Generation, direct observation, and kinetics of triplet 9-(1,1,4,4,5,5,8,8-octamethyl-1,2,3,4,5,6,7,8-octahydroanthryl)phenylcarbene

Article abstract of DOI:10.1021/ol026119p

(matrix presented) Reactivities of the title triplet carbene were compared with those of the "untied" counterpart, [2,4,6-tris(tert)butylphenyl](phenyl)carbene. An appreciable increase in the stability of triplet carbene is noted by tuning the distance be

Full text of DOI:10.1021/ol026119p

ORGANIC
LETTERS
2002
Vol. 4, No. 13
2261-2264
Generation, Direct Observation, and
Kinetics of Triplet 9-(1,1,4,4,5,5,8,8-
Octamethyl-1,2,3,4,5,6,7,8-octahydro-
anthryl)phenylcarbene
Takashi Iikubo, Katsuyuki Hirai, and Hideo Tomioka*
Chemistry Department for Materials, Faculty of Engineering and Instrumental Analysis
Center, Mie UniVersity, Tsu, Mie 514-8507 Japan
tomioka@chem.mie-u.ac.jp
Received May 1, 2002
ABSTRACT
Reactivities of the title triplet carbene were compared with those of the “untied” counterpart, [2,4,6-tris(tert)butylphenyl](phenyl)carbene. An
appreciable increase in the stability of triplet carbene is noted by tuning the distance between the o-hydrogen and the carbene center.
Triplet carbenes are highly reactive organic radicals that are
also notoriously difficult to stabilize.1 This is partly because
they can abstract hydrogen even from very poor sources of
electrons such as C-H bonds. For instance, the tert-butyl
group is known as one of the most effective kinetic protectors
and many reactive molecules have been stabilized and
isolated by using this group. However, [2,4,6-tris(tert)-
butylphenyl](phenyl)carbene (2) decayed due to intra-
molecular abstraction of hydrogen from the methyl group
of o-tert-butyl to form indane (3) and the lifetime of 2 was
estimated to be 125 µs,2 which is only 60 times longer than
the “parent” diphenylcarbene (DPC) under identical condi-
tions. The observations suggest that tert-butyl groups, which
have been successfully used to protect many reactive centers,
are almost useless to stabilize a triplet carbene center.
One way to retard intramolecular H transfer is to adjust
the distance between hydrogen and the carbene center. We
thus prepared a precursor (4) of hindered DPC in which the
two o-tert-butyl groups have been “tied back” by incorpora-
tion into a six-membered ring.3 If the DPC (5) derived from
4 were to decay intramolecularly, a primary hydrogen would
have to move in a different manner than that in 2 and the
carbene should be more persistent than 2. Inspection of the
structure of triplet carbenes optimized by PM3 indicates that
the carbene bond angle in 5 (166.8°) is smaller than that in
2 (179.5°) and that the distance between the methyl and
carbene carbon in 5 (2.9 and 3.3 Å) is shorter than that in 2
(3.0, 3.1, and 4.4 Å).
9-(1,1,4,4,5,5,8,8-Octamethyl-1,2,3,4,5,6,7,8-octahydro-
anthryl)phenyldiazomethane (4 )4 was prepared by the
Scheme 2
Scheme 1
10.1021/ol026119p CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/30/2002

treatment of the corresponding ketimine with N2O4, followed
by the reduction of the resulting N-nitrosoketimine with
LiAlH4. An X-ray diffraction study of 4 confirmed the
structure (Figure 1).5 Irradiation (λ > 300 nm) of 4 in a
Figure 2. Molecular structure (ORTEP) of 6 as determined from
X-ray diffraction analysis (see Supporting Information).
Figure 1. Molecular structure (ORTEP) of 4 as determined from
X-ray diffraction analysis (see Supporting Information).
3). Signals at 4790, 4820, and 6850 G are assigned to a set
of the high-field x, y, and z transitions from which the zero-
field splitting (ZFS) parameters were obtained as D ) 0.332
cm-1 and E ) 0.001 cm-1. Similar irradiation of 4 in
3-methylpentane (3-MP) at 77 K also gave ESR signals with
essentially the same ZFS parameters (D ) 0.333 cm-1 and
E ) 0.001 cm-1). When sterically congested triplet DPCs
are generated in a soft matrix, e.g., 3-MP, the x and y lines
degassed benzene solution at room temperature gave indan
derivatives (6)6 as a mixture of isomers. It is probable that
6 is produced from the photolytically generated carbene 5,
which underwent insertion into the C-H bonds of methyl
groups at either the 1- and/or 8-position. A structural analysis
was performed for 6 (Figure 2).5
The observation is essentially the same as that reported
for tert-butylated DPC 2. However, spectroscopic studies
reveal the somewhat different nature between the two
carbenes.
Irradiation (λ > 300 nm) of 4 in a 2-methyltetrahydrofuran
(2-MTHF) glass at 77 K gave ESR signals with typical fine
structure patterns for unoriented triplet species,7 i.e., 5 (Figure
(1) 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.
(2) (a) Hirai, K.; Komatsu, K.; Tomioka, H. Chem. Lett. 1994, 503. (b)
Tomioka, H.; Okada, H.; Watanabe, T.; Banno, K.; Komatsu, K.; Hirai, K.
J. Am. Chem. Soc. 1997, 119, 1582. (c) Hirai, K.; Yasuda, K.; Tomioka,
H. Chem. Lett. 2000, 398.
(3) (a) Brunton, G.; Griller, D.; Barclay, L. R. C.; Ingold, K. U. J. Am.
Chem. Soc. 1976, 98, 6803. (b) Brunton, G.; Gray, J. A.; Griller, D.; Barclay,
L. R. C.; Ingold, K. U. J. Am. Chem. Soc. 1978, 100, 4197.
(4) Red crystal; mp 133-150 °C dec; 1HNMR (CDCl3, 300 Hz) δ 1.00
(s, 4H), 1.31 (s, 12H), 1.32 (s, 12H), 1.50-1.75 (m, 4H), 6.20 (d, J ) 8.08
Hz, 1H), 6.87 (d, J ) 8.45 Hz, 1H), 6.97 (t, J ) 7.90 Hz, 1H), 7.11 (t, J
) 7.90 Hz, 1H), 7.34 (d, J ) 7.72 Hz, 1H), 7.48 (s, 1H); IR (KBr) 2046
cm-1
.
(5) See Supporting Information for full details of the X-ray crystal
structure of compounds 4 and 6.
(6) Major isomer: white crystal; mp 120-121 °C; 1H NMR (CDCl3,
300 Hz) δ 0.861 (s, 3H), 0.96 (s, 3H), 1.16 (s, 3H), 1.23 (s, 3H), 1.30 (s,
3H), 1.33 (s, 3H), 1.41 (s, 3H), 1.51-1.67 (m, 7H), 1.96-2.06 (m, 1H),
2.09 (d, J ) 11.76 Hz, 1H), 2.33 (dd, J ) 8.82, 11.76 Hz, 1H), 4.77 (d, J
) 8.82 Hz, 1H), 7.21 (s, 1H), 6.90-7.37 (m, 5H); EIMS m/z (%) 386.0
(M+), 371.0 (100).
Figure 3. ESR signals obtained by photolysis (λ > 300 nm) of 4
in 2-methyltetrahydrofuran at 77 K.
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Org. Lett., Vol. 4, No. 13, 2002

of the spectrum become closer, thus resulting in a smaller
E/D value than those in a harder matrix.
Since the E value, when weighted by D, depends on the
magnitude of the central C-C-C bond angle,7 this is
explained as an indication that the carbenes relax to a
thermodynamically more stable structure with an expanded
C-C-C angle, different from those of the nascent carbene,
presumably to gain relief from steric compression.8 The small
change in the parameters of 5 between two different matrices
of significantly different rigidity suggests that in 5, there is
little change between the geometry at birth, which may be
somewhat dictated by that of the precursor diazomethane,
and the geometry of thermodynamically most stable form.
It is interesting to compare the results with that observed
for 2. The E/D value of 5 is larger than that of 2, indicating
that the carbene bond angle increases in going from 2 to 5,
which is in accord with the PM3 prediction.
To determine the thermal stability of these carbenes, they
were generated in a more viscous matrix, i.e., propanetriol
triacetate, and the matrix containing them was warmed
gradually in 10 K increments. The ESR signals of 2
disappeared at around 170 K, while those of 5 started to
decrease only at 190 K, suggesting that 5 is appreciably less
reactive than 2.
Irradiation of 4 in 2-MTHF was then monitored by UV/
vis spectroscopy, which revealed the rapid appearance of new
absorption bands at the expense of the original absorption
due to 4 (Figure 4). The new spectrum consists of two
identifiable features, an intense UV band with a maximum
at 281 nm and weak structured ones at 321 and 338 nm.
These features are usually present in the spectra of triplet
DPCs.9 Moreover, since strong ESR signals ascribable to a
triplet carbene are observed under the identical conditions,
the absorption spectrum can be assigned to triplet carbene
5.
When the 2-MTHF matrix containing 5 was gradually
warmed, the absorption bands due to 5 started to decompose
at around 145 K. The bands due to 2 were shown to disappear
at around 95 K under identical conditions. The greater
thermal stability of 5 relative to that of 2 is again noted.
Laser flash photolysis10 of 4 in a degassed benzene solution
at room temperature with a 10 ns, 70-90 mJ, 308 nm pulse
from a XeCl laser produced a transient species showing an
apparent maximum at 335 nm (Figure 5). As the major
absorption bands of carbene 5 are overlapping with the
precursor diazo compound 4 and the samples were therefore
not sufficiently transparent for adequate monitoring in this
region, the whole spectrum was not observed. However, the
band at 335 nm coincides with that observed during the
Figure 4. UV/vis spectra obtained by irradiation of 4 in 2-meth-
yltetrahydrofuran. (a) Spectra obtained by irradiation (λ > 300 nm)
of 4 at 77 K. (b) Same sample after 10 min of irradiation. (c) Same
sample after the matrix was thawed to room temperature and
refrozen to 77 K.
photolysis of 4 in a 2-MTHF matrix at 77 K. The decay of
the transient band due to 5 was found to be first order (k )
4.3 × 103 s-1), in accordance with the product analysis study,
indicating that intramolecular H abstraction is the main decay
pathway for 5, and the lifetime was determined to be 233
µs.
(7) For reviews of the EPR spectra of triplet carbenes, see: (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.
(8) Tomioka, H. In AdVances in Strained and Interesting Organic
Molecules; Halton B., Ed.; JAI Press: Greenwich, CT, 2000; Vol. 8, pp
83-112.
Figure 5. Absorption spectrum of transient products formed during
the irradiation of 4 in degassed benzene recorded 1 µs after
excitation. Inset shows the time course of the absorption at 335
nm (oscillogram trace).
(9) (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. II, p 185.
(10) Details of our LFT equipment are reported elsewhere; see ref 2b.
Org. Lett., Vol. 4, No. 13, 2002
2263

The increase in the stability of triplet DPC was achieved
by tuning the distance between the o-hydrogens and the
carbene center. The effect, however, is not large. Inspection
of the X-ray structure of the precursor diazomethane 4
(Figure 1) and final product 6 (Figure 2) is expected to
provide a more quantitative insight into the effect. Since the
position of hydrogen in 4 cannot be determined absolutely,
the bond distances between the 1- and 8-methyl and diazo
carbons are used to roughly estimate the distance between
hydrogen and the carbene carbon. As expected in the
somewhat flexible octahydroanthracene structure, not all of
the methyl groups in 4 are the same distance from the diazo
carbon. The two methyl groups are noticeably closer (2.89
and 2.96 Å) than the other two (3.52 and 3.36 Å). As we
have not determined the X-ray structure of 1, we cannot
evaluate the effect of fixation of one of the methyl groups
into a six-membered ring. The most relevant compound we
could find in the database11 was 2-tert-butylbenzophenone.12
The shortest bond distance between the methyl and carbonyl
carbons in the benzophenone is 3.016 Å. This means that at
least two methyl groups in 4 are forced to move close to the
carbon that will become the carbene center. On the other
hand, all bond lengths and angles in 6 are within the expected
limits for these types of compounds,13 but a slight distortion
of the fused five-membered ring is noted by slight elongation
of two bonds in this ring (1.55 and 1.56 Å). These
observations mean that there might be a slight steric strain
in a cyclic transition state for the H atom abstraction reaction
of carbene 5 leading to 6. Presumably, the stability of the
carbene will be further increased if o-tert-butyl groups are
tied back by incorporation into a smaller ring. Perdeuteration
of all the primary hydrogens on the 1- and 8-methyl groups
will also result in a more persistent triplet DPC. This research
is currently in progress in our laboratory.
Acknowledgment. The authors are grateful to the Min-
istry 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).
Supporting Information Available: X-ray structure
determination for compounds 4 and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL026119P
(13) (a) Pitt, C. G.; Rector, D. H.; White, M. C.; McPhail, A. T.; Miller,
R. C. J. Chem. Soc., Perkin Trans. 1 1976, 2374. (b) Ghosh, A. C.; Hazra,
B. G.; Karup-Nielsen, I.; Kane, M. J.; Hawke, D.; Duax, W. L.; Weeks, C.
M. J. Org. Chem. 1979, 44, 683. (c) Dobbs, T. K.; Hertzler, D. V.; Keen,
G. W.; Eisenbraun, E. D.; Fink, R.; Hossain, M. B.; van der Helm, D. J.
Org. Chem. 1980, 45, 4769. (d) Wilson, R. M.; Dietz, J. G.; Shepherd, T.
A.; Ho, D. M.; Schnapp, K. A.; Elder, R. C.; Watkins, J. W., II; Geraci, L.
S.; Campana, C. F. J. Am. Chem. Soc. 1989, 111, 1749.
(11) Cambridge Structural Data Base; Cambridge Crystallographic Data
Center: Cambridge, UK, 2001.
(12) Wagner, P. J.; Giri, B. P.; Scaiano, J. C.; Ward, D. L.; Gabe, E.;
Lee, F. L. J. Am. Chem. Soc. 1985, 107, 5483.
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