In conclusion, herein we described a new supramolecular
method for the reversible modulation of the catalytic activity
of a photoredox complex through its reversible encapsulation
and release. The photocatalyst is in the on-state as long as it
operates free in solution, while it is switched off when sequestered
in the self-assembled, hydrogen-bonded capsule. The reversibility
of the process is achieved through addition of a suitable compe-
titive guest at proper concentrations.
Notes and references
Fig. 2 Variation of the residual sulfide 4 during reversible switching
of the photoredox catalytic activity of 1 through sequential additions
of 2 (6 eq., 100 min), competitive guest 3 (10 eq., 165 min) and 2
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2 U. Luning, Angew. Chem., Int. Ed., 2012, 51, 8163.
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4
3, 5503; (c) M. S. Masar III, N. C. Gianneschi, C. G. Oliveri,
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likely that visible light with 1 as a photosensitizer transforms
in the wet solvent employed into H that directly
oxidizes the electron rich substrate 4.
O
2
2 2
O
2
The catalytic system is sensitive to the electron density of the
thioether. In fact, after 3 h with methyl phenyl sulfide the
reaction provides the corresponding sulfoxide in only 5%
yield. This again supports the formation of H O as a true
(
5
1
2
2
6 J.-M. Zen, S.-L. Liou, A. Senthil Kumar and M.-S. Hsia, Angew.
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oxidant without participation of 1 in the oxygen transfer step.
The reversible control of the catalytic activity of 1 was
demonstrated by performing the sulfoxidation experiment
described in Fig. 2. The reaction was started following the
7
J. M. R. Narayanam and C. R. J. Stephenson, Chem. Soc. Rev.,
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2
8
(a) L. R. MacGillivray and J. L. Atwood, Nature, 1997, 389, 469;
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oxidation of 4 with 1 under visible light with O
eq. of 2 were added to the system leading to the encapsulation of
@2 O) thus causing a halt of the sulfide consumption and
Á(H
2
. After 100 minutes
9
(a) E. S. Barrett, T. J. Dale and J. Rebek, Jr., J. Am. Chem. Soc.,
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6
2
1
6
2
8
4
, 4365.
formation of oxidation products. The concentration of the former
species remained unchanged until when, after 165 minutes from
the beginning of the reaction, 10 eq. of 3 were added leading to
release of catalyst 1 into solution, formation of 3@26 and
consequent restoration of the catalytic activity (Fig. 2). The
increased rate of the reaction observed in this second part seems
to be related to the presence of an excess of 3 in solution that, as
observed in Table 1, entries 1 and 6, slightly promotes oxidation
of 4. A second stopping of the reaction was achieved by
addition of a second amount of 2 (33 eq.) after 230 min from
the beginning of the reaction. Confinement of the photocatalyst
10 (a) Y. Aoyama, Y. Tanaka, Y. H. Toi and H. Ogoshi, J. Am.
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2
2
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1
3 6 2
2 Ru(bpy) (PF ) with resorcin[4]arene bearing ethyl side chains was
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1
3 (a) E. Baciocchi, T. Del Giacco, F. Elisei, M. F. Gerini, M. Guerra,
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1
6
within the supramolecular host 2 does not alter its absorption
(
b) R. S. Davidson and J. E. Pratt, Tetrahedron Lett., 1983,
properties, therefore the lack of catalytic activity in the presence
of the capsule is likely due to the interrupted energy transfer
24, 5903; (c) T. Pigot, T. Arbitre, H. Martinez and S. Lacombe,
Tetrahedron Lett., 2004, 45, 4047.
4 It is known that water is not a suitable guest for the hexamer as
observed in the hydration reaction of alkynes mediated by an
encapsulated Au(I) catalyst: A. Cavarzan, A. Scarso, P. Sgarbossa,
G. Strukul and J. N. H. Reek, J. Am. Chem. Soc., 2011, 133, 2848.
1
from the Ru(II) center to O , the latter probably not being a
2
suitable co-guest for the cavity of 2 , or due to the lack of water
6
1
4
within the cavity.
1
2084 Chem. Commun., 2012, 48, 12082–12084
This journal is c The Royal Society of Chemistry 2012