Triplet diphenylcarbenes protected by (trimethylsilyl)ethynyl groups

Article abstract of DOI:10.1021/jo049718d

(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1a) was shown to be stable enough to survive Sonogashira coupling reaction conditions at an elevated temperature and gave not only a para-monosubstituted product, (4-trimethylsilylethyny

Full text of DOI:10.1021/jo049718d

Tr ip let Dip h en ylca r ben es P r otected by (Tr im eth ylsilyl)eth yn yl
Gr ou p s
Tetsuji Itoh,† Yoshinobu J inbo,† Katsuyuki Hirai,‡ and Hideo Tomioka*,†
Chemistry Department for Materials, Faculty of Engineering, and Instrumental Analysis Facilities,
Life Science Research Center, Mie University, Tsu, Mie 514-8507, J apan
tomioka@chem.mie-u.ac.jp
Received February 17, 2004
(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1a ) was shown to be stable
enough to survive Sonogashira coupling reaction conditions at an elevated temperature and gave
not only a para-monosubstituted product, (4-trimethylsilylethynyl-2,6-dibromophenyl)(4-tert-butyl-
2,6-dimethylphenyl)diazomethane (1b), but also a disubstituted one, [2,4-bis(trimethylsilylethynyl)-
6-bromophenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1c), and a trisubstituted product,
[2,4,6-tris(trimethylsilylethynyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1d ). Triplet
diphenylcarbenes (DPCs) generated by photolysis of those ethynylated diphenyldiazomethanes were
characterized by ESR and UV-vis spectroscopies at low temperature and laser flash photolysis
techniques in solution at room temperature. Although ESR data indicated that ethynyl groups at
the ortho positions are likely to stabilize triplet DPCs both sterically and electronically more
effectively than o-bromine groups, kinetic studies suggested that the stability of triplet DPCs is
not increased by o-ethynyl groups, as opposed to o-bromine groups. It is likely that triplet DPCs
decay by interacting with the o-ethynyl groups.
The stabilization of a triplet carbene has emerged as
a challenging target1 since the recent syntheses2,3 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. Recent growing
interest in triplet carbenes as potential organic ferro-
magnets4 adds practical meaning to the project.
To isolate the triplet carbene with its electronic integ-
rity (one centered diradical) intact, steric protection is
the ideal method.1,5 The first step is then to prepare a
precursory diazo compound properly functionalized around
precarbenic diazo carbon. To this aim, a series of precur-
sor diphenyldiazomethanes bearing various substituents
at the ortho positions as kinetic protectors have been
prepared. Triplet diphenylcarbenes (DPCs) generated by
photolysis of these precursory diazomethanes are actually
fairly stable, and some of them can survive a day in
solution at room temperature.1 A species surviving a day
is very stable for a triplet carbene but still ephemeral as
a real molecule.
This approach has a serious pitfall. Because diazo
functional groups are generally labile, these groups
should be introduced at the last step of synthesis. Scheme
1 outlines the procedure employed to prepare those
sterically congested diphenyldiazomethanes.6 This indi-
cates that kinetic protectors are introduced in the main
framework before diazo groups are constructed. This
automatically requires that the kinetic protectors should
not be large enough to prevent a subsequent chemical
procedure for constructing the diazo group. In other
words, the carbene generated from such a diazo precursor
should also have a space to be quenched by accepting a
reagent externally. This synthetic dilemma needs to be
overcome so that a stable triplet carbene can be realized.
We found that a precursory diphenyldiazomethane for
a persistent triplet carbene is also persistent for a diazo
compound 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 sur-
vives under Sonogashira coupling reaction7 conditions
and undergoes substitution with trimethylsilylacetylene
at the para positions leading to bis(2,6-dibromo-4-tri-
methylsilylethynylphenyl)diazomethane.8 This is obvi-
† Chemistry Department for Materials.
‡ Instrumental Analysis Facilities.
(1) See for reviews: (a) Tomioka, H. Acc. Chem. Res. 1997, 30, 315.
(b) Tomioka, H. In Advances in Carbene Chemistry; Brinker, U., Ed.;
J AI 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.
(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.
(6) Procedures to prepare sterically congested diphenyldiazomethanes
were originally invented by Zimmerman and Paskovich. See: Zim-
merman, H. E.; Paskovich, D. H. J . Am. Chem. Soc. 1964, 86, 2149.
(7) Sonogashira, K. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 3, pp
521-549.
(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.
(8) Tomioka, H.; Hattori, M.; Hirai, K.; Sato, K.; Shiomi, D.; Takui,
T.; Itoh, K. J . Am. Chem. Soc. 1998, 120, 1106.
(5) Kirmse, W. Angew. Chem., Int. Ed. 2003, 42, 2117.
10.1021/jo049718d CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/08/2004
4238
J . Org. Chem. 2004, 69, 4238-4244

Protected Triplet Diphenylcarbenes
SCHEME 1
SCHEME 2
ously due to the fact that the kinetic protectors intro-
duced to protect the carbene center from an external
reagent are also able to protect the diazo carbon.
Whether even the bromine group at the ortho position
can also be replaced by a substituent with the diazo group
intact is still questionable. If this were possible, it might
be possible to introduce a new (possibly more effective)
kinetic protector after the diazo group is introduced. To
test this idea, we have attempted exhaustive ethynylation
of polybrominated diphenyldiazomethane.
nyldiazomethanes was then spectroscopically carried out.
Irradiation (λ > 300 nm) of the monosubstituted one (1b)
in a 2-methyltetrahydrofuran (2-MTHF) glass at 77 K
gave rise to ESR spectra (Figure 1a) characteristic of
randomly oriented triplet molecules.10 The zero-field
splitting (ZFS) parameters were D ) 0.419 cm-1 and E
) 0.0317 cm-1, showing unequivocally that triplet signals
are due to triplet (4-trimethylsilylethynyl-2,6-dibromophe-
nyl)(4-tert-butyl-2,6-dimethylphenyl)carbene (32b) gener-
ated by the photodissociation of 1b.
Essentially identical ESR signals attributable to triplet
carbenes are observed in the similar photolysis of other
precursor diazomethanes (1c as well as 1d ).11 The signals
are analyzed in terms of D and E values, which are
reported in Table 1. The parameters of triplet diphenyl-
carbene (32a )9 before ethynylation are also included for
the sake of comparison.
(b ) UV-vis St u d ies in a R igid Ma t r ix a t Low
Tem p er a tu r e. Photolysis (λ > 300 nm) of 1b in 2-MTHF
glass at 77 K resulted in the appearance of new bands
at the expense of the original absorption due to 1b
(Figure 2). The new bands consist of two identifiable
features, a strong and sharp maximum at 308 and 330
nm and a broad and weak band tailing from 400 to 500
nm.10 These features are usually present in the spectrum
of triplet diarylcarbenes. The glassy solution did not
exhibit any changes for hours when kept at 77 K, but it
disappeared completely after a brief thawing to room
temperature. On the basis of these observations, coupled
with the fact that ESR signals due to triplet diarylcar-
bene are observed under identical conditions, the absorp-
tion spectrum is attributable to the triplet carbene (32b).
The thermal stability is then estimated by raising the
sample temperature in 10 K increments to the desired
temperature, allowing the sample to stand for 5 min, and
Resu lts
Rea ction of Br om in a ted Dip h en yld ia zom eth a n e
w ith Tr im eth ylsilyla cetylen e. The substrate chosen
for the attempt was (2,4,6-tribromophenyl)(4-tert-butyl-
2,6-dimethylphenyl)diazomethane (1a ),9 in which bro-
mine groups on one side of the phenyl ring are replaced
with alkyl groups in order to decrease the possible
complexity due to multisubstitution. The reaction of the
diazomethane with 2 equiv of trimethylsilylacetylene in
the presence of (PPh3)2PdCl2/CuI in NEt3 at room tem-
perature gave a para-monosubstituted product, (4-tri-
methylsilylethynyl-2,6-dibromophenyl)(4-tert-butyl-2,6-
dimethylphenyl)diazomethane (1b), in 90% yield, along
with a small amount of a disubstituted one, [2,4-bis-
(trimethylsilylethynyl)-6-bromophenyl](4-tert-butyl-2,6-
dimethylphenyl)diazomethane (1c), no trisubstituted
product being isolated. When 1b was treated with 7 equiv
of the acetylene at 45 °C for 1 day, 1c was formed in 30%
yield along with a small amount of trisubstituted product,
[2,4,6-tris(trimethylsilylethynyl)phenyl](4-tert-butyl-2,6-
dimethylphenyl)diazomethane (1d ). On the other hand,
when 1c was reacted with 10 equiv of the acetylene at
45 °C for 1 day, 1d was obtained in 17% yield. Reactions
at a temperature higher than this resulted in a signifi-
cant loss of the diazo functional group.
All of the diazomethanes were purified by alumina
column chromatography at -10 °C followed by repeated
chromatography on a gel permeation column. They were
rather stable for substituted diazomethanes and could
be stored in a refrigerator for several months without any
appreciable decomposition. The structure of these diazo
1
compounds was characterized by H and 13C NMR and
(10) See for reviews of the EPR and UV-vis spectra of triplet
carbenes: (a) Sander, W.; Bucher, G.; Wierlacher, S. Chem. Rev. 1993,
93, 1583. (b) Trozzolo, A. M.; Wasserman, E. In Carbenes; J ones, M.,
J r., Moss, R. A., Eds.; Wiley: New York, 1975; Vol. 2, pp 185-206. (c)
Tomioka, H. In Advances in Strained and Interesting Organic Mol-
ecules; Halton, B., Ed.; J AI Press: Greenwich, CT, 2000; Vol. 8, pp
83-112.
IR.
Sp ectr oscop ic Stu d ies. (a ) ESR Stu d ies in Rigid
Ma tr ix a t Low Tem p er a tu r e. The characterization of
triplet carbenes generated from these ethynylated diphe-
(11) In the ESR spectra obtained by the photolysis of 1c, the bands
not marked with X, Y, and Z could not be assigned properly, except
that at 333.6 mT attributable to fortuitous doublet species. As these
signals are only observed for 1c, in which two ortho substituents on
one side of the phenyl ring are not same, they may be signals due to
a conformer of triplet carbene 32c. However, we could not properly
analyze the signals in terms of ZFS parameters.
(9) (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.
J . Org. Chem, Vol. 69, No. 12, 2004 4239

Itoh et al.
10 ns, 10 mJ , and 308 nm pulse from a XeCl laser
produced a transient species showing a band with a
maximum at 310 and extending to 500 nm, which
appeared coincident with the pulse (Figure 3). Because
of the overlaps with the absorption maxima of the diazo
precursor 1b, the samples are not sufficiently transparent
for adequate monitoring in the 320-360 nm region.
Therefore direct comparison of the spectrum with that
3
of 2b observed in the photolysis of 1b in matrix at low
temperature was not possible. It is likely that we are
observing the tailing part of the absorption band due to
32b. This assignment was supported by trapping experi-
ments (vide infra). All absorption bands decayed in a
similar manner. The inset in Figure 3 shows the decay
of 32b in the absence of trapping reagents, which is found
to be of the second-order (2k/ꢀl ) 1.27 s-1). The rough
lifetime of 32b is estimated in the form of half-life, t1/2, to
be 1.3 ( 0.1s.
Support is lent to this assignment by trapping experi-
ments using oxygen. When LFP was carried out on a
3
nondegassed benzene solution of 1b, the half-life of 2b
decreased dramatically, and a broad absorption band
with a maximum at 420 nm appeared at the expense of
the absorption due to 32b (Figure 4). The spent solution
was found to contain the corresponding benzophenone
(4b) as the main product. It is well documented12,13 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. Thus, the observations can be
3
interpreted as indicating that 2b is trapped by oxygen
to form the carbonyl oxide (3b), which confirms that the
3
transient absorption quenched by oxygen is due to 2b.
The apparent built-up rate constant, kobs, of the car-
bonyl oxide (3b) is essentially identical with that of the
decay of 2b (inset in Figure 4), and kobs is expressed as
F IGURE 1. EPR spectra obtained after irradiation of (a) 1b,
(b) 1c, and (c) 1d in 2-MTHF at 77 K.
3
TABLE 1. D a n d E Va lu es for Tr ip let Dia r ylca r ben esa
kobs ) k0 + k [O2]
(1)
carbenes
D (cm-1
)
E (cm-1
)
E/D
O2
2a
2b
2c
2d
0.423
0.419
0.328
0.348
0.0326
0.0317
0.0093
0.011
0.0771
0.0756
0.0285
0.0316
where k0 represents the rate of decay of 32b in the
absence of oxygen and kO is the quenching rate constant
2
3
of 2b by oxygen. The plot of the observed pseudo-first-
order rate constant of the formation of the oxide against
a
Measured in 2-methyltetrahydrofuran at 77 K.
[O2] is linear (Figure 5). From the slope of this plot, kO
2
was determined to be 1.8 × 107 M-1 s-1, which is
measuring the absorption bands. The characteristic
bands due to 2b started to decay slowly at 120 K and
decayed rather sharply at 160 K. Similar irradiation of
approximately 2 orders of magnitude smaller than that
3
3
observed with the “parent” DPC (kO ) 5.0 × 109 M-1
2
s-1).13
1c and 1d also gave absorption bands ascribable to the
When a degassed benzene solution of 1b containing
1,4-cyclohexadiene (CHD) was excited, a new species was
formed, showing an absorption with λmax ) 390 nm, which
3
corresponding triplet carbenes, 32c and 2d , respectively
3
(Figures S1 and S2, Table 2). The spectrum of 2c and
32d exhibited rather sharp bands at 390 and 405 nm,
respectively, in addition to a sharp and strong band at
315 and 416 nm and a weak, broad band in the visible
region.
3
formed as the 310 nm signal of 2b decayed (Figure 6).
The decay of 32b was again found to be kinetically
correlated with the growth of the new species. Thus, this
new signal was attributable to the diphenylmethyl radi-
3
3
3
cal (5b) formed as a result of the H abstraction of 2b
Upon thawing the matrix, those bands for 2c and 2d
also disappeared irreversibly at around 160 K, indicating
similar thermal stability.
from the diene, since it is now well documented14,15 that
triplet arylcarbenes, generated in good hydrogen-donor
solvents, undergo H abstraction leading to the corre-
sponding radicals, which show transient absorptions at
To determine the stability of the present carbenes more
accurately, the lifetime is estimated at room temperature
in degassed benzene, in which we measured the lifetime
of a series of sterically congested diarylcarbenes.1
Laser flash photolysis (LFP) of 1b (1.6 × 10-4 M) in a
degassed benzene solution at room temperature with a
(12) See for review: Sander, W. Angew. Chem., Int. Ed. Engl. 1990,
29, 344. (b) Bunnelle, W. Chem. Rev. 1991, 91, 336.
(13) Scaiano, J . C.; McGimpsey, W. G.; Casal, H. L. J . Org. Chem.
1989, 54, 1612.
4240 J . Org. Chem., Vol. 69, No. 12, 2004

Protected Triplet Diphenylcarbenes
F IGURE 2. UV-vis spectral changes observed in the photolysis of 1b. (a) UV-vis spectrum of 1b in 2-MTHF at 77 K. (b) Same
sample after 5-min irradiation (λ > 300 nm). (c, d) Same sample after thawing the matrix at 130 (c) and 300 K (d).
TABLE 2. UV-Vis Sp ectr a of Tr ip let Dia r ylca r ben es
a
b
carbenes
λmax (nm)
Td (K)
2a
2b
2c
2d
327, 348
308, 330
315, 390
416, 405
160
160
160
160
a
b
Measured in 2-methyltetrahydrofuran at 77 K. Temperature
at which the UV-vis spectra of triplet diarylcarbenes disappeared.
F IGURE 4. Transient absorption spectra obtained in LFP of
1b in nondegassed benzene with a 308-nm excimer laser
recorded 10 µs after the pulse. The inset shows a plot of the
growth of the oxide (3b) monitored at 440 nm as a function of
time.
first-order rate constant of the formation of the radical
against [CHD] is linear (Figure 7), and the slope of this
plot yields the absolute rate constant for the reaction of
32b with the diene, kCHD ) 1.7 × 102 M-1 s-1, which is
some 5 orders of magnitude smaller than that observed
F IGURE 3. Transient absorption spectra obtained in LFP of
1b in degassed benzene with a 308-nm excimer laser recorded
100 µs after the pulse. Inset shows oscillogram traces moni-
tored at 310 nm.
3
with DPC (kCHD ) 1 × 107 M-1 s-1).14c
Again, similar measurements have been done for other
3
3
carbenes 2c and 2d (Figures S3-S8), and the results
are summarized in Table 3, which also includes the data
of 2a for the sake of comparison.
wavelengths longer than those of the precursor carbenes.
The excellent hydrogen-donor properties of CHD have
been well recognized.16 The plot of the observed pseudo-
3
Discu ssion
(14) For reviews see: (a) Platz, M. S. Kinetics and Spectroscopy of
Carbenes and Biradicals; Plenum: New York, 1990. (b) J ackson, J .
E.; Platz, M. S. In Advances in Carbene Chemistry; Brinker, U., Ed.;
J AI 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.
Effect of o-Eth yn yl Gr ou p on th e Str u ctu r e of
Tr ip let Dip h en ylca r b en es. The structure of triplet
(15) Hadel, L. M.; Maloney, V. M.; Platz, M. S.; McGimpsey, W. G.;
Scaiano, J . C. J . Phys. Chem. 1986, 90, 2488.
(16) Encinas, M. V.; Scaiano, J . C. J . Am. Chem. Soc. 1981, 103,
6393.
J . Org. Chem, Vol. 69, No. 12, 2004 4241

Itoh et al.
F IGURE 6. Transient absorption spectra obtained in LFP of
1b 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 of the radical (5b)
monitored at 400 nm as a function of time.
F IGURE 5. Plot of the growth rate of the oxide (3) monitored
at 440 nm for 3b (a), 3c (b), and 3d (c) as a function of the
concentration of oxygen.
SCHEME 3
carbenes is characterized by ESR ZFS parameters D and
E. The D value is related to the separation between the
unpaired electrons.10 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.
Because 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 delocal-
ized.
F IGURE 7. Plot of the growth rate of the radicals (5)
monitored at 400 nm for 5b (a), at 410 nm for 5c (b), and at
440 nm for 5d (c) as a function of the concentration of the
diene.
TABLE 3. Kin etic Da ta for Tr ip let Dia r ylca r ben esa
carbenes
k (s-1
)
τ (s)
kO (M-1 s-1
)
kCHD (M-1 s-1
)
2
2a
2b
2c
2d
(9.7)b
(0.55)c
(1.3)c
3.0
3.1 × 107
1.8 × 107
3.5 × 107
1.4 × 108
3.6 × 102
1.7 × 102
1.0 × 102
3.4 × 102
(1.27)b
0.33
The slight decrease in D values on going from 32a to
32b is interpreted as indicating that unpaired electrons
are delocalized into the ethynyl group at the para
position. A significant decrease in both D and E/D values
3.95
0.25
a
All kinetic measurements were carried out in benzene at 20
°C. In 2k/ꢀl. c Half-life.
b
3
on going from 2b to 32c then suggests that, as one more
ethynyl group is introduced at the ortho position, un-
paired electrons are more delocalized into the substitu-
ents simply because of the increasing number of the
accepting group. This may also indicate that the central
bond angle is expanded as a result of an increased ortho
interaction induced by the trimethylsilylethynyl group.
However, the D and E/D values increased slightly on
scopic features provide no evidence for significant through-
space interaction between carbene centers and o-ethynyl
substituents.
Theoretical calculations provided information on the
effect of the o-ethynyl group on the structure of triplet
diphenylcarbenes. Thus, optimized geometries calculated
for three model triplet (2-X-6-Y-phenyl)(2,6-dimethylphe-
nyl)carbene systems, where the substituents X/Y are
varied from Br/Br (2a ′) to Br/ethynyl (2c′) to ethynyl/
ethynyl (2d ′), by DFT(UB3LYP/6-31G(d)) suggest that
the angle of the carbene center (R) decreases as the
substituents X/Y are changed from Br/Br (R ) 171.5°) to
3
going from 32c to 2d , which was not expected in this
interpretation. It is interesting to note here in this
connection that significant changes are observed in UV-
3
vis spectra on going from 2c to 32d . It may be, then, that
the additional effect of the o-ethynyl group is not large
enough to affect the ESR ZFS parameters. The spectro-
4242 J . Org. Chem., Vol. 69, No. 12, 2004

Protected Triplet Diphenylcarbenes
SCHEME 4
carried out in a degassed C6D6 solution sealed in an NMR
tube and was monitored by 1H NMR (300 MHz), new
signals appeared at the initial stage of irradiation but
did not grow significantly even though the starting
compound decomposed. The product signals gradually
disappeared when the sample was allowed to stand
overnight. More interestingly, an ESR measurement of
the mixture showed the presence of paramagnetic species
even at room temperature.
It has been reported that triplet carbenes react with a
carbon-carbon triple bond quite efficiently. For instance,
triplet diphenylcarbene reacts with acetylenes in a step-
wise fashion to give a triplet diradical, which eventually
results in the formation of indene derivatives.17 Triplet
fluorenylidene reacts with phenylacetylene and other
terminal acetylenes even at 77 K to give the correspond-
ing triplet vinylcarbenes.18 A more electrophilic triplet
carbene, e.g., 4-oxo-2,3,5,6-tetrafluorocyclohexa-2,5-di-
enylidene, reacts with acetylene even at temperatures
as low as 35 K.19
Br/ethynyl (R ) 163.2°) to ethynyl/ethynyl (R ) 160.6°)
(Figures S9-S11). This indicates that the steric interac-
tion decreases as the o-bromine group is replaced with
the o-ethynyl group. The bond distance between the
aromatic ortho carbon and the o-bromine atom in 2a ′ is
1.905-1.906 Å, while that between the ortho carbon and
o-ethynyl carbon in 2d ′ is 1.423-1.424 Å, indicating that
the o-ethynyl carbon is located closer to the carbenic
carbon than the o-bromine group. However, the van der
Waals radius of Br is larger than that of C. These
differences result in the observed difference in the
geometrical parameters between 2a ′ and 2d ′. It is to be
noted here that, although the steric repulsion of the
o-trimethylsilylethynyl group with the methyl group on
the opposite side of the benzene ring may not be as severe
in comparison with that of the o-bromine groups, the
large TMS group located at the end of the ethynyl carbon
must overhang the reactive site and would, hence, be
expected to prevent external reagents from approaching
the carbene center.
Therefore, it is likely that the triplet carbene center
3
3
in 2c and 2d can be trapped by the ethynyl bond at the
ortho position to generate a triplet diradical. Probably,
the diradical cannot give a stable final product in a simple
step, unlike in the case of intermolecular reaction, and
hence leads to a complex reaction mixture. 2-(2-Ethy-
nylphenyl)phenylcarbene undergoes intramolecular ad-
dition to the ethynyl group to give a cyclopropene
derivative.20 In the present case, an indenylidene in its
triplet state may be formed as the cyclopropene from 2c
and 2d would be highly strained. However, it was not
possible to obtain evidence for the indene substructure
as NMR signals of the product mixtures were very broad
and weak.
Effect of o-Eth yn yl Gr ou p s on th e Sta bility of
Tr ip let Dip h en ylca r ben es. Inspection of the data in
Tables 2 and 3 suggests that replacement of the o-
bromine group with the TMS ethynyl group would not
result in a significant change in the stability of triplet
DPCs. Thus, neither the thermal stability in the matrix
at low temperature (Td in Table 2) nor the half-life and/
or lifetime in solution at room temperature (τ or t1/2 in
Table 3) show a particular trend as the bromine sub-
stituent is successively replaced with ethynyl groups. It
is important to note here that the decay mode of the
triplet carbene changed from second order to first order
once an ethynyl group was introduced at the ortho
position.
As it is clear that there is a significant difference in
the decay pathways between DPC having o-bromine
groups and that having o-ethynyl groups, it is more useful
to compare the reactivity toward typical triplet quench-
ers, i.e., oxygen and CHD, to examine the role of a kinetic
protector on the reactivity of triplet DPC. Inspection of
the data in Table 3 again indicated that there are no clear
trends in the quenching rate constant to evaluate the
effect of the ethynyl group on the reactivity of triplet
DPC, as opposed to the bromine group.
Con clu sion
The present investigation revealed that sterically
congested diphenyldiazomethanes, precursors for persis-
tent triplet carbenes, are also fairly persistent and can
be further modified with the diazo group intact. Although,
in the present study, a new kinetic protector introduced
by using this method is reactive with the triplet carbene
center and hence cannot effectively protect the carbenic
center, it will be basically possible to introduce a bulkier
This suggests that carbenes 32c and 32d most likely
decayed by reacting with the o-ethynyl group. Product
analysis study supports the suggestion. Most persistent
triplet DPCs decay by undergoing dimerization at the
carbene center to give tetra(aryl)ethene.1 Photolysis of
1b (having no o-ethynyl groups) in degassed benzene also
gave dimer, tetraarylethene (7), along with anthracene
(8) (Scheme 4). The latter product is known to be
obviously derived from o-quinodimethane derivative (9).9c,d
These observations indicate that the decay pattern is not
affected by a change in the substituent at the para
position.
(17) Hendrickson, M. E.; Baron, W. J .; J ones, M., J r. J . Am. Chem.
Soc. 1971, 93, 1554.
(18) Lee, M. S.; J ackson, J . E. Res. Chem. Intermed. 1994, 20, 223.
(19) Kotting, C.; Sander, W.; Senzlober, M. Chem. Eur. J . 1998, 4,
2360.
In marked contrast, similar irradiation of 1c and 1d
(having ethynyl gropus at ortho positions) resulted in the
formation of a tarry matter, from which no particular
products could be isolated. When the photolysis was
(20) Mykytka, J . P.; J ones, W. M. J . Am. Chem. Soc. 1975, 97, 6393.
(21) Prisinzano, T.; Law, H.; Dukat, M.; Slassi, A.; MaClean, N.;
Demchyshyn, L.; Glennon, R. A. Bioorg. Med. Chem. 2001, 9, 613.
(22) Farrar, J . M.; Sienkowska, M.; Kaszynski, P. Synth. Commun.
2000, 30, 4039.
J . Org. Chem, Vol. 69, No. 12, 2004 4243

Itoh et al.
ments 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 determi-
nation was within (0.5 mT.
and less reactive protector so as to generate a more
persistent triplet carbene. A study along this line is in
progress in this laboratory.
Low -Tem p er a tu r e UV-vis Sp ectr a . Low-temperature
spectra at 77 K were obtained by using a variable-temperature
liquid-nitrogen cryostat 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 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 care-
fully controlling the matrix temperature with a temperature
controller.
F la sh P h otolysis. All flash measurements were made on
a 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 µs pulse duration), (ii) a 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) a 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 150-W xenon short arc lamp (L 2195) was used as the
probe source, and the monitoring beam, guided using an optical
fiber scope, was arranged in an orientation perpendicular to
the excitation source. The probe beam was monitored with a
photomultiplier tube through a linear image sensor (512
photodiodes used). The timing of the excitation pulse, the probe
beam, and the detection system was achieved through a digital
synchroscope interfaced to a computer. This allowed for rapid
processing and storage of the data and provided printed
graphic capabilities. Each trace was also displayed on a
monitor.
A sample was placed in a long-necked Pyrex tube with a
sidearm connected to a quartz fluorescence cuvette and de-
gassed 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. For experiments
in which the rate constant for reaction of oxygen with carbenes
was determined, varying concentration of oxygen in nitrogen
were bubbled through the solution.13
Exp er im en ta l Section
P r ep a r a t ion of {2,6-Dib r om o-4-(t r im et h ylsilyl)et h y-
n ylp h en yl}(2,6-d im eth yl-4-ter t-bu tylp h en yl)d ia zom eth -
a n e (1b). A mixture of (2,4,6-tribromophenyl)(2,6-dimethyl-
4-tert-butylphenyl)diazomethane (1a , 93 mg, 0.18 mmol),
(Ph3P)2PdCl2 (10 mg, 0.032 mmol), and CuI (5 mg, 0.03 mmol)
in anhydrous triethylamine (1 mL) was stirred at room
temperature under an Ar atmosphere for 1 h. Trimethylsilyl-
acetylene (50 µL, 0.36 mmol) was added to the mixture, and
stirring was continued for 1 day. The solution was evaporated,
and the residue was chromatographed on a silica gel column
eluted with n-hexane. The resulting crude product was purified
by repeated chromatography on a gel permeation column with
CHCl3. Diazomethane 1b was obtained as an orange viscous
liquid in 89% yield: 1H NMR (CDCl3) δ 7.66 (s, 2H), 7.07 (s,
2H), 2.14 (s, 6H), 1.30 (s, 9H), 0.25 (s, 9H); 13C NMR (CDCl3)
δ 151.2, 137.7, 136.4, 132.8, 126.2, 124,9, 124.8, 124.6, 101.8,
98.2, 63.9, 34.6, 31.5, 21.4, 0.0; IR (NaCl) ν 2066 cm-1
.
P r ep a r a tion of [2-Br om o-4,6-bis{(tr im eth ylsilyl)eth y-
n ylp h en yl}](2,6-d im eth yl-4-ter t-bu tylp h en yl)d ia zom eth -
a n e (1c). A mixture of monotrimethylsilylethynylated diazo-
methane (1b, 25 mg, 0.047 mmol), (Ph3P)2PdCl2 (3 mg, 0.009
mmol), and CuI (2 mg, 0.01 mmol) in anhydrous triethylamine
(300 µL) was stirred at room temperature under an Ar
atmosphere for 1 h. Trimethylsilylacetylene (50 µL, 0.36 mmol)
was added to the mixture, and stirring was continued at 45
°C for 1 day. The solution was evaporated and purified by
repeated chromatography on a gel permeation column with
CHCl3. Diazomethane 1c was obtained as an orange solid in
30% yield: 1H NMR (CDCl3) δ 7.62 (d, J ) 1.83 Hz, 2H), 7.56
(d, J ) 1.79 Hz, 2H), 2.15 (s, 6H), 1.31 (s, 9H), 0.24 (s, 9H),
0.14 (s, 9H); 13C NMR (CDCl3) δ 151.0, 137.9, 137.1, 136.8,
134.4, 125.8, 125.5, 123.8, 122.9, 122.5, 102.8, 102.5, 100.9,
96.9, 61.9, 34.5, 31.5, 21.3, 0.0; IR (NaCl) ν 2055 cm-1
.
P reparation of[2,4,6-Tris{(trimethylsilyl)ethynylphenyl}]-
(2,6-d im eth yl-4-ter t-bu tyl-p h en yl)d ia zom eth a n e (1d ). A
mixture of bis(trimethylsilyl)ethynylated diazomethane (1c,
12 mg, 0.038 mmol), (Ph3P)2PdCl2 (3 mg, 0.009 mmol), and
CuI (2 mg, 0.01 mmol) in anhydrous triethylamine (200 µL)
was stirred at room temperature under an Ar atmosphere for
1 h. Trimethylsilylacetylene (50 µL, 0.36 mmol) was added to
the mixture, and stirring was continued at 45 °C for 1 day.
The solution was evaporated and purified by repeated chro-
matography on a gel permeation column with CHCl3. Diazo-
methane 1d was obtained as an orange viscous liquid in 17%
yield: 1H NMR (CDCl3) δ 7.52 (s, 2H), 7.07 (s, 2H), 2.17 (s,
6H), 1.31 (s, 9H), 0.23 (s, 9H), 0.11 (s, 18H); 13C NMR (CDCl3)
δ 150.7, 138.2, 137.8, 131.2, 125.7, 125.6, 121.9, 120.8, 105.3,
101.2, 95.8, 61.1, 34.5, 31.5, 29.3, 0.0; IR (NaCl) ν 2066 cm-1
Diazomethane 1c was recovered in 33% yield.
.
Ir r a d ia tion for P r od u ct An a lysis. 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 of 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.
Ack n ow led gm en t. The authors are grateful to the
Ministry of Education, Culture, Sports, Science, and
Technology of J apan for support of this work through a
Grant-in-Aid for Scientific Research for Specially Pro-
moted Research (no. 12002007). The support from the
Mitsubishi Foundation and the Nagase Science and
Technology Foundation is also appreciated.
EP R Mea su r em en ts. The diazo compound was dissolved
in 2-methyltetrahydrofuran (10-3 M), and the solution was
degassed in a quartz cell by three freeze-degas-thaw cycles.
The sample was cooled in an optical transmission EPR cavity
at 77 K and irradiated with a 500-W Xe or Hg lamp using a
Pyrex filter. EPR spectra were measured on an ESR spec-
trometer (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 digital temperature
indicator/controller, which provided accuracy of the measure-
Su p p or tin g In for m a tion Ava ila ble: UV-vis spectra of
2c and 2d (Figures S1 and S2), time-resolved UV-vis spectra
of 2c and 2d in the absence and presence of oxygen or 1,4-
cyclohexadiene (Figures S3-S8), optimized geometries of 2a ′,
2c′, and 2d ′ (Figures S9-S11), Gaussian archives entries
(Table S1), and NMR spectra of 1a , 1b, 1c, and 1d . This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O049718D
4244 J . Org. Chem., Vol. 69, No. 12, 2004