Triplet diphenylcarbene protected by iodine and bromine groups

Article abstract of DOI:10.1002/poc.1651

Diphenyldiazomethane having two iodine and two bromine groups at the ortho positions, (2, 6-dibromo-4-tertbutylphenyl) (2, 6-diiodo-4-tert-butylphenyl) diazomethane (1a-N₂), has been synthesized. Triplet diphenylcarbene₃ 1a is generated from the precursor and is characterized by UV-Vis spectroscopy at low temperature and laser flash photolysis (LFP) techniques at room temperature. Irradiation of 1a-N₂ in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K gives the UV-Vis spectrum of ³1a, but ESR signals that can be assigned to a triplet species are not detected. The LFP of 1a-N₂ in a degassed benzene solution produces the transient absorption ascribable to ³1a at 351 nm, which decays in a second-order process with a rate constant of 0.17 sS¹. The first half-life of ³1a is estimated to be 24 s. Triplet carbene ³1a is trapped by oxygen to generate benzophenone oxide 1a-O, with maximum absorbance at 415 nm, with a rate constant of 7.3T10⁶MS ¹sS¹, and also by 1, 4-cyclohexadiene (CHD) to produce the diphenylmethyl radical (1a-H₂, lmax 1/4 380 nm) with a rate constant of 39MS1sS1. The carbene is also trapped by methanol with a rate constant of 0.30MS²sS¹. Steady-state irradiation of 1a-N₂ in degassed benzene affords only a small amount of the phenanthrene 2a as an assignable product. Copyright 2010 John Wiley & Sons, Ltd.

Full text of DOI:10.1002/poc.1651

Special Issue Paper
Received: 31 August 2009,
Revised: 16 October 2009,
Accepted: 21 October 2009,
Published online in Wiley InterScience: 25 January 2010
(www.interscience.wiley.com) DOI 10.1002/poc.1651
Triplet diphenylcarbene protected by iodine
and bromine groups
a
b
b
*
Katsuyuki Hirai , Kana Bessho , Toshikazu Kitagawa
and Hideo Tomiokac,d
Diphenyldiazomethane having two iodine and two bromine groups at the ortho positions, (2,6-dibromo-4-tert-
butylphenyl)(2,6-diiodo-4-tert-butylphenyl)diazomethane (1a-N2), has been synthesized. Triplet diphenylcarbene 31a
is generated from the precursor and is characterized by UV–Vis spectroscopy at low temperature and laser flash
photolysis (LFP) techniques at room temperature. Irradiation of 1a-N2 in a 2-methyltetrahydrofuran (2-MTHF) matrix
at 77 K gives the UV–Vis spectrum of 31a, but ESR signals that can be assigned to a triplet species are not detected. The
3
LFP of 1a-N2 in a degassed benzene solution produces the transient absorption ascribable to 1a at 351 nm, which
decays in a second-order process with a rate constant of 0.17 sS1. The first half-life of 31a is estimated to be 24 s. Triplet
carbene 31a is trapped by oxygen to generate benzophenone oxide 1a-O, with maximum absorbance at 415 nm, with a
rate constant of 7.3 T 106 MS1 S1
, and also by 1,4-cyclohexadiene (CHD) to produce the diphenylmethyl radical
s
(1a-H2, lmax ¼ 380 nm) with a rate constant of 39 MS1 S1
s
. The carbene is also trapped by methanol with a rate constant
of 0.30 MS2 S1
. Steady-state irradiation of 1a-N2 in degassed benzene affords only a small amount of the phenan-
s
threne 2a as an assignable product. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: diazo compound; kinetics; steric protection; triplet carbene
orders of magnitude greater than ‘parent’ diphenylcarbene.
INTRODUCTION
Product analysis studies indicate that the main decay pathway is
dimerization at the carbenic center followed by photocycliza-
tion.[13–16] This indicates that the carbene center is not
completely shielded and hence the carbene still has a space in
which the carbenes are coupled with each other. To realize stable
triplet carbene, we need to explore kinetic protectors that are
bulkier than bromine.
Triplet carbenes are highly reactive organic radicals that are
notoriously difficult to stabilize.[1–7] To isolate the triplet carbene
with its electronic integrity (one center diradical) intact, steric
protection is the ideal conceptual method. Attempts to stabilize
triplet diphenylcarbenes by introducing various substituents at
the ortho positions have been reported.[8–11] The strategy,
however, encounters
a
limitation when alkyl groups are
Iodine atom has been regarded as a better kinetic protector of
triplet carbene than bromine atom because the van der Waals
radius (200 pm) is bigger than bromine group (185 pm).[17] In
addition, according to the theoretical calculations,[22,23] the C—I
bond length (212 pm) of iodobenzene is longer than the C—Br
(189 pm) of bromobenzene. More persistent triplet diphenylcar-
employed as protecting groups. Triplet carbenes abstract a
hydrogen even from very poor sources of electrons such as CH
bonds. Thus, tert-butyl groups, which have been successfully used
to protect many reactive centers, are almost powerless to
stabilize triplet carbene center.[12]
The halogen group was found to be a very effective
protector.[13–16] While the van der Waals radius of bromine atom
is similar to that of methyl (Br, 185 pm; Me, 175 pm),[17] the C—Br
bond length (181 pm) is longer than the C—C (Me) (177 pm),[17]
suggesting that the o-bromine groups overhang the reactive site
more effectively than o-methyl groups. Moreover, halides are
usually reactive toward singlet carbenes to form halonium ylides,
but they are not reactive with the triplet state.[18–20] In addition,
the value of j1 which gives a measure of the strength of the
spin–orbit interaction is large (2460 cmꢀ1) for bromine,[21]
suggesting that the intersystem crossing from the nascent
singlet carbene to the triplet should be accelerated by
introducing a bromine atom. Based on these viewpoints, we
prepared bis(2,6-dibromo-4-tert-butylphenyl)diazomethane (1b-N2)
and demonstrated that triplet bis(2,6-dibromo-4-tert-butylphenyl)
carbene (31b) generated therefrom was shown to have a half-life
of 16 s in solution at room temperature (Scheme 1),[15] ca. eight
*
Correspondence to: K. Hirai, Instrumental Analysis Facilities, Life Science
Research Center, Mie University, Tsu, Mie 514-8507, Japan.
E-mail: hirai@chem.mie-u.ac.jp
a
b
c
d
K. Hirai
Instrumental Analysis Facilities, Life Science Research Center, Mie University,
Tsu, Mie 514-8507, Japan
K. Bessho, T. Kitagawa
Department of Chemistry for Materials, Graduate School of Engineering, Mie
University, Tsu, Mie 514-8507, Japan
H. Tomioka
Department of Applied Chemistry, Aichi Institute of Technology, Toyota, Aichi
470-0392, Japan
H. Tomioka
Nagoya Industrial Research Institute, Nagoya, Aichi 464-0819, Japan
J. Phys. Org. Chem. 2010, 23 347–356
Copyright ß 2010 John Wiley & Sons, Ltd.

K. HIRAI ET AL.
Scheme 3. Preparation of (2,6-dibromo-4-tert-butylphenyl)(2,6-diiodo-4-
tert-butylphenyl)diazomethane (1a-N2)
Scheme 1. Generation of diarylcarbenes 1
purified by repeated chromatography on a gel permeation
column. Diazomethane 1a-N2 was rather stable for diazomethane
and could be stored in a refrigerator for several months without
any appreciable decomposition.
benes than the tetrabromodiphenylcarbenes may be realized, if
one can generate a diphenylcarbene having four iodine groups at
all of the ortho positions. However, we cannot prepare the
desired diazomethane at present. We were able to prepare
diphenyldiazomethane having two iodine groups in addition to
two bromine groups at all of the ortho positions, that is,
(2,6-dibromo-4-tert-butylphenyl)(2,6-diiodo-4-tert-butylphenyl)-
diazomethane (1a-N2). Herein, we wish to report that triplet
diphenylcarbene 31a generated from the precursor 1a-N2 is more
Spectroscopic studies
ESR studies in rigid matrix at low temperature
Irradiation (l > 300 nm) of 1b-N2 in 2-methyltetrahydrofuran
(2-MTHF) at 77 K gave rise to ESR signals with a typical
fine structure of randomly oriented triplet carbene.[15,27,28]
However, similar irradiation of 1a-N2 in 2-MTHF at 77 K gave
no triplet signals of carbene 1a (Fig. 1(a)). Not only 1b but also
most sterically hindered diphenylcarebenes generally gave rise to
the stable triplet signals under the same conditions.[12–16,29–33] It
has been observed that the ESR signals sometimes become
sharper and the signal intensities increase 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 a single preferred orientation
rather than several slightly distorted orientations dictated by the
trapping of the precursor in the frozen matrix.[34–39] We examined
whether or not triplet signals appeared by warming the matrix.
3
persistent than the tetrabromo analog 1b (Scheme 1).
RESULTS AND DISCUSSION
Preparation of precursor diazomethane
(2,6-Dibromo-4-tert-butylphenyl)(2,6-diiodo-4-tert-butylphenyl)-
diazomethane (1a-N2) was prepared according to the reaction
procedure outlined in Schemes 2 and 3. Thus, 2,6-diiodo-
4-tert-butyltoluene (3)[24] was converted to the acetate 4 by
oxidizing with KMnO4 in a mixture of acetic anhydride, acetic
acid, and sulfuric acid, which was then hydrolyzed and oxidized to
2,6-diiodo-4-tert-butylbenzaldehyde (6) with PCC (Scheme 2).
1-tert-Butyl-3,5-dibromo-4-iodobenzene (7)[25] was lithiated by
simply treating with butyl lithium (BuLi), which was then reacted
with aldehyde 6 to give the diphenylmethanol 8. The alcohol was
then converted to the corresponding diazomethane (1a-N2) by
way of a carbamate[26] (Scheme 3). The diazomethane was
Figure 1. (a) ESR spectra obtained by irradiation of diazo compound
1a-N2 in 2-methyltetrahydrofuran at 77 K. (b)–(f) The same sample
measured at 77 K after thawing to 160 K (b), 170 K (c), 180 K (d), 190 K
(e), and 300 K (f)
Scheme 2. Preparation of 4-tert-butyl-2,6-diiodobenzaldehyde (6)
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 347–356

STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE
The ESR measurements were carried out after warming the matrix
involving the photoproducts and recooling again to 77 K. This
procedure can compensate weakening of signals due to the Curie
law.[40–42] The photolyzed sample was gradually warmed up to
150 K, but no ESR signals appeared. However, when the sample
was warmed to 160 K, a new doublet signal appeared around
327 mT (Fig. 1(b)). The signal intensity increased gradually as the
sample was warmed up to 190 K (Fig. 1(c)–(e)) and the signal
could be observed even at 300 K (Fig. 1(f)). The ESR signals of
carbene 1b were stable at 77 K and disappeared irreversibly
when the matrix was warmed to 170 K.[43]
It has been reported that the sharp peak due to the radical
species around 330 mT is found when some triplet signals in
2-MTHF at 77 K decrease by thawing the matrix.[29,30,44] This
means that the triplet diarylcarbenes decayed by abstracting a
hydrogen from 2-MTHF to form the corresponding diarylmethyl
radicals. In the case of the irradiation of 1a-N2, the radical signal
may be due to (2,6-dibromo-4-tert-butylphenyl)(2,6-diiodo-
4-tert-butylphenyl)methyl radical generated by a hydrogen
abstraction of triplet carbene 31a because the radical signal
appears upon warming the matrix. The observation indicates that
the signals due to carbene 31a are invisible, but that the carbene
exists in the matrix. We assumed the temperature (Td) at which
signals due to triplet carbene disappeared as a measure of the
thermal stability of triplet carbenes.[11] If the temperature at
Figure 2. UV–Vis spectra obtained by irradiation of diazo compound
1a-N2. (A) (a) Spectrum of 1a-N2 in 2-methyltetrahydrofuran at 77 K.
(b) The same sample after irradiation (l > 300 nm). (c) The same sample
after thawing to 100 K. (B) UV–Vis spectral changes measured upon
thawing the same sample from 100 to 300 K.
3
which 1a abstracts a hydrogen from the solvent is assumed to
the Td, that of 31a is higher than that (130 K) for bis(2,4,6-
tribromophenyl)carbene[15] and is similar to that (170 K) for
bis(2,6-dibromo-4-phenylphenyl)carbene.[16] The UV–Vis spec-
trum due to 31a also disappeared at the same temperature (vide
infra).
broad band from 440 to 520 nm. These features, rather strong
absorption bands in the UV region and a weak and broad band in
the visible region, are usually present in the spectra of triplet
diphenylcarbenes.[27,28] The glassy solution did not exhibit any
changes for hours when kept at 77 K, but disappeared irreversibly
when it was allowed to warm to room temperature and cooled to
77 K. On the basis of similarity with the spectrum and thermal
behavior of 31b,[15] the absorption spectrum can be attributed to
3
Influence of iodine atom on ESR signals. Triplet carbene 1a was
observed by UV–Vis spectroscopy at low temperature and LFP at
room temperature (vide infra) but not by ESR spectroscopy.
Sander and co-workers reported that the radical pair of
2,3,5,6-tetrafluorophenylnitren-4-yl and iodine atom was pro-
duced by the photolysis of 2,3,5,6-tetrafluoro-4-iodophenylnitrene
in Ar at 4 K, but that the iodine atoms formed during the
photolysis could not be observed in the ESR spectra.[45] It is
explained that no detection of the iodine signals is attributed to
the orbital degeneracy and strong spin–orbit coupling in the
iodine atoms. Moreover, they found that triplet species having an
iodine atom at the ortho position afforded almost no the ESR
signals in Ar matrix at cryogenic temperature.[46] For instance,
2,3,4,5-tetrafluoro-6-iodophenylnitrene has broad ESR signals
at 4 K but no signal at 15 K. 2-Iodophenylnitrene and
2-iodophenyl(phenyl)carbene have no signals even at 4 K. We
performed the ESR measurement of 1a at 5 K, but no triplet
signals were observed. Since carbene 1a possesses two iodine
groups at the ortho positions, the influence of the iodine atoms
3
triplet carbene 1a.
The thermal stability is 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
3
bands. The characteristic bands due to 1a shifted the maxima
from 336 and 367 nm to 348 and 369 nm at 100 K, respectively
(Fig. 2A(c)). Similar changes in the UV–Vis spectra upon thawing
the matrix were also observed for other sterically congested
diarylcarbenes and were generally interpreted in terms of
geometrical changes of the carbenes (vide infra).[27,28,39–42] The
bands started to decay slowly at 150 K and decayed rather
sharply at around 180 K (Fig. 2(B)). The temperature at which
the bands of 31a start to decrease is roughly consistent with
the temperature at which the ESR signal due to the
(2,6-dibromo-4-tert-butylphenyl)(2,6-diiodo-4-tert-butylphenyl)
methyl radical appeared.
3
on ESR spectrum of 1a may be larger than those of the mono
iodo-substituted species.
It should be noted here that absorption bands of most triplet
diphenylcarbenes having no kinetic protectors at the ortho
positions in 2-MTHF disappear in the temperature range of
100–105 K[28,29] where the viscosity of the matrix changes
dramatically from 107 to 103 P. [39] Even the absorption bands
of triplet tetrabromo analog 31b disappear up to 170 K in
2-MTHF.[15] This observation demonstrates that the thermal
UV–Vis studies in rigid matrix at low temperature
Photolysis (l > 300 nm) of 1a-N2 in a 2-MTHF matrix at 77 K
resulted in the appearance of new bands at the expense of the
original absorption due to the starting diazo compound
(Fig. 2A(b)). The new bands consist of two identifiable features,
strong and sharp maxima at 336 and 367 nm and a weak and
3
3
stability of 1a is slightly larger than that of 1b.
J. Phys. Org. Chem. 2010, 23 347–356
Copyright ß 2010 John Wiley & Sons, Ltd.
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K. HIRAI ET AL.
Figure 4. Transient absorption spectra obtained in LFP of 1a-N2 (a) in
degassed benzene and (b) in O2 saturated benzene with a 308 nm
excimer laser recorded after 100 ms. The inset shows an oscillogram trace
in O2 saturated benzene monitored at 415 nm.
Figure 3. Absorption of transient products formed during laser irradia-
tion (308 nm) of diazo compound 1a-N2 in degassed benzene at room
temperature recorded from immediately after irradiation to 240 s after
excitation. The inset shows the time course of the absorption at 351 nm.
The half-life is just a measure of lifetime and cannot be
regarded as a quantitative scale for reactivity. In this respect, the
rate constant of the triplet carbene with a typical triplet quencher
can be employed as a more quantitative scale of the reactivity. It
is well-documented that carbenes with triplet ground states are
readily trapped with oxygen or a good hydrogen donor such as
1,4-cyclohexadiene (CHD).[47] Therefore, the rate constants of the
trapping reactions by O2 and CHD are used as a more quantitative
scale to estimate the reactivities of the triplet carbenes.
Laser flash photolysis (LFP) studies in solution at room temperature
The lifetime of carbene 1a was estimated in degassed benzene at
room temperature, in which we have measured the lifetime of a
series of sterically congested diarylcarbenes.[8–11] The lifetime of
carbene 1a was too long to monitor by the LFP technique, which
has been routinely used for the determination of the decay rate
constants of triplet diarylcarbenes. Thus, a conventional UV–Vis
spectroscopic method was more conveniently employed in this
case.[29,30]
LFP of 1a-N2 in O2 saturated benzene resulted in a dramatic
decrease in the lifetime of 31a and a concurrent appearance of a
new broad absorption band at 415 nm (Fig. 4(b)). The spent
solution was found to contain the corresponding benzophenone.
It is well-documented[48–55] that diarylcarbenes with triplet
ground states are readily trapped by oxygen to generate the
corresponding diaryl ketone oxides (Scheme 4), which show a
broad absorption band centered at 390–450 nm. Thus, our
observations can be interpreted as indicating that the transient
absorption quenched by oxygen is due to 31a. The rate of
increase in the band at 415 nm is the same as the rate of decrease
Brief irradiation of 1a-N2 in degassed benzene at 25 8C with a
150 mJ, 308 nm pulse from XeCl excimer laser produced transient
absorption bands, showing strong maxima at 351 and 366 nm
(Fig. 3). The maxima of the bands were shifted slightly from those
observed in the photolysis of 1a-N2 in 2-MTHF at 100 K. However,
the transient bands were markedly quenched by oxygen to give
the corresponding ketone oxide (vide infra), and hence we
assigned the bands to triplet 31a. The absorption bands decayed
very slowly, and did not disappear completely even after 2 min
under these conditions. The decay was found to be a second
order (2k/el ¼ 0.17 sꢀ1) (Table 1). The approximate half-life (t1/2) of
3
of the peak due to 1a within experimental error, showing that
31a is quenched with oxygen to form carbonyl oxide 31a-O2. The
rate constant (kO2) for the quenching of 31a by O2 is determined
to be 7.3 ꢁ 106 Mꢀ1sꢀ1 from a plot of the observed pseudo-
first-order growth rate of 31a-O2 as a function of [O2] (Fig. 5,
Table 1). This is some three orders of magnitude smaller than that
3
31a was estimated to be 24 s. The data indicated that 1a was
more persistent than the tetrabromo analog 31b (2k/el ¼ 0.35 sꢀ1
,
t1/2 ¼ 16 s).[15]
Table 1. Kinetic data of carbenes 1
a
Carbenes
2k/el (sꢀ1
)
t1/2 (s)
kO (Mꢀ1sꢀ1
)
kCHD (Mꢀ1sꢀ1
)
KkMeOH (Mꢀ2sꢀ1
)
2
1a
1bb
0.17
0.35
24
16
7.3 ꢁ 106
2.1 ꢁ 107
3.9 ꢁ 10
0.30
0.42c
5.3 ꢁ 102
a K ¼ kTS/kST.
b Reference
c Reference
[15]
[43]
.
.
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 347–356

STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE
Figure 6. Transient absorption spectra obtained in LFP of 1a-N2 in
degassed benzene in the presence of 1,4-cyclohexadiene (0.265 M) with
a 308 nm excimer laser recorded after 1 ms. The inset shows a plot of the
decay of triplet carbene 31a monitored at 350 nm as a function of the
concentration of 1,4-cyclohexadiene
Scheme 4. Photolysis of diazo compounds in the presence of carbene
quenchers
of the carbene overlapped, the growth curves of the radical were
not observed exactly. However,
a plot of the observed
pseudo-first-order rate constants of the decay of the carbene
monitored at 350 nm against [CHD] is linear (Fig. 6 inset), and the
slope of this plot yields the absolute rate constant for the reaction
of 31a with the diene, kCHD ¼ 39 Mꢀ1sꢀ1, which is approximately
six orders of magnitude smaller than that observed with the
‘parent’ triplet diphenylcarbene (kCHD ¼ 1.0 ꢁ 107 Mꢀ1sꢀ1 [63]
)
one order of magnitude smaller than that observed with triplet
and
3
tetrabromo analog 1b (5.3 ꢁ 102 Mꢀ1sꢀ1) (Table 1).[15]
LFP of 1a-N2 in a degassed benzene solution containing
3
methanol afforded absorption bands due to triplet carbene 1a
detected in the absence of methanol. The decay curves of
31a measured are shown in Fig. 7. The decay rate (kobs) of 1a
3
increased upon the addition of methanol. However, the plot of
kobs versus methanol concentration was not linear for the
concentration range of 2.47–9.22 M.[44,67] A plot of better linearity
was observed when the kobs was plotted against the dimer
concentration of methanol. The quenching rate constant
(KkMeOH) by methanol was estimated to be 0.30 Mꢀ2sꢀ1 from
Figure 5. A plot of the growth of the diaryl ketone oxide 1a-O2
monitored at 415 nm as a function of the concentration of oxygen
observed with the ‘parent’ triplet diphenylcarbene
(kO2 ¼ 5.0 ꢁ 109 Mꢀ1sꢀ1 [55]
)
and three times smaller than that
3
observed with triplet tetrabromo analog 1b.[15]
When a degassed benzene solution of 1a-N2 containing CHD
was excited, a new species was formed, showing an absorption
with lmax ¼ 380 nm (Fig. 6). The decay of 31a was again found to
be kinetically correlated with the growth of the new species.
Thus, this new signal was attributed to the corresponding
diarylmethyl radical (1a-H) formed as a result of a hydrogen
abstraction of 31a from the diene, because it is now well-
documented[47,56–65] that triplet diarylcarbenes, generated in
good hydrogen donor solvents, undergo a hydrogen abstraction
leading to the corresponding radicals (Scheme 4). These radicals
typically have transient absorptions at longer wavelengths than
those of the precursor carbenes. The excellent hydrogen donor
properties of CHD have been well recognized.[66] The spent
solution was found to contain the diarylmethane as the main
product. Since the absorption of the radical and the tailing band
Figure 7. The decay curves of triplet carbene 31a in degassed benzene
in the presence of methanol monitored at 350 nm
J. Phys. Org. Chem. 2010, 23 347–356
Copyright ß 2010 John Wiley & Sons, Ltd.
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K. HIRAI ET AL.
Scheme 5. Photolysis of diazo compound 1a-N2 in degassed benzene at
25 8C
afforded the corresponding 9,10-diarylphenanthrene deriva-
tives.[15,16] We demonstrated that bis(2,6-dibromo-4-phenylphenyl)
carbene dimerized to form tetrakis(2,6-dibromophenylphenyl)
ethene as an initial product which underwent photocyclization to
give the corresponding phenanthrene.[16]
Figure 8. A plot of the decay of triplet carbene 31a monitored at 350 nm
as a function of the concentration of methanol.
Photolysis (l > 350 nm) of 1a-N2 (5.0 mg) in degassed
benzene-d6 (0.5 ml) in a sealed NMR tube at 25 8C was monitored
by 1H NMR spectroscopy. The irradiation (18 min) gave only very
weak signals at 8.94, 8.77, 7.48, 6.14, 1.22, and 1.07 ppm along
with weak complex peaks from 1.5 to 0.7 ppm at the expense of
the signals of 1a-N2. 1H NMR spectrum of a fraction obtained by a
recycled GPC of the photomixture showed the phenanthrene
derivative 2a (a brown oil in 18% yield) (Scheme 5). Carbene 1a
presumably decayed through the same pathway, namely by
dimerization followed by photocyclization, to provide 2a. 1H NMR
spectra of the other GPC fractions showed no detectable peaks.
Photolysis (l > 300 nm) of 1b-N2 in an oxygen-saturated
benzene solution afforded the corresponding diaryl ketone 1b-O
in 66% yield (Scheme 6). Irradiation of 1a-N2 under the same
conditions gave ketone 1a-O in 77% yield.
the slope of this plot (Fig. 8, Table 1). This is smaller than that
(0.42 Mꢀ2sꢀ1 [43]
3
observed with 1b.
)
The mechanism of the reaction of triplet carbenes with
methanol to form methyl ether is of considerable interest.[68]
Insertion reaction of carbenes with alcohols to form ethers is
usually considered to occur from the singlet states of these
species (Scheme 4). However, LFP studies have shown that the
ground state of diphenylcarbene is efficiently quenched by
methanol.[59,69]
The reaction of the triplet carbenes with the O—H bonds of
alcohols is explained either by pre-equilibrium[70] or by
surface-crossing mechanisms.[71,72] In the pre-equilibrium mech-
anism, triplet to singlet interconversion occurs prior to O—H
insertion of the singlet carbene, whereas, in the surface-crossing
mechanism, triplet states can react with alcohols, with surface
crossing occurring after the carbene begins to interact with the
O—H bond. In either of the mechanisms, the decrease in KkMeOH
is considered to reflect a larger free-energy difference between
the ground triplet states and their singlet states.[71–73] In other
words, the difference of the rate constants, KkMeOH, in Table 1
suggests that the free-energy difference between the ground
triplet and singlet states for 1a is larger than that for 1b.
The values of t1/2, kO2, kCHD, and KkMeOH are summarized in
Table 1. Inspection of the data immediately indicates that
carbene 31a showed not only the larger stability estimated in
Irradiation (l > 300 nm) of 1a-N2 in degassed benzene
containing CHD resulted in the formation of a mixture of two
products, both showing methylene protons at 4.31 and 4.70 ppm,
respectively. It has been well known that triplet diarylcarbenes
produce the corresponding diarylmethanes when generated in
good hydrogen donors such as the diene. Unfortunately, the
reaction mixture could be separated by neither TLC nor GPC.
1
From the H NMR spectrum of the mixture, however, the minor
product is assigned to diarylmethane 1a-H2 since the aromatic
and tert-butyl protons are similar to those of diazo 1a-N2
(Scheme 6). On the other hand, the major product is assigned
to (2,6-dibromo-4-tert-butylphenyl)(2-iodo-4-tert-butylphenyl)-
methane 1a(-H2 in which an iodine in 1a-H2 is replaced by a
3
degassed benzene compared to 1b but also essentially lower
reactivities toward typical carbene quenchers such as oxygen,
CHD, and methanol than 31b. Thus, the half-life and the
quenching rate constants of 31a clearly suggest that iodine
groups are better kinetic protectors than bromine groups, as
expected.
hydrogen since a aromatic signal (7.92 ppm) of diiodo-
tert-butylphenyl group in 1a-N2 is converted into a double
doublet at 7.18 ppm and two doublets at 7.86 and 6.48 ppm
(Scheme 6). The similar irradiation at long wavelength
(l > 350 nm) gave only 1a-H2 in 75% yield.
In the photolysis of 1a-N2 in degassed benzene containing
methanol, two methyl ethers were formed. The irradiation with
l > 300 nm of 1a-N2 afforded a mixture of the methyl ethers,
1a-HOMe (44%) and 1a(-HOMe (38%), and the longer wave-
length irradiation (l > 350 nm) gave 1a-HOMe (48%) as a sole
product. The methyl ethers are presumably produced as a result
of the interaction of carbene 1a with O—H bond of methanol.
It has been shown that the irradiation (l > 300 nm) of 1b-N2 in
the presence of CHD afforded only 1b-H2 in 59% yield and the
similar irradiation (l > 300 nm) in the presence of methanol gave
1b-HOMe in 48% yield (Scheme 6).[15]
Product analysis studies
Most of the sterically congested triplet diphenylcarbenes decay
mainly by undergoing dimerization at the carbenic center to give
tetra-arylethenes as the main product, when generated in
degassed benzene at room temperature in the absence of
appropriate carbene quenchers. For instance, bis(2,4,6-
trichlorophenyl)carbene in benzene produced tetrakis(2,4,6-
trichlorophenyl)ethene in >80% yield.[26,31] On the other hand,
bis(2,6-dibromophenyl)carbenes under the same conditions
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STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE
Scheme 6. Photoproducts obtained by irradiation of diazo compound (1-N2) at 25 8C. (a) Reference [15]. (b) Relative yields determined by 1H NMR
The bond dissociation energy between carbon and halogen for
Ph-I (<72.1 kcal/mol) is >11.4 kcal/mol smaller than that of Ph-Br
(83.5 kcal/mol).[74] The facility of dehalogenation of iodine atom
compared to bromine atom is plausible result. It is well known
that in a solvent with easily abstractable hydrogen atoms
photolysis of iodoarenes gives the corresponding deiodinated
arenes by photodissociation of C—I bond followed by hydrogen
transfer from the solvent.[75] Therefore, the formation of
diarylmethane 1a(-H2 and methyl ether 1a(-HOMe may be
formed by the over-irradiation of the initial photoproducts 1a-H2
and 1a-HOMe. In the degassed benzene, the corresponding
monoiodo-ketone was not formed because of the absence of
good hydrogen sources.
conversions are rather severe and cannot be applied to diazo
compounds which are usually thermally unstable. Recently, the
method of converting iodoarenes into the diaryliodonium salts
using mild conditions was developed.[76] If one can convert the
ortho iodine group on the present diphenyldiazomethane into
the corresponding diaryliodonium salt by using this method, a
carbene generated therefrom will be highly shielded by the ortho
aryliodonium group.
EXPERIMENTAL SECTION
General information
1H and 13C NMR spectra were determined with JEOL
JNM-AL300 FT/NMR spectrometer in CDCl3 with Me4Si as an
internal reference. IR spectra were measured with films on NaCl
plate (oils and semisolids) or KBr pellets (solids). IR spectra were
recorded on a JASCO FT/IR-410 spectrometer, and UV–Vis spectra
were recorded on a JASCO V-560 spectrometer or a Shimadzu
MaltiSpec-1500. The mass spectra were recorded on a JEOL
JMS-600H mass spectrometer or an Applied Biosystems Voyager
DE-Pro MALDI-TOF mass spectrometer. Thin layer chromatog-
raphy for diazo compounds was performed using Merck
aluminum oxide 60 PF254 on a glass plate (Type E). Column
chromatography was performed using silica gel (63–210 mm) or
neutral alumina (Act. I, inactivated with 5% water) for diazo
compounds. Recycle GPC was undertaken with a JASCO PU-2086
chromatograph with a UV-2070 UV–Vis detector using a Shodex
GPC H-2001 (20 mm ꢁ 50 cm).
CONCLUSIONS
In the present study, diphenyldiazomethane 1a-N2 having two
iodine and two bromine groups at the ortho positions has been
synthesized and triplet carbene 31a was generated by the
photolysis of the diazomethane. Triplet carbene 31a in degassed
benzene at room temperature decayed in a second-order process
with a rate constant of 0.17 sꢀ1, which was half of that of the
tetrabromo analog 31b. The reaction rate constants of 1a with the
carbene quenchers were also smaller than those of 1b,
respectively. As expected, an iodine group shielded the carbene
center better than a bromine group.
One may expect that a triplet diphenylcarbene having four
iodine groups at all of the ortho positions is more persistent than
the present carbene. At present, the synthesis of the precursor of
the carbene is difficult. We are making an effort to synthesize it
now.
Preparation of 4-tert-butyl-2,6-diiodobenzyl acetate (4)
It is well known that some iodoarenes can be converted to the
corresponding diaryliodonium salts. One can convert an iodine
group into an aryliodonium group by using certain synthetic
methods. Unfortunately, the conditions employed for such
To a stirred solution of the 4-tert-butyl-2,6-diiodotoluene (3)[24]
(4.85 g, 12.1 mmol) in H2SO4 (3.3 ml), AcOH (20 ml), and Ac2O
(40 ml) was slowly added KMnO4 (2.49 g, 15.8 mmol) at 40 8C over
a period of 3 h and the mixture was stirred for 1 day at the same
J. Phys. Org. Chem. 2010, 23 347–356
Copyright ß 2010 John Wiley & Sons, Ltd.
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K. HIRAI ET AL.
temperature. The mixture was poured into H2O and extracted
with Et2O. The ethereal layer was washed with 10% NaOH
solution and brine, dried over anhydrous Na2SO4, and the solvent
was removed under reduced pressure. The residue was
131.4, 125.4, 98.3, 83.2, 34.6, 34.1, 31.0, 30.9; EI-MS (m/z) 708
(Mþ4, 5.4), 706 (Mþ2, 15.5), 704 (Mþ, 7.4), 458 (100), 443 (96.3),
413 (28.0); HRMS calcd for C21H24Br2I2O 703.8283, found m/z
703.8334.
chromatographed through
a column of silica gel using
hexane-CHCl3 (1:1) as an eluent to give 4-tert-butyl-2,
6-diiodobenzyl acetate (4) as a yellow viscous liquid (1.13 g,
65%): H NMR (300 MHz, CDCl3, ppm) d 7.86 (s, 2H), 5.40 (s, 2H),
Preparation of ethyl N-[(2,6-dibromo-4-tert-butylphenyl)
(2,6-diiodo-4-tert-butylphenyl)methyl]carbamate (10)
1
2.13 (s, 3H), 1.28 (s, 9H); 13C NMR (75.5 MHz, CDCl3, ppm) d 170.7,
155.7, 137.7, 136.4, 100.3, 75.7, 34.4, 30.9, 20.7; EI-MS (m/z) 458
(Mþ, 3.6), 331 (56.8), 289 (100); HRMS calcd for C13H16I2O2
457.9240, found m/z 457.9313.
A solution of alcohol 8 (0.70 g, 1.0 mmol) and SOCl2 (10 ml) was
stirred at room temperature overnight, and the mixture was
evaporated under reduced pressure to give (2,6-dibromo-4-tert-
butylphenyl)(2,6-diiodo-4-tert-butylphenyl)chloromethane (9) as
yellow viscous solids (0.73 g, >99%), which was rather unstable
1
and hence used without further purification: H NMR (300 MHz,
Preparation of 4-tert-butyl-2,6-diiodobenzyl alcohol (5)
CDCl3, ppm) d 7.97 (s, 2H), 7.56 (s, 2H), 6.80(s, 1H), 1.29 (s, 9H)
1.28(s, 9H). A mixture of silver tetrafluoroborate (290 mg,
1.49 mmol) and ethyl carbamate (1.15 g, 12.9 mmol) was heated
at 60 8C. To the mixture was added a solution of chloromethane 9
(0.73 g, 1.0 mmol) in 1,4-dioxane (10 ml), and the resulting
mixture was heated at 100 8C overnight. After cooling at room
temperature, the mixture was filtered, and CHCl3 (60 ml) were
added to the filtrate. The organic layer was washed well with
water, dried over anhydrous Na2SO4, and the solvent was
removed under reduced pressure. The residue was chromato-
graphed through a column of silica gel using hexane-CH2Cl2 (1:1)
as an eluent to give ethyl N-[(2,6-dibromo-4-tert-butylphenyl)
(2,6-diiodo-4-tert-butylphenyl)methyl]carbamate (10) as yellow
solids (156 mg, 21%): mp 91.7–93.8 8C; 1H NMR (300 MHz, CDCl3,
ppm) d 7.91 (s, 2H), 7.54 (s, 2H), 6.50 (d, J ¼ 9.0 Hz, 1H), 5.26 (d,
J ¼ 9.0 Hz, 1H), 4.22 (q, J ¼ 7.2 Hz, 2H), 1.32–1.26 (m, 3H), 1.28 (s,
9H), 1.26 (s, 9H); 13C NMR (75.5 MHz, CDCl3, ppm) d 154.2, 153.3,
153.1, 139.4, 137.0, 132.6, 131.3, 125.9, 99.1 66.9, 61.1, 34.3, 33.8,
30.81, 30.79, 14.8; EI-MS (m/z) 698 (Mþ2-Br, 23.3), 696 (M-Br, 23.1),
651 (53.4), 649 (100), 647 (49.5), 621 (25.9); HRMS calcd for
C24H29BrI2NO2 (M-Br) 695.9471, found m/z 695.9416.
To a stirred solution of acetate 4 (1.39 g, 3.14 mmol) in MeOH
(25 ml) was added K2CO3 (114 mg, 1.05 mmol) and the mixture
was stirred at room temperature for 3 h. The reaction mixture was
quenched by the addition of 2 M hydrochloric acid. The mixture
was extracted with ether, and the organic solution was washed
with brine, dried over anhydrous Na2SO4, and the solvent was
removed under reduced pressure to give 4-tert-butyl-2,6-diiodobenzyl
alcohol (5) as a yellow viscous liquid (1.21 g, 93%): 1H NMR
(300 MHz, CDCl3, ppm) d 7.82 (s, 2H), 5.00 (d, J ¼ 6.9 Hz, 2H), 2.10
(t, J ¼ 6.9 Hz, 1H), 1.27 (s, 9H); 13C NMR (75.5 MHz, CDCl3, ppm) d
154.8, 140.4, 137.4, 99.6, 74.2, 34.1, 30.9; EI-MS (m/z) 416(Mþ, 12.7),
415 (100), 400 (57.1), 245 (39.3); HRMS calcd for C11H14I2O
415.9135, found m/z 415.9158.
Preparation of 4-tert-butyl-2,6-diiodobenzaldehyde (6)
To a stirred solution of benzyl alcohol 5 (1.03 g, 2.48 mmol) in
CH2Cl2 (20 ml) was added PCC (1.54 g, 7.20 mmol) and the
mixture was stirred at room temperature overnight. The solvent
was removed under reduced pressure and the residue was
chromatographed through
a column of silica gel using
hexane-CH2Cl2 (1:1) as an eluent to give 4-tert-butyl-2,6-
diiodobenzaldehyde (6) as yellow solids (1.03 g, >99%): mp
69.8–72.2 8C; 1H NMR (300 MHz, CDCl3, ppm) d 9.76 (s, 1H), 8.00 (s,
2H), 1.32 (s, 9H); 13C NMR (75.5 MHz, CDCl3, ppm) d 195.2, 158.7,
131.9, 138.9, 98.1, 34.8, 30.7; EI-MS (m/z) 414 (Mþ, 30.6), 413 (100),
398 (61.0); HRMS calcd for C11H12I2O 413.8971, found m/z
413.9001.
Preparation of (2,6-dibromo-4-tert-butylphenyl)
(2,6-diiodo-4-tert-butylphenyl)diazomethane (1a-N2)
To a solution of carbamate 10 (215 mg, 279 mmol) in Ac2O (15 ml)
and AcOH (7.5 ml) was added NaNO2 (579 mg, 8.36 mmol) in small
portions at 0 8C over a period of 3 h, and the mixture was stirred at
room temperature for 1 day after the addition was complete. The
mixture was poured into water and extracted with ether. The
ethereal layer was washed with 5% NaHCO3 aqueous solution,
water, dried over anhydrous Na2SO4, and the solvent was
removed under reduced pressure to give ethyl N-nitroso-N
[(2,6-dibromo-4-tert-butylphenyl)(2,6-diiodo-4-tert-butylphenyl)
methyl]carbamate (11) as a yellow viscous oil (207 mg, 92%); 1H
NMR (300 MHz, CDCl3, ppm) d 7.89 (s, 2H), 7.54 (s, 2H), 6.87 (s, 1H)
4.40 (q, J ¼ 7.2 Hz, 2H), 1.32–1.26 (m, 3H), 1.29 (s, 9H), 1.27 (s, 9H).
To a solution cooled at ꢀ15 8C of nitroso compound 11 in
anhydrous THF (24 ml) was added potassium tert-butoxide
(116 mg, 1.04 mmol) at once under an atmosphere of argon.
After stirring at room temperature overnight, the mixture was
poured into water and extracted with ether. The ethereal layer
was washed with water, dried over anhydrous Na2SO4, and
evaporated. The residue was purified by recycle GPC (16 cycles,
CHCl3) to give (2,6-dibromo-4-tert-butylphenyl)(2,6-diiodo-4-
tert-butylphenyl)diazomethane (1a-N2) as orange solids (88 mg,
48%): mp 150.8–153.5 8C (dec.); 1H NMR (300 MHz, CDCl3, ppm) d
7.92 (s, 2H), 7.57 (s, 2H), 1.30 (s, 9H), 1.28 (s, 9H); 13C NMR
Preparation of (2,6-dibromo-4-tert-butylphenyl)(2,6-
diiodo-4-tert-butylphenyl)methanol (8)
To a solution of 1-tert-butyl-3,5-dibromo-4-iodobenzene (7)[25]
(1.00 g, 2.43 mmol) in dry Et2O (15 ml) was added BuLi (2.60 M in
hexane, 0.96 ml, 2.50 mmol) at ꢀ78 8C. The solution was stirred for
5 h, and then a solution of aldehyde 6 (1.04 g, 2.50 mmol) in dry
ether (5 ml) was added dropwise at ꢀ78 8C. The mixture was
stirred at room temperature overnight and then was quenched
by the addition of saturated aqueous ammonium chloride. The
mixture was extracted with ether, washed with water, dried over
anhydrous Na2SO4, and the solvent was removed under reduced
pressure. The residue was chromatographed through a column of
silica gel using hexane-CHCl3 (1:2) as an eluent to give
(2,6-dibromo-4-tert-butylphenyl)(2,6-diiodo-4-tert-butylphenyl)
methanol (8) as colorless solids (1.10 g, 67%): mp 69.8–72.2 8C; 1H
NMR (300 MHz, CDCl3, ppm) d 7.93 (s, 2H), 7.53 (s, 2H) 6.39 (d,
J ¼ 9.4 Hz, 1H), 2.99 (d, J ¼ 9.4 Hz, 1H), 1.29 (s, 9H), 1.28 (s, 9H); 13
C
NMR (75.5 MHz, CDCl3, ppm) d 153.9, 153.6, 138.6, 139.4, 134.4,
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STERIC PROTECTION OF TRIPLET DIPHENYLCARBENE
(75.5 MHz, CDCl3, ppm) d 154.0, 153.4, 138.5, 133.2, 130.9 127.0,
125.6, 100.8, 76.8, 34.7, 34.3, 31.0; IR (KBr, cmꢀ1) 2061 (nCN2).
measured on a JEOL JES TE 200 spectrometer (X-band microwave
unit, 100 kHz field modulation). The signal positions were read by
using a gaussmeter. The temperature was controlled by a 9650
microprocessor-based digital temperature indicator/controller,
provided the measurements accuracy is within ꢂ0.1 K and the
control ability is within ꢂ0.2 K. Errors in the measurements of
component amplitudes did not exceed 5%, the accuracy of the
resonance fields determination was within ꢂ0.5 mT.
Product analysis
Photolysis (l > 350 nm) of 1a-N2 (5.0 mg, 7.0 mmol) in degassed
benzene-d6 (0.5 ml) gave the phenanthrene 2a: a brown oil
(0.7 mg, 18%); 1H NMR (300 MHz, CDCl3, ppm) d 8.82 (d, J ¼ 2.0 Hz,
2H), 8.49 (d, J ¼ 2.0 Hz, 2H), 7.31 (s, 4H), 1.49 (s, 18H), 1.29 (s, 18H);
MALDI-TOF-mass calcd for C42H45Br4I2 (MþH) 1118.83, found m/z
1118.77.
Low-temperature UV–Vis spectra
Photolysis (l > 300 nm) of 1a-N2 (5.0 mg, 7.0 mmol) in
nondegassed benzene (5.0 ml) gave the ketone 1-O: a yellow
oil (3.8 mg, 77%); H NMR (300 MHz, CDCl3, ppm) d 7.91 (s, 2H),
7.58 (s, 2H), 1.32 (s, 9H), 1.30 (s, 9H); 13C NMR (75.5 MHz, CDCl3,
ppm) d 193.9, 157.0, 156.5, 141.7, 138.5, 134.4, 130.8, 124.1, 95.3,
35.1, 34.6, 30.8; IR (KBr, cmꢀ1) 1684 (nCO); EI-MS (m/z) 706 (Mþ4,
51.2), 704 (Mþ2, 100), 702 (Mþ, 51.6), 413 (67.5), 319 (54.0), 57
(33.6); HRMS calcd for C21H22Br2I2O 701.8127, found m/z
701.8136.
Photolysis (l > 350 nm) of 1a-N2 (5.0 mg, 7.0 mmol) in
degassed benzene (0.4 ml) in the presence of CHD (0.1 ml) gave
the double-hydrogen abstraction product 1a-H2: a colorless oil
(3.4 mg, 75%); 1H NMR (300 MHz, CDCl3, ppm) d 7.83 (s, 2H), 7.49
(s, 2H), 4.70 (s, 2H), 1.29 (s, 9H), 1.27 (s, 9H); 13C NMR (100 MHz,
CDCl3, ppm) d 153.0, 152.8, 140.0, 138.0, 134.6, 130.3, 126.5, 100.9,
52.4, 34.7, 34.2, 31.13, 31.10; EI-MS (m/z) 692 (Mþ4, 42.8), 690
(Mþ2, 82.0), 688 (Mþ, 40.8), 677 (26.0), 675 (51.9), 673 (27.7), 57
(100); HRMS calcd for C21H24Br2I2 687.8334, found m/z 687.8282.
Photolysis (l > 300 nm) of 1a-N2 (5.0 mg, 7.0 mmol) in
degassed benzene (0.4 ml) in the presence of CHD (0.1 ml) gave
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 (1.5 ꢁ 10ꢀ3 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 10ꢀ5 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 using a filter (l > 300 nm), and the
spectral changes were 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).
1
LFP
All flash photolysis measurements were made on a Unisoku
TSP-601 flash spectrometer. The excitation source for the LFP was
a Lambada Physik LEXTRA XeCl excimer laser. A Hamamatsu
150-W xenon short arc lamp (L2195) 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 linear image sensor (512 photodiodes used). Timing
of the laser excitation pulse, the probe beam, and the detection
system was achieved through an Iwatsu model DS-8631 digital
synchroscope, which was interfaced to a NEC 9821 RA266
computer. This allowed for rapid processing and storage of the
data and provided printed graphics capabilities. Each trace was
also displayed on a Mitsubishi CRT RDT152A monitor.
A sample was placed in a long-necked Pyrex tube that had a
sidearm connected to a quartz fluorescence cuvette and
degassed using a minimum of five freeze–degas–thaw cycles
at a pressure near 10ꢀ5 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.
a
mixture of 1a-H2 and (2,6-dibromo-4-tert-butylphenyl)
(2-iodo-4-tert-butylphenyl)methane (1a(-H2). 1a(-H2: 1H NMR
(300 MHz, CDCl3, ppm) d 7.86 (d, J ¼ 2.0 Hz, 1H), 7.57 (s, 2H), 7.18
(dd, J ¼ 2.0, 8.3 Hz, 1H), 6.48 (d, J ¼ 8.3 Hz, 1H), 4.31 (s, 2H), 1.32
(s, 9H), 1.27 (s, 9H).
Photolysis (l > 350 nm) of 1a-N2 (5.0 mg, 7.0 mmol) in
degassed benzene (0.4 ml) in the presence of methanol
(0.1 ml) gave the OH insertion product 1-HOMe along with an
1
unreacted 1a-N2 (2.6 mg): pale orange solids (1.4 mg, 48%); H
NMR (300 MHz, CDCl3, ppm) d 7.96 (s, 2H), 7.54 (s, 2H), 5.80 (s, 1H),
3.48 (s, 3H), 1.30 (s, 9H), 1.28 (s, 9H); 13C NMR (100 MHz, CDCl3,
ppm) d 154.0, 153.9, 139.4, 135.8, 132.4, 131.5, 126.1, 99.5, 91.2,
58.7, 34.7, 34.3, 31.1, 31.0; EI-MS (m/z) 722 (Mþ4, 28.7), 720 (Mþ2,
53.2), 718 (Mþ, 26.9), 641 (81.7), 639 (81.6), 595 (50.7), 593 (100),
591 (52.4), 412 (93.0), 57 (53.4); HRMS calcd for C22H26 Br2I2O
717.8440, found m/z 717.8382.
Photolysis (l > 300 nm) of 1a-N2 (5.0 mg, 7.0 mmol) in
degassed benzene (0.4 ml) in the presence of methanol
(0.1 ml) gave a mixture of 1a-HOMe and (2,6-dibromo-4-tert-
butylphenyl)(2-iodo-4-tert-butylphenyl)methyl methyl ether
1
(1a(-HOMe). 1a(-HOMe: H NMR (300 MHz, CDCl3, ppm) d 7.90
(d, J ¼ 1.8 Hz, 1H), 7.61 (s, 2H), 7.29–7.22 (m, 1H), 6.98 (d, J ¼ 8.3 Hz,
1H), 5.93 (s, 1H), 3.44 (s, 3H), 1.33 (s, 9H), 1.26 (s, 9H).
Acknowledgements
ESR measurements
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 Pro-
moted Research (No. 21550044). They also thank Professor W.
Sander of Ruhr-Universita¨t Bochum for sharing his results prior to
publication.
The diazo compound was dissolved in 2-MTHF (1.5 ꢁ 10ꢀ2 M) and
the solution was degassed in a quartz cell by three free-
ze–degas–thaw cycles. The sample was cooled in an optical
transmission ESR cavity at 77 K and irradiated with a Wacom
500 W Xe lamp using a filter (l > 300 nm). ESR spectra were
J. Phys. Org. Chem. 2010, 23 347–356
Copyright ß 2010 John Wiley & Sons, Ltd.
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K. HIRAI ET AL.
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