Singlet oxygen generation from poly[4-diacetoxyiodo]styrene and hydrogen peroxide

Article abstract of DOI:10.3906/kim-1602-13

Treatment of hydrogen peroxide with a polymer-supported hypervalent iodine compound, poly[4-diacetoxyiodo] styrene (PDAIS), generates singlet molecular oxygen (¹O2). Singlet oxygen generation was proved by trapping with typical organic compounds such as conjugated dienes, aromatic dienes, and electron-rich alkene. When compared to monomer analogue, the use of PDAIS in peroxidation of substrates gave slightly better yields (45%–96%). Regeneration and reuse of PDAIS showed similar activity. The mechanism underlying generation of singlet oxygen and reaction scope was examined.

Full text of DOI:10.3906/kim-1602-13

Turk J Chem
Turkish Journal of Chemistry
(2017) 41: 467 – 475
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1602-13
Research Article
Singlet oxygen generation from poly[4-diacetoxyiodo]styrene and hydrogen
peroxide
Mustafa C¸ ATIR∗
Department of Chemistry, Faculty of Arts and Science, Erzincan University, Erzincan, Turkey
Received: 02.02.2016
•
Accepted/Published Online: 04.03.2016
•
Final Version: 05.09.2017
Abstract:Treatment of hydrogen peroxide with a polymer-supported hypervalent iodine compound, poly[4-diacetoxyiodo]
styrene (PDAIS), generates singlet molecular oxygen (1 O2). Singlet oxygen generation was proved by trapping with
typical organic compounds such as conjugated dienes, aromatic dienes, and electron-rich alkene. When compared to
monomer analogue, the use of PDAIS in peroxidation of substrates gave slightly better yields (45%–96%). Regeneration
and reuse of PDAIS showed similar activity. The mechanism underlying generation of singlet oxygen and reaction scope
was examined.
Key words: Hydrogen tetroxide, oxidation, peroxyiodane, polymer-supported hypervalent iodine, singlet oxygen
1. Introduction
Singlet molecular oxygen (1 O2), an unstable, energy-rich active oxygen species, was first reported in the 1960s.
Since then, peroxidation of organic molecules to endoperoxides or alkyl hydroperoxides by singlet oxygen has
received much attention because of its unique reactivity and high selectivity toward organic substrates such
as olefins, conjugated dienes, heterocyles, sulfides, and phenols.1,2 1 O2 also plays diverse, important roles in
intracellular signaling mechanisms, in reactions leading to cell death,3−5 and, remarkably, in photodynamic
therapy, which is a noninvasive procedure used for the cure of malignant tumors and age-related macular
degeneration.6−9 Among the various methods available for the production of 1 O2 , the photochemical method
involving the irradiation of oxygen gas in the presence of an organic dye such as rose bengal, methylene blue,
or porphyrins is the most commonly used.10,11 In addition, a number of chemical methods for singlet oxygen
generation from hydrogen peroxide have been reported.12−14 For example, disproportionation of hydrogen
peroxide catalyzed by molibdate ions affords 1 O2 in aqueous alkaline media.15,16 However, this method is
restricted to low-molecular-weight substrates or substrates bearing hydrophilic groups.17,18
Hypervalent iodine reagents are reactive, highly selective, and compatible with a range of functional
groups, because of which they are popular in natural product synthesis. These reagents are suitable alter-
natives to heavy metal-based reagents used in industry.19 In particular, bis(trifluoroacetoxy)iodobenzene 1a,
(diacetoxy)iodobenzene 1b, iodosylbenzene 1c, iodylbenzene 1d, and Dess–Martin periodinane 1e have been
extensively used in a variety of useful oxidative transformations (Figure).20−22 However, oxidation of organic
substrates with these reagents results in a stoichiometric amount of iodobenzene as waste, which is troublesome
to recover and reuse because it is volatile and more soluble in typical organic solvents.23 On the other hand,
∗Correspondence: mcatir@erzincan.edu.tr
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polymer-supported reagents are generally easy to regenerate, less toxic, and pose a reduced risk of explosion.
For this reason, a number of research groups have attempted to develop iodobenzene-based polymer-supported
hypervalent iodine reagents having (diacetoxy)iodo groups (2b),24−27 hydroxy(tosyloxy)iodo groups (2c),27
and 1,2-benziodoxol-3-one groups (2d)28 (Figure). PDAIS (2b) was successfully used in several organic trans-
formations including oxidation of alcohols,29 α-hydroxylation of ketones,29 oxidative 1,2-aryl migration of alkyl
aryl ketones,30 oxidative esterification of primary alcohols,31,32 and oxidative ligand-directed acetoxylation of
arene and alkane C–H bonds.33 Togo showed that reactivity of PDAIS (2b) is similar to that of their monomeric
compound 1b.30
Figure. Examples of hypervalent and polymer-supported hypervalent iodine compounds.
We previously described the decomposition of hydrogen peroxide by hypervalent aryl-λ3 -iodane PIFA
1a to generate singlet oxygen, which can be trapped by the addition of singlet oxygen acceptors.34,35 Recently,
we reported a new method for the generation of 1 O2 in which iodosylarene (ArIO) is generated in situ from
an iodoarene (ArI) and H2 O2 in the presence of methyltrioxorhenium as a catalyst. Further reaction of ArIO
with H2 O2 produced singlet oxygen.36 Herein, I introduce the first example of poly[4-diacetoxyiodo]styrene
(PDAIS-2b)-induced activation of hydrogen peroxide to generate singlet oxygen for the peroxidation of organic
substrates, regeneration and reuse of 2b for the same reactions, and comparison of the product yield with that
obtained using our previously reported PIFA/H2 O2 system (Scheme 1).34
Scheme 1. Generation of singlet oxygen in organic media.
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2. Results and discussion
2.1. Preparation of PDAIS (2b)
Polymer-supported hypervalent(III) reagent PDAIS (2b) was synthesized by the oxidation of commercially
available iodopolystyrene (PS-I) with peracetic acid, which was freshly prepared from acetic anhydride and
hydrogen peroxide.37,38 The results of IR spectroscopy, elemental analysis, and iodometry analysis33 performed
on 2b suggested that most of the iodine atoms in the polystyrene ring were oxidized to the corresponding
(diacetoxy)iodo groups, which is in agreement with the literature yield (83%).37
2.2. Peroxidation of organic substrates by PDAIS and H2 O2
Treatment of a suspension of PDAIS in dichloromethane with 35% hydrogen peroxide led to the release of
oxygen bubbles and yielded a solid, which was filtered off from the reaction medium and washed with ether. IR
spectroscopy confirmed that the product was iodopolystyrene. To determine whether the PDAIS/H2 O2 system
could generate singlet oxygen or not, α-terpinene 3a was selected as the singlet oxygen acceptor. Compound
3a can be aromatized by PDAIS to give p-cymene 5 or peroxidized with 1 O2 to form ascaridole 4a, rendering
it a good diagnostic tool for product distribution. Oxidation of 3a with the PDAIS-hydrogen peroxide system
was first studied as the model reaction in CH2 Cl2 at room temperature for 3 h. The decreasing of 3a and the
appearance of the signals belonging to 4a and 5 were observed by the 1 H NMR spectra of the reaction mixture
that was separated from the polymer by filtration, providing evidence for the generation of singlet oxygen. To
examine the solvent effect on the peroxidation of model substrate 3a and the product ratio, tetrahydrofuran
(THF), ether, acetone, dichloromethane (DCM), 1,2-dichloroethane, and carbon tetrachloride were selected for
solvent screening. Two equivalents of PDAIS were sufficient for the full conversion of the starting reactant 3a in
all the tested solvents, although the yields of 4a and the product ratios (4a:5) varied according to the solvent
(Table 1). The results presented in Table 1 indicate that, while the best product ratio (4a:5) was observed
in DCM or ether, the optimum yield of 4a was obtained in DCM (Table 1, entry 2). The poor solubility of
hydrogen peroxide and excellent dispersion of PDAIS in DCM allowed for controllable singlet oxygen production
with the use of the PDAIS/H2 O2 system. Therefore, DCM was selected as the optimal reaction solvent and
the optimal reaction conditions were determined to be as follows: ratio of PDAIS/H2 O2 to 3a, 2:10; room
temperature; reaction time, approximately 3 h. In the control experiments, 4a was not observed in the absence
of PDAIS or H2 O2 , while the reaction of 3a with PDAIS in the absence of H2 O2 afforded 5. Thus, it was
confirmed that the use of both PDAIS and H2 O2 is essential to generate singlet oxygen.
Table 1. Peroxidation of 3a with the PDAIS/H2 O2 system in various solventsa .
Entry Solvent
Conv. [%]b Yield 4a [%]c (4a:5)b
1
2
3
4
5
6
CCl4
CH2Cl2
(CH2)2Cl2 > 98
CH3CN
Et2O
THF
> 98
> 98
40
80
40
49
63
51
73:27
85:15
54:56
72:28
86:14
77:23
> 98
> 98
> 98
a Conditions: 3a (0.3 mmol), PDAIS (0.6 mmol), 35% H2 O2 (3 mmol). b Conversions and product ratios were
determined by 1 H NMR spectra of the reaction mixture. c NMR yields determined with 4-methoxyacetophenone as
internal standard.
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To demonstrate the utility of this method for the peroxidation of different types of organic substrates, two
conjugated dienes 3b and 3c; three aromatic hydrocarbons 3d, 3e, and 3f; and one electron-rich alkene 3g were
oxidized using the PDAIS/H2 O2 system under the optimal reaction conditions. The results are given in Table
2. 1,3-Cyclohexadiene 3b afforded the desired endoperoxide 4b in 45% chemical yield and 90% conversion,
Table 2. Peroxidation of various substrates (3b–g) by the PDAIS/H2 O2 systema .
[%]f
Entry
Substrate
Product(s)
Conv. %]b,c Yield [%]a
Lit. Yield
40d
90
(75:25)
45d
49b,e
1
3b
4b
4c
5b
73d
2
3
>98
>98
30d
80b,e
3c
93d
96e
90d
3d
4d
4
65
61d
85d,g
3e
4e
5
6
>99
88d
80d
3f
4f
75
(95:5)
48d
63b,e
30d
3g
4g
6
a Conditions: Substrate (0.5 mmol), PDAIS (1 mmol) and 35% H2 O2 (5 mmol); b The conversions, product ratios, and
yields were determined by 1 H NMR analysis with 4’-methoxyacetophenone as internal standard; c Values in parentheses
e
correspond to product ratios obtained in CDCl3 as a solvent; d Isolated yields after column chromatography; CDCl3
was used as solvent; f Previously reported yields obtained from the PIFA-H2 O2 system: Substrate/PIFA/H2 O2
(1/2.5/6); g Substrate/PIFA/H2 O2 (1/4/10).
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along with benzene as the byproduct in 15% NMR yield (Table 2, entry 1). Similarly, 1,3-cycloheptadiene 3c
afforded endoperoxide 4c in good isolated yield (73%) (Table 2, entry 2). The reaction of dimethylanthracene
3d under identical reaction conditions provided the corresponding endoperoxide 4d in a high yield of 93%
(Table 2, entry 3). The relatively less reactive diphenylanthracene 3e was also oxygenated to 4e, albeit with a
slightly lower conversion of 65% (Table 2, entry 4). Oxygenation of 3f proceeded smoothly to produce 4f in
88% yield (Table 2, entry 5). The electron-rich alkene 2,3-dimethyl-2-butene 3g participated in the ene reaction
with singlet oxygen. Oxygenation of 3g resulted in the formation of allylic hydroperoxide 4g and allylic alcohol
6 in a 95:5 ratio (Table 2, entry 6). Hydroperoxide 4g was isolated as a colorless oil in 48% yield at 75%
conversion. The formation of 6 was attributed to the reaction of a small amount of 4g with PDAIS under the
given reaction conditions. The oxidation products obtained using the present system were purified by short
column chromatography and characterized by 1 H and 13 C NMR spectroscopy. The spectra of the products
were identical to the literature data.36
For all the reactions considered, PDAIS 2b as the hypervalent iodine(III) source slightly outperformed
its previously reported monomer analogue PIFA 1a in terms of reactivity and peroxidation yield.34 Probably
because of the relatively poor solubility of PDAIS in the solvent its slow reaction with hydrogen peroxide
prevents loss of singlet oxygen through bubbles. An exception was 9,10-diphenylanthracene 3e, which required
a greater amount of 1a for full conversion (Table 2, footnote g). Poly(4-iodostyrene) (PS-I) generated after
the reaction with PDAIS could easily be recovered in quantitative yield, and PDAIS could be regenerated by
the reaction of PS-I in acetic anhydride with hydrogen peroxide in the usual manner. The regenerated PDAIS
showed no loss of activity, as was confirmed by its use in the oxygenation of 3a and 3d; endoperoxides 4a and
4d were isolated in 78% and 93% yield, respectively.
Based on the above-mentioned experimental results and our previous reports, a plausible mechanism
for the generation of 1 O2 from the PDAIS/H2 O2 system is proposed (Scheme 2). First, PDAIS reacts with
H2 O2 to give reactive intermediate peroxyiodane 7 via the exchange of two acetyl groups for two hydroperoxyl
groups. Homolytic cleavage of the iodine(III)-peroxy bonds in 7 generates PS-I and hydroperoxyl radicals,
which undergo coupling to form hydrogen tetroxide (H2 O4). Homolytic dissociation of the H2 O4 intermediate
generates singlet oxygen and hydrogen peroxide.
Scheme 2. Proposed mechanism of 1 O2 generation from the PDAIS/H2 O2 system.
To confirm the assumption that PDAIS was synthesized from the reaction of iodopolystyrene (PS-I)
with hydrogen peroxide and acetic anhydride, a catalytic amount of PS-I (10% mol) was used in the control
experiment for the oxidation of DMA 3d to endoperoxide 4d in the presence of acetic anhydride and hydrogen
peroxide. After stirring the reaction mixture for 48 h, 4d was isolated in 65% yield. In this catalytic process,
peracetic acid oxidizes PS-I to 2b, which then reacts with hydrogen peroxide to generate singlet oxygen and
PS-I (Scheme 3).
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Scheme 3. Proposed mechanism for catalytic oxidation of 3d to 4d with the PDAIS/H2 O2 system.
In conclusion, this report presents a convenient method for chemical singlet oxygen generation in organic
media for the oxygenation of organic substrates using the PDAIS/H2 O2 system. PDAIS can be regenerated
quantitatively from the iodopolystyrene formed after the peroxidation reaction and reused without loss of
activity, thus making this system both synthetically practical and environmentally benign.
3. Experimental
3.1. General
All reagents used were commercially available unless otherwise stated, and all solvents were used as received.
Compound 2b was prepared according to the method reported in the literature. Varian 400 and Bruker 400
spectrometers were used to acquire 400 MHz 1 H NMR and 100 MHz 13 C NMR spectra. Chemical shifts are
reported in ppm relative to tetramethylsilane (TMS) and chloroform-d. All the peroxidation products are known
and were confirmed by comparison with the previously reported NMR data.39−44
3.2. Preparation of poly[4-diacetoxyiodo]styrene (2b)33
To an acetic anhydride (23 mL, 243 mmol) was slowly added hydrogen peroxide (35%, 5.34 mL, 58.7 mmol)
at 0 ◦ C. The solution was left to warm to ambient temperature and stirred overnight. Iodopolystyrene (PS-I)
(920 mg, 4 mmol) was then added to this solution. The mixture was stirred for 16 h at 40 ◦ C. At the end
of the reaction, the crude mixture was cooled to room temperature and diethyl ether (60 mL) was added to
the solution to precipitate the polymer. The collected polymer was washed with ether (3 × 20 mL) and ether
was removed under reduced pressure to afford PDAIS-2b (1.16 g, 3.36 mmol) as an off-white solid. IR (KBr,
cm−1): 1628, 1571, 1481, 1448, 1405, 1270, 1183, 1004, 766. Anal. calcd for PDAIS: C, 36.33; H, 3.66, found:
C, 34.84; H, 3.37.
3.3. General procedure for oxygenation of substrates (3a–g) with the 2b/H2 O2 system
To a suspension of substrates (3a–g, 0.5 mmol) and 35% hydrogen peroxide (443 µL, 5 mmol) in dichloromethane
(4 mL) was added PDAIS (2b) (348 mg, 1 mmol) dropwise within 1 h at room temperature. The mixture was
further stirred for 2 h. After removal of the solvent, ether was added to the reaction mixture to precipitate
iodopolystyrene (PS-I). The formed PS-I was recovered by washing with ether (2 × 15 mL). After the addition
of 4-methoxyacetophenone as an internal standard, the organic phase was washed with water (2 × 10 mL)
and brine, dried over Na2 SO4 , and concentrated. The conversion of 3a–g, formation of 4a–g, and product
distribution were determined directly by 1 H NMR analysis of the crude mixture. Then the resulting mixture
was chromatographed on a short column (silica gel, 13–15 g) using a mixture of hexane and Et2 O (90:10) as
an eluent to afford the desired oxygenated products.
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3.3.1. 1-Methyl-4-(propan-2-yl)-2,3-dioxabicyclo[2.2.2]oct-5-ene (4a)39
Colorless oil, 80%. 1 H NMR (400 MHz, CDCl3): δ = 6.50 (d, J = 8.6 Hz, A part of AB system, 1H), 6.41 (d,
J = 8.6 Hz, B part of AB system, 1H), 2.05–1.98 (m, 2H), 1.96–1.87 (m, 1H), 1.57–1.51 (m, 2H), 1.37 (s, 3H),
1.0 (d, J = 7.0 Hz, 6H). 13 C NMR (100 MHz, CDCl3): δ = 136.6, 133.3, 80.0, 74.6, 32.3, 29.7, 25.8, 21.6,
17.5, 17.4.
3.3.2. 2,3-Dioxabicyclo[2.2.2]oct-5-ene (4b)40
Colorless oil, 45%. 1 H NMR (400 MHz, CDCl3): δ = 6.69 (dd, J = 4.4, 3.3 Hz, 2H), 4.69–4.66 (m, 2H),
2.33–2.27 (m, 2H, AA′ part of AA′BB′ system), 1.53–1.47 (m, 2H, BB′ part of AA′BB′ system). 13 C NMR
(100 MHz, CDCl3): δ = 132.2, 70.9, 21.6.
3.3.3. 6,7-Dioxabicyclo[3.2.2]non-8-ene (4c)41
Colorless oil, 73%. 1 H NMR (400 MHz, CDCl3): δ = 6.39 (dd, J = 4.9, 3.2 Hz, 2H), 4.76 (m, 2H), 2.01–1.87
(m, 4H), 1.62–1.56 (m, 1H), 1.46–1.33 (m, 1H). 13 C NMR (100 MHz, CDCl3): δ = 127.9, 77.4, 31.7, 18.5.
3.3.4. 9,10-Dimethyl-4a,9,9a,10-tetrahydro-9,10-epidioxyanthracene (4d)42
Recrystallized from DCM-hexane, white powder, 93%, mp 217–219 ◦ C, lit. 218.5–220 ◦ C. 1 H NMR (400 MHz,
CDCl3): δ = 7.40–7.36 (m, 4H, AA′ part of AA′BB′ system), 7.29–7.24 (m, 4H, BB′ part of AA′BB′ system),
2.14 (s, 6H). 13 C NMR (100 MHz, CDCl3): δ = 141.0, 127.6, 120.9, 79.7, 13.9.
3.3.5. 9,10-Diphenyl-9,10-dihydro-9,10-epidioxyanthracene (4e)42
Recrystallized from DCM-hexane, white powder, 61%, mp 193–195 ◦ C (decomposition), lit. 194–196 ◦ C. 1 H
NMR (400 MHz, CDCl3): δ = 7.78–7.75 (m, 4H), 7.71–7.67 (m, 4H), 7.61–7.56 (m, 2H), 7.26–7.19 (m, 8H).
13 C NMR (100 MHz, CDCl3): δ = 140.2, 133.0, 128.4, 128.3, 127.7, 127.6, 123.5, 84.1.
3.3.6. Benzene-1,2-diylbis(phenylmethanone) (4f)43
Recrystallized from ether-hexane, white powder, 88%, mp 143.5–145 ◦ C, lit. 144–147 ◦ C. 1 H NMR (400 MHz,
CDCl3): δ = 7.71–7.35 (m, 14H). 13 C NMR (100 MHz, CDCl3): δ = 196.6, 140.0, 137.2, 133.0, 130.3, 129.8,
129.7, 128.3.
3.3.7. 2,3-Dimethylbut-3-en-2-yl hydroperoxide (4g)44
Isolated as a colorless oil, 48%. 1 H NMR (400 MHz, CDCl3): δ = 7.40 (s, 1H), 5.01–5.00 (m, 1H), 4.97–4.95
(m, 1H), 1.81–1.80 (m, 3H), 1.36 (s, 6H). 13 C NMR (100 MHz, CDCl3): δ = 147.9, 111.9, 84.4, 23.9, 18.7.
3.3.8. 2,3-Dimethylbut-3-en-2-ol (6)44
Colorless oil, 2%–3%. 1 H NMR (400 MHz, CDCl3): δ = 5.00 (m, 1H), 4.77 (m, 1H), 1.81 (m, 3H), 1.54 (bs,
1H), 1.35 (s, 6H). 13 C NMR (100 MHz, CDCl3): δ = 151.9, 108.4, 73.1, 28.8, 19.2.
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Acknowledgments
Financial support from Erzincan University is gratefully acknowledged. I would like to thank Editage for English
language editing.
References
1. Wasserman, H. H.; Ives, J. L. Tetrahedron 1981, 37, 1825-1852.
2. Prein, M.; Adam, W. Angew. Chem. Int. Ed. Eng. 1996, 35, 447-494.
3. Ogilby, P. R. Chem. Soc. Rev. 2010, 39, 3181-3209.
4. Picard, S.; Clermont, G.; Genin, E.; Blanchard-Desce, M. Tetrahedron 2015, 71, 1088-1094.
5. Pedersen, S. K.; Holmehave, J.; Blaikie, F. H.; Gollmer, A.; Breitenbach, T.; Jensen, H. H.; Ogilby, P. R. J. Org.
Chem. 2014, 79, 3079-3807.
6. Oleinick, N. L.; Morris, R. L.; Belichenko. T. Photochem. Photobiol. Sci. 2002, 1, 1-21.
7. Dougherty, T. J. J. Clin. Laser Med. Surg. 2002, 20, 3-7.
8. Triesscheijn, M.; Baas, P.; Schellens, J. H. M.; Stewart, F. A. Oncologist 2006, 11, 1034-1044.
9. Ozlem, S.; Akkaya, E. U. J. Am. Chem. Soc. 2009, 131, 48-49.
10. Midden, W. R.; Wang, S. Y. J. Am. Chem. Soc. 1983, 105, 4129-4135.
11. Nowakowska, M.; Sustar, E.; Guillet, J. E. J. Photochem. Photobiol. A 1994, 80, 369-376.
12. Aubry, J. M. J. Am. Chem. Soc. 1985, 107, 5844-5849.
13. Nardello, V.; Marko, J.; Vermeersch, G.; Aubry, J. M. Inorg. Chem. 1995, 34, 4950.
14. Aubry, J. M.; Cazin, B. Inorg. Chem. 1988, 27, 2013-2014.
15. Clague, M. J.; Butler, A. J. Am. Chem. Soc. 1995, 117, 3475-3484.
16. Caron, L.; Nardello, V.; Alsters, P. L.; Aubry, J. M. J. Mol. Catal. A-Chem. 2006, 251, 194-199.
17. Nardello, V.; Bouttemy, S.; Aubry, J. M. J. Mol. Catal. A-Chem. 1997, 117, 439-447.
18. Aubry, J. M.; Cazin, B.; Duprat, F. J. Org. Chem. 1989, 54, 726-728.
19. Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2008, 108, 5299-5358.
20. Moriarty, R. M. J. Org. Chem. 2005, 70, 2893-2903.
21. Ladziata, U.; Zhdankin, V. V. ARKIVOC 2006, 9, 26-58.
22. Richardson, R. D.; Wirth, T. Angew. Chem. Int. Ed. 2006, 45, 4402-4404.
23. Yusubov, M. S.; Funk, T. V.; Chi, K. W.; Cha, E. H. Kim, G. H.; Kirschning, A.; Zhdankin, V. V. J. Org. Chem.
2008, 73, 295-297.
24. Tohma, H.; Takizawa, S.; Maegawa, T.; Kita, Y. Angew. Chem. Int. Ed. 2000, 39, 1306-1307.
25. Tohma, H.; Maegawa, T.; Kita, Y. Synlett 2003, 723-725.
26. Sakuratani, K.; Togo, H. ARKIVOC 2003, 6, 11-20.
27. Shang, Y.; But, T. Y. S.; Togo, H.; Toy, P. H. Synlett 2007, 67-70.
28. Jang, H. S.; Chung, W. J.; Lee, Y. S. Tetrahedron Lett. 2007, 48, 3731-3734.
29. Ley, S. V.; Thomas, A. W.; Finch, H. J Chem. Soc. Perk. T. 1 1999, 669-671.
30. Togo, H.; Sakuratani, K. Synlett 2002, 1966-1975.
31. Liu, X. L.; Lin, S. Y.; Sheng, S. R.; Wei, M. H.; Gong, B. J. Chin. Chem. Soc. 2007, 54, 1119-1122.
32. Abe, S; Sakuratani, K; Togo, H. J. Org. Chem. 2001, 66, 6174-6177.
33. Kalberer, E. W.; Whitfield, S. R.; Sanford, M. S. J. Mol. Catal. A-Chem. 2006, 251, 108-113.
474

C¸ATIR/Turk J Chem
34. Catir, M.; Kilic, H. Synlett 2003, 1180-1182.
35. Catir, M.; Kilic, H.; Nardello-Rataj, V.; Aubry, J. M.; Kazaz, C. J. Org. Chem. 2009, 74, 4560-4564.
36. Kalay, E.; Kilic, H.; Catir, M.; Cakici, M.; Kazaz, C. Pure Appl. Chem. 2014, 86, 945-952.
37. Togo, H.; Nogami, G.; Yokoyama, M. Synlett 1998, 534-536.
38. Huang, X.; Zhu, Q. Synth. Commun. 2001, 31, 111-115.
39. Zhang, K.; Kopetzki, D.; Seeberger, P. H.; Antonietti, M.; Vilela, F. Angew. Chem. Int. Ed. 2013, 52, 1432-1436.
40. Paddock, V. L.; Phipps, R. J.; Conde-Angulo, A.; Blanco-Martin, A.; Giro-Manas, C.; Martin, L. J.; White, A. J.
P.; Spivey, A. C. J. Org. Chem. 2011, 76, 1483-1486.
41. Pearson, A. J.; Lai, Y. S.; Lu, W. Y.; Pinkerton, A. A. J. Org. Chem. 1989, 54, 3882-3893.
42. Donkers, R. L.; Workentin, M. S. J. Am. Chem. Soc. 2004, 126, 1688-1698.
43. Nandakumar, M.; Sivasakthikumaran, R.; Mohanakrishnan, A. K. Eur. J. Org. Chem. 2012, 3647-3657.
44. Dang, H. S.; Davies, A. G.; Davison, I. G. E.; Schiesser, C. H. J. Org. Chem. 1990, 55, 1432-1438.
475