Electronic substituent effects on the EPR-spectral D parameter of trimethylenemethane (TMM) triplet diradicals compared with localized cyclopentane-1,3-diyls

Article abstract of DOI:10.1039/a708236c

The EPR-spectral D parameter for a series of aryl-substituted trimethylenemethane (TMM) derivatives 2a-g has been determined in a 2-methyltetrahydrofuran (MTHF) matrix at 77 K. Although the substituent effects on the D parameter are relatively small compa

Full text of DOI:10.1039/a708236c

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Electronic substituent effects on the EPR-spectral D parameter of
trimethylenemethane (TMM) triplet diradicals compared with
localized cyclopentane-1,3-diyls
Manabu Abe and Waldemar Adam*
Institute of Organic Chemistry, University of Würzburg, Am Hubland D-97074 Würzburg,
Germany
The EPR-spectral D parameter for a series of aryl-substituted trimethylenemethane (TMM) derivatives
2a–g has been determined in a 2-methyltetrahydrofuran (MTHF) matrix at 77 K. Although the substituent
effects on the D parameter are relatively small compared with those of the localized 1,3-diradicals 4, the
good correlation between the D values of TMMs 2 and those established for 4 signifies that electronic
substituent effects may also be adequately monitored in this way for cross-conjugated triplet species. Thus,
the p-NO2 group acts as a spin acceptor, while the p-F substituent behaves as a spin donor. Significant
steric effects in the aryl-substituted TMMs 2a–m impose a twisted conformation and, thus, reduce
conjugation at the benzyl-radical center and are responsible for the relatively small substituent effects on
the D parameter in these TMM triplets. Moreover, good linearity is observed between the AM1/AUHF-
calculated spin densities at the benzyl position for the twisted conformation and the experimental D values
of the TMM triplet diradicals 2.
triplet diradicals between the experimental D value and the
Introduction
theoretical spin densities ρA and ρB (readily computed by
semiempirical calculations) at the radical centers A and B,
separated by the interradical distance dAB [eqn. (1)]. Indeed, an
Paul Dowd1 first confirmed in 1966 the theoretically predicted2
triplet ground state of a cross-conjugated diradical species,†
namely the parent trimethylenemethane (TMM 1), by means of
3 µ0g2µB ρAρB
2
D/cmϪ1
=
ͩ ͪ
R1
R2
(1)
H
H
3
16π
dAB
H
H
excellent (r2 = 0.963) linear correlation between the D value and
the spin density (ρ) at the cumyl radical site was demonstrated
for a large number of substituents.8e For the first time electronic
effects such as spin delocalization and radical stabilization in
benzyl-type radicals could be quantitatively described in terms
of the EPR-spectral D parameter of triplet diradicals; in fact, a
good correspondence with the chemical σrad scales9 was shown
to apply.8e
The question arises as to whether electronic effects in radicals
may be sensed through appropriately substituted Berson-type
TMM triplet diradicals 2. A few substituted derivatives of 2
(R1 = H, R2 = Ar) have been reported,4a but no clear-cut correl-
ation between the D values and the electronic character of the
examined substituents was discernible. Consequently, we
decided to cover the full range of substituents in the TMMs 2
(R1 = H, R2 = Ar) from p-NH2 to p-NO2, in order to assess
whether in these cross-conjugated triplet diradicals electronic
substituent effects parallel those established for the localized
ones of the cyclopentane-1,3-diyl type 4.‡ Herein we report the
results of our study.
•
H
•
H
•
•
1
2
D/cm–1
0.024
0.0265
(R1 = R2 = H)
X
A
B
•
Ph
•
R
R
•
•
4
3
0.084 (R = H)
0.045 (R = Ph)
0.0506 (X = H)
EPR spectroscopy under matrix isolation. A major contribu-
tion in this area was achieved by Berson and co-workers,4 who
devised a convenient synthesis of the cyclic TMM derivatives
2, which enabled them to explore in detail the chemical and
physical properties of these unusual diradicals.5
Another class of related cyclic diradicals are the localized
ones, of which the parent species 3 was reported by Closs6 in
1975, the triplet ground state of which was again established
by EPR measurements at 4 K. Through the facile Hünig iso-
pyrazole route7 for the preparation of the azoalkanes as pre-
cursors, a large variety of aryl-substituted derivatives of the
bicyclic type 4 was made available by us, to assess electronic
substituent effects in such localized triplet diradicals through
the EPR-spectral D parameter.8 Theoretical consideration8b
provided a simple functional relationship for these localized
Results and discussion
The diazenes 5, the precursors of the TMM derivatives 2, were
synthesized by analogy to the reported methods (Scheme 1).4a
The fulvenes 8, prepared according to the literature,11,12 were
converted into the corresponding [4 ϩ 2] cycloadducts 7 with
ethyl azodicarboxylate (DEAD) and reduced catalytically to
‡ We also examined some unsymmetrical diaryl-substituted derivatives
of 2 (R1 = H, R2 = Ar), which would feature Berson-type TMMs
with different aryl group on one and the same radical site to assess
captodative effects10 through the highly sensitive D parameter.
† Such cross-conjugated species were first prepared by Schlenk and
Brauns.3
J. Chem. Soc., Perkin Trans. 2, 1998
1063

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Table 1 EPR D parameters for the triplet TMM derivatives 2a–g and
the corresponding localized cyclopentane-1,3-diyls 4a–g
X
X
R1
R1
|D/hc|/cmϪ1
DEAD (1.1 equiv.)
X
2a
4b
cyclohexane, ca. 20 °C, 12 h
N
COOEt
NO2
CN
NH2
OMe
Cl
H
F
(2g)
(2f)
(2e)
(2d)
(2c)
(2a)
(2b)
0.0162
0.0182
0.0184
0.0195
0.0196
0.0196c
0.0201
0.0454
0.0477
0.0493
0.0509
0.0502
0.0506
0.0514
N
COOEt
8
7
H2, Pd–C
EtOAc
X
X
a Measured in MTHF matrix at 77 K, the D values were determined
from the experimental EPR spectra, error 0.0001 cmϪ1, except for 2b
(X = CN), whose D value was estimated by simulation, error 0.0002
R1
R1
(a) KOH (5 equiv.)
propan-2-ol,
reflux, 3 h
cmϪ1 b Taken from ref. 8(b). c Taken from ref. 4(a).
.
(b) 2 M CuCl2
N
N
COOEt
N
N
COOEt
6
5
hν
333 nm
2.1 W
MTHF
77 K
g
a
c
e
2
b
H
F
d
f
3–5 min
R1
X
H
H
H
H
H
H
H
Cl OMe
CN
X
NH2
NO2
R1
m
j
2
h
i
k
l
R1
X
p-Cl-C6H4
Ph Ph Ph Ph
Ph
H
F
Cl OMe CN
OMe
2
the hydrazides 6. Their saponification and subsequent oxid-
ation afforded the relatively labile diazenes 5. In the case of 5e
(R1 = H, X = NH2), the hydrazide 6e was prepared by the reduc-
tion of the nitro group of hydrazide 6g [eqn. (2)]. The nitro-
NO2
NH2
Fig. 1 |D/hc|/cmϪ1 values of the localized diradicals 4a–g versus those
of the TMMs 2a–g
H
H
Zn
EtOH, reflux, 2 h
As can be seen in Table 1, although the substituents effects on
the D values are relatively small for the TMM derivatives 2
compared with those of the localized analogs 4,8b a good linear
correspondence (r = 0.955, slope m = 1.556) is obeyed (Fig. 1).
Thus, as for the localized diradicals 4, also for this set of sub-
stituents, p-NO2 is the best spin acceptor and p-F is the worst;
in fact, the D value of p-F is slightly higher than that for the
parent (p-H), and acts as a spin donor. Interestingly, the p-NH2
group also behaves as spin acceptor for the TMM triplet diradi-
cals 2, analogously to the localized derivatives 4.8b
For the set of diaryl-substituted triplets 2h–m (Scheme 1), all
the measured D values ranged between 0.0176 and 0.0178 cmϪ1,
i.e. within experimental error, and the values are indistinguish-
able from the parent system 2h (0.0178 cmϪ1). In view of these
negligible electronic substituent effects, all further work on
these systems was abandoned.
As is evident in Table 1, the electronic substituent effects on
the D parameter of the delocalized (cross-conjugated) TMMs
2a–g are relatively small compared with those of the localized
triplet diradicals 4. In simplistic terms, in the localized di-
radicals 4, one of the two unpaired spins resides at each of the
two radical sites, while in the TMMs 2, two spins are delocal-
ized over three radical sites. Effectively, two-thirds of the spin
density is available in TMMs 2 for electronic perturbation by
the substituents. In fact, the slope of 1.556 in Fig. 1 bears out
this reduced efficacy of electronic substituent effects for the
localized versus delocalized diradicals. Additionally, in the
TMM structures steric factors must also be considered. While
(2)
N
COOEt
N
COOEt
N
COOEt
N
COOEt
6g
6e
substituted diazene 5g (R1 = H, X = NO2) was synthesized by
the oxidation of 5e (R1 = H, X = NH2) with dimethyldioxirane
[eqn. (3)].
NO2
NH2
H
H
O
(2 equiv.)
O
(3)
N
CH2Cl2, 0.5 h
N
N
N
5g
5e
The triplet diradicals 2 were generated from the diazenes 5 by
irradiation with 333 nm laser line (2.1 W) in a 77 K 2-methyl-
tetrahydrofuran (MTHF) matrix. The zero-field-splitting (zfs)
parameters D and E were determined from the X and Y signals
of the EPR spectra13 because the Z signals were detected only
as shoulders of the Y signals (Table 1); the results were checked
by simulation. In the case of derivative 2f (X = CN), the D
value had to be estimated by simulation (error 0.0002 cmϪ1),
since the Z and Y signals were indistinguishable.
1064
J. Chem. Soc., Perkin Trans. 2, 1998

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Table 2 AM1/AUHF-calculated spin densities at the benzylic radical
site of the TMMs 2a–g for both the twisted (ρtw) and planar (ρpl)
conformation
a
b
∆ρtw
×102
∆ρpl
×102
X
ρtw
ρpl
NO2
CN
NH2
OMe
Cl
F
H
(2g)
(2f)
(2e)
(2d)
(2c)
(2b)
(2a)
0.4300
0.4328
0.4340
0.4343
0.4343
0.4345
0.4347
0.47
0.19
0.07
0.04
0.04
0.02
0.00
0.5637
0.5683
0.5664
0.5690
0.5704
0.5701
0.5719
0.82
0.36
0.55
0.29
0.15
0.18
0.00
a ∆ρtw = ρtw(H) Ϫ ρtw(X). b ∆ρpl = ρpl(H) Ϫ ρpl(X).
Fig. 2 AM1-calculated optimized structure of the parent TMM 2a
derivative
Fig. 3 AM1/AUHF-calculated spin densities (ρtw) at the benzylic
radical site versus |D/hc|/cmϪ1 values of the TMMs 2a–g
the benzylic radical site in the localized triplets 4 is perfectly
planar,8b severe steric repulsion between the ortho-H atoms of
the phenyl ring and the hydrogen atoms at the radical sites of
the cyclopentane-diyl ring is expected to afford a twisted con-
formation, and delocalization of the spin into the phenyl ring
at the benzylic site should additionally be smaller than in the
planar conformation. To confirm this point, the optimized
conformation for the 2a derivative was assessed by the semi-
empirical AM114 method (Fig. 2). As expected, the twisted
conformation is lower in energy with the twist angles of ca.
28Њ for the phenyl group and ca. 6Њ around the exomethylene
bond. For the other TMMs 2b–g, the twist angles were to
within one degree, the same.
triplet diradicals in view of the approximations (neglect of two-
center interactions) made in its derivation.8b Presumably the
spin distribution over the cross-conjugated TMM moiety and
spin polarization at the central quaternary carbon atom are
relatively insensitive to electronic substituent effects at the
benzyl-radical site in these TMM triplet states. It should be
instructive to probe this through higher-level computations. At
this point we conclude that the electronic substituent effects in
the delocalized triplet diradicals 2 parallel those established
localized diradicals 4.
Experimental
Next, in order to assess quantitatively how much the spin
delocalization into the phenyl ring is diminished in the twisted
conformation, the spin densities (ρ) at the benzylic radical
site were calculated by the AM1/AUHF method for both the
twisted (ρtw) and planar (ρpl) conformations of the TMMs 2
(Table 2). As expected, the electronic substituent effects on the
spin delocalization in the twisted conformation of the TMMs 2,
as expressed by ∆ρtw = ρtw(H) Ϫ ρtw(X), are much smaller (only
a half) than those of the planar conformation, as expressed
by ∆ρpl = ρpl(H) Ϫ ρpl(X). These results unequivocally establish
that the twisted conformation of the phenyl ring in the cross-
conjugated TMM triplet diradicals 2 is one of the reasons for
relatively small electronic substituent effects on the D values, as
observed experimentally (Table 1, Fig. 1).
General aspects
1H and 13C NMR spectra were measured on a Bruker AC
200 (1H, 200 MHz; 13C, 50 MHz) spectrometer with deuterio-
chloroform as an internal standard. J values are given in Hz. IR
spectra were recorded on a Perkin-Elmer 1600 FT-IR spectro-
photometer. Elemental analyses were carried out by the Micro-
analytical Division of the Institute of Inorganic Chemistry,
University of Würzburg. Melting points were taken on a Büchi
apparatus B-545, and are not corrected. TLC analyses were
conducted on precoated silica-gel foils Polygram SIL G/UV254
(40 × 80 mm) from Macherey & Nagel. Spots were identified on
a UV-light exposure. Silica gel (63–200 µm; Woelm) was used
for column chromatography.
For the localized triplet diradicals 4, we have shown that the
simple expression in eqn. (1) relates the experimental EPR-
spectral D parameter directly to the theoretical spin densities ρA
and ρB at the benzylic radical sites.8b In fact, this convenient
relationship allows us to assess the degree of spin delocalization
in benzyl-type radicals as a function of the electronic nature of
the substituent on the phenyl group.8b Of course, it was of inter-
est to check whether such a functional correlation also applies
to the cross-conjugated TMMs 2. Consequently, the computed
spin densities (ρtw) at the benzylic radical sites were plotted
against the experimental D values for the TMMs 2 (Fig. 3).
Indeed, quite good linearity (r2 = 0.910) is observed, with the
largest deviations notable for p-F and p-NH2. For the former,
semiempirical calculations are notoriously less satisfactory for
fluorine substituents, while for the para-amino group correc-
tions for polar effects probably need to be made.8e However,
more surprising is the fact that such a good linear correlation is
observed at all, because eqn. (1) should be limited to localized
Preparation of the fulvene
The fulvenes 8b–d11c and 8f,g12 were synthesized by the
reported methods.
Preparation of the cycloadducts 6b–d,f–m
The corresponding fulvene (10 mmol) was dissolved in cyclo-
hexane (50 cm3) and 1.1 equiv. of diethyl azodicarboxylate (11
mmol) was slowly added at ca. 20 ЊC. The mixture was stirred
for 12 h, after which the precipitate was collected by filtration.
The white powder, the [4 ϩ 2] cycloadduct 7, was dissolved in
ethyl acetate (100 cm3), and 10 mg of palladium-on-charcoal
catalyst were added. The mixture was stirred under a hydrogen-
gas atmosphere (for 7b–d,f,g 0.5 h, for 7h–m 1.5 h), the catalyst
was removed by the filtration over Celite, and the ethyl acetate
was removed at reduced pressure (40 ЊC at 240 Torr). The solid,
except for 6b, was recrystallized from MeOH (50 cm3) at Ϫ5 ЊC
to give the pure hydrazides 6, of which derivatives 6d15 and 6h4a
J. Chem. Soc., Perkin Trans. 2, 1998
1065

View Article Online
are known compounds. The hydrazide 6b was purified by silica-
gel chromatography with EtOAc–n-hexane (25%) as the eluent.
Diethyl 7-(4-fluorophenylmethylene)-2,3-diazabicyclo[2.2.1]-
heptane-2,3-dicarboxylate (6b). Viscous oil (2.60 g, 75%)
(Found: C, 61.95; H, 6.09; N, 8.24. C18H21FN2O4 requires C,
62.06; H, 6.08; N, 8.04%); νmax(liquid film)/cmϪ1 2981, 2874,
1747, 1694, 836; δH(200 MHz; CDCl3) 1.15–1.47 (6 H, m,
CH3CH2), 1.75–2.25 (4 H, m, 5- and 6-H), 4.05–4.45 (4 H, m,
CH3CH2), 4.52–4.80 (1 H, br, 1-H), 4.95–5.35 (1 H, br, 4-H),
6.34 (1 H, s, vinyl), 6.99–7.09 (2 H, br, ArH), 7.26–7.29 (2 H, br,
ArH); δC(50 MHz; CDCl3) 14.5 (2 × br q, OCH2CH3), 26.8
(2 × br t, 5- and 6-CH2), 57.8 (2 × br d, 1- and 4-CH), 62.5
(2 × br t, OCH2CH3), 115.5 (2 × dd, JCF 21.5, Ar), 116.3 (d,
C24H25ClN2O4 requires C, 65.23; H, 5.80; N, 6.26%); νmax(KBr)/
cmϪ1 2980, 1746, 1705, 1317; δH(200 MHz; CDCl3) 1.15–1.45
(6 H, m, CH3CH2), 1.80–2.23 (4 H, m, 5- and 6-H), 4.05–4.35
(4 H, m, CH3CH2), 4.71–4.95 (2 H, br, 1- and 4-H), 7.07–7.12
(4 H, m, ArH), 7.26–7.32 (5 H, m, ArH); δC(50 MHz; CDCl3)
14.4 (2 × br q, OCH2CH3), 25.9 (2 × br t, 5- and 6-CH2), 60.2
(2 × br d, 1- and 4-CH), 62.3 (2 × br t, OCH2CH3), 127.9
(2 × d, Ar), 128.3 (2 × d, Ar), 128.4 (2 × d, Ar), 129.3 (2 × d,
Ar), 130.1 (s, C᎐C or Ar), 130.7 (d, Ar), 133.7 (s, Ar), 137.8 (s,
᎐
Ar or C᎐C), 138.5 (s, C᎐C), 139.6 (s, Ar), 157.1 (2 × s, C᎐O).
᎐
᎐
᎐
Diethyl
7-(4-methoxydiphenylmethylene)2,3-diazabicyclo-
[2.2.1]heptane-2,3-dicarboxylate (6k). Colorless powder (2.27 g,
52%), mp 52–53 ЊC (Found: C, 68.51; H, 6.58; N, 6.52.
C25H28N2O5 requires C, 68.79; H, 6.47; N, 6.42%); νmax(KBr)/
cmϪ1 2980, 2908, 1749, 1702, 1607, 1510, 1248, 1058, 827;
δH(200 MHz; CDCl3) 1.16–1.26 (6 H, m, CH3CH2), 1.96–2.10
(4 H, m, 5- and 6-H), 3.75 (3 H, s, OCH3), 4.06–4.15 (4 H, m,
CH3CH2), 4.80 (2 H, br s, 1- and 4-H), 6.77–6.85 (2 H, m, ArH),
7.02–7.13 (4 H, m, ArH), 7.24–7.27 (3 H, m, ArH); δC(50 MHz;
CDCl3) 14.2 (2 × br q, OCH2CH3), 28.2 (2 × br t, 5- and
6-CH2), 54.9 (q, OCH3), 60.3 (2 × br d, 1- and 4-CH), 62.0
(2 × br t, OCH2CH3), 113.3 (2 × d, Ar), 127.5 (2 × d, Ar), 127.9
CH᎐C), 129.8 (2 × d, Ar), 131.7 (s, CH᎐C), 139.3 (s, Ar), 155.2
᎐
᎐
(2 × s, C᎐O), 162.1 (d, JCF 246).
᎐
Diethyl 7-(4-chlorophenylmethylene)-2,3-diazabicyclo[2.2.1]-
heptane-2,3-dicarboxylate (6c). Colorless powder (3.03 g, 83%),
mp 85–86 ЊC (Found: C, 59.16; H, 5.68; N, 7.63. C18H21ClN2O4
requires C, 59.26; H, 5.80; N, 7.68%); νmax(KBr)/cmϪ1 2988,
2939, 1742, 1685, 1492, 1101; δH(200 MHz; CDCl3) 1.16–1.45
(6 H, m, CH3CH2), 1.62–2.25 (4 H, m, 5- and 6-H), 4.04–4.20
(4 H, m, CH3CH2), 4.55–4.85 (1 H, br, 1-H), 4.95–5.38 (1 H, br,
4-H), 6.233 (1 H, s, vinyl), 7.22–7.38 (4 H, m, ArH); δC(50 MHz;
CDCl3) 14.4 (2 × br q, OCH2CH3), 26.8 (2 ×br t, 5- and
6-CH2), 57.7 (2× br d, 1- and 4-CH), 62.6 (2 × br t, OCH2CH3),
(2 × d, Ar), 129.1 (2 × d, Ar), 130.4 (s, C᎐C or Ar), 130.6 (d,
᎐
Ar), 132.2 (s, Ar or C᎐C), 136.1 (s, C᎐C), 140.2 (s, Ar), 156.4
᎐
᎐
(2 × s, C᎐O), 158.9 (s, Ar).
᎐
116.2 (d, CH᎐C), 128.6 (2 × d, Ar), 129.2 (2 × d, Ar), 133.4 (s,
Diethyl
7-(4-cyanodiphenylmethylene)-2,3-diazabicyclo-
᎐
Ar), 134.0 (S, CH᎐C), 140.0 (s, Ar), 154.5 (2 × s, C᎐O).
[2.2.1]heptane-2,3-dicarboxylate (6l). Colorless powder (2.33 g,
54%), mp 168–169 ЊC (Found: C, 69.35; H, 5.77; N, 9.62.
C25H25N3O4 requires, C, 69.59; H, 5.84; N, 9.74%); νmax(KBr)/
cmϪ1 2980, 2226, 1692, 1319, 1008; δH(200 MHz; CDCl3) 1.20–
1.45 (6 H, m, CH3CH2), 1.80–2.22 (4 H, m, 5- and 6-H), 4.10–
4.45 (4 H, m, CH3CH2), 4.86 (2 H, br s, 1- and 4-H), 7.07–7.12
(2 H, m, ArH), 7.27–7.37 (5 H, m, ArH), 7.37–7.66 (2 H, m,
ArH); δC(50 MHz; CDCl3) 14.5 (2 × br q, OCH2CH3), 27.5
(2 × br t, 5- and 6-CH2), 60.5 (2 × br d, 1- and 4-CH), 62.5
(2 × br t, OCH2CH3), 111.5 (s, Ar), 118.4 (s, CN), 128.3 (2 × d,
᎐
᎐
Diethyl 7-(4-cyanophenylmethylene)-2,3-diazabicyclo[2.2.1]-
heptane-2,3-dicarboxylate (6f). Colorless powder (2.95 g, 83%),
mp 126–127 ЊC (Found: C, 64.24; H, 5.87; N, 11.96. C19H21-
N3O4 requires C, 64.21; H, 5.96; N, 11.82%); νmax(KBr)/cmϪ1
2981, 2226, 1736, 1705, 1245, 1311, 892; δH(200 MHz; CDCl3)
1.23–1.30 (6 H, m, CH3CH2), 1.68–2.31 (4 H, m, 5- and 6-H),
4.05–4.38 (4 H, m, CH3CH2), 4.61–4.81 (1 H, br, 1-H), 4.95–
5.32 (1 H, br, 4-H), 6.40 (1 H, s, vinyl), 7.27–7.50 (2 H, br, ArH),
7.61–7.66 (2 H, m, ArH); δC(50 MHz; CDCl3) 14.3 (2 × br q,
OCH2CH3), 26.7 (2 × br t, 5- and 6-CH2), 57.7 (2 × br d, 1- and
4-CH), 62.0 (2 × br t, OCH2CH3), 111.1 (s, CN), 115.9 (d,
Ar), 128.5 (2 × d, Ar), 129.3 (2 × d, Ar), 129.6 (s, C᎐C or Ar),
᎐
130.1 (d, Ar), 132.1 (2 × d, Ar), 139.1 (s, C᎐C), 139.4 (s, Ar),
᎐
CH᎐C), 118.5 (s, Ar), 128.6 (2 × d, Ar), 132.3 (2 × d, Ar), 140.2
144.8 (s, Ar), 155.8 (2 × s, C᎐O).
᎐
᎐
(s, CH᎐C), 142.8 (s, Ar), 156.5 (2 × s, C᎐O).
Diethyl 7-(4-chloro-4Ј-methoxydiphenylmethylene)-2,3-diaza-
bicyclo[2.2.1]heptane-2,3-dicarboxylate (6m). Colorless powder
(2.40 g, 51%), mp 59–60 ЊC (Found: C, 63.89; H, 5.91; N, 5.81.
C25H27ClN2O5 requires C, 63.76; H, 5.78; N, 5.95%); νmax(KBr)/
cmϪ1 2980, 2873, 1749, 1702, 1607, 1511, 1248, 836, 817; δH(200
MHz; CDCl3) 1.15–1.38 (6 H, m, CH3CH2), 1.85–2.21 (4 H, m,
5- and 6-H), 3.61 (3 H, s, OCH3), 4.18–4.25 (4 H, m, CH3CH2),
4.85 (2 H, br s, 1- and 4-H), 6.81–6.89 (2 H, m, ArH), 7.02–7.10
(4 H, m, ArH), 7.28–7.33 (2 H, m, ArH); δC(50 MHz; CDCl3)
14.8 (2 × br q, OCH2CH3), 26.5 (2 × br t, 5- and 6-CH2), 55.7
(q, OCH3), 60.9 (2 × br d, 1- and 4-CH), 62.8 (2 × br t,
᎐
᎐
Diethyl 7-(4-nitrophenylmethylene)-2,3-diazabicyclo[2.2.1]-
heptane-2,3-dicarboxylate (6g). Colorless powder (1.32 g, 35%),
mp 89–90 ЊC (Found: C, 57.31; H, 5.77; N, 11.21. C18H21N3O6
requires C, 57.59; H, 5.64; N, 11.19%); νmax(KBr)/cmϪ1 2978,
1742, 1702, 1594, 1512, 891; δH(200 MHz; CDCl3) 1.17–1.45
(6 H, m, CH3CH2), 1.65–2.25 (4 H, m, 5- and 6-H), 4.01–4.45
(4 H, m, CH3CH2), 4.52–4.90 (1 H, br, 1-H), 4.95–5.35 (1 H, br,
4-H), 6.44 (1 H, s, vinyl), 7.28–7.58 (2 H, br, ArH), 8.16–8.21
(2 H, m, ArH); δC(50 MHz; CDCl3) 14.4 (2 × br q, OCH2CH3),
26.4 (2 × br t, 5- and 6-CH2), 57.6 (2 × br d, 1- and 4-CH), 62.7
(2 × br t, OCH CH ), 115.5 (d, CH᎐C), 123.9 (2 × d, Ar), 128.8
OCH CH ), 114.1 (2 × d, Ar), 129.0 (2 × d, Ar), 130.1 (s, C᎐C),
᎐
2 3
᎐
2
3
(2 × d, Ar), 142.2 (s, Ar or CH᎐C), 143.5 (s, CH᎐C or Ar),
130.8 (s, Ar), 131.2 (2 × d, Ar), 132.6 (2 × d, Ar), 134.1 (s, Ar),
᎐
᎐
146.9 (s, Ar), 156.1 (2 × s, C᎐O).
137.3 (s, Ar), 139.4 (s, C᎐C), 158.4 (2 × s, C᎐O), 159.8 (s, Ar).
᎐
᎐
᎐
Diethyl
7-(4-fluorodiphenylmethylene)-2,3-diazabicyclo-
[2.2.1]heptane-2,3-dicarboxylate (6i). Colorless powder (3.78 g,
89%), mp 95–96 ЊC (Found: C, 67.69; H, 6.20; N, 6.64.
C24H25FN2O4 requires C, 67.91; H, 5.94; N, 6.60%); νmax(KBr)/
cmϪ1 2990, 2908, 1748, 1705, 1603; δH(200 MHz; CDCl3) 1.02–
1.41 (6 H, m, CH3CH2), 1.70–2.22 (4 H, m, 5- and 6-H), 4.05–
4.38 (4 H, m, CH3CH2), 4.71 (1 H, br s, 1-H), 4.84 (1 H, br s,
4-H), 6.91–7.12 (7 H, m, ArH), 7.22–7.30 (2 H, m, ArH);
δC(50 MHz; CDCl3) 14.3 (2 × br q, OCH2CH3), 26.7 (2 × br t,
5- and 6-CH2), 60.3 (2 × br d, 1- and 4-CH), 62.3 (2 × br t,
OCH2CH3), 115.1 (2 × dd, JCF 21.3, Ar), 127.8 (2 × d, Ar),
Preparation of cycloadduct 6e
To a solution of cycloadduct 6g (1.00 g, 2.9 mmol) in 78%
EtOH (50 cm3) was added zinc dust (9.50 g, 50 equiv.) and
calcium chloride (0.230 g, 0.7 equiv.) at ca. 20 ЊC. The mixture
was allowed to reflux for 2 h, the excess zinc metal was removed
by filtration, and the solvent removed under reduced pressure
(30 ЊC, 100 Torr). The desired cycloadduct 6e was purified by
silica-gel chromatography with EtOAc–n-hexane (30%) as the
eluent.
Diethyl 7-(4-aminophenylmethylene)-2,3-diazabicyclo[2.2.1]-
heptane-2,3-dicarboxylate (6e). Yellow powder (810 mg, 74%),
mp 64–65 ЊC (Found: C, 62.31; H, 6.88; N, 12.21. C18H23N3O4
requires C, 62.59; H, 6.71; N, 12.17%); νmax(KBr)/cmϪ1 3462,
3368, 2981, 1739, 1518; δH(200 MHz; CDCl3) 1.20–1.30 (6 H,
m, CH3CH2), 1.60–2.02 (4 H, m, 5- and 6-H), 3.64 (2 H, br s,
NH2), 4.04–4.30 (4 H, m, CH3CH2), 4.45–4.75 (1 H, br, 1-H),
128.2 (2 × d, Ar), 129.2 (2 × d, Ar), 130.1 (s, C᎐C or Ar), 131.0
(d, Ar), 136.1 (s, Ar or C᎐C), 137.3 (C᎐C), 139.9 (s, Ar), 156.8
᎐
᎐
᎐
(2 × s, C᎐O), 162.2 (d, JCF 246, Ar).
᎐
Diethyl
7-(4-chlorodiphenylmethylene)-2,3-diazabicyclo-
[2.2.1]heptane-2,3-dicarboxylate (6j). Colorless powder (3.00
mg, 68%), mp 95–96 ЊC (Found: C, 65.38; H, 5.71; N, 6.35.
1066
J. Chem. Soc., Perkin Trans. 2, 1998

View Article Online
5.25–5.32 (1 H, br, 4-H), 6.22 (1 H, s, vinyl), 6.58–6.64 (2 H, br,
ArH), 6.98–7.25 (2 H, m, ArH); δC(50 MHz; CDCl3) 14.3
(2 × br q, OCH2CH3), 28.0 (2 × br t, 5- and 6-CH2), 57.9
(2 × br d, 1-, and 4-CH), 62.2 (2 × br t, OCH2CH3), 114.8
m, ArH), 7.58–7.62 (2 H, m, ArH); δC(50 MHz; CDCl3) 20.7 (t,
5- or 6-CH2), 21.4 (t, 6- or 5-CH2), 72.4 (d, 1- or 4-CH), 77.9 (d,
1- or 4-CH), 110.9 (s, Ar), 116.1 (d, CH᎐C), 118.5 (s, CN), 129.1
᎐
(2 × d, Ar), 132.2 (2 × d, Ar), 139.9 (s, CH᎐C), 150.2 (s, Ar).
᎐
(2 × d, Ar), 117.3 (d, CH᎐C), 125.7 (s, Ar), 129.2 (2 × d, Ar),
7-(4-Fluorodiphenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-
ene (5i). Yellow powder (904 mg, 65%), mp 106–107 ЊC with
decomposition (Found: C, 77.59; H, 5.69; N, 10.09. C18H15FN2
requires C, 77.68; H, 5.43; N, 10.06%); νmax(KBr)/cmϪ1 2989,
᎐
135.8 (s, CH᎐C), 146.2 (S, Ar), 156.5 (2 × s, C᎐O).
᎐
᎐
Preparation of the diazenes 5b–f,h–m4a
Ϫ1
Under reflux, to a solution of KOH (25 mmol, 5 equiv.) in
degassed propan-2-ol (15 cm3) was added dropwise a solution
of cycloadduct 6 (5 mmol) in degassed propan-2-ol (5 cm3)
under a nitrogen-gas atmosphere. The mixture was stirred for
3 h, cooled to 0 ЊC, diluted by addition of 100 g of ice–water,
and acidified with 5 cm3 of conc. hydrochloric acid (pH ca. 1–2).
The pH was adjusted to ca. 5–6 by addition of 12% aqueous
NH3 solution, 10 cm3 of 1  aqueous CuCl2 solution were added
and the resulting suspension was recovered by filtration. The
dark powder was dissolved in a mixture of 12% aqueous
NH3 solution (50 cm3) and CH2Cl2 (50 cm3), and the mixture
was stirred for 30 min at 0 ЊC. The organic layer was separated
and dried over MgSO4 at 0 ЊC. The diazene 5 was purified by
silica-gel chromatography with EtOAc–n-hexane (15%) as the
eluent. The spectral data for diazenes 5 are given below, except
for derivatives 5d15 and 5h,4a which are known.
1601, 1507, 1221, 826; λmax(benzene)/nm 341 (ε/M cmϪ1 112),
334 (100); δH(200 MHz; CDCl3) 1.16–1.24 (2 H, m, 5- and 6-H),
1.81–1.86 (2 H, m, 5- and 6-H), 5.35 (1 H, br s, 4-H), 5.37 (1 H,
br s, 1-H), 6.97–7.05 (7 H, m, ArH), 7.26–7.30 (2 H, m, ArH);
δC(50 MHz; CDCl3) 21.19 (t, 5- or 6-CH2), 21.23 (t, 6- or
5-CH2), 75.2 (d, 1- or 4-CH), 75.3 (d, 4- or 1-CH), 115.1
(2 × dd, JCF 21, Ar), 127.8 (2 × d, Ar), 128.1 (2 × d, Ar), 129.2
(2 × d, Ar), 130.1 (s, C᎐C), 130.9 (d, Ar), 135.6 (s, Ar), 139.4 (s,
᎐
C᎐C), 143.6 (s, Ar), 162.1 (d, JCF 246, Ar).
᎐
7-(4-Chlorodiphenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-
ene (5j). Colorless powder (988 mg, 67%), mp 98–100 ЊC with
decomposition (Found: C, 73.47; H, 4.84; N, 9.63. C18H15ClN2
requires C, 73.34; H, 5.13; N, 9.50%); νmax(KBr)/cmϪ1 3030,
Ϫ1
1489, 822; λmax(benzene)/nm 343 (ε/M cmϪ1 102), 331 (115);
δH(200 MHz; CDCl3) 1.20–1.28 (2 H, m, 5- and 6-H), 1.83–1.91
(2 H, m, 5- and 6-H), 5.38 (1 H, br s, 4-H), 5.40 (1 H, br s, 1-H),
6.98–7.07 (4 H, m, ArH), 7.27–7.36 (5 H, m, ArH); δC(50 MHz;
CDCl3) 21.27 (t, 5- or 6-CH2), 21.34 (t, 6- or 5-CH2), 75.3 (d,
1- or 4-CH), 75.4 (d, 4- or 1-CH), 127.9 (d, Ar), 128.3 (2 × d,
7-(4-Fluorophenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-
ene (5b). Colorless plates (525 mg, 52%), mp 68–69 ЊC with
decomposition (Found: C, 71.05; H, 5.40; N, 13.59. C12H11FN2
requires C, 71.27; H, 5.48; N, 13.85%); νmax(KBr)/cmϪ1 2950,
Ar), 128.4 (2 × d, Ar), 129.4 (2 × d, Ar), 130.3 (s, C᎐C), 130.7
᎐
Ϫ1
1601, 1509, 1226, 1162, 1098, 830; λmax(benzene)/nm 342 (ε/M
(2 × d, Ar), 133.8 (s, Ar), 136.1 (s, Ar), 139.2 (s, C᎐C), 144.4 (s,
᎐
cmϪ1 202), 331 (200); δH(200 MHz; CDCl3) 1.19–1.27 (2 H, m,
5- and 6-H), 1.77–1.84 (2 H, m, 5- and 6-H), 5.26 (1 H, br s,
4-H), 5.69 (1 H, br s, 1-H), 6.13 (1 H, s, vinyl), 6.99–7.07 (2 H,
m, ArH), 7.13–7.21 (2 H, m, ArH); δC(50 MHz; CDCl3) 20.8 (t,
5- or 6-CH2), 21.7 (t, 6- or 5-CH2), 72.6 (d, 1- or 4-CH), 78.1 (d,
Ar).
7-(4-Methoxydiphenylmethylene)-2,3-diazabicyclo[2.2.1]hept-
2-ene (5k). Colorless powder (218 mg, 15%), mp 66–67 ЊC with
decomposition (Found: C, 78.39; H, 6.46; N, 9.87. C19H18N2O
requires C, 78.59; H, 6.25; N, 9.65%); νmax(KBr)/cmϪ1 2942,
Ϫ1
4- or 1-CH), 115.6 (2 × dd, JCF 22.4, Ar), 116.5 (d, CH᎐C),
1751, 1607, 1509, 1248, 1033, 826; λmax(benzene)/nm 342 (ε/M
᎐
129.6 (2 × d, Ar), 129.8 (s, Ar), 131.7 (s, CH᎐C), 161.5 (d, J
295, Ar).
cmϪ1 193), 330 (226); δH(200 MHz; CDCl3) 1.18–1.26 (2 H, m,
5- and 6-H), 1.82–1.91 (2 H, m, 5- and 6-H), 3.82 (3 H, s,
OCH3), 5.38 (1 H, br s, 4-H), 5.45 (1 H, br s, 1-H), 6.83–6.89
(2 H, m, ArH), 6.96–7.09 (3 H, m, ArH), 7.26 (4 H, m, ArH);
δC(50 MHz; CDCl3) 21.3 (t, 5- or 6-CH2), 21.4 (t, 6- or 5-CH2),
55.3 (q, OCH3), 75.4 (d, 1- or 4-CH), 75.5 (d, 4- or 1-CH), 113.5
(2 × d, Ar), 127.7 (d, Ar), 128.1 (2 × d, Ar), 129.4 (2 × d, Ar),
᎐
CF
7-(4-Chlorophenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-
ene (5c). Colorless powder (350 mg, 32%), mp 79–81 ЊC with
decomposition (Found: C, 65.95; H, 5.18; N, 12.65. C12H11ClN2
requires C, 65.91; H, 5.07; N, 12.81%); νmax(KBr)/cmϪ1 2949,
Ϫ1
1492, 1279, 1095, 821; λmax(benzene)/nm 340 (ε/M cmϪ1 144),
331 (123); δH(200 MHz; CDCl3) 1.20–1.24 (2 H, m, 5- and 6-H),
1.78–1.83 (2 H, m, 5- and 6-H), 5.24 (1 H, br s, 4-H), 5.67 (1 H,
br s, 1-H), 6.11 (1 H, s, vinyl), 7.05–7.15 (2 H, m, ArH), 7.21–
7.29 (2 H, m, ArH); δC(50 MHz; CDCl3) 20.7 (t, 5- or 6-CH2),
21.7 (t, 6- or 5-CH2), 72.6 (d, 1- or 4-CH), 78.0 (d, 4- or 1-CH),
130.7 (2 × d, Ar), 130.9 (s, C᎐C), 132.2 (s, Ar), 140.0 (s, C᎐C),
142.8 (s, Ar), 159.1 (s, Ar).
᎐ ᎐
7-(4-Cyanodiphenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-
ene (5l). Yellow powder (314 mg, 22%), mp 98–99 ЊC with
decomposition (Found: C, 79.71; H, 5.35; N, 14.91. C19H15N3
requires C, 79.96; H, 5.30; N, 14.73%); νmax(KBr)/cmϪ1 3028,
2943, 2230, 1605, 1118, 828; λmax(benzene)/nm 343 (ε/MϪ1 cmϪ1
187), 329 (201); δH(200 MHz; CDCl3) 1.24–1.31 (2 H, m, 5- and
6-H), 1.86–1.92 (2 H, m, 5- and 6-H), 5.33 (1 H, br s, 4-H), 5.41
(1 H, br s, 1-H), 6.99–7.04 (2 H, m, ArH), 7.16–7.20 (2 H, m,
ArH), 7.26–7.36 (3 H, m, ArH), 7.36–7.65 (2 H, m, ArH); δC(50
MHz; CDCl3) 21.2 (t, 5- or 6-CH2), 21.3 (t, 6- or 5-CH2), 75.2
(d, 1- or 4-CH), 75.4 (4- or 1-CH), 111.5 (s, Ar), 118.5 (s, CN),
128.3 (d, Ar), 128.5 (2 × d, Ar), 129.3 (2 × d, Ar), 129.9 (s,
116.4 (d, CH᎐C), 128.7 (2 × d, Ar), 129.3 (2 × d, Ar), 133.3 (s,
᎐
CH᎐C), 133.9 (s, Ar), 147.3 (s, Ar).
᎐
7-(4-Aminophenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-
ene (5e). Yellow powder (508 mg, 51%), mp 77–78 ЊC with
decomposition (Found: C, 72.05; H, 6.81; N, 21.19. C12H13N3
requires C, 72.34; H, 6.58; N, 21.09%); νmax(KBr)/cmϪ1 3457,
3364, 2974, 2944, 1619, 1517, 1297, 1180, 1110, 831; λmax
-
Ϫ1
(benzene)/nm 359 (ε/M cmϪ1 151); δH(200 MHz; CDCl3)
0.88–1.23 (2 H, m, 5- and 6-H), 1.35–1.50 (2 H, m, 5- and 6-H),
3.14 (2 H, br s, NH2), 4.90 (1 H, d, J 2.2, 4-H), 5.52 (1 H, d,
J 2.2, 1-H), 5.73 (1 H, s, vinyl), 6.24–6.32 (2 H, m, ArH), 6.77–
6.83 (2 H, m, ArH); δC(50 MHz; CDCl3) 19.9 (t, 5- or 6-CH2),
20.9 (t, 6- or 5-CH2), 71.8 (d, 1- or 4-CH), 77.5 (d, 4- or 1-CH),
C᎐C), 130.1 (2 × d, Ar), 132.1 (2 × d, Ar), 138.5 (s, C᎐C), 144.3
᎐
᎐
(s, Ar), 146.3 (s, Ar).
7-(4-Chloro-4Ј-methoxydiphenylmethylene)-2,3-diazabicyclo-
[2.2.1]hept-2-ene (5m). Colorless powder (439 mg, 27%), mp
81–82 ЊC with decomposition (Found: C, 70.26; H, 5.08; N,
8.33. C19H17ClN2O requires C, 70.26; H, 5.28; N, 8.62%);
νmax(KBr)/cmϪ1 3029, 2947, 1508, 1247, 837, 818; λmax(benzene)/
nm 341 (ε/MϪ1 cmϪ1 110), 329 (123); δH(200 MHz; CDCl3) 1.02–
1.27 (2 H, m, 5- and 6-H), 1.82–1.88 (2 H, m, 5- and 6-H), 3.82
(3 H, s, OCH3), 5.35 (1 H, br s, 4-H), 5.43 (1 H, br s, 1-H), 6.83–
6.87 (2 H, m, ArH), 6.93–7.02 (4 H, m, ArH), 7.26–7.33 (2 H,
m); δC(50 MHz; CDCl3) 21.3 (t, 5- or 6-CH2), 21.4 (t, 6-,
5-CH2), 55.3 (q, OCH3), 75.4 (2 × d, 1- and 4-CH), 113.7
(2 × d, Ar), 128.4 (2 × d, Ar), 129.8 (s, Ar), 130.6 (2 × d, Ar),
113.9 (2 × d, Ar), 116.6 (d, CH᎐C), 127.4 (s, Ar), 128.5 (2 × d,
᎐
Ar), 142.1 (s, CH᎐C), 145.4 (s, Ar).
᎐
7-(4-Cyanophenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-
ene (5f). Yellow powder (272 mg, 26%), mp 78–79 ЊC with
decomposition (Found: C, 74.38; H, 5.31; N, 20.29. C13H11N3
requires C, 74.62; H, 5.30; N, 20.08%); νmax(KBr)/cmϪ1 2935,
Ϫ1
2225, 1603, 1505, 1416, 884, 830; λmax(benzene)/nm 322 (ε/M
cmϪ1 114), 300 (201); δH(200 MHz; CDCl3) 1.16–1.27 (2 H, m,
5- and 6-H), 1.77–1.86 (2 H, m, 5- and 6-H), 5.25 (1 H, br s,
4-H), 5.65 (1 H, br s, 1-H), 6.17 (1 H, s, vinyl), 7.25–7.29 (2 H,
J. Chem. Soc., Perkin Trans. 2, 1998
1067

View Article Online
2 (a) W. E. Moffit, cited in C. Coulson, J. Chem. Phys., 1948, 45, 243;
130.7 (2 × d, Ar), 131.7 (s, C᎐C), 133.7 (s, Ar), 138.4 (s, C᎐C),
143.3 (s, Ar), 159.3 (s, Ar).
᎐
᎐
(b) A. Gold, J. Am. Chem. Soc., 1969, 91, 4961.
3 (a) W. Schlenk and M. Brauns, Chem. Ber., 1915, 48, 661; (b)
W. Schlenk and M. Brauns, Chem. Ber., 1915, 48, 716.
4 (a) M. S. Platz, J. M. McBride, R. D. Little, J. Harrison, A. Shaw,
S. E. Potter and J. A. Berson, J. Am. Chem. Soc., 1976, 98, 5725;
(b) J. A. Berson, in Diradicals, Ed. W. T. Borden, Wiley, 1982,
pp. 152–194.
5 (a) T. Hirano, T. Kumagai, T. Miyashi, K. Akiyama and Y. Ikegami,
J. Org. Chem., 1991, 56, 1907; (b) T. Hirano, T. Kumagai,
T. Miyashi, K. Akiyama and Y. Ikegami, J. Org. Chem., 1991, 56,
1907.
Preparation of diazene 5g
To a solution of diazene 5e (160 mg, 0.8 mmol) in dry CH2Cl2
(30 cm3) was added slowly a solution of dimethyldioxirane16
(1.6 mmol, 2 equiv.) in acetone (21 cm3) at ca. 20 ЊC. The mix-
ture was stirred for 30 min, after which the solvent was removed
at reduced pressure (30 ЊC, 100 Torr) and the desired diazene 5g
(156 mg, 85%) was purified by recrystallization from MeOH at
Ϫ5 ЊC.
6 (a) S. L. Buchwalter and G. L. Closs, J. Am. Chem. Soc., 1975, 97,
3857; (b) S. L. Buchwalter and G. L. Closs, J. Am. Chem. Soc., 1979,
101, 3857.
7-(4-Nitrophenylmethylene)-2,3-diazabicyclo[2.2.1]hept-2-ene
(5g). Yellow powder, mp 85–86 ЊC with decomposition (Found:
C, 62.56; H, 4.89; N, 18.33. C12H11N3O2 requires C, 62.87; H,
4.84; N, 18.33%); νmax(KBr)/cmϪ1 2946, 1599, 1516, 1344, 1108,
7 K. Beck and S. Hünig, Chem. Ber., 1987, 120, 477.
8 (a) W. Adam. L. Fröhlich, W. M. Nau, H.-G. Korth and
R. Sustmann, Angew. Chem., Int. Ed. Engl., 1993, 32, 1339; (b)
W. Adam, F. Kita, H. M. Harrer, W. M. Nau and R. Zipf, J. Org.
Chem., 1996, 61, 7056; (c) W. Adam, H. M. Harrer, F. Kita and
W. M. Nau, Pure Appl. Chem., 1997, 69, 91; (d) W. Adam,
O. Emmert and H. M. Harrer, J. Chem. Soc., Perkin Trans. 2, 1997,
687; (e) W. Adam, H. M. Harrer, F. Kita, H.-G. Korth and W. M.
Nau, J. Org. Chem., 1997, 62, 1419; ( f ) W. Adam, H. M. Harrer,
F. Kita and W. M. Nau, Adv. Photochem., in the press.
Ϫ1
883; λmax(benzene)/nm 348 (ε/M cmϪ1 121), 331 (115); δH(200
MHz; CDCl3) 1.24–1.33 (2 H, m, 5- and 6-H), 1.75–1.89 (2 H,
m, 5- and 6-H), 5.29 (1 H, br s, 4-H), 5.69 (1 H, br s, 1-H), 6.25
(1 H, s, vinyl), 7.30–7.41 (2 H, m, ArH), 8.16–8.22 (2 H, m),
δC(50 MHz; CDCl3) 20.8 (t, 5- or 6-CH2), 21.5 (t, 6- or 5-CH2),
72.5 (d, 1- or 4-CH), 78.1 (d, 4- or 1-CH), 115.8 (d, CH᎐C),
᎐
123.9 (2 × d, Ar), 128.6 (2 × d, Ar), 141.9 (s, CH᎐C), 146.9 (s,
᎐
9 (a) X. Creary, M. E. Mehrsheikh-Mohammadi and S. McDonald,
J. Org. Chem., 1987, 52, 3254; (b) X. Creary, in Substituent Effects in
Radical Chemistry, NATO ASI Ser. C, Vol. 189, Eds. H. G. Viehe,
Z. Janousek and R. Merényi, Riedel, Dordrecht, Netherlands,
1986, pp. 145–262; (c) X. Creary and M. E. Mehrsheikh-
Mohammadi, Tetrahedron Lett., 1988, 29, 749; (d) X. Creary, M. E.
Mehrsheikh-Mohammadi and S. McDonald, J. Org. Chem., 1989,
54, 2904; (e) D. R. Arnold, in Substituent Effects in Radical
Chemistry, NATO ASI Ser. C, Vol. 189, Eds. H. G. Viehe,
Z. Janousek and R. Merényi, Riedel, Dordrecht, Netherlands,
1986, pp. 171–188; ( f ) H. Agirbas and R. A. Jackson, J. Chem. Soc.,
Perkin Trans. 2, 1983, 739.
Ar), 151.1 (s, Ar).
Computations
Full geometry optimization of the TMM derivatives 2 was
carried out by use of the AM1 method and AUHF wave-
function,17 which are provided in the VAMP 5.0 program,18
and run on an IRIS INDIGO Silicon Graphics Workstation.
Excellent spin expectation 〈S2〉 values of 2.00 were obtained for
the spin densities of these radicals.
10 (a) H. G. Viehe, Z. Janousek and R. Merényi, Acc. Chem. Res., 1985,
18, 148; (b) W. J. Leigh, D. R. Arnold, R. W. Humphreys and P. C.
Wong, Can. J. Chem., 1980, 58, 2537; (c) L. Sylvander, L. Stella,
H.-G. Korth and R. S. Sustmann, Tetrahedron Lett., 1985, 26, 749.
11 (a) M. Neuenschwander, in The Chemistry of Double-bonded
Functional Groups, Ed. S. Patai, Wiley, London, 1989, pp. 1131–
1268; (b) N. P. Marullo and J. A. Alford, J. Org. Chem., 1968, 33,
2368; (c) K. J. Stone and R. D. Little, J. Org. Chem., 1984, 49, 1849.
12 M. Neuenschwander and R. Iseli, Helv. Chem. Acta, 1977, 60, 1061.
13 (a) J. A. Weil, J. R. Bolton and J. E. Wertz, in Electronic Paramagnetic
Resonance, Wiley, New York, 1994, pp. 167–171; (b) D. A.
Dougherty, in Kinetics and Spectroscopy of Carbenes and Diradicals,
Ed. M. S. Platz, Plenum, New York, 1990, pp. 117–142.
EPR spectroscopy
A sample (1.5 × 10Ϫ2 mmol) of the diazene 5 was dissolved in
0.4 cm3 of degassed MTHF and transferred to a 3 × 25 mm
quartz EPR tube which was then sealed and, to prepare the
matrix sample, cooled at 77 K by liquid nitrogen. The triplet
diradicals 2 were generated by irradiation with the 333 nm line
of an INNOVA-100 CW argon-ion laser (2.1 W, 3–5 min) at
77 K. The EPR spectrum was recorded on a Bruker ESP-300
spectrometer (9.34 GHz, spectra accumulation with the Bruker
1620 data system). The D values were determined by a manual
analysis of the X, Y signals or the Z signals and checked by
simulation with Bruker’s WIN-EPR SimFonia Version 1.2. The
error limits are 0.0002 cmϪ1.
14 M. J. S. Dewar, J. Am. Chem. Soc., 1985, 107, 3902.
15 R. D. Little and M. G. Venegas, J. Org. Chem., 1978, 43, 2921.
16 W. Adam, J. Bialas and L. Hadjiarapoglou, Chem. Ber., 1991, 124,
2377.
17 T. Kovar, Diploma Thesis, University of Erlangen-Nürnberg, 1987.
18 G. Rauhut, J. Chandrasekhar, A. Alex, T. Steinke and T. Clark,
VAMP 5.0, University of Erlangen-Nürnberg, 1993.
Acknowledgements
We express our gratitude to the Deutsche Forschungsgemein-
schaft and Volkswagen Foundation for generous financial sup-
port. M. A. thanks the Alexander-von-Humboldt Foundation
for a fellowship (1997–1998).
References
Paper 7/08236C
Received 17th November 1997
Accepted 23rd December 1997
1 (a) P. Dowd, J. Am. Chem. Soc., 1966, 88, 2587; (b) P. Dowd, Acc.
Chem. Res., 1972, 5, 242.
1068
J. Chem. Soc., Perkin Trans. 2, 1998