lifetime (1.69 µs) was very similar to that obtained for (S)-
Kp (1.70 µs). By contrast, the transient absorption spectra
obtained for dyads 2 and 3 under the same conditions did
not correspond to their triplet excited states but instead to
the corresponding biradicals. A safe assignment of these
transient species was based on the typical bands with maxima
s-1). All together, the above facts are consistent with
intersystem crossing concurrent with product formation as
the key factor determining biradical lifetimes14 in the case
of 2 and 3. The photophysical data obtained for dyads 1-3
correlated well with the results from the steady-state studies.
Thus, photolysis of 2 and 3 gave rise to photocyclization
products arising from the reaction of the Bz-derived triplet
excited states with the 7-allylic position of the cholesterol
skeleton by an intramolecular hydrogen abstraction process
(Scheme 2). The more reactive (R)-Kp-R-Ch system in the
steady-state photolysis (Figure 1) produced also the shorter-
lived biradical in the laser flash photolysis experiments
(Figure 3B and Table 1). Besides, the nonreactive triplet
excited state of dyad 1 was actually detected, whereas the
T-T absorption of 2 and 3 was not clearly identified due to
the short triplet lifetimes (10-12 ns, as revealed by indirect
measurements based on energy transfer to naphthalene).9,16
In conclusion, the Kp-Ch dyads studied in the present
work seem to be appropriate model systems to generate the
allylic 7-Ch radical in an efficient and selective fashion.
Oxygen quenching of the Bz-derived triplets would only be
expected for (S)-Kp-â-Ch, in view of its sufficiently long
lifetime; by contrast, this process would not compete with
the very fast intramolecular H abstraction in (S)- and (R)-
Kp-R-Ch. The biradicals generated from the last two
compounds, however, live long enough to be quenched by
oxygen, to give the 7- hydroperoxide and the 7-keto
compounds. Thus, dyad 1 would be a suitable model to study
Type II Ch photooxidation, whereas dyads 2 and 3 appear
to be excellent systems to investigate clean Ch photooxida-
tion via the Type I mechanism. Systematic work is currently
being performed in our group to explore the potential of this
concept.
at ca. 330 and 545 nm and on their relative intensities (ꢀ545
/
ꢀ330, ca. 0.17).5,12 Figure 3A shows the biradical generated
from 2; a similar spectrum was obtained in the case of 3.
The decay kinetics of these transient intermediates was
monitored at 330 nm, and the biradical lifetimes found for
dyads 2 and 3 were 280 and 220 ns, respectively (Figure 3B
and Table 1). As expected for related 1-hydroxy-1,n biradi-
Table 1. Photophysical Parameters of Dyads 1-3
parameters (S)-Kp-â-Ch (1) (S)-Kp-R-Ch (2) (R)-Kp-R-Ch (3)
τT (µs)
kiq (s-1
1.690
0.010
0.012
a
)
<1.0 × 104
1.0 × 108
0.8 × 108
0.2 × 108
0.8 (0.7)d
0.28 (0.55)d
1.2 × 108
1.0 × 108
0.2 × 108
0.8 (0.7)d
0.22 (0.49)d
kH (s-1),b (%)
kπ (s-1),b (%)
Φketyl radical
τbiradical (µs)
c
0.2
a The intramolecular quenching rate constants of (S)-Kp-R-Ch and (R)-
Kp-R-Ch were obtained by using the equation kiq ) 1/τi - 1/τ0, where τi
is the lifetimes of the ketone triplets in compounds 2 and 3 and τ0 is the
(S)-Kp triplet lifetime (1.7 µs). b The rate constants for hydrogen abstraction
and physical quenching by the π system were obtained using the following
equations: kΗ ) kiq × Φketyl radical and kiq ) kΗ + kπ. c The quantum yield
of ketyl radical formation was determined by the comparative method, using
Bz as the standard. d In the presence of 1.2 M pyridine.
cals, the lifetimes were significantly longer in the presence
of added pyridine as a Lewis base14,15 (see Table 1); however,
they were not affected by the temperature and were quenched
by oxygen very quickly (rate constant of ca. 3.6 × 109 M-1
Acknowledgment. We would like to thank the financial
support given by the Spanish Government (Grant CTQ2004-
03811) and Juan de la Cierva contract to I.A.
(10) Pischel, U.; Abad, S.; Domingo, L. R.; Bosca´, F.; Miranda, M. A.
Angew. Chem., Int. Ed. 2003, 42, 2531-2532.
(11) Jung, M. E.; Johnson, T. W. J. Org. Chem. 1999, 64, 7651-7653.
(12) Mart´ınez, L. J.; Scaiano, J. C. J. Am. Chem. Soc. 1997, 119, 11066-
11070.
(13) Carmichael, L.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15,
1-250.
(14) Wagner, P. J. Acc. Chem. Res. 1989, 22, 83-91.
(15) Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252-258.
Supporting Information Available: Spectroscopic data
and experimental details. This material is available free of
OL061854C
(16) Scaiano, J. C.; McGimpsey, W. G.; Leigh, W. J.; Jacobs, S. J. Org.
Chem. 1987, 52, 4540-4544.
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Org. Lett., Vol. 8, No. 20, 2006