CH2Cl2, 95:5).
The first red fraction was collected and evaporated. A
green powder of the 11 was obtained 0.049 g, yield 64 %,
mp 189 °C.
solution of KOH in ethanol (0.5 mL), and yellow HgO
(1.0 g, 4.90 mmol) were added. The color of the solution
changed gradually from yellow to pink. After 3 h of stir-
ring at 0 °C, the reaction was completed; the solution was
filtered and the residue was washed with Et2O. The com-
bined organic fractions were evaporated under reduced
pressure to give 13 0.101 g as a pink powder, yield 58 %,
mp 135 °C (lit. [27] 140 °C).
1
IR: m = 2058.6 cm-1. H-NMR (d, ppm): 7.351–7.417
(m, 2H), 7.470–7.552 (m, 3H), 7.745 (d, 3J = 8.4 Hz, 1H),
3
1H-NMR (d, ppm): 7.165 (td, 3J = 7.5 Hz, 4J = 1.5 Hz,
7.958 (d, J = 7.7 Hz, 1H), 7.994–8.054 (m, 3H).
3
4
4J = 1.0 Hz, H9), 7.302 (ddd, J = 7.4 Hz, J = 1.0 Hz,
13C-NMR (d, ppm): 64.19 (C=N), 118.68 (C–H), 119.43
(C–H), 120.90 (C–H), 121.67 (C–H), 124.77 (C–H), 125.35
(C–H), 125.50 (C–H), 125.63 (C–H), 126.22 (C–H), 126.39
(C), 127.89 (C), 128.33 (C), 129.65 (C–H), 131.99 (C),
132.88 (C), 134.52(C).
5J = 0.5 Hz, H7), 7.342 (td, 3J = 7.0 Hz, 4J = 1.5 Hz,
H8), 7.367 (td, J = 7.0 Hz, J = 1.5 Hz, H3), 7.517 (d,
3
4
3
5
3J = 8.5 Hz, H6), 7.554 (ddd, J = 7.5 Hz, J = 0.5 Hz,
3
3
4
H10), 7.569 (td, J = 8.3 Hz, J = 7.5 Hz, J = 1.5 Hz,
H2), 7.722 (dd, 3J = 7.5 Hz, 4J = 1.0 Hz, 5J = 0.5 Hz,
H4), 7.908 (d, J = 8.0 Hz, H5), 9.544 (ddd, J = 8.5 Hz,
(E)-11-(11H-benzo[a]fluorene-11-ylidene)-11H-
3
3
4J = 1.0 Hz, H1).
benzo[a]fluorene ((E)-bisbenzo[a]fluorenylidene, E-6)
13C-NMR (d, ppm): 117.83 (C6), 118.72 (C7), 123.47
(C10), 124.91 (C1), 125.91 (C3), 129.01 (C4), 129.18 (C9),
130.67 (C11b), 131.15 (C2), 132.36 (C11a), 133.09 (C8),
135.21 (C4a), 135.99 (C5), 140.58 (C10a), 142.58 (C6b),
147.22 (C6a), 229.61 (C11).
To a stirred solution of diazo 13 (0.25 g, 1.02 mmol) in dry
toluene (40 mL) protected by CaCl2 tube, freshly prepared
thioketone 11 (0.25 g, 1.02 mmol) was added at 100 °C.
After refluxing for 12 h, the reaction mixture was cooled to
rt, and the solvent was removed under reduced pressure.
The crude product was purified by column chromatography
on silica gel (eluted by PE). The first violet fraction was
collected and evaporated. A violet powder of the 6 was
obtained, 0.014 g, yield 10 %. Violet single crystals of E-6
were obtained by recrystallization from CH2Cl2, mp
272–275 °C.
11H-Benzo[a]fluorene-11-hydrazone (12)
Thioketone 11 (0.11 g, 0.45 mmol) was dissolved in eth-
anol (8 mL), benzene (6 mL), and hydrazine hydrate
(0.4 mL, 6.22 mmol) were added. The red solution chan-
ged immediately to yellow. The reaction mixture was
refluxed for 2 h. The solvent was evaporated under reduced
pressure; the crude product treated several times with
ethanol and the solvent evaporated. A yellow powder of 12
was obtained, 0.095 g, yield 84 %, mp 164 °C.
UV/Vis: k = 490 nm (sh), 507 nm (lit. [28] k = 500
1
3
3
nm). H-NMR (d, ppm): 6.855 (td, J = 8.0 Hz, J = 7.
0
6 Hz, 4J = 1.0 Hz, H9, H9 ), 7.159 (td, 3J = 8.0 Hz,
0
3J = 7.50Hz, 4J = 1.0 Hz, H8, H8 ), 7.200 (d, 3J = 8.0 Hz,
1H-NMR (d, ppm): 6.581 (s, 2H), 7.317 (td,
3J = 7.5 Hz, 4J = 1.5 Hz, 1H), 7.415 (t, 3J = 8.0 Hz,
H10, H10 ), 7.3022 (td, J = 7.7 Hz, J = 7.2 Hz, J = 1.
3
3
4
5 Hz, H2, H2 ), 7.378 (td, 3J = 7.5 Hz, 3J = 1.5 Hz,
0
0
3
4
1H), 7.427 (td, J = 8.0 Hz, J = 7.5 Hz, 1H), 7.536 (t,
3J = 8.0 Hz, 4J = 1.5 Hz, 4J = 1.0 Hz, 1H), 7.745 (d,
4J = 1.0 Hz, H3, H3 ), 7.664 (d, 3J = 8.5 Hz, H7, H7 ),
40
7.896 (d, 3J = 8.0 Hz, H4, H ), 7.917 (d, 3J0 = 8.0 Hz, H6,
0
3
3J = 8.0 Hz, 2H), 7.792 (d, J = 8.0 Hz, 1H), 7.826 (d,
H6 ), 7.967 (d, 30J = 8.5 Hz, H5, H5 ), 8.306 (d,
3J = 8.5 Hz, H1, H1 ).
3
3J = 8.5 Hz, 1H), 7.897 (d, J = 7.5 Hz, 1H), 9.229 (d,
0
0
3J = 8.5 Hz, 1H).
13C-NMR (d, ppm): 118.75 (C6, C6 ), 119.43 (C7, C7 ),
0
0
0
13C-NMR (d, ppm): 117.82 (C–H), 120.01 (C–H),
125.34 (C–H), 125.44 (C–H), 125.84 (C–H), 127.40 (C–H),
127.53 (C–H), 128.72 (C–H), 129.42 (C–H), 129.43 (C–H),
129.50 (C–H), 130.31 (C), 131.05 (C), 134.11 (C), 137.47
(C), 141.19 (C), 148.33 (C).
125.05 (C3, C3 ), 126.88 (C10, C10 ), 127.12 (C9, C9 ),
0
0
0
127.98 (C2, C2 ), 128.02 (C8, C8 ), 128.78 (C1, C10 ), 129.38
0
0
(C4, C4 ), 130.52 (C11b, C11b0 ), 131.91 (C5, C5 ), 132.72
0
0
(C11a, C11a0), 133.96 (C4a, C4a 0), 139.73 (C6b, C6b ), 140.66
0
(C10a, C10a ), 141.86 (C11, C11 ), 142.70 (C6a, C6a ).
11-Diazo-11H-benzo[a]fluorene (13)
X-ray diffraction study
To a stirred solution of hydrazone 12 (0.17 g, 0.69 mmol)
in dry Et2O (30 ml) protected by a CaCl2 tube, anhydrous
Na2SO4 (0.35 g, 2.43 mmol), a freshly prepared saturated
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
123

Struct Chem (2013) 24:1229–1240
1233
˚
(k = 0.71073 A). Single crystals were attached to glass
11-diazo-11H-benzo[a]fluorene (13) [27]. Coupling of
thione 11 with diazo 13 in boiling toluene gave E-6 (after
column chromatography) as violet crystals (k(UV/Vis) =
507 nm) in 10 % yield. Compound E-6 was allegedly
obtained by oxidation of bisbenzo[a]fluorenyl (synthesized
from 9-bromo-11H-benzo[a]fluorene and NaI in boiling
acetone) with O2 and KOEt in pyridine as dark red plates
(mp 317 °C from xylene) [28, 42], but was characterized
only by the UV spectrum (kmax = 500 nm) [43]. This
absorption band is different from the absorption band of the
isomeric E/Z-7, 530 nm [44, 45]. The structure of E-6 was
fibers, with either epoxy glue or mineral oil. Data were
collected at 173 K. Low temperature was maintained with
a Bruker KRYOFLEX nitrogen cryostat. The diffractom-
eter was controlled by a pentium-based PC running the
SMART software package [29]. Immediately after collec-
tion, the raw data frames were transferred to a second PC
computer for integration and reduction by the SAINT
program package [30]. The structures were solved and
refined using the SHELXTL software package [31] by full
matrix least-squares on F2o with anisotropic displacement
parameters for non-hydrogen atoms. The supplementary
crystallographic data can be retrieved from the Cambridge
Crystallographic Data Centre1.
1
established by H-NMR and 13C-NMR spectroscopy and
by X-ray analysis (vide infra). The synthesis affords the
chiral E-6 as a racemate.
Computational details
Molecular and crystal structures
The quantum mechanical calculations were performed with
the Gaussian 03 [32] package. Becke’s [33] three-param-
eter hybrid density functional B3LYP, with the non-local
correlation functional of Lee et al. [34] was used. The split
valence 6–311G(d,p) basis set was employed. All struc-
tures were fully optimized under the symmetry constraints
specified. Vibrational frequencies were calculated to verify
the natures of the stationary points. Non-scaled thermal
corrections to Gibbs’ free energy were used.
The crystal structures of BAEs have been reviewed [1, 2].
PAE E-6 crystallizes in the monoclinic space group
C2/c. Figure 3 gives an ORTEP diagram of E-6, as
determined by X-ray analysis (anisotropic displacement
parameters are drawn at the 50 % probability level). The
crystallographic data are given in Table 1 [see footnote 1].
Table 2 gives the conformations and selected geomet-
rical parameters derived from the crystal structures of E-6
and BAEs 2 [22], 3 [46], and 4 [47]. Structural parameters
are defined as follows [14]: pure ethylenic twist x around
0
0
C11=C11 (E-6) or C9=C9 (2, 3, and 4) is the average value
0
0
Results and discussion
of the torsion angles C11a–C11–C11 –C10a and C10a–0C11–
0
0
0
C11 –C11a (E-6)0 or the torsion angles C9a–C9–C9 –C9a and
0
Synthesis
C8a–C9–C9 –C8a (2, 3, and 4), respectively; fold angle u is
the dihedral angle between the least-square planes formed
by the rings A and B, see Fig. 2; dihedral twist m is the
dihedral angle between the least-square planes formed by
the rings GAEB and CFDH (E-6) or the rings AEB and
PAE E-6 was synthesized by Barton’s double extrusion
diazo-thione coupling method (aka Barton–Kellog olefin-
ation) (Scheme 1) [35–37]. The starting material for the
synthesis of E-6 was 11H-benzo[b]fluoren-11-one (8) [25].
Haller–Bauer cleavage (a base-induced cleavage of a car-
bon–carbon bond in a non-enolizable ketone) [38, 39] of
the C10a–C11 bond of 8 (KOH, toluene) gave 2-(2-naph-
thyl)benzoic acid (9) [26]. Intramolecular Friedel–Crafts
acylation of 9 in PPA at 75 °C (4 h) gave the kinetically
controlled 11H-benzo[a]fluoren-11-one (10) [26]. The key
synthon, 11H-benzo[a]fluoren-11-thione (11) [40] was
prepared by treating 10 with Lawesson’s reagent [41] in
boiling benzene. Thione 11 was converted to the hydrazone
12 with hydrazine hydrate in boiling ethanol/benzene.
Oxidation of 12 with HgO/KOH in diethyl ether gave
CFD (2, 3, and 4), see Figs. 1 an0d 2; pyramidalization
0
angles v(C11)/v(C11 ) and v(C9)/v(C9 ) are the improper tor-
0
0
0
0
sion angles C10a–C101–C11 –C11a and C10a –C110 –C11–C11a
0
0
(E-6) or C9a–C9–C9 –C8a and C9a –C9 –C9–C8a (2, 3, and
4), respectively, -180°.
The overall conformations of the BAEs are primarily
0
characterized by the pure twist of the central C9=C9 bond
and by the folding dihedral of the tricyclic moieties [1, 2,
14]. Bifluorenylidene (2) adopts a twisted conformation
with a pure twist angle of x = 32°–33°. Dixanthylene (3)
and bianthrone (4) adopt anti-folded conformations with a
zero pure twist angle and folding dihedral angles of
u = 40°–43°. The definitions of the BAE conformations
[14] can also be extended to PAEs. Like 2, E-6 adopts a
twisted conformation. The most important feature of the
1
CCDC-891703 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge
CB2 1EZ, UK; fax: ?44(0)1223-336033; email: deposit@ccdc.
cam.ac.uk]
crystal structure of PAE E-6 is the pure twist of the central
C11=C11 bond, x = 39.1°, substantially higher than 33.0°
(a-modification) and 31.9° (b-modification) in 2. This
0
123

1234
Struct Chem (2013) 24:1229–1240
Scheme 1 Preparation of E-6
by Barton’s double extrusion
diazo-thione coupling method
Synthesis
6
7
5
4
3
PPA
KOH
8
2 C7H8
11
9
HO
10
1
O
9
O
O
8
10
LR
C6H6
N2H4*H2O
EtOH, C6H6
S 11
12 N
HgO, Na2SO4
NH2
EtOH, KOH
Et2O
N
+
S
2 13
C7H8
100°C
6
11
benzo[b]annelated E/Z-7 the corresponding UV/Vis high-
est wavelength absorption is shifted even further, kmax
=
530 nm [44, 45], in spite of their lower pure ethylenic twist
angle, 34°–35° (calculated). It appears that the batochromic
shift due to the doubly benzo[b]annelation is larger than the
batochromic shift due to the doubly benzo[a]annelation.
For comparison, in tetrakisbenzo[a,i, a0, i0]bifluorenylidene,
the highest wavelength absorption are at 525 and 546 nm
[44]. The dihedral angle between the two averaged planes
of the benzo[a]fluorenylidene moieties of E-6 is very high,
61.1°, as compared with the respective dihedral angle
between the two fluorenylidene planes of 2 (43°–42°). The
benzo[a]fluorenylidene moieties of E-6 are only slightly
syn-folded (u = 4°–6°), similarly to 2. The central
0
C11=C11 bond length 138.6 pm of E-6 is elongated, as
0
compared with the C9=C9 bond in 2 (136.4–136.7 pm).
Fig. 3 ORTEP drawing of bis-11H-benzo[a]fluorenylidene (E-6)
0
0
The 0 internal bond angles C11a–C11–C10a and C11a –C11
–
C10a of the five-membered rings in E-6 (105.9° and
105.7°) are close to the corresponding angles in 2 (105.2°).
An unusual feature in the molecular structure of E-6 is the
enhanced twist is primarily attributed to the double
benzo[a]annelation of the bifluorenylidene moiety at the
fjord regions. It is also responsible for the violet color of
E-6 (507 nm). It is interesting to note, that in the doubly
0
large syn-pyramidalization angles at C11 and C11 : 12.6°
123

Struct Chem (2013) 24:1229–1240
1235
The unit cell of the crystal structure E-6 contains eight
molecules. They are arranged as four pairs of enantiomers.
The GAEB moieties of the enantiomers in each pair lie in
parallel planes with the interplanar distance of 52.5 pm.
The HDFC moieties of the enantiomers lie in parallel
planes with the interplanar distance of 692.2 pm. The
GAEB moiety of one enantiomer in each pair and the
HDFC moiety of the other enantiomer exhibit a C–Hꢀꢀꢀp
interactions, with the shortest distance between an aromatic
hydrogen and a ring centroid of 289.3 pm (H2(1.5 - x,
-0.5 ? y, 0.5 - z)ꢀꢀꢀCgC(x, y, z)).
Table 1 Crystallographic data for compound E-6
E-6
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C34H20
428.50
295(1)
Monoclinic
C2/c
˚
a (A)
26.737(4)
16.8695(9)
23.979(3)
90
˚
b (A)
˚
c (A)
The GAEB moieties of the same neighboring enantio-
mers are stacked, and their planes form the dihedral angle
of 14.4°. The distances between the centroids of the
respective five- and six-membered rings are A(x, y, z)ꢀꢀꢀ
A0(1.5 - x, -0.5 ? y, 0.5 - z) = 399.3 pm, CgAꢀꢀꢀCgE0 =
403.0 pm, CgEꢀꢀꢀCgA0 = 419.0 pm, CgBꢀꢀꢀCgG0 = 456.4 pm,
CgGꢀꢀꢀCgB0 = 470.2 pm. The HDFC moieties of the same
neighboring enantiomers form the dihedral angle of 64.8°.
Thus, in the crystal of E-6, the GAEB moieties of E-6 lie in
parallel or nearly parallel planes, while its HDFC moieties
are arranged in a ‘‘herringbone’’ pattern [49]. The unit cell
of E-6 is shown in Fig. 4.
a (°)
b (°)
c (°)
94.888(2)
90
3
˚
Volume (A )
4388(1)
Z
8
Density (calculated) (Mg/m3)
1.297
Absorption coeff. (mm-1
)
0.074
F(000)
Crystal size (mm3)
Crystal color
1792
0.35 9 0.31 9 0.16
Purple
Theta range (°)
3.06 to 27.00
23507
Reflections collected
Independent reflections
Absorption correction
4778 [Rint = 0.0407]
None
6
7
4
Data/restraints/parameters
Goodness-of-fit on F2
4778/0/387
1.356
5
5a
6
7
8
3
5
4a
4b
6a6b
G
8
4a
A
A
B
B
11a
Final R indices [I [ 2sigma(I)]
R1 = 0.1149
wR2 = 0.2278
R1 = 0.1257
wR2 = 0.2327
4
9
E
2
E
9a
9
11
11
3
G11b
11a
10a
10a
10
10
1
R indices (all data)
1
2
1'
1'
8'
8'
8a'
9a'
9a'
8a'
9'
9'
7'
6'
2'
2'
3'
7'
F
F
D
C
D
C
4b'
4a'
4a'
4b'
3'
6'
5'
4'
5'
4'
and 14.8°, as compared with the much smaller pyrami-
14
15
0
dalization angles at C9 and C9 of 2 (5.9° and 3.9° in a-
form, 5.7° and 2.2° in b-form). This syn-pyramidalization
indicates an enhanced strain in the fjord regions of the
NMR spectroscopy
molecule. The overcrowding in E-6 is noticeable, but not
significant, reaching 8 % penetration for the C1ꢀꢀꢀC10
0
Table 3 gives the experimental 1H-NMR chemical shifts of
E-6, (E)-bis-11H-benzo[b]fluorenylidene (E-7), (Z)-bis-
11H-benzo[b]fluorenylidene (Z-7), 9-(11H-benzo[a]fluo-
ren-11-yliden)-9H-fluorene (14) [40], and 9-(11H-
benzo[b]fluoren-11-yliden)-9H-fluorene (15) [40], and the
calculated (GIAO-B3LYP/6-311G(d,p)) chemical shifts of
E-6 and Z-6. Table 4 gives the 13C-NMR chemical shifts
of E-6 and of the relevant BAEs. 1H- and 13C-NMR
chemical shifts of E-6 are shown in Fig. 5. Complete
assignments were made through two-dimensional correla-
tion spectroscopy (DQF-COSY, HSQC, and HMBC).
In homomerous BAEs such as 2–4, it is possible to
distinguish qualitatively among the twisted conformation,
/
0
0
0
C1 ꢀꢀꢀC10 and H10ꢀꢀꢀC11b /H10 ꢀꢀꢀC11b contact distances,
based on the respective Van der Waals radii of hydrogen
(115 pm) an0d carbon (171 pm) [48]). Fjord region protons
0
H10 and H9 are located above the plane of the aromatic
ring G. The angles0 between the normal of plane G and the
0
vectors CgGꢀꢀꢀH10 and CgGꢀꢀꢀH9 are 42.7° and 29.3°,
respectively (CgG is the centroid of the aromatic ring G).
0
0
The distances H10 ꢀꢀꢀCgG and H9 ꢀꢀꢀCgG are 324.5 and
505.6 pm, respectively. The geometrical parameters of the
molecular structure of E-6 agree very well with the
respective parameters of the calculated global minimum C2
conformation of E-6 at B3LYP/6-311G(d,p) (vide infra).
123

1236
Struct Chem (2013) 24:1229–1240
Table 2 The X-ray characteristic geometrical parameters of E-6, 2 [23], 3 [46], 4 [47]
0
0
0
0
0
0
x (°)
u (°)
m (°)
v C11/C11 (°)
C11=C11
(pm)
C1ꢀꢀꢀC10
H1ꢀꢀꢀC10
H10ꢀꢀꢀC11b
H1ꢀꢀꢀH10
0
0
0
0
C1 ꢀꢀꢀC10
H1 ꢀꢀꢀC10
H10 ꢀꢀꢀC11b
H1 ꢀꢀꢀH10
(pm)
(pm)
(pm)
(pm)
E-6
39.1
5.8
4.1
61.1
-12.6
-14.8
138.6
315.5
314.3
289.0
290.5
269.8
273.1
263.6
281.1
0
0
0
0
0
0
0
0
x (°)
u (°)
m (°)
vC9/C9 (°)
C9=C9
(pm)
C1ꢀꢀꢀC1
C8ꢀꢀꢀC8
(pm)
H1ꢀꢀꢀC1
H8ꢀꢀꢀC8
(pm)
H1ꢀꢀꢀC9a
H1ꢀꢀꢀH8
0
0
H8ꢀꢀꢀC8a
H1 ꢀꢀꢀH8
(pm)
(pm)
a-2
b-2
a-3
b-3
4
33.0
31.9
0.0
5.2
4.2
5.9
3.9
136.7
136.4
137
317.6
314.7
297
261
253
269
282
281
226
199
271
292
304
4.5
5.7
2.7
2.2
43.0
-1.2
1.2
0.0
40.1
40.0
2.7
140
302
-2.7
-3.6
0.0
136.4
294
x the pure ethylenic twist angle, u the fold angle, m the dihedral twist angle, v the pyramidalization angle
Fig. 4 The unit cell of E-6
(view along Z axis)
the anti-folded conformation and the syn-folded confor-
1
from the respective shifts in 2
(dH8 = 8.39 ppm,
mation in solution, using H-NMR chemical shifts of the
dH7 = 7.21 ppm) and in 14 (dH10 = 8.44 ppm,
dH9 = 7.14 ppm). In a strong magnetic field, nuclei loca-
ted over an aromatic ring experience a diminished mag-
netic field as a result of the induced magnetic field caused
by the circulating p-electrons [51]. This shielding effect
0
0
fjord region protons H1, H8, H1 , and H8 [50]. Thus, in
twisted 2, the fjord region protons appear at 8.39 ppm
(primarily due0 to the diamagnetic anisotropy of the twisted
central C9=C9 bond), while in anti-folded 3 and 4, these
0
0
protons appear at 7.15 and 70.06 ppm, respectively. The
indicates that, in solution, H9/H9 and H10/H10 in E-6 are
positioned above the planes of the opposing naphthalene
rings. It is consistent with the X-ray structure (vide supra)
and the DFT calculations (vide infra). The experimental
1H-NMR chemical shifts of E-6 are consistent with the
calculated chemical shifts of E-6, but significantly differ
from the calculated chemical shifts Z-6 (Table 3). The
excellent agreement between the experimental and calcu-
0
neighboring protons H2, H7, H2 , and H7 in 3 are even more
shielded, appearing at 6.88 ppm. PAE E-6, like BAE 2,
adopts a twisted conformation, with the downfield chemi-
0
cal shift of H1/H1 (8.31 ppm) and0 the absence of
the downfield chemical s0hift of H2/H2 (7.32 ppm). The
downfield shift of H1/H1 in E-6 closely resembles the
chemical shifts of the fjord regions protons of 2
(dH1 = 8.39 ppm) and of 14 (dH10 = 8.44 ppm). The
1
lated H-NMR chemical shifts of E-6 rule out the alter-
native characterization of the product of the synthesis as
0
0
upfield shifts of H10/H10 (d = 7.20 ppm) and H9/H9
(d = 6.86 ppm) in E-6, however, are significantly different
Z-6 or E/Z-mixtures of 6.
123

Struct Chem (2013) 24:1229–1240
1237
Table 3 The experimental
1H-NMR chemical shifts
(d, ppm) of E-6, 2, 14, and 15
and the respective calculated
(GIAO-B3LYP/6-311G(d,p))
chemical shifts of E-6 and Z-6
Exp
Exp
Calc
Calc
Calc–exp
Exp
Exp
dE-6 (ppm) d2 (ppm) dE-6 (ppm) dZ-6 (ppm) DdE-6 (ppm) d14 (ppm) d15 (ppm)
H1
H2
H3
H4
H5
H6
H7
H8
H9
8.306
7.322
7.378
7.896
7.967
7.917
7.664
7.159
6.855
8.386
7.211
7.332
7.709
8.50
7.46
7.48
7.98
8.03
8.03
7.74
7.20
6.87
7.29
7.90
6.82
7.14
7.74
7.94
7.98
7.84
7.36
7.24
8.52
0.19
0.14
0.10
0.09
0.06
0.11
0.08
0.04
0.02
0.09
7.993
7.205
7.321
7.846
7.918
7.866
7.700
7.260
7.140
8.436
7.205
6.894
7.205
7.671
7.763
7.401
7.321
8.407
8.397
7.249
7.358
7.872
8.102
7.872
7.453
7.358
7.731
8.848
8.360
7.205
7.327
7.731
7.731
7.327
7.205
8.538
H10 7.200
0
H1
0
H2
0
H1
0
H1
0
H5
0
H6
0
H7
0
H8
Table 4 The experimental
13C-NMR chemical shifts (d,
ppm) of E-6, 2, 3, 14, and 15
d2 (ppm) d3 (ppm) dE-6 (ppm) d14 (ppm) d15 (ppm)
d14 (ppm) d15 (ppm)
0
0
0
0
C1
C2
C3
C4
C4a
C4b
C5
C5a
C6
C6a
C6b
C7
C8
C8a
C9
C9a
C10
C10a
C11
C11a
C11b
126.73
126.85
129.15
119.89
141.31
141.31
119.89
128.31
122.40
128.12
117.08
155.48
155.48
117.08
128.78
127.98
125.05
129.40
133.96
129.00
127.34
124.93
129.43
134.29
126.77
127.48
129.48
120.60
141.14
138.55
117.96
134.04
128.36
C1
C2
C3
C4
128.03
126.75
128.83
119.62
140.55
142.30
120.18
126.26
126.90
128.89
119.89
141.14
141.16
119.96
0
C4a
0
131.91
131.76
C5
0
129.15
128.12
118.75
142.70
139.73
119.43
128.02
118.47
142.33
140.29
119.74
128.60
C6
129.50
128.96
6a0
C
6b0
C
0
126.85
126.73
138.38
141.01
138.38
122.40
128.31
124.92
121.44
124.92
126.98
125.86
C7
C8
126.75
128.78
138.53
142.62
139.56
126.78
125.85
138.65
139.71
138.33
0
0
C8a
0
127.12
127.44
129.45
133.08
126.52
137.00
140.99
140.09
C9
100
126.88
140.66
141.86
132.72
130.52
124.80
139.99
140.52
132.24
129.95
C
C
C
C
C
10a0
110
11a0
11b0
Table 4 gives the 13C-NMR chemical shifts of E-6 as
compared with those of BAEs 2, 3, 14, and 15. The
(dC11 = 141.86 ppm),
2
(dC9 = 141.11 ppm), 14
0
(dC11 = 140.52 ppm and dC9 = 142.62 ppm, and 15
0
similarity among the 13C-NMR chemical shifts of the
(dC11 = 140.99 ppm and dC9 = 139.71 ppm) is noted. In
0
carbon atoms of the central C11=C11 in E-6
general, the 13C-NMR chemical shifts of aromatic carbons
123

1238
Struct Chem (2013) 24:1229–1240
7.664
7.917
in E-6 are very close to those of the respective carbons in
14.
7.159
7.967
7.896
6.855
DFT study
7.200
7.378
8.306
128.0
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 [52, 53].
Recently, the B3LYP hybrid functional was successfully
employed to treat overcrowded BAEs [19, 40, 54]. PAEs
E-6, Z-6, E-7, and Z-7 were subjected to a computational
DFT study. The relative B3LYP/6-311G(d,p) energies
(DG298) of the global minima of these PAEs and their
selected calculated and experimental geometrical
7.322
128.8
125.1
129.4
126.9
140.7
132.7 130.5
141.9
127.1
128.0
134.0
131.9
139.7
142.7
118.8
119.4
(E
)-6
Fig. 5 1H- and 13C-NMR chemical shifts of E-6 (d, ppm)
Table 5 Relative energies and selected geometrical parameters of PAEs E-6, Z-6, 2, E-7, Z-7, 14, and 15
0
0
0
0
DG298 (kJ/mol)
x (°)
u (°)
m (°)
v
C
(°)
C1ꢀꢀꢀC10
H1ꢀꢀꢀC10
H10ꢀꢀꢀC11b
H1ꢀꢀꢀH10
110
10
10
100
10
11/C
C ꢀꢀꢀC10
H ꢀꢀꢀC10
H
(pm)
ꢀꢀꢀC11b
H ꢀꢀꢀH10
(pm)
(pm)
(pm)
E-6
E-6a
C2
C2
0.00
39.8
39.1
3.5
5.8
4.1
2.5
60.6
61.1
12.8
318.4
315.5
314.3
321.0
288.2
289.0
290.5
265.6
269.8
263.6
258.6b
285.0
273.1
281.1
225.1
-12.6
-14.8
0.2
E-7
-39.73
34.5
43.9
260.4
0
0
0
0
DG298 (kJ/mol)
x (°)
u (°)
m (°)
v
C
(°)
C10ꢀꢀꢀC10
H10ꢀꢀꢀC10
H1ꢀꢀꢀC11b
H10ꢀꢀꢀH10
0
0
0
0
11/C11
(pm)
H10 ꢀꢀꢀC10
H1 ꢀꢀꢀC11b
(pm)
(pm)
(pm)
Z-6
Z-7
C2
6.18
42.3
34.4
1.3
2.0
52.8
43.4
3.6
0.1
320.5
322.0
262.5
259.8
285.3
258.4d
233.7
224.0
C2
-39.91
0
0
0
0
DG298 (kJ/mol)
x (°)
u (°)
m (°)
v
C
(°)
C1ꢀꢀꢀC1
H10ꢀꢀꢀC8
H8 ꢀꢀꢀC10
H10ꢀꢀꢀH80
0
0
11/C9
C10ꢀꢀꢀC8
(pm)
(pm)
H1ꢀꢀꢀH1
(pm)
(pm)
14
15
C1
C1
22.45c
0.00
37.1
34.2
3.1
3.4
2.2
2.7
51.1
43.0
-10.6
-5.4
-0.2
-0.1
314.5
322.2
320.5
321.4
0
250.5
260.0
0
270.9
259.3
0
224.3
289.4
224.8
225.3
0
x (°)
u (°)
m (°)
v
C1ꢀꢀꢀC1
H1ꢀꢀꢀC1
H1ꢀꢀꢀC9a
H1ꢀꢀꢀH1
0
0
0
0
0
C9/C9
C8ꢀꢀꢀC8
H8ꢀꢀꢀC8
H8ꢀꢀꢀC8a
H8 ꢀꢀꢀH8
(°)
(pm)
(pm)
(pm)
(pm)
2
a-2a
D2
33.9
33.0
2.6
5.2
4.2
4.5
2.7
42.5
0.0
5.9
3.9
5.7
2.2
320.9
317.6
259.3
261
284.9
224.9
226
b-2a
31.9
314.7
253
199
x the pure ethylenic twist angle, u the fold angle, m the dihedral twist angle, v the pyramidalization angle
a
X-ray structure
10 100
b
c
d
H10ꢀꢀꢀC /H ꢀꢀꢀC1
Relative to DG298 of 15
10 10
H1ꢀꢀꢀC /H ꢀꢀꢀC1
123

Struct Chem (2013) 24:1229–1240
1239
Diastereoselectivity
parameters are presented in Table 5. Each of these PAEs
adopts a twisted C2-conformation as their minima. E-6 is
6.2 kJ/mol more stable than Z-6, which corresponds to
Keq(E:Z) = 92:8 (at 298 K). By contrast, the free energies of
E-7 and Z-7 are almost equal: DDG298 = 0.18 kJ/mol.
However, the constitutional isomers 6 and 7 differ consid-
erably in their relative free energies: E-6 is 39.7 kJ/mol
higher in energy than E-7 and Z-6 is 46.1 kJ/mol less stable
than Z-7. The angularly benzo[a]annelated bifluorenylid-
enes are destabilized relative to the corresponding linearly
benzo[b]annelated bifluorenylidenes. This destabilization
The formation of E-6 in the synthesis without any indication
of the formation of Z-6 may be rationalized by assuming that
the reaction is kinetically controlled. A more plausible
explanation, however, is that the reaction is thermodynam-
ically controlled and in the event that the less stable Z-6 is
also formed, it rapidly undergoes Z ? E diastereomeriza-
tion via an orthogonally twisted diradical transition state, to
give the more stable E-6. This scenario is based on the
plausible assumption that the energy barrier for the
Z ? E diastereomerization is low. This assumption is sup-
ported by the reported fast Z-7 ? E-7 diastereomerization
at room temperature (DG298 = 23.5 kcal/mol) [44]. The
energy barrier for Z-6 ? E-6 is expected to be even lower,
due to the enhanced destabilization of the ground states of the
angularly benzo[a]annelated E-6 and Z-6, relative to the
linearly benzo[b]annelated E-7 and Z-7. Bearing in mind
that the 11H-benzo[a]fluorenyl radical, is 2.9 kJ/mol more
stable than the 11H-benzo[b]fluorenyl radical, the relative
stabilization of the diradical transition states (orthogonal
benzo[a]fluorenyl radicals vs. orthogonal benzo[b]fluorenyl
radicals) of the two diastereomerization processes is
expected to be a minor factor. Indeed, the B3LYP/6-
311G(d,p) calculated diastereomerization barrier for
Z-7 ? E-7 is DG298 = 57.0 kJ/mol (13.6 kcal/mol), which
is notably lower than the experimental value for the
Z-7 ? E-7 diastereomerization. The calculated equilibrium
constant Keq(E-6 ? Z-6) = 92:8 is indicative of a marked
thermodynamically controlled diastereoselectivity of the
reaction leading to E-6, but does not rule out the formation of
effect is attributed primarily to the higher pure twist angles of
0
the central C11=C11 bonds (vide infra) and the enhanced
0
pyramidalization of C11 and C11 in E-6 (13–15°) relative
to E-7 and Z-7 (0°). Thus, the deviation from planarity in
these annelated PAEs due to overcrowding in the fjord
regions, are energetically more costly in the doubly angu-
larly benzo[a]annelated derivatives than in the doubly line-
arly benzo[b]annelated derivatives. The cost of deviation
from planarity is nearly twice as lower when only one
benzannelated fluorenyl moiety is present: 14 is higher
in energy than 15 by only 22.5 kJ/mol. Planar 11H-
benzo[a]fluorenyl radical, however, is only 2.9 kJ/mol more
stable than planar 11H-benzo[b]fluorenyl radical.
The calculated pure twist angles around C11=C11 in
0
E-6 and Z-6 are 39.8° and 42.3°, respectively. For com-
parison, the respective pure twist angles are 33.9° in 2
0
(around C9=C9 ), 37.1 and 34.2° in 14 and 15 (around
0
C9=C11 ), and 34.5° and 34.4° in E-7 and Z-7 (around
0
C11=C11 ). Thus, angular benzo[a]annelation steadily
increases the twist around the double bond, contrary to
linear benzo[b]annelation. This trend is also manifested in
the values of the dihedral angles between the fluorenyl
moieties. The dihedral angle m in E-6 is 60.6°, signifi-
cantly higher than the respective dihedral angle in Z-6,
52.8°. In the parent BAE 2, m = 42.5°, in 14 and 15
1
Z-6 as a minor product. However, the H-NMR spectrum
of the crude product of the reaction indicates that the pres-
ence of Z-6 as an impurity could be no more than (NMT)
2 %, if any.
m = 51.1° and 43.0°, respectively, and in E-7 and Z-7
0
m = 43.9° and 43.4°, respectively. The C11=C11 bond
lengths in E-6 and Z-6 are 138.8 and 139.0 pm, respec-
Conclusions
0
tively, slightly longer than C9=C9 in 2, 137.8 pm. The
overcrowding in the calculated structure of E-6 is similar
In conclusion, the diastereoselective synthesis of the
overcrowded highly twisted violet E-6,
a doubly
0
0
to that in the X-ray structure: 0 r(C1ꢀꢀꢀC10 )/r(C1 ꢀꢀꢀC10) =
benzo[a]annelated bifluorenylidene at the fjord regions,
provides a new, highly twisted violet platform for the
generation of electron acceptors in organic electronics and
calls for the development of multiply benzannelated
bifluorenylidenes for this purpose.
0
318.4 pm, r(H10ꢀꢀꢀC11b )/r(H10 ꢀꢀꢀC11b) = 265.6 pm. In
Z-6, however,0 short nonbonding distances of 262.5 pm
0
0
(C10 0ꢀꢀꢀH10 /C10 ꢀꢀꢀH10) and 244.1 ppm (C11a ꢀꢀꢀH1/C11aꢀꢀꢀ
H1 ) are noted.
The very good agreement in the geometrical parameters
between the experimental X-ray structure and the calcu-
lated structure of E-6 is remarkable (Table 5). The calcu-
lated (GIAO-B3LYP/6-311G(d,p)) chemical shifts of E-6
(vide supra) are also in a good agreement with the exper-
imental chemical shifts (Table 3), contrary to the calcu-
lated chemical shifts of Z-6.
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