Preparation of bis(diazo) compounds incorporated into butadiyne and thiophene units and generation and characterization of their photoproducts

Article abstract of DOI:10.1039/b409095k

(2,6-Dimethyl-4-tert-butylphenyl)(2,4,6-tribromophenyl)diazomethane(1- N₂) was found to be stable enough to survive under Sonogashira coupling reaction conditions, and aryldiazomethyl substituents were introduced at the 1,4-positions of butadiy

Full text of DOI:10.1039/b409095k

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A R T I C L E
Preparation of bis(diazo) compounds incorporated into butadiyne
and thiophene units and generation and characterization of their
photoproducts
Fumika Morisaki,a Masakuni Kurono,a Katsuyuki Hiraib and Hideo Tomioka*a
a Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu, Mie,
514-8507, Japan
b Instrumental Analysis Facilities, Life Science Research Center, Mie University, Tsu, Mie,
514-8507, Japan
Received 16th June 2004, Accepted 3rd December 2004
First published as an Advance Article on the web 21st January 2005
(2,6-Dimethyl-4-tert-butylphenyl)(2,4,6-tribromophenyl)diazomethane(1-N2) was found to be stable enough to
survive under Sonogashira coupling reaction conditions, and aryldiazomethyl substituents were introduced at the
1,4-positions of butadiyne (4-2N2) and the 2,5-positions of thiophene(5-2N2). Irradiation of those bis(diazo)
compounds generated bis(carbenes), which were characterized by using ESR and UV/vis spectroscopic techniques in
a matrix at low temperature as well as time-resolved UV/vis spectroscopy in solution at room temperature. These
studies revealed that both of the bis(carbenes), 4 and 5, have singlet quinoidal diradical ground states with a very
small singlet–triplet energy gap of less than 1 kcal mol−1. A remarkable increase in the lifetime of bis(carbenes), as
opposed to that of the monocarbene (2), was noted and was interpreted to indicate that bis(carbenes) are
thermodynamically stabilized as a result of delocalization of unpaired electrons throughout the p net framework. In
spite of the stability, both bis(carbenes) are readily trapped by molecular oxygen to afford bis(ketones). Presumably,
the reaction of the upper-lying localized quintet states with oxygen is much faster than that for lower-lying states.
of the TMS group with NaOH proceeded smoothly to give an
ethynyl derivative (3-N2) in quantitative yield. Stirring a mixture
of 3-N2 and CuCl(OH)–TMEDA in CH2Cl2 under air for 6 h
at room temperature gave a bis(diazo)butadiyne compound
Introduction
Diazo compounds1 are excellent precursors for carbenes and,
hence, have been widely employed to generate and study
carbenes2 because they decompose very efficiently and cleanly
(4-2N2) in 94% yield.
upon either heating or irradiation to eliminate nitrogen gas,
The diyne (4-2N2) was treated with Na2S·9H2O in THF at
leaving the carbene. Moreover, the reaction occurs even at
50 ◦C for 20 h to result in the formation of a thiophene derivative
very low temperatures in the solid phase, which makes the
spectroscopic study of carbenes very easy. However, due to the
inherent sensitivity of the diazo functional group to heat, light,
acids, and metals, this group is usually introduced at the last step
(5-2N2) in 60% yield. All the procedures were carried out in
the dark, and compounds were purified either by silica gel
chromatography at ca. 0 ◦C and/or by GPC. No appreciable
decomposition of the diazo group was noted in all cases, at least
of the synthesis, which sometimes makes the versatility of the
under the conditions used here.
group less variable. For instance, a precursor diazo compound
cannot always be prepared without restraint.
The results showed that the protected diazo compound is
fairly persistent and can be used as a building block to construct
We found, almost by chance, that a diphenyldiazomethane
a new poly diazo derivative from which a new magnetic system
prepared to generate a persistent triplet carbene3 is also persis-
may be derived.
tent for a diazo compound and, hence, can be further modified
The characterization of the structure of the bis(carbene)
into a more complicated diazo compound while leaving the
expected to be generated upon photolysis of those bis(diazo)
diazo group intact. For instance, bis(2,4,6-tribromophenyl)-
precursors is of great importance in view of recent growing
diazomethane survives under Sonogashira coupling reaction4
interest in spin molecules.9–12 Thus, the photolysis of 4-2N2 and
conditions, leading to bis(2,6-dibromo-4-trimethylsilylethynyl-
5-2N2 was monitored by UV/vis and ESR spectroscopy.
phenyl)diazomethane.5 It is very interesting and important to
explore the applicability of this method, as it may open a window
to a new route to organic magnetic materials. We report here that
Spectroscopic studies
ESR studies. Irradiation of monodiazomethane 2-N2 in 2-
methyltetrahydrofuran (2MTHF) at 77 K gave ESR signals with
typical fine structure patterns for unoriented triplet species, i.e.,
32 (Fig. 1). The signals were analyzed in terms of zero-field
splitting (ZFS) parameters to be D = 0.455 cm−1 and E =
0.0335 cm−1. The values were similar with those observed for
a series of brominated triplet diphenylcarbenes.3,13
Similar irradiation of the bis(diazo) compound (5-2N2) con-
nected by a thiophene ring in 2-MTHF at 70 K gave ESR signals
(Fig. 2a) that were completely different from those observed for
the corresponding monocarbene, 32. The signals observed from
200 to 400 and around 710 mT were too weak to be unequivo-
cally assigned to a species that is expected to be produced in the
photolysis of 5-2N2, such as a triplet monocarbene, quinoidal
diradical, or quintet bis(carbene). However, a rather strong
a diphenyldiazo unit can be connected by using an oxidative
coupling reaction6 of acetylene to the butadiyne unit, which
is then converted to the thiophene ring by the reaction with
sodium sulfide.7 The characterization of bis(carbene) generated
from those bis(diazo) compounds is also reported.
Results
Preparation of bis(diazo) compounds
Treatment of 2,6-dimethyl-4-tert-butylphenyl)(2,4,6-tribromo-
phenyl)diazomethane (1-N2) with trimethylsilylacetylene in the
presence of (Ph3P)2PdCl2 and CuI at 40 ◦C for 2 days gave
(2,6-dimethyl-4-tert-butylphenyl)(2,6-dibromo-4-trimethylsilyl-
ethynyphenyl)diazomethane (2-N2) in 63% yield. Deprotection8
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0
4 3 1
©

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Scheme 1
due to a thermally populated excited state. Actually, the plot
of the intensity of the signals at 160 mT vs. the temperature
from 10–70 K for the diradical before relaxation (Fig. 3a) and
from 15–110 K for that after relaxation (Fig. 3b) deviated from
the Curie–Weiss law, confirming that the diradical was a singlet
ground state with a low-lying excited triplet state.
The temperature dependence of ESR signal intensity for
35 indicates that the quinoidal form of bis(carbene) has a
singlet ground state. To determine the singlet–triplet energy
gap (DET = 2J) of quinoidal bis(carbene), the observed data
Fig. 1 ESR spectrum obtained by irradiation of monodiazo compound
2-N2.
–
S
were analyzed in terms of the Bleaney–Bowers-type thermal
distribution (1)17
signal at 160 mT can be assignable to the DmS = 2 transitions
of a triplet species. Usually, this transition in triplet carbene is
very weak and often unobserved. Therefore, it is likely that this
forbidden transition is due to a triplet state of bis(carbene), that
is best represented by its quinoidal diradical form. Cooling the
sample to 10 K resulted in a marked decrease of the signal at
160 mT (Fig. 2b), but the signal was regained when the matrix
was thawed again to 80 K (Fig. 2c). When the matrix temperature
was further raised to 100 K, the shape of the signals around
220 to 400 mT changed rather dramatically to produce a rather
well-resolved spectrum along with the increase of the signal due
to the forbidden transition (Fig. 2d). These changes in ESR
signals upon thawing the matrix are usually interpreted in terms
C
3 exp(−2J/kT)
I =
(1)
T 1 + 3 exp(−2J/kT)
where I is the intensity of the ESR active triplet state while T,
k, and C are the absolute temperature, Boltzman constant, and
an arbitrary constant, respectively. Fitting the parameters in
eqn. (1) to observed intensity data for thermally populated
triplets gave an exchange integral, with J/k values of −117 K
(J = −223 cal mol−1) and −94 K (J = −187 cal mol−1) for
nascent and relaxed diradicals, respectively.
Similar irradiation of bis(diazo) compounds connected by
butadiyne (4-2N2) in 2-MTHF at 70 K gave an ESR spectrum
(Fig. 4a) showing a main signal at 257.3 mT. The signals
disappeared when the temperature was lowered to 4 K (Fig. 4b)
but were reproduced upon thawing up to 80 K (Fig. 4c). The
plot of the intensity of the signal at 257 mT vs. the temperature
from 10–80 K (where the shape of the signals is unchanged)
deviated from the Curie–Weiss law (Fig. 5a), again confirming
that the diradical was a singlet ground state with a low-lying
excited triplet state. When the temperature was raised up to
90 K, a significant change in the spectrum was noted with a
shift of main signals to 135.8 mT (Fig. 4d). This is interpreted in
terms of the geometrical relaxation of the species accompanied
13c
of the geometrical change of the paramagnetic species as the
matrix softens, allowing the carbene to assume a single preferred
orientation rather than several slightly distorted orientations
dictated by the trapping of the precursor in the frozen matrix.
We assigned the new fine structured peaks to a triplet species with
D = 0.180 cm−1 and E = 0.0121 cm−1. The observed spectrum
was reproduced reasonably well by computer simulation,14 using
these zfs parameters (Fig. 2f).
Recooling the sample to 15 K resulted in a marked decrease
in the well-resolved triplet signal at 220 to 400 mT and the
forbidden transition at 160 mT (Fig. 2e). Warming the sample to
90 K reproduced the original signals observed at 100 K (Fig. 2f),
the intensity of which increased up to 110 K and started to
decrease above 110 K. All the signals disappeared irreversibly
when the temperature was raised up to 180 K and brought down
to 100 K (Fig. 2g).
13c
by softening of the matrix. The intensity of the new signal also
decreased as the temperature was lowered but increased upon
warming to 110 K (Fig. 4e–f). The signals started to decrease
above 110 K and disappeared completely at 200 K (Fig. 4g). The
plot of intensity of the signal at 136 mT vs. the temperature from
10–110 K, where the shape of the signals is unchanged, deviated
from the Curie–Weiss law (Fig. 5b). The nonlinear least-squares
The decreasing intensity of the spectra at <30 K can make
them difficult to observe and clearly shows that the spectra are
4 3 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0

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Fig. 4 (a) ESR spectrum obtained by irradiation of bis(diazo) com-
pound 4-2N2 in 2-methyltetrahydrofuran at 70 K. (b–g) The same spectra
observed at 4 K (b), 80 K(c), 90 K (d), 10 K (e). (f–g) The same spectra
taken at 100 K after thawing the matrix to 140 (f) and 200 K (g).
the rapid disappearance of the original absorption due to 2 and
the concurrent growth of sharp and strong absorption bands
at 308 and 330 nm with weak bands at 440, 459, and 477 nm
(Fig. 6). Characteristic absorption bands usually observed for
triplet diarylcarbenes consist of a rather sharp and strong band
in the UV region along with weak, broad, and structured bands
in the visible region. Since the ESR signals ascribable to triplet
carbene are observed under identical conditions, the absorption
spectrum can be safely assigned to triplet carbene 32. When the
matrix was thawed gradually, the bands became sharper and
shifted slightly to shorter wavelengths of 307 and 327 nm. Upon
further thawing, the bands started to disappear at around 110 K
and completely vanished at around 160 K. This is presumably
related to the geometrical changes associated with the softening
Fig. 2 (a) ESR spectrum obtained by irradiation of bis(diazo) com-
pound 5-2N2 in 2-methyltetrahydrofuran at 70 K. (b–e) The same sample
observed at 10 K (b), 80 K (c), 100 K (d), 15 K (e), 90 K (f). (g) The same
sample taken at 10 K after thawing to 180 K. (h) Simulated spectrum of
35 with S = 1, g = 2.000, |D/hc| = 0.0566 cm−1, |E/hc| = 0.0121 cm−1
and line width of 35 G.
13c
of the matrix.
Similar irradiation of 5-2N2 in a 2-MTHF matrix at 77 K
resulted in the rapid disappearance of the original absorption
due to 5-2N2 and the concurrent growth of sharp and strong
absorption bands at 596 nm along with sharp and weak bands
at 548 and 404 nm (Fig. 7). The matrix took on a distinct
green color. The bands were found to be thermally quite stable.
Thus, when the matrix was thawed gradually, the bands became
sharper and shifted slightly to 501, 542, and 589 nm, while the
404 nm band disappeared at around 145 K. This is presumably
related to the geometrical changes associated with the softening
curve fitting of the parameters of eqn. (1) to the data in Fig. 5
yields J/k values of −58 K (J = −115 cal mol−1) and −132 K
(J = −262 cal mol−1) for the nascent and relaxed diradicals,
respectively.
UV/Vis spectroscopic study in a matrix at low temperature.
Irradiation of 2-N2 in a 2-MTHF matrix at 77 K resulted in
Fig. 3 Plots of the intensities of the ESR signals of thermally populated 35 at 160 mT vs. reciprocal of temperature (a) before relaxation (10–70 K)
and (b) after relaxation (15–110 K).
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0
4 3 3

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Fig. 5 Plots of the intensities of the ESR signals of thermally populated 34 (a) at 267 mT vs. reciprocal of temperature (10–80 K) before relaxation
and (b) at 136 mT vs. reciprocal of temperature (10–110 K) after relaxation.
The bands observed in the photolysis of 5-2N2 are obviously
not ascribable to a mono-carbene but rather to a bis(carbene).
The strong absorption bands in the visible regions indicate the
presence of the extended p-system.18 Therefore, the bis(carbene)
is likely to exist in a delocalized quinoidal form rather than in
a localized one. It is interesting to note here that the bands
ascribable to a mono-carbene expected to be generated by
elimination of one diazo group were not appreciably detected
even at the initial stage of irradiation, suggesting that two
nitrogen molecules are likely eliminated following absorption
of a single photon.19
Similar irradiation of the butadiyne derivative (4-2N2) gave
essentially identical results. Thus, irradiation in 2-MTHF at
77 K gave a species showing a sharp and strong band at 574 nm
along with weak broad bands at 390 and 363 nm (Fig. 8).
Upon thawing the matrix, the band shifted slightly to 570 nm
13c
at around 145 K, again due to the geometrical changes, and
disappeared irreversibly at around 230 K. Thus, the biscarbene is
significantly and thermally more stable than the corresponding
mono-carbene but much less stable than the thiophene analogue.
Fig. 6 UV/Vis spectra obtained by irradiation of monodiazo com-
pound 2-N2. Spectra of (a) 2-N2 in 2-methyltetrahydrofuran at 77 K,
(b) the same sample after irradiation (k > 350 nm), and (c–d) the same
sample after thawing to 95 K (c) and 160 K (d).
Fig. 8 UV/Vis spectra obtained by irradiation of bis(diazo) compound
4-2N2. Spectra of (a) 4-2N2 in 2-methyltetrahydrofuran at 77 K, (b) the
same sample after irradiation (k > 350 nm), and (c–d) the same sample
after thawing to 145 (c) and 230 K (d).
The absorption bands are again completely different from
those observed for the corresponding mono-carbene (obtained
by irradiation of 2-N2 under identical conditions) and, thus, are
assignable to bis(carbene). The strong bands in the visible region
suggest that this bis(carbene) also exists as a delocalized form.
Fig. 7 UV/Vis spectra obtained by irradiation of monodiazo com-
pound 5-N2. Spectra of (a) 5-N2 in 2-methyltetrahydrofuran at 77 K,
(b) the same sample after irradiation (k > 350 nm), and (c–d) the same
sample after thawing to 145 K (c) and 300 K (d).
13c
of the matrix. Upon further thawing, the bands started to
disappear at around 150 K but did not completely disappear
even at 300 K. The anomalous thermal stability of the diradical
is noted here.
Time-resolved UV/Vis spectroscopic study in solution at room
temperature. In order to know the stability of the present
carbenes more accurately, the lifetime was estimated in a
4 3 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0

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degassed benzene solution at room temperature, in which we
have measured the lifetime of a series of sterically congested
diarylcarbenes.3
(2-O2), which confirms that the transient absorption quenched
by oxygen is due to 32.
The apparent build-up rate constant, kobs, of the carbonyl
oxide (2-O2) is essentially identical with that of the decay of 32,
and kobs is expressed as given in eqn. (2)
Laser flash photolysis (LFP) of 2-N2 (1 × 10−4 M) in a
degassed benzene solution at room temperature with a 10 ns,
70–90 mJ, 308 nm pulse from a XeCl excimer laser produced a
transient species showing a strong absorption at 310 nm, which
appeared coincident with the pulse (Fig. 9). On the basis of the
low-temperature spectrum, we assigned the transient product
kobs = k0 + kO [O2]
(2)
2
3
where k0 represents the rate of decay of 2 in the absence of
oxygen and kO is the quenching rate constant of 32 by oxygen.
3
3
to 2. The inset in Fig. 9 shows the decay of 2 in the absence
2
A plot of the observed pseudo-first-order rate constant of the
formation of the oxide against [O2] is linear. From the slope of
of trapping reagents, which is found to be of the second order
3
(2k/el = 1.27 s−1). The rough lifetime of 2 is estimated in the
this plot, kO was determined to be 1.1 × 107 M−1 s−1, which is
form of the half-life, t1/2, to be 1.3 s.
2
approximately 2 orders of magnitude smaller than that observed
3
with the “parent” triplet diphenylcarbene DPC (kO = 5.0 ×
2
109 M−1 s−1).21
The lifetime of the transient species generated in the photolysis
of bis(diazo) compounds was too long to monitor with the laser
flash photolysis technique, which has been routinely used for
such studies, and a conventional UV/Vis spectroscopic method
was more convenient in this case.
As shown in Fig. 10, the UV/Vis spectrum obtained just
after brief irradiation of 5-2N2 in degassed benzene at 20 ◦C
is essentially the same as that observed in the photolysis in 2-
MTHF at 77 K and, hence, attributable to 5. The absorption
bands decayed very slowly; the transient bands did not disappear
completely even after 2 h under these conditions.
The decay was analyzed in terms of the combination of the
first and second orders. The decay curve was analyzed by Igor
using the following equation.
Fig. 9 Absorption of transient products formed during irradiation of
monodiazo compound 2-N2 in degassed benzene at room temperature
recorded (a) 1 ls, (b) 100 ls and (c) 10 ms after excitation. Inset shows
the time course of the absorption at 310 nm (oscillogram trace).
O.D. = A1exp(−k1t) + 1/(1/A2 + k2t)
(3)
Support is lent to this assignment by trapping experiments
using oxygen. When LFP was carried out on a non-degassed
benzene solution of 2-N2, the half-life of 32 decreased dramati-
cally, and a broad absorption band with a maximum at 410 nm
appeared at the expense of the absorption due to 32. The spent
solution was found to contain the corresponding benzophenone
(2-O) as the main product. It is well documented20,21 that the
diarylcarbenes with triplet ground states are readily trapped
by oxygen to generate the corresponding diaryl ketone oxides,
which are easily observed directly either by matrix isolation or
by flash photolysis, and show a broad absorption band centered
at 396–450 nm. Thus, the observations can be interpreted to
indicate that 32 is trapped by oxygen to form the carbonyl oxide
The decay was fit to a first-order decay (k1 = 1.2 × 10−4 s−1, s =
139 min) and the second order (2k/el = 6.2 × 10−2 s−1, t1/2
=
4 min) in the ratio of 0.21 : 0.31.
The decay rate of the transient bands increased dramatically
when the irradiation was carried out in the presence of oxygen.
The spent solution was found to include the corresponding
diketone 5-2O as the main product. The observations indicate
that the transient species is trapped by oxygen. The quenching
rate constant was estimated to be 1.1 × 107 M−1 s−1.
The transient absorption bands generated by photolysis of
4-2N2 decayed much more quickly than those generated from 5-
2N2. The irradiation of 4-2N2 generated absorption bands that
were similar to those observed at low temperature (Fig. 11).
Fig. 10 Absorption of transient products formed during irradiation of monodiazo compound 5-2N2 in degassed benzene at room temperature
recorded (a) 0.1, (b) 30, (c) 60, (d) 300, (e) 1200 and (f) 3600 s after excitation. Inset shows the time course of the absorption at 599 nm.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0
4 3 5

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Fig. 11 Absorption of transient products formed during irradiation of bis(diazo) compound 4-2N2 in degassed benzene at room temperature
recorded (a) immediately, (b) 10, (c) 30, (d) 60, (e) 120, (f) 180, (g) 240 and (h) 300 s after excitation. Inset shows the time course of the absorption at
571 nm.
The absorption bands decayed within 300 s in a second-order
manner (2k/el = 8.8 × 10−2 s−1). The half-life was estimated to
be 30 s. Again, the transient bands were quenched by oxygen at
a rate constant of 7.4 × 106 M−1 s−1.
The effect of a coupling linker has been extensively studied,
not for the systems with bis(carbene) units,27 but for those with
bis(nitrene) units. For instance, two nitrene centers introduced
in conjugative manner into a coupling linker group, such
as benzene,28 biphenyl,29 stilbene,30 diphenylbutadiene,
phenyloctatetraene,
di-
30a,b
diphenylacetylene,31 diphenylbutadi-
30a,b
Discussion
yne,31 tetraphenylallene,32 benzophenone and 1,1-diphenyl-
ethylene,33 diphenyl sulfide,34 azobenzene,34 and thiophene,35
have been generated and characterized by ESR; it was revealed
that all have a singlet ground state with a delocalized diradical
structure having a very small singlet–triplet energy gap of 150 to
600 cal mol−1.
Numerous studies have been done to construct high-spin
systems built around triplet carbene subunits linked across m-
11b,22,23
phenylene.
In these systems, the covalent interaction be-
tween the triplet centers is topologically prohibited, but commu-
nication via a shared p-space is possible, thereby generating high-
spin states. Considerably less is known about their conjugated
cousins, p-phenylene poly-carbenes, where spin-pairing between
the triplet carbene units leads to lower spin states.22
The present findings that two persistent diarylcarbene units
introduced either in thiophene or in butadiyne systems have a
singlet quinoidal biradical ground state with a very small singlet–
triplet energy gap of less than 1 kcal mol−1 are in full accord with
those from previous observations and predictions.
For instance, p-phenylenebis(methylene) is predicted to have a
singlet ground state of Aꢀ symmetry with a delocalized diradical
structure.24 The lower-lying excited state is computed to be the
Most studies on those species have been done simply by using
ESR since the measurements have been crucial to characterize
those paramagnetic species and successfully employed in many
systems, but almost no information is available on the electronic
properties and reactivities of those interesting species. In the
present study, we have measured the UV/Vis spectra and also
revealed their reactivity in solution at room temperature.
Both bis(carbenes), 4 and 5, feature very sharp and strong
absorption bands in the visible region. These features are usually
noted for some diradical species incorporated into quinoidal
cumulated p conjugating systems.18 These observations provide
further evidence for the contribution of a fully delocalized
diradical structure. The surprising thermal stability of the
absorption bands is noted. Thus, the bands due to 5 survive
even at room temperature. In accord with this, 5 survives
for hours in solution at room temperature, being stable enough
to be observable by the conventional spectroscopic means. The
remarkable increase in the lifetime of biscarbenes 4 and 5, as
opposed to the isolated local diradical system, suggests that
the biscarbenes are thermodynamically stabilized as a result
of delocalization of unpaired electrons throughout the p net
framework.
5
3Aꢀ one at about 2 kcal mol−1, and the Aꢀquintet state lies at
about 27 kcal mol−1 above the 1Aꢀ state. Although the 3Aꢀ triplet
state was undetected by ESR even at low temperature, probably
due to its high reactivity, the species has been generated in an
inert gas matrix at 10 K and characterized by the trapping
reaction with O2 and HCl under these conditions.24 The triplet
state of p-phenylenebis(phenylmethylene) was detected by ESR
and shown to be a thermally populated one lying ca. 1 kcal mol−1
above the ground state singlet diradical.16,25
A series of p-phenylenebis(diradicals), in which the local
diradical centers are changed from phenylnitrene having a triplet
ground state with a rp singlet lying 18 kcal mol−1 high in energy
to phenylchlorocarbene having a r2 singlet ground state with a
triplet lying 7–9 kcal mol−1 high in energy, have been generated in
an inert gas matrix at low temperature and characterized by IR
in combination with theoretical calculations.22,26 These studies
have revealed that all the systems have an 1Aꢀ singlet ground state
1
and that the relative energies between three Aꢀ states, singlet,
triplet, and quintet, change very little regardless of the nature of
the local diradical subunits.
It is to be noted here that the relative energy between those
possible multiplicities must be dependent upon the nature of a
coupling linker. For instance, in a p-phenylene system, maximum
bonding occurs for the quinoidal diradicals S1 and T1, but this is
achieved with loss of aromaticity. The situation must be changed
if the linking p-phenylene system is replaced with a simple double
bond.
In spite of their stability, both bis(carbenes) 4 and 5 are readily
trapped by oxygen. The quenching rate constants observed for 4
and 5 are very similar to that for the isolated local carbene unit 2.
Moreover, the main oxidation products from both bis(carbenes)
are diketones, obviously formed as a result of the interaction of
oxygen with the local carbene center rather than with the other
4 3 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0

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Scheme 2
parts where spin densities must build up. These observations
suggest that, although the contribution of extensively delocal-
ized diradical forms is important in bis(carbenes), they still keep
their carbenic character toward a typical carbene quencher.
Presumably, the reaction of the upper-lying localized quintet
states with oxygen is much faster than that of other low-lying
states.
by column chromatography (silica gel, hexane–chloroform =
1 : 1) to give the methanol (2.0 g, 74%) as a white solid: mp
91.5–93.4 ◦C; 1H-NMR (300 MHz, CDCl3, ppm) d 7.73 (s,
2H, Ph–H), 6.97 (s, 2H, Ph-H), 6.41 (d, J = 8.82 Hz, 1H,
HO–C–H), 2.55 (d, J = 8.82 Hz, 1H, -OH), 2.28 (s, 6H, -Me),
1.29 (s, 9H, -tBu); 13C-NMR (75.5 MHz, CDCl3, ppm) d 150.4,
139.5, 136.6, 136.0, 133.8, 126.8, 124.8, 121.2, 76.1, 34.1, 31.2,
21.7; MS m/z (relative intensity) 508 (M + 6, 5.0), 506 (M +
4, 14.1), 504 (M + 2, 14.6), 502 (M+, 4.8), 343 (30.2), 57 (100);
HRMS calcd for C19H21OBr3 m/z 501.9141, found 501.9136.
Conclusion
The present investigation revealed that sterically congested di-
phenyldiazomethanes can be used as building blocks to prepare
functionalized bis(diazo) compounds. Fairly stable bis(carbenes)
with fully delocalized diradical structures are realized by using
this method, which enabled us to study the reactivity of these
molecules for the first time. Although these molecules are not
candidates for practical use in the development of organo-
ferromagnetic materials, they are of considerable importance in
understanding the nature of open-shell organic molecules. The
present method can be easily extended to construct precursor
polydiazo compounds, which can generate high-spin polycar-
benes with considerable stability.
(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)chlo-
romethane. A solution of the methanol (5.5 g, 11 mmol) in
thionyl chloride (20 mL) was stirred at room temperature over-
night. Excess thionyl chloride was distilled from the reaction
mixture and the mixture was dried under reduced pressure to
give the chloromethane (2.0 g, 74%) as a brown semi-solid:
1H-NMR (300 MHz, CDCl3, ppm) d 7.75 (s, 2H, Ph–H), 6.97
(s, 2H, Ph–H), 6.84 (s, 1H, Cl–C–H), 2.38 (s, 6H, -Me), 1.28 (s,
9H, -tBu); 13C-NMR (75.5 MHz, CDCl3, ppm) d 150.6, 137.6,
137.2, 136.4, 130.9, 127.2, 125.7, 122.1, 62.0, 34.0, 31.1, 22.8.
Since the chloride was found to be unstable, it was used without
further purification.
Experimental
Ethyl N-[(2,4,6-tribromophenyl)(4-tert-butyl-2,6-dimethylphe-
nyl)methyl]carbamate. To a heated mixture of ethyl carba-
mate (13.5 g, 152 mmol) and silver tetrafluoroborate (2.5 g,
13 mmol) in 1,4-dioxane (20 mL) was added a solution of the
chloromethane (5.7 g, 11 mmol) in dioxane (30 mL) at 60 ◦C and
the mixture was refluxed overnight. After cooling to room tem-
perature, the mixture was filtered and the filtrate was extracted
with chloroform, the organic layer was washed with water, dried
over anhydrous sodium sulfate and concentrated under reduced
pressure to leave a crude product which was purified by column
chromatography (silica gel, hexane–dichloromethane = 1 : 1)
to g◦ive the carbamate (2.0 g, 74%) as a brown solid: mp 101–
103 C; 1H-NMR (300 MHz, CDCl3, ppm) d 7.71 (s, 2H, Ph–H),
6.96 (s, 2H, Ph–H), 6.39 (d, J = 8.64 Hz, 1H, N–C–H), 5.24 (d,
J = 8.64 Hz, 1H, -NH), 4.16 (q, J = 3.03 Hz, 2H, -CH2-),
2.19(s, 6H, -Me), 1.28–1.23 (m, 12H, -tBu + -CH3); 13C-NMR
(75.5 MHz, CDCl3, ppm) d 154.9, 150.6, 137.5, 136.8, 136.3,
132.3, 126.9, 125.0, 120.9, 61.2, 57.9, 34.1, 31.2, 21.3, 14.7; MS
m/z (relative intensity) 579 (M + 6, 2.7), 577 (M + 4, 7.8), 575
(M + 2, 7.6), 573 (M+, 2.8), 496 (82.6), 57 (100); HRMS calcd
for C22H26NO2Br3 m/z 572.9512, found 572.9516.
General methods
Reagents were obtained commercially and used without further
purification unless otherwise noted. In the case of experiments,
tetrahydrofuran, ether, toluene, dioxane were purified by distilla-
tion from sodium/benzophenone and dichloromethane, carbon
tetrachloride, triethylamine were from calcium hydride. Gel
permeation chromatography (GPC) was performed on a Shodex
GPC H-2001 using UV-1570 as a detector, or Shodex GPC H-
20025 using UV-970. 1H- and 13C-NMR spectra were recorded
on a JEOL DATUM LTD. JNM-AL300 FT-NMR and were
referenced against internal tetramethylsillane on the d scale in
parts per million. Infrared spectra were recorded on a JASCO
corporation FT-IR 410 infrared spectrophotometer. UV/Vis
spectra were recorded on a JASCO corporation CT-560 UV-
vis spectrophotometer on the k scale in nanometer.
(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)me-
thanol. To a mixture of magnesium ribbon (0.8 g, 33 mg atoms)
and a crystal of iodine in anhydrous ether (10 mL) was added
a solution of 2-bromo-5-tert-butyl-m-xylene36 (7.2 g, 30 mmol)
in dry ether (15 mL) and the mixture was refluxed overnight
under nitrogen atmosphere. After cooling to 0 ◦C, a solution
of 2,4,6-tribromobenzaldehyde37 (10.3 g, 30 mmol) in dry
tetrahydrofuran (25 mL) was added to that Grignard reagent,
and the mixture was refluxed for 5 h. Saturated aqueous
ammonium chloride was added carefully and the mixture was
extracted with ether. The ethereal layer was washed with water,
dried over anhydrous sodium sulfate and concentrated under
reduced pressure to leave a crude product which was purified
Ethyl N-nitroso-N-[(2,4,6-tribromophenyl)(4-tert-butyl-2,6-
dimethylphenyl)methyl]carbamate. To a stirred solution of
the carbamate (4.3 g, 7.5 mmol) in acetic acid (30 mL) and
acetic anhydride (40 mL) was added sodium nitrite (10.0 g,
149 mmol) portionwise at 0 ◦C. After stirring at room tem-
perature overnight, the reaction mixture was poured into ice
and extracted with ether. The ethereal layer was washed with
water, saturated aqueous sodium carbonate and water again,
and the organic layer was dried over anhydrous sodium sulfate,
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0
4 3 7

View Article Online
and concentrated under reduced pressure to give the nitroso-
carbamate (6.2 g, 56%) as a brown solid: 1H-NMR (300 MHz,
CDCl3, ppm) d 7.69 (s, 2H, Ph–H), 6.91 (brs, 3H, Ph–H +
N–C–H), 4.35 (q, J = 7.17 Hz, 2H, -CH2-), 1.96 (s, 6H, -Me),
1.27 (s, 9H, -tBu), 1.22 (t, J = 7.17 Hz, 3H, -CH3). The nitroso
compound was found to be rather unstable and hence used
without further purification.
7.09 (s, 4H, Ph–H), 2.16 (s, 12H, -Me), 1.31 (s, 18H, -tBu);
13C-NMR (75.5 MHz, CDCl3, ppm) d 151.1, 137.5, 136.6,
133.9, 126.0, 124.55, 124.53, 122.3, 79.7, 76.2, 34.3, 31.3, 21.1;
IR (NaCl, cm−1) m 2139, 2056.
2,5-Bis{3,5-dibromo-4-(4-tert-butyl-2,6-dimethylphenyldiazo-
methyl)phenyl}thiophene (5-2N2). To a stirred solution of
4-2N2 (7 mg, 7.6 lmol) in tetrahydrofuran (2 mL) was added
Na2S(H2O)9 (7.3 mg, 35 lmol) under nitrogen atmosphere and
the mixture was stirred at 50 ◦C for 20 h. The mixture was
concentrated, pored into water, and extracted with ether. The
ethereal layer was dried and evaporated under reduced pressure
to leave a crude product which was purified by GPC (chloroform,
monitored at 350 nm) to give 5-2N2 (4.0 mg, 55%) as an orange
viscous liquid: 1H-NMR (300 MHz, CDCl3, ppm) d 7.82 (s, 4H,
Ph–H), 7.30 (s, 4H, Ph–H), 7.09 (s, 2H, Thiophene-H), 2.20 (s,
12H, -Me), 1.32 (s, 18H, -tBu); 13C-NMR (75.5 MHz, CDCl3,
ppm) d 150.9, 141.0, 137.4, 135.0, 131.2, 129.9, 126.0, 125.7,
125.5, 124.8, 63.5, 34.3, 31.3, 21.1; IR (NaCl, cm−1) m 2055.
(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)dia-
zomethane (1-N2). To a stirred solution of the nitroso-
carbamate (1.4 g, 2.2 mmol) in dry tetrahydrofuran (15 mL)
was added potassium tert-butoxide (0.5 mg, 4.7 mmol) at
◦
−20 C under argon atmosphere. After stirring overnight, the
reaction mixture was poured into ice, extracted with ether, and
the ethereal layer was washed with water, dried over anhydrous
sodium sulfate, concentrated and dried under reduced
pressure to leave a crude product which was purified by column
chromatography (alumina, hexane at −20 ◦C), followed by GPC
(chloroform, monitored at 350 nm) to give the diazomethane
◦
1
(0.6 g, 48%) as an orange solid: mp 117.3–118.0 C; H-NMR
(300 MHz, CDCl3, ppm) d 7.73 (s, 2H, Ph–H), 7.08 (s, 2H, Ph–
H), 2.15 (s, 6H, -Me), 1.30 (s, 9H, -tBu); 13C-NMR (75.5 MHz,
CDCl3, ppm) d 150.9, 137.3, 135.6, 131.6, 126.0, 125.7, 124.5,
121.1, 63.1, 34.3, 31.3, 21.2; EIMS m/z (relative intensity) 518
(M + 6, 0.6), 516 (M + 4, 1.4), 514 (M + 2, 1.4), 512 (M+,
0.6), 490(5.5), 488(11.5), 486(8.6), 484(3.2), 227(23.0), 57(100);
HRMS Calcd for C19H19Br3N2 511.9097; Found m/z 511.9096;
IR (NaCl, cm−1) m 2054.
Irradiation for product analysis. In a typical run, a solution
of the diazo compound (1, ca. 5 mg) in solvent was placed in a
Pyrex tube and irradiated with a high-pressure, 300 W mercury
lamp until all the diazo compound was destroyed. The irradia-
tion mixture was then concentrated on a rotary evaporator below
20 ◦C. Individual components were isolated by column chro-
matography or by preparative TLC and identified by NMR and
MS. The following products were isolated and characterized.
Butadiyne-diketone (4-2O). A white solid (90%); mp 312.0–
(2,6-Dibromo-4-trimethylsilylethynylphenyl)(4-tert-butyl-2,6-
dimethylphenyl)diazomethane (2-N2). To a stirred mixture of
1-N2 (124.0 mg, 0.24 mmol), bis(triphenylphosphino)palladium
dichloride (catalytic amount) and copper iodide (catalytic
amount) in dry triethylamine (4 mL) and dry tetrahydrofuran
(2 mL) was added trimethylsilylacetylene (100.0 ll, 0.72 mmol)
under nitrogen atmosphere. After heating overnight at 35 ◦C,
the reaction mixture was concentrated under reduced pressure,
and the residue was dissolved in ether, filtered, and the filtrate
was concentrated and dried under reduced pressure to leave a
crude product which was purified GPC (chloroform, monitored
at 350 nm) to afford 2-N2 (95.0 mg, 74%) as an orange viscous
liquid: 1H-NMR (300 MHz, CDCl3, ppm) d 7.66 (s, 2H, Ph–H),
7.07 (s, 2H, Ph–H), 2.14 (s, 6H, -Me), 1.30 (s, 9H, -tBu), 0.24 (s,
9H, -TMS); 13C-NMR (75.5 MHz, CDCl3, ppm) d 150.9, 137.4,
136.2, 132.6, 125.9, 124.7, 124.6, 124.3, 101.5, 97.9, 63.7, 34.3,
31.3, 21.1, -0.2. IR (NaCl, cm−1) m 2054.
◦
1
313.4 C; 300 MHz H NMR (CDCl3) d 7.71 (s, 4H), 7.06 (s,
4H), 2.29 (s, 12H), 1.31 (s, 18H); 75.5 MHz 13C NMR (CDCl3) d
195.6, 155.0, 143.8, 138.7, 136.2, 134.1, 126.8, 124.8, 121.2, 79.5,
77.2, 34.7, 31.0, 22.0; IR (KBr) m
1662 cm−1; MS (MALDI-
=
C
O
TOF) m/z calcd for C42H37Br4O2 (M − H+) 888.9522, found
888.9316.
Thiophene-diketone (5-2O). A pale yellow solid (72%); mp
◦
1
261.7–263.0 C; 300 MHz H NMR (CDCl3) d 7.80 (s, 4H),
7.38 (s, 2H), 7.06 (s, 4H), 2.32 (s, 12H), 1.32 (s, 18H); 75.5 MHz
13C NMR (CDCl3) d 196.0, 154.7, 141.6, 141.3, 138.5, 136.8,
134.7, 129.5, 126.6, 126.5, 122.0, 34.7, 31.1, 22.0; IR (KBr) m
=
C
O
1672 cm−1; MS (MALDI-TOF) m/z calcd for C42H39Br4O2S
(M − H+) 922.9399, found 922.9509.
EPR Measurements. The diazo compound was dissolved in
2-methyltetrahydrofuran (10−3 M) and the solution was degassed
in a quartz cell by three freeze–degas–thaw cycles. The sample
was cooled in an optical transmission EPR cavity usually at
77 K and irradiated with a Wacom 500 W Xe lamp using
a Pyrex filter. EPR spectra were measured on a JEOL JES
TE 200 spectrometer (X-band microwave unit, 100 kHz field
modulation). The signal positions were read by the use of a
gaussmeter. A JEOL liquid helium transfer system was attached
for the low temperature measurement. The temperature was
controlled by a 9650 Microprocessor-based Digital Tempera-
ture Indicator/Controller, provided the measurements accuracy
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.
(2,6-Dibromo-4-ethynylphenyl)(4-tert-butyl-2,6-dimethylphe-
nyl)diazomethane (3-N2). To a solution of 2-N2 (16.8 mg,
0.03 mmol) in tert-butyl alcohol (1 mL) was added 10% aqueous
sodium hydroxide (0.3 mL). After stirring overnight at room
temperature, the reaction mixture was extracted with ether, the
ethereal layer was washed with water, dried over anhydrous
sodium sulfate, concentrated and dried under reduced pressure
to leave a crude product which was purified by GPC (chloroform,
monitored at 350 nm) to afford 3-N2 (11.7 mg, 80%) as an orange
solid: mp 48.5–49.5.0 ◦C; 1H-NMR (300 MHz, CDCl3, ppm) d
7.69 (s, 2H, Ph–H), 7.08 (s, 2H, Ph–H), 3.18 (s, 1H, ≡CH), 2.15
(s, 6H, -Me), 1.30 (s, 9H, -tBu); 13C-NMR (75.5 MHz, CDCl3,
ppm) d 151.0, 137.5, 136.4, 133.1, 125.9, 124.6, 123.2, 80.4,
80.2, 63.7, 34.3, 31.3, 21.1; IR (NaCl, cm−1) m 2162, 2053.
Low-temperature UV/Vis spectra. Low-temperature spectra
at 77 K were obtained by using an Oxford variable-temperature
liquid-nitrogen cryostat (DN 1704) equipped with a quartz outer
window and a sapphire inner window. The sample was dissolved
in dry 2-MTHF, 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 Halos 300 W high-pressure
mercury lamp using a Pyrex filter, and the spectral changes were
1,4-Bis{3,5-dibromo-4-(4-tert-butyl-2,6-dimethylphenyldiazo-
methyl)phenyl}butadiyne (4-2N2). To
a solution of 3-N2
(10.6 mg, 0.02 mmol) in dichloromethane (2 mL) was
added CuCl(OH)·TMEDA (catalytic amount) under oxygen
atmosphere. After stirring overnight at room temperature, the
reaction mixture was filtered, and the filtrate was concentrated
and dried under reduced pressure to leave a crude product
which was purified by GPC (chloroform, monitored at 350 nm)
to give 4-2N2 (6.0 mg, 57%) as an orange viscous liquid:
1H-NMR (300 MHz, CDCl3, ppm) d 7.71 (s, 4H, Ph–H),
4 3 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 3 1 – 4 4 0

View Article Online
recorded at appropriate time intervals. The spectral changes
upon thawing were also monitored by carefully controlling
the matrix temperature with an Oxford Instrument Intelligent
Temperature Controller (ITC 4).
9 See for reviews (a) A. Rajca, Chem. Rev., 1994, 94, 871; (b) H.
Iwamura, Adv. Phys. Org. Chem., 1990, 26, 179; (c) Magnetic
Properties of Organic Materials, ed. P. M. Lahti, Marcel Dekker,
New York, 1999.
10 (a) O. Kahn, Molecular Magnetism, VCH Publishers, Inc., Weinheim,
1993; (b) D. Gatteschi, Adv. Mater., 1994, 6, 635.
Flash photolysis. All flash measurements were made on a
Unisoku TSP-601 flash spectrometer. Three excitation light
sources were used depending on the precursor absorption bands
and lifetime of the transient species. They were (i) a cylindrical
150 W Xe flash lamp (100 J flash−1 with 10 ls pulse duration), (ii)
a Quanta-Ray GCR-11 Nd:YAG laser (355 nm pulses of up to
40 mJ pulse−1 and 5–6 ns duration; 266 nm pulses of up to 30 mJ
pulse−1 and 4–5 ns duration), and (iii) a Lamda Physik LEXTRA
XeCl excimer laser (308 nm pulses of up to 200 mJ pulse−1 and
17 ns duration). The beam shape and size were controlled by a
focal length cylindrical lens.
11 (a) J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed. Engl., 1994,
33, 385; (b) J. S. Miller and A. J. Epstein, Chem. Eng. News, 1995,
October 2, 30.
12 (a) Magnetism: A Supramolecular Function, ed. O. Kahn, NATO ASI
Series C, Kluwer, Dordrecht, 1996; (b) Molecule-Based Magnetic
Materialsed. M. M. Turnbull, T. Sugimoto, L. K. Thompson, ACS
Symp. Ser. 644, ACS, Washington, 1996; (c) D. Gatteschi, Curr. Opin.
Solid State Mater Sci., 1996, 1, 192; (d) MRS Bulletin, ed. J. S. Miller
and A. J. Epstein, 2000, vol. 25, p. 21; (e) Magnetism: Molecules
to Materials II, ed. J. S. Miller and M. Drillon, Wiley-VCH, 2001;
(f) Molecular Magnetism, ed. K. Itoh and M. Kinoshita, Kodansha–
Gordon and Breach: Tokyo, 2000.
13 See for reviews of the EPR spectra of triplet carbenes: (a) W.
Sander, G. Bucher and S. Wierlacher, Chem. Rev., 1993, 93, 1583;
(b) A. M. Trozzolo and E. Wasserman, in Carbenes, ed. M. Jones,
Jr. and R. A. Moss, , New York, 1975, vol. 2, pp 185–206; (c) H.
Tomioka, in Advances in Strained and Interesting Organic Molecules,
ed. B. Halton, JAI Press, Greenwich, CT, 2000, vol. 8, pp. 83–112.
14 (a) Y. Teki, Thesis, Osaka City University, Osaka, Japan, 1985;
(b) K. Takui, in Molecular Magnetism in Organic-Based Materials,
ed. P. M. Lahti, Marcel Dekker, New York, 1999.
A Hamamatsu 150 W xenon short arc lamp (L 2195) was used
as the probe source, and the monitoring beam, guided using an
optical fiber scope, was arranged in an orientation perpendicular
to the excitation source. The probe beam was monitored with a
Hamamatsu R2949 photomultiplier tube through a Hamamatsu
S3701-512Q MOS linear image sensor (512 photodiodes used).
Timing of the excitation pulse, the probe beam, and the detection
system was achieved through an Iwatsu Model DS-8631 digital
synchro scope which was interfaced to a NEC 9801 RX2
computer. This allowed for rapid processing and storage of the
data and provided printed graphic capabilities. Each trace was
also displayed on a NEC CRT N5913U monitor.
A sample was placed in a long-necked Pyrex tube which
had a side arm connected to a quartz fluorescence cuvette and
degassed using a minimum of four freeze–degas–thaw cycles at
a pressure near 10−5 Torr immediately prior to being flashed.
The sample system was flame-sealed under reduced pressure,
and the solution was transferred to the quartz cuvette which
was placed in the sample chamber of the flash spectrometer. A
cell holder block of the sample chamber was equipped with a
thermostat and allowed to come to thermal equilibrium. The
concentration of the sample was adjusted so that it absorbed a
significant portion of the excitation light.
15 K. Itoh, T. Takui, Y. Teki and T. Kinoshita, Mol. Cryst. Liq. Cryst.,
1989, 176, 49.
16 K. Itoh, Pure Appl. Chem., 1978, 50, 1251.
17 B. Bleaney and K. D. Bowers, Proc. R. Soc. London, 1982, A214, 451.
18 (a) T. Harder, J. Bendig, G. Scholz and R. Sto¨sser, J. Am. Chem. Soc.,
1996, 118, 2497; (b) T. Harder, R. Sto¨sser, P. Wessig and J. Bendig,
J. Photochem. Photobiol. A, Chem., 1997, 103, 105; (c) H. Tomioka
and S. Sawai, Org. Biomol. Chem., 2003, 1, 4441.
19 It is to be noted here that, in the photolysis of polydiazo compounds
in 2-MTHF matrix at low temperatures, photo-de-diazoniation
processes are sensitive to experimental conditions such as wavelength
and intensities of the light and concentration of the precursor. See
(a) Y. Teki, T. Takui, H. Yagi, K. Katoh and H. Iwamura, J. Chem.
Phys., 1985, 83, 539; (b) K. Matsuda, N. Nakamura, K. Inoue,
N. Koga and H. Iwamura, Bull. Chem. Soc. Jpn., 1996, 69, 1483;
(c) N. Nakamura, K. Inoue and H. Iwamura, Angew. Chem., Int. Ed.
Engl., 1993, 32, 872; (d) T. Sugawara, M. Inada and H. Iwamura,
Tetrahedron Lett., 1983, 24, 1723.
20 See for reviews: (a) W. Sander, Angew. Chem., Int. Ed. Engl., 1990,
29, 344; (b) W. Bunnelle, Chem. Rev., 1991, 91, 336.
21 J. C. Scaiano, W. G. McGimpsey and H. L. Casal, J. Org. Chem.,
1989, 54, 1612.
Acknowledgements
The authors are grateful to the Ministry of Education, Culture,
Sports, Science and Technology of Japan for support of this work
through a Grant-in-Aid for Scientific Research for Specially
Promoted Research (No. 12002007). The support from the
Mitsubishi Foundation and the Nagase Science and Technology
Foundation is also appreciated.
22 See for reviews (a) P. Zuev and R. S. Sheridan, Tetrahedron, 1995,
42, 1337; (b) A. Nicolaides and H. Tomioka, in Photochemistry of
Organic Molecules in Isotropic and Anisotropic Media, ed. V. Rama-
murthy and K. S. Schanze, Marcel Dekker, Inc., New York, 2003,
pp. 133–184; (c) A. Nicolaides and H. Tomioka, J. Photosci., 2003,
10, 165.
23 N. Koga and H. Iwamura, in Carbene Chemistry, ed. G. Bertrand,
FontisMedia, Lausanne, 2002, pp. 271–296.
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27 The role of bridging heteroatoms such as oxygen, sulfur, sulfoxide,
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