C O M M U N I C A T I O N S
3
as indicating that 5 is trapped by oxygen. The quenching rate
3
constant of the reaction of 5 by oxygen, kO , was obtained from
2
the slope of a plot of the observed rate constant of the formation
of the oxide against [O2] and was determined to be 5.5 × 104 M-1
s-1, which is about 1/15 of that observed for triplet (2,6-dibromo-
4-tert-butylphenyl)[2,6-bis(trifluoromethyl)-4-iso-propylphenyl]car-
bene (39), the most stable triplet diphenylcarbene (kO2 ) 8.6 × 105
M-1 s-1).10
On the other hand, when a degassed solution of 4 containing
1,4-cyclohexadiene (CHD) was excited, a new species, with its
3
maxima at 382 and 407 nm, was formed as the bands due to 5
decayed. The decay was again found to be kinetically correlated
with the growth of the new species. Thus, this new band was
attributable to the dianthrylmethyl radical 811 formed as a result of
H abstraction of 35 from the diene, because it is also well-
documented that triplet arylcarbenes generated in good hydrogen
donor solvents, such as the diene, undergo H abstraction, leading
to the corresponding radicals.12 The absolute rate constant for the
Figure 1. ESR spectra obtained by irradiation of 4 in a 2MTHF matrix (a)
at 110 K and (b) same sample after warming the matrix to room temperature
and refreezing to 110 K.
3
reaction of 5 with the diene, kCHD, was also obtained from the
slope of a plot of the pseudo-first-order rate constant of the
formation of the radical against [CHD] and determined to be 0.02
M-1 s-1, which is some 3 orders of magnitude smaller than that
3
observed for 9 (kCHD ) 1.0 × 10 M-1 s-1).10
A simple modification of triplet di(anthryl)carbene, thus, results
in a rather unexpectedly large increase in the stability.
Acknowledgment. The authors are grateful to the Ministry of
Education, Science, and Culture of Japan for support of this work
through a Grant-in-Aid for Scientific Research for Specially
Promoted Research (No. 12002007).
Figure 2. UV/vis spectra obtained by irradiation of 4 in benzene at room
temperature. (a) Spectra of 4 in benzene at room temperature. (b) Same
sample after irradiation (λ > 350 nm). (c) Same sample after standing at
room temperature for approximately 15 days.
Supporting Information Available: Experimental details, figures
showing decay of 35 at 75 °C, Arrhenius plot for decay of 35, trapping
of 5 by oxygen and 1,4-cyclohexadiene, and X-ray crystal structure
3
of 7 (PDF and CIF). This material is available free of charge via the
(Figure 2). Photolysis of 4 (5 × 10-5 M) resulted in the appearance
of new absorption bands (330, 360, and 452 nm), which are assigned
3
References
to the triplet carbene 5 because essentially the same bands were
3
observed when 5 was generated in 2MTHF at 77 K.
(1) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV.
2000, 100, 39. (b) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913.
(2) Kirmse, W. Angew. Chem., Int. Ed. 2003, 42, 2117.
The bands ascribable to 35 were very stable under these
conditions; the characteristic absorption bands were clearly observ-
able even after standing for 1 week at room temperature (Figure
2c). The decay was too slow to determine the exact kinetics at this
temperature. The bands did not decay at a significant rate until the
temperature was raised to 70 °C. The decay curve under these
conditions was analyzed by a combination of first- and second-
order kinetics. At 70 °C, the main decay (77%) was fitted to first-
order kinetics (k ) 1.08 × 10-2 min-1, τ ) 93 min), while a minor
one (23%) was found to be second-order kinetics (2k/ꢀl ) 4.24 ×
10-2 min-1, t1/2 ) 68 min). The Arrhenius plot of the main (first-
order) decay rate is linear, which gives the following kinetic
parameters: A ) 1.72 × 1011 s-1, ∆E ) 23.7 kcal/mol. From the
(3) See for review: Iwamura, H. AdV. Phys. Org. Chem. 1990, 26, 179.
(4) (a) Tomioka, H. Acc. Chem. Res. 1997, 30, 315. (b) Tomioka, H. In
AdVances in Carbene Chemistry; Brinker, U., Ed.; JAI Press: Greenwich,
CT, 1998; Vol. 2, pp 175-214. (c) Tomioka, H. In Carbene Chemistry;
Bertrand, G., Ed.; Fontis Media S. A.: Lausanne, 2002; pp 103-152.
(5) Tomioka, H.; Iwamoto, E.; Itakura, H.; Hirai, K. Nature 2001, 412, 626.
(6) (a) 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.
(b) Astles, D. J.; Girard, M.; Griller, D.; Kolt, J.; Wayner, D. D. J. Org.
Chem. 1988, 53, 6053. (c) Takahashi, Y.; Tomura, M.; Yoshida, K.;
Murata, S.; Tomioka, H. Angew. Chem., Int. Ed. 2000, 39, 3478.
(7) Inspection of the X-ray crystal structure of the corresponding ketone (7)
clearly indicates that the phenyl group at the C10 position is perpendicular
to the anthryl ring and hence methyl groups at the ortho positions are
hanging over the C10 position of the anthryl ring where a significant
amount of nonbonded electrons resides in 35.
plot, the decay rate at 25 °C is estimated to be 4.8 × 10-5 min-1
,
(8) Sander, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 344.
3
the lifetime of 5 at 25 °C being 14.5 days.
(9) Scaiano, J. C.; McGimpsey, W. G.; Casal, H. L. J. Org. Chem. 1989, 54,
3
The marked stability of 5 is also shown in its reaction toward
1612
3
typical triplet carbene quenchers. The decay rate of 5 increased
(10) Hirai, K.; Tomioka, H. J. Am. Chem. Soc. 1999, 121, 10213. The difference
is not so large when compared with 31b (kO2 ) 9.5 × 104 M-1 s-1 and
kCHD ) 0.047 M-1 s-1). This can be interpreted as indicating that the
origin of the increased lifetime of benzene is not mainly due to an intrinsic
decrease in the reactivity of the carbene center but to the decrease in the
decay from the C10 position.
(11) Adam, W.; Schneider, K.; Stapper, M.; Steeken, S. J. Am. Chem. Soc.
1997, 119, 3280.
(12) For reviews, see: Platz, M. S., Ed. Kinetics and Spectroscopy of Carbenes
and Biradicals; Plenum: New York, 1990.
dramatically when it was generated in the presence of oxygen, and
a new broad band with a maximum at 505 nm appeared at the
3
expense of the absorption band due to 5. The spent solution was
found to contain dianthryl ketone (7) as the main product. It is
well-documented that diarylcarbenes with a triplet ground state are
readily trapped by oxygen to generate the corresponding diaryl
ketone oxides (e.g., 6 ), which show a broad absorption band
centered at 400-500 nm.8,9 Thus, the observation can be interpreted
JA038423Z
9
J. AM. CHEM. SOC. VOL. 125, NO. 48, 2003 14665