Triplet diphenylcarbenes protected by o-aryl groups

Article abstract of DOI:10.1021/ja047738r

Diphenyldiazomethanes (2a-N₂) having phenyl groups at the ortho positions were prepared and the corresponding diphenylcarbenes (2a) photolytically generated from them were characterized not only by the traditional product analysis method but also by ESR and UV/vis spectroscopy in a rigid organic matrix at low temperature and in laser flash photolysis in solution at room temperature. Product analysis indicated that fluorenes 4a are formed almost exclusively. Fluorene is most likely produced by the attack of singlet carbene ¹2a on an ortho carbon of the phenyl substituent to generate isofluorene, followed by 1,5-H shift. Irradiation of 2a-N₂ in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K gave ESR signals ascribable to triplet carbenes (³2a). UV/vis spectra of ³2a were obtained by irradiating 2a-N₂ under identical conditions. However, laser flash photolysis (LFP) of 2a-N₂ in degassed benzene at room temperature showed transient absorption bands completely different from those observed in photolysis in the 2-MTHF matrix at 77 K. The transient band decayed in first-order with a rate constant of 7.6 × 10⁴ s ^-1, which was not retarded by deuterium substitution of o-phenyl hydrogens. Essentially the same transient band was observed in LFP of fluorene 4a. The LFP of 2a-N₂ in nondegassed benzene gave transient absorption bands ascribable to triplet carbene ³2a and the corresponding carbonyl oxide. The quenching rate constant of ³2a by oxygen and the lifetime of ³2a in the absence of oxygen were estimated by plotting the observed pseudo-first-order rate constant of the formation of the carbonyl oxide against the concentration of oxygen. They were 1.9 × 10⁷ M^-1s^-1 and 16 μs, respectively. Similar studies with diphenyldiazomethanes having a series of substituents (4-Ph, 3,5-(CF ₃)₂, and 2-F) on the ortfto-phenyl rings gave essentially identical results, indicating that those substituents exhibit little effect on overall reaction pathway. From those studies, it is suggested that triplet carbenes ³2 are also trapped by the ortho-phenyl ring to give eventually fluorenes.

Full text of DOI:10.1021/ja047738r

Published on Web 09/04/2004
Triplet Diphenylcarbenes Protected by o-Aryl Groups
Kyoko Monguchi,§ Tetsuji Itoh,§ Katsuyki 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
Received April 19, 2004; E-mail: tomioka@chem.mie-u.ac.jp
Abstract: Diphenyldiazomethanes (2a-N2) having phenyl groups at the ortho positions were prepared
and the corresponding diphenylcarbenes (2a) photolytically generated from them were characterized not
only by the traditional product analysis method but also by ESR and UV/vis spectroscopy in a rigid organic
matrix at low temperature and in laser flash photolysis in solution at room temperature. Product analysis
indicated that fluorenes 4a are formed almost exclusively. Fluorene is most likely produced by the attack
1
of singlet carbene 2a on an ortho carbon of the phenyl substituent to generate isofluorene, followed by
1,5-H shift. Irradiation of 2a-N2 in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K gave ESR signals
ascribable to triplet carbenes (32a). UV/vis spectra of 32a were obtained by irradiating 2a-N2 under identical
conditions. However, laser flash photolysis (LFP) of 2a-N2 in degassed benzene at room temperature
showed transient absorption bands completely different from those observed in photolysis in the 2-MTHF
matrix at 77 K. The transient band decayed in first-order with a rate constant of 7.6 × 104 s-1, which was
not retarded by deuterium substitution of o-phenyl hydrogens. Essentially the same transient band was
observed in LFP of fluorene 4a. The LFP of 2a-N2 in nondegassed benzene gave transient absorption
bands ascribable to triplet carbene 32a and the corresponding carbonyl oxide. The quenching rate constant
of 32a by oxygen and the lifetime of 32a in the absence of oxygen were estimated by plotting the observed
pseudo-first-order rate constant of the formation of the carbonyl oxide against the concentration of oxygen.
They were 1.9 × 107 M-1s-1 and 16 µs, respectively. Similar studies with diphenyldiazomethanes having
a series of substituents (4-Ph, 3,5-(CF3)2, and 2-F) on the ortho-phenyl rings gave essentially identical
results, indicating that those substituents exhibit little effect on overall reaction pathway. From those studies,
it is suggested that triplet carbenes 32 are also trapped by the ortho-phenyl ring to give eventually fluorenes.
Triplet carbenes are highly reactive organic radicals that are
also notoriously difficult to stabilize.1 Since the recent synthe-
ses2,3 of stable singlet carbenes ruled out the long-standing view
that carbenes are not stable enough to be isolated in a
macroscopic scale at room temperature, the stabilization of a
triplet carbene has emerged as a challenging target.1 Recent
growing interest in triplet carbenes as potential organic ferro-
magnets4 adds practical meaning to the project.
has been prepared and has led to fairly stable triplet diphenyl-
carbenes, some of which can survive a day in solution at room
temperature.5,6 A species surviving a day is very stable for a
triplet carbene, but is still very ephemeral for a real molecule.
To realize this, one needs to explore a more effective kinetic
protector.
The phenyl group is an attractive protector, although a singlet
carbene is known to react with benzene.7 triplet states are usually
unreactive toward benzene. However, it was difficult to prepare
the precursor diphenyldiazomethane having a phenyl group at
the ortho position. For instance, 2,2′,6,6′-tetraphenylated ben-
zophenones do not form hydrazones, even under rigorous
conditions. We found that a diphenyldiazomethane prepared to
To isolate the triplet carbene with its electronic integrity (one
centered diradical) intact, steric protection is the ideal method.
Thus, a series of precursor diphenyldiazomethanes bearing
various substituents at the ortho positions as kinetic protectors
§ Chemistry Department for Materials, Faculty of Engineering.
¶ Instrumental Analysis Facilities, Life Science Research Center.
(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.
(5) 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.
(6) More recently, a triplet carbene surviving a week has been realized by
using anthryl groups that stabilize triplet carbenes both kinetically and
thermodynamically. See Tomioka, H.; Iwamoto, E.; Itakura, H.; Hirai, K.
Nature 2001, 412, 626. (e) Iwamoto, E.; Hirai, K.; Tomioka, H. J. Am.
Chem. Soc. 2003, 125, 14664.
(2) Igau, A.; Gru¨tzmacher, H.; Baceiredo, A.; Bertrand, M. J. Am. Chem. Soc.
1988, 110, 6463. See also Bourissou, D.; Guerret, O.; Gabbai, F. P.;
Bertrand, G. Chem. ReV. 2000, 100, 39.
(3) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,
113, 361. See also Arduengo, A. J., III Acc. Chem. Res. 1999, 32, 913.
(4) See for review (a) Rajca, A. Chem. ReV. 1994, 94, 871. (b) Iwamura, H.
AdV. Phys. Org. Chem. 1990, 26, 179. (c) Magnetic Properties of Organic
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999.
(7) For reviews of general reactions of carbenes, see (a) Kirmse, W. Carbene
Chemistry, 2nd edition; Academic Press: New York, 1971. (b) Carbenes;
Moss, R. A., Jones, M., Jr., Eds.; Wiley: New York, 1973 and 1975; Vol.
1 and 2. (c) Carbene(oide), Carbine; Regitz, M., Ed.; Thieme; Stuttgart,
1989.
9
11900
J. AM. CHEM. SOC. 2004, 126, 11900-11913
10.1021/ja047738r CCC: $27.50 © 2004 American Chemical Society

Diphenylcarbenes Protected by o-Aryl Groups
A R T I C L E S
Scheme 1
Scheme 2
desired phenylated compound as a yellow solid in ca. 90% yield
with the carbamate group intact. Nitrosation of the carbamate
with NaNO2, followed by treatment of the resulting N-
nitrosocarbamate with t-BuOK gave [2,4,6-tris(phenyl)phenyl]-
(2,6-dimethyl-4-phenylphenyl)diazomethane (2a-N2) as an
orange solid.
Similar arylation using substituted benzene boronic acids,
where substituents were 4-phenyl, 3,5-bis(trifluoromethyl), and
2-fluoro groups all resulted in the o-arylation although more
rigorous conditions were required, and the corresponding diazo
compounds were then again obtained as orange solids.
All the diazo compounds were purified by TLC followed by
GPC. Appreciable decomposition of the diazo group was not
noted in all cases, at least under the conditions employed here.
The diazo compounds were fairly stable and could be stored in
a refrigerator for several months without any appreciable
decomposition.
B. Comparison between [2,4,6-Tris(phenyl)phenyl](2,6-
dimethyl-4-phenylphenyl)carbene (2a) and (2,6-Dibromo-4-
phenylphenyl)(2,6-dimethyl-4-phenylphenyl)carbene (1). To
know the effect of the o-phenyl group on the reactivities of
triplet diphenylcarbenes (DPCs) as opposed to that of the
o-bromo group, we first photolyzed [2,4,6-tris(phenyl)phenyl]-
(2,6-dimethyl-4-phenylphenyl)diazomethane (2a-N2) and the
data are compared with those obtained by irradiation of
(2,6-dibromo-4-phenylphenyl)(2,6-dimethyl-4-phenylphenyl)-
diazomethane (1-N2).
generate a persistent triplet carbene is also relatively stable and
hence can be further modified with the diazo group intact to a
more complicated diazo compound. For instance, bis(2,4,6-
tribromophenyl)diazomethane survives under Sonogashira cou-
pling reaction conditions leading to bis(2,6-dibromo-4-tri-
methylsilylethynylphenyl)diazomethane.8 This is obviously due
the fact that the kinetic protectors introduced to protect the
carbene center from external reagent also are able to protect
the diazo carbon. It is potentially possible to prepare the desired
precursor by extending this idea.9 We wish to report here the
preparation of o-phenylated diphenyldiazomethanes and char-
acterization of carbenes generated from them.
Results
A. Preparation of Precursor Diazomethanes. The diazo
compound we chose for this study is (2,6-dibromo-4-phenyl-
phenyl)(2,6-dimethyl-4-phenylphenyl)diazomethane (1-N2). Since
we found that bromine groups at the para-position could be
replaced with a phenyl group by using Suzuki coupling
reaction,10 we first reacted the diazo compound with benzene
boronic acid. However, all attempts to obtain o-phenylated
diphenyldiazomethane under similar conditions and under more
rigorous conditions were unsuccessful. Therefore, we were
forced to find a route to introduce o-phenyl group at a previous
stage. We found that a diarylmethylcarbamate, a precursor for
the diaryldiazomethane, was smoothly o-phenylated and then
the resulting carbamate was converted into the corresponding
diazomethane (Scheme 1).
Product Analysis Studies. Irradiation of 1-N2 in a degassed
benzene resulted in the formation of tetraarylethylene (3) as the
main product (Scheme 2). It has been now well documented
that sterically congested triplet DPCs undergo coupling at the
carbenic center to give a dimer.5 The formation of the ethylene
is thus explained as indicating that the carbene center is well
shielded by kinetic protectors and triplet carbene concentration
builds up to the point at which dimerization occurs.12
In marked contrast, similar irradiation of 2a-N2 under
identical conditions gave a fluorene derivative (4a) as a sole
isolable product, neither dimeric products nor anthracene
derivatives being detected (Scheme 3). It is very interesting to
note that 4a was obtained when photolysis was carried out even
in methanol (vide infra). Methanol is known as an efficient
trapping reagent for carbene and is shown to react with both
singlet and triplet diarylcarbenes.13
Treatment of (2,6-dibromo-4-phenylphenyl)(2,6-dimethyl-4-
phenylphenyl)methylcarbamate with benzene boronic acid in
the presence of the Pd(OAc)2/P(t-Bu)2(2-BP) catalyst system,
with KF as the base in tetrahydrofuran (THF)11 at room-
temperature overnight, followed by the usual workup, gave the
A notable difference in the decay pathway between the two
carbenes is thus observed.
(8) Tomioka, H.; Hattori, M.; Hirai, K.; Sato, K.; Shiomi, D.; Takui, T,; Itoh,
K. J. Am. Chem. Soc. 1998, 120, 1106.
(12) (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.
(13) For review of O-H insertion reactions of carbenes, see (a) Kirmse, W. In
AdVances in Carbene Chemistry; Brinker, U. H., Ed.; JAI Press: Stamford,
1994; Vol. 1, pp 1-57. (b) Kirmse, W. In AdVances in Carbene Chemistry;
Brinker, U., Ed.; JAI Press: Greenwich, CT, 2001; Vol. 3, pp 1-51.
(9) Ohtsuka, Y.; Itoh, T.; Hirai, K.; Takui, T.; Tomioka, H. Org. Lett. 2004,
6, 847.
(10) Itoh, T.; Hirai, K.; Tomioka, H. J. Am. Chem. Soc. 2004, 126, 1130.
(11) For reviews of Suzuki coupling reaction, (a) Suzuki, A. In Metal-Catalyzed
Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH:
Weinheim, 1998; pp 49-97. (b) Stanforth, S. P. Tetrahedron 1998, 54,
263.
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J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11901

A R T I C L E S
Monguchi et al.
Figure 1. ESR spectra obtained by irradiation of (a) diazo compound 1-N2
and (b) 2a-N2 in 2-methyltetrahydrofuran at 77 K.
Figure 2. UV/vis spectra obtained by irradiation of diazo compound 1-N2.
(a) Spectra of 1-N2 in 2-methyltetrahydrofuran at 77 K. (b) the same sample
after irradiation (λ > 350 nm). (c-d) the same sample after thawing to (c)
100 and (d) 170 K.
Scheme 3
A significant decrease in both E and E/D values on going
3
3
from 1 to 2a suggests that, as phenyl groups are introduced
at ortho position, the central bond angle is expanded due to
increased ortho-interaction induced by the phenyl group. A slight
3
3
decrease in D values on going from 1 to 2a can then be
interpreted as indicating that unpaired electrons are delocalized
into phenyl groups at the ortho positions not only because of
the increasing number of the accepting group but also because
of the expansion of the central bond angle. However, the extent
of the increase is rather small when taking into account the
amount of increase in phenyl rings. This indicates that o-phenyl
rings are not coplanar with the carbenic phenyl ring (vide infra).
The ESR signals were not only stable at this temperature but
disappeared irreversibly when the matrix was thawed to room
temperature. 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
Currie law.15 The triplet signals of 32a disappeared around 130
K. However, the triplet signals of brominated DPCs are
generally weak and broad. The signals of 31 were also not strong
enough to allow such estimation. Instead, thermal stability of
31 was estimated in more accurately by monitoring the UV/vis
Spectroscopic Studies. ESR Studies in Rigid Matrixes at
Low Temperature. Irradiation (λ > 300 nm) of 1-N2 in a
2-methyltetrahydrofuran (2-MTHF) glass at 77 K gave rise to
ESR spectra (Figure 1a) characteristic of randomly oriented
triplet molecules. The zero field splitting (ZFS) parameters were
D ) 0.348 cm-1 and E ) 0.0326 cm-1 (E/D ) 0.0937), showing
unequivocally that triplet signals are due to triplet (2,6-dibromo-
4-phenylphenyl)(2,6-dimethyl-4-phenylphenyl)carbene(31) gen-
erated by photodissociation of 1-N2. Similar irradiation of 2a-
N2 gave triplet signals ascribable to [2,4,6-tri(phenyl)phenyl](2,6-
dimethyl-4-phenylphenyl)carbene(32a) with ZFS parameters of
D ) 0.300 cm-1 and E ) 0.00486 cm-1 (E/D ) 0.0162) (Figure
1b).
The structure of triplet carbenes is characterized by ESR ZFS
parameters D and E.14 The D value is related to the separation
between the unpaired electrons. The E value, on the other hand,
when weighed by D, is a measure of the deviation from axial
symmetry. For diarylcarbenes, this value will thus depend on
the magnitude of the central C-C-C angle. Since the E value
depends on the magnitude of the central angle, the reduction in
E indicates that the carbene adopts a structure with an expanded
C-C-C angle upon annealing. This interpretation is supported
by the observation that the substantial reduction of E is usually
accompanied by a significant reduction in D, indicating that
the electrons are becoming more delocalized.
3
spectra of 1 as a function of temperature.
UV/Vis Studies in Rigid Matrixes at Low Temperature.
Photolysis (λ > 300 nm) of 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 1-N2 (Figure 2a). The new bands
showed rather strong and broad absorption bands centered at
349 nm along with apparent maxima at 315 and 391 nm. Since
ESR signals ascribable to triplet carbene are observed under
the identical conditions, the absorption spectrum can be at-
tributable to triplet carbene (31). The bands are to be compared
with those observed for (2,4,6-tribromophenyl)(2,6-dimethyl-
4-tert-butylphenyl)carbene,12c a nonphenylated analogue of 31,
which exhibited rather weak bands at 337 and 349 nm. Rather
strong bands at longer wavelength region exhibited by 31 clearly
showed the role of phenyl groups at para position.
The absorption bands were stable for hours when kept at this
low temperature, but upon thawing the matrix, the bands became
(14) See for reviews of the ESR 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. (c) Tomioka, H. In AdVances in
Strained and Interesting Organic Molecules; Halton, B., Ed.; JAI Press:
Greenwich, CT, 2000; Vol. 8, pp 83-112.
(15) Platz, M. S. In Diradicals; Borden, W. T., Ed.; Wiley: New York, 1982;
pp 195-258. Wasserman, E.; Hutton, R. S. Acc. Chem. Res. 1977, 10, 27.
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11902 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Diphenylcarbenes Protected by o-Aryl Groups
A R T I C L E S
Figure 4. Absorption spectra of the transient products obtained upon 308-
nm excitation of 1-N2 in degassed benzene recorded after 10 µs. Insets
show the decay of the transient products monitored at 365 nm.
Figure 3. UV/vis spectra obtained by irradiation of diazo compound 2a-
N2. (a) Spectra of 2a-N2 in 2-methyltetrahydrofuran at 77 K. (b) the same
sample after irradiation (λ > 350 nm). (c-d) the same sample after thawing
to (c) 100 and (d) 130 K.
sharp with an apparent shift of the maximum from 354 to 349
nm at around 100 K (Figure 2b-d). This change was not reversed
when the samples were recooled. Changes of this kind have
often been observed for sterically congested triplet diarylcar-
benes and are usually interpreted in terms of geometrical
changes.14c,16 Thus, when a carbene is generated in rigid matrixes
at very low temperature, its initial geometry should be dictated
by that of the precursor. Even if the thermodynamically most
stable geometry of the carbene is different from the geometry
at birth, the rigidity of the matrixes prevents the carbene from
achieving its minimum-energy geometry. But, when the matrix
is softened by annealing, the carbene is allowed to relax to its
preferred geometry probably to gain relief from steric compres-
sion. In marked contrast, it has been shown that the ZFS
parameters of sterically unperturbed triplet DPCs show little
sensitivity toward the temperature of the matrix. The absorption
bands due to relaxed carbenes disappeared irreversibly at around
170 K.
Figure 5. Transient absorption spectra obtained in LFP of 1-N2 in
nondegassed benzene with a 308-nm excimer laser recorded 200 µs after
the pulse. The inset shows the growth of the oxide (1-O2) monitored at
440 nm.
Laser flash photolysis (LFP) of 1-N2 in degassed benzene
produced transient absorption bands showing maxima at 365
nm (Figure 4), which is essentially identical with that observed
in photolysis of 1-N2 in a matrix at 77 K. Thus, we assigned
3
the band to triplet carbene 1. The transient band decayed in
Similar irradiation of 2a-N2 under identical conditions gave
rather strong and broad absorption centered at 356 nm with
apparent maxima at 326 and 387 nm (Figure 3a). Again, since
ESR signals ascribable to triplet carbene are observed under
the identical conditions, the absorption spectrum can be at-
tributable to triplet carbene (32a). The absorption due to 32a is
second-order fashion. This is in accord with the product study
indicating that triplet carbene 31 mainly decayed by undergoing
dimerization to give 3. The rate constant was estimated to be
2k/ꢀl ) 1.7 s-1, where ꢀ and l are the extinction coefficient and
the optical length, respectively. The lifetime was estimated in
the form of a half-life (t1/2) to be 0.5 s.
3
thus very similar but a little bit broader than that of 1. This
When LFP was carried out on a nondegassed benzene solution
of 1-N2, the half-life of 31 decreased dramatically, and a broad
absorption band with a maximum at 410 nm appeared at the
expense of the absorption due to 31 (Figure 5). The spent
solution was found to contain hexabromobenzophenone
(1-O) as the main product. It is well-documented17,18 that
diarylcarbenes with triplet ground states are readily trapped by
oxygen to generate the corresponding diaryl ketone oxides,
which show a broad absorption band centered at 396-450 nm.
The oxides eventually formed the corresponding ketones
(Scheme 4). The spent solution showed the presence of ketone
may be due to the difference of molecular distortion between
the two. The bands became sharp with an apparent shift of the
maximum from 356 to 360 nm at around 100 K and disappeared
irreversibly at around 130 K (Figure 3b-d).
This temperature is to be compared with that observed for
31 before o-phenylation, which is shown to survive up to 170
K. A marked decrease in the thermal stability upon replacement
of o-bromine group with phenyl group is thus noted.
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.5,6
(17) See for review: Sander, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 344.
(b) Bunnelle, W. Chem. ReV. 1991, 91, 336.
(18) See LFP study of trapping of triplet carbene with oxygen, (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.
(16) (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.
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A R T I C L E S
Monguchi et al.
Figure 6. Plot of the growth rate of the oxide (1-O2) monitored at 440
Figure 7. Transient absorption spectra obtained in LFP of 1-N2 in degassed
benzene in the presence of 1,4-cyclohexadiene (CHD) with a 308-nm
excimer laser recorded 100 ms after the pulse. The inset shows a plot of
the growth rate of the radical (1-H) monitored at 420 nm as a function of
the concentration of the diene.
nm as a function of the concentration of oxygen.
Scheme 4
(1-O). Thus, the observations can be interpreted as indicating
that 31 is trapped by oxygen to form the carbonyl oxide
(1-O2), which confirms that the transient absorption quenched
3
by oxygen is due to 1.
The apparent built-up rate constant, kobs, of the carbonyl oxide
(1-O2) is expressed as given in eq 1
Figure 8. Absorption spectra of the transient products obtained upon 308-
nm excitation of 2a-N2 in degassed benzene recorded after 1,10, and 100
µs. Insets show the decay of the transient products monitored at 410 nm.
kobs ) k0 + kO2 [O2]
(1)
wavelengths than those of the precursor carbenes. The excellent
hydrogen donor properties of CHD have been well recognized.21
A plot of the observed pseudo-first-order rate constant of the
formation of the radical against [CHD] is linear (Inset in Figure
7), and the slope of this plot yields the absolute rate constant
3
where k0 represents the rate of decay of 1 in the absence of
oxygen and kO is quenching rate constant of 1 by oxygen. A
3
2
plot of the observed pseudo-first-order rate constant of the
formation of the oxide against [O2] is linear (Figure 6). From
3
for the reaction of 1 with the diene, kCHD ) 1.2 × 102 M-1
the slope of this plot, kO was determined to be 2.1 × 107 M-1
s-1, which is some five orders of magnitude smaller than that
2
s-1, which is approximately two orders of magnitude smaller
3
observed with DPC (kCHD ) 1 × 107 M-1 s-1).20b
3
than that observed with the “parent” DPC (kO ) 5.0 × 109
2
LFP of 2a-N2 gave completely different results. Thus, LFP
of 2a-N2 in a degassed benzene solution at room temperature
produced a transient species showing a rather broad band with
apparent maxima at 420 and 500 nm extending up 550 nm,
which appeared coincident with the pulse (Figure 8). Due to
the overlaps with the absorption maxima of the diazo precursor
2a-N2, the samples are not sufficiently transparent for adequate
M-1 s-1).18b
When a degassed benzene solution of 1-N2 containing 1,4-
cyclohexadiene (CHD) was excited, a new species was formed,
showing an absorption with λmax ) 400 nm, formed as the 365
nm signal of 31 decayed (Figure 7). The decay of 31 was again
found to be kinetically correlated with the growth of the new
species. Thus, this new signal was attributable to the hexabro-
modiphenylmethyl radical (1-H) formed as a result of H
(20) For LFP study of H abstraction by triplet carbenes, see (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.
3
abstraction of 1 from the diene (Scheme 4), since it is now
well-documented19,20 that triplet arylcarbenes, generated in good
hydrogen donor solvents, undergo H abstraction leading to the
corresponding radicals that show transient absorptions at longer
(19) For reviews, see (a) Kinetics and Spectroscopy of Carbenes and Biradicals;
Platz, M. S., Ed.; 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. See also, (c) Closs, G. L.;
Rabinow, B. E. J. Am. Chem. Soc. 1976, 98, 8190.
(21) Encinas, M. V.; Scaiano, J. C. J. Am. Chem. Soc. 1981, 103, 6393.
9
11904 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Diphenylcarbenes Protected by o-Aryl Groups
A R T I C L E S
Figure 10. Plot of the growth rate of the oxide (2a-O2) monitored at 440
nm (red) and the decay of triplet carbene (32a) monitored at 390 nm (blue)
as a function of the concentration of oxygen.
Figure 9. Transient absorption spectra obtained in LFP of 2a-N2 in
nondegassed benzene with a 308-nm excimer laser recorded 1 µs after the
pulse. The inset shows the growth of the oxide (2a-O2) monitored at 440
nm.
3
carbene 2a, which is efficiently trapped by oxygen, is not
trapped by a less efficient trapping reagent, CHD.
Again notable difference in reactivity between the two
carbenes is observed.
monitoring in the 300-360 nm region. The two bands decayed
in a similar manner. The inset in Figure 8 shows the decay of
the transient species in the absence of trapping reagents, which
is found to be first order with the rate constant of k ) 7.6 ×
104 s-1, the lifetime being 13 µs. The transient species are
significantly different from that observed for 2a in matrix at
low temperature (Figure 3a) and hence are not assignable to
32a.
C. Assignment of Transient Product in LFP of 2a-N2 in
Degassed Solution. The absorption bands observed in LFP of
2a-N2 in solution at room temperature are not assigned to 32a.
What then is the origin of the absorption? Since the analysis of
the spent solution showed the presence of fluorene as an almost
exclusive product and since the transient bands decay by first-
order kinetics, the band may be ascribable to a species leading
to the fluorene from photolytically generated carbene 2a. A
singlet state of 12a is highly unlikely since singlet diarylcarbenes
are mostly invisible even in a time resolution in picoseconds.22
3
Trapping experiments were carried out to gain more informa-
tion on the intermediate. When LFP was carried out on a
nondegassed benzene solution of 2a-N2, transient bands
showing apparent maximum at 390 nm that are quite different
from that observed in LFP under degassed conditions were
observed (Figure 9). Again due to the overlaps with the
absorption maximum of the diazo precursor 2a-N2, it is not
possible to observe whole absorption bands. However, the bands
are obviously shifted to a shorter wavelength than that observed
in LFP of 2a-N2 in degassed benzene and can be regarded as
the tail part of the absorption bands due to triplet carbene 32a.
1
Fluorene (4a) is not likely formed directly from carbene 2a,
but rather is produced by way of intermediates.
Photochemistry of 2-diazomethylbiphenyl systems has been
studied. Photolysis of 2-diazomethylbiphenyl in solution affords
fluorene almost exclusively.23,24 When the irradiation is carried
out in methanol, methyl ether, obviously formed as a result of
O-H insertion of biphenyl-2-ylcarbene, is obtained competi-
tively.25 Therefore, the carbene is proposed to be involved in
the reaction leading to fluorene. Since the deuterium isotope
This assignment was actually supported by monitoring the
decay of the band more carefully. As the band decayed, a new
band appeared at 440 nm. The growth rate of the new band
was essentially identical with that of the decay rate (Inset in
Figure 10), indicating that the transient product was trapped by
oxygen to generate a new transient product, which is most likely
carbonyl oxide (2a-O2). Moreover, analysis of the spent
solution showed the presence of ketone (2a-O) in 10% along
with fluorene 4a.
effect of the formal C-H insertion is very small (kH/kD
)
1.12),23 rate-controlling cyclization of biphenyl-2-ylcarbene to
give a diradical or o-quinodimethane (isofluorene) followed by
migration of hydrogen is proposed. From the product ratio of
fluorene and methyl ether, the rate constants for cyclization are
estimated to be 2 × 108 s-1 and ∼1011 s-1 for triplet and singlet
states, respectively.25 Similar irradiation of (biphenyl-2-yl)-
phenyldiazomethane also produces the formal C-H insertion
product, 9-phenylfulorene, as the main product. Again, methyl
ether is competitively formed when irradiated in methanol.26
From these studies on related systems, several intermediates
can be assumed. Initial reaction of carbene center with an
o-phenyl ring will lead to either tetracyclic cyclopropanes (5,
6) or isofluorene (7) (vide infra). The former intermediates are
A plot of the observed pseudo-first-order rate constant of the
formation of the oxide against [O2] is linear (Figure 10). From
the slope of this plot, kO was determined to be 1.9 × 107 M-1
2
s-1. The intercept gave k0, the rate of decay of 32a in the absence
of oxygen, to be 6.3 × 104 s-1 (τ ) 16 µs). Essentially the
same data are obtained by monitoring the decay of the band at
390 nm (kO ) 1.7 × 107 M-1 s-1 and k0 ) 6.0 × 104 s-1).
2
LFP of 2a-N2 in degassed benzene in the presence of a large
excess of CHD, however, resulted in the formation of the same
transient bands observed in LFP of 2a-N2 in the absence of
oxygen, not the other bands being detected. Product analysis
studies also indicated that fluorene 4a was formed almost
exclusively even in the presence of large excess of CHD. Triplet
(22) See for instance, Portella-Oberli, M. T.; Jeannin, C.; Soep, B.; Zerza, G.;
Chergui, M. Chem. Phys. Lett. 1998, 296, 323.
(23) Denny, D. B.; Klemchuk, P. P. J. Am. Chem. Soc. 1958, 80, 3289.
(24) Palik, E. C.; Platz, M. S. J. Org. Chem. 1983, 48, 963.
(25) Kirmse, W.; Kund, K.; Ritzer, E.; Dorigo, A. E.; Houk, K. N. J. Am. Chem.
Soc. 1986, 108, 6045.
(26) Dorra, M.; Gomann, K.; Guth, M.; Kirmse, W. J. Phys. Org. Chem. 1996,
9, 598. We thank a referee for notifying us of the relevant works by Kirmse.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11905

A R T I C L E S
Monguchi et al.
Table 1. Energies (∆E), Relative to 2A, of Intermediates
Calculated Using DFT at the (U)R3BLYP/6-31G(d) Level of Theory
Figure 11. B3LYP/6-31G* relative energies (kcal/mol) of species involved
in the reaction of ortho-biphenylcarbene 2A leading to fluorene 4A by way
of isofluorene 7A.
supposed to be rather very high in energy in the light of ring
strain and hence are less likely. Moreover, spectroscopical
features of the observed species match with those for quino-
dimethane structure in isofluorene (7) rather than those for the
former. For example, o-quinodimethanes usually exhibit broad
and strong absorption bands centereted around 350-450 nm.27
DFT Calculations. To gain some insights into intermediates
involved in the present photolysis, we performed DFT calcula-
tions28,29 of potential energy surface for the possible reaction
of biphenyl-2-ylcarbene 2A (Figure 11, Table 1).
Absorption maximum (λ, nm) and oscillator strength (f), computed using
time-dependent DFT calculation at the R3LYP/6-31 G(d) level of theory,
are also shown.
itself and the transition state for formation of isofluorene 7A.
This suggests that 7A is, of these three possible cyclization
products of the carbene, by far the lowest in energy. Transition
1
state energy leading from singlet carbene 1A to isofluorene
7A is estimated to be only 0.5 kcal/mol, while that from
isofluorene 7A to fluorene 4A is 2.6 kcal/mol.
To assist in the experimental identification of the reactive
intermediates, the eletronic spectra of tetracyclic intermediates
and isofluorene were calculated, using time-dependent (TD)
DFT theory.30 As shown in Table 1, isofluorene 7A is predicted
to have relatively intense absorption bands at 310 and 490 nm,
while tetracyclic intermediates 5A and 6A are predicted to have
a very weak absorption band at 338 and 383 nm at the longest,
respectively.
The (U)B3LYP calculations predicted carbene 2A has a triplet
ground state with the singlet lying 6.9 kcal/mol higher in energy.
The dihedral angles between the two phenyl rings in triplet and
singlet states are 41.8 and 35.0 degrees, respectively.
Kinetic Deuterium Isotope Effects. If the transient absorp-
tion band is ascribable to isofluorene 7a, the decay process
corresponds to the 1,5-H shift process leading to fluorene 4a.
One may then expect the kinetic deuterium isotope effect
(KDIEs) in this process (vide infra). Thus, we prepared desired
precursors (2a-N2-d) according to the essentially identical
procedure employed to prepare the protio analogue by using
penta(deutrio)phenylboronic acid.
Irradiation of 2a-N2-d in degassed benzene again resulted
in the formation of fluorene (4a-d) as the exclusive product, no
additional products being detected. Irradiation of 2a-N2-d in
2-MTHF matrix at 77 K produced ESR signals (having D )
Carbenes can cyclize either by forming σ bonds to two
carbons of the phenyl substituent to produce tetracyclic cyclo-
propanes (5A and 6A) or by attack of carbenic carbon on an
ortho carbon of the phenyl substituent to afford isofluorene-
(7A). B3LYP calculations indicated that isofluorene 7A is 38.6
kcal/mol lower in energy than singlet carbene. Tetracyclic
compounds 5A and 6A were calculated to be significantly higher
in energy, much higher in energy even than the singlet carbene
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, Foresman, J. B.; K.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. W. M.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. GAUSSIAN 98; Gaussian Inc.:
Pittsburgh, PA, 1998.
(27) (a) de Fonscka, K. K.; Manning, C.; McCullough, J. J.; Yarwood, A. J. J.
Am. Chem. Soc. 1977, 99, 8257. (b) de Fonscka, K. K.; McCullough, J. J.;
Yarwood, A. J. J. Am. Chem. Soc. 1979, 101, 3277. (c) McCullough, J. J.
Acc. Chem. Res. 1980, 13, 270. (d) Wintgens, V.; Netto-Ferreira, J. C.; H.
L. Casal, H. L.; Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 2363. (e)
Johnston, L. T.; Scaiano, J. C. Chem. ReV. 1989, 89, 521. (f) Netto-Ferreira,
J. C.; Wintgens, V.; Scaiano, J. C. Can. J. Chem. 1994, 72, 1565.
(28) (a) Lee, C.; Yang, W.; Paar, R. G. Phys. ReV. B 1980, 37, 785. (b) Becke,
A. D. Phys. ReV. A 1988, 38, 3098. (c) Becke, A, D. J. Chem. Phys. 1993,
98, 5648.
(30) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys.
1998, 108, 4439.
9
11906 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Diphenylcarbenes Protected by o-Aryl Groups
A R T I C L E S
0.304 cm-1, E ) 0.00454 cm-1) and UV/vis spectra (showing
maxima at 323, 353, and 390 nm), which are essentially identical
with those observed in the photolysis of the protio analogue,
2a-N2. Thermal stability of those signals and absorption bands
was also similar to those of 2a. Therefore, those transient
species were easily assigned to the corresponding triplet carbene
32a-d.
LFP of 2a-N2-d in degassed benzene produced again
transient absorption showing the maxima at 420 and 490 nm,
which decayed in essentially the same first-order manner with
the rate constant of k ) 7.6 × 104 s-1 (τ ) 13 µs), which is
almost identical with that observed by LFP of 2a-N2. Almost
no KDIE was observed.
sigmatropic rearrangement of cis-1,3-pentadiene36 and also with
kH/kD of 3.7 for the shift of aromaticity-driven isoindene to
indene conversion.27b It is thus highly unlikely that the transient
absorption band is due to isofluorene.
3
LFP of Fluorene 4a. Kirmse et al.26 have observed, in LFP
of (biphenyl-2-yl)phenyldiazomethane, a transient absorption
band with λmax ) 370 nm whose rates of decay are 2.3-3.4 ×
105 s-1 and are nearly independent of the solvent. They also
noted that the decrease is associated with an increase in the
band due to 9-phenylfluorene and that the decay rate is strongly
accelerated by the presence of oxygen. They assigned the
transient band to triplet 9-phenylfluorene since the same
absorption is generated by laser excitation of the fluorene.
LFP of fluorene 4a in degassed benzene produced a transient
band with a sharp band with maxima at 350 nm along with a
rather broad band with apparent maxima at 415 and 500 nm
extending up to 550 nm. All the bands decayed in first order.
The decay rate monitored at 420 nm is found to be 6.37 × 104
s-1 (τ ) 16 µs). The decacy rate is strongly accelerated by the
presence of oxygen with the quenching rate constant of 2.10 ×
109 M-1 s-1. Although the strong band at 350 nm was not
apparent in the transient absorption spectra obtained in LFP of
2a-N2 due to the overlap with the absorption maxima of the
diazo precursor, the spectroscopic features in the 400 to 550
nm region and decay profile are in good agreement with that
observed in LFP of 4a. It is thus likely that the transient band
observed in LFP of diazo compound 2a-N2 is ascribable to
triplet fluorene 4a.
Kinetics and Thermodynamic of 1,5-Hydrogen Shift. The
1,5-shift in simple cyclic and acyclic compounds is relatively
slow as a consequence of substantial activation energies. For
instance, the activation energy (∆Hq) for the 1,5-shift in 1,3-
pentadiene is 36 kcal/mol and even higher (∆Hq ) 41.7 kcal/
mol, ∆Gq ) 40.1 kcal/mol) in 1,3-cyclohexadiene.31 However
it was recently found that the 1,5-shift of isoindene to
indene27a-c,32 and 8a, 9-dihydrophenanthrene to 9, 10-dihydro-
phenanthrene33 proceeds rapidly. This indicates that formation
of an aromatic ring in the products leading to high exergonicity
of the 1,5-shift is responsible for the lowering of the activation
energy. The effects of aromatic stabilization on the rates of 1,5-
shifts has been estimated and examined by B3LYP/6-31G**
computations.34 The activation barriers ∆Hq (and ∆Gq) for 1,5-
shift of isoindene27a-c,32 and 8a, 9-dihydrophenanthrene33 are
calculated to be 18.4 (and 17.1) kcal/mol and 14.8 (and 13.8)
kcal/mol, respectively. This indicates that formation of an
aromatic ring in the products leading to high exergonicity of
the 1,5-shift is responsible for the lowering of the activation
energy. In the present system, two benzene rings are regenerated
in the isomerization of 7a to 4a compared with the above two
systems. Isofluorene (7A)-fluorene(4A) conversion is estimated
to be the most exothermic (54.5 kcal/mol) and hence to have
the smallest barrier (∆Hq ) 2.6 kcal/mol and ∆Gq ) 3.2 kcal/
mol). This is another example of the Bell-Evans-Polanyi
It should be noted here that no new band was observed during
the quenching reaction of triplet flourene by oxygen. This is to
be compared with the observation that the transient band is
observed in LFP of 2a-N2 in the presence of oxygen (vide
supra).
D. [2,6-Bis(aryl)-4-phenylphenyl](2,6-dimethyl-4-phenylphe-
nyl)carbene (2b-d). Effect of Substituents on ortho-Phenyl
Groups. Observations indicate that singlet carbene 12a is trapped
exclusively and very rapidly by the o-phenyl group in solution
at room temperature to form fluorene before it undergoes
intersystem crossing to the triplet. This means that if one can
35
principle.
The rate constant of 7.6 × 104 s-1 observed for the present
system is more than four and two orders of magnitude larger
than that observed for isoindene to indene (k ) 3.1 s-1) and
9-phenyl-8a, 9-dihydrophenanthrene to 9,10-dihydrophenan-
threne processes (k ) 1.6 × 102 s-1), respectively. However,
this rate is very small for the process with ∆Gq ) 3.2 kcal/
mol, which corresponds to 4 × 1010 s-1. The rate constants
observed for 1,5-H shift in isoindene and 9-phenyl-8a, 9-dihy-
drophenanthrene are in good agreement with those estimated
based on ∆Gq ) 17.1 (2.1 s-1) and 13.8 kcal/mol (5.5 × 102
s-1), respectively. Moreover, essentially zero kinetic deuterium
isotope effect (kH/kD ) 1) observed for the present system is in
sharp contrast with kH/kD of ca. 5 observed for the 1,5-
1
retard the initial trapping reaction of 2, it may be possible to
observe a triplet carbene in solution. One way to realize this is
to introduce substituents on the o-phenyl ring. So, we prepared
precursor diphenyldiazomethanes in which ortho-phenyl rings
carry such substituents as 4-phenyl (2b-N2), 1,3-bis(trifluo-
romethyl) (2c-N2), and 2-fluoro groups (2d-N2). Trifluoro-
methyl groups are expected to reduce electron density on the
phenyl ring while the 2-fluoro group will increase steric
1
repulsion and hence retard the initial attack of 2.
ESR ZFS parameters and UV/vis absorption maxima of triplet
3
carbenes 2b-d observed in the photolysis of the precursory
diazomethanes 2-N2 in the 2-MTHF matrix as well as Td are
summarized in Table 2. The data in Table 2 indicate that the
structures of triplet carbenes (D and E values and absorption
maxima) and thermal stability (Td) are practically unaffected
by those substituents.
(31) Hess, B. A., Jr.; Baldwin, J. E. J. Org. Chem. 2002, 67, 6025.
(32) Roth, W. R. Tetrahedron Lett. 1964, 1009.
(33) (a) de Violet, Ph. F.; Bonneau, R.; Lapouyade, R.; Koussini, R.; Ware, W.
R. J. Am. Chem. Soc. 1978, 100, 6683. (b) Lewis, F. D.; Zuo, X.;
Gevorgyan, V.; Rubin, M. J. Am. Chem. Soc. 2002, 124, 13664.
(34) Alabugin, I. V.; Manoharan, M.; Breiner, B.; Lewis, F. D. J. Am. Chem.
Soc. 2003, 125, 9329.
LFP of 2b-N2-2d-N2 in degassed benzene again produced
the transient absorption bands similar to those observed in LFP
(35) (a) Bell, R. P. Proc. R. Soc. London 1936, 154A, 414. (b) Evans, M. G.;
Polanyi, M. Trans. Faraday Soc. 1938, 34, 11. (c) Shaik, S. S.; Schlegel,
H. B.; Wolfe, S. Theoretical Aspects of Physical Organic Chemistry;
Wiley: New York, 1992.
(36) Roth, W. R.; Konig, J. Liebigs Ann. Chem. 1966, 699, 24. See also Liu,
Y.-P.; Lynch, G. C.; Truong, T. N.; Lu, D.-H.; Truhlar, D. G.; Garrett, B.
C. J. Am. Chem. Soc. 1993, 115, 2408.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11907

A R T I C L E S
Monguchi et al.
Table 2. D and E Values and UV/Vis Spectra of Triplet
Diarylcarbenes (31 and 32)a
system can explain why putative intermediates are not detected
in nano-second time resolution.
1
1
carbenes
|
D|
(cm-
)
|
E
|
(cm-
)
E/D
λmax (nm)
Td (K)b
A crucial difference between the present system and (bi-
phenyl-2-yl)phenylcarbene is found in the reactivity toward
methanol. While significant amount of O-H insertion product
is formed in the reaction of the latter, the former carbene is not
quenched by methanol at all. This suggests that the singlet state
is also protected from external reagent in the former carbene.
Second, triplet carbene is observed in matrix photolysis at
low temperature although this is not observed in LFP in
degassed solution at room temperature. This is somewhat
unexpected as, in most cases, the transient bands due to triplet
carbene are observed both in matrix photolysis and in LFP. This
can be explained by assuming the effect of the matrix on the
reaction pathway of the singlet state. If, as is suggested by DFT
calculations, o-phenyl groups are not coplanar with the carbenic
phenyl group, cyclization of the carbenes will require molecular
motions. Such motion can be blocked in a rigid matrix to some
extent; hence, the decay reaction in the singlet state leading to
fluorene is retarded. Thus, the triplet state comes to be
observable as the singlet state has enough lifetime to intersystem-
cross to the triplet under these conditions. It is interesting to
note here that product analysis of the spent solution of the
photolysis in the matrix at 77 K showed that fluorene is still
formed almost exclusively even under these conditions.37 This
may mean that triplet states also react with the o-phenyl group
(vide infra).
The effect of oxygen is also interesting. A triplet carbene
absorption band is observed in solution only in LFP of 2-N2
in the presence of oxygen, along with the band due to carbonyl
oxide. This can be interpreted in terms of external perturbation
of intersystem crossing by oxygen. It is known that molecular
oxygen enhances the overall intersystem crossing.38 Therefore,
the observation of triplet carbene absorption band in the presence
of oxygen can be interpreted as indicating that intersystem
crossing either in the excited state of diazo compound (S1 to
T1) and/or in the carbenes (singlet to triplet states) is enhanced
by oxygen to the point where detection of the transient bands
is possible (vide infra).
However, product analysis indicates that fluorene 4 is still
obtained as the major product even in the presence of oxygen
while ketone is formed only as a minor product (∼10%),
although the triplet carbene band is trapped by oxygen and the
band due to the carbonyl oxide is apparent. This suggests that
fluorene is still formed as the main product either from the
singlet and/or from the triplet. Therefore, a part of the reason
that the triplet state 2a is observable in the presence of oxygen
is due to the fact that the bands of triplet fluorene (34) become
invisible in the presence of oxygen as a result of efficient
quenching and hence the absorption band due to triplet carbene
32 becomes apparent under these conditions. However, since
the bands due to carbonyl oxide (2a-O2) are also seen only in
the presence of oxygen and the oxidation product 2a-O is
formed only in the presence of oxygen, external perturbation
1
0.348
0.300
0.304
0.308
0.324
0.335
0.0326
0.0937
0.0162
0.0149
0.0181
0.0269
0.0175
315, 349, 391
326, 356, 387
323, 353, 390
316, 341, 376
320, 357, 400
320, 351, 380
170
130
130
140
130
140
2a
2a-d
2b
2c
0.00486
0.00454
0.00557
0.00873
0.00586
2d
a Measured in 2-methyltetrahydrofuran at 77 K. b Temperature at which
the UV/vis spectra of triplet diarylcarbenes disappeared.
Table 3. Kinetic Data Obtained in LFP of Diazomethanes (2-N2)a
1
diazomethanes
λmax (nm)
k (s-
)
τ (µs)
2a-N2
2a-N2-d
2b-N2
2c-N2
2d-N2
410
420
480
400
420
7.6 × 104
7.6 × 104
6.9 × 104
7.1 × 104
8.7 × 104
13
13
15
14
12
a All kinetic measurements were carried out in benzene at 20 °C.
of 2a-N2, which decayed in first-order kinetics. No transient
bands ascribable to the triplet states were detected. The rate
constants summarized in Table 3 apparently indicate that the
decay rates change very little by the substituents on o-phenyl
groups. Finally, product analysis of the spent solution indicated
that fluorenes 4 were produced as almost exclusive products in
all cases (Scheme 3).
The observations clearly indicate that the substituents on the
o-phenyl rings exhibit little influence on overall reaction initiated
by photoexcitation of 2-N2.
Discussion
We have prepared precursor diphenyldiazomethanes 2-N2
for DPC having o-aryl groups as a potential kinetic protector
of carbene center. Carbenes 2 are then generated by irradiating
the precursors 2-N2.
The observations in the photolysis of 2-N2 are different from
those in the photolysis of sterically congested diaryldiazo-
methanes in several respects. First, main products observed in
the photolysis of sterically congested diphenyldiazomethanes
in benzene solution at room temperature are, as is seen in the
photolysis of 1-N2, mostly dimers of triplet DPCs. In the
present case, however, fluorene 4 is always obtained as the
almost exclusive product, no carbenic dimers being detected.
It is clear that the nascent singlet state of the carbene is almost
exclusively trapped by the o-phenyl group to give fluorene 4.
Computational studies suggest that the initial attack of carbene
on an ortho carbon of the phenyl group at the ortho position
proceeds with a very low activation barrier. It is then likely
that the singlet state is almost instantaneously trapped before it
undergoes intersystem crossing to the triplet state. This process
is not affected by substituents on the ortho phenyl groups. Thus,
it is not possible to observe the triplet state in solution at room-
temperature directly.
Although isofluorene is likely to be involved in the reaction
leading to fluorene, this species is also not detected by LFP
with time resolution in nanoseconds. Computational studies
predict that the rate for a “double”-aromaticity-driven 1,5-H shift
process in isofluorene-fluorene conversion is 4 × 1010 s-1 for
the model system. These data from calculations for the model
(37) Intermolecular H atom abstractions of triplet carbenes have been shown to
proceed by a QMT mechanism at very low temperature. See for review:
(a) Platz, M. S. Acc. Chem. Res. 1988, 21, 236. (b) Tomioka, H. Res. Chem.
Intermed. 1994, 20, 605. (c) Platz, M. S. In Kinetics and Spectroscopy of
Carbenes and Biradicals; Platz, M. S.; Ed.; Plenum: New York, 1990; pp
143-212.
(38) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings;
Menlo Park, 1978; p 193.
9
11908 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Diphenylcarbenes Protected by o-Aryl Groups
A R T I C L E S
of ISC by oxygen still plays a role. If the ISC is on a time scale
of picoseconds (as with diphenylcarbene), on the other hand, it
is not understandable why this should be increased by the
presence of triplet oxygen (the increase should be limited by
the diffusion of oxygen in solution). It is more reasonable to
assume that triplet state 32a is also generated to some extent in
the absence of oxygen but undergoes cyclization leading to
fluorene, since this is only reaction available to the triplet but
mechanism, triplet to singlet interconversion occurs prior to
O-H insertion of the singlet, while triplet carbenes can react
directly with alcohol with surface crossing occurring after the
carbene has begun to interact with the O-H bond in the latter
mechanism. In light of the rather large gap between singlet and
triplet states calculated for 2A and also the fact that fluorene 4
is obtained as a main product even in the matrix at low
temperature, surface crossing mechanism is likely one in this
case.
3
that 2a comes to be trapped by oxygen in the presence of
oxygen in competition with the intramolecular reaction.
How well is the carbene center in the triplet state protected
by the ortho phenyl groups? Since the transient bands due to
the triplet state were not seen under ambient conditions, the
lifetime was estimated from a Stern-Volmer plot in the oxygen
quenching experiments. This value of 16 µs is unexpectedly
small. It is interesting to note here that the quenching rate
The present observation is to be compared with the isocar-
bazole to carbazole conversion, a nitrogen analogue of the
present system, observed in LFP of ortho-biphenyl azide.42 The
activation and reaction energies for this process are calculated
at the B3LYP/6-31G* level of theory to be 7.5 and 53.7 kcal/
mol, respectively, suggesting that the activation energy is larger
than that of the carbon analogue while the reaction energy is
similar. It is pointed out that introduction of heteroatoms
significantly increases the activation energy for the hydrogen
shift.34 However, the rate constant observed for isocarbazole to
carbazole conversion in pentane is reported to be 1.4 × 107
s-1, more than three orders of magnitude smaller than that
predicted on the present system. A considerable kinetic
deuterium isotope effect (kH/kD ) 3.4) is observed in this case.
3
constant of 2a by oxygen is essentially identical with that
3
observed for 1 (kO ) 1.11 × 107 Μ-1 s-1), whose lifetime
2
3
under ambient conditions is ca. 1 s. Moreover, 1 was shown
to abstract hydrogen from CHD with a rate constant of k ) 7.4
3
× 102 Μ-1s-1, while 2a was not quenched by CHD. This
3
means that the carbenic center in 2a is equally well shielded
by the o-phenyl groups from external reagents as in 31.
3
Extremely shorter lifetime of 2a (τ ) 16 µs) as opposed to
31(τ ) 1 s) is then interpreted only in terms of reaction of the
triplet state with the o-phenyl groups. This is in accordance with
the product study showing that ketone is produced only in 10%
yield even in the irradiation in the presence of oxygen (vide
supra).
Conclusion
We have explored procedures to prepare precursors for
diphenylcarbenes having aryl groups at the ortho position.
Diphenylcarbenes generated from those precursors are not stable,
but exhibited an interesting chemistry. The finding that the
carbenic center is almost completely shielded by an o-aryl group
from external reagents indicates that once one can introduce an
aryl group that can prevent the reaction of carbene at the ortho
carbon, one will be able to generate fairly stable triplet carbene.
Perfluorophenyl group is an attractive candidate in this respect
since C-F bonds are only the bond utterly unreactive toward
carbene.
The reaction of sterically congested triplet carbenes with
benzene is usually considered to be very slow. Indeed, almost
no products from such reaction have been detected in the
photolysis of sterically congested diphenyldiazomethanes. How-
ever, methylene is shown to react with benzene both in singlet
and triplet states.39 In the triplet reaction, C-H abstraction is
proposed from kinetic studies, but C-C bond formation to
generate a diradical intermediate is suggested by theoretical
calculations. It is then possible that triplet states can interact
with the adjacent ortho carbon of the o-phenyl group that is in
close vicinity. Such interaction will eventually lead to fluorene
by way of isofluorene.
A reaction of triplet carbene leading to a typical singlet
product is seen in the reaction with alcohols.13 Generally, singlet
carbenes insert readily into the O-H bond of alcohol while
hydrogen abstraction from the C-H bonds of alcohol is a
characteristic reaction of the triplet ground state of carbene. The
behavior of aryl and diarylcarbenes seemingly violates this
simple rule. Upon addition of alcohol in the LFP experiments,
Experimental Section
General Methods. Reagents were obtained commercially and used
without further purification unless otherwise noted. Tetrahydrofuran,
ethyl ether, toluene, and dioxane were dried over benzophenone ketyl
(sodium/benzophenone) and dichloromethane, carbon tetrachloride and
triethylamine were dried over calcium hydride. The 1H NMR (300
MHz) and 13C NMR (75 MHz) spectra were recorded on a JEOL JNM-
AL300FT/NMR spectrometer in CDCl3 with SiMe4 as an internal
reference. IR spectra were measured on a JASCO FT/IR-410 spec-
trometer, and UV/vis spectra were recorded on a JASCO CT-560
spectrophotometer. ESR spectra were measured on a JEOL JES-
TE200D ESR spectrometer. The mass spectra were recorded on a JEOL
JMS-600H mass spectrometer and AB Voyager-DE PROMALDI-TOF
mass spectrometer. Gel permeation chromatography (GPC) was carried
out on a JASCO PU-1586 chromatograph with a UV-1570 UV/vis
detector using a Shodex GPC H-2001 column (20 mm × 50 cm). Thin-
layer chromatography was carried out on a Merck Kieseigel 60 PF
254. Column chromatograph was carried out on a silica gel (Merck
for dry column chromatography) or on an aluminum oxide, active
neutral (activity I) (ICN for column chromatography).
for instance, quenching of both singlet (kS ) 1010 Μ-1 s-1
)
and triplet diphenylcarbene(kT ) 107 Μ-1 s-1) is observed.40
A reaction from the triplet state to a singlet product must
involve surface crossing. Two mechanisms, preequilibrium and
surface crossing, are proposed for the relatively slow reaction
of triplet diarylcarbenes.41 According to the preequilibrium
(39) Hartz, N.; Surya Prakash, G. K.; Olah, G. A. J. Am. Chem. Soc. 1993,
115, 901.
(40) (a) Griller, D.; Nazran, A. S.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106,
198. (b) Griller, D.; Nazran, A. S.; Scaiano, J. C. Tetrahendron 1985, 41,
1525.
Preparation of (2,6-Dibromo-4-phenylphenyl)(2,6-dimethyl-4-
phenylphenyl)diazomethane (1-N2). A solution of 3,5-dimethylbi-
(41) For discussion on this issue, see Platz, M. S.; Maloney, V. M. In Kinetics
and Spectroscopy of Carbenes and Biradicals; Platz, M. S., Ed.; Plenum:
New York, 1990; pp 320-348.
(42) Tsao, M.-L.; Gritsan, N.; James, T. R.; Platz, M. S.; Hrovat, D. A.; Broden,
W. T. J. Am. Chem. Soc. 2003, 125, 9343.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11909

A R T I C L E S
Monguchi et al.
phenyl43 (4.92 g, 27.0 mmol) in CCl4 was cooled in an ice bath to 0
°C and Br2 (1.35 mL, 27.0 mmol) was added dropwise under vigorous
stirring over a period of 15 min and the mixture was stirred overnight
at room temperature. The mixture was neutralized with 10% NaOH
aq, and the residue was extracted with CCl4. The organic layer was
washed with H2O, dried (Na2SO4) and evaporated to leave 4-bromo-
3,5-dimethylbiphenyl as a white solid in 97% yield, which was used
for the next step without further purification: mp 52.2-54.5 °C; H
NMR (CDCl3) δ 2.48 (s, 6 H), 7.29 (s, 2 H), 7.45-7.32 (m, 3 H), 7.54
(dd, J ) 1.65, 6.80 Hz, 2 H).
brownish viscous oil was chromatographed on a silica gel column eluted
with CHCl3-n-hexane (3:1). Ethyl N-(2,6-dibromo-4-phenylphenyl)-
(2,6-dimethyl-4-phenylphenyl)methylcarbamate was obtained as a
1
yellow solid in 63% yield; mp 202.6-204.8 °C; H NMR (CDCl3) δ
1.29 (t, J ) 7.17 Hz, 3 H), 2.32 (s, 6 H), 4.18-4.24 (m, 2 H), 5.35 (d,
J ) 8.82 Hz, 1 H), 6.59 (d, J ) 8.82 Hz, 1 H), 7.23 (s, 2 H), 7.34-
7.29 (m, 2 H), 7.44-7.35 (m, 4 H), 7.58-7.52 (m, 4 H), 7.80 (s, 2 H);
13C NMR (CDCl3) δ 14.8, 21.3, 58.1, 61.3, 124.9, 127.0, 127.3, 128.4,
128.5, 128.7, 129.0, 132.6, 135.0, 136.5, 137.5, 137.7, 138.0, 140.3,
140.4, 142.3, 155.0; EIMS m/z (relative intensity) 595 (M+4, 1.5),
593 (M+2, 2.2), 591 (M+, 1.6), 514 (100), 512 (94.6), 344 (40.9),
183(100); HRMS Calcd for C30H27Br2NO2 591.0408, Found m/z
591.0413.
1
To a solution of the bromide (4.47 g, 17.1 mmol) in Et2O (30 mL)
was added nBuLi (7.08 mL, 2.66 M in hexane, 18.8 mmol) dropwise
at -78 °C. After stirring for 3 h at -78 °C, a solution of DMF (3.97
mL, 51.3 mmol) in Et2O (7 mL) was added to the solution. The resulting
solution was gradually warmed to room temperature and then stirred
overnight. Ammonium chloride aq. was added and the resulting mixture
was extracted with Et2O. The ethereal layer was washed with water
and brine, dried over Na2SO4, and evaporated under reduced pressure.
The resulting brownish viscous oil was chromatographed on a silica
gel column eluted with CH2Cl2-n-hexane (1:1). 3,5-Dimethyl-4-
formylbiphenyl was obtained as a white solid in 79% yield; mp 41.3-
43.9 °C; 1H NMR (CDCl3) δ 2.69 (s, 6 H), 7.31 (s, 2 H), 7.40 (tt, J )
1.65, 7.17 Hz, 1 H), 7.46 (dd, J ) 6.98, 7.17 Hz, 2 H), 7.61 (dd, J )
6.98, 1.65 Hz, 2 H), 10.65 (s, 1 H); 13C NMR (CDCl3) δ 20.7, 127.2,
To a stirred and cooled solution of the carbamate (88.9 mg, 0.15
mmol) in Ac2O (3 mL) and AcOH (2 mL) was added NaNO2 (207.0
mg, 3.0 mmol) in a small portion at 0 °C over a period of 3 h and the
mixture was stirred for 1 day at room temperature after addition was
completed. The mixture was poured into H2O and extracted with Et2O.
The ethereal layer was washed with 5% Na2CO3 aq solution, H2O, dried
(Na2SO4), and evaporated to dryness. Ethyl N-nitroso-N-(2,6-dibromo-
4-phenylphenyl)(2,6-dimethyl-4-phenylphenyl)methylcarbamate was
obtained in 50% yield as a yellow solid, which was used without further
1
purification; H NMR (CDCl3) δ 1.29 (t, J ) 6.62 Hz, 3 H), 2.09 (s,
6 H), 4.40 (q, J ) 6.62 Hz, 2 H), 7.08 (s, 1 H), 7.23 (s, 2 H), 7.47-
7.39 (m, 6 H), 7.61-7.54 (m, 4 H), 7.79 (s, 2 H).
128.2, 128.4, 128.9, 131.2, 139.8, 141.8, 145.5, 193.0; IR (NaCl, cm-1
)
ν 1685; EIMS m/z (relative intensity) 210 (M+, 100), 209 (88.0); HRMS
Calcd for C15H14O 210.1045, Found m/z 210.1036.
All the following operations were carried out in a dark room. To a
stirred and cooled solution of the nitrosocarbamate (48.5 mg, 0.078
mmol) in anhydrous THF (3 mL) was added t-BuOK (17.5 mg, 0.16
mmol) at 0 °C under Ar atmosphere and the mixture was stirred for 1
day at room temperature. The mixture was poured into H2O and
extracted with Et2O. The ethereal layer was washed with H2O, dried
(Na2SO4), and evaporated. The resulting orange solid was chromato-
graphed on a cooled alumina column eluted with n-hexane at 0 °C.
The diazo compound obtained as a fast-moving fraction from the
column was fairly pure but was further purified by repeated chroma-
tography on a gel permeation column with CHCl3. (2,6-Dibromo-4-
phenylphenyl)(2,6-dimethyl-4-phenylphenyl)diazomethane (1-N2)
was obtained as an orange solid in 57% yield; mp 108.5-111.3 °C;
1H NMR (CDCl3) δ 2.27 (s, 6 H), 7.34 (s, 2 H), 7.48-7.38 (m, 6 H),
7.62-7.54 (m, 4 H), 7.84 (s, 2 H); 13C NMR (CDCl3) δ 21.2, 47.8,
125.8, 126.9, 127.0, 127.2, 127.6, 128.3, 128.5, 128.7, 129.1, 130.5,
131.7, 137.5, 138.3, 140.4, 140.6, 142.7; IR (NaCl, cm-1) νCdN2 2054.
Preparation of (2,4,6-Triphenylphenyl)(2,6-dimethyl-4-phenyl-
phenyl)diazomethane (2a-N2) To a solution of ethyl N-(2,6-dibromo-
4-phenylphenyl)(2,6-dimethyl-4-phenylphenyl)methylcarbamate (200.0
mg, 0.337 mmol) in THF (10 mL) were added phenylboronic acid
(123.3 mg, 1.01 mmol), palladium acetate (7.6 mg, 0.0337 mmol), di-
tert-butylphosphinobiphenyl (30.2 mg, 0.10 mmol), and potassium
fluoride (129.2 mg, 2.22 mmol) under Ar atmosphere and the mixture
was stirred at room temperature for 1 day. The mixture was then filtered
and the filtrate was evaporated under reduced pressure. The resulting
brownish viscous oil was purified by the use of a thin layer chromato-
graph eluted with CH2Cl2-n-hexane (1:1). Ethyl N-(2,4,6-tri-
phenylphenyl)(2,6-dimethyl-4-phenylphenyl)methylcarbamate was
obtained as an yellow solid in 90% yield; mp 90.4-92.1 °C; 1H NMR
(CDCl3) δ 1.06 (t, J ) 7.17 Hz, 3 H), 1.90 (s, 6 H), 3.95-3.74 (m, 2
H), 4.48 (d, J ) 9.55 Hz, 1 H), 6.59 (d, J ) 9.55 Hz, 1 H), 6.83 (s, 2
H), 7.21 (m, 4 H), 7.32-7.27 (m, 3 H), 7.35 (m, 2 H), 7.47-7.36 (m,
9 H), 7.61 (d, J ) 7.17 Hz, 4 H); 13C NMR (CDCl3) δ 14.4, 21.3,
53.6, 60.6, 126.4, 126.8, 126.9, 127.0, 127.5, 127.7, 127.9, 128.6, 128.7,
130.0, 136.5, 136.7, 138.2, 138.6, 139.5, 139.6, 141.0, 142.8, 143.0,
154.7; EIMS m/z (relative intensity) 587 (M+, 78.3), 558 (47.0), 498
(44.4), 483 (58.5), 376(45.1), 332 (100), 317 (96.0), 208 (48.9); HRMS
Calcd for C42H37NO2 587.2824, Found m/z 587.2893.
To a solution of the 4-iodo-3,5-dibromobiphenyl44 (4.75 g, 10.9
mmol) in anhydrous Et2O (30 mL) was added nBuLi (4.53 mL, 2.64
M in hexane, 12.0 mmol) dropwise at -78 °C. After stirring for 5 h at
-78 °C, a solution of the aldehyde (2.29 g, 10.9 mmol) in anhydrous
Et2O (15 mL) was added to the solution. The resulting solution was
gradually warmed to room temperature and stirred for 1 day. Am-
monium chloride aq. was added to the solution and the resulting mixture
was extracted with Et2O. The ethereal layer was washed with water
and brine, dried over Na2SO4, and evaporated under reduced pressure.
The resulting brownish viscous oil was chromatographed on a silica
gel column eluted with CH2Cl2-n-hexane (2:1). (2,6-Dibromo-4-
phenylphenyl)(2,6-dimethyl-4-phenylphenyl)methanol was obtained
as a white solid in 85% yield; mp 103.2-105.1 °C; 1H NMR (CDCl3)
δ 2.39 (s, 6 H), 2.73 (d, J ) 8.82 Hz, 1 H), 6.61 (d, J ) 8.82 Hz, 1 H),
7.24 (s, 2 H), 7.33-7.31 (m, 2 H), 7.47-7.39 (m, 4 H), 7.61-7.54
(m, 4 H), 7.82 (s, 2 H); 13C NMR (CDCl3) δ 21.7, 76.3, 124.7, 126.84,
126.87, 127.1, 128.3, 128.4, 128.6, 129.0, 132.1, 136.4, 137.2, 137.7,
138.4, 140.0, 140.4, 142.4; EIMS m/z (relative intensity) 524 (M+4,
5.7), 523(19.8), 522 (M+2, 11.9), 521(39.2), 520 (M+, 7.2), 519 (19.7),
508 (15.9), 506(31.9), 504(16.6), 183(100); HRMS Calcd for C27H22-
Br2O 520.0037, Found m/z 520.0087.
The methanol (4.39 g, 8.41 mmol) was added to SOCl2 (50 mL)
and the mixture was stirred at 35 °C for 1 day. The excess SOCl2 was
evaporated to give (2,6-dibromo-4-phenylphenyl)(2,6-dimethyl-4-
phenylphenyl)chloromethane as a brown solid quantitatively, which
1
was used without further purification; H NMR (CDCl3) δ 2.51 (s, 6
H), 7.01 (s, 1 H), 7.25 (s, 2 H), 7.35-7.30 (m, 2 H), 7.48-7.39 (m, 4
H), 7.55 (d, J ) 6.80 Hz, 2 H), 7.60 (d, J ) 7.17 Hz, 2 H), 7.84 (s, 2
H).
A solution of the chloromethane (4.50 g, 8.41 mmol) in anhydrous
dioxane (60 mL) was added slowly to a mixture of ethyl carbamate
(18.7 g, 210.2 mmol) and AgBF4 (2.46 g, 12.6 mmol) heated at 60 °C
under vigorous stirring and the mixture was stirred at 100 °C for 1
day. The mixture was then filtered and the filtrate was extracted with
CHCl3. The organic layer was washed with H2O to remove dioxane,
dried (Na2SO4), and evaporated under reduced pressure. The resulting
(43) Old, D. W.; Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1998, 120,
9722.
(44) Udayakumar, B. S.; Gary, B. J. Org. Chem. 1992, 57, 348.
To a stirred and cooled solution of the carbamates (185.7 mg) in
Ac2O (6.0 mL) and AcOH (4.0 mL) was added NaNO2 (436.0 mg,
9
11910 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Diphenylcarbenes Protected by o-Aryl Groups
A R T I C L E S
6.32 mmol) in small portions at 0 °C over a period of 3 h and the
mixture was stirred for 1 day at room temperature after addition was
completed. The mixture was poured into H2O and extracted with Et2O.
The ethereal layer was washed with 5% NaHCO3 aq solution, H2O,
dried (Na2SO4) and evaporated to dryness. Ethyl N-nitroso-N-(2,4,6-
triphenylphenyl)(2,6-dimethyl-4-phenylphenyl)methylcarbamate was
obtained as a mixture in 99% yield as a yellow semisolid, which was
used for the next step without isolation.
methylcarbamate was isolated as an yellow solid in 79%: mp 89.1-
91.5 °C; H NMR (CDCl3) δ 1.17-1.00 (t, J ) 7.17 Hz, 3 H), 2.06,
1
2.02 (s, 6 H), 3.99-3.55 (m, 2 H), 4.92, 4.84, 4.79 (d, J ) 9.37 Hz, 1
H), 6.68, 6.57, 6.46 (d, J ) 9.37 Hz, 1 H), 6.84, 6.80, 6.78 (s, 2 H),
7.24-7.03 (m, 3 H), 7.35-7.30 (m, 2 H), 7.39 (s, 2 H), 7.44-7.37
(m, 6 H), 7.60 (d, J ) 7.17 Hz, 2 H); 13C NMR (CDCl3) δ 14.3, 14.4,
14.7, 21.4, 21.6, 21.8, 53.2, 53.6, 54.2, 60.59, 60.64, 60.7, 114.7, 115.0,
115.1, 115.3, 115.4, 123.5, 123.6, 123.7, 126.9, 127.0, 127.7, 127.8,
128.2, 128.3, 128.4, 128.6, 128.8, 130.2, 130.4, 130.6, 130.7, 136.2,
136.47, 136.53, 136.8, 137.6, 138.16, 138.24, 138.7, 139.0, 139.2, 139.3,
141.1, 154.7, 154.8; EIMS m/z (relative intensity) 623 (M+, 69.0), 593
(36.1), 549 (16.3); HRMS Calcd for C42H35F2NO2 623.2636; Found
m/z 623.2597. Diazomethane (2d-N2) was prepared by using essentially
All the following operations were carried out in a dark room. To a
stirred and cooled solution of the nitrosocarbamates (194.9 mg) in
anhydrous THF (20 mL) was added t-BuOK (70.9 mg, 0.632 mmol)
at 0 °C under Ar atmosphere and the mixture was stirred for 1 day at
room temperature. The mixture was poured into ice and extracted with
Et2O. The ethereal layer was washed with H2O, dried (Na2SO4), and
evaporated. The resulting orange solid was chromatographed on a
cooled alumina column eluted with n-hexane at 0 °C. The diazo
compound obtained as a fast-moving fraction from the column was
fairly pure but was further purified by repeated chromatography on a
gel permeation column with CHCl3. (2,4,6-Triphenylphenyl)(2,6-
dimethyl-4-phenylphenyl)diazomethane (2a-N2) was obtained as an
1
the same method described above; yield 8%; mp 68.1-71.0 °C; H
NMR (CDCl3) δ 2.18 (s, 6 H), 6.93 (s, 2 H), 7.09-6.96 (m, 2 H),
7.22-7.13 (m, 2 H), 7.34-7.29 (m, 4 H), 7.47-7.37 (m, 7 H), 7.54
(s, 2 H), 7.66-7.61 (m, 4 H); 13C NMR (CDCl3) δ 21.4, 59.2, 115.2,
115.5, 123.7, 126.9, 127.0, 127.2, 127.3, 127.5, 127.8, 128.5, 128.55,
128.63, 128.7, 128.78, 128.81, 129.06, 129.14, 129.24, 130.4, 130.89,
130.93, 1334.5, 138.2, 139.5, 139.9, 141.0, 158.0, 161.2; IR (NaCl,
cm-1) νCdN 2043.
1
orange solid in 22%: mp 78.2-81.6 °C; H NMR (CDCl3) δ 2.10 (s,
2
6 H), 6.91 (s, 2 H), 7.22-7.16 (m, 5 H), 7.34-7.26 (m, 6 H), 7.43-
7.39 (m, 7 H), 7.53 (s, 2 H), 7.66 (d, J ) 7.17 Hz, 2 H); 13C NMR
(CDCl3) δ 21.7, 59.4, 126.80, 126.85, 127.0, 127.1, 127.3, 127.5, 127.9,
128.4, 128.6, 128.8, 129.2, 129.4, 129.6, 137.2, 138.5, 139.7, 139.8,
Preparation of [2,6-Bis{3,5-bis(trifluoromethyl)phenyl}-4-
phenylphenyl](2,6-dimethyl-4-phenylphenyl)diazomethane (1d-N2).
To a solution of ethyl N-(2,6-dibromo-4-phenylphenyl)(2,6-dimethyl-
4-phenylphenyl)methylcarbamate (300.0 mg, 0.506 mmol) in toluene
(15 mL) were added 3,5-bis(trifluoromethyl)phenylboronic acid (391.2
mg, 1.517 mmol), palladium acetate (11.4 mg, 0.0506 mmol), di-tert-
butylphosphinobiphenyl (45.3 mg, 0.152 mmol), and potassium fluoride
(194.0 mg, 3.340 mmol) under Ar atmosphere and the mixture was
refluxed for 1 day. The mixture was then filtered and the filtrate was
evaporated under reduced pressure. The resulting brownish viscous oil
was purified by the use of a thin layer chromatograph eluted with CH2-
Cl2-n-hexane (1:1). Ethyl N-[2,6-bis{3,5-bis(trifluoromethyl)}-4-
phenyl]phenyl](2,6-dimethyl-4-phenylphenyl)methylcarbamate (3d)
was isolated as an yellow solid in 59%; mp 108.0-112.0 °C; 1H NMR
(CDCl3) δ 0.99 (t, J ) 7.17 Hz, 3 H), 1.97 (s, 6 H), 3.92-3.55 (m, 2
H), 4.43 (d, J ) 8.82 Hz, 1 H), 6.54 (d, J ) 8.82 Hz, 1 H), 6.90 (s, 2
H), 7.45-7.32 (m, 6 H), 7.52 (s, 4 H), 7.59 (m, 4 H), 7.74 (s, 4 H),
7.83 (s, 2 H); 13C NMR (CDCl3) δ 13.9, 21.2, 53.2, 61.0, 126.8, 127.0,
127.3, 127.7, 128.1, 128.3, 128.5, 129.0, 130.8, 136.2, 136.7, 137.0,
138.4, 139.6, 139.9, 140.0, 140.8, 144.6, 154.6; MS(MALDI-TOF) 882
(M+Na, 53.5); HRMS Calcd for C46H33F12NO2Na 882.2212; Found
m/z 882.2233. The carbamate was converted to ethyl N-nitroso-N-
[2,6-bis{3,5-bis(trifluoromethyl)phenyl}-4-phenylphenyl](2,6-dimethyl-
140.8, 141.0, 141.4; IR (NaCl, cm-1) νCdN 2041.
2
Preparation of [{2,6-Di(phenyl-d5)-4-phenyl}phenyl](2,6-dimeth-
yl-4-phenylphenyl)diazomethane (2a-N2-d) Diazomethane (1b-d) was
prepared by using essentially the same method described above by way
of the following compounds.
Ethyl N-[{2,6-Di(phenyl-d5)-4-phenyl}phenyl](2,6-dimethyl-4-
phenylphenyl)methylcarbamate. Yield 74%; yellow solid; mp 103.1-
1
105.0 °C; H NMR (CDCl3) δ 1.06 (t, J ) 7.17 Hz, 3 H), 1.90 (s, 6
H), 4.00-3.71 (m, 2H), 4.48 (d, J ) 9.55 Hz, 1 H), 6.59 (d, J ) 9.55
Hz, 1 H), 6.83 (s, 2 H), 7.38 (s, 2 H), 7.31 (tt, J ) 1.47, 7.17 Hz, 2 H),
7.35 (s, 2 H), 7.42-7.37 (m, 4 H), 7.46 (dd, J ) 1.29, 8.08 Hz, 2 H),
7.61 (dd, J ) 1.47, 8.08 Hz, 2H); 13C NMR (CDCl3) δ 14.4, 21.3,
53.6, 60.6, 126.85, 126.90, 127.0, 127.5, 127.9, 128.6, 128.7, 129.6,
136.5, 136.7, 138.2, 138.6, 139.5, 139.6, 140.9, 142.7, 142.9, 154.7;
EIMS m/z (relative intensity) 598 (M+1, 47.2), 597 (M+, 100.0), 568
(50.5), 524 (29.0), 508 (34.3); HRMS Calcd for C42H27D10NO2
597.3442; Found m/z 597.3446.
Ethyl N-Nitroso-N-[{2,6-di(phenyl-d5)-4-phenyl}phenyl](2,6-di-
methyl-4-phenylphenyl)methylcarbamate. Yield 99%; yellow solid;
1H NMR (CDCl3) δ 1.13 (t, J ) 7.17 Hz, 3 H), 1.91 (s, 6 H), 4.37-
3.95 (m, 2 H), 6.81 (s, 2 H), 7.16 (s, 2 H), 7.30 (s, 2 H), 7.43-7.27
(m, 6 H), 7.47 (dd, J ) 1.47, 7.17 Hz, 2 H), 7.60 (dd, J ) 1.47, 6.98
Hz, 2 H). [{2,6-Di(phenyl-d5)-4-phenyl}phenyl](2,6-dimethyl-4-
phenylphenyl)-diazomethane (2a-N2-d). Yield 10%; orange solid;
1
4-phenylphenyl)methylcarbamate; yield 99%, H NMR (CDCl3) δ
1.25 (t, J ) 7.17 Hz, 3H), 1.85 (s, 6 H), 4.30-4.19 (m, 2 H), 6.75 (s,
1 H), 6.90 (s, 2 H), 7.44-7.30 (m, 10 H), 7.53 (s, 2 H), 7.79 (s, 4 H),
7.88 (s, 2 H). Diazomethane (2d-N2) was prepared by using essentially
1
the same method described above; yield 31%, mp 52.4-54.9 °C; H
1
NMR (CDCl3) δ 2.12 (s, 6 H), 6.97 (s, 2 H), 7.48-7.29 (m, 6 H), 7.53
(s, 2 H), 7.71-7.62 (m, 4 H), 7.76 (s, 4 H), 7.79 (s, 2 H); 13C NMR
(CDCl3) δ 21.1, 67.8, 126.1, 126.9, 127.5, 128.5, 128.6, 128.9, 129.0,
mp 65.3-67.0 °C; H NMR (CDCl3) δ 2.10 (s, 6 H), 4.37-3.95 (m,
2 H), 6.92 (s, 2 H), 7.35-7.30 (m, 6 H), 7.46-7.37 (m, 5 H), 7.54 (s,
2 H), 7.67 (dd, J ) 2.02, 6.98 Hz, 2 H); 13C NMR (CDCl3) δ 141.2,
137.2, 127.3, 141.0, 129.4, 127.0 (C14), 140.8, 128.8, 126.9, 139.8,
129.1, 129.4, 130.6, 137.5, 140.5, 142.9, 154.7; IR (NaCl, cm-1) νCdN
2
128.6, 126.8, 139.6, 127.8, 59.7, 138.6, 127.5, 21.8; IR (NaCl, cm-1
)
2045.
νCdN 2048.
Preparation of (2,6-Bisbiphenyl-4-yl-4-phenylphenyl)(2,6-dimeth-
yl-4-phenylphenyl)diazomethane (1e). Diazomethane 1e was prepared
with using essentially the same method described above by way of the
following compounds. Ethyl N-(2,6-bisbiphenyl-4-yl-4-phenylphen-
yl)(2,6-dimethyl-4-phenylphenyl)methylcarbamate; yield 21%, mp
120.3-122.5 °C; 1H NMR (CDCl3) δ 0.93 (t, J ) 7.17 Hz, 3 H), 1.98
(s, 6 H), 3.93-3.64 (m, 2 H), 4.57 (d, J ) 9.37 Hz, 1 H), 6.67 (d, J )
9.37 Hz, 1 H), 6.85 (s, 2 H), 7.46-7.27 (m, 21 H), 7.54 (s, 2 H) 7.56-
7.51 (m, 4 H), 7.65-7.62 (m, 3 H); 13C NMR (CDCl3) δ 14.3, 21.4,
53.7, 60.7, 126.3, 126.8, 127.0, 127.2, 127.6, 127.8, 128.1, 128.6, 128.8,
129.1, 129.7, 136.3, 136.6, 136.8, 138.4, 138.8, 139.3, 139.5, 139.6,
140.7, 140.9. 142.2, 142.5, 143.7, 154.7; EIMS m/z (relative intensity)
2
Preparation of [2,6-Bis{2-fluorophenyl}-4-phenylphenyl](2,6-
dimethyl-4-phenylphenyl)diazomethane (2d-N2). To a solution of
ethyl N-(2,6-dibromo-4-phenylphenyl)(2,6-dimethyl-4-phenylphenyl)-
methylcarbamate (300.0 mg, 0.506 mmol) in DME (6 mL) were added
2-fluorophenylboronic acid (426.0 mg, 3.03 mmol), Pd(PPh3)4 (35.1
mg, 0.0303 mmol), and aqueous sodium carbonate (2 M, 1.0 mL) and
the mixture was refluxed under Ar atmosphere for 1 day. The mixture
was then filtered and the filtrate was evaporated under reduced pressure.
The resulting brownish viscous oil was purified by the use of a thin
layer chromatograph eluted with CH2Cl2-n-hexane (1:1). Ethyl N-[2,6-
bis(2-flurophenyl)-4-phenylphenyl](2,6-dimethyl-4-phenylphenyl)-
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11911

A R T I C L E S
Monguchi et al.
739 (M+, 100), 710 (51.1), 666 (38.5), 649 (93.2), 635 (57.6), 557
(36.3), 484 (99.5), 468 (60.9), 208 (41.1); HRMS Calcd for C54H45-
NO2 739.3450, Found m/z 739.3455. Ethyl N-(2-bromo-4-phenyl-6-
biphenyl-4-ylphenyl)(2,6-dimethyl-4-phenylphenyl)methylcarbam-
ate; yield 75%, mp 105.3-107.0 °C; 1H NMR (CDCl3) δ 1.20 (t, J )
6.98 Hz, 3 H), 2.17, 2.06 (s, 6 H), 4.11 (q, J ) 6.98 Hz, 2 H), 5.07 (d,
J ) 8.82 Hz, 1 H), 6.55 (d, J ) 8.82 Hz, 1 H), 7.03 (s, 2 H), 7.44-
7.28 (m, 13 H), 7.52-7.49 (m, 4 H), 7.61-7.58 (m, 3 H), 7.89 (d, J
) 2.02 Hz, 1 H); 13C NMR (CDCl3) δ 14.7, 56.0, 61.1, 124.7, 126.3,
126.9, 126.99, 127.01, 127.2, 127.3, 128.0, 128.7, 128.8, 128.9, 129.5,
131.9, 132.5, 136.6, 137.7, 138.5, 139.8, 139.9, 140.2, 140.5, 140.6,
140.8, 144.4, 155.0; EIMS m/z (relative intensity) 667 (M+2, 2.8),
665 (M+, 2.8), 585 (100), 497 (36.2); HRMS Calcd for C42H36BrNO2
665.1929; Found m/z 665.1935. Diazomethane (1e); yield 6%, mp
94.6-96.6 °C; 1H NMR (CDCl3) δ 2.15 (s, 6 H), 6.92 (s, 2 H), 7.57-
7.27 (m, 22 H), 7.59 (d, J ) 8.08 Hz, 2 H), 7.61 (s, 2 H), 7.69 (d, J
) 8.08 Hz, 4 H); 13C NMR (CDCl3) δ 21.9, 68.7, 126.7, 126.8, 126.9,
127.2, 127.3, 127.6, 127.7, 128.6, 128.7, 128.8, 128.9, 129.2, 129.4,
129.7, 137.4, 138.6, 139.8, 139.9, 140.0, 140.5, 140.6, 140.7, 140.8,
minor: 7.17 (d, J ) 2.02 Hz,), 7.07 (d, J ) 2.02 Hz,), 2.27 (s, 6 H),
2.11 (s, 6 H); 13C NMR (CDCl3) δ 22.9, 23.3, 23.7, 24.6, 126.1, 126.4
126.7, 126.8, 127.1, 127.8, 128.5, 128.6, 128.9, 129.1, 129.3, 131.5,
131.8, 132.4, 132.9, 137.1, 138.5, 139.0, 139.7, 140.2, 141.2,
142.5, 142.9; HRMS Calcd for C54H40Br4 1003.9858; Found m/z
1003.9865.
1,3-Diphenyl-9-(2,6-dimethyl-4-phenylphenyl)fluorene
(4a).
90%: white solid; mp 80.2-82.0 °C; 1H NMR (CDCl3) δ 1.26 (s, 3H,
-Me), 2.30 (s, 3H, -Me), 5.56 (s, 1H,), 6.89 (s, 1H), 7.00 (s, 1H),
7.12-7.03 (m, 5H) 7,25-7.20 (m) 7.34-7.27 (m), 7.38 (d, J ) 7.53
Hz, 2H), 7.42 (d, J ) 1.65 Hz, 1H), 7.51-7.44 (m, 4H), 7.74 (d, J )
7.72 Hz, 1H), 7.93 (d, J ) 7.72 Hz, 1H), 8.09 (d, J ) 1.65 Hz, 1H);
13C NMR (CDCl3) δ 18.8, 21.5, 49.2, 117.7, 120.0, 124.3, 126.5, 126.7,
126.9, 127.0, 127.3, 127.3, 127.5, 127.8, 128.1, 128.6, 128.8, 136.2,
137.5, 137.8, 139.0, 140.4, 140.5, 140.7, 141.1, 141.2, 142.21, 142.22,
143.9, 147.0; EIMS m/e (relative intensity) 498 (M+, 43.4), 497 (100),
482 (10.5), 420 (7.5), 317 (12.8), 249 (10.6); HRMS Calcd for C39H30
498.2347; Found m/z 498.2397.
1-(Phenyl-d5)-3-phenyl-5,6,7,8-tetradeuterio-9-(2,6-dimethyl-4-
phenylphenyl)fluorene (4a-d). 90%; white solid; mp 74.0-76.3 °C;
1H NMR (CDCl3) δ 1.27 (s, 3H, -Me), 2.29 (s, 3H, -Me), 6.89 (s,
1H), 6.99 (s, 1H), 7.34-7.28 (m, 2H), 7.38 (d, J ) 7.53 Hz, 2H), 7.42
(d, J ) 1.65 Hz, 1H), 7.51-7.44 (m, 4H), 7.75 (dd, J ) 1.29, 6.98 Hz,
2H), 8.09 (d, J ) 1.65 Hz, 1H); 13C NMR (CDCl3); δ 18.8, 21.5, 117.7,
126.4, 126.9, 127.28, 127.33, 127.8, 128.6, 128.8, 136.2, 137.5, 137.8,
138.9, 140.4, 140.5, 141.2; EIMS m/z (relative intensity) 510 (M+2,
10.4), 509 (M+1, 32.5), 508.1 (M+, 73.0), 493 (12.0), 369 (11.0), 236
(48.1); HRMS Calcd for C39H20D10 508.2965; Found m/z 508.2945.
1-Biphenylyl-3,7-diphenyl-9-(2,6-dimethyl-4-phenylphenyl)fluo-
rene (4b). 82%; white solid; mp 98.1-100.2 °C; 1H NMR (CDCl3) δ
1.33 (s, 3 H), 2.37 (s, 3 H), 5.63 (s, 1 H), 6.93 (d, J ) 8.82 Hz, 1 H),
7.07 (d, J ) 8.08 Hz, 1 H), 7.50-7.26 (m, Ar), 7.58 (d, J ) 7.17 Hz,
2 H), 7.78 (d, J ) 7.35 Hz, 2 H), 8.01 (d, J ) 8.08 Hz, 1 H), 8.13 (d,
J ) 1.65 Hz, 1 H); 13C NMR (CDCl3) δ 19.0, 22.6, 51.1, 117.8, 120.5,
123.0, 126.5, 126.9, 127.0, 127.2, 127.3, 127.4, 127.8, 128.2, 128.6,
128.7, 128.9, 129.6, 134.5, 137.7, 138.5, 139.3, 139.9, 140.8, 141.0,
141.3, 142.5, 143.6, 144.9, 147.0; EIMS m/z (relative intensity) 650
(M+, 100), 469 (8.5), 325 (7.5); HRMS Calcd for C51H38 650.2973;
Found m/z 650.2981.
140.9; IR (NaCl, cm-1) νCdN 2045.
2
Preparation of [(2,4,6-Triphenyl)phenyl][(2,6-dimethyl-4-phenyl)-
phenyl]ketone. A solution of (2,6-dibromo-4-phenylphenyl)(2,6-di-
methyl-4-phenylphenyl)methanol (50.0 mg, 0.096 mmol) in acetone
was stirred, CrO3 (191.5 mg, 1.91 mmol) in concentrated H2SO4 (0.2
mL) and H2O (0.5 mL)was added dropwise under vigorous stirring
over a period of 15 min, and the mixture was stirred for 2 h at room
temperature. To the mixture was added iPrOH and the residue was
extracted with Et2O. The organic layer was washed with H2O, dried
(Na2SO4), and evaporated to leave {(2,6-bibromo-4-phenyl)phenyl}-
{(2,6-dimethyl-4-phenyl)phenyl}ketone as a white solid quantitatively,
which was used for the next step without further purification: 1H NMR
(CDCl3) δ 2.40 (s, 6 H), 7.31 (s, 2 H), 7.51-7.42 (m, 6 H), 7.58 (dd,
J ) 1.65, 8.27 Hz, 2 H), 7.82 (dd, J ) 1.47, 8.45 Hz, 2 H), 7.82 (s, 2
H); IR (KBr, cm-1) νCdO 1666. To a solution of the ketone (20.0 mg,
0.0038 mmol) in THF (1.0 mL) were added phenylboronic acid (14.1
mg, 0.115 mmol), palladium acetate (0.9 mg, 0.00384 mmol), di-tert-
butylphosphinobiphenyl (14.1 mg, 0.0115 mmol), and potassium
fluoride (14.7 mg, 0.254 mmol) under Ar atmosphere and the mixture
was stirred at 90 °C for 1 day. The mixture was then filtered and the
filtrate was evaporated under reduced pressure. The resulting brownish
viscous oil was purified by the use of thin layer chromatograph eluted
with CH2Cl2-n-hexane (1:1). [(2,4,6-Triphenyl)phenyl][(2,6-dimethyl-
4-phenyl)phenyl]ketone was obtained as an white solid in 90% yield;
1-(3,5-Trifluoromethylphenyl)-3-phenyl-6,8-bis(trifluoromethyl)-
9-(2,6-dimethyl-4-phenylphenyl)fluorene (4c). 96%; white solid; mp
1
92.5-95.0 °C; H NMR (CDCl3) δ 1.55 (s, 3 H), 2.17 (s, 3 H), 5.58
(s, 1 H), 7.31 (s, 1 H), 7.37 (s, 1 H), 7.43 (d, J ) 1.65 Hz, 1 H),
7.75-7.60 (m, Ar), 7.93-7.84 (m, Ar), 8.07 (s, 1 H), 8.14 (d, J )
1.65 Hz, 2 H), 8.26 (s, 1 H), 8.53 (s, 1 H); 13C NMR (CDCl3) δ 18.8,
22.4, 51.2, 117.7, 120.2, 120.3, 122.4, 126.9, 127.1, 127.2, 127.9, 128.2,
128.6, 129.0, 130.3, 130.8, 131.7, 132.7, 137.4, 139.1, 139.9, 140.6,
141.1, 142.9, 143.7, 143.9; EIMS m/z (relative intensity) 770 (M+, 48.7),
769 (100), 700 (10.7), 588 (43.8), 181 (30.3); HRMS Calcd for
C43H26F12 770.1842; Found m/z 770.1809.
1
mp 51.0-53.2 °C; H NMR (CDCl3) δ 2.09 (s, 6H, -Me), 6.87 (s,
2H), 7.23-7.21 (m, 6H), 7.47-7.29 (m, 12H), 7.59 (s, 2H), 7.70 (dd,
J ) 1.47, 8.45 Hz, 2H); 13C NMR (CDCl3) δ 22.1, 127.0, 127.1, 127.2,
127.5, 127.6, 128.1, 128.7, 128.86, 128.92, 129.0, 129.1, 137.8, 138.2,
138.8, 139.4, 140.4, 141.0, 142.0, 142.2, 142.7, 198.8; EIMS m/z
(relative intensity) 514 (M+, 47.1), 437 (100), 361 (21.7), 333 (12.8),
209 (23.3); HRMS Calcd for C39H30O 514.2296; Found m/z 514.2296;
UV/vis (toluene, nm) λmax 254 (ꢀ 25402), 296 (ꢀ 19262); IR (NaCl,
cm-1) νCdO 1658.
1-(2-Fluorophenyl)-3-phenyl-5-fluoro-9-(2,6-dimethyl-4-phenyl-
1
phenyl)fluorene (4d). 86%; white solid; mp 92.7-94.0 °C; H NMR
Irradiation for Product Analysis. In a typical run, a solution of
the diazo compound (1, ca. 10 mg) in solvent was placed in a Pyrex
tube and irradiated with a high-pressure, 300-W mercury lamp until
all the diazo compound was destroyed. The irradiation mixture was
then concentrated on a rotary evaporator below 20 °C. Individual
components were isolated by preparative TLC and identified by NMR
and MS.
(CDCl3) δ 1.25 (s, 3H, -Me), 2.39 (s, 3H, -Me), 5.63 (s, 1H), 6.82
(d, J ) 6.98 Hz, 1H), 6.86 (s, 1H), 6.94 (s, 1H), 7.00 (d, J ) 7.35 Hz,
2H), 7.24-7.05 (m, 3H), 7.32-7.27 (m, 2H), 7.43-7.37 (m, 5H), 7.45
(s, 1H), 7.50-7.47 (m, 4H), 7.75 (dd, J ) 1.47, 7.17 Hz, 2H,), 8.33
(d, J ) 1.65 Hz, 1H); 13C NMR (CDCl3) δ 18.8, 21.3, 49.8, 113.9,
114.2, 114.5, 114.9, 122.0, 123.3, 126.4, 126.9, 127.4, 127.7, 127.8,
128.6, 128.7, 128.8, 129.0, 140.8; EIMS m/z (relative intensity) 536
(M+2, 10.0), 535 (M+1, 37.0), 534 (M+, 90.6), 519 (14.3), 368 (16.6),
236 (35.9); HRMS Calcd for C39H28F2 534.2159; Found m/z 534.2152.
ESR 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 ESR cavity at 77 K and irradiated with a
Wacom 500 W Xe or Hg lamp using a Pyrex filter. ESR spectra were
In this way, the following products were isolated and characterized.
1,2-Bis(2,6-dibromo-4-pheylphenyl)-1,2-bis(2,6-dimethyl-4-
phenylphenyl)ethene (3). 75%: yellow solid; mp 238.3-240.8 °C;
mixture of cis and trans component (major: minor ) 1:0.3); 1H NMR
(CDCl3) major: δ 7.79 (d, J ) 2.02 Hz, 2 H), 7.68 (d, J ) 2.02 Hz,
2 H), 7.54-7.61 (m, 10 H), 7.37-7.44 (m, 10 H), 7.17 (d, J ) 2.02
Hz, 2 H), 7.00 (d, J ) 2.02 Hz, 2 H), 2.16 (s, 6 H), 2.09 (s, 6 H),
9
11912 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Diphenylcarbenes Protected by o-Aryl Groups
A R T I C L E S
measured on a JEOL JES TE 200 spectrometer (X-band microwave
unit, 100 kHz field modulation). The signal positions were read by the
use of a gaussmeter. The temperature was controlled by a 9650
Microprocessor-based Digital Temperature Indicator/Controller, which
provided accuracy of the measurements within (0.1 K and of the
control ability within (0.2 K. Errors in the measurements of component
amplitudes did not exceed 5%, and the accuracy of the resonance field
determination was within (0.5 mT.
Low-Temperature UV/Vis Spectra. Low-temperature spectra at 77
K were obtained by using an Oxford variable-temperature liquid-
nitrogen cryostat (DN 1704) equipped with a quartz outer window and
a sapphire inner window. The sample was dissolved in dry 2-MTHF,
placed in a long-necked quartz cuvette of 1-mm path length, and
degassed thoroughly by repeated freeze-degas-thaw cycles at a
pressure near 10-5 Torr. The cuvette was flame-sealed under reduced
pressure, placed in the cryostat, and cooled to 77 K. The sample was
irradiated for several minutes in the spectrometer with a Halos 300-W
high-pressure mercury lamp using a Pyrex filter, and the spectral
changes were recorded at appropriate time intervals. The spectral
changes upon thawing were also monitored by carefully controlling
the matrix temperature with an Oxford Instrument Intelligent Temper-
ature Controller (ITC 4).
Flash Photolysis. All flash measurements were made on a Unisoku
TSP-601 flash spectrometer. Three excitation light sources were used
depending on the precursor absorption bands and lifetime of the transient
species. They were (i) a cylindrical 150-W Xe flash lamp (100 J/flash
with 10-ms pulse duration), (ii) a Quanta-Ray GCR-11 Nd:YAG laser
(355 nm pulses of up to 40 mJ/pulse and 5-6-ns duration; 266 nm
pulses of up to 30 mJ/pulse and 4-5-ns duration), and (iii) Lamda
Physik LEXTRA XeCl excimer laser (308 nm pulses of up to 200 mJ/
pulse and 17-ns duration). The beam shape and size were controlled
by a focal length cylindrical lens.
A Hamamatsu 150 W xenon short arc lamp (L 2195) was used as
the probe source; the monitoring beam, guided using an optical fiber
scope, was arranged in an orientation perpendicular to the excitation
source. The probe beam was monitored with a Hamamatsu R2949
photomultiplier tube through a Hamamatsu S3701-512Q MOS linear
image sensor (512 photodiodes used). The timing of the excitation pulse,
the probe beam, and the detection system was achieved through an
Iwatsu Model DS-8631 digital synchroscope interfaced to an NEC 9801
RX2 computer. This allowed for rapid processing and storage of the
data and provided printed graphic capabilities. Each trace was also
displayed on an NEC CRT N5913U monitor.
A sample was placed in a long-necked Pyrex tube with 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 flame-
sealed under reduced pressure, and the solution was transferred to the
quartz cuvette that was placed in the sample chamber of the flash
spectrometer. A cell holder block of the sample chamber was equipped
with a thermostat and allowed to come to thermal equilibrium. The
concentration of the sample was adjusted so that it absorbed a significant
portion of the excitation light.
Computational Procedures. DFT calculations were carried out
using the GAUSSIAN 94,27 programs. Optimized geometries were
obtained at the B3LYP/6-31G(d)28 levels of theory. Vibrational
frequencies obtained at the B3LYP level of theory were scaled by 0.961
and zero-point energies (ZPE) by 0.981.45 The electronic spectra of
tetracyclic intermediates and isofluorene were calculated using time-
dependent (TD) DFT theory.30
Acknowledgment. The authors are grateful to the Ministry
of Education, Culture, Sports, Science, and Technology of Japan
for support of this work through a Grant-in-Aid for Scientific
Research for Specially Promoted Research (No. 12002007). The
support from the Mitsubishi Foundation and the Nagase Science
and Technology Foundation is also appreciated.
Supporting Information Available: Variable temperature
UV/vis spectra of 2a-d, 2b, 2c, and 2d (Figures S1-S4), time-
resolved UV/vis spectra obtained by LFP of 2a-d-N2, 2b-N2,
2c-N2, 2d-N2 and 4a (Figures S5-9), optimized geometries
1
3
of 2A, 2A, 4A, 5A, 6A, and 7A (Figures S10-S15), and
Gaussian archives entries (Table S1). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA047738R
(45) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
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