Photolysis of regioisomeric diazides of 1,2-diphenylacetylenes studies by matrix-isolation spectroscopy and DFT calculations.

Article abstract of DOI:10.1039/b308138a

A series of diazides of 1,2-diphenylacetylenes was photolyzed in matrices at low temperature and transient photoproducts were characterized by using IR, UV/vis methods combined with ESR studies. Theoretical calculations were also used to understand the experimental findings. The introduction of phenylethynyl groups on phenyl azides has little effect on the photochemical pathway. Thus, upon photoexcitation, (phenylethynyl)phenyl azides afforded the corresponding triplet nitrene, which is in photoequilibrium with the corresponding azacycloheptatetraene. In marked contrast, azidophenylethynyl groups exhibited a dramatic effect not only on the photochemical pathway of phenyl azides but also on the electronic and molecular structure of the photoproducts. The patterns of the effect depended upon the relative position of azide groups in the diphenylacetylene unit. Whenever two azide groups were situated in a conjugating position with respect to each other, as in p,p'-, o,o'-, and p,o'-bis(azides), the azides always resulted in the formation of a quinoidal diimine diradical in which unpaired electrons were extensively delocalizedin the pi-conjugation. The situation changed rather dramatically when azide groups were introduced in the meta position. Thus, the formation of azacycloheptatetraene was noted in the photolysis of the m.m'-isomer. ESR studies indicated the generation of a quintet state that was shown to be a thermally populated state with a very small energy gap of ca. 100 cal mol(-1). The m,p'-isomer was shown to be an excellent precursor for the high-spin quintet dinitrene.The IR spectra of the photoproduct showed no bands ascribable to azacycloheptatetraene. The observed spectra were in good agreement with that calculated for the quintet state. Strong EPR signals assignable to the quintet state were observed, along with rather weak signals due to mononitrenes. Moreover, the quintet bis(nitrene) was rather photostable under these conditions.

Full text of DOI:10.1039/b308138a

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Photolysis of regioisomeric diazides of 1,2-diphenylacetylenes
studied by matrix-isolation spectroscopy and DFT calculations†
Hideo Tomioka* and Shinji Sawai
Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu,
Mie 514-8507, Japan
Received 18th July 2003, Accepted 7th October 2003
First published as an Advance Article on the web 30th October 2003
A series of diazides of 1,2-diphenylacetylenes was photolyzed in matrices at low temperature and transient
photoproducts were characterized by using IR, UV/vis methods combined with ESR studies. Theoretical calculations
were also used to understand the experimental findings. The introduction of phenylethynyl groups on phenyl azides
has little effect on the photochemical pathway. Thus, upon photoexcitation, (phenylethynyl)phenyl azides afforded
the corresponding triplet nitrene, which is in photoequilibrium with the corresponding azacycloheptatetraene. In
marked contrast, azidophenylethynyl groups exhibited a dramatic effect not only on the photochemical pathway of
phenyl azides but also on the electronic and molecular structure of the photoproducts. The patterns of the effect
depended upon the relative position of azide groups in the diphenylacetylene unit. Whenever two azide groups were
situated in a conjugating position with respect to each other, as in p,pЈ-, o,oЈ-, and p,oЈ-bis(azides), the azides always
resulted in the formation of a quinoidal diimine diradical in which unpaired electrons were extensively delocalized
in the π-conjugation. The situation changed rather dramatically when azide groups were introduced in the meta
position. Thus, the formation of azacycloheptatetraene was noted in the photolysis of the m,mЈ-isomer. ESR studies
indicated the generation of a quintet state that was shown to be a thermally populated state with a very small energy
Ϫ1
gap of ca. 100 cal mol . The m,pЈ-isomer was shown to be an excellent precursor for the high-spin quintet dinitrene.
The IR spectra of the photoproduct showed no bands ascribable to azacycloheptatetraene. The observed spectra
were in good agreement with that calculated for the quintet state. Strong EPR signals assignable to the quintet state
were observed, along with rather weak signals due to mononitrenes. Moreover, the quintet bis(nitrene) was rather
photostable under these conditions.
7
g–j
Introduction
to a singlet phenylnitrene and relaxes to a triplet counterpart.
The triplet nitrene within inert matrices at low temperature is
shown to undergo ring expansion to give the tetraene upon
The photochemistry of phenyl azides has attracted interest in
view of the useful applications of azides in heterocyclic syn-
5a,h
photoexcitation.
1
1,2
theses, photoresist techniques and the biochemical method
The photochemistry of phenyl polyazides has started to
attract ever-increasing interest in view of the modern appli-
cations of polyazides in organo-ferromagnetic materials. Since
experimental studies found that m-phenylenedinitrene is a
ground-state quintet, many attempts have been made to gen-
1,3,4
of photo-affinity labeling.
The complete picture of the chemical reactions initiated by
the elimination of molecular nitrogen has recently begun to
unfold. A preparative study has revealed various chemical paths
that require the involvement of different intermediates and
matrix isolation, and a flash-photolytic investigation has
provided direct evidence for the intermediates (Scheme 1). The
photolysis of phenyl azide generates a singlet phenylnitrene,
which isomerizes to benzazirine. Benzazirine can be intercepted
9
10
erate and characterize polynitrenes, which are connected to
appropriate linkers in a ferromagnetic fashion. For instance,
two nitrene centers are introduced into a coupling linker group
5,6
7
11
12
13
13c,d
such as benzene, biphenyl, stilbene, diphenylbutadiene,
13c,d
14
diphenyloctatetraene,
diphenylacetylene,
diphenylbuta-
8
with thiol before it opens to form azacycloheptatetraene. This
14
15
16
diyne, tetraphenylallene, benzophenone and 1,1-diphenyl-
ethylene, diphenyl sulfide, azobenzene and thiophene to
construct a π-conjugated system with the general structure of
species can be intercepted with diethylamine to form an azepine
16
17
17
18
7a,c
adduct. In the absence of amine, the tetraene reverts slowly
:N–Ph–X–Ph–N: and various possible connectivity types.
ESR measurements have been crucial in characterizing
high-spin species, and they have been used successfully in many
systems. However, the method has a serious disadvantage.
Basically, it affords almost no information on the diamagnetic
species that may be formed during irradiation. The fact that a
triplet phenylnitrene easily undergoes ring-expansion upon
photo-excitation, for example, clearly suggests that the high-
spin species may not be photostable even at very low temper-
ature. An independent investigation is thus desirable to reveal
the overall reactions occurring upon photo-irradiation of
polyazide compounds.
This paper is the report of an investigation in which aromatic
diazides were photolyzed in matrices at low temperature and
transient intermediates were characterized by using IR and UV/
vis methods combined with ESR studies. Theoretical calcu-
lations were also used to understand the experimental findings.
Scheme 1
†
(
Electronic supplementary information (ESI) available: Tables S1–S7
observed and calculated IR bands) and Figs. S1–3. See http://
www.rsc.org/suppdata/ob/b3/b308138a/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 4 1 – 4 4 5 0
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Two phenylazide units linked by an acetylene bond (2) were
selected as the bisazides for this study. In order to learn the
effect of the acetylene bond on the photochemistry of phenyl
azide, the monoazide counterparts (1) were also investigated.
Results
Photolysis of ethynylphenyl azides (1)
The deposition of m-ethynylphenyl azide (m-1a-N ) in Ar at
3
2
0 K gives an IR spectrum with absorptions at 2116 and 2098
Ϫ
1
Ϫ
1
cm arising from the azide group and at 3318 cm due to the
acetylenic C–H bond. Broadband irradiation (λ > 350 nm) of
the sample at 12 K resulted in a rapid decrease in the bands
attributable to the starting material and the concurrent appear-
ance of new bands in the IR (Fig. 1). Analysis of these product
absorption bands by plotting their intensities as a function of
the irradiation time and wavelength of light suggested that
Fig. 2 (a) UV/vis spectrum of 3-ethynylphenyl azide (m-1a-N3)
matrix-isolated in argon at 13 K. (b) UV/vis spectrum obtained by
irradiation of (a) at λ > 420 nm. (c) UV/vis spectrum obtained by
irradiation of (b) at λ > 300 nm.
there were at least two photoproducts: A (3324 s, 1476 w, 972 w,
Ϫ1
8
64 m, 720 m, 646 s, and 610 s cm ) and B (3318 s, 1888 s, 1380
Ϫ1
m, 1230 m, 796 s, 752 m, 694 s, and 606 s cm ). Control
experiments showed that those two products were inter-
convertible upon irradiation. Thus, irradiation of the initial
photomixtures with light at λ > 420 nm resulted in the dis-
appearance of the bands ascribable to B and in the concurrent
growth of the bands due to A. Irradiation of the mixture with
light of λ > 300 nm then reproduced the IR bands ascribable to
B at the expense of those of A. The UV/vis spectral changes
were not so prominent as those of IR, but they exhibited similar
photo-interconversion (Fig. 2).
shift of the acetylenic C–H absorption band in going from 1 to
B, then, suggested that the ethynyl group was intact during this
transformation. Thus, 4-ethynyl-1,2,4,6-azacycloheptatetraene
(m-1a-AZA) was proposed as the most probable structure for
B, which can be formed by loss of nitrogen from m-1-N , fol-
3
lowed by ring expansion of the resulting nitrene (m-1a-N). The
product A, which is interconvertible with B upon irradiation
under these conditions, is then likely assignable to 3-ethynyl-
phenylnitrene (m-1a-N) in its triplet state (Scheme 2).
Scheme 2
Further assignment of the IR spectra of m-1a-AZA and
m-1a-N was accomplished by comparison with the DFT calcu-
19–21
lation
at the UB3LYP/6-31G* level of theory (Table S1
and Fig. 1). The vibrational frequencies were calculated for
those compounds and were compared with the experimental
vibrational frequencies observed with m-1a-AZA and m-1a-N,
which indicated that the calculated frequencies match the
experimental data reasonably well.
Fig. 1 Difference IR spectrum showing the photochemistry of 3-
ethynylphenyl azide (m-1a-N ) matrix-isolated in argon at 13 K.
3
(
a) Bands pointing upward are that of m-1a-N and bands pointing
3
downward are appearing upon irradiation at λ > 300nm. (b) Bands
pointing downward are appearing upon subsequent irradiation at λ
Support is lent to this assignment by ESR studies. When the
photolysis (λ > 350 nm) of m-1-N in 2-methyltetrahydrofuran
>
420 nm of the initial photoproduct from m-1a-N upon irradiation at
3
3
λ > 300 nm. (c) Calculated IR spectrum for triplet 3-ethynylphenyl-
nitrene (m-1a-N). (d) Calculated IR spectrum for 4-ethynyl(aza)-
cycloheptatetraene (m-1a-AZA).
(2-MTHF) matrix at 77 K was monitored by ESR, a sharp
signal appeared at around 700 mT. On the basis of the calcu-
Ϫ1
lated zero-field splitting (ZFS) parameter (|D/hc| = 1.15 cm ) in
22
comparison with that of triplet phenylnitrene, this signal is
assigned to triplet 3-ethynylphenylnitrene (m-1a-N). The
intensity of the signal was decreased upon irradiation with light
at λ > 420 nm but was recovered upon irradiation with light at
λ > 300 nm. Thus, the interconversion between a triplet nitrene
(m-1a-N) and the tetraene (m-1a-AZA) is suggested.
What are these photoproducts? Analysis of the major IR
bands provided some insights into their structures. First, the
appearance of an intense 1888 cm band in B suggested the
presence of a strained heterocumulene bond, indicating an
aza-1,2,4,6-cycloheptatetraene structure for B. Only a slight
Ϫ1
4
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Essentially, the same chemistry was observed in the
photolysis of 1-(3-azidophenyl)-2-phenylacetylene (m-1b-N ), a
3
phenylated derivative of 1a. Thus, the irradiation generated
triplet 4-(phenylethynyl)phenylnitrene (m-1b-N) and 4-(phenyl-
ethynyl)-1,2,4,6-azacycloheptatetraene (m-1b-AZA), which
were again shown to be photointerconvertible under these
conditions (Scheme 2, Fig. S1).
Irradiation of the p-isomers (p-1-N ) afforded similar results.
3
Thus, the irradiation resulted in the formation of triplet nitrene
(
p-1-N) and azacycloheptatetraene (p-1-AZA), which were
interconvertible upon irradiation (Scheme 3). Again, the spectra
calculated for m-1b-AZA and m-1b-N fit the experimental
spectra (Fig. 3). An intriguing difference was observed, how-
ever, between the structures of the two nitrenes, m-1-N and p-1-
N. The UV/vis spectra (Fig. 4) and ESR showed that spin
delocalization into the ethynyl groups is more significant in the
p-isomer than in the m-isomer. Thus, the ZFS parameters for
Fig. 4 (a) UV/vis spectrum of 4-ethynylphenyl azide (p-1a-N ) matrix-
3
isolated in argon at 13 K. (b) UV/vis spectrum obtained by irradiation
Ϫ1
p-1a-N and p-1b-N are estimated to be 1.01 and 0.93 cm ,
of (a) at λ > 420 nm. (c) UV/vis spectrum obtained by irradiation of
(
b) at λ > 366 nm.
23
respectively. More prominently, the UV/vis spectra of p-1-N
exhibited a series of sharp absorptions in the visible region,
which are to be compared with that of the m-isomer. Thus, the
smaller D values in ESR as well as the structured bands in the
visible region both suggest that, in p-1-N, the contribution of
the quinoidal structure as a result of the delocalization of the
unpaired electrons is important.
It is noteworthy that those sharp absorptions are red-shifted
from 444, 468, and 487 to 506, 522, and 566 nm, respectively, on
going from p-1a-N to p-1b-N. This can be interpreted to indi-
cate that, in p-1b-N, the delocalization of the unpaired electrons
onto the terminal phenyl ring is also important.
Photolysis of bis(azidophenyl)acetylenes (2)
Scheme 3
p,pЈ-Isomer. Irradiation (λ > 390 nm) of p,pЈ-2-N ,N matrix-
3
3
isolated in Ar at 10 K resulted in the rapid and complete dis-
appearance of strong bands characteristic of azide groups,
indicating that both azide groups were destroyed to generate
bis(nitrene). New bands formed upon irradiation showed a
Ϫ1
rather strong and sharp absorption band at 822 cm along with
Ϫ1
small bands at 962, 1082, and 1230 cm , but they exhibited no
Ϫ1
absorption in the region of 1780 to 1990 cm , where bands
ascribable to strained cumulenic bonds appeared (Fig. 5). The
initial photoproduct was photostable under these conditions.
This suggests that nitrene generated from p,pЈ-2-N ,N3 is
3
photostable and does not undergo ring-enlargement to form
azacycloheptatetraene, e.g., p,pЈ-2-AZA, even upon photo-
excitation. The matrix also took on a distinct pink color, and a
series of sharp strong absorptions was observed at 333, 398,
4
01, 420, and 475 nm in the UV/vis spectrum. These strong
absorptions in the visible region indicate the presence of the
extended π-system (Fig. 6a). Therefore, bisnitrene exists as a
delocalized bisquinoimine structure (p,pЈ-2-Q) rather than
localized bis(nitrene) (p,pЈ-2-N,N) (Scheme 4). Similar struc-
tured absorptions were observed in the photolysis of 4,4Ј-
diazidostilbene, in which the bisquinimine structure was
13a
also proposed. Actually, the vibrational frequencies calcu-
lated for the delocalized bis(iminoquinone) diradical (triplet)
match the experimental vibrational frequencies observed for the
photoproduct from p,pЈ-2-N ,N fairly well (Fig. 5).
3
3
In order to gain more insights into the structure of the
bisnitrene, the photolysis was monitored by ESR in 2-MTHF at
77 K, which indicated a sharp signal at 642 mT ascribable to a
Fig. 3 Difference IR spectrum showing the photochemistry of
4
-ethynylphenyl azide (p-1a-N ) matrix-isolated in argon at 13 K.
3
Ϫ1
(
a) Bands pointing upward are those of p-1a-N and bands pointing
triplet mononitrene (|D/hc| = 0.92 cm ) along with weak sig-
3
downward are appearing upon irradiation at λ > 420nm. (b) Bands
nals at 472 and 63 mT and a signal in the gЈ= 2 region (Fig. 7a).
When the photolysis of p,pЈ-2-N ,N was monitored by the UV/
pointing downward are appearing upon subsequent irradiation at
3
3
λ > 366 nm of the initial photoproduct from p-1a-N upon irradiation
3
vis spectra in the same matrix at 77 K, strong absorptions in the
region of 400 to 650 nm were observed (Fig. S2). Comparison
of the absorptions with those observed in argon matrices at
at λ > 420nm. (c) Calculated IR spectrum for triplet 4-ethynylphenyl-
nitrene (p-1a-N). (d) Calculated IR spectrum for 5-ethynyl(aza)cyclo-
heptatetraene (p-1a-AZA).
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Scheme 4
Fig. 5 (a) IR spectrum of 1,2-bis(4-azidophenyl)acetylene (p,pЈ-2-
N ,N ) matrix-isolated in argon at 13 K. (b) IR spectrum obtained by
Fig. 7 ESR spectrum obtained by irradiation (λ > 390 nm) of (a) 1,2-
bis(4-azidophenyl)acetylene (p,pЈ-2-N ,N ), (b) 1-(2-azidophenyl)-2-(4-
azidophenyl)acetylene (p,oЈ-2-N ,N ) and (c) 1,2-bis(2-azidophenyl)-
acetylene (o,oЈ-2-N ,N ) in 2-methyltetrahydrofuran at 77 K.
3
3
irradiation of (a) at λ > 390 nm. (c) Changes in the IR bands upon
irradiation (a Ϫ b). (d) Calculated (UB3LYP/6-31G*) IR spectrum for
triplet quinoidal diradical (p,pЈ-2-Q).
3
3
3
3
3
3
abstraction of nitrenes from matrices, which eventually results
in the formation of the amino-nitreno compound p,pЈ-2-
1
3,14
N,NH2.
It has been shown that 4Ј-amino-4-nitrenostilbene
13b
exhibits weak absorptions around 500–700 nm. Thus, the
species showing a sharp signal at 642 mT is assigned as
4
Ј-amino-4-nitrenotolan (p,pЈ-2-N,NH ). The peak at ca. 335
2
mT varies with the irradiation conditions and is due to radical
impurities that are often observed in the irradiation of aryl
14
azides.
The origin of the weak signals at 472 and 63 mT is not clear
at present. They may be some of the signals ascribable to the
triplet state of the quinoidal diradical (p,pЈ-2-Q). It is to be
noted here that the quinoidal triplet diradical form of 1,4-
1
1
12
phenylene dinitrene, 4,4-biphenyl dinitrene and 4,4Ј-stilbene
13a
dinitrene show signals at 138, 243 and 416 mT (|D/hc| = 0.169
cm ), 116, 205 and 411 mT (|D/hc| = 0.02 cm ), and 148, 258
and 389 mT(|D/hc| =0.122 cm ), respectively.
Ϫ1
Ϫ1
Fig. 6 UV/vis spectrum obtained by irradiation (λ > 390 nm) of
Ϫ1
(
a) 1,2-bis(4-azidophenyl)acetylene (p,pЈ-2-N ,N ), (b) 1-(2-azido-
3 3
phenyl)-2-(4-azidophenyl)acetylene and (p,oЈ-2-N ,N ), and (c) 1,2-
bis(2-azidophenyl)acetylene (o,oЈ-2-N ,N ) in argon at 13 K.
3
3
p,oЈ-Isomer. Similar results are observed in the irradiation of
p,oЈ-isomer (p,oЈ-2-N ,N ). Thus, no absorptions ascribable to a
3
3
3
3
1
0 K indicated that the bands at 389, 421, and 445 nm are
strained cumulenic double bond were observed in the IR
spectra of the photoproducts from those bisazide compounds.
Again, the vibrational frequencies calculated for the quinoidal
diradical (triplet state) fit the experimental spectrum nicely
(Fig. 8). A series of sharp absorptions was observed in the
region of 400 to 600 nm in the UV/vis spectra (Fig. 6b). These
similar to those generated in the noble gas matrix, although
they are broad and red-shifted. However, bands at 492, 532, and
6
products are produced under these conditions. One of the
probable reactions under these conditions is the hydrogen atom
48 nm are not seen in the argon matrix, suggesting that new
4
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Scheme 5
Fig. 8 (a) IR spectrum of 1-(4-azidophenyl)-2-(2-azidophenyl)acetyl-
ene (p,oЈ-2-N , N ) matrix-isolated in argon at 13 K. (b) IR spectrum
3
3
obtained by irradiation of (a) at λ > 390 nm. (c) Changes in the IR
bands upon irradiation (a Ϫ b). (d) Calculated (UB3LYP/6-31G*) IR
spectrum for triplet quinoidal diradical (p,oЈ-2-Q).
Fig. 9 (a) IR spectrum of 1,2-bis(2-azidophenyl)acetylene (o,oЈ-2-
N ,N ) matrix-isolated in argon at 13 K.(b) IR spectrum obtained by
irradiation of (a) at λ > 390 nm. (c) Changes in the IR bands upon
irradiation (a Ϫ b). (d) and (dЈ) Calculated (UB3LYP/6-31G*) IR
spectrum for triplet quinoidal diradical (o,oЈ-2-Q).
observations are consistent with the assumption that the species
exist as quinoidal diradical structures (p,oЈ-2-Q) rather than as
localized dinitrene species (p,oЈ-2-N,N) (Scheme 5).
3
3
ESR studies again showed the presence of signals attri-
butable to triplet mononitrenes (Fig. 7b). Photolysis of the
p,oЈ-isomer (p,oЈ-2-N ,N ) in 2-MTHF at 77 K, for instance,
3
3
Ϫ1
Only one mononitrene signal (679 mT, |D/hc| = 1.03 cm )
resulted in the formation of signals at 666 mT (|D/hc| = 1.01
Ϫ1
Ϫ1
cm ) and 645 mT (|D/hc| = 0.93 cm ), both attributable to
triplet mononitrenes. Assuming again that triplet nitrenes
undergo a hydrogen-abstraction reaction, these two mono-
nitrenes are assigned to o- and p-amino(nitreno)tolans (p,oЈ-2-
was observed in this case, as expected from the structure. Very
weak signals at 458 and 45 mT, presumably attributable to
the triplet state of quinoidal diradicals, are also observable
(Fig. 7c).
N,NH ). The signals in a low-field region were too weak to be
observable in this case.
2
m,mЈ-Isomer. Photolysis of the m,mЈ-isomer (m,mЈ-2-N3,
gave somewhat different results. Thus, irradiation (λ > 350 nm)
of m,mЈ-2-N ,N matrix-isolated in Ar at 10 K gave a product
N3)
o,oЈ-Isomer. The irradiation o,oЈ-isomer (o,oЈ-2-N ,N ) gave
3
3
3
3
Ϫ1
similar results (Scheme 6). The experimental vibrational fre-
quencies observed for photoproducts match those calculated
for quinoidal diradicals (triplet) fairly well (Fig. 9).
exhibiting an absorption band at 1892 cm in the IR spectra, as
the strong bands due to azide groups disappeared (Fig. 10).
These spectroscopic features suggest that the species formed as
a result of the photodenitrogenation of m,mЈ-2-N ,N is mainly
A series of sharp absorptions is noted in the region of 400 to
3
3
6
00 nm (Fig. 6c). It is noteworthy that those bands in the visible
azacycloheptatetraene (m,mЈ-2-AZA) rather than bis(nitrene)
(m,mЈ-2-N,N) (Scheme 7). Unlike its mononitrene analogue, i.e.,
m-1-N, the species was photostable under these conditions. The
UV/vis spectra of the photoproduct exhibited rather broad
region are notably shifted in going from the p,pЈ- to p,oЈ- to
o,oЈ- isomer. This is in accord with the increasing length of the
π-conjugation system.
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Scheme 6
Fig. 11 UV/vis spectrum obtained by irradiation (λ > 390 nm)
of (a) 1-(4-Azidophenyl)-2-(3-azidophenyl)acetylene (p,mЈ-2-N ,N );
3
3
(
b) 1,2-bis(3-azidophenyl)acetylene (m,mЈ-2-N ,N ) in argon at 13 K.
3
3
Fig. 10 (a) IR spectrum of 1,2-bis(3-azidophenyl)acetylene (m,mЈ-2-
N ,N ) matrix-isolated in argon at 13 K.(b) IR spectrum obtained by
3
3
irradiation of (a) at λ > 390 nm. (c) Changes in the IR bands upon
irradiation (a Ϫ b). (d) Calculated (UB3LYP/6-31G*) IR spectrum for
quintet bis(nitrene) (m,mЈ-2-N,N).
Scheme 7
bands at 382 and 336 nm without showing structured absorp-
tion bands in the visible region (Fig. 11a).
The ESR spectra obtained by the photolysis (λ > 350 nm) of
m,mЈ-2-N ,N in 2-MTHF at 77 K were essentially similar to
Fig. 12 ESR spetrum obtained by irradiation (λ > 390 nm) of (a) 1,2-
bis(3-azidophenyl)acetylene (m,mЈ-2-N ,N ). (b) 1-(4-Azidophenyl)-2-
3
3
3
3
14
those observed in the same matrix at 10 K (Fig. 12a). Thus,
weak signals at 15, 272, 311, and 860 mT were assignable to the
quintet states of bisnitrene m,mЈ-2-N,N. It has been demon-
strated that bisnitrene (m,mЈ-2-N,N) has singlet ground states
with thermally populated triplet and quintet states above the
(3-azidophenyl)acetylene (p,mЈ-2-N ,N ) in 2-methyltetrahydrofuran at
3 3
7
7 K.
Ϫ1
cm ) can be assignable to a mononitrene. Since a signal assign-
able to a doublet free radical was not prominent in this case and
since the formation of azacycloheptatetraene is clearly shown
14
ground state. A rather strong signal at 692 mT (|D/hc| = 1.15
4
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by IR experiments, aza-nitrene rather than amino-nitrene is
suggested to be a product in this case.
(m-aminophenyl) acetylenes. Again, taking into account the
effect of para substituents on the D value, the mononitrene with
the lower D value is assigned to the nitrene with the ethynyl
substituent at the para position.
p, mЈ-Isomer. Irradiation of the p,mЈ-isomer (p,mЈ-2-N ,N ),
3
3
on the other hand, led to different results from those observed
in the photolysis of the m,mЈ-isomer. Thus, irradiation (λ > 350
nm) of p,mЈ-2-N ,N matrix-isolated in Ar gave a species show-
Discussion
3
3
Ϫ1
ing no absorptions in 2000–1800 cm , indicating that aza-
cycloheptatetraene is not formed (Fig. 13). The spectra of the
initial photoproduct changed very little upon continued irradi-
ation under these conditions, indicating that the product is
photostable. The observed IR bands fit those calculated for
bis(nitrene) with the quintet state fairly well (Scheme 8).
The present investigation on the characterization of transient
photoproducts from mono- and diazides employing versatile
spectroscopic methods in combination with theoretical calcu-
lations revealed the nature of the photochemical process and
the transient species in some detail.
Due to the limitation of the experimental setup in this labor-
atory, it was impossible to obtain all the spectra in the same
medium. Therefore the comparison of the IR and UV/vis data
(
obtained in argon matrix) with ESR data (observed in organic
matrix) should be made with care by taking into account
possible reactions with the solvents in the latter.
On the other hand, matching of calculated bands and
experimental ones are not always very good. This is partly
because some of the product IR bands were extremely weak
which prevented us from obtaining apparent overall spectrum.
Also, the limitation of DFT with open-shell diradicals and
24
cumulenes should be taken into account.
The introduction of phenylethynyl groups on phenyl azides
shows little effect on the photochemical pathway. Thus, upon
photoexcitation, (phenylethynyl)phenyl azides afforded the
corresponding triplet nitrene, which is in photoequilibrium with
the corresponding azacycloheptatetraene.
In marked contrast, azidophenylethynyl groups exhibited a
dramatic effect not only on the photochemical pathway of
phenyl azides but also on the electronic and molecular structure
of the photoproducts. The patterns of the effect depend upon
the relative position of the azide groups in the diphenylacetyl-
ene unit.
Whenever two azide groups were situated in the conjugating
position with respect to each other, as in p,pЈ-, o,oЈ-, and o,pЈ-
bis(azides), the azides always resulted in the formation of a
quinoidal diimine diradical in which unpaired electrons were
extensively delocalized in the π-conjugation framework. Geom-
etries optimized by UB3LYP/6-31G(d) for the triplet biradical
states clearly displayed bond alternation in the benzene ring
and the ethynyl bond, suggesting the contribution of an exten-
sively delocalized form. Even though quinoidal dinitrenes with
triplet biradical states were detected, the ground state multi-
plicities of the diradical are not characterized in the present
study. It has been shown that analogous quinoidal dinitrenes,
Fig. 13 (a) IR spectrum of 1-(4-azidophenyl)-2-(3-azidophenyl)-
acetylene (p,mЈ-2-N ,N ) matrix-isolated in argon at 13 K. (b) IR
spectrum obtained by irradiation of (a) at λ > 390 nm. (c) Changes in
the IR bands upon irradiation (a Ϫ b). (d) Calculated (UB3LYP/
3
3
6
-31G*) IR spectrum for quintet bis(nitrene) (p,mЈ-2-N,N).
ESR spectra obtained by irradiation of the bisazide in
2
-MTHF at 77 K are again essentially the same as those
obtained in the photolysis at 10 K (Fig. 12b). Thus, they con-
sisted of weak signals at 90–150 mT and around 834 mT and an
intense signal at 303 mT, all ascribable to the dinitrene in quin-
11
12
such as 1,4-phenylenedinitrene, 4,4-biphenyldinitrene, 4,4Ј-
1
3
13c,d
stilbenedinitrene, 1,4-bis(p-nitrenophenyl)-1,3-butadiene,
Ϫ1
13c,d
tet states. Two signals at 650 (|D/hc| = 0.95 cm ) and 683 mT
and 1,8-bis(p-nitrenophenyl)-1,3,5,7-octatetraene,
all have a
Ϫ1
(
|D/hc| =1.08 cm ) ascribable to mononitrene are observed in
singlet biradical ground state with a very small singlet-triplet
Ϫ1
this case. Since a signal due to a doublet free radical was also
energy gap of 150 to 600 cal mol . Therefore, it is likely that
detected, these signals are assignable to amino-nitrenes, i.e.,
the triplet biradical detected in the present system is in the
thermally excited state.
(
p-aminophenyl) (m-nitrenophenyl) and (p-nitrenophenyl)
Scheme 8
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 4 1 – 4 4 5 0
4447

View Article Online
The situation changed rather dramatically when the azide
groups were introduced in the meta position. Thus, the form-
ation of azacycloheptatetraene was noted in the photolysis of
the m,mЈ-isomer. ESR studies indicated the generation of the
quintet state, which is shown to be a thermally populated
Merck Kieselgel 60 PF254. Column chromatography was per-
formed on silica gel (Fuji Davidson) for column chromato-
graphy or ICN for dry column chromatography.
Materials
Ϫ1
state with a very small energy gap of ca. 100 cal mol . It is
All the azides prepared below showed one spot in TLC.
interesting to note here that neither the quintet state nor the
corresponding lower-lying singlet state of the dinitrene was
characterized by the matrix-IR study. It is rather difficult to
estimate the relative ratio of two species formed under two very
different conditions. It is possible that nitrene generated in a
rare-gas matrix at a very low temperature has more opportun-
ities to undergo a ring-expansion reaction than nitrene in an
organic matrix at 77 K. This is so because photochemical pro-
cesses in noble gas matrices sometimes result in the generation
of reactive intermediates in a vibrationally excited state and
undergo subsequent reactions faster than when they cool off to
4
-Ethynylphenyl azide (p-1a-N ). To a cooled solution of
3
4
-iodoaniline (1.00 g, 4.57 mmol) and conc. HCl (8 ml) in H O
2
(
8 ml) was slowly added a solution of NaNO (0.378 g, 5.36
2
mmol) in H O (5 ml) at 0 ЊC and the mixture was stirred for 1 h
2
in the dark. The precipitate was filtered and the filtrate was then
added to a cooled solution of NaN (0.594 g, 9.14 mmol) in
3
H O (5 ml) at 0 ЊC. After stirring for 1 h at 0 ЊC in the dark, the
2
precipitate formed was filtered and purified by preparative TLC
to afford 4-iodophenyl azide (0.98 g, 81%) as a yellowish solid:
1
25
mp 30.4–31.3 ЊC; H-NMR (CDCl ) δ 7.62 (d, J = 8.91 Hz, 2 H),
6
thermally relaxed states. However, in organic matrices, the
intermediates can be easily relaxed by increasing the number of
vibrational modes of the host molecules (see below, however).
Strong EPR signals ascribable to mononitrene can be assigned
to aza-nitrene rather than amino-nitrene. The intensity of
this signal is stronger than those ascribable to the quintet state
even though the population of this state is fairly high at this
high temperature. The observation suggests that aza-nitrene
formation is dominant even under these conditions.
3
Ϫ1
.77 (d, J = 8.91 Hz, 2 H); IR (KBr) ν 2128 cm . A solution of
the azide (200 mg, 0.81 mmol) and trimethylsilylacetylene
0.113 ml, 0.81 mmol) in anhydrous NEt (3 ml) was carefully
(
3
degassed by repeated cycles of evacuation and purge with
argon. To the mixture was added (Ph P) PdCl (5 mg) and CuI
3
2
2
(
4 mg) and the resulting mixture was stirred for 24 h at room
temp. Solvent was evaporated to leave the residue which was
purified by TLC to give 4-trimethylsilylethynylphenyl azide
1
(
55 mg, 31%) as a reddish liquid: H-NMR (CDCl ) δ 7.44 (d,
The p,mЈ-isomer is shown to be an excellent precursor
for high-spin quintet dinitrene. The IR spectra of the photo-
product showed no bands ascribable to azacycloheptatetraene,
and the observed spectra are in good agreement with that calcu-
lated for the quintet state. Strong EPR signals assignable to the
quintet state have been observed along with rather weak signals
due to mononitrenes. Moreover, the quintet bis(nitrene) is
rather photostable under these conditions; IR bands due to the
quintet dinitrene showed no change even upon prolonged
irradiation under these conditions.
3
J = 8.58 Hz, 2 H), 6.94 (d, J = 8.58 Hz, 2 H), 0.25 (s, 9 H); IR
Ϫ1
(
KBr) ν 2112 cm . The azide (40 mg, 0.196 mmol) was added
to a solution of 1 M NaOH aq (1 ml) in MeOH (3 ml) and
stirred for 3 h at room temp. The mixture was evaporated and
extracted with Et O (20 ml × 3). The organic layer was dried
2
and evaporated. The residue which was purified by preparative
TLC (n-hexane) to give 4-ethynylphenyl azide (20 mg, 71%) as a
1
yellow solid: mp 23–24 ЊC; H-NMR (CDCl ) δ 7.47 (d, J = 8.58
3
Hz, 2 H), 6.98 (d, J = 8.58 Hz, 2 H), 3.08 (s, 1 H); IR (KBr) ν
Ϫ1
2
104 cm .
Finally, monoazide derivatives to be formed by step-wise
extrusion of a nitrogen molecule were not observed in the
photolysis of bis(azides) in the Ar matrix. Only one photon was
consumed for the extrusion of dinitrogen in the photolysis of
the bis(diazo) precursor to generate dicarbene in the solid states
3-Ethynylphenyl azide (m-1a-N
). To a degassed solution of
3
3-iodonitrobenzene (160 mg, 0.65 mmol) and trimethylsilyl-
acetylene (90 µl, 0.65 mmol) in anhydrous NEt (2 ml) was
added a catalytic amount of (Ph P) PdCl and CuI. The mix-
3
26
at a cryogenic temperature. Thus, the present observations
suggest that similar one-photon processes took place in the
photolysis of bis(diazides) at low temperature.
3
2
2
ture was worked up as described above to give 3-trimethylsilyl-
nitrobenzene (129 mg, 89%) as a yellow solid: mp 52.3–53.1 ЊC;
1
In 2-MTHF matrix at 77 K, on the other hand, the exact
nature of the photo-deazetation process is not clear. However it
is rather sensitive to the structure of the precursor bis(azides).
For instance, in the case of the m,mЈ isomer, one photon double
deazetation is probably a major process, while in the case
of p,mЈ isomer, stepwise deazetation is the more likely pro-
cess. It is to be noted here that, in the photolysis of polydiazo
compounds in 2-MTHF matrix at low temperatures, photo-
deazetation processes are sensitive to experimental conditions
such as wavelength and intensities of the light and concen-
H-NMR (CDCl ) δ 8.30 (s, 1 H), 8.15 (d, J = 8.25 Hz, 1 H),
3
7.95 (d, J = 7.91 Hz, 1 H), 7.48 (dd, J = 7.91, 8.25 Hz, 1 H),
0.25 (s, 9 H). To a solution of the nitrobenzene (206 mg, 0.92
mmol) in AcOH (3 ml) was slowly added Fe (250 mg) under
vigorous stirring at 60–70 ЊC and the mixture was vigorously
stirred for 5 min. After adding Fe (125 mg), the mixture was
stirred for 12 h. After cooling to room temp, the mixture was
made alkaline with 20% NaOH aq and extracted with Et
(3 × 20 ml). The organic layer was washed with H O, dried
(Na SO ) and evaporated to dryness. The crude 3-trimethylsilyl-
lethynylaniline (162 mg) was diazotized, followed by addition
of NaN as described above. After usual work-up, followed by
O
2
2
2
4
27
tration of the precursor. Further work is needed to make the
process clearer.
3
preparative TLC (n-hexane) 3-(trimethylsily)lethynylphenyl
1
azide was obtained as a yellow liquid (67 mg, 40%): H-NMR
Experimental section
(CDCl ) δ 7.33–7.23 (m, 2 H), 7.12 (d, J = 1.66 Hz, 1 H), 6.96
3
Ϫ1
(
dd, J = 7.26, 1.66 Hz, 1 H), 0.25 (s, 9 H); IR (KBr) ν 2112 cm .
General methods
The azide (67 mg, 0.34 mmol) was desilylated as described
above to give 3-ethynylphenyl azide (m-1a-N ) as a yellow liquid
) δ 7.38–7.25 (m, 2 H), 7.15 (d,
1
H-NMR spectra were recorded on a JEOL JNM-AC300FT/
3
1
NMR spectrometer in CDCl with Me Si as an internal refer-
(27 mg, 54%): H-NMR (CDCl
3
3
4
ence. IR spectra were measured on a JASCO-Herschel FT/IR-
00H spectrometer and UV/vis spectra were recorded on a
J = 1.65 Hz, 1 H), 7.00 (dd, J = 7.26, 1.65 Hz, 1 H), 3.08 (s, 1 H);
Ϫ1
6
IR (KBr) ν 2112 cm .
JASCO CT-560 spectrophotometer. The mass spectra were
recorded on a JEOL JMS-600H mass spectrometer. Gel per-
meation chromatography (GPC) was carried out on a JASCO,
Model HLC-01 instrument. The GPC column was a Shodex
H-2001. Thin layer chromatography was carried out on a
2-Ethynylphenyl azide (o-1a-N ). In the manner described for
3
p-1a-N , 2-iodoaniline was converted to 2-iodophenyl azide
3
1
(60%, orange liquid, H-NMR (CDCl ) δ 788 (dd, J = 1.32,
3
7.98 Hz, 1 H), 7.42–7.36 (m, 1 H), 7.13 (dd, J = 1.32, 7.92 Hz,
4
448
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 4 1 – 4 4 5 0

View Article Online
Ϫ1
1
H), 6.89–6.83 (m, 1 H); IR (KBr) ν 2128cm ). The azide was
ESR measurements. The diazo compound was dissolved in
Ϫ4
then coupled with trimethylsilylacetylene, followed by desilyl-
2-methyltetrahydrofuran (2-MTHF) (5 × 10 M), and the solu-
ation with base to give 2-ethynylphenyl azide as a white solid
tion was degassed in a quartz cell by four freeze-degas-thaw
cycles. The sample was cooled in a optical transmission ESR
cavity at 77 K and irradiated with a Wacom 500 W Xe lamp
using a Pyrex filter. ESR spectra were measured on a JEOL JES
TE200 spectrometer (X-band microwave unit, 100 kHz field
modulation). The signal positions were read by the use of a
gaussmeter.
1
in 60% yield: mp 25.3–27.1 ЊC; H-NMR (CDCl ) δ 7.32 (dd,
3
J = 1.32, 7.79 Hz, 1 H); 7.23–7.18 (m, 1 H), 6.99–6.90 (m, 2 H),
Ϫ1
3
.24 (s, 1 H); IR (KBr) ν 2112 cm .
1
-(4-Azidophenyl)-2-phenylacetylene (p-1b-N ). To a degassed
3
solution of 4-ethynylphenyl azide (100 mg, 0.41 mmol) and
phenylacetylene (45 µl, 0.41 mmol) in NEt (2 ml) was added a
3
catalytic amount of (Ph P) PdCl and CuI. The mixture was
3
2
2
Low-temperature UV/vis spectra. Low-temperature spectra at
7 K were obtained by using an Oxford variable-temperature
worked up as described above to give the desired acetylene
p-1b-N ) as a yellow solid (78 mg, 85%): mp 60.3–61.1 ЊC;
7
(
3
liquid-nitrogen cryostat (DN 2704) equipped with a quartz
outer window and a sapphire inner window. The sample was
dissolved in dry 2-MTHF, placed in a long-necked quartz
cuvette of 1 mm path length, and degassed by four freeze-
1
H-NMR (CDCl ) δ 7.53–7.50 (m, 5 H), 7.35–7.33 (m, 2 H),
3
Ϫ1
7
.01 (d, J = 8.58 Hz, 2 H); IR (KBr) ν 2118 cm .
Ϫ5
1
-(3-Azidophenyl)-2-phenylacetylene(m-1b-N ).
1-(3-Nitro-
3
degas-thaw cycles at pressure near 10 Torr. The cuvette was
phenyl)-2-phenylacetylene was prepared by Sonogashira
coupling of 3-iodonitrobenzene (100 mg, 0.41 mmol) with
flame-sealed, under reduced pressure, placed in the cryostat,
and cooled to 77 K. The sample was irradiated for several
minutes in the spectrometer with a Halos 500 W high-pressure
mercury lamp using a Pyrex filter and the spectral changes were
recorded at appropriate time intervals. The spectral changes
upon thawing were also monitored by carefully controlling the
matrix temperature with an Oxford Instrument Intelligent
Temperature Controller (ITC 4).
phenylacetylene (45 µl, 0.41 mmol) as a pale yellow solid
1
(
63 mg, 69%): mp 102.5–104.0 ЊC; H-NMR (CDCl ) δ 8.36 (s,
3
1
7
H), 8.17 (d, J = 8.58 Hz, 1 H), 7.81 (d, J = 7.59 Hz, 1 H), 7.57–
.35 (m, 6 H). The nitro compound (63 mg, 0.28 mmol) was
treated with Fe in AcOH to give 1-(3-aminophenyl)-2-phenyl-
acetylene, which was diazotized followed by NaN treatment.
3
The crude product was purified by preparative TLC to give
1
-(3-azidophenyl)-2-phenylacetylene (m-1b-N , 29 mg, 47%) as a
3
1
Acknowledgements
yellow liquid: H-NMR (CDCl ) δ 7.56–7.52 (m, 2 H), 7.36–
3
7
.23 (m, 6 H), 6.99 (dd, J = 7.58, 1.32 Hz, 1 H); IR (KBr) ν 2118
The authors are grateful to the Ministry of Education, Science,
Sports and Culture of Japan for support of this work through a
Grant-in-Aid for Scientific Research for Specially Promoted
Research (NO. 12002007), and, Nagase Science and Technol-
ogy Foundation and Mitsubishi Foundation for partial
support.
Ϫ1
cm .
1
,2-Bis(4-azidophenyl)acetylene (p,pЈ-2-N ,N ). 1,2-Bis(4-
3
3
azidophenyl)acetylene was prepared by coupling 4-iodophenyl
azide with 4-ethynylphenyl azide in the presence of (Ph P) -
3
2
PdCl /CuI in NEt in 21% yield after GPC purification as an
2
3
1
orange solid: mp 101.2–102.6 ЊC; H-NMR (CDCl ) δ 7.50
d, J = 8.58 Hz), 7.01 (d, J = 8.58 Hz); IR (KBr) ν 2124 cm .
3
Ϫ1
Notes and References
(
1
(a) Azides and Nitrenes, Reactivity and Utility, ed. E. F. V. Scriven,
Academic Press, New York, 1984; (b) Nitrenes, ed. W. Lwowski,
Interscience, New York, 1970; (c) B. Iddon, O. Meth-Cohn, E. F. V.
Scriven, H. Suschitzky and P. T. Gallagher, Angew. Chem., Int. Ed.
Engl., 1979, 18, 900; (d ) E. F. V. Scriven, in Reactive Intermediates,
ed. R. A. Abramovitch, Plenum, New York, 1981, Vol. 2, Chapter 1;
1
,2-Bis(2-azidophenyl)acetylene (o,oЈ-2-N ,N ). 1,2-Bis(2-
3
3
azidophenyl)acetylene was prepared by coupling 2-idophenyl-
azide with 2-ethynylphenyl azide in 33% yield as a pale yellow
solid: mp 90.4–91.8 ЊC; H-NMR (CDCl ) δ 7.55 (dd, J = 1.32,
.98 Hz, 2 H), 7.40–7.34 (m, 2 H), 7.16–7.10 (m, 4 H); IR (KBr)
1
3
7
(
e) W. Lwowski, in Reactive Intermediates, ed. M. Jones, and
Ϫ1
ν 2112 cm .
R. A. Moss, Wiley, New York, 1981, Chapter 8; ( f ) W. T. Borden,
N. P. Gristan, C. M. Hadad, W. L. Karney, C. R. Kemnitz and
M. S. Platz, Acc. Chem. Res., 2000, 33, 765.
1
-(2-Azidophenyl)-2-(4-azidophenyl)acetylene (p,oЈ-2-N ,N ).
3
3
1
-(2-Azidophenyl)-2-(4-azidophenyl)acetylene was prepared by
2 (a) E. W. Meijer, S. Nijhuis and F. C. B. M. van Vroonhoven, J. Am.
Chem. Soc., 1988, 110, 7209; (b) M. Yan, S. X. Cai, M. N. Wybourne
and J. F. W. Keana, J. Am. Chem. Soc., 1993, 115, 814.
(a) K. Kanakarajan, R. Goodrich, M. J. T. Young, S. Soundararajan
and M. S. Platz, J. Am. Chem. Soc., 1988, 110, 6536; (b) N. W.
Shaffer and M. S. Platz, Tetrahedron Lett., 1989, 30, 6465;
(c) N. Soundararajan and M. S. Platz, J. Org. Chem., 1990, 55, 2034;
(d ) M. S. Platz and D. S. Watt, Photochem. Photobiol., 1991, 54, 329;
(e) S. X. Cai and J. F. W. Keana, Tetrahedron Lett., 1989, 30, 5409;
coupling 4-iodophenyl azide with 2-ethynylphenyl azide in 47%
yield as a yellowish solid: mp 95.5–96.9 ЊC; H-NMR (CDCl )
δ 7.56–7.47 (m, 3 H), 7.38–7.32 (m, 1 H), 7.15–7.09 (m, 2 H),
7
1
3
3
Ϫ1
.02 (d, J = 8.91 Hz, 2 H); IR (KBr) ν 2124 cm .
1
,2-Bis(azidophenyl)acetylene (m,mЈ-2-N ,N ) and 1-(4-azido-
3 3
phenyl)-2-(3-azidophenyl)acetylene (p,mЈ-2-N ,N ) were pre-
pared according to the literature procedures.
3
3
14
(
f ) J. F. W. Keana and S. X. Cai, J. Org. Chem., 1990, 55, 3640.
V. Chowdhry and F. H. Westheimer, Annu. Rev. Biochem., 1979, 48,
93.
4
5
Matrix-isolation spectroscopy. Matrix experiments were per-
2
28,29
formed by means of standard techniques
using an Iwatani
(a) O. L. Chapman and J. P. Le Roux, J. Am. Chem. Soc., 1978, 100,
282; (b) O. L. Chapman and R. S. Sheridan, J. Am. Chem. Soc.,
1979, 101, 3690; (c) I. R. Dunkin and P. C. P. Tompson, J. Chem.
Soc., Chem. Commun., 1980, 499; (d ) I. R. Dunkin and P. C. P.
Thompson, J. Chem. Soc., Chem. Commun., 1982, 1192; (e) I. R.
Dunkin, T. Donelly and T. S. Lockhart, Tetrahedron Lett., 1985,
Cryo Mini closed-cycle helium cryostat. For IR experiments, a
CsI window was attached to the copper holder at the bottom of
the cold head. Two opposing ports of a vacuum shroud sur-
rounding the cold head were fitted with KBr with a quartz plate
for UV irradiation and a deposition plate for admitting the
sample and matrix gas. For UV experiments, a sapphire cold
window and a quartz outer window were used. The temperature
of the matrix was controlled by an Iwatani TCU-1 controller
3
6
59; ( f ) C. Wentrup and H. W. Winter, J. Am. Chem. Soc., 1980, 102,
159; (g) C. Wentrup, C. Thètaz, E. Tagliaferri, H. J. Linder,
B. Kitscke, H. W. Winter and H. P. Reisenauer, Angew. Chem., Int.
Ed. Engl., 1980, 19, 566; (h) J. C. Hayes and R. S. Sheridan, J. Am.
Chem. Soc., 1990, 112, 5879.
(a) R. S. Sheridan, in Organic Photochemistry, ed. A. Padwa, Marcel
Dekker Inc, New York, 1987, Vol. 8, pp. 159–248; (b) I. R. Dunkin,
Chem. Soc. Rev., 1980, 9, 1.
(
gold vs. chromel thermocouple).
6
7
Irradiations were carried out with a Wacom 500W xenon
high-pressure arc lamp. For broad-band irradiation Toshiba
cutoff filters were used (50% transmittance at the specified
wavelength).
(a) W. v. E. Doering and R. A. Odum, Tetrahedron, 1966, 22, 81;
(b) R. J. Sundberg and R. W. Heintzelman, J. Org. Chem., 1974, 39,
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 4 1 – 4 4 5 0
4449

View Article Online
2
546; (c) B. A. DeGraff, D. W. Gillespie and R. J. Sundberg, J. Am.
20 (a) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785;
(b) B. Miehlich, A. Savin, H. Stoll and H. Preuss, Chem. Phys. Lett.,
1989, 157, 200; (c) A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
21 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone,
M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo,
S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui,
K. Morokuma, D. K. Malick, A. D. Rabuck, J. B. Raghavachari,
K. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. W. M. Gill, B. Johnson,
W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-
Gordon, E. S. Replogle, J. A. Pople, GAUSSIAN 98, Gaussian Inc.:
Pittsburgh, PA, 1998.
Chem. Soc., 1974, 95, 7491; (d ) M. Sumitani, S. Nagakura and
K. Yoshihara, Bull. Chem. Soc. Jpn., 1976, 49, 2995; (e) P. E. Nielsen
and O. Buchard, Photochem. Photobiol., 1982, 35, 317; ( f ) C. J.
Shields, D. R. Chrisope, G. B. Schuster, A. J. Dixon, M. Poliakoff
and J. J. Turner, J. Am. Chem. Soc., 1987, 109, 4723; (g) Y. Z. Li, J. P.
Kirby, M. W. George, M. Poliakoff and G. B. Schuster, J. Am. Chem.
Soc., 1988, 110, 8092; (h) A. K. Schrock and G. B. Schuster, J. Am.
Chem. Soc., 1984, 106, 5234–5229; (i) C. G. Younger and R. A. Bell,
J. Chem. Soc., Chem. Commun., 1982, 1359; (j) T. -Y. Liang and
G. B. Schuster, J. Chem. Soc., Chem. Commun., 1987, 109, 7803;
(
k) A. Albini, G. Bettinetti and G. Minoli, J. Chem. Soc., Chem.
Commun., 1991, 113, 6928.
8
9
S. E. Carroll, B. Nay, F. F. v. Scriven, H. Saschitzky and D. R.
Thomas, Tetrahedron Lett., 1977, 175.
See for review: (a) A. Rajca, Chem. Rev., 1994, 94, 871;
(
b) H. Iwamura, Adv. Phys. Org. Chem., 1990, 26, 179; (c) Magnetic
Properties of Organic Materials, ed. P. M. Lahti, Marcel Dekker,
22 G. Smolinsky, E. Wasserman and W. A. Yager, J. Am. Chem. Soc.,
1962, 84, 3220.
New York, 1999.
1
1
0 E. Wassermann, R. W. Murray, W. A. Yager, A. M. Trozzolo and
G. Smolinsky, J. Am. Chem. Soc., 1967, 89, 5076.
23 It is well known that the substitution at the para position of
phenylnitrene results in the delocalization of the spin density to
decrease the zero-field splitting parameter D: J. H. Hall, J. M.
Fargher and M. R. Gisler, J. Am. Chem. Soc., 1978, 100, 2029.
24 It has been pointed out that DFT calculations have problems with
cumulenes. For instance, Schreiner and Schaefer mentioned that
comparison of geometries obtained using various DFT approaches
showed good agreement with MP2/cc-pVTZ, CCSD(T)/cc-pVTZ,
and the experiment, but that DFT energetics displayed significant
differences when compared to the explicitly correlated theoretical
and experimental results. See: H. Lee, H. F. Schaefer, III and P. R.
Schreiner, J. Phys. Chem. A, 2002, 106, 11923 . Also, concerning the
limitations of DFT on open-shell singlet diradicals, see: T. Bally and
W. T. Borden, Reviews in Computational Chemistry, ed. K. B.
Lipkowitz and D. B. Boyd, VCH-Wiley, New York, 1999, vol. 13,
p. 1.
1 (a) A. Nicolaides, H. Tomioka and S. Murata, J. Am. Chem. Soc.,
1
1
998, 120, 11530; (b) A. Singh and J. S. Brinen, J. Am. Chem. Soc.,
971, 93, 540.
1
2 (a) C. Ling and P. M. Lahti, Chem. Lett., 1993, 769; (b) T. Ohana,
M. Kaise, S. Nimura, O. Kikuchi and A. Yabe, Chem. Lett.,
1
1
993, 765; (c) T. Ohana, M. Kaise and A. Yabe, Chem. Lett., 1992,
397; (d ) T. Ohana, A. Ouchi, H. Moriyama and A. Yabe,
J. Photochem. Photobiol. A, Chem., 1993, 72, 83; (e) M. Minato,
P. M. Lahti and H. van Willigen, J. Am. Chem. Soc., 1993, 115,
4
532.
1
3 (a) T. Harder, J. Bendig, G. Scholz and R. Stösser, J. Am. Chem.
Soc., 1996, 118, 2497; (b) T. Harder, R. Stöβer, P. Wessig and
J. Bendig, J. Photochem. Photobiol. A, Chem., 1997, 103, 105;
(c) M. Minato and P. M. Lahti, J. Phys. Org. Chem., 1993, 6, 483;
(d ) M. Minato and P. M. Lahti, J. Am. Chem. Soc., 1997, 119, 2187;
(e) S. Mimura, O. Kikuchi, T. Ohana, A. Yabe and M. Kaise, Chem.
25 B. F. LeBlanc and R. S. Sheridan, J. Am. Chem. Soc., 1988, 110,
7250.
Lett., 1996, 125.
26 W. Subhan, P. Rempala and R. S. Sheridan, J. Am. Chem. Soc., 1998,
120, 11528.
1
1
1
1
4 S. Murata and H. Iwamura, J. Am. Chem. Soc., 1991, 113,
5
547.
27 Y. Teki, T. Takui, H. Yagi, K. Katoh and H. Iwamura, J. Chem.
Phys., 1985, 83, 539. See also: K. Matsuda, N. Nakamura, K. Inoue,
N. Koga and H. Iwamura, Bull. Chem. Soc. Jpn., 1996, 69, 1483;
N. Nakamura, K. Inoue and H. Iwamura, Angew. Chem., Int. Ed.
Engl., 1993, 32, 872; T. Sugawara, M. Inada and H. Iwamura,
Tetrahedron Lett., 1983, 24, 1723.
28 H. Tomioka, N. Ichikawa and K. Komatsu, J. Am. Chem. Soc., 1992,
114, 6045.
29 R. J. McMahon, O. L. Chapman, R. A. Hayes, T. C. Hess and
H. P. Krimmer, J. Am. Chem. Soc., 1985, 107, 7597.
5 T. Yamagata, H. Tukada and K. Kobayashi, Chem. Lett., 1998,
29.
1
6 C. Ling, M. Minato, P. M. Lahti and H. van Willigen, J. Am. Chem.
Soc., 1992, 114, 9959.
7 S. Mimura, O. Kikuchi, T. Ohana, A. Yabe and M. Kaise,
Chem. Lett., 1994, 1679.
8 A. Ling and P. M. Lahti, J. Am. Chem. Soc., 1994, 116, 8784.
9 B3LYP/6-31G(d)
1
1
2
0
optimized geometries and vibrational
2
1
frequencies were obtained with GAUSSIAN 98
.
4
450
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 4 1 – 4 4 5 0