C U 2O-Catalyzed C(SP 3)-H/C(SP 3)-H Cross-Coupling Using TEMPO: Synthesis of 3-(2-Oxoalkyl)-3-hydroxyoxindoles

Article abstract of DOI:10.1080/00397911.2015.1111383

A simple, convenient, and efficient oxidative cross-coupling reaction of oxindoles with ketones toward a variety of 3-(2-oxoalkyl)-3-hydroxyoxindoles in moderate to excellent yields has been developed. This transformation proceeds via a tandem oxidative cross-coupling by using 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) in air as an environmentally benign oxidant. This methodology provides an alternative approach for the direct generation of all-carbon quaternary centers at the C3 position of oxindoles.

Full text of DOI:10.1080/00397911.2015.1111383

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
CU2O-Catalyzed C(SP3)-H/C(SP3)-H Cross-Coupling
Using TEMPO: Synthesis of 3-(2-Oxoalkyl)-3-
hydroxyoxindoles
Baoshuang Wang, Jiayi Zhu, Yang Wei, Guotian Luo, Hongen Qu & Liang-Xian
Liu
To cite this article: Baoshuang Wang, Jiayi Zhu, Yang Wei, Guotian Luo, Hongen Qu & Liang-
Xian Liu (2015) CU2O-Catalyzed C(SP3)-H/C(SP3)-H Cross-Coupling Using TEMPO: Synthesis
of 3-(2-Oxoalkyl)-3-hydroxyoxindoles, Synthetic Communications, 45:24, 2841-2848, DOI:
10.1080/00397911.2015.1111383
To link to this article: http://dx.doi.org/10.1080/00397911.2015.1111383
View supplementary material
Accepted author version posted online: 11
Nov 2015.
Submit your article to this journal
Article views: 11
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=lsyc20
Download by: [Radcliffe Infirmary]
Date: 17 December 2015, At: 03:24

Synthetic Communications1, 45: 2841–2848, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2015.1111383
3
3
CU O-CATALYZED C(SP )-H/C(SP )-H CROSS-
2
COUPLING USING TEMPO: SYNTHESIS OF
3-(2-OXOALKYL)-3-HYDROXYOXINDOLES
Baoshuang Wang, Jiayi Zhu, Yang Wei, Guotian Luo,
Hongen Qu, and Liang-Xian Liu
Key Laboratory of Jiangxi University for Functional Materials Chemistry,
College of Chemistry and Chemical Engineering, Gannan Normal University,
Ganzhou, Jiangxi, China
GRAPHICAL ABSTRACT
Abstract A simple, convenient, and efficient oxidative cross-coupling reaction of oxindoles
with ketones toward a variety of 3-(2-oxoalkyl)-3-hydroxyoxindoles in moderate to
excellent yields has been developed. This transformation proceeds via a tandem oxidative
cross-coupling by using 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) in air as an
environmentally benign oxidant. This methodology provides an alternative approach for
the direct generation of all-carbon quaternary centers at the C3 position of oxindoles.
Keywords Cross-coupling; 3-hydroxyindole; methyl ketone; TEMPO
INTRODUCTION
The direct oxidative cross-coupling reaction of two individual CꢀH bonds has
been recognized as an ultimately ideal goal for the formation of carbonꢀcarbon
bonds.[1] Over the past few years the transition-metal-catalyzed oxidative cross-coup-
ling reaction through cleavage of CꢀH bonds represent a more environmentally and
economically attractive strategy and led to the emergence of new protocols.[2] Impor-
tantly, this strategy is not only advantageous with respect to the overall minimization
of by-product formation but also allows for streamlining organic syntheses.[3]
Received August 26, 2015.
Address correspondence to Hongen Qu and Liang-Xian Liu, Key Laboratory of Jiangxi University
for Functional Materials Chemistry, College of Chemistry and Chemical Engineering, Gannan Normal
University, Ganzhou, Jiangxi 341000, P. R. China. E-mail: quhongen-2003@163.com; lxliu@xmu.edu.cn
2841

2842
B. WANG ET AL.
Figure 1. Naturally occurring and biologically bioactive 3-substituted 3-hydroxy-2-oxindoles.
However, the field has still many formidable issues, which continue to challenge the
chemistry community. In particular, the direct oxidative C(sp3)-H=C(sp3)-H bond
cross-coupling reactions represent the biggest challenge.
Many biologically active compounds and natural products possess an oxindole
framework with a hydroxy-bearing tetrasubstituted stereogenic center at C3 (Fig. 1).
Such motifs represent the substructures of many natural products, which have gar-
nered interest because of their wide spectrum of biological activities, including anti-
oxidant, anticancer, and neuroprotective properties.[4,5] Accordingly, much effort has
been devoted in the past years to the preparation of these compounds.[6] These stra-
tegies mainly relied on the direct hydroxylation of 3-alkyl-substituted oxindoles,[7]
nucleophilic additions to isatins,[8,9] and intramolecular arylation reactions.[10] Thus,
the development of the direct construction of 3-substituted 3-hydroxy-2-oxindoles
is still in high demand. However, the direct construction of 3-(2-oxoalkyl)-3-
hydroxyoxindoles from oxindoles still remained elusive. Here, we report that
2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO)[11] could effectively catalyzed aerobic
oxidation of oxindoles for synthesis of 3-(2-oxoalkyl)-3-hydroxyoxindoles under
mild reaction conditions.
RESULTS AND DISCUSSION
Initially, we used TEMPO (10 mol%) as the oxidant, K3PO4 as the base, and
different copper catalysts for the model reaction of indolin-2-one 4a and acetone
5a at room temperature. As shown in Table 1, all copper catalysts could promote
the coupling reaction and Cu2O was the best catalyst, providing the product 6a in
52% yield (Table 1, entry 5). Further investigations revealed that there was an obvi-
ous increase in yield when catalyst loading was increased to 25 mol % (Table 1, entry
8). Among the bases tested, K3PO4 was still found to be the most effective for this
transformation. Other bases, such as NaHCO3, Na2CO3, LiOH, NaOH, and
K2CO3, exhibited lower efficiency than K3PO4 (Table 1, entries 10–14). After screen-
ing TEMPO loading (Table 1, entries 15–18), the greatest yield of 6a was achieved
when the reaction was carried out with Cu2O (0.25 equiv), TEMPO (0.2 equiv),
and K3PO4 (0.1 equiv) in acetone (0.5 mL) at room temperature (Table 1, entry 17).
With the optimized reaction conditions in hand, the scope and generality of the
method were examined by varying the structures of the indolin-2-ones 4 and ketones
5. As shown in Table 2, various valuable 3-(2-oxoalkyl)-3-hydroxyoxindoles 6 can be
conveniently and efficiently obtained in moderate to good yields with high regioselec-
tivity by this novel TEMPO-catalyzed oxidative cross-coupling reaction, indicating
that this method is general and practically useful. In general, methyl ketones were
suitable for this method, and the best yield in this work (89%) was obtained when

OXIDATIVE CROSS-COUPLING REACTIONS
2843
Table 1. Optimization of the reaction conditionsa
Entry
Catalyst (mmol)
TEMPO (mmol)
base (mmol)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
CuSC4ꢁ5H2O(0.02)
CuCI2 (0.02)
Cu(OAc)2 (0.02)
Cul (0.02)
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.01
0.03
0.04
0.05
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
NaHCO3 (0.02)
Na2CO3 (0.02)
LiOH (0.02)
NaOH (0.02)
K2CO3 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
K3PO4 (0.02)
30
13
20
35
52
60
66
69
69
32
38
45
40
56
46
75
85
83
Cu2O (0.02)
Cu2O (0.03)
Cu2O (0.04)
Cu2O (0.05)
Cu2O (0.06)
Cu2O (0.05)
Cu2O (0.05)
Cu2O (0.05)
Cu2O (0.05)
Cu2O (0.05)
Cu2O (0.05)
Cu2O (0.05)
Cu2O (0.05)
Cu2O (0.05)
aReaction conditions: oxindole (0.2 mmol) and acetone (0.5 mL) at rt for 4 h.
bIsolated yield.
acetone 5a reacted with 5-fluoroindolin-2-one 4b. The yields decreased when ketones
with large steric bulk, such as 2-pentanone 5b, 2-hexanone 5c, and 2-heptanone 5d,
were used as the donors. Various aryl-substituted methyl ketones with electron-
donating groups reacted under the present protocol to give the desired products
6e–6k in good yields. The steric properties of the aryl ring did not appear to signifi-
cantly affect the yield, as ortho-functionalized aryl substrates 1-(2-methoxyphenyl)
ethanone 5i and 1-(naphthalen-1-yl)ethanone 5j performed equally well in the
cross-coupling. Unexpectedly, cycloheptanone 5 l reacted with 4a smoothly and pro-
duced the bis(3-hydroxyindolin-2-one) 6 l in 72% yield.
Interestingly, when evaluating the effect of different substituents at C5 position
of oxindoles on reactivity, we observed that oxindoles with electron-withdrawing
groups, such as F and Cl, reacted smoothly with ketones, affording the correspond-
ing products in better yields as compared with unsubstituted oxindole 4a.
In order to better understand the reaction mechanism, some control experi-
ments were carried out. First, we tested the reaction in the absence of the ketone
as coupling partner (Scheme 1). The oxidative product 7 was obtained in 86% yield,
indicating that 7 may serve as an intermediate in the catalytic cycle. Second, 15%
yield of 6a was obtained in the absence of TEMPO, indicating that TEMPO is cru-
cial for the oxidative cross-coupling. Third, the oxidative cross-coupling reaction of

2844
B. WANG ET AL.
Table 2. Reactions of different indolin-2-ones with ketonesa
aReaction conditions: 4 (0.2 mmol), 5 (0.5 mL), Cu2O (0.05 mmol), TEMPO (0.04 mmol), and K3PO4
(0.02 mmol) at room temperature. Isolated yield based on 4.
4a with acetone was attempted under an argon atmosphere (in the absence of mol-
ecular oxygen). The result showed that the reaction was carried out inefficiently,
and the desired product 6a was obtained with a poor yield of 12%. The result
demonstrated that dioxygen is necessary for an efficient oxidative cross-coupling
reaction. Fourthly, the reaction of 4a with acetone in the absence of Cu2O resulted
in the formation of 6a in a lower yield (49%) and a longer reaction time (24 h). Fifth,
we also carried out this reaction in the absence of K3PO3 but no expected product
was observed.
Based on the previous studies[12] and these results, a hypothesized mechanism
of this oxidative cross-coupling reaction is shown in Scheme 2. Initially, tautomer 8

OXIDATIVE CROSS-COUPLING REACTIONS
2845
Scheme 1. Control experiments.
Scheme 2. Plausible reaction path.
of indole 4 was oxidized in the presence of TEMPO, Cu2O, and O2 into intermediate
9. Then, a rapid dehydration in 9 gave intermediate 10, which tautomerizes to 7.
Finally, the nucleophilic addition of ketones to 7 afforded the corresponding
products.
CONCLUSIONS
In summary, we have developed an efficient oxidative cross-coupling reaction
of oxindoles with ketones using Cu2O as catalyst and TEMPO in air as oxidant with
excellent regioselectivity under mild conditions. This reaction provides a novel
method for the generation of all-carbon quaternary centers at the C3 position of oxi-
ndoles. Moreover, it has several advantages: (1) an inexpensive and environmentally
friendly TEMPO has been used in air as oxidant, (2) the operationally simple and
broad substrate make it potentially useful, (3) it is highly regioselective (1,30-linkage),
and (4) this oxidative cross-coupling reaction proceeds without exclusion of moisture
or air from the reaction mixture and allows the isolation of the desired 3-(2-
oxoalkyl)-3-hydroxyoxindoles in moderate to excellent yields.
EXPERIMENTAL
Oxindole (0.2 mmol) was added to a solution of TEMPO (0.04 mmol), Cu2O
(0.05 mmol), and K3PO4 (0.02 mmol) in ketone (0.5 mL) under an air atmosphere
and the mixture was stirred at room temperature for 4–48 h. The reaction mixture

2846
B. WANG ET AL.
was concentrated under reduced pressure. The residue was purified by flash
chromatography on silica gel (eluent–EtOAc=PE ¼ 1:4) to yield the corresponding
product 6.
FUNDING
The authors are grateful to the NSF of China (Nos: 21162001 and 21462002),
Natural Science Foundation of Jiangxi Province (No: 20132BAB203007) and Gradu-
ate Student Innovation Project of Jiangxi Province (No: YC2013-S216) for financial
support.
SUPPLEMENTAL MATERIAL
Experimental procedures and full spectroscopic data for all new compounds
can be accessed on the publisher’s website.
REFERENCES
1. For reviews, see (a) McGlacken, G. P.; Bateman, L. M. Recent advances in aryl-aryl bond
formation by direct arylation. Chem. Soc. Rev. 2009, 38, 2447–2464; (b) Ashenhurst, J. A.
Intermolecular oxidative cross-coupling of arenes. Chem. Soc. Rev. 2010, 39, 540–548; (c)
Han, W.; Mayer, P.; Ofial, A. R. Palladium-catalyzed dehydrogenative cross couplings
of benzazoles with azoles. Angew. Chem. Int. Ed. 2011, 50, 2178–2182; (d) Kuhl, N.;
Hopkinson, M. N.; Wencel-Delord, J.; Glorious, F. Beyond directing groups: Transition
metal-catalyzed CꢀH activation of simple arenes. Angew. Chem. Int. Ed. 2012, 51,
10236–10254.
2. For selected reviews, see (a) Yu, J.; Giri, R.; Chen, X. s-Chelation-directed CꢀH functio-
nalizations using Pd(II) and Cu(II) catalysts: Regioselectivity, stereoselectivity, and cata-
lytic turnover. Org. Biomol. Chem. 2006, 4, 4041–4047; (b) Giri, R.; Shi, B.; Engle, K.;
Maugel, N.; Yu, J. Transition-metal-catalyzed CꢀH activation reactions: Diastereoselec-
tivity and enantioselectivity. Chem. Soc. Rev. 2009, 38, 3242–3272; (c) Li, C. Cross-dehy-
drogenative coupling (CDC): Exploring CꢀC bond formations beyond functional group
transformations. Acc. Chem. Res. 2009, 42, 335–344; (d) Chen, X.; Engle, K. M.; Wang,
D.; Yu, J. Palladium(II)-catalyzed C–H activation=C–C cross-coupling reactions: Versa-
tility and practicality. Angew. Chem. Int. Ed. 2009, 48, 5094–5115; (e) Ackermann, L.;
Vicente, R.; Kapdi, A. R. Transition-metal-catalyzed direct arylation of (hetero)arenes
by CꢀH bond cleavage. Angew. Chem. Int. Ed. 2009, 48, 9792–9826; (f) Yeun, C. S.;
Dong, V. M. Catalytic dehydrogenative cross-coupling: Forming carbonꢀcarbon bonds
by oxidizing two carbonꢀhydrogen bonds. Chem. Rev. 2011, 111, 1215–1292; (g) Liu,
C.; Zhang, H.; Shi, W.; Lei, A. Bond formations between two nucleophiles: Transition-
metal-catalyzed oxidative cross-coupling reactions. Chem. Rev. 2011, 111, 1780–1824;
(h) Engle, K. M.; Mei, T.; Wasa, M.; Yu, J. Weak coordination as powerful means for
developing broadly useful C-H functionalization reactions. Acc. Chem. Res. 2012, 45,
788–802.
3. (a) Trost, B. M. The chemistry reaction and atom economy. Science 1991, 254, 1471–1477; (b)
Trost, B. M. On inventing reactions for atom economy. Acc. Chem. Res. 2002, 35, 695–705.
4. For reviews, see (a) Galliford, C. V.; Scheidt, K. A. Pyrrolidinyl-spirooxindole natural
products as inspirations for the development of potential therapeutic agents. Angew.
Chem. Int. Ed. 2007, 46, 8748–8758; (b) Marti, C.; M Carreira, E. Construction of

OXIDATIVE CROSS-COUPLING REACTIONS
2847
spiro[pyrrolidine-3,30-oxindoles]: Recent applications to the synthesis of oxindole alka-
loids. Eur. J. Org. Chem. 2003, 2209–2219.
5. (a) Kagata, T.; Saito, S.; Shigemori, H.; Ohsaki, A.; Ishiyama, H.; Kubota, T.;
Kobayashi, J. Paratunamides AꢀD, oxindole alkaloids from Cinnamodendron axillare.
J. Nat. Prod. 2006, 69, 1517–1521; (b) Kohno, J.; Koguchi, Y.; Nishio, M.; Nakao,
K.; Kuroda, M.; Shimizu, R.; Ohnuki, T.; Komatsubara, S. J. Org. Chem. 2000, 65,
990–995; (c) Kobayashi, J.; Suzuki, H.; Shimbo, K.; Takeya, K.; Morita, H. Celogentins
AꢀC, new antimitotic bicyclic peptides from the seeds of Celosia argentea. J. Org. Chem.
2001, 66, 6626–6633.
6. Recent reviews, see (a) Shen, K.; Liu, X.; Lin, L.; Feng, X. Recent progress in enantiose-
lective synthesis of C3-functionalized oxindoles: Rare earth metals take action. Chem. Sci.
2012, 3, 327–334; (b) Zhou, F.; Liu, Y.-L.; Zhou, J. Catalytic asymmetric synthesis of
oxindoles bearing a tetrasubstituted stereocenter at the C-3 position. Adv. Synth. Catal.
2010, 352, 1381–1407.
7. (a) Ishimaru, T.; Shibata, N.; Nagai, J.; Nakamura, S.; Toru, T.; Kanemasa, S. Lewis
acid–catalyzed enantioselective hydroxylation reactions of oxindoles and b-keto esters
using DBFOX ligand. J. Am. Chem. Soc. 2006, 128, 16488–16489; (b) Sano, D.; Nagata,
K.; Itoh, T. Catalytic asymmetric hydroxylation of oxindoles by molecular oxygen using a
phase-transfer catalyst. Org. Lett. 2008, 10, 1593–4595; (c) Bui, T.; Candeias, N. R.;
Barbas III, C. F. Dimeric quinidine-catalyzed enantioselective aminooxygenation of oxi-
ndoles: An organocatalytic approach to 3-hydroxyoxindole derivatives. J. Am. Chem. Soc.
2010, 132, 5574–5575; (d) Zhang, Z.; Zheng, W.; Antilla, J. C. Highly enantioselective
catalytic benzoyloxylation of 3-aryloxindoles using chiral VAPOL calcium phosphate.
Angew. Chem. Int. Ed. 2011, 50, 1135–1138.
8. For selected examples using metal catalysis, see (a) Shintani, R.; Inoue, M.; Hayashi, T.
Angew. Chem. Int. Ed. 2006, 45, 3353–3356; (b) Hanhan, N. V.; Sahin, A. H.; Chang, T.
W.; Fettinger, J. C.; Franz, A. K. Catalytic asymmetric synthesis of substituted
3-hydroxy-2-oxindoles. Angew. Chem. Int. Ed. 2010, 49, 744–747; (c) Tomita, D.; Yamat-
sugu, K.; Kanai, M.; Shibasaki, M. Enantioselective synthesis of SM-130686 based on the
development of asymmetric Cu(I)F catalysis to access 2-oxindoles containing a tetrasub-
stituted carbon. J. Am. Chem. Soc. 2009, 131, 6946–6948.
9. For selected examples using organocatalysis, see (a) Nakamura, S.; Hara, N.; Nakashima,
H.; Kubo, K.; Shibata, N.; Toru, T. Enantioselective synthesis of (R)-convolutamydine:A
with new N-heteroarylsulfonylprolinamides. Chem. Eur. J. 2008, 14, 8079–8081; (b) Itoh,
T.; Ishikawa, H.; Hayashi, Y. Asymmetric aldol reaction of acetaldehyde and isatin deri-
vatives for the total syntheses of ent-convolutamydine E and CPC-1 and a half fragment
of madindoline A and B. Org. Lett. 2009, 11, 3854–3857; (c) Liu, Y.-L.; Wang, B.-L.; Cao,
J.-J.; Chen, L.; Zhang, Y.-X.; Wang, C.; Zhou, J. Organocatalytic asymmetric synthesis of
substituted 3-hydroxy-2-oxindoles via MoritaꢀBaylisꢀHillman reaction. J. Am. Chem.
Soc. 2010, 132, 15176–15178; (d) Zhang, K.; Yin, C.-K.; Liu, X.-H.; Lin, L.-L.; Feng,
X.-M. Catalytic asymmetric addition of alkyl enol ethers to 1,2-dicarbonyl compounds:
Highly enantioselective synthesis of substituted 3-alkyl-3-hydroxyoxindoles. Angew.
Chem. Int. Ed. 2011, 50, 2573–2577; (e) Liu, Y.-L.; Zhou, J. Organocatalytic asymmetric
synthesis of 3-difluoroalkyl 3-hydroxyoxindoles. Chem. Commun. 2012, 48, 1919–1921; (f)
Hara, N.; Nakamura, S.; Funahashi, Y.; Shibata, N. Organocatalytic enantioselective
decarboxylative addition of malonic acids half thioesters to isatins. Adv. Synth. Catal.
2011, 353, 2976–2980; (g) Zhong, F.-R.; Yao, W.-J.; Dou, X.-W.; Lu, Y.-X. Enantioselec-
tive construction of 3-hydroxy oxindoles via decarboxylative addition of b-ketoacids to
isatins. Org. Lett. 2012, 14, 4018–4021.
10. (a) Jia, Y.-X.; Hillgren, J. M.; Watson, E. M.; Marsden, S. P.; Kündig, E. P. Chiral N-het-
erocyclic carbene ligands for asymmetric catalytic oxindole synthesis. Chem. Commun.

2848
B. WANG ET AL.
2008, 4040–4042; (b) Yin, L.; Kanai, M.; Shibasaki, M. A facile pathway to enantiomeri-
cally enriched 3-hydroxy-2-oxindoles: Asymmetric intramolecular arylation of a-keto
amides catalyzed by a palladium–difluorPhos complex. Angew. Chem. Int. Ed. 2011, 50,
7620–7623; (c) Wei, W.-T.; Zhou, M.-B.; Fan, J.-H.; Liu, W.; Song, R.-J.; Liu, Y.; Hu,
M.; Xie, P.; Li, J.-H. Synthesis of oxindoles by iron-catalyzed oxidative 1,2-alkylarylation
of activated alkenes with an aryl C(sp2)-H bond and a C(sp3)-H bond adjacent to a het-
eroatom. Angew. Chem. Int. Ed. 2013, 52, 3638–3641; (d) Fan, J.-H.; Wei, W.-T.; Zhou,
M.-B.; Song, R.-J.; Li, J.-H. Palladium-catalyzed oxidative difunctionalization of alkenes
with a-carbonyl alkyl bromides initiated through a Heck-type insertion: A route to indo-
lin-2-ones. Angew. Chem. Int. Ed. 2014, 53, 6650–6654; (c) Zhou, M.-B.; Wang, C.-Y.;
Song, R.-J.; Liu, Y.; Wei, W.-T.; Li, J.-H. Chem. Commun. 2013, 49, 10817–10819.
11. For reviews on the application of TEMPO and its derivatives in organic synthesis, see (a)
Sheldon, R. A.; Arends, I. W. C. E.; Brink, G. J. T.; Dijksman, A. Acc. Chem. Res. 2002,
35, 774–781 (b) Sheldon, R. A.; Arends, I. W. C. E. Organocatalytic oxidations mediated
by nitroxyl radicals. Adv. Synth. Catal. 2004, 346, 1051–1071; (c) Tebben, L.; Studer, A.
Nitroxides: Applications in synthesis and in polymer chemistry. Angew. Chem. Int. Ed.
2011, 50, 5034–5068; (d) Ciriminna, R.; Pagliaro, M. Industrial oxidations with organo-
catalyst TEMPO and its derivatives. Org. Process Res. Dev. 2010, 14, 245–251; (e) Piera,
J.; Bäckvall, J.-E. Catalytic oxidation of organic substrates by molecular oxygen and
hydrogen peroxide by multistep electron transfer—a biomimetic approach. Angew. Chem.
Int. Ed. 2008, 47, 3506–3523; (f) Zhan, B.-Z.; Thompson, A. Recent developments in the
aerobic oxidation of alcohols. Tetrahedron 2004, 60, 2917–2935; (e) Zhou, Z.-G.; Liu, L.-X.
TEMPO and its derivatives: Synthesis and applications. Curr. Org. Chem. 2014, 18,
459–474.
12. (a) Ganachaud, C.; Garfagnoli, V.; Tron, T.; Iacazio, G. Trimerisation of indole through
laccase catalysis. Tetrahedron Lett. 2008, 49, 2476–2478; (b) Bergman, J.; Bergman, S.;
Lindström, J. O. Formation of 6,13-dimethyl-5,12-diazachrysene by oxidative coupling
of 2-methylindole followed by base-induced ring-expansion. Tetrahedron Lett. 1998, 39,
4119–4122.