Bicyclo[6.3.0]undeca-1(11),2,4,6,8-pentaen-10-ylidene: An Aromatic Carbene with Ambiphilic Properties

Article abstract of DOI:10.1002/cplu.201700069

The title carbene (4) was generated as a highly reactive species in solution by photoirradiation of 10-diazobicyclo[6.3.0]undecapentaene (5) using a high-pressure mercury lamp. Carbene 4 reacts with benzene to afford two isomeric adducts, 10-phenylbicyclo[6.3.0]undecapentaene (10) and tricyclo[9.3.0^{3, 10}.0]heptadeca-1,3(10),4,6,8,12,14,16-octaene (11). The reactivity toward benzene is a characteristic of an electrophilic aromatic carbene analogous to cyclopentadienylidene 1. In contrast, the reaction of 4 with methanol produces 7-methoxybicyclo[6.3.0]undeca-1,3,5,8,10-pentaene (15). When [D₁]methanol was employed as a reactant, the 10-deuterated analogue was formed. The results clearly indicate the formation of bicyclo[6.3.0]undecapentaenyl cation (7) as a novel 10 π-electronic compound by protonation of 4. Theoretical calculations indicate that the 2- and 7-positions of the cation have the largest positive charge in the cation. Moreover, the carbene was generated in the presence of tert-butyl hydroperoxide in aqueous tetrahydrofuran to afford azulene through oxidation of 7, followed by decarbonylation. The nucleophilic property of carbene 3 is similar to that of cycloheptatrienylidene 2. Thus, 4 can be regarded as a novel ambiphilic aromatic carbene.

Full text of DOI:10.1002/cplu.201700069

Accepted Article
Title: Bicyclo[6.3.0]undeca-1(11),2,4,6,8-pentaen-10-ylidene: An
aromatic carbene having ambiphilic properties
Authors: Takeshi Kawase, Hiroyuki Ishikawa, Jun-ichi Nishida, John W.
Jones, Jr., and Lawrence T. Scott
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Bicyclo[6.3.0]undeca-1(11),2,4,6,8-pentaen-10-ylidene: An
aromatic carbene having ambiphilic properties
Hiroyuki Ishikawa[a], Jun-ichi Nishida[a], John W. Jones, Jr.[b], Lawrence T. Scott[b,c]*, and
Takeshi Kawase[a]*
Dedication ((optional))
Abstract: The title carbene (4) was generated as a highly reactive
species in solutions by photoirradiation of 10-diazobicyclo[6.3.0]-
undecapentaene (5) using a high pressure mercury lamp. The
carbene 4 reacts with benzene to afford two isomeric adducts, 10-
basis of the resonance contribution 1A, which is consistent with
the one-step insertion mechanism for alcoholic O-H bond.[2] In
resonance contribution 1A, the filled p orbital of the carbene is
conjugated with the two  bonds, forming a cyclopentadienyl
anion, which is aromatic in character. On the other hand,
cycloheptatrienylidene (2) displays nucleophilic character
consistent with the resonance contribution 2C.[3] In this resonance
contributor, the empty p orbital is conjugated with three  bonds,
completing a six-electron seven-center cyclic system like the
tropylium cation. The filled HOMO orbital of the carbene remains
localized and imparts nucleophilic character. Thus, carbene 2
reacts readily with electrophilic alkenes but not with benzene.[3]
Recent theoretical studies on 2 reveal that the energy minimum
for this C7H6 species is not the carbene 2, but cyclohepta-1,2,4,6-
tetraene 3 possessing a highly strained allene moiety.[4]
phenylbicyclo[6.3.0]undecapentaene
(10)
and
tricyclo[9.3.03,10.0]heptadeca-1,3(10),4,6,8,12,14,16-octaene
(11).
The reactivity toward benzene is a characteristic of an electrophilic
aromatic carbene analogous to cyclopentadienylidene 1. On the other
hand, the reaction of
4
with methanol produces 7-
(15). When
methoxybicyclo[6.3.0]undeca-1,3,5,8,10-pentaene
methanol-d1 was employed as a reactant, the 10-dueterated analogue
was formed. The results clearly indicate the formation of
bicyclo[6.3.0]undecapentaenyl cation (7) as a novel 10-electronic
compound by protonation of 4. Theoretical calculations indicate that
the 2 and 7 positions of the cation have the largest positive charge in
the cation. Moreover, the carbene was generated in the presence of
t-butyl hydroperoxide in aqueous THF to afford azulene via oxidation
of 7, followed by decarbonylation. The nucleophilic property of the
carbene 3 is similar to that of cycloheptatrienylidene 2. Thus, 4 can
be regarded as a novel ambiphilic aromatic carbene.
Introduction
Fig. 1 Aromatic carbenes 1 and 2, and their resonance contributions
Aromatic carbenes constitute an interesting class of reactive
chemical species. In general, they are divided into two categories,
nucleophilic and electrophilic carbenes, on the basis of their
structural and electronic features. Cyclopentadienylidene (1) and
cycloheptatrienylidene (2) are typical aromatic carbenes (Fig. 1).
ESR analysis reveals that 1 is a ground state triplet 1B.[1] On the
other hand, 1 can be regarded as an electrophilic carbene on the
[a]
H. Ishikawa, Prof. J. Nishida, Prof. T. Kawase
Graduate School of Engineering
Scheme 1 Molecular structure of 3‒5, and resonance contributions of 4
University of Hyogo
2167 Shosha, Himeji, Hyogo 671-2280, Japan
E-mail: kawase@eng.u-hyogo.a.cjp
Dr. J. W. Jones, Jr. Prof. L. T. Scott
Department of Chemistry and Center for Advanced Study,
University of Nevada
Reno, Nevada, 89557-0020, USA
Prof. L. T. Scott
Merkert Chemistry Center, Department of Chemistry,
Boston College,
Bicyclo[6.3.0]undeca-1(11),2,4,6,8-pentaen-10-ylidene (4) is a
[b]
[c]
derivative
of
cyclopentadienylidene
1,
fused
with
cyclooctatetraene (COT). From the view point of its reactivity as
an aromatic carbene, 4 might be expected to exhibit electrophilic
properties, as suggested by a partial cyclopentadienylidene
moiety 4A in Scheme 1, or nucleophilic properties, as suggested
by the 10π electronic periphery in 4C. Characterization of 4
generated by photoirradiation of 10-diazobicyclo[6.3.0]undeca-
1(11),2,4,6,8-pentaene (5) in 2-methyltetrahydrofuran matrix at
17 K has been published previously.[5] The ESR spectrum of 4
exhibits intense signals characteristic of a triplet species, the zero-
Chestnut Hill, Massachusetts, 02467-3860, USA
Supporting information for this article is given via a link at the end of
the document.
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field splitting parameters of which were evaluated to be D =
0.2401 cm‒1 and E = 0.0044 cm‒1. The spectroscopic properties
of 4, namely the red shift of the maximum in the UV-vis spectrum
and the much smaller D value in the ESR spectrum compared to
1 (D = 0.4089 cm‒1 and E = 0.0120 cm‒1)[1a], imply a large degree
of biradical character (4B) in the electronic structures of 4
(Scheme 1). However, the reactivity of 4 has not been well
explored thus far.
thermal lability. A subsequent C—C bond cleavage of the three
membered ring of 12 and a proton transfer lead to the compound
10, whereas thermal sigmatropic shift of 12 affords 11.
Cycloheptatrienylidene 2, a nucleophilic carbene, does not react
with benzene. These results indicate that carbene 4 acts as an
electrophilic carbene analogous to 1.
Recently, we reported the synthesis, reactivity and magnetic
properties of bicyclo[6.3.0]undecapentaenyl anion 6.[6] The anion
1
shows large chemical shift changes in its H-NMR spectra that
can be ascribed to ion-pairing effects. The 1H-NMR analysis
reveals that relatively small conformational changes of the COT
ring appear to cause large changes in the paratropic properties.
The counterbalance between the aromatic character of the five-
membered ring and the antiaromatic character of the 12-electron
periphery determines the overall electronic and magnetic
properties of 6.
Scheme 3 Photoreaction of 5 in benzene, and molecular structure of 13.
Analogous to 6, the electronic property of carbene 4 should be
highly susceptible to external stimuli. Therefore, we have
investigated the reactivity of 4 under several conditions, and have
found that 4 possesses ambiphilic properties: both electrophilic
and nucleophilic properties in the same molecule. More
specifically, the reactions of 4 in hydroxylic solvents give products
via bicyclo[6.3.0]undecapentaenyl cation 7 as another reactive
Treatment of 11 with sodium hydride in THF and quenching the
generated anion with acetic acid furnishes a mixture of 11 and a
new hydrocarbon 14 as an eight-membered ring vinylogue of
fluorene in 47 and 50% yields, respectively (Scheme 4).
intermediate. Cation
7
can be regarded as
isoelectronic
a
novel
with
[4n]annuleno[4n]annulene,[7]
bicyclo[6.2.0]decapentaene 8.[8] Although the formation of
triphenyl derivative 9 was reported in a paper by Dürr and
Scheppers,[9] neither the electronic nor the chemical properties of
9 were mentioned. Moreover, the parent cation 7 has been
heretofore elusive. Herein, we describe and discuss the
generation and reactions of aromatic carbene 4 and cation 7.
Scheme 4 Basic treatment and subsequent acidic quenching of 11
To our surprise, when the photoreaction of 5 was performed in
methanol (CH3OH), we obtained 7-methoxybicyclo[6.3.0]undeca-
1,3,5,8,10-pentaene (15) in 64% yield as the sole isolated product
(Scheme 5). When deuterated methanol (CH3OD) was employed
for this reaction, the 10-deuterio derivative 15d was obtained. The
1H-NMR spectra reveal that the deuteration occurred at the 10-
position of 15 (Fig. 2). The results clearly indicated the formation
of cation 7 as another reactive intermediate, generated by
nucleophilic proton abstraction from the hydroxyl group of
methanol by 4.[2] The optimized structure and the LUMO of 7 have
been calculated by the PM3 method.[12,13] The cation has a twisted
structure (Fig. 3a and 3b). The 9- and 11-carbons have the largest
Scheme 2 Molecular structures of 6‒9.
Results and Discussion
Irradiation of 5 with a high pressure mercury lamp (HPML)
promotes the extrusion of N2, generating the carbene 4. The
reaction was carried out in benzene at room temperature to afford
10-phenylbicyclo[6.3.0]undecapentaene (10) and tricyclic
compound 11 in 27 and 29% yields, respectively (Scheme 3). The
structures were confirmed by 1H-NMR spectral and elemental
analyses. The reaction providing these products is analogous to
the reaction of triphenylcyclopentadienylidene with benzene.[9,10]
Spiro compound 12 is likely formed as the initial product, because
the reaction of cyclopentadienylidene 1 with benzene produces
spiro compound 13 as an isolable product.[11] Compound 12,
however, cannot be isolated under the conditions, owing to its
Scheme 5 Photoreaction of 5 in methanol and methanol-d1.
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It has been known that oxidation of tropylium cation 18 with
hydrogen peroxides afforded benzene and/or benzaldehyde, but
not tropone 19 (Scheme 7).[15] Consequently, formation of ketone
20 as a tropone vinylogue would not be anticipated in the reaction.
A proposed reaction mechanism is outlined in Scheme 7.
Scheme 6 The reaction of 5 in the presence of t-butyl hydroperoxide in aqueous
THF affording azulene 16 and 4-formylazulene 17
Fig. 2 1H-NMR spectra of 15 (down) and 15d (upper) in CDCl3.
a)
c)
b)
d)
Scheme 7 Molecular structures of 18—21, and a proposed reaction mechanism
of the reaction of 4 with tBuOOH in aqueous THF affording azulene 16 and 4-
formylazulene 17 via cation 7.
Carbinols derived from stable carbenium ions are normally quite
labile,[16] because disproportionation of the carbinols to the
corresponding ketone and hydrocarbon occur in protic media. For
example, quenching of tropylium cation 18 with water affords
tropone 19 and cycloheptatriene.[16a] Unfortunately, the reaction
of 5 in aqueous THF led to a complex mixture, and neither 20 nor
21 could be detected. Because 21 is an isolable material,[17] the
results indicate relatively low stability of cation 7.
Fig. 3 Optimized structures of the cation 7 with PM3 calculations; a) top and b)
side views, and c) the LUMO coefficients of 7. d) Electron density of 7 obtained
by HMO calculation.
LUMO coefficients; however, the reaction took place at the 2(7)-
position. A Hückel molecular orbital (HMO) calculation predicts
the lowest electron density at the 2- and 7-carbons of 7 (Fig. 3d).
The selective formation of 15 strongly supports the formation of 7.
Further evidence for the generation of 7 was provided by the
reaction of 5 with hydroperoxides (R-OOH), such as t-butyl
hydroperoxide (tBuOOH), hydrogen peroxide (HOOH), and m-
chloroperbenzoic acid (mCPBA), affording azulene 16. When the
photoreaction of 5 was carried out in the presence of t-butyl
hydroperoxide in aqueous THF [THF:H2O:tBuOOH (70 wt.% in
H2O) = 15:5:5], azulene 16 and 4-formylazulene 17[14] were
produced in 31 and 7% yields, respectively (Scheme 6). The
reactions of 5 with HOOH or mCPBA in aqueous THF also
afforded 16, albeit in lower yields (~15%). On the other hand, the
reaction of 5 and mCPBA in dry THF afforded no azulenic
compounds. Therefore, the presence of both the hydroperoxides
and water is critical for the formation of azulenes. The results
strongly suggest the formation of the cation 7 as an intermediate.
Conclusions
The title carbine 4 was generated by photoreaction of the
corresponding diazo compound 5. The reaction of 4 with benzene
produces two compounds 10 and 11 via the spiro adduct 12. On
the other hand, the reaction of 4 in methanol affords adduct 15 via
the bicyclo[6.3.0]undecapentaenyl cation 7 as another reactive
intermediate. These results indicate that the carbene 4 acts as an
electrophilic
carbene
in
benzene,
analogous
to
cyclopentadienylidene 1, and as a nucleophilic carbene in
methanol, analogous to cycloheptatrienylidene 2. Thus, 4 can be
regarded as a novel ambiphilic carbene, whose reactivity is highly
susceptible to external stimuli. Moreover, the reaction of 4 was
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t
performed in the presence of BuOOH in aqueous THF to afford
the azulenes 16 and 17. The azulenes are most likely formed by
oxidation of 7 at the 2- or 7-position with hydroperoxides. The
results are well consistent with the theoretical prediction.
1H), 6.57 (s, 1H), 7.19 (t, J = 7.3 Hz, 1H), 7.31 (t, J = 7.7 Hz, 2H),
7.44 (d, J = 7.6 Hz, 2H) ppm;13C NMR (125 MHz, CDCl3)  = 44.7,
124.8, 126.9, 128.6, 129.6, 130.2, 130.8, 132.1, 132.6, 133.1,
133.6, 135.8, 142.7, 144.1, 147.5 ppm; Anal. Found C 93.37; H
6.45%; Calcd. For C17H14: C 93.54; H 6.46%; HRMS (FAB, NBA)
m/z: calcd. for [C17H14]: 218.1096; found: 218.1092.
Tricyclo[9.3.03,10.0]heptadeca-1,3(10),4,6,8,12,14,16-octaene 11:
MS (EI) m/z (%) = 218 (100) [M]+, 217 (95) [M–1]+; IR (KBr):  =
2995 (s), 1403 (m), 1339 (m), 850 (m), 655 (s), 639 (s) cm‒1;
UV/Vis (cyclohexane): max (log ) = 265 sh (4.01), 297 (3.74), 378
(3.68); 1H NMR (500 MHz, CDCl3)  = 3.47 (s, 3H), 4.18 (dd, J =
6.8, 1.4 Hz, 1H), 5.59 (dd, J = 10.9, 6.8 Hz, 1H), 5.95 (ddt, J =
10.9, 6.8, 1.4 Hz, 1H), 6.39 (dd, J = 5.1, 1.7 Hz, 1H), 6.44 (dd, J
= 12.5, 6.9 Hz, 1H), 6.46 (dd, J = 2.3, 1.4 Hz, 1H), 6.55 (dd, J =
12.5, 4.7 Hz, 1H), 6.79 (dd, J = 5.1, 2.3 Hz, 1H), 6.87 (d, J = 6.9
Hz, 1H) ppm; 13C NMR (125 MHz, CDCl3) = 57.2, 78.3, 122.6,
123.22, 124.4, 127.6, 131.2, 132.1, 133.5, 134.5, 137.1, 147.5
ppm; HRMS (FAB, NBA) m/z: calcd. for [C17H14]: 218.1096; found:
218.1097.
Experimental Section
General: All reactions of air- or moisture-sensitive compounds
were carried out in a dry reaction vessel under a positive pressure
of nitrogen. Air- and moisture-sensitive liquids and solutions were
transferred via syringe. Analytical thin-layer chromatography was
performed using glass plates pre-coated with Merck Art. 7730
Kiesel-gel 60 GF-254. Thin layer chromatography plates were
visualized by exposure to UV light. Organic solutions were
concentrated by using rotary evaporation at ca. 15 Torr obtained
with a diaphragm pump. Column chromatography was performed
with Merck Kiesel-gel 60. All reagents were commercially
available and used without further purification unless otherwise
noted. THF was purchased from Wako Chemical Co. and distilled
from lithium aluminum hydride at 760 Torr under a nitrogen
atmosphere before use.
Isomerization of 11.
A solution of 11 (70 mg) in THF (2 mL) was added to a suspension
of sodium hydride (100 mg) in THF (5 mL), and the mixture was
stirred at 0 ˚C for 30 min under N2. The purple anion solution was
cooled to ‒50 ˚C, and quenched with acetic acid (1 mL). The
solution was stirred at that temperature for 5 min, and then
allowed to warm at RT. The reaction mixture was diluted with
water and extracted with hexane. The organic layer was washed
with water and brine, and dried over anhydrous Na2SO4. The
solvent was evaporated under reduced pressure. The residue
was charged on silica gel and purified by column chromatography
to give 11 (32 mg, 46%) from the first fraction and 14 (35 mg,
50%) as a pale yellow oil from the second fraction of hexane
Melting points were recorded on a Yanaco MP–S3 apparatus and
are reported uncorrected. Mass spectral analyses (EI) were
performed on a JEOL JMS-O1SG-2 instrument. FAB- and HR-
mass spectra were recorded with a JEOL JMS-700. FAB-mass
spectrometer in a positive mode with 3-nitrobenzyl alcohol (NBA)
as the matrix. Elemental analyses were obtained from Yanaco
MT5 CHN corder. 1H- and 13C-NMR spectra were recorded on a
Bruker–Biospin DRX-500, spectrometer, and measured at 20 ˚C.
IR spectra were obtained using
a Shimadzu FTIR-8400
spectrometer. Electronic (UV-vis) and fluorescence spectra in
solution were recorded with a JASCO V650. DFT calculations for
7 were carried out by using the B3LYP/6-31G(d) method with the
Gaussian 09 program package.[12]
elutions.
Tricyclo[9.3.03,10.0]heptadeca-
1(11),3(10),4,6,8,12,14,16-octaene 14: MS (EI) m/z (%) = 218
(100) [M]+, 217 (42) [M–1]+; UV/Vis (cyclohexane) : max (log ) =
261 (3.97), 374 (3.35); IR (KBr):  = 3000 (s), 2920 (s), 1380 (m),
708 (s), 650 (s) cm‒1; 1H NMR (500 MHz, CDCl3)  = 2.94 (s, 2H),
5.61–5.68 (m, 4H), 5.74–5.81 (m, 4H), 5.85–5.89 (m, 2H), 5.94 (d,
J = 11.5 Hz, 2H) ppm; 13C NMR (125 MHz, CDCl3)  = 46.8, 129.2,
130.7, 132.0, 132.8, 133.1, 134.0, 144.0, 144.6 ppm; HRMS (FAB,
NBA) m/z: calcd. for [C17H14]: 218.1096; found: 218.1096.
Synthesis:
pentaene (
10-Diazobicyclo[6.3.0]undeca-1(11),2,4,6,8-
) was prepared by the reported procedure.[6]
5
Photoreaction of 5 in benzene:
Photoreaction of 5 in methanol:
A solution of 5 (280 mg, 1.7 mmol) in benzene (150 mL) was
irradiated using a HPML through a Pyrex filter at RT for 30 min.
The reaction mixture was concentrated under reduced pressure.
The residue was charged on silica gel and purified by column
chromatography to afford 10 as an orange solid (88 mg, 29%), 11
as a yellow oil (81 mg, 27%) and recovered 5 (46 mg) from
A solution of 5 (280 mg, 1.7 mmol) in methanol (100 mL) was
irradiated using a HPML through a Pyrex filter at 0 ˚C for 30 min.
The reaction mixture was diluted with water and extracted with
hexane. The organic layer was washed with water and brine, and
dried over anhydrous Na2SO4. The solvent was evaporated under
reduced pressure. The residue was charged on silica gel and
purified by column chromatography to give the methoxy derivative
15 (182 mg, 64%) as a red oil from a hexane/ethyl acetate (95:5)
elution. 7-Methoxybicyclo[6.3.0]undeca-1,3,5,8,10-pentaene 15:
MS (EI) m/z (%) = 172 (25) [M]+, 171 (17) [M–1]+, 141 (100) [M–
OCH3]+; IR (KBr):  = 2931 (m), 2828 (m), 1684 (s), 1456 (m),
1198 (m), 1111 (s), 979 (m), 768 (m), 755 (m) cm‒1; UV/Vis
(cyclohexane): max (log ) = 232 (4.10), 261 (3.95), 319 (3.81),
subsequent
three
hexane
elusions.
10-
mp
phenylbicyclo[6.3.0]undeca-1(8),2,4,6,10-pentaene
10:
121.0―121.5 ºC; MS (EI) m/z (%) = 218 (100) [M+], 217 (85) [M–
1]+; IR (KBr):  = 2995 (m), 1490 (m), 1450 (m), 1370 (m), 755 (s),
696 (s) cm‒1; UV/Vis (cyclohexane): max (log ) = 225 (4.23), 298
(4.10), 350 (3.71); 1H NMR (500 MHz, CDCl3):  = 3.40 (br. s, 2H),
5.63–5.69 (m, 2H), 5.82 (dd, J = 11.3, 3.0 Hz, 1H), 5.90 (dd, J =
11.5, 3.5 Hz, 1H), 5.99 (d, J = 11.7 Hz, 1H), 6.03 (d, J = 11.7 Hz,
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430 (2.45); 1H NMR (500 MHz, CDCl3)  = 3.47 (s, 3H), 4.18 (dd,
J = 6.8, 1.4 Hz, 1H), 5.59 (dd, J = 10.9, 6.8 Hz, 1H), 5.95 (ddt, J
= 10.9, 6.8, 1.4 Hz, 1H), 6.39 (dd, J = 5.1, 1.7 Hz, 1H), 6.44 (dd,
J = 12.5, 6.9 Hz, 1H), 6.46 (dd, J = 2.3, 1.4 Hz, 1H), 6.55 (dd, J =
12.5, 4.7 Hz, 1H), 6.79 (dd, J = 5.1, 2.3 Hz, 1H), 6.87 (d, J = 6.9
Hz, 1H) ppm; 13C NMR (125 MHz, CDCl3)  = 57.2, 78.3, 122.6,
123.2, 124.4, 127.6, 131.2, 132.1, 133.5, 134.5, 137.1, 147.5
ppm; HRMS (FAB, NBA) m/z: calcd. for [C12H12O]: 172.0888;
found:172.0887.
[4]
[5]
[6]
[7]
M. Christl in Modern Allene Chemistry, Vol. 1 (Eds.: N. Krause, A.
Stephane, K. Hashmi), Wiley-VCH, Weinheim, 2002, pp. 185–242
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H. Ozoe, Y. Uno, C. Kitamura, H. Kurata, M. Oda, J. W. Jones Jr., L. T.
Scott, T. Kawase, Chem. Asian J. 2014, 9, 893–900.
[4n]Annuleno[4n]annulenes with 10π electrons were also reported; a) M.
Oda, T. Watabe, T. Kawase, Tetrahedron Lett. 1981, 22, 4249–4252; b)
T. Kawase, M. Oda, Tetrahedron Lett. 1982, 23, 2677–2678. c) M. Oda,
Pure & Appl. Chem. 1986, 58, 7–14.
[8]
a) M. Oda, H. Oikawa, N. Fukazawa, Y. Kitahara, Tetrahedron Lett. 1977,
4409–4412; b) C. Kabuto, M. Oda, Tetrahedron Lett. 1980, 103-116; c)
M. Oda, H. Oikawa, Tetrahedron Lett. 1980, 107–110; d) D. Kawka, P.
Mues, E. Vogel, Angew. Chem. 1983, 95, 1006–1007; Angew. Chem. Int.
Ed. Engl. 1983, 22, 1003–1004; e) W. R. Roth, H.-W. Lennartz, E. Vogel,
M. Leiendecker, M. Oda, Chem. Ber. 1986, 119, 837–843.
H. Dürr, G. Scheppers, Tetrahedron Lett. 1968, 6059–6062.
Photoreaction of 5 with tBuOOH in aqueous THF:
To a solution of 5 (255 mg, 1.5 mmol) in THF/H2O (15 mL/5 mL)
was added tBuOOH (70 wt.% in H2O, 5 mL), and the mixture was
irradiated using a HPML through a Pyrex filter at 0 ˚C for 1 hour.
The reaction mixture was extracted with hexane. The organic
layer was washed with water and brine, and dried over anhydrous
Na2SO4. The solvent was evaporated under reduced pressure.
The residue was charged on silica gel and purified by column
chromatography to give azulene 16 (35 mg, 31%) as a blue solid
and unreacted 5 (105 mg) from hexane elutions, and 4-
formylazulene 17 (10 mg, 7%) as a dark blue solid from a
[9]
[10] H. Dürr, H. Kober, I. Halberstadt, U. Neu, T. T. Coburn, T. Mitsuhashi, W.
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:
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1H), 8.45 (d, J = 9.4 Hz, 1H), 10.91 (s, 1H) ppm.
Acknowledgements
This study was supported in Japan by Hyogo prefecture and a
JSPS Grant-in-Aid for Scientific Research (C) (JP16K05896 and
JSPS KAKENHI grant No. JP15H00959). Financial support from
the US National Science Foundation and Department of Energy
is also gratefully acknowledged. We especially thank Dr. Akihito
Konishi at Osaka University for the measurements of mass and
HRMS spectra.
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Keywords: aromatic carbenes • diazo compound •
photoreaction • azulenes • carbocations
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10.1002/cplu.201700069
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Bicyclo[6.3.0]undeca-1(11),2,4,6,8-
pentaen-10-ylidene: An aromatic
carbene having ambiphilic properties
The title carbene was generated by photodecomposition of the corresponding diazo
compound. The carbene reacts with benzene in an electrophilic manner and with
methanol in a nucleophilic manner, respectively. The reaction with methanol
generates a heretofore unknown cationic species as a reactive intermediate.
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