95
Chemistry Letters 2000
The data clearly suggest that the reactivity of kinetically
stabilized triplet diphenylcarbenes toward typical triplet
quenchers is significantly decreased by the spin-delocalizing
substituent at the para positions. Accordingly, the persistency
of the triplet carbenes in solution at room temperature is
increased. These observations suggest that the kinetically sta-
bilized triplet DPC will be further stabilized more effectively
by stronger spin-delocalizing substituents at para positions.
The further studies in this direction are in progress in this
laboratory.
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. The help of
Ms. Yui Ishikawa with the EPR measurements is also
acknowledged.
References and Notes
1
See for reviews a) H. Tomioka, Acc. Chem. Res., 30, 1315
(1997). b) H. Tomioka, Adv. Carbene Chem., 2, 175 (1998).
Phosphinocarbene and imidazol-2-ylidene were prepared as
“bottle-able” singlet carbenes in 1988 and 1991, respectively.
See a) A. Igau, H. Grützmacher, A. Baceiredo, and G.
Bertrand, J. Am. Chem. Soc., 110, 6463 (1988). b) A. J.
Arduengo, III, R. L. Harlow, and M. Kline, J. Am. Chem.
Soc., 113, 361 (1991).
2
Laser flash photolysis (LFP)7 of 1a in a degassed benzene
solution at 20 °C with a 10 ns, 70-90 mJ, 308 nm pulse from a
XeCl excimer laser produced a transient species showing a
weak absorption around 480 nm. On the basis of the low-tem-
perature spectrum, we assign the absorption to triplet 2a.
Unfortunately, the strong absorption around 300-330 nm,
which are observed in the low-temperature spectrum, was in a
region of very difficult detection since the region falls under-
neath that of the precursor diazomethane and hence the sam-
ples are not sufficiently transparent for adequate monitoring.
However, the absorption at 480 nm was sufficiently strong to
run kinetic measurements. The decay of 2a is found to be first
order (k = 1.7 s-1) and the lifetime is determined to be 0.59 s
(Figure 1).
3
4
5
D. Bourisson, O. Guerret, F. D. Gabbai, and G. Bertrand,
Chem. Rev., 100, in press (2000).
K. Hirai and H. Tomioka, J. Am. Chem. Soc., 121, 10123
(1999).
For a review: X-K. Jiang, Acc. Chem. Res., 30, 283 (1997).
See also R. W. Humphreys and D. R. Arnold, Can. J. Chem.,
57, 2652 (1979).
The diazomethane (1a) was prepared by the acyl cleavage of
the corresponding N-alkyl-N-nitroso-urethane by potassium
tert-butoxide. It was purified by gel permeation chromatogra-
phy to afford an orange solid; mp 209-210 °C; 1HNMR
(CDCl3) δ 2.11 (s, 12 H), 7.38 (s, 4 H) ppm; IR (NaCl) 2048
and 2224 cm-1.
For detail of the spectroscopic work, see, H. Tomioka, H.
Okada, T. Watanabe, K. Banno, K. Komatsu, and K. Hirai, J.
Am. Chem. Soc., 119, 1582 (1997).
See for reviews of the EPR and UV/vis spectra of triplet car-
bene, a) W. Sander, G. Bucher, and S. Wierlacher, Chem.
Rev., 93, 1583 (1993). b) A. M. Trozzolo, and E.
Wasserman, “Carbenes,” ed by M. Jones, Jr. and R. A. Moss,
Wiley, New York (1975), Vol. 2, pp. 185-206.
a) H. Tukada, T. Sugawara, S. Murata, and H. Iwamura,
Tetrahedron Lett., 27, 235 (1986). b) A. S. Nazran, E. J.
Gabe, Y. LePage, D. J. Northcott, J. M. Park, and D. Griller,
J. Am. Chem. Soc., 105, 2912 (1983). c) H. Tomioka, T.
Watanabe, K. Hirai, K. Furukawa, T. Takui, and T. Itoh, J.
Am. Chem. Soc., 117, 6376 (1995).
6
7
8
The reactivities of 2a toward typical triplet quenchers, i.e.,
oxygen and cyclohexa-1,4-diene (CHD), were then investigat-
ed by using LFP. Thus, when LFP was carried out on a non-
degassed benzene solution of 1a, the lifetime of the absorption
due to 2a decreased dramatically, and a broad absorption band
with a maximum at 400 nm appeared as the band due to 2a
decayed. The observation can be interpreted as indicating that
the triplet 2a is trapped by oxygen to generate carbonyl oxide
(3a)12, thus confirming that the transient absorption quenched
by oxygen is due to 2a. A plot of the observed pseudo-first-
order rate of carbonyl oxide formation vs oxygen concentration
9
yields the quenching rate constant kO = 3.1 × 107 M-1 s-1.
2
10 Y-M. Hu, K. Hirai, and H. Tomioka, J. Chem. Phys, A, 103,
9280 (1999).
Similarly, LFP of 1a on a degassed benzene solution in the
presence of CHD generated a new signal at 380 nm attributa-
ble to the diarylmethyl radicals (4) as the signals of 2a
decayed, showing that triplet 2a abstracts H from the diene.11
The rate constant (kCHD) of H abstraction of 2 from CHD is
determined from a plot of the apparent build-up rate constant
of the radical vs [CHD]. The data are summarized in Table 1
along with those obtained in the reaction of 2b.
11 a) J. M. Dust and D. R. Arnold, J. Am. Chem. Soc., 105, 1221
and 6531 (1983). b) D. D. M. Wayner and D. R. Arnold,
Can. J. Chem., 62, 1164 (1984), and 63, 2378 (1985).
12 M. S. Platz and V. M. Maloney “Kinetics and Spectroscopy
of Carbenes and Biradicals,” ed by M. S. Platz, Plenum Press,
New York (1990), pp. 239-352.