Coupling of nitrogen heteroaromatics and alkanes without transition metals: A new oxidative cross-coupling at C-H/C-H bonds

Article abstract of DOI:10.1002/chem.200801893

The coupling of nitrogen heteroaromatics and alkanes as an example of new oxidative cross-coupling at two different C-H bonds in the absence of a transition metal was reported. On the basis of nitrogen-atom activation, it is found that pyridine N-oxide de

Full text of DOI:10.1002/chem.200801893

COMMUNICATION
DOI: 10.1002/chem.200801893
Coupling of Nitrogen Heteroaromatics and Alkanes without Transition
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Metals: A New Oxidative Cross-Coupling at C H/C H Bonds
Guojun Deng,[b] Kirika Ueda,[a] Shuichi Yanagisawa,[a] Kenichiro Itami,*[a] and
Chao-Jun Li*[b]
À
À
The direct conversion of C H bonds into C C bonds can
potentially lead to more efficient synthesis and provide ben-
efits in terms of resource conservation and environmental
sustainability.[1] Since the seminal work reported by Murai,[2]
À
extensive research has been carried out on C C bond for-
Scheme 1. Transition-metal-free oxidative cross-coupling of N-heteroaro-
matics and alkanes.
À
mation through transition-metal-catalyzed C H bond func-
tionalization.[3,4] We[5] and others[6] have developed various
À
methods to generate C C bonds directly from two different
À
C H bonds (cross-dehydrogenative-coupling, CDC) in the
Very recently, we demonstrated that the biaryl coupling
of electron-deficient nitrogen heteroaromatics and haloar-
presence of an oxidizing reagent and a transition metal.
À
À
Since these reactions proceed best with activated C H
bonds, including those of 1,3-dicarbonyls, alkynes, and nitro-
enes (C H bond arylation) can be promoted by potassium
tert-butoxide alone, without the addition of any exogenous
transition-metal species.[8] A typical example using pyrazine
is shown in Scheme 2. Several control experiments indicate
that the coupling involves the KOtBu-induced generation of
an aryl radical from haloarene, followed by homolytic aro-
matic substitution[9] or SRN1 reaction.[10] Furthermore, we ob-
served the formation of 2-cyclohexylpyrazine when the reac-
alkanes, such cross-coupling involving an unactivated sp3-hy-
bridized C H bond has been a challenging target. Very re-
cently, we have established that the CDC reactions of
simple unactivated alkanes with 1,3-dicarbonyl compounds
or 2-pyridylbenzene derivatives can be promoted by iron
and ruthenium catalysts.[7] However, in all previous reports
À
À
À
of C C bond formations by C H activation of alkanes, the
use of a transition-metal catalyst is essential both for activat-
À
ing the alkyl C H bond and for subsequent coupling to
À
form C C bond. Herein, we report on the coupling of nitro-
gen heteroaromatics and alkanes as an example of new oxi-
À
dative cross-coupling at two different C H bonds in the ab-
sence of a transition metal (Scheme 1).
[a] K. Ueda, S. Yanagisawa, Prof. Dr. K. Itami
Department of Chemistry
Graduate School of Science, Nagoya University
and
PRESTO, Japan Science and Technology Agency (JST)
Chikusa, Nagoya 464-8602 (Japan)
Fax : (+81)52-788-6098
E-mail: itami@chem.nagoya-u.ac.jp
[b] Dr. G. Deng, Prof. Dr. C.-J. Li
Department of Chemistry, McGill University, Montreal
QC, H3A 2 K6 (Canada)
Fax : (+1)514-398-3797
E-mail: cj.li@mcgill.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801893.
Scheme 2. KOtBu-promoted arylation and alkylation of nitrogen hetero-
aromatics.
Chem. Eur. J. 2009, 15, 333 – 337
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333

tion was conducted in the presence of cyclohexane
(Scheme 2). This last reaction most likely proceeds through
cyclohexyl radical, generated by the radical exchange reac-
tion of phenyl radical and cyclohexane.[11]
The successful coupling of nitrogen heteroaromatics and
alkanes not only supports the radical nature of previous
KOtBu-promoted biaryl coupling,[8] but also serves as a
starting point toward the development of new transition-
In view of the importance of pyridine N-oxide derivatives
in medicinal chemistry[14] as well as their facile conversion
(deoxygenation)[13] to the more fertile pyridine deriva-
tives,[15] further optimization of reaction conditions was con-
ducted (Table 1). In this study, pyridine N-oxide (1a) and
À
metal-free cross-coupling at two different C H bonds. How-
ever, at the early stage of investigation, we became aware of
two critical drawbacks of the present KOtBu/PhI system: 1)
indirect generation of alkyl radical and 2) lack of regioselec-
tivity for some heteroaromatics (e.g., the reaction of pyri-
dine and cyclohexane produces 2-, 3-, and 4-cyclohexylpyri-
dines in 5, 5, and 10% yield, respectively). In an effort to
find a more efficient and direct method to generate alkyl
radical from alkane, peroxides have been chosen as a poten-
tial promoter. However, the cross-coupling of pyridine and
cyclohexane does not proceed under the influence of perox-
ides such as tBuOOtBu [Eq. (1)]. The coordination of nitro-
Table 1. Optimization of reaction conditions.[a]
Peroxide (equiv)
T [oC]
Yield [%][b]
1
135
40 (5a trace)
2
3
4
5
3a (2.0)
3a (3.0)
3a (2.0)
3a (2.0)
135
135
120
100
77 (3.5:1)
66 (1:2.3)
59 (11:1)
trace
6
135
6 (5a trace)
gen to potassium ion has been suspected as an additional
promoting factor for KOtBu in the previous two coupling
reactions (Schemes 2 and 3).[12]
7
8
9
135
135
135
48 (7.0:1)
16 (5a trace)
trace
[a] Conditions: 1a (0.5 mmol), 2a (9.2 mmol, 1.0 mL), 3 (0.5–1.5 mmol),
100–1358C, 15 h, under air. [b] Determined by 1H NMR spectroscopy
using 1,2-dichloroethane as an internal standard.
Scheme 3. Enhancement of reactivity and selectivity by “nitrogen atom
activation” of pyridine ring.
cyclohexane (2a) were used as model substrates. As men-
tioned earlier, 2,6-dicyclohexylated pyridine N-oxide (4a) is
obtained in 40% yield when the reaction is conducted with
tBuOOtBu (3a) at 1358C for 15 h under air (Table 1,
entry 1). Increasing the amount of 3a (2–3 equiv) results in
the formation of both di- and trialkylation products (4a and
5a) in good combined yield (entries 2 and 3). In particular,
5a becomes a major product when three equivalents of 3a
are employed. Although the coupling occurs at 1208C with
reasonable efficiency and selectivity, further decrease of re-
action temperature to 1008C results in nearly no reaction
(entries 4 and 5). The choice of peroxide promoter is also
important. The reactions using cumyl tert-butyl peroxide
(3b), dicumyl peroxide (3c), and tert-butyl peroxybenzoate
(3d) result in much lower product yields (entries 6–8). Only
trace amount of coupling product is obtained with tert-butyl
hydroperoxide (3e) (entry 9).
On the basis of our hypothesis that “nitrogen-atom activa-
tion” might be a key to achieving the desired coupling, we
found that pyridine N-oxide derivatives are superior sub-
strates for the radical-based arene/alkane cross-coupling.
For example, the reaction of pyridine N-oxide (1.0 equiv)
and cyclohexane (18 equiv) in the presence of tBuOOtBu
(1.0 equiv) at 1358C for 15 h furnishes 2,6-dialkylated prod-
uct in 40% yield [Eq. (2)], whereas only trace product was
obtained when pyridine reacts with cyclohexane under simi-
lar conditions and no change was observed by increasing the
amount of peroxide to three equivalents [Eq. (1)]. The addi-
tional N-oxide moiety clearly enhances both the reactivity
and regioselectivity (Scheme 3). It should be noted that effi-
cient alkylation method of pyridine N-oxide is still rare in
comparison to the recently emerging arylation/alkenylation
method using transition metal catalysts.[13]
With the optimized conditions in hand, the scope of the
reaction with respect to pyridine N-oxide derivatives and cy-
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Chem. Eur. J. 2009, 15, 333 – 337

Cross-Coupling Reactions
COMMUNICATION
cloalkanes was investigated (Tables 2 and 3). Under the in-
fluence of tBuOOtBu (3a), various electronically and struc-
turally diverse pyridine N-oxide derivatives (1a–1h) react
with cyclohexane (2a) to give the corresponding alkylated
products (4 and 5) in good yields (Table 2). The coupling
takes place with various electron-donating and withdrawing
substituents on the pyridine ring of 1 without affecting the
efficiency (entries 2–7). With a substituent at 4-position, the
2,6-dialkylation product (4) is predominantly produced (en-
tries 2–4). With a substituent at 2-position, the monoalkyla-
tion product becomes the major product (entries 5–7). The
N-oxides of other nitrogen heteroaromatics such as quino-
line N-oxide derivative 1h also react smoothly with 2a
(entry 8). It should be noted that the reaction always takes
place on the oxygenated N-heteroaromatic ring leaving
other aromatic rings on the substrate intact (entries 4, 6–8).
À
À
The present C H/C H cross-coupling reaction occurs
3
À
with a range of sp -hybridized C H bonds. Representative
results using cycloalkanes (2) as coupling partners for pyri-
dine N-oxide (1a) are shown in Table 3. In a similar manner
to the reaction of 2a, cyclooctane (2b) and cycloheptane
(2c) react smoothly with 1a (entries 1 and 2). Presumably
because of the low boiling point of cyclopentane (2d), its re-
action furnishes 2,6-dialylation product 4k selectively
(entry 3). The reactions also take place with norbornane
(2e) and 1,4-dioxane (2 f) to give structurally interesting
molecules in good yields (entries 4 and 5).
Table 2. Cross-coupling of nitrogen heterocycles (1) with cyclohexane (2a).[a]
N-Heterocycles
Products
Isolated
yield
[%]
1
70[b]
In summary, transition-metal-free systems for the cross-
coupling reactions of nitrogen heteroaromatics and alkanes
Table 3. Cross-coupling of pyridine N-oxide (1a) with cycloalkanes (2).[a]
2
3
55
66
Alkanes
Products
Isolated
yield [%]
1
71[b]
4
5
65
75
2
3
4
61[c]
40[d]
56[e]
6
7
8
81[c]
71[d]
72[e]
5
61[f]
[a] Conditions: 1a (0.5 mmol), 2 (9.2 mmol), 3a (1.0 mmol), 1358C, 15 h,
under air. [b] 4i:5i=6:1. [c] 4j:5j=4:1. [d] 5k trace. [e] 4l:5l=6:1.
[f] 4m:5m=2.2:1.
[a] Conditions: 1 (0.5 mmol), 2a (9.2 mmol, 1.0 mL), 3a (1.0 mmol), 1358C,
15 h, under air. [b] 4a:5a=4.8:1. [c] 4 f: 5 f=2.2:1. [d] 5g trace. [e] 4h:5h=
2:1.
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C.-J. Li, K. Itami et al.
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[3] For reviews on the reactions of C H bonds, see: a) V. Ritleng, C.
Sirlin, M. Pfeffer, Chem. Rev. 2002, 102, 1731; b) G. Dyker, Angew.
Chem. 1999, 111, 1808; Angew Chem. Int. Ed. 1999, 38, 1698; c) T.
Naota, H. Takaya, S. I. Murahashi, Chem. Rev. 1998, 98, 2599; d) D.
Alberico, M. E. Scott, M. Lautens, Chem. Rev. 2007, 107, 174;
e) L. C. Campeau, D. R. Stuart, K. Fagnou, Aldrichimica Acta 2007,
40, 35; f) K. Godula, D. Sames, Science 2006, 312, 67.
are described. Under the influence of tBuOOtBu, pyridine
N-oxide derivatives react with alkanes to furnish the corre-
sponding cross-coupling products (alkylated nitrogen hetero-
cycles) in good yields. We believe that the present oxidative
À
cross-coupling reactions at two different C H bonds not
only contributes to the realization of “greener” synthesis,
but also unlocks opportunities for markedly different strat-
egies in chemical synthesis. Moreover, in view of the current
strict guidelines limiting transition-metal levels in pharma-
[4] For selected recent examples, see: a) L. Ackermann, A. Althammer,
R. Born, Angew. Chem. 2006, 118, 2681; Angew. Chem. Int. Ed.
2006, 45, 2619; b) C. Lenges, M. Brookhart, J. Am. Chem. Soc. 1999,
121, 6616; c) F. Kakiuchi, S. Kan, K. Igi, N. Chatani, S. Murai, J.
Am. Chem. Soc. 2003, 125, 1698; d) Y. G. Lim, J. A. Ahn, C. H. Jun,
Org. Lett. 2004, 6, 4687; e) R. K. Thalji, J. A. Ellman, R. G. Berg-
man, J. Am. Chem. Soc. 2004, 126, 7192; f) A. Lazareva, O. Daugu-
lis, Org. Lett. 2006, 8, 5211; g) M. D. K. Boele, G. P. F. van Strij-
donck, A. H. M. de Vries, P. C. J. Kamer, J. G. de Vries, P. W. N. M.
van Leeuwen, J. Am. Chem. Soc. 2002, 124, 1586; h) S. J. Tremont,
H. ur Rahman, J. Am. Chem. Soc. 1984, 106, 5759; i) V. G. Zaitsev,
O. Daugulis, J. Am. Chem. Soc. 2005, 127, 4156; j) D. Kalyani, N. R.
Deprez, L. V. Desai, M. S. Sanford, J. Am. Chem. Soc. 2005, 127,
7330; k) A. R. Dick, K. L. Hull, M. S. Sanford, J. Am. Chem. Soc.
2004, 126, 2300; l) R. Giri, X. Chen, J. Q. Yu, Angew. Chem. 2005,
117, 2150; Angew. Chem. Int. Ed. 2005, 44, 2112; m) R. Giri, J.
Liang, J. G. Lei, J. J. Li, D. H. Wang, X. Chen, I. C. Naggar, C. Guo,
B. M. Foxman, J. Q. Yu, Angew. Chem. 2005, 117, 7586; Angew.
Chem. Int. Ed. 2005, 44, 7420; n) S. Yanagisawa, T. Sudo, R. Noyori,
K. Itami, J. Am. Chem. Soc. 2006, 128, 11748; o) S. I. Murahashi, N.
Komiya, H. Terai, T. Nakae, J. Am. Chem. Soc. 2003, 125, 15312;
p) S. I. Murahashi, N, Komiya, H. Terai, Angew. Chem. 2005, 117,
7091; Angew. Chem. Int. Ed. 2005, 44, 6931; q) X. Chen, J. J. Li,
X. S. Hao, C. E. Goodhue, J. Q. Yu, J. Am. Chem. Soc. 2006, 128,
78; r) X. Chen, C. E. Goodhue, J. Q. Yu, J. Am. Chem. Soc. 2006,
128, 12634; s) R. Giri, N. Maugel, J. J. Li, D. H. Wang, S. P. Breazza-
no, L. B. Saunders, J. Q. Yu, J. Am. Chem. Soc. 2007, 129, 3510; t) O.
Daugulis, V. G. Zaitsev, Angew. Chem. 2005, 117, 4114; Angew.
Chem. Int. Ed. 2005, 44, 4046; u) V. G. Zaitsev, D. Shabashov, O.
Daugulis, J. Am. Chem. Soc. 2005, 127, 13154; v) H. A. Chiong, O.
Daugulis, Org. Lett. 2007, 9, 1449.
ceuticals,[16] the realization of C C bond-forming reactions
À
without using a transition metal is noteworthy. The elucida-
tions of reaction mechanism and full scope of present meth-
odology are the focus of ongoing research efforts.
Experimental Section
A representative experimental procedure (4a and 5a): A reaction vessel
was charged with pyridine N-oxide (1a, 47.5 mg, 0.5 mmol), tert-butyl
peroxide (3a, 146 mg, 1.0 mmol) and cyclohexane (2a, 1.0 mL,
9.2 mmol). Then the reaction vessel was sealed and the resulting solution
was stirred at 1358C for 15 h. After cooling to room temperature, the re-
sulting mixture was removed in vacuo and the residue was purified by
column chromatography (SiO2, hexane/ethyl acetate 6:1) to give 4a
(75 mg, 58%) and 5a (21 mg, 12%) as pale yellow oils.
Data for 4a: 1H NMR (400 MHz, CDCl3): d=7.16–7.12 (m, 1H), 7.03–
7.02 (m, 2H), 3.55–3.47 (m, 2H), 2.04–2.00 (m, 4H), 1.83–1.73 (m, 6H),
1.53–1.41ACHTUNGTRENNUNG
(m, 4H), 1.29–1.17 ppm (m, 6H); 13C NMR (100 MHz, CDCl3):
d=156.7, 125.4, 120.2, 38.1, 31.2, 26.7, 26.6 ppm; IR (liquid film): n˜ =
3076, 2912, 2846, 1520, 1442, 1389, 1230, 835, 749 cmÀ1; MS (EI): m/z
(%): 259, 242 (100), 214, 188, 175, 158, 144, 119, 91, 77; HRMS calcd for
C17H25NO: 259.1936; found: 259.1933.
Data for 5a: 1H NMR (400 MHz, CDCl3): d=6.85 (s, 2H), 3.56–3.48 (m,
2H), 2.45–2.41 (m, 1H), 2.05–2.01 (m, 4H), 1.84–1.72 (m, 11H), 1.58–
1.42 (m, 4H), 1.32–1.18 ppm (m, 11H); 13C NMR (100 MHz, CDCl3): d=
155.9, 146.1, 118.4, 43.8, 38.2, 33.9, 31.4, 26.8, 26.6, 26.1 ppm; IR (liquid
film): n˜ =2925, 2846, 1553, 1454, 1415, 1270, 1237, 845 cmÀ1; MS (EI):
m/z (%): 341, 324 (100), 296, 270, 257, 214; HRMS calcd for C23H35NO:
341.2719; found: 341.2716.
[5] a) Z. Li, C.-J. Li, J. Am. Chem. Soc. 2004, 126, 11810; b) Z. Li, C.-J.
Li, Org. Lett. 2004, 6, 4997; c) Z. Li, C.-J. Li, Eur. J. Org. Chem.
2005, 3173; d) Z. Li, C.-J. Li, J. Am. Chem. Soc. 2005, 127, 6968;
e) Z. Li, C.-J. Li, J. Am. Chem. Soc. 2005, 127, 3672; f) Z. Li, C.-J.
Li, J. Am. Chem. Soc. 2006, 128, 56; g) Y. Zhang, C.-J. Li, J. Am.
Chem. Soc. 2006, 128, 4242; h) Y. Zhang, C.-J. Li, Angew. Chem.
2006, 118, 1983; Angew. Chem. Int. Ed. 2006, 45, 1949; i) O. Baslꢁ,
C.-J. Li, Green Chem. 2007, 9, 1047; j) Z. Li, D. S. Bohle, C.-J. Li,
Proc. Natl. Acad. Sci. USA 2006, 103, 8928; k) Z. Li, L. Cao, C.-J.
Li, Angew. Chem. 2007, 119, 6625; Angew. Chem. Int. Ed. 2007, 46,
6505.
The experiments in Tables 2 and 3 were carried out analogously. All
products were purified by column chromatography and characterized by
NMR spectroscopy and standard/high-resolution mass spectrometry.
[6] a) L. Eberson, L. Gomez-Gonzalez, Acta Chem. Scand. 1973, 27,
1249; b) H. Yoshimoto, H. Itatani, Bull. Chem. Soc. Jpn. 1973, 46,
2490; c) K. Masui, H. Ikegami, A. Mori, J. Am. Chem. Soc. 2004,
126, 5074; d) D. R. Stuart, K. Fagnou, Science 2007, 316, 1172;
e) D. R. Stuart, E. Villemure, K. Fagnou, J. Am. Chem. Soc. 2007,
129, 12072; f) T. Dohi, M. Ito, K. Morimoto, M. Iwata, Y. Kita,
Angew. Chem. 2008, 120, 1321; Angew Chem. Int. Ed. 2008, 47,
1301; g) T. A. Dwight, N. R. Rue, D. Charyk, R. Josselyn, B.
DeBoef, Org. Lett. 2007, 9, 3137; h) B. Li, S. Tian, Z. Fang, Z. Shi,
Angew. Chem. 2008, 120, 1131; Angew Chem. Int. Ed. 2008, 47,
1115; i) K. L. Hull, M. S. Sanford, J. Am. Chem. Soc. 2007, 129,
11904; j) J.-B. Xia, S.-L. You; Organometallics 2007, 26, 4869; k) G.
Cai, Y. Fu, Y. Li, X. Wan, Z.-J. Shi, J. Am. Chem. Soc., 2007, 129,
7666.
Acknowledgements
We are grateful to PRESTO program of Japan Science and Technology
Agency (to K.I.), a Grant-in-Aid for Scientific Research from MEXT,
Japan (to K.I.), Canada Research Chair (Tier I) foundation (to C.J.L.),
the NSF-EPA joint program for a Sustainable Environment (to C.J.L.),
and NSERC (to C.J.L.) for support of our research. S.Y. is a recipient of
the JSPS Predoctoral Fellowships for Young Scientists.
À
À
Keywords: alkanes · C C coupling · C H activation ·
nitrogen heterocycles
[7] a) Y. Zhang, C.-J. Li, Eur. J. Org. Chem. 2007, 4654; b) G. Deng, L.
Zhao, C.-J. Li, Angew. Chem. 2008, 120, 6374; Angew. Chem. Int.
Ed. 2008, 47, 6278.
[1] P. T. Anastas, J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, New York, 1998.
[8] S. Yanagisawa, K. Ueda, T. Taniguchi, K. Itami, Org. Lett. 2008, 10,
4673.
[2] S. Murai, F. Kakiuchi, S. Sekine, Y. Tanaka, A. Kamatani, M.
Sonoda, N. Chatani, Nature 1993, 366, 529.
336
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Cross-Coupling Reactions
COMMUNICATION
[9] A. Studer, M. Bossart, in Radicals in Organic Synthesis Vol. 2 (Eds.:
P. Renaud, M. P. Sibi), Wiley-VCH, Weinheim, 2001, p. 62.
[10] J. F. Bunnett, Acc. Chem. Res. 1978, 11, 413.
[11] a) R. G. Kryger, J. P. Lorand, N. R. Stevens, N. R. Herron, J. Am.
Chem. Soc. 1977, 99, 7589; b) J. C. Scaiano, L. C. Stewart, J. Am.
Chem. Soc. 1983, 105, 3609.
[12] Such a coordination effect might be closely related to the Minisciꢂs
seminal work demonstrating the enhanced reactivity of protonated
pyridines toward nucleophilic radicals; for reviews, see: a) F. Minis-
ci, Synthesis 1973, 1; b) F. Minisci, Top. Curr. Chem. 1976, 62, 1; c) F.
Minisci, E. Vismara, F. Fontana, Heterocycles 1989, 28, 489.
[13] a) L.-C. Campeau, S. Rousseaux, K. Fagnou, J. Am. Chem. Soc.
2005, 127, 18020; b) J.-P. Leclerc, K. Fagnou, Angew. Chem. 2006,
118, 7945; Angew. Chem. Int. Ed. 2006, 45, 7781; c) K. S. Kanyiva, Y.
Nakao, T. Hiyama, Angew. Chem. 2007, 119, 9028; Angew. Chem.
Int. Ed. 2007, 46, 8872; d) L.-C. Campeau, M. Bertrand-Laperle, J.-P.
Leclerc, E. Villemure, S. Gorelsky, K. Fagnou, J. Am. Chem. Soc.
2008, 130, 3266; e) S. H. Cho, S. J. Hwang, S. Chang, J. Am. Chem.
Soc. 2008, 130, 9254.
[14] J. Balzarini, E. Keyaerts, L. Vijgen, F. Vandermeer, M. Stevens,
E. D. Clercq, H. Egberink, M. V. Ranst, J. Antimicrob. Chemother.
2006, 57, 472.
[15] T. Henkel, R. M. Brunne, H. Mꢃller, F. Reichel, Angew. Chem.
1999, 111, 688; Angew. Chem. Int. Ed. 1999, 38, 643.
[16] a) C. E. Garrett, K. Prasad, Adv. Synth. Catal. 2004, 346, 889;
b) C. M. Crudden, M. Sateesh, R. Lewis, J. Am. Chem. Soc. 2005,
127, 10045.
Received: September 15, 2008
Published online: November 26, 2008
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