Generation and reactivities of triplet diphenylcarbenes protected by bulky groups as para substituents

Article abstract of DOI:10.1002/poc.1898

Bis(2,6-dimethylphenyl)diazomethane (1b-N₂) having 10-(4-tert-butyl-2,6-dimethylphenyl)-2,7-di(phenylethynyl)-9-anthrylethynyl groups at the 4,4′-positions was synthesized. Triplet diphenylcarbene ³1b was generated by photolysis of t

Full text of DOI:10.1002/poc.1898

Research Article
Received: 28 February 2011,
Revised: 10 May 2011,
Accepted: 16 May 2011,
Published online in Wiley Online Library: 18 July 2011
(wileyonlinelibrary.com) DOI 10.1002/poc.1898
Generation and reactivities of triplet
diphenylcarbenes protected by bulky
groups as para substituents
Katsuyuki Hiraia*, Keiji Hatanakab, Tsuyoshi Yamaguchib, Akiko Miyajimab,
Toshikazu Kitagawab* and Hideo Tomiokac,d
Bis(2,6‐dimethylphenyl)diazomethane (1b‐N2) having 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐di(phenylethynyl)‐9‐
anthrylethynyl groups at the 4,4′‐positions was synthesized. Triplet diphenylcarbene 31b was generated by photol-
ysis of the precursor and was characterized by electron spin resonance and UV–Vis spectroscopies at low tempera-
ture, and laser flash photolysis techniques at room temperature. Irradiation of 1b‐N2 in a 2‐methyltetrahydrofuran
3
3
matrix at 77 K gave electron spin resonance signals ascribable to triplet carbene 1b. UV–Vis spectra of 1b were
obtained by irradiating 1b‐N2 under identical conditions. The laser flash photolysis of 1b‐N2 in a degassed benzene
solution gave transient absorption bands ascribable to 31b, which decayed in a first‐order fashion with a rate
constant of 0.46 s–1. The carbene was shown to be less reactive than the triplet bis(2,6‐dimethylphenyl)carbene
(31a) having 9‐anthrylethynyl groups at the 4,4′‐positions, which decayed in a second‐order fashion with a rate
3
constant (2 k/εl) of 0.69 s–1. Triplet carbene 1b was trapped by oxygen to generate ketone oxide 1b‐O with a rate
constant of 1.9 × 107 M–1 s–1 and also by 1,4‐cyclohexadiene with a rate constant of 1.2 M–1 s–1. Similar studies
with diphenyldiazomethanes (1c‐N2 and 1 d‐N2) having bulky substituents, 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐
2,7‐bis[3,5‐di(phenylethynyl)phenylethynyl]‐9‐anthrylethynyl and 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis(9‐
triptycylethynyl)‐9‐anthrylethynyl groups at the 4,4′‐positions respectively indicated that the corresponding triplet
carbenes 31c and 31 d were more persistent than 31b. Steady‐state irradiation of 1a‐N2, 1c‐N2, and 1 d‐N2 in degassed
benzene afforded tetraarylethenes 2a, 2c, and 2d in 83, 50, and 51% yield, respectively. Copyright © 2011 John
Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: diazo compound; kinetics; steric protection; triplet carbene
[3,5‐di(phenylethynyl)phenylethynyl]‐9‐anthrylethynyl group at
INTRODUCTION
the para position was optimized by DFT calculations.[21,22] The
optimized geometry at B3LYP/3‐21G* level of theory indicated
that the two terminal benzene rings are located at both sides
of the carbene carbon and that even a hydrogen on the phenyl
group that is closest to the carbene carbon is 603 pm away from
the carbon (Fig. 1). Therefore, the para substituent may block the
Triplet carbenes are highly reactive organic radicals that are also
notoriously difficult to stabilize.[1–7] To isolate the triplet carbene
with its electronic integrity (one centered diradical) intact, steric
protection is the ideal method. According to this concept,
attempts to stabilize triplet diphenylcarbenes by introducing var-
ious substituents at the ortho positions have been made.[8–11] For
example, triplet bis(2‐bromo‐4‐phenyl‐6‐trifluoromethylphenyl)
carbene was shown to have a half‐life of 40 min in solution at
room temperature,[12] about 10 orders of magnitude greater
than the ‘parent’ diphenylcarbene. The strategy, however,
encounters a limitation when bulky substituents are employed
as protecting groups. Persistent triplet diphenylcarbenes are
generated by photolysis of the nitrogen precursors, diphenyldi-
azomethanes, which are almost always synthesized from the
corresponding benzhydrols via the carbamate pathway.[12–20]
However, synthesis of a carbamate derivative having substitu-
ents bulkier than a trifluoromethyl group at the ortho position
was not successful. That is, generation of a diphenylcarbene
having the bulkier substituents is difficult synthetically.
* Correspondence to: K. Hirai, Instrumental Analysis Facilities, Social Cooperation
Research Center, Mie University, Tsu, Mie 514–8507, Japan.
E‐mail: hirai@chem.mie‐u.ac.jp
a K. Hirai
Instrumental Analysis Facilities, Social Cooperation Research Center, Mie Uni-
versity, Tsu, Mie 514–8507, Japan
b K. Hatanaka, T. Yamaguchi, A. Miyajima, T. Kitagawa
Department of Chemistry for Materials, Graduate School of Engineering, Mie
University, Tsu, Mie 514–8507, Japan
c H. Tomioka
Department of Applied Chemistry, Aichi Institute of Technology, Toyota, Aichi
470–0392, Japan
Is it possible to introduce a substituent at the para position
that is sufficiently bulky to extend around the carbene carbon?
To confirm the possibility of steric protection of carbene by para
substituent, triplet 2,6‐dimethylphenylcarbene having 2,7‐bis
d H. Tomioka
Nagoya Industrial Science Research Institute, Department of Research, Nagoya,
Aichi 464–0819, Japan
J. Phys. Org. Chem. 2011, 24 909–920
Copyright © 2011 John Wiley & Sons, Ltd.

K. HIRAI ET AL.
easy to introduce bulky substituents at the para positions of
sterically hindered diphenyldiazomethanes.
6.03 Å
To test the validity of this idea, we attempted to protect
diphenylcarbene using the para substituents. We employed
the following approach to introduce bulky substituents to
diphenyldiazomethane. First, an anthracene ring was selected
as a base where steric protectors are put on. To prevent
unfavorable reactions on the anthracene base because of a leak
of the unpaired electrons of the carbene, 4‐tert‐butyl‐2,6‐
dimethylphenyl group was introduced at the 10‐position of the
base. Protecting groups were introduced at the 2,7‐positions of
the base using coupling reaction. Second, to the base having
the protectors in place, an ethynyl linker was introduced at the
9‐position. Finally, the resulting bases were introduced at the
para positions of diphenyldiazomethane.
H
Structures projected into both sides from the center of an air-
port building are called a ‘wing’. Because the substituents at 2,7‐
positions of the anthracene in Fig. 1 are similar in appearance to
the wing, we also refer to protecting groups at these positions as
‘wings’. As ‘wing,’ phenylethynyl (PE), 3,5‐bis(phenylethynyl)phe-
nylethynyl (BPEPE), and triptycylethynyl (TrpE) groups were cho-
sen for the synthetic ease. We wish to report here that triplet
diphenylcarbenes 31b, 31c, and 31d with ‘wings,’ generated from
the precursor diazo compounds, were more persistent than the
Figure 1. Optimized structure of triplet 4‐{2,7‐bis[3,5‐di(phenylethynyl)
phenylethynyl]‐9‐anthrylethynyl}‐2,6‐dimethylphenylcarbene at B3LYP/
3‐21G* level of theory
carbene center from external reagents without intramolecular
reaction with carbene.
3
triplet diphenylcarbene without ‘wings,’ 1a (Scheme 1).
In benzene, most sterically hindered diphenylcarbenes decay
by dimerization at the carbene center to give either tetrapheny-
lethenes[13,14,23] or phenanthrenes, which are formed by photo-
cyclization of the ethenes followed by dehalogenation.[17,18,20]
If dimerization can be prevented by the para substituent, the
carbene might become more persistent.
We found that a diphenyldiazomethane derivative, synthe-
sized as a precursor to a persistent triplet carbene, itself is also
stable and hence can be further modified at the para position
with the diazo group intact. For instance, (2,4,6‐tribromophenyl)
(2,6‐dibromo‐4‐tert‐butylphenyl)diazomethane survives under
Sonogashira coupling reaction conditions and undergoes
substitution with trimethylsilylacetylene at the para positions,
leading to (2,6‐dibromo‐4‐trimethylsilylethynylphenyl)(2,6‐dibromo‐
4‐tert‐butylphenyl)diazomethane.[24] That is, it may be relatively
RESULTS AND DISCUSSION
Preparation of para substituents
The para substituents having steric protectors were prepared
according to the reaction procedures outlined in Schemes 2
and 3. Thus, treatment of 3,5‐diiodo‐1‐trimethylsilylethynylben-
zene (3)[25] with phenylacetylene in the presence of (Ph3P)2PdCl2
and CuI in NEt3 at 40 °C for one day, followed by gel permeation
chromatography (GPC) gave 3,5‐bis(phenylethynyl)‐1‐trimethyl-
silylethynylbenzene (4) in 93% yield. The deprotection of the
trimethylsilyl group in 4 proceeded smoothly by treating it with
K2CO3 in MeOH–tetrahydrofuran (THF) at room temperature. 10‐
(4‐tert‐Butyl‐2,6‐dimethylphenyl)‐2,7‐bis[3,5‐bis(phenylethynyl)-
N2
X
X
X
X
hν
Y
Y
Y
Y
1
X
X
X
X
1-N2
a: X = Y = H
b: X =
, Y =
, Y =
c: X =
, Y =
d: X =
Scheme 1. Generation of bis[4‐(9‐anthrylenthnyl)‐2,6‐dimethylphenyl]carbenes 1
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STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE BY PARA SUBSTITUENTS
of the phenylacetylene 5 in 68% yield. Sonogashira coupling of
10 with an excess of trimethylsilylacetylene followed by depro-
tection of the trimethylsilyl group afforded the ethynyl derivative
12 in moderate yield. Ethynylanthracenes 9 and 16 were synthe-
sized in a similar manner from 6, and the corresponding acety-
lenes in 20 and 29% total yields, respectively (Scheme 3).
I
I
i
TMS
X
Coupling of diazomethane with para substituents
4: X = TMS (93%)
5: X = H (95%)
3
ii
Sonogashira coupling of bis(4‐iodo‐2,6‐dimethylphenyl)diazo-
methane (17)[27] with 2.8 equiv of ethynylanthracene 12 resulted
in the formation of a desired diazo compound 1c‐N2 as red crys-
tals in 85% yield under mild conditions (Scheme 4). Diazo-
methanes 1a, b, and d were obtained by similar procedures in
40–75% yields. All of the diazomethanes were purified by re-
peated chromatography on a gel permeation column. The char-
TMS = trimethylsilyl
(i) Pd(PPh3)2Cl, CuI, Et3N, rt. (ii) K2CO3, MeOH-THF, rt.
Scheme 2. Preparation of 3,5‐bis(phenylethynyl)phenylacetylene 5
phenylethynyl]‐9‐bromoanthracene (10) was synthesized by
Sonogashira cross coupling reaction of tribromide 6[26] and 3 equiv
1
acterization of diazo compounds 1a‐d was mainly based on H‐
Br
X
Br
Br
i
6
7: X = Br (35%)
i
8: X =
9: X =
TMS (79%)
H (71%)
ii
13
v
iii
5
X
X
14: X = Br (71%)
v
ii
15: X =
16: X =
TMS (52%)
H (79%)
10: X = Br (68%)
iv
ii
11: X =
12: X =
TMS (61%)
H (79%)
(i) Pd(PPh3)2Cl, CuI, Et3N, rt. (ii) K2CO3, MeOH-THF, rt. (iii) Pd(PPh3)2Cl, CuI, Et3N-THF, 40 °C.
(iv) Pd(PPh3)2Cl, CuI, iPr2NH-benzene, rt. (v) Pd(PPh3)2Cl, CuI, Et3N-benzene, rt.
Scheme 3. Preparation of para substituents 9, 12, and 16
J. Phys. Org. Chem. 2011, 24 909–920
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K. HIRAI ET AL.
unequivocally showing the formation of triplet carbene 31b
(Table 1). The ESR signals were stable at this temperature. The
thermal stability of the triplet carbenes could be estimated by
thawing the matrix containing triplet carbenes gradually and
recooling to 77 K to measure the signals. This procedure can
compensate for a weakening of signals because of the Currie
law.[30–32] When the sample was warmed up to 90 K, a new set
of triplet signals with zfs parameters of D = 0.221 cm–1 and
E = 0.0010 cm–1 (E/D = 0.0046) appeared at the expense of the
original signals (Fig. 2(b), Table 1). ESR signals of sterically hin-
dered diarylcarbenes often become sharper when the measure-
ments are made after the sample is warmed and recooled. This is
partly because as the temperature is raised, the matrix softens
and allows the carbene to assume the most stable geometry
rather than several slightly distorted orientations dictated by
the trapping of the precursor in the frozen matrix.[33–38] The sig-
nal intensity decreased gradually as the sample was warmed up
to 150 K (Fig. 2(c)) and the signal disappeared at 190 K irrevers-
ibly (Fig. 2(d)).
N2
9
1a-N2
(40%)
i
I
I
17
12
ii
1b-N2
(85%)
16
iii
i
1c-N2
(52%)
1d-N2
(75%)
(i) Pd(PPh3)2Cl, CuI, Et3N, 40 °C. (ii) Pd(PPh3)4, CuI, iPr2NH-
benzene, 40 °C. (iii) Pd(PPh3)2Cl, CuI, iPr2NH-benzene, 40 °C.
Scheme 4. Preparation of diazo compounds 1‐N2
Similar irradiation of the other diazomethanes 1a‐N2, 1c‐N2,
and 1 d‐N2 in 2‐MTHF glass at 77 K also gave rise to ESR spectra
characteristic of randomly oriented triplet carbenes (i.e., 31a, 31c,
and 31 d) (Figs. S1–S3 and Table 1).[28,29] When the matrices were
warmed up, signals of 1a, 1c, and 1 d started to decrease at 140,
170, and 160 K, respectively.
1
NMR and infrared (IR) spectroscopy. For example, the H NMR
spectrum of 1c showed three singlet peaks at 1.47, 1.79 and
2.29 ppm, which were assigned to tert‐butyl, inner and outer
methyl groups, respectively, and five aromatic singlet peaks in
the ratio of 1:2:1:1:1 at 7.32, 7.29, 7.48, 7.61, and 8.93 ppm, in
agreement with the calculated values. In the IR spectrum a
strong and sharp band assigned to a diazo group was observed
at 2046 cm–1.
The D and E values of the carbenes 1a–d are significantly
smaller than those of triplet bis(2,6‐dimethylphenyl)carbene
(D = 0.377 cm–1 and E = 0.0101 cm–1),[39] indicating that the two
unpaired electrons of the triplet carbenes 1a–d are delocalized
to the anthracene rings. The zfs parameters of 1a–d are almost
constant regardless of not only the presence or absence of the
‘wings’ but also the types of the ‘wings,’ suggesting that the un-
paired electrons are delocalized only to the anthracene rings.
However, the temperature (Td) at which the ESR signals because
31 started to disappear became higher in the other 1a < 1b < 1
d < 1c. The different tendencies observed for zfs and Td can be
explained in terms of the steric effect of the ‘wings’. We have
reported that the triplet diphenylcarbenes decay in 2‐MTHF at
low temperature by abstracting a hydrogen from the solvent to
form the corresponding diphenylmethyl radicals.[12,19,20] Thus,
large ‘wings’ would retard the approach of 2‐MTHF to the car-
bene center, resulting in the high Tds.
Spectroscopic studies
Electron spin resonance studies in rigid matrix at low temperature
Irradiation (λ > 300 nm) of diazomethane 1b‐N2 having PE
groups in 2‐methyltetrahydrofuran (2‐MTHF) at 77 K gave rise
to electron spin resonance (ESR) signals with a fine structure pat-
[28,29]
tern typical of an unoriented triplet species (Fig. 2(a)).
The
signals were analyzed in terms of zero‐field splitting (zfs) para-
meters to be D = 0.273 cm–1 and E = 0.0075 cm–1 (E/D = 0.0269),
(a)
Z
X
Y
UV–Vis studies in rigid matrices at low temperature
Z
Photolysis (λ > 300 nm) of 1b‐N2 in a 2‐MTHF matrix at 77 K
resulted in the appearance of new bands at the expense of the
original absorption because of the diazo compound (Fig. 3
(Ab)). The new bands consisted of two identifiable features,
strong and sharp maxima at 340 and 353 nm and a broad band
from 530 to 590 nm. These features, that is, strong absorption
bands in the UV region and a broad band in the visible region,
are usually present in the spectra of triplet diphenylcar-
benes.[28,29] The glassy solution exhibited no change for hours
when kept at 77 K, but disappeared irreversibly when it was
allowed to warm to room temperature and was then cooled to
77 K. On the basis of these observations, coupled with the ESR
data, the absorption spectrum can be safely assigned to triplet
carbene 31b.
(b)
X
Y
(c)
(d)
300
400
500
600
Magnetic Field /mT
The thermal stability is then estimated by raising the sample
temperature in 10 K increments to the desired temperature,
allowing it to stand for 15 min, and measuring the absorption
Figure 2. (a) ESR spectrum obtained by irradiation of diazo compound
1b‐N2 in 2‐methyltetrahydrofuran at 77 K. The same sample measured
at 77 K after thawing to 90 K (b), 150 K (c), and 190 K (d)
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STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE BY PARA SUBSTITUENTS
Table 1. ESR and UV–Vis spectroscopic data of carbenes 1
Carbenes
Temp.a (K) |D| (cm–1)
ESR |E| (cm–1)
|E|/|D|
Td (K) Temp.a (K)
UV–Vis λmax (nm)
Td (K)
1a
77
90→77b
77
0.224
0.224
0.273
0.221
0.234
0.234
0.280
0.223
0.0030
0.0018
0.0075
0.0010
0.0013
0.0017
0.0084
0.0015
0.0134
0.0080
0.0269
0.0046
0.0054
0.0074
0.0300
0.0067
140
150
170
160
77
100
77
90
77
100
77
100
342, 437, 480, 516
341, 437, 484, 517
340, 353, 413, 562
140
1b
1c
1d
140
170
160
90→77b
77
344, 352, 392, 421, 531, 572
345, 360, 395, 421, 568
348, 360, 426, 535, 576
313, 328, 341, 385, 512, 549
312, 331, 344, 391, 521, 559
90→77b
77
90→77b
aTemperature at which the spectrum was measured.
bMeasured at 77 K after warming up to 90 K.
bands. When the matrix was warmed gradually to 90 K, the
broad absorption band at 562 nm changed rather dramatically
to a sharp and strong band with its maximum shifting to a longer
wavelength at 572 nm, while the strong band at 353 nm shifted
very little (Fig. 3(Ac), Table 1). Similar changes in the UV–Vis spec-
tra upon thawing the matrix were also observed for sterically
hindered diarylcarbenes and are generally interpreted in terms
of geometrical changes of the carbenes.[28,29,33–38] The bands
started to decay at 140 K and disappeared irreversibly at 230 K
(Fig. 3(B)). The decay‐initiating temperature was almost consis-
tent with that observed in the ESR study.
absorption maxima because of 31 started to disappear are sum-
marized in Table 1. The Tds are almost the same as those deter-
mined by ESR and the general trends are similar. Thus, a rather
high Td is noted for 1c (170 K) and 1d (160 K) compared with
the other carbenes (1a and 1b) (Table 1), which is attributed to
effective steric protection by the BPEPE and TrpE groups at the
para positions.
Laser flash photolysis studies in solution at room temperature
For quantitative evaluation of the stabilities of the present car-
benes, the lifetimes were estimated in a degassed benzene at
room temperature, in which we have measured the lifetime of
a series of other sterically congested diarylcarbenes.[8–10] Laser
flash photolysis (LFP) of 1b‐N2 (5.0 × 10−5 M) in degassed ben-
zene at room temperature with a 10 ns, 100 mJ, 308 nm pulse
from a XeCl laser produced a transient species showing a maxi-
mum at 560 nm, which appeared coincidentally with the pulse
(Fig. 4). The maximum of the band was shifted to a shorter wave-
length than that observed in the photolysis of 1b‐N2 in 2‐MTHF
at 77 K. However, the transient band was markedly quenched by
oxygen to give the corresponding diaryl ketone oxide (vide infra),
Similar spectra were obtained in the photolysis of other diazo
precursors 1‐N2 in 2‐MTHF at 77 K (Figs. S4–S6). The absorption
3
maxima observed for 1 and the temperature (Td) at which the
1.5
(A)
(a) Before irradiation
(b) After irradiation
(c) 90 K
1.0
0.5
0.0
3
and hence we assigned the band to 1b. The absorption bands
decayed very slowly, and did not disappear completely even
300
400
500
600
700
2 s
Wavelength / nm
0.4
1.5
1.0
0.5
0.0
(B)
90 K
140 K
230 K
260 K
0.2
0
2
4
Time / s
0.0
-0.2
300
400
500
600
700
350
400
450
500
550
600
Wavelength / nm
Wavelength / nm
Figure 3. UV–Vis spectra obtained by irradiation of diazo compound
1b‐N2. (A) (a) Spectrum of 1b‐N2 in 2‐methyltetrahydrofuran at 77 K. (b)
The same sample after irradiation (λ > 300 nm). (c) The same sample after
thawing to 90 K. (B) UV–Vis spectral changes measured at 90, 140, 230,
and 260 K, respectively
Figure 4. Transient absorption spectrum obtained in the LFP of diazo
compound 1b‐N2 in degassed benzene at room temperature with a
308 nm excimer laser recorded after 10 µs. The inset shows the time
course of the absorption at 550 nm
J. Phys. Org. Chem. 2011, 24 909–920
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K. HIRAI ET AL.
after 5 s under the employed conditions. The decay was found to
be a first‐order process (k = 0.46 s–1) with a lifetime (τ) of 2.2 s
(Table 2).
observed pseudo‐first‐order growth rate of 1b‐O2 as a function
of [O2] (Fig. 5, Table 2). This is one order of magnitude smaller
than that observed with triplet bis(2,6‐dimethylphenyl)carbene
(kO2 = 1.5 × 108 M–1 s–1).[39] The rate constants kO2 of the other
carbenes (1a, b, and d) were measured and are listed in Table 2,
Photolysis of the other diazomethanes 1a‐N2, 1c‐N2, and 1 d‐N2
was carried out under similar conditions. LFP of 1a‐N2 produced a
3
transient band at 510 nm, which was assigned to 1a (Fig. S7).
which indicate that there is not much difference in the kO2.
The spectrum of 31a decayed with a second‐order kinetics
When a degassed benzene solution of 1b‐N2 containing
1,4‐cyclohexadiene (CHD) was excited, the decay of 31b was ac-
celerated compared with that in the absence of the diene. It is
now well documented that triplet diarylcarbenes, generated
(2 k/εl = 0.69 s–1) and the approximate half‐life (t1/2) was esti-
3
3
mated to be 0.41 s. Because the lifetimes of 1c and 1 d were
too long to monitor by the LFP technique, a conventional UV–
Vis spectroscopic method was more conveniently employed in
these cases. Laser photolysis of these diazomethanes also pro-
duced transient bands at 550 and 552 nm, respectively, which
were assigned to 31c and 31 d, respectively (Figs. S8 and S9).
in good hydrogen donor solvents, undergo
a hydrogen
abstraction leading to the corresponding radical species (1‐H)
(Scheme 5).[40,49–58] The excellent hydrogen donor properties of
CHD have been well recognized.[59] In the photolysis of diazo-
methane 1b‐N2, because the absorption band of the radical
3
The absorption band of 1c decayed slightly more slowly than
3
that of 31b. The decay was also found to be a first‐order process
(k = 0.40 s–1, τ = 2.5 s). The lifetimes of 31b and 31c were five
times longer than the half‐life of 31a. Triplet carbene 31d having
triptycyl groups also decayed in a first‐order fashion. The decay
(k = 0.10 s–1, τ =10 s) was the slowest among the four carbenes
studied (Table 2). The parent 1a decayed in a second‐order fash-
ion, while the carbenes having the ‘wings’ (i.e., 1b, 1c, and 1d)
exhibited in first‐order kinetics, which suggests that the PhE,
BPEPE, and TrpE groups on the para substituents prevented
the dimerization of the carbenes under the LFP conditions.
The rate constants of the decays cannot be compared directly
because of the differing reaction order of the decays of 31a and
the others. The rate constant of the reactions of triplet carbenes
with a typical triplet quencher can be employed as a more quan-
titative scale of reactivity. In general, carbenes with triplet
ground states are readily trapped with oxygen or a good hydro-
gen donor such as 1,4‐cyclohexadiene (CHD).[40] Therefore, the
rate constants for the trapping by O2 (kO2) and CHD (kCHD) serve
as more quantitative scales for the estimation of the reactivities
of triplet carbenes. The LFP of 1b‐N2 in O2 saturated benzene
resulted in a dramatic decrease in the lifetime of 31b and a con-
current appearance of a new broad absorption band at approxi-
mately 470 nm. The spent solution was found to contain the
corresponding benzophenone. It is well‐documented[41–48] that
diarylcarbenes with triplet ground states are readily trapped by
oxygen to generate the corresponding diaryl ketone oxides,
which show a broad absorption band from 360 to 480 nm
(Scheme 5). Thus, our observations can be interpreted as indicat-
ing that the transient absorption quenched by oxygen is due to
31b. The rate of increase in the band at 470 nm is practically the
1b‐H overlapped the tail band of the carbene 1b, the growth
curve of the radical could not be observed precisely. However,
a plot of the observed pseudo‐first‐order rate constants of the
decay of 31b monitored at 550 nm against the diene concentra-
tions [CHD] is linear (Fig. 6), and the slope of this plot yields the
absolute rate constant for the reaction of 31b with the diene,
kCHD = 1.2 M–1 s–1, which is approximately 1/80 of that observed
with triplet bis(2,6‐dimethylphenyl)carbene (kCHD = 94 M–1 s–1).[39]
The decay rates of the other carbenes 1a, 1c, and 1d were also
accelerated in the presence of the diene (Table 2), indicating
that these carbenes were also quenched by the diene. The
data indicate that the rate constants of the bimolecular reactions
of 31b, 31c, and 31d with the diene (1.2, 1.5, and 7.3 M–1 s–1,
respectively) are approximately one order smaller than kCHD of
31a (48 M–1 s–1).
Although carbenes 1a–d showed similar bimolecular reaction
rate constants (kO2) with oxygen, the rate constants with CHD
(kCHD) varied on the order of 1a = 1b < 1d < < 1c. The difference
in the trend of kO2 and kCHD is explained in terms of the reactiv-
ities and sizes of oxygen and the diene. Because oxygen is much
more reactive than the diene toward triplet carbenes, differences
among the rate constants kO2 are negligibly small compared
with kCHD. In addition, an oxygen molecule is too small to be
blocked by the ‘wings’. However, because 1,4‐cyclohexadiene is
evidently a larger molecule than oxygen, the ‘wings’ can protect
the carbenic carbon from the diene. On the basis of the observed
values of kCHD, the TrpE group is a less efficient protecting group
than the PE and BPEPE groups. This may be explained in terms of
poor electronic stabilization of the carbenes by the TrpE groups.
3
same as the rate of decrease of the peak because of 1b, show-
Product analysis studies
ing that 31b is quenched with oxygen to form ketone oxide
1b‐O2. The rate constant (kO2) for the quenching of 31b by
oxygen is determined to be 1.9 × 107 M–1 s–1 from a plot of the
Most of the sterically congested triplet diphenylcarbenes decay
mainly by undergoing dimerization at the carbene center to give
Table 2. Kinetic data of carbenes 1
Carbenes
k (s–1)
τ (s)
kO2 (M–1 s–1)
kCHD (M–1 s–1)
1a
1b
1c
1d
0.69a
0.46
0.40
0.10
0.41b
2.2
2.5
2.4 × 107
1.9 × 107
2.0 × 107
1.4 × 107
48
1.2
1.5
7.3
10
a2 k/εl,
bt1/2.
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STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE BY PARA SUBSTITUENTS
0.8
0.6
0.4
0.2
O
O
O
N2
Ar
1-N2
Ar
Ar
Ar
Ar
1-O
Ar
1-O2
h
–N2
kO2
O2
ISC
ISC: intersystem
crossing
Ar
Ar
Ar
Ar
11
31
kCHD
Ar
Ar
1-H2
Ar
Ar
1-H
0.0
0.1
0.2
0.3
0.4
[CHD] / M
Figure 6. A plot of the decay rate constants of 31b monitored at 550 nm
as a function of the concentration of 1,4‐cyclohexadiene
Scheme 5. Photolysis of diazo compounds 1‐N2 in the presence of
carbene quenchers
tetraarylethenes as the main product, when generated in
degassed benzene at room temperature in the absence of ap-
propriate carbene quenchers (Scheme 6).[13,14,23] To determine
the decay pathway of the present carbenes, we carried out prod-
uct analysis of the photolysis of 1a‐N2, 1c‐N2, and 1d‐N2 in
degassed benzene at room temperature.[60]
Photolysis (>300 nm) of 1d‐N2 (5.0 mg) in degassed benzene‐
d6 (0.5 mL) in a sealed NMR tube at 25 °C was monitored by
1H‐NMR spectroscopy. The irradiation for 6 min gave new
peaks at 9.71, 5.09, 1.93, 1.65, 1.51, and 1.46 ppm along with
weak complex peaks at the expense of the signals of 1d‐N2.
The 1H‐NMR spectrum of a fraction obtained by recycled
GPC of the photoproducts showed the dimerization product,
tetraarylethene 2d (yellow crystals in 51% yield) (Scheme 6). In
the photolysis, it is reasonable that ethene 2d was formed as a
result of dimerization at the carbene center. Similar irradiations
of 1a‐N2 and 1c‐N2 also afforded the corresponding ethenes
2a and 2c in 83 and 50% yields, respectively. It is rather
surprising that even heavily congested carbenes 1c and 1d gave
a considerable amount of dimers.
degassed benzene. The dimerization of carbenes with huge
‘wings’ should be very slow compared with 1a. Furthermore,
the concentration of 1 (~10–5 M) in the LFP measurements is
three orders of magnitude lower than in the product study
(~10–2 M). Although highly pure special grade benzene was used
in the LFP study, the observed first‐order‐decays in the LFP could
be because of the quenching of carbenes 1c and 1d with a
residual amount of impurities in the solvent. Alternatively, in
the LFP conditions, the carbenes may decay by an intramo-
lecular C–H insertion to the ortho methyl groups.
To check the reaction mode of 1, we also carried out an
analysis of the photoproducts from 1d‐N2, which generates
the longest‐lived carbene 1d, in the presence of the
carbene quenchers, oxygen and 1,4‐cyclohexadiene. Photolysis
(>300 nm) of 1d‐N2 in an oxygen‐saturated benzene solution
afforded diaryl ketone 1d‐O in 89% yield (Scheme 5). Formation
of the ketone suggests that the intermediate observed in
the oxygen‐saturated solution in the LFP experiments is a
ketone oxide 1‐O2. On the other hand, irradiation (>300 nm)
of 1d‐N2 in degassed benzene containing 1,4‐cyclohexadiene
afforded diarylmethane 1d‐H2 in 75% yield, as a result of the
double‐hydrogen abstraction reaction of triplet 31d from the
diene (Scheme 5). A diphenyl ketone and a diphenylmethane
are well‐known products frequently obtained by reactions of
triplet diphenylcarbene with O2 and 1,4‐cyclohexadiene,
respectively.
This result seems inconsistent with the fact that in the LFP
experiments 1c and 1d decayed with first‐order kinetics in
1.6
1.2
0.8
0.4
0.0
N2
h
Ar
Ar
Ar
Ar
1-N2
benzene
–N2
1
Ar
Ar
Ar
1
0
2
4
6
8
10
Ar
[O2] / mM
2
Figure 5. A plot of the growth rate constants of the diaryl ketone oxide
1b‐O2 monitored at 470 nm as a function of the concentration of oxygen
Scheme 6. Photolysis of diazo compounds 1‐N2 in degassed benzene
at 25 °C
J. Phys. Org. Chem. 2011, 24 909–920
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K. HIRAI ET AL.
374 (M+, 57), 359 (100), 309 (15), 180 (18); HRMS calcd for
C27H22Si 374.1491, found m/z 374.1464.
CONCLUSION
In the present study, diphenyldiazomethanes 1‐N2 having either
the PE, BPEPE, or TrpE groups on the para substituents were syn-
thesized, and triplet carbenes generated by their photolysis were
characterized by ESR and UV–Vis spectroscopies at low tempera-
Preparation of 1‐ethynyl‐3,5‐bis(phenylethynyl)benzene (5)
A mixture of 4 (190 mg, 0.51 mmol), K2CO3 (245 mg, 1.78 mmol),
MeOH (6 mL), and THF (6 mL) was stirred at room temperature
for 30 min. After workup of the reaction mixture, the crude prod-
uct was purified by GPC (5 cycles, chloroform, monitored at
260 nm) to afford 1‐ethynyl‐3,5‐bis(phenylethynyl)benzene (5)
as yellow crystals (145 mg, 95%): mp 96.1–96.7 °C; IR (KBr, cm–1)
3
3
3
tures. Triplet carbenes 1b, 1c, and 1d decayed in degassed
benzene at room temperature in a first‐order fashion with rate
constants of 0.46, 0.49, and 0.10 s–1, respectively, which were
3
smaller than that of the parent 1a. The rate constants of the
3
3
3
3
reaction of 1b, 1c, and 1d with oxygen were similar to 1a
having no ‘wings’. On the other hand, the rate constants with
1,4‐cyclohexadiene were greatly changed depending on the
type of the ‘wings’. The major photoproducts from 1c‐N2 and
1d‐N2 in degassed benzene were tetraarylethenes 2c and 2d,
respectively, in contrast to the kinetics of decay derived from
LFP experiments. The results of the present study demonstrated
that the introduction of large ‘wings’ on the para substituents is
a promising new strategy for the kinetic stabilization of
diphenylcarbene.
1
3303 (s, νC≡C‐H), 2211 (w, νC≡C); H‐NMR (300 MHz, CDCl3, ppm)
δ 7.67 (t, J = 1.6 Hz, 1H), 7.60 (d, J = 1.5 Hz, 2H), 7.54–7.51
(m, 4H), 7.38–7.36 (m, 6H), 3.11 (s, 1H); 13 C‐NMR (75.5 MHz,
CDCl3, ppm) δ 134.6, 134.5, 131.7, 128.7, 128.4, 124.1, 122.8,
122.7, 90.6, 87.6, 82.0, 78.3; EI‐MS (m/z) 302 (M+, 100), 300 (22);
HRMS calcd for C24H14 302.1096, found m/z 302.1134.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
(phenylethynyl)‐9‐bromoanthracene (7)
To a mixture of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7,9‐tribro-
moanthracene (6)[26] (30 mg, 52 µmol), bis(triphenylphosphine)
palladium(II) dichloride (0.5 mg, 0.7 µmol), and copper(I) iodide
(0.4 mg, 2.0 µmol) in anhydrous triethylamine (0.5 mL) was added
phenylacetylene (23 μL, 210 µmol) at room temperature under
an atmosphere of a 1:1 mixture of nitrogen and hydrogen, and
the mixture was stirred at 35 °C for 13 h. After evaporation of
the solvent, the residue was passed through a short column
(aluminum oxide deactivated with 5% water, chloroform at
0 °C) and then purified by preparative TLC (silica gel, hexane) to
give 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis(phenylethynyl)‐
9‐bromoanthracene (7) as yellow crystals (11 mg, 35%): mp
EXPERIMENTAL SECTION
General information
1H and 13C‐NMR spectra were determined with a FT NMR spec-
trometer (JNM-AL300, JEOL) in CDCl3 with Me4Si as an internal
reference. IR spectra were measured either with films on a NaCl
plate (oils and semisolids) with KBr pellets (solids). IR spectra
were recorded on an IR spectrometer (FT/IR400, JASCO) and
UV–Vis spectra were recorded on a UV-Vis spectrometer (V-560,
JASCO or MultiSpec-1500, Shimadzu). The mass spectra were
recorded on an EI mass spectrometer (JMS-600H, JEOL) or a
MALDI-TOF mass spectrometer (Voyager DE-Pro, Applied Biosys-
tems). Thin layer chromatography (TLC) was performed using
Merck silica gel 60 F254 or Merck aluminum oxide 60 PF254 on a
glass plate (Type E) for diazo compounds. Column chromatogra-
phy was performed using silica gel (63–210 µm) or neutral alu-
mina (Act. I, inactivated with 5% water) for diazo compounds.
Recycle GPC was undertaken with a HPLC pump (PU-2086,
JASCO) with a UV-Vis detector (UV-2070, JASCO) using a GPC col-
umn (H-2001, 20mm× 50 cm, Shodex) .
1
278.9–281.9 °C; IR (KBr, cm–1) 2210 (w, νC≡C); H‐NMR (300 MHz,
CDCl3, ppm) δ 8.87 (s, 2H), 7.62–7.61 (m, 4H), 7.43–7.42 (m, 6H),
7.40–7.37 (m, 4H), 7.25 (s, 2H), 1.74 (s, 6H), 1.45 (s, 9H); 13 C‐NMR
(75.5 MHz, CDCl3, ppm) δ 151.0, 136.9, 136.8, 136.1, 133.3,
131.8, 130.6, 130.1, 128.6, 128.5, 128.4, 126.8, 124.7, 123.1,
122.3, 121.8, 91.4, 89.8, 34.5, 31.5, 20.4; EI‐MS (m/z) 618
(M + 2, 99), 616 (M+, 100), 522 (23), 368 (22), 236 (27); HRMS calcd
for C42H33Br 616.1765, found m/z 616.1667.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
(phenylethynyl)‐9‐trimethylsilylethynylanthracene (8)
Preparation of 3,5‐bis(phenylethynyl)‐1‐trimethylsilylethy-
nylbenzene (4)
To a mixture of 9‐bromoanthracene 7 (10.6 mg, 17 µmol), bis
(triphenylphosphine)palladium(II) dichloride (0.5 mg, 0.7 µmol),
and copper(I) iodide (0.2 mg, 1.0 µmol) in anhydrous triethylamine
(0.5mL) was added trimethylsilylacetylene (24 μL, 170 µmol) at
room temperature under an atmosphere of a 1:1 mixture of nitro-
gen and hydrogen, and the mixture was stirred at 50°C for 16 h.
After evaporation of the solvent, the residue was passed through
a short column (aluminum oxide, chloroform) and then purified
by GPC (14 cycles, chloroform, monitored at 300 nm) to give
10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis(phenylethynyl)‐9‐
trimethylsilylethynylanthracene (8) as yellow crystals (8.5 mg,
79%): mp 262.5–264.6 °C; IR (KBr, cm–1) 2208 (w, νC≡C), 2140
(w, νC≡C); 1H‐NMR (300 MHz, CDCl3, ppm) δ 8.82 (s, 2H), 7.62–
7.60 (m, 4H), 7.42–7.37 (m, 10H), 7.25 (s, 2H), 1.73 (s, 6H), 1.44
(s, 9H), 0.50 (s, 9H); 13 C‐NMR (75.5 MHz, CDCl3, ppm) δ 150.9,
137.8, 136.8, 133.4, 132.7, 131.8, 131.0, 129.0, 128.5, 128.4,
126.7, 124.6, 123.2, 121.6, 116.7, 108.0, 101.3, 91.1, 90.2, 34.5,
To a mixture of 3,5‐diiodo‐1‐trimethylsilylethynylbenzene (3)[25]
(231mg, 0.54 mmol), bis(triphenylphosphine)palladium(II) dichlor-
ide (8.1mg, 11µmol), and copper(I) iodide (4.0mg, 21 µmol) in
anhydrous triethylamine (6mL) was added phenylacetylene
(155μL, 1.63 mmol) at room temperature under an atmosphere
of a 1:1 mixture of nitrogen and hydrogen and the mixture was
stirred for 3 h. After evaporation of the solvent, the residue was
passed through a short column (aluminum oxide deactivated
with 5% water, chloroform) at 0 °C and then purified by pre-
parative TLC (silica gel, hexane) to give 3,5‐bis(phenylethynyl)‐
1‐rimethylsilylethynylbenzene (4) as
a yellow viscous oil
(190 mg, 93%): IR (NaCl, neat, cm–1) 2214 (w, νC≡C); 1H‐NMR
(300 MHz, CDCl3, ppm) δ 7.62 (t, J = 1.6 Hz, 1H), 7.58 (d,
J = 1.6 Hz, 2H), 7.53–7.50 (m, 4H), 7.36–7.34 (m, 6H), 0.26 (s, 9H);
13 C‐NMR (75.5 MHz, CDCl3, ppm) δ 134.5, 134.4, 131.8, 128.7,
128.5, 124.1, 124.0, 122.9, 103.4, 95.8, 90.6, 87.9, 0.0; EI‐MS (m/z)
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STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE BY PARA SUBSTITUENTS
31.5, 20.3, 0.3; EI‐MS (m/z) 634 (M+, 100), 618 (5.6), 309 (5.4), 288
(8.6); HRMS calcd for C47H42Si 634.3056, found m/z 634.3107.
trimethylsilylethylylanthracene 11 as yellow crystals (50 mg,
61%): mp 164.4–165.0 °C; IR (KBr, cm–1) 2212 (w, νC≡C), 2142 (w,
C≡C); 1H‐NMR (300 MHz, CDCl3, ppm) δ 8.83 (s, 2H), 7.73 (d,
ν
J = 1.5 Hz, 4H), 7.69 (t, J = 1.5 Hz, 2H), 7.57–7.54 (m, 8H), 7.44‐
7.43 (m, 4H), 7.39–7.36 (m, 12H), 7.26 (s, 2H), 1.74 (s, 6H), 1.45
(s, 9H), 0.53 (s, 9H); 13 C‐NMR (75.5 MHz, CDCl3, ppm) δ 150.7,
137.6, 133.9, 133.8, 133.0, 132.3, 131.4, 131.0, 128.8, 128.3,
128.1, 126.5, 124.4, 123.8, 123.7, 122.5, 120.9, 116.7, 108.0,
100.8, 90.9, 90.3, 89.3, 87.5, 34.2, 3und m/z 1034.57.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐9‐ethynyl‐
2,7‐bis(phenylethynyl)anthracene (9)
A mixture of 8 (7.0 mg, 11 µmol), K2CO3 (5.2 mg, 38 µmol), MeOH
(0.5 mL), and THF (0.5 mL) was stirred at room temperature for 25
min. After evaporation of the solvent, the residue was extracted
with chloroform. The organic layer was washed with brine and
dried over anhydrous Na2SO4, and the solvent was removed un-
der reduced pressure. The crude product was purified by GPC
(6 cycles, chloroform, monitored at 300 nm) to afford 10‐(4‐
tert‐butyl‐2,6‐dimethylphenyl)‐9‐ethynyl‐2,7‐bis(phenylethynyl)
anthracene (9) as yellow crystals (4.4 mg, 71%): mp 286.3–289.1 °C;
IR (NaCl, neat cm–1) 3301 (s, νC≡C‐H), 2207 (w, νC≡C), 2139 (w, νC≡C);
1H‐NMR (300 MHz, CDCl3, ppm) δ 8.85 (s, 2H), 7.62–7.61 (m, 4H),
7.44–7.37 (m, 10H), 7.24 (s, 2H), 4.14 (s, 1H), 1.74 (s, 6H), 1.45
(s, 9H); 13 C‐NMR (75.5 MHz, CDCl3, ppm) δ 151.0, 138.2, 136.8,
133.3, 133.0, 131.8, 131.0, 129.0, 128.6, 128.5, 128.4, 126.8,
124.6, 123.1, 121.8, 115.6, 91.2, 90.0, 89.6, 80.0, 34.5, 31.5, 20.3;
EI‐MS (m/z) 562 (M+, 48), 561 (100), 368 (10), 273 (11), 83 (11);
HRMS calcd for C44H34 562.2660, found m/z 562.2678.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
[3,5‐bis(phenylethynyl)phenylethynyl]‐9‐ethynylanthracene
(12)
A mixture of 11 (22 mg, 21 µmol), K2CO3 (7.3 mg, 53 µmol), MeOH
(2 mL), and THF (2 mL) was stirred at room temperature for 1 h.
After evaporation of the solvent, the residue was extracted with
chloroform. The organic layer was washed with water and dried
over anhydrous Na2SO4, and the solvent was removed under re-
duced pressure. The crude product was purified by GPC (5 cycles,
chloroform, monitored at 300 nm) to afford ethynylanthracene
12 as yellow crystals (16 mg, 79%): mp 159.8–161.6 °C; IR (KBr,
cm–1) 3303 (w, νC≡C‐H), 2252 (m, νC≡C), 2213 (w, νC≡C); 1H‐NMR
(300 MHz, CDCl3, ppm) δ 8.87 (s, 2H), 7.74 (d, J = 1.5 Hz, 4H),
7.68 (t, J = 1.5 Hz, 2H), 7.56–7.53 (m, 8H), 7.47 (d, J = 9.0 Hz, 2H),
7.43 (dd, J = 9.0, 1.3 Hz, 2H), 7.39–7.36 (m, 8H), 7.35 (s, 4H), 7.27
(s, 2H), 4.16 (s, 1H), 1.76 (s, 6H), 1.45 (s, 9H); 13 C‐NMR (75.5 MHz,
CDCl3, ppm) δ 151.1, 138.3, 136.8, 134.3, 134.2, 133.2, 133.0,
131.7, 131.1, 129.1, 128.6, 128.4, 128.3, 126.9, 124.7, 124.1,
123.9, 122.8, 121.4, 115.9, 91.2, 90.6, 89.9, 89.7, 87.8, 79.9, 34.5,
31.5, 20.3; MALDI‐TOF‐MS calcd for C76H50 962.39, found
m/z 962.52.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
[3,5‐bis(phenylethynyl)phenylethynyl]‐9‐bromoanthracene (10)
To a mixture of tribromoanthracene 6 (92 mg, 0.16 mmol), bis(tri-
phenylphosphine)palladium(II) dichloride (10 mg, 14 µmol), and
copper(I) iodide (5.0 mg, 26 µmol) in anhydrous triethylamine
(10 mL) was added a solution of 5 (145 mg, 0.48 mmol) in anhy-
drous tetrahydrofuran (7 mL) at room temperature under an
atmosphere of a 1:1 mixture of nitrogen and hydrogen, and the
mixture was stirred at 40 °C for 13 h. After evaporation of the sol-
vent, the residue was passed through a short column (aluminum
oxide deactivated with 5% water, chloroform) and then purified
by preparative GPC (6 cycles, chloroform, monitored at 300 nm)
to give bromoanthracene 10 as yellow crystals (110 mg, 68%):
mp 157.9–160.7 °C; IR (KBr, cm–1) 2252 (m, νC≡C), 2211 (w, νC≡C);
1H‐NMR (300 MHz, CDCl3, ppm) δ 8.83 (s, 2H), 7.74 (d, J = 1.7 Hz,
4H), 7.69 (t, J = 1.7 Hz, 2H), 7.56–7.53 (m, 8H), 7.48–7.42 (m, 4H),
7.39–7.36 (m, 12H), 7.27 (s, 2H), 1.76 (s, 6H), 1.46 (s, 9H); 13 C‐
NMR (75.5 MHz, CDCl3, ppm) δ 151.1, 137.0, 136.8, 134.3, 134.2,
133.2, 132.2, 131.7, 130.6, 130.3, 128.6, 128.5, 128.4, 127.0,
124.7, 124.2, 123.8, 122.8, 122.1, 121.9, 90.9, 90.7, 89.9, 87.8,
34.5, 31.5, 20.4; MALDI‐TOF‐MS calcd for C74H49Br 1016.30, found
m/z 1016.41.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
(9‐triptycylethynyl)‐9‐bromoanthracene (14)
To a mixture of tribromoanthracene 6 (131 mg, 0.23 mmol),
9‐ethynyltriptycene (13)[61] (152 mg, 0.60 mmol), bis(triphenyl-
phosphine)palladium(II) dichloride (16 mg, 23 µmol), and copper
(I) iodide (4.3 mg, 23 µmol) were added anhydrous triethylamine
(13 mL) and anhydrous benzene (13 mL) at room temperature
under an atmosphere of a 1:1 mixture of nitrogen and hydrogen,
and the mixture was refluxed for 16 h. After evaporation of the
solvent, the residue was passed through a short column (silica
gel, chloroform) and then purified by preparative GPC (6
cycles, chloroform, monitored at 300 nm) to give bromoanthra-
cene 14 as yellow crystals (152 mg, 71%): mp 307.3–309.4 °C;
IR (KBr, cm–1) 2232 (w, νC≡C); 1H‐NMR (300 MHz, CDCl3, ppm)
δ 9.14 (s, 2H), 7.91–7.88 (m, 6H), 7.75(dd, J = 1.5, 9.0 Hz, 2H),
7.62 (d, J = 9.0 Hz, 2H), 7.43 (dd, J = 6.8, 1.3 Hz, 6H), 7.32 (s, 2H),
7.14–7.04 (m, 12H), 5.48 (s, 2H), 1.85 (s, 6H), 1.48 (s, 9H); 13 C‐
NMR (75.5 MHz, CDCl3, ppm) δ 151.2, 144.4, 144.3, 137.3, 136.9,
133.2, 132.3, 130.8, 130.5, 129.2, 127.2, 125.8, 125.3, 124.8,
123.5, 122.6, 122.1, 93.1, 86.1, 53.7, 53.3, 34.6, 31.5, 29.7, 20.4;
MALDI‐TOF‐MS calcd for C70H49Br 968.30, found m/z 968.33.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
[3,5‐bis(phenylethynyl)phenylethynyl]‐9‐trimethylsilylethy-
nylanthracene (11)
To a mixture of the 9‐bromoanthracene 10 (58 mg, 57 µmol), bis
(triphenylphosphine)palladium(II) dichloride (7.0 mg, 6.0 µmol),
copper(I) iodide (1.0 mg, 5.0 µmol), anhydrous diisopropylamine
(3 mL), and anhydrous benzene (3 mL) was added trimethylsilyla-
cetylene (10 μL, 0.14 mmo l) at room temperature under an ar-
gon atmosphere, and the mixture was refluxed for 15 h. After
evaporation of the solvent, the residue was passed through a
short column (aluminum oxide, chloroform) and then purified
by GPC (7 cycles, chloroform, monitored at 300 nm) to give
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
(9‐triptycylethynyl)‐9‐trimethylsilylethynylanthracene (15)
To a mixture of 9‐bromoanthracene 14 (78 mg, 84 µmol), bis
(triphenylphosphine)palladium(II) dichloride (12 mg, 17 µmol),
J. Phys. Org. Chem. 2011, 24 909–920
Copyright © 2011 John Wiley & Sons, Ltd.
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K. HIRAI ET AL.
copper(I) iodide (3.2 mg, 17 µmol), anhydrous triethylamine
(15 mL), and anhydrous benzene (15 mL) was added trimethylsi-
lylacetylene (60 μL, 424 µmol) at room temperature under an at-
mosphere of a 1:1 mixture of nitrogen and hydrogen, and the
mixture was refluxed for 19 h. After evaporation of the solvent,
the residue was passed through a short column (silica gel, chlo-
roform) and then purified by GPC (17 cycles, chloroform, moni-
tored at 300 nm) to give trimethylsilylethylylanthracene 15 as
yellow crystals (45 mg, 52%): mp 318.3–322.0 °C; IR (KBr, cm–1)
2142 (w, νC≡C); 1H‐NMR (300 MHz, CDCl3, ppm) δ 9.16 (s, 2H),
7.92–7.89 (m, 6H), 7.74 (dd, J = 9.0, 1.4 Hz, 2H), 7.60 (d,
J = 9.0 Hz, 2H), 7.44–7.42 (m, 6H), 7.32 (s, 2H), 7.13‐7.03 (m,
12H), 5.47 (s, 2H), 1.83 (s, 6H), 1.48 (s, 9H), 0.51 (s, 9H); 13 C‐NMR
(75.5 MHz, CDCl3, ppm) δ 151.1, 144.4, 144.4, 138.1, 136.8,
133.3, 132.8, 131.8, 129.3, 128.9, 127.1, 125.8, 125.2, 124.7,
123.5, 122.6, 122.4, 116.9, 108.3, 101.2, 93.3, 85.6, 53.7, 53.3,
34.6, 31.5, 20.3, 0.22; MALDI‐TOF‐MS calcd for C75H58Si 986.43,
found m/z 986.45.
Preparation of bis{4‐[10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐
2,7‐bis(phenylethynyl)‐9‐anthrylethynyl]‐2,6‐dimethylphe-
nyl}diazomethane (1b‐N2)
To a mixture of diazomethane 17 (6.5 mg, 13 µmol), 10‐(4‐tert‐
butyl‐2,6‐dimethylphenyl)‐9‐ethynyl‐2,7‐bis(phenylethynyl)an-
thracene (9) (14 mg, 25 µmol), bis(triphenylphosphine)palladium
(II) dichloride (0.8 mg, 1.2 µmol), and copper(I) iodide (0.4 mg,
2.4 µmol) was added anhydrous triethylamine (1 mL) at room
temperature under an atmosphere of a 1:1 mixture of nitrogen
and hydrogen, and the mixture was stirred at 40 °C for 11 h. After
evaporation of the solvent, the residue was passed through a
short column (aluminum oxide deactivated with 5% water,
chloroform) at 0 °C and then purified by GPC (9 cycles, chloro-
form, monitored at 300 nm) to give diazomethane 1b‐N2 as a
red semisolid (7.2 mg, 40%): IR (NaCl, neat, cm–1) 2204 (νC≡C),
2041 (s, νC=N2); 1H‐NMR (300 MHz, CDCl3, ppm) δ 8.91 (s, 4H),
7.63–7.61 (m, 12H), 7.45 (m, 4H), 7.38–7.34 (m, 16H), 7.25 (s, 4H),
2.30 (s, 12H), 1.77 (s, 12H), 1.46 (s, 18H); 13 C‐NMR (75.5 MHz,
CDCl3, ppm) δ 150.9, 137.6, 137.5, 136.8, 133.4, 132.5, 132.3,
131.7, 130.8, 130.0, 129.1, 128.6, 128.5, 128.4, 126.8, 124.6, 123.2,
122.2, 121.5, 117.0, 101.9, 91.2, 90.2, 86.5, 59.5, 34.5, 31.5, 20.9, 20.4.
Preparation of 10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis
(9‐triptycylethynyl)‐9‐ethynylanthracene (16)
A mixture of 15 (84 mg, 81 µmol), K2CO3 (39 mg, 282 µmol),
MeOH (30 mL), and THF (40 mL) was stirred at room temper-
ature for 4 h. After evaporation of the solvent, the residue
was extracted with chloroform. The organic layer was
washed with water, and dried over anhydrous Na2SO4, and
the solvent was removed under reduced pressure. The crude
product was purified by GPC (3 cycles, chloroform, monitored
at 300 nm) to afford ethynylanthracene 16 as yellow crystals
(16 mg, 79%): mp 142.2–144.5 °C; IR (KBr, cm–1) 3300 (w, νC≡C‐H),
2239 (w, νC≡C); 1H‐NMR (300 MHz, CDCl3, ppm) δ 9.17 (s, 2H),
7.91–7.89 (m, 6H), 7.76 (dd, J = 9.0, 1.7 Hz, 2H), 7.63 (d,
J = 8.5 Hz, 2H), 7.43–7.41 (m, 6H), 7.33 (s, 2H), 7.13–7.03 (m,
12H), 5.47 (s, 2H), 4.25 (s, 1H), 1.85 (s, 6H), 1.48 (s, 9H); 13 C‐NMR
(75.5 MHz, CDCl3, ppm) δ 151.1, 144.4, 144.3, 138.5, 136.8,
133.2, 133.1, 131.1, 129.3, 128.2, 127.2, 125.8, 125.2, 124.7,
123.5, 122.6, 121.6, 115.9, 93.2, 90.1, 85.9, 79.9, 53.7, 53.2, 34.6,
31.5, 20.4; MALDI‐TOF‐MS calcd for C76H50 914.39, found m/z
914.42.
Preparation of bis[4‐(10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐
2,7‐bis(3,5‐bis(phenylethynyl)phenylethynyl)‐9‐anthrylethy-
nyl)‐2,6‐dimethylphenyl]diazomethane (1c‐N2)
To a mixture of diazomethane 17 (3.0 mg, 6.0 µmol), anthracene
12 (16.3 mg, 17.0 µmol), tetrakis(triphenylphosphine)palladium
(II) (2.0 mg, 2.0 µmol), and copper(I) iodide (1.0 mg, 5.0 µmol)
were added anhydrous diisopropylamine (1 mL) and anhydrous
benzene (1 mL) at room temperature under an argon atmo-
sphere, and the mixture was stirred at 40 °C for 20 h. After evap-
oration of the solvent, the residue was passed through a short
column (aluminum oxide deactivated with 5% water, chloro-
form) at 0 °C and then purified by GPC (13 cycles, chloroform,
monitored at 300 nm) to give diazomethane 1c‐N2 as red crystals
(11 mg, 85%): mp 126.5–130.2 °C (dec.); IR (KBr, cm–1) 2212 (w,
1
ν
C≡C), 2046 (s, νC=N2); H‐NMR (300 MHz, CDCl3, ppm) δ 8.93 (s,
4H), 7.72 (d, J = 1.7 Hz, 8H), 7.64 (d, J = 1.6 Hz, 4H), 7.61 (s, 4H),
7.53–7.43 (m, 24H), 7.35–7.32 (m, 24H), 7.29 (s, 4H), 2.29 (s,
12H), 1.79 (s, 12H), 1.47 (s, 18H); 13 C‐NMR (75.5 MHz, CDCl3,
ppm) δ 151.0, 137.7, 137.6, 136.9, 134.2, 134.1, 133.4, 132.4,
132.3, 131.7, 131.3, 130.1, 129.3, 128.7, 128.4, 128.3, 127.0,
124.7, 124.2, 123.9, 122.8, 122.2, 121.1, 117.3, 91.4, 90.7, 89.8,
89.7, 87.8, 86.3, 59.6, 34.6, 31.6, 20.9, 20.4.
Preparation of bis[4‐(9‐anthrylethynyl)‐2,6‐dimethylphenyl]
diazomethane (1a‐N2)
Toamixtureofbis(4‐iodo‐2,6‐dimethylphenyl)diazomethane17[27]
(25 mg, 50 µmol), 9‐ethynylanthracene[62] (31 mg, 153 µmol), bis
(triphenylphosphine)palladium(II) dichloride (4.0mg, 5.7 µmol),
and copper(I) iodide (2.0mg, 11 µmol) was added anhydrous
triethylamine (1mL) at room temperature under an atmo-
sphere of a 1:1 mixture of nitrogen and hydrogen, and the
mixture was stirred at 40 °C for 15 h. After evaporation of the
solvent, the residue was passed through a short column (alu-
minum oxide deactivated with 5% water, chloroform) at 0 °C
and then purified by GPC (12 cycles, chloroform, monitored
at 300nm) to give diazomethane 1a‐N2 as red crystals (17 mg,
52%): mp 120.3–123.8 °C (dec.); IR (KBr, cm–1) 2194 (w, νC≡C);
2041 (s, νC=N2); 1H‐NMR (300 MHz, CDCl3, ppm) δ 8.66 (d,
J = 8.6 Hz, 4H), 8.43 (s, 2H), 8.02 (d, J = 8.6 Hz, 4H), 7.64–7.58 (m,
4H), 7.54–7.49 (m, 8H), 2.23 (s, 12H); 13 C‐NMR (75.5 MHz, CDCl3,
ppm) δ 137.4, 132.6, 132.1, 131.2, 129.7, 128.7, 127.7, 126.8, 126.6,
125.7.
Preparation of bis[4‐(10‐(4‐tert‐butyl‐2,6‐dimethylphenyl)‐
2,7‐bis(9‐triptycylethynyl)‐9‐anthrylethynyl)‐2,6‐dimethyl-
phenyl]diazomethane (1d‐N2)
To a mixture of 17 (13 mg, 25 µmol), anthracene 16 (69 mg,
75 µmol), bis(triphenylphosphine)palladium(II) dichloride (5.3 mg,
7.5µmol), and copper(I) iodide (2.9mg, 15µmol) was added
anhydrous diisopropylamine (9mL) and anhydrous benzene
(9mL) at room temperature under an argon atmosphere, and
the mixture was stirred at 40 °C for 22h. After evaporation of
the solvent, the residue was passed through a short column
(aluminum oxide deactivated with 5% water, chloroform) at 0 °C
and then purified by GPC (7 cycles, chloroform, monitored at
320 nm) to give diazomethane 1d‐N2 as orange crystals (39 mg,
75%): mp 121.3–123.6 °C (dec.); IR (KBr, cm–1) 2187 (w, νC≡C),
1
2042 (s, νC=N2); H‐NMR (300 MHz, CDCl3, ppm) δ 9.24–9.23 (m,
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Copyright © 2011 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 909–920

STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE BY PARA SUBSTITUENTS
4H), 7.92–7.89 (m, 12H), 7.76 (dd, J = 9.0, 1.7 Hz, 4H), 7.63 (d,
J = 9.0 Hz, 4H), 7.59 (s, 4H), 7.39–7.36 (m, 12H), 7.33 (s, 4H),
7.08–6.98 (m, 24H), 5.43 (s, 4H), 2.13 (s, 12H), 1.87 (s, 12H), 1.49
(s, 18H); 13 C‐NMR (75.5 MHz, CDCl3, ppm) δ 151.1, 144.5, 144.4,
137.9, 137.6, 136.9, 133.4, 132.5, 132.2, 131.6, 130.1, 129.5,
129.0, 127.2, 125.8, 125.2, 124.7, 123.5, 122.5, 122.2, 121.3,
117.3, 102.5, 93.4, 86.6, 85.9, 59.5, 53.8, 53.3, 34.6, 31.6, 20.9, 20.4.
131.8, 131.7, 129.4, 129.0, 127.2, 127.1, 125.7, 125.2, 124.7,
123.5, 122.5, 121.2, 120.6, 114.8, 99.8, 93.4, 85.7, 85.5, 53.7, 53.3,
34.6, 31.5, 21.0, 20.4; MALDI‐TOF‐MS calcd for C161H116 2048.91,
found m/z 2048.93.
Electron spin resonance measurements
The diazo compound was dissolved in 2‐methyltetrahydrofuran
(1.5 × 10–2 M), and the solution was degassed in a quartz cell by
three freeze–degas–thaw cycles. The sample was cooled in an
optical transmission ESR cavity at 77 K and irradiated with a
500 W Xe lamp (Wacom) through a filter (λ > 300 nm). ESR spec-
tra were measured on an ESR spectrometer (JES TE 200, X‐band
microwave unit, 100 kHz field modulation, JEOL). The signal posi-
tions were read by the use of a gaussmeter. The temperature
was controlled by a 9650 microprocessor‐based digital tempera-
ture indicator/controller, which provided the measurements ac-
curacy within 0.1 K and the control ability within 0.2 K. Errors
in the measurements of component amplitudes did not exceed
5%; the accuracy of the resonance fields determination was
within 0.5 mT.
Product analysis
Photolysis (λ > 300 nm) of 1a‐N2, 1c‐N2, and 1d‐N2 (5.0 mg) in
degassed benzene‐d6 (0.5 mL) at 25 °C gave tetraarylethenes
2a, 2c, and 2 d, respectively.
2a: yellow crystals (4.1 mg, 83%); mp 389.0–390.0 °C; IR (KBr,
1
cm–1) 2357 (w, νC≡C); H‐NMR (300 MHz, CDCl3, ppm) δ 8.66 (d,
J = 8.3 Hz, 8H), 8.42 (s, 4H), 8.01 (d, J = 7.7 Hz, 8H), 7.61–7.56 (m,
8H), 7.52–7.47 (m, 8H), 7.37–7.33 (m, 8H), 2.21 (s, 12H), 1.99 (s,
12H); MALDI‐TOF‐MS calcd for C98H68 1244.53, found m/z
1244.52.
2c: yellow crystals (2.4 mg, 50%); mp 328.0–331.0 °C; IR (KBr,
cm–1) 2357 (w, νC≡C); 1H‐NMR (300 MHz, C6D6, ppm) δ 9.38
(s, 8H), 7.65 (d, J = 9.0 Hz, 16H), 1.98 (s, 12H), 1.87 (s, 12H), 1.74
(s, 12H), 1.44 (s, 36H); 1H‐NMR (300 MHz, CDCl3, ppm) δ 8.77
(s, 8H), 7.53 (d, J = 1.5 Hz, 16H), 7.40 (s, 4H), 7.36 (s, 4H), 2.14
(s, 12H), 1.91 (s, 12H), 1.83 (s, 24H), 1.47 (s, 36H); 13 C‐NMR
(75.5 MHz, CDCl3, ppm) δ 136.9, 134.0, 131.7, 128.5, 128.4,
126.5, 124.7, 123.9, 122.8, 120.9, 91.4, 90.5, 89.7, 88.0, 34.6, 31.6,
26.6, 30.0, 20.5; MALDI‐TOF‐MS calcd for C338H228 4285.78, found
m/z 4285.40.
Low‐temperature UV–Vis spectra
UV–Vis spectra at 77 K were obtained by using a variable‐
temperature liquid-nitrogen cryostat (DN 1704, Oxford Inst.)
equipped with a quartz outer window and a sapphire inner win-
dow. The sample was dissolved in dry 2‐MTHF (1.5 × 10–3 M),
placed in a long‐necked quartz cuvette of 1‐mm path length,
and degassed thoroughly by repeated freeze–degas–thaw cycles
at a pressure near 10–5 Torr. The cuvette was flame‐sealed under
reduced pressure, placed in the cryostat, and cooled to 77 K. The
sample was irradiated for several minutes in the spectrometer
with a Wacom 500 W Xe lamp through a filter (λ > 300 nm), and
the spectral changes were recorded at appropriate time inter-
vals. The spectral changes upon thawing were also monitored
by carefully controlling the matrix temperature with a intelligent
temperature controller (ITC 4, Oxford Inst.).
2d: yellow crystals (2.5 mg, 51%); mp 234.8–240.1 °C; IR (KBr,
1
cm–1) 1605 (w, νC=C); H‐NMR (300 MHz, CDCl3, ppm) δ 9.18 (s,
8H), 7.83–7.80 (m, 24H), 7.76 (dd, J = 9.0, 1.5 Hz, 8H), 7.57 (d,
J = 9.0 Hz, 8H), 7.36 (s, 4H), 7.32 (s, 4H), 7.29–7.23 (m, 48H),
6.96–6.89 (m, 48H), 5.31 (s, 8H), 1.92 (s, 12H), 1.82 (s, 24H), 1.71
(s, 12H), 1.46 (s, 36H); 13 C‐NMR (75.5 MHz, CDCl3, ppm) δ 149.3,
130.9, 126.5, 126.3, 125.9, 125.5, 124.9, 121.3, 121.1, 120.7,
120.6, 118.9, 118.6, 117.1, 115.9, 115.5, 115.1, 114.1, 113.9,
113.6, 113.4, 112.3, 93.5, 90.1, 84.0, 82.6, 58.3, 58.0, 43.0, 40.6,
33.3, 31.7, 15.4; MALDI‐TOF‐MS calcd for C322H228 4093.84, found
m/z 4093.81.
Laser flash photolysis
Photolysis (λ > 300 nm) of 1d‐N2 (5.0 mg, 2.4 µmol) in oxygen‐
saturated benzene (1.0 mL) at 25 °C gave the ketone 1d‐O: yel-
low crystals (4.4 mg, 89%); IR (KBr, cm–1) 1662 (νC=O); 1H‐NMR
(300 MHz, CDCl3, ppm) δ 9.22 (s, 4H), 7.90 (m, 12H), 7.77 (dd,
J = 8.9, 1.6 Hz, 4H), 7.64 (d, J = 9.0 Hz, 4H), 7.56 (s, 4H), 7.39–7.36
(m, 12H), 7.33 (s, 4H), 7.07–6.98 (m, 24H), 5.43 (s, 4H), 2.19 (s,
12H), 1.87 (s, 12H), 1.49 (s, 18H); 13 C‐NMR (75.5 MHz, CDCl3,
ppm) δ 200.3, 151.1, 144.4, 144.3, 140.0, 137.2, 136.8, 133.3,
132.6, 132.2, 131.6, 131.5, 129.4, 129.1, 127.3, 125.8, 125.2,
124.8, 123.5, 122.5, 122.2, 121.5, 116.7, 101.9, 93.3, 87.5, 86.0,
53.7, 53.3, 34.6, 31.5, 20.9, 20.4; MALDI‐TOF‐MS calcd for
All flash photolysis measurements were made on a flash spec-
trometer (TSP-601, Unisoku). The excitation source for the laser
flash photolysis was a XeCl excimer laser (LEXTRA50, Lambda
Physik). A 150 W xenon short arc lamp (L2195, Hamamatsu)
was used as the probe source, and the monitoring beam guided
using an optical fiber scope was arranged in an orientation per-
pendicular to the excitation source. The probe beam was moni-
tored with a photomultiplier tube (R2949, Hamamatsu) through
a linear image sensor (S3701-512Q, 512 photodiodes used,
Hamamatsu). Timing of the laser excitation pulse, the probe
beam, and the detection system was achieved through a digital
synchroscope (DS-8631, Iwatsu), which was interfaced to a com-
puter (PC‐9821 RA266, NEC).
C
161H114O 2062.89, found m/z 2062.93.
Photolysis (λ > 300 nm) of 1d‐N2 (5.0 mg, 2.4 µmol) in degassed
benzene (0.8 mL) in the presence of 1,4‐cyclohexadiene (0.2 mL,
2.1 mmol) gave the double‐hydrogen abstraction product 1d‐
H2: yellow crystals (3.4 mg, 75%); mp 153.0–156.0 °C; IR (KBr,
cm–1) 2355 (w, νC≡C);1H‐NMR (300 MHz, CDCl3, ppm) δ 9.25
(s, 4H), 7.91–7.88 (m, 12H), 7.76 (dd, J = 1.4, 8.7 Hz, 4H), 7.62
(d, J = 9.0 Hz, 4H), 7.50 (s, 4H), 7.37 (dd, J = 6.7, 1.6 Hz, 12H), 7.33
(s, 4H), 7.07–6.98 (m, 24H), 5.43 (s, 4H), 4.13 (s, 2H), 2.15
(s, 12H), 1.87 (s, 12H), 1.56 (s, 18H); 13 C‐NMR (75.5 MHz, CDCl3,
ppm) δ 151.0, 144.41, 144.36, 137.5, 137.4, 136.9, 133.4, 132.4,
A sample was placed in a long‐necked Pyrex tube that had a
sidearm connected to a quartz fluorescence cuvette and was
degassed using a minimum of five freeze–degas–thaw cycles at
a pressure near 10–5 Torr immediately prior to being flashed.
The sample system was sealed, and the solution was transferred
to the quartz cuvette, which was placed in the sample chamber
of the flash spectrometer. The concentration of the sample
was adjusted so that it absorbed a significant portion of the
laser light.
J. Phys. Org. Chem. 2011, 24 909–920
Copyright © 2011 John Wiley & Sons, Ltd.
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K. HIRAI ET AL.
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