Oxidation of allenes bearing 1,8-diphenoxy or diaryloxyacridene moieties

Article abstract of DOI:10.1002/poc.3665

New allene compounds bearing 1,8-diphenoxy or bis(3,5-di-tert-butylphenyl) oxyacridene (diaryloxyacridene) moieties were synthesized. These compounds were more easily oxidized than Cl-substituted compounds. The allene compounds were oxidized to generate thermally stable triplet carbenes. Electron spin resonance and electron spin transient nutation spectra of the oxidation reaction products suggested the generation of triplet species; however, the main isolated product from the 1,8-diphenoxy allene was a cyclized compound that was characterized by X-ray analysis. The product derived from diaryloxy allene was also characterized by X-ray analysis as a singlet species, which included a vinyl cation stabilized by a neighboring O(3,5-di-tert-butylphenyl) group. This singlet species was stable even at room temperature and did not show any electron spin resonance signals in solution, indicating that the triplet species was not in equilibrium with the singlet. Copyright

Full text of DOI:10.1002/poc.3665

Special issue article
Received: 30 June 2016,
Revised: 19 September 2016,
Accepted: 10 October 2016,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.3665
Oxidation of allenes bearing 1,8-diphenoxy or
diaryloxyacridene moieties
Shin-ichi Fuku-ena, Junki Yamamotoa, Ko Furukawab, Daisuke Hashizumec,
Naomi Kawatad and Yohsuke Yamamotoa*
New allene compounds bearing 1,8-diphenoxy or bis(3,5-di-tert-butylphenyl) oxyacridene (diaryloxyacridene) moieties were
synthesized. These compounds were more easily oxidized than Cl-substituted compounds. The allene compounds were oxi-
dized to generate thermally stable triplet carbenes. Electron spin resonance and electron spin transient nutation spectra of
the oxidation reaction products suggested the generation of triplet species; however, the main isolated product from the
1,8-diphenoxy allene was a cyclized compound that was characterized by X-ray analysis. The product derived from diaryloxy
allene was also characterized by X-ray analysis as a singlet species, which included a vinyl cation stabilized by a neighboring
O(3,5-di-tert-butylphenyl) group. This singlet species was stable even at room temperature and did not show any electron
spin resonance signals in solution, indicating that the triplet species was not in equilibrium with the singlet. Copyright ©
2016 John Wiley & Sons, Ltd.
Keywords: ESR; Preparation; triplet carbene
carbene 6a had no steric protecting groups and was highly reac-
tive, with its rapid decomposition preventing observation. How-
INTRODUCTION
Triplet carbenes have 2 unpaired electrons on a carbon atom
and are much more reactive than the corresponding singlet
carbenes.[1] Characterization of triplet carbenes is typically
performed by electron spin resonance (ESR) at low temperatures,
and more detailed structural information on these species re-
mains limited. Triplet carbenes are typically stabilized by placing
inert bulky protecting groups around carbene center.[2–6] In
2003, Tomioka et al reported a di(9-anthryl) carbene 2 having
aryl groups at 10 positions (Scheme 1).[5] This molecule showed
the longest measured carbene lifetime (circa 2 weeks) in
degassed benzene at room temperature. Although the triplet
carbene could not be isolated as a pure solid, the Tomioka group
succeeded in measuring the X-ray diffraction of a triplet carbene,
which was partially generated by photolysis of a crystalline diazo
compound.[6]
ever, these results implied that stable triplet carbenes may be
synthesized by introducing suitable steric protecting groups
around the carbenic center. Because the system I bearing elec-
tron withdrawing Cl could not be easily oxidized (Fig. 1),[10]
we introduced electron donating groups such as methoxy
groups. However, oxidation of the allene compound bearing
1,8-dimethoxyacridene moieties II did not give the desired
triplet carbenes. Instead, an unexpected tetracation dimer
species was generated after cleavage of the O-C bond of the
OMe group, although the dimer was an interesting compound
with singlet diradical character.[9]
Herein, we report the synthesis and oxidation of allene com-
pounds bearing phenoxy and aryloxy groups III to prevent from
the O-C bond(s) cleavage. The electron-donating phenoxy and
aryloxy groups should facilitate the double oxidation reaction,
and we anticipated that steric repulsion among the 4 aryl groups
We designed a new strategy to generate triplet carbenes,
based on 2-electron oxidation of allenic precursors bearing het-
erocyclic moieties at their terminal positions (Scheme 2).[7–10]
Because this atomic arrangement does not change before and
after oxidation, it is possible to introduce protecting groups X
around the central carbon atom.
* Correspondence to: Y. Yamamoto, Department of Chemistry, Graduate School
of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-
8526, Japan.
E-mail: yyama@sci.hiroshima-u.ac.jp
First, we oxidized allenic compound
3 bearing 1,8-
dimethoxythioxanthene moieties (Scheme 3).[7] However, a sta-
ble triplet carbene was not obtained, and we instead generated
the demethylated compound 4, which likely formed via a singlet
oxonium compound. Then, we attempted to oxidize an allene
compound bearing acridene moieties. The nitrogen-bridged
allenes were expected to oxidize more easily than the analogous
sulfur-bridged system. The two-electron oxidation of 5a resulted
in the formation of dimer 7 (Scheme 4).[8] This result indicated
that the intermediate was carbene 6a. Furthermore, trapping ex-
periments with tetramethylpiperidine N-oxide (TEMPO) to afford
the carbonyl compound 8a, and theoretical calculations sug-
gested that intermediate 6a had a triplet ground state. Triplet
a S.- Fuku-en, J. Yamamoto, Y. Yamamoto
Department of Chemistry, Graduate School of Science, Hiroshima University,
Higashi-Hiroshima 739-8526, Japan
b K. Furukawa
Center for Instrumental Analysis, Institute for Research Promotion, Niigata
University, Niigata 950-2181, Japan
c D. Hashizume
Materials Characterization Support Unit, RIKEN Center for Emergent Matter
Science (CEMS), Saitama 351-0198, Japan
d N. Kawata
Technical Center, Hiroshima University, Higashi-Hiroshima 739-8524, Japan
J. Phys. Org. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

S.-I. FUKU-EN ET AL.
should prevent their interaction with the carbenic center,
inhibiting decomposition reactions.
RESULTS AND DISCUSSION
Synthesis of the allene compounds 5b and 5c
First, we synthesized the acridone compounds 13b and 13c
bearing phenoxy or aryloxy substituents. The amine 9[9] was
selectively dilithiated by refluxing in n-hexane followed by treat-
ment with BrCl2CCCl2Br to afford 10. The demethylation of 10 with
1-dodecanthiol and t-BuOK gave 11. The metal-free coupling
Scheme 1. Reported triplet carbene measured by Tomioka et al (2013)
ꢀ
reaction of 11 with Ph2I+OTfꢀ or Ar2I+BF4 (Ar = 3,5-(t-Bu)2C6H3)
Scheme 2. Oxidation of allenic precursors
Scheme 3. Oxidation of allene bearing thioxanthene moieties
Scheme 4. Oxidation of nonsubstituted allene
Figure 2. Molecular structure of 5b. Hydrogen atoms have been omit-
Figure 1. Modification of the protection groups
ted for clarity. Thermal ellipsoids are drawn at the 50% probability level
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OXIDATION OF ALLENES
Oxidation of the allene
compound bearing
phenoxy groups
The reaction of 5b with 2
equivalents of (4-BrC6H4)3N•
+SbCl6 gave the decompo-
ꢀ
sition product 16b·SbClX
(X = 4 or 6) in a yield circa
60% (Scheme 6). The pro-
duct, 16b, was also obtained
when using only 1 equiva-
ꢀ
lent of (4-BrC6H4)3N•+SbCl6 .
Analysis
by
1H
NMR
indicated that both crude
samples contained almost
the same amount of 16b,
suggesting that the interme-
diate 15b was generated af-
ter 1-electron oxidation, and
then cyclization took place
(Fig. 3).
To confirm whether the
desired triplet dication spe-
cies 17b was generated as
a minor species, we mea-
sured ESR spectra at 5 K after
Scheme 5. Synthesis of 5b and 5c
ꢀ
the reaction of 5b with 2 equivalents of (4-BrC6H4)3N•+SbCl6 in
degassed CH2Cl2 at low temperature (Fig. 4).
In addition to a large doublet signal from impurities such as
oxidants and monoradicals (0.342 T), small signals (0.333 and
0.354 T) were observed at ꢀ85°C (Fig. 4). Although the side sig-
nals remained at ꢀ50°C, these signals disappeared on heating
to room temperature.
The observed side peaks may be assigned to the desired trip-
let dication species; however, the intensity of the side peaks was
much weaker than that of the doublet impurities. Furthermore, a
forbidden transition Δm = 2, which is a characteristic signal of
triplet diradicals, was not observed in the half-field region.
Oxidation of allene bearing aryloxy groups
Because the intermediate from the phenoxy system was not sta-
ble, substituents were introduced into the phenyl groups to sup-
press decomposition reactions such as intramolecular radical
cyclization. Because the 4 substituted aryloxy groups in the
allene compound repel each other, the aryl groups were not ex-
pected to interact with the carbenic center.
Scheme 6. Oxidation of allenic precursor 5b
afforded 12b and 12c. A halogen lithium exchange reaction
followed by treatment with methyl chloroformate provided the
desired acridones 13b and 13c in 4 steps with 11% yield for
both. Reaction of 13b and 13c with MeLi followed by the treat-
ment with p-TsOH successfully provided the desired olefin 14b
and 14c, respectively. The reaction of 13b and 14b (or 13c
and 14c) with Tf2O followed by treatment with DBU afforded
the desired allenes 5b and 5c in yields of 42% and 13%,
respectively.
Initially, we tried to introduce halogens at the 2,6-positions of
the phenyl groups. However, the steric bulkiness of these groups
prevented the synthesis of allene compounds bearing 2,
6-substituted aryloxy groups. We then introduced 3,5-substituted
aryloxy groups at 1,8 positions of the acridene moieties.
The 13C NMR spectra of 5b and 5c exhibited resonances attrib-
uted to the allenic carbon at 216.6 and 216.4 ppm, which were
similar to that of I (213.5 ppm). Single crystals of 5b suitable
for X-ray analysis were obtained by crystallization from CH2Cl2,
and their structure is shown in Figure 2. The angle of the allenic
moiety was more closer to a linear arrangement [C-C-C = 176.7
(2)°] than that of I [164.9(3)-165.6(2)°]. The dihedral angle of the
2 acridene moieties was 79° and close to an orthogonal
geometry.
ꢀ
The reaction of 5c with (4-BrC6H4)3N•+SbCl6 in CH2Cl2 was
performed, while measuring ESR spectra. Small side peaks were
observed together with major doublet peaks, as was the case
for the phenoxy system. Spectra of the frozen solution changed
with reaction time and depended on reaction temperature. After
30 seconds at ꢀ80°C, the solution of 5c with (4-BrC6H4)3N•
+SbCl6ꢀ became brown. The solution color changed immediately
after adding solvent, indicating that the first oxidation took place
rapidly at low temperature. In addition to the large doublet
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S.-I. FUKU-EN ET AL.
signal from impurities such as oxidants and radicals, small side
signals (circa 0.332 and 0.353 T) were observed at ꢀ80°C
(Fig. 5). This spectrum pattern was the same as that of 5b ob-
served at ꢀ80°C. For the reaction at ꢀ20°C, the solution color be-
came black after 10 minutes, suggesting that the reaction
proceeded further.
Also in this case, the Δm = 2 signal could not be observed. But
the signal intensity of the Δm = 2 peak may be low because the
intensity depends on the D-value.[11] Therefore, to determine the
spin multiplicity of the species detected by ESR measurements,
we measured the electron spin transient nutation (ESTN) spec-
trum of the oxidized 5c in dichloromethane at 5 K (Fig. 6).
Equation 1 gives the nutation frequency (ωn).
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ωn ¼ SðS þ 1Þ ꢀ mðm ꢀ 1Þω1
(1)
The nutation frequency for doublet species (ω1) depends on
the instrument used, and in this case, ω1 = 14.3 MHz. The nuta-
pffiffiffi
tion frequency for the triplet species (ω2) was 20.2 ( 2 ω1)
MHz. Side signals were observed at 0.323 and 0.345 T in the ESTN
measurement. The nutation frequency of 19.8 MHz confirmed
the triplet nature of the product detected as minor peaks by
ESR measurements.
Because the minor species was determined to be a triplet,
we attempted to isolate the oxidation product for X-ray analysis.
A 2-electron oxidation reaction of compound 5c with 3 equiva-
lents of SbCl5 afforded a dark purple product. Single crystals
for X-ray analysis were obtained from THF/n-hexane, and their
structure is shown in Figure 7. Owing to efflorescence of the
crystals, the data quality was too low to allow a detailed discus-
sion of the structural parameters; however, the configuration of
the nonhydrogen atoms was roughly determined. Because 1
oxygen atom was coordinated to the central carbon atom, the
structure was assigned as a singlet species 18c (Fig. 7).
An ESTN measurement of the isolated product was also
performed (Fig. 8). The nutation frequency for the doublet spe-
cies was ω1 = 17.3 MHz and that of the triplet species was
ω2 = 24.5 MHz. Signals from triplet species were not observed,
and only minor impurities from doublet species were observed.
Therefore, we conclude that there was no equilibrium between
the triplet species observed in Figure 6 and the major product
18c (Fig. 7). The triplet species was formed only as a minor
product and likely decomposed or was lost in the isolation
process.
Figure 3. Molecular structure of 16b·SbCl6. Hydrogen atoms, 4-t-BuC6H4
groups, phenyl groups, counter anion, and solvent molecules have been
omitted for clarity. Thermal ellipsoids are drawn at the 50% probability
level
Figure 4. Electron spin resonance spectra for the oxidization of 5b in
degassed CH2Cl2. These spectra were measured at 5 K: A-D, for the reac-
tion conditions. A, ꢀ85°C after 15 seconds; A, ꢀ50°C after 5 minutes; C, at
ꢀ50°C after an additional 15 minutes; D, after rising to room tempera-
ture, for an additional 2 minutes
Figure 5. Electron spin resonance spectra for the oxidization of 5c in
degassed CH2Cl2 measured at 5 K. Reaction conditions indicated in the
figure. Side peaks were observed at 0.332 and 0.353 T
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OXIDATION OF ALLENES
Figure 6. A, 2-D ESTN spectrum for the oxidized 5c in degassed CH2Cl2 measured at 5 K. The dotted line shows ωn/ω1 = 1.4. B, ESTN spectrum as a
function of the magnetic field at ωn/ω1 = 1.4. ESTN, electron spin transient nutation
Scheme 7. Oxidation of 5c
Figure 7. Molecular structure of 18c. Hydrogen atoms, 4-t-BuC6H4
groups on nitrogen atoms, t-Bu groups on aryloxy groups, counter
anions, and solvent molecules have been omitted for clarity
Scheme 8. Products of the oxidation reaction
CONCLUSION
To establish a system for generating stable triplet carbenes by
2-electron oxidation of allene compounds bearing acridene
moieties, we introduced phenoxy or aryloxy groups as steric
protection for triplet carbenes. We expected that these
systems could be easily oxidized.
In the phenoxy system, oxidation of allene 5b resulted in phe-
nyl groups interacting with the central carbon atom, which had a
radical character, and radical cyclization occurred to afford the
Figure 8. Electron spin transient nutation spectrum for 18c in degassed
CH2Cl2 measured at 5 K
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S.-I. FUKU-EN ET AL.
decomposition product 16b. To prevent phenyl groups from
interacting with the central carbon, we introduced t-Bu groups
at the m-positions of the phenyl groups. Although ESTN mea-
surements of the crude solution indicated that a triplet species
was generated, the main isolated product was the singlet species
18c. ESTN measurement of 18c suggested that no equilibrium
existed between the singlet and the triplet.
CDCl3) δ 153.89 (C), 146.44 (C), 145.28 (C), 144.54 (C), 123.64
(CH), 125.74 (CH), 121.61 (CH), 120.46 (CH), 111.95 (CH), 110.38
(C), 34.18 (C), 31.30 (CH3). HRMS (ESI) m/z calcd for
[C22H21O2NBr2 + H]+ 490.0017 (79Br-79Br) 491.9997 (79Br-81Br)
493.9976 (81Br-81Br), found 490.0009 (79Br-79Br) 491.9987
(79Br-81Br) 493.9967 (81Br-81Br).
Further experimental and computational analyses are under-
way to generate thermally stable triplet carbene as the ground
state by using slightly modified structures.
Synthesis of bis(3,5-di-tert-butylphenyl)iodonium
tetrafluoroborate
To the solution of m-CPBA (65%, 2.9 g, 11 mmol) in CH2Cl2
(30 mL) under argon, 1-iodo-3,5-tert-butylbenzene (3.2 g,
10 mmol) and BF3·OEt2 (3.1 mL, 25 mmol) was added and stirred
for 30 minutes at room temperature. To the mixture, (3,5-tert-
butylphenyl)boronic acid (2.3 g, 10 mmol) was added at 0°C
and then stirred for 15 minutes at room temperature. Column
chromatography (silica gel, CH2Cl2, and then CH2Cl2:MeOH = 20:1)
followed by washing ether gave bis(3,5-tert-butylphenyl)
iodonium tetrafluoroborate (4.60 g, 7.76 mmol, 78%) as a white
solid.
EXPERIMENTAL SECTION
General procedures
The 1H NMR (400 MHz) chemical shifts (δ) are given in parts
per million downfield from Me4Si, determined by residual
CHCl3 (δ
=
7.26 ppm), residual protonated DMSO-d6
(δ = 2.50 ppm). The 13C NMR (100 MHz) chemical shifts (δ)
are given in parts per million downfield from Me4Si, deter-
mined by CDCl3 (δ = 77.0 ppm), DMSO-d6 (δ = 39.52 ppm).
CH2Cl2 was purified by distillation from CaH2 under N2 atmo-
sphere. Ether, hexane, and THF were purified by passing
through a Glass Contour solvent dispensing system under an
N2 atmosphere.
Mp. 65.0°C-68.0°C (decomp.). 1H NMR (400 MHz, CDCl3) δ 7.70
(s, 4H), 7.60 (s, 2H), 1.31 (s, 36H). 13C NMR (400 MHz, CDCl3) δ
156.10 (C), 129.08 (CH), 126.77 (CH), 122.28 (C), 35.72 (C), 31.04
(CH3). 19F NMR (376 MHz, CDCl3), δ [ꢀ148.45, ꢀ148.50] (4F).
HRMS (ESI) m/z calcd for [C28H42I]+ 505.2326, found 505.2326.
Anal. calcd for C28H42BF4I: C 56.78, H 7.15, N 0.00%. Found: C
56.68, H 7.25, N 0.04%.
Synthesis of 10
At room temperature, added dropwise to a stirring solution of 9
(22.3 g, 61.7 mmol) in dry n-hexane (180 mL) under argon was
n-BuLi (1.58 M in n-hexane, 85.0 mL, 134 mmol). The mixture
was refluxed for 4.5 hours. Added at ꢀ78°C to the mixture was
dry THF (90 mL) and 1,2-dibromotetrachloroethane (42.0 g,
129 mmol). After stirring overnight at room temperature, the re-
action was quenched with saturated NaHCO3 aq. and extracted
with CH2Cl2. The combined organic layer was washed with brine
and dried (K2CO3). Removal of the solvent followed by column
chromatography (silica gel, AcOEt:n-hexane = 1:3) afforded 10
(16.9 g, 32.5 mmol, y. 53%) as a white solid.
Synthesis of 12b (Ar′ = Ph)
The solution of 11 (348 mg, 0.709 mmol) and t-BuOK (175 mg,
1.56 mmol) in dry THF (2.0 mL) under argon was stirred for 15 mi-
nutes at 0°C. To the mixture, Ph2I+OTfꢀ (700 mg, 1.63 mmol) was
added at 0°C and then stirred for 1 hour at 40°C. After cooling to
room temperature, the reaction mixture was treated with H2O
and the mixture was extracted with CH2Cl2 and dried over
Na2SO4. Removal of the solvent followed by column chromatog-
raphy (silica gel, CH2Cl2:n-hexane = 1:4 → CH2Cl2:n-hexane = 1:1)
afforded 12b (271 mg, 0.421 mmol, y. 59%) as a white solid.
1
1
Mp. 135.0°C (decomp.) H NMR (400 MHz, CDCl3) δ 7.18 (m,
Mp. 109.0°C (decomp.). H NMR (400 MHz, CDCl3) δ 7.34 (m,
4H), 6.69 (m, 6H), 3.91 (s, 6H), 1.29 (s, 9H). 13C NMR (100 MHz,
CDCl3) δ 157.26 (C), 147.13 (C), 144.74 (C), 144.20 (C), 127.89
(CH), 125.38 (CH), 120.90 (CH), 111.38 (C), 107.54 (CH), 110.38
(C), 56.12 (CH3), 33.88 (C), 31.25 (CH3). HRMS (APCI) m/z calcd
for [C24H25O2NBr2 + H]+ 518.033 (79Br-79Br) 520.031 (79Br-81Br)
522.0289 (81Br-81Br), found 518.0333 (79Br-79Br) 520.0307
(79Br-81Br) 522.0283 (81Br-81Br).
4H), 7.25 (d, 2H, J = 8 Hz), 7.19 (t, 2H, J = 8 Hz), 7.11 (t, 2H,
J = 8 Hz), 7.00 (d, 4H, J = 8 Hz), 6.90 (d, 2H, J = 8 Hz), 6.77 (d,
4H, J = 8 Hz), 1.30 (s, 9H). 13C NMR (400 MHz, CDCl3) δ 156.76
(C), 155.45 (C), 147.85 (C), 145.00 (C), 144.62 (C), 129.68 (CH),
128.20 (CH), 125.75 (CH), 123.79 (CH), 123.28 (CH), 121.41 (CH),
118.05 (CH), 116.23 (CH), 115.05 (C), 34.10 (C), 31.39 (CH3). HRMS
(APCI) m/z calcd for [C34H29O2NBr2 + H]+ 642.0638 (79Br-79Br)
644.0617 (79Br-81Br) 646.0597 (81Br-81Br), found 642.0648
(79Br-79Br) 644.0623 (79Br-81Br) 646.06 (81Br-81Br).
Synthesis of 11
To the solution of amine 10 (8.19 g, 15.8 mmol) in DMF
(60 mL), t-BuOK (5.30 g, 47.3 mmol), 1-dodecanthiol (10 mL,
42 mmol) was added at 0°C and heated to 100°C for 3 hours.
After cooling to room temperature, the reaction mixture was
poured into ice water. To the flask, 10% HCl aq. was added
to bring the pH to 1, and the mixture was extracted with
AcOEt, and organic extracts were washed with brine and dried
over Na2SO4. After removing solvents followed by washing with
hexane, the desired 11 (4.41 g, 8.98 mmol, 57%) as a white
solid was obtained.
Synthesis of 12c (Ar′ = 3,5-(t-Bu)2C6H3)
The solution of 11 (105 mg, 0.213 mmol) and t-BuOK (71 mg,
0.63 mmol) in dry THF (5.0 mL) under argon was stirred for 30 mi-
nutes at 0°C. To the mixture, Ar2I+OTfꢀ (Ar = (m-t-Bu)2C6H3)
(296 mg, 0.500 mmol) was added at 0°C and then stirred for
2 hours at 40°C. After cooling to room temperature, the reaction
mixture was treated with H2O and the mixture was extracted
with CH2Cl2 and dried over K2CO3. Removal of the solvent
followed by column chromatography (silica gel, CH2Cl2 → AcOEt)
afforded 12c (195 mg, 0.222 mmol, y. 88%) as a white solid.
1H NMR (400 MHz, CDCl3) δ 7.23 (d, 2H, J = 8 Hz), 7.14-7.18 (m,
4H), 6.87 (dd, 2H, J = 1, 8 Hz), 6.84 (d, 2H, J = 2 Hz), 6.77 (d, 2H,
Mp. 157.0°C (decomp.). 1H NMR (400 MHz, CDCl3) δ 7.22 (d, 2H,
J = 8 Hz), 7.13 (t, 2H, J = 8 Hz), 6.85 (d, 2H, J = 8 Hz), 6.70 (d, 2H,
J = 8 Hz), 6.60 (d, 2H, J = 8 Hz), 1.29 (s, 9H). 13C NMR (100 MHz,
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OXIDATION OF ALLENES
J = 8 Hz), 6.73 (dd, 2H, J = 1, 7 Hz), 1.29 (s, 45H). 13C NMR
(400 MHz, CDCl3) δ 156.12 (C), 156.03 (C), 152.64 (C), 147.85 (C),
144.94 (C), 144.86 (C), 128.16 (CH), 125.77 (CH), 123.29 (CH),
121.39 (CH), 177.53 (CH), 115.47 (CH), 114.48 (C), 112.98 (CH),
34.99 (C), 34.18 (C), 31.41 (CH3), 31.38 (CH3). HRMS (APCI) m/z
calcd for [C50H61O2NNr2 + Na]+ 88802961, found 88802970. Anal.
calcd for C50H61Br2NO2: C 69.20, H 7.19, N 1.61%. Found: C 69.23,
H 7.27, N 1.56%.
evaporator; p-toluenesulfonic acid (10 mg, 0.053 mmol) and
CH2Cl2 (1.0 mL) were then added to the residue. After stirring
for 30 minutes at room temperature, the reaction mixture was
purified by column chromatography (silica, CH2Cl2) to afford
the desired olefin 14b (4.0 mg, 0.0078 mmol, 20%).
5b (Ar′ = Ph) To a solution of the ketone 13b (285 mg,
0.559 mmol) and freshly prepared olefin 14b (286 mg,
0.559 mmol) in the dry CH2Cl2 (3.0 mL) under argon, Tf2O
(0.11 mL, 0.67 mmol) was added at room temperature and
heated to reflux for 2 hours. DBU (0.84 mL, 5.6 mmol) was added
to the mixture. The reaction mixture was stirred for 3 hours at
room temperature. The reaction was quenched with saturated
NaHCO3 aq. and extracted with CH2Cl2. The combined organic
layer was washed with brine and dried (K2CO3). Removal of the
solvent followed by washing with CH3CN afforded the allene
compound 5b (234 mg, 0.233 mmol, 42%) as a pale yellow solid.
Synthesis of 13b (Ar′ = Ph)
To a stirring solution of 12b (165 mg, 0.256 mmol) in dry THF
(4.0 mL) under argon, t-BuLi (1.65 M in n-pentane, 0.65 mL,
1.1 mmol) was added at ꢀ78°C. The mixture was stirred for
1 hour at ꢀ78°C. To the mixture, ClCO2Me (0.05 mL, 0.6 mmol)
was added at ꢀ78°C. After stirring for 1 hour at ꢀ78°C and then
for 24 hour at room temperature, the reaction was quenched
with saturated NaHCO3 aq. and extracted with CH2Cl2. The com-
bined organic layer was washed with brine and dried (K2CO3).
Removal of the solvent followed by column chromatography
(silica gel, CH2Cl2 → AcOEt:n-hexane = 1:1) afforded 13b
(66 mg, 0.13 mmol, y. 50%) as a pale yellow solid.
1
Mp. 261.0°C-264.0°C. H NMR (400 MHz, C6D6) δ 7.43 (d, 4H,
J = 8 Hz), 6.99-7.10 (m, 20H), 6.82 (t, 4H, J = 8 Hz), 6.52 (t, 4H,
J = 8 Hz), 6.37 (d, 4H, J = 8 Hz), 5.94 (d, 4H, J = 8 Hz), 1.27 (s,
1
18H). H NMR (400 MHz, CD2Cl2) δ 7.61 (d, 4H, J = 8 Hz), 7.07-
7.12 (m, 12H), 6.95 (t, 4H, J = 8 Hz), 6.74 (d, 8H, J = 8 Hz), 6.63
(d, 4H, J = 8 Hz), 6.12 (d, 4H, J = 8 Hz), 5.64 (d, 4H, J = 8 Hz),
1.41 (s, 18H). 13C NMR (100 MHz, CDCl3) δ 216.64 (C = C = C),
156.96 (C), 155.01 (C), 150.72 (C), 142.54 (C), 138.37 (C), 130.07
(CH), 128.47 (CH), 127.24 (CH), 125.88 (CH), 121.72 (CH), 119.79
(CH), 111.09 (C), 109.76 (CH), 108.15 (CH), 94.07 (C), 34.45 (C),
31.15 (CH3). HRMS (APCI) m/z calcd for [C71H58O4N2 + H]+
1003.4478, found 1003.4469. Anal. calcd for C72H59NO4: C
85.00, H 5.83, N 2.79%. Found: C 85.26, H 5.75, N 2.76%.
1
Mp. 259.0°C-260.0°C. H NMR (400 MHz, CDCl3) δ 7.70 (d, 2H,
J = 8 Hz), 7.30 (m, 6H) 7.23 (t, 2H, J = 8 Hz), 7.08 (m, 6H), 6.55
(d, 2H, J = 8 Hz), 6.37 (d, 2H, J = 8 Hz) 1.46 (s, 9H). 13C NMR
(400 MHz, CDCl3) δ 176.61 (C), 158.43 (C), 157.21 (C), 152.61 (C),
144.96 (C), 137.11 (C), 132.26 (CH), 129.42 (CH), 129.33 (CH),
127.97 (CH), 123.18 (CH), 119.79 (CH), 115.69 (C), 111.14 (CH),
111.08 (CH), 34.91 (C), 31.36 (CH3). HRMS (ESI) m/z calcd for
[C35H30O3N]+ 512.2220, found 512.2220.
Synthesis of 5c (Ar′ = 3,5-(t-Bu)2C6H3)
Synthesis of 13c (Ar′ = 3,5-(t-Bu)2C6H3)
14c (Ar′ = 3,5-(t-Bu)2C6H3): To a solution of acridone 13c
(38 mg, 0.052 mmol) in the dry THF (1.5 mL) under argon, MeLi
(1.14 M in THF, 0.090 mL, 0.10 mmol) was dropwise added at
ꢀ78°C. After the reaction mixture was stirred for 3 hours, the re-
action mixture was treated with saturated NH4Cl aq. The reaction
mixture was extracted with CH2Cl2. The combined organic layer
was washed with brine and dried (K2CO3). The solvent was re-
To a stirring solution of 12c (195 mg, 0.222 mmol) in dry THF
(7.0 mL) under argon, t-BuLi (1.65 M in n-pentane, 0.54 mL,
0.89 mmol) was added at ꢀ78°C. The mixture was stirred for
1 hour at ꢀ78°C. To the mixture, ClCO2Me (0.045 mL, 0.58 mmol)
was added at ꢀ78°C and then stirred overnight at room temper-
ature. The reaction was quenched with saturated NaHCO3 aq.
and extracted with CH2Cl2. The combined organic layer was
washed with brine and dried (K2CO3). Removal of the solvent
followed by column chromatography (silica gel, CH2Cl2:n-
hexane = 1:2 → CH2Cl2) afforded 7b (72 mg, 0.098 mmol, y. 44%)
as a pale yellow oil.
moved using
a rotary evaporator; p-toluenesulfonic acid
(13 mg, 0.068 mmol) and CH2Cl2 (3.0 mL) were then added to
the residue. After stirring for 15 minutes at room temperature,
the reaction mixture was purified by column chromatography
(silica, CH2Cl2) to afford the desired olefin 14c (19 mg,
0.026 mmol, 50%) as a pale yellow oil.
1H NMR (400 MHz, CDCl3) δ 7.68 (d, 2H, J = 8 Hz), 7.29 (d, 2H,
J = 8 Hz), 7.19 (m, 4H,), 7.00 (d, 4H, J = 2 Hz), 6.49 (d, 2H,
J = 8 Hz), 6.28 (d, 2H, J = 8 Hz), 1.43 (s, 9H), 1.29 (s, 36H). 13C
NMR (400 MHz, C6D6) δ 176.24 (C), 159.67 (C), 157.55 (C), 152.73 (C),
152.15 (C), 145.63 (C), 138.01 (C), 132.09 (CH9, 130.00 (CH), 117.51
(CH), 116.80 (C), 110.73 (CH), 110.54 (CH), 35.00 (C), 34.71 (C), 31.47
(CH3), 31.29 (CH3). HRMS (APCI) m/z calcd for [C51H61O3N + H]+
73604724, found 736.4735.
5c (Ar′ = 3,5-(t-Bu)2C6H3): To a solution of the ketone 13c
(19 mg, 0.026 mmol) and freshly prepared olefin 14c (19 mg,
0.026 mmol) in the dry CH2Cl2 (3.0 mL) under argon, Tf2O
(0.0050 mL, 0.030 mmol) was added at room temperature and
heated to reflux for 2 hours. DBU (0.05 mL, 0.33 mmol) was
added to the mixture. The reaction mixture was stirred for
3 hours at room temperature. The reaction was quenched with
saturated NaHCO3 aq. and extracted with CH2Cl2. The combined
organic layer was washed with brine and dried (K2CO3). Removal
of the solvent followed by purification by column chromatogra-
phy (silica, CH2Cl2:hexane = 1:1, 1% NEt3) followed by washing
with hexane afforded the allene 55c (5.0 mg, 0.0034 mmol, y.
13%) as a white solid.
Synthesis of 5b (Ar′ = Ph)
14b (Ar′ = Ph): To a solution of acridone 13b (19.8 mg,
0.0387 mmol) in the dry THF (2.0 mL) under argon, MeLi
(1.14 M in THF, 0.10 mL, 0.11 mmol) was dropwise added at
ꢀ78°C. After the reaction mixture was stirred for 3 hours,
the reaction mixture was treated with saturated NH4Cl aq.
The reaction mixture was extracted with CH2Cl2. The com-
bined organic layer was washed with brine and
dried (K2CO3). The solvent was removed using a rotary
Mp. >300°C. 1H NMR (400 MHz, CDCl3) δ 7.49 (d, 4H, J = 8 Hz),
7.09 (t, 4H, J = 2 Hz), 6.99 (d, 8H, J = 2 Hz), 6.71 (d, 4H, J = 8 Hz),
6.60 (t, 4H, J = 8 Hz), 6.28 (d, 4H, J = 8 Hz), 5.44 (d, 4H, J = 8 Hz),
1.44 (s, 18H) 1.20 (s, 72H). 13C NMR (400 MHz, CDCl3) δ 216.39
J. Phys. Org. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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S.-I. FUKU-EN ET AL.
Table 1. Crystallographic data
Compound
5b
16b·SbCl6
16b·SbCl4
Formula
C71H58N2O4
1003.19
Triclinic
P-1
Pale yellow
block
C83H64Cl6N2O4Sb
1487.81
C95H81Cl4N2O4Sb
1578.17
Molecular weight
Crystal system
Space group
Color
Triclinic
P-1
Dark purple
plate
Triclinic
P-1
Dark purple
plate
Habit
Crystal dimension, mm
a, Å
b, Å
c, Å
α, deg
0.07 × 0.06 × 0.06
10.2353(16)
11.8589(19)
24.222(4)
79.771(2)
82.932(2)
67.411(2)
2666.5(7)
2
0.14 × 0.04 × 0.02
14.052(9)
14.764(9)
20.666(13)
85.465(8)
77.962(8)
70.523(9)
3953(4)
0.1 × 0.04 × 0.04
14.902(8)
16.856(9)
18.712(10)
67.309(7)
76.969(7)
65.316(6)
3926(4)
β, deg
γ, deg
V, Å3
Z
2
2
Dcalc, g/cm3
Abs coeffecient, mmꢀ1
F(000)
1.249
0.077
1060
1.250
0.600
1522
1.334
0.542
1522
Radiation; λ, Å
Temperature, K
Data collected
Data/restrains/parameter
R1 [I > 2σ (I)]
wR2 (all data)
GOF
Mo Kα, 0.710 73
173(2)
Mo Kα, 0.710 73
173(2)
Mo Kα, 0.710 73
173(2)
+h, +k, l
13 786/0/913
0.0671
+h, +k,
l
+h, +k,
l
12 197/0/700
0.0567
13 780/0/862
0.0824
0.1648
1.032
0.1918
0.887
0.1629
1.015
Solv. for crystallization
CH2Cl2/n-hexane
CH2Cl2
benzene/hexane
ꢀ
Reaction of 5b with 1 equivalent of (4-BrC6H4)3N•+SbCl6
(C = C = C), 157.70 (C), 155.49 (C), 151.11 (C), 150.68 (C), 142.01
(C), 138.64 (C), 130.69 (CH), 127.00 (CH), 125.74 (CH),
155.67(CH), 113.85 (CH), 112.18 (C), 110.83 (CH), 108.35 (CH),
34.70 (C), 34.66 (C), 31.45 (CH3), 31.37 (CH3). HRMS (ESI) m/z
calcd for [C103H122O4N2 + H]+ 1451.9465, found 1451.9477. Anal.
calcd for C103H122N2O4 + H2O: C 84.15, H 8.50, N 1.91%. Found: C
84.37, H 8.52, N 1.95%.
The solution of 5b (20 mg, 0.020 mmol) and (4-BrC6H4)3N•+SbCl6
ꢀ
(16 mg, 0.020 mmol) in dry CH3CN (1.0 mL) was stirred for 15 mi-
nutes at ꢀ40°C. After quenching through H2O (0.1 mL) at ꢀ40°C,
the mixture was stirred for 15 minutes at room temperature. The
solvent was removed in vacuo, and then the residue was washed
with hexane to afford dark brown solid (15 mg). Observed by 1H
NMR measurement was 16b·SbClx (X = 4 or 6).
Reaction of 5b with 2 equivalents of (4-BrC6H4)3N•+SbCl6
ꢀ
ꢀ
The solution of 5b (48 mg, 0.048 mmol) and (4-BrC6H4)3N•+SbCl6
Reaction of 5c with 3 equivalents of SbCl5
(78 mg, 0.096 mmol) in dry CH3CN (1.0 mL) was stirred for 15 mi-
nutes at ꢀ40°C. After quenching by H2O (0.2 mL) at ꢀ40°C, the
mixture was stirred for 15 minutes at room temperature. The sol-
vent was removed in vacuo, and then the residue was recrystal-
lized from benzene to afford 16b·SbClx (36 mg, X = 4;
0.028 mmol, 59%, X = 6; 0.027 mmol, 56%) as dark brown
needles.
To a solution of 5c (243 mg, 0.167 mmol) in dry CH2Cl2 (20 mL)
under N2 atmosphere, SbCl5 (1.0 M in CH2Cl2, 0.55 mL,
0.550 mmol) was dropwise added at ꢀ78°C and the reaction
mixture was stirred for 40 minutes. Removal of the solvent
followed by washing with dry Et2O afforded 18c (294 mg,
0.139 mmol, y. 83%) as a dark purple solid. The structure was de-
termined by X-ray analysis.
1
Mp. 230.0°C-231.0°C (decomp.). H NMR (400 MHz, CD3CN) δ
7.99 (dd, 1H, J = 8, 2 Hz), 7.94 (dd, 1H, J = 8, 2 Hz), 7.83 (t, 1H,
J = 8 Hz), 7.65-7.73 (m, 4H, J = 8, 2 Hz), 7.39-7.41 (m, 2H), 7.32-
7.34 (m, 2H), 7.27 (t, 2H, J = 8 Hz), 6.84-7.16 (m, 14H), 7.39-7.41
(m, 2H), 6.69-6.70 (m, 2H), 6.51 (d, 1H, J = 8 Hz), 6.47 (d, 1H,
J = 8 Hz), 6.26 (d, 2H, J = 8 Hz), 6.02 (d, 2H, J = 8 Hz), 5.96
(d, 1H, J = 8 Hz), 5.93 (d, 1H, J = 8 Hz), 5.77-5.80 (m, 2H) 1.50
(s, 9H), 1.39 (s, 9H). HRMS (ESI) m/z calcd for [C71H57O4N2]+
1001.4313, found 1001.4301.
General procedure for ESR (ESTN) measurement
To the mixture of an allenic precursor and 2 equivalents of
ꢀ
(4-BrC6H4)3N•+SbCl6 , degassed CH2Cl2 was added through
vaccuum transfer. Performed were ESR and ESTN analyses of
the solution out.
X-ray crystal structural analysis
UV-vis (CH2Cl2) of a single crystal 16b·SbCl4 795, 563, 536,
373 nm (a single crystal 16b·SbCl6 showed the same absorption
bands.)
Crystals suitable for X-ray structure determination were mounted
on a charge-coupled device diffractometer and irradiated with
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Copyright © 2016 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2016)

OXIDATION OF ALLENES
114, 8747; (l) A. F. M. Noisier, M. A. Brimble, Chem Rev. 2014,
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graphite monochromated Mo-Kα radiation (λ = 0.710 73 Å) at
173 K for data collection. The structures were solved by a direct
method using the SHELX-97 program.[12] Refinement on F2 was
performed using full-matrix least squares with the SHELX-97 pro-
gram. All nonhydrogen atoms were refined using anisotropic
thermal parameters. Selected crystallographic parameters are
summarized in Table 1.
[2] For leading reviews on triplet carbenes see:(a) H. Tomioka, Acc Chem
Res. 1997, 30, 315; (b)H. Tomioka, in Reactive Intermediate Chemistry
(Eds. R. A. Moss, M. S. Platz, M. Jones, Jr.) 2004, Ch. 9, pp. 375-461
(Wiley-VCH: New Jersey, NJ). (c) K. Hirai, T. Itoh, H. Tomioka, Bull
Chem Soc Jpn. 2007, 80, 138; (d) K. Hirai, T. Itoh, H. Tomioka, Chem
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[3] For factors for stabilizing triplet carbenes see:(a) H. L. Woodcock, D.
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14664.
[5] (a) K. Hirai, H. Tomioka, J Am Chem Soc. 1999, 121, 10213; (b)
H. Tomioka, E. Iwamoto, H. Itakura, K. Hirai, Nature. 2001, 412, 626.
[6] M. Kawano, K. Hirai, H. Tomioka, Y. Ohashi, J Am Chem Soc. 2007,
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Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research
on Priority Areas (No. 24109002, Stimuli-Responsive Chemical
Species) and for Science Research B (No. 26288017) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. In addition, we acknowledge the Research Center for
Computational Science, Okazaki, Japan, for the machine time
on their computer system.
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J. Phys. Org. Chem. (2016)
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