Edge Article
Chemical Science
of the peak was lower than in the case of PFH+ and TEOA
‡‡‡ Protonation: kdec ¼ 1.8 ꢃ 103 sꢀ1 Mꢀ1; Angew. Chem., 1987, 99, 572; reduction:
kred ꢂ 2 ꢃ 103 sꢀ1 Mꢀ1
.
solution and its lifetime shortened to ꢂ3 ms caused by the
§§§ And partially (10%) from 3[PFH+]*; Chem. Phys. Lett., 1980, 69, 61–65.
{{{ kred ¼ 2.5 ꢃ 1010 sꢀ1 Mꢀ1; S. Solar; W. Solar; N. Getoff, Z. Naturforsch., A:
Phys., Phys. Chem., Kosmophys., 1982, 37, 1077.
electron transfer from the PFcꢀ to Rhcat
.
Rhcat itself does not exhibit any transient species and no
product from PET with TEOA is detected. Unlike its iridium
analogue, Rh(III)–H is not photoactive.42
kkk kred ꢂ 1010 sꢀ1 Mꢀ1; estimated value, based on: S. Solar; W. Solar; N. Getoff, Z.
Naturforsch., A: Phys., Phys. Chem., Kosmophys., 1982, 37, 1077.
**** Redox potential of [PFHc]2+ is E0 ¼ +1.07 V vs. SCE, Fig. S6.† DGꢁ ¼ ꢀe ꢃ
The quantum yield of the product formation was determined
to be F ¼ (0.14 ꢄ 0.05)% at 455 nm measured at low light
intensity (Pabsorbed ¼ 9.3 mW, see ESI† for details). The low
quantum yield is caused by loss of excitation by uorescence
(ꢂ39%),28 low triplet yield (ꢂ10%),29 disproportionation of the
RhII species (two moles of PFcꢀ for one mole of RhI)33 and partial
Rh(III)–H decomposition (ꢂ50% of Rh(III)–H lost to H2).
(ꢀ0.76 V + 1.07 V) ꢀ 0.08 eV ¼ ꢀ0.39 eV ꢂ ꢀ37 kJ molꢀ1
.
1 G. A. Tolstikov, V. N. Odinokov, R. I. Galeeva, R. S. Bakeeva
and V. R. Akhunova, Tetrahedron Lett., 1979, 20, 4851–4854.
2 R. O. Hutchins and D. Kandasamy, J. Am. Chem. Soc., 1973,
95, 6131–6133.
3 D. E. Ward and C. K. Rhee, Synth. Commun., 1988, 18, 1927–
1933.
Conclusions
4 D. E. Ward and C. K. Rhee, Can. J. Chem., 1989, 67, 1206–
1211.
In summary, the selective photocatalytic reduction of aldehydes
over ketones was achieved employing in situ generated Rh(III)–H
as the reduction reagent. Contrary to a formate-based aqueous
reduction, the Rh(III)–H is formed in the photocatalytic protocol
slowly and allows therefore to kinetically distinguish between
aldehydes and ketones. The photoreduction proceeds both via
photoinduced electron transfer from the proavine triplet and
by oxidative quenching with Rhcat. The former pathway is
oxygen sensitive and the latter is light intensity dependent. The
light intensity inuences directly the reaction mechanism and
the reaction rate. A change of the light source (high-power LED
vs. uorescence light bulb) affects the product yield and the
photocatalytic mechanism.
5 Y. Maki, K. Kikuchi, H. Sugiyama and S. Seto, Tetrahedron
Lett., 1977, 18, 263–264.
6 C. Adams, Synth. Commun., 1984, 14, 1349–1353.
7 B. Zeynizadeh and F. Shirini, J. Chem. Res., 2003, 2003, 334–
339.
8 K. Tanemura, T. Suzuki, Y. Nishida, K. Satsumabayashi and
T. Horaguchi, Synth. Commun., 2005, 35, 867–872.
9 S. Chandrasekhar and A. Shrinidhi, Synth. Commun., 2014,
44, 2051–2056.
10 D. J. Raber, W. C. Guida and D. C. Shoenberger, Tetrahedron
Lett., 1981, 22, 5107–5110.
11 G. W. Gribble and D. C. Ferguson, J. Chem. Soc., Chem.
Commun., 1975, 535–536.
12 C. F. Nutaitis and G. W. Gribble, Tetrahedron Lett., 1983, 24,
4287–4290.
13 Y. Kuroiwa, S. Matsumura and K. Toshima, Synlett, 2008,
2008, 2523–2525.
14 C. P. Casey, N. A. Strotman, S. E. Beetner, J. B. Johnson,
D. C. Priebe and I. A. Guzei, Organometallics, 2006, 25,
1236–1244.
15 M. Zhang, W. D. Rouch and R. D. McCulla, Eur. J. Org. Chem.,
2012, 2012, 6187–6196.
Acknowledgements
We thank the Deutsche Forschungsgemeinscha (GRK 1626)
for nancial support. We thank the Laboratory of Organic
Photochemistry of Faculty of Science at Masaryk University in
Brno, Czech Republic for transient absorption spectroscopic
measurements, which were supported by the Czech Ministry of
Education (LO1214). We thank Malte Hansen for help in prep-
aration the graphical abstract and Prof. O. Reiser for helpful
discussions.
16 D. H. Nam and C. B. Park, ChemBioChem, 2012, 13, 1278–
1282.
17 R. W. Armstrong and N. M. Panzer, J. Am. Chem. Soc., 1972,
94, 7650–7653.
18 A. I. Krasna, Photochem. Photobiol., 1979, 29, 267–276.
19 M. T. Youinou and R. Ziessel, J. Organomet. Chem., 1989, 363,
197–208.
Notes and references
§ PF is weakly uorescent till pH ¼ 11.5 which corresponds to the pKa of the
singlet excited state. K. Kalyanasundaram; D. Dung, J. Phys. Chem., 1980, 84, 2551.
{
3[PFH+]* + 3[PFH+]* / 1[PFH+]* + 1[PFH+].
20 F. Hollmann, A. Schmid and E. Steckhan, Angew. Chem., Int.
Ed., 2001, 40, 169–171.
k This value corresponds well with the published potential (+1.36 V). M. P. Pileni;
¨
M. Gratzel, J. Phys. Chem., 1980, 84, 2402.
** E0 ¼ +0.80 V vs. Ag/AgCl.
21 H. C. Lo, O. Buriez, J. B. Kerr and R. H. Fish, Angew. Chem.,
Int. Ed., 1999, 38, 1429–1432.
22 C. Leiva, H. C. Lo and R. H. Fish, J. Organomet. Chem., 2010,
695, 145–150.
23 Y. Himeda, N. Onozawa-Komatsuzaki, H. Sugihara,
H. Arakawa and K. Kasuga, J. Mol. Catal. A: Chem., 2003,
195, 95–100.
†† DGꢁ ¼ ꢀe ꢃ (ꢀ0.76 V + 1.44 V) ꢀ 0.08 eV ¼ ꢀ0.76 eV ꢂ ꢀ73.3 kJ molꢀ1
according to J. Am. Chem. Soc., 1999, 121, 1681–1687.
,
‡‡ 300 eq. of Rh catalyst vs. PFH+, Stern–Volmer quenching constant is (2260 ꢄ
30) Mꢀ1
.
§§ Measured molar absorptivities are: 3455(PFH+) ¼ 28 600; 3455(RhIIIcat) ¼ 120.
{{ These side reactions have similar rate constants.
kk The ketone reduction does not efficiently compete with the decomposition.
*** pKa(PFHc) ¼ 4.5; J. Chem. Soc., Chem. Comm., 1979, 1137–1138.
¨
¨
24 U. Kolle and M. Gratzel, Angew. Chem., Int. Ed. Engl., 1987,
††† kprot ¼ 1.6 ꢃ 106 sꢀ1 Mꢀ1; U. Kolle; M. Gratzel, Angew. Chem., 1987, 99, 572.
99, 572–574.
¨
¨
This journal is © The Royal Society of Chemistry 2015
Chem. Sci., 2015, 6, 2027–2034 | 2033