Preparation of sterically congested Di(4-pyridyl)diazomethanes and characterization of triplet carbenes from them

Article abstract of DOI:10.1021/ol047467p

(Chemical Equation Presented) Di(4-pyridyl)diazomethanes having two and four ortho chlorine groups were prepared, and the triplet carbenes generated from them were characterized by ESR and UV/vis at low temperature and time-resolved UV/vis at room tempera

Full text of DOI:10.1021/ol047467p

ORGANIC
LETTERS
2
005
Vol. 7, No. 5
11-814
Preparation of Sterically Congested
Di(4-pyridyl)diazomethanes and
Characterization of Triplet Carbenes
from Them
8
†
†
‡
,†
Tetsuji Itoh, Akira Takada, Katsuyuki Hirai, and Hideo Tomioka*
Chemistry Department for Materials, Faculty of Engineering, Mie UniVersity,
Tsu, Mie 514-8507, Japan, and Instrumental Analysis Facilities,
Life Science Research Center, Mie UniVersity, Tsu, Mie 514-8507, Japan
tomioka@chem.mie-u.ac.jp
Received December 9, 2004
ABSTRACT
Di(4-pyridyl)diazomethanes having two and four ortho chlorine groups were prepared, and the triplet carbenes generated from them were
characterized by ESR and UV/vis at low temperature and time-resolved UV/vis at room temperature. An appreciable increase in the stability
of triplet carbenes is achieved by introducing ortho chlorine groups.
Many attempts have been made to prepare purely organic
spin systems. Such approaches have several problems, which
prevent the formation of usable systems. For instance, a
between radical centers and metal ions can be realized
through a pyridyl ligand to generate a high spin unit (Scheme
1). This system allows us to extend the dimension of the
1
“
bottom-up” synthetic approach to obtaining molecular
magnets is faced with synthetic limitations, especially when
the orders of the alignment of spins become higher. On the
other hand, efforts to increase the number of aligned spins
are hampered by the development of antiferromagnetic intra
and/or interchain interactions between the radical centers.
Scheme 1
To overcome these problems, heterospin systems compris-
ing of 2p spins of organic radicals and 3d spins of magnetic
2,3
metal ions have been proposed. The strategy is based on
the supramolecular chemistry exhibited by pyridine- and
4
polypyridine-metal ions. For instance, magnetic interaction
spin network from one (1D) to two (2D) and three (3D) by
simple self-assembly between pyridyl groups and metal ions.5
Pyridines having a triplet carbene unit have been success-
fully employed to generate high heterospin polycarbenes with
†
Faculty of Engineering.
‡
Life Science Research Center.
(
1) (a) Iwamura, H. AdV. Phys. Org. Chem. 1990, 26, 179. (b) Dougherty,
D. A. Acc. Chem. Res. 1991, 24, 88. (c) Rajca, A. Chem. ReV. 1994, 94,
71. (d) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel
Dekker: New York, 1999.
2) Koga, N.; Iwamura, H. In Carbene Chemistry; Bertrand, G., Ed.;
Fontis Media: Lausanne, 2002; pp 271-296.
8
(3) (a) Koga, N.; Iwamura, H. In ref 1c, pp 629-659. (b) Koga, N.;
Iwamura, H. Mol. Cryst. Liq. Cryst. 1997, 305, 415. (c) Iwamura, H.; Koga,
N. Mol. Cryst. Liq. Cryst. 1999, 334, 437. (d) Iwamura, H.; Koga, N. Pure
Appl. Chem. 1999, 71, 231.
(
1
0.1021/ol047467p CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/01/2005

D- and/or 3D-spin networks.2,6 However, the chemical
Irradiation (λ > 300 nm) of 1c-N
(2.0 × 10 M) in
-2
2
2
9
stability of the triplet carbene in these systems is ignored.
In this respect, it is desirable to design and prepare a
persistent triplet pyridylcarbene. We report here our attempts
propanetriol triacetate (PT) glass at 77 K gave ESR signals
with typical fine structure patterns for unoriented triplet
1
0,11
3
species,
i.e., 1c. Signals at 109.0, 476.9, and 531.8 mT
to generate and characterize triplet di(4-pyridyl)carbenes
are assigned to a low-field z and a set of high-field x and y
3
(
DPyCs, 1b and c) having two and four chlorine groups at
transitions, respectively, from which the zero-field splitting
7
-1
the ortho positions as a kinetic protector.
(ZFS) parameters were obtained as D ) 0.409 cm and E
-
1
Most sterically congested diaryldiazomethanes have been
successfully prepared by base treatment of the corresponding
) 0.013 cm (E/D ) 0.032).
The thermal stability of the triplet carbenes could be
estimated by thawing the matrix containing triplet carbenes
gradually and recooling again to 77 K to measure the signal.
This procedure can compensate for the weakening of signals
8
N-(nitroso)methylcarbamates. However, all attempts to pre-
pare the desired precursor diazomethanes, [4-(3,5-dichloro)-
2
pyridyl](4-pyridyl)diazomethane (1b-N ) and bis[4-(3,5-
3
dichloro)pyridyl]diazomethane (1c-N ), were unsuccessful by
2
due to Curie’s Law. When the matrix containing 1c was
this procedure. We thus chose the second route, which is
the nitrosation followed by the reduction of the corresponding
ketimines by lithium aluminum hydride (LAH). The desired
warmed gradually in 10 K increments, the ESR signals of
1c disappeared at around 200 K.
Similar measurements were carried out with 1b-N and
2
3
8
[
4-(3,5-dichloro)pyridyl](4-pyridyl)ketimine was prepared by
the data are summarized in Table 1. Data for the “parent”
triplet di(4-pyridyl)carbene ( 1a) are included for the sake
of comparison.
3
the conventional method. Thus, treatment of 3,5-dichloro-
pyridine with lithium diisopropylamine (LDA) followed by
addition of 4-cyanopyridine gave the ketimine in good yield.
Nitrosation of the ketimine followed by reduction with LAH
2
gave the corresponding diazo compound (1b-N ) as a yellow
solid (Scheme 2). Tetrachlorinated ketimine could not be
Table 1. Spectroscopic Data for Di(4-pyridyl)carbenesa
carbenes |D| (cm-1) |E| (cm-1
)
E/D
λmax (nm) Tdb (K)
1
1
1
a
b
c
0.437
0.421
0.409
0.020
0.019
0.013
0.046 300, 500
0.043 512, 467
0.032 360, 500
160
180
200
Scheme 2
a
Measured in PT at 77 K. b Temperature at which the ESR spectra of
triplet di(4-pyridyl)carbenes disappeared.
The zero-field splitting (ZFS) parameters D and E give
information on the molecular and electronic structures of
1
0,11
triplet carbenes.
The D value is related to the separation
between the unpaired electrons. The E value, when weighted
by D, (i.e., E/D), is a measure of the deviation from axial
symmetry. For diarylcarbenes, this value thus depends on
the magnitude of the central C-C-C angle. Put more simply,
the more the two electrons are delocalized in carbenes with
a conjugated carbene π-system, the smaller the value of the
repulsive interaction D will be. On the other hand, increasing
the bond angle at the carbene center leads to a higher
p-orbital contribution and a smaller value for E. Although
the values D and E depend on the electronic distribution, it
has been shown that there is a good correlation between the
E/D ratio and the bond angle at the divalent carbon atom.
Inspection of the data in Table 1 reveals the effect of ortho
chlorine groups on the structures and stabilities of triplet
prepared similarly but was obtained by treatment of the
corresponding methanesulfonate with sodium azide. Nitro-
sation of the ketimine followed by reduction gave 1c-N as
2
a yellow solid (Scheme 2). All diazomethanes were purified
by repeated cycles of gel permeation chromatography fol-
lowed by preparative TLC.
(4) (a) Lehn, J.-M. Supramolecular Chemistry; VCH Publisher: New
York, 1995. (b) Nierengarten, J.-F.; Dietrich-Buchecker, C. O.; Sauvage,
J.-P. J. Am. Chem. Soc. 1994, 116, 375. (c) Leininger, J.-F.; Olenyuk, B.;
Stang, P. J. Chem. ReV. 2000, 100, 853.
3
3
DPyC. Both D and E/D values decrease from 1a to 1b to
(
5) Iwamura, H.; Inoue, K.; Koga, N. New J. Chem. 1998, 201.
3
1
c. The observed decrease in the D and E/D values is then
(6) (a) Karasawa, S.; Kumada, H.; Iwamura, H.; Koga, N. J. Am. Chem.
Soc. 2001, 123, 9685. (b) Karasawa, S.; Koga, N. Polyhedron 2001, 20,
387.
7) For reviews of persistent triplet carbenes, see: (a) Tomioka, H. Acc.
explained as indicating that the carbene bond angle expands
as more chlorine groups are introduced at the ortho positions,
1
(
Chem. Res. 1997, 30, 315. (b) Tomioka, H. In AdVances in Carbene
Chemistry; Brinker, U., Ed.; JAI Press: Greenwich, CT, 1998; Vol. 2. pp
(9) Because of inherent instable nature of triplet di(4-pyridyl)carbenes,
a viscous matrix such as propanetriol triacetate was required to obtain ESR
signal at 77 K.
1
75-214. (c) Tomioka, H. In AdVances in Strained and Interesting Organic
Molecules; Halton, B., Ed.; JAI Press: Greenwich, CT, 2000; Vol. 8, pp
3-112. (d) Tomioka, H. In Carbene Chemistry; Bertrand, G., Ed.; Fontis
Media, S. A.: Lausanne, 2002; pp 103-152.
8) (a)Zimmerman, H. E.; Paskovich, D. H. J. Am. Chem. Soc. 1964,
6, 2149. (b) Regitz, M.; Maas, G. Diazo Compounds-Properties and
Synthesis; Academic Press: Orlando, 1986.
8
(10) For reviews of the EPR and UV/vis 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
(11) Wasserman, E.; Hutton, R. S. Acc. Chem. Res. 1977, 10, 27.
812
Org. Lett., Vol. 7, No. 5, 2005

most probably as a result of steric repulsion between the
ortho substituents and increased delocalization of the elec-
trons. In other words, as more chlorine groups are introduced
at ortho positions, triplet DPyCs are more thermodynamically
stabilized. The ortho chlorine groups are also expected to
stabilize triplet DPyCs kinetically, by protecting the carbenic
center from external reagents.12 In accord with this inter-
pretation, the thermal stability of triplet DpyCs increased
significantly in this order, as judged from the temperature
at which ESR signals and UV/Vis absorption bands (vide
infra) disappear (Td).
observed under identical conditions, the absorption spectrum
3
can be assigned to triplet carbene 1c. When the PT matrix
3
containing 1c was gradually warmed, the absorption bands
3
due to 1c started to decompose at around 210 K and
disappeared completely at 230 K. Similar measurements were
carried out with 1a-N and 1b-N and the absorption maxima
2 2
are summarized in Table 1.
Laser flash photolysis(LFP) of 1-N was then carried out.13
2
LFP of 1c-N in a degassed benzene solution at room
2
temperature with a 10 ns, 70-90 mJ, 308 nm pulse from a
XeCl laser produced no apparent new transient absorption
band. This is obviously because of the inherent weak nature
of the absorption band due to 1c (vide supra). However,
when LFP was carried out on a nondegassed benzene solution
2
containing 1c-N , a new broad absorption band at 370 nm
To determine the effects of o-chlorine groups on the
kinetics of triplet DPyCs in solution at room temperature,
3
time-resolved UV/Vis spectroscopic measurements of 1-N
2
were carried out. Irradiation of 1 in PT matrix at 77 K was
first monitored by UV/Vis spectroscopy, to determine the
spectroscopic features of triplet DPyCs. Irradiation (λ > 300
appeared (Figure 2). The spent solution was found to contain
-
2
nm) of 1c-N
appearance of new absorption bands centered at 500 and 560
nm at the expense of the original absorption due to 1c-N
Figure 1). The new bands are very weak and could not be
2
(2.0 × 10 M) in PT at 77 K resulted in the
2
(
Figure 2. Transient absorption spectrum obtained in LFP of diazo
compound 1c-N
laser recorded 1 µs after the pulse. The inset shows a plot of the
growth rate of the oxide (1c-O ) monitored at 370 nm as a function
of the concentration of oxygen.
2
in nondegassed benzene with 308-nm excimer
2
Figure 1. UV-vis spectral changes observed in the photolysis of
diazo compound 1c-N . (a) UV-vis spectrum of 1c-N in PT at
bis[4-(3,5-dichloro)pyridyl]ketone (1c-O) as the main prod-
uct. It is well documented that diarylcarbenes with triplet
ground states are readily trapped by oxygen to generate the
corresponding diaryl ketone oxides, which are readily
observed directly either by matrix isolation spectroscopy or
by flash photolysis techniques, and show a broad absorption
2
2
7
7 K. (b) Same sample after irradiation (λ > 300 nm). (c-e) Same
sample after warming the matrix to (c) 210, (d) 220, and (e) 230
K.
detected when the initial concentration of diazo compound
was adjusted for usual UV/vis measurements, i.e., in the order
14,15
band centered at 390-450 nm.
Thus, the observations can be interpreted to indicate that
-
4
of 10 M. Generally, UV/vis spectra of triplet diphenyl-
carbenes consist of two identifiable features, an intense UV
band with a maximum at around 290-320 nm and weak
structured bands at around 400-500 nm.10 The observed
absorption bands are appreciably different from those
observed for typical triplet diphenylcarbenes. However, since
strong ESR signals ascribable to a triplet carbene are
3
the triplet carbene 1c is trapped by oxygen to form the
2
carbonyl oxide (1c-O ). The apparent build-up rate constant,
k
obs, of the carbonyl oxide increases as the concentration of
oxygen increases. The rate constant kobs is expressed as given
in eq 1
kobs ) k + k [O ]
(1)
0
O
2
2
(
12) Tomioka, H.; Hirai, K.; Fujii, C. Acta Chim. Scand. 1993, 46, 680.
b) Tomioka, H.; Hirai, K.: Nakayama, T. J. Am. Chem. Soc. 1993, 115,
285. (c)Kawano, M.; Hirai, K.; Tomioka, H.; Ohashi, Y. J. Am. Chem.
(
3
0
where k represents the rate of decay of 1c in the absence
1
3
Soc. 2001, 123, 6904.
of oxygen and kO2 is the quenching rate constant of 1c with
Org. Lett., Vol. 7, No. 5, 2005
813

oxygen. As shown in the inset of Figure 2, the plot of the
observed pseudo-first-order rate constant for the formation
carbene, i.e., oxygen,14,15 is also significantly retarded when
four ortho chlorine groups are introduced.
of 1c-O
2
against [O
2
] is linear. From the slope of this plot,
It is interesting to compare the effect of chlorine groups
on triplet DPyCs with that on triplet diphenylcarbenes
3
the rate constant for the quenching of 1c by oxygen was
7
-1 -1
12
determined to be 6.5 × 10 M s . The intercept of the
(DPCs). It has been shown that the lifetime of triplet bis-
4
-1
12b
slope (k
0
) was determined to be 1.7 × 10 s . The lifetime
(2,4,6-trichlorophenyl)carbene is 18 ms, which is some 4
3
16
of 1c was thus estimated to be 60.0 µs.
orders of magnitude greater than that of the “parent” triplet
Similar measurement was carried out with other di(4-
diphenylcarbene (τ ) 1 µs). Thus, the effect of o-Cl on
DPyCs is notably smaller than that on DPCs.
3
3
pyridyl)diazomethanes (1a-N
2
2
and 1b-N ) and kinetic data
are summarized in Table 2.
It is interesting to note here that the ZFS parameter of
3
-1
parent DPC (D ) 0.4055 cm ) is smaller than that of parent
3
3
-1
DPyC, 1a (D ) 0.437 cm ). In addition, the rate of the
decrease in D values on introduction of four o-Cl groups in
Table 2. Kinetic Data for Reaction of Triplet
3
3
-1
3
DPyC, i.e., on going from 1a (D ) 0.437 cm ) to 1c (D
Di(4-pyridyl)carbenes with Oxygena
-
1
)
0.409 cm ), is ca. 6%, which is apparently smaller than
carbenes λmaxb (nm)
ko (s-1
)
τcarbene (µs)
c
kO (M s-1
-1
)
3
2
the corresponding decrease in DPC, i.e., on going from DPC
(D ) 0.4055 cm ) to chlorinated DPC (D ) 0.364 cm )
(ca 10%). These results suggest that in DPyCs, unpaired
6
5
4
9
-1
3
-1 12
1a
1b
1c
425
410
370
1.9 × 10
7.9 × 10
1.7 × 10
0.53
1.3
60
1.9 × 10
8
3
6.6 × 10
7
6.5 × 10
electrons are more localized on the carbene center than in
3
a
b
DPCs, and are not effectively delocalized to the aromatic
Measured in benzene at 20 °C. Absorption maxima of the corre-
c
sponding carbonyl oxide (1-O2). τcarbene ) 1/ko.
ring, even on introduction of o-Cl groups.
We have shown that the triplet DpyC, a unit for high
heterospin polycarbenes, can be stabilized by steric protection
to some extent, but that this carbene is more difficult to
stabilize than triplet DPCs.
Inspection of the data in Table 2 indicates that chlorine
groups at the ortho positions effectively stabilize DPyCs only
when they are introduced at all the ortho positions. The half-
life is increased only slightly when two chlorine groups are
introduced, but it is increased by 2 orders of magnitude when
all the ortho hydrogens are replaced with chlorine groups.
The reaction with the most efficient quencher of triplet
Acknowledgment. We 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 for
Specially Promoted Research (No. 12002007).
(
13) For details of our LFP equipments, see; Tomioka, H.; Okada, H.;
Supporting Information Available: EPR spectra of 1a,
Watanabe, T.; Banno, K.; Komatsu, K.; Hirai, K. J. Am. Chem. Soc. 1997,
19, 1582.
1
b, and 1c (Figures S1, S2, and S3), UV/vis spectra of 1a
and 1b (Figures S4 and S5), time-resolved UV-vis spectra
of 1a-O and 1b-O (Figures S6 and S7). Preparation of 1b-
and 1c-N . This material is available free of charge via
1
2
1
(14) For review, see: (a) Sander, W. Angew. Chem., Int. Ed. Engl. 1990,
9, 344. (b) Bunnelle, W. Chem. ReV. 1991, 91, 336.
2
2
(15) Scaiano, J. C.; McGimpsey, W. G.; Casal, H. L. J. Org. Chem.
N
2
2
989, 54, 1612.
16) Carbonyl oxide formation has been used to monitored carbenes that
(
the Internet at http://pubs.acs.org.
are transparent in the useful UV region. See, for instance; Morgan, S.; Platz,
M. S.; Jones, M., Jr.; Myers, D. R. J. Org. Chem. 1991, 56, 1351.
OL047467P
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Org. Lett., Vol. 7, No. 5, 2005