A triplet carbene that can almost be bottled [2]

Full text of DOI:10.1021/ja991387c

J. Am. Chem. Soc. 1999, 121, 10213-10214
10213
A Triplet Carbene That Can Almost Be Bottled
Katsuyuki Hirai and Hideo Tomioka*
Chemistry Department for Materials
Faculty of Engineering, Mie UniVersity
Tsu, Mie 514-8507, Japan
ReceiVed April 27, 1999
ReVised Manuscript ReceiVed August 5, 1999
The recent synthesis of stable singlet carbenes1-3 seems to upset
the long-standing view that carbenes are not stable enough to be
isolated on a macroscopic scale at room temperature. However,
this view still holds true since those singlet carbenes are strongly
stabilized by two heteroatom substituents most likely due to ylidic
contributions.4 In addition, more importantly, triplet counterparts
have not been isolated yet. To isolate a carbene with its electronic
integrity (one centered diradical) intact, steric protection is the
ideal method. This is especially true for triplet carbene since a
protecting group, when introduced near the carbene center, not
only blocks the carbene center from external reagents but also
results in thermodynamic stabilization by increasing the carbene
angle.
Attempts to isolate triplet diphenylcarbene by introducing
various substituents at the ortho positions have been made, where
triplet carbenes with lifetimes over minutes under normal condi-
tions are realized.5,6 This is very long-lived for triplet carbenes
but would still be an ephemeral existence for “real” molecules.
The crucial point for realizing stable triplet carbenes is to
explore effective kinetic protectors which are bulky yet unreactive
toward carbenes, since carbenes have only two modifiable
substituents and are reactive enough to attack even very poor
sources of electrons such as C-H bonds.
In this respect, the trifluoromethyl (CF3) group has been
regarded as an ideal kinetic protector of carbenes since it is much
bigger than methyl and bromine groups and, more importantly,
C-F bonds are known to be almost the only type of bond
unreactive toward carbenic centers.7 However, since it is almost
impossible to prepare the precursor diphenyldiazomethane (DDM)
bearing four ortho CF3 groups, we decided to use the sterically
less demanding protectors in combination with CF3 groups. The
precursor which we were able to prepare is DDM (1) having two
ortho bromine groups in addition to two CF3 groups. The triplet
carbene (2) generated from this precursor was found to be
extremely stable; it is the first carbene that can survive more than
1 h in solution at room temperature and can also be stored in a
bottle cooled at -40 °C.
Figure 1. (a) EPR spectra obtained by irradiation of 1 in MTHF at 77
K. (b) Same sample after annealing to 110 K.
Irradiation (λ > 300 nm) of 1 in a 2-methyltetrahydrofuran
(MTHF) glass at 77 K gave a fine-structured EPR line shape
(Figure 1). This shape is 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 (2).
Inspection of the spectrum reveals that there are at least two sets
of triplet signals. When the matrix containing the carbene was
thawed gradually, another new set of triplet peaks appeared at
the expense of the original peaks. These changes were not
reversible. These changes of E/D values upon thawing matrices
have often been observed in the EPR spectrum of sterically
congested triplet diarylcarbenes and are usually interpreted in
terms of geometrical changes.5,8
More importantly, the triplet signals were found to be persistent
even at a significantly higher temperature. For instance, a
significant decay of the triplet signals began only at 210 K in
MTHF. In more viscous media, triacetin, no appreciable changes
were observed even at 230 K. Measurable decay of the signals
was observed only at 273 K (0 °C), where the “first-order” half-
life (t1/2) was approximately 10 min.
The dramatic stability of 2 was also shown by monitoring the
UV/vis spectra of 2 at a low temperature as a function of
temperature. Irradiation of 1 in a triacetin matrix at 110 K resulted
in the appearance of new bands at the expense of the original
absorption due to 1. As shown in Figure 2, the new spectrum
consists of two identifiable features, intense UV bands with
maxima at 318, 329, and 343 nm and weak broad bands with
apparent maxima at 465 and 489 nm. These features are usually
present in the spectra of triplet diarylcarbenes in organic matrices
at cryogenic temperatures.9 The absorption spectrum can be
attributed to triplet carbene (2) more unequivocally as triplet EPR
signals are observed under the identical conditions. The absorption
maxima shifted slightly but distinctly when the matrix was
warmed from 225 to 230 K. These observations are again
(1) Igau, A.; Grutzmacher, H.; Baceiredo, A.; Bertrand G. J. Am. Chem.
Soc. 1988, 110, 6463. See also: Alcaraz, G.; Wecker, U.; Baceiredo, A.;
Dahan, F.; Bertrand, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1246 and
references therein.
(2) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,
113, 361. See also: Arduengo, A. J., III; Krafczyk, R. Chem. Z. 1998, 32, 6,
and references therein.
(3) For a recent review, see: Herrmann, W. A.; Ko¨cher, C. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2162.
(4) (a) Regitz, M. Angew. Chem., Int. Ed. Engl. 1991, 30, 674. (b) Dagani,
R. Chem. Eng. News 1991, Jan 28, 19. Dagani, R. Chem. Eng. News 1994,
May 2, 20. (c) Heinemann, C.; Mu¨ller, T.; Apeloig, Y.; Schwarz, H. J. Am.
Chem. Soc. 1996, 118, 2023. (d) Boehme, C.; Frenking, G. J. Am. Chem.
Soc. 1996, 118, 2039.
(5) (a) Tomioka, H.; Hattori, M.; Hirai, K.; Murata, S. J. Am. Chem. Soc.
1996, 118, 8723. (b) See for a review: Tomioka, H. Acc. Chem. Res. 1997,
30, 315. Tomioka, H. In AdVances in Carbene Chemistry; Brinker, U., Ed.;
JAI Press: Greenwich, CT, 1998; Vol. 2, pp 175-214.
(6) (a) Tomioka, H.; Nakajima, J.; Mizuno, H.; Sone, T.; Hirai, K. J. Am.
Chem. Soc. 1995, 117, 11355. (b) Tomioka H.; Hattori, M.; Hirai, K.; Sone,
K.; Shiomi, D.; Takui, T.; Ithoh, K. J. Am. Chem. Soc. 1998, 120, 1106. (c)
Tomioka, H.; Mizuno. H.; Itakura, H.; Hirai. K. J. Chem. Soc., Chem. Commun.
1997, 2261.
(8) Wasserman, E.; Kuck, V. J.; Yager, W. A.; Hutton, R. S.; Greene, F.
D.; Abegg, V. P.; Weinshenker, N. M. J. Am. Chem. Soc. 1971, 93, 6335.
Tukada, H.; Sugawara, T.; Murata, S.; Iwamura, H. Tetrahedron Lett. 1986,
27, 235.
(9) See for reviews: (a) Trozzolo, A. M. Acc. Chem. Res. 1968, 1, 329.
(b) Trozzolo, A. M.; Wasserman, E. In Carbenes; Moss, R. A., Jones, M.,
Jr., Eds.; Wiley: New York, 1975; Vol. 2, Chapter 5. (c) Sander, W.; Bacher,
G.; Weirlacher, S. Chem. ReV. 1993, 93, 1593.
(7) See for instance: Tomioka, H.; Taketsuji, K. J. Chem. Soc., Chem.
Commun. 1997, 1745.
10.1021/ja991387c CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/25/1999

10214 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Communications to the Editor
Figure 2. UV/vis spectra obtained by irradiation of 1 in triacetin: (a)
spectrum of 1 in triacetin at 110 K; (b) same sample after 15 min
irradiation (λ > 350 nm); and (c) same sample after warming to 273 K.
The inset shows a plot of the absorbance at 345 nm as a function of
time at 0 °C.
Figure 3. UV/vis spectrum obtained by irradiation of 1 in benzene at
20 °C. Inset shows a plot of the absorbance at 345 nm as a function of
time.
interpreted in terms of the geometrical changes of 2 upon
annealing (vide supra). A significant decay of the absorption was
not observed until the matrix temperature was raised to 273 K (0
°C), where the half-life (t1/2) was estimated to be 10 min. These
observations are in excellent agreement with the EPR studies (vide
supra).
Table 1. Kinetics and ESR Data for Triplet Carbene
a
b
a
2k/ꢀla
t1/2
kO
kCHD
Dc
Ec
2
carbenes
(s-1
)
(s) (M-1 s-1
)
(M-1 s-1
)
(cm-1
)
(cm-1
)
32
1.7 × 10-3 940 8.6 × 105 1.0 × 10
0.383
0.0247
35
3.5 × 10-1
16 2.1 × 107 5.3 × 102 0.442 ∼0
To estimate the lifetime of triplet carbene 2 under our standard
conditions, i.e., in degassed benzene at 20 °C, laser flash
photolysis (LFP) of 1 was carried out. However, the lifetime of
32 was too long to be monitored by the LFP system, and a
conventional UV-vis spectroscopic method was more conve-
niently employed in this case. As shown in Figure 3, the UV-
vis spectrum obtained just after photolysis of 1 in benzene at 20
°C exhibits characteristic features of 32, i.e., an intense absorption
in the UV region along with weak, broad bands in visible regions,
which decay very slowly. The transient signals did not disappear
completely even after 1 h under these conditions. The decay was
found to be second order (2k/ꢀl ) 1.7 × 10-3 s-1). The
3
approximate half-life (t1/2) of 2 is estimated to be 16 min.
3
The reactivities of 2 toward typical triplet quenchers, i.e.,
oxygen and 1,4-cyclohexadiene (CHD), were then investigated
using LFP. Thus, LFP of 1 in a nondegassed benzene solution
resulted in a dramatic decrease in the lifetime of 32 and a
concurrent appearance of a new absorption band at 410 nm. The
rate of increase in the band at 410 nm is practically the same as
3
3
that of the peak due to 2, showing that 2 is quenched with
di(2,4-dibromo-6-tert-butylphenyl)carbene (35)5a (Table 1). It is
oxygen to form carbonyl oxide (3).10 The rate constant (kO ) for
2
clear that reactivities decrease by approximately 2 orders of
the quenching of 32 by O2 is determined to be 8.6 × 105 M-1 s-1
from a plot of the observed pseudo-first-order growth rate of 3
as a function of [O2]. Similarly, LFP of 2 in degassed benzene in
the presence of CHD generated a new signal attributable to the
3
magnitude as two o-bromine groups in 5 are replaced with two
CF3 groups. CF3 is thus shown to be an excellent kinetic protector
of triplet carbene.
3
diarylmethyl radical (4) as the signals of 2 decayed, showing
Acknowledgment. This work was supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science and Culture,
Japan and Nagase Science and Technology Foundation.
that 2 abstracts H from the diene.11 The rate constant (kCHD) of
3
H abstraction of 32 from CHD is determined to be 1.0 × 10 M-1
s-1 from a plot of the apparent build-up rate constant of the radical
vs [CHD].
JA991387C
The kinetic and ESR data for 32 are to be compared with those
observed for the longest-lived triplet carbene thus far known, i.e.,
(11) It is also well-documented that triplet diarylcarbenes generated in the
presence of good hydrogen donor undergo H abstraction leading to the
corresponding radical showing transient absorption at longer wavelength than
the corresponding carbene. For reviews see: Platz, M. S., Ed. Kinetics and
Spectroscopy of Carbenes and Biradicals; Plenum: New York, 1990. Jackson,
J. E.; Platz, M. S. In AdVances in Carbene Chemistry; Brinker, U., Ed.; JAI
Press: Greenwich, CT, 1994; Vol. 1, pp 87-160.
(10) It is well-documented that diarylcarbenes with triplet ground states
are readily trapped by oxygen to generate the corresponding diaryl ketone
oxides. For a review, see: Sander W. Angew. Chem., Int. Ed. Engl. 1990, 20,
344.