Diastereoselective synthesis of cyclopropanes bearing trifluoromethyl-substituted all-carbon quaternary centers from 2-trifluoromethyl-1,3-enynes beyond fluorine elimination

Article abstract of DOI:10.1039/c9cc00785g

Herein, a one-pot, two-step procedure for the diastereoselective synthesis of cyclopropanes bearing trifluoromethyl-substituted all-carbon quaternary centers has been described. Trifluoromethyl-activated 1,3-enynes undergo cyclopropanation reactions with sulfur ylides under mild reaction conditions without fluoride elimination, which affords the cis-isomer mainly. Interestingly, a sequential TBAF-mediated deprotection of the triisopropylsilyl group results in a diastereoenriched epimerization which gives rise to the trans-cyclopropanes as the sole isomers.

Full text of DOI:10.1039/c9cc00785g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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DOI: 10.1039/C9CC00785G
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Diastereoselective synthesis of cyclopropanes bearing
trifluoromethyl-substituted all-carbon quaternary centers from 2-
trifluoromethyl-1,3-enynes beyond fluorine elimination
Received 00th January 20xx,
Accepted 00th January 20xx
Shujie Chen,a,b Jinhui Zhang,a Mingfeng Yang,a Fenggang Liu,a Zhipeng Xie,a Yunlin Liu,*a
Wenxia Lin,a Daru Wang,a Xiangrui Li,a and Jiahai Wang*a
DOI: 10.1039/x0xx00000x
Here, a one-pot two-step procedure for the diastereoselective this field.7 In current approaches, incorporating a strong
synthesis of cyclopropanes bearing trifluoromethyl-substituted all- electron-withdrawing group (e.g., ketone, cyano, sulfone) to
a) Cyclopropanation via heteroatom-derived ylides (N, S, P, As, Te)
carbon quaternary centers was described. Trifluoromethyl-
activated 1,3-enynes undergo cyclopropanation reactions with
CH LG
H
EWG
LG
EWG
H
H
sulfur ylides under mild reaction conditions without fluoride
elimination, which affords the cis-isomer mainly. Interestingly,
sequential TBAF-mediated deprotection of the triisopropylsilyl
group results in diastereoenriched epimerization which gives rise to
the trans-cyclopropanes as the sole isomer.
H
or
EWG
EWG = -COR, -CN, -PO(OR)2, -SO2R
CF
EWG
How about
?
3
b) Trifluoromethyl alkenes as synthetic intermediates
CF2
Nu- or radical
CF3
3 + 2
SN2' type
F3C
gem-difluoroalkene
Cyclopropane is a basic structural subunit found in a diverse
range of naturally occurring secondary metabolites,1 as well as
many therapeutic agents,2 in which the cyclopropane plays an
important role in biological activity. Furthermore, the high
strain energy of three-membered ring makes them versatile
synthetic intermediates for the synthesis of more complex
(hetero)cyclic systems and acyclic alkanes.3 Over the years,
increasingly synthetic efforts have been focused on the
development of new strategies and methodologies to generate
functionalized cyclopropanes stereo-selectively, as well as
employing them as key intermediates to access challenging
target molecules.4 Among the existing methods for
cyclopropane generation, cyclopropanations of olefins
represent the most common strategies, such as
Simmons−Smith reactions, carbenoid-mediated reactions with
electron-neutral/-rich alkenes,5 and Michael-initiated ring
closure reactions with electron-deficient alkenes. Ylide acting as
cyclopropanation reagent undergoing Michael-initiated ring
closure reactions with enones was firstly showcased by Corey
and Chaykovsky.6 Since then, great progress has been made in
2 + 1
This Work
F3C
Scheme
1 Typical ylide cyclopropanation reactions and
background of trifluoromethyl alkenes
the alkenes was necessary in order to decrease the LUMO
energy level of the olefins (Scheme 1, a). Due to the highly
electron-withdrawing effects of the trifluoromethyl group,8 we
question whether the CF3-substituted alkenes are capable of
undergoing ylide cyclopropanation.
Molecules incorporating fluorine always fascinate chemists
owing to the positive effects of fluorine in biochemical
sciences.9 Due to their ready availability and unique reactivity,
trifluoromethyl alkenes have received considerable attention
and become promising fluorine-containing building blocks in
past years. Typically, trifluoromethyl alkenes are subjected to
addition reactions with nucleophiles or radicals concomitant
with cleavage of a C-F bond to afford gem-difluoroalkenes (SN2′-
type reaction) (Scheme 1, b, right side).10 By contrast,
transformations incorporating a trifluoromethyl group from
trifluoromethyl alkenes into products were still rare.
Preliminary experimental results from Béguéet and co-workers
proved the reactivity of trifluoromethyl alkenes in Diels-Alder
reactions with Danishefsky's diene, as well as in 1,3-dipolar
cycloadditions with azomethine ylide or nitrone.11 Furthermore,
a. School of Chemistry and Chemical Engineering, Guangzhou University, 230 Wai
Huan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, P.
R. China.
b. Key Laboratory of Functional Molecular Engineering of Guangdong Province
(South China University of Technology), Guangzhou 510640, P. R. China.
E-mail: ylliu@gzhu.edu.cn, jiahaiwang@gzhu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Trost et al. also realized
a palladium-catalyzed [3+2]-
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cycloaddition of trimethylenemethane with trifluoromethyl a Reaction conditions: 1 (0.33 mmol), 2a (0.30 mmVoiel)wiAnrti2clemOLnlionef
DOI: 10.1039/C9CC00785G
alkenes to furnish cyclopentanes bearing CF3 group (Scheme 1, MeCN at room temperature for 48 h; Isolated yields; dr values
b, left side).12 To the best of our knowledge, trifluoromethyl were determined by 1H NMR.
olefins engaging in Michael-initiated ring closure reactions with Table 2 Scope of sulfur ylidesa
O
TIPS
ylides has never been reported. Here, we report a mild one-pot
O
Me
S
CF3
R
procedure for the diastereoselective cyclopropanation of
trifluoromethyl-enynes using sulfur ylide reagents.13 The key to
our success is to figure out the relationship between the
reactivity and the substitution fashion of trifluoromethyl
olefins.
MeCN, rt, 48 h,
then TBAF wrokup
R
p-FPh
F3C
1e
2
3
O
3ea
3eb
3ec
3ed
3ee
(R = Br): 77%, dr > 20:1
(R = Cl): 85%, dr > 20:1
(R = H): 76%, dr > 20:1
(R = i-Pr): 82%, dr > 20:1
(R = CN): 65%, dr > 20:1
O
F3C
3ef
F3C
R
b
In initial experiments, we performed the cyclopropanation
reaction of benzoyl sulfur ylide 2a with different types of 2-
(trifluoromethyl)-1-alkenes 1a-e in MeCN at room temperature
for 2 days (Table 1). Among the tested 2-(trifluoromethyl)-1-
alkenes, substrates bearing aryl (1a), styryl (1b), and alkyl (1c)
substituents all failed to afford the desired product under the
reaction condition and even at elevated temperature (for more
details, see the Table S1). To our delight, trifluoromethyl-enyne
1d displayed enhanced reactivity towards cyclopropanation
reaction, which yielded the desired cyclopropane product 3da
in 92% yield with 74:26 dr. No defluorinative byproduct was
detected by GC-MS analysis. We speculated that the high level
of s-orbital character enhanced the electron-withdrawing
character of the alkynyl group which further lowered the LUMO
energy level of trifluoromethyl-enyne. Similarly, the reaction of
triisopropylsilyl (TIPS)-protected trifluoromethyl-enyne 1e with
2a occurred smoothly to form 3ea in 85% yield, 84:16 dr. Quite
unexpectedly, an attempt to remove the TIPS group by in situ
treatment with TBAF gave rise to a terminal alkyne with greatly
diastereoselectivity enhancement (>20:1 dr). Similarly, 3da was
also obtained as a sole isomer by treating the corresponding
reaction mixture with TBAF (for more details, see the Table S1).
Further optimization of the reaction solvents showed that
MeCN and THF were both appropriate reaction solvents, while
alcoholic solvents proved to be unfavorable (for more details,
see the Table S2).
: 77% (74%) , dr > 20:1
O
O
F
F3C
NO2
F3C
3eh
3eg
: 58%, dr >20:1
(X-ray)
: 80%, dr >20:1
O
O
O
OMe
OMe
O
F3C
F3C
Cl
F3C
3ej
O
CF3
OMe
3ei
3ek:
: 65%, dr >20:1
: 70%, dr >20:1
Me
83%, dr >20:1
O
O
O
S
Me
F3C
F3C
3em
F3C
3el
3en
:40%, dr >20:1
: 82%, dr >20:1
: 46%, dr >20:1
a Reaction conditions: 1e (0.33 mmol), 2 (0.30 mmol) in 2 mL of
MeCN at room temperature for 48 h, then TBAF (0.33 mmol, 1
M in THF), 0.5 h; Isolated yields; dr values were determined by
1H NMR. b Reaction was performed at 2 mmol scale.
position were converted into the cyclopropanes smoothly (3ea-
3ed: 76-85% yields, and >20:1 dr). Those with electron-
withdrawing substituents (e.g., CN, NO2) also tolerated the mild
cyclopropanation conditions with excellent stereocontrol,
albeit in decreased yield (3ee and 3eg). The relative
configuration of 3eg was determined by single crystal X-ray
diffraction analysis, which showed the trans-relationship
between the CF3 and the carbonyl group. Notably, the
transformation of naphthyl sulfur ylide with 1e ran smoothly
even at 2 mmol scale (3ef). Introducing a fluorine atom to the
2-position of benzene ring did not impact on the reaction’s
efficiency and selectivity (3eh). In the case of substrates with
electron donating groups, transformations also proceeded well
and yielded the products 3ej and 3ek in 70% and 83% yields
respectively in a completely stereoselective fashion. Aromatic
hetereocyclic precursor 2l was also tolerated and gave the
corresponding product in high yield and with excellent
diastereoselectivity (3el, 82% yiled, and >20:1 dr). Aliphatic
sulfur ylides bearing cyclopropyl and i-butyl proved to be less
reactive, delivering the corresponding cyclopropanes in
decreased yields, both with >20:1 dr (3em and 3en).
With the optimal conditions in hand, the scope of the sulfur
ylides was explored. As summarized in Table 2, moderate to
good yields and excellent diastereoselectivity were obtained
using sulfur ylides with various arene substitution patterns.
Substrates bearing bromo, chloro, and isopropyl at the para-
Table 1 Substituent effects for cyclopropanation reactions of
trifluoromethyl alkenesa
O
Me
S
O
R
F3C
R
CF3
p-FPh
+
MeCN, rt, 48 h
Br
Br
1
2a
3aa ea
-
CF3
CF3
CF3
To prove the generality of the present method, other types of
trifluoromethylenynes were evaluated (Table 3).14 Gratifyingly,
aromatic (1d, i) and heteroaromatic (1j) trifluoromethylenynes
underwent cyclopropanation reactions and subsequent
diastereoenriched process smoothly to afford the products in
moderate to good yields, again with excellent
MeO
1a
1b
1c
, 0% yield
, 0% yield
Ph
, 0% yield
TIPS
CF3
CF3
1d
1e
, 85% yield, 84:16 dr
, 92% yield, 74:26 dr
2 | J. Name., 2012, 00, 1-3
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CF3
diastereoselectivity (3db, 3ia and 3ja). Noteworthily, this
protocol was successfully used in late-stage functionalization of
drug-scaffolds. For example, dehydroepiandrosterone (1k) and
ethynodiol diacetate-derived 1,3-enynes (1l) could undergo
cyclopropanations under standard conditions, affording
compounds of particular interest for the medicinal chemistry.
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DOI: 10.1039/C9CC00785G
N
O
N
N
a)
BnN3
CuI
R
c)
Ph
R
HetAr-I
O
O
F3C
[Pd]
d)
[Pd]
N
6
4
5
CF3
N
b)
F3C
O
O
Ph
Ph
Br
Benzaldoxime
3ef
R
PIFA
R
F3C
O
Ph
7
Scheme 2. The synthetic transformations of product 3ef.
Table 3 Scope of 2-Trifluoromethyl-1,3-enynesa
Reagents and conditions: a) CuI (0.5 equiv), benzyl azide (4
R
o
O
equiv), MeOH, 80 C, Ar, 48 h, 95%; b) benzaldehyde oxime (4
R
O
Me
S
Ar
MeCN, rt, 48 h,
then TBAF workup
equiv), PIFA (4 equiv), MeOH/H2O (5:1 v/v), rt, 24 h, 61%; c) 6-
iodoquinoline (1.2 equiv), PdCl2(PPh3)2 (0.05 equiv), CuI (0.1
equiv), i-Pr2NH (10 equiv), THF, 50 oC, Ar, 24 h, 98%; d)
(bromoethynyl)benzene (2 equiv), Pd(dba)2 (0.05 equiv), CuI
(0.05 equiv), Et3N (2 equiv), THF, rt, Ar, 12 h, 63%. PIFA =
(bis(trifluoroacetoxy)iodo)benzene.
Ar
p-FPh
F3C
CF3
1
2
3
MeO2C
S
Ph
O
O
O
Ar
Ar
Ar
F3C
F3C
3ja
F3C
3ia
Ar = p-BrPh:
3db
Ar = p-ClPh:
Ar = p-BrPh:
63%b, dr > 20:1
90% yield, dr > 20:1
68%, dr > 20:1
Finally, to gain further insight into the process of
stereoselectivity enrichment, several common bases were
evaluated for their ability in such transformation, including
KOH, K2CO3 and Et3N. Inorganic base KOH and K2CO3 delivered
the cyclopropane 3da in 77% and 85% isolated yields,
respectively, both as the sole isomer. However, Et3N failed to
facilitate such process (for more details, see the Table S3). In
addition, such a desilylation of TIPS-protected alkyne was
conducted by treatment with AgF instead of TBAF, which
selectively delivered the terminal alkyne without deprotonation
on the carbon α to the carbonyl group.19 Such conversion gave
rise to diastereosiomeric mixtures, in which the major isomer
could be isolated by common column chromatography in 69%
O
F3C
H
O
O
O
H
Ar
F3C
H
Ar
O
O
O
O
3kf
3la
(Ar = 2-Naphthyl): 45%, dr = 1:1
from Dehydroepiandrosterone
(Ar = p-BrPh): 47%, dr = 1:1
from Ethynodiol diacetate
a Reaction conditions: 1 (0.30 mmol), 2 (0.33 mmol) in 2 mL of
MeCN at room temperature for 48 h, then TBAF (0.33 mmol, 1
M in THF), 0.5 h; Isolated yields; dr values were determined by
1H NMR. b 0.6 mmol TBAF and 2 h was required in the workup
procedure.
yield
(eq
1).
Quite
unexpectedly,
To further demonstrate the synthetic utility of the resulting
products, compound 3ef was subjected to a series of chemical
transformations. Obviously, functional group manipulation of
the terminal triple bond is straightforward. The 3ef can act as a
good dipolarophile undergoing 1,3-dipolar cycloaddition with
1,3-dipolar compounds. For instance, the CuI-catalyzed azide-
alkyne Huisgen cycloaddition of benzyl azide with 3ef generated
1,2,3-triazole-containing CF3-quaternary center in high yield,
which was not easy to access previously (Scheme 2a).15
Similarly, isoxazole 5 can be prepared by the addition of 3ef to
oxime, which proceed via a nitrile oxide intermediate (Scheme
2b).16 In addition, the terminal alkynyl moiety can undergo
transition metal-catalyzed cross-coupling reactions. For
example, a typical Sonogashira reaction of 3ef with 6-
iodoquinoline delivered the heterocycle-containing product 6 in
O
O
Ha
Ha
i) MeCN, rt, 48 h
1e
+
2a
+
(1)
ii) AgF (150 mol%), rt, 3 h,
then HCl workup
(110 mol%) (100 mol%)
CF3
CF3
4ea
Br
Br
3ea
(69% yield)
almost quantitative yield (Scheme 2c). Furthermore,
a
palladium-catalyzed cross-coupling reaction allowed for the
introduction of a unsymmetrical 1,3-diyne 7 (Scheme 2d).17
These transformations furnished various cyclopentanes bearing
CF3-substituted quaternary center. Owing to the beneficial
effect of CF3 group on many bioactive pharmaceuticals, the
present transformations will raise the interest of medicinal
chemists.18
1
Figure 1. H NMR studies of the stereochemistry (illustrating
from 1.5-3.5 ppm). Spectrum (a) refers to the diastereoisomeric
mixture resulting from eq 1; Spectrum (b) refers to major
isomer from eq 1 after flash column chromatography; Spectrum
(c) refers to the product 3ea resulting from standard conditions.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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an epimerization was observed by 1H NMR spectroscopy
analysis of the diastereoisomeric mixtures, major isomer 4ea
and 3ea (Figure 1a, 1b, 1c, respectively). Combined with the X-
ray crystallographic analysis of 3eg, we may reasonably come to
the conclusion that the cyclopropanation reactions delivered
the cis-cyclopropane as the major isomer originally, and then a
base-promoted thermodynamic epimerization took place,
driving the carbonyl group from cis-position to the trans-
position relative to the CF3 group (Figure 2). Such type of
epimerization was also observed in the case of 1d (for more
details, see the Figure S1).
4
Reviews and recent examples, see: (a) L. Dian, I. Marek,
Chem. Rev., 2018, 118, 8415; (b) C. EbDnOerI:,1E0..1M03.9VCi/eCawr9rACertCiicr0lae0,O78nl5inGe
Chem. Rev., 2017, 117, 11651; (c) D. Qian, J. Zhang, Chem.
Soc. Rev., 2015, 44, 677; (d) H. Lebel, J. Marcoux, C.
Molinaro, A. B. Charette, Chem. Rev., 2003, 103, 977; (e) Y.
Guo, T. Liang, S. W. Kim, H. Xiao, M. J. Krische, J. Am. Chem.
Soc., 2017, 139, 6847; (f) A. Tinoco, Viktoria. Steck, V. Tyagi,
R. Fasan, J. Am. Chem. Soc., 2017, 139, 5293.
5
6
(a) S. Chanthamath, S. Iwasa, Acc. Chem. Res., 2016, 49, 2080;
(b) L. R. Collins, M. Gastel, F. Neese, A. Fürstner, J. Am. Chem.
Soc. 2018, 140, 13042.
(a) E. J. Corey, M. Chaykovsky, J. Am. Chem. Soc., 1962, 84,
867; (b) E. J. Corey, M .Chaykovsky, J. Am. Chem. Soc., 1965,
87, 1353; (c) Yu. G. Gololobov, A. N. Nesmeyanov, V. P.
Lysenko, I. E. Boldeskul, Tetrahedron, 1987, 43, 2609.
Review and selected examples; see: (a) X. Sun, Y. Tang, Acc.
Chem. Res., 2008, 41, 937; (b) S. L. Riches, C. Saha, N. F.
Filgueira, E. Grange, E. M. M. McGarrigle, V. K. Aggarwal, J.
Am. Chem. Soc., 2010, 132, 7626; (c) C. C. C. Johansson, N.
Bremeyer, S. V. Ley, D. R. Owen, S. C. Smith, M. J. Gaunt,
Angew. Chem. Int. Ed., 2006, 45, 6024.
7
O
O
O
- H+
+ H+
+ H+
- H+
R
R
R
F3C
F3C
F3C
Figure 2. The mechanism of thermodynamic epimerization.
8
9
J. Nie, H. Guo, D. Cahard, J. Ma, Chem. Rev., 2011, 111, 455.
(a) P. Kirsch, Modern Fluoroorganic Chemistry. Synthesis,
Reactivity, Applications, Wiley-VCH, Weinheim, 2nd edn,
2013; (b) K. Müller, C. Faeh, F. Diederich, Science, 2007, 317,
1881; (c) S. Purser, P. R. Moore, S. Swallowb, V. Gouverneur,
Chem. Soc. Rev., 2008, 37, 320; (d) J. Wang, M. Sánchez-
Roselló, J. L. Aceña, C. del Pozo, A. E. Sorochinsky, S. Fustero,
V. A. Soloshonok, H. Liu, Chem. Rev., 2014, 114, 2432; (e) E. P.
Gillis, K. J. Eastman, M. D. Hill, D. J. Donnelly, N. A. Meanwell,
J. Med. Chem., 2015, 58, 8315.
In summary, we have developed the first highly
diastereoselective
cyclopropanation
reactions
of
trifluoromethyl-enynes with sulfur ylides via a maneuverable
one-pot, two-step procedure, in which the CF3 group acts as a
novel electron-withdrawing group to enhance the
nucleophilicity of the olefin. A base-triggered thermodynamic
epimerization took place during the process, resulting
stereoselectivity enrichments. This approach allows for the
access of cyclopropanes bearing CF3-substituted all-carbon
10 Transition-metal catalyzed SN2’-type addition: (a) X. Lu, X.
Wang, T. Gong, J. Pi, S. He, Y. Fu, Chem. Sci., 2019, 10, 809; (b)
Y. Lan, F. Yang, C. Wang, ACS Catal., 2018, 8, 9245; (c) P. Gao,
C. Yuan, Y. Zhao, Z. Shi, Chem, 2018, 4, 2201; Transition-metal
free SN2’-type addition: (d) K. Fuchibe, H. Hatta, K. Oh, R. Oki,
J. Ichikawa, Angew. Chem. Int. Ed., 2017, 56, 5890; (e) K.
Hirotaki, T. Hanamoto, Org. Lett., 2013, 15, 1226; SN2’-type
addition/cyclization: (f) K. Fuchibe, M. Takahashi, J. Ichikawa,
Angew. Chem., 2012, 124, 12225; (g) X. Zhou, C. Huang, Y.
Zeng, J. Xiong, Y. Xiao, J. Zhang, Chem. Commun., 2017, 53,
1084; Radical addition: (h) S. B. Lang, B. J. Wiles, C. B. Kelly, G.
A. Molander, Angew. Chem. Int. Ed., 2017, 56, 15073.
11 D. Bonnet-Delpon, J. Begue, T. Lequeux, M. Ourevitch,
Tetrahedron, 1996, 52, 59.
12 B. M. Trost, L. Debien, J. Am. Chem. Soc., 2015, 137, 11606.
13 Reviews on sulfur ylides, see: (a) L. Lu, T. Li, Q. Wang, W.
Xiao, Chem. Soc. Rev., 2017, 46, 4135; (b) J. D. Neuhaus, O.
Rik, J. Merad, N. Maulide, Top. Curr. Chem., 2018, 376, 1.
14 Relative stereochemistry of products was assigned in analogy
to that determined for 3eg.
quaternary
centers.
The
resulting
CF3-substituted
cyclopropanes incorporating an alkynyl group could also
contribute to the diversity-oriented synthesis of fluoroalkylated
compounds. The successful application of trifluoromethyl
olefins as Michael-type acceptors in this work improves the
scope of typical cyclopropanation reactions of ylides. Further
studies including exploring chiral sulfur ylides as well as new
methodology based on trifluoromethyl olefins are in progress.
We are grateful to the National Natural Science Foundation
of China (No. 21575078, 21805049 and 21801050), the Open
Fund of the Key Laboratory of Functional Molecular Engineering
of Guangdong Province (No. 2018kf03, South China University
of Technology). Dr. Meng Yang is kindly acknowledged for single
crystal structural analyses.
15 M. Meldal, C. W. Tornøe, Chem. Rev., 2008, 108, 2952.
16 (a) A. M. Jawalekar, E. Reubsaet, F. P. J. T. Rutjes, F. L. van
Delft, Chem. Commun., 2011, 47, 3198; (b) W. Guo, Z. Luo, W.
Zeng, X. Zhang, ACS Catal., 2017, 7, 896.
17 W. Shi, Y. Luo, X. Luo, L. Chao, H. Zhang, J. Wang, A. Lei, J.
Am. Chem. Soc., 2008, 130, 14713.
Conflicts of interest
There are no conflicts to declare.
18 Example about metabolic stability of CF3-substituted
cyclopropane: D. Barnes-Seeman, M. Jain, L. Bell, S. Ferreira,
S. Cohen, X. Chen, J. Amin, B. Snodgrass, P. Hatsis, ACS Med.
Chem. Lett., 2013, 4, 514.
Notes and references
1
(a) J. Pietruszka, Chem. Rev., 2003, 103, 1051; (b) D. Y.-K.
Chen, R. H. Pouwer, J.-A. Richard, Chem. Soc. Rev., 2012, 41,
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19 S. Kim, B. Kim, J. In, Synthesis, 2009, 12, 1963.
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