Peterson olefination: Unexpected rearrangement in the overcrowded polycyclic aromatic ene series

Article abstract of DOI:10.1002/ejoc.201100789

An attempted synthesis of the angularly annelated 9-(11′-benzo[a] fluoren-11′-ylidene)-9H-fluorene (3) through a Peterson olefination reaction between (9H-fluoren-9-yl)trimethylsilyl anion (5a) and 11H-benzo[a]fluoren-11-one (6) led to the linearly annelated 9-(11′-benzo[b]fluoren-11′-ylidene)-9H-fluorene (4), due to an unexpected rearrangement. The proposed mechanism of the rearrangement is illustrated. The core of the mechanism is an intramolecular Haller-Bauer cleavage of the naphthyl C^11a′-C^11′ bond in the β-oxysilane anion 11 (formed from 5a and 6) to give the 1-naphthyl anion (E)-12, followed by E/Z isomerization to (Z)-12 and by proton migration to give the 3-naphthyl anion (Z)-14. The intramolecular nucleophilic addition of the naphthyl anion at C-3′ of (Z)-14 to the carbonyl carbon gives the β-oxysilane anion 15, a benzo[b]fluorenylidene derivative. The mechanism is supported by the results of DFT calculations. The synthesis of 3 was achieved by application of Barton's double extrusion diazo-thione coupling method.

Full text of DOI:10.1002/ejoc.201100789

FULL PAPER
DOI: 10.1002/ejoc.201100789
Peterson Olefination: Unexpected Rearrangement in the Overcrowded
Polycyclic Aromatic Ene Series
Naela Assadi,[a] Sergey Pogodin,[a] and Israel Agranat*[a]
Keywords: Reaction mechanisms / Rearrangement / Olefination / Polycycles / Arenes / Density functional calculations
An attempted synthesis of the angularly annelated 9-(11Ј-
anion 11 (formed from 5a and 6) to give the 1-naphthyl anion
benzo[a]fluoren-11Ј-ylidene)-9H-fluorene (3) through a Pe- (E)-12, followed by E/Z isomerization to (Z)-12 and by proton
terson olefination reaction between (9H-fluoren-9-yl)trimeth-
ylsilyl anion (5a) and 11H-benzo[a]fluoren-11-one (6) led to
the linearly annelated 9-(11Ј-benzo[b]fluoren-11Ј-ylidene)-
9H-fluorene (4), due to an unexpected rearrangement. The
proposed mechanism of the rearrangement is illustrated. The
core of the mechanism is an intramolecular Haller–Bauer
cleavage of the naphthyl C11aЈ–C11Ј bond in the β-oxysilane
migration to give the 3-naphthyl anion (Z)-14. The intramo-
lecular nucleophilic addition of the naphthyl anion at C-3Ј of
(Z)-14 to the carbonyl carbon gives the β-oxysilane anion 15,
a benzo[b]fluorenylidene derivative. The mechanism is sup-
ported by the results of DFT calculations. The synthesis of 3
was achieved by application of Barton’s double extrusion di-
azo–thione coupling method.
Introduction
The Peterson olefination reaction[1–5] (Scheme 1) is a
two-step synthesis of alkenes involving the addition of α-
silyl carbanions to carbonyl groups to form β-hydroxysilyl
intermediates, followed by the elimination of silols to form
alkenes. The reaction is considered to be the silicon varia-
tion of the wide class of the Wittig-type reactions, involving
additions of α-heteroatom-stabilized carbanions to alde-
hydes and ketones.[4] The initial condensations are not
stereoselective and the β-hydroxysilyl intermediates are
formed as roughly 1:1 mixtures of erythro and threo iso-
mers.[3] The product stereochemistries are remarkably in-
sensitive to the effects of counterion, solvent, added salts,
variation of the carbanion-forming base, and tempera-
Scheme 1. Peterson olefination reaction.
Peterson olefination reactions have been extensively ap-
ture.[6,7] However, the β-hydroxysilyl intermediates undergo plied by Mills et al. in the synthesis of overcrowded bis-
anti elimination under acidic conditions and syn elimination tricyclic aromatic enes (BAEs, 1), including heteromerous
under basic conditions, so both E and Z isomers can be bifluorenylidenes (1, X ϶ Y) and diphenylmethylidene fluor-
obtained from a single β-hydroxysilyl diastereomer.[8,9] The enes.[13–15] The 3,6-dimethyl derivative of the parent bifluor-
final eliminations might proceed either in a stepwise man- enylidene 2, for example, was synthesized by addition of
ner[10,11] or by a concerted mechanism through the forma- trimethylsilyl carbanion to 9H-fluorene to give (9H-fluoren-
tion of oxasiletanide[12] intermediates, although there ap- 9-yl)trimethysilane, which upon treatment with nBuLi and
pears to be overwhelming evidence in support of a stepwise 3,6-methyl-9H-fluoren-9-one gave 3,6-dimethylbifluorenyl-
mechanism.[4]
idene.[15]
In the course of our studies of overcrowded bistricyclic
aromatic enes (BAEs),[16,17] we have recently directed our
attention to naphthologous analogues of BAEs.[18] We at-
tempted the synthesis of 9-(11Ј-benzo[a]fluoren-11Ј-ylid-
ene)-9H-fluorene (3) by application of a Peterson ole-
fination. Here, though, we report an unexpected rearrange-
ment in the course of a Peterson olefination, in which in-
stead of the target – the angularly annelated 3 – its consti-
[a] Organic Chemistry, Institute of Chemistry, The Hebrew
University of Jerusalem,
Philadelphia Bldg. #201/205, Edmond J. Safra Campus,
Jerusalem 91904, Israel
E-mail: isria@vms.huji.ac.il
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100789.
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tutional isomer, the linearly annelated 9-(11Ј-benzo[b]fluor-
en-11Ј-ylidene)-9H-fluorene (4), was formed. The synthesis
of 3 was eventually achieved by application of Barton’s
double extrusion diazo–thione coupling method,[19–21] also
known as the Barton–Kellogg olefination.[21]
Scheme 2. Attempted synthesis of the BAE 3 and formation of the
polycyclic aromatic ene (PAE) 4.
Results and Discussion
The attempted synthesis of 3 and the formation of the
rearrangement product 4 are shown in Scheme 2. Treatment
of a THF solution of (9H-fluoren-9-yl)trimethylsilane (5)
[22]
with nBuLi gave the corresponding deep yellow carban-
ion 5a. Addition of 11H-benzo[a]fluoren-11-one (6)[23,24] at
–78 °C and subsequent running of the reaction at room
temperature for 14 hours gave a crude mixture of products
after workup. Purification by column chromatography on
silica gel gave the linearly annelated rearranged product 4,
identical to an authentic sample,[25] as a red powder (m.p.
173–175 °C) in 4.5% yield. The structure of 4 was verified
by 1H NMR and 13C NMR spectroscopy (vide infra). Com-
pounds 2[26] (12% yield), 5 (45%), and 6 (4%) were also
identified and characterized among the reaction products.
There was no indication of the formation of the expected
product 3, neither in the crude product mixture, nor in the
eluted chromatography fractions, as concluded from the ab-
uct from the coupling reaction between 10 and 9 was puri-
fied by column chromatography on silica gel to give 3 as a
red powder (9% yield, m.p. 155–160 °C). The mass spec-
trum of 3 showed a signal at m/z = 378 [M]+. The structure
of 3 was established by 1H NMR and 13C NMR spec-
troscopy (vide infra).
1
sence of any H NMR signal due to 2-H (vide infra) at δ =
6.87 ppm. Likewise, (E)- and (Z)-11-(11ЈH-benzo[b]fluoren-
11Ј-ylidene)-11H-benzo[b]fluorene [(E)-7 and (Z)-7,
below][25,27] were not formed, as was concluded from the
absence of the 1H NMR signals of their fjord region[17] sing-
lets [δ(10-H) = 8.90 for (E)-7 and 9.11 ppm for (Z)-7]. The
mass spectrum of the crude reaction product showed a sig-
nal at m/z = 592, which could be assigned to a C47H28 spe-
cies formed from two benzo[a]fluorenyl (C17H11) units and
one fluorenylidene (C13H8) units, possibly derived from 9,9-
bis(11H-benzo[a]fluorenyl)-9H-fluorene (8); it was not fur-
ther characterized.
The angularly annelated 3 was eventually synthesized
from 9-diazo-9H-fluorene (9) and 11H-benzo[a]-
fluoren-11-thione (10) by Barton’s double extrusion diazo–
thione coupling method (Scheme 3).[19–21] The crude prod- Scheme 3. Synthesis of the PAE 3.
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Rearrangement in Crowded Polycyclic Aromatic Enes
NMR Spectroscopy
Table 1. Experimentally measured and calculated 1H and 13C chem-
ical shifts for the PAEs 2–4 and 7.
1
The H NMR chemical shifts of 3 and 4 are depicted in
2[a] 3[a]
3[b]
4[a]
4[b]
(E)-7[a]
(Z)-7[a]
1
Figure 1. The H NMR spectrum of 2 is characterized by a
1Ј-H
2Ј-H
7.993
8.13
7.35
8.397
7.249
9.05
7.84
low-field chemical shift of the fjord region protons 1-H, 8-
H, 1Ј-H, and 8Ј-H (δ = 8.39 ppm), which is characteristic of
a twisted conformation in BAEs.[25] The 1H NMR chemical
shifts of the fjord region protons of 2–4, (E)-7, and (Z)-7
are all given in Table 1. In 4 the corresponding protons ap-
pear at 8.54 (8-H), 8.36 (1-H), 8.40 (1Ј-H), and 8.85 ppm
(10Ј-H), indicating a twisted conformation.[25] A 2D-
NOESY experiment indicated NOE interactions between 1-
7.188–
7.222
7.300–
7.343
7.846
7.918
7.866
7.700
7.260
7.140
3Ј-H
7.43
7.358
7.54
4Ј-H
5Ј-H
6Ј-H
7Ј-H
8Ј-H
9Ј-H
7.92
7.97
7.97
7.78
7.32
7.18
7.872
8.102
7.872
7.453
7.358
7.718–
7.743
8.848
8.360
7.61
7.99
8.19
8.01
7.47
7.31
1
H and 10Ј-H and between 8-H and 1Ј-H. The H NMR
spectrum of 3 differs from that of 4. The fjord region pro-
tons 8-H and 10Ј-H appear, as expected, at δ = 8.41 and
8.44 ppm, respectively, similarly to the corresponding pro-
tons 1-H and 1Ј-H of 4 and 1Ј-H, 8-H of 2. However, the
other two fjord region protons of 3 appear at δ = 7.20 (1-
H) and 7.99 ppm (1Ј-H), unlike the corresponding protons
8-Hand10Ј-Hof4.Thedifferenteffectsoftheangularbenzo[a]-
annelation in 3 versus the linear benzo[b] annelation of 4
on the fjord region protons are noted.
10Ј-H
1-H
8.436
8.39 7.188–
7.222
8.56
7.30
8.61
8.73
8.64
8.46
2-H
3-H
4-H
5-H
7.23 6.894
6.90
7.27
7.76
7.86
7.185–
7.225
7.327
7.27
7.43
7.86
7.84
7.35 7.188–
7.222
7.72 7.671
7.718–
7.743
7.718–
7.743
7.327
7.185–
7.225
8.538
7.72 7.763
6-H
7-H
7.35 7.401
7.23 7.300–
7.343
7.45
7.35
7.41
7.26
8-H
8.39 8.407
8.51
8.53
10-H
8.90
9.11
[a] Experimentally measured (500 MHz, CHCl3) chemical shifts.
[b] Calculated (GIAO) chemical shifts at B3LYP/6-311G(d,p) ge-
ometry.
pared with 7.23 and 7.20 ppm in 2 and 4, respectively. The
pronounced upfield shifts of 2-H and 1-H in 3 are probably
due to shielding by the diamagnetic ring current of the op-
posite naphthalene ring.[28] In a strong magnetic field, nu-
clei located over an aromatic ring experience a diminished
magnetic field as a result of the induced magnetic field
caused by the circulating π electrons.[28] This shielding effect
indicates that in solution, 1-H and 2-H of 3, in contrast to
the corresponding 1-H and 2-H of 4, are positioned some-
what above the plane of the naphthalene ring, which is con-
sistent with the results of the DFT calculations (vide infra).
Mechanism of Rearrangement
Scheme 4 depicts a proposed mechanism of the re-
arrangement accompanying the Peterson olefination reac-
tion between 5a and 6 leading to 4. Nucleophilic addition
of the trimethylsilyl carbanion 5a to the ketone 6 (step a)
1
Figure 1. H NMR chemical shifts of the PAEs 3 and 4.
In 4, the naphthalene fjord region proton 10Ј-H is shifted gives the β-oxysilane anion 11. The crucial step of the pro-
downfield (relative to 1-H, 8-H, and 1Ј-H) to δ = 8.85 ppm posed mechanism is an intramolecular Haller–Bauer cleav-
[similarly to 10-H in (E)-7, δ = 8.90 ppm[26]]. In 3, the age (a base-induced cleavage of a carbon–carbon bond in a
naphthalene fjord region 1Ј-H is shifted upfield [δ(1Ј-H) = non-enolizable ketone)[29,30] of the C11aЈ–C11Ј bond of 11 to
7.99 ppm] relative to δ(8-H) and δ(10Ј-H). Interestingly, the form the 1-naphthyl anion (E)-12 (step b). The Haller–
fjord region 1-H of 3 facing the naphthalene ring is shifted Bauer reaction would be expected to cleave the 1-naphthyl
considerably upfield [δ(1-H) = 7.20 ppm]. More strikingly, C11aЈ–C11Ј bond of 11 to give the 1-naphthyl anion (E)-12
the signal due to 2-H of 3 appears at δ = 6.89 ppm as com- in preference to the phenyl C10aЈ–C11Ј bond to give the
Eur. J. Org. Chem. 2011, 6773–6780
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N. Assadi, S. Pogodin, I. Agranat
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Scheme 4. Proposed mechanism of the Peterson olefination and rearrangement leading to 4.
phenyl anion (E)-13, because the resulting 1-naphthyl anion catalyzed intramolecular migration of a silyl group from a
(E)-12 would be expected to be more stable than the alter- carbon atom to an oxygen atom by a mechanism involving
native phenyl anion (E)-13.[24,31,32]
a hypervalent pentacoordinate silicon species).[33,34] Elimi-
The (E)-12 diastereomer then undergoes E/Z isomeriza- nation of the silanol group from 16 and the formation of
tion to give the more stable (vide infra) (Z)-12 diastereomer the C11Ј=C9 double bond yields the final product: the PAE
(step c). The E and Z stereodescriptors in 12 refer to the 4 (step g). Alternatively, the elimination of the silanol group
conformations in which the carbonyl group and the anionic from 15 could proceed through the formation of the four-
carbon are cis and trans (to each other), respectively. The membered oxasiletanide intermediate 17 (step h), which
rearrangement occurs when the 1-naphthyl anion (Z)-12 could lose the silanol group to give 4 (step i). Step f and
isomerizes to its constitutional isomer, the 3-naphthyl anion step g are consistent with the reported mechanism of the
(Z)-14, by proton migration (step d). The anion (Z)-14 is Peterson olefination reaction.[4] The proposed mechanism
well positioned for the following step: the intramolecular of the rearrangement accompanying the Peterson ole-
nucleophilic addition of the naphthyl anion at C-3Ј of (Z)- fination of 6 leading to 4 is in agreement with the results of
14 to the carbonyl carbon to give the β-oxysilane anion 15, the DFT study (vide infra).
a constitutional isomer of 11 (step e). The anion 15 then
undergoes a migration of the trimethylsilyl group to the
DFT Study
oxygen atom at C-11Ј to give the carbanion 16 (step f). This
DFT methods are capable of generating a variety of iso-
step is essentially the Brook rearrangement (i.e., a base- lated molecular properties quite accurately, especially with
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Rearrangement in Crowded Polycyclic Aromatic Enes
the aid of the hybrid functional, and in a cost-effective Table 2. Relative free energies (ΔG298, kJmol–1) of the PAEs 3 and
way.[35,36] The B3LYP hybrid functional has been success-
4 and related species.
fully employed to treat BAEs[37,38] and overcrowded
[a]
B3LYP/6-311G(d,p) B3LYP/6-311+G(d,p)
naphtho analogues of mono-bridged tetraarylethylenes.[18]
3
4
t
t
C1
C1
22.45
0.00
The PAEs 3 and 4, the siloxane anions 18 and 16, the β-
hydroxysilanes 19 and 20 (the O-protonated derivatives of
the β-oxysilanes 11 and 15, respectively), the lithium salts
21 and 22 of 11 and 15, respectively, the 1-naphthyl anion
(Z)-12, the 3-naphthyl anion (Z)-14, the phenyl anion (Z)-
13, and the parent 1-naphthalenyl anion 23 and 2-naph-
thanenyl anion 24 were subjected to a computational DFT
study of their conformations. The B3LYP/6-311G(d,p) rela-
tive Gibbs free energies (ΔG298) of these species are pre-
sented in Table 2. Table 3 gives selected geometrical param-
eters of these species: the torsion angles (τ1 and τ2) around
the central C11Ј–C9 bond at the fjord region, the torsion
angle (ꢀ) O–C11Ј–C9–Si, describing the rotational conform-
ers around C11Ј–C9 single bond, the dihedral angles be-
tween the benzene and the naphthalene rings of the benzo-
fluorenyl moiety (υ1), between the benzene rings of the
fluorenyl moiety (υ2), and between the benzofluorenyl and
fluorenyl moieties (υ3), and the pyramidalization angles (χ)
at C-11Ј, C-9, C-11aЈ, C-10aЈ, C-9a, and C-8a. In addition,
Table S1 in the Supporting Information shows the shortest
contact distances in the compounds under study.
12
13
14
18
16
Z
Z
Z
sc
sc
C1
C1
C1
C1
C1
0.00
23.54
11.06
0.00[b]
9.57[b]
–269.78
–275.55
19
19
20
20
sc
ac
sc
C1
C1
C1
C1
27.91
57.98
0.00
ac
30.18
21
22
sc
sc
C1
C1
24.85
0.00
23
24
–
–
Cs
Cs
0.00
6.69
0.00
5.49
[a] t: twisted conformation; sc: synclinal conformation; ac: anti-
clinal conformation. [b] Single-point energy calculation on the
B3LYP/6-311G(d,p) geometry.
Table 3. Selected dihedral and torsion angles in the PAEs 3 and 4
and in related species.
χ
χ
χ
τ1
τ2
[º]
C-11Ј
C-9
[º]
C-11aЈ C-10aЈ
ꢀ
[º]
C-9a
[º]
C-8a
[º]
υ1
[º]
υ2
[º]
υ3
[º]
[a]
3
4
t
–34.5
–39.7
–34.1
–34.2
–33.9
–
–
–
–10.6
–5.5
0.2
–0.1
0.0
11.1
9.7
–11.4
–10.5
–10.1
–10.5
6.1 3.4
1.7 2.7
51.1
43.0
t
10.3
10.7
–10.6
10.2
–0.1
–9.9
–0.1
5.5
2
12
t
Z
2.6 2.6
34.6 6.0
42.5
–
–87.5 –29.6
4.8
60.5
–
–2.1
–
14
19
20
21
22
18
16
Z
sc
sc
sc
sc
sc
sc
–86.8 –29.5 –5.3
–60.9
–58.6 –58.1 –67.9
–59.0
–62.7 –60.4 65.9
–61.9 –66.9
–59.3 –60.7 –72.3
–64.1 67.5
–66.2 –63.1 70.0
–63.1
50.7
114.9
23.8
43.2 6.1
17.7 1.4
12.4 0.4
18.4 0.9
12.2 0.7
0.5 1.0
2.2 4.0
–
2.2
0.2
–2.4
5.3
1.9
3.6
–1.5
–8.0
0.8
–1.2
0.4
0.5
–0.1
46.4
58.2
44.9
58.1
83.9
68.9
67.5
–3.0
–1.2
2.3
7.7
–1.9
–6.4
1.5
1.2
0.6
–67.0
–65.6
1.3
64.3
–6.9
–1.5
–2.6
81.2
[a] t: twisted conformation; sc: synclinal conformation; ac: anti-
clinal conformation.
and sc-21. Indeed, a comparison of the geometries of 3 and
Figure 2 depicts the DFT calculated structures of 3 and 4 shows that the fjord torsion angle τ2 (C11aЈ–C11Ј–C9–C9a)
4. The benzo[b]fluorenylidene PAE 4 was found to be of 3 is 5.5° greater than the corresponding τ2 of 4. More-
22.5 kJmol–1 more stable than the benzo[a]fluorenylidene over, the dihedral angles υ1 and υ3 of 3 are 4.4° and 8.1°
PAE 3. Likewise, the β-hydroxysilane sc-20, its lithium salt greater than the corresponding angles of 4. Most striking
sc-22, and the siloxane sc-16 were more stable than their are the pronounced pyramidalization angles of the
constitutional isomers sc-19, sc-21, and sc-18 by 27.9, 24.9, carbon atoms of the central C11Ј=C9 double bond of 3 –
and 5.8 kJmol–1, respectively. The benzo[b]fluorenylidene χ(C-11Ј) = –10.6° and χ(C-9) = –5.5° – whereas the corre-
derivatives 4, sc-20, and sc-22 are less overcrowded than the sponding angles in 4 are negligible (0.2° and –0.1°). The syn
corresponding benzo[a]fluorenylidene derivatives 3, sc-19, pyramidalization of the C11Ј=C9 bond in 3 is notable. The
Eur. J. Org. Chem. 2011, 6773–6780
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N. Assadi, S. Pogodin, I. Agranat
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large dihedral angles (υ3) and pyramidalization (χ) values in the steric effect of electron–electron repulsion even more
19–22 are expected, due to the sp3 hybridization of C-9 and pronounced, which is reflected in greater stabilization of
C-11Ј, and do not reflect the degree of overcrowding. How- (Z)-12 than of (Z)-14. Note that the HOMA aromaticity
ever, the dihedral angles (υ1) in sc-19 and sc-21, describing indices[41] calculated for the five- and six-membered rings
the loss of conjugation between phenyl and naphthyl aro- of (Z)-12, (Z)-14, 19, 20, 3, and 4 did not indicate any sig-
matic systems, are larger than the corresponding dihedral nificant differences between the benzo[a]fluorenylidene and
angles in sc-20 and sc-22 (by 5.3° and 6.2°), showing the benzo[b]fluorenylidene series. The dihedral angles between
greater steric strain in the former species. The nonbonding the phenyl and the naphthalene planes in (Z)-12 and (Z)-14
C···H distances in 4, sc-20, and sc-22 are also somewhat are 34.6° and 43.2°, respectively, indicating more effective
larger than the corresponding distances in 3, sc-19, and sc- conjugation between the aromatic moieties in (Z)-12 than
21 (Table S1 in the Supporting Information). The siloxanes in (Z)-14.
sc-16 and sc-18 are not overcrowded, due to very large dihe-
The stereochemistry of the β-hydroxysilanes 19 and 20
dral angles (υ3). It is known that upon oxidation or re- deserves a comment. In each case, two conformers were lo-
duction of the parent BAE 2 the twist angle between its two cated with the following ꢀ torsion angles: –58.1° in synclinal
bistricyclic ring systems increases, reaching 58° in 22– and 19 (sc-19), 132.8° in anticlinal 19 (ac-19) (see Figure 3), and
64° in 22+, in comparison with 33.9° in 2.[39,40]
–60.4° in sc-20 and 177.1° in antperiplanar 20 (ap-20) (see
Figure 4). The conformer sc-19 was more stable than ac-
19 by 24.5 kJmol–1; sc-20 was more stable than ap-20 by
31.3 kJmol–1. In the synclinal conformers, the oxygen and
silicon atoms are well positioned for intramolecular cycliza-
tion to the four-membered oxasiletanides such as 17.
Figure 2. Calculated structures of 3 and 4 at B3LYP/6-311G(d,p).
It is noteworthy that according to the proposed mecha-
nism of the rearrangement, the 1-naphthyl anion (Z)-12 re-
arranges to the 3-naphthyl anion (Z)-14 even though (Z)-
12 is 11.1 kJmol–1 more stable than (Z)-14. The (Z)-
12Ǟ(Z)-14 rearrangement (step d) is probably not the rate-
determining step of the reaction leading to 4. The course
of the reaction is dictated by the relative stabilizations of
the benzo[b]fluorenylidene constitutional isomers (4 vs. 3 by
22.5 kJmol–1, 20 vs. 19 by 27.9 kJmol–1, and 16 vs. 18 by
5.8 kJmol–1). Single-point calculations of (Z)-12 and (Z)-
14 with diffuse functions on the heavy atoms [at B3LYP/6-
311+(d,p)//B3LYP/6-311(d,p)] indicated a similar trend,
with ΔETot = 9.6 kJmol–1, relative to ΔETot = 13.7 kJmol–1
at B3LYP/6-311G(d,p). At these levels, the 1-naphthalenyl
anion 23 is more stable than the 2-naphthalenyl anion 24:
ΔG298 = 6.7 kJmol–1 [B3LYP/6-311G(d,p)] and 5.5 kJmol–1
[B3LYP/6-311+G(d,p)]. The literature B3LYP/DZP++
ZPVE-corrected energy of 24 relative to 23 is 5.3 kJmol–1.
This difference is attributed both to steric effects of repul-
sion between the lone electron pair and the adjacent hydro-
gen atoms and to the hybridization effects of the reduction
in the bond angle at the anionic center.[32] NBO calculations
for the 1-naphthyl anion (Z)-12 and the 3-naphthyl anion
Figure 3. Calculated structures of sc-19 and ac-19 at B3LYP/6-
311G(d,p).
Figure 4. Calculated structures of sc-20 and ap-20 at B3LYP/6-
311G(d,p).
Conclusions
The synthesis of the PAE 3 can be achieved by applying
(Z)-14 indicate that most of the negative charge is concen- the Barton diazo–thione coupling method to diazofluorene
trated on the naphthyl moieties: –0.824 [(Z)-12] and –0.855 (9) and benzo[a]fluorenethione (10). In contrast, the Pe-
[(Z)-14]. As might be expected, in (Z)-12 a substantial part terson olefination reaction between the fluorene derivative
of the charge is located at C-1 (–0.313), whereas in (Z)-14 5a and benzo[a]fluorenone (6) leads to the PAE 4, a consti-
a substantial part of the charge is located at C-3 (–0.351). tutional isomer of 3. This rearrangement sheds new light
These values are higher than the corresponding charges at on the mechanism of Peterson olefination in the series of
C-1 of 23 (–0.261) and at C-2 of 24 (–0.244). This renders polycyclic aromatic ketones.
6778
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Eur. J. Org. Chem. 2011, 6773–6780

Rearrangement in Crowded Polycyclic Aromatic Enes
4, C-5), 126.73 (C-1, C-8), 126.85 (C-2, C-7), 129.15 (C-3, C-6),
138.28 (C-8a, C-9a), 141.01 (C-9, C-9Ј), 141.31 (C-4a, C-4b) ppm.
Experimental Section
Melting points are uncorrected. All NMR spectra were recorded
with a Bruker DRX 500 spectrometer. H NMR spectra were re-
9-(11ЈH-Benzo[b]fluoren-11Ј-ylidene)-9H-fluorene (4): Red powder
1
0.017 g, yield 4.5%, m.p. 173–175 °C (ref.[25] 175–180 °C). 1H
corded at 500.2 MHz with CDCl3 as solvent and as internal stan-
dard [δ(CHCl3) = 7.26 ppm]. 13C NMR spectra were recorded at
125.78 MHz with CDCl3 as solvent and as internal standard
[δ(CDCl3) = 77.0 ppm]. Complete assignments were made through
two-dimensional correlation spectroscopy (DQF-COSY, HSQC,
NOESY and HMBC). Mass spectrometry was performed with a
Voyager-DE™ PRO workstation and the matrix-assisted laser de-
sorption/ionization (MALDI) technique (2,5-dihydroxybenzoic
acid matrix). Petroleum ether (PE, b.p. 40–60 °C), THF, and ben-
zene were dried on sodium and freshly distilled.
3
NMR (CDCl3): δ = 7.185–7.225 (m, 2 H, 2-H, 7-H), 7.249 (td, J
4
3
3
4
= 7.5, J = 1.0 Hz, 1 H, 2Ј-H), 7.327 (2ϫtd, J = 7.5, J = 6.5, J
= 1.0 Hz, 2 H, 3-H, 6-H), 7.358 (2ϫtd, 2 H, 3Ј-H, 8Ј-H), 7.453
(td, 3J = 7.4, 3J = 7.3, 4J = 1.5, 4J = 1.0 Hz, 1 H, 7Ј-H), 7.718–
3
5
7.743 (m, 3 H, 4-H, 5-H, 9Ј-H), 7.872 (ddd, J = 7.0, J = 0.5 Hz,
2 H, 4Ј-H, 6Ј-H), 8.102 (s, 1 H, 5Ј-H), 8.360 (d, J = 8.0 Hz, 1 H,
3
1-H), 8.397 (d, 3J = 8.0 Hz, 1 H, 1Ј-H), 8.538 (d, 3J = 8.0 Hz, 1 H,
8-H), 8.848 (s, 1 H, 10Ј-H) ppm. 13C NMR (CDCl3): δ = 117.96
(C-5Ј), 119.89 (C-4), 119.96 (C-5), 120.60 (C-4Ј), 125.85 (C-8),
125.86 (C-8Ј), 126.26 (C-1), 126.52 (C-10Ј), 126.77 (C-1Ј), 126.78
(C-7), 126.90 (C-2), 126.98 (C-7Ј), 127.48 (C-2Ј), 128.36 (C-6Ј),
128.89 (C-3), 128.96 (C-6), 129.45 (C-9Ј), 129.48 (C-3Ј), 133.08 (C-
9aЈ), 134.04 (C-5aЈ), 137.00 (C-10aЈ), 138.33 (C-9a), 138.55 (C-4bЈ),
138.65 (C-8a), 139.71 (C-9), 140.09 (C-11aЈ), 140.99 (C-11Ј), 141.14
(C-4a, C-4aЈ), 141.16 (C-4b) ppm. 2D NMR NOESY experimenta-
tion indicated NOE interaction betweens 1-H and 10Ј-H and like-
wise between 8-H and 1Ј-H.
9H-Fluoren-9-yltrimethylsilane (5): Compound 5 was prepared by
a literature procedure with some modifications.[13] Fluorene (0.5 g,
3.0 mmol) was dissolved in dry THF (20 mL) in a round-bottomed
flask containing a magnetic stirrer and fitted with a septum. The
reaction mixture was cooled to –78 °C under argon. nBuLi
(2.4 mL, 3.9 mmol, 1.6 m in hexane) was added. The color of the
reaction mixture turned orange. After 0.5 h, the reaction mixture
was allowed to warm to 0 °C and stirred for an additional 10 min.
The reaction mixture was cooled again to –78 °C and trimethylsilyl
chloride (0.8 mL, 6.02 mmol) was added; the color turned yellow.
The reaction mixture was stirred for 0.5 h and then allowed to
warm to 0 °C and stirred for 2 h. It was quenched with saturated
aqueous NH4Cl and extracted with CH2Cl2, the organic fraction
was dried on MgSO4 and filtered, and the solvent was evaporated
under reduced pressure. Trituration of the crude product with PE
gave 5 as a yellow powder, 0.58 g, yield 81%, m.p. 100 °C (ref.[22]
97.5–99.5 °C). 1H NMR (CDCl3): δ = –0.063 [s, 9 H, Si(CH3)],
11H-Benzo[a]fluoren-11-one (6): Orange powder, 0.020 g, yield 4%.
9-(11ЈH-benzo[a]fluoren-11Ј-ylidene)-9H-fluorene (3) was not iden-
tified among the reaction products.
9-(11ЈH-Benzo[a]fluoren-11Ј-ylidene)-9H-fluorene (3): The diazo
compound 9[42] (0.08 g, 0.41 mmol) was added to a stirred solution
of the thione 10 (0.10 g, 0.14 mmol, prepared analogously to 11H-
benzo[b]fluorene-11-thione,[25] from 6 and Lawesson’s reagent) in
boiling benzene (20 mL) protected by a CaCl2 tube. The color of
the reaction mixture was red. After having been heated at reflux
for 12 h, the reaction mixture was allowed to cool to room temp.,
and the solvent was removed under reduced pressure. The crude
product was purified by column chromatography on silica gel,
eluted by PE. The desired product 3 was isolated and obtained as
a red powder (0.058 g, yield 9%). M.p. 155–160 °C. 1H NMR
3
4
3.868 (s, 1 H, 9-H), 7.295 (td, J = 7.5, J = 1.0 Hz, 2 H, 2-H, 7-
3
3
4
H), 7.346 (t, J = 7.5 Hz, 2 H, 3-H, 6-H), 7.506 (d, J = 7.0, J =
1.0 Hz, 2 H, 1-H, 8-H), 7.858 (d, 3J = 7.5 Hz, 2 H, 4-H, 5-H) ppm.
13C NMR (CDCl3): δ = –2.69 [Si(CH3)], 42.74 (C-9), 119.87 (C-4,
C-5), 123.98 (C-1, C-8), 125.16 (C-3, C-7), 125.93 (C-2, C-6),
140.40 (C-4a, C-4b), 145.69 (C-8a, C-9a) ppm.
3
4
(CDCl3): δ = 6.865 (td, J = 7.5, J = 1.0 Hz, 1 H, 2-H), 7.148 (td,
3J = 7.5, J = 1.5 Hz, 1 H, 9Ј-H), 7.178 (dd, J = 6.5, J = 1.5 Hz,
4
3
4
9-(11ЈH-Benzo[b]fluoren-11Ј-ylidene)-9H-fluorene (4): In a round-
bottomed flask containing a magnetic stirrer and fitted with a sep-
tum, 5 (0.46 g, 1.9 mmol) was dissolved in dry THF (30 mL). The
reaction mixture was cooled to –78 °C under argon and nBuLi
(1.5 mL, 2.5 mmol, 1.6 m in hexane) was added. The color of the
reaction mixture turned deep yellow. After 0.5 h, the reaction mix-
ture was allowed to warm to 0 °C and stirred for 10 min. The reac-
tion mixture was then cooled again to –78 °C and a solution of the
ketone 6[23,24] (0.5 g, 1.9 mmol) in THF (20 mL) was added drop-
wise. The color turned orange. After 0.5 h, the reaction mixture
was allowed to warm to 0 °C and stirred overnight. The reaction
mixture was quenched with saturated aqueous NH4Cl, extracted
with CH2Cl2, dried on MgSO4, and filtered, and the solvent was
evaporated under reduced pressure. The crude product was ob-
tained as an orange powder (0.25 g). NMR spectroscopy of the
crude product indicated the presence of the following compounds:
2, 4, 5, and 6. The crude product was purified by column
chromatography on silica gel, with elution with PE/CH2Cl2 (99:1).
The following fractions were eluted:
3
3
1 H, 1-H), 7.194–7.231 (m, 2 H, 2Ј-H, 3-H), 7.272 (td, J = 8.0, J
= 7.5 Hz, 1 H, 8Ј-H), 7.309–7.348 (m, 2 H, 3Ј-H, 7-H), 7.406 (td,
3J = 7.5, J = 1.0 Hz, 1 H, 6-H), 7.689 (d, J = 7.5 Hz, 1 H, 4-H),
4
3
7.718 (d, 3J = 7.0 Hz, 1 H, 7Ј-H), 7.778 (d, 3J = 7.5 Hz, 1 H, 5-H),
3
3
7.861 (d, J = 8.0 Hz, 1 H, 4Ј-H), 7.887 (d, J = 8.0 Hz, 1 H, 6Ј-
3
3
H), 7.936 (d, J = 8.0 Hz, 1 H, 5Ј-H), 7.982 (d, J = 8.5 Hz, 1 H,
1Ј-H), 8.414 (d, J = 7.5 Hz, 1 H, 8-H), 8.442 (d, 3J = 8.0 Hz, 1 H,
3
10Ј-H) ppm. 13C NMR (CDCl3): δ = 118.47 (C-6Ј), 119.62 (C-4),
119.74 (C-7Ј), 120.18 (C-5), 124.80 (C-10Ј), 124.93 (C-3Ј), 126.75
(C-2, C-7), 127.34 (C-2Ј), 127.44 (C-9Ј), 128.03 (C-1), 128.60 (C-
8Ј), 128.78 (C-8), 128.83 (C-3), 129.00 (C-1Ј), 129.43 (C-4Ј), 129.50
(C-6), 129.95 (C-11bЈ), 131.76 (C-5Ј), 132.24 (C-11aЈ), 134.29 (C-
4aЈ), 138.53 (C-8a), 139.56 (C-9a), 139.99 (C-10aЈ), 140.29 (C-6bЈ),
140.52 (C-11), 140.55 (C-4a), 142.30 (C-4b), 142.33 (C-6aЈ), 142.62
(C-9) ppm. Mass spectrometry showed a peak at m/z = 378 [M]+.
11H-Benzo[a]fluoren-11-one (6): Orange powder, 0.082 g, yield
18%, m.p. 129 °C (ref.[24] 132 °C), also identified among the reac-
tion products. 1H NMR (CDCl3): δ = 7.259 (t, 3J = 8.2, 3J = 6.
4 Hz, 1 H, 9-H), 7.405–7.478 (m, 3 H, 3-H, 7-H, 8-H), 7.586 (td,
3J = 8.4, 4J = 1.0, 5J = 0.6 Hz, 2 H, 2-H, 6-H), 7.623 (d, 3J =
9H-Fluoren-9-yltrimethylsilane (5): Yellow crystals, 0.21 g, yield
45%.
3
3
8.2 Hz, 1 H, 10-H), 7.771 (d, J = 8.3 Hz, 1 H, 4-H), 7.971 (d, J
= 8.2 Hz, 1 H, 5-H), 8.974 (d, 3J = 8.5 Hz, 1 H, 1-H) ppm. 13C
NMR (CDCl3): δ = 118.02 (C-6), 119.92 (C-7), 123.79 (C-10),
124.26 (C-1), 126.38 (C-8), 126.83 (C-11a), 128.49 (C-4), 129.22 (C-
9), 129.40 (C-2), 130.15 (C-11b), 134.16 (C-3), 134.39 (C-4a),
Bifluorenylidene (2): Red-orange crystals, m.p. 190 °C (ref.[26] m.p.
191–198°), 0.079 g, yield 12%. H NMR (CDCl3): δ = 7.211 (t, 2
1
H, 2-H, 9-H), 7.332 (t, 2 H, 3-H, 6-H), 7.709 (d, 2 H, 4-H, 5-H),
8.386 (d, 2 H, 1-H, 8-H) ppm. 13C NMR (CDCl3): δ = 119.89 (C-
Eur. J. Org. Chem. 2011, 6773–6780
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6779

N. Assadi, S. Pogodin, I. Agranat
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Received: June 2, 2011
Published Online: September 22, 2011
6780
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 6773–6780