Syntheses, Structures and Optoelectronic Properties of Spiroconjugated Cyclic Ketones

Article abstract of DOI:10.1002/ejoc.201600235

Synthetic investigations towards spiroconjugated ketones and derivatives thereof are presented. These studies led to the development of a short procedure for the synthesis of spiroconjugated tetraketone 1 in two steps with 10 % overall yield and the first-time synthesis of spirocyclic trindone 2. The optoelectronic properties of these compounds based on spectroscopic and electrochemical measurements in combination with DFT calculations as well as their molecular structures in the solid state are presented and discussed.

Full text of DOI:10.1002/ejoc.201600235

DOI: 10.1002/ejoc.201600235
Full Paper
Spiroconjugation
Syntheses, Structures and Optoelectronic Properties of
Spiroconjugated Cyclic Ketones
Jennifer Wilbuer,[a] Gregor Schnakenburg,[b] and Birgit Esser*[a][‡]
Dedicated to Professor Rolf Gleiter on the occasion of his 80th birthday
Abstract: Synthetic investigations towards spiroconjugated optoelectronic properties of these compounds based on spec-
ketones and derivatives thereof are presented. These studies
led to the development of a short procedure for the synthesis
of spiroconjugated tetraketone 1 in two steps with 10 % overall
yield and the first-time synthesis of spirocyclic trindone 2. The
troscopic and electrochemical measurements in combination
with DFT calculations as well as their molecular structures in
the solid state are presented and discussed.
Introduction
serve a strong effect of spiroconjugation, both halves of the
molecule must possess fragment orbitals that are antisymmetric
The concept of spiroconjugation was first postulated in 1967[1,2] with respect to the two perpendicular planes of the π-systems
and can be described as the homoconjugative through-space since only molecular orbitals of the same symmetry can inter-
interaction between two π-systems arranged perpendicularly to act. In case of a positive interaction, molecular orbitals are ener-
each other and connected by a common tetrahedral atom.[1–6] getically shifted, which, for frontier molecular orbitals, leads to
Spiroconjugation can lead to a lowering or increase in energy a change in the HOMO–LUMO gap compared to a non-spiro-
of occupied or unoccupied molecular orbitals relative to the conjugated reference compound.[1–4] A variety of spirocyclic
half-molecules connected by the spirocenter. In order to ob- compounds has been suggested, and some have been synthe-
Figure 1. Spiroconjugated tetraketone 1, spirocyclic trindone 2, 9,9′-spirobifluorene (3), schematic drawing of co-polymer 4 incorporating tetraketone 1, and
dibrominated derivative 5.
[a] Kekulé-Institut für Organische Chemie und Biochemie,
sized that demonstrate the phenomenon of spiroconjugation
as shown by photoelectron spectroscopy or other spectroscopic
techniques.[3–5,7–10] The extent of spiroconjugation depends on
the orbital overlap between the two π-systems and, hence, on
the substitution pattern around the spirocenter. One example
displaying a significant amount of spiroconjugation is spirocy-
clic tetraketone 1 (Figure 1).[11–13] Spiroconjugation in 1 leads
to a lowering of its LUMO by 0.31 eV relative to a hypothetical
non-spiroconjugated model.[11,14] Another feature that makes
spiroconjugated molecules attractive candidates to be incorpo-
Rheinische Friedrich-Wilhelms-Universität Bonn
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
[b] Institut für Anorganische Chemie,
Rheinische Friedrich-Wilhelms-Universität Bonn
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
[‡] Current address: Institut für Organische Chemie,
Albert-Ludwigs-Universität Freiburg
Albertstr. 21, 79104 Freiburg, Germany
E-mail: besser@oc.uni-freiburg.de
www.esser-lab.uni-freiburg.de
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201600235.
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rated into organic electronic materials is their three-dimen-
sional structure; π–π stacking and excimer formation in conju-
gated polymer films, leading to undesired long-wavelength
emissions, can be effectively suppressed through the three-
dimensional structure of spiroconjugated monomer units. For
instance, 9,9′-spirobifluorene (3; Figure 1) has been incorpo-
rated into a variety of conjugated polymers and significantly
improved their performance in devices such as organic light-
emitting diodes (OLEDs).[15–20] The three-dimensional structure
of 3 suppresses excimer formation, and the substitution pattern
provides a higher oxidative stability compared to its fluorene
analogues.[21,22] However, spiroconjugation in 3, leading to an
increase in HOMO energy by 0.1 eV relative to a non-spiroconju-
gated model,[7,8,23] is less pronounced than in 1.
We are interested in incorporating spiroconjugated mono-
mer units into conjugated polymer architectures and in investi-
gating the effect of spiroconjugation on the electronic proper-
ties of the resulting materials. As such, due to its significant
extent of spiroconjugation, spirocyclic tetraketone 1 is an inter-
esting target enabling facile access to conjugated polymer ar-
chitectures such as 4 (Figure 1). For the synthesis of these poly-
mers by cross-coupling reactions with suitable co-monomers, a
functionalized derivative such as 5 is required. Our efforts to
generate dibrominated spirocyclic tetraketone 5 led to the de-
velopment of a new two-step synthetic route to tetraketone 1
and the synthesis of novel spirocyclic trindone 2, which are
reported in this article as well as spectroscopic, electrochemical,
structural, and theoretical investigations into 1, 2, and related
derivatives.
Scheme 1. Synthesis of diketones 8 and 9 followed by oxidative bromination
of the methylene groups in 8 leading to 12, 13 and 14 as well as the oxid-
ation of 8 to tetraketone 1 as reported by Maslak and co-workers.[12]
A retrosynthetic analysis of 1 according to Scheme 2 reveals
two bond-breaking possibilities to access 1 in a straightforward
manner. According to option A the starting materials would be
2,2-dibromo-1,3-indandione (18) as the electrophile and syn-
thon 19, resulting in an Umpolung approach involving phthal-
aldehyde as the nucleophilic reagent. In option B, reasonable
starting materials invoked 1,3-indandione (20) as the nucleo-
phile and phthaloyl dichloride (21) as an appropriate electro-
phile.
Results and Discussion
The synthesis of dibrominated spirocyclic tetraketone 5 and
parent tetraketone 1 as a model compound was first attempted
according to the procedure reported by Maslak and co-workers
for 1[12] (Scheme 1). This approach involves a five-step synthesis
starting from benzyl chloride (6), during which the spirocyclic
core is installed using a Friedel–Crafts acylation of 7 leading to
diketone 8. Oxidation and hydrolysis lead to tetraketone 1. After
modification of the reported procedures to achieve higher
yields, spirocyclic diketone 8 as well as the novel dibrominated
diketone 9 were synthesized starting from 6 and 10, respec-
tively, in three steps as precursors to 1 and 5 (Scheme 1). In the
literature, the following oxidative bromination of the methylene
groups in 8 to yield tetrabromide 11 was reported using carbon
tetrachloride as solvent (Scheme 1).[12] Due to its adverse health
effects, the availability of carbon tetrachloride is very limited
nowadays, and hence a variety of reaction conditions were
tested for the bromination of 8 that avoided its use. However,
only mixtures of compounds were formed, of which 12, 13, and
14 could be isolated in small quantities. Due to these difficulties
in effecting oxidation of diketone 8, the oxidation of the brom-
Scheme 2. Retrosynthetic analysis of 1.
According to route A thioacetal 22 was synthesized from 23
as the dianionic synthon 19, and 2,2-dibromo-1,3-indandione
(18) was accessed from indanedione (20) (Scheme 3). Reaction
of 22 with 18, however, did not yield the desired species 24,
likely due to steric hindrance associated with the requisite two-
fold nucleophilic attack of 22-derived anionic species on 18.
To evaluate route B (Scheme 2), a number of reaction condi-
inated derivative 9 was not attempted. Instead, we set out to tions for the condensation of 1,3-indanedione (20) with phthal-
find an alternative and shorter synthetic route to spirocyclic oyl dichloride (21) were evaluated. Under acidic or neutral pH
tetraketone 1, which could then be applied to the generation conditions, no formation of tetraketone 1 was observed; only
of dibrominated derivative 5.
bindone (25) and tautomers of bindone could be isolated. Con-
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Scheme 3. Attempted synthesis of 24 as a precursor to 1.
ducting the reaction at basic pH also failed to afford tetraketone
1. However, under these conditions, a new compound was
formed as the main product and was identified as spirocyclic
pentaketone 2 (“trindone”; Scheme 4). Compound 2 was likely
formed by reaction of bindone (25), the product of self-conden-
sation of 20, with phthaloyl dichloride (21). Even after adjust-
ment of the reaction conditions, direct formation of tetraketone
1 from 20 and 21 was never observed under basic reaction
conditions. After optimization of the reaction conditions, 2 was
synthesized in 23 % yield by treating 1 equiv. of 21 with
2 equiv. of 20 and 3 equiv. of NaH. Trindone (2) was character-
ized by NMR spectroscopy, mass spectrometry and X-ray crys-
tallography. The 1H NMR signal of the ortho-hydrogen atom
shown in Scheme 4 is observed at low field (δ = 9.81 ppm) due
to an H-bond, formed with the proximal carbonyl oxygen atom.
These types of H-bonds, and the resulting downfield shifts of
the proton resonances, have been observed in bindone (25)
and derivatives thereof.[24]
Scheme 4. Reaction of 1,3-indanedione (20) with phthaloyl dichloride (21)
under basic conditions to form trindone (2).
Scheme 5. Oxidative cleavage of the C–C double bond in 2 to lead to spiro-
cyclic tetraketone 1.
tribromotrindone 29 in 20 % yield as a mixture of diastereo-
mers, which were inseparable by column chromatography. The
Trindone (2), already possessing the desired spirocyclic attempted cleavage of the C–C double bond in 29 with sodium
framework, was envisioned after oxidative cleavage of the C–C periodate and ruthenium trichloride, envisioned to furnish 5,
double bond, to furnish tetraketone 1. A variety of conditions remained unsuccessful.
for the oxidative cleavage were tested, the best of which in-
volved a combination of sodium periodate and ruthenium tri-
chloride in a solvent mixture of acetonitrile/dichloromethane/
water (2:2:3),[25] which afforded 1 in 45 % yield (Scheme 5).
In summary, although the synthesis of dibrominated tetra-
ketone 5 was not possible, a new, short synthetic procedure
was developed for the synthesis of spirocyclic tetraketone 1.
The procedure by Maslak and co-workers led to 1 in five steps
This new synthetic protocol was then applied to the synthe- and 14 % overall yield (see Scheme 1).[12] However, using this
sis of dibrominated derivative 5. 5-Bromo-1,3-indanedione (26) new procedure, spirocyclic tetraketone 1 can now be accessed
and 4-bromophthaloyl dichloride (27) were synthesized from 4- in two synthetic steps with 10 % overall yield. In addition, novel
bromophthalic anhydride (28) according to Scheme 6. Reaction spirocyclic trindone (2) was synthesized and characterized for
of 26 and 27 under the previously optimized conditions led to the first time.
Scheme 6. Reaction of bromo-1,3-indanedione (26) with 4-bromophthaloyl dichloride (27) under basic conditions to afford tribromotrindone 29.
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In the following, the results of X-ray crystallographic, spectro- bond of the same nature is present with a length of 2.159 or
scopic, electrochemical, and theoretical investigations will be 2.176 Å (probably of a racemic twin).
reported and discussed. The molecular structures of 13, 14, and
9 in the crystal are shown in Figure 2. The spiro-angles (angle
between the two parts of the molecule connected by the spiro-
cyclic carbon atom) were found to be 85.0°, 87.0°, and 78.7° for
compounds 13, 14, and 9, respectively.
Novel spirocyclic trindone (2) was further investigated with
respect to its optoelectronic properties. The absorption spec-
trum of trindone (2) is shown in Figure 4 in comparison with
the spectra of tetraketone 1 and bindone (25). Compound 25
was synthesized by self-condensation of 1,3-indanedione
(20).[28] Here, it was important to exclude the smallest impuri-
ties of isobindone,[28,29] which are readily detected in the UV/
Vis spectra by a strong absorption at ca. 520 nm (CH2Cl2) even
if it is present in concentrations not detectable by NMR spectro-
scopy. The main absorption band in 2, likely corresponding to
a π–π* transition, is found at 247 nm, bathochromically shifted
relative to tetraketone 1 (238 nm). Compound 2 features a sec-
ond strong absorption at ca. 350 nm, likely correlating to an n–
π* transition, which is also present in 25 (341 nm). This batho-
chromic shift of the n–π* transition in 2 relative to 25 indicates
a lowering of its LUMO energy caused by the spirocyclic struc-
ture and a spiroconjugative interaction. Notably, both com-
pounds are bright yellow; this coloration is caused by the
shared n–π* transition in both compounds.
Figure 2. Molecular structures of 13, 14, and 9 in the solid state (most
hydrogen atoms are omitted for clarity, ORTEP ellipsoids are shown at 50 %
probability).
The molecular structures of spirocyclic tetraketone 1 and
trindone (2), which are reported here for the first time, as well
as bindone (25)[26] are shown in Figure 3.
Figure 4. Absorption spectra of trindone (2), tetraketone 1, and bindone (25)
in CH2Cl2.
Figure 3. Molecular structures of spirocyclic tetraketone 1, trindone (2), and
bindone (25) in the solid state (most hydrogen atoms are omitted for clarity,
ORTEP ellipsoids are shown at 50 % probability).
Cyclic voltammograms were measured to determine the re-
The spiro-angle in 1 amounts to 89.1°, and the lengths of duction potentials[30] and LUMO energy of trindone (2) relative
the C–C bonds around the spirocenter lie between 1.53 and to tetraketone 1, diketone 8, and bindone (25) (Figure 5 and
1.55 Å. In the molecular structure of trindone (2), the proximity Table 1). For diketone 8, two quasi-reversible[31] reduction peaks
of oxygen atom O5 to the hydrogen atom attached to C6, re- corresponding to one-electron reductions of the two keto
sulting in an H-bond with a length of 2.12 Å, becomes apparent. groups were observed at –2.50 and –2.90 V (vs. Fc/Fc+). For
This distance as well as the C–H···O angle with 142.9° lie within tetraketone 1, three partly reversible reduction peaks were
the accepted range for C–H···O hydrogen bonds of 2.0–2.8 Å found at –1.28, –1.93 and –2.28 V (vs. Fc/Fc+). At fast scan rates,
and 110–180°.[24,27] Due to the indanedione moiety present in the first reduction becomes reversible. Maslak et al. reported
2, the right half of the spirocyclic system is slightly tilted down- only the first reduction peak for 1 at –0.9 V [vs. a saturated
ward resulting in a spiro-angle of 86.4°. In bindone (25), an H- calomel electrode (SCE) in CH3CN], which is in good agreement
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with our result, assuming that the oxidation of ferrocene occurs
at 0.38 V vs. SCE in CH3CN[32] and considering that a different
solvent was used. Compared to diketone 8, all potentials are
significantly shifted to less negative potentials for spirocyclic
tetraketone 1, indicating an energetic lowering of unoccupied
orbitals due to spiroconjugation.[11]
Figure 6. Calculated LUMOs of 1, 2, and 25 (PW6B95-D3/def2-QZVP, orca_
plot,[33] Chimera,[40] further information can be found in the Supporting Infor-
mation).
Finally, DFT calculations were employed to investigate the
energy and shape of the frontier molecular orbitals of tetra-
ketone 1, trindone (2), and bindone (25) and to compare their
structures with those measured in the solid state. Calculations
were performed using the ORCA 3.0.2 program package.[33] The
structures were optimized using TPSS[34]-D3[35,36]/def2-TZVP.[37]
Single-point energy calculations were performed using
PW6B95[38]-D3/def2-QZVP.[39] Further information can be found
in the Supporting Information. The calculated structures are in
very good agreement with those determined by X-ray crystal-
lography. The mean deviations for bond lengths are 0.007 Å
(gaseous phase, abs.: 0.011 Å) and 0.006 Å (COSMO, abs.:
0.009 Å); further description is shown in Table S7 of the Sup-
porting Information. The calculated difference in LUMO ener-
gies between 2 and 25 is in excellent agreement with the esti-
mated difference of the measured first reduction potentials
Figure 5. Cyclic voltammograms of 8, 1, 2, and 25 (1 mM* in THF, 0.1 M
E
1 pc, which both differ by 0.2 eV. The LUMOs calculated for 1,
nBu4NPF6, scan rate 0.2, 1, 0.1 and 0.02 V/s, respectively; 8, 25 with GC-
electrode, and 1, 2 with Pt-electrode). *Due to solubility issues the concentra-
tion of 2 was lower.
2, and 25 are shown in Figure 6. In tetraketone 1, the LUMO is
symmetrical with respect to the two halves of the molecule. In
spite of the connection of these two halves by a tetrahedral
carbon center, the orbital lobes of the same sign can overlap,
indicating the presence of spiroconjugation. This spiroconjuga-
tion causes an energetic lowering of the LUMO in 1. In 2, the
LUMO is predominately localized on the bindone part of the
molecule, as mentioned above, and hence the first reduction
appears here. In bindone (25), the LUMO is completely localized
on the upper half of the molecule.
Table 1. Electrochemical data for 8, 1, 2, and 25 (potentials in V vs. Fc/Fc+).[a]
[b]
[c]
[b]
[c]
[b]
[c]
E1 pc
E1 1/2
E2 pc
E2 1/2
E3 pc
E3 1/2
8
1
2
–2.60
–1.34
–1.30
–1.46
–2.50
–1.28
–1.25
–
–3.11
–2.02
–1.85
–1.76
–2.90
–1.93
–
–
–
–2.36
–3.10
–2.27
–2.28
–2.99
–2.22
25
–1.67
[a] From cyclic voltammograms (conditions see caption of Figure 5). [b] Cath-
odic peak potential. [c] Half-wave potential.
Conclusions
The first reduction of trindone (2) appears at an even higher
potential of –1.25 V, and two further reductions were found at
–1.85 V (cathodic peak potential) and –2.99 V. The first two
reductions likely appear in the bindone part of the molecule
(see the calculated LUMO in Figure 6, which is localized on this
part of the molecule). The second reduction could lead to a
rotation of the upper indanedione ring, which might be the
reason for the irreversibility of this process. The third reduction,
on the other hand, is quasi-reversible and may involve the inda-
nedione part of the molecule. For bindone (25) three reduction
waves were found, of which only the third one is reversible. The
first reduction has a peak potential of –1.46 V. In comparison,
the first reduction peak of trindone (2) appears at –1.30 V
(cathodic peak potential), indicating an energetic lowering of
its LUMO compared to non-spiroconjugated 25 by 0.2 eV (esti-
mated from the onset of the first reduction peaks).
Synthetic routes to dibrominated spirocyclic tetraketone 5 were
investigated. A short synthetic procedure was developed for
the synthesis of unsubstituted spirocyclic tetraketone 1, which
allows its synthesis in two steps and a 10 % overall yield. As an
intermediate in the synthesis of 1, spirocyclic trindone (2) was
synthesized for the first time. Spectroscopic and electrochemi-
cal measurements indicate that spiroconjugation is present in
2. Moreover, the structures of both 1 and 2 in the solid state
as well as those of three other spirocyclic ketones are reported
for the first time.
Experimental Section
General: Commercially available chemicals were obtained from
Acros-Organics, Alfa Aesar, Fisher, Grüssing, KMF, Merck, Roth,
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Sigma–Aldrich, TCI-Europe, and VWR and used without further puri-
fication. Experiments with moisture- or oxygen-sensitive substances
were carried out under argon using standard Schlenk techniques.
ring at room temperature for 16 h, the yellow-brownish suspension
was again cooled to 0 °C, and a third portion of P4O10 (1.00 g,
3.52 mmol) was added. After stirring at room temperature for an-
Anhydrous solvents were obtained from an M.Braun solvent purifi- other 6 h, the reaction was quenched by addition of aq. NaOH (3 %,
cation system (MB-SPS) and stored under argon over molecular
w/w; 100 mL). The white suspension was diluted with brine (50 mL)
and the organic phase was separated. The aqueous phase was ex-
sieves (3 or 4 Å) for a minimum duration of 48 h before use. All
1
other solvents were distilled prior to use. H and 13C{1H}NMR spec- tracted with CH2Cl2 (3 × 100 mL), and the combined organic ex-
tra were recorded with Bruker Avance dpx 300 and 400 MHz spec- tracts were washed with brine (3 × 100 mL), dried (Na2SO4), and
trometers at room temperature and referenced to the residual pro-
ton or the carbon resonance of the deuterated solvent.[41] NMR
spectra were analyzed using MestReNova software.[42] Electrospray
ionization mass spectra (ESI-MS) were recorded with a Bruker
the solvent was removed under reduced pressure. Column chroma-
tography (silica gel; cyclohexane/CH2Cl2, 1:0 to 0:1) afforded 8
(278 mg, 32 %) as beige solid. Rf (CH2Cl2) = 0.30. 1H NMR (400 MHz,
CDCl3): δ = 7.77 (d, J = 7.7 Hz, 2 H), 7.66 (ddd, J = 7.5, 7.5, 1.2 Hz,
micrOTOF-Q time-of-flight spectrometer, and electron-impact ioni- 2 H), 7.58–7.55 (m, 2 H), 7.42 (ddd, J = 7.4, 7.4, 1.0 Hz, 2 H), 3.73 (d,
zation mass spectra (EI-MS) were recorded using a Thermo Finnigan
(Bremen) MAT 90 or MAT 95 XL sector-field device. Only the struc-
J = 16.9 Hz, 2 H), 3.20 (d, J = 16.9 Hz, 2 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 202.8, 153.9, 135.5, 135.4, 127.9, 126.5, 125.0, 65.4,
turally significant and the strongest fragmentation peaks are re- 38.2 ppm. MS (EI+): m/z = 248 [M]+, 221 [M – CO + H]+. UV/Vis
ported. Silica gel layered alumina plates (Merck, Silica Gel 60 F254)
were used for TLC, and compounds were detected using UV light
(λ = 366 and 254 nm). For flash column chromatography, silica gel
(Merck 60, 40–63 μm) was used as the stationary phase. Eluents are
described in the respective synthetic procedures. Melting points
were measured with a Reichert Thermovar and are uncorrected. IR
spectra were recorded with a Nicolet 380 FT-IR spectrometer (Smart
Orbit, Thermo Electron Cooperation) neat. Absorption spectra were
recorded with a Lambda 18 UV/Vis spectrometer (PerkinElmer). Cy-
clic voltammograms were measured inside a glovebox using a po-
tentiostat (PGSTAT128N) by Metrohm Autolab. As working electrode
a glassy carbon disc electrode (2 mm diameter) or a platinum disc
electrode (2 mm diameter) was used, as counter electrode a plati-
num rod, and as reference electrode an Ag/AgNO3 electrode con-
(CH2Cl2): λabs (ε [rel]) = 249 (1), 290 (0.19), 297 (0.18), 331 (0.008)
nm.
General Procedure for the Bromination of 2,2′-Spirobiindane-
1,1′-dione (8): Diketone 8 was suspended in argon-saturated H2O,
and the respective brominating species was added. The suspension
was stirred under irradiation by the respective lightbulb for the
indicated time. For solvation reasons CHCl3 was added. The reaction
was quenched with aq. Na2S2O3 (3 %, w/w), diluted with water and
extracted with CH2Cl2. The combined organic extracts were washed
with brine, dried (Na2SO4), and the solvent was removed under re-
duced pressure. Column chromatography (silica gel; cyclohexane/
CH2Cl2, 1:0 to 1:3) afforded the brominated compounds 12, 13, and
14.
3,3′-Dibromo-2,2′-spirobiindane-1,1′-dione (14): According to
the general procedure, a mixture of diketone 8 (100 mg, 403 μmol),
H2O (1.5 mL), CHCl3 (0.5 mL) and NBS (290 mg, 1.61 mmol) was
irradiated using a 500 W bulb for 2.5 h. After addition of another
portion of NBS (290 mg, 1.61 mmol), irradiation was continued for
19.5 h. Workup and purification yielded 9 mg (5 %) of 14 as a
slightly yellow solid. Rf = 0.68 (CH2Cl2). 1H NMR (400 MHz, CDCl3):
δ = 7.88–7.79 (m, 4 H), 7.78–7.74 (m, 2 H), 7.58–7.51 (m, 2 H), 6.20
(s, 1 H), 6.11 (s, 1 H) ppm. MS (EI+): m/z = 325 [M – Br]+, 246 [M –
2 Br]+.
taining a silver wire immersed in an inner chamber filled with 0.1
AgNO3 and 0.1 nBu4NPF6 in THF. The analyte solution contained
about 10 mL of solvent (THF) with 0.1 nBu4NPF6 and about 1 m
M
M
M
M
analyte, except for 2, which was less soluble and an exact concen-
tration was not determined. The ferrocene/ferrocenium redox cou-
ple was used as internal reference. Details of the X-ray crystallo-
graphic studies can be found in the Supporting Information. CCDC
1451516 (for 1), 1451517 (for 13), 1451518 (for 14), 1451519 (for 9),
1451520 (for 2), and 1456538 (for 25) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
3,3,3′-Tribromo-2,2′-spirobiindane-1,1′-dione (12): According to
the general procedure, a mixture of diketone 8 (80 mg, 322 μmol),
H2O (0.6 mL) and NBS (116 mg, 644 μmol) was irradiated using a
200 W bulb for 48 h. After addition of H2O (1 mL), CHCl3 (0.5 mL)
and NBS (118 mg, 663 μmol), irradiation was continued using a
500 W bulb for 3 h. Workup and purification yielded 88 mg (56 %)
of 12 as a mixture of two diastereomers as a white solid and traces
of 14. Analytical data for 12: Rf (CH2Cl2) = 0.8. 1H NMR (400 MHz,
CDCl3): δ = 8.20–8.09 (m, 2 H), 7.93–7.74 (m, 6 H), 7.71–7.57 (m, 4
H), 7.57–7.44 (m, 4 H), 5.96 (s, 1 H), 5.78 (s, 1 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = 193.3, 190.2, 189.7, 186.8, 157.5, 157.4, 154.8,
154.4, 137.3, 136.9, 136.7, 136.4, 132.4, 132.1, 131.2, 131.1, 130.2,
130.1, 129.6, 129.0, 126.4, 126.0, 125.9, 125.9, 125.7, 125.3, 125.1,
124.8, 84.5, 81.9, 56.9, 54.9, 49.5, 47.8 ppm. MS (EI+): m/z = 481 [M
– H]+, 403 [M – Br]+. HRMS (EI+): calcd. for C17H8Br3O2 [M – H]
480.8074, found 480.8073.
Diethyl 2,2-Dibenzylmalonate (15): Prepared as described in the
literature.[12] Column chromatography (silica gel, CH2Cl2) afforded
1
15 as a colorless oil. Rf = 0.74 (CH2Cl2). H NMR (400 MHz, CDCl3):
δ = 7.31–7.19 (m, 10 H), 4.12 (q, J = 7.1 Hz, 4 H), 3.25 (s, 4 H), 1.17
(t, J = 7.1 Hz, 6 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 171.1, 136.5,
130.3, 128.3, 127.0, 61.3, 60.3, 39.3, 14.0 ppm. MS (EI+): m/z = 340
[M]+, 295 [M – OEt]+, 249 [M – Bn]+, 203 [M – OEt – MePh]+, 91
[Bn]+.
2,2-Dibenzylmalonic Acid (7): Prepared as described in the litera-
ture.[12] Removal of the solvent under reduced pressure afforded 7
as a white solid. No further purification was necessary. 1H NMR
(400 MHz, [D4]MeOH): δ = 7.29–7.19 (m, 10 H), 3.21 (s, 4 H) ppm.
MS (EI+): m/z = 284 [M]+, 240 [M – CO2]+, 193 [M – Bn]+, 149 [M –
CO2 – Bn]+, 91 [Bn]+.
2,2′-Spirobiindane-1,1′-dione (8): Synthesized using a modified
literature procedure.[12] A suspension of malonic acid 7 (1.00 g,
3.52 mmol) in a mixture of nitrobenzene/anhydrous n-hexane (1:9;
100 mL) was cooled to 0 °C, and P4O10 (1.00 g, 3.52 mmol) was
added. The mixture was warmed to room temperature and stirred
for 2 h. The slightly yellow suspension was cooled to 0 °C, and a
second portion of P4O10 (1.00 g, 3.52 mmol) was added. After stir-
3,3-Dibromo-2,2′-spirobiindane-1,1′,3′-trione (13): According to
the general procedure, diketone 8 (70 mg, 282 μmol) was sus-
pended in H2O (0.2 mL), and HBr (aq., 48 %; 64 μL, 564 μmol) was
added. H2O2 (aq., 30 %; 115 μL, 1.13 mmol) was added dropwise,
and the resulting suspension was irradiated using a 200 W bulb for
48 h. After addition of H2O (1 mL), CHCl3 (0.5 mL), HBr (aq., 48 %;
128 μL, 1.13 mmol) and H2O2 (aq., 30 %; 230 μL, 2.26 mmol), irradia-
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tion was continued using a 500 W bulb for 3 h. Workup and purifica-
tion yielded 9 mg (8 %) of 13 as yellowish crystalline solid, 61 mg
(45 %) of 12 and traces of 14. Analytical data for 13: Rf = 0.6
tinued for 30.5 h. Nitrobenzene (6 mL) was added to the white
suspension, and the resulting dark red mixture was stirred at room
temperature for 18.5 h. The reaction was quenched by addition of
aq. NaOH (3 % w/w; 20 mL). The organic phase was separated and
washed with H2O (30 mL). The aqueous phase was extracted with
CH2Cl2 (3 × 30 mL). The combined organic extracts were washed
with brine (100 mL), dried (MgSO4), and the solvent was removed
under reduced pressure. Column chromatography (silica gel; cyclo-
1
(CH2Cl2). H NMR (400 MHz, CDCl3): δ = 8.13 (dd, J = 5.7, 3.1 Hz, 2
H), 8.05 (ddd, J = 7.9, 0.8, 0.8 Hz, 1 H), 7.95 (dd, J = 5.7, 3.1 Hz, 2
H), 7.88 (ddd, J = 7.9, 7.3, 1.2 Hz, 1 H), 7.75 (ddd, J = 7.7, 1.2, 0.7 Hz,
1 H), 7.59 (ddd, J = 7.7, 7.2, 1.0 Hz, 1 H) ppm. 13C NMR (101 MHz,
CDCl3): δ = 191.6, 191.4, 157.4, 143.3, 137.1, 136.7, 136.5, 132.1,
131.5, 126.2, 124.8, 124.7, 124.1, 78.6, 51.4 ppm. MS (EI+): m/z = 339 hexane/CH2Cl2, 1:0 to 2:3) afforded 9 (25.8 mg, 23 %) as a white
[M – Br]+, 261 [M – 2 Br + H]+.
solid. Rf (cyclohexane/CH2Cl2, 2:3) = 0.43. 1H NMR (400 MHz, CDCl3):
δ = 7.88 (d, J = 1.9 Hz, 2 H), 7.76 (dd, J = 8.1, 1.9 Hz, 2 H), 7.45 (d,
J = 8.2 Hz, 2 H), 3.65 (d, J = 17.1 Hz, 2 H), 3.13 (d, J = 17.1 Hz, 2 H)
ppm. 13C NMR (75 MHz, CDCl3): δ = 200.7, 152.3, 138.3, 137.1, 128.0,
122.3, 66.4, 37.5 ppm. MS (EI+): m/z = 404 [M]+, 376 [M – CO]+, 325
[M – Br]+, 397 [M – CO – Br]+, 246 [M – 2 Br]+, 218 [M – CO – 2 Br]+.
HRMS (EI+): calcd. for C17H10Br2O2 403.9048 [M], found 403.9050.
Diethyl 2,2-Bis(4-bromobenzyl)malonate (16): Compound 16
was prepared in a fashion highly analogous to that used to gener-
ate 15, and the analytical data were compared to those in the litera-
ture.[43] To a solution of NaOEt [generated in situ from Na (793 mg,
34.5 mmol) in anhydrous EtOH (25 mL)] diethyl malonate (2.30 mL,
15.0 mmol) was added. The solution was stirred at room tempera-
ture for 30 min and then cooled to 0 °C, resulting in precipitation 1,2-Bis(1,3-dithian-2-yl)benzene (22): Prepared in analogy to a lit-
of a white solid. 4-Bromobenzyl chloride (10; 6.16 g, 30.0 mmol)
erature procedure.[44] To a solution of o-phthalaldehyde (23;
was added in one portion, and stirring was continued for 1 h before 274 mg, 2.00 mmol) in CH2Cl2 (5 mL) was added 1,3-propanedithiol
the temperature was allowed to rise to room temperature. After (414 μL, 4.00 mmol). After addition of BF3·OEt2 (48 %; 99 μL,
stirring at room temperature overnight, the milky white suspension
800 μmol), the solution was stirred at room temperature for 5 d.
The reaction was quenched with satd. aq. NaHCO3 (15 mL) and
diluted with EtOAc (50 mL). The organic phase was washed with
brine (20 mL) and dried (Na2SO4). The solvent was removed under
reduced pressure affording 22 (560 mg, 89 %) as a colorless foam.
Rf (cyclohexane/EtOAc, 1:1) = 0.50. 1H NMR (300 MHz, CDCl3): δ =
7.59 (dd, J = 5.8, 3.4 Hz, 2 H), 7.29 (dd, J = 5.8, 3.4 Hz, 2 H), 5.66 (s,
2 H), 3.19–3.09 (m, 4 H), 2.97–2.90 (m, 4 H), 2.25–2.16 (m, 2 H), 2.03–
1.88 (m, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 136.7, 129.3, 129.0,
was acidified with 1 HCl, diluted with CH2Cl2 (30 mL), and the
M
aqueous phase was extracted with CH2Cl2 (3 × 30 mL). The com-
bined organic extracts were washed with brine (100 mL), dried
(Na2SO4), and the solvent was removed under reduced pressure.
The colorless oil solidified to a white solid (16, 7.35 g, 98 %). No
1
further purification was necessary. Rf (CH2Cl2) = 0.68–0.77. H NMR
(400 MHz, CDCl3): δ = 7.40–7.37 (m, 4 H), 7.04–7.01 (m, 4 H), 4.10
(q, J = 7.1 Hz, 4 H), 3.15 (s, 4 H), 1.15 (t, J = 7.1 Hz, 6 H) ppm. 13C
NMR (101 MHz, CDCl3): δ = 170.7, 135.3, 131.9, 131.5, 121.2, 61.6, 49.0, 32.7, 25.4 ppm. MS (EI+): m/z = 314 [M]+, 207 [M – C3H7S2]+.
60.0, 39.1, 14.0 ppm. MS (EI+): m/z = 496 [M]+, 451 [M – OEt]+, 348
[M – 2 CO2 – 2 C2H6]+, 327 [M – BnBr]+, 281 [M – OEt – MePhBr]+,
169 [BnBr]+. HRMS (EI+): calcd. for C21H22Br2O4 [M] 495.9885, found
495.9883.
HRMS (EI+): calcd. for C14H18S4 [M] 314.0291, found 314.0295.
2,2-Dibromo-1,3-indanedione (18): To a mixture of 1,3-indanedi-
one (20; 151 mg, 1.00 mmol) and KBr (601 mg, 5.00 mmol) in tolu-
ene (5 mL) were added 1
M HCl (5.00 mL, 5.00 mmol) and H2O2 (aq.,
2,2-Bis(4-bromobenzyl)malonic Acid (17): Diethyl 2,2-bis(4-
bromobenzyl)malonate (16; 1.00 g, 2.00 mmol) and NaOH (332 mg,
8.23 mmol) were ground using a mortar and pestle and heated to
105 °C in a Schlenk tube. After reaching this temperature, EtOH
(2 mL) and H2O (1 mL) were added, and the white suspension was
stirred for 5.5 h. Further portions of EtOH (2 mL) and H2O (1 mL)
were added, and stirring was continued at 100 °C for 3 h. The reac-
35 %; 1.72 mL, 20.0 mmol). After stirring at room temperature for
30 min, the reaction was quenched by addition of aq. Na2S2O3 (3 %
w/w; 15 mL) and satd. aq. NaHCO3 (10 mL). The organic phase was
separated, and the aqueous phase was extracted with EtOAc (2 ×
20 mL). The combined organic extracts were washed with brine
(20 mL), dried (Na2SO4), and the solvent was removed under re-
duced pressure. Column chromatography (silica gel; cyclohexane/
tion mixture was stirred at room temperature overnight and at CH2Cl2, 1:1 to 0:1) afforded 18 (183 mg, 60 %) as a colorless, crystal-
100 °C for 7 h. The resulting white suspension was cooled to room
temperature, diluted with H2O (60 mL) and washed with CH2Cl2
(2 × 10 mL). Ice was added to the aqueous phase, before it was
acidified with concd. HCl and extracted with CH2Cl2 (4 × 50 mL).
The combined organic extracts were washed with brine (50 mL),
dried (Na2SO4), and the solvent was removed under reduced pres-
sure (at 32 °C). Recrystallization from CH2Cl2/cyclohexane yielded
line solid. Rf (cyclohexane/EtOAc, 1:1) = 0.44. 1H NMR (300 MHz,
CDCl3): δ = 8.14–8.08 (m, 2 H), 8.04–7.98 (m, 2 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 187.4, 137.9, 135.9, 125.9, 51.4 ppm. HRMS
(EI+): calcd. for C9H4Br2O2 [M] 301.8578, found 301.8581.
2-(2,3-Dihydro-3-oxo-1H-inden-1-ylidene)-1H-indene-1,3(2H)-di-
one (25): Prepared as described in the literature.[28] Column
chromatography (silica gel; cyclohexane/ethyl acetate, 1:2) and re-
crystallization (CH2Cl2) afforded 25 as a yellow, crystalline solid; m.p.
1
17 as a white solid (592 mg, 67 %). H NMR (500 MHz, [D4]MeOH):
δ = 7.43–7.40 (m, 4 H), 7.17–7.14 (m, 4 H), 3.14 (s, 4 H) ppm. 13C
NMR (126 MHz, [D4]MeOH): δ = 174.4, 137.0, 133.1, 132.3, 121.9,
61.1, 39.9 ppm. MS (ESI–): m/z = 439 [M – H]–, 395 [M – H – CO2]–.
HRMS (ESI–): calcd. for C17H13Br2O4 [M – H]– 438.9186, found
438.9190.
1
210–212 °C. Rf (cyclohexane/EtOAc, 1:2) = 0.53. H NMR (300 MHz,
CDCl3): δ = 9.68 (dt, J = 8.1, 0.9 Hz, 1 H), 8.05–8.01 (m, 1 H), 8.00–
7.93 (m, 2 H), 7.90–7.84 (m, 1 H), 7.84–7.79 (m, 2 H), 7.75 (td, J =
7.4, 1.0 Hz, 1 H), 4.17 (s, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
201.1, 191.2, 189.5, 155.5, 146.1, 141.8, 141.4, 140.6, 135.6, 135.5,
6,6′-Dibromo-2,2′-spirobiindane-1,1′-dione (9): P4O10 (120 mg, 135.5, 134.3, 131.8, 126.0, 123.7, 123.6, 123.2, 43.6 ppm. IR: ν = 2899
˜
423 μmol) was added to a suspension of 2,2-bis(4-bromobenzyl)ma-
lonic acid (17; 120 mg, 271 μmol) in anhydrous n-hexane (20 mL),
and the resulting mixture was stirred at room temperature for 5.5 h.
(m), 1712 (s), 1682 (s), 1608 (m), 1579 (s), 1567 (s), 1460 (m), 1370
(w), 1347 (m), 1320 (m), 1299 (w), 1269 (m), 1246 (s), 1218 (s), 1154
(m), 1089 (w), 1044 (m), 1021 (w), 985 (m), 920 (w), 884 (w), 868
Another portion of P4O10 (120 mg, 423 μmol) was added to the (w), 815 (w), 799 (w), 755 (m), 733 (s), 702 (m), 654 (w), 635 (s), 595
white suspension, and stirring was continued for 17 h. A third por-
tion of P4O10 (120 mg, 423 μmol) was added, and stirring was con-
(m), 565 (m), 527 (m), 504 (w), 488 (w), 446 (m), 421 (w), 411 (w) cm–1
MS (EI+): m/z = 274 [M]+, 246 [M – CO]+; 218 [M – 2 CO]+, 189 [M –
.
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3 CO – H]+. UV/Vis (CH2Cl2): λabs (ε [rel]) = 240 (0.71), 259 (0.68), 266 25 mL). The combined organic extracts were washed with brine
(0.72), 341 (1), 254 (sh, 0.84), 412 (0.02) nm.
(25 mL), dried (Na2SO4), and the solvent was removed under re-
duced pressure. Recrystallization from acetone yielded 542 mg
(55 %) of 26 as a slightly brown solid. Rf (CH2Cl2) = 0.61. 1H NMR
(400 MHz, CDCl3): δ = 8.09 (d, J = 1.7 Hz, 1 H), 7.94 (dd, J = 8.0,
1.7 Hz, 1 H), 7.83 (d, J = 8.1 Hz, 1 H), 3.24 (s, 2 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 196.2, 196.0, 144.6, 141.9, 138.9, 131.4, 126.5,
124.7, 45.1 ppm. MS (EI+): m/z = 224 [M]+, 196 [M – CO]+. HRMS
(EI+): calcd. for C9H5BrO2 [M] 223.9473, found 223.9474.
Spirocyclic Trindone (2): NaH (60 %, w/w, in mineral oil; 738 mg,
18.5 mmol) was suspended in anhydrous THF (100 mL) und cooled
to 0 °C. 1,3-Indanedione (20; 1.85 g, 12.3 mmol) was added, and
the resulting red suspension was stirred for 15 min until gas evolu-
tion had ceased. o-Phthaloyl dichloride (21; 943 μL, 6.15 mmol) was
added dropwise at 0 °C. The mixture was stirred at room tempera-
ture for 20 h. The reaction was quenched by addition of H2O
(15 mL). The solvent was removed under reduced pressure and the
residue filtered. The filter cake was washed with H2O (ca. 600 mL)
and small quantities of cold MeOH and recrystallized from EtOAc.
The resulting yellow solid was further purified by column chroma-
tography (silica gel; cyclohexane/EtOAc/CH2Cl2, 4:1:0 to 0:0:1) and
recrystallization from CH2Cl2 affording 584 mg (23 %) of 2 as a yel-
low crystalline solid; m.p. 348–350 °C. Rf (CH2Cl2) = 0.38. 1H NMR
(500 MHz, CD2Cl2): δ = 9.81 (ddd, J = 8.2, 0.8, 0.8 Hz, 1 H), 8.11–
8.07 (m, 2 H), 8.06 (ddd, J = 7.6, 0.9, 0.9 Hz, 1 H), 8.02–7.97 (m, 3
H), 7.94 (ddd, J = 7.6, 1.4, 0.7 Hz, 1 H), 7.87–7.81 (m, 2 H), 7.80–7.74
(m, 2 H) ppm. 13C NMR (126 MHz, CD2Cl2): δ = 193.8, 191.4, 189.8,
188.8, 149.7, 147.6, 143.5, 142.7, 140.6, 138.6, 136.9, 136.6, 136.2,
135.9, 135.1, 132.1, 127.5, 125.3, 124.3, 124.0, 123.9 ppm (spiro-C
4-Bromophthaloyl Dichloride (27): Prepared in analogy to a litera-
ture procedure.[46] 4-Bromophthalic anhydride (28; 1.50 g,
6.67 mmol) and PCl5 (1.47 g, 7.07 mmol) were mixed under argon
and heated to 160 °C with stirring. After refluxing for 2.5 h, the
temperature was gradually raised to 250 °C. At the same time, the
resulting POCl3 was distilled off. A second distillation (130 °C,
0.5 mbar) afforded 1.46 g (78 %) of 27 as a colorless oil. 1H NMR
(300 MHz, CDCl3): δ = 7.98–7.95 (m, 1 H), 7.88–7.84 (m, 2 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 166.8, 166.3, 136.0, 132.9, 132.4, 132.2,
132.1, 129.5 ppm. MS (EI+): m/z = 245 [M – Cl]+, 201 [M – Br]+, 182
[M – 2 Cl – CO]+. HRMS (EI+): calcd. for C8H3BrClO2 [M – Cl] 244.9005,
found 244.9002.
Brominated Spirocyclic Pentaketone 29: Prepared in analogy to
2. NaH (60 %, w/w, in mineral oil; 80 mg, 3.33 mmol) was suspended
in anhydrous THF (15 mL) und cooled to 0 °C. To the white suspen-
sion 5-bromo-1,3-indanedione (26; 250 mg, 1.11 mmol) was added,
and the mixture was stirred until gas eruption had ceased.
4-Bromophthaloyl dichloride (27; 313 mg, 1.11 mmol) was added
dropwise, and to insure complete addition the syringe was rinsed
with THF (5 mL). The mixture was stirred at room temperature over-
night. The reaction was quenched through addition of H2O (6 mL),
and the solvent was removed under reduced pressure. The aqueous
phase was extracted with CH2Cl2 (3 × 25 mL). The combined or-
ganic extracts were washed with brine, dried (Na2SO4), and the sol-
vent was removed under reduced pressure. Column chromatogra-
phy (silica gel; CH2Cl2) afforded 29 (72 mg, 20 % relating to 26) as
a red-orange solid (mixture of isomers). MS (EI+): m/z = 638 [M]+.
signal not detected). IR: ν = 1709 (s), 1678 (s), 1605 (m), 1590 (m),
˜
1578 (m), 1558 (s), 1457 (m), 1352 (m), 1326 (m), 1286 (w), 1230 (s),
1150 (s), 1049 (m), 988 (m), 959 (s), 900 (m), 880 (m), 852 (m), 778
(s), 733 (s), 699 (s), 685 (s), 674 (s), 632 (s) cm–1. HRMS (EI+): calcd.
for C26H12O5 [M] 404.0685, found 404.0685. UV/Vis (CH2Cl2): λabs (ε
[rel]) = 247 (1), 350 (0.59), 365 (sh, 0.51), 427 (0.02) nm.
2,2′-Spirobiindanetetraone (1): Synthesized using conditions de-
scribed by Carlsen et al.,[25] analytical data were compared to those
in the literature.[12] Compound 2 (100 mg, 247 μmol) was sus-
pended in a mixture of CH2Cl2/CH3CN (1:1; 4 mL) and sonicated for
10 min. To the resulting yellow suspension, NaIO4 (325 mg,
1.52 mmol), RuCl3·xH2O (30 mg) and H2O (3 mL) were added. The
flask was immersed in a water bath preheated to 45 °C, and the
dark mixture was stirred at this temperature for 7.5 h and at room
temperature overnight. The reaction mixture was filtered, washed
with CH2Cl2, and the organic phase was washed with H2O. The
aqueous phase was extracted with CH2Cl2 (2 × 10 mL). The com-
bined organic extracts were washed with brine (20 mL), dried
(Na2SO4), and the solvent was removed under reduced pressure.
Recrystallization from EtOAc yielded 31 mg (45 %) of 1 as a white
solid; m.p. 240–246 °C. Column chromatography is possible if done
in a quick manner, but it results in partial decomposition of 1. Rf
(CH2Cl2) = 0.59. 1H NMR (300 MHz, CDCl3): δ = 8.10 (dd, J = 5.7,
3.1 Hz, 4 H), 7.93 (dd, J = 5.7, 3.1 Hz, 4 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 191.7, 144.9, 136.5, 124.7 ppm (spiro-C signal not de-
tected). MS (EI+): m/z = 276 [M]+, 248 [M – CO]+, 220 [M – 2 CO]+.
Acknowledgments
J. W. thanks M. E. Speer for support with cyclic voltammetry
measurements, R. Sure for advice in DFT calculations and C.
Rödde of the single-crystal X-ray diffraction service. Generous
support by the Chemical Industry Trust (Liebig Fellowship,
Li 189/11) and the German Research Foundation (Emmy Noe-
ther program, ES 361/2-1) is gratefully acknowledged.
Keywords: Spiroconjugation · Spiro compounds · Through-
IR: ν = 1715 (s), 1698 (s), 1585 (m), 1468 (w), 1350 (w), 1257 (m),
space interactions · Ketones · Hydrogen bonds
˜
1238 (s), 1167 (m), 998 (m), 964 (w), 888 (w), 779 (s), 771 (s), 726
(m), 688 (w), 628 (s), 606 (m), 589 (s) cm–1. HRMS (EI+): calcd. for
C17H8O4 [M] 276.0423, found 276.0421. UV/Vis (CH2Cl2): λabs (ε
[rel]) = 238 (1), 255 (sh, 0.38), 298 (0.03), 354 (0.004) nm.
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M HCl (7 mL). The result-
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Received: March 1, 2016
Published Online: April 24, 2016
Eur. J. Org. Chem. 2016, 2404–2412
www.eurjoc.org
2412
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim