Recyclable silica-supported iodoarene-RuCl3 bifunctional catalysts for oxidation of alcohols and alkylarenes

Article abstract of DOI:10.1016/j.tetlet.2011.08.097

Novel, efficient, and recyclable bifunctional catalysts bearing SiO ₂-supported RuCl₃ and iodoarene moieties were developed and used for environmentally benign oxidation of alcohols or alkylarenes at the benzylic position. These reactions in the presence of oxone as stoichiometric oxidant afforded the corresponding carbonyl compounds in high yields under mild conditions and convenient work-up. Furthermore, these SiO₂-supported bifunctional catalysts can be recovered by simple filtration and directly reused.

Full text of DOI:10.1016/j.tetlet.2011.08.097

Tetrahedron Letters 52 (2011) 5652–5655
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Recyclable silica-supported iodoarene–RuCl3 bifunctional catalysts
for oxidation of alcohols and alkylarenes
Xiao-Mei Zeng a, Jiang-Min Chen a,b, , Kyle Middleton a, Viktor V. Zhdankin a,
⇑
⇑
a Department of Chemistry and Biochemistry, University of Minnesota Duluth, 1039 University Dr., Duluth, MN 55812, USA
b College of Biological and Chemical Engineering, Jiaxing University, 56 South Yuexiu Rd., Jiaxing 314001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel, efficient, and recyclable bifunctional catalysts bearing SiO2-supported RuCl3 and iodoarene moieties
were developed and used for environmentally benign oxidation of alcohols or alkylarenes at the benzylic
position. These reactions in the presence of oxone as stoichiometric oxidant afforded the corresponding
carbonyl compounds in high yields under mild conditions and convenient work-up. Furthermore, these
SiO2-supported bifunctional catalysts can be recovered by simple filtration and directly reused.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 27 July 2011
Revised 14 August 2011
Accepted 16 August 2011
Available online 23 August 2011
Keywords:
Oxidation
Alcohols
Aromatic hydrocarbons
Hypervalent iodine
Ruthenium
Selective oxidation of alcohols and aromatic hydrocarbons into
the corresponding carbonyl compounds is one of the most funda-
mental and important transformations in synthetic organic chemis-
try.1,2 Environmentally benign procedures are especially important
for the production of pharmaceuticals, flavors and fragrances, and
agrochemicals. Thus, there is an increasing demand for oxidation
process that is catalytic and uses green, economic, and efficient
oxidants.
As oxidants, hypervalent iodine reagents have recently received
much attention due to their low toxicity, mild reactivity, ready
availability, high stability, easy handling, etc.3,4 However, their
low atom economy remains a major drawback, as stoichiometric
amount of aryliodides or similar waste products is produced,
which complicates isolation and purification of the products.5,6
Therefore, the catalytic utilization of hypervalent iodine reagents,
in view of economical and environmental requirements, is the
most attractive strategy, and new reaction systems based on cata-
lytic iodine(V) species and a stoichiometric oxidant have recently
been developed,4 including publications from our group.2b,7
However, synthetic value of the iodine(V)-based catalytic cycles
is limited by the reoxidation step of iodine(I) or iodine(III) to the
iodine(V) species, which proceeds relatively slowly even at tem-
peratures above 70 °C.8 Recently, we have reported an extremely
mild and efficient iodine(V)/RuCl3 tandem catalytic system
for the oxidation of alcohols and alkylarenes using oxone (2KHSO5
ÁKHSO4ÁK2SO4) as a stoichiometric oxidant,2b in which ruthe-
nium catalyst dramatically accelerates the reaction under mild
conditions.
However, the homogeneously catalyzed organic reactions com-
monly suffer from disadvantages, such as difficult separation of the
product and recovery of the catalyst. These problems can be
addressed by the immobilization of homogeneous catalysts onto
solid supports. Recently, silica-supported catalytic metallic species
have been successfully employed in aqueous media,9 since silica
displays many advantageous properties—excellent stability (both
chemical and thermal), high surface area, good accessibility, and
organic groups can be robustly anchored to the surface, to provide
catalytic centers.10
The analysis of literature data encouraged us to develop effi-
cient bifunctional hybrid-type catalysts by immobilization of both
hypervalent iodine and RuCl3 onto solid silica gel supports. Hybrid-
ization of these two moieties should result in more efficient oxida-
tion of organic substrates under mild conditions. Furthermore, the
combination of both co-catalysts in one reagent should make the
recovery and reuse of both hypervalent iodine and ruthenium
much easier. As a part of our studies2b,7,11 toward the development
of practical and environmentally benign oxidation reactions, we
herein, report the synthesis of SiO2-supported iodoarene–RuCl3
bifunctional catalysts 5 and 6 (Scheme 1), which are useful for
environmentally benign oxidation of alcohols or alkylarenes in
acetonitrile–water mixture at room temperature.
Our approach to the preparation of such SiO2-supported
iodoarene–RuCl3 bifunctional catalysts 5 and 6 consists of building
a suitable ligand structure 3 or 4 on the surface of commercial
⇑
Corresponding authors.
E-mail addresses: chemcjm@yahoo.com.cn (J.-M. Chen), vzhdanki@d.umn.edu
(V.V. Zhdankin).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.097

X.-M. Zeng et al. / Tetrahedron Letters 52 (2011) 5652–5655
5653
O
I
COCl
NH2
NH2
HN
I
SiO2
SiO2
Et3N, CH2Cl2,
RT, overnight
NH2
2
1 (AMPS)
N
MeO OMe
toluene,
O
NMe2
reflux, 24 h
O
I
O
I
MeOH, reflux, 3 h
HN
N
HN
SiO2
SiO2
N
NMe2
3
4
N
RuCl3
RuCl3
MeOH,
MeOH,
reflux, 3 d
reflux, 3 d
O
I
O
I
HN
N
HN
N
SiO2
SiO2
NMe2
Cl3Ru
Cl3Ru
N
5
6
Scheme 1. Synthesis of SiO2-supported iodoarene–RuCl3 bifunctional catalysts 5 and 6.
aminopropyl silica (AMPS, 1), followed by complexation with RuCl3
Table 1
Effect of oxidant and catalyst 5 on the oxidation of 1-phenylethanola
(Scheme 1). The synthesized catalysts 5 and 6 were characterized by
FT-IR and elemental analysis. The amount of the RuCl3 and iodine
loaded on the surface of silica gel was determined by elemental anal-
yses of chlorine and iodine. The loadings of iodine and ruthenium for
catalyst 5 are 0.60 mmol/g and 0.06 mmol/g, respectively, and for
catalyst 6 0.62 mmol/g and 0.05 mmol/g, respectively. For compar-
ison, the ratio of catalytic iodine to ruthenium in the previously re-
ported tandem catalytic system PhI/RuCl3 was 31:1.2b
Oxidant, catalyst 5 (3-14 mol% of I
OH
O
and 0.3-1.4 mol% of Ru)
Ph
CH3CN/H2O (1:1), RT
Ph
7
8
Entry Cat. 5
Oxidant
(equiv)
Time
(h)
Conversionb
(%)
Yieldc
(%)
(mg)
1
2
3
4
5
6
7
None
10
15
30
45
Oxone (3)
Oxone (1.5)
Oxone (3)
Oxone (3)
Oxone (3)
24
12
2
0
0
To investigate catalytic activity of the new bifunctional catalysts
in the oxidation of alcohols, the reaction was first optimized using
1-phenylethanol 7 as a model substrate, polymer 5 as a catalyst,
and several common stoichiometric oxidants (Table 1). As ex-
pected, in the absence of catalyst 5 the oxidation of substrate 7 at
room temperature proceeded extremely slowly (Table 1, entry 1)
using oxone as the oxidant. The combined application of 5 and ox-
one (1.5–3 equiv) resulted in fast oxidation of 7 to acetophenone 8
with up to 100% conversion (Table 1, entries 2–5). The yield of prod-
uct reached maximum at 15–30 mg of catalyst per 0.2 mmol of
alcohol (Table 1, entries 3 and 4). Further increases in the amount
of catalysts (Table 1, entry 5) resulted in a lower yield, probably
due to the rapid decomposition of oxidant before oxidizing the sub-
strate (see Ref. 2b). Other oxidants, such as MCPBA and peracetic
acid showed slightly lower reactivity and lack of selectivity as com-
pared to oxone (Table 1, entries 6 and 7). Previously, oxone has been
employed as environmentally safe co-oxidant for hypervalent io-
dine-catalyzed oxidations;8 moreover, oxone is commercially avail-
able and an inexpensive reagent.12 The optimized reaction required
at least 3 mol equiv of oxone and 15 mg (5 mol % of I and 0.5 mol %
of Ru) of catalyst 5 for the oxidation of alcohols.
86
100
100
92
87
86
85
98
98
92
87
47d
2
4.5
3.5
4
10
10
MCPBA (1.5)
AcOOH (1.5)
a
All reactions were performed at room temperature using 1-phenylethanol
(0.2 mmol), catalyst 5 (10–45 mg; 0.006–0.027 mmol of I and 0.0006–0.0027 mmol
Ru), and oxidant (1.5–3.0 equiv) in MeCN/H2O (1:1 v/v) unless otherwise noted.
b
Based on GC–MS analysis.
Yields of isolated products.
In addition to product 8, 1-acetoxy-1-phenylethane was isolated in 38% yield.
c
d
(Table 2, entries 5–13). However, depending on the substituent
on the benzene ring, different types of products were obtained in
the reactions of primary benzylic alcohols; for example, the oxida-
tion of benzyl alcohol afforded benzoic acid (Table 2, entries 1 and
2), while 4-nitrobenzyl alcohol gave exclusively 4-nitrobenzalde-
hyde (Table 2, entries 3 and 4) in excellent yields. These results dif-
fered from the results reported by Ishihara and co-workers8a and
by our group.2b Both catalysts 5 and 6 showed similar catalytic
activity in the oxidation of alcohols.
Various alcohols 9 were smoothly oxidized to corresponding
products 10 or 11 under optimized reaction conditions in excellent
yields at room temperature (Table 2). Similar to the high-temper-
ature ArI/oxone procedure,8a secondary benzylic alcohols and ali-
cyclic alcohols were converted to the respective ketones 10
Selective oxidation of the benzylic C–H bonds is of particular
interest for organic chemists. Recently, Vinod and co-workers re-
ported the oxidation of benzylic C–H bonds performed under

5654
X.-M. Zeng et al. / Tetrahedron Letters 52 (2011) 5652–5655
Table 2
Table 3
Oxidation of alcohols catalyzed by SiO2-supported bifunctional catalysts 5 and 6a
Effect of oxidant and catalyst 5 on the oxidation of ethylbenzenea
Oxone (3 eq.), catalyst 5 or 6
Oxidant, catalyst 5 (3-9 mol% of I
O
O
OH
O
(5 mol% of I and 0.5 mol% of Ru)
CH3CN/H2O(1:1), RT
and 0.3-0.9 mol% of Ru)
or
Ph
R2
R2
R1
R1
R1
Ph
OH
CH3CN/H2O (1:1), RT
12
8
11
9
10
Entry Cat. 5
Oxidant
(equiv)
Time
(h)
Conversionb
(%)
Yieldc
(%)
Entry
Catalyst
Alcohol
CH2OH
Product
Time (h)
2
Yieldb (%)
88
(mg)
CO2H
1
2
3
4
5
None
10
30
10
10
Oxone (3.0)
Oxone (1.5)
Oxone (6.0)
MCPBA (3.0)
AcOOH (3.0)
24
24
12
24
24
0
0
17
99
0.18
0
16
98
0
1
2
5
CH2OH
CH2OH
CO2H
CHO
0
a
6
5
2.5
9.5
90
96
All reactions were performed at room temperature using ethylbenzene
(0.2 mmol), catalyst 5 (10–30 mg; 0.006–0.018 mmol of I and 0.0006–0.0018 mmol
Ru), and oxidant (1.5–6.0 equiv) in MeCN/H2O (1:1 v/v) unless otherwise noted.
b
Based on GC–MS analysis.
Yields of isolated products.
c
3
4
24 h (Table 3, entries 1, 4, and 5). The reaction also proceeded very
slowly when smaller quantities of oxone were used (Table 3, entry
2). The optimized reaction conditions led to 99% conversion of sub-
strate 12 to product 8 and required 6.0 mol equiv of oxone over
12 h and the use of 30 mg (9 mol % of I and 0.9 mol % of Ru) catalyst
5 (Table 3, entry 3).
NO2
NO2
CHO
CH2OH
6
12
94
Results of the oxidation of several other aromatic hydrocarbons
under optimized catalytic conditions using catalyst 5 or 6 were
summarized in Table 4. In general, moderate to high isolated yields
of aromatic ketones were obtained in these oxidations under very
mild reaction conditions (Table 4, entries 1–10). Compared with
the high-temperature IBX/oxone procedure,2a our protocol was
much more selective and generally did not afford products of C–
C bond cleavage, similarly to the results reported previously for
the ArI/RuCl3 tandem catalytic system.2b Interestingly, toluene
was oxidized in 2 h with the major product being benzaldehyde;
however, with the reaction time extended to 24 h, benzaldehyde
is almost totally converted to benzoic acid (Table 4, entries 11
and 12). Moreover, catalyst 6 showed noticeably higher catalytic
reactivity in the oxidation of anthrancene and indane compared
to catalyst 5 (Table 4, entries 6–9). Recycled catalyst 6 showed
slightly lower activity compared to the fresh catalyst (Table 4,
NO2
OH
NO2
O
5
6
7
8
9
5
2
98
97
86
79
96
Ph
Ph
O
OH
OH
OH
OH
6
1.5
1.5
5
Ph
Ph
Ph
Ph
Ph
O
5c
5
Ph
O
Ph
O
6
1
Ph
O
O
OH
OH
10
11
5
6
3.5
2.5
92
88
Table 4
Oxidation of aromatic hydrocarbons catalyzed by SiO2-supported bifunctional
catalysts 5 and 6a
Oxone (6 eq.), catalyst 5 or 6
OH
OH
12
13
5
6
O
O
1.5
1.5
92
81
O
O
(9 mol% of I and 0.9 mol% of Ru)
or
Ar
R
Ar
Ar
R
OH
CH3CN/H2O(1:1), RT
13
15
14
a
All reactions were performed at room temperature using alcohol (0.2 mmol),
Entry Catalyst Substrate
Product
Time
(h)
Yieldb
(%)
catalyst 5 or 6 (15 mg, 0.009 mmol of I and 0.0009 mmol Ru), and oxone (0.6 mmol)
in MeCN/H2O (1:1 v/v) unless otherwise noted.
b
Yields of isolated products.
Recycled catalyst 5 was used.
O
Ph
c
1
2
3
4
5
5
12
12
12
24
12
98
94
71
59
58
Ph
O
Ph
Ph
Ph
Ph
catalytic conditions by IBX formed in situ using oxone as a terminal
oxidant at 70–80 °C for 8–48 h.2a Our group has also reported the
oxidation of C–H bonds by using the ArI/RuCl3 tandem catalytic
system at room temperature under mild conditions.2b We investi-
gated the catalyzed oxidation of C–H bonds using the SiO2-
supported iodoarene–RuCl3 bifunctional catalyst 5. The reaction
was optimized using ethylbenzene 12 as a model substrate
(Table 3). As expected, in the absence of catalyst 5 and in the
presence of oxone, or in the presence of catalyst 5 and MCPBA or
peracetic acid the oxidation of substrate 12 at room temperature
did not occur or did not show any measurable conversion after
6
Ph
O
6c
5
Ph
O
Ph
O
6
Ph

X.-M. Zeng et al. / Tetrahedron Letters 52 (2011) 5652–5655
5655
support of his research program (F.C.P., G.K. 02.740.11.5211;
Zayavka 2010-1.5-000-010-044). This work was supported by a re-
search grant from the National Science Foundation (CHE-1009038).
Table 4 (continued)
Entry Catalyst Substrate
Product
Time
(h)
Yieldb
(%)
O
O
Supplementary data
6
7
5
6
24
12
68
92
Supplementary data (experimental procedures and 1H and 13C
NMR spectra for representative products) associated with this Let-
ter can be found, in the online version, at doi:10.1016/j.tetlet.
2011.08.097.
O
References and notes
8
5
6
6
24
25
83
77
1. (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic Compounds;
Academic Press: New York, 1981; (b) Hudlicky, M. Oxidations in Organic
Chemistry; American Chemical Society: Washington, DC, 1990; (c)Bäckvall, J.-E.,
Ed.Modern Oxidation Methods; Wiley-VCH: Weinheim, Germany, 2004.
2. (a) Ojha, L. R.; Kudugunti, S.; Maddukuri, P. P.; Kommareddy, A.; Gunna, M. R.;
Dokuparthi, P.; Gottam, H. B.; Botha, K. K.; Parapati, D. R.; Vinod, T. K. Synlett
2009, 117–121; (b) Yusubov, M. S.; Zagulyaeva, A. A.; Zhdankin, V. V. Chem. Eur.
J. 2009, 15, 11091–11094; (c) Stang, P. J.; Zhdankin, V. V. Chem. Rev. 1996, 96,
1123–1178; (d) Shim, W. G.; Jung, S. C.; Seo, S. G.; Kim, S. C. Catal. Today 2011,
164, 500–506.
3. (a) Varvoglis, A. Hypervalent Iodine in Organic Synthesis; London: Academic
Press, 1997; (b)Hypervalent Iodine Chemistry; Wirth, T., Ed.; Springer: Berlin,
2003; (c) Koser, G. F Aldrichimica Acta 2001, 34, 89–102; (d) Koser, G. F. Adv.
Heterocycl. Chem. 2004, 86, 225–292; (e) Moriarty, R. M. J. Org. Chem. 2005, 70,
2893–2903; (f) Quideau, S.; Pouysegu, L.; Deffieux, D. Synlett 2008, 467–495;
(g) Ladziata, U.; Zhdankin, V. V. ARKIVOC 2006, 26–58; (h) Ciufolini, M. A.;
Braun, N. A.; Canesi, S.; Ousmer, M.; Chang, J.; Chai, D. Synthesis 2007, 3759–
3772; i Zhdankin, V. V. Science of Synthesis 2007, 31a, 161–234. Chapter 31.4.1;
(j) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2008, 108, 5299–5358; (k) Ladziata,
U.; Zhdankin, V. V. Synlett 2007, 527–537; (l) Ochiai, M.; Miyamoto, K. Eur. J.
Org. Chem. 2008, 4229–4239; (m) Ngatimin, M.; Lupton, D. W. Aust. J. Chem.
2010, 63, 653–658; (n) Yusubov, M. S.; Nemykin, V. N.; Zhdankin, V. V.
Tetrahedron 2010, 66, 5745–5752; (o) Quideau, S.; Wirth, T. Tetrahedron 2010,
66, 5737–5738; (p) Uyanik, M.; Ishihara, K. Aldrichimica Acta 2010, 43, 83–91;
(q) Merritt, E. A.; Olofsson, B. Angew. Chem., Int. Ed. 2009, 48, 9052–9070; (r)
Merritt, E. A.; Olofsson, B. Synthesis 2011, 517–538; (s) Zhdankin, V. V. J. Org.
Chem. 2011, 76, 1185–1197.
O
O
9
6
O
O
10
12
O
15
O
d
11
12
5
6
PhCH3
PhCH3
PhCO2H
PhCHO
PhCO2H
PhCHO
24
24
85
d
80
4. (a) Richardson, R. D.; Wirth, T. Angew. Chem., Int. Ed. 2006, 45, 4402–4404; (b)
Zhdankin, V. V. ARKIVOC 2009, 1–62; (c) Dohi, T.; Kita, Y. Chem. Commun. 2009,
2073–2085; (d) Uyanik, M.; Ishihara, K. Chem. Commun. 2009, 2086–2099; (e)
Mueller, P.; Godoy, J. Tetrahedron Lett. 1981, 22, 2361–2364; (f) Dohi, T.; Kita, Y.
Kagaku (Kyoto, Japan) 2006, 61, 68–69; (g) Yakura, T.; Yamauchi, Y.; Tian, Y.;
Omoto, M. Chem. Pharm. Bull. 2008, 56, 1632–1634; (h) Yakura, T.; Ozono, A.
Adv. Synth. Catal. 2011, 353, 855–859; (i) Miura, T.; Nakashima, K.; Tada, N.;
Itoh, A. Chem. Commun. 2011, 47, 1875–1877.
a
All reactions were performed at room temperature using alkylarene (0.2 mmol),
catalyst 5 or 6 (30 mg, 0.018 mmol of I and 0.0018 mmol of Ru), and oxone
(1.2 mmol) in MeCN/H2O (1:1 v/v) unless otherwise noted.
b
Yields of isolated products.
Recycled catalyst 6 was used.
c
d
Preparative yield of benzoic acid; benzaldehyde was present as a minor product
(under 10% as estimated by 1H NMR of reaction mixture).
5. Lenoir, D. Angew. Chem., Int. Ed. 2006, 45, 3206–3210.
6. Constable, D. J. C.; Curzons, A. D.; Cunningham, V. L. Green Chem. 2002, 4, 521–
527.
7. (a) Yusubov, M. S.; Chi, K.-W.; Park, J. Y.; Karimov, R.; Zhdankin, V. V.
Tetrahedron Lett. 2006, 47, 6305–6308; (b) Koposov, A. Y.; Karimov, R. R.;
Pronin, A. A.; Skrupskaya, T.; Yusubov, M. S.; Zhdankin, V. V. J. Org. Chem. 2006,
71, 9912–9914.
8. (a) Uyanik, M.; Akakura, M.; Ishihara, K. J. Am. Chem. Soc. 2009, 131, 251–262;
(b) Thottumkara, A. P.; Bowsher, M. S.; Vinod, T. K. Org. Lett. 2005, 7, 2933–
2936; (c) Schulze, A.; Giannis, A. Synthesis 2006, 257–260; (d) T; Yakura, T.;
Konishi, T. Synlett 2007, 765–768; (e) Yakura, T.; Tian, Y.; Yamauchi, Y.; Omoto,
M.; Konishi, T. Chem. Pharm. Bull. 2009, 57, 252–256.
9. (a) Minakata, S.; Komatsu, M. Chem. Rev. 2009, 109, 711–724; (b) Shamim, T.;
Paul, S. Catal. Lett. 2010, 136, 260–265; (c) Gu, Y.; Ogawa, C.; Kobayashi, J.; Mori,
Y.; Kobayashi, S. Angew. Chem., Int. Ed. 2006, 45, 7217–7220.
10. (a) Paul, S.; Clark, J. H. J. Mol. Catal. A Chem. 2004, 215, 107–111; (b) Choudhary,
D.; Paul, S.; Gupta, R.; Clark, J. H. Green Chem. 2006, 8, 479–482; (c) Shamim, T.;
Gupta, M.; Paul, S. J. Mol. Catal. A Chem. 2009, 302, 15–19; (d) Shamim, T.;
Choudhary, D.; Mahajan, S.; Gupta, R.; Paul, S. Catal. Commun. 2009, 10, 1931–
1935.
entry 3), which was similar to the activity of recovered catalyst 5
(Table 2, entry 7).
In summary, we designed and synthesized novel bifunctional
SiO2-supported iodoarene–RuCl3 catalysts bearing two catalytic
sites, the iodoarene and RuCl3 moieties. These catalysts were dem-
onstrated to be useful for the efficient and environmentally benign
oxidation of alcohols and aromatic hydrocarbons to corresponding
carbonyl compounds using oxone as a stoichiometric oxidant. Due
to the mild reaction conditions, our protocol is highly selective and
generally does not afford products of the C–C bond cleavage.
Moreover, these bifunctional catalysts can be recovered by simple
filtration and directly reused.
11. (a) Chen, J.-M.; Zeng, X.-M.; Middleton, K.; Zhdankin, V. V. Tetrahedron Lett.
2011, 52, 1952–1955; (b) Chen, J.-M.; Zeng, X.-M.; Zhdankin, V. V. Synlett 2010,
2771–2774; (c) Yusubov, M. S.; Zhdankin, V. V. Mendeleev Commun. 2010, 20,
185–191; (d) Chen, J.-M.; Huang, X. Synthesis 2004, 2459–2462; (e) Chen, J.-M.;
Huang, X. Synthesis 2004, 1577–1580.
Acknowledgments
J.-M. Chen thanks the financial Foundation of Jiaxing University
for supporting his visit to the University of Minnesota Duluth.
Viktor Zhdankin is thankful to the Government of Russia for
12. Marcotullio, M. C.; Epifano, F.; Curini, M. Trends Org. Chem. 2003, 10, 21–34.