The quantum yields for oxetane formation in various solvents
are listed in Table 1.
photoexcited quinone by stilbene in dioxane on the picosecond
timescale, chloranil was excited at 406 nm (to avoid the
unintentional photoexcitation of stilbene). The excitation at
406 nm was achieved with a Ti:sapphire laser system consisting
of a Photonics Industries Ti:sapphire oscillator coupled to a
Coherent Innova 310 argon-ion laser and two consecutive
Photonics Industries Ti:sapphire amplifiers pumped by a
Continuum Surelite I Nd:YAG laser at 10 Hz.45
Formation constants of solvent complexes with chloranil
In a typical procedure, a 1.5 mL aliquot of a 0.1 mM solution
of chloranil in dichloromethane was transferred to a 1 cm
quartz cuvette. An equimolar (0.1 mM) solution of chloranil in
a donor solvent (dioxane or tetrahydrofuran) was added incre-
mentally. The absorption changes were measured at the spectral
maxima as well as at other wavelengths close to the absorption
maxima. From the growth of the new absorption band at 312
nm with increasing molar fraction of either dioxane or tetra-
hydrofuran, the equilibrium constants (Ks) for the formation of
the chloranil–solvent complexes were determined by the appli-
cation of the Benesi–Hildebrand method.28 Thus, the plot of
[Q]/As vs. [solvent]Ϫ1 was linear with a correlation coefficient
R > 0.97, and the extinction coefficients (εs) and the formation
constants (Ks) were calculated from the intercept and the
slope.28 The Ks values were substantially smaller for oxygen-
donor solvents relative to those evaluated for aromatic solvents,
such as benzene, toluene, and p-xylene reported earlier29 (see
Table 2).
Acknowledgements
We thank the R. A. Welch Foundation and the National
Science Foundation for financial support.
References
1 J.-H. Xu, L.-C. Wang, J.-W. Xu, B.-Z. Yan and H.-C. Yuan, J. Chem.
Soc., Perkin Trans. 1, 1994, 571.
2 D. Sun, S. M. Hubig and J. K. Kochi, J. Org. Chem., in the press.
3 E. Paterno and G. Chieffi, Gazz. Chim. Ital., 1909, 39, 341; G. Büchi,
C. G. Inman and E. S. Lipinsky, J. Am. Chem. Soc., 1954, 76, 4327.
4 Spectroscopic studies on various Paterno–Büchi couplings have
revealed several reactive intermediates including exciplexes,5,6
ion-radical pairs,6–10 and 1,4 biradicals. 8–12
5 R. A. Caldwell, D. C. Hrncir, T. Muñoz, Jr. and D. J. Unett, J. Am.
Chem. Soc., 1996, 118, 8741; R. A. Caldwell, G. W. Sovocool and
R. P. Gajewski, J. Am. Chem. Soc., 1973, 95, 2549; N. E. Schore and
N. J. Turro, J. Am. Chem. Soc., 1975, 97, 2482.
6 S. Hu and D. C. Neckers, J. Org. Chem., 1997, 62, 6820.
7 J. Gersdorf, J. Mattay and H. Görner, J. Am. Chem. Soc., 1987, 109,
1203.
8 S. C. Freilich and K. S. Peters, J. Am. Chem. Soc., 1985, 107, 3819;
J. Am. Chem. Soc., 1981, 103, 6255.
9 A. G. Griesbeck, H. Mauder and S. Stadtmüller, Acc. Chem. Res.,
1994, 27, 70.
10 G. Eckert and M. Goez, J. Am. Chem. Soc., 1994, 116, 11999.
11 D. I. Schuster, G. Lem and N. A. Kaprinidis, Chem. Rev., 1993, 93,
3.
12 S. Hu and D. C. Neckers, J. Org. Chem., 1997, 62, 564.
13 For reviews, see: G. Jones II, in Organic Photochemistry, ed. A.
Padwa, Dekker, New York, 1981, vol. 5, p. 1; A. G. Griesbeck,
in CRC Handbook of Organic Photochemistry and Photobiology,
ed. W. M. Horspool and P.-S. Song, CRC Press, Boca Raton, FL,
1995, p. 522.
14 R. Rathore, S. M. Hubig and J. K. Kochi, J. Am. Chem. Soc., 1997,
119, 11468, and references therein.
15 E. Bosch, S. M. Hubig and J. K. Kochi, J. Am. Chem. Soc., 1998,
120, 386.
Photocoupling of (Z)-stilbene with chloranil
An equimolar solution of chloranil and (Z)-stilbene in dioxane
was irradiated at λexc = 436 nm, and the simultaneous disap-
pearance of chloranil and (Z)-stilbene [as well as the appear-
ance of trans-spirooxetane and (E)-stilbene] was periodically
monitored by HPLC analysis (up to 8 h). This kinetics study
was carried out with the aid of a medium-pressure (500 W)
mercury lamp that was focused through an aqueous IR filter
followed by an aqueous NaNO2–CuSO4 solution filter
(440 30 nm). The 1 cm cuvette fitted with a Schlenk adapter
was filled with a solution of chloranil (0.05 M) and (Z)-stilbene
(0.05 M) in 3 mL dioxane under argon. At various times during
the irradiation, a 20 µL aliquot was extracted and diluted with 5
mL of methanol. The four components [chloranil, (Z)-stilbene,
(E)-stilbene and spirooxetane] of the reaction mixture were
quantified by HPLC with biphenyl as an internal standard. The
overall conversion of the reactants was kept below 15% in order
to avoid the competitive quenching of excited chloranil by (E)-
stilbene formed during the photoreaction. Table 3 shows the
change in concentration of all reactants and products over time,
and from the slope of the linear (R > 0.97) concentration–time
plots versus the intensity of the light source (determined by
ferrioxalate actinometry24), the quantum yields for the con-
sumption of chloranil and (Z)-stilbene [and for the production
of trans-spirooxetane and (E)-stilbene] were determined to be
0.18, 0.48, 0.18 and 0.27, respectively. Thus, the consumption
of chloranil and the production of spirooxetane occurred at the
same rate and quantum efficiency. Furthermore, the consump-
tion of (Z)-stilbene occurred with a quantum efficiency equal
to the sum of (E)-stilbene and trans-spirooxetane efficiencies.
16 R. Gschwind and E. Haselbach, Helv. Chim. Acta, 1979, 62, 941.
17 S. M. Hubig, T. M. Bockman and J. K. Kochi, J. Am. Chem. Soc.,
1997, 119, 2926.
18 The reduction potential of the triplet quinone is E*red = 2.15 V vs.
SCE. See ref. 14.
19 The oxidation (peak) potential of stilbene is EPox ≅ 1.56 V vs. SCE.
See ref 23.
20 The triplet ion-radical pair in eqn. (2) then follows various reaction
pathways including back-electron transfer,21a,b ion dissociation,21c
and the critical coupling to the triplet 1,4-biradical2 which
subsequently undergoes ring closure to form the final oxetane
product.8–12
21 (a) K. S. Peters, Adv. Electron Transfer Chem., 1994, 4, 27; (b) M. A.
Fox, Adv. Photochem., 1986, 13, 237; (c) H. Knibbe, D. Rehm and
A. Weller, Ber. Bunsenges. Phys. Chem., 1968, 72, 257.
22 The chloranil anion radical absorbs at λmax = 450 nm and λmax = 320
nm. See J. J. André and G. Weill, Mol. Phys., 1968, 15, 97.
23 The stilbene cation radical absorbs at λmax = 480 and 760 nm. See:
F. D. Lewis, A. M. Bedell, R. E. Dykstra, J. E. Elbert, I. R. Gould
and S. Farid, J. Am. Chem. Soc., 1990, 112, 8055; T. Shida,
Electronic Absorption Spectra of Radical Ions, Elsevier, New York,
1988, p. 112.
Time-resolved absorption spectra from photoexcitation of
chloranil with stilbene
Laser excitation of chloranil at ëexc ؍
355 nm. The nano-
second/microsecond time-resolved absorption measurements
were carried out with a kinetic spectrometer including a
Quantel (YG580-10) Q-switched Nd3ϩ:YAG laser (10 ns
pulsewidth).44 The third (355 nm) harmonic output was used
for the excitation of chloranil. The solutions of chloranil and
(E)-stilbenes were prepared under an argon atmostphere in a
1 cm cuvette fitted with a Schlenk adapter. The concentrations
of the components were adjusted for absorbances in the range
0.5–0.8 at the excitation wavelength of λexc = 355 nm.
24 C. G. Hatchard and C. A. Parker, Proc. R. Soc. London, Ser. A,
1956, 235, 518. See also: J. G. Calvert and J. N. Pitts, Jr.,
Photochemistry, Wiley, New York, 1996, p. 786.
25 C. Reichardt, Solvent Effects in Organic Chemistry, Verlag Chemie,
New York, 1979.
26 R. C. Weast, CRC Handbook of Chemistry and Physics, 53rd edn.,
CRC Press, Boca Raton, FL, 1975, p. E73.
27 D. Bryce-Smith, B. F. Connett and A. Gilbert, J. Chem. Soc. (B),
1968, 816; M. A. Slifkin, Spectrochim. Acta, Part A, 1969, 25, 1037.
Laser excitation at 406 nm. To monitor the quenching of
J. Chem. Soc., Perkin Trans. 2, 1999, 781–788
787