Photocatalytic oxidation of alkanes with dioxygen by visible light and copper(II) and iron(III) chlorides: Preference oxidation of alkanes over alcohols and ketones

Article abstract of DOI:10.1246/bcsj.77.2251

Visible light irradiation of alkanes in acetonitrile with CuCl₂ and FeCl₃ catalysts under atmospheric dioxygen gave the corresponding alcohols and ketones effectively; in these reactions, the total selectivity of the products did not decrease so much with increase of alkane conversion. For example, cyclohexanol and cyclohexanone were formed with ca. 70% selectivity at 50% conversion, because overoxidation of the products took place more slowly than cyclohexane oxidation. The relative reactivity values of cycloalkanes increased as their ring-sizes decreased. In the oxidation of hexane, the reactivity ratio of C¹-/C²-/C³-H was found to be 1.0/1.4/1.8 with CuCl₂ and 1.0/4.6/6.6 with FeCl₃, respectively. Toluene and diphenylmethane were more reactive than cyclohexane with FeCl₃, as expected, whereas the alkane was oxidized faster than the benzylic compounds in the separate reaction with CuCl₂. Moreover, the alkane oxidation could be comparably performed by sunlight instead of an artificial lamp.

Full text of DOI:10.1246/bcsj.77.2251

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 2251–2255 (2004) 2251
Photocatalytic Oxidation of Alkanes with Dioxygen by Visible Light
and Copper(II) and Iron(III) Chlorides: Preference Oxidation of Alkanes
over Alcohols and Ketones
ꢀ
Ken Takaki, Jun Yamamoto, Kimihiro Komeyama, Tomonori Kawabata, and Katsuomi Takehira
Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University,
Kagamiyama, Higashi-Hiroshima 739-8527
Received June 16, 2004; E-mail: ktakaki@hiroshima-u.ac.jp
Visible light irradiation of alkanes in acetonitrile with CuCl2 and FeCl3 catalysts under atmospheric dioxygen gave
the corresponding alcohols and ketones effectively; in these reactions, the total selectivity of the products did not de-
crease so much with increase of alkane conversion. For example, cyclohexanol and cyclohexanone were formed with
ca. 70% selectivity at 50% conversion, because overoxidation of the products took place more slowly than cyclohexane
oxidation. The relative reactivity values of cycloalkanes increased as their ring-sizes decreased. In the oxidation of hex-
ane, the reactivity ratio of C1-/C2-/C3-H was found to be 1.0/1.4/1.8 with CuCl2 and 1.0/4.6/6.6 with FeCl3, respec-
tively. Toluene and diphenylmethane were more reactive than cyclohexane with FeCl3, as expected, whereas the alkane
was oxidized faster than the benzylic compounds in the separate reaction with CuCl2. Moreover, the alkane oxidation
could be comparably performed by sunlight instead of an artificial lamp.
Oxidation of alkanes, particularly cyclohexane, with dioxy-
25
20
15
10
5
100
80
60
40
20
0
gen is an important reaction in industry. However, the conver-
sion should be kept at a very low level to get high product se-
lectivity, as can be seen in the Du Pont process.1 Much atten-
tion has been paid to photocatalytic reactions towards im-
provement of the oxidation, because they are potentially
performable under environmentally benign conditions. Many
photocatalysts have been used in the liquid-phase oxidation
of cyclohexane. These include ammonium cerium(IV) nitrate,2
titanium(IV) oxide,3 polyoxotungstate,4 iron(III) porphyrins,5
various transition metal chlorides,6 and alumina-supported va-
nadium(V) oxide.7 However, most of these studies were usual-
ly carried out under UV light irradiation,8 and focused on the
initial stage of the reaction at very low conversion, which sug-
gests that a photocatalytic approach also suffers from the di-
lemma of selectivity and conversion. Previously, we disclosed
an efficient oxidation of cyclohexane and adamantane with di-
oxygen by visible light/CuCl2 and FeCl3 systems, wherein
fairly good selectivity for the corresponding alcohols and ke-
tones was attained at over 50% conversion.9 We studied this
system further using various alkanes and benzylic compounds
and found a unique feature, particularly with CuCl2, in that al-
kanes are usually oxidized faster than alcohols, ketones, and
benzylic substrates. Moreover, comparable results were ob-
tained by the reaction under sunshine. We would like to report
herein these results.
selec
OH
O
Cl
0
0
2
4
6
8
10
12
Time / h
Fig. 1. Oxidation of cyclohexane by visible light/CuCl2.
ed by GC in the liquid phase, though nonvolatile materials like
dicarboxylic acids were not analyzed. Very little carbon diox-
ide was generated in the gas phase (ca. 3% yield based on the
alkane), which was probably derived from the solvent, not
from the alkane.9 Conversion of cyclohexane reached 55% af-
ter 10 h, but total selectivity of the alcohol and ketone was kept
nearly unchanged at 70%. Various alkanes and benzylic sub-
strates were oxidized under the standard conditions [Eq. 1],
and these results are summarized in Table 1.
visible light
CuCl2 or FeCl3 (5 mM)
OH
R1 CH R2
O
C
Results and Discussion
R1 CH2 R2
(100 mM)
+
R1
R2
O
2 (1 atm), CH3CN (100 mL)
Irradiation of cyclohexane in acetonitrile by visible light
(halogen lamp, >400 nm) with CuCl2 catalysts (5 mol%) un-
der atmospheric dioxygen at 13 ꢁC gave cyclohexanol and cy-
clohexanone together with negligible amounts of cyclohexyl
chloride, as shown in Fig. 1.10 Other products were not detect-
13 2 °C, 10 - 20 h
ð1Þ
As can be seen in Fig. 1, the most striking feature of the
present system is that the total selectivity of the alcohol and
ketone does not diminish significantly as the conversion in-
Published on the web December 10, 2004; DOI 10.1246/bcsj.77.2251

2252 Bull. Chem. Soc. Jpn., 77, No. 12 (2004)
Photocatalytic Oxidation of Alkanes
Table 1. Initial Reaction Rate and Selectivity on the Oxidation of Various Substrates
CuCl2
FeCl3
Formation Selectivityb) Consumption Formation
Formation Selectivityb)
Substrate
Consumption Formation
of substratea) of alcohola) of ketonea) /%
of substratea) of alcohola) of ketonea) /%
Cyclopentane
Cyclohexane
Cyclooctane
Cyclododecanec)
Hexane
6.46
5.45
4.34
2.07
4.71
0.93
1.60
1.20
0.75
0.95
1.09
26
47
53
80
54
5.10
5.78
4.47
1.99
6.94
0.38
0.86
0.34
0.73
1.18
1.30
22
35
37
70
26
1.05
0.60
0.46
0.93
0.23 (C1)
0.39 (C2)
0.58 (C3)
0.10
0.58 (C1)
0.38 (C2)
0.38 (C3)
0.16
0.02 (C1)
0.26 (C2)
0.23 (C3)
0.17
0.19 (C1)
0.38 (C2)
0.70 (C3)
2.41d)
Toluene
Diphenylmethane 0.71
Tetralin 1.38
0.53
49
21
76
3.37
3.09
4.30
77
55
51
0.07
0.08
0.16
0.34 (ꢀ)
0
1.03
1.54
1.86 (ꢀ)
0
1.62
0.37 (ꢀ)
0.22 (ꢁ)
2.46
0.34 (ꢀ)
0.12 (ꢁ)
2.36
(ꢁ)
(ꢁ)
Cyclododecanec,e) 6.30
77
4.81
55
a) Unit: mM/h. b) Formation rate of (alcohols + ketones)/consumption rate. c) Cyclododecane (50 mM) and the catalysts (2.5 mM).
ꢁ
d) Calculated from the results during the first 5 h. e) Carried out under sunshine at 20–40 C.
Cyclohexane Cyclohexanol Cyclohexanone
CuCl2
FeCl3
1.0
1.0
0.5
0.6
0.6
0.7
CuCl2
FeCl3
1.3
2.2
1.0
1.0
0.6
0.6
0.3
0.4
( / H)
( / H)
Scheme 1. Relative reaction rate on the oxidation of cyclo-
hexane, cyclohexanol, and cyclohexanone.
Scheme 2. Relative reactivity of cycloalkanes.
creases, in contrast to the normal tendency observed for the al-
kane functionalization. These promising phenomena could be
interpreted by the fact that overoxidation of the products took
place more slowly than alkane oxidation (Scheme 1).11,12
Since the reaction rates were measured separately, the sub-
strate alcohol and ketone may deactivate the catalysts to exhib-
it seemingly lower reactivities. However, when cyclohexanol,
cyclohexanone, adipic acid, and hexanoic acid (2–3 equiv of
the catalysts) were added to the reaction mixtures containing
the Cu and Fe catalysts, no noticeable change in their UV–
vis spectra was observed in the visible light region.13 More-
over, the consumption rate of cyclohexane and the formation
rate of the products were not altered by these additives. Thus,
these unusual results would be caused by the characteristics of
the present system.14
Cyclopentane, cyclooctane, and cyclododecane were oxi-
dized similarly to give the corresponding alcohols and ketones
with preference for the former by CuCl2 and for the latter
by FeCl3 as shown in Table 1, along with small amounts of
chloroalkanes (<5% of the products). The relative reactivity
of these cycloalkanes was determined by the competitive reac-
tions using an equimolar mixture of all substrates (Scheme 2).11
Interestingly, the normalized reactivity increases as the ring-
size decreases with both catalysts, which is, though via a dif-
ferent mechanism, in agreement with the oxidative addition
reaction with Ir catalyst.15 Of course, the reactivity order did
not change in the separate reactions.
C1
C2
C3
CuCl2
FeCl3
1.0
1.0
1.4
4.6
1.8
6.6
( / H)
( / H)
Scheme 3. Relative reactivity of the C–H bonds of hexane.
7.1 reported for the typical radical reactions with hydroxyl,
alkoxyl, alkylperoxyl, and chloro radicals, the present value
seems to be small.16 The relative reactivity of secondary to pri-
mary C–H bond was investigated using hexane (Scheme 3).11
With FeCl3, C2- and C3-alcohols and ketones were totally pro-
duced 4.6 and 6.6 times faster than the C1-products, respec-
tively. On the other hand, these ratios were decreased to 1.4
and 1.8 with CuCl2, which indicates that the terminal products
are comparably formed as internal ones. The ratio of all secon-
dary compounds to primary compounds with Cu is similar to
that of hydroxylation of hexane with manganese(III) porphy-
rins.17
The present system was applied to the oxidation of the alkyl
chains of aromatic compounds. Toluene was effectively oxi-
dized with FeCl3 to give benzaldehyde predominantly within
5 h, together with a small amount of benzyl alcohol, and then
formation of the aldehyde was flattened and benzoic acid was
produced as a major product (Fig. 2). The total selectivity of
the three products was maintained at about 70%, but other
products formed by the aromatic C–H bond oxidation and
chlorinated compounds were not detected. On the other hand,
the reaction with CuCl2 was very slow; here conversion of the
The normalized relative reactivity of tertiary to secondary
C–H bonds found by the present system was 1.5 and 1.7 with
the Cu and Fe catalysts, respectively; these were obtained in
the reaction of adamantane.9 Compared to the ratios of 2.1–

K. Takaki et al.
Bull. Chem. Soc. Jpn., 77, No. 12 (2004) 2253
CuCl
FeCl
3
2
5
4
3
2
1
0
100
80
60
40
20
0
15
10
5
100
80
60
40
20
0
CH
3
CO H
2
CH
3
selec
selec
CHO
CHO
CH OH
2
CH OH
2
0
0
5
10
15
20
0
5
10
15
20
Time / h
Time / h
Fig. 2. Oxidation of toluene by visible light/CuCl2 and FeCl3.
hydrogen from alkane to give an alkyl radical. The subsequent
reaction of the radical with dioxygen, followed by reduction
with Fe(II) and protonation yields alkyl hydroperoxide. Then,
the alcohol and ketone are derived from the peroxide. Alterna-
tively, the initial stage of the reaction would include an elec-
tron transfer process from alkane to photo-excited FeCl3 to af-
ford an alkyl radical cation and Fe(II). However, these scenar-
ios could not fully explain the regioselectivity and relative re-
activity with CuCl2 catalyst, which is a subject for further
investigation.
Finally, the oxidation of cyclododecane with CuCl2 was car-
ried out under sunlight, and the results are compared with
those obtained by a halogen lamp (Fig. 3). Sunlight showed
better performance than the halogen lamp with respect to con-
sumption rate of the alkane and formation rate of the products,
wherein the alcohol and ketone were formed in nearly equal
amounts. The selectivity was, however, decreased gradually
as the reaction proceeded. Similar results were obtained with
the Fe catalyst.
In summary, we have demonstrated that liquid-phase partial
oxidation of alkanes with dioxygen can be effectively induced
with CuCl2 and FeCl3 catalysts under visible light irradiation.
A remarkable feature of the present system is that the corre-
sponding alcohols and ketones were generally less reactive
than the starting alkanes, and thus, product selectivities did
not decrease so much despite high conversion. The relative re-
activity of cycloalkanes increased with decrease of their ring-
size. Particularly with the Cu catalyst, reactivity ratios of ter-
tiary to secondary to primary C–H were smaller than those ex-
pected by normal radical reactions. Alkanes were oxidized
faster than benzylic compounds in the separate reactions with
CuCl2. In addition, the present system was simply conducted
under sunshine, which is potentially useful for the exploitation
of an environmentally benign process.
Cyclohexane Toluene
Diphenylmethane
CuCl2
FeCl3
1.0
1.0
0.4
2.3
0.8
3.2
( / H)
( / H)
Scheme 4. Relative reactivity of sp3 C–H bond of cyclohex-
ane, toluene, and diphenylmethane.
substrate reached only 8% after 20 h under the standard condi-
tions. Bezaldehyde and benzyl alcohol were formed together
with a little benzyl chloride (8% of the two products), but ben-
zoic acid was not formed. Similar results were obtained in the
reaction of diphenylmethane, wherein benzophenone and
benzhydrol were obtained in preference for the ketone without
chlorinated compounds.
Reaction rates of toluene and diphenylmethane were com-
pared with that of cyclohexane, separately measured under
the identical conditions (Scheme 4).11 With the Fe catalyst, tol-
uene and diphenylmethane were oxidized 2.3 and 3.2 times
faster than cyclohexane, respectively. Surprisingly, benzylic
C–H bonds of the two aromatic compounds were less reactive
than those of the alkane with CuCl2. Competitive reaction of
cyclohexane and diphenylmethane gave, however, different re-
sults: the normalized ratio of diphenylmethane to cyclohexane
was 1.4 and 3.5 with the Fe and Cu catalysts, respectively.
That is, the order was the same with Fe, whereas the order re-
versed with Cu. The difference observed for the Cu-catalyzed
reaction may be interpreted to result from the disturbing effect
of the aromatic rings for alkane oxidation. However, this
seems less likely, because the reaction rate of cyclohexane
with CuCl2 decreased slightly in the presence of benzene (5
equiv of CuCl2). Instead, some intermediate like an alkyl radi-
cal, initially generated from cyclohexane, would be equilibrat-
ed with diphenylmethane. Moreover, oxidation of tetralin with
FeCl3 gave ꢀ-tetralol and ꢀ-tetralone with the preference for
the latter, but the corresponding ꢁ-products were not detected.
In the reaction with CuCl2, all four products were formed; here
the ratio of total ꢀ- to ꢁ-products was 2.1.
Experimental
General. All photoreactions were carried out in a three-necked
cylindrical Pyrex glass vessel (74 mm ꢂ, ca. 610 mL, net volume:
380 mL), containing an Eikohsha EHC-300 halogen arc (300 W,
>400 nm) with double jackets of quartz (52 mm o.d.) for cooling
water. The vessel was connected to a dioxygen balloon. The appa-
ratus was placed in a thermostatic water bath with magnetic stir-
The reaction mechanism of the present system was suggest-
ed previously by us9 and by another group.6 Irradiation of
Fe(III) chloride would yield Fe(II) and Clꢂ, which abstracts

2254 Bull. Chem. Soc. Jpn., 77, No. 12 (2004)
Photocatalytic Oxidation of Alkanes
Sunlight
Halogen Lamp
20
100
80
60
40
20
0
20
15
10
5
100
80
60
40
20
0
O
15
selec
OH
selec
10
OH
O
5
0
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time / h
Time / h
Fig. 3. Oxidation of cyclododecane with CuCl2 by sunlight and halogen lamp.
ring. GC analyses were performed on a Shimadzu GC-17A equip-
ped with an FID by using a 30 m ꢃ 0.25 mm i.d. J & W capillary
column (DB5.625) and a Shimadzu GC-14B with FID by using a
3 m ꢃ 3 mm i.d. column of 15% of OV-17 on Chromosorb W.
UV–vis spectra were recorded with a JASCO V-500 spectropho-
tometer. Mass spectra (EI) were obtained at 70 eV on a Shimadzu
GCMS-QP5050. The solvent, substrates, and authentic samples for
the reaction products of high purity were commercially available
and were used as received.
Schuchardt, W. A. Carvalho, and E. V. Spinace, Synlett, 1993,
713.
2 E. Baciocchi, T. D. Giacco, and G. V. Sebastiani, Tetrahe-
dron Lett., 28, 1941 (1987).
3
a) W. Mu, J.-M. Herrmann, and P. Pichat, Catal. Lett., 3,
73 (1989). b) J. M. Herrmann, W. Mu, and P. Pichat, ‘‘Heteroge-
neous Catalysis and Fine Chemicals II,’’ ed by M. Guisnet, Elsev-
ier, Amsterdam (1991), pp. 405–412.
4
A. Molinari, A. Maldotti, R. Amadelli, A. Sgobino, and
V. Carassiti, Inorg. Chim. Acta, 272, 197 (1998).
A. Maldotti, C. Bartocci, R. Amadelli, E. Polo, P. Battioni,
and D. Mansuy, J. Chem. Soc., Chem. Commun., 1991, 1487.
G. B. Shul’pin, G. V. Nizova, and Y. N. Kozlov, New J.
Chem., 20, 1243 (1996), and references therein.
K. Teramura, T. Tanaka, M. Kani, T. Hosokawa, and
T. Funabiki, J. Mol. Catal. A: Chem., 208, 299 (2004).
A few examples of gas phase oxidation of alkanes promot-
General Procedure for the Photocatalytic Oxidation. A so-
lution of the substrate (10 mmol) and CuCl2 2H2O (85 mg, 0.5
5
ꢄ
mmol, 5 mol%) or FeCl3 6H2O (135 mg, 0.5 mmol, 5 mol%)
ꢄ
in acetonitrile (100 mL) was stirred for a while under dioxygen
(1 atm), then irradiated by a halogen lamp with stirring at 13 ꢅ
6
ꢁ
2 C as standard conditions. During the reaction, aliquots of the
7
mixture (0.5 mL) were withdrawn with a micropipette at 1 h inter-
vals over 10–20 h, and analyzed by GC with adequate internal
standards such as mesitylene, p-xylene, benzene, and methyl ben-
zoate. A more dilute solution of cyclododecane (842 mg, 5 mmol)
in the solvent (100 mL) containing the catalyst (5 mol%) was used
because of its low solubility. In the competitive reactions, a mix-
ture of 5-, 6-, 8-, and 12-membered cycloalkanes (5 mmol each)
with the catalyst (0.5 mmol), and that of diphenylmethane and cy-
clohexane (10 mmol each) with the catalyst (0.5 mmol) were irra-
diated. Oxidation of cyclohexanol and cyclohexanone was per-
formed under the identical conditions to that of cyclohexane.
The reaction of cyclododecane under sunshine was simply carried
out by stirring of the standard solution in 300 mL Pyrex flask at-
tached with dioxygen balloon outside in September, wherein the
temperature of the mixture rose to 40 ꢁC. The initial reaction rates
in Table 1 were obtained by the least squares method, and the rel-
ative reactivity and selectivity described in the text were deter-
mined based on these rates.11
8
ed by visible light have been reported, see: a) H. Sun, F. Blatter,
and H. Frei, J. Am. Chem. Soc., 118, 6873 (1996). b) S. Takenaka,
T. Tanaka, T. Funabiki, and S. Yoshida, Catal. Lett., 44,
67 (1997). c) H. Yamashita, K. Yoshizawa, M. Ariyuki, S.
Higashimoto, M. Che, and M. Anpo, J. Chem. Soc., Chem. Com-
mun., 2001, 435. d) (Liquid phase reaction) G. B. Shul’pin, M. M.
Bochkova, and G. V. Nizova, J. Chem. Soc., Perkin Trans. 2,
1995, 1465.
9
K. Takaki, J. Yamamoto, Y. Matsushita, H. Morii, T.
Shishido, and K. Takehira, Bull. Chem. Soc. Jpn., 76, 393 (2003).
10 The reaction was carried out using more dilute solution
than the standard conditions indicated in Eq. 1, i.e., cyclohexane
(20 mM) and CuCl2 (1 mM) in CH3CN (100 mL), which gave
higher selectivity in general.
11 Relative reactivities in Schemes 1–4 were calculated as
follows: Scheme 1, (consumption rate of cyclohexanol or cyclo-
hexanone)/(that of C6H12), obtained by separate reaction;
Scheme 2, (consumption rate of cycloalkane/number of C–H)/
(that of C6H12/12), competitive reaction; Scheme 3, (total forma-
tion rate of Cn-alcohol and ketone/4)/(that of C1-alcohol and al-
dehyde/6); Scheme 4, (consumption rate of toluene or diphenyl-
methane/number of sp3 C–H)/(that of C6H12/12), separate reac-
tion.
This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology.
References
1
a) C. A. Tolman, J. D. Druliner, M. J. Nappa, and N.
12 Cyclohexanol and cyclohexanone were oxidized to uniden-
tified nonvolatile materials directly, though a very small amount
of the ketone was formed from the alcohol.
Herron, ‘‘Activation and Functionalization of Alkanes,’’ ed by
C. L. Hill, Wiley, New York (1989), pp. 303–360. b) U.

K. Takaki et al.
Bull. Chem. Soc. Jpn., 77, No. 12 (2004) 2255
13 Absorption light at 462 nm for CuCl2 and its shoulder over
400 nm for FeCl3 are responsible for the present oxidation, see:
Ref. 9.
14 Preference photochemical oxidation of cyclooctane over 2-
butanol with polyoxotungstate was documented in the presence of
acids, see: C. H. Hill, R. F. Renneke, and L. A. Combs, New J.
Chem., 13, 701 (1989).
16 a) G. W. Smith and H. D. Williams, J. Org. Chem., 26,
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J. Morzycki, R. S. Hay-Motherwell, W. B. Motherwell, N.
Ozbalik, and K. M. Schwartzentrvber, J. Chem. Soc., Perkin
Trans. 1, 1986, 947. d) Ref. 1a, and references therein.
17 B. R. Cook, T. J. Reinert, and K. S. Suslick, J. Am. Chem.
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15 R. G. Bergman, Science, 223, 902 (1984).