On the electronic character of localized singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals: Substituent effects on the lifetime

Article abstract of DOI:10.1021/ja026301l

Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measure

Full text of DOI:10.1021/ja026301l

Published on Web 05/15/2002
On the Electronic Character of Localized Singlet
2,2-Dimethoxycyclopentane-1,3-diyl Diradicals: Substituent Effects on the
Lifetime
Manabu Abe,*,† Waldemar Adam,‡ Michihiro Hara,§ Masanori Hattori,† Tetsuro Majima,§
Masatomo Nojima,† Kei Tachibana,† and Sachiko Tojo§
Department of Materials Chemistry, Graduate School of Engineering, Osaka UniVersity,
Suita 565-0871, Osaka, Japan, Institut fu¨r Organische Chemie der UniVersita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany, and Institute of Scientific and Industrial Research, Osaka UniVersity,
Ibaraki 567-0047, Osaka, Japan
Received March 24, 2002
Our present day understanding of the electronic nature of
localized triplet diradicals is well-documented by experimental as
well as theoretical studies;1 however, this is not the case for the
corresponding singlet diradicals. The major reason resides in the
fact that these species are quite short-lived and difficult to handle
experimentally.2 A breakthrough was the spectral detection and
characterization of the first localized singlet diradical, namely the
2,2-difluoro-substituted 1,3-diradical 1 (X ) F; Y, Z ) H; λmax
530 nm; τ293 K ) 80 ns in n-pentane),3 whose singlet ground state
Figure 1. (a) Transient absorption spectrum for the singlet diradical 3a
measured immediately after the laser pulse (λexc 355 nm); (b) absorption
was predicted by Borden’s computational work4 for the parent
cyclopentane-1,3-diyl (1,3-DR, X ) F). More recently, we have
found that an alkoxy group (X ) OR in 1,3-DR) stabilizes
sufficiently such transient species to place the singlet below the
triplet state, as confirmed by the long-lived singlet diradical 2
(X ) OEt; Y, Z ) H; λmax 550 nm; τ293K ) 880 ns in benzene).5
spectrum in a MCP matrix at 77 K; (c) fluorescence spectrum (λmax 740
nm) in the MCP matrix at 77 K (λexc 590 nm); (d) transient decay trace at
580 nm and 20 °C.
Table 1. Substituent Effects on the Lifetime of the Singlet
Diradicals 3 (X ) OMe)a
entry
3
τ
293K (ns)b
1
2
3
4
5
6
7
8
3a (Y, Z ) OMe)
3b (Y, Z ) Me)
1050 ( 80
460 ( 30
320 ( 15
490 ( 30
625 ( 25
600 ( 35
470 ( 30
740 ( 60
3c (Y, Z ) H)
3d (Y, Z ) Cl)
3e (Y, Z ) CN)
3f (Y ) OMe, Z ) H)
3g (Y ) CN, Z ) H)
3h (Y ) OMe, Z ) CN)
In this study, we have examined the electronic substituent effects
on the lifetime of the singlet diradicals 3a-h (X ) OMe), for which
electron-donating (Y, Z ) Me, OMe) and/or electron-withdrawing
groups (Y, Z ) Cl, CN) were introduced at the para position of
the phenyl rings. The experimental results have provided valuable
information on the electronic character of localized singlet diradi-
cals.
For the generation of the diradicals 3, we have chosen the
photodenitrogenation of the azoalkanes 4a-h (λmax ∼360 nm, ꢀ ∼
100) by photolysis (>320 nm) in benzene, which afforded
quantitatively the housanes 5a-h (>95%, Scheme 1) by intramo-
a The diradicals 3a-h were generated during laser flash photolysis (355
nm, 5-ns pulse width) of the diazenes 4a-h. b In benzene solution at 20
°C; errors are standard deviations of the mean (5 data points).
diradicals 3 were measured in benzene by means of laser-flash
photolysis (λexc ) 355 nm, 5 ns pulse). Strong absorption was
observed in the visible region (ca. 600 nm), which decayed with
clean first-order kinetics, as exemplified for the diazene 4a (Y, Z
) OMe) in Figure 1 (spectrum a and time profile d). A similar
absorption band was observed in a methylcyclopentane (MCP)
matrix at 77 K (Figure 1, spectrum b). The transient species were
assigned to the diradicals 3a-h on the following experimental
evidence: (i) the absorption maxima (around 600 nm) are similar
to those of the singlet diradicals 1 and 2; (ii) the persisting species
3a,c at 77 K are EPR silent; (iii) the lifetimes (Table 1) are not
affected by the presence of molecular oxygen;6 and (iv) the large
(log A ) 11.2) preexponential Arrhenius factor for 3c (Y ) H, Ea
) 25.9 kJ/mol) suggests spin-allowed ring closure to housane 5.
Additionally, significant fluorescence (λmax 740 nm at λexc 590 nm)
was observed for the localized singlet diradical 3a in the MCP
matrix at 77 K (Figure 2c). As may be clearly seen from the lifetime
(τ293K) data of the singlet diradicals 3 in Table 1, both electron-
donating and electron-withdrawing groups stabilize the singlet
Scheme 1
lecular cyclization of the singlet diradicals 3a-h. The transient
absorption spectra and decay traces of the intermediary singlet
* Corresponding author. E-mail: abe@ap.chem.eng.osaka-u.ac.jp.
† Department of Materials Chemistry, Osaka University.
‡ Institut fu¨r Organische Chemie der Universita¨t Wu¨rzburg.
§ Institute of Scientific and Industrial Research, Osaka University.
9
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J. AM. CHEM. SOC. 2002, 124, 6540-6541
10.1021/ja026301l CCC: $22.00 © 2002 American Chemical Society

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
Internet at http://pubs.acs.org.
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