Functionalization of the benzobicyclo[3.2.1]octadiene skeleton via photocatalytic and thermal oxygenation of a furan derivative

Article abstract of DOI:10.1016/j.tetlet.2011.09.076

A simple and efficient protocol is utilized for the synthesis of novel functionalized benzobicyclo[3.2.1]octadiene derivatives by photocatalytic oxygenation of a furan derivative using an anionic free-base porphyrin as well as cationic and anionic mangane

Full text of DOI:10.1016/j.tetlet.2011.09.076

Tetrahedron Letters 52 (2011) 6255–6259
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Tetrahedron Letters
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Functionalization of the benzobicyclo[3.2.1]octadiene skeleton via
photocatalytic and thermal oxygenation of a furan derivative
Ilijana Kikaš a, Ottó Horváth b, , Irena Škoric
⇑
a,
⇑
´
a
´
Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, 10000 Zagreb, Croatia
b Department of General and Inorganic Chemistry, Institute of Chemistry, Faculty of Engineering, University of Pannonia, PO Box 158, Veszprém H-8201, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2011
Revised 3 September 2011
Accepted 16 September 2011
Available online 22 September 2011
A simple and efficient protocol is utilized for the synthesis of novel functionalized benzobicyclo[3.2.1]oct-
adiene derivatives by photocatalytic oxygenation of a furan derivative using an anionic free-base porphyrin
as well as cationic and anionic manganese(III) porphyrins under different reaction conditions. The course
and yields of these reactions were compared to those of the thermal reaction using m-chloroperbenzoic acid
as the oxidizing agent. The deviating reaction pathways with anionic and cationic metalloporphyrins may
be attributed to simultaneous electronic and steric effects. Application of free-base and metalated water-
soluble porphyrins for photocatalytic oxygenation of the furan ring fused to the rigid methano-bridged
skeleton proved to be regioselective and flexible compared to the thermal reactions with mCPBA, giving
at the same time novel potentially biologically active bicyclo[3.2.1]octenes with the basic skeleton of which
is incorporated in many natural compounds.
Keywords:
Benzobicyclo[3.2.1]octadienes
Furan derivative
Photocatalytic oxygenation
Porphyrins
Ó 2011 Elsevier Ltd. All rights reserved.
Earlier studies indicated that photochemical reactions for the
synthesis of rigid methano-bridged heteroaromatics 1–3 (Fig. 1) also
offered suitable pathways toward oxygenated derivatives 4–61
which cannot be produced (at least with sufficient selectivity) via
thermal processes.
manganese(III) porphyrins proved to be effective for catalytic oxy-
genation of -pinene. In aqueous systems, at relatively low sub-
a
strate:catalyst ratio (S/C = 500) its selective epoxidation was
observed, strongly deviating from the case of aprotic organic sol-
vents such as benzene or toluene, where allylic hydroxylation
products were formed.6 Photocatalytic epoxidation of cyclooctene
was achieved in acetonitrile with various metalloporphyrins.8
These precedents inspired us to study the photocatalytic activ-
ity of water-soluble manganese(III) porphyrins for the oxygenation
of benzobicyclo[3.2.1]octadiene derivative 1, having a basic skele-
ton similar to that of previously studied naturally occurring cyclo-
alkenes. Our aim was to compare the oxygenation pathways of this
furo bicyclic structure 1 in both a thermal reaction involving
mCPBA and a photocatalytic processes mediated by free-base and
manganese(III) porphyrins. Anionic and cationic metalloporphy-
rins were used in our experiments (Fig. S1). Differences and simi-
larities in these pathways may shed light on the mechanisms of
the various oxygenation processes, giving hints for appropriate
choice of the catalyst and other conditions to achieve efficient con-
version with high selectivity. The charge on the ligand, influencing
its Lewis basicity may affect the catalytic activity of the complex
through the metal center.
In continuation of our interest in the preparation of new benzobi-
cyclo[3.2.1]octadiene derivatives, we turned our attention to the
utilization of photocatalytic oxygenation of bicycloalkenes contain-
ing a fused furan ring. In this case the starting substrate was com-
pound 1 ( Fig. 1). Functionalization of this type of furo polycycle
affords new polycyclic epoxides, enediones, ketones, alcohols, and/
or hydroperoxides. Their structures represent basic frameworks of
many biologically active and important substances isolated from
Nature.2,3 On the other hand, the fused heteroaromatic benzobicy-
clo[3.2.1]octadiene 1 can be easily obtained from the corresponding
o-vinyl substituted hetero-stilbene and proved to be a convenient
substrate for efficient one-step photochemical ring closure.4,5
Both metalated and free-base porphyrins were found to be effi-
cient in photocatalytic oxygenation of cycloalkenes6–8 and other
unsaturated heteroaromatics.9 While free-base porphyrins can be
utilized as sensitizers for the generation of singlet oxygen,9–13
metalloporphyrins are much more versatile as photocatalysts.
Their application can result in autooxidation reactions, hydroxyl-
ations, or direct oxygen transfer yielding epoxides.14,15 Cationic
In the thermal reaction of 1 using mCPBA (Scheme 1) the enedi-
one 8 is thought to be produced via rearrangement of intermediate
epoxide 7 formed initially from 1.16 Since there are two epoxida-
tion sites on furan 1, we concur with the literature suggestion that
the initial epoxidation occurs at the substituted cyclohexyl side of
the furan ring as indicated in the left sequence in Scheme 1. This is
reasonable on the basis of the concept that epoxidation should
⇑
Corresponding authors. Tel.: +36 88 624 159; fax: +36 88 624 548 (O.H.); tel.:
+385 1 4597 241; fax: +385 1 4597 250 (I.Š.).
E-mail addresses:
(I. Škoric´).
otto@vegic.uni-pannon.hu (O. Horváth),
iskoric@fkit.hr
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.076

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I. Kikaš et al. / Tetrahedron Letters 52 (2011) 6255–6259
neither in the thermal oxidation with mCPBA, nor with manga-
R
R
nese(III) porphyrin photocatalysts. This observation clearly indi-
cates that this is the only case when singlet oxygen is the
oxidative agent, the reaction pathway leading to a final product
deviating from all the others. According to the previous studies
on oxygenation of cycloalkenes, the mechanism of the reaction is
quite complicated, involving at least 3–4 elementary steps.9,10,13
In the presence of anionic Mn(III)TSPP3ꢀ, under similar condi-
tions as in the case of the corresponding free base (air-saturated
solution, pH 7), epoxide 13 and furan ring-opened 8 and 14 prod-
ucts were detected in appreciable amounts (Scheme 2, Table 1).
This observation differs significantly from that published by Hen-
nig et al.15 in which this anionic manganese(III) porphyrin did
not display any photocatalytic activity regarding the oxygenation
O
O
O
1
4
3
2
R = H, methyl, vinyl
OH
OOH
O
O
O
O
5
6
Figure 1. Structures of known rigid methano-bridged heteroaromatics 1–6.
occur at the double bond bearing the more electron-donating
group.17 Therefore, we propose the formation of 8 from the rear-
rangement reaction of epoxide 7. Further oxygenation to 9 did
not take place. As described in our previous paper,18 substituted
styryl derivatives 2a–c (Scheme 1) are easily available from ben-
zo-furo bicyclic structure 1. These compounds were also submitted
to additional thermal transformations using mCPBA, giving all
trans-11a–c (formed via intermediates 10a–c) in good yields
(71–88%). Further oxygenation to compounds 12 was not observed.
Scheme 2 shows the products 13–17 of light-induced oxygena-
tion of 1, using different porphyrins as photocatalysts. Blank exper-
iments were also carried out. No permanent change was observed
in the dark (after stirring air-saturated solutions containing both
substrate and catalyst for 2 h), nor upon irradiation in the absence
of porphyrin. These observations clearly indicated that the sub-
strate does not undergo any thermal reaction even in the presence
of ground-state porphyrins.
of a-pinene in a similar system. We had the same experience dur-
ing the preliminary investigation of endo-6-phenyl-6,9-dihydro-
5H-5,9-methano-benzocycloheptene,21 a compound similar to 1,
but without a furan ring, using Fe(III)TSPP3ꢀ photocatalyst. How-
ever, in the presence of this catalyst, 1 was also photooxygenated
to the same products as those observed with the manganese(III)
porphyrin. These results clearly indicate that the double bond in
the furan ring is more reactive than that in a saturated hydrocar-
bon environment. This phenomenon may be attributed to the mes-
omeric effect of the free electron pairs on the oxygen atom.
Nevertheless, in the presence of cationic manganese(III) porphy-
rins an epoxy-derivative was also obtained as the main product
of the oxygenation of
a
-pinene.15 This observation suggests that
the positively charged porphyrin ligand is significantly better able
to promote oxygenation on the double bond of a cycloalkene than
the negatively charged example.
Since under the experimental conditions applied19 [irradiation
with polychromatic light (k >380 nm)] the porphyrin photocata-
lysts absorb exclusively in these systems, the chemical changes ob-
served can be attributed unambiguously to photoinduced reactions
involving the excited states of these macrocyclic compounds.
The longer-lived triplet state plays a role in the photoinduced
reactions if a free-base porphyrin (H2TSPP4ꢀ in our case) is the
photoactive species in the system. Thus, it functions as a sensitizer
generating singlet oxygen via triplet quenching by the dissolved
ground-state oxygen molecules.20 The product in this case is
hydroxybutenolide derivative 15 (Scheme 2), which cannot be
detected in any other oxygenation process studied in this work,
Modification of the experimental conditions (pH or oxygen con-
centration) resulted in changes in the ratio of the products but did
not cause formation of further new species (Table 1). Thus, increas-
ing the pH (to 10) enhanced the quantity of the epoxidized product
13 from 35% to 87%. This phenomenon suggests that the higher pH
apparently hinders further oxidation of this derivative, strongly
diminishing the amounts of ring-opened products. This effect
may be attributed to the role of protons in the disproportionation
reaction giving an Mn(V) species (see later, Eq. 3). Hence, forma-
tion of this highly reactive intermediate is reduced at higher pH,
decreasing the chance of further oxidation of 13, and thus promot-
ing its accumulation. Bubbling oxygen instead of air significantly
increased the amounts of the products 13 and 14 from 35% to
70% and from 2% to 10%, respectively (Table 1). As Scheme 2 indi-
cates, application of cationic manganese(III) porphyrin [Mn(III)TM-
PyP5+] did not lead to the formation of different products, but the
hydroxy 16 and hydroperoxy 17 derivatives became the main
products (formed via intermediates 18 and 19), while the epoxi-
dized compound 13 was formed only in traces. This phenomenon
indicates that a change in the sign of the ligand charge strongly
modifies the distribution of the oxygenation products. The main
products with the anionic photocatalyst become trace products
with the cationic metalloporphyrin and vice versa, that is, compe-
tition of the simultaneous oxygenation reactions is governed by
the ligand charge. Besides, as mentioned before, the cationic com-
plex proved to be effective in oxygenation of various cycloalkenes,
while the anionic example was less efficient or inactive.6,15 Appar-
ently, the lower Lewis basicity of the porphyrin ligand promotes
oxygenation on C@C bonds. This was confirmed by flash photolysis
experiments with manganese(III) porphyrins possessing substitu-
ents of various electron demand.22,23
R
O
O
1
2a: R = H; 2b: R = methyl; 2c: R = vinyl
mCPBA
O
mCPBA
R
O
O
7
O
10a-c
O
O
O
R
O
8 (77%)
trans-11a-c (71-88%)
As earlier studies6 and more recent publications8,23 have indi-
cated, in the case of manganese(III) porphyrins as photocatalysts,
(P)MnIV@O and (P)MnV@O intermediates play the key role of the
in situ generated reactive species in the oxygenation of cycloalk-
enes. The formation of (P)MnV@O in primary photochemical reac-
O
O
O
R
O
O
O
9
12
Scheme 1. Proposed reaction pathways for the thermal reactions of 1 and 2.

I. Kikaš et al. / Tetrahedron Letters 52 (2011) 6255–6259
6257
O
1
> 380 nm
h
ν, λ
cationic
Mn(III)porphyrin
hν, λ > 380 nm
anionic
hν, λ > 380 nm
non-metalated
anionic porphyrin
Mn(III)porphyrin
1O2
O
O
O
O
O
O
O
18
7
13 (35-87%)
OH
OH
O
O
O
O
O
O
16 (47%)
19
8 (10-63%)
OOH
O
OH
14 (2-10%)
O
17 (17%)
13 (in traces)
(pH 7, air-saturation unless otherwise stated; see Table
O
OH
16 and 17 (in traces)
15 (73%)
Scheme 2. Proposed reaction pathways for the photocatalytic oxygenation reactions of
1
1
for anionic
Mn(III)porphyrin).
several orders of magnitude higher for manganese(V)-oxo
porphyrins than for the corresponding Mn(IV) species. Hence,
(P)MnV@O can be considered as the major oxidant in the photocat-
alytic oxygenations in our systems.
Table 1
Photoproducts with yields (%) obtained using anionic manganese(III) porphyrin under
various relevant experimental conditions.a
Compound
8
13
14
16
17
As Scheme 2 indicates, the positively charged porphyrin ligand
promotes electrophilic attack on the inner double bond of the furan
ring, while the anionic catalyst favors the outer C@C bond. This
phenomenon may be attributed to simultaneous electronic and
steric effects. Compared to the outer double bond, the accessibility
of the inner double bond to the oxygen atom coordinated to the
bulky macrocyclic complex is sterically hindered, due to its bicyclic
environment. From an electronic point of view, this bond is more
favored for electrophilic attack as a consequence of the electron-
donating effect of the adjacent hydrocarbon (bicycloalkyl) parts
of the molecule. Since the lower Lewis basicity of the cationic com-
plex makes the corresponding manganese(V)-oxo intermediate
much more electrophilic than the anionic one, in the case of the
previous metalloporphyrin the electronic effect governs the reac-
tion pathway, leading to hydroxylation or hydroperoxylation on
the inner carbon atoms via an epoxy intermediate. For the less
electrophilic anionic complex steric hindrance is the predominant
effect, promoting attack at the more accessible outer bond.
All the photoproducts, 8 and 13–17 (Scheme 2), were isolated by
repeated thin-layer chromatography and characterized by spectro-
scopic methods.19 From the NMR spectra, using different techniques
(COSY, NOESY, HSQC, and HMBC) and taking as reference our previ-
ous results,21,24 the structures of all the compounds were deter-
mined. All the photoproducts have recognizable patterns in their
1H NMR spectra (Fig. 2) but different numbers of signals in their ali-
phatic regions, depending on the structure (See Supplementary
data). These suggested results were confirmed from their 13C NMR
spectra which clearly revealed the structures with characteristic for-
myl, keto, hydroxy, and/or hydroperoxy groups. Molecular ions m/z
212 (8, 13, 14), m/z 228 (15), m/z 214 (16), and m/z 230 (17) indicated
that one or two oxygen atoms were incorporated in the starting sub-
strate 1. The IR spectra of 8 and 13–17 showed characteristic signals
for the formyl, keto, hydroxy and/or hydroperoxy groups. The ther-
malproducts11a–c wereisolatedby repeatedcolumnchromatogra-
phy and characterized by spectroscopic methods19 (Fig. S2).
% (pH 7, air-saturation)
% (pH 10, air-saturation)
% (pH 7, oxygen-saturation)
63
10
20
35
87
70
2
3
10
Traces
Traces
Traces
Traces
Traces
Traces
a
In the cases of the free base and cationic manganese(III) porphyrin Scheme 2
unambiguously displays the products and yields, hence they are not provided in
this Table.
tions was observed in acetonitrile, as a consequence of heterolytic
cleavage of the O–Cl bond in the axially coordinated perchlorate
counterion.22,23 (P)MnIV@O was produced by photoinduced homol-
ysis of the O–Cl or O–N bond of axially coordinated chlorate or ni-
trate, respectively, in acetonitrile,22,23 or via a ligand-to-metal
charge-transfer process (also a photoinduced homolysis but that
of the metal-ligand bond) with chloride or hydroxide axial ligands
in aqueous systems.6,15 In the latter case the Mn(II) species was
formed in the primary photochemical step (Eq. 1) followed by
the coordination of oxygen (Eq. 2).
ðPÞMnIIIOH þ h
m
! ðPÞMnIIþÅOH
ð1Þ
ð2Þ
2ðPÞMnII þ O2 ! 2ðPÞMnIV@O
Eq. 2 is an overall reaction comprising several steps.15 Since our
experiments were carried out in water–acetone solvent mixture,
hydroxide or water was axially coordinated to the Mn(III) center.
Accordingly, hydroxyl radicals formed in the primary photochem-
ical step most probably react with the organic solvent. The Mn(IV)
complexes readily disproportionate, giving highly reactive manga-
nese(V)-oxo species (Eq. 3).22,23
2ðPÞMnIV@O þ Hþ ꢀ ðPÞMnV@O þ ðPÞMnIIIOH
Disproportionation is much faster than synproportionation in
this equilibrium system, moreover, a polar solvent promotes the
previous process, thus it occurs with nearly a diffusion-controlled
rate constant.22 The rate constants for epoxidation of olefins are
ð3Þ

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I. Kikaš et al. / Tetrahedron Letters 52 (2011) 6255–6259
Hf
Hf
O
1
H1
H1
O
O
8
H1
H1
O
O
OH
15
OH
Hf
Hf
O
16
PPM 6.0
5.6
5.2
4.8
4.4
4.0
3.6
3.2
2.8
2.4
2.0
Figure 2. 1H NMR spectra of benzobicyclo[3.2.1]octadienes 1, 8, 15, and 16 (CDCl3, 600 MHz).
In conclusion, a simple and efficient protocol is utilized for the
synthesis of functionalized benzobicyclo[3.2.1]octadienes from
furan derivative 1 having no characteristic functional groups but
reactive double bonds.
Application of free-base and metalated water-soluble porphy-
rins for photocatalytic oxygenation of furan derivative 1 proved
to be flexible compared to the thermal reactions with mCPBA.
Excitation of the free-base porphyrin (H2TSPP4ꢀ) produces singlet
oxygen, the reaction of which with the substrate leads to a
hydroxybutenolide derivative. Use of the corresponding manga-
nese(III) porphyrin [Mn(III)TSPP3ꢀ] as a photocatalyst results in
the formation of epoxidized and furan ring-opened derivatives
as the main products, indicating that the preferred target of oxy-
genation in this case is the outer double bond of the furan ring. In
contrast, photocatalysis with cationic manganese(III) porphyrin
leads to oxygenation on the inner double bond of the furan ring,
forming hydroperoxy- and hydroxy-derivatives. These results
demonstrate that the charge on the porphyrin ligand determines
the pathway of the oxygenation reaction in this system, offering
the possibility of controlling the types and distributions of the
final products. Application of water-soluble free-base and metal-
loporphyrins for oxygenation of organic substrates in water–
acetone solvent mixture promotes the easy separation of the
products from the catalyst, ensuring the efficient reuse of the

I. Kikaš et al. / Tetrahedron Letters 52 (2011) 6255–6259
6259
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Acknowledgments
This research was funded by a grant from the Ministry of Science,
Education and Sports of the Republic of Croatia (125-0982933-
2926) and also in the framework of the Hungarian-Croatian
Intergovernmental S&T Cooperation Program for 2009-2011 jointly
financed by the Hungarian National Office of Research and
Technology (OMFB-01247/2009). Our work was also supported by
the National Development Agency (TÁMOP 4.2.2.-08/1/2008-0018,
Livable environment and healthier people – Bioinnovation and
Green Technology research at the University of Pannonia, the project
is co-financed by the European Social Fund with the support of the
European Union).
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19. Typical experimental procedure for the photocatalytic oxygenation of 1. A solvent
mixture prepared by addition of the same volumes of acetone and water was
used in the photocatalytic experiments. A solution of 40 ml of 1 and the Mn(III)
porphyrin (see Fig. S1) was irradiated with
a 70 W tungsten halogen
immersion lamp (Philips, kir >380 nm) in a thermostated 50 ml cylindrical
photoreactor. A stream of air or oxygen was passed through the solution at rt
over 2 h, also ensuring vigorous stirring. The concentration conditions were: S/
C = 100, Mn(III)porphyrin = 0.00144 mol and 1 = 0.144 mol. After termination
of the photolysis, acetone was removed by vacuum distillation. The remaining
two phases were separated by standard methods. The water-insoluble
oxygenation products remained in the organic phase and there was no
unreacted substrate. The photoproducts 8 (10–64% yield) and 13–17 (13: 35–
87%, 14:2–10%, 15: 73%, 16: 47%, 17: 17%) (Scheme 2), were isolated by
repeated thin-layer chromatography and characterized by spectroscopic
methods.
Supplementary data
Supplementary data associated with this Letter can be found, in
the online version, at doi:10.1016/j.tetlet.2011.09.076.
Typical experimental procedure for the thermal oxygenation of
1 and 2. To
References and notes
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