Protonation of Tetraphenyl- and 2,3,4-Triphenylcyclopentadienone – An NMR and X-ray Crystallographic Study

Article abstract of DOI:10.1002/ejoc.201700019

The protonation of 2,3,4,5-tetraphenylcyclopentadienone (tetracyclone) yields 6,11-diphenyl-5H-benzo[a]fluoren-5-one (17) and 2,3,4,5-tetraphenylcyclopent-2-en-1-one (18) as the major products. This contrasts with the pyrolysis of tetracyclone, which yields 5,6-diphenyl-11H-benzo[a]fluoren-11-one (22), a structural isomer of 17. Mechanisms are presented that rationalize these observations. The protonation of 4-hydroxy-2,3,4-triphenylcyclopent-2-en-1-none, the precursor to 2,3,4-triphenylcyclopentadienone, generates 2-(2′-oxo-3′,4′,5′-triphenylcyclopent-3′-enyl)-3,4,5-triphenylcyclopenta-2,4-dienone (30), which has been unequivocally characterized by X-ray crystallography. The establishment of the conformation of molecule 30 in the solid state, which correlates with the conformation in solution, provides a rationale for the subsequent formation of the tetrahydro-pentaphenyl-as-indacene-3,4-dione (32) upon further protonation.

Full text of DOI:10.1002/ejoc.201700019

A Journal of
Accepted Article
Title: Protonation of Tetraphenyl- and 2,3,4-Triphenyl-
cyclopentadienone: An NMR and X-Ray Crystallographic Study
Authors: Laura Harrington, James Britten, Michael Casey, John
Grealis, Yannick Ortin, Helge Muller-Bunz, and Michael
James McGlinchey
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700019
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10.1002/ejoc.201700019
European Journal of Organic Chemistry
Protonation of Tetraphenyl- and 2,3,4-Triphenylcyclopentadienone:
an NMR and X-Ray Crystallographic Study
Laura E. Harrington,a James F. Britten,a Michael Casey,b John Grealis,b
Yannick Ortin,b Helge Müller-Bunz,b and Michael J. McGlinchey*b
a. Department of Chemistry, McMaster University, Hamilton ON L8S 4M1,
Canada
b. School of Chemistry, University College Dublin, Belfield, Dublin 4,
Ireland
Email: Michael J. McGlinchey* - michael.mcglinchey@ucd.ie
Abstract:
The
protonation
of
2,3,4,5-tetraphenylcyclopentadienone
(tetracyclone) yields 6,11-diphenyl-5H-benzo[a]fluoren-5-one (17) and 2,3,4,5-
tetraphenylcyclopent-2-en-1-one (18) as the major products. This contrasts
with the pyrolysis of tetracyclone that yields 5,6-diphenyl-11H-benzo[a]fluoren-
11-one (22) a structural isomer of 17. Mechanisms are presented that
rationalize these observations. The protonation of 4-hydroxy-2,3,4-triphenyl-
cyclopentadienone, the precursor to 2,3,4-triphenylcyclopentadienone,
generates
2-(2′-oxo-3′,4′,5′-triphenylcyclopent-3′-enyl)-3,4,5-triphenylcyclo-
penta-2,4-dienone, (30) which has been unequivocally characterized by X-ray
crystallography. The establishment of the conformation of molecule 30 in the
solid state, which correlates with its structure in solution, provides a rationale
for the subsequent formation of the tetrahydro-pentaphenyl-as-indacene-3,4-
dione, 32, upon further protonation.
Keywords: cyclopentadienones, protonation, X-ray crystallography, molecular
rearrangements, NMR spectroscopy.
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Introduction
The study of potential precursors to cyclopentadienyl cations has a long and
checkered history.[1] In 1925, Ziegler and Schnell reported that a solution of
pentaphenylcyclopentadienol (1) in concentrated sulfuric acid contained a
purple species that was considered to be the pentaphenylcyclopentadienyl
cation (2), that yielded bis(pentaphenylcyclopentadienyl) ether (3) when
hydrolyzed.[2] Spectroscopic studies by Bloom and Krapcho in 1959 were
interpreted in support of this assertion.[3]
O
C6H5
C6H5
C6H5
OH
Ph5C5 C5Ph5
C6H5
C6H5
C6H5
conc
+
3
H2SO4
H2O
C6H5
C6H5
H
C6H5
C6H5
C6H5
C6H5
1
2
C6H5
CH2Cl2
BF3
H2O
H2O
O
C6H5
C6H5
(25%)
4
C6H5
+
C6H5
H
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
6 (5%)
7
(48%)
5
Scheme 1: Products of the protonation and aqueous quench of C5Ph5OH.
However, this view was challenged by Breslow and Chang who showed
that an aqueous quench of the C5Ph5OH / H2SO4 reaction led to
pentaphenylcyclopentadiene, (4), 9,10,11-triphenyl-9H-cyclopenta[l]phen-
anthrene (5), 2,2,3,4,5-pentaphenylcyclopenta-3-en-1-one (6), and other
unidentified products.[4] Moreover, they also found that the deep blue solution
resulting from treatment of a dichloromethane solution of 1 with boron
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trifluoride exhibited an ESR spectrum entirely typical of a triplet state, as
represented by 7 and other canonical forms (Scheme 1).[5] In subsequent
work by Volz, whereby C5Ph5Cl was treated with antimony pentachloride, it
was claimed that a crystalline sample of [C5Ph5]+ [SbCl6]- was obtained, and
its ESR spectrum was again observed as a triplet. However, no X-ray
crystallographic data were reported.[6]
As part of more recent attempts to prepare a cyclopentadienyl cation, a
crystalline sample supposedly composed of [C5Me5]+ [B(C6F5)4]- was assigned
a planar geometry with unequal bond lengths.[7] Subsequently, however, the
cation was reassigned as the pentamethylcyclopentenyl cation [C5Me5H2]+,
exhibiting its expected planar structure.[8-10]
A different approach involved the reaction of tetraphenylcyclopenta-
dienone (tetracyclone) with aluminum trichloride in the expectation that
coordination of the electrophile to the ketonic oxygen would generate the
required cation. In the first such report, by Allen and VanAllan in 1943,[11] it
was claimed that the product was 2,3,4,5-tetraphenylcyclopentan-1-one (8);
once again, this claim was rebutted,[12] and the product was instead
established as 9,11-diphenyl-9H,11H-cyclopenta[l]phenanthren-10-one (9),
whereby the initially generated positive charge was delocalized onto the most
distant phenyl ring, as in 10, thus initiating formation of the tetracyclic
framework (Scheme 2). However, the relative orientation of the phenyl rings in
9 was undetermined until 2002 when Pascal unequivocally characterized both
the cis and trans isomers in an X-ray crystallographic study.[13]
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O
O
H
H
AlCl3
C6H5
C6H5
C6H5
C6H5
H
C6H5
H
C6H5
benzene
C6H5
AlCl3
C6H5
benzene
8
-
AlCl3
O
O
H
H
C6H5
C6H5
C6H5
C6H5
+
10
9
Scheme 2: Reaction of tetracyclone with AlCl3.
Another interesting result is that reported by Carpentier,[14] who treated
1,2,4-trihydroxycyclopent-1-en-3,5-dione (11) with sulfuric acid, and claimed in
a UV-spectroscopic study that the positive charge resides on the oxygens in
12a rather than forming the delocalised pentahydroxycyclopentadienyl cation
(12b) (Scheme 3).
H
OH
H
OH
OH
+
+
+
O
O
- H
O
HO
H
O
HO
OH
HO
OH
HO
OH
HO
11
+
H
OH
+
OH
+
OH
HO
OH
HO
OH
HO
OH
HO
12b
12a
Scheme 3: Protonation of 11 does not produce the pentahydroxy-
cyclopentadienyl cation (12b).
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Analogously, protonation of 3-ferrocenyl-2,4,5-triphenylcyclopenta-
dienone (13) generates the ferrocenyl-hydroxyfulvene cation, 14, whereby the
positive charge is delocalized onto the iron atom (Scheme 4). This assertion is
supported not only by the restricted rotation of the ferrocenyl moiety, but also
by DFT calculations.[15]
H
O
O
CF3CO2H
Fe
+
Fe
14
13
Scheme 4: Protonation of 13 yields the ferrocenyl-hydroxytriphenylfulvene
cation, 14.
Likewise, protonation of the diphenylindenol, 15, bearing a tetrahedral
alkynyl-dicobalt hexacarbonyl cluster substituent leads to the cation 16a in
which the ligand adopts a benzofulvene-type structure and the charge
deficiency is delocalized onto a cobalt tricarbonyl vertex (Scheme 5). This
structure was secured not only spectroscopically, but also by replacing the
[Co(CO)3]+ moiety with an isolobal Fe(CO)3 unit to form the neutral iron-cobalt
cluster, 16b, that was characterized by X-ray crystallography.[16]
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Ph
Ph
Ph
Ph
Ph
OH
Ph
HBF4
Et2O
+
Fe(CO)3
Co(CO)3
Co(CO)3
TMS
TMS
TMS
Co
(CO)3
Co
(CO)3
Co
(CO)3
16b
16a
15
Scheme 5: Formation of a metal cluster stabilized indenyl cation, 16a, and a
neutral analogue, 16b.
We here describe the protonation of tetracyclone with triflic acid, the
characterization of the rearrangement products and also offer a mechanistic
rationale. In addition, a previously reported product (30, see later, in Scheme
12) of the protonation of 2,3,4-triphenylcyclopentadienone has been isolated
and unequivocally characterized by X-ray crystallography.
Results and Discussion
Protonation of tetraphenylcyclopentadienone
An initial attempt to protonate tetracyclone with trifluoroacetic acid in an NMR
tube was unsuccessful; in contrast, addition of triflic acid to a dichloromethane
solution of tetracyclone yielded a complex mixture that was separated with
considerable difficulty, initially by flash column chromatography, and
subsequently by preparative HPLC. The major contributor (62%) was
recovered tetracyclone, but the largest new product (21%, based on
tetracyclone consumed) was identified by NMR spectroscopy, mass
spectrometry, and finally by X-ray crystallography as 6,11-diphenyl-5H-
benzo[a]fluoren-5-one (17), whose molecular structure is depicted in Figure 1.
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As shown in Scheme 6, the reduction product 2,3,4,5-tetraphenylcyclopent-2-
en-1-one (18) was also formed (18% yield), and was identified by comparison
of its 1H and 13C NMR and mass spectra with previously reported data.[17] The
X-ray crystal structure of 18 has already been reported, and in the present
instance the trans isomer was obtained, in accord with the literature
characterization.[17,18].
O
O
O
O
O
Ph
Ph
Ph
H
Ph
Ph
Ph
Ph
CF3SO3H
CH2Cl2
+
+
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
18 (18%)
17 (21%)
19 (trace)
???
Scheme 6: Products of the protonation of tetracyclone with triflic acid.
O1
C17
C4
C5
C15
C6
C3
C13
C6a
C2
C1
C7
C11
C19
C8
C18
C10
C9
C21
C22
Figure 1: Molecular structure of 6,11-diphenyl-5H-benzo[a]fluoren-5-one (17).
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We are aware of only one other reference to 6,11-diphenyl-5H-
benzo[a]fluoren-5-one (17); it was tentatively identified as a product of the
hydrolysis of the hydrazone 20 under drastic conditions that led to cyclization
with elimination of hydrazine (Scheme 7).[19] Unfortunately, the report did not
include any spectroscopic data, or any other characterization of this product
for comparison.
O
OH
+
H3O
??
NNH2
20
17
Scheme 7: Previously reported possible formation of 17.
Although the diphenyl derivative, 17, has not been conclusively
characterized previously, the parent 5H-benzo[a]fluoren-5-one framework has
been reported as a product of the flow pyrolysis of chrysene-6,12-dione
through a quartz tube at 960 °C (Scheme 8).[20]
O
O
960 oC
O
Scheme 8: Pyrolysis of chrysene-6,12-dione yields 5H-benzo[a]fluoren-5-one
upon loss of CO.
Three other products of the protonation of tetracyclone were isolated,
but only in minuscule quantities. They exhibited mass spectra with molecular
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masses of 602, 578, and 544; however, at present they remain unidentified.
The accurate mass of the molecular ion of the largest minor fraction indicated
a formula of C45H30O2, and its 13C NMR spectrum exhibited peaks at 201.1
and 185.6 ppm, indicating the presence of two different carbonyl
environments. A possible assignment as 2,3,4,5,6,7a-hexaphenyl-1H-indene-
1,7(7aH)-dione, 19, arising from dimerization and elimination of a
diphenylcarbene unit, could fit the data but, evidently, without an X-ray crystal
structure this must remain speculative.
The products from protonation of tetracyclone differ somewhat from
those found by McNelis upon pyrolysis at 425 ºC.[21] Under these conditions,
the major products are 2,3,4,5-tetraphenylcyclopent-2-en-1-one (18) (13%),
1,2,4-tetraphenylnaphthalene (21) (25-42%), and 5,6-diphenyl-11H-
benzo[a]fluoren-11-one (22) (17-28%), as shown in Scheme 9. It is particularly
interesting that the thermolysis product 22 is an isomer of the protonation
product 17.
Ph
O
O
O
Ph
H
Ph
Ph
Ph
Ph
+
+
Ph
H
425 oC
Ph
Ph
Ph
Ph
Ph
22 (17 - 28%)
Ph
18 (~13%)
21 (25 - 42%)
Scheme 9: Products from the pyrolysis of tetracyclone (data from Ref. 21).
The mechanistic proposal suggested by McNelis[21] invoked heterolytic
cleavage of the C(1)-C(5) linkage, rotation about the C(2)-C(3) bond in 23, two
cyclizations to generate the tetracyclic system, 24, and finally aerial oxidation
to form the observed product 22 (or transfer of two hydrogen atoms to
tetracyclone to yield 18). While formation of the five-membered ring was
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suggested to proceed via electrophilic attack by an acylium ion, the other
cyclization would involve nucleophilic attack by a vinyl anion on a phenyl ring
as in route (i) of Scheme 10.[21] However, one can envisage another, possibly
more realistic, proposal as indicated in route (ii). This process could proceed
via carbene formation, as in 25, prior to bond rotation and the two cyclizations.
Of course, these are merely resonance structures of the same intermediates,
but, in either case, the C(2)-C(3) linkage has primarily single bond character
and so is free to rotate.
O
O
O
H
+
Ph
-
+
O
-
Ph
H
Ph
(i)
24 Ph
Ph
Ph
Ph
23
[O]
Ph
O
O
(ii)
O
Ph
Ph
Ph
22Ph
Ph
Ph
25
Scheme 10: Mechanistic proposals (i) from Ref. 21, and (ii) this work, for the
pyrolytic route from tetracyclone to 22.
In the present case, the process leading to 6,11-diphenyl-5H-
benzo[a]fluoren-5-one (17) can be rationalized as occurring by protonation at
C(5), cleavage to form the acylium ion 26, rotation about the C(3)-C(4) bond,
and two electrophilic cyclizations onto phenyl rings to form 27 (Scheme 11).
As before, oxidation yields the final product, 17. Thus, the essential difference
between the pyrolytic and protonation routes apparently involves the different
rotation modes available prior to the two cyclizations.
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O
O
O
O
H
H
+
+
Ph
Ph
+
+
Ph
Ph
Ph
Ph
H
Ph
Ph
H
H
26
O
O
O
O
H
H
Ph
H
Ph
Ph
Ph
+
[O]
-H
+
Ph
Ph
Ph
Ph
+
H
H
H
17
27
Scheme 11: Mechanistic proposal for the protonation route from tetracyclone
to 17.
Protonation of 2,3,4-triphenylcyclopentadienone
The acid-catalyzed dehydration of 4-hydroxy-2,3,4-triphenylcyclopent-2-en-1-
one (28), which leads to the short-lived species 2,3,4-triphenyl-
cyclopentadienone (29), has been extensively studied.[22-25] Many of the
1
reported products were characterized from their H NMR, UV and mass
spectra. A crucial intermediate was suggested to be the dimer, 30, on the
basis of its spectra and Diels-Alder reaction with DMAD to form 31.[25] We now
report the isolation and X-ray crystal structure of 30 that not only confirms its
identity, but also provides a rationale for its conversion to the tetracyclic
system 32 (Scheme 12).
In our hands, when 4-hydroxy-2,3,4-triphenylcyclopent-2-en-1-one (28)
was heated at reflux for 20 hours with a solution of hydrochloric acid in dry
dioxane, then cooled to room temperature and chromatographed on silica, it
yielded a dark red solid identified by X-ray crystallography as 2-(2′-oxo-3′,4′,5′-
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triphenylcyclopent-3′-enyl)-3,4,5-triphenyl-cyclopenta-2,4-dienone (30), along
with traces of the dihydro-pentaphenyl-as-indacene-3,4-dione, 32, and the
intensely blue oxidation product 33.[26]
O
OH
O
O
Ph
Ph
Ph
HCl
Ph
Ph
Ph
OH
Ph
dry CH2Cl2
Ph
Ph
Ph
Ph
dry dioxane
33
29
28
reflux
20 h.
DDQ
O
O
O
H
Ph
O
E
H
E
O
Ph
Ph
E-C C-E
Ph
H2SO4
AcOH
Ph
Ph
Ph
Ph
Ph
Ph
-CO
Ph
H
Ph
H
H
Ph
H
Ph
Ph
H
Ph
Ph
(E = CO2Me)
31
30
32
Scheme 12: Products from the protonation of 4-hydroxy-2,3,4-
triphenylcyclopent-2-en-1-one (28).
The molecular structure of 30 appears as Figure 2 and shows that the
planar five-membered ring systems are almost orthogonal to each other (87°).
The phenyl groups attached to C(3), C(4), C(5), C(3') and C(4') are twisted at
an average angle of 50° to the central rings, thus finding a compromise
between conjugative coplanarity and steric interactions. The only exception is
the phenyl at C(5′), which is effectively orthogonal to the central ring (86°);
since C(5′) is a tetrahedral carbon and not part of a π-system, the phenyl
adopts an orientation such as to minimise steric repulsions. The hydrogen
atoms attached to C(1′) and C(5′) are trans to each other, and make a
dihedral angle of 50°.
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C31'
C3'
O2
O1
C51
C1
C2'
C5
C41'
C1'
C4'
C5'
C2
C3
C41
C4
C31
C51'
Figure 2: The molecular structure of 2-(2′-oxo-3′,4′,5′-triphenylcyclopent-3′-
enyl)-3,4,5-triphenyl-cyclopenta-2,4-dienone (30).
In the original report of the isolation of the dienone 30, identification
1
was based mainly on its IR and UV spectra as well as H NMR data acquired
at 60 MHz.[25] Fortunately, with the availability of a 600 MHz spectrometer, a
1
more complete analysis is now achievable (for H, 13C, HSQC, HMBC and
1
TOCSY spectra, see Supporting Information). In the H NMR spectrum the
protons at C(1′) and C(5′) appear as 4 Hz doublets at 3.61 and 4.75 ppm,
respectively. This small coupling constant is in accordance with the crystal
structure whereby the dihedral angle between these two protons is 50°. In the
13C NMR spectrum, C(1′) and C(5′) are now readily distinguishable (at 54.8
and 54.9 ppm, respectively), and the carbonyl resonances at C(1) and C(2′)
are seen at 200.9 and 204.7 ppm, typical of cyclic ketones.
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Knowing the structure of 30, one can now rationalize the proton NMR
spectrum of 32 which exhibits CH environments at δ 4.81 (d, J = 7 Hz), δ 4.37
(d, J = 1.5 Hz) and δ 3.96 (dd, J = 7 Hz and 1.5 Hz). This indicates that the
proton at 3.96 ppm is sited trans to its neighbour at 4.81 ppm, and cis to its
counterpart at 4.37 ppm. Since the protons at C(1′) and C(5′) in 30 are known
to be trans, the incoming proton at C(2) must approach cis to the one at C(1′).
As indicated in Scheme 13, the orientation of the cyclopentadienone ring in
30, which is almost orthogonal to its attached cyclopentenone, favours exo
attack at C(2) by the incoming proton, thus aligning the phenyl at C(5′) for ring
closure onto C(3). The resulting cyclohexene ring thus adopts the favoured
half-chair conformation.
O
O
O
O
Ph
Ph
+
H
H
Ph
Ph
Ph
H
Ph
Ph
H
H
Ph
H
Ph
Ph
30
32
Scheme 13: Proposed mechanism for the formation of the tetrahydro-
pentaphenyl-as-indacene-3,4-dione, 32.
Conclusion
The protonation of 2,3,4,5-tetraphenylcyclopentadienone (tetracyclone) yields
6,11-diphenyl-5H-benzo[a]fluoren-5-one (17) and 2,3,4,5-tetraphenylcyclo-
pent-2-en-1-one (18) as the major products. This contrasts with the pyrolysis
of tetracyclone that yields 5,6-diphenyl-11H-benzo[a]fluoren-11-one (22), a
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structural isomer of 17. Mechanisms are presented that rationalize these
observations in terms of the ability of particular carbon-carbon linkages, which
have essentially single bond character, to rotate prior to double ring closure.
An important intermediate, 2-(2′-oxo-3′,4′,5′-triphenylcyclopent-3′-enyl)-
3,4,5-triphenyl-cyclopenta-2,4-dienone (30) arising from the protonation of
2,3,4-triphenylcyclopentadienone has been unequivocally characterized by
X-ray crystallography and NMR spectroscopy. The establishment of the
conformation of molecule 30 in the solid state, which correlates with its
structure in solution, provides a rationale for the subsequent formation of the
tetrahydro-pentaphenyl-as-indacene-3,4-dione, 32, upon further protonation.
Experimental Section
General Methods: All reactions were carried under a nitrogen atmosphere,
1
and solvents were dried by standard procedures. H and 13C NMR spectra
were recorded on 500 or 600 MHz spectrometers. Assignments were based
1
on standard two-dimensional NMR techniques (1H-1H COSY, H-13C HSQC
and HMBC, NOESY, TOCSY). Infrared spectra were recorded on a Perkin-
Elmer Paragon 1000 FT-IR spectrometer and were calibrated with
polystyrene. Electrospray mass spectrometry was performed on a Micromass
Quattro microinstrument. Merck silica gel 60 (230-400 mesh) was used for
flash chromatography. HPLC separations (100 μL injection volume) were
performed on an HP 1090 Series II liquid chromatograph equipped with an HP
diode array detector (200 – 600 nm) using an Envirosap PAH column (10 mm
x 250 mm, 5 μm C18; Phenonemex, Torrence, CA) at a flow rate of 3.5 mL
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min-1. The solvent gradient program went from 85% CH3CN / 15% H2O, to
100% CH3CN, to 25% CH2Cl2 / 75% CH3CN. Melting points were determined
on an Electrothermal ENG instrument and are uncorrected. Elemental
analyses were carried out by the Microanalytical Laboratory at University
College Dublin.
Protonation of tetraphenylcyclopentadienone: In an initial survey, 2,3,4,5-
tetraphenylcyclopentadienone (~50 mg) was dissolved in CD2Cl2 in an NMR
tube and cooled to -78 ºC; two drops of trifluoromethylsulfonic (triflic) acid
were added, the tube was shaken and the spectra were immediately recorded
1
at -80 ºC. H NMR (500 MHz, CD2Cl2): δ 7.99, 7.84, 7.70, 7.61, 7.50, 7.43,
7.30, 7.17, 6.85, 5.63; 13C NMR (125 MHz, CD2Cl2): δ 204.7, 194.2, 170.3,
161.4, 157.0, 148.3, 140.5, 135.4, 135.1, 133.6-127.4, 121.9, 119.4, 116.9,
114.4, 59.8.
The process was then repeated on a larger scale (1 g) and the
products were separated initially by flash chromatography, and then by HPLC
as described above, to yield recovered tetracyclone (62%), 6,11-diphenyl-5H-
benz[a]fluoren-5-one (17) (21% based on tetracyclone recovered), 2,3,4,5-
tetraphenylcyclopent-2-en-1-one (18) (18%), and three unidentified trace
products. Data for 17 (numbering is in accord with the crystal structure): δH
NMR (500 MHz, CD2Cl2): 8.07 (1H, d, J 7.8 Hz, H4), 7.61 (2H, dd, J 7.2 Hz,
6.9 Hz, phenyl-m H), 7.58 (1H, m, phenyl-p H), 7.55 (1H, m, phenyl-p H), 7.53
(2H, m, phenyl-m H), 7.51 (2H, dd, J 7.5 Hz, 1.5 Hz, phenyl-o H), 7.38 (2H,
dd, J 7.5 Hz, 2.1 Hz, phenyl-o H), 7.29 (1H, dd, J 7.7 Hz, 7.0 Hz, H3), 7.27
(1H, d, J 7.2 Hz, H1), 7.22 (1H, dd, J 7.6 Hz, 7.0 Hz, H2), 7.05 (1H, dd, J 7.6
Hz, 7.5 Hz, H9), 6.82 (1H, d, J 7.4 Hz, H10), 6.81 (1H, dd, J 7.5 Hz, 7.1 Hz,
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10.1002/ejoc.201700019
European Journal of Organic Chemistry
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H8), 6.39 (1H, d, J 7.9 Hz, H7); δC NMR (125 MHz, CD2Cl2): 186.0 (C=O),
149.0, 148.6, 147.0, 137.1, 136.5, 134.1, 130.8, 130.6 (C4a,6,6a,6b,10a,11,11a,11b),
135.4, 135.2 (C12,18), 132.8 (C2), 130.7 (C9), 130.2, 129.1 (phenyl o-C), 130.0,
128.9 (phenyl m-C), 129.6, 129.0 (phenyl p-C), 128.3 (C3, C8), 128.2 (C4),
125.9 (C1, C7), 122.8 (C10). HRMS (EI): found 382.1666. C29H18O: requires
382.1671. Data for 18: (δH NMR (600 MHz, CD2Cl2): 7.38 - 7.34 (5H, m), 7.33
- 7.30 (3H, m), 7.26 - 7.23 (2H, m), 7.23 - 7.18 (5H, m), 7.18 - 7.14 (5H, m),
4.61 (1H, s, br, cyclopentenone H), 3.73 (1H, d, J 2.0 Hz, cyclopentenone H);
δC NMR (150 MHz, CD2Cl2): 205.7 (C=O), 169.7, 142.0, 139.9, 135.2, 132.7
(phenyl-ipso C and cyclopentenone C=C), 130.4, 129.6, 129.5, 129.4, 128.9,
128.8, 128.6, 128.3 (phenyl o,m-carbons), 130.0, 128.6, 127.8, 127.6 (phenyl
p-carbons), 63.9 (cyclopentenone CH), 58.0 (cyclopentenone CH). HRMS
(EI): found: 386.1666. C29H22O requires 386.1671.
Data for unidentified fraction 1: (δH NMR (600 MHz, CD2Cl2): 7.97 (1H, dd, J
2.4 Hz, 0.6 Hz), 7.59 - 7.56 (2H, m), 7.55 - 7.50 (4H, m), 7.48 - 7.47 (2H, m),
7.43 - 7.31 (15H, m), 7.22 (1H, dd, J 8.4 Hz, 0.6 Hz), 7.11 (1H, dd, J 9.0 Hz,
2.4 Hz), 7.05 (1H, ddd, J 7.6 Hz, 7.2 Hz, 0.6 Hz), 6.82 (2H, ddd, J 7.0 Hz, 6.6
Hz, 1.2 Hz), 6.38 (1H, dd, J 7.8 Hz, 1.2 Hz); δC NMR (150 MHz, CD2Cl2):
201.1, 185.6, 152.7, 149.7, 148.6, 146.9, 138.4, 138.0, 137.1, 136.5, 135.4,
135.1, 133.5, 133.3, 131.7, 130.8, 130.7, 130.1, 130.0, 129.6, 129.5, 129.4,
129.2, 129.1, 129.0, 128.8, 128.7, 128.5, 128.2, 128.0, 126.5, 126.2, 122.9.
HRMS (EI): [M+H]+: found: 603.2315. C45H31O2 requires 603.2324.
Unidentified fraction 2: HRMS (EI) [M+H]+: found: 545.4568. Unidentified
fraction 3: HRMS (EI): found: 578.5683.
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European Journal of Organic Chemistry
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Protonation of 2,3,4-triphenylcyclopentadienone: 4-Hydroxy-2,3,4-triphenyl-
cyclopenten-1-one (653 mg. 2.00 mmol) dissolved in dry dichloromethane (10
mL) under nitrogen was treated with HCl (7.50 mL of a 4 M solution in dry
dioxane, 30 mL) and heated at reflux for 20 h. The red solution was allowed to
cool to room temperature and the solvent removed under vacuum.
Chromatography on silica using dichloromethane/pentane (1:4) as eluent
gave a highly coloured blue solid (4 mg), now known to be 33,[24,25] and 2-(2′-
oxo-3′,4′,5′-triphenylcyclopent-3′-enyl)-3,4,5-triphenylcyclopenta-2,4-dienone
(30) (100 mg, 0.162 mmol; 16%), m.p. 208-209 ºC (lit. 207-209 ºC) [23] as a
red solid: calcd. for C46H32O2•CHCl3 C 76.58, H 4.65; found C 76.13, H 4.64.
Recrystallization from trichloromethane/hexane (2:1) furnished a sample
suitable for an X-ray study. Data for 30: δH NMR (600 MHz, CDCl3): 7.27 (2H,
m, H33',35'), 7.26 (4H, m, H43',45',52,56), 7.25 (4H, m, H42',46',53',55'), 7.21 (1H, m,
H34'), 7.20 (1H, m, H44), 7.19 (1H, H54'), 7.17 (6H, m, H32',36',44',53,54,55), 7.16 (2H,
m, H52',56'), 7.15 (1H, m, H34), 7.11 (2H, m, H43,45), 6.96 (2H, t, J 7.5 Hz, H33,35),
6.88 (2H, d, J 7.0 Hz H42,46), 6.55 (2H, t, J 7.5 Hz, H32,36), 4.75 (1H, d, J 4 Hz,
H5'), 3.61 (1H, d, J 4 Hz, H1'). δC NMR (125 MHz, CDCl3): 204.7 (C2'), 200.9
(C1), 169.1 (C4'), 159.0 (C3), 153.9 (C4), 141.0 (C51'), 139.9 (C41'), 134.8 (C3'),
132.9 (C41), 132.0 (C31), 131.8 (C31'), 130.6 (C51), 130.1 (C42',46'), 130.0
(C32',36' 52,56), 129.5 (C42,46), 129.0 (C43,45), 128.9 (C53',55'), 128.8 (C34), 128.7
,
(C44), 128.6 (C32,36), 128.3 (C52',56'), 128.2 (C54,34'), 128.0 (C43',45',53,55), 127.9
(C33',35'), 127.8 (C33,35), 127.5 (C44'), 127.0 (C54'), 125.7 (C5), 124.0 (C2), 54.9
(C5'), 54.8 (C1'). IR (solid, KBr): 3422, 3052, 1701 (CO), 1618, 1443 cm-1.
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10.1002/ejoc.201700019
European Journal of Organic Chemistry
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X-Ray Crystallography Diffraction Measurements for 17 and 30:
Data listed in Table 1 were collected using a Bruker SMART APEX CCD area
detector diffractometer using graphite monochromated Cu Kα radiation for 17
(λ = 1.54184 Ǻ) and Mo Kα radiation for 30 (λ = 0.71073 Ǻ). A full sphere of
reciprocal space was scanned by phi-omega scans. Pseudo-empirical
absorption correction based on redundant reflections was performed by the
program SADABS. The structures were solved by direct methods using
SHELXS-97, and refined by full-matrix least-squares on F2 for all data using
SHELXL-97.[27] Hydrogen atoms were added at calculated positions and
refined using a riding model. Their isotropic temperature factors were fixed to
1.2 times the equivalent isotropic displacement parameters of the parent
atom. The available crystals of 17 were very small and the unit cell contained
three disordered molecules; thus, only an isotropic model is presented.
Although it is not possible to claim precise geometric parameters, the
chemical identity of the molecule is evident.
CCDC-1492339 (for 30) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
We thank the Natural Sciences and Engineering Council of Canada (NSERC)
and Science Foundation Ireland (SFI), for generous financial support. We also
thank University College Dublin and the Centre for Synthesis and Chemical
Biology (CSCB) for the use of analytical facilities, and the reviewers for their
helpful comments.
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10.1002/ejoc.201700019
European Journal of Organic Chemistry
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Table 1. Crystallographic data for 17 and 30.
17
30
formula
C29H18O
382.45
1.54184
monoclinic
Pc (#7)
9.6253(8)
27.256(2)
11.8024(10)
90
C46H32O2•CHCl3
736.08
M
wavelength [Å]
0.71073
triclinic
crystal system
space group
a [Å]
Pī (#2)
10.4878(10)
12.7118(12)
14.3259(13)
97.751(2)
97.563(2)
102.334(2)
1823.2(3)
2
b [Å]
c [Å]
α [º]
β [º]
106.924(3)
90
γ [º]
V [Å3]
2962.3(4)
6
Z
ρcalcd [g cm-3]
T [K]
1.225
1.341
173(2)
100(2)
–1
0.059
0.292
μ [mm
F(000)
]
1032
764
θ range for data collection 3.24 to
1.45 to 26.00°
52.45°
index ranges
-9<=h<=9
-12<=h<=12
-15<=k<=15
-17<=ℓ<=17
15874
-23<=k<=22
-9<=ℓ<=8
8086
reflections measured
independent reflections
Rint
3979
7132
0.0660
0.0228
data/restraints/parameters 3979/122/377 7132/0/601
final R values [I>2θ (I)]:
R1
0.1241
0.3176
0.0510
0.1331
wR2
R values (all data):
R1
0.1890
0.3600
1.156
0.0598
0.1403
1.032
wR2
2
GOF on F
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10.1002/ejoc.201700019
European Journal of Organic Chemistry
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European Journal of Organic Chemistry
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10.1002/ejoc.201700019
European Journal of Organic Chemistry
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Graphical Abstract
Thermolysis and protonation of tetracyclone (1) yield isomeric rearrangement
products 22 and 17, respectively. These observations are rationalized in terms
of specific rotations about different single bonds prior to double ring closure.
O
O
O
Ph
Ph
CF3SO3H
CH2Cl2
425 oC
Ph
Ph
22
17
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