8
486
H. Ikeda et al. / Tetrahedron Letters 42 (2001) 8485–8488
†
methoxyphenyl)bicyclo-[3.1.0]hexane (5, Fig. 1) and
with 8 as a minor product. Of course, prolonged irradi-
ation of 6 gives 8 quantitatively under oxygen.
1
,5-bis(4-methoxyphenyl)-6,7-diazabicyclo[3.2.1]oct-6-
ene (6) under various electron-transfer conditions and
found that it is also the case with the reactivity of
†
Interestingly, one-electron oxidation reactions of 5 and
1
,3-bis(4-methoxyphenyl)cyclohexane-1,3-diyl
cation
6
by tris(4-bromophenyl)aminium hexachloroanti-
+
monate (Ar N SbCl ) give rapidly 4,4¦-dimethoxy-m-
+ −
radical (7 ).
3
6
terphenyl (9) under nitrogen and even under oxygen. As
shown in Table 1, the cerium(IV) tetra-n-butylammo-
nium nitrate [Ce(n-Bu N) (NO ) , CBN] -catalyzed
Diazene 6 was prepared by the BF ·Et O-catalyzed
3
2
4
7
reaction of 1,5-bis(4-methoxyphenyl)hex-5-en-1-one
tosylhydrazone. Bicyclohexane 5 was obtained quanti-
tatively by pyrolysis, direct irradiation, and benzophe-
none (BP)-sensitized photoreaction of 6. The halfwave
4
2
2 6
reaction of 5 and 6 under nitrogen forms 9 similarly but
slowly. Under oxygen, CBN-catalyzed reaction gives 9
together with 8 as a minor product.
ox
1
‡
oxidation potentials (E
) of 5 (+0.77 V versus SCE
/2
in acetonitrile) and 6 (+1.28 V) are low enough to
Scheme 3 shows plausible mechanistic connections
among all reactions. The DCA-sensitized photoreac-
quench the excited singlet state of DCA exergonically,
tions of 5 and 6 initially form cation radicals 5 and
+
+
as suggested by the calculated free energy change for
§
6 , which undergo cleavage and deazetation, respec-
+ −
electron transfer, DG =−1.19 and −0.68 eV in acetoni-
trile, respectively. In agreement with the calculation, 5
and 6 quench the fluorescence of DCA efficiently with
for 5,
for 6, respectively,
in aerated acetonitrile, dichloromethane, and benzene.
In spite of large k close to the diffusion control rate,
the DCA-sensitized photoreactions of 5 (u>360 nm)
under nitrogen in acetonitrile, dichloromethane, or ben-
zene result in a quantitative recovery, but 1,5-bis(4-
et
tively, to form 7 . Then, a facile BET from DCA to
+
7
succeeds to form diyl 7 under nitrogen and oxygen.
10
−1 −1
rate constants, k =1.7, 1.6, and 1.1×10
M
s
The inertness of 5 under nitrogen is likely due to the
degeneracy of 5. The degeneracy of 5 and formation of
5 from 6 are completed by cyclization of 7. The fact
that pyrolysis, direct irradiation, or BP-sensitized pho-
toreaction of 6 forms 5 supplements the intervention of
7 in the DCA-sensitized photoreactions of 5 and 6.
Under oxygen, however, relatively slow oxygenation of
q
1
0
−1 −1
and 1.4, 1.1, and 0.84×10
M
s
q
+
+ ¶
methoxyphenyl)-6,7-dioxabicyclo[3.2.1]octane
(8,
7
competes with BET, forming 8 through 10 . On
†
Scheme 2) is efficiently formed under oxygen in polar
acetonitrile and dichloromethane (Table 1). Similar
irradiation (u>410 nm) of DCA with 6 under nitrogen
quantitatively in acetonitrile, dichloro-
methane, and benzene. Deazetation of 6 also occurs
quantitatively under oxygen, giving rise to 5 together
the other hand, one-electron oxidant-catalyzed reac-
tions of 5 and 6 similarly form 7 , but BET to form 7
is energetically unfavorable under these one-electron
oxidation conditions. Thus, cation radical 7 formed
from 5 and 6 then gives rise to 9 under nitrogen and a
mixture of 8 and 9 through 11 under oxygen, as shown
in Scheme 3.
+
+
affords
5
Time-resolved absorption spectroscopy upon laser flash
ꢀꢀ
photolysis (LFP) and g-ray irradiation** directly
observed cation radicals derived from 5 and 6. Under
+
−
N-methylquinolinium tetrafluoroborate (NMQ BF )–
4
toluene-cosensitized conditions in dichloromethane,
nanosecond LFP of 5 and 6 exhibit nearly the same
spectra with an intense absorption band with umax at
5
65 nm together with a weak band at 495 nm, as shown
†† + +
in Fig. 2. These results suggest that 5 and 6 afford
the same transient species. It is noteworthy that upon
¶
The fact that the DCA-sensitized reaction of 5 slowly afforded 8
even in nonpolar benzene (Table 1) suggests that molecular oxygen
capture of 7 may occur concurrently (Scheme 3).
Nanosecond absorption spectroscopy upon LFP was carried out
with a pulsed Xe arc lamp (150 W) and a XeCl excimer laser
ꢀ
ꢀ
(
Lumonics EX600, u =308 nm, 100 mJ) at the Advanced Instru-
ex
mental Laboratory for Graduate Research of Department of
Chemistry, Graduate School of Science, Tohoku University.
A sample solution (10 mM) was degassed by repeating five freeze
Scheme 2.
*
*
‡
The values of Eox were estimated as Epa (anodic peak potentials)
−3
(77 K)–pump (10 mmHg)–thaw (ambient temperature) cycles and
1
/2
−
3
−
0.03 V, which were measured by cyclic voltammetry at a platinum
sealed at 10 mmHg at 77 K. This matrix was irradiated at 77 K
+
−
60
electrode in dry acetonitrile with 0.1 M Et N ClO as a supporting
for 8 h with g-ray from a 5.1 TBq Co source at the Cobalt 60 g
4
4
electrolyte.
Ray Irradiation Facility, Tohoku University.
§
††
The values of DG were estimated by using the Rehm–Weller
Similarly, absorption bands with umax at 549 and 485 nm were
observed by LFP (Continuum Surelite-10 YAG laser, Nd, THG,
uex=355 nm, 55 mJ) of 5 in acetonitrile under the DCA–biphenyl-
cosensitized conditions.
et
equation: DG (eV)=Eox (sub)−E
5
red
(DCA)−E0-0 (DCA)–e /
2
et
red
1
1/2
1/2
mr, where E
(DCA)=−0.95 V and E
(DCA)=2.91 eV in
/2
0-0
2
6
acetonitrile and the coulombic term (e /mr) is disregarded.
Ikeda, Hiroshi
