Effect of para substituants on the molecular and electronic structures of sterically congested triplet diphenylcarbenes

Article abstract of DOI:10.1021/jp991576h

A series of triplet di(2,6-dimethylphenyl)carbenes (³2) bearing nine symmetrical para di-substituents with well-distributed electronic properties have been generated by the irradiation of the corresponding diazo precursors and studied using ele

Full text of DOI:10.1021/jp991576h

9280
J. Phys. Chem. A 1999, 103, 9280-9284
Effect of Para Substituents on the Molecular and Electronic Structures of Sterically
Congested Triplet Diphenylcarbenes
Yingmo Hu, Katsuyuki Hirai, and Hideo Tomioka*
Chemistry Department for Materials, Faculty of Engineering, Mie UniVersity, Tsu, Mie 514-8507, Japan
ReceiVed: May 14, 1999; In Final Form: September 21, 1999
A series of triplet di(2,6-dimethylphenyl)carbenes (32) bearing nine symmetrical para di-substituents with
well-distributed electronic properties have been generated by the irradiation of the corresponding diazo
precursors and studied using electron paramagnetic resonance spectroscopy. The zero field splitting parameters,
D and E, were measured in matrices of different viscosities and are analyzed in terms of a sigma-dot (σ•)
scale of spin-delocalization substituent constants. Fairly good correlation (r > 0.9) with σ• based on the spin
delocalization in the absence of polar effects was found for the D values of 32 in its minimum energy geometry,
3
but not for those in the metastable state. The magnitude of F for 2 was larger than that for o-unsubstituted
diphenylcarbenes and is discussed in terms of the difference in the geometry between the two carbene systems.
Introduction
SCHEME 1
While the history of stable and persistent radicals is long and
impressive,1 that of persistent triplet carbenes is short, and triplet
carbenes capable of surviving under normal conditions have not
been realized yet.2,3 Two basic strategies, i.e., thermodynamic
and kinetic stabilization, are possible for the stabilization of
reactive species. Studies examining the relationship between
structure and reactivity have shown that thermodynamic stabi-
lization (electronic conjugation effect) usually plays an important
role in stabilizing the singlet state and that the singlet state
undergoing the stabilization becomes less reactive due to the
contribution of the conjugation to such an extent that the species
can be isolated under ambient conditions.4,5 On the other hand,
kinetic stabilization (steric protection) is more effective in
stabilizing the triplet counterpart since the introduction of
sterically bulky groups around the carbenic center results in an
increase in the carbene bond angle. In this case, the triplet state
is thermodynamically stabilized with respect to the singlet.
To this end, attempts have been made whereby triplet
diphenylcarbenes (DPCs) were stabilized by introducing a series
of substituents at the ortho positions,3,4 and triplet DPCs
surviving more than an hour in solution at room temperature
were realized.6 Even though this is a long lifetime for a triplet
carbene, it is still ephemeral.
To realize a stable triplet carbene, it is necessary to explore
kinetic protectors which are bulky yet unreactive toward the
carbenic center. This approach encounters a limitation when
diazo compounds are used as a precursor since introduction of
the diazo group becomes more difficult as more bulky groups
are introduced at the ortho positions.
The other strategy we have not examined extensively is the
electronic (thermodynamic) effect of para substituents on the
stability of kinetically protected triplet diphenylcarbenes. To
explore this effect, we generated a series of triplet di(2,6-
dimethylphenyl)carbenes (2) bearing nine symmetrical para di-
substituents with diverse electronic properties by photolysis of
the diazo precursor (1) and studied their structure and reactivities
by using electron paramagnetic resonance (EPR) spectroscopy.
The zero-field splitting (ZFS) parameters were analyzed in terms
of the spin delocalizing power of the para substituents.
Experimental Section
A. Synthesis. All of the diazo compounds (1) used in this
study were prepared by the reduction of the corresponding
N-nitrosodiaryl ketimine with LiAlH4 according to the procedure
developed by Zimmerman and Paskovich.7 Full details of the
procedure and their spectroscopic data will be published
elsewhere.8 The diazomethanes were purified by repeated
chromatography on a Shodex GPC H-2001 (20 mm × 50 cm)
gel permeation column eluted with CHCl3 immediately before
the irradiation.
B. ESR Measurements. Diazo compounds were dissolved
in the appropriate organic solvent (10-3 M), and the solution
was degassed in a quartz EPR tube by three freeze-degas-thaw
cycles. The sample was cooled in an quartz Dewar filled with
liquid nitrogen and irradiated with a Wacom 500 W Xe lamp
using a Pyrex filter in an optical transmission EPR cavity at 77
K. EPR spectra were measured on a JEOL JES TE 200
spectrometer (x-band microwave unit, 100 kHz field modula-
tion). The signal positions were read by the use of a gauss meter.
For samples used in the temperature dependence studies, the
EPR tube was placed in the spectrometer equipped with a JEOL
ES-DVT3 liquid nitrogen transfer system and was irradiated at
110 K. The temperature of the sample was monitored and
controlled by a PID controller. The sample temperature was
raised in 10 K increments to the desired temperature, allowed
to stand for 1 min, and recooled to 110 K to measure the signal.
All solvents employed were of spectroscopic grade and
purified by distillation just prior to use.
10.1021/jp991576h CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/30/1999

Para Substituents on Triplet Diphenylcarbenes
J. Phys. Chem. A, Vol. 103, No. 46, 1999 9281
Figure 2. ESR spectra of di(2,6-dimethyl-4-tert-butyl-phenyl)carbene
(2c) in 1,2,3-propanetriol triacetate (PT). (a) Sample at 110 K. (b) Same
sample after annealing to 200 K and recooling to 110 K.
The E value, when weighted 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.9,10
The ZFS parameters observed for all of the carbenes (2) are
significantly smaller than those of “parent” DPC and for most
4,4′-disubstituted DPCs (3) (vide infra).
Figure 1. ESR spectra of di(2,6-dimethyl-4-tert-butylphenyl)carbene
(2c) in 2-methyltetrahydrofuran (2-MTHF) (a) and 3-methylpentane
(3-MP) (b) at 77 K.
TABLE 1: D and E Values for
Di(2,6-dimethylphenyl)carbenes (2) in
2-Methyltetrahydrofuran (2-MTHF) at 77 K
carbenes (2) 4,4′-substituents D (cm-1
)
E (cm-1
)
Td (K)a
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
0.337
0.368
0.373
0.377
0.354
0.353
0.368
0.340
0.335
0.0101
0.0117
0.0134
0.0129
0.0144
0.0117
0.0134
0.0093
0.0081
140
120
170
120
120
120
120
180
170
Me
tBu
F
Cl
Br
OMe
CN
NO2
Effects of Temperatures and Matrices. Similar irradiation
of 1 in a more viscous matrix, i.e., 1,2,3-propanetriol triacetate
(PT), at 110 K also gave rise to a typical set of triplet signals
3
due to 2 with ZFS parameters similar to those obtained in
3
2-MTHF at 77 K, as shown in Figure 2a for 2c.
a Temperature at which the triplet signal disappeared.
When the PT glass containing carbene 2 was warmed
gradually to 200 K in 10 K increments, another new set of triplet
peaks appeared at the expense of the original peaks which
disappeared rapidly around 180 K (see Figure 2b). These
changes were not reversible; when the sample was cooled to
110 K, no change took place except that the signal intensity
increased according to the Curie law. A signal due to a doublet
species also started to appear around 180 K (see Figure 2b).
The triplet signals eventually disappeared around 210 K and
were taken over by the signal due to doublet species.
In the new set of triplet peaks, the x and y lines moved closer
together, resulting in a substantial reduction in E. Smaller but
distinct shifts in the z lines were also noted, indicating that D
had also diminished.
Since the E value, when weighted by D, depends on the
magnitude of the central angle, it 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 accompanied by a
significant reduction in D, indicating that the electrons are
becoming more delocalized. Thus, the interpretation is consistent
with the concept of angular expansion.
Results and Discussion
The Zero-Field Splitting Parameters. Irradiation (λ > 300
nm) of precursor diazomethanes (1) in 2-methyltetrahydrofuran
(2-MTHF) at 77 K gave rise to a paramagnetic species readily
characterized from its EPR spectrum as a derivative of triplet
methylene (32). Figure 1a shows the spectra observed for triplet
di(4-tert-butyl-2,6-dimethylphenyl)carbene (32c). As shown in
Figure 1, the spectra were sufficiently intense that four lines
could be observed (Hz1, Hx2, Hy2, Hz2) under these conditions.
The EPR spectra of all the other diphenylcarbenes investigated
in this study exhibited similar spectroscopic features.
Carbene triplet signals were assigned in terms of the Hamil-
tonian9
1
H ) gâH‚S + D Sz2 - S2 + E(Sx2 - Sy2)
(
)
3
The zero-field splitting (ZFS) parameters, D and E, which were
obtained from the observed spectra by employing an iterative
computer program, are reported in Table 1. The D value, which
is related to the separation between the unpaired electrons, varies
considerably with the nature of the triplet. While the overall
magnitude of D is characteristic of a particular class of triplets,
smaller variations in this parameter, which reflect structural
differences between various triplet carbenes, are seen within a
given class.
Changes of this kind have been often observed for sterically
congested triplet diarylcarbenes and are usually interpreted in
terms of geometrical changes.11 Thus, when a carbene is
generated in rigid matrices at very low temperature, it should
have the geometry dictated by that of the precursor. Even if the

9282 J. Phys. Chem. A, Vol. 103, No. 46, 1999
Hu et al.
TABLE 2: D and E Values for
indicates that the triplet carbene undergoes hydrogen abstraction
from 3-MP even at 77 K to form a radical species. In the case
of 4,4′-dicyano- (2h) and 4,4′-dinitro derivatives (2i), major
signals were due to doublet species and triplet signals were so
weak that assignment of the field position was impossible.
Analysis of the ZFS Parameters. It is interesting to compare
the ZFS parameters of di(2,6-dimethylphenyl)carbenes (32) with
“parent” DPC10 under identical conditions. As expected, a
marked reduction both in the E and D values is noted as four
methyl groups are introduced at all the ortho positions of DPC,
Di(2,6-dimethylphenyl)carbenes (2) in 1,2,3-Propanetriol
Triacetate (PT) at 110 K
carbenes (2) 4,4′-substituents D (cm-1
)
E (cm-1
)
Td (K)b
a
a
2a
2b
2c
2d
2e
2f
H
0.354
(0.335)
0.381
(0.331)
0.371
(0.328)
0.377
(0.344)
0.354
(0.326)
0.348
0.0093
240
(2.8 × 10-6
0.0121
)
)
)
)
)
)
Me
tBu
F
210
210
220
220
220
200
230
220
(2.6 × 10-6
0.0128
(2.6 × 10-6
0.0116
i.e., continuously 3DPC (D ) 0.5055 cm-1, E ) 0.0194 cm-1
)
(2.8 × 10-6
0.0120
3
Cl
to 2a, for instance, in 2-MTHF at 77 K. Thus, both of these
parameters decrease with increasing steric crowding in the
carbenes and reflect the steric influence of o-methyl groups
which force an expansion of the central angle to some extent
even in rigid matrix. The ZFS parameters of 32 undergo further
reduction upon annealing the matrix to gain relief from steric
congestion. In marked contrast, the parameters of 3DPC exhibit
little sensitivity to the temperature and/or the softness of the
matrix. This indicates that the minimum energy geometry of
3DPC has a considerably smaller C-C-C angle than that in 32
and that the matrix provides sufficient space for it to take that
geometry even at low temperature.
(2.5 × 10-6
0.0118
Br
(0.327)
0.370
(2.5 × 10-6
0.0129
2g
2h
2i
OMe
CN
NO2
(-)c
(-)c
0.344
0.0093
(0.283)
0.330
(0.259)
(2.7 × 10-6
0.0078
)
(2.7 × 10-6
)
a The values in parentheses refer to those observed upon anneling
the matrix. b Temperature at which the triplet signals disappeared. c The
triplet signals disappeared before the relaxation.
It is then worth examining how these structural differences
affect the influence of para substituents on the electronic
structures of triplet diphenylcarbene systems.
TABLE 3: D and E Values for
Di(2,6-dimethylphenyl)carbenes (2) in 3-Methylpentane
(3-MP) at 77 K
D (cm-1
)
E (cm-1
)
The effect of p-substituents on the D values of o-unsubstituted
DPCs (3) has been systematically investigated by Arnold and
co-workers.13 Although the substituent effects on the D value
of 3 are not large, those authors revealed two trends. First,
p-substitution generally causes a decrease in D over that in the
parent DPC. Second, the decrease in D is largest when DPC is
substituted with one para electron withdrawing group (e.g., NO2)
and one para′ electron donating group (e.g., NMe2). The second
effect is interpreted in terms of merostabilization,14 in which
the contribution of the charge-separated resonance structures is
assisted by those unsymmetrical substituents and allows the
unpaired electron in the π-orbital of the diphenylcarbenes to
be delocalized. Since this type of substitution pattern is not used
in the present study, we will focus on the first effect here.
In the case of o-unsubstituted DPCs (3), both mono- and
symmetrical di-substitution both cause a decrease in D. No trend,
however, is consistent with the Hammett substituent parameters
(σ, σ+, etc.). This is in accord with the results obtained for the
substituent effects on the ESR hyperfine splittings of substituted
benzyl radicals.15
carbenes (2)
4,4′-substituents
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
0.349
0.347
0.347
0.361
0.337
0.333
0.343
0.0049
0.0059
0.0061
0.0081
0.0052
0.0054
0.0087
Me
tBu
F
Cl
Br
OMe
CN
NO2
a
a
-
-
a
a
-
-
a Triplet signals were so weak that the ZFS parameters could not be
assigned.
thermodynamically most stable geometry of the carbene is
different from that at birth, the rigidity of the matrices prevents
the carbene from achieving its minimum-energy geometry.
When matrices are softened by annealing, the carbene undergoes
relaxation to the preferred geometry probably to gain relief from
steric compression. It has been shown that the ZFS parameters
of sterically unperturbed diphenylcarbenes show little sensitivity
toward the temperature of the matrix.10b,11c
Similar changes in the D and E values upon annealing were
also observed for other carbenes employed in this study,
although the temperatures (Td) at which the triplet signals of
the relaxed species disappeared were somewhat sensitive to the
para substituents as summarized in Table 2. In the case of 4,4′-
dimethoxy derivative (2f), the triplet signals disappeared around
200 K before the carbene relaxed to its minimum-energy
geometry. As a result, the ZFS parameters for the relaxed
geometry were not estimated.
When the carbenes were generated in a soft matrix, i.e.,
3-methylpentane (3-MP),12 the ZFS parameters observed at 77
K were significantly smaller than those observed in 2-MTHF
at 77 K and were similar to those of the relaxed geometry in
TA at around 200 K (see Figure 1b and Table 3). This indicates
that the 3-MP glass is not sufficiently rigid to restrict the motion
of the carbene, even at 77 K, and that the carbene can easily
assume its minimum-energy geometry under these conditions.
In this matrix, however, the typical set of triplet signals was
always accompanied by a signal due to a doublet species. This
The dominant interaction of the unpaired electron in a
π-radical is with the electrons paired in the π-bonds. Such
interactions are characterized by the delocalization of the spin
throughout the π-system. To estimate the relative abilities of
substituents to delocalize the spin, sigma-dot substituent con-
stants, (σ•), have been proposed.16 Among the various ap-
proaches to the definition of a σ• scale, Arnold’s σR• scale17 is
the most suitable for the analysis of the substituent effect on
the D values of DPCs since this scale is a nonkinetic measure
of radical stabilizing effects based on hyperfine coupling
constants in the benzylic radical. Creary’s σc• scale18 is also
useful since, although this scale is based on the thermal
rearrangement rate of substituted methylenecyclopropanes to the
isopropylidenecyclopropanes, the rearrangement is shown to be
a radical process that is devoid of significant polar character in
the transition state. Actually the σc• scale is shown18 to correlate
reasonably well with σR•.
3
Figure 3 shows an attempt to correlate the D values13 of -
DPCs (3a∼e) with σR•. Since the data available for both D and

Para Substituents on Triplet Diphenylcarbenes
J. Phys. Chem. A, Vol. 103, No. 46, 1999 9283
Figure 5. The plot of the D values for di(2,6-dimethylphenyl)carbenes
(2) against σc•: O denotes the data in 2-methyltetrahydrofuran
(2-MTHF) at 77 K; 0 denotes the data in 1,2,3-propanetriol triacetate
(PT) at 110 K; 4 denotes the data in 3-methylpentane (3-MP) at 77 K;
b denotes the data in PT at 200 K.
Figure 3. The plot of the D values for 4,4′-disubstituted diphenyl-
carbenes (3a∼e) against σR• (O) and σc• (0).
employed. Thus, the D values in TA at 200 K correlate relatively
well (F ) 0.589, r ) 0.900) with σR• (Figure 4) and fairly well
(F ) 0.0654, r ) 0.969) with σc• (Figure 5). A poor correlation
with σR• obviously stems from the lack of the largest constant,
i.e., σR•, for the p-NO2 group. The correlation of D values in
3-MP at 77 K is rather poor with either σR• (r ) 0.68) (Figure
4) or σc• (r ) 0.51) (Figure 5), but this is also due to the lack
of necessary data.
Electronic Effects on the Structure of Sterically Congested
Triplet Diphenylcarbenes. The present study has revealed the
following facts concerning the effect of substituents on the
electronic and molecular structures of sterically congested
diarylcarbenes.
First, the linear free energy relationship between the ZFS
parameters of sterically congested triplet diphenylcarbenes and
the stabilization energy due to spin delocalization holds for the
most thermodynamically stable geometry, not for the initial
unrelaxed geometry. This is reasonable since the geometry at
birth should be influenced by that of the precursor to some
extent, and the spin delocalization in the system is not ideally
developed. Once the temperature is raised and the matrix
becomes soft, the carbene can relax to the most stable geometry
which is electronically stabilized by para substituents depending
on the radical stabilizing power. If one assumes that the
magnitude of the steric strain in 2 is essentially the same
regardless of the para substituents, the magnitude of the shift
of the D values upon annealing should reflect the delicate
balance between the spin delocalization ability and the polar
effect of a substituent. In the case of o-unsubstituted DPCs, the
geometrical change from an initially formed geometry to one
of minimum energy is so small that the change can be
accomplished even in rigid matrices at low temperatures.
Second, the difference in the dependency of the D values on
Figure 4. The plot of the D values for di(2,6-dimethylphenyl)carbenes
(2) against σR•: O denotes the data in 2-methyltetrahydrofuran
(2-MTHF) at 77 K; 0 denotes the data for 1,2,3-propanetriol triacetate
(PT) at 110 K; 4 denotes the data for 3-methylpentane (3-MP) at 77
K; b denotes the data for PT at 200 K.
σR• are rather limited, the correlation cannot be assessed
accurately. However, the D values correlate reasonably well
(F ) 0.264, r ) 0.977) with the σR• scale (Figure 3). Similar
attempts with σc• also give an analogous correlation (F )
0.0273, r ) 0.944) (Figure 3). This is not unreasonable in the
light of the nature of the D values and σ• employed.
Our attempts to correlate the D values of sterically congested
3
triplet carbene 2 with σR• and σc• show that correlations are
very sensitive to the geometric structures of the carbenes. Thus,
the D values observed for carbenes at birth in rigid matrices at
low temperature, i.e., those obtained in 2-MTHF at 77 K and
in PT at 110 K, where carbenes retain their metastable
geometries, give a highly scattered plot when correlated against
both σR• and σc• (Figures 4 and 5). A general trend for D values
to decrease with increasing the power of radical stabilization
of a substituent is seen. However, the correlations are disap-
pointingly poor (r < 0.5).
σ• between DPCs (33) and 2 is worthy of comment. The
comparison of the F value between the two carbene systems
clearly indicates that the D values of 32 are approximately two
times more sensitive to σ• than those of 3DPC. This is reasonable
since carbenes 2 have a more linear and perpendicular geometry
3
3
3
than 3, as evidenced by the large decrease in both D and E
3
Surprisingly, improved correlations are found when the D
values observed for the carbenes in their minimum-energy
geometries, attained upon annealing and/or in soft matrices, are
values relative to that of 3, and hence the unpaired electron
can be delocalized more effectively in the π-system of aromatic
rings including substituents.

9284 J. Phys. Chem. A, Vol. 103, No. 46, 1999
Hu et al.
DPCs exhibit significant E values of 0.016-0.019 cm-1
regardless of the para substituents.10,13 This indicates that the
two half-filled orbitals at the divalent carbon consist of a
p-orbital perpendicular to the phenyl rings and an orbital
occupying the plane of the molecule with considerable s
character. The unpaired electron in the p-orbital is delocalized
into the phenyl rings while the unpaired electron in the σ-orbital
is localized at the divalent carbon.
of Ms. Yui Ishikawa with the ESR measurements is also
acknowledged.
References and Notes
(1) (a) Forrester, A. R.; Hay, J. M.; Thomson, R. H. Organic Chemistry
of Stable Radicals; Academic Press: London, 1968. (b) Griller, D.; Ingold,
K. U. Acc. Chem. Res. 1976, 9, 13. (c) Ballester, M. Acc. Chem. Res. 1985,
18, 380.
(2) See for reviews: (a) Tomioka, H. Acc. Chem. Res. 1997, 30, 1315.
(b) Tomioka, H. In AdVances in Carbene Chemistry; Brinker, U., Ed.; JAI
Press: Greenwich, CT, 1998; Vol. 2, pp 175-214.
(3) (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.; Itoh, K. J. Am. Chem. Soc. 1998, 120,
1106. (c) Tomioka, H.; Mizuno, H.; Itakura, H.; Hirai, K. J. Chem. Soc.,
Chem. Commun. 1997, 2261.
(4) Phosphinocarbene and imidazol-2-ylidene were prepared as “bottle-
able” singlet carbenes in 1988 and 1991, respectively. See: Igau, A.;
Gru¨tzmacher, H.; Baceiredo, A.; Bertrand, G. J. Am. Chem. Soc. 1988, 110,
6463. Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361.
3
On the other hand, the E values observed for the relaxed 2
are essentially zero (E < 0.00001 cm-1) regardless of the para
substituents. A zero E value in the ZFS parameters is generally
interpreted as indicating that DPCs have a linear structure with
the phenyl planes perpendicular to each other. Each unpaired
electron is expected to be in the p-orbital and can be delocalized
into the phenyl rings, leaving no localized unpaired electron at
the divalent carbon atom.
Finally, the effect of the electronic (thermodynamic) stabiliza-
3
tion on the stability of 2 was examined by measuring the
(5) For reviews, see: (a) Herrman, W. A.; Ko¨cher, C. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2162. (b) Arduengo, A. J., III; Krafczyk, R. Chem.
Unserer Zeit. 1998, 32, 6. (c) Bourisson, D.; Guerret, O.; Gabbai, F. P.;
Bertrand, G., Chem. ReV., in press.
(6) Hirai, K.; Tomioka, H., J. Am. Chem. Soc., in press.
(7) Zimmerman, H. E.; Paskovich, D. H. J. Am. Chem. Soc. 1964, 86,
2149. See also: Tomioka, H.; Okada, H.; Watanabe, T.; Banno, K.;
Komatsu, K.; Hirai, K. J. Am. Chem. Soc. 1997, 119, 1582.
(8) Hu, Y.-M.; Hirai, K.; Tomioka, H., to be published.
(9) For more detailed descriptions of the triplet state and zero-field
splittings, see: (a) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance;
McGraw-Hill: New York, 1972. (b) McGlynm, S. P.; Azumi, T.; Kinoshita,
M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: New Jersey,
1969.
(10) See for reviews of the EPR spectra of triplet carbenes: (a) Sander,
W.; Bucher, G.; Wierlacher, S. Chem. ReV. 1993, 93, 1583. (b) Trozzolo,
A. M.; Wasserman, E. In Carbenes; Jones, M., Jr., Moss, R. A., Eds.;
Wiley: New York, 1975; Vol. 2, pp 185-206.
(11) (a) Tukada, H.; Sugawara, T.; Murata, S.; Iwamura, H. Tetrahedron
Lett. 1986, 27, 235. (b) Nazran, A. S.; Gabe, E. J.; LePage, Y.; Northcott,
D. J.; Park, J. M.; Griller, D. J. Am. Chem. Soc. 1983, 105, 2912. (c) Gilbert,
B. C.; Griller, D.; Nazran, A. S. J. Org. Chem. 1985, 50, 4738. (d) Nazran,
A. S.; Lee, F. L.; Lepage, Y.; Northcott, D. J.; Park, J. M.; Griller, O. J.
Phys. Chem. 1984, 88, 5251. (e) Tomioka, H.; Watanabe, T.; Hirai, K.;
Furukawa, K.; Takui, T.; Itoh, K. J. Am. Chem. Soc. 1995, 117, 6376.
(12) Viscosity of 3-MP, η, relative to 2-MTHF at 77 K is reported to
be 1.44 × 10-6. See: Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook
of Photochemistry; Marcel Dekker: New York, 1993.
(13) (a) Humphreys, R. W. R.; Arnold, D. R. Can. J. Chem. 1979, 57,
1979. (b) Arnold, D. R.; Humphreys, R. W. R. J. Chem. Soc., Chem.
Commun. 1978, 181.
(14) Baldock, R. W.; Hudson, P.; Katritzky, A. R. J. Chem. Soc., Perkin
Trans 1 1974, 1422. Katritzky, A. R. J. Chem. Soc., Perkin Trans. 2 1974,
1427.
(15) Neta, P.; Schuler, R. H. J. Phys. Chem. 1973, 77, 1368.
(16) For a pertinent overview see: Jiang, X.-K. Acc. Chem. Res. 1997,
30, 283.
(17) (a) Dust, J. M.; Arnold, D. R. J. Am. Chem. Soc. 1983, 105, 1221
and 6531. (b) Wayner, D. D. M.; Arnold, D. R. Can. J. Chem. 1984, 62,
1164. (c) Wayner, D. D. M.; Arnold, D. R. Can. J. Chem. 1985, 63, 2378.
(18) Creary, X. J. Org. Chem. 1980, 45, 280. Creary, X.; Mehrsheik-
Mohammadi, M. E.; McDonald, S. J. Org. Chem. 1987, 52, 3254.
(19) Tomioka, H.; Hattori, M.; Hirai, K.; Murata, S. J. Am. Chem. Soc.
1996, 118, 8723.
temperature (Td) at which the triplet signals completely disap-
peared not only in 2-MTHF (Table 1) but also in PT (Table 2).
The data in PT appear to level off to some extent, presumably
because Td in PT are close to the limit of the inherent stability
3
of the carbenes 2. Therefore the data in 2-MTHF are used to
evaluate the thermodynamic effect on the stability of 32.
Inspection of the data in Table 1 indicates that 4-cyano and
nitro groups exhibit significant stabilizing effects on 32. Taking
into account the increased ability of those substituents to
delocalize the unpaired electron, this indicates that triplet
carbenes kinetically stabilized by the ortho substituents can be
further stabilized thermodynamically by spin-delocalizing para
substituents.
Interestingly, the para-tert-Bu group is found to exert an
equally significant effect on Td. Since the σ• of this group is
not large, the effect is explained in terms of a steric factor.
Sterically congested DPCs usually decay by dimerization to form
tetra(aryl)ethylenes in an inert solvent when other intramolecular
reaction channels are not available.2 However, it has been
demonstrated that triplet DPCs also decay by undergoing
coupling at the para positions, especially when the carbene
center is effectively blocked by the ortho substituents.19
In this connection, the rather high Td observed for the para,
para′-unsubstituted derivative (2a) is surprising since hydrogens
at the para position have neither a spin-delocalizing effect nor
steric protecting ability.
To obtain more insight into the combination of kinetic and
thermodynamic factors on the stability of triplet carbenes, both
kinetic studies using laser flash photolysis and product analysis
studies in solution at room temperatures are required. Such
studies are in progress in this laboratory.
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. The help