ChemComm
Cite this: Chem. Commun., 2012, 48, 299–301
COMMUNICATION
Convenient photooxidation of alcohols using dye sensitised zinc oxide in
combination with silver nitrate and TEMPOw
Vineet Jeena and Ross S. Robinson*
Received 19th September 2011, Accepted 31st October 2011
DOI: 10.1039/c1cc15790f
A novel photooxidative system using dye sensitised zinc oxide in
combination with silver nitrate and 2,2,6,6-tetramethylpiperidine-
1-oxyl radical (TEMPO) in an aqueous solution is described.
Under visible light irradiation the selective oxidation of alcohols
to their corresponding aldehydes and ketones was effected in good
to excellent yields.
Photocatalysis, by semiconductor metal oxides, has been identified
as a promising route for organic chemistry in the 21st century.1
Of the semiconductors available, TiO2 (the most studied
Fig. 1
semiconductor) and ZnO have emerged as the semiconductors
of choice due to their low cost, stability under irradiation and
environmental friendliness.2 From a synthetic chemistry point
was sensitised with alizarin red to allow the system to be
activated by visible light irradiation. Under the above conditions,
of view, the most appealing aspect of semiconductor mediated
the dye and not the semiconductor will be activated preventing
photocatalysis is their ability to oxidise alcohols to their
carbonyl derivatives.3
As part of our interest in tandem coupling reactions,4 we
the formation of strongly oxidising hydroxyl radicals.
Upon visible light irradiation, the activated dye transfers
an electron into the conduction band of the semiconductor
aimed to carry out a photocatalysed tandem oxidation coupling
(see Fig. 1, step a). The injected electron is readily consumed
reaction which involves the oxidation of an alcohol to its
by silver(I) which is reduced to metallic silver (Fig. 1, step b).
aldehyde, followed by a trapping of the generated aldehyde by
The activated dye returns to its ground state oxidising TEMPO
an appropriate nucleophile without the isolation of the inter-
from its radical form into the oxoammonium salt (the active
mediate aldehyde. While the TiO2 mediated alcohol oxidation
oxidising species of TEMPO) (Fig. 1, step c). The oxoammonium
has been well documented the photocatalytic procedure is
salt oxidises the alcohol to its carbonyl derivative (Fig. 1, step d),
unselective, due to the high oxidative potential of the holes
created in the valence band,5 resulting in a variety of products in
forming the hydroxylamine which can regenerate the TEMPO
addition to the desired carbonyl compound.6 Thus, before
embarking on a photocatalysed tandem coupling reaction, the
photooxidative procedure needed to be refined. Recently,
Zhao et al.7a and Nagata et al.7b have demonstrated the light
activated oxidation of 2,2,6,6-tetramethylpiperidine-1-oxyl
radical (TEMPO) to the oxoammonium salt (TEMPO+) which
selectively oxidises alcohols to their carbonyl compounds.
However, due to the small scale (300 mmol–0.1 mmol) and long
reaction times (18–24 h) these procedures are unlikely to find
application in traditional organic synthesis.
radical via oxidation by the dye radical (Fig. 1, step e) (see ESIw
for table of standard redox potentials).
In an effort to validate the proposed mechanism, we
attempted a test reaction which involved mixing a portion of
dye sensitised TiO2 or ZnO, silver nitrate, TEMPO and benzyl
alcohol in water under visible light irradiation. The reactions
were conducted on the 0.1 mmol scale to aid comparison with
the previously mentioned photooxidative systems.7 Preliminary
investigations revealed that the quantity of silver nitrate was
vital for the photooxidative system to proceed (see ESIw for
details) with 18 equivalents shown to be optimum. While the
quantity of silver nitrate may be deemed to be excessive, there
are a number of beneficial effects relating to the use of silver(I)
as a sacrificial electron acceptor. Firstly, dye sensitised photo-
catalysts are unstable in aqueous solution as the dye can easily
detach from the photocatalyst.8 Concentrated silver solutions
are known to stabilise dye sensitised photocatalysts preventing
the detachment of the dye.9 Secondly, the superior electron
accepting ability of silver(I) would drive the photooxidative
Herein, we report an improved strategy for the oxidation of
alcohols to aldehydes using a dye sensitised metal oxide/silver/
TEMPO system in an aqueous solution. The semiconductor
Department of Chemistry, University of KwaZulu-Natal, Scottsville,
Pietermaritzburg, 3209, South Africa. E-mail: robinsonr@ukzn.ac.za
w Electronic supplementary information (ESI) available: Analytical
data, experimental procedures and NMR data. See DOI: 10.1039/
c1cc15790f
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 299–301 299