Generation, reactions, and kinetics of sterically congested triplet diphenylcarbenes. Effects of bromine groups

Article abstract of DOI:10.1021/ja010658s

A series of polybrominated diphenylcarbenes (DPCs) are generated by irradiation of the corresponding precursor diazomethanes, and their reactivities are investigated by means of low-temperature spectroscopies as well as laser flash photolysis. Triplet bis

Full text of DOI:10.1021/ja010658s

Published on Web 12/27/2001
Generation, Reactions, and Kinetics of Sterically Congested
Triplet Diphenylcarbenes. Effects of Bromine Groups
Hideo Tomioka,* Tetsuya Watanabe, Makoto Hattori, Naoki Nomura, and
Katsuyuki Hirai
Contribution from the Chemistry Department for Materials, Faculty of Engineering,
Mie UniVersity, Tsu, Mie 514-8507, Japan
Received March 12, 2001. Revised Manuscript Received October 3, 2001
Abstract: A series of polybrominated diphenylcarbenes (DPCs) are generated by irradiation of the
corresponding precursor diazomethanes, and their reactivities are investigated by means of low-temperature
spectroscopies as well as laser flash photolysis. Triplet bis(2,4,6-tribromophenyl)carbene was shown to
decay by undergoing dimerization and to have a half-life of 1 s in a degassed benzene solution at room
temperature, some 6 orders of magnitude longer-lived than the parent DPC. Anomalous effects of para
substituents on the stability of the triplet are noted. Thus, while the replacement of a 4-bromine group with
a methyl group resulted in a sharp decrease in the lifetime, introduction of a tert-butyl group resulted in a
dramatic increase in the lifetime; triplet bis(2,6-dibromo-4-tert-butylphenyl)carbene was shown to have a
half-life of 16 s in solution at room temperature. Attempts to increase the stability of these polybrominated
DPCs by buttressing effects of a m-bromine group and by the synergetic effect of bromine and methyl
groups are also described.
For a long time, carbenes were believed not to be stable
enough to be isolated in macroscopic amounts at room tem-
perature. However, the recent syntheses of stable singlet
carbenes,1,2 i.e., phosphinocarbenes and imidazol-2-ylidenes,
have upset this extreme view. These carbenes are obviously
thermodynamically stabilized by the π-donating ability of the
heteroatom substituent directly attached to the carbenic carbon.
Naturally, interpretation of these species as “free” carbene has
been a topic of debate,3 since the possible contribution of the
ylidic character as a result of pπ-pπ delocalization should not
be neglected.
The stabilization of a triplet carbene emerges as a challenging
target since triplet carbene is less susceptible to conjugative
stabilization and hence is more likely to keep the electronic
integrity as a free carbene, i.e., one centered diradical. Steric
protection (kinetic stabilization) is an ideal method for stabilizing
the triplet since a protecting group, when introduced near the
carbene center, not only blocks the carbenic center from external
reagents but also results in thermodynamic stabilization with
respect to the singlet by increasing the central angle.4
Attempts have been made along this line whereby triplet
diphenylcarbenes (DPCs) were stabilized by introducing a series
of substituents at the ortho positions.5-9 However, a high affinity
of carbenes for electrons makes it very difficult to explore the
usual protecting groups for this extremely reactive center with
only two modifiable substituents; carbenes can react even with
very poor sources of electrons, such as C-H bonds.10 For
instance, the tert-butyl group, which has been successfully
employed as one of the most effective protecting groups for
many other reactive centers in organic chemistry, was found to
be ineffective,8b,c much less effective even than methyl and
chlorine groups in protecting triplet DPCs.
In this light, it is crucial to develop a protecting group which
is sterically congesting and yet unreactive toward triplet
carbenes. Bromine atoms appear to be promising as a protecting
group toward the triplet carbene center because, while the van
der Waals radius is similar to that of methyl (Br, 185 pm; Me,
175 pm),11 the C-Br bond length (181 pm) is longer than the
C-C (Me) (177 pm),11 suggesting that the o-bromo groups must
(5) Hirai, K.; Tomioka, H. J. Am. Chem. Soc. 1999, 121, 10213.
(6) (a) Tomioka, H.; Nakajima, J.; Mizuno, H.; Sone, T.; Hirai, K. J. Am. Chem.
Soc. 1995, 117, 11355. (b) Tomioka, H.; Nakajima, J.; Mizuno, H.; Iiba,
E.; Hirai, K. Can. J. Chem. 1999, 77, 1066.
(7) Tomioka, H.; Mizuno, H.; Itakura, H.; Hirai, K. J. Chem. Soc., Chem.
Commun. 1997, 2261.
(8) (a) Tomioka, H.; Okada, H.; Watanabe, T.; Hirai, K. Angew. Chem., Int.
Ed. Engl. 1994, 33, 873. (b) Tomioka, H.; Okada, H.; Watanabe, T.; Banno,
K.; Komatsu, K.; Hirai, K. J. Am. Chem. Soc. 1997, 119, 1582. (c) Hirai,
K.; Komatsu, K.; Tomioka, H. Chem. Lett. 1994, 503. (d) Hirai, K.; Yasuda,
K.; Tomioka, H. Chem. Lett. 2000. 398.
(9) Zimmerman, H. E.; Paskovich, D. H. J. Am. Chem. Soc. 1964, 86, 2149.
(10) For reviews, see: (a) Regitz, M., Ed. Carbene (oide), Carbine; Houben-
Weyl, Thieme: Stuttgart, 1989; Vol. E19b. (b) Moss, R. A., Jones, M.,
Jr., Eds. Carbenes; Wiley: New York, 1973, 1975; Vols. I and II. (c)
Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press: New York, 1971.
(11) Rowland, R. S.; Taylor, R. J. Phys. Chem. 1996, 100, 7384.
(1) Phosphinocarbene and imidazol-2-ylidene were prepared as “bottleable”
singlet carbenes in 1988 and 1991, respectively. See: Igau, A.; Gru¨tzma-
cher, H.; Baceiredo, A.; Bertrand, G. J. Am. Chem. Soc. 1988, 110, 6463.
Aruduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,
113, 361.
(2) See for reviews: (a) Bourissou, D.; Guerret, O.; Gabba¨i, F. P.; Bertrand.
G. Chem. ReV. 2000, 100, 39. (b) Aruduengo, A. J., III. Acc. Chem. Res.
1999, 32, 913.
(3) (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.
(4) See for reviews: (a) Tomioka, H. Acc. Chem. Res. 1997, 30, 1315. (b)
Tomioka, H. In AdVances in Carbene Chemistry; Binker, U., Ed.; JAI
Press: Greenwich, CT, 1998; Vol. 2, pp 175-214.
9
474 VOL. 124, NO. 3, 2002 J. AM. CHEM. SOC.
10.1021/ja010658s CCC: $22.00 © 2002 American Chemical Society

Sterically Congested Triplet Diphenylcarbenes
A R T I C L E S
Scheme 1
overhang the reactive site more effectively. Moreover, halides
are usually reactive toward singlet carbenes to form halonium
ylides, but they are not reactive with the triplet state.10 In
addition, the value of êl, which gives a measure of the strength
of the spin-orbit interaction, is increased rather dramatically
on going from chlorine (586 cm-1) to bromine (2460 cm-1),12
suggesting that the intersystem crossing from the nascent singlet
carbene to the triplet should be accelerated by introducing a
bromine atom.
Thus, we generated polybrominated diphenylcarbenes by
photolysis of the precursor diazomethane and found that these
carbenes exhibited an enormously prolonged existence even in
solution at room temperature, most probably due to steric
shielding of the carbene center by bromine groups at the ortho
positions.13
Results and Discussion
A. Bis(2,4,6-tribromophenyl)carbene. Irradiation of bis(2,4,6-
tribromophenyl)diazomethane (1a) in 2-methyltetrahydrofuran
(2-MTHF) glass at 77 K gave a fine-structure ESR line shape
characteristic of randomly oriented triplet molecules with a large
D value attributable to one-center nπ spin-spin interaction at
a divalent carbon of diarylcarbene (Scheme 1, Figure 1A-(a)).14
The fine-structure constants were |D| ) 0.3958 cm-1, |E| )
0.0295 cm-1, showing unequivocally that the triplet signals are
due to triplet hexabromodiphenylcarbene (2a) generated by
photodissociation of 1a. The ESR signals not only were stable
at this temperature but also survived even at 130 K. However,
as the samples were warmed, the x and y lines of the spectrum
moved closer together, resulting in essentially zero E value
(Figure 1A-(b)). Cooling the sample did not reverse this change.
This is interpreted in terms of geometrical change of carbenes
often observed for sterically congested carbenes.14c,15,16 The ESR
spectra of carbenes are characterized by two parameters, D and
E. 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 carbene from axial symmetry, or, more plainly, it
describes the extent to which the molecule is bent. Therefore,
the substantial reduction in the E value upon annealing the
Figure 1. (A) ESR spectra of bis(2,4,6-tribromophenyl)carbene (2a) in
2-methyltetrahydrofuran (2-MTHF) (a) at 77 K and (b) at 110 K. (B) UV/
vis spectra obtained by irradiation of 1a in 2-MTHF: (a) spectra of 1a in
2-MTHF at 77 K; (b) same sample after 10 min irradiation (λ > 350 nm);
(c) same sample after warming to 95 K; (d) same sample after warming
the matrix to room temperature and refreezing to 77 K.
matrix indicates that the carbene relaxes to a structure with an
expanded C-C-C angle, presumably to gain relief from steric
compression. Thus, when a carbene is formed at low temper-
ature, it should have the bent geometry and the conformation
dictated by those of the precursor. Even if the thermodynami-
cally 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.
However, when the matrix is softened upon annealing, the
carbene can relax to a stabler geometry, depending on the
softness of the media.
(12) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Marcel Dekker: New York, 1993.
(13) Preliminary reports of the present study were published: (a) Tomioka, H.;
Watanabe, T.; Hirai, K.; Takui, T.; Itoh, K. J. Am. Chem. Soc. 1995, 117,
6376. (b) Tomioka, H.; Hattori, M.; Hirai, K.; Murata, S. J. Am. Chem.
Soc. 1996, 118, 8723.
(14) See for reviews of the ESR 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. (c) Tomioka, H. In
AdVances in Strained and Interesting Organic Molecules; Halton, B., Ed.;
JAI Press: Greenwich, CT, 2000; Vol. 8, pp 83-112.
It should be interesting to note here that the fine-structure
constants observed for 2a in a softer matrix, i.e., 3-methylpen-
tane (3-MP),17 were essentially the same as those observed for
the relaxed carbenes even at 77 K and that the spectrum did
not change upon annealing in this case. This suggests that the
initial geometry is perturbed by the softness of environment,
not by temperatures.
(15) (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) Gilbert, B.
C.; Griller, D.; Nazran, A. S. J. Org. Chem. 1985, 50, 4738. (d) Nazran,
A. S.; Lee, F. L. LePage, Y.; Northcott, D. J.; Park, J. M.; Griller, D. J.
Phys. Chem. 1984, 88, 5251.
(16) (a) Hu, Y.-M.; Hirai, K.; Tomioka, H. J. Phys. Chem. A 1999, 103, 9280.
(b) Hu, Y.-M.; Hirai, K.; Tomioka, H. Chem. Lett. 2000, 94.
(17) Viscosity of 3-MP, η, relative to 2-MTHF at 77 K is reported to be 1.44
× 10-6. See ref 11.
9
J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002 475

A R T I C L E S
Tomioka et al.
Optical spectroscopical monitoring in the frozen medium gave
analogous results. Irradiation (λ > 350 nm) of 1 (1 × 10-4 M)
in 2-MTHF glass at 77 K resulted in the appearance of new
absorption bands at the expense of the original absorption due
to 1. As shown in Figure 1B-(b), the new spectrum consists of
two identifiable features, two intense UV bands with maxima
at 337 and 349 nm and weak and broad bands with apparent
maxima around 480 nm. These features are usually present in
the spectra of triplet diarylcarbenes in organic matrixes at 77
K.14,18 The glassy solution did not exhibit any changes for
several hours when kept at 77 K. However, when it was allowed
to warm to room temperature and then cooled to 77 K, the
characteristic bands disappeared (Figure 1B-(d)). On the basis
of these observations coupled with ESR data (vide supra), the
absorption spectrum can be attributed to triplet carbene (32).
Monitoring of the spectral changes as a function of temper-
ature shows two interesting features. First, the absorption
maxima shifted slightly but distinctly from 337 and 349 nm to
339 and 353 nm when the matrix was warmed from 90 to 95 K
(Figure 1B-(c)). This change was not reversed when the samples
were cooled. This change should be ascribable to the geometrical
change of the triplet carbene associated with changes in viscocity
of the matrixes upon annealing, as has been revealed in ESR
experiments. It should be noted here that such a distinct shift
in a UV/vis spectrum has not previously been observed for other
relatively congested diarylcarbenes undergoing similar geo-
metrical changes. Second, while absorption bands of most triplet
diarylcarbenes disappear in the temperature range 100-105 K,
where the viscocity of the matrix changes dramatically from
107 to 103 P,19 no appreciable changes were observed for the
absorption bands ascribable to 32 in 2-MTHF up to 120 K, where
the samples were almost completely fluid. Significant decom-
position began only at 130 K (viscocity is 1.6 P),19 where the
“first-order” half-life (t1/2) was approximately 15 min. In more
viscous solvents, 2 was able to survive at much higher
temperatures: 190 K in dibutylphthalate (t1/2 ) 37 min) and
210 K in triacetine (t1/2 ) 33 min).
Figure 2. Absorption spectrum of the transient products formed during
the irradiation of 1a in degassed benzene solution at 20 °C, recorded 50 µs
after excitation. Inset (a) shows an oscillogram trace monitored at 355 nm.
Inset (b) shows oscillogram traces monitored at 360 and 410 nm during
the irradiation in a non-degassed benzene solution.
generate the corresponding diaryl ketone oxides, which are easily
observed directly either by matrix isolation or by flash photolysis
and show a broad absorption band centered at 396-450 nm.
Thus, the observations can be interpreted as indicating that 32a
is trapped by oxygen to form the carbonyl oxide (3), which
confirms that the transient absorption quenched by oxygen is
3
due to 2a.
The apparent built-up rate constant, kobs, of the carbonyl oxide
3
(3) is essentially identical with that of the decay of 2a (inset
(b) in Figure 2), and kobs is expressed as given in eq 1, where
kobs ) k0 + kO [O2]
(1)
2
k0 represents the rate of decay of 32a in the absence of oxygen
Laser flash photolysis (LFP) of 1a (1 × 10-4 M) in a degassed
benzene solution at room temperature with a 10 ns, 70-90 mJ,
308 nm pulse from a XeCl excimer laser produced a transient
species showing a strong absorption at 355 nm and a weak
absorption extending from 420 to 500 nm, which appeared
coincident with the pulse (Figure 2). The transient signals
disappeared rather gradually, persisting for at least 30 s under
those conditions. On the basis of the low-temperature spectrum,
and kO is the quenching rate constant of 32a by oxygen. A plot
2
of the observed pseudo-first-order rate constant of the formation
of the oxide against [O2] is linear (Figure 3). From the slope of
this plot, kO was determined to be 1.1 × 107 M-1 s-1, which
2
is approximately 2 orders of magnitude smaller than that
observed with the “parent” 3DPC (kO ) 5.0 × 109 M-1 s-1).20b
2
When a degassed benzene solution of 1a containing 1,4-
cyclohexadiene (CHD) was excited, a new species was formed,
showing an absorption with λmax ) 359 nm, formed as the 355
nm signal of 32a decayed. The decay of 32a was again found to
be kinetically correlated with the growth of the new species.
Thus, this new signal was attributable to the hexabromodiphe-
nylmethyl radical (5) formed as a result of H abstraction of 32a
from the diene, since it is now well-documented21,22 that triplet
arylcarbenes, generated in good hydrogen donor solvents,
undergo H abstraction leading to the corresponding radicals
which show transient absorptions at longer wavelengths than
those of the precursor carbenes. The excellent hydrogen donor
properties of CHD have been well-recognized.23 A plot of the
observed pseudo-first-order rate constant of the formation of
3
we assign the transient product to 2a. The inset (a) in Figure
3
2 shows the decay of 2a in the absence of trapping reagents,
which is found to be second order (2k/ꢀl ) 8.9 s-1). The rough
lifetime of 32a is estimated in the form of half-life, t1/2, to be 1
( 0.1 s.
Support is lent to this assignment by trapping experiments
using oxygen. When LFP was carried out on a non-degassed
3
benzene solution of 1a, the half-life of 2a decreased dramati-
cally, and a broad absorption band with a maximum at 410 nm
appeared at the expense of the absorption due to 32a. The spent
solution was found to contain hexabromobenzophenone (4) as
the main product. It is well-documented20 that the diarylcarbenes
with triplet ground states are readily trapped by oxygen to
(20) (a) Sander, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 344. (b) Scaiano, J.
C.; McGimpsey, W. G.; Casal, H. L. J. Org. Chem. 1989, 54, 1612. (c)
Bunnelle, W. Chem. ReV. 1991, 91, 336.
(18) Trozzolo, A. M. Acc. Chem. Res. 1968, 1, 329.
(19) Ling, A. C.; Willard, J. E. J. Phys. Chem. 1968, 72, 1918.
9
476 J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002

Sterically Congested Triplet Diphenylcarbenes
A R T I C L E S
Thus, we prepared bis(4-methyl-2,6-dibromophenyl)- (1b) and
bis(4-tert-butyl-2,6-dibromophenyl)diazomethanes (1c) and in-
vestigated the reactivities of carbenes (2b,c) therefrom, which
revealed that the p-alkyl groups exhibited unexpectedly large
effects on the lifetime of triplet polybrominated diphenyl-
carbenes (32).
Anomalous effects of para substituents on the reactivities of
3
32 were shown by monitoring the UV/vis spectra of 2 at low
temperature as a function of temperature. Irradiation (λ > 350
nm) of 1c in 2-MTHF glass at 77 K resulted in the appearance
of new bands with maxima at 331 and 344 nm at the expense
of the original absorption due to 1c. The new bands exhibited
no change when kept at 77 K but disappeared upon thawing
the matrix to room temperature. On the basis of these observa-
tions coupled with EPR data (vide supra), the absorption
spectrum can be attributed to 32c. The stability of carbene was
then examined by slowly warming the matrix containing 32c (5
°C/15 min) from 77 K. No appreciable changes were observed
3
for the absorption bands due to 2c in MTHF up to 160 K.
Figure 3. Plot of the growth rate of the benzophenone oxide (3a) as a
function of oxygen concentration.
Significant decomposition began only at 170 K, where the “first-
order” half-life was approximately 48 min, which should be
the radical against [CHD] is linear, and the slope of this plot
3
compared with that observed for 2a, whose t1/2 was 40 min
3
yields the absolute rate constant for the reaction of 2a with
already at 120 K, In marked contrast, the introduction of a
methyl group at the para positions caused a significant desta-
the diene, kCHD ) 7.4 × 102 M-1 s-1, which is some 5 orders
of magnitude smaller than that observed with 3DPC (kCHD ) 1
× 107 M-1 s-1).21b
3
3
bilization in 2. Thus, in MTHF matrix, 2b decayed very fast
even below 100 K.
Thus, the half-life (t1/2) and the quenching rate constants (kO
LFP studies also indicated that the stability of 32 was
dramatically affected by substituents at the para positions. LFP
of 1c in a degassed benzene produced a transient species
showing a strong absorption at 340 nm and a weak absorption
2
and kCHD) of triplet hexabromodiphenylcarbene (32a) as opposed
to those of triplet dimesitylcarbene clearly suggest that bromine
groups are better kinetic protectors than methyl groups, as
expected.
3
extending from 380 to 400 nm, which we assigned to 2c on
B. Anomalous Effects of Para Substituents. Reactions of
carbenes are shown to be sensitive to electronic perturbation,
probably due to the fact that a small shift in energy may invert
the energy difference between the singlet and triplet states
(∆GST). Generally speaking, electron donor groups stabilize the
electrophilic singlet carbene more than they do the radical-like
triplet and lead to smaller ∆GST.24 The two phenyl rings in the
triplet diphenylcarbene do not lie in the same plane in the solid
state25 and probably not in solution due to unfavorable steric
interactions. This is most likely true for singlet diphenylcarbene
as well. Therefore, the electronic effects on the diphenylcarbene
reactivities are not straightforward. Actually, the electronic effect
on the triplet diphenylcarbene reactivities is not well-character-
ized.26,27
the basis of the UV/vis spectrum and ESR measurement at low
temperature. The transient signals due to 32c decayed very
3
slowly, much slower than that of 2a; it took more than 200 s
before all the signals disappeared completely under these
conditions. The decay was found to be second order (2k/ꢀl )
0.36 s-1). The rough lifetime of 32c is estimated in the form of
a half-life, t1/2, to be 16 s. These values clearly indicate that
32c is some 20 times longer-lived than 32a. In marked contrast,
the transient absorption due to 32b obtained by LFP of 1b under
identical conditions decayed rather rapidly within 1 s. The decay
was found to be best fit to a first-order decay (k ) 4.7 s-1) in
this case, and the lifetime is estimated to be 0.21 s (t1/2 ) 0.22
s).
The results clearly suggest that the lifetime of triplet
polybrominated DPC (32) was dramatically affected by the
remote para substituents, which are not expected to exert a
significant effect on the steric congestion around the carbenic
center. To know the origin of these puzzling effects, the
following studies were carried out.
(21) (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.
(22) For reviews, see: (a) Platz, M. S., Ed. Kinetics and Spectroscopy of
Carbenes and Biradicals; Plenum: New York, 1990. (b) Tackson, J. E.;
Platz, M. S. In AdVances in Carbene Chemistry; Brinker, U., Ed.; JAI
Press: Greenwich, CT, 1994; Vol. 1, pp 87-160.
(23) Encinas, M. V.; Scaiano, J. C. J. Am. Chem. Soc. 1981, 103, 6393.
(24) See for review: Schuster, G. B. AdV. Phys. Org. Chem. 1986, 22, 311.
(25) Hutchinson, C. A., Jr.; Kohler, B. E. J. Chem. Phys. 1969, 51, 3327.
(26) Hadel, L. M.; Maloney, V. M.; Platz, M. S.; McGimpsey, W. G.; Scaiano,
J. C. J. Phys. Chem. 1986, 90, 2488.
3
ESR spectra of 2a-c were obtained in 2-MTHF glasses at
77 K by subjecting a frozen solution of 1 to a short period of
photolysis, and the spectra were analyzed in terms of D and E
values, which are reported in Table 1. Inspection of the data in
the table clearly indicates that there are no significant change
in E/D values as one changes the para substituents. These
observations indicate that the para substituents exhibit little effect
3
on the geometries of 2, as expected. PM3 calculations28 also
(27) See for review: 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.
(28) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209, 221. MOPAC version
6.01 (JCPE No. 9044) was used.
9
J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002 477

A R T I C L E S
Tomioka et al.
Table 1. D and E Values for Triplet
Bis(2,6-dibromophenyl)carbenes (2) in 2-Methyltetrahydrofuran
(2-MTHF) at 77 Ka
Scheme 2
-1
carbene
D/cm
E/cm-1
E/D
2a
0.396
(0.368)
0.396
(-)
0.397
(0.442)
0.484
(0.600)
0.0295
(0.00026)
0.0275
(-)
0.0745
(0.0007)
0.0695
(-)
2b
2c
2d
0.0311
(∼0)
0.0784
(∼0)
at the other positions. The most probable reactive sites are the
aromatic rings, where spin can be delocalized.
0.0151
(0.0119)
0.0311
(0.020)
In this light, it is important to note that trityl radicals are
known to undergo either methyl-para or para-para coupling,
depending on the substitution patterns.30 Thus, it is likely that
the brominated diphenylcarebenes (2) also undergo similar
coupling. The coupling reactions of trityl radicals are not
suppressed by “reactive” substituents. The anomalous reactivity
of halogen at the para position is noted. For instance, (p-
bromophenyl)diphenylmethyl undergoes methyl-para cou-
pling.31 On the other hand, tri(p-tolyl)methyl undergoes rapid
disproportionation to yield tri(p-tolyl)methane and a quinoid
compound which rapidly polymerizes.32 However, the coupling
at the para positions is retarded by a tert-butyl group at this
position.33
a The values in parentheses refer to those observed in 3-methylpentane
(3-MP) at 77 K. No ESR signals ascribable to 2b were observed in 3-MP
at 77 K.
Table 2. UV/Vis Spectra and Kinetic Data for Triplet
Bis(2,6-dibromophenyl)Carbenes (2)
λmax
(nm)
2k/ꢀl
(s-1
t1/2
(s)
kO
kCHD
(M-1 s-1
)
2
carbene
)
(M-1 s-1
)
2a
2b
2c
2d
339, 353
335, 350
335, 349
343, 357
8.9
1.0
0.2
16
16
1.1 × 107
2.3 × 107
2.1 × 107
4.6 × 106
7.4 × 102
3.2 × 102
5.3 × 102
2.5 × 102
(4.7)
0.35
0.35
a UV/vis spectra were obtained in 2-MTHF matrix at 77 K. b All
measurements were done in benzene at ca. 20 °C.
Thus, rather large stabilizing effects of tert-butyl groups at
3
the para positions on the lifetime of 2 compared to those of
bromo and methyl groups are compatible with the effect of
substituents observed in the coupling reaction of trityl radicals.
The complexity of the products observed in the reaction of 2
indicate that both the central angle (R) and the dihedral angle
(θ) of the two aryl rings are essentially the same between the
three carbenes (32a-c).
3
may be partly due to the complexity associated with the coupling
reactions. The R-para coupling of triplet DPCs, for instance,
unlike that of trityls, does not lead to a final stable product and
gives rise to an intermediate open-shell molecule whose
subsequent reactions will be complicated by possible ambient
reactivity.
C. Effects of Meta Substituents. It is well-known that, in
1,2-disubstituted benzene derivatives, introduction of substitu-
ents in the 3-position exerts a very large effect on the rate of
appropriate reactions, which are considered in light of the
importance of bond bending: the 3-substituents “buttress” the
2-substituents.34 One would assume then that protection of the
carbenic center by ortho substituents will be greatly strengthened
by introduction of another group at the 3-position and that the
lifetime of such carbenes must be prolonged.35
The reactivities of 2 toward typical triplet quenchers, i.e.,
oxygen and 1,4-cyclohexadiene (CHD), were then investigated
3
by using LFP, and the rate constant (kO ) for quenching 2 by
2
oxygen and the rate constant (kCHD) for H abstraction of 32 from
CHD are reported in Table 2. Inspection of the data summarized
in the table suggests that the reactivities of 32 toward those triplet
quenchers are again not significantly affected by the para
substituents.
These observations clearly support a naive idea based on
intuition that the para substituents exert almost no steric effects
3
around the remote carbenic center of 2, at least in terms of
steric congestion. Electronic effects,29 which usually affect the
singlet state energy more effectively than the triplet,24 on the
other hand, seem also not so important as to change the reactivity
3
of 2 so significantly.
This has been shown in the reaction of polymethylated
diphenylcarbenes;8 the lifetime of triplet states increased on
going from dimesityl- to didurylcarbenes.
What is the origin of the marked para effect on the stability
of 32 in benzene, then? Benzene is known to be one of the least
reactive solvents toward triplet carbenes. Thus, most of the
persistent triplet diphenylcarbenes undergo dimerization in
benzene to form tetra(aryl)ethylenes as almost exclusive isolable
product.4 Carbene 32a also underwent dimerization, as evidenced
by the second-order decay kinetics in benzene. However, product
analysis of the spent solution showed that carbenic dimer was
formed in surprisingly smaller amount (vide infra) as opposed
to the yield of carbene dimer from other sterically congested
diphenylcarbenes, e.g., dimesitylcarbene.8,13 This suggests that
simple dimerization of 32 at the carbene center must suffer from
severe steric repulsion, and hence the carbene is forced to react
Bis(2,3,6-tribromo-4-tert-butylphenyl)diazomethane (1d) again
generated carbene 2d (Scheme 2) as a result of facile dissocia-
tion of N2 upon photoexcitation not only in solution at ambient
temperature but also in matrix at low temperature, which enabled
3
us to observe and investigate triplet carbene 2d in detail. The
results are somewhat contrary to that expected, however. Thus,
(30) Reviews of triarylmethyls: (a) Sholle, V. D.; Rozantsev, E. G. Russ. Chem.
ReV. 1973, 42, 1011. (b) McBride, J. M. Tetrahedron 1974, 30, 2009.
(31) Bowden, S. T.; Watkins, T. F. J. Chem. Soc. 1940, 1249.
(32) Marvel, C. S.; Rieger, W. H.; Mueller, M. B. J. Am. Chem. Soc. 1939, 61,
2769, Marvel, C. S.; Mueller, M. B.; Himel, C. M.; Kaplan, J. F. J. Am.
Chem. Soc. 1939, 61, 2771.
(33) Neumann, W. P.; Uzick, W.; Zarkadis, A. K. J. Am. Chem. Soc. 1986,
108, 3762. Rajca, A.; Utamupanya, S. J. Org. Chem. 1992, 113, 2552.
(34) Westheimer, F. H. In Steric Effects in Organic Chemistry; Newman, M.
S., Ed.; Wiley: New York, 1956; pp 523-555.
(35) For buttressing effect in carbene chemistry, see: Tomioka, H.; Kimoto,
K.; Murata, H.; Izawa, Y. J. Chem. Soc., Perkin Trans. 1 1991, 471.
+
11
(29) Hammett-type polar substituent constants σp(σp
)
for Br, Me, and t-Bu
groups are 0.23 (0.15), -0.17 (-0.31), and -0.11 (-0.26), respectively,
and spin-delocalization substituent constants σc• devoid of polar effects for
the three groups are 0.13, 0.11, and 0.13, respectively. (a) Creary, X. J.
Org. Chem. 1980, 45, 280. Creary, X.; Mehrsheik-Mohammadi, M. E.;
McDonald, S. J. Org. Chem. 1987, 52, 3254.
9
478 J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002

Sterically Congested Triplet Diphenylcarbenes
A R T I C L E S
irradiation of 1d in 2-MTHF matrix at 77 K produced a UV/
vis absorption band similar to that observed for 32c. Monitoring
of the spectral change as a function of temperature indicated
ring in 32d is distorted, probably due to steric repulsion between
the two adjacent bromine groups at the 2 and 3 positions, and
that the two bromine groups are tilted, one up, one down, with
respect to the benzene plane by ca. 2°. Therefore, the buttressed
2-bromine group is forced to move away from the carbenic
center and hence cannot block it from external reagents more
effectively than the unbuttressed 6-bromine group.
3
that the absorption ascribable to 2d was also persistent up to
170 K, where the half-life of 32d was essentially identical with
3
that of 2c.
Kinetic measurements in the solution phase made by using
LFP provided more quantitative data. LFP of 1d in a degassed
benzene produced transient absorption ascribable to 32d, which
also decayed very slowly in second order. The rate constant
was, however, essentially the same as that of 32c under the same
condition. Quenching rate constants of 32d by oxygen and CHD
were determined and are reported in Table 2, which also indicate
that the reactivity of 32d is not significantly changed compared
It may be intriguing to examine the difference in buttressing
effects between polymethylated and polybrominated diphenyl-
carbenes. The Taft steric substituent constants (Es) of bromine
and methyl groups are the same.36 The difference, then, can be
ascribable to the difference in other more specific natures
between methyl and bromine groups. Bromine groups are
spherically symmetrical and therefore interact with each other
with little directional factor. Therefore, a benzene ring is forced
to be distorted when two bromine groups are introduced at
adjacent position. On the other hand, in the case of methyl
groups, a hydrogen atom, bound to carbon, is only conically
symmetrical, and the interaction potential of this hydrogen atom
with another atom will depend on the particular point on the
carbon-hydrogen bond to which the other atom approaches.
Therefore, the two methyl groups at adjacent position can be
accepted on the benzene ring without causing severe distortion
by simply rotating each other to minimize the interaction
potential. This will result in the restraint of free rotation of the
o-methyl group and bring the methyl group closer and tighter
around the carbenic center. Electronic repulsion between the
two adjacent substituents should also be more significant for
electronegative bromine groups than for neutral methyl groups.
D. Possible Synergetic Effects of Bromine and Methyl
Groups. The results show that bromine and methyl groups can
both serve as effective kinetic protectors toward triplet carbene,
but they seem to shield the center in different fashion, as
expected from the differences in shape and size. In the case of
dimesitylcarbene, for instance, the directional factor is an
important one, and hence van der Waals potential functions for
the interaction between methyl groups at the ortho positions
are not simple. They can rotate each other harmonically to
diminish the interaction potential between the hydrogens. Such
a directional factor is less important in polybrominated diphen-
ylcarbenes, as we saw already in the previous section. On the
other hand, the electronic properties of the two groups are
different.29 From these considerations, it is very intriguing to
generate 3DPC protected by the two substituents and investigate
reactivities in order to examine possible synergetic effects of
3
to that of 2c.
The observations suggest that the buttressing effect, which
exerted a signficant effect on the reactivity of triplet poly-
methylated diphenylcarbenes,8 shows little influence on the
reactivity of triplet polybrominated diphenylcarbenes. To get
some insight into the origin of this rather unexpected result,
structural information was collected by means of ESR measure-
ments and theoretical calculations. Zero-field splitting param-
eters observed for 32d in 2-MTHF at 77 K were similar to those
3
obtained for 2c under the same conditions. Since 2-MTHF is
known to be a hard matrix, it is evident that these data do not
reflect the thermodynamically stable geometry of the carbene
(vide supra).15,16 In a soft matrix, i.e., 3-MP,12 where carbenes
can relax to a more stable geometry even at 77 K, the D and E
values are distinctly different between 32c and 32d. Thus, the E
value of 32c was estimated to be essentially zero, while in 32d,
E was still significant (Table 1). This is exactly the reverse of
what was observed with polymethylated DPCs,8 where E values
decrease as more methyl groups are introduced. It may be that
the m-bromo substituents intrinsically bias the electronic
structure of carbene 2d in a manner that destroys the near-axial
symmetry of carbene 2c. Thus, one may not deduce the bond
angle difference from consideration of the E values as in the
case of polymethylated DPCs.
Semiempirical calculations provided intriguing results. The
optimized geometries calculated by PM3/UHF suggested that
32c has almost linear (R ) 179.82°) and perpendicular (θ )
3
89.9°) structure. Similar optimization of 2d resulted in three
main conformers, i.e., I (R ) 167.9°, θ ) 75.7°), II (R ) 170.0°,
θ ) 80.8°), and III (R ) 179.4°, θ ) 89.7°), that are very close
in energy ((0.04 kcal/mol). Although two conformers (32d, I
3
the two kinetic protectors on the stability of DPC. Above all
3
and 2d, II) have a definitely smaller bond angle (R) than in
things, almost nothing is known concerning the reactivity of
sterically congested DPCs bearing unsymmetrical kinetic pro-
tectors.
32c, the third one (32d, III) has essentially the same R and θ
3
values as for 2c. However, the distances from the carbene
carbon to the o-bromo groups are notably different between the
Irradiation of (2,6-dibromo-4-tert-butylphenyl)(2,6-dimethyl-
4-tert-butylphenyl)diazomethane (1e) in 2-MTHF glasses at 77
3
two carbene systems. This distance in 2c is 3.057 Å. On the
other hand, the distances to the two bromine groups are
marginally different in all three conformers of 32d; the distances
to the 2-bromine group, which should be buttressed by
3-bromine, in I, II, and III are 3.236, 3.228, and 3.200 Å,
respectively, while those to 2-bromine are 2.996, 3.005, and
3.054 Å. This means that 2-bromine groups, which are expected
to get closer to the carbenic center than 6-bromine, are actually
moving in the opposite direction. Therefore, the carbenic center
in 32d is considered to be not protected as tightly as expected.
A closer inspection of the geometry revealed that the benzene
3
K gave typical ESR signals ascribable to triplet carbene 2e
(Scheme 3). The signals were analyzed in terms of D and E
values and are reported in Table 3, which also includes the
3
values of 2c and bis(2,4-dimethyl-4-tert-butylphenyl)carbene
(2f)16 for comparison purpose. It is interesting to note that the
values of 32e are not intermediate between those for 32c and 2f
but are very much like those of 32c, indicating that the geometry
(36) Taft, R. W., Jr. In Steric Effects in Organic Chemistry; Newman, M. S.,
Ed.; Wiley: New York, 1956; pp 556-695.
9
J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002 479

A R T I C L E S
Tomioka et al.
Scheme 3
Scheme 4
Table 3. ESRa and Kineticsb Data for Triplet Diphenylcarbenes (2)
at 370 nm was formed as the carbene absorption at 340 nm
decayed. Similar secondary products are observed in LFP of
dimesitylcarbene8 and are assigned to o-quinodimethane, pre-
sumably formed as a result of intramolecular 1,4-H shift from
the o-methyl to the carbene center. The growth rate of
o-quinodimethane, which is equal to the intramolecular H
carbene
D/cm-1
E/cm-1
E/D
2k/ꢀl (s-1
)
k (s-1 c
)
t1/2 (s)
i
2cd
2e
0.397
0.423
0.373
0.031
0.0324
0.0134
0.0784
0.0765
0.0381
0.35
2.4
5.8
16
1.8
0.5
0.58
0.62
2fe
a ESR data were obtained in 2-MTHF at 77 K. b All measurements were
done in degassed benzene at ca. 20 °C. c Intramolecular H migration rate
constant. d The data in Tables 1 and 2 are included for comparison purpose.
e Reference 16.
3
abstraction rate (ki) of 2e, was estimated to be 0.58 s-1. This
3
value is also comparable to that observed with 2f (ki ) 0.62
s-1).37 Thus, 32e is shown to decay, at least in part, by
abstracting hydrogen from the o-methyl group, while the main
decay pathway for 32c is dimerization. This intramolecular
3
reaction channel available for 2e explains the difference in
lifetimes between the two carbenes. Product analysis results
(vide infra) also supported this idea.
The results suggest that DPC protected by unsymmetrical
groups of different nature undergoes little extra stabilization as
a result of possible synergetic effects of the two groups but
exhibits a hybrid character depending on the conditions; the
configurations and stabilities of 32e in a low-temperature matrix
are dominantly affected by the bromine substituents, while the
kinetics in solution phase are controlled by the methyl substit-
uents. This can be explained in terms of the dependence of
quenching processes possible for 32e on the conditions. Presum-
ably, at low temperatures, the main quenching pathway for 32e
is dimerization to form the carbene dimer. The carbene center
3
in 2e is well-protected by the two o-bromine groups and the
Figure 4. Absorption spectrum of the transient products formed during
the irradiation of 1e in degassed benzene solution at 20 °C, recorded 50 µs
after excitation. The inset shows oscillogram traces monitored at 340 and
360 nm.
two o-methyl groups. The methyl groups in 32e are fixed more
tightly than those in 32f, as the substituents on the opposite side
are spherically symmetrical bromine groups, and hence 2e
3
3
becomes thermally more stable than 2f. On the other hand,
3
of 2e is dominantly influenced by the bromophenyl side of
when the temperature is raised, intramolecular H abstraction,
which usually requires higher activation energy,9 becomes also
allowed and starts to compete with the dimerization. Thus, the
reactivity of 2e is affected by the reactive methyl groups at
higher temperature.
E. Product Analysis Studies. The reaction patterns observed
for polybrominated diphenylcarbenes (2) were analogous to
those observed with other sterically congested diphenylcarbenes.
These carbenes did not react with the diazo precursors to form
ketazines in a relatively inert solvent, e.g., benzene. Instead they
underwent dimerization. However, the structure and yield of
the dimer were notably different from those observed for other
persistent 3DPCs, i.e., hexachlorodiphenylcarbenes9 and di-
mesitylcarbenes,8a,b,9 which produced tetra(aryl)ethylenes in
fairly good yield (∼80%). Thus, generation of 2a-c in a
degassed benzene solution by irradiating 1a-c resulted in the
formation of a tarry matter, from which a relatively small
the two groups. Monitoring UV/vis spectral changes as a
function of temperature showed that absorption bands ascribable
to 32e started to disappear significantly only at 170 K in
3
3
2-MTHF. Thus, the thermal stability of 2e is comparable to
that of 32c but is much higher than that of 32f,16 which
decomposed significantly already at 130 K.
Despite the notable similarity of the properties of 32e to those
of 32c in low-temperature matrices, the behavior of 32e in
solution at room temperature was found to be more or less
3
comparable to that of 2f (Table 3). LFP of 1e in a degassed
benzene generated transient absorption due to 32e which decayed
3
within 10 s, considerably faster than 2c (Figure 4). A rough
lifetime was estimated in the form of a half-life, t1/2, to be 1.8
3
s. This is 1 order of magnitude smaller than that for 2c and
3
comparable to that for 2f (t1/2 ) 0.5 s).37 The decay kinetics
of 32e indicated that a new species with an absorption maximum
1
amount of dimeric product was isolated. H NMR analysis of
the dimer exhibited the presence of four aromatic protons
(37) Tomioka, H.; Yingmo, Y.-H.; Ishikawa, Y.; Hirai, K. Bull. Chem. Soc.
Jpn. 2001, 74, 2207.
9
480 J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002

Sterically Congested Triplet Diphenylcarbenes
A R T I C L E S
Scheme 5
Scheme 6
notably shifted downfield in addition to the four protons in the
“normal” region. MS analysis showed no peak corresponding
to the M+ ion calculated for simple tetra(aryl)ethylene (7), but
the peak corresponding to the fragment formed as a result of
loss of two bromine units from 7. The structure of the product
which explains the data best is phenanthrene derivatives (9)
(Scheme 5). A presumable possibility is that triplet carbenes
(32) undergo dimerization to form tetra(aryl)ethylenes (7), which
subsequently cyclize to lead to phenanthrene (9), since the
photocyclization of stilbene derivatives to give the corresponding
phenanthrenes has been well-documented.38 However, all at-
tempts to detect or isolate 7 were unsuccessful. It may be that
7 is very sensitive under these conditions and is consumed to
form 9 before it accumulates.
life over a second is realized. Taking into account the fact that
the van der Waals radius of bromine is similar to that of methyl,
this is in part due to the longer bond length of the C-Br bond
than the C-C (Me), whereby the o-bromo groups overhang the
reactive site more effectively than the o-methyl groups. More
importantly, it is likely that this difference is mostly due to the
difference in reactivities toward triplet carbene between the two
groups.
One may wonder whether a through-space interaction between
the triplet carbene center and the o-bromine groups may play a
role in stabilizing the triplet carbene. ESR data clearly indicate,
however, that there is no such interaction, at least in a rigid
matrix at a low temperature. PM3 calculations also support this
idea. A possible role of the heavy atom effect in stabilizing the
triplet state is not clear, as the present techniques do not allow
us to observe the singlet counterpart and hence the intersystem
crossing process. The kinetic studies using a faster kinetic
window will be required to clarify these effects.
Rather anomalous effects of para substituents are noted on
the reactivities of polybrominated triplet DPCs to such an extent
that have not been noticed in the reactions of any other
analogous sterically congested triplet DPCs. This means that
the carbenic center in polybrominated DPCs is so effectively
shielded by the o-bromine groups that the aromatic positions
become significant reactive sites to decay. Triplet carbene
surviving more than a minute is realized for the first time by
quenching this leaking process.
Irradiation of 1e under identical conditions gave rise to a new
product along with the phenanthrene-type dimer (9e). The new
product was easily assigned as the anthracene derivative (10)
on the basis of spectroscopic data. The formation of 10 is
interpreted in terms of intramolecular hydrogen migration in
3
triplet carbene 2e, from the o-methyl group to the carbenic
center, to generate o-quinodimethane (11), followed by subse-
quent cyclization and dehydrobromination (Scheme 6). The
formation of 10 is in accord with the spectroscopic observation
(vide supra). The structure of phenanthrene 9e suggests that
debromination is crucial for the formation of phenanthrene under
these conditions.
On the other hand, the buttressing effect is found to be
ineffective in this case, probably because repulsion between two
adjacent bromine groups on a benzene ring is so strong that the
benzene ring is distorted to such an extent that the protecting
bromine groups are forced to move away from the carbenic
center.
Summary
The present study has revealed unique properties of the
bromine group as a kinetic protector toward the triplet carbene
center. As expected, bromine groups shield the carbene center
better than methyl groups. Thus, triplet carbene having a half-
Those observations give a clue to a longer-lived triplet
carbene and a hope to isolate a stable triplet carbene in the near
future.
(38) (a) Mallory, F. B.; Mallory C. W. Org. React. 1984, 30, 1. (b) Gilbert, A.
In CRC Handbook of Organic Photochemistry and Photobiology; Horspool,
W. M., Song, P.-S., Ed.; CRC Press: Boca Roton, FL, 1994; pp 291-300.
9
J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002 481

A R T I C L E S
Tomioka et al.
1-Bromo-3,6-di-tert-butyl-8-methylanthracene (10): 16%; pale
yellow solid; mp 174.5-175.6 °C; 1H NMR (CDCl3) δ 1.44 (s, 18 H),
2.85 (s, 3 H), 7.42 (s, 1 H), 7.73 (s, 1 H), 7.81 (s, 1 H), 7.85 (s, 1 H),
8.31 (s, 1 H), 8.75 (s, 1 H); MS m/e (relative intensity) 384 (M + 2,
100), 382 (M+, 89.0), 367 (44.1), 57 (64.0); HRMS calcd for C23H27-
Br m/e 382.1296, found 382.1242.
Experimental Section
General Methods. 1H NMR spectra were recorded on a JEOL JNM-
AC300FT/NMR spectrometer in CDCl3 with Me4Si as an internal
reference. IR spectra were measured on a JASCO-Herschel FT/IR-
600H spectrometer, and UV-vis spectra were recorded on a JASCO
CT-560 spectrophotometer. The mass spectra were recorded on a JEOL
JMS-600H mass spectrometer. Gel permeation chromatography (GPC)
was carried out on a JASCO model HLC-01 instrument. The GPC
column was a Shodex H-2001. Thin-layer chromatography was carried
out on a Merck Kieselgel 60 PF254. Column chromatography was
performed on silica gel (Fuji Davidson) for column chromatography
or ICN for dry column chromatography.
Preparation of Polybrominated Diphenyldiazomethanes (1). All
diazomethanes employed in this study were prepared by treating diaryl-
(chloro)methane with ethyl carbamate in the presence of AgBF4,
followed by nitrosation of the resulting ethyl (diarylmethyl)carbamate
and treatment of the resulting ethyl N-nitroso-N-(diarylmethyl)carbamate
with t-BuOK, developed by Zimmerman and Paskovich.9 The desired
chlorides were obtained by nitration of benzophenones, followed by
reduction of the resulting 3,3′-dinitrobenzophenone to 3,3′-diamino-
benzhydrol, bromination with Br2, and dediazoniation of the poly-
bromo-3,3′-diaminobenzhydrols. The following diazomethanes were
prepared.
ESR Measurements. The diazo compound was dissolved in
2-methyltetrahydrofuran (2-MTHF, 5 × 10-4 M), and the solution was
degassed in a quartz cell by four 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 lamp using a Pyrex filter. ESR
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.
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 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).
Bis(2,4,6-tribromophenyl)diazomethane (1a): orange solid; mp
129-131 °C; 1H NMR (CDCl3) δ 7.76 (s, 4 H); IR (KBr) 2064 cm-1
.
Bis(2,6-dibromo-4-methylphenyl)diazomethane (1b): orange solid;
1
mp 115-117 °C (dec); H NMR (CDCl3) δ 2.31 (s, 6 H), 7.41 (s, 4
H); IR (KBr) 2064 cm-1
.
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,
and the detection system was achieved through an Iwatsu model DS-
8631 digital synchroscope, which was interfaced to an 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 an NEC CRT N5913U monitor.
Bis(2,6-dibromo-4-tert-butylphenyl)diazomethane (1c): orange
solid; mp 150-151 °C (dec); 1H NMR (CDCl3) δ 1.30 (s, 18 H), 7.57
(s, 4 H); IR (KBr) 2064 cm-1
.
Bis(2,3,6-tribromo-4-tert-butylphenyl)diazomethane (1d): orange
solid; mp 169-170 °C (dec); 1H NMR (CDCl3) δ 1.53 (s, 18 H), 7.67
(s, 2 H); IR (KBr) 2064 cm-1
.
(2,6-Dibromo-4-tert-butylphenyl)(2,6-dimethyl-4-tert-butylphen-
yl)diazomethane (1e): orange solid; mp 121-122 °C; 1H NMR
(CDCl3) δ 1.29 (s, 9 H), 1.30 (s, 9 H), 2.17 (s, 6 H), 7.07 (s, 2 H), 7.57
(s, 2 H); IR (KBr) 2048 cm-1
.
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 column chromatography or by preparative
TLC and identified by NMR and MS.
In this way, the following products were isolated and characterized.
1,3,6,8-Tetrabromo-9,10-bis(2,4,6-tribromophenyl)phen-
anthrene (9a): 30%; white solid; mp 334.3-334.7 °C; 1H NMR
(CDCl3) δ 7.60 (d, J ) 1.65 Hz, 2 H), 7.65 (d, J ) 2.02 Hz, 2 H), 8.15
(d, J ) 1.65 Hz, 2 H), 8.84 (d, J ) 2.02 Hz, 2 H).39
A sample was placed in a long-necked Pyrex tube which 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.
1,8-Dibromo-3,6-di-tert-butyl-9,10-bis(2,6-dibromo-4-tert-
butylphenyl)phenanthrene (9c): 63%; pale yellow solid; mp 129.1-
131.3 °C; H NMR (CDCl3) δ 1.28 (s, 18 H), 1.50 (s, 18 H), 7.28 (s,
4 H), 8.01 (d, J ) 1.84 Hz, 2 H), 8.79 (d, J ) 1.84 Hz, 2 H); HRMS
calcd for C42H44Br6 m/e 1021.8543, found 1021.8536.
1,8-Dibromo-3,6-di-tert-butyl-9,10-bis(2,6-dimethyl-4-tert-
Acknowledgment. The authors are grateful to the Ministry
of Education, Science, Sports and Culture of Japan for support
of this work through a Grant-in-Aid for Scientific Research for
Specially Promoted Research (NO. 12002007) and the Nagase
Science and Technology Foundation and Mitsubishi Foundation
for partial support.
1
butylphenyl)phenanthrene (9e): 25%; pale yellow solid; mp 196.4-
1
199.3 °C; H NMR (CDCl3) δ 1.23 (s, 18 H), 1.49 (s, 18 H), 1.68 (s,
12 H), 6.66 (s, 4 H), 7.96 (d, J ) 2.02 Hz, 2 H), 8.79 (d, J ) 2.02 Hz,
2 H); MS m/e (relative intensity) 770 (M + 4, 57.5), 768 (M + 2,
100), 766 (M+, 50.8); HRMS calcd for C46H56Br2 m/e 766.2748, found
766.2712.
Supporting Information Available: Experimental descrip-
tions for 1a-e; geometries of 32d (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(39) This compound was insoluble in most solvents and showed no volatility.
Partly due to this difficult nature, all attempts to measure high-resolution
mass spectra failed.
JA010658S
9
482 J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002