9,10-Dicyanoanthracene photosensitized oxidation of aryl alkanols: Evidence for an electron transfer mechanism

Article abstract of DOI:10.1016/S0040-4039(03)01549-1

9,10-Dicyanoanthracene (DCA) photosensitizes the oxidation of a series of para substituted aryl alkanols in oxygen-saturated acetonitrile. Product analysis and Hammett correlations support an electron transfer mechanism for the title reaction.

Full text of DOI:10.1016/S0040-4039(03)01549-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6247–6251
9
,10-Dicyanoanthracene photosensitized oxidation of aryl
alkanols: evidence for an electron transfer mechanism
Ioannis N. Lykakis, Stellios Lestakis and Michael Orfanopoulos*
Department of Chemistry, University of Crete, 71409 Iraklion, Crete, Greece
Received 23 April 2003; revised 13 June 2003; accepted 20 June 2003
Abstract—9,10-Dicyanoanthracene (DCA) photosensitizes the oxidation of a series of para substituted aryl alkanols in oxygen-sat-
urated acetonitrile. Product analysis and Hammett correlations support an electron transfer mechanism for the title reaction.
©
2003 Elsevier Ltd. All rights reserved.
Dicyanoanthracene (DCA) is a fluorescent molecule
This useful photochemical property of DCA has been
applied extensively in photosensitized oxidations of
’
−
with reduction potential DCA /DCA=−0.97 V versus
1
2,4–6
7
8
SCE in CH CN. The oxidation of various organic
alkenes,
aromatic alkanes, cyclopropanes, 1,2-
9 10 11,12
3
compounds photosensitized by 9,10-dicyanoanthracene
diaryl oxirane, aryl disilane, and anisyl ether.
(
DCA) in the presence of molecular oxygen has been
1
3
extensively studied over the past thirty years. Two
competing mechanisms have been reported: (a) an elec-
tron transfer (ET) to the excited singlet DCA and (b)
the formation of singlet oxygen ( O ) by energy trans-
fer. The relative contribution of these two pathways
depends on the solvent polarity and the nature of the
substrate. For example in the presence of singlet oxygen
acceptors, the singlet oxygen adducts may be the only
Our previous long-standing mechanistic studies on
Type II sensitized photooxygenation reactions
prompted us to investigate for the first time the DCA
sensitized photooxidation of a series of aryl alkanols to
aryl ketones. Photooxidation of aryl alkanols is
chemoselective and produces mainly the corresponding
aryl ketones. Apart from the mechanistic interest, this
reaction is of synthetic use. In light of these results we
also discuss mechanistic possibilities for the title
reaction.
1
2
2
1
,2
observable product (Scheme 1).
Most of the work has focused on Type I reactions. In
non-polar solvents an exciplex emission is observed
In this study we have used a number of properly
designed 1-aryl-1-alkanols, compounds 1–10 (Table 1),
and 2-aryl-1-alkanols 11 and 12 (Table 2). These sub-
strates are totally inert to singlet oxygen (Type II
mechanism), however they provide competition
3
(
Scheme 2). However, in acetonitrile and other polar
solvents no emission is observed because the ion pairs
diffuse apart to give solvent-separated radical ions,
which can react further.
Scheme 1. Type I and Type II photooxidation mechanisms.
Keywords: 9,10-dicyanoanthracene; photooxidation sensitizer; aryl alkanols.
*
Corresponding author. Fax: +30-2810-393601; e-mail: orfanop@chemistry.uoc.gr
0
040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01549-1

6
248
I. N. Lykakis et al. / Tetrahedron Letters 44 (2003) 6247–6251
ucts. It is interesting to note that the conversions in
these photooxidations (based on the remaining starting
materials), increase proportionally with increasing elec-
tron-donating ability of the corresponding para sub-
stituent. For example, the conversion of 3, (X=CH O)
3
is three times higher than that of 8 (X=CH ). In the
3
case of an electron-withdrawing substituent such as
Scheme 2. A radical cation intermediate by an electron trans-
fer mechanism from excited DCA.
para trifluoromethyl (X=CF ), substrate 2, no reaction
3
product was detected after 20 min of irradiation (Table
1
).
between C –H and C –C oxidative bond cleavage in
the side-chain (Scheme 3).
h
h
i
1
4,15
However, besides electronic effects, steric factors play
an important role in the transition state of these pho-
tooxidation reactions. For example, in the case of sub-
strate 8 where the para substituent is a methyl group,
the conversion of the photoreaction is three times
higher (15%), compared to that of substrate 10 (5%),
with an isopropyl group as the para substituent. This
result may indicate that formation of an exciplex
between the excited DCA and the substrate 8 is of
lower energy than that with substrate 10 due to steric
reasons.
In a 4-mL Pyrex cell, a 0.05 M solution of aryl alkanol
and DCA, 5.5×10 M, as the sensitizer in oxygen
saturated acetonitrile was irradiated (>300 nm) with a
−
5
3
00 Watt Xenon lamp as the light source. The results of
the photooxidation of 1-aryl-1-alkanols 1–10 are shown
in Table 1.
After 20 min irradiation of the aryl alkanols 1, 3, 4, 8,
9
and 10 the corresponding aryl ketones 1a, 3a, 4a, 8a,
a and 10a, were produced as the only oxidation prod-
9
Table 1. DCA sensitized photooxidation of secondary 1-aryl-1-alkanols 1–10
Substrate
Irrad. time (min)
% Conversiona,b
Relative product yieldb (%)
ArCOR
ArCHO
1
2
3
3
3
4
4
5
5
6
7
8
8
9
9
20
20
20
60
90
20
90
20
60
10
20
20
60
20
60
20
60
2
–
100
–
100
>99
>98
94
95
78
80
30
25
100
100
100
>99
100
>99
Ndc
–
Nd
<1
<2
6
47
67
97
60
98
25
63
74
30
15
41
10
35
5
5
22
20
31
d
d
e
e
65
Nd
Nd
Nd
<1
Nd
<1
1
1
0
0
20
a
b
c
Aryl alkanols 0.05 M, DCA 5.5×10−4 M, in oxygen saturated CH CN, 5–10°C, irradiation with Xenon Lamp, 300 W (>300 nm).
Determined by gas chromatography. The error was ±1%.
3
Not detected.
d
e
In the case of substrate 6, benzaldehyde 1b 34% and 4-methoxybenzoic acid 3c 5%, were determined by 1H NMR.
1
An additional 10% of 4-methoxybenzoic acid was determined by H NMR.

I. N. Lykakis et al. / Tetrahedron Letters 44 (2003) 6247–6251
Table 2. DCA sensitized photooxidation of aryl alkanols 11 and 12
6249
Substrate
Irrad. time (min)
% Conversiona,b
Relative product yieldb (%)
ArCOR
ArCHO
11
11
12
10
20
20
3
7
10
Traces
Traces
Traces
>97
>99
>98
a
Aryl alkanol 0.05M, DCA 5.5×10−4 M, in oxygen saturated CH CN in a 4 mL Pyrex cell was irradiated (>300 nm), 5–10°C, irradiation with
3
Xenon Lamp, 300 W.
Determined by gas chromatography. The error was ±1%.
b
All these results indicate the formation of a radical
cation intermediate of aryl alkanols whose C –H and/
h
or C –C side-chain cleavage leads to the corresponding
h
i
1
6
aryl ketones and/or aryl aldehydes (Scheme 3). In
similar studies, Baciocchi and co-workers have shown
previously that the side-chain oxidation of aromatic
1
4,15
alcohols proceeds via a radical cation intermediate.
In order to study further the C –H versus C –C bond
h
h
i
cleavage of this reaction, we examined the photooxida-
tion of aryl alkanols 11 and 12. Both of these substrates
have the hydroxyl group on the carbon next to benzylic
(
C ). The results of the photooxidations are summa-
i
rized in Table 2. Oxidation of both aryl alkanols leads
to C –C bond scission, producing mainly 4-methoxy
h
i
Scheme 3. Two possible side-chain oxidative cleavages.
benzaldehyde. These results suggest also that the OH
group on C increases the stability of the newly forming
i
cation or radical fragment. Subsequent heterolytic or
homolytic C –C bond cleavage produces almost exclu-
h
i
sively the arylaldehyde in more than 97% relative yield.
These results also suggest that DCA photosensitized
oxidations of 2-aryl-1-alkanols proceed via an electron
transfer mechanism.
It is interesting to emphasize here that in the transition
state the C –Ci bond cleavage leading to the aryl
h
aldehyde, may be facilitated by increasing the stability
’
of the newly forming radical fragment R . For example,
the ratio of aryl ketone : aryl aldehyde decreases from
A reasonable mechanistic rationalization for the sensi-
tized photooxidation of aryl alkanols is presented in
Scheme 5. An electron is transferred from the aryl
substrate to the photoexcited sensitizer to form the
radical ions. In the case of a stable cation or radical
9
4:6 to 78:22 to 30:31 to 25:65 with the increase in
radical stability in going from ethyl to isopropyl, to
benzyl and t-butyl radicals in substrates 4, 5, 6 and 7,
respectively (Table 1). We point out here that in the
case of substrate 7 where R is a t-butyl group (forming
a stable leaving t-butyl radical), the oxidative cleavage
of C –C leading to the corresponding 4-methoxybenz-
+/
’
R
fragment, a C –C scission is the exclusive or the
h
i
predominant path, followed by oxygen capture of the
intermediate radical which consequently decomposes to
the observed aryl aldehyde (ArCHO). However, when
h
i
aldehyde 3b, is the predominant pathway.
the R fragment is relatively unstable (Me or Et) a C –H
a
In addition to these results a Hammett correlation in
the competition of para (X) substituted 1-aryl-1-
scission is greatly preferred to C –C bond cleavage,
h
i
leading to the aryl ketone (substrates 1–4 and 8–10).
The photoselective oxidation of these substrates to the
aryl ketone makes this reaction, at least for these
examples, preparatively useful. The two pathways are
always in competition. The C –C bond cleavage is
ethanols (X=CH O, CH , Et, i-Pr and CF ) versus
3
3
3
1
-phenyl-1-ethanol, for the photooxidation reaction,
2
gave a negative slope of z=−1.12, R =0.9932, for
example k /k =15.84±0.30 and k /k =0.50±0.02
CH O
H
CF3
H
h
i
3
+
/
’
(
Scheme 4). This result indicates the development of
dictated by the stability of the newly forming R . The
more stable R fragment leads to more extensive C –C
bond scission. The subsequent oxidation of DCA by
electron transfer to molecular oxygen producing the
positive charge (e.g. a radical cation) in the transition
state, which is better stabilized by electron-donating
substituents.
h
i
−
.

6
250
I. N. Lykakis et al. / Tetrahedron Letters 44 (2003) 6247–6251
1
-aryl-1-alkanols the nature of the R substituent on the
b-carbon C , dictates the ratio of C –H to C –C bond
i
h
h
i
cleavage and consequently the corresponding ratio of
aryl ketone versus aryl aldehyde. However, the pho-
tooxidation of 2-aryl-1-alkanols, gave the correspond-
ing aryl aldehydes, as the only products. The results
support the view that electron transfer to DCA from
the substrate takes place to form the radical cation
intermediate, which undergoes heterolytic C –C bond
h
i
cleavage.
Acknowledgements
We thank the Ministry of Education (B–EPEAEK
graduate program) and the Greek Secretariat of
Research and Technology (Greek–French collaborative
research program Platon 2001) for financial support
and graduate fellowships to I. Lykakis.
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