C O M M U N I C A T I O N S
diradicals 3 relative to the parent case 3c (Y, Z ) H). From the
symmetrical derivatives (entries 1-5) it is evident that a strong
electron-donating group (3a: Y, Z ) OMe, entry 1) increases
significantly the lifetime; actually, derivative 3a possesses the
longest lifetime of all diradicals examined. In the unsymmetrical
cases (entries 6-8), there is also a definite prolongation of the
lifetime, but this is clearly less than that for the symmetrical 3a.7
What electronic character of these singlet diradicals accounts
for these unusual and unprecedented lifetime data? To rationalize
the experimental substituent effects, we shall consider the possible
mesomeric structures A-D depicted in Figure 2 for the singlet 2,2-
dimethoxycyclopentane-1,3-diyls.
Figure 3. Pertinent bond orders (BO) in the model structure 6.
Furthermore, also hyperconjugation operates through the meso-
meric structure D, as expressed by the finding of BO2 > 1.0 [1.15
(Y ) OH), 1.08 (Y ) H), 1.00 (Y ) CN)] and BO3 < 1.0 [0.82
(Y ) OH), 0.90 (Y ) H), 0.98 (Y ) CN)]. Hyperconjugation
accounts for the notable stabilization through electron donation to
the allylic cation by the two terminal p-MeO substituents, which
should prolong significantly the lifetime of the singlet diradical 3a.
Such electron donation through hyperconjugation also has been
proposed in 1,3-DR (X ) F) by Borden et al.11 Nevertheless, the
electron-withdrawing substituents in the symmetrically disubstituted
singlet diradical 3e (Y, Z ) CN) should destabilize the hypercon-
jugative structure D and lower the lifetime compared to unsym-
metrically monosubstituted derivative 3g (Y ) CN; Z ) H), which
is contrary to our experimental results (compare entries 5 and 7).
As already explained in point 1 (radical character), the radical-
stabilizing effect of the p-CN group comes to bear.
In summary, the unprecedented substituent effects on the lifetime
of the singlet diradicals reported herein provide valuable insight
into the electronic character of these species. From the localized
singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 we have
learned that the lifetime of such short-lived intermediates may be
prolonged through the synergistic stabilization of radical zwitter-
ionic, π-bonding and hyperconjugative structures.
Figure 2. Possible mesomeric structures to express the electronic character
in singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals.
(1) Radical character: Our previous study5 disclosed that the
singlet diradical 2 reacts with molecular oxygen at a significant
rate, i.e., k is ∼105 M-1 s-1, which substantiates the radical character
in such diradicals. Thus, the singlet-diradical structure A should
significantly contribute in the stabilization of the radical site by
the para-substituted phenyl group. For benzyl-type radicals, the
stabilization power8 of para substituents has the following order:
CN (σR‚8a ) 0.040, σC‚8b ) 0.46, ∆D8c ) 0.54) . Cl (0.011, 0.12,
0.04) ∼ Me (0.015, 0.11, 0.02) ∼ OMe (0.018, 0.24, -0.05) > H
(0.00). Evidently, the prolonged lifetime of the dicyano derivative
3e (Y, Z ) CN) is well accounted for by such radical stabilization;
however, it fails for the dimethoxy case 3a (Y, Z ) OMe), which
is actually the longest lived singlet diradical in Table 1. Addition-
ally, other electronic effects must operate to rationalize the observed
substituent-dependent lifetimes of these singlet diradicals.
(2) Zwitterionic character: As suggested by Salem and Rowland,9
the singlet structure A should have some dipolar contribution, as
expected by the structure B. Our experimental work clearly
manifests the importance of such dipolar character since the
unsymmetrically substituted derivatives stabilize the singlet diradical
in the order 3h (Y ) OMe; Z ) CN) > 3f (Y ) OMe; Z ) H) >
3g (Y ) CN; Z ) H) > 3c (Y, Z ) H) (entries 3 and 6-8). Thus,
in 3h the electron-donating p-MeO group stabilizes the positive
charge of the dipole and the electron-withdrawing p-CN group the
negative one. However, this zwitterionic character does not explain
why the symmetrical derivatives 3a (Y, Z ) OMe; 1050 ns) and
3e (Y, Z ) CN; 625 ns) possess longer lifetimes than the
unsymmetrical ones 3e (Y ) OMe; Z ) H; 600 ns) and 3f (Y )
CN; Z ) H; 470 ns). In the case of 3a, the methoxy group should
destabilize the anionic center, while in the case of 3e the cyano
group should destabilize the cationic site. Thus, these symmetrical
singlet diradicals should have shorter lifetimes than all the unsym-
metrical ones. Again, still other factors must play a role.
Acknowledgment. The work was supported by the Ministry
of Education, Science, Sports and Culture of Japan, Volkswagen
Foundation, Deutsche Forschungsgemeinschaft, and an A. v.
Humboldt fellowship to M.A. (1997-98).
Supporting Information Available: Experimental and computa-
tional details (PDF). This material is available free of charge via the
References
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(7) Of note is the substantial geminal substituent effect on the lifetime of
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3a and 3e, the Mulliken bond orders (BO) were computed for the
model diradicals 6 at the UB3LYP/6-31G* level of theory (Figure
3).10 The BO1 values reveal substantial π bonding [BO1 > 0; 0.35
(Y ) OH), 0.58 (Y ) H), 0.40 (Y ) CN)] between the C1 and C3
radical sites, which implies that the mesomeric structure C is
important. Thus, the stronger this π bonding in the singlet diradical,
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lifetime. The calculated BO1 trend follows qualitatively very nicely
the observed order (Table 1).
(9) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92-111.
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