Synthesis of oxazoles through copper-mediated aerobic oxidative dehydrogenative annulation and oxygenation of aldehydes and amines

Article abstract of DOI:10.1002/anie.201206382

A fragment-assembling strategy is used to form oxazoles from aryl acetaldehydes, amines, and molecular oxygen under mild conditions (see scheme). The transformation is highly efficient with the removal of six hydrogen atoms, including the cleavage of four C(sp³)-H bonds. Copyright

Full text of DOI:10.1002/anie.201206382

Angewandte
Chemie
DOI: 10.1002/anie.201206382
Oxygenation
Synthesis of Oxazoles through Copper-Mediated Aerobic Oxidative
Dehydrogenative Annulation and Oxygenation of Aldehydes and
Amines**
Zejun Xu, Chun Zhang, and Ning Jiao*
Oxazoles with their 2 and 5 positions substituted with aryl or
alkyl groups are regarded as privileged heterocyclic motifs in
numerous pharmacologically active synthetic molecules.[1]
They are also widespread in bioactive natural products.[2]
Subsequently, a lot of new methods have been developed to
form oxazoles.[3–6] Among these methods, the intramolecular
cyclization of acyclic precursors,[3] the oxidative coupling of
amines and prefunctionalized aldehydes or ketones,[4] the
oxidation of oxazolines,[5] and other methods were widely
used.[6] Practical and efficient approaches to oxazoles from
readily available starting material are still desirable.[3–6]
Recently, an elegant work for the formation of oxazoles
that are substituted at the 2 and 5 positions was reported by
Zhang and co-workers [Eq. (1)].[7] Using the fragment-
assembling strategy, they realized the [2+2+1] annulation of
trisubstituted dihydrooxazoles from prefunctionalized N-
alkylamidines [Eq. (2)]. Herein, we present an aerobic
dehydrogenative annulation and dioxygen activation
approach to oxazoles from simple and readily available
aldehydes, amines, and molecular oxygen [Eq. (3)]. This
transformation is highly efficient with the removal of six
3
À
hydrogen atoms, including the cleavage of four C(sp ) H
bonds.[10] Furthermore, the dehydrogenative coupling strat-
egy[11] and the dioxygen activation of molecular oxygen
(1 atm) make this transformation very efficient and practical.
Our studies commenced with the reactions of 2-phenyl-
acetaldehyde (1a) and benzylamine (2a) in the presence of
CuBr2 and K2CO3 in toluene at 808C under oxygen atmos-
phere (1 atm). Interestingly, 2,5-diphenyloxazole (3aa) was
produced in 65% yield (Table 1, entry 1). Other copper
salts[12] and bases exhibited lower efficiencies (see Table 1,
entries 3 and 4, respectively, and the Supporting Information).
Further studies indicated that the presence of a ligand could
promote the efficiency of this transformation (Table 1,
entries 1 and 5). Among these ligands, pyridine (20 mol%)
performed well and significantly increased the yield of 3aa to
82% (see Table 1, entry 5; for other ligands, see the Support-
ing Information). Subsequently, the effect of different sol-
vents was surveyed. The reactions in DCE or other solvents
resulted in lower yields (see Table 1, entry 8, and the
Supporting Information). Significantly, even when this reac-
tion was performed under air, it also worked well and
afforded 3aa in 70% yield (Table 1, entry 9). In contrast, this
reaction did not work under argon atmosphere (Table 1,
entry 10).
a terminal alkyne, a nitrile, and a pyridine/quinoline N-oxide
as the oxygen source. In light of the increasing demand for
environmentally benign organic synthesis and green chemis-
try, molecular oxygen is probably the ideal terminal oxidant
and oxygen atom source for oxygenation, because of its
remarkable advantages, such as being inexpensive, having
a high atom efficiency, and in most cases with water as the by-
product.[8] More recently, Chiba and co-workers[9] developed
a significant intramolecular oxygenation approach to 2,5,5-
[*] Z. Xu,[+] C. Zhang,[+] Prof. Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
Xue Yuan Rd. 38, Beijing 100191 (China)
E-mail: jiaoning@bjmu.edu.cn
Homepage: http://sklnbd.bjmu.edu.cn/nj
Prof. Dr. N. Jiao
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
[+] These authors contributed equally to this work.
[**] Financial support from the National Basic Research Program of
China (973 Program; grant 2009CB825300) and the National
Science Foundation of China (grant 21172006) are greatly appre-
ciated. We thank Hang Yin for reproducing the results of 3 fa and
3ar.
Under these optimized conditions, the scope of substi-
tuted amines (2) was investigated (Scheme 1). The substitu-
ents at the phenyl ring of benzylamines did not affect the
efficiency of this transformation, and the desired oxazole
products were produced in moderate to good yields. Notably,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206382.
Angew. Chem. Int. Ed. 2012, 51, 11367 –11370
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11367

.
Angewandte
Communications
Table 1: The oxidative dehydrogenative annulation and oxygenation of
1a with 2a.[a]
respectively. The naphthyl-substituted amine was also toler-
ated in this transformation, generating 2-(naphthalen-1-yl)-5-
phenyloxazole 3am in 89% yield. It is noteworthy that alkyl-
substituted amines were also tolerated in this reaction. For
example, amines that contained long alkyl chains gave the
products in moderate yields (3ar–3aw). Substituted phene-
thylamines could be smoothly transformed into the desired
products with good yields (3an–3ap). Furthermore, the steric
hindrance did not affect the efficiency (3aq, 3dq, and 3 fq).
The present method provides a simple and easily operable
protocol for the preparation of 2,5-diaryl- or 2,5-dialkyl-
substituted oxazole derivatives from simple and readily
available starting materials by employing molecular oxygen
as the oxygen source.
This aerobic oxidative dehydrogenative annulation was
further expanded to a range of substituted aldehydes
1 (Scheme 2). The results indicate that reactions of aldehydes
with electron-donating groups, such as methoxy, methyl, and
ethyloxy, at the aryl ring, proceeded well with moderate to
good yields. Moreover, substituents at different positions of
Entry
Cu salt
Base
Ligand
Solvent
Yield%[b]
1
CuBr2
CuBr2
CuCl2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
K2CO3
K2CO3
K2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
–
–
–
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCE
65
10
64
21
82
74
46
60
70
0
2[c]
3
4
5
pyridine
pyridine
2,2’-bipyridine
pyridine
pyridine
pyridine
6[d]
7
8
9[e]
10[f]
toluene
toluene
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), copper salt
(0.3 mmol), ligand (0.04 mmol), base (0.4 mmol), solvent (3.0 mL), O2
(1 atm), 808C, 16 h. [b] Yields of isolated products. [c] CuBr2
(0.03 mmol) was used. [d] Pyridine (0.01 mmol) was used. [e] The
reaction was carried out under air. [f] The reaction was carried out under
argon atmosphere (1 atm). Entry in bold highlights optimized reaction
conditions. DCE=dichloroethane.
Scheme 2. Oxidative dehydrogenative annulation and oxygenation of
different aldehydes (1) with 2a. [a] Reaction conditions: see Table 1,
entry 5. [b] Yields of isolated products.
the arene ring (para, meta, and ortho position) did not affect
the efficiency obviously (3ba–3da). Unfortunately, aliphatic
aldehydes did not work under these reaction conditions
(3ha).
Some control experiments were carried out in order to
probe the mechanism of this transformation. The reaction of
1a and 2a in the presence of 18O2 (1 atm) generated 18O-
labeled product [18O]-3aa in 80% yield under the standard
conditions [Eq. (4), yield determined by HRMS, see the
Supporting Information]. In contrast, the similar reaction
under 16O2 atmosphere but in the presence of H218O
(5.0 equiv) did not afford [18O]-3aa [Eq. (5), determined by
HRMS, see the Supporting Information]. Both these results
Scheme 1. Oxidative dehydrogenative annulation and oxygenation of
different amines (2) with 1a. [a] Reaction conditions: see Table 1,
entry 5. [b] Yields of isolated products. [c] The reaction was stirred for
11 h.
reactions of fluoro-, chloro-, and bromo-substituted benzyl-
amines with 1a proceeded well and gave the desired oxazole
derivative 3ah, 3ai, 3aj, and 3ak (which could be used for
further transformations) in 76%, 66%, 57%, and 74% yield,
11368 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 11367 –11370

Angewandte
Chemie
could be easily oxidized to the desired oxazole product 3aa
under oxygen atmosphere.[5f]
In conclusion, we have demonstrated the synthesis of
oxazoles through a Cu-mediated aerobic oxidative dehydro-
genative annulation of aldehydes, amines, and molecular
À
oxygen by C H functionalization and dioxygen activation.
This transformation is highly efficient with the removal of six
indicate that the oxygen atom of the oxazole product
originated from molecular oxygen. H2O did not participate
in this intermolecular cyclization process.
Furthermore, the reaction of 2-phenylacetaldehyde (1a)
and benzamide (4) under the standard conditions was
investigated. Although desired product 3aa was detected,
the yield was only 13% [Eq. (6)], which is much lower than
that of the model reaction (82%, see Table 1, entry 5). The
hydrogen atoms, including the functionalization of four
C(sp ) H bonds. Furthermore, the dehydrogenative coupling
3
À
strategy and the dioxygen activation of molecular oxygen
(1 atm) make this transformation very practical and atom-
economical. Further studies to clearly understand the reac-
tion mechanism and the synthetic applications are ongoing in
our laboratory.
Received: August 8, 2012
Published online: October 9, 2012
Keywords: annulation · copper · dehydrogenation · oxazoles ·
.
oxygen · oxygenation
reaction of 2-phenylacetaldehyde (1a) and benzonitrile (5)
under the standard conditions could not generate 3aa
[Eq. (7)], which might exclude 5 as the intermediate of this
[1] a) P. Wipf, Chem. Rev. 1995, 95, 2115; b) N. Desroy, F. Moreau, S.
Briet, G. Le Fralliec, S. Floquet, L. Durant, V. Vongsouthi, V.
Gerusz, A. Denis, S. Escaich, Bioorg. Med. Chem. 2009, 17, 1276;
c) S. Heng, K. R. Gryncel, E. R. Kantrowitz, Bioorg. Med.
Chem. 2009, 17, 3916; d) W. Wang, D. Yao, M. Gu, M. Fan, J. Li,
Y. Xing, F. Nan, Bioorg. Med. Chem. Lett. 2005, 15, 5284.
[2] a) V. Yeh, Tetrahedron 2004, 60, 11995; b) D. S. Dalisay, E. W.
Rogers, A. S. Edison, T. F. Molinski, J. Nat. Prod. 2009, 72, 732;
c) Z. Jin, Nat. Prod. Rep. 2006, 23, 464; d) Z. Jin, Nat. Prod. Rep.
2009, 26, 382; e) Z. Jin, Nat. Prod. Rep. 2011, 28, 1143.
[3] a) K. C. Nicolaou, J. Hao, M. V. Reddy, P. B. Rao, G. Rassias,
S. A. Snyder, X. Huang, D. Y.-K. Chen, W. E. Brenzovich, N.
Giuseppone, P. Giannakakou, A. OꢀBrate, J. Am. Chem. Soc.
2004, 126, 12897; b) T. Lechel, D. Lentz, H. U. Reissig, Chem.
Eur. J. 2009, 15, 5432; c) H. Takeuchi, S. Yanagida, T. Ozaki, S.
Hagiwara, S. Eguchi, J. Org. Chem. 1989, 54, 431; d) M. Keni, J. J.
Tepe, J. Org. Chem. 2005, 70, 4211; e) P. Wipf, C. P. Miller, J. Org.
Chem. 1993, 58, 3604; f) A. E. Wendlandt, S. S. Stahl, Org.
Biomol. Chem. 2012, 10, 3866.
[4] a) C. Wan, J. Zhang, S. Wang, J. Fan, Z. Wang, Org. Lett. 2010, 12,
2338; b) C. F. Wan, L. F. Gao, Q. Wang, J. T. Zhang, Z. Y. Wang,
Org. Lett. 2010, 12, 3902; c) W.-J. Xue, Q. Li, Y.-P. Zhu, J.-G.
Wang, A.-X. Wu, Chem. Commun. 2012, 48, 3485; d) J. Xie, H.
Jiang, Y. Cheng, C. Zhu, Chem. Commun. 2012, 48, 979.
[5] a) A. I. Meyers, F. Tavares, Tetrahedron Lett. 1994, 35, 2481;
b) A. J. Phillips, Y. Uto, P. Wipf, M. J. Reno, D. R. Williams, Org.
Lett. 2000, 2, 1165; c) A. I. Meyers, F. X. Tavares, J. Org. Chem.
1996, 61, 8207; d) D. R. Williams, D. P. Lowder, G.-Y. Gu, D. A.
Brooks, Tetrahedron Lett. 1997, 38, 331; e) Y. Wang, Z. Li, Y.
Huang, C. Tang, X. Wu, J. Xu, H. Yao, Tetrahedron 2011, 67,
7406; f) Y. Huang, L. Ni, F. Gan, Y. He, J. Xu, X. Wu, H. Yao,
Tetrahedron 2011, 67, 2066.
oxidative transformation. These results indicate that benza-
mide (4) may be involved in this process, but they are not the
main pathways of this dehydrogenative annulation process.
Although intensive studies of the redox reaction on the
copper salt are still ongoing, a possible reaction pathway for
the dioxygen activation and oxazole formation is proposed in
Scheme 3 on the basis of the above-mentioned results.
Aldehydes (1a) and anilines (2a) initially dehydrate to form
imines (6).[13] Under basic conditions, imines (6) should be
oxidized by molecular oxygen to form radical intermediate 7,
a process that should be facilitated by copper salt.[14] After-
ward, the 1,5-hydrogen atom abstraction would provide
intermediate 8.[9,15] Subsequent intramolecular radical cou-
pling affords the 4,5-dihydrooxazole intermediate 9, which
[6] a) I. Cano, E. ꢁlvarez, C. M. Nicasio, P. J. Pꢂrez, J. Am. Chem.
Soc. 2011, 133, 191; b) J. C. Lee, S. Kim, Y. C. Lee, Synth.
Commun. 2003, 33, 1611; c) R. Nesi, S. Turchi, D. Giomi, J. Org.
Chem. 1996, 61, 7933; d) B. A. Mendelsohn, S. Lee, S. Kim, F.
Teyssier, V. S. Aulakh, M. A. Ciufolini, Org. Lett. 2009, 11, 1539;
e) C. L. Zhong, B. Y. Tang, P. Yin, Y. Chen, L. He, J. Org. Chem.
2012, 77, 4271; f) P. G. Ferrini, A. Marxer, Angew. Chem. 1963,
75, 165; Angew. Chem. Int. Ed. Engl. 1963, 2, 99; g) E. Merkul,
T. J. J. Mꢃller, Chem. Commun. 2006, 4817; h) G. L. Young, S. A.
Smith, R. J. K. Taylor, Tetrahedron Lett. 2004, 45, 3797; i) J.
Scheme 3. Possible reaction pathway for dioxygen activation and
oxazole formation.
Angew. Chem. Int. Ed. 2012, 51, 11367 –11370
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11369

.
Angewandte
Communications
Lister, R. Robinson, J. Chem. Soc. 1912, 1297; j) M. P. Kumar,
R.-S. Liu, J. Org. Chem. 2006, 71, 4951; k) C. Kison, T. Opatz,
Chem. Eur. J. 2009, 15, 843; l) B. Shi, A. J. Blake, W. Lewis, I. B.
Campbell, B. D. Judkins, C. J. Moody, J. Org. Chem. 2010, 75,
152; m) H. Jiang, H. Huang, H. Cao, C. Qi, Org. Lett. 2010, 12,
5561; n) Y. M. Pan, F. J. Zheng, H. X. Lin, Z. P. Zhan, J. Org.
Chem. 2009, 74, 3148; o) J. J. Lee, J. Kim, Y. M. Jun, B. M. Lee,
B. H. Kim, Tetrahedron 2009, 65, 8821; p) P.-Y. Coqueron, C.
Didier, M. A. Ciufolini, Angew. Chem. 2003, 115, 1451; Angew.
Chem. Int. Ed. 2003, 42, 1411; q) During the submission, a paper
describing Cu-catalyzed [2+2+1] cycloaddition of alkynes and
nitriles for the synthesis of 1,3-oxazoles was published: X. Li, L.
Huang, H. Chen, W. Wu, H. Huang, H. Jiang, Chem. Sci. 2012,
DOI: 10.1039/C2SC21041J.
Y.-F. Wang, E. P. J. Ng, S. Chiba, J. Am. Chem. Soc. 2011, 133,
13942; f) J. Wang, J. Wang, Y. Zhu, P. Lu, Y. Wang, Chem.
Commun. 2011, 47, 3275; g) L. Zhang, G. Y. Ang, S. Chiba, Org.
Lett. 2011, 13, 1622; h) A. Hꢄusser, M. Trautmann, E. Roduner,
Chem. Commun. 2011, 47, 6954; i) C. Wꢃrtele, O. Sander, V.
Lutz, T. Waitz, F. Tuczek, S. Schindler, J. Am. Chem. Soc. 2009,
131, 7544; j) H. R. Lucas, L. Li, A. A. Narducci Sarjeant, M. A.
Vance, E. I. Salomon, K. D. Karlin, J. Am. Chem. Soc. 2009, 131,
3230; k) I. Garcia-Bosch, A. Company, J. R. Frisch, M. Torrent-
Sucarrat, M. Cardellach, I. Gamba, M. Gꢃell, L. Casella, L.
Que, Jr., X. Ribas, J. M. Luis, M. Costas, Angew. Chem. 2010,
122, 2456; Angew. Chem. Int. Ed. 2010, 49, 2406; l) S. Palavicini,
A. Granata, E. Monzani, L. Casella, J. Am. Chem. Soc. 2005, 127,
18031; m) C. Zhang, L. Zhang, N. Jiao, Adv. Synth. Catal. 2012,
354, 1293; n) Q. Liu, P. Wu, Y. Yang, Z. Zeng, J. Liu, H. Yi, A.
Lei, Angew. Chem. 2012, 124, 4744; Angew. Chem. Int. Ed. 2012,
51, 4666; o) C. Zhang, N. Jiao, J. Am. Chem. Soc. 2010, 132, 28;
for mechanistic studies, see: p) A. E. King, L. M. Huffman, A.
Casitas, M. Costas, X. Ribas, S. S. Stahl, J. Am. Chem. Soc. 2010,
132, 12068; q) A. E. King, T. C. Brunold, S. S. Stahl, J. Am.
Chem. Soc. 2009, 131, 5044; r) Y.-H. Zhang, J.-Q. Yu, J. Am.
Chem. Soc. 2009, 131, 14654; s) N. Decharin, S. S. Stahl, J. Am.
Chem. Soc. 2011, 133, 5732; t) Y. Wei, H. Zhao, J. Kan, W. Su, M.
Hong, J. Am. Chem. Soc. 2010, 132, 2522; u) E. Boess, D.
Sureshkumar, A. Sud, C. Wirtz, C. Fares, M. Klussmann, J. Am.
Chem. Soc. 2011, 133, 8106; v) P. Chen, E. I. Solomon, J. Am.
Chem. Soc. 2004, 126, 4991; w) M. P. Lanci, V. V. Simirnov, C. J.
Cramer, E. V. Gauchenova, J. Sundermeyer, J. P. Roth, J. Am.
Chem. Soc. 2007, 129, 14697; x) M. Rolff, J. Schottenheim, G.
Peters, F. Tuczek, Angew. Chem. 2010, 122, 6583; Angew. Chem.
Int. Ed. 2010, 49, 6438; y) K. Schrçder, B. Join, A. J. Amali, K.
Junge, X. Ribas, M. Costas, M. Beller, Angew. Chem. 2011, 123,
1461; Angew. Chem. Int. Ed. 2011, 50, 1425.
[7] W. He, C. Li, L. Zhang, J. Am. Chem. Soc. 2011, 133, 8482.
[8] For some reviews, see: a) T. Punniyamurthy, S. Velusamy, J.
Iqbal, Chem. Rev. 2005, 105, 2329; b) S. S. Stahl, Angew. Chem.
2004, 116, 3480; Angew. Chem. Int. Ed. 2004, 43, 3400; c) M. S.
Sigman, D. R. Jensen, Acc. Chem. Res. 2006, 39, 221; d) K. M.
Gligorich, M. S. Sigman, Angew. Chem. 2006, 118, 6764; Angew.
Chem. Int. Ed. 2006, 45, 6612; e) A. E. Wendlandt, A. M. Suess,
S. S. Stahl, Angew. Chem. 2011, 123, 11256; Angew. Chem. Int.
Ed. 2011, 50, 11062; f) Z. Shi, C. Zhang, C. Tang, N. Jiao, Chem.
Soc. Rev. 2012, 41, 3381; g) C. Zhang, C. Tang, N. Jiao, Chem.
Soc. Rev. 2012, 41, 3464.
[9] Y.-F. Wang, H. Chen, X. Zhu, S. Chiba, J. Am. Chem. Soc. 2012,
134, 11980.
À
[10] For some reviews about C H functionalization, see: a) C.-L.
Sun, B.-J. Li, Z.-J. Shi, Chem. Rev. 2011, 111, 1293; b) D. A.
Colby, R. G. Bergman, J. A. Ellman, Chem. Rev. 2010, 110, 624;
c) C. Copꢂret, Chem. Rev. 2010, 110, 656; d) T. W. Lyons, M. S.
Sanford, Chem. Rev. 2010, 110, 1147; e) I. A. I. Mkhalid, J. H.
Barnard, T. B. Marder, J. M. Murphy, J. F. Hartwig, Chem. Rev.
2010, 110, 890; f) M. P. Doyle, R. Duffy, M. Ratnikov, L. Zhou,
Chem. Rev. 2010, 110, 704; g) A. Gunay, K. H. Theopold, Chem.
Rev. 2010, 110, 1060; h) D. Balcells, E. Colt, O. Eisenstein, Chem.
Rev. 2010, 110, 749; i) F. Bellina, R. Rossi, Chem. Rev. 2010, 110,
1082; j) F. W. Patureau, F. Glorius, Angew. Chem. 2011, 123,
2021; Angew. Chem. Int. Ed. 2011, 50, 1977; k) K. M. Engle, T.-S.
Mei, M. Wasa, J.-Q. Yu, Acc. Chem. Res. 2012, 45, 788.
[13] K. Saito, K. Harada, Tetrahedron Lett. 1989, 30, 4535.
[14] a) N. A. Milas, Chem. Rev. 1932, 10, 295; b) A. Rieche, E. Hoeft,
H. Schultze, Chem. Ber. 1964, 97, 195; c) Science of Synthesis.
À
Compounds with One Saturated Carbon Heteroatom Bond.
Peroxides, Vol. 38 (Ed.: A. Berkessel), Thieme, Stuttgart, 2009,
pp. 9 – 141; d) A. G. Davies, R. V. Foster, R. Nery, J. Chem. Soc.
1954, 2204; e) H. Yamamoto, M. Akutagawa, H. Aoyama, Y.
[11] For some reviews of cross-dehydrogenative coupling (CDC),
see: a) C.-J. Li, Acc. Chem. Res. 2009, 42, 335; b) C. S. Yeung,
V. M. Dong, Chem. Rev. 2011, 111, 1215; c) C.-J. Li, Z. Li, Pure
Appl. Chem. 2006, 78, 935; d) C. Scheuermann, Chem. Asian J.
2010, 5, 436.
[12] For reviews on multicopper oxygenases, see: a) E. I. Solomon, P.
Chen, M. Metz, S.-K. Lee, A. E. Palmer, Angew. Chem. 2001,
113, 4702; Angew. Chem. Int. Ed. 2001, 40, 4570; b) E. I.
Solomon, U. M. Sundaram, T. E. Machonkin, Chem. Rev. 1996,
96, 2563; for copper-catalyzed or copper-mediated reactions
including dioxygen activation in recent years, see: c) S. Chiba, L.
Zhang, J.-Y. Lee, J. Am. Chem. Soc. 2010, 132, 7266; d) H. Wang,
Y. Wang, D. Liang, L. Liu, J. Zhang, Q. Zhu, Angew. Chem. 2011,
123, 5796; Angew. Chem. Int. Ed. 2011, 50, 5678; e) K. K. Toh,
Omote, J. Chem. Soc. Perkin Trans. 1 1980, 2300; f) H.
Yamamoto, M. Hirayama, M. Akutagawa, T. Nishio, C.
Kashima, Y. Omote, Bull. Chem. Soc. Jpn. 1988, 61, 2678;
g) H. Nakamura, T. Goto, Bull. Chem. Soc. Jpn. 1988, 61, 3776;
h) C. Zhang, Z. Xu, L. Zhang, N. Jiao, Angew. Chem. 2011, 123,
11284; Angew. Chem. Int. Ed. 2011, 50, 11088; i) B. Song, S.
Wang, C. Sun, H. Deng, B. Xu, Tetrahedron Lett. 2007, 48, 8982;
j) C. Zhang, Z. Xu, T. Shen, G. Wu, L. Zhang, N. Jiao, Org. Lett.
2012, 14, 2362.
[15] a) R. Kundu, Z. T. Ball, Org. Lett. 2010, 12, 2460; b) J.
Robertson, J. Pillai, R. K. Lush, Chem. Soc. Rev. 2001, 30, 94;
ˇ
´
c) Z. Cekovic, Tetrahedron 2003, 59, 8073; d) G. Majetich, K.
Wheless, Tetrahedron 1995, 51, 7095.
11370 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 11367 –11370