by 1H and 13C NMR spectroscopy and by UV spectroscopy.
All of the diazirines had UV absorptions in the 325-390
nm region. They have been previously described,5 except
for 2d (see the Supporting Information). LFP of diazirines
2a-e with a xenon fluoride excimer laser at 351 nm (A2a-e
) 0.5 at 310 nm) generated carbenes 3a-e. The UV spectra
of 3a and 3b in 1 M MeCN-THF and of 3c-e in CCl4 are
shown in Figure S1 of the Supporting Information; their
absorption maxima are collected in Table 1.7
and carbanion 4a appear in Figure S2 (Supporting Infor-
mation); absorption maxima of the carbanions appear in
Table 1.
MeCN-THF was not an appropriate solvent for similar
reactions of carbenes 3c-e because these form transient
ylides with THF (see Figure S3, Supporting Information).
The THF ylide of carbene 3e has been reported previously.8
On the other hand, reactions of carbenes 3c-e with TBACl
could be visualized by LFP in CCl4.9 Figure S2 (Supporting
Information) shows the LFP-UV spectra of carbenes 3c-e
and carbanions 4c-e in CCl4; absorption maxima appear in
Table 1. Although phenylchlorocarbene abstracts Cl from
CCl4, the reaction is rather slow (k ) 3.8 × 104 M-1 s-1 in
MeCN)10 and does not interfere with the much more rapid
(µs to ns) processes that we examine here.
Kinetics of the carbene-chloride reactions were followed
by LFP, monitoring the apparent rate of decay of each
carbene with added TBACl. Rate constants for the carbene-
chloride reactions were obtained from the slopes of correla-
tions of the apparent quenching rate constants as a function
of the concentration of TBACl. The rate constants (kCl) are
collected in Table 2; Figure S4 of the Supporting Information
Table 1. UV Absorptions of Carbenes (3) and Carbanions (4)a
X
solvent
λmax (3)
λmax (4)
H
Cl
CF3
CN
NO2
MeCN-THF
MeCN-THF
CCl4
CCl4
CCl4
320
320
312
312
312
410
408
400
400
416
a Reported in nm at 25 °C in 1 M MeCN-THF (a and b) and in CCl4
(c-e).
In the presence of added tetrabutylammonium chloride
(TBACl), LFP of the diazirines revealed not only carbenes
3a-e but also carbanions 4a-e formed by reactions of the
carbenes with chloride1 (see Scheme 1). Figure 1 shows the
Table 2. Rate Constants for Reactions of Carbenes and
Carbanions
carbene
kCl (M-1 s-1
)
carbanion
kACN (M-1 s-1
)
3aa
3ba
3cb
3db
3eb
1.61 × 108
3.45 × 108
5.45 × 108
7.00 × 108
8.68 × 108
4aa
4ba
4cb
4db
4eb
2.50 × 107
1.07 × 107
1.62 × 106
9.18 × 105
1.16 × 104
a Solvent 1 M MeCN-THF, 25 °C. b Solvent CCl4, 25 °C.
depicts the correlations.
The capture of carbenes 3a-e by chloride ion to form
carbanions 4a-e is rapid, with kCl ranging from 1.6 × 108
M-1 s-1 for 3a to 8.7 × 108 M-1 s-1 for 3e. The rate
constants increase with increasing electron-withdrawing
power of the carbene’s para-substituent; i.e., as the carbene
is destabilized and the carbanion product is stabilized.11
However, the range of the rate constants is small, with kCl
12
(3e)/kCl (3a) ) 5.4. A Hammett correlation of log kCl vs σP
(Figure 2) affords F ) +0.86 (r ) 0.985).
Figure 1. UV absorptions of carbene 3b (320 nm) and carbanion
4b (408 nm) in 1 M MeCN-THF. The data points were recorded
400 ns after the laser pulse.
(8) Celebi, S.; Tsao, M.-L.; Platz, M. S. J. Phys. Chem. A 2001, 105,
1158.
(9) Carbanions 4a and 4b, however, were unstable in CCl4 and not readily
visualized in this solvent.
(10) Jones, M. B.; Maloney, V. M.; Platz, M. S. J. Am. Chem. Soc. 1992,
114, 2163. Electron-withdrawing substituents on ArCCl further slow Cl
abstraction.
UV spectra of carbene 3b and carbanion 4b formed upon
LFP of diazirine 2b in the presence of 0.2 M TBACl in 1 M
MeCN-THF. Analogous LFP-UV spectra of carbene 3a
(11) p-Methylphenylchlorocarbene did not appear to give the corre-
sponding dichloromethide carbanion (LFP-UV)) with TBACl.
(12) Substituent constants are taken from: Smith, M. B.; March, J.
March’s AdVanced Organic Chemistry, Reactions, Mechanisms, and Stuc-
ture, 5th ed.; Wiley: New York, 2001; p 370. σp- is used in the Hammett
correlation of Figure 3 for the p-nitro substituent because there is direct
resonance interaction of the carbanionic charge and the substituent (cf. this
reference, p 375). In this case, the use of σp leads to a poor correlation.
(6) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
(7) UV spectra of 3a-c at 77 K (3-methylpentane glass) and at ambient
temperature (isooctane solution) are described in: Gould, I. R.; Turro, N.
J.; Butcher, J., Jr.; Doubleday, C., Jr.; Hacker, N. P.; Lehr, G. F.; Moss, R.
A.; Cox, D. P.; Guo, W.; Munjal, R. C.; Perez, L. A.; Fedorynski, M.
Tetrahedron 1985, 41, 1587.
Surprisingly, however, the rate data for the cyano-substituted carbanion
-
correlates much better with σp than with σp
.
1246
Org. Lett., Vol. 8, No. 6, 2006