Lone selectivity of the decatungstate-sensitized photooxidation of 1-substituted cycloalkenes

Article abstract of DOI:10.1055/s-2004-832803

Decatungstate-sensitized oxidation of alkyl and phenyl substituted cycloalkenes in the presence of molecular oxygen shows a general preference for hydrogen abstraction on the congested side of the double bond.

Full text of DOI:10.1055/s-2004-832803

LETTER
2131
Lone Selectivity of the Decatungstate-Sensitized Photooxidation of
1-Substituted Cycloalkenes
D
ecatung
o
state-Sensit
a
n
xidation of
1
n
-Substituted
i
Cycloa
s
lkenes N. Lykakis, Michael Orfanopoulos*
Department of Chemistry, University of Crete, 71409 Iraklion, Crete, Greece
Fax +30(2810)393601; E-mail: orfanop@chemistry.uoc.gr
Received 6 July 2004
shown that both electron transfer (ET) and hydrogen atom
transfer (HAT) mechanisms may operate.3,14 In a recent
study of this reaction a deviation from both Type I and
Type II mechanisms was reported.3a A ‘free radical path-
way’ mechanism designated as Type I¢ was proposed in-
stead (Equation 1).
Abstract: Decatungstate-sensitized oxidation of alkyl and phenyl
substituted cycloalkenes in the presence of molecular oxygen shows
a general preference for hydrogen abstraction on the congested side
of the double bond.
Key words: photooxidation, cycloalkenes, hydroperoxides, deca-
tungstate, polyoxometalate
O2
hv
XH
4-
4- *
W10O32
W10O32
wO
(XOOH)I'
X
Type I'
(HAT)
Photosensitized oxidation plays a major role in many nat-
ural and industrial processes. Numerous reports have dis-
cussed the mechanisms for sensitized oxidations in the
presence of molecular oxygen, with two possible mecha-
nisms having been distinguished, namely Type I and Type
II (Scheme 1).1 In Type I the triplet state of the sensitizer
interacts with the substrate (XH = alkene) affording a pair
of free radicals by electron-transfer,2 or hydrogen atom-
transfer.3 Type II reactions involve the direct interaction
of the excited sensitizer with molecular oxygen, produc-
ing by energy transfer singlet oxygen (1O2). This species
adds directly to a variety of unsaturated hydrocarbons fur-
nishing the allylic hydroperoxides or endoperoxides via
‘ene’ or [4+2] reactions, respectively.4
XH = alkene
HAT = Hydrogen Atom Transfer
Equation 1
Following our general interest in the singlet oxygen pho-
tosensitized oxidation of alkenes,4,15 we present herein de-
tails of the decatungstate sensitized oxidation of 1-
substituted cycloalkenes in the presence of molecular
oxygen. This decatungstate-photosensitized reaction is re-
gioselective, a feature not previously recognized, and it
may shed light on certain factors controlling the formation
of oxygenated products. The reaction shows a general
preference for hydrogen abstraction from the lone,16 more
congested side of the double bond (Figure 1). This lone-
side selectivity, in comparison to the singlet oxygen gem-
inal-selectivity for similar substrates, reflects a complete
change in mechanism between the two different oxidative
systems.
Type II
O2
XH
1O2
(XOOH)II
hv
ISC
3Sens*
1Sens*
Sens
twin-selectivity
R
Sens = Sensitizer
XH = Alkene
ET = Eletron Transfer
ISC = Intersystem Crossing
O2
XH
(XOOH)I
Radicals
lone-selectivity
twix- or geminal-selectivity
(ET)
(CH2)n
n = 1, 2
Type I
Figure 1
Scheme 1 Type I and Type II mechanisms in photosensitized oxi-
dations
The decatungstate photosensitized oxygenations of 1-sub-
A variety of sensitizers or photocatalysts have been used stituted cycloalkenes were carried out as follows: a solu-
4–
in order to sensitize a large number of different chemical tion of alkene (0.02 M) and W10O32 (5.5 × 10–5 M) in
reactions. Among them, the decatungstate anion5–14 oxygen-saturated acetonitrile was irradiated (15–20 min)
(W10O324–) photocatalyzed oxidations of various sub- with a Variac Eimac 300 Watt Xenon lamp as the light
strates, such as alkanes,8–11 aryl alcohols,12,13 amines13 and
source. The product distribution was measured by com-
alkenes,3,14 have attracted considerable attention. For ex-
1
parison of integrals of the appropriate peaks in the H
ample, steady-state and time-resolved studies on the deca- NMR spectra and by GC analysis of the allylic alcohols
tungstate-sensitized photooxidation of alkenes have produced after reduction (using PPh3) of initially formed
allylic hydroperoxides. The results are summarized in
Table 1.
SYNLETT 2004, No. 12, pp 2131–2134
0
1
.1
0
.2
0
0
4
Advanced online publication: 03.09.2004
DOI: 10.1055/s-2004-832803; Art ID: G21904ST
© Georg Thieme Verlag Stuttgart · New York

2132
I. N. Lykakis, M. Orfanopoulos
LETTER
Table 1 Decatungstate-Sensitized Photooxidation of 1-Substituted Cycloalkenes 1–6
twin
R
R
R
R
OOH
4-
hν/W10O32
O2/CH3CN
twix or gem
lone
HOO
O
(CH2)n
(CH2)n
(CH2)n
(CH2)n
c
a
b
n
R
1
2
3
4
1
1
2
2
t-Bu
Ph
lone-products
R' = H, Ph
CHR'
t-Bu
Ph
R
R
HOO
5
6
2
2
CH3
PhCH2
OOH
O
(CH2)n
(CH2)n
(CH2)n
d
e
f
twix-(gem)-products
twin-product
Relative Product Yield (%)b
twix (gem)-Selectivity
Alkenea Conversion (%)b
lone/gem- Selectivity (%)b
lone-Selectivity
twin-Product
a
b
c
d
e
5
f
1
2
3
4
5
6
40
55
50
50
45
40
27
63
41
64
19
27
2
37
14
39
22
3
29
20
15
12
32
28
–
66:34
80:20
85:15
88:12
63:37
65:35
2
5
1
–
–
Ndc
Ndc
2
2
–
36
20
8
6
1
18
a Alkenes (0.02 M) was irradiated (15 min, Xenon lamb, 300 W) in the presence of [Bu4N]4W10O32 (5.5 × 10–4 M) in 4 mL MeCN, at 5–10 °C.
b Determined by 1H NMR and GC after reduction with PPh3. The errors were 4% and 1%, respectively.
c Not detected.
Irradiation of 1–6 for 15 minutes at ca. 5 °C afforded the lecular oxygen to give again W10O324– and the correspond-
corresponding allylic hydroperoxides (a, b, d and f) and ing hydroperoxide products.3,11 Examination of the
a,b-unsaturated cyclic ketones (c and e) in 40–55% con- possible transition states leading to the major and minor
versions (Table 1). As seen from Table 1, photooxygen- products provides a reasonable explanation for the lone-
ation of a series of phenyl- and alkyl-substituted selectivity. In transition state TS2 leading to the minor
cycloalkenes, 1–6, shows a preference for hydrogen ab- product, the non-bonded interactions involving the R
straction from the more congested lone methylene side of group (alkyl-, phenyl- and benzyl-) and the incoming de-
the double bond. 1-tert-Butyl-substituted cyclopentene catungstate reactive intermediate (wO), are expected to be
and cyclohexene substrates 1 and 3 illustrate this point. stronger than those in transition state TS1 where this steric
Similarly, cycloalkenes 5 and 6 again give as the major interaction must be negligible (Scheme 3). However, it is
products those, derived from hydrogen abstraction from difficult to evaluate the relative contributions of non-
the lone, more congested methylene group. The presence bonded interactions from those of other effects, such as
of a phenyl substituent does not affect the lone-selectivity. electronic. For example, when the substituent is phenyl
This is impressively demonstrated with substrates 2 and 4 (substrates 2 and 4), electronic factors may be more im-
where the lone-selectivity was found to be 80% and 88% portant than steric in controlling the stereoselectivity. In
relatively to the twix (geminal) selectivity (20% and the intermediate corresponding to RI2, the conjugation be-
12%).
tween the phenyl group and the double bond is destroyed.
Thus, RI2 will be less stable than the intermediate corre-
sponding to RI1, in which the conjugation between the
phenyl group and the double bond is preserved (relative
yield of secondary hydroperoxide a 63% and 64%, re-
spectively).17
A reasonable mechanistic explanation that may rational-
ize these results is shown in Scheme 2. In the first step,
under irradiation conditions, the decatungstate anion con-
verts into the established relatively long-lived excited in-
termediate wO.11 Allylic hydrogen abstraction from the
alkene14 produces the one-electron-reduced species It is clearly demonstrated by the present results that alkyl
H+W10O32 or/and HW10O32 and radical intermediates or phenyl substituents activate the C-H bond in the lone-
RI1, RI2 and RI3 (Scheme 2). The one-electron-reduced position roughly by a factor of 1.7–7.3, depending on the
species of decatungstate reoxidizes in the presence of mo-
5–
4–
Synlett 2004, No. 12, 2131–2134 © Thieme Stuttgart · New York

LETTER
Decatungstate-Sensitized Photooxidation of 1-Substituted Cycloalkenes
2133
substrate, over the corresponding twix (geminal) C-H po-
sition.
O
H
O
H
O
O
R
R
O
O
3b
3a
O
O
O
O
W
O
H
lone (CH2)n
O
W
H
Equation 2
(CH2)n
O
O
TS1
TS2
stereoelectronic effects, the existence of an equilibrium
favors the thermodynamically more stable secondary hy-
droperoxide.
more favorable
R
less favorable
R
R
Cycloketones c and e were formed due to the further
oxidation of the initially formed hydroperoxides a and d,
respectively (Scheme 3). As shown in Table 1, when the
R substituent of the cycloalkenes is a tert-butyl or a phe-
nyl group (entries 1–4), the ratio of the cycloketones c/e
increases. Examination of the possible transition states
leading to the major and minor cycloketones again pro-
vides a reasonable explanation for this increase. In transi-
tion state TS4 leading to the minor cycloketone e, the non-
bonded interactions involving the R group (alkyl-, phenyl-
and benzyl-) and the incoming decatungstate reactive
intermediate (wO), are expected to be stronger than those
in transition state TS3 where this steric interaction must be
negligible (Scheme 3).
(CH2)n
RI2
(CH2)n
(CH2)n
RI1
RI3
5-
W10O32 H+
W10O32 H+
5-
O2
O2
4-
4-
W10O32
W10O32
R
R
OOH
R
HOO
OOH
(CH2)n
a
(CH2)n
b
(CH2)n
d
minor
major
Scheme 2 Possible transition state of the decatungstate-photosensi-
tized oxygenation of the cycloalkenes
It is interesting to note that when the substituent R was t-
butyl- or phenyl- (substrates 1–4), a small amount, 2–5%
of tertiary hydroperoxide b was formed (Table 1). How-
ever, when R was methyl or benzyl (substrates 5 and 6)
the amount of tertiary hydroperoxide b increased substan-
tially (relative yields of 36% and 20%, respectively). Ex-
amination of the possible transition states leading to the
major tertiary hydroperoxide (when R is methyl, substrate
5) and to the minor (when R is t-butyl, substrate 3) pro-
vides an insight into the course of the reaction (Figure 2).
In transition state TS3, leading to the minor product (when
R is t-butyl), the non-bonded interactions between the
large t-butyl group and the incoming oxygen as well as the
1,2-gauche interactions are expected to be stronger than in
transition state TS4, where this steric interaction is small-
er.7,18 Therefore, because of non-bonded interactions,
transition state TS4 is expected to be favored over the TS3.
R
R
O
O
HOO
OOH
O
O
O
O
W
O
H
O
W
H
lone (CH2)n
(CH2)n
O
O
TS3
TS4
more favorable
less favorable
R
R
+ W10O324- + H2O
O
O
+ W10O324- + H2O
(CH2)n
(CH2)n
e
minor
c
major
Scheme 3 Formation of cycloketones by decatungstate photo-
sensitized oxidation of the corresponding allylic hydroperoxides
For reasons of comparison, we mention here that in a pre-
vious work,15 we found that the singlet oxygen-sensitized
photooxygenation of cycloalkenes 1–6, unlike the deca-
tungstate photooxygenation of such substrates, shows a
strong preference for hydrogen abstraction from the meth-
ylene group that is geminal to the large alkyl- or phenyl-
substituent of the cycloalkenes (designated as twix selec-
tivity, Table 2).15 This result is opposite to that observed
in the decatungstate-sensitized oxygenations and was at-
tributed to non-bonded interactions involving the large
substituent R of cycloalkenes and the incoming molecular
oxygen at the formation of the proposed perepoxide inter-
mediate.4a The opposite measured regioselectivities in the
decatungstate oxidation, compared to the singlet oxygen
photooxygenation, support further the proposed mecha-
nism.
O2
O2 steric effect
H
H
H
CH3
CH3
H
H
H
H
H
H
H
H
H
H
CH3
H
H
1,2 gauche
interaction
TS3
TS4
more favorable
less favorable
Figure 2 Steric effect associated with planar conformations of
tertiary radicals
It was further observed that hydroperoxide 3b, upon
standing at room temperature for several hours, rear-
ranged to the more stable secondary hydroperoxide iso-
mer 3a (Equation 2).19 Therefore, regardless of
Synlett 2004, No. 12, 2131–2134 © Thieme Stuttgart · New York

2134
I. N. Lykakis, M. Orfanopoulos
LETTER
Table 2 Regioselectivities in the Decatungstate and Singlet
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wO (1O2)b
wO (1O2)
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Ph
12 (85)
(15) 88
(27) 66
34 (73)
4
1
2
8 (43)
CH3
Ph
(0) 80
20 (100)
(44) 58
(55) 53
34 (13)
5
4
18 (15)
Ph
(23) 83
17 (77)
29 (23)
3
a In the case of singlet oxygen numerical values in parenthesis
represent percent of hydrogen abstraction.
b Taken from ref.15b
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In conclusion, we have shown here that in the decatung-
state-sensitized photooxidation of 1-substituted cycloalk-
enes the oxidation products are preferentially formed by
the O2 trapping of the radical which is formed mainly by
hydrogen abstraction in the lone-congested side of the
double bond.
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Acknowledgment
We thank Greek Secretariat of Research and Technology (Greek–
French collaborative research program Platon 2001) for financial
support and graduate fellowships to I. Lykakis. We also thank Dr.
T. Montangon for valuable comments and discussions.
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Synlett 2004, No. 12, 2131–2134 © Thieme Stuttgart · New York