Palladium-Catalyzed Dearomatizing Difunctionalization of Indoles and Benzofurans

Article abstract of DOI:10.1002/ejoc.201600194

A palladium-catalyzed dearomatizing difunctionalization of N-Boc-indoles and benzofurans to give tricyclic indolines and 2,3-dihydrobenzofurans with a fully substituted carbon center is described. Product formation occurs under mild conditions at room temperature with commercially available aryl boronic acids and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a mild oxidant in good yields. 2-Carboxyalkyl-substituted indoles and benzofurans react under Pd-catalysis with aryl boronic acids and TEMPO through dearomatizing arylation/cyclization to the corresponding indolines and dihydrobenzofurans containing a lactone moiety bearing a fully substituted carbon center.

Full text of DOI:10.1002/ejoc.201600194

DOI: 10.1002/ejoc.201600194
Full Paper
Synthetic Methods
Palladium-Catalyzed Dearomatizing Difunctionalization of
Indoles and Benzofurans
Vincenzo Ramella,[a] Zhiheng He,[a] Constantin G. Daniliuc,[a] and Armido Studer*[a]
Abstract: A palladium-catalyzed dearomatizing difunctionali- conditions at room temperature with commercially available
zation of N-Boc-indoles and benzofurans to give tricyclic indo-
lines and 2,3-dihydrobenzofurans with a fully substituted car-
bon center is described. Product formation occurs under mild
aryl boronic acids and 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) as a mild oxidant in good yields.
Introduction
would comprise a more straightforward direct approach to
indolines or 2,3-dihydrobenzofurans, respectively, bearing fully
substituted carbon centers at the C2 and/or C3 position. These
dearomatization reactions provide easy access to complex
structures in a single operation. Although dearomatization of
indoles is a highly active and well-investigated research area,[7]
there are only a few protocols for the dearomatization of benzo-
furans.[7g,8] Therefore, the development of new methods for the
dearomatizing difunctionalization of indoles and, in particular,
of benzofurans remains of high importance.
Along these lines, we already succeeded in the Pd-catalyzed
dearomatizing 2-arylation-3-aminoalkoxylation of benzofur-
ans[9] and indoles[10] by using 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO)[11] and arylboronic acids as reaction partners
(Scheme 1). TEMPO acted both as an oxidant and as a trapping
reagent in these reactions. Recently, we established another
three-component coupling including a carboxylic acid as an ad-
ditional reaction partner. This process allows indolines and 2,3-
dihydrobenzofurans bearing a fully substituted C-center at the
The indoline[1] and 2,3-dihydrobenzofuran core[2] are two im-
portant structural entities that can be found as substructures in
many biologically active natural products and pharmaceutically
relevant compounds (Figure 1). Despite great efforts in recent
years towards their preparation, methods for the stereoselective
construction of 2,3-disubstituted indolines and 2,3-dihydro-
benzofurans remain limited. Established methods include cyclo-
propanation/ring-opening/iminium cyclization cascades,[3] tran-
sition-metal-catalyzed aryl aminations[4] and Fischer indol-
izations[5] for the construction of indolines and intramolecular
alkylation of phenols,[6] transition-metal-catalyzed aryl oxygen-
ations[6] and cyclization via carbene intermediates[6] for the
preparation 2,3-dihydrobenzofurans.
Figure 1. Natural products bearing an indoline or a 2,3-dihydrobenzofuran
substructure.
The 2,3-difunctionalization of readily available indoles and
benzofurans involving dearomatization of the heteroarene
[a] Organic Chemistry Institute, Westfälische Wilhelms-Universität,
Corrensstraße 40, 48149 Münster, Germany
E-mail: studer@uni-meunster.de
http://www.uni-muenster.de/chemie.oc/studer
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600194.
Scheme 1. Pd-catalyzed cyclizing 1,2-difunctionalization of indoles and
benzofurans.
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C2 position to be constructed.[12] Herein, we disclose a dearo- 2.0 to 1.0 equiv. keeping Et2O as the solvent provided a worse
matizing arylation/cyclization sequence of readily prepared 2- result (45 %; entry 8). Among the tested carboxylic acids, TFA,
indolyl or 2-benzofuryl-substituted carboxylic acids to give tri- with the lowest pKa in the series, turned out to be best suited
cyclic indolines and 2,3-dihydrobenzofurans. Reactions occur as an additive (entries 9–12). The amount of phenylboronic acid
under mild conditions with complete regiocontrol and com- and TEMPO was varied between 1.5 and 4.0 equiv. and the best
plete 2,3-stereocontrol.
result was obtained with 2.0 equiv. of phenylboronic acid and
2.5 equiv. of TEMPO (entries 13–15). Surprisingly, decreasing the
amount of Pd-catalyst to 5.0 mol-% led to a further increase of
the yield (83 %; entry 16). Notably, in the absence of TFA, only
5 % 2a was formed, and with other oxidants such as O2 and
1,4-benzoquinone, the addition/cyclization did not work.
With the optimized conditions in hand, we investigated the
reaction scope by systematically varying the benzofuran and
the aryl boronic acid (Scheme 2). To ensure high conversion in
all experiments, the reaction time was extended to 6 h. Varia-
tion of the boronic acid was studied first. The sterically more
hindered 2-methylphenyl boronic acid afforded 2b in only 13 %
yield. Protodeborylation was the dominant reaction pathway for
this aryl donor. A moderate yield was also achieved with meta-
methylphenyl boronic acid (see 2c). However, phenylboronic
acids bearing an electron-donating substituent at the para-po-
sition gave the corresponding 2,3-dihydrobenzofurans 2d–f in
good yields. para-Substituted phenylboronic acids bearing an
electronic-withdrawing group were also well tolerated, and lact-
ones 2g–i were isolated in good yields. The lower yield ob-
Results and Discussion
We first investigated the cyclizing arylation of 2-(benzofuran-2-
yl)acetic acid (1a) with phenylboronic acid under Pd-catalysis
and TEMPO to give lactone 2a (Table 1). The preparation of acid
1a and of all other heteroaryl-substituted acids used in this
study is described in the Supporting Information. All com-
pounds prepared in this study were formed as racemates and in
the schemes only one enantiomer is drawn. Experiments were
conducted by using an excess of phenylboronic acid (1.5 to
4.0 equiv.), TEMPO (2.5 to 4.0 equiv.), and Pd(OAc)2 (5 or 10 mol-
%) as the catalyst. Additive and solvent were systematically var-
ied. In the absence of any additive, in neat acetic acid, the tar-
get product 2a was obtained in 53 % yield (Table 1, entry 1). A
slightly worse result was achieved in propionic acid (49 %; en-
try 2). We then tested trifluoroacetic acid (TFA) as an additive
in combination with a nonprotic polar organic solvent. In tetra-
hydrofuran (THF) with TFA (2.0 equiv.) the yield dropped to
29 % (entry 3). Changing the solvent to dichloromethane, di-
oxane, toluene, or diethyl ether led to a gradual increase of the
yield to 75 % (entries 4–7). Reducing the amount of TFA from
Table 1. Reaction optimization.
Entry
Additive
Solvent
Yield (%)[a]
1[b]
CH3CO2H
CH3CH2CO2H
THF
CH2Cl2
dioxane
toluene
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
53
49
29
60
67
74
75
45
7
11
12
4
81
67
80
83
5
2[b]
3[b]
TFA
TFA
TFA
TFA
TFA
TFA
2-ClCH2CO2H
CH3CO2H
CH3CH2CO2H
pivalic acid
TFA
TFA
TFA
TFA
4[b]
5[b]
6[b]
7[b]
8[c]
9[c]
10[c]
11[c]
12[c]
13[d]
14[e]
15[f]
16[g]
17
[a] Determined by 1H NMR spectroscopic analysis with CH2Br2 as internal
standard. [b] PhB(OH)2 (4.0 equiv.) and TEMPO (4.0 equiv.) were used; reaction
time: 24 h. [c] Additive (1.0 equiv.) was used. [d] PhB(OH)2 (2.0 equiv.) and
TEMPO (4.0 equiv.) were used. [e] PhB(OH)2 (1.5 equiv.) and TEMPO
(4.0 equiv.) were used. [f] PhB(OH)2 (2.0 equiv.) and TEMPO (2.5 equiv.) were
used. [g] Pd(OAc)2 (5.0 mol-%) was used.
Scheme 2. Scope of the Pd-catalyzed dearomatizing cyclization of 2-carboxy-
alkyl-benzofurans.
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tained for (4-bromophenyl)boronic acid (see 2j) is likely caused
by a solubility problem. An alkylboronic acid (nBuB(OH)2) and
(2-furyl)boronic acid as a member of a heteroaromatic system
did not provide the desired product; starting 1a remained unre-
acted in these cases. We continued the studies by looking at
various C5 and C7-substituted benzofurans bearing electron-
donating and -withdrawing groups with phenylboronic acid as
the reaction partner. The corresponding 2,3-dihydrobenzo-
furans 2k–o were formed in good to excellent yields.
All compounds prepared so far contain a γ-lactone moiety
(five-membered lactone). We wondered whether the reaction
could also be used to prepare benzofurans with annulated six-
and seven-membered lactones. To this end, the propanoic and
butanoic acid derivatives of 1a were tested. Pleasingly, the pro-
panoic acid congener afforded δ-lactone 2p in excellent yield
(95 %), and the homologous ε-lactone 2q was also formed in
good yield.
The studies were continued by investigating the induced
stereoselectivity using chiral α-substituted 2-(benzofuran-2-
yl)acetic acids, α-substituted and ꢀ-substituted 2-(benzofuran-
2-yl)propanoic acids (Scheme 3). Under the optimized condi-
tions, the α-benzyl derivative of 1a afforded the target lactone
2r in disappointing yield (8 %) with a 5.8:1 diastereoselectivity.
The low yield is likely due to steric effects caused by the addi-
tional α-substituent at the carboxylic acid. The stereoselectivity
was determined by 1H NMR analysis on the crude product.
Changing the α-substituent to n-propyl did not improve the
outcome of the reaction, and the γ-lactone 2s was isolated in
low yield. To reduce steric effects, we switched to the homolo-
gous α-substituted propanoic acid derivatives. Indeed, aryl-
ation/cyclization with the α-benzyl or α-propyl derivative of 1g
gave the target products 2t and 2u in improved yields (62 and
65 %); however, the diastereoselectivity dropped to 2:1. To in-
crease selectivity, we examined the ꢀ-phenyl- and ꢀ-butyl-sub-
stituted derivatives of 1g. For the phenyl-derivative, diastereo-
selectivity was excellent (> 25:1) but the yield dropped slightly
(see 2v). For the acid bearing the smaller ꢀ-butyl substituent,
both diastereoselectivity and yield dropped (see 2w). For the ꢀ-
phenyl-substituted 2-(benzofuran-2-yl)propanoic acid, the aryl
boronic acid was varied and high stereoselectivity was obtained
in the formation of 2x and 2y. Whereas para-tolyl boronic acid
provided a high yield (71 %), the yield dropped significantly in
the reaction with the para-trifluoromethylphenyl boronic acid.
Electronic effects of the ꢀ-aryl-substituent at the acid side chain
also influence both the yield and selectivity. The electron-rich
para-tolyl-derivative gave a good yield and high selectivity (see
2z) and the para-trifluoromethylphenyl-congener afforded a
lower yield and a lower selectivity in the reaction with phenyl
boronic acid (see 2aa). The relative configuration of the major
isomers of 2r and 2t was assigned by X-ray analysis and 2u was
assigned in analogy (Figure 2). The relative configuration in 2v
was assigned by 2D-NMR spectroscopy and all other δ-lactones
bearing ꢀ-substituents were assigned in analogy.
Scheme 3. Scope of the Pd-catalyzed dearomatizing cyclization of benzo-
furans with substituents at the α- or ꢀ-position (only major isomer drawn).
Figure 2. Crystal structures of the major isomers of 2r (top) and 2t (bottom).
We then tested the dearomatizing arylative lactonization on
zofurans, lactone 4a was obtained in 77 % yield. Additional ex-
indoles (Scheme 4). The reaction was first investigated with 2- perimentation revealed that the yield could be further im-
(N-Boc-indol-2-yl)acetic acid and phenyl boronic acid. Under the proved upon running the reaction in the presence of KF
optimized conditions identified for functionalization of the ben- (2.5 equiv.) with PhB(OH)2 (2.5 equiv.), TEMPO (3.0 equiv.), and
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Pd(OAc)2 (5.0 mol-%) in a 2:1 mixture of 2-chloroacetic acid/THF ones by using the propanoic and butanoic acid derivatives of
to provide 4a in 88 % yield as a single diastereoisomer. para- 3a. The δ-lactone 4k was obtained in 86 % yield. However, for
Substituted arylboronic acids bearing electron-donating and the homologous system, lactonization did not occur and acid 5
-withdrawing groups afforded, in all cases, the desired aryl- was isolated in 84 % yield and moderate diastereoselectivity as
ation/lactonization products 4b–f in high yields (75–87 %). Elec- the only detectable product. Pleasingly, 2-(2-hydroxyethyl)- and
tron-withdrawing substituents on the indole core at positions 2-(3-hydroxyproyl)-substituted N-Boc-indoles were suitable sub-
C5 or C6 were tolerated and the corresponding indolines 4h strates for phenylation/cyclization, and the corresponding
and 4i were isolated in moderate to good yields. A higher yield ethers 4j and 4l were formed, albeit in slightly lower yields as
was obtained for the indole bearing an electron-donating compared to the acid derivatives. Notably, arylation and subse-
methyl group at the C5 position (81 %, see 4g). Next, we stud- quent ether formation could not be achieved in the benzofuran
ied the arylative cyclization to six- and seven-membered lact- series.
To study the stereoinduction exerted by a substituent at the
α- or ꢀ-position of the alkanoic acid side chain in these indoles,
we examined phenylation/lactonization of indoles 3i and 3j
(Scheme 5). Both indolines 4m and 4n were formed with excel-
lent diastereoselectivity (> 25:1). However, compared with ind-
oles bearing unsubstituted side chain acids, the yields dropped.
The relative configuration of 4m and 4n was unambiguously
assigned by X-ray analysis (Figure 3). Interestingly, the relative
Scheme 4. Scope of the Pd-catalyzed dearomatizing cyclization of N-Boc ind-
oles.
Scheme 5. Stereoselective Pd-catalyzed dearomatizing cyclization of N-Boc
indoles bearing substituents at the α- or ꢀ-position.
Figure 3. Crystal structures of 4m (top) and 4n (bottom).
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der an argon atmosphere and were performed by using standard
Schlenk techniques. All solvents for extraction and flash chromatog-
raphy were distilled before use. Propionic acid and chloroacetic acid
were purchased from Acros and used as received. Pd(OAc)2 (reagent
grade, 98 %), potassium fluoride (99+ %) were purchased from Al-
drich and used as received. TEMPO was purchased from CIBA Speci-
ality Chemicals and used as received. Other chemicals were pur-
chased from ABCR, Acros Organics, Alfa Aesar, Fluka and Sigma–
Aldrich and used as received. Tetrahydrofuran was distilled from
potassium under argon directly before using. Dichloromethane was
distilled from P2O5 under argon directly before using. Diethyl ether
was distilled from sodium and potassium under argon directly be-
fore using.
stereochemistry obtained for the indolines differs from the rela-
tive stereochemistry of the major isomers of the corresponding
dihydrobenzofurans 2s and 2v (see Scheme 3).
The following mechanism is proposed to explain the reaction
outcome (Scheme 6). Firstly, transmetalation of the aryl group
from ArB(OH)2, in case of the benzofurans, and from
[ArBF(OH)2]K, for the indoles, to a PdII species will generate an
aryl-Pd complex I. Palladation of the C3 position of the hetero-
arene with I provides complex II. Electrophilic C3-palladation is
followed by a 1,2-Pd-migration to give an arylPd complex III.
Intramolecular trapping of the thus generated carbocation by
the acid moiety and deprotonation provide intermediate IV. Re-
ductive elimination eventually leads to the product lactone and
Pd0. Oxidation of Pd0 to PdII by 2.0 equiv. TEMPO regenerates
the catalyst. Generation of C2-palladated intermediates of type
III through electrophilic palladation at the C3 position and sub-
sequent metallotropic 1,2-migration is well documented for
indoles.[10,13] Although less explored, electrophilic palladation
was previously reported to initially occur at the C3-position also
for benzofurans,[9,14] in agreement with our suggested mecha-
nism.
1H, 13C, and 19F NMR spectra were recorded with a Bruker DPX 300,
a Bruker AV 300, or a Bruker AV 400 spectrometer. Chemical shifts
are referred to the solvent (CDCl3) residual peak (1H: δ = 7.26 ppm,
13C: δ = 77.2 ppm) and to an external standard (CFCl3: δ = 0.0 ppm)
for 19F NMR spectra. Multiplicities of NMR signals are described as
s (singlet), d (doublet), t (triplet), q (quartet), sept (septet), m (mul-
tiplet) or br (broad signal). All C2 esterifications were carried out in
a photoreactor equipped with six high power (5 W) Lambertian
LEDs (HPB8b-48K5BF/WPCB, blue). Thin-layer chromatography (TLC)
was carried out on Merck silica gel 60 F254 plates; detection by UV
(254 nm) or treatment with NaHCO3 (5.0 g), KMnO4 (1.5 g) and H2O
(0.20 L) stain followed by heating or treatment with 4-anisaldehyde
(0.50 mL), H2SO4 (1.0 mL), acetic acid (50 mL) and ethanol (150 mL)
stain followed by heating. Flash chromatography (FC) was carried
out on Merck silica gel 60 (40–63 μm) or Acros silica gel 60 A (60–
200 μm) with an argon pressure up to 0.5 bar or with a Grace
Davison Reveleris IES flash chromatography system with Reveleris
C18 reversed-phase flash cartridges (4 g or 12 g) with a flow rate
of 18 or 30 mL/min and CH3CN/H2O as a solvent mixture (UV detec-
tion: 210 nm and 230 nm). HRMS ESI were performed with a Bruker
MicroTof. Melting points (MP) were determined with a Stuart SMP10
or a Stuart SMP3 and are uncorrected. IR spectra were recorded
with a Digilab Varian 3100 FTIR Excalibur series. IR signals are de-
scribed as w (weak), m (medium), or s (strong).
General Procedure for the Arylation/Cyclization of Benzofurans
(GP1): A solution of benzofuran (0.25 mmol, 1.0 equiv.), Pd(OAc)2
(2.8 mg, 13 μmol, 5.0 mol-%), arylboronic acid (0.50 mmol,
2.0 equiv.), TFA (39 μL, 2.0 mmol, 2.0 equiv.), and TEMPO (98 mg,
0.63 mmol, 2.5 equiv.) in diethyl ether (1.0 mL) was stirred at room
temperature for 6 h. The reaction mixture was filtered through silica
gel and washed with a mixture of dichloromethane (200 mL) and
acetic acid (2 mL). The volatiles were removed under reduced pres-
sure. The residue was purified by FC (CH3CN/H2O, 0.0 min/5 %
CH3CN, 30 min/90 % CH3CN, 10 min/90 % CH3CN).
Scheme 6. Proposed catalytic cycle.
Conclusions
We have developed a palladium-catalyzed dearomatizing di-
functionalization of benzofurans and indoles, affording tricyclic
indolines or 2,3-dihydrobenzofurans containing a lactone moi-
ety. Simple 1,2-diastereselectivity is complete for all examples
tested, leading to the syn-cyclization products. Induced selectiv-
ity exerted by a substituent at the acid side chain ranges from
2:1 to > 25:1. Arylative dearomatization occurs at room temper-
ature in 4 to 6 h by using commercially available arylboronic
acids and TEMPO as a mild oxidant. For the indole series, alco-
hols can be used as internal nucleophiles, leading to ethers.
General Procedure for the Synthesis of Indolines (GP2): An ind-
ole (0.25 mmol, 1.0 equiv.), Pd(OAc)2 (2.8 mg, 13 μmol, 5.0 mol-%),
arylboronic acid (0.38 mmol, 1.5 equiv.), potassium fluoride (22 mg,
0.38 mmol, 1.5 equiv.), and TEMPO (0.12 g, 0.75 mmol, 3.0 equiv.)
in a mixture of 2-chloroacetic acid/THF (2:1, wt, 0.50 mL) was stirred
at room temperature for 4 h. Aq. sat. Na2CO3 (5.0 mL) was added
and the mixture was extracted with CH2Cl2 (3 × 10 mL). The com-
bined organic layers were dried with MgSO4 and the volatiles were
removed under reduced pressure. The residue was purified by FC.
X-ray Crystal Data: CCDC 1454734 (for 2r), 1454733 (for 2t),
1454732 (for 4m), and 1454735 (for 4n) contain the supplementary
Experimental Section
General: All reactions involving air- or moisture-sensitive reagents crystallographic data for this paper. These data can be obtained
or intermediates were carried out in heat-gun-dried glassware un- free of charge from The Cambridge Crystallographic Data Centre.
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[8] a) Y.-C. Xiao, C.-Z. Yue, P.-Q. Chen, Y.-C. Chen, Org. Lett. 2014, 16, 3208–
3211; b) N. Chopin, H. Gérard, I. Chataigner, S. R. Piettre, J. Org. Chem.
2009, 74, 1237–1246; c) A. S. Kyei, K. Tchabanenko, J. E. Baldwin, R. M.
Adlington, Tetrahedron Lett. 2004, 45, 8931–8934; d) P. A. Caruana, A. J.
Frontier, Tetrahedron 2004, 60, 10921–10926; e) D. Qin, R. X. Ren, T. Siu,
C. Zheng, S. J. Danishefsky, Angew. Chem. Int. Ed. 2001, 40, 4709–4713;
Angew. Chem. 2001, 113, 4845–4849; f) G. Stork, A. Yamashita, J. Adams,
G. R. Schulte, R. Chesworth, Y. Miyazaki, J. J. Farmer, J. Am. Chem. Soc.
2009, 131, 11402–11406; g) S. France, J. Boonsombat, C. A. Leverett, A.
Padwa, J. Org. Chem. 2008, 73, 8120–8123; h) P. J. Parsons, C. S. Penkett,
A. J. Shell, Chem. Rev. 1996, 96, 195–206.
Acknowledgments
The authors thank the University of Münster and the Chinese
Scholarship Council (stipend to Z. H.) for supporting our work.
Keywords: Nitrogen heterocycles · Oxygen heterocycles ·
Dearomatization · Palladium · Diastereoselectivity · Lactones
[1] a) P. A. Cockrum, S. M. Colegate, J. A. Edgar, K. Flower, D. Gardner, R. I.
Willing, Phytochemistry 1999, 51, 153–157; b) Y. Sasaki, D. Kato, D. L.
Boger, J. Am. Chem. Soc. 2010, 132, 13533–13544; c) J. Yang, H. Wu, L.
Shen, Y. Qin, J. Am. Chem. Soc. 2007, 129, 13794–13795.
[9] S. Kirchberg, R. Fröhlich, A. Studer, Angew. Chem. Int. Ed. 2010, 49, 6877–
6880; Angew. Chem. 2010, 122, 7029–7032.
[10] S. Kirchberg, R. Fröhlich, A. Studer, Angew. Chem. Int. Ed. 2009, 48, 4235–
4238; Angew. Chem. 2009, 121, 4299–4302.
[2] a) T. Hayashi, R. H. Thomson, Phytochemistry 1975, 14, 1085–1087; b) F.-
N. Ko, T.-S. Wu, M.-J. Liou, T.-F. Huang, C.-M. Teng, Eur. J. Pharmacol. 1992,
218, 129–135.
[3] D. Zhang, H. Song, Y. Qin, Acc. Chem. Res. 2011, 44, 447–457.
[4] D. Liu, G. Zhao, L. Xiang, Eur. J. Org. Chem. 2010, 3975–3984.
[5] A. Martínez, M. J. Webber, S. Müller, B. List, Angew. Chem. Int. Ed. 2013,
52, 9486–9490; Angew. Chem. 2013, 125, 9664–9668.
[11] a) T. Vogler, A. Studer, Synthesis 2008, 1979–1993; b) L. Tebben, A. Studer,
Angew. Chem. Int. Ed. 2011, 50, 5034–5068; Angew. Chem. 2011, 123,
5138–5174; c) S. Wertz, A. Studer, Green Chem. 2013, 15, 3116–3134.
[12] a) K. Hata, Z. He, C. G. Daniliuc, K. Itami, A. Studer, Chem. Commun. 2014,
50, 463–465; b) V. Ramella, Z. He, C. G. Daniliuc, A. Studer, Org. Lett. 2015,
17, 664–667.
[13] a) F. Focante, I. Camurati, D. Nanni, R. Leardini, L. Resconi, Organometal-
lics 2004, 23, 5135–5141; b) B. S. Lane, M. A. Brown, D. Sames, J. Am.
Chem. Soc. 2005, 127, 8050–8057; c) N. P. Grimster, C. Gauntlett, C. R. A.
Godfrey, M. J. Gaunt, Angew. Chem. Int. Ed. 2005, 44, 3125–3129; Angew.
Chem. 2005, 117, 3185–3189.
[6] T. D. Sheppard, J. Chem. Res. 2011, 35, 377–385.
[7] a) W. Zi, Z. Zuo, D. Ma, Acc. Chem. Res. 2015, 48, 702–711; b) A. Acharya,
D. Anumandla, C. S. Jeffrey, J. Am. Chem. Soc. 2015, 137, 14858–14860;
c) Z. Chai, J.-N. Chen, Z. Liu, X.-F. Li, P.-J. Yang, J.-P. Hu, G. Yang, Org.
Biomol. Chem. 2016, 14, 1024–1030; d) M. C. DiPoto, R. P. Hughes, J. Wu,
J. Am. Chem. Soc. 2015, 137, 14861–14864; e) L. Han, W. Zhang, X.-X. Shi,
S.-L. You, Adv. Synth. Catal. 2015, 357, 3064–3068; f) C. N. Rao, C. Bentz,
H.-U. Reissig, Chem. Eur. J. 2015, 21, 15951–15954; g) G. Ramachandran,
K. I. Sathiyanarayanan, Current Organocatalysis 2015, 2, 14–26.
[14] T. A. Dwight, N. R. Rue, D. Charyk, R. Josselyn, B. DeBoef, Org. Lett. 2007,
9, 3137–3139.
Received: February 20, 2016
Published Online: April 19, 2016
Eur. J. Org. Chem. 2016, 2268–2273
www.eurjoc.org
2273
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim