Generation of “Sumanenylidene”: A Ground-State Triplet Carbene on a Curved π-Conjugated Periphery

Article abstract of DOI:10.1002/asia.201801802

We have observed the generation of sumanenylidene (2), a divalent, neutral-carbon species at the benzylic position of sumanene (1). We also clarified both experimentally and theoretically that the ground state of compound 2 was a triplet state and that its singlet–triplet energy gap (ΔE_ST) was similar to that in fluorenylidene. The curved structure of compound 2 led to slightly better spin delocalization over the two adjacent aromatic rings than in planar systems, because of the unpaired spins on the σ and π orbitals. Synthetic application of the carbene precursor, diazosumanene (5), with a variety of thiocarbonyl compounds revealed its utility for the preparation of tetrasubstituted alkene compounds (e.g., that contain a strongly electron-donating unit) that are directly conjugated to the sumanene (1) moiety.

Full text of DOI:10.1002/asia.201801802

CHEMISTRY
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Accepted Article
Title: Generation of “Sumanenylidene”: A Ground-State Triplet
Carbene on a Curved pi-Conjugated Peripher
Authors: Yumi Yakiyama, Yufeng Wang, Sayaka Hatano, Manabu Abe,
and Hidehiro Sakurai
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Generation of “Sumanenylidene”: A Ground-State Triplet Carbene
on a Curved -Conjugated Periphery
Yumi Yakiyama,[1] Yufeng Wang,[1] Sayaka Hatano,[2] Manabu Abe,[2] and Hidehiro Sakurai*[1]
Abstract: We have observed the generation of sumanenylidene (2),
a divalent, neutral carbon species on the benzylic position of
sumanene (1), We have also clarified experimentally and
theoretically that the ground state of 2 was triplet and that its singlet-
triplet energy gap (EST) value was similar to that of fluorenylidene.
The curved structure of 2 yields slightly better spin delocalization
onto the two adjacent aromatic rings than planar systems do
because of the unpaired spins on - and -orbitals. The synthetic
application of the carbene precursor, diazosumanene (5), with
various thiocarbonyl compounds revealed its usefulness to prepare
tetra-substituted alkene compounds (e.g. having a strong electron
donor unit) directly conjugated with the 1 moiety.
we show the preparation of sumanene-based carbene,
sumanenylidene (2), having clarified its structural and electronic
information by zero-field parameters obtained from ESR
measurement, density functional theory (DFT) calculation, and
reactivity against cyclohexane. In addition, the applicability of the
carbene precursor, diazosumanene (5), for Barton–Kellogg
reaction and the details of the resulting olefins were discussed.
Results and Discussion
Preparation of 5, a precursor of 2, was conducted from
sumanenone (3)[9] through the conventional route, including
hydrazination, followed by MnO2 oxidation, as shown in Scheme
1. The successful preparation of 5 was confirmed by IR
measurement. The clear N-N stretching mode was observed at
2045 cm−1, which is almost the same value as that of
diazofluorene (2042 cm−1, see Supporting information).[10] The
stability of 5 in the solution state was low when initiating
decomposition within 1.5 hour, and it was stable under air in the
solid state.
Introduction
Carbene is a divalent, neutral carbon species with two unshared
valence electrons. It works as an effective reaction intermediate
in organic synthesis. The structures and reactivity of carbenes
have been investigated for more than 50 years.[1] Carbenes can
take two possible spin states at the ground state, (i.e., singlet
and triplet), and especially in the case of ground-state triplet
carbenes, strong relationships exist between zero-field
parameters obtained by electron spin resonance (ESR) and the
geometry around the carbene center and the substituent
effect.[2,3] In addition, carbene reactivity is influenced from its
singlet-triplet energy gap (EST).[4] Recent work has focused on
much more complicated systems, such as polycarbenes, to
show the unique properties of new type of carbenes.[5]
Buckybowls represented by corannulene and sumanene (1),
the partial structure of C60, have been studied in terms of both
fundamental and material chemistry.[6] Their curved structures
and bent -orbitals give them unique properties that cannot be
observed in the planar -conjugated molecules, especially in
spin chemistry.[7] This motivated us to generate the carbene
moiety on the curved  molecule and to investigate the effects of
the curved structure on the structural and electronic relationship.
To achieve this challenging work, we focused on sumanene (1)
Scheme 1. Structure of sumanene 1 and the preparation of sumanenylidene
(2) via diazosumanene (5).
(Scheme 1),
which
possesses the
well-investigated
fluorenylidene skeleton[2a,4b,8] as a partial structure. In this paper
[1]
[2]
Prof. Dr. Y. Yakiyama, Y. Wang, Prof. Dr. H. Sakurai
Division of Applied Chemistry, Graduate School of Engineering
Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871, JAPAN
E-mail: hsakurai@chem.eng.osaka-u.ac.jp
Dr. S. Hatano, Prof. Dr. M. Abe
Department of Chemistry, Graduate School of Science
Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526,
JAPAN
Figure 1. ESR spectrum of 2 in 77 K MTHF glassy matrix measured at 9.40
GHz.
Supporting information for this article is given via a link at the end of
the document.
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The generation of 2 from 5 was confirmed by in-situ ESR
measurement at 77 K (Figure 1). The degassed solution of 5 in
0.1 mM 2-methyltetrahydrofuran (MTHF) by Ar bubbling was
sealed and was irradiated by a 365 nm LED lamp for 1 min (see
Supporting Information). This afforded a total of six signals at
825, 1540, 2798, 4530, 5522, and 7565 G, which corresponded
to the spin transitions along the x, y, and z axes, indicating that 2
Indeed, the spin density, especially at the peripheral carbons on
the fluorene skeleton in 2, showed a higher value than that of
fluorenylidene, also indicating better spin delocalization for 2.
Furthermore, the E/hc value, which is relates to the bond angle
at the carbene center of 2, was much larger than that of
diphenylcarbene and was close to that of fluorenylidene. This
tendency well reflected bond angle differences.
was
a
ground-state triplet carbene, as observed in
Next, the photo reaction of 5 by low-temperature matrix
absorption spectroscopy was carried out (Figure 3). The 380 ±
10 nm pulse light irradiation immediately caused a decrease in
the absorption band at 393 nm, which was attributable to the
disappearance of 5. Concurrently, the new absorption band
generation at 410~440 nm was found to have a clear isosbestic
point, indicating that only two species exist in the system,
decreasing 5 and increasing 2. These results also supported the
effective generation of 2 from 5.
diphenylcarbene and fluorenylidene.[2] Notably, the signal
positions were similar to reported ESR data on diphenylcarbene,
which comprises part of the structure for 2.[11] The spin
distribution of
2
was also investigated both from the
experimentally obtained the zero-field parameters and from
calculations (Table 1 and Figure 2). The slightly smaller D/hc
value of 2, when compared with the other two, may suggest that
the curved structure of 2 yields slightly better spin delocalization
onto the two adjacent aromatic rings than planar
diphenylcarbene and fluorenylidene do. This is because of better
overlap of spin-existing - and -orbitals at the carbene centre of
2 than those of planar molecules.
Table 1. Zero-field parameters and the structural information of
diphenylcarbene, fluorenylidene and 2, as obtained from ESR measurements
and DFT calculations.
Diphenylcarbene
0.4055[8b]
0.0194[8b]
0.0478[8b]
143.2[a]
Fluorenylidene
0.4078[2a]
0.0283[2a]
0.0693[2a]
112.1[a]
2
D/hc (cm−1
)
0.39
E/hc (cm−1
)
0.0253
0.065
112.0[a]
5.80[b]
E/D
Bond angle (°)
EST (kcal/mol)
Figure 3. Low-temperature matrix absorption spectroscopy of the 0.1 mM
MTHF solution of 5 with the irradiation of 380 ± 10nm at 77 K. The bottom
dotted line shows the absorption spectrum of 5.
7.22[b]
5.59[b]
[a] Calculated at B3LYP/6-311+G(d,p) level of theory. [b] As density
functionals do not include a Coulomb correlation term, nor do they treat
nondynamic electron correlation accurately, they intrinsically misestimate the
EST values.[12] The EST values have been adjusted by the 3.68 kcal/mol
difference between the experimental (EST = 9.05 kcal/mol)[13] and computed
EST for methylene (the correction is 3.68 kcal/mol at B3LYP/6-311+G(d,p)).
Scheme 2. Reactivity confirmation of sumanenylidene 2 prepared by the 365
nm photo irradiation of 5 in cyclohexane.
As mentioned above, a carbene’s reactivity is strongly
affected by the energy relationship between the singlet and the
triplet states. During a reaction with hydrocarbons, if the singlet
carbene is predominant under the reaction condition, a C-H
Figure 2. Spin density map of A) fluorenylidene and B) 2 calculated at
UB3LYP/6-31G++(d,p) level of theory.
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bond-inserted product (a hydrocarbon adduct) is obtained.
Conversely, if the triplet carbene is more stable, the dimer
product is observed in addition to C-H bond-inserted product.
For example, diphenylcarbene gives the dimer as the main
product,[14] while fluorenylidene mainly gives cyclohexane’s C-H
adduct instead of the dimer, despite the fact that both of the
carbenes are ground-state triplets. The difference in reactivity
strongly suggests independence from the spin state of the
ground state and is well explained by each carbene’s EST
values.[14a] Therefore, next we investigated the reactivity of 2
against cyclohexane. DFT calculations also supported the
reactivity of 2 via its EST value (Table 1).
The 12 mM cyclohexane solution of 2 was stirred at rt for 1 h
under an irradiating 365-nm LED lamp, giving the cyclohexane
adduct (6a) as the main product (65% yield) together with the 1
dimer (6b) (18% yield) and 1 (3% yield). It is known that the EST
value and the bond angle of the carbene center have a strong
relationship.[15] If the bond angle of the carbene center is small,
the EST value tends to become small, which makes
intercrossing from triplet to singlet easy. With 2, the calculated
bond angle at the triplet state and the EST value were 112.0°
and 5.80 kcal/mol, respectively, which were nearly the same as
those of fluorenylidene (112.1° and 5.59 kcal/mol, respectively),
and were significantly smaller than those of diphenylcarbene
(143.2° and 7.22 kcal/mol, respectively). Thus, the reaction
product of 2 with cyclohexane and these parameters clearly
indicate that the reaction proceeded through the singlet state,
even though 2 was a ground-state triplet species. This result
also shows the high similarity of 2 to fluorenylidene.
Figure 4. Crystal structure of 7a. A) ORTEP drawing of 7a with thermal
ellipsoid at 50% probability. B) Bowl depth of the sumanene skeleton in 7a. C)
Packing structure viewed from the c axis. D) Intermolecular interactions both
within and between the columns. Blue dotted lines: CH··· interaction; red
dotted lines: - interaction. Hydrogen atoms in B)-D) are omitted for clarity.
Figure 5. Crystal structure of 7b. A) ORTEP drawing of 7b with thermal
ellipsoid at 50% probability. B) Bowl depth of the sumanene skeleton in 7b. C)
Packing structure viewed from the c axis with intermolecular interactions. Blue
dotted lines: CH··· interaction; red dotted lines: - interaction. Grey, C; red,
O; green, Cl. Hydrogen atoms in B) and C) are omitted for clarity.
Finally, the synthetic application of the carbene precursor, 5,
was investigated. The results of the Barton–Kellogg reaction of 5
with various thiocarbonyl species (either isolated or generated in
situ from ketones with Lawesson’s reagent) are shown in
Scheme 3. Through the Barton–Kellogg reaction, the sterically
crowded tetra-substituted alkenes (7a)-(7d) were prepared,
which were directly conjugated to the 1 moiety in moderate
yields. From the stable thiocarbonyl compounds, 1,3-dithiole-2-
thione and 4,5- ethylenedithio-1,3-dithiole-2-thione, the
corresponding adducts, (8a) and (8b), were also obtained in
54% and 46% yields, respectively. Within these products, 7a, 7b
and 8a exhibited good crystallinity to afford single crystals of
sufficient quality for X-ray analysis (Figures 4-6).
Scheme 3. Barton-Kellogg reaction between diazosumanene (5) and
thiocarbonyl species that were isolated or generated from corresponding
ketones using Lawesson’s reagent.
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structures were CH- and - stacking interactions. The
distances were 3.50~3.79 Å and 3.27~3.38 Å respectively in 7a,
3.44~3.86 Å and 3.33 Å respectively in 6b and 3.57~3.60 Å and
3.34~3.40 Å respectively in 8a.
Since the 1,3-dithiol skeleton is the partial structure of
tetrathiafulvalelne (TTF), 8a was expected to be a strong donor
molecule, having a curved- structure. Figure 7 shows the cyclic
voltammogram measured in CH2Cl2. One reversible redox wave
and an irreversible oxidation peak were observed at 0.32 V and
0.80 V (vs. Fc/Fc+), respectively. The reversible redox wave is
attributable to the redox reaction at the one ring structure of the
TTF moiety (0.30 V vs. Fc/Fc+).[17] As expected, 8a has high a
potential to be a non-planar electron donor molecule.
Conclusions
Figure 6. Crystal structure of 8a. A) ORTEP drawing of 8a with thermal
ellipsoid at 50% probability. B) Bowl depth of the sumanene skeleton in 8a. C)
For the first time we have generated a curved -system-based
carbene 2. And we have clarified experimentally and
theoretically that the ground state of 2 was triplet carbene and its
EST value resembled to that of fluorenylidene, which strongly
affected its reactivity to mainly give C-H inserted product in the
reaction with cyclohexane. We also demonstrated the synthetic
utility of the precursor 5 with Barton-Kellogg reaction to afford
various tetra-substituted alkenes. We hope that the research will
trigger the new carbene chemistry at the curved -system and
now plan to expand the system into the multiple carbene
systems.
Packing structure viewed from the
b axis with inter-columnar CH···
interaction. D) Intermolecular interactions both within the columns. Blue dotted
lines: CH··· interaction; red dotted lines: - interaction. Grey, C; orange, S.
Hydrogen atoms in B)-D) are omitted for clarity.
In the crystal structure, all the molecules showed different
structural features. The bowl depths at the peripheral aromatic
carbon were 1.20 Å for 7a, 1.13 and 1.16 Å for 7b (in two
crystallographically independent units), and 1.11 Å for 8a, while
the depth of pristine 1 is 1.11 Å. When the packing structures of
the three were compared, 7a and 8a provided columnar
structures. Unlike already reported functionalized symmetrical
sumanene derivatives, which form the well-stacked columnar
structure for sumanene skeletons,[16] 7a formed the head-tail
stack type, in which sumanene and fluorene skeletons stacked
alternatively. With its smaller group, 8a provided slipped-stacked
columns for sumanene skeletons. Within 7b, no columnar
stacking structure was found, which may be due to the presence
of the non-planar xanthene skeleton and CH2Cl2 as a crystalline
solvent. The main intermolecular interactions in all the packing
Experimental Section
Low temperature ESR measurement: Diazosumanene 5 (0.8 mg, 2.8
mmol) in
3
mm quartz ESR tube was dissolved in 2-
mehtyltetrahydrofurane (3.0 mL) at 27 °C. The mixture was degassed by
Ar bubbling for 30 min. The degassed solution was sealed and cooled to
77 K in an optical transmission ESR cavity and irradiated 365 nm light
with ARK TECH Awill LED light source for 1 min. ESR spectrum was
measured on a Bruker X-band E-500 spectrometer (9.400840 GHz).
Acknowledgements
This study was supported by a Grant-in-Aid for Scientific
Research on Innovative Area ’ System Figuration’ from MEXT
(No. JP26102002), and JSPS KAKENHI (JP26288020,
JP15H06357, JP18K05080). Single crystal X-ray diffraction
studies with synchrotron radiation were performed at SPring-8
(BL02B1, 2017A1396, 2017B1569), Pohang Accelerator
Laboratory (2016-3rd-2D-021) and High Energy Accelerator
Research Organization, KEK (PF-BL-8A, 20142S-001). Y. Y.
acknowledges Ogasawara Foundation for the Promotion of
Science & Engineering and Iketani Science and Technology
Foundation for the support. We thank Dr. Yasutaka Kitagawa for
the great help for quantum chemical calculations.
Figure 7. Cyclic voltammogram of 8a. Ag/AgNO3 reference electrode,
measured at a glassy-carbon working electrode in a CH2Cl2 solution (1 × 10–4
M) containing [n-Bu4N][ClO4] (1 × 10–4 M) as a supporting electrolyte with Pt
wire as a counter electrode. Scan Rate = 50 mV/s.
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Chem. Sci. 2018, 9, 5100-5105.
Keywords: Buckybowls • Carbenes • Curved- systems • Diazo
compounds • Olefination
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