Generation, direct observation and kinetics of triplet (2,4,6-triisopropylphenyl)phenylcarbene

Article abstract of DOI:10.1246/cl.2000.398

The title carbene (2a) was generated and studied not only by product analysis but also by spectroscopic means. The results are compared with those observed for the corresponding terr-butyl analogue (2b), which revealed intriguing and unique natures required for kinetic protectors of triplet carbene.

Full text of DOI:10.1246/cl.2000.398

398
Chemistry Letters 2000
Generation, Direct Observation and Kinetics of Triplet (2,4,6-
Triisopropylphenyl)phenylcarbene
Katsuyuki Hirai, Kentaro Yasuda, and Hideo Tomioka*
Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu, Mie 514-8507
(Received January 5, 2000; CL-000002)
afforded phenylindan (3a)7 and diphenylmethane (4)7 in 10:1
ratio. It is probable that 3a is produced from the photolytically
generated carbene (2a), which underwent insertion into the C-H
bonds of isopropyl methyl groups at the ortho positions. The
formation of 4, on the other hand, can be explained in terms of
hydrogen abstraction of 2a from the methine of o-isopropyl
group, followed by H migration in the resulting diradical. It
may be that 3a is also formed by hydrogen-abstraction mecha-
nism. Similar irradiation of 1a in methanol, however, resulted
in the exclusive formation of methyl ether at the complete
expense of 3a and 4. This is not surprising since methanol is
known8 as a particularly excellent scavenger for most of the
carbenes. These results are in contrast with that observed for
the tert-butyl counterpart (2b) where the exclusive formation of
the corresponding indan (3b) is observed even in methanol.5
Irradiation (λ>300 nm) of 1a in a 2-methyltetrahydrofuran
(2-MTHF) glass at 77 K gave a fine structured EPR line shape
characteristic of randomly oriented triplet molecules with a
large D value attributable to a one-center nπ spin-spin interac-
tion at the divalent carbon of diarylcarbene.9 The zero-field
splitting (ZFS) parameters were D=0.4076 cm-1 and
E=0.01495 cm-1, showing unequivocally that the triplet sig-
nals are due to diphenylcarbene (32a) generated from 1a as a
result of N2 elimination upon irradiation.
The title carbene (2a) was generated and studied not only
by product analysis but also by spectroscopic means. The
results are compared with those observed for the corresponding
tert-butyl analogue (2b), which revealed intriguing and unique
natures required for kinetic protectors of triplet carbene.
Since stable singlet carbenes have been synthesized and
characterized in macroscopic scale,1,2 the stabilization and iso-
lation of a triplet counterpart has emerged as a next challenging
target.3 Recent growing interest in triplet carbenes as potential
organic ferromagnets4 adds more practical meanings on the
project.
The stabilization of a triplet carbene is shown3 to be better
accomplished by steric protection than thermodynamic effects
since the latter strategy not only result in the stabilization of the
singlet but also poses the issue of electronic configuration as a
pure carbene (one-centered diradical) as a result of conjugation.
While steric protection (kinetic stabilization) is therefore a
much better method of stabilizing the triplet, due to the highly
reactive nature of carbenes, this strategy encounters a limitation
when alkyl groups are employed as protecting groups. For
instance, tert-butyl group, which has been recognized as one of
the most effective protecting group, is shown to be ineffective
in protecting triplet carbene center, since it is attacked by proxi-
mate carbene center instantaneously.5
Isopropyl group appears to be an attractive kinetic protec-
tor for carbene since it is not expected to be in too much close
contact with carbene center when introduced at ortho position
of diphenylcarbene, but is still able to block the center from
external reagents. Thus, we generated, characterized and stud-
ied the title carbene (2a) and compared its features with that of
the tert-butyl counterpart (2b).
Similar irradiation of 1a in a soft matrix, i.e., 3-methylpen-
tane (3-MP) glass at 77 K also gave a structured EPR spectrum
3
due to 2a. However, the x and y lines of the spectrum were
closer than those observed in 2-MTHF matrix at 77 K and
analysis of the signals gave markedly smaller ZFS parameters
(D=0.3749 cm-1, E=0.00935 cm-1). Since the E value, when
weighted by D, depends on the magnitude of the central C-C-C
angle, it indicates that, in soft matrices, the carbene relaxes to a
Irradiation of a degassed benzene solution of (2,4,6-triiso-
propylphenyl)phenyldiazomethane (1a)6 at room temperature
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
399
structure with an expanded C-C-C angle presumably to gain
relief from steric compression.10
The results are also to be compared with those observed for
32b, which exhibits essentially zero E value in both matrices at
77 K.11
reagents, although it can not protect the carbenic center from
efficient trapping reagents such as methanol as opposed to tert-
butyl groups.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports, and
Culture, Japan.
Irradiation of 1a in 2-MTHF glass at 77 K was then moni-
tored by UV/vis spectroscopy, which revealed the rapid appear-
ance of new absorption bands at the expense of the original
absorption due to 1a (Figure 1). The new spectrum consists of
two identifiable features, intense UV band with maxima at 316
nm and weak, broad and structured ones with apparent maxima
at 389, 414, 428, and 446 nm. These features are usually pres-
ent in the spectra of triplet diarylcarbenes in organic glasses at
77 K.9b,12 The glassy solution did not exhibit any changes for
several hours when kept at 77 K, but the characteristic bands
disappeared irreversibly when the matrix was allowed to warm
to 100 K. On the basis of these observations, coupled with the
EPR data, the absorption spectrum can be safely assigned to
triplet carbene 2a.
Laser flash photolysis (LFP) of a degassed benzene solu-
tion of benzophenone (BP, 1×10-3 M) as a triplet sensitizer at
room temperature with the third harmonic (355 nm) of a
Nd/YAG laser (ca. 40 mJ, 5-6 ns pulses) produced transient
species showing maxima at 320 and 530 nm apparently due to
triplet BP.13 When LFP of BP was carried out in the presence
of 1a, essentially the same absorption was observed. However,
the absorption band at 530 nm disappeared much faster than
that at 320 nm and the rate of the 530 nm decay increased with
the increasing concentration of 1a.
References and Notes
1
(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. Kine, J.
Am. Chem. Soc., 113, 361 (1991).
2
3
See for reviews, W. A. Herrmann and C. Köcher,
Angew. Chem., Int. Ed. Engl., 36, 2162 (1997).
See for reviews, H. Tomioka, Acc. Chem. Res., 30,
315 (1997), H. Tomioka, "Advances in Carbene
Chemistry," ed by U. Brinker, JAI Press, Stamford
(1998), vol. 2, pp. 176-214.
4
5
For reviews, A. Rajca, Chem. Rev., 94, 871 (1994); H.
Iwamura, Adv. Phys. Org. Chem., 26, 179 (1990).
(a) K. Hirai, K. Komatsu, and H. Tomioka, Chem.
Lett., 1994, 503. (b) H. Tomioka, H. Okada, T.
Watanabe, K. Banno, K. Komatsu, and K. Hirai, J.
Am. Chem. Soc., 119, 1582 (1997).
6
The diazomethane 1a was prepared by the treatment
of (2,4,6-triisopropylphenyl)phenylketimine with
N2O4 followed by the reduction of the resulting N-
nitroso ketimine with LiAlH4; 1H NMR (CDCl3) δ
7.28 (t, J=7.26 Hz, 2 H), 7.01 (t, J=7.26 Hz, 1 H),
7.12 (s, 2 H), 6.79 (d, J=7.26 Hz, 2 H), 3.07 (sept,
J=6.93 Hz, 2 H), 2.95 (sept, J=6.93 Hz, 1 H), 1.30 (d,
J=6.93 Hz, 6 H), 1.19 (d, J=6.93 Hz, 6 H), 1.11 (d,
J=6.93 Hz, 6 H); IR (NaCl) 2024 cm-1.
This suggests that triplet BP is quenched by 1a. The
bimolecular rate constant for triplet BP quenching by 1a was
obtained from a plot of the rate of decay for triplet BP moni-
tored at 530 nm vs. concentration of 1a to be kBP=4×109 M-1 s-1,
which is nearly diffusion-controlled. These observations indi-
cate that energy transfer from triplet BP to 1a occurs.
7
3a (geometrical mixture): a colorless oil; 1H NMR
(CDCl3) δ 7.28-7.10 (m, 3 H), 6.99-6.82 (m, 4 H),
4.45-4.37 (m, 1 H), 3.33-3.12 (m, 1 H), 2.94-2.22 (m,
4 H), 1.30-0.83 (m, 15 H); HRMS for C22H28
: Calcd
m/z 292.2191; obsd 292.2182. 4: a colorless oil; 1H
NMR (CDCl3) δ 7.20 (dd, J=7.35, 7.53 Hz, 2 H), 7.12
(t, J=7.53 Hz, 1 H), 7.00 (d, J=7.35 Hz, 2 H), 6.874
(s, 1 H), 6.868 (s, 1 H), 5.04 (s, 1 H), 4.74 (s, 1 H),
4.09 (s, 2 H), 2.95 (sept, J=6.80 Hz, 1 H), 2.90 (sept,
J=6.98 Hz, 1 H), 1.91 (s, 3 H), 1.28 (d, J=6.98 Hz, 6
The residual spectrum around 320 nm is similar to that
observed during the photolysis of 1a in 2-MTHF glass at 77 K
(vide supra). Thus, we assign the transient product showing the
absorption maximum around 320 nm from the sensitized pho-
H), 1.07 (d, J=6.80 Hz, 6 H); HRMS for C22H28
Calcd m/z 292.2191; obsd 292.2097.
:
3
tolysis of 1a to triplet carbene 2a which must be generated
8
9
For review of O-H insertion of carbenes, see W.
Kirmse, "Advances in Carbene Chemistry," ed by U.
Brinker, JAI Press, Stamford (1994), vol. 1, pp. 1-57.
(a) A. Trozzolo and E. Wasserman, "Carbenes," ed by
M. Jones, Jr. and R. A. Moss, Wiley, New York
(1975), vol. 2, pp. 185-206. (b) W. Sander, G.
Bucher, and S. Wierlacher, Chem. Rev., 93, 1583
(1993).
from triplet excited state of 1a. The decay of the transient band
due to 2a was found to be first order, in accordance with the
3
product analysis data, showing that intramolecular H abstrac-
tion leading to 3 and 4 is the main decay pathway for triplet
carbene (32a) under these conditions, and the lifetime was
determined to be 129 µs, which is essentially the same with that
of 32b ( τ=120 µs ).5
10 a) H. Tukada, T. Sugawara, S. Murata, and H.
Iwamura, Tetrahedron Lett., 27, 235 (1986). b) A. S.
Nazran, F. L. Lee, E. J. Gabe, Y. Lepage, D. J.
Northcott, J. M. Park, and D. Griller, J. Phys. Chem.,
88, 5251 (1984). c) B. C. Gilbert, D. Griller, and A.
S. Nazran, J. Org. Chem., 50, 4738 (1985). d) H.
Tomioka, T. Watanabe, K. Hirai, K. Furukawa, T.
Takui, and K. Itoh, J. Am. Chem. Soc., 117, 6376
(1995).
The comparison of the data between the two carbene sys-
tems reveals intriguing and unique natures required for kinetic
protectors of triplet carbenes.
3
The significant E/D values of 2a as opposed to the essen-
tially zero E value for 32b under the identical conditions clearly
3
suggest that steric congestion around the carbene center in 2a
3
is less severe than that in 2b, as expected. This suggests that
3
the two methyls of o-isopropyl groups in 2a are directing
11 ZFS parameters for 32b are; D=0.3408 cm-1, E~0
cm-1 and D=0.3390 cm-1, E~0 cm-1, in 2MTHF
and 3-MP at 77 K, respectively.
opposite to the carbene center in order to avoid steric interac-
tion at least in matrices at low temperature.
12 A. M. Trozzolo, Acc. Chem. Res., 1, 329 (1968).
13 M. V. Encinas and J. C. Scaiano, J. Am. Chem. Soc.,
103, 6393 (1981).
The observations reveal that o-isopropyl groups can act as
kinetic protector of triplet carbene comparable to sterically
more bulky tert-butyl group in the absence of efficient external