RESEARCH FRONT
Thermal Denitrogenation of Cyclic Azoalkanes
1617
r ϭ 0.965
r ϭ 0.988
(a) 0.6
(b) 0.6
0.5
0.5
0.4
0.3
0.2
0.1
0.0
0.4
0.3
0.2
0.1
0.0
0.00
0.01
0.02
0.03
0.04
0.0
0.1
0.2
0.3
0.4
0.5
σα•
σC•
Fig. 2. Logarithmic plot of the relative denitrogenation rate (krel) for azoalkane 2 versus (a) Arnold’sparameter sa and (b) Creary’s sC parameter, see Table 1.
was weakly correlated with the lifetime t293 of DR2 (Table 1).
These results strongly suggest that the denitrogenation of 2
occurs by stepwise C–N bond cleavage via DZ2 (Scheme 1).
In summary, the results of the present study, which investi-
gated the thermal denitrogenation of compound 2, showed that
the substituent effect on the denitrogenation rate (krel) was not
correlated with the lifetime of the resulting singlet diradical
DR2, but was highly correlated with the radical-stabilizing
parameters sꢀa and sꢀC. These strong correlations provide the
first strong experimental evidence that thermal denitrogenation
of 7,7-dialkoxy-2,3-diazabicyclo[2.2.1]hept-2-ene derivatives
proceeds via stepwise C–N bond cleavage. This remarkable
finding should stimulate future theoretical and experimental
investigations of azo-chemistry, which is both mechanistically
and synthetically fascinating.
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Experimental
Thermolysis of 2
A sample of approximately 70 mg (0.21 mmol) of 2[9] was dis-
solved in 7 mL of d6-benzene.[13] The solution was divided
into seven samples; thus, each sample contained 1 mL of the
solution. Each sample in an NMR-tube was degassed by
three freeze-and-thaw cycles, and was sealed under vacuum at
,0.01 mmHg. The thermolysis was performed at 420 ꢁ 0.5 K in
silicone oil. The disappearance of 2 was determined at regular
time intervals. The relative amounts of 2 that remained were
directly determined by quantitative 1H NMR spectroscopy
(600 MHz). Triphenylmethane (Ph3CH) was used as an internal
standard. The first-order plots were linear (Fig. 1). The absolute
thermolysis rate constants (k) were determined from the slope of
the plots (Table 1).
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Acknowledgements
This work was supported by a Grant-in-Aid for Science Research on Inno-
vative Areas (No. 21108516, ‘pi-Space’) and (B) (No. 19350021) from the
Ministry of Education, Culture, Sports, Science and Technology, Japan, and
by the Tokuyama Science Foundation.
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