Nonstatistical dynamic effects in the thermal C2-C6 Diels-Alder cyclization of enyne-allenes

Article abstract of DOI:10.1021/jo302524a

The Diels-Alder (DA) reaction channel of the thermal C²-C ⁶ (Schmittel) cyclization of enyne-allenes is studied computationally and experimentally evaluating the influence of temperature on product ratios. Remote substituents at the alkyne terminus influence the mechanism of the C ²-C⁶/DA cyclization steering it either to a stepwise or a concerted course. Temperature independent product ratios, selectivity of product formation, and computational results obtained at (U)BLYP/6-31G(d) level unveil a mechanism that is strongly controlled by nonstatistical dynamics.

Full text of DOI:10.1021/jo302524a

Article
pubs.acs.org/joc
Nonstatistical Dynamic Effects in the Thermal C2−C6 Diels−Alder
Cyclization of Enyne−Allenes
Debabrata Samanta, Mehmet Emin Cinar, Kalpataru Das, and Michael Schmittel*
Department of Chemistry and Biology, Universitat Siegen, Adolf-Reichwein-Strasse, D-57068 Siegen, Germany
̈
S
* Supporting Information
ABSTRACT: The Diels−Alder (DA) reaction channel of the thermal C2−C6
(Schmittel) cyclization of enyne−allenes is studied computationally and exper-
imentally evaluating the influence of temperature on product ratios. Remote
substituents at the alkyne terminus influence the mechanism of the C2−C6/DA
cyclization steering it either to a stepwise or a concerted course. Temperature
independent product ratios, selectivity of product formation, and computational
results obtained at (U)BLYP/6-31G(d) level unveil a mechanism that is strongly
controlled by nonstatistical dynamics.
cyclization, denoted as the C2−C6 or Schmittel cyclization of
INTRODUCTION
■
enyne−allenes.15,16 It has been extensively studied16 both
mechanistically17 and theoretically18 due to its value to afford
complex carbocycles19 and to effect DNA cleavage.20 σ,π-
Diradical intermediate 2 (Scheme 1) was successfully trapped
with 1,4-cyclohexadiene17a and ultrafast radical clocks (diphe-
nylcyclopropyl clock),21 and computational results as well as
the aforementioned observation of DNA double-strand
cleavage20 furnish strong evidence for the cyclization to evolve
via a diradical intermediate. The postulated diradical may
undergo intramolecular follow-up reactions to furnish the
formal Diels−Alder (DA) product 3,17b,20a,22 ene product 4,15
or [2 + 2] cycloadduct 523 depending on the nature of
substituents at the allene terminus (Scheme 1).
The elucidation of a reaction mechanism is one of the classical
tasks and challenges in physical organic chemistry. Tradition-
ally, our conceptual approach to understand kinetically
controlled reaction schemes is based on well-founded textbook
knowledge of kinetics and guided either by transition state
theory (TST)1 applying a classical statistical ansatz or by the
RRKM (Rice−Ramsperger−Kassel−Marcus) theory.2 Accord-
ing to the latter, which is a microcanonical theory, the energy
obtained from collisions of molecules has to be concentrated in
proper vibrational modes to cross the activation barrier. In a
multistep process, the various intermediates will thus have
initially those vibrations populated that had been selected by
the preceding transition-state passage. If intramolecular vibra-
tional energy redistribution (IVR) of kinetic energy takes place
in the ensuing intermediate prior to undergoing the follow-up
reaction, then the intermediate will obey statistical kinetics.
However, if the follow-up reaction is faster than IVR, then the
intermediate will show non-RRKM behavior that is denoted as
nonstatistical dynamics. There is a wide variety of systems
reacting via SN2 ion−dipole complexes,3 organic diradicals,4
radical cations,5 radicals,5c,6 and carbocations7 that cannot be
explained on the basis of classical TST. Particularly at the
boundary of highly asynchronous concerted and stepwise
mechanisms, processes have been demonstrated to be subject
to nonstatistical dynamic effects.3c,8,9
Substituents at the alkyne and allene termini may additionally
influence the mechanism of the C2−C6 cyclization with regard
to a stepwise vs concerted course. Experimental studies24,25
based on radical clock opening and intramolecular kinetic
isotope effects (KIEs) as well as theoretical calculations26 at the
(U)B3LYP/6-31G(d) level suggest that the C2−C6/ene
cyclization proceeds via a diradical mechanism when a
radical-stabilizing group such as an aryl unit is located at either
the alkyne or allene terminus (Scheme 2).
Singleton and Lipton27 argue, based on experimental
intermolecular KIEs as well as theoretical and dynamic
trajectory calculations, that both the stepwise and concerted
ene reactions proceed via a single TS and that dynamic effects
at a post-transition-state valley−ridge inflection point28 decide
about concerted against stepwise trajectories. The occurrence
of dynamic effects has received further experimental backup by
Thermal cycloaromatizations of enediynes (Bergman reac-
tion)10 and of enyne−allenes (Myers−Saito/C2−C7 cycliza-
tion)11 have received ample attention over the last two
decades12,13 as the resultant diradicals represent key inter-
mediates in the mode of action of natural enediyne antitumor
antibiotics.14 In 1995, Schmittel et al. disclosed a competing
thermal reaction mode to the well-known Myers−Saito
Received: November 19, 2012
Published: February 5, 2013
© 2013 American Chemical Society
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Scheme 1. Formal Diels−Alder (3), Ene (4), and [2 + 2] Cycloaddition (5) Products from Diradical Intermediate 2
Scheme 2. Mechanistic Options for the Thermal C2−C6/Ene
Cyclization of Enyne−Allenes24
at the alkyne terminus a stepwise mechanism will prevail and
nonstatistical dynamic effects become important. These
mechanistic interpretations are in good agreement with earlier
trapping reactions.17a,21 On the other hand, if the enyne−allene
is equipped with aryl groups at both termini (R1, R2 = aryl), the
diradical intermediate finds itself in a deeper energy well and
the overall reaction follows a classical stepwise mechanism as
evidenced by significant intramolecular KIEs.
An alternative thermal cyclization mode of enyne−allenes is
the C2−C6/DA reaction that requires an additional double
bond, generally embedded in a benzene ring, at the allene
terminus of 1 (R2 or R3 = Ar). When the process was first
described,17b the almost identical reaction rates for 10 → 11
and 12 → 13 were taken as convincing evidence for a stepwise
diradical mechanism (Scheme 3) because concerted DA
cycloaddition of dienes involving aromatic subunits are usually
dramatically decelerated by ortho-substituents. Finally, rear-
omatization of 13 via stepwise [1,5]-hydrogen shifts provides
the fluorene derivative 14. In the case of DA product 11, the
[1,5]-shift is prohibited leading to an overall reaction stop.17b
In contrast, DFT calculations from Ho et al.30 on the parent
enyne−allene 1 (with R1 = R2 = H, R3 = vinyl, non-
benzannulated) support a concerted mechanism for the C2−
C6/DA reaction (Scheme 4). The new data presented here will
furnish evidence that the thermal C2−C6/DA reaction takes
place at the borderline between a stepwise and concerted
process, with the stepwise mechanism being strongly influenced
by nonstatistical dynamic effects. Moreover, we demonstrate
our inter- and intramolecular KIE studies on the C2−C6/ene
cyclization of enyne−allenes possessing tBu, TMS, TIPS, and p-
anisyl groups as substituents4 because the experimental inter-
and intramolecular KIEs clearly deviate from the statistically
expected ratios. For example, near-unity intramolecular KIEs,
constant over a wide range of temperature, are observed for
enyne−allenes with a radical-stabilizing group at the allene
terminus. The observed near-unity intramolecular KIE is
explained using Carpenter’s dynamic model8a,29 suggesting
that an intermediate with large energy excess may directly
collapse into the products. Thus, with radical-stabilizing groups
Scheme 3. Thermolyses of Mesityl- and Phenyl-Substituted Enyne−allenes17b
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Scheme 4. Mechanistic Options for the Thermal C2−C6/DA
Reaction of Enyne−Allenes
Moreover, in the reaction of 25a to 19a the alkyne carbon
signal (C2) shifts from 102.3 to 210.4 ppm, the latter shift
being diagnostic for enyne−allenes. The structural identity of
all other enyne−allenes 19b−f was confirmed analogously.
Equally typical for 19 is an IR CCC stretching absorption
around 1910−1931 cm−1.
Thermolysis. As stated above, a temperature-independent
product ratio may be taken as an important experimental
indicator for dynamic behavior.8a,36 Thermolysis of enyne−
allenes 19a−f was thus performed in dry degassed toluene in a
sealed tube at various temperatures using a thermostat
operating at a temperature stability of 0.1 °C. Temperatures
were selected on the basis of differential scanning calorimetry
(DSC) onset temperatures of the exothermic signal responsible
for cyclization. For example, with the onset temperature of 19f
being 53 °C, partial conversion was studied up to 90 °C. At this
temperature, the conversion is 98% after 7 min of thermolysis.
For the other cases (19a−e), the onset temperatures range
from 86 to 103 °C, allowing partial conversion between 50 and
120 °C to be achieved. For 19a−e, the thermolyses were
initially performed at 50 °C for 60 min leading to 4−12%
conversion, which is sufficient to measure integrations in the 1H
NMR with the desired accuracy. The conversion was 86−94%
after 15 min at 120 °C. The thermal stability of products 20
and 21 obtained in the thermolysis was checked by heating
them in dry toluene for sufficient time at the highest used
temperature. All of them proved to be stable.
that substituents at the alkyne terminus affect nonstatistical
aspects like in the case of C2−C6/ene reaction.4,27
RESULTS
■
Synthesis. As model compounds for our study we selected
enyne−allenes 19. The presence of an aryl substituent at the
alkyne terminus is expected to steer the reaction toward the
C2−C6 diradical pathway, whereas the m-tolyl subunit at the
allene terminus should allow the diradical to divert into two
constitutionally different benzofluorenes. If dynamic effects are
present, the product ratio 20/21 (Scheme 5) should be
temperature-independent.8a
It proved to be very difficult to separate the constitutionally
isomeric products 20 and 21, but finally at least 20f and 21f
were separated by preparative TLC. Their full characterization
The required enyne−allenes 19 for our experimental studies
were obtained via a convenient four-step synthesis, starting out
with a Sonogashira coupling of the corresponding alkyne with
o-iodobenzaldehyde 2231 in the presence of a catalytic amount
of palladium(II) affording 23a−e.32 The latter were treated
with trimethylsilylacetylene in a Grignard reaction furnishing
propargyl alcohols 24a−e in good to excellent yields17d,33
(Scheme 6). Compounds 24a−e were treated with acetic
anhydride in the presence of DMAP and NEt3 to afford the
corresponding propargyl acetates 25a−e34 that were sub-
sequently reacted with m-tolylmagnesium bromide in the
presence of ZnCl2 and Pd(0) to yield 19a−e in good to
excellent yields.35
As an additional model compound, we selected enyne−allene
19f in order to evaluate the effect of P(O)Ph2, a radical-
stabilizing unit (Scheme 7). Preparation of 19f was achieved by
reacting aldehyde 23a with m-tolylacetylene to furnish
propargyl alcohol 26 in very good yield. Finally, 26 was
reacted with chlorodiphenylphosphine in the presence of NEt3
to afford the diphenylphosphine oxide substituted enyne−
allene 19f in moderate yield.17d
was accomplished by IR, HR-MS, H, 13C, and H−1H COSY
NMR. In the 1H−1H COSY NMR, the single correlation of the
methyl protons 8-H (see Scheme 8) at 2.18 ppm with the
aromatic doublet for 9-H at 8.05 ppm confirms the formation
of 21f, whereas the methyl protons 1-H at 2.05 ppm correlate
with a doublet of the aromatic proton 2-H at 7.13 ppm, thus
supporting the formation of 20f. All other constitutionally
isomeric products, 20 and 21, obtained after thermal cyclization
of enyne−allenes 19a−e, were isolated by preparative TLC and
characterized in the mixture utilizing the same techniques. In
the 1H−1H COSY spectra of the pure mixture of 20d and 21d,
for example, a correlation of the singlet for proton 8-H at 2.38
ppm with the singlet signal of 9-H situated at 8.27 ppm arising
from the naphthalene ring confirms the identity of 21d (see the
1
1
1
Supporting Information for complete H−1H correlation). By
additionally considering integration, it is clear that singlet
signals in 1H NMR at 3.99 ppm and 4.01 ppm originate due to
fulvenyl protons (5-H and 10-H) of 20d and 21d, respectively.
Hence, the product ratios after thermolysis were determined by
utilizing the integration of these two fulvenyl proton signals. In
an analogous way, product ratio analysis and characterization
were performed for other mixtures.
All compounds were purified by flash column chromatog-
1
raphy and characterized by H, 13C NMR, IR, and either HR-
MS or elemental analysis. For example, the disappearance of
1
the CH3CO2− signal in the H NMR of 25a at 2.09 ppm and
subsequent manifestation of a singlet allene proton signal (3-H)
An evaluation of the product ratios 20a−c/21a−c suggests
that with increasing donor substitution at the phenyl group of
at 7.37 ppm confirms the formation of enyne−allene 19a.
Scheme 5. Probing the Diels−Alder Cyclization of 19 for Nonstatistical Dynamic Effects via Formation of Two Products
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Scheme 6. Synthesis of Enyne−allenes 19a−e
Scheme 7. Synthesis of Enyne−Allene 19f
Scheme 8. Thermolyses of Enyne−Allenes 19a−f
the alkyne terminus temperature independence is manifest at
higher temperature. For 20d,e/21d,e the product ratios are no
longer constant within the given temperature window.
were verified by analyzing the harmonic vibrational frequencies
via their analytical second derivatives because they should have
NIMAG = 0 and 1, respectively. IRC calculations were carried
out at the same level of theory to show that optimized
transition states are connected to two minima on the energy
hypersurface. Free energies including unscaled zero point
vibrational energies after thermal correction are given in kcal
mol−1 relative to the corresponding starting materials.
Computations. To further understand the thermal enyne−
allene cyclization via stepwise and concerted pathways, reaction
profiles were computed using DFT. By comparing the most
common DFT methods in regard to accuracy and computa-
tional cost for the Bergman and Myers-Saito biradical
cyclizations, Schreiner et al.37 recommended the pure DFT
functional (MPWLYP, G96LYP, and BLYP) as the most
suitable methods with a 6-31G (d) basis set. Therefore, the
BLYP38 method with a 6-31G (d)39 basis set was chosen for the
stationary point calculations in the present study. Geometry
optimizations of all gas-phase stationary points were performed
with a 6-31G (d) basis set using Becke’s pure gradient-
corrected exchange functional in conjunction with the Lee−
Yang−Parr nonlocal correlation functional (BLYP) as imple-
mented in Gaussian 03.40 Unrestricted calculations with a
broken spin symmetry approach (BS-UBLYP), involving the
mixing of the frontier molecular orbitals (HOMO and LUMO)
to break the spin and spatial symmetries, were performed for
the open-shell singlet state transition state (TS) structures and
intermediates. In contrast, the restricted method was applied
for all closed-shell molecules. It is well-known that the broken
spin approach in combination with DFT provides good results
for the cyclization of unsaturated systems.17e,18e,f,41 The spin
projection method of Yamaguchi et al.42 was applied for
molecules possessing spin contaminated wave functions.
Calculated minima and first-order saddle point structures
DISCUSSION
■
Statistical Kinetics Vs Nonstatistical Dynamics. Accord-
ing to previous experimental and theoretical studies, enyne-
allenes 19a−f are expected to undergo Schmittel cyclization via
the corresponding diradical intermediate.17,18 The diradical may
either attack onto the ortho or ortho′ positions of the m-tolyl
subunit at the allene terminus along two pathways that
according to statistical kinetic models should exhibit different
activation enthalpies because of their constitutional and steric
difference. After cyclization, the two constitutionally different
benzofluorenes 20a−f and 21a−f should form.
Experimentally, the following picture arises: as shown in
Table 1, the product ratios 20a,b/21a,b prove to be
temperature independent in the range 50−70 or 80 °C but
decrease gently upon increasing the temperature up to 120 °C.
Temperature independence of the product ratio is only to be
explained in two ways: (1) Due to the relation ln (k20/k21) =
−ΔΔH⧧/RT + const it requires ΔΔH⧧ = 0. An accidential
occurrence of ΔΔH⧧ = 0, however, is extremely unlikely as the
two competing reactions are of the same kind, with one being
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that C-7 is always closer to the ortho than the ortho′ center
during the stepwise reaction course of 19a−c (See Figure 2).
Thus, the close proximity of the σ-radical center (C-7 atom) to
the more hindered ortho-position is already preinstated in the
C2−C6 transition state and kept in the σ,π-diradical
intermediate. Importantly, the diradical with the opposite
orientation, i.e., with the less hindered ortho′ carbon center
being closer to C-7, does not show up as energetic minimum.
Therefore, intramolecular radical combination occurs to a larger
extent at the more hindered ortho site although it goes along
with a higher transition state energy barrier than the less
hindered attack.
In the concerted pathway, the two transition states have an
energy barrier of 17.25 and 18.37 kcal mol−1 for the less
(19ag⧧) and more (19ah⧧) hindered attack, i.e., ortho′ and
ortho attack, respectively. While these barriers are slightly
higher than those of the stepwise mechanism, it seems likely
that with increasing temperatures (>70 °C) small amounts of
reactant molecules are able to overcome them. As a
consequence, the product ratio becomes increasingly temper-
ature dependent. In agreement of theory and experiment,
product 21a, which results from the less hindered ortho′ attack,
increases in the product ratio at >70 °C (see Table 1).
Replacing the phenyl group by a p-fluorophenyl group in 19b
gives rise to a similar minimum energy path (MEP) as for 19a.
In this case, the more and less hindered concerted transition
states lie 0.95 and 0.66 kcal mol−1 higher in energy,
respectively, than those of 19a. Notably, the difference between
transition state energies of the C2−C6 (19ba⧧) and low-lying
concerted route (19bg⧧) is 0.72 kcal mol−1 higher than that for
19a. Hence, the molecules are expected to need a higher
temperature to realize some trajectories through the concerted
pathway. Indeed, formation of a temperature-dependent
product ratio starts at >80 °C.
Compound 19c with a p-tolyl group at the alkyne terminus
shows a different behavior because it exhibits a temperature-
dependent product ratio from 50 to 100 °C. As such, one is led
to assume that 19c is not subjected to nonstatistical dynamics.
Notably, though, product 20c, which arises from the more
hindered ortho-attack, forms in major amounts suggesting a
nonclassical behavior. In this system, the difference between
transition state energy of the C2−C6 (19ca⧧) and the less
hindered concerted route (19cg⧧) is only 0.22 kcal mol−1. Such
finding is in full agreement with a temperature-dependent
product ratio even at lower temperature.
Although no further computational study have been
performed, the temperature-dependent product outcome
from 19d,e in the investigated temperature window can now
readily be rationalized: (1) The intermediate wells are deeper
for both cases, because the p-anisyl group (19d) stabilizes the
intermediate to a larger extent than others and the 2,6-
dimethylphenyl group (19e) increases the second TS barriers
along the stepwise pathway by imparting an additional steric
hindrance due its two methyl groups. These affect the product
outcome at lower temperatures. (2) The energy gaps between
C2−C6 TS and concerted TSs are expected to be small, which
lead to the temperature dependent product formation also at
higher temperatures.
Table 1. Product Ratios (20/21) from the Thermolysis of
Enyne−Allenes 19a−f
a
a
temp
(°C)
time
(min)
ratio
temp
(°C)
time
(min)
ratio
20/21
20/21
19a
19c
19e
50
60
60
55
45
35
30
25
20
15
60
55
45
35
30
25
20
15
60
55
45
35
30
25
20
15
1.76
1.76
1.76
1.66
1.64
1.61
1.58
1.56
1.93
1.81
1.77
1.70
1.65
1.58
1.58
1.58
1.88
1.84
1.78
1.76
1.71
1.68
1.63
19b
19d
19f
50
60
60
55
45
35
30
25
20
15
60
55
45
35
30
25
20
15
25
20
15
10
7
1.71
1.71
1.71
1.71
1.68
1.66
1.58
1.51
1.88
1.81
1.73
1.70
1.61
1.59
1.57
1.56
0.82
0.82
0.82
0.83
0.83
70
70
80
80
90
90
100
110
120
50
100
110
120
50
60
60
70
70
80
80
90
90
100
110
120
50
100
110
120
50
60
60
70
70
80
80
90
90
100
110
120
1.61
a
1
Obtained from the crude H NMR spectra. Tonset (DSC) = 86 °C
(19a), 103 °C (19b), 85 °C (19c), 87 °C (19d), 90 °C (19e), and 53
°C (19f). Standard deviation of ratios = 0.01−0.03.
clearly sterically disfavored in the product forming step. (2) As
the ring-closure reactions should have different activation
barriers (ΔΔH⧧ ≠ 0), the constant value may rather be
attributed to dynamic effects.
To shed some light onto the various pathways, the energetics
of the cyclization was calculated along the two possible
pathways, stepwise and concerted, using the DFT method.
In the stepwise pathway of 19a, the C2−C6 cyclization
transition state, 19aa⧧, is located at 16.27 kcal mol−1 and thus
lies 3.97 kcal mol−1 above the open-shell singlet diradical
intermediate 19ab (Figure 1). In the next step, the diradical
may close in an intramolecular fashion either at the more
hindered ortho-position with its nearby methyl group or less
hindered ortho-position of the m-tolyl ring. The transition states
corresponding to the more (19ad⧧) and less (19ac⧧) hindered
ortho attack lie only 0.88 and 0.51 kcal mol−1, respectively,
above the σ,π-diradical intermediate 19ab. Thus, when
reactants pass from the highly energetic initial transition state
into the shallow minima of 19ab on the potential energy
hypersurface, reaction dynamics may affect the selectivity due
to the shallow minimum. Product formation is now a result of
direct continuation of the trajectories that pass through the
initial transition state.9a,43 Molecules possessing enough energy
along the initial vibration in the C2−C6 transition state will
overcome the follow-up DA TS before IVR takes place, thus
furnishing a product under nonstatistical dynamic conditions.
In 19a, the distance between C-7 and ortho and ortho′ carbon
centers is 4.180 and 5.568 Å, respectively, which changes to
3.689 and 4.333 at the C2−C6 TS as well as to 3.359 and 3.399
Å for the diradical intermediate 19ab, respectively. It is seen
Experimentally, replacement of the TMS by a POPh2 group
results in a preference for ortho′ attack and a constant product
ratio at 50−90 °C. For the POPh2-substituted enyne−allene,
we compute the two singlet diradical intermediates 19fb and
19fb1,44 but only one preceding C2−C6 TS. This TS is
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Figure 1. Reaction profiles for the thermal cyclization of 19a,b,c,f in the gas phase at the (BS)-(U)/BLYP/6-31G(d) level. Bond distances (Å) and
relative free energies after thermal correction are shown in second and third brackets, respectively. Numerical values in black, red, blue, and green
correspond to 19a,b,c,f.
Figure 2. Three-dimensional presentation of the stationary points (C2−C6 transition states and intermediates) of 19a−c,f showing distances in Å.
characterized by the more hindered ortho carbon center being
closer to the C-7 atom. In contrast, the singlet diradical 19fb
with the less hindered ortho′ carbon center being closer to C-7
is 0.65 kcal mol−1 lower in energy than 19fb1 that has the more
hindered ortho carbon center closer to the C-7 atom. This
finding suggests that there is a bifurcation and that the C2−C6
TS serves both intermediates. The computed relative energies
of both intermediates are in good accordance with the
experimentally detected product ratio. Moreover, as the
POPh2 unit is a radical stabilizing group, the C2−C6 TS to
diradicals 19fb and 19fb1 is significantly lower than the TS of
the concerted route. As a result, the reaction proceeds
exclusively via the stepwise pathway, showing no temperature
dependence even at higher temperatures.
Stability and Character of Diradical. The DFT
calculations show that the α-SOMO representing the σ radical
is located at the aryl group of the alkyne terminus and the β-
SOMO representing the π-radical is located at the fulvenyl
moiety including the probing aryl ring. Because the energy
difference between the singlet and triplet diradical states of
19ab amounts to only 0.25 kcal mol−1 we see an almost 50%
spin-contaminated state. The easiness of spin contamination
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Figure 3. Two singly occupied molecular orbitals and spin density of 19ab (singlet).
tolylethynyl)benzaldehyde (23c),17d 2-(4-anisylethynyl)benzaldehyde-
(23d),47 and 3-(trimethylsilyl)-1-[2-(2-phenylethynyl)phenyl]prop-2-
yn-1-ol (24a)48 were synthesized according to reported procedures.
2-((2,6-Dimethylphenyl)ethynyl)benzaldehyde (23e). 2-
Ethynyl-1,3-dimethylbenzene (800 mg, 6.14 mmol) was added to a
mixture of 2-iodobenzaldehyde (22) (713 mg, 3.07 mmol), CuI (41.0
mg, 215 μmol), [Pd(PPh3)2]Cl2 (60.0 mg, 86.2 μmol), and 2 mL of
triethylamine. After being stirred for 16 h at room temperature, the
reaction mixture was quenched with saturated aqueous ammonium
chloride solution. The aqueous layer was washed three times with n-
pentane. The combined organic layers were dried over sodium sulfate
and evaporated under reduced pressure. After purification by flash
column chromatography (silica gel, n-hexane/ethyl acetate = 98:2, Rf =
0.28) compound 19e (651 mg, 91%) was isolated as a white solid: mp
can be visualized from the spin density diagram in Figure 3.
The amount of spin contamination of 19ab was checked in
solvents of different polarity, such as acetonitrile, acetone,
dichloromethane and toluene, using PCM model as described
by Houk et al..45 The almost constant spin value of 1.029 in
solvents of different polarity suggests that the diradical under
investigation is nonpolar.
CONCLUSIONS
■
Herein we present examples in which the changeover between
temperature-dependent and -independent product ratios in the
thermal Schmittel (C2−C6)/DA cyclization of various sub-
stituted enyne-allenes occurs as a function of an remote
electronic substituent. The scenario is being investigated
experimentally and computationally with DFT. The cyclization
process occurs through a nonpolarized σ,π-diradical inter-
mediate for enyne allenes containing an aryl group at the alkyne
terminus. In most cases, i.e. 19a−c, nonstatistical dynamics
explains the selectivity of product formation and the constant
product ratio at low temperature (50 to 70 or 80 °C). At
medium temperature range (70 or 80 °C to 120 or 130 °C), the
product ratios are not constant any more and the selectivity of
product formation decreases, because of increasing amounts of
product molecules taking the classical concerted pathway.
̃
= 59 °C; IR (KBr) ν 3061, 2858, 2823, 2388, 2279, 1699, 1618, 1595,
1
1550, 1454, 1388, 1330, 1162, 813, 772 cm−1; H NMR (400 MHz,
C6D6) δ 2.36 (s, 6H), 6.82−6.86 (m, 3H), 6.92−6.97 (m, 2H), 7.31
(ddd, J = 7.8, 1.2, 0.4 Hz, 1H), 7.88 (ddd, J = 7.8, 1.2, 0.4 Hz, 1H),
10.77 (d, J = 0.7 Hz, 1H) ppm; 13C NMR (100 MHz, C6D6) δ 21.5,
94.2, 94.8, 123.0, 127.5, 127.5, 127.9, 128.8, 129.1, 133.6, 133.7, 136.6,
141.2, 190.7 ppm. Anal. Calcd for C17H14O: C, 87.15; H, 6.02. Found:
C, 87.11; H, 6.00.
General Procedure for Preparation of Propargyl Alcohol
(24a−e). A solution of ethylmagnesium bromide (6.98 mmol),
prepared from ethyl bromide (0.520 mL, 6.98 mmol) and magnesium
turnings (167 mg, 6.98 mmol) in 50 mL of dry tetrahydrofuran was
added to a solution of the monosubstituted acetylene (7.68 mmol) in
20 mL of dry tetrahydrofuran under nitrogen atmosphere at 0 °C. The
resulting reaction mixture was stirred at room temperature for 15 min
to exchange the Grignard with acetylene followed by the addition of a
solution of aldehyde (6.28 mmol) in dry tetrahydrofuran (10 mL).
After being stirred for another 15 min at room temperature, the
reaction mixture was quenched with water. The aqueous layer was
extracted three times with diethyl ether. The combined organic layers
were dried over MgSO4 and concentrated to furnish the crude
product. The crude products were then subjected for flash column
chromatography on silica gel to yield the pure products.
EXPERIMENTAL SECTION
■
General Methods. All commercially available reagents and NMR
solvents were used without further purification. Reactions were carried
out under nitrogen atmosphere using freshly distilled, anhydrous
solvents. Diethyl ether was first dried over calcium hydride and then
distilled over sodium. Tetrahydrofuran (THF) and toluene were
distilled over potassium. Triethylamine (NEt3) and dichloromethane
(DCM) were distilled over calcium hydride. All reactions were
followed by thin-layer chromatography (TLC) on silica gel plates
(60F-254). Silica gel (0.035−0.070 mm) was used for column
chromatography. Preparative TLC was carried out by using silica gel
1-(2-((4-Fluorophenyl)ethynyl)phenyl)-3-(trimethylsilyl)-
prop-2-yn-1-ol (24b). Column chromatography, n-hexane/ethyl
acetate (17:3), Rf = 0.51, colorless oil: 94% yield (5.90 mmol, 1.90
̃
1
plates (60 F254). H and 13C NMR spectra were recorded on a 400
g); IR (KBr) ν 3541, 3401, 3068, 2960, 2899, 2173, 1597, 1509, 1408,
1
MHz spectrometer using the deuterated solvent as the lock and
residual solvent as internal reference. Chemical shifts (δ) are reported
in ppm. The following abbreviations were utilized to describe NMR
peak patterns: s = singlet, d = doublet, t = triplet td = triplet of
doublets, dd = doublet of doublets, ddd = doublet of doublet of
doublets, m = multiplet, and brs = broad singlet. Coupling constants
refer to proton−proton coupling unless mentioned otherwise. The
numbering of the carbon skeleton of molecular formulas in the
manuscript does not comply with IUPAC nomenclature rules; it is
only used for assignments of the NMR spectra. Compounds 23e,
1232, 1156, 1094, 1038, 982, 847, 760 cm−1. H NMR (400 MHz,
C6D6) δ = 0.09 (s, 9H), 2.26 (d, J = 5.8 Hz, 1H), 6.00 (d, J = 5.8 Hz,
1H), 6.64 (t, J = 8.8 Hz, J H−F = 8.8 Hz, 2H), 6.89 (td, J = 7.8, 1.2 Hz,
1H), 7.01 (td, J = 7.8, 1.2 Hz, 1H), 7.24 (dd, J = 8.8 Hz, JH−F = 5.4 Hz,
2H), 7.41 (dd, J = 7.8, 1.2 Hz, 1H), 7.74 (ddd, J = 7.8, 1.2, 0.4 Hz, 1H)
ppm; 13C NMR (100 MHz, C6D6) δ −0.2, 63.8, 87.2, 90.7, 94.4, 106.0,
115.9 (d, JC−F = 22.2 Hz), 119.5 (d, JC−F = 3.3 Hz), 121.7, 127.0,
128.3, 129.2, 132.5, 133.8 (d, JC−F = 8.1 Hz), 143.2, 163.0 (d, JC−F
=
247.9 Hz) ppm; HRMS-EI (m/z) for C20H19FOSi [M]+ calcd 322.119,
found 322.119.
1
24a−e, 25a−e, and 26 were characterized by IR, H, 13C NMR, and
1-(2-(4-Tolylethynyl)phenyl)-3-(trimethylsilyl)prop-2-yn-1-ol
(24c). Column chromatography, n-hexane/ethyl acetate (9:1), Rf =
HR-MS (linear ion trap analyzer) or elemental analysis, whereas 19a−
1
f, 20a−f, and 21a−f were characterized using H−1H COSY NMR in
̃
0.47, yellow oil: 92% yield (5.78 mmol, 1.84 g); IR (KBr) ν 3544,
addition. 2-Iodobenzaldehyde (22),31 2-(phenylethynyl)benzaldehyde
(23a),32a 2-((4-fluorophenyl)ethynyl)benzaldehyde (23b),46 2-(p-
3420, 3066, 3029, 2960, 2898, 2216, 2173, 1598, 1511, 1250, 1038,
982, 851, 759 cm−1; 1H NMR (400 MHz, C6D6) δ = 0.10 (s, 9H), 1.99
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Article
(s, 3H), 2.18 (d, J = 5.6 Hz, 1H), 6.05 (d, J = 5.6 Hz, 1H), 6.85 (d, J =
8.0 Hz, 2H), 6.88 (td, J = 7.8, 1.2 Hz, 1H), 7.00 (td, J = 7.8, 1.2 Hz,
1H), 7.44 (dd, J = 7.8, 1.2 Hz, 1H, semicovered), 7.46 (d, J = 8.0 Hz,
2H, semicovered), 7.74 (ddd, J = 7.8, 1.2, 0.4 Hz, 1H) ppm; 13C NMR
(100 MHz, C6D6) δ −0.1, 21.3, 63.9, 87.0, 90.5, 95.9, 106.2, 120.6,
122.1, 126.8, 128.2, 128.8, 129.4, 131.9, 132.4, 138.8, 143.1 ppm;
HRMS-EI (m/z) for C21H22OSi [M]+ calcd 318.144, found 318.143.
1-(2-((4-Methoxyphenyl)ethynyl)phenyl)-3-(trimethylsilyl)-
prop-2-yn-1-ol (24d). Column chromatography, n-pentane/di-
chloromethane (4:1), Rf = 0.56, yellow oil: 74% yield (4.65 mmol,
2.49 g); IR (KBr) ν
̃
3069, 2960, 2181, 1743, 1597, 1509, 1369, 1224,
1
1159, 1044, 839, 760 cm−1; H NMR (400 MHz, C6D6) δ = 0.10 (s,
9H), 1.58 (s, 3H), 6.63 (t, J = 8.8 Hz, J H−F = 8.8 Hz, 2H), 7.91 (td, J =
7.8, 1.2 Hz, 1H), 7.04 (td, J = 7.8, 1.2 Hz, 1H), 7.40−7.45 (m, 3H),
7.48 (s, 1H), 8.02 (dd, J = 7.8, 1.2 Hz, 1H) ppm; 13C NMR (100
MHz, C6D6) δ −0.3, 20.6, 64.4, 86.6, 92.6, 94.7, 102.2, 115.9 (d, JC−F
=
21.8 Hz), 119.3 (d, JC−F = 3.6 Hz), 123.3, 128.6, 128.9, 129.2, 132.5,
134.1 (d, JC−F = 8.4 Hz), 138.9, 163.0 (d, JC−F = 248.3 Hz), 169.1
ppm; HRMS-EI (m/z) for C22H21FO2Si [M]+ calcd 364.129, found
364.130.
̃
1.55 g); IR (KBr) ν 3442, 3068, 2959, 2899, 2838, 2215, 2172, 1607,
1-(2-(4-Tolylethynyl)phenyl)-3-(trimethylsilyl)prop-2-ynyl
Acetate (25c). Column chromatography, n-hexane/ethyl acetate
(20:1), Rf = 0.54, yellow oil: 92% yield (6.56 mmol, 2.36 g); IR (KBr)
1511, 1250, 1175, 1033, 844, 760 cm−1; 1H NMR (400 MHz, C6D6) δ
= 0.11 (s, 9H), 2.26 (d, J = 5.9 Hz, 1H), 3.18 (s, 3H), 6.08 (d, J = 5.9
Hz, 1H), 6.64 (d, J = 8.8 Hz, 2H), 6.89 (td, J = 7.8, 1.2 Hz, 1H), 7.00
(td, J = 7.8, 1.2 Hz, 1H), 7.45−7.48 (m, 3H), 7.75 (ddd, J = 7.8, 1.2,
0.4 Hz, 1H) ppm; 13C NMR (100 MHz, C6D6) δ −0.1, 54.7, 64.0,
86.3, 90.5, 95.8, 106.2, 114.4, 115.6, 122.3, 126.9, 128.2, 128.7, 132.3,
133.4, 143.0, 160.3 ppm. Anal. Calcd for C21H22O2Si: C, 75.41; H
6.63. Found: C, 75.18; H 6.66.
̃
ν 3052, 2960, 2849, 2216, 2180, 1745, 1511, 1368, 1251, 1222, 1044,
845, 760 cm−1; 1H NMR (400 MHz, C6D6) δ = 0.11 (s, 9H), 1.60 (s,
3H), 1.96 (s, 3H), 6.84 (d, J = 8.0 Hz, 2H), 6.90 (td, J = 7.8, 1.2 Hz,
1H), 7.03 (td, J = 7.8, 1.2 Hz, 1H), 7.46 (dd, J = 7.8, 1.2 Hz, 1H), 7.56
(s, 1H), 7.62 (d, J = 8.0 Hz, 2H), 8.04 (dd, J = 7.8, 1.2 Hz, 1H) ppm;
13C NMR (100 MHz, C6D6) δ −0.3, 20.3, 21.3, 64.6, 86.4, 92.5, 96.2,
1-(2-((2,6-Dimethylphenyl)ethynyl)phenyl)-3-(trimethyl-
silyl)prop-2-yn-1-ol (24e). Column chromatography, n-pentane/
dichloromethane (4:1), Rf = 0.64, yellow oil: 47% yield (2.95 mmol,
102.4, 120.4, 123.8, 128.5, 128.6, 129.1, 129.5, 132.1, 132.5, 138.8
(2C), 169.1 ppm; HRMS-EI (m/z) for C23H24O2Si [M]+ calcd
360.155, found 360.154.
1-(2-((4-Methoxyphenyl)ethynyl)phenyl)-3-(trimethylsilyl)-
prop-2-ynyl Acetate (25d). Column chromatography, n-pentane/
dichloromethane (5:1), Rf = 0.58, colorless oil: 90% yield (6.42 mmol,
̃
0.980 g); IR (KBr) ν 3540, 3442, 3092, 2960, 2918, 2893, 2215, 2172,
1
1607, 1511, 1250, 1175, 1033, 844, 760 cm−1; H NMR (400 MHz,
C6D6) δ = 0.07 (s, 9H), 2.03−2.05 (m, 1H), 2.50 (s, 6H), 6.07 (d, J =
5.5 Hz, 1H), 6.90 (d, J = 7.6 Hz, 2H, semicovered), 6.91 (td, J = 7.8,
1.2 Hz, 1H, semicovered), 6.95−6.99 (m, 1H), 7.03 (td, J = 7.8, 1.2
Hz, 1H), 7.42 (dd, J = 7.8, 1.2 Hz, 1H), 7.80 (ddd, J = 7.8, 1.2, 0.6 Hz,
1H) ppm; 13C NMR (100 MHz, C6D6) δ −0.2, 21.4, 63.8, 90.7, 93.3,
95.7, 106.1, 122.4, 123.3, 126.7, 127.2, 128.2, 128.4, 128.9, 132.7,
140.7, 142.6 ppm. Anal. Calcd for C22H24OSi: C, 79.47; H, 7.28.
Found: C, 79.40; H, 7.31.
̃
2.42 g); IR (KBr) ν 3070, 2960, 2838, 2215, 2180, 1743, 1607, 1513,
1289, 1251, 1223, 1035, 845, 761 cm−1; 1H NMR (400 MHz, C6D6) δ
= 0.11 (s, 9H), 1.61 (s, 3H), 3.16 (s, 3H), 6.61 (d, J = 8.8 Hz, 2H),
6.93 (td, J = 7.8, 1.2 Hz, 1H), 7.04 (td, J = 7.8, 1.2 Hz, 1H), 7.48 (dd, J
= 7.8, 1.2 Hz, 1H), 7.57 (s, 1H), 7.63 (d, J = 8.8 Hz, 2H), 8.06 (dd, J =
7.8, 1.2 Hz, 1H) ppm; 13C NMR (100 MHz, C6D6) δ −0.3, 20.4, 54.7,
64.6, 85.7, 92.5, 96.2, 102.4, 114.5, 115.4, 124.1, 128.3, 128.5, 129.1,
132.3, 133.7, 138.6, 160.4, 169.2 ppm. Anal. Calcd for C23H24O3Si: C,
73.37; H, 6.42. Found: C, 73.44; H, 6.20.
1-(2-(Phenylethynyl)phenyl)-3-m-tolylprop-2-yn-1-ol (26).
Column chromatography, n-hexane/ethyl acetate (3:2), Rf = 0.75,
̃
viscous yellow oil: 88% yield (5.53 mmol, 1.78 g); IR (KBr) ν 3391,
1-(2-((2,6-Dimethylphenyl)ethynyl)phenyl)-3-(trimethyl-
silyl)prop-2-ynyl acetate (25e). Column chromatography, n-
pentane/dichloromethane (5:1), Rf = 0.52, colorless oil: 94% yield
3059, 3032, 2920, 2860, 2226, 1600, 1493, 1448, 1378, 1270, 1094,
1
1033, 998, 972, 908, 785, 757, 733; H NMR (400 MHz, C6D6) δ =
̃
(6.70 mmol, 2.51 g); IR (KBr) ν 3072, 2960, 2852, 2179, 1749, 1487,
1.89 (s, 3H), 2.39 (brs, 1H), 6.27 (d, J = 5.5 Hz, 1H), 6.77 (d, J = 7.6
Hz, 1H), 6.87 (t, J = 7.6 Hz, 1H), 6.92 (td, J = 7.8, 1.3 Hz, 1H), 6.96−
7.00 (m, 3H), 7.05 (td, J = 7.8, 1.3 Hz, 1H), 7.18 (s, 1H), 7.24 (d, J =
7.6 Hz, 1H), 7.47−7.51 (m, 3H), 7.81 (d, J = 7.8 Hz, 1H) ppm; 13C
NMR (100 MHz, C6D6) δ 20.6, 63.9, 86.6, 87.6, 89.5, 95.6, 121.9,
123.2, 123.6, 127.0, 128.2, 128.4, 128.6(2C), 129.1, 129.1, 129.4,
131.9, 132.6, 132.8, 138.0, 143.6 ppm; HRMS-EI (m/z) for C24H18O
[M]+ calcd 322.136, found 322.133.
General Procedure for the Preparation of Propargyl
Acetates 25a−e. 4-(Dimethylamino)pyridine (174 mg, 1.43
mmol), acetic anhydride (0.840 mL, 8.91 mmol), and dry triethyl-
amine (2 mL) were added to a stirred solution of propargyl alcohol
(7.13 mmol) in dry dichloromethane. After being stirred for 30 min at
room temperature, the reaction mixture was quenched and washed
with saturated NaHCO3 solution. The aqueous layer was extracted
with dichloromethane three times. The combined organic layers were
dried over MgSO4 and concentrated under reduced pressure to
produce the crude product. The resulting crude products were
subsequently purified by flash silica gel column chromatography.
1-(2-(Phenylethynyl)phenyl)-3-(trimethylsilyl)prop-2-ynyl
acetate (25a). Column chromatography, n-hexane/ethyl acetate
(9:1), Rf = 0.54, dark yellow oil: 83% yield (5.92 mmol, 2.05 g); IR
1
1369, 1222, 1041, 957, 845, 761 cm−1; H NMR (400 MHz, C6D6) δ
= 0.05 (s, 9H), 1.55 (s, 3H), 2.57 (s, 6H), 6.88 (d, J = 7.8 Hz, 2H),
6.92 (td, J = 7.8, 1.2 Hz, 1H), 6.93−6.97 (m, 1H), 7.04 (td, J = 7.8, 1.2
Hz, 1H), 7.43 (dd, J = 7.8, 1.2 Hz, 1H), 7.46 (s, 1H), 7.92 (d, J = 7.8
Hz, 1H) ppm; 13C NMR (100 MHz, C6D6) δ −0.3, 20.3, 21.4, 64.8,
92.2, 93.6, 95.1, 102.6, 123.2, 123.6, 127.2 (2C), 128.5, 128.7, 128.9,
132.9, 138.9, 140.8, 168.8 ppm. Anal. Calcd for C24H26O2Si: C, 76.96;
H, 7.00. Found: C, 76.64; H 6.87.
General Procedure for Preparation of Enyne−Allenes 19a−
e. m-Tolylmagnesium bromide solution (23.4 mmol, prepared by
standard Grignard reaction) in dry tetrahydrofuran (50 mL) was
added dropwise to a solution of zinc chloride (2.22 g, 16.4 mmol) in
20 mL of dry diethyl ether at room temperature under nitrogen
atmosphere. The reaction mixture was stirred for 30 min to complete
formation of the diarylzinc. Subsequently, the reaction mixture was
cooled down to −60 °C followed by addition of tetrakis-
(triphenylphosphine)palladium(0) (0.674 g, 0.584 mmol) in 10 mL
of dry tetrahydrofuran. After stirring for 15 min at the same
temperature, propargyl acetate (5.84 mmol) in dry tetrahydrofuran
(10 mL) was added to the mixture. Finally, the solution was allowed to
stir for 14 h to complete the reaction. The reaction mixture was
quenched with saturated ammonium chloride solution. The aqueous
layer was extracted three times with diethyl ether. The combined
organic layers were dried over Na2SO4 and concentrated under
reduced pressure. Flash silica gel column chromatography was utilized
to obtain the pure compound.
̃
(KBr) ν 3064, 2961, 2900, 2217, 2181, 1744, 1495, 1370, 1225, 1045,
1
958, 911, 847 cm−1; H NMR (400 MHz, C6D6) δ 0.10 (s, 9H), 1.59
(s, 3H), 6.91 (td, J = 7.8, 1.2 Hz, 1H), 6.94−7.05 (m, 4H), 7.43 (dd, J
= 7.8, 1.2 Hz, 1H), 7.50 (s, 1H), 7.65 (dd, J = 8.2, 1.5 Hz, 2H), 8.01
(dd, J = 7.8, 1.2 Hz, 1H) ppm; 13C NMR (100 MHz, C6D6) δ −0.3,
20.3, 64.5, 86.9, 92.5, 95.6, 102.3, 123.3, 123.5, 128.5, 128.7, 128.8
(2C), 129.1, 132.1, 132.6, 138.9, 169.1 ppm; HRMS-EI (m/z) for
C22H22O2Si [M]+ calcd 346.139, found 346.139.
Trimethyl(3-(2-(phenylethynyl)phenyl)-1-m-tolylpropa-1,2-
dienyl)silane (19a). Column chromatography, n-hexane, Rf = 0.55,
̃
yellow oil: 91% yield (5.31 mmol, 2.01 g); IR (KBr) ν 3056, 3028,
2958, 2113, 1911, 1597, 1489, 1445, 1252, 1118, 1084, 949, 844, 756
cm−1. 1H NMR (400 MHz, C6D6) δ 0.29 (s, 9H, 9-H), 2.08 (s, 3H, 14-
H), 6.86 (td, J = 7.8, 1.2 Hz, 1H, 5-H), 6.89 (d, J = 7.8 Hz, 1H, 11-H),
1-(2-((4-Fluorophenyl)ethynyl)phenyl)-3-(trimethylsilyl)-
prop-2-ynyl Acetate (25b). Column chromatography, n-hexane/
ethyl acetate (20:1), Rf = 0.47, colorless oil: 96% yield (6.84 mmol,
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6.96−7.00 (m, 3H, 1, 2-H), 7.01 (td, J = 7.8, 1.2 Hz, 1H, 6-H), 7.11 (t,
J = 7.8 Hz, 1H, 12-H), 7.28 (s, 1H, 8-H), 7.29 (d, J = 7.8 Hz, 1H, 13-
H), 7.37 (s, 1H, 10-H), 7.42 (dd, J = 6.8, 2.4 Hz, 2H, 3-H), 7.52 (ddd,
J = 7.8, 1.2, 0.4 Hz, 1H, 4-H), 7.65 (dd, J = 7.8, 1.2 Hz, 1H, 7-H) ppm;
13C NMR (100 MHz, C6D6) δ −0.1, 21.4, 88.5, 90.0, 95.0, 105.2,
121.2, 123.7, 125.4, 126.0, 126.5, 127.9, 128.4, 128.6, 128.9, 129.0,
129.1, 131.9, 133.1, 136.6, 137.0, 138.5, 210.4 ppm. Anal. Calcd for
C27H26Si: C, 85.66; H 6.92. Found: C, 85.73; H 6.84.
1H, 12-H), 7.32 (d, J = 8.0 Hz, 1H, 13-H), 7.36 (s, 1H, 8-H), 7.38−
7.40 (m, 3H, 3,10-H), 7.58 (ddd, J = 7.8, 1.2, 0.4 Hz, 1H, 4-H), 7.69
(dd, J = 7.8, 1.2 Hz, 1H, 7-H) ppm; 13C NMR (100 MHz, C6D6) δ
−0.1, 21.4, 54.7, 87.2, 90.2, 95.2, 105.2, 114.4, 115.9, 121.7, 125.5,
126.0, 126.5, 127.9, 128.7, 128.9, 129.1, 133.0, 133.4, 136.7, 136.8,
138.5, 160.2, 210.4 ppm. Anal. Calcd for C28H28OSi: C, 82.30; H, 6.91.
Found: C, 82.05; H, 6.95.
[3-[2-(2,6-Dimethylphenylethynyl)phenyl]-1-m-tolylpropa-
1,2-dienyl]trimethylsilane (19e). Column chromatography, n-
pentane/dichloromethane (49:1), Rf = 0.51, yellow oil: 80% yield
(3-(2-((p-Fluorophenyl)ethynyl)phenyl)-1-m-tolylpropa-1,2-
dienyl)trimethylsilane (19b). Column chromatography, n-hexane/
ethyl acetate (49:1), Rf = 0.61, yellow oil: 72% yield (4.20 mmol, 1.67
̃
(4.65 mmol, 1.89 g); IR (KBr) ν 3064, 2960, 2208, 2179, 1931, 1487,
̃
g); IR (KBr) ν 3058, 2956, 2924, 2855, 2215, 2179, 1911, 1601, 1508,
1230, 1155, 836, 755 cm−1; 1H NMR (400 MHz, C6D6) δ 0.29 (s, 9H,
8-H), 2.06 (s, 3H, 13-H), 6.58 (t, J = 8.8 Hz, JH−F = 8.8 Hz, 2H, 1-H),
6.85 (td, J = 7.8, 1.2 Hz, 1H, 4-H), 6.89 (d, J = 7.8 Hz, 1H, 10-H), 7.00
(td, J = 7.8, 1.2, Hz, 1H, 5-H), 7.10 (t, J = 7.8 Hz, 1H, 11-H), 7.12 (dd,
J = 8.8, 3.6 Hz, 2H, 2-H), 7.25 (s, 1H, 7-H), 7.31 (d, J = 7.8 Hz, 1H,
12-H), 7.39 (s, 1H, 9-H), 7.52 (dd, J = 7.8, 1.2 Hz, 1H, 3-H), 7.68 (dd,
J = 7.8, 1.2 Hz, 1H, 6-H) ppm; 13C NMR (100 MHz, C6D6) δ −0.1,
1369, 1222, 1045, 957, 845, 760 cm−1; H NMR (400 MHz, C6D6) δ
0.28 (s, 9H, 9-H), 2.07 (s, 3H, 14-H), 2.46 (s, 6H, 3-H), 6.86−6.90
(m, 4H, 2,5,11-H), 6.94−6.97 (m, 1H, 1-H), 7.01 (td, J = 7.8, 1.2 Hz,
1H, 6-H), 7.10 (t, J = 7.8 Hz, 1H, 12-H), 7.31 (d, J = 7.8 Hz, 1H, 13-
H), 7.33 (s, 1H, 8-H), 7.38 (s, 1H, 10-H), 7.52 (dd, J = 7.8, 1.2 Hz,
1H, 4-H), 7.70 (dd, J = 7.8, 1.2 Hz, 1H, 7-H) ppm; 13C NMR (100
MHz, C6D6) δ 0.0, 21.5, 21.6, 90.2, 92.8, 97.0, 105.3, 121.9, 123.7,
125.6, 126.2, 126.7, 127.2, 128.1, 128.3, 128.9, 129.1, 129.2, 133.2,
136.7, 136.9, 138.7, 140.6, 210.4 ppm. Anal. Calcd for C29H30Si: C,
85.66; H, 7.44. Found: C, 85.32; H 7.45.
1
21.4, 88.0, 89.9, 93.8, 105.3, 115.7 (d, JC−F = 22.1 Hz), 119.7 (d, JC−F
=
3.4 Hz), 121.0, 125.4, 126.0, 126.5, 128.9, 129.0, 129.0, 133.0 (2C),
133.7 (d, JC−F = 8.2 Hz), 136.6, 137.0, 138.6, 162.8 (d, JC−F = 247.7
Hz), 210.2 ppm; HRMS-EI (m/z) for C27H25FSi [M]+ calcd 396.171,
found 396.171.
Trimethyl(1-m-tolyl-3-(2-(p-tolylethynyl)phenyl)propa-1,2-
dienyl)silane (19c). Column chromatography, n-hexane/ethyl
acetate (49:1), Rf = 0.48, yellow oil: 71% yield (4.15 mmol, 1.63 g);
1-(Diphenylphosphinoyl)-1-(m-tolyl)-3-(2-phenylethynyl-
phenyl)propa-1,2-diene (19f). Chlorodiphenylphosphine (452 mg,
2.05 mmol) in 5 mL of tetrahydrofuran was added to a solution of 26
(550 mg, 1.70 mmol) and NEt3 (207 mg, 2.05 mmol) in 25 mL of
tetrahydrofuran over 15 min at −78 °C. The reaction mixture was
stirred for 1 h at the same temperature and then allowed to warm to 0
°C slowly. After being stirred for 1 h, the mixture was hydrolyzed with
water and extracted with ethyl acetate three times. The combined
organic layers were dried over magnesium sulfate and concentrated
under reduced pressure. After purification by flash column
chromatography using cold eluant (silica gel, n-hexane/ethyl acetate
= 3:2, Rf = 0.25), 19f was isolated in 46% yield (398 mg, 786 μmol) as
an orange flurry solid. This thermolabile compound (Tonset = 53 °C)
IR (KBr) ν 3052, 2959, 2214, 1910, 1600, 1509, 1481, 1250, 944, 842,
̃
1
754 cm−1; H NMR (400 MHz, C6D6) δ 0.28 (s, 9H, 9-H), 1.97 (s,
3H, 1-H), 2.07 (s, 3H, 14-H), 6.80 (d, J = 8.0 Hz, 2H, 2-H), 6.85 (ddd,
J = 8.0, 7.8, 1.0 Hz, 1H, 5-H), 6.89 (d, J = 7.8 Hz, 1H, 11-H), 7.00
(ddd, J = 8.0, 7.8, 1.0 Hz, 1H, 6-H), 7.11 (t, J = 7.8 Hz, 1H, 12-H),
7.31 (d, J = 7.8 Hz, 1H, 13-H), 7.34 (s, 1H, 8-H), 7.38−7.40 (m, 3H,
3,10-H), 7.56 (dd, J = 7.8, 1.0 Hz, 1H, 4-H), 7.68 (dd, J = 8.0, 1.0 Hz,
1H, 7-H) ppm; 13C NMR (100 MHz, C6D6) δ −0.1, 21.3, 21.4, 87.9,
90.1, 95.2, 105.2, 120.9, 121.5, 125.5, 126.0, 126.5, 127.9, 128.8, 128.9,
129.1, 129.4, 131.9, 133.1, 136.7, 136.9, 138.4, 138.5, 210.4 ppm;
HRMS-EI (m/z) for C28H28Si [M]+ calcd 392.196, found 392.196.
̃
was used for thermolysis immediately: IR (KBr) ν 3056, 2920, 2862,
2279, 1924, 1599, 1493, 1437, 1196, 1117, 756, 694 cm−1; H NMR
(400 MHz, C6D6) δ 1.97 (s, 3H, 13-H), 6.71−7.03 (m, 14H, Ar-H),
7.08 (d, J = 10.5 Hz, 1H, 8-H), 7.36−7.39 (m, 4H, Ar-H), 7.95−8.00
(m, 5H, 7, 14-H) ppm; 13C NMR (100 MHz, C6D6) δ 21.3, 87.9, 95.2,
96.7 (d, JC−P = 12.0 Hz), 106.9 (d, JC−P = 96 Hz), 126.3 (d, JC−P = 4.3
Hz), 127.8, 128.5, 128.7 (d, JC−P = 4.3 Hz), 128.9, 129.1, 129.2, 129.5,
129.6, 131.7 (d, JC−P = 7.1 Hz), 131.8 (d, JC−P = 7.1 Hz), 131.9, 132.0
(d, JC−P = 9.4 Hz), 132.1 (d, JC−P = 9.4 Hz), 132.3 (d, JC−P = 9.4 Hz),
132.9, 133.5 (d, JC−P = 105 Hz), 134.0 (d, JC−P = 105 Hz), 134.3 (d,
JC−P = 7.1 Hz), 138.7, 213.5 (d, JC−P = 6.0 Hz) ppm. Further
characterization was not possible due to the thermal lability.
1
[3-[2-(p-Anisylethynyl)phenyl]-1-m-tolylpropa-1,2-dienyl]-
trimethylsilane (19d). Column chromatography, n-pentane/di-
chloromethane (49:1), Rf = 0.38, yellow oil: 98% yield (5.72 mmol,
̃
2.34 g); IR (KBr) ν 3061, 2956, 2840, 2280, 2213, 1911, 1606, 1510,
1288, 1249, 1174, 1033, 841, 756 cm−1; 1H NMR (400 MHz, C6D6) δ
0.29 (s, 9H, 9-H), 2.07 (s, 3H, 14-H), 3.16 (s, 3H, 1-H), 6.58 (d, J =
8.8 Hz, 2H, 2-H), 6.86 (td, J = 7.8, 1.2 Hz, 1H, 5-H), 6.89 (d, J = 8.0
Hz, 1H, 11-H), 7.01 (td, J = 7.8, 1.2 Hz, 1H, 6-H), 7.11 (t, J = 8.0 Hz,
1459
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Article
General Procedure for Thermolysis of Enyne−Allenes 19a−f
Generating Cyclized Products 20a−f and 21a−f. A solution of
100 mg of enyne−allene in dry and degassed toluene was taken in a
sealed tube and heated for 10−15 min at 140 °C to complete the
cyclization. After removal of toluene under reduced pressure and
Thermolysis of Trimethyl(3-(2-(phenylethynyl)phenyl)-1-m-
tolylpropa-1,2-dienyl)silane (19a) Yielding 20a and 21a.
Preparative TLC, n-pentane, Rf = 0.16, yellow oil: 88% yield (232
̃
μmol, 88.0 mg); IR (KBr) ν 3059, 3027, 2952, 2931, 2901, 2866, 1601,
1
1492, 1464, 1443, 1397, 1264, 1251, 904, 840, 762, 702 cm−1; H
NMR (400 MHz, C6D6) δ 0.57 (s, 9H, 9-H), 0.60 (s, 9H, 22-H), 2.13
(s, 3H, 13-H), 2.36 (s, 3H, 24-H), 3.99 (s, 2H, 8-H), 4.02 (s, 2H, 21-
H), 6.26 (d, J = 8.0 Hz, 1H, 7-H), 6.75 (d, J = 7.8 Hz, 1H, 20-H), 6.93
(td, J = 8.0, 1.0 Hz, 1H, 6-H, semicovered), 6.96 (td, J = 7.8, 1.0 Hz,
1H, 19-H, semicovered), 7.06 (td, J = 7.8, 1.0 Hz, 1H, 5-H,
semicovered), 7.07−7.10 (m, 3H, 11, 18, 25-H), 7.17−7.40 (m, 13H,
1, 2, 3, 4, 12, 14, 15, 16, 17-H), 7.76 (d, J = 8.8 Hz, 1H, 26-H), 8.26 (s,
1H, 23-H), 8.37 (d, J = 8.5 Hz, 1H, 10-H) ppm; 13C NMR (100 MHz,
C6D6) δ 3.37, 3.51, 21.9, 26.8, 39.7, 39.8, 121.3, 124.1, 124.7, 124.8,
125.5, 126.7, 126.8, 127.1, 127.2 (2C), 127.9, 128.0, 128.2, 128.9 (2C),
129.5, 129.8, 130.4, 130.8 (2C), 130.9, 131.6, 131.7, 132.7, 134.4,
135.8, 136.7, 136.8, 137.2, 137.5, 138.6, 138.9, 140.2, 141.5, 141.9,
143.9, 144.5, 144.6, 147.9, 148.9 ppm; HRMS-EI (m/z) for C27H26Si
[M]+ calcd 378.180, found 378.180.
purification by chromatography (preparative TLC, silica gel 60 F254
)
the two constitutional isomers were purified and characterized either
in mixture or in separated form.
(6-Methyl-5-phenyl-11H-benzo[b]fluoren-10-yl)diphenyl-
phosphine Oxide (20f). Preparative TLC, n-pentane/ethylacetate
(3:2), Rf = 0.46, white semisolid: 36% yield (71 μmol, 36.0 mg); IR
̃
(KBr) ν 3056, 2963, 2930, 2855, 2359, 2338, 1732, 1437, 1365, 1261,
1
1188, 1116, 1100, 953, 800, 759, 744, 722, 697 cm−1; H NMR (400
MHz, CD2Cl2) δ 2.05 (s, 3H, 12-H), 4.03 (s, 2H, 8-H), 5.85 (d, J = 7.6
Hz, 1H, 7-H), 6.88 (td, J = 7.6, 1.0 Hz, 1H, 6-H), 7.07 (t, J = 7.8 Hz,
1H, 10-H), 7.13 (d, J = 7.8 Hz, 1H, 11-H), 7.14 (t, J = 7.6 Hz, 1H, 5-
H), 7.31 (d, J = 7.6 Hz, 1H, 4-H), 7.47−7.52 (m, 6H, 2,14-H), 7.55−
7.63 (m, 5H, 1, 3, 15-H), 7.78 (ddd, JH−P = 15.6 Hz, J = 8.4 Hz, J = 1.3
Hz, 4H, 13-H), 8.29 (d, J = 7.8 Hz, 1H, 9-H) ppm; 13C NMR (100
MHz, CD2Cl2) δ 26.5, 39.2 (d, JC−P = 3.1 Hz), 123.5 (d, JC−P = 102
Hz), 124.3, 124.6, 125.3, 126.5, 127.2 (d, JC−P = 7.3 Hz), 127.6, 128.4,
129.1 (d, JC−P = 12.0 Hz), 129.3, 130.3, 131.9 (d, JC−P = 8.4 Hz),
132.1, 132.2, 132.2 (d, JC−P = 9.5 Hz), 134.4 (d, JC−P = 9.2 Hz), 135.5
(d, JC−P = 102 Hz), 137.3, 139.7 (d, JC−P = 2.9 Hz), 140.0 (d, JC−P
=
12.4 Hz), 140.3, 142.8, 144.8, 149.3 (d, JC−P = 8.3 Hz) ppm; HRMS-EI
(m/z) for C36H27OP [M]+ calcd 506.180, found 506.180.
Thermolysis of (3-(2-((p-Fluorophenyl)ethynyl)phenyl)-1-m-
tolylpropa-1,2-dienyl)trimethylsilane (19b) Yielding 20b and
21b. Preparative TLC, n-pentane, Rf = 0.46, colorless oil: 85% yield
̃
(214 μmol, 85.0 mg); IR (KBr) ν 3044, 2953, 2900, 1603, 1512, 1464,
1252, 1222, 1157, 907, 855, 763 cm−1; H NMR (400 MHz, C6D6) δ
0.56 (s, 9H, 8-H), 0.59 (s, 9H, 20-H), 2.02 (s, 3H, 12-H), 2.37 (s, 3H,
22-H), 3.96 (s, 2H, 7-H), 3.99 (s, 2H, 19-H), 6.22 (d, J = 8.0 Hz, 1H,
6-H), 6.69 (d, J = 7.8 Hz, 1H, 18-H), 6.83 (t, J = 8.8 Hz, JH−F = 8.8 Hz,
2H, 1-H), 6.91−6.96 (m, 3H, 5, 13-H), 6.98 (td, J = 7.8, 0.8 Hz, 1H,
17-H), 7.03−7.13 (m, 8H, 2, 4, 10, 14, 16, 23-H), 7.25−7.30 (m, 3H,
3, 11, 15-H), 7.62 (d, J = 8.0 Hz, 1H, 24-H), 8.26 (s, 1H, 21-H), 8.35
(d, J = 8.0 Hz, 1H, 9-H) ppm; 13C NMR (100 MHz, C6D6) δ 3.3, 3.4,
21.9, 26.9, 39.6, 40.7, 115.6 (d, JC−F = 19.3 Hz) 116.4 (d, JC−F = 19.3
Hz), 123.8, 124.4, 124.7, 124.8 (2C), 124.9, 126.8, 126.9, 127.2, 127.2,
127.3, 127.5, 128.2, 129.9, 131.3, 131.6, 131.8, 132.0 (d, JC−F = 7.7
Hz), 132.3 (d, JC−F = 7.7 Hz), 133.0, 134.5, 134.5, 135.5, 135.8 (d, JC−F
= 3.4 Hz), 136.9, 136.9, 137.4, 138.5, 139.1, 139.6 (d, JC−F = 3.7 Hz),
141.2, 141.6, 144.4, 144.7, 147.9, 148.8, 162.9 (d, JC−F = 244.8 Hz)
163.0 (d, JC−F = 245.3 Hz) ppm; HRMS-EI (m/z) for C27H25FSi [M]+
calcd 396.171, found 396.171.
1
(8-Methyl-5-phenyl-11H-benzo[b]fluoren-10-yl)diphenyl-
phosphine Oxide (21f). Preparative TLC, n-pentane/ethylacetate
(3:2), Rf = 0.43, white semisolid: 41% yield (80.9 μmol, 41.0 mg); IR
̃
(KBr) ν 3058, 2922, 2852, 2360, 2341, 1733, 1620, 1589, 1510, 1437,
1394, 1329, 1184, 1117, 1101, 1071, 1028, 920, 905, 822, 768, 763,
1
729, 697 cm−1; H NMR (400 MHz, CD2Cl2) δ 2.18 (s, 3H, 10-H),
4.05 (d, JH−P = 1.6 Hz, 2H, 8-H), 6.31 (d, J = 7.8 Hz, 1H, 7-H), 6.95
(td, J = 7.8, 0.8 Hz, 1H, 6-H), 7.13−7.18 (m, 2H, 5, 11-H), 7.33 (d, J =
7.8 Hz, 1H, 4-H), 7.40 (dd, J = 7.6, 1.8 Hz, 2H, 3-H), 7.45 (dd, J = 8.3
Hz, JH−P = 1.8 Hz, 1H, 12-H), 7.50 (tdd, J = 8.0 Hz, JH−P = 3.0 Hz, J =
1.2 Hz, 4H, 14-H), 7.57−7.66 (m, 5H, 1,2,15-H), 7.78 (ddd, JH−P
=
15.6 Hz, J = 8.0 Hz, J = 1.3 Hz, 4H, 13-H), 8.05 (d, J = 0.6 Hz, 1H, 9-
H) ppm; 13C NMR (100 MHz, CD2Cl2) δ 21.8, 39.3 (d, JC−P = 2.7
Hz), 122.1 (d, JC−P = 99.2 Hz), 123.7, 124.7, 126.6, 127.0 (d, JC−P
=
5.7 Hz), 127.2, 127.6, 127.7, 128.4, 129.1(d, JC−P = 12.0 Hz), 129.6,
129.9, 131.8 (d, JC−P = 8.9 Hz), 132.1 (d, JC−P = 9.9 Hz), 132.2 (d,
JC−P = 2.8 Hz), 133.4 (d, JC−P = 9.3 Hz), 135.4 (d, JC−P = 101.8 Hz),
135.9, 137.6 (d, JC−P = 11.9 Hz), 138.6 (d, JC−P = 2.6 Hz), 139.1,
139.8, 144.7, 150.2 (d, JC−P = 9.3 Hz) ppm; HRMS-EI (m/z) for
C36H27OP [M]+ calcd 506.180, found 506.180.
Thermolysis of Trimethyl(1-m-tolyl-3-(2-(p-tolylethynyl)-
phenyl)propa-1,2-dienyl)silane (19c) Yielding 20c and 21c.
Preparative TLC, n-pentane, Rf = 0.57, yellow oil: 76% yield (194
1460
dx.doi.org/10.1021/jo302524a | J. Org. Chem. 2013, 78, 1451−1462

The Journal of Organic Chemistry
Article
μmol, 76.0 mg); IR (KBr) ν
̃
3058, 2958, 2155, 1930, 1758, 1598, 1483,
141.5, 141.7, 142.6, 144.4, 144.6, 148.6, 149.3 ppm. Anal. Calcd for
C29H30Si: C, 85.66; H, 7.44. Found: C, 85.36; H, 7.66.
1445, 1249, 843, 758 cm−1; 1H NMR (400 MHz, C6D6) δ 0.57 (s, 9H,
9-H), 0.60 (s, 9H, 22-H), 2.19 (s, 6H, 1, 14-H), 2.23 (s, 3H, 13-H),
2.36 (s, 3H, 24-H), 4.00 (s, 2H, 8-H), 4.02 (s, 2H, 21-H), 6.42 (d, J =
8.0 Hz, 1H, 7-H), 6.90 (d, J = 8.0 Hz, 1H, 20-H), 6.99−7.00 (m, 2H,
11, 17-H), 7.03−7.06 (m, 4H, 3, 16-H), 7.07−7.17 (m, 3H, 4, 6, 12-
H), 7.26−7.34 (m, 8H, 2, 5, 15, 18, 19, 25-H), 7.83 (d, J = 8.0 Hz, 1H,
26-H), 8.26 (s, 1H, 23-H), 8.37 (d, J = 8.0 Hz, 1H, 10-H) ppm; 13C
NMR (100 MHz, C6D6) δ 3.4, 3.5, 21.4, 21.9, 26.9 (2C), 39.7, 39.8,
124.1, 124.6, 124.7, 124.8, 124.8, 126.7, 126.8, 127.1, 127.1, 127.9,
128.1, 129.7 (2C), 129.8, 130.2 (2C), 130.6 (2C), 130.8, 131.9, 132.0,
132.6, 134.3, 135.9, 136.8, 136.8, 137.2, 137.3, 137.3, 137.4, 137.5,
138.6, 139.1, 140.8, 141.6, 142.0, 144.4, 144.6, 148.0, 148.9 ppm;
HRMS-EI (m/z) for C28H28Si [M]+ calcd 392.196, found 392.196.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of all relevant compounds and
computational data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: schmittel@chemie.uni-siegen.de.
Notes
Thermolysis of [3-[2-(p-Anisylethynyl)phenyl]-1-m-tolylpro-
pa-1,2-dienyl]trimethylsilane (19d) Yielding 20d and 21d.
Preparative TLC, n-pentane, Rf = 0.31, yellow oil: 47% yield (115
The authors declare no competing financial interest.
̃
μmol, 47.0 mg); IR (KBr) ν 3424, 2954, 2280, 1609, 1515, 1283, 1245,
ACKNOWLEDGMENTS
1034, 906, 852, 763 cm−1; 1H NMR (400 MHz, C6D6) δ 0.58 (s, 9H,
9-H), 0.61 (s, 9H, 22-H), 2.19 (s, 3H, 13-H), 2.38 (s, 3H, 24-H), 3.34
(s, 3H, 1-H), 3.38 (s, 3H, 14-H), 3.99 (s, 2H, 8-H), 4.01 (s, 2H, 21-
H), 6.47 (d, J = 7.8 Hz, 1H, 7-H), 6.83 (d, J = 8.6 Hz, 2H, 2-H), 6.90−
7.01 (m, 5H, 6, 15, 19, 20-H), 7.07 (td, J = 7.8, 1.0 Hz, 1H, 5-H,
semicovered), 7.09 (td, J = 7.8, 1.0 Hz, 1H, 18-H, semicovered), 7.11−
7.13 (m, 2H, 12, 25-H), 7.21 (d, J = 8.6 Hz, 2H, 3-H), 7.27−7.32 (m,
5H, 4, 11, 16, 17-H), 7.85 (d, J = 8.0 Hz, 1H, 26-H), 8.27 (s, 1H, 23-
H), 8.37 (d, J = 8.0 Hz, 1H, 10-H) ppm; 13C NMR (100 MHz, C6D6)
δ 3.4, 3.5, 22.0, 26.9, 39.7, 39.8, 54.7, 54.8, 114.3, 115.0, 124.1, 124.6,
124.7, 124.7, 124.8, 126.8, 126.9, 127.1 (2C), 127.9, 128.1, 129.8 (2C),
130.8, 131.4, 131.7, 132.1, 132.2, 132.3, 132.6, 134.3, 135.7, 135.8,
136.6 (2C), 137.1, 137.3, 137.5, 138.6, 139.5, 141.6, 142.1, 144.4,
144.6, 148.0, 148.9, 159.7, 159.8 ppm. Anal. Calcd for C28H28OSi: C,
82.30; H, 6.91. Found: C, 82.48; H, 7.15.
■
We are indebted to the Deutsche Forschungsgemeinschaft and
University of Siegen for continued support of our research.
REFERENCES
■
(1) Steinfeld, J. I.; Francisco, J. S.; Hase, W. L. Chemical Kinetics and
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Thermolysis of [3-[2-(2,6-Dimethylphenylethynyl)phenyl]-1-
m-tolylpropa-1,2-dienyl]trimethylsilane (19e) Yielding 20e
and 21e. Preparative TLC, n-pentane, Rf = 0.45, yellow oil: 34%
̃
yield (83.6 μmol, 34.0 mg); IR (KBr) ν 3062, 2951, 2930, 2855, 2361,
1
2342, 2279, 1464, 1251, 904, 870, 840, 764 cm−1; H NMR (400
MHz, C6D6) δ 0.55 (s, 9H, 9-H), 0.58 (s, 9H, 22-H), 1.90 (s, 6H, 3-
H), 1.92 (s, 6H, 16-H), 2.15 (s, 3H, 13-H), 2.36 (s, 3H, 24-H), 4.03 (s,
2H, 8-H), 4.05 (s, 2H, 21-H), 6.36 (d, J = 8.0 Hz, 1H, 7-H), 6.77 (d, J
= 7.8 Hz, 1H, 20-H), 6.97 (t, J = 8.0 Hz, 1H, 6-H, semicovered), 6.99
(t, J = 7.8 Hz, 1H, 19-H, semicovered), 7.06 (d, J = 8.0 Hz, 2H, 2-H,
semicovered), 7.07−7.11 (m, 3H, 5, 18, 25-H), 7.11 (d, J = 8.0 Hz,
1H, 12-H, semicovered), 7.18 (d, J = 8.0 Hz, 2H, 15-H), 7.26−7.32
(m, 5H, 1, 4, 11, 14, 17-H), 7.66 (d, J = 7.8 Hz, 1H, 26-H), 8.28 (s,
1H, 23-H), 8.38 (d, J = 8.0 Hz, 1H, 10-H) ppm; 13C NMR (100 MHz,
C6D6) δ 3.4, 3.5, 20.1, 20.5, 21.9, 25.0, 39.8, 39.9, 122.7, 123.4. 124.7
(2C), 124.8, 126.5, 127.3, 127.4, 127.4, 127.5, 127.6, 127.9, 128.1,
128.4, 128.4, 128.8, 128.6, 129.7, 130.4, 130.8, 131.1, 132.5, 132.6,
134.2, 134.7, 135.2, 136.4, 136.9, 137.3, 137.8, 137.9, 138.6, 140.0,
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