Triplet diphenylcarbenes protected by trifluoromethyl and bromine groups. A triplet carbene surviving a day in solution at room temperature

Article abstract of DOI:10.1021/ja056575j

Two types of diphenyldiazomethanes having two trifluoromethyl and two bromine groups at the ortho positions, either in unsymmetrical or in symmetrical fashion, that is, (2,6-dibromo-4-phenylphenyl)[4-phenyl-2,6- bis(trifluoromethyl)phenyl]diazomethane (U-

Full text of DOI:10.1021/ja056575j

Published on Web 12/31/2005
Triplet Diphenylcarbenes Protected by Trifluoromethyl and
Bromine Groups. A Triplet Carbene Surviving a Day in
Solution at Room Temperature
Tetsuji Itoh,† Yoshimaru Nakata,† Katsuyuki Hirai,*,‡ and Hideo Tomioka*,†,§
Contribution from the Chemistry Department for Materials, Faculty of Engineering, and
Instrumental Analysis Facilities, Life Science Research Center, Mie UniVersity, Tsu,
Mie 514-8507, Japan, and Department of Applied Chemistry, Aichi Institute of Technology,
Toyota, Aichi 470-0392, Japan
Received September 25, 2005; E-mail: hirai@chem.mie-u.ac.jp
Abstract: Two types of diphenyldiazomethanes having two trifluoromethyl and two bromine groups at the
ortho positions, either in unsymmetrical or in symmetrical fashion, that is, (2,6-dibromo-4-phenylphenyl)-
[4-phenyl-2,6-bis(trifluoromethyl)phenyl]diazomethane (U-1-N2) and bis(2-bromo-4-phenyl-6-trifluorometh-
ylphenyl)diazomethane (S-1-N2), are prepared. Triplet diphenylcarbenes (U-31 or S-31) are generated from
those precursors and are characterized by ESR, UV/vis spectroscopy at low temperature, as well as time-
resolved UV/vis spectroscopy at room temperature. Those carbenes are shown to be at least 2 orders of
magnitude less reactive than the most stable triplet diphenylcarbene thus far known, that is, bis(2,6-dibromo-
4-phenylphenyl)carbene. It has been also shown that S-31 is significantly more stable than U-31 even though
both have the same two kinds of substituents. It is suspected that the perpendicular alignment of the two
most bulky groups is a more effective way to shield the carbenic center than the planar one. By this way,
triplet substituted diphenylcarbene surviving nearly a day in solution at room temperature is realized for
the first time.
Triplet carbenes are highly reactive organic radicals that are
also notoriously difficult to stabilize.1 To isolate the triplet
carbene with its electronic integrity (one centered diradical)
intact, steric protection is the ideal method. Attempts to stabilize
triplet diphenylcarbenes by introducing various substituents at
the ortho positions have been made.2 This strategy, however,
encounters a limitation when alkyl groups are employed as
protecting groups. Triplet carbenes abstract 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 useless to stabilize triplet carbene
center.3 The bromine group was found to be a very effective
protector.4 Triplet bis(2,6-dibromo-4-tert-butylphenyl)carbene
was shown to have a half-life of 16 s in solution at room
temperature,4c ca. 8 orders of magnitude greater than “parent”
diphenylcarbene (DPC). This is partly because the van der Waals
radius of bromine groups is similar to that of methyl while the
C-Br bond length is longer than C-C(Me). More importantly,
halides are usually reactive toward singlet carbene, but they are
not reactive with the triplet state.5 Product analysis studies
indicate that the main decay pathway is dimerization at the
carbenic center.4 This indicates that the carbene center is not
completely shielded and hence carbene still has a space in which
to undergo coupling with each other. To realize stable triplet
carbene, we need to explore kinetic protectors that are bulky
yet unreactive toward triplet carbenes.
The trifluoromethyl (CF3) group has been regarded as an ideal
kinetic protector of carbene because it is much bigger than
methyl and bromine groups and C-F bonds are known to be
almost the only type of bond unreactive toward carbenic center.6
Ideally stable triplet DPC may be realized if one can generate
DPC having four CF3 groups at all of the ortho positions.
However, it is synthetically almost impossible to prepare the
precursor diphenyldiazomethane (DDM). Sterically congested
DDMs are usually prepared from the corresponding diphenyl-
methyl halides by way of the methylcarbamates. The four ortho
CF3 groups introduced on the diphenylmethyl halides are
† Chemistry Department for Materials, Mie University.
‡ Instrumental Analysis Facilities, Mie University.
§ Aichi Institute of Technology.
(1) (a) Regitz, M. Angew. Chem., Int. Ed. Engl. 1991, 30, 674. (b) Dagani, R.
Chem. Eng. News 1991, Jan. 28, 19; 1994, May 2, 20. (c) Heinemann, C.;
Mu¨ller, T.; Apeloig, Y.; Schwartz, H. J. Am. Chem. Soc. 1996, 118, 2023.
(d) Beohme, C.; Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039. (e)
Wentrup, C. Science 2001, 292, 1846. (f) Roth, H. D. Nature 2001, 412,
598. (g) Kirmse, W. Angew. Chem., Int. Ed. 2003, 42, 2117.
(2) See for reviews: (a) Tomioka, H. Acc. Chem. Res. 1997, 30, 315. (b)
Tomioka, H. In AdVances in Carbene Chemistry; Brinker, U., Ed.; JAI
Press: Greenwich, CT, 1998; Vol. 2, pp 175-214. (c) Tomioka, H. In
Carbene Chemistry; Bertrand, G., Ed.; Fontis Media S. A.: Lausanne, 2002;
pp 103-152.
(3) Tomioka, H.; Okada, H.; Watanabe, T.; Banno, K.; Komatsu, K.; Hirai,
K. J. Am. Chem. Soc. 1997, 119, 1582.
(4) (a) Tomioka, H.; Watanabe, T.; Hirai, K.; Furukawa, K.; Takui, T.; Itoh,
K. J. Am. Chem. Soc. 1995, 117, 6376. (b) Tomioka, H.; Hattori, M.; Hirai,
K. J. Am. Chem. Soc. 1996, 118, 8723. (c) Tomioka, H.; Watanabe, T.;
Hattori, M.; Nomura, N.; Hirai, K. J. Am. Chem. Soc. 2002, 124, 474. (d)
Hirai, K.; Iikubo, T.; Tomioka, H. Chem. Lett. 2002, 1226.
(5) For reviews of general reactions of carbenes, see: (a) Kirmse, W. Carbene
Chemistry, 2nd ed.; Academic Press: New York, 1971. (b) Carbenes; Moss,
R. A., Jones, Jr., M., Eds.; Wiley: New York, 1973 and 1975; Vols. 1 and
2. (c) Carbene(oide), Carbine; Regitz, M., Ed.; Thieme: Stuttgart, 1989.
(6) Tomioka, H.; Taketsuji, H. J. Chem. Soc., Chem. Commun. 1997, 1745.
9
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J. AM. CHEM. SOC. 2006, 128, 957-967
957

A R T I C L E S
Scheme 1
Itoh et al.
Scheme 2
expected to shield the methyl carbon so tightly that the carbon
has little space to accept a reagent for further modification. Bis-
[2,6-bis(trifluoromethyl)phenyl]methanol, a precursor for the
desired diazomethane, was found to be completely inert to a
further treatment. Thus, we decided to use bromine group as
an auxiliary group and to attempt to prepare a precursor DDM
having as many CF3 groups as possible. We were able to prepare
two kinds of DDMs having two CF3 groups in addition to two
bromine groups at all of the ortho positions. They are unsym-
metrically substituted, having each of those two protectors in
each of the phenyl rings, that is, (2,6-dibromo-4-phenylphenyl)-
[4-phenyl-2,6-bis(trifluoromethyl)phenyl]diazomethane (U-1-
N2), and symmetrically substituted, having both groups in each
one of the phenyl rings, that is, bis(2-bromo-4-phenyl-6-tri-
fluoromethylphenyl)diazomethane (S-1-N2). We wish to report
here that DPCs generated from those precursor DDMs were very
persistent, surviving a day in solution at room temperature.7
diarylmethyl alcohol (7). The alcohol was then converted to
the corresponding DDM (U-1-N2) by way of a carbamate8
(Scheme 1).
To prepare the symmetrical one, that is, S-1-N2, 2-bromo-
6-trifluoromethylphenyllithium was desired. The isomerization
of 2-bromo-5-phenyl-3-trifluoromethylphenyllithium to 2-bromo-
4-phenyl-6-trifluoromethylphenyllithium (10) at low tempera-
ture, a reaction originally reported by Schlosser,9 worked quite
well. Thus, treatment of 4-bromo-3-trifluoromethylbiphenyl (9)
with lithium tetramethylpiperidine (LTMP), followed by the
addition of methyl formate, gave 2-bromo-4-phenyl-6-trifluo-
romethylbenzaldehyde (11). The reaction of aldehyde 11 with
a mixture of biphenyl 9 with LTMP gave the desired diaryl-
methyl alcohol (12). The alcohol was converted to S-1-N2 by
way of chloride and carbamate (13) as summarized in Scheme
2.
Results
Preparation of Precursor Diazomethanes. The diazo-
methanes that we used in this study are prepared according to
the reaction procedure outlined in Schemes 1 and 2. The
preparation of the unsymmetrical one was rather straightforward.
Thus, 1,3-bis(trifluoromethyl)benzene having a phenyl group
at position 5, prepared from the corresponding bromide (3) by
the Suzuki coupling reaction, was converted to the 2-lithio
derivative by simply treating with butyllithium/N,N,N′,N′-
tetramethylethylenediamine (BuLi/TMEDA), which was then
reacted with 2,6-dibromo-4-phenylbenzaldehyde to give the
All of the diazomethanes were purified by silica gel chro-
matography at -10 °C followed by repeated chromatography
on a gel permeation column. They were rather stable for
diazomethane and could be stored in a refrigerator for several
months without any appreciable decomposition.
We thus generated triplet carbenes (U-31 or S-31) by irradia-
tion of the precursor diazo compound (1-N2) and characterized
them by ESR, UV/vis spectroscopy at low temperature, as well
as time-resolved UV/vis spectroscopy at room temperature. The
structures and reactivities of 1 will be compared to the most
stable triplet diphenylcarbene thus far known, that is, bis(2,6-
dibromo-4-phenylphenyl)carbene 24d (Scheme 3).
(7) (a) Preliminary reports on the present study were published; Hirai, K.;
Tomioka, H. J. Am. Chem. Soc. 1999, 121, 10213. (b) Here, we used a
phenyl group instead of a tert-butyl group at the para-positions. The effect
of the phenyl group on the stability of persistent triplet diphenylcarbenes
has been investigated. See ref 4d.
(8) Zimmmerman, H. E.; Paskovich, D. H. J. Am. Chem. Soc. 1964, 86, 2149.
(9) Mongin, F.; Desponds, O.; Schlosser, M. Tetrahedron Lett. 1996, 37, 2767.
9
958 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Triplet Diphenylcarbenes
A R T I C L E S
Table 1. ESRa and UV/Visb Spectral Data
1
1
triplet carbenes
D (cm-
)
E (cm-
)
E/D
λmax (nm)
Tdc (K)
U-31
0.358
(0.327
0.0281
0.0266
0.079
0.081)
d
0.0838
0.0109)
357, 506
240
S-31
358, 494
352, 497
260
170
32
0.359
(0.402
0.0301
0.0044
a Measured in 2-MTHF at 77 K. Values in the parentheses are those
measured after thawing to 100 K. b Measured in 2-MTHF. c Temperature
at which absorption bands due to triplet carbenes disappeared in 2-MTHF.
d ZFS parameters could not be determined (see text).
(Figure 1c) characteristic of randomly oriented triplet mol-
ecules.10 The signal at 23.0 mT is assigned to a low-field Z
transition, but those ascribable to high-field X, Y, and Z
transitions are extremely weak.
Figure 1. (a) ESR spectrum obtained by irradiation of diazo compound
U-1-N2 in 2-methyltetrahydrofuran at 77 K. (b) The same sample measured
at 77 K after thawing to 110 K. (c) ESR spectrum obtained by irradiation
of diazo compound S-1-N2 in 2-methyltetrahydrofuran at 77 K. (d) The
same sample measured at 77 K after thawing to 110 K.
ESR signals sometimes become sharper when the measure-
ments are made after the sample is warmed and recooled. This
is partly because as the temperature is raised, the matrix softens
and allows the carbene to assume a single preferred orientation
rather than several slightly distorted orientations dictated by the
trapping of the precursor in the frozen matrix.11,12 Also, in the
case of sterically congested triplet diarylcarbenes, a new set of
triplet signals often appears at the expense of the original peaks,
as in the case of U-31. However, the measurements after
warming the sample to 110 K did not result in an increase in
signal intensity to allow us to confidently determine the zfs
parameters. Nevertheless, the signal due to the low-field Z
transition suggests that triplet signals are due to triplet bis(2-
bromo-4-phenyl-6-trifluoromethylphenyl)carbene (S-31) gener-
ated by photodissociation of S-1-N2.
Scheme 3
Spectroscopic Studies. (a) ESR Studies in Rigid Matrixes
at Low Temperature. Irradiation (λ > 300 nm) of (2,6-dibro-
mo-4-phenylphenyl)[4-phenyl-2,6-bis(trifluoromethyl)phenyl]-
diazomethane (U-1-N2) in 2-methyltetrahydrofuran (2-MTHF)
at 70 K gave rise to ESR signals with a typical fine structure
pattern for unoriented triplet species10 (Figure 1a). The signals
are analyzed in terms of the zero-field splitting (zfs) parameters
to be D ) 0.358 cm-1 and E ) 0.0281 cm-1 (E/D ) 0.079),
showing unequivocally that triplet signals are due to triplet
(2,6-dibromo-4-phenylphenyl)[4-phenyl-2,6-bis(trifluorometh-
yl)phenyl]carbene (U-31).
The EPR signals not only were stable at this temperature but
also survived even at 110 K. However, as the samples were
warmed, a new set of triplet peaks appear at the expense of the
original peaks (Figure 1b) with the zfs parameters of D ) 0.327
cm-1 and E ) 0.0266 cm-1 (E/D ) 0.081). Cooling the sample
to 77 K did not reverse this change. This is usually interpreted
in terms of geometrical changes (vide infra).11,12
The D and E values are reported in Table 1, which includes
D and E values observed for bis(4-phenyl-2,6-dibromophenyl)-
3
carbene 24d for the sake of comparison.
The thermal stability of the triplet carbenes could be estimated
by thawing the matrix containing triplet carbenes gradually and
recooling again to 77 K to measure the signal. This procedure
can compensate weakening of signals due to the Currie law.13
However, the triplet signals of brominated DPCs are generally
weak and broad. The signals of 31 were thus not strong enough
3
to allow such estimation. Instead, thermal stability of 1 was
estimated in more accurately by monitoring the UV/vis spectra
3
of 1 as a function of temperature (vide infra).
(b) UV/Vis Studies in Rigid Matrixes at Low Tempera-
ture. Photolysis (λ > 300 nm) of U-1-N2 in 2-MTHF glass at
77 K resulted in the appearance of new bands at the expense of
the original absorption due to the starting diazo compound
(Figure 2Ab). The new bands consist of two identifiable features,
a strong and sharp maximum at 357 nm and a broad and weak
band with apparent maximum at around 506 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
spectrum of triplet DPCs.10 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 these observations coupled
with ESR data, the absorption spectrum can be attributed to
triplet carbene (U-31).
Similar irradiation of other precursor diazomethanes, bis(2-
bromo-4-phenyl-6-trifluoromethylphenyl)diazomethane (S-1-
N2), in 2-MTHF glass at 20 K also gave rise to ESR spectra
(10) See for reviews of the EPR and UV/vis spectra of triplet carbenes: (a)
Sander, W.; Bucher, G.; Wierlacher, S. Chem. ReV. 1993, 93, 1583. (b)
Trozzolo, A. M.; Wasserman, E. In Carbenes; Jones, M., Jr., Moss, R. A.,
Eds.; Wiley: New York, 1975; Vol. 2, pp 185-206.
(11) (a) Tukada, H.; Sugawara, T.; Murata, S.; Iwamura, H. Tetrahedron Lett.
1986, 27, 235. (b) Nazran, A. S.; Gabe, F. J.; LePage, Y.; Northcott, D. J.;
Park, J. M.; Griller, D. J. Am. Chem. Soc. 1983, 105, 2912. (c) Nazran, A.
S.; Griller, D. J. Chem. Soc., Chem. Commun. 1983, 850. (d) Gilbert, B.
C.; Griller, D.; Nazran, A. S. J. Org. Chem. 1985, 50, 4738. (e) Nazran,
A. S.; Lee, F. L.; LePage, Y.; Northcott, D. J.; Park, J. M.; Griller, D. J.
Phys. Chem. 1984, 88, 5251.
(13) (a) Platz, M. S. In Diradicals; Borden, W. T., Ed.; Wiley: New York,
1982; pp 195-258. (b) Wasserman, E.; Hutton, R. S. Acc. Chem. Res.
1977, 10, 27. (c) Breslow, R.; Chang, H. W.; Wasserman, E. J. Am. Chem.
Soc. 1967, 89, 1112.
(12) See also: Tomioka, H. In AdVances in Strained and Interesting Organic
Molecules; Halton, B., Ed.; JAI Press: Greenwich, CT, 2000; Vol. 8, pp
83-112.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 959

A R T I C L E S
Itoh et al.
Figure 3. UV/vis spectra obtained by irradiation of diazo compound S-1-
N2. (A) (a) Spectrum of S-1-N2 in 2-methyltetrahydrofuran at 77 K. (b)
The same sample after irradiation (λ > 300 nm). (c) The same sample after
thawing to 100 K. (B) UV/vis spectral change measured at 10 K increments
upon warming the same sample from 100 to 300 K. For the sake of
simplicity, the spectral changes are shown in 20 K increments.
Figure 2. UV/vis spectra obtained by irradiation of diazo compound U-1-
N2. (A) (a) Spectrum of U-1-N2 in 2-methyltetrahydrofuran at 77 K. (b)
The same sample after irradiation (λ > 350 nm). (c) The same sample after
thawing to 100 K. (B) UV/vis spectral change measured at 10 K increments
upon thawing the same sample from 100 to 300 K. For the sake of simplicity,
the spectral changes are shown in 20 K increments.
The thermal stability is then estimated by raising the sample
temperature in 10 K increments to the desired temperature,
allowing it to stand for 5 min, and measuring the absorption
bands. The characteristic bands due to U-31 became sharp at
100 K. Similar changes in the UV/vis spectra upon thawing
the matrix were also observed for other sterically congested
diarylcarbenes and are generally interpreted in terms of geo-
metrical changes of the carbenes (vide infra).10-12 The bands
started to decay slowly at 120 K, decayed rather sharply at
around 200 K, and were observable up to 240 K.
Similar absorption but increased stabilities was noted for S-31.
Photolysis (λ > 300 nm) of S-1-N2 in 2-MTHF glass at 77 K
resulted in the appearance of new bands showing a strong and
sharp maximum at 358 nm and a broad and weak band with
apparent maximum at around 494 nm, ascribable to S-31 (Figure
3Ab). The characteristic bands due to S-31 became sharp with
a slight shift of the maximum from 358 to 368 nm at 100 K.
The bands started to decay slowly at around 140 K, decayed
rather sharply at 220 K, but were observable up to 260 K (Figure
3B).
It should be noted here that absorption bands of most triplet
DPCs in 2-MTHF disappear in the temperature range 100-
105 K10 where the viscosity of the matrix changes dramatically
from 107 to 103 P.14 Even the absorption bands of relatively
stable triplet diphenylcarbene 32 disappear only up to 130 K in
2-MTHF.4d
This observation clearly demonstrates anomalous stability of
31 for a triplet diphenylcarbene, where S-31 is slightly more
stable than U-31.
(c) Laser Flash Photolysis Studies in Solution at Room
Temperature. To know the stability of the present carbenes
more accurately, the lifetime is estimated in a degassed benzene
at room temperature, in which we have measured the lifetime
of a series of sterically congested diarylcarbenes.2 The lifetime
of the present carbenes 1 was too long to monitor by the laser
flash photolysis (LFP) technique, which has been routinely used
for such a study. Thus, a conventional UV/vis spectroscopic
method was more conveniently employed in this case.
Brief irradiation of U-1-N2 in degassed benzene at 20 °C
with a 70-90 mJ, 308 nm pulse from XeCl excimer laser
produced transient absorption bands, showing a strong maximum
at 340 nm (Figure 4). The maximum of the band was shifted to
a shorter wavelength than that observed in the photolysis of
U-1-N2 in 2-MTHF at 77 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 U-31.
The absorption bands decay very slowly, and the transient bands
did not disappear completely even after 2 h under these
conditions. The decay was found to be second order (2k/ꢀl )
3.26 × 10-3 s-1). The approximate half-life (t1/2) of U-31 is
estimated to be 11 min.
Similar irradiation of S-1-N2 also produced transient bands
at 344 nm (Figure 5). Although the maximum was again shifted
to a shorter wavelength than that observed in photolysis of S-1-
N2 in 2-MTHF at 77 K, the bands were markedly quenched by
oxygen to give the corresponding ketone oxide (vide infra) and
hence were assigned to U-31. The spectrum attributable to S-31
decayed much slower than that of the unsymmetrical one, U-31,
and did not disappear completely even after standing a day.
(14) Ling, A. C.; Willard, J. E. J. Phys. Chem. 1968, 72, 1918.
9
960 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Triplet Diphenylcarbenes
A R T I C L E S
Figure 4. Absorption of transient products formed during irradiation of
diazo compound U-1-N2 in degassed benzene at room temperature recorded
from immediately after irradiation to 300 min after excitation. The inset
shows the time course of the absorption at 340 nm.
Figure 6. Transient absorption spectra obtained in LFP of U-1-N2 (a) in
degassed benzene and (b) in nondegassed benzene with a 308-nm excimer
laser recorded 500 µs after the pulse. The inset shows oscillogram traces
monitored at 420 nm.
Scheme 4
Figure 5. Absorption of transient products formed during irradiation of
diazo compound S-1-N2 in degassed benzene at room temperature recorded
from immediately after irradiation to 300 min after excitation. The inset
shows the time course of the absorption at 344 nm.
broad absorption band centered at 390-450 nm (Scheme 4).
Thus, our observations can be interpreted as indicating that the
transient absorption quenched by oxygen is due to U-31. The
rate of increase in the band at 420 nm is practically the same
as the rate of decrease of the peak due to U-31, showing that
U-31 is quenched with oxygen to form carbonyl oxide (U-1-
Triplet substituted diphenylcarbene surviving a day is realized
for the first time. The decay was also found to be second order
(2k/ꢀl ) 2.88 × 10-4 s-1). The approximate half-life (t1/2) of
S-31 is estimated to be 40 min.
O2). The rate constant (kO ) for the quenching of U-31 by O2 is
2
The half-life (t1/2) 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).15 Therefore,
the rate constants of the trapping reactions by O2 and CHD,
that is, kO2 or kCHD, respectively, are used as a more quantitative
scale to estimate the reactivities of the triplet carbenes.
LFP of U-1-N2 in a nondegassed benzene solution resulted
in a dramatic decrease in the lifetime of U-31 and a concurrent
appearance of a new absorption band at 420 nm (Figure 6).
The spent solution was found to contain the corresponding
benzophenone. It is well-documented16,17 that diarylcarbenes
with triplet ground states are readily trapped by oxygen to
generate the corresponding diaryl ketone oxides, which show a
determined to be 4.6 × 105 M-1 s-1 from a plot of the observed
pseudo-first-order growth rate of U-1-O2 as a function of [O2]
(Figure 7a). This is some 4 orders of magnitude smaller than
3
that observed with the “parent” DPC (kO ) 5.0 × 109 M-1
2
s-1 17f
and 1 order of magnitude smaller than that observed with
)
the most stable triplet diphenylcarbene thus far known, that is,
bis(4-phenyl-2,6-dibromophenyl)carbene 2.4d
When a degassed benzene solution of U-1-N2 containing
1,4-cyclohexadiene (CHD) was excited, a new species was
formed, showing an absorption with λmax ) 400 nm, formed as
the 344 nm signal of U-31 decayed (Figure 8). The decay of
U-31 was again found to be kinetically correlated with the
growth of the new species. Thus, this new signal was attributable
to the corresponding diarylmethyl radical (U-1-H) formed as
(17) See for LFP study: (a) Sugawara, T.; Iwamura, H.; Hayashi, H.; Sekiguchi,
A.; Ando, W.; Liu, M. T. H. Chem. Lett. 1983, 1261. (b) Casal, M. L.;
Sugamori, S. E.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 7623. (c)
Casal, H. L.; Tanner, M.; Werstiuk, N. H.; Scaiano, J. C. J. Am. Chem.
Soc. 1985, 107, 4616. (d) Barcus, R. L.; Hadel, L. M.; Johnston, L. J.;
Platz, M. L.; Savino, T. G.; Scaiano, J. C. J. Am. Chem. Soc. 1986, 108,
3928. (e) Fujiwara, Y.; Tanimoto, Y.; Itoh, M.; Hirai, K.; Tomioka, H. J.
Am. Chem. Soc. 1987, 109, 1942. (f) Scaiano, J. C.; McGrimpsey, W. G.;
Casal, H. L. J. Org. Chem. 1989, 54, 1612.
(15) See for reviews of the general reaction of triplet carbenes: Tomioka, H.
In ReactiVe Intermediate Chemistry; Moss, A. M., Platz, M. S., Jones, M.,
Jr., Eds.; Wiley: New York, 2004; pp 375-461.
(16) See for review of carbonyl oxides: (a) Sander, W. Angew. Chem., Int. Ed.
Engl. 1990, 29, 344. (b) Bunnelle, W. Chem. ReV. 1991, 91, 336.
9
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A R T I C L E S
Itoh et al.
Figure 7. Plot of the growth of the diaryl ketone oxides (a: red) U-1-O2
and (b: blue) S-1-O2 monitored at 420 nm as a function of the concentration
of oxygen.
Figure 9. Plot of the growth of the diarylmethyl radicals (a: red) U-1-H
and (b: blue) S-1-H monitored at 410 nm as a function of the concentration
of 1,4-cyclohexadiene.
Table 2. Kinetic Dataa
1
1
1
triplet carbenes
2k/
ꢀ
l (s-
)
t1 2 (min)
/
kO2 (M-1 s-
)
k
CHD (M-1 s-
)
U-31
S-31
32
3.26 × 10-3
2.88 × 10-4
3.5 × 10-1
11
40
0.2
4.6 × 105
5.4 × 105
7.5 × 106
6.7
6.7
2.8 × 102
a Measured in degassed benzene at room temperature.
Product Analysis Studies. Most of the sterically congested
triplet DPCs decay mainly by undergoing dimerization at the
carbenic center to give tetra(aryl)ethenes as the main product,
when generated in degassed benzene at room temperature in
the absence of appropriate triplet carbene quenchers such as
oxygen and hydrogen donors, for instance, bis(2,4,6-trichloro-
phenyl)carbene produced tetrakis(2,4,6-trichlorophenyl)ethyl-
ene in ∼80% yield when generated in benzene. In other words,
unless this simple coupling process is not prevented, it is not
possible to realize really stable triplet carbene. Triplet carbene
32 also decayed by dimerization to give tetrakis(2,6-dibromo-
4-phenylphenyl)ethylene, which underwent cyclization to give
phenanthrene derivatives.
Figure 8. Transient absorption spectra obtained in LFP of U-1-N2 in
degassed benzene in the presence of 1,4-cyclohexadiene with a 308-nm
excimer laser recorded 500 ms after the pulse. The inset shows oscillogram
traces monitored at 410 nm.
a result of H abstraction of U-31 from the diene, because it is
now well-documented15,18,19 that triplet arylcarbenes, generated
in good hydrogen donor solvents, undergo H abstraction leading
to the corresponding radicals (Scheme 4), which show transient
absorptions at longer wavelengths than those of the precursor
carbenes. The excellent hydrogen donor properties of CHD have
been well recognized.20 The spent solution was found to contain
the diarylmethane as the main product. A plot of the observed
pseudo-first-order rate constant of the formation of the radical
against [CHD] is linear (Figure 9a), and the slope of this plot
yields the absolute rate constant for the reaction of U-31 with
the diene, kCHD ) 6.7 M-1 s-1, which is approximately 6 orders
of magnitude smaller than that observed with the “parent” 3DPC
(kCHD ) 1.0 × 107 M-1 s-1).19f
Photolysis of 1-N2 in degassed benzene gave a complex
1
mixture. H NMR spectra of the irradiation mixtures were too
complicated to further characterize the structure. However, some
of the fractions obtained by GPC of the photomixture showed
a mass peak corresponding to the dimer of carbene 1 (Figures
1
S3 and S4). H NMR spectra of the fraction (Figures S5 and
S6) were also extremely complicated, but are not likely to be
assignable to those expected to be formed as a result of
dimerization at the carbene center (or phenanthrene derivative
to be derived from the dimer).
Again, similar measurements were done for the other carbenes
(S-31) (Figures S1 and S2), and the results are summarized in
Table 2. It is interesting to note here that carbenes (S-31) showed
essentially identical reactivities toward those typical triplet
quenchers, that is, oxygen and CHD even though the stability
of S-31 estimated in degassed benzene and in 2-MTHF is
significantly larger than its positional isomer U-31.
The complexity of the spectra can be explained in terms of
possible isomerism in the carbene dimer even if the simple
carbene dimer, that is, tetra(aryl)ethene, is formed. For instance,
two isomers, syn and anti, can be expected for the carbene dimer
produced in the photolysis of U-1-N2. Close inspection of 1H
NMR spectra of the dimer suggested that the spectra could be
best explained in terms of a 6:4 mixture of two isomers. On
the other hand, such geometrical isomerism is not expected for
the carbene dimer from photolysis of S-1-N2. The inspection
of the molecular models however indicated that rotational
isomerism (atropisomerism) is plausible due to the steric
repulsion between two bulky ortho-substituents, that is, bromine
and trifluoromethyl groups in this case. Unfortunately, aromatic
proton signals in this case are too complex to allow us to
estimate the ratio of possible isomers.
(18) For reviews, see: (a) Platz, M. S., Ed. Kinetics and Spectroscopy of
Carbenes and Biradicals; Plenum: New York, 1990. (b) Jackson, J. E.;
Platz, M. S. In AdVances in Carbene Chemistry; Brinker, U., Ed.; JAI
Press: Greenwich, CT, 1994; Vol. 1, pp 87-160.
(19) See for LFP study: (a) Moritani, I.; Murahashi, S.; Ashitake, H.; Kimura,
K.; Tsubomura, H. J. Am. Chem. Soc. 1968, 90, 5918. (b) Closs, G. L.;
Rabinow, B. E. J. Am. Chem. Soc. 1976, 98, 8190. (c) Grasse, P. G.; Brauer,
B.-E.; Zupancic, J. J.; Kaufmann, K. J.; Schuster, G. B. J. Am. Chem. Soc.
1983, 105, 6833. (d) Sugawara, T.; Iwamura, H.; Hayashi, H.; Sekiguchi,
A.; Ando, W.; Liu, M. T. H. Chem. Lett. 1983, 1259. (e) Lapin, S. C.;
Brauer, B. E.; Schuster, G. B. J. Am. Chem. Soc. 1984, 106, 2092. (f) Hadel,
L. M.; Platz, M. S.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 283. (g)
Griffin, G. W.; Horn, K. A. J. Am. Chem. Soc. 1987, 109, 4491. (h) Doss,
S. H.; Abdel-Wahab, A. A.; Fruhof, E. M.; Du¨rr, H.; Gould, I. R.; Turro,
N. J. J. Org. Chem. 1987, 52, 434.
(20) Encinas, M. V.; Scaiano, J. C. J. Am. Chem. Soc. 1981, 103, 6393.
9
962 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Triplet Diphenylcarbenes
A R T I C L E S
Figure 10. Selected geometrical parameters (distance in Å and angle in deg) for triplet bis(2,6-dibromophenyl)carbene (32a), [2,6-bis(trifluoromethyl)-
phenyl](2,6-dibromophenyl)carbene (U-31a), and bis(2-bromo-6-trifluoromethylphenyl)carbene (S-31a) at B3LYP/6-31G* level of theory.
It has been also shown that triplet arylcarbenes start to decay
by undergoing coupling at aromatic rings where considerable
spin density must build up, as the carbenic center is sterically
more tightly shielded. This usually resulted in a complex
mixture.
in this case, ω is increased to 80.8° while θ is still 127.8°. This
indicates that the CF3 group shields the carbon more ef-
fectively.21
Even though the distance between C(aromatic) and C(triflu-
oromethyl) is significantly shorter (1.6 Å) than that of C-Br
(1.9 Å),22 three fluorine atoms make the van der Waals radius
of the trifluoromethyl group much larger than those of the
methyl and bromine groups. Therefore, the phenyl rings are
forced to rotate around the C(dN2)-C(Ar) bond to relieve the
compression, which results in a larger dihedral angle. This
motion will result in the more effective shielding of procarbenic
carbon.
Discussion
Comparison between 31 and 32. Effect of Trifluoromethyl
Group As Opposed to Bromine Group as a Kinetic Protec-
3
tor. The kinetic and ESR data for 1 are to be compared to
those observed for the longest-lived triplet carbene thus far
known, that is, bis(2,6-dibromo-4-phenylphenyl)carbene (32) 4d
(Tables 1 and 2).
It is clear that reactivities toward typical triplet quenchers,
that is, oxygen and CHD, decrease by approximately 1 and 2
orders of magnitude, respectively, as two o-bromine groups in
32 are replaced with two CF3 groups. Similarly, the lifetimes in
degassed benzene also increase by approximately 2 orders of
magnitude. CF3 is thus shown to be an excellent kinetic protector
of triplet carbene.
We have shown that the inspection of X-ray crystal structures
of precursory diphenyldiazomethanes (DDMs) shows how steric
shielding of the ortho substituents affects the structures and
stabilities of triplet DPCs.21 It is noted that, as ortho substituents
are introduced on DDMs, the dihedral angle (ω) between two
phenyl rings increases, while the diazo carbon angle (θ) changes
very little. For instance, θ/ω values of bis(4-bromophenyl)-
diazomethane and bis(2,4,6-tribromophenyl)diazomethane are
126.9°/45.6° and 127.0°/70°, respectively.21 In this way, the
procarbenic carbon is shielded more effectively from external
reagents by the protector substituents. Unfortunately, we were
not able to determine the crystal structure of either S- or U-1-
N2 due to disorder issues. However, the crystal structure of (2,6-
dibromo-4-tert-butylphenyl)[2,6-bis(trifluoromethyl)-4-tert-bu-
tylphenyl]diazomethane, a p-tert-butyl analogue of U-1-N2,
gave some insights on how the structure is affected when two
bromine groups are replaced with trifluoromethyl groups. Thus,
To obtain more direct insights into the difference between
the structures of three triplet carbenes, triplet bis(2,6-dibro-
mophenyl)carbene (32a), (2,6-dibromophenyl)[2,6-bis(trifluo-
romethyl)phenyl]carbene (U-31a), and bis(2-bromo-6-trifluo-
3
romethylphenyl)carbene (S-31a) (as models for 2, U-31, and
S-31, respectively) are optimized by DFT calculations.23,24 The
optimized geometries at UB3LYP/6-31G* level of theory
indicate that the carbene angle (θ)/the dihedral angle (ω)
between two phenyl rings of tetrabrominated DPC 32a is 156.6°/
77.0° but those for both U-31a and S-31a are 158.8°/85.7° and
162.9°/81.4°, respectively (Figure 10).25
These predictions indicate that both U-31a and S-31a have
bent structures with a slightly wider angle of the carbene carbon
3
and dihedral angles than those of 2. The inspection of the
optimized geometries also indicates that, while both U- and
S-31a have nearly perpendicular structures, that of 32 is twisted
from the perpendicular to a staggered one. Thus, the carbene
3
3
center is better shielded in 1a than in 2.
(22) Rowland, R. S.; Taylor, R. J. Phys. Chem. A 1996, 100, 7384.
(23) Frisch, M. J.; et al. Gaussian 94; Gaussian, Inc.: Pittsburgh, PA, 1995.
(24) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b) Miehlich,
B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (c)
Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(25) Schaefer and co-workers optimized the structure of triplet (2,6-dibromo-
4-tert-butylphenyl)[4-iso-propyl-2,6-bis(trifluoromethyl)phenyl]carbene, a
4-alkylated analogue of U-31, and also showed that this carbene has a bent
structure with the central bond angle of 167° at the B3LYP/6-311G(d,p)
level. Woodcock, H. L.; Moran, D.; Schleyer, P. von R.; Schaefer, H. F.,
III. J. Am. Chem. Soc. 2001, 123, 4331.
(21) Iikubo, T.; Itoh, T.; Hirai, K.; Takahashi, Y.; Kawano, M.; Ohashi, Y.;
Tomioka, H. Eur. J. Org. Chem. 2004, 3004.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 963

A R T I C L E S
Itoh et al.
The ESR data are not in complete agreement with the
theoretical prediction. The ESR spectra of carbenes are char-
acterized by two parameters D and E.10,26 The D value is related
to the separation between the unpaired electrons. The value of
E, when weighted by D, is a measure of the deviation of the
carbene from axial symmetry, or, more plainly, it describes the
extent to which the molecule is bent. The calculations predict
that opening of the carbene-carbon bond angle strongly
destabilizes the singlet but requires very little additional energy
for the triplet, thus making the singlet-triplet splitting larger.27,28
The E/D value for U-31 in 2-MTHF at 77 K is slightly smaller
(vide supra). However, it is not possible to compare the value
between the two, as the values for S-31 were not determined.
It would be very informative to inspect the X-ray crystal
structure of precursor diazomethanes. Unfortunately, X-ray
crystal structural analysis of both U-1-N2 and S-1-N2 showed
a slightly disordered structure about the ortho substituents. The
structural parameters ω/θ for U-1-N2 and S-1-N2 obtained
from the partially disordered crystal structure are 83.9°/128.3°
and 78.7°/128.7°, respectively (Figure S7). DFT calculations
of S-1a-N2 and U-1a-N2 (Figure S8) show, however, that the
two DDMs have a similar bond angle (θ ) 127.5°) and dihedral
angle (ω ) ∼70°). Although the disordered structure indicates
that ω is slightly larger in U-1-N2 than in S-1-N2, it is likely
that those two groups are too large to further affect the structure
of DDMs.
Inspection of the optimized geometries of U-31a and S-31a
(Figure 10) also indicates that there is only a slight difference
between the two carbenes. The carbene bond angle is slightly
larger for S-31a (162.9°) than for U-31a (158.8°), while the
dihedral angle is larger for U-31a (ω ) 85.7°) than for S-31a
(ω ) 81.4°). The C-C distances between the aromatic and
carbene carbon (d1 and d2) are slightly shorter for U-31a (1.376
and 1.380 Å) than for S-31a (1.375 and 1.375 Å). Spin densities
on the carbene carbon are slightly smaller for S-31a (1.377) than
for U-31a (1.397).
This is in accord with the observation that both carbenes
showed essentially identical reactivities toward oxygen and
CHD. A significant difference in the stability observed in
benzene is ascribable to the difference in the decay pathway
under these conditions. In the absence of proper trapping
reagents, triplet carbenes decay mainly by dimerization. So, the
difference in the stability is due to that in the rate of
dimerization. If one assumes 31 has nearly perpendicular
geometry, in U-31, the two carbon atoms of the most effective
kinetic protector, CF3 groups, are fixed in the same plane, while
in S-32 they are located in a perpendicular manner. Presumably,
the perpendicular alignment of the two most bulky groups is a
more effective way to shield the carbenic center than the planar
one from the dimerization. This change in the alignment may
not be important for the attack of a smaller and reactive reagent
such as oxygen and 1,4-cyclohexadiene.
3
than that for 2 under identical conditions (Table 1).
However, it has been well-documented that sterically con-
gested diarylcarbenes generated in rigid matrix often have the
bent geometry and the conformation dictated by those of the
precursor and that even if the thermodynamically most stable
geometry and/or conformation of the carbene is different from
those at the birth, the rigidity of the matrix prevents it from
assuming its minimum energy structure. When the matrix is
softened upon annealing, the carbene can relax to a more stable
geometry depending on the softness of the media.11,12,29
Therefore one needs to compare the ZFS parameters of triplet
3
DPC in its relaxed state. When the matrix containing 2 was
warmed, X and Y lines of the spectrum moved closer together,
resulting in a smaller E value.4d The substantial reduction in
the E value upon annealing the matrix indicates that the carbene
relaxes to a structure with a wider C-C-C angle presumably
to gain relief from steric compression.
On the other hand, changes in ESR signals for 31 upon
warming the matrix are small (Figure 1). Only a slight shift of
X and Y lines in the ESR signals upon annealing the matrix to
110 K is observed for U-31. These observations are not expected
3
from the assumption that steric compression in 1 is expected
3
to be much severer than that in 2 in the light of the bulkiness
of trifluoromethyl group as opposed to bromine. Part of the
reason for this is the fact that the ESR spectra of brominated
carbenes are notoriously broad and weak, and hence ZFS
parameters may not be precise enough to allow one to examine
the structural feature in details.
Comparison between U-31 and S-31. Positional Effect of
Trifluoromethyl Group as a Kinetic Protector. It is interesting
to note here that S-31 with bromine and trifluoromethyl groups
at the ortho positions of each of two phenyl rings showed
significantly greater stability than U-31 having two bromine
groups at the ortho positions of one phenyl ring and two
trifluoromethyl groups at the ortho positions of the other phenyl
ring in degassed benzene solution. This indicates that, even if
the same two kinds of o-substituents are introduced, the effect
on the stability of triplet DPCs depends on how those groups
are introduced.
Conclusion
The present study revealed that CF3 groups are an effective
kinetic protector for triplet diphenylcarbenes. From a synthetic
viewpoint, it is probably impossible to prepare precursor
diphenyldiazomethane with four CF3 groups at all of the ortho
positions. Auxiliary protector is shown to be successfully
employed to solve this problem. In this case, a manner to
introduce the two kinds of protector is found to be crucial to
further stabilize triplet carbene toward self-dimerization. In this
way, triplet substituted diphenylcarbene surviving nearly a day
in solution at room temperature is realized for the first time.
This is the most persistent triplet diphenylcarbene thus far
reported.
What is the origin for this difference? The structure of triplet
diarylcarbene is usually characterized by the ZFS parameters
(26) A large number of triplet carbenes have been characterized by ESR
spectroscopy since the pioneering work of Wasserman et al.: (a) Wasser-
man, E.; Hutton, R. S. Acc. Chem. Res. 1977, 10, 27. (b) Wasserman, E.;
Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964, 41, 1763.
(27) Harrison, J. F. J. Am. Chem. Soc. 1971, 93, 4112.
(28) Sulzbach, H. M.; Bolton, E.; Lenoir, D.; Schleyer, P. v. R.; Schaefer, H.
F., III. J. Am. Chem. Soc. 1996, 118, 9908.
(29) Tomioka, H.; Hu, Y.; Ishikawa, Y.; Hirai, K. Bull. Chem. Soc. Jpn. 2001,
74, 2207. Hu, Y.; Hirai, K.; Tomioka, H. J. Phys. Chem. A 1999, 103,
9280.
Experimental Section
General Information. IR spectra were measured on a JASCO FT/
IR-410 spectrometer, and UV/vis spectra were recorded on a JASCO
V-560 spectrometer. The mass spectra were recorded on a JEOL JMS-
1
600H mass spectrometer. H and 13C NMR spectra were determined
9
964 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Triplet Diphenylcarbenes
A R T I C L E S
with a JEOL JNM-AL300 FT/NMR spectrometer in CDCl3 or C6D6
with Me4Si as an internal reference. Thin-layer chromatography was
done on a Merck Kieselgel 60 PF254. Column chromatography was
carried out on a silica gel (Merck for dry column chromatography) or
on an aluminum oxide (ICN, neutral activity grade I, inactivated with
5% H2O). GPC was undertaken with a JASCO PU-986 chromatograph
with a UV-970 UV/vis detector using a Shodex GPC H-2001 (20 mm
× 50 cm).
10H), 6.86 (d, J ) 6.61 Hz, 1H), 3.49 (d, J ) 6.61 Hz, 1H); 13C NMR
(75.5 MHz, CDCl3, ppm) δ 143.1, 141.6, 138.1, 137.9, 137.3, 136.9,
132.7, 132.2 (q, J ) 30.5 Hz), 129.2, 129.1, 129.0, 128.9, 128.6, 127.1,
127.0, 124.5, 124.1 (q, J ) 274.7 Hz), 72.5; EI-MS (m/z) 632 (M + 4,
5.1%), 630 (M + 2, 9.7%), 628 (M+, 5.2%), 341 (18.5%), 339 (17.1%),
317 (base), 152 (14.1%); HRMS calcd for C32H28NBrO2 627.9471,
found m/z 627.9470.
A solution of 7 (364 mg, 0.58 mmol) and SOCl2 (20 mL) was stirred
at room temperature overnight, and the mixture was evaporated under
reduced pressure to give (3,5-dibromobiphenyl-4-yl)[3,5-bis(trifluo-
romethyl)biphenyl-4-yl]chloromethane as a brown viscous oil (375 mg,
quantitative yield), which was rather unstable and hence used without
further purification: 1H NMR (300 MHz, CDCl3, ppm) δ 8.13 (s, 2H),
7.82 (s, 2H), 7.69-7.43 (m, 10H), 7.13 (s, 1H). A mixture of silver
tetrafluoroborate (321 mg, 1.65 mmol) and ethyl carbamate (7.50 g,
17.7 mmol) was heated at 60 °C. To the mixture was added a solution
of the chloromethane (1.07 g, 1.65 mmol) in dioxane (50 mL), and it
was heated at 100 °C overnight. The mixture was filtered, and water
(50 mL) and CHCl3 (50 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
chromatographed on silica gel using n-hexane-CH2Cl2 (1:1) as an
eluent to give ethyl N-{(3,5-dibromobiphenyl-4-yl) [3,5-bis(trifluoro-
methyl)biphenyl-4-yl]methyl}carbamate (8) as a yellow semisolid (51
mg, 4.4%): 1H NMR (300 MHz, CDCl3, ppm) δ 8.12 (s, 2H), 7.80 (s,
2H), 7.70-7.40 (m, 10H), 6.93 (d, J ) 7.17 Hz, 1H), 5.49 (d, J )
7.17 Hz, 1H), 4.16 (q, J ) 7.17 Hz, 2H), 1.26 (t, J ) 7.17 Hz, 3H);
13C NMR (75.5 MHz, CDCl3, ppm) δ 155.7, 143.0, 142.8, 142.5, 141.6,
141.4, 137.9, 137.3, 136.0, 133.1, 132.1, 131.8 (q, J ) 31.1 Hz), 129.2,
129.1, 128.6, 127.1, 126.9 (q, J ) 6.2 Hz), 124.2 (q, J ) 275.3 Hz),
61.4, 54.9, 14.7; EI-MS (m/z) 703 (M + 4, 1.8%), 701 (M + 2, 5.8%),
699 (M+, 3.2%), 621 (42.0%), 619 (40.9%), 576 (base), 504 (16.9%);
HRMS calcd for C38H20FNO2Br2 698.9844, found m/z 698.9835.
Materials. Preparation of [3,5-Bis(trifluoromethyl)-1,1′-biphenyl-
4-yl](3,5-dibromo-1,1′-biphenyl-4-yl)diazomethane (U-1-N2). To a
mixture of 4-amino-3,5-dibromo-1,1′-biphenyl (3.27 g, 10 mmol)31 in
water (4 mL) and acetic acid (6.5 mL) was added concentrated sulfuric
acid (2.8 g, 28 mmol) under stirring. The mixture was heated to
complete dissolution and then cooled to 10 °C. The aniline sulfate was
diazotized at 10 °C with an aqueous solution of sodium nitrite (0.77 g,
11 mmol). To a mixture of copper(II) sulfate (1.92 g, 12 mmol) in
water (7.5 mL) and ice (5.0 g) was added KCN (3.25 g, 50 mmol) in
water (7.5 mL), while the temperature was kept below 20 °C by adding
more ice. The voluminous precipitate that initially formed dissolved.
Finally, NaHCO3 (6.7 g, 80 mmol) and benzene (10 mL) were added.
To this mixture was added dropwise the above solution of the diazonium
compound with vigorous stirring at 50-55 °C over 20 min. After an
additional 30 min of stirring, the mixture was extracted with benzene,
and washed with 2 N NaOH solution and brine. The organic phase
was concentrated, and the crude product was chromatographed on silica
gel using n-hexane-CH2Cl2 (2:1) as an eluent to give 3,5-dibromo-4-
cyanobiphenyl (14) as a brown solid (1.75 g, 52%): mp 131-133 °C;
1H NMR (300 MHz, CDCl3, ppm) δ 7.84 (s, 2H), 7.57-7.48 (m, 5H);
13C NMR (75.5 MHz, CDCl3, ppm) δ 147.7, 136.7, 130.3, 129.7, 129.4,
127.2, 126.9, 116.9, 116.1; EI MS (m/z) 339 (M + 2, 49.6%), 337
(M+, base), 335 (40.7%), 117 (43.9%), 88 (12.5%), HRMS calcd for
C13H7NBr2 334.8944, found m/z 334.8947; IR (KBr, cm-1) 2229 (νCN).
A solution of DIBAL-H in hexane (0.93 M, 5.4 mL, 5.0 mmol)
was added dropwise to a solution of 1.41 g (4.19 mmol) of 14 in 12
mL of CH2Cl2. The solution was stirred under Ar at ambient temperature
for 30 min and was then diluted with 10 mL of Et2O and cooled in ice.
Ten milliliters of 3 N HCl was carefully added, and the mixture was
vigorously stirred at 40 °C for 1 h. The organic fraction was washed
with brine, dried over Na2SO4, and evaporated. The crude product was
chromatographed on silica gel using n-hexane-CH2Cl2 (1:1) as an
eluent to give 3,5-dibromobiphenyl-4-carboaldehyde (6) as a light brown
Into a stirred solution of 8 (67 mg, 0.096 mmol) in anhydrous CCl4
(5 mL) at 0 °C was bubbled N2O4 gas (1.25 g, 13.6 mmol), and AcONa
(2.23 g, 27.2 mmol) was added. The mixture was stirred at room
temperature overnight, poured onto cold aqueous Na2CO3, and extracted
with CCl4. The organic layer was washed with 5% Na2CO3 solution
and water, dried over anhydrous Na2SO4, and evaporated to ethyl
N-nitroso-N-{(3,5-dibromobiphenyl-4-yl)[3,5-bis(trifluoromethyl)bi-
phenyl-4-yl]methyl}carbamate as a yellow oil. To a solution of the
nitroso compound in anhydrous THF (10 mL) was added potassium
tert-butoxide (23.8 mg, 0.21 mmol) at once under an atmosphere of
argon. The mixture was stirred at room temperature overnight, poured
onto water, and extracted with Et2O. The ethereal layer was washed
with water, dried over anhydrous Na2SO4, and evaporated. The reaction
mixture was passed through a column (aluminum oxide deactivated
with 5% water, n-hexane), and the first fraction was further purified
by GPC (20 cycles, CHCl3) followed by TLC (n-hexane) to give
(3,5-dibromobiphenyl-4-yl)[3,5-bis(trifluoromethyl)biphenyl-4-yl]di-
azomethane (U-1-N2) as an orange solid (10 mg, 14.5%): mp 134.4-
135.2 °C (dec); 1H NMR (300 MHz, CDCl3, ppm) δ 8.17 (s, 2H), 7.82
(s, 2H), 7.66-7.38 (m, 10H); 13C NMR (75.5 MHz, CDCl3, ppm) δ
66.0, 124.3, 125.7 (q, J ) 275.3 Hz), 126.8, 126.9, 127.0, 127.2, 128.7
(q, J ) 31.5 Hz), 129.1, 129.4, 129.5 (q, J ) 5.6 Hz), 132.2, 137.4,
1
solid (1.25 g, 88%): mp 93.5-95.0 °C; H NMR (300 MHz, CDCl3,
ppm) δ 10.42 (s, 1H), 8.07 (s, 1H), 8.06 (s, 1H), 7.62 (d, J ) 7.72 Hz,
2H), 7.54-7. 26 (m, 3H); 13C NMR (75.5 MHz, CDCl3, ppm) 190.8,
147.1, 136.8, 132.1, 130.5, 129.4, 129.2, 127.2, 125.5; EI-MS (m/z)
342 (M + 4, 51.9%), 340 (M + 2, base), 338 (M+, 50.1%), 232
(17.0%), 230 (17.0%), 151 (33.2%); HRMS calcd for C13H8OBr2
337.8941, found m/z 337.8949.
To a solution of 0.77 g (2.66 mmol) of 3,5-bis(trifluoromethyl)-
biphenyl (4)32 and 0.80 mL (5.32 mmol) of N,N,N′,N′-tetramethyleth-
n
ylenediamine in dry Et2O (2.5 mL) was added dropwise BuLi (2.66
M in hexane, 2.00 mL, 5.32 mmol) at -20 °C. The solution was stirred
overnight at room temperature and refluxed for 2 h, and then a solution
of 6 (0.90 g, 2.66 mmol) in dry THF (3 mL) was added dropwise at 0
°C. The mixture was refluxed for 4 h and was quenched by addition of
saturated aqueous ammonium chloride. The mixture was extracted with
ether, washed with water, dried over Na2SO4, and the solvent was
removed under reduced pressure. The residue was chromatographed
on silica gel using n-hexane-CH2Cl2 (2:1) as an eluent to give (3,5-
dibromobiphenyl-4-yl)[3,5-bis(trifluoromethyl)biphenyl-4-yl]metha-
137.6, 140.7, 141.3, 142.4, 142.8; IR (KBr, cm-1) 2068 (νCN ).
2
Preparation of Bis(3-bromo-5-trifluoromethyl-1,1′-biphenyl-4-
yl)diazomethane (S-1-N2). To a vigorously stirred mixture of 4-amino-
3-trifluoromethylbiphenyl (3.68 g, 15.4 mmol)30 and 47% hydrobromic
acid (50 mL) was added dropwise a solution of sodium nitrite (2.12 g,
30.8 mmol) in water (5 mL) at 0 °C. After the mixture was stirred at
0 °C for 1 h, copper powder (2.5 g) was added to the solution and the
mixture was stirred at room temperature overnight. The mixture was
heated at 60 °C for 2 h, diluted with water (200 mL), and filtered. The
filtrate was extracted with ether, washed with water and aqueous Na2-
CO3, and dried over anhydrous Na2SO4. The organic layer was
1
nol (7) as a white solid (1.04 g, 46%): mp 81.0-84.5 °C; H NMR
(300 MHz, CDCl3, ppm) δ 8.16 (s, 2H), 7.80 (s, 2H), 7.70-7.41 (m,
(30) Pettit, M. R.; Tatlow, T. C. J. Chem. Soc. 1954, 1071.
(31) Miura, Y.; Kurokawa, S.; Nakatsuji, M.; Ando, K.; Teki, Y. J. Org. Chem.
1998, 63, 8295.
(32) Grasa, G. A.; Hillier, A. C.; Nolan, S. P. Org. Lett. 2001, 3, 1077.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 965

A R T I C L E S
Itoh et al.
evaporated to leave crude product, which was chromatographed on silica
gel eluted with n-hexane to afford 3.39 g of 4-bromo-3-trifluorometh-
ylbiphenyl (9) as a colorless liquid in 72% yield: 1H NMR (300 MHz,
CDCl3, ppm) δ 7.89 (d, J ) 2.20 Hz, 1H), 7.77 (d, J ) 8.27 Hz, 1H),
7.60-7.40 (m, 6H); 13C NMR (75.5 MHz, CDCl3, ppm) δ 140.7, 138.7,
135.3, 131.4, 130.5 (q, J ) 31.1 Hz), 129.1, 128.4, 127.0, 126.5 (q,
J ) 5.6 Hz), 123.0 (q, J ) 273.0 Hz), 118.7; EI-MS (m/z) 302 (M +
2, 99.3%), 300 (M+, base), 228 (26.4%), 185 (19.3%); HRMS calcd
for C13H8F3Br 299.9761, found m/z 299.9758.
(m, 10H), 7.20 (d, J ) 9.00, 1H), 5.58 (d, J ) 9.00 Hz, 1H), 4.19 (q,
J ) 7.17 Hz, 2H), 1.27 (t, J ) 7.17 Hz, 3H); 13C NMR (75.5 MHz,
CDCl3, ppm) δ 155.0, 142.0, 137.5, 136.9, 136.3, 136.3, 131.5 (q, J )
30.5 Hz), 129.2, 128.8, 127.0, 126.0 (q, J ) 6.3 Hz), 123.6 (q, J )
275.3 Hz), 61.5, 56.8, 14.6; EI-MS (m/z) 703 (M + 4, 8.5%), 701
(M + 2, 12.9%), 699 (M+, 7.4%), 671 (29.8%), 622 (base), 620
(96.1%), 576 (28.1%), 453 (20.2%); HRMS calcd for C38H20FNO2Br2
698.9844, found m/z 698.9843.
Into a stirred solution of 13 (230 mg, 0.33 mmol) in anhydrous CCl4
(5 mL) at 0 °C was bubbled N2O4 gas (1.24 g, 13.5 mmol), and AcONa
(2.40 g, 29.3 mmol) was added. The mixture was stirred at room
temperature overnight, poured onto cold aqueous Na2CO3, and extracted
with CCl4. The organic layer was washed with 5% Na2CO3 solution
and water, dried over anhydrous Na2SO4, and evaporated to give
N-nitroso compound as a yellow oil. To a solution of the nitroso
compound in anhydrous THF (10 mL) was added potassium tert-
butoxide (78 mg, 0.69 mmol) at once under an atmosphere of argon.
The mixture was stirred at room temperature overnight, poured onto
water, and extracted with Et2O. The ethereal layer was washed with
water, dried over anhydrous Na2SO4, and evaporated. The reaction
mixture was passed through a column (aluminum oxide deactivated
with 5% water, n-hexane), and a first fraction was further purified by
GPC (15 cycles, CHCl3) followed by TLC (n-hexane) to give bis(3-
bromo-5-trifluoromethylbiphenyl-4-yl)diazomethane (S-1-N2) as an
orange solid (26 mg, 12%): mp 148.5-150.2 °C (dec); 1H NMR (300
MHz, CDCl3, ppm) δ 7.62-7.42 (m, 6H), 7.60 (d, J ) 7.72 Hz, 4H),
7.94 (d, J ) 2.04 Hz, 2H), 8.07 (d, J ) 2.04 Hz, 2H); 13C NMR (300
MHz, CDCl3, ppm) δ 62.0, 125.6 (q, J ) 8.1 Hz), 126.9 (q, J ) 275.3
Hz), 126.9, 127.4, 128.4 (q, J ) 30.4 Hz), 128.8, 129.2, 136.1, 137.5,
To a solution of tetramethylpiperidine (2.35 g, 16.6 mmol) in dry
THF (20 mL) was added n-BuLi (2.66 M in hexane, 6.24 mL, 16.6
mmol) at 0 °C. The solution was stirred for 0.5 h, and then a solution
of 9 (5.0 g, 16.6 mmol) in THF (20 mL) was added dropwise at -78
°C. After the mixture was stirred for 2 h at this temperature, excess
methyl formate was added and stirred overnight at room temperature.
The reaction mixture was quenched by addition of saturated aqueous
ammonium chloride. The mixture was extracted with ether, washed
with water, dried over Na2SO4, and the solvent was removed under
reduced pressure. The residue was chromatographed on silica gel using
n-hexane-CH2Cl2 (1:1) as an eluent to give 3-bromo-5-trifluorometh-
ylbiphenyl-4-carbaldehyde (11) as a light yellow solid (2.50 g, 46%):
mp 65.4-66.9 °C; 1H NMR (300 MHz, CDCl3, ppm) δ 10.42 (s, 1H),
8.06 (d, J ) 2.02 Hz, 1H), 7.95 (d, J ) 2.02 Hz, 1H), 7.63-7.48 (m,
5H); 13C NMR (75.5 MHz, CDCl3, ppm) δ 190.2, 146.1, 137.1, 135.6,
131.3 (q, J ) 33.0 Hz), 129.5, 129.4, 129.3, 129.0, 127.3, 124.8 (q,
J ) 5.6 Hz), 122.8 (q, J ) 275.0 Hz); EI-MS (m/z) 330 (M + 2, 97.8%),
328 (M+, base), 220 (14.3%), 219 (13.0%), 201 (12.4%); HRMS calcd
for C14H8BrF3O 327.9710, found m/z 327.9714.
To a solution of tetramethylpiperidine (1.21 g, 8.63 mmol) in dry
n
141.7; IR (KBr, cm-1) 2068 (νCN ).
2
THF (10 mL) was added BuLi (2.66 M in hexane, 3.25 mL, 8.65
Product Analysis. Photolysis of U- and S-1-N2 in degassed benzene
gave the following products.
mmol) at 0 °C. The solution was stirred for 0.5 h, and then a solution
of 9 (2.60 g, 8.63 mmol) in THF (20 mL) was dropwise added at -78
°C. After the mixture was stirred for 2 h at this temperature, 11 (2.84
g, 8.63 mmol) was added and stirred overnight at room temperature.
The reaction mixture was quenched by addition of saturated aqueous
ammonium chloride. The mixture was extracted with ether, washed
with water, dried over Na2SO4, and the solvent was removed under
reduced pressure. The residue was chromatographed on silica gel using
n-hexane-CH2Cl2 (2:1) as an eluent to give bis(3-bromo-5-trifluoro-
methylbiphenyl-4-yl)methanol (12) as a white solid (2.50 g, 46%): mp
48.0-50.0 °C; 1H NMR (300 MHz, CDCl3, ppm) δ 8.02 (d, J ) 11.13
Hz, 2H), 7.68 (d, J ) 11.13 Hz, 2H), 7.63-7.40 (m, 10H), 6.77 (d,
J ) 9.37 Hz, 1H), 3.12 (d, J ) 9.37 Hz, 1H); 13C NMR (75.5 Hz,
CDCl3, ppm) δ 142.2, 141.7, 138.7, 137.9, 137.6, 136.6, 135.8, 131.7
(q, J ) 31.1 Hz), 129.2, 128.0, 126.8, 124.0 (q, J ) 274.0 Hz), 72.8;
EI-MS (m/z) 632 (M + 4, 10.2%), 630 (M + 2, 16.8%), 628 (M+,
10.4%), 329 (96.7%), 327 (base), 311 (32.4%), 309 (32.2%), 201
(57.9%); HRMS calcd for C27H16F6OBr2 627.9471, found m/z 627.9466.
Carbene-dimer from U-1-N2: A 6:4 mixture of two isomers, A
1
and B; yellow semisolid (38%); H NMR (300 MHz, CDCl3, ppm) δ
7.81 (d, J ) 1.8 Hz, 1.2H, A), 7.73 (d, J ) 1.7 Hz, 1.2H, A), 7.70-
7.62 (m, 2.4H, B), 7.60 (s, 2H), 7.57 (s, 2H), 7.50-7.30 (m, 16.4H),
7.07 (s, 0.8H, B), 7.05 (s, 0.8H, B), 6.78 (s, 1.2H, A); MALDI-TOF-
MASS calcd for C54H28Br4F12 (carbene dimer) 1219.87, found m/z
1219.91.
Carbene-dimer from S-1-N2: A mixture of rotational isomers;
yellow semisolid (52%); 1H NMR (300 MHz, CDCl3, ppm) δ 8.01 (s,
0.3H), 8.02 (d, J ) 1.7 Hz, 1.2H), 7.93-7.75 (m, 2H), 7.69 (d, J )
2.0 Hz, 1.2H), 7.62-7.21 (m, 20H), 7.12 (s, 0.3H), 7.10 (s, 1.2H),
7.03 (s, 0.3H), 6.72 (s, 1.2H), 6.60 (s, 0.3H); MALDI-TOF-MASS calcd
for C54H29Br4F12 (carbene dimmer + H) 1220.88, found m/z 1220.90.
Photolysis of U- and S-1-N2 in degassed benzene in the presence
of 1,4-cyclobutadiene gave the double-hydrogen abstraction product
1-H2.
U-1-H2: White semisolid (59%); 1H NMR (300 MHz, CDCl3, ppm)
δ 8.10 (s, 2H), 7.75 (s, 2H),7.66-7.35 (m, 10H), 4.76 (s, 2H); EI-MS
(m/z) 616 (M + 4, 2.8%), 614 (M + 2, 5.2%), 612 (M+, 2.7%), 536
(35.0%), 534 (32.0%), 456 (base); HRMS calcd for C27H16F6Br2
611.9522, found m/z 611.9627.
S-1-H2: White solid (32%); mp 121-126 °C; 1H NMR (300 MHz,
CDCl3, ppm) δ 8.08 (s, 1H), 7.96 (s, 1H), 7.92 (s, 2H), 7.64-7.38 (m,
10H), 4.61 (s, 2H); EI-MS (m/z) 616 (M + 4, 0.6%), 614 (M + 2,
1.2%), 612 (M+, 0.6%), 536 (77.0%), 534 (82.8%), 456 (base); HRMS
calcd for C27H16F6Br2 611.9522, found m/z 611.9557.
A solution of 12 (2.23 g, 3.54 mmol) and SOCl2 (20 mL) was stirred
at room temperature overnight, and the mixture was evaporated under
reduced pressure to give bis(2-bromo-4-phenyl-6-trifluoromethylphe-
nyl)chloromethane as a brown viscous oil (2.30 g, quantitative yield),
which was rather unstable and hence used without further purifica-
tion: 1H NMR (300 MHz, CDCl3, ppm) δ 6.77 (s, 1H), 7.40-8.05
(m, 14H). A mixture of silver tetrafluoroborate (691 mg, 3.55 mmol)
and ethyl carbamate (7.5 g, 35 mmol) was heated at 60 °C. To the
mixture was added a solution of the chloromethane (2.30 g, 3.54 mmol)
in 1,4-dioxane (50 mL), and it was heated at 100 °C overnight. The
mixture was filtered, and water (50 mL) and CHCl3 (50 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 chromatographed on silica gel using
n-hexane-CH2Cl2 (1:1) as an eluent to give ethyl N-[bis(3-bromo-5-
trifluoromethylbiphenyl-4-yl)methyl]carbamate (13) as a yellow semi-
solid (231 mg, 9.3%): 1H NMR (300 MHz, CDCl3, ppm) δ 7.79 (dd,
J ) 7.90, 2.02 Hz, 2H), 7.75 (dd, J ) 7.90, 2.02 Hz, 2H), 7.60-7.30
Photolysis of U- and S-1-N2 in nondegassed benzene gave the
following ketones 1-O.
U-1-O: White solid (90%); mp 152-154 °C; 1H NMR (300 MHz,
CDCl3, ppm) δ 8.16 (s, 2H), 7.91 (s, 2H), 7.69-7.44 (m, 10H);13C
NMR (75.5 MHz, CDCl3, ppm) δ 189.6, 146.2, 143.9, 137.3, 136.7,
136.1, 134.3, 132.8, 130.6 (q, J ) 32.1 Hz), 129.4, 129.4, 129.2, 128.6
(q, J ) 5.3 Hz), 127.3, 127.2, 125.9, 125.2, 123.0 (q, J ) 275.1 Hz);
IR (KBr, cm-1) 1684 (νCO); EI-MS (m/z) 630 (M + 4, 48.2%), 628
9
966 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Triplet Diphenylcarbenes
A R T I C L E S
(M + 2, 92.6%), 626 (M+, 47.0%), 341 (44.7%), 339 (92.6%), 337
(47.3%), 317 (base); HRMS calcd for C27H14F6OBr2 625.9315, found
m/z 625.9313.
and the detection system was achieved through an Iwatsu model DS-
8631 digital synchroscope, which was interfaced to a NEC 9801 RX2
computer. This allowed for rapid processing and storage of the data
and provided printed graphics capabilities. Each trace was also displayed
on a NEC CRT N5913U 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 four freeze-degas-thaw
cycles at a pressure near 10-5 Torr immediately prior to being flashed.
The sample system was sealed, and the solution was transferred to the
quartz cuvette, which was placed in the sample chamber of the flash
spectrometer. The concentration of the sample was adjusted so that it
absorbed a significant portion of the laser light.
1
S-1-O: White solid (80%); mp 76-79 °C; H NMR (300 MHz,
CDCl3, ppm) δ 8.05 (d, J ) 1.70 Hz, 2H), 8.00 (d, J ) 1.70 Hz, 2H),
7.64-7.46 (m, 10H); 13C NMR (75.5 MHz, CDCl3, ppm) δ 190.5,
145.2, 137.1, 136.1, 135.6, 133.2 (q, J ) 32.4 Hz), 129.4, 129.3, 127.2,
126.1 (q, J ) 6.1 Hz), 122.9 (q, J ) 275.3 Hz), 121.9; IR (KBr, cm-1
)
1685 (νCO); EI-MS (m/z) 630 (M + 4, 30.8%), 628 (M + 2, 58.0%),
626 (M+, 29.7%), 329 (base), 327 (99.6%); HRMS calcd for C27H14F6-
OBr2 625.9315, found m/z 625.9315.
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-TE200 spectrometer (X-band microwave unit,
100 kHz field modulation). The signal positions were read by the use
of a gaussmeter. For samples used in the temperature dependence
studies, the EPR tube was placed in the spectrometer equipped with a
JEOL ES-DVT3 liquid nitrogen transfer system and was irradiated at
77 K. The temperature of the sample was monitored and controlled by
a PID controller. The sample temperature was raised in 10 K increments
to the desired temperature, allowed to stand for 1 min, and recooled to
77 K to measure the signal. A JEOL liquid helium transfer system
was attached for the low-temperature measurement. The temperature
was controlled by a 9650 Microprocessor-based Digital Temperature
Indicator/Controller, which provided the measurements accuracy within
(0.1 K and the control ability within (0.2 K. Errors in the measure-
ments of component amplitudes did not exceed 5%; the accuracy of
the resonance fields determination was within (0.5 mT. All solvents
employed were of spectroscopic grade and purified by distillation just
prior to use.
Low-Temperature UV/Vis Spectra. Low-temperature spectra at 77
K were obtained by using an Oxford variable-temperature liquid-
nitrogen cryostat (DN 2704) 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 by four 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 500-W high-pressure mercury
lamp using a Pyrex filter, 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).
Laser Flash Photolysis. All flash photolysis measurements were
made on a Unisoku TSP-601 flash spectrometer. The excitation source
for the laser flash photolysis was a 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,
Crystallographic Data. Crystallographic data were recorded on a
Rigaku RAXIS-RAPID imaging plate area detector with graphite-
monochromated Mo KR radiation (λ ) 0.71070 Å) at a temperature of
-150 °C. The structure were solved by a direct method with SIR9233
and refined by the full-matrix least-squares method.34 All of the non-
hydrogen atoms were refined anisotropically, and hydrogen atoms were
refined by a rigid model. A total of 5339 (for S-1-N2) and 5296 (for
U-1-N2) reflections were collected. The final cycle of full-matrix least-
squares refinement on F2 was based on all reflections and 335 variable
parameters and converged with R ) 0.131 (for S-1-N2) and 0.106
(for U-1-N2) and Rw ) 0.344 (for S-1-N2) and 0.296 (for U-1-N2).
All calculations were performed using the CrystalStructure crystal-
lographic software package.35
Acknowledgment. We are grateful to the Ministry of Educa-
tion, 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).
Support from the Mitsubishi Foundation and the Nagase Science
and Technology Foundation is also appreciated.
Supporting Information Available: Transient absorption
spectra obtained in LFP of S-1-N2 in the presence of oxygen
(Figure S1) and 1,4-cyclohexadiene (Figure S2), MALDI-TOF-
MASS spectra of carbene dimers from S-1-N2 and U-1-N2
1
(Figures S3 and S4), H NMR spectra of carbene dimers from
S-1-N2 and U-1-N2 (Figures S5 and S6), ORTEP drawings
of S-1-N2 and U-1-N2 (Figure S7), selected geometrical
parameters for U-1a-N2 and S-1a-N2 at B3LYP/6-31G* level
of theory (Figure S8), Cartesian coordinates, absolute energies
of all calculated compounds at the B3LYP/6-31G* level of
theory, an X-ray crystallographic file (CIF), and complete ref
23. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA056575J
(33) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M.;
Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(34) DIRDIF99: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W.
P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-99 program
system, Technical Report of the Crystallography Laboratory, University
of Nijmegen, The Netherlands, 1999.
(35) CrystalStructure 2.00: Crystal Structure Analysis Package, Rigaku and
MSC. CRYSTALS Issue 10; Watkin, D. J.; Prout, C. K.; Carruthers, J. R.;
Betteridge, P. W. Chemical Crystallography Laboratory, Oxford, UK, 2001.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 967