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Scheme 2 A possible reaction mechanism.
On the other hand, the ratio of fluorescence intensity (I0/I)
turned out to show first-order dependence on the concentration
of enamine 1a and duroquinone 3a, respectively. Besides it was
found that 3a quenched the fluorescence more efficiently than
1a did. These results suggest that electron transfer occurs from
Ru* to duroquinone 3a first. A cyclic voltammogram for 3a
exhibited a broad reversible redox wave at ꢀ1.20 V (vs. Cp2Fe).
Addition of LiBF4 (10 eq.) caused an anodic shift of the redox
potential by ꢀ0.99 V, indicating lowering of the LUMO level
of 3a. The irreversible oxidation wave for 1a was observed at a
potential lower than that for 2a. On the basis of these
experimental results, a possible reaction mechanism is presented
in Scheme 2. First, visible-light irradiation of Ru(II) by either a Xe
lamp or the sun causes excitation to Ru(II)*. Then, Ru(II)* under-
goes 1e-transfer to 3a activated by Li+ and converts into Ru(III)
demanding an electron. Oxidation of enamine 1 by the resultant
Ru(III) followed by addition to 2 gives radical intermediate 6, and
subsequent 1e-oxidation11 furnishes the coupling product 4, which
is converted to g-diketone product 5 by hydrolysis.
In conclusion, we have developed photocatalytic synthesis
of g-diketones via oxidative coupling of enamines with silyl
enol ethers using the [Ru(bpy)3]2+ catalyst under irradiation of
visible light. Sunlight can be successfully used for this photo-
reaction. A 2e-oxidation process involved in this transforma-
tion is achieved by the electron relay system composed of the
SET photoredox catalyst and the suitable electron acceptor,
duroquinone.
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8 A 1H NMR spectrum for 4aa shows a characteristic multiplet
signal at 4.84 ppm, assignable to the a-methine proton (see ESIw).
9 Identified byproducts are the photoinduced coupling product of 2a
with 3a. Homocoupling products of 1a and 2a were not observed.
10 For the sunlight-driven reactions, see: (a) P. Esser, B. Pohlmann
and H.-D. Scharf, Angew. Chem., Int. Ed. Engl., 1994, 33, 2009;
The financial support from the Japanese government
(Grants-in-Aid for Scientific Research: No. 23750174) is grate-
fully acknowledged. Y. Y. also gratefully acknowledges financial
support from the GCOE program ‘‘Education and Research
Center for Emergence of New Molecular Chemistry’’.
(b) C. Schiel, M. Oelgemoller, J. Ortner and J. Mattay, Green
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Notes and references
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Chem., Int. Ed., 2009, 48, 1360; (c) T. P. Yoon, M. A. Ischay and
ꢀ
11 We cannot exclude the pathway that 3aꢁ directly oxidizes 6.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5355–5357 5357