Kinetic Resolution of β-Sulfonyl Ketones through Enantioselective β-Elimination using a Cation-Binding Polyether Catalyst

Article abstract of DOI:10.1002/anie.201508127

Reported herein is the first enantioselective β-elimination reaction catalyzed by a chiral cation-binding polyether. By using this catalytic protocol, a wide range of β-sulfonyl ketones could be effectively resolved with high stereoselectivity (S up to >300). Key to the success of this process is the favorable secondary interactions of the catalyst with the Lewis basic groups on the sulfone substrate. The enone product of this process can be easily converted into the racemic starting material, and allows an effective recycling and overall synthesis of chiral β-sulfonyl ketones in high yield and excellent enantioselectivity.

Full text of DOI:10.1002/anie.201508127

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201508127
German Edition: DOI: 10.1002/ange.201508127
Asymmetric Catalysis
Kinetic Resolution of b-Sulfonyl Ketones through Enantioselective
b-Elimination using a Cation-Binding Polyether Catalyst
Liang Li+, Yidong Liu+, Yang Peng, Lei Yu, Xiaoyan Wu, and Hailong Yan*
Abstract: Reported herein is the first enantioselective b-
elimination reaction catalyzed by a chiral cation-binding
polyether. By using this catalytic protocol, a wide range of b-
sulfonyl ketones could be effectively resolved with high
stereoselectivity (S up to > 300). Key to the success of this
process is the favorable secondary interactions of the catalyst
with the Lewis basic groups on the sulfone substrate. The enone
product of this process can be easily converted into the racemic
starting material, and allows an effective recycling and overall
synthesis of chiral b-sulfonyl ketones in high yield and
excellent enantioselectivity.
tiomer of a racemic substrate to produce the enone product,
an efficient kinetic resolution[5] of such functionalized mole-
cules can be realized. Herein, we report the first example of
such a reaction to access a wide range of b-sulfonyl ketones in
high enantiopurity, and it is promoted by a chiral ion-pairing
catalyst.[6]
In our studies, we chose chiral sulfones as the target,
because they are known to possess a wide spectra of biological
activities,[7] such as cancer agents,[7d] secretase inhibitors for
the treatment of Alzheimerꢀs disease,[7f] and antibacterial
agents.[7g] Previous syntheses of these functionalities mainly
focused on metal-catalyzed hydrogenation,[8] substitution,[9]
[11]
icinal elimination[1] of a hydrogen and a leaving group, such
as halogen, carboxylate, sulfone, etc., is one of the most
fundamental organic transformations for the construction of
cycloaddition,[10] and C H insertion. Lewis acid mediated
À
V
Diels–Alder reactions have also been developed to produce
the desired targets in high enantioselectivity.[12] More specif-
ically, organocatalytic methods have also been successfully
applied to the preparation of chiral sulfones, most notably
from the groups of Alexakis, Deng, Tian, Chen, and Chi.[13]
Our group has been interested in the use of chiral polyethers
as cation-binding catalysts in asymmetric catalysis,[14] and we
were curious whether these catalysts could be applicable to
asymmetric b-elimination reactions. As proposed in
Scheme 2, while the polyether catalyst can capture KF by
=
C C bonds and has been widely employed in the synthesis of
natural products as well as medicines.[2] In particular, b-
elimination of ketones provides efficient access to enones
which are valuable functionalities and intermediates in
organic synthesis (Scheme 1a). Although various catalytic
Scheme 2. Proposed reaction transition state.
Scheme 1. Base-promoted b-elimination of ketones.
a phase-transfer pathway, its Brønsted-acidic moieties may be
involved in secondary interactions, such as hydrogen bonding,
with the Lewis basic groups on the sulfonyl ketone substrate,
and preferentially with one of the enantiomers over the other.
The activated fluoride anion would then act as a base to
deprotonate the a-proton. The elimination of b-sulfone would
follow to afford the a,b-unsaturated ketone and lead to
a kinetic resolution of the racemic starting sulfonyl ketones.
We initiated our studies by examining the elimination of
racemic 2a in the presence of potassium fluoride and
a BINOL-based cation-binding catalyst (Table 1). Different
catalysts were examined at 10 mol% loading in 1,4-dioxane at
258C. Based on our knowledge of the catalytic performance
of chiral bis(hydroxy) polyethers, we systematically modified
the length of the polyether chain, which is known to be
responsible for the formation of a suitable chiral coordination
cage for a potassium cation. Indeed, the length of the
systems have been developed to achieve excellent efficiency
of this process,[3] to the best of our knowledge, an enantio-
selective variant of the b-elimination reaction of a b-func-
tional ketone has never been reported in the literature.[4] As
shown in Scheme 1b, one can imagine that, if a chiral
environment can induce selective elimination of one enan-
[*] L. Li,[+] Y. Liu,[+] Y. Peng, L. Yu, X. Wu, Prof. H. Yan
Innovative Drug Research Centre, Chongqing University
Chongqing 401331 (China)
E-mail: yhl198151@cqu.edu.cn
[+] These authors contributed equally to this work.
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201508127.
Angew. Chem. Int. Ed. 2016, 55, 331 –335
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
331

Angewandte
Communications
Chemie
Table 1: Optimization of the reaction conditions.[a]
pletely inactive and the free terminal hydroxy group at the
2,2’-position of the BINOL backbone played an essential role
in maintaining the superior catalytic performance of the
multifunctional catalyst. In further experiments, the enantio-
selectivity was found to be highly dependent on the choice of
solvent, with 1,4-dioxane remaining the optimal one
(entries 7–10). Further optimization of the reaction condi-
tions on potassium fluoride loading and concentration
identified the optimal reaction conditions for both conversion
and enantioselectivity (entry 11).[15]
With the optimal catalytic conditions in hand, we turned
to the investigation of substrate scope. As shown in Table 2,
the high selectivity achieved by (R)-B could be extended to
a wide range of aryl and heteroaryl sulfones. Substrates with
aryl groups were well-tolerated, including those bearing
either electron-donating or electron-withdrawing groups
[(R)-2a–l; S value from 10 to 327]. Additionally, heteroaryl
substrates proved to be excellent substrates for the reaction,
thus affording the unreacted sulfones with high enantioselec-
tivity as well as selectivity factors [(R)-2m–o; S value from 24
to 327]. The position of the substituents, however, affected the
selectivity of the reaction. As a general trend, substrates with
substituents on R2 showed higher selectivity [(R)-2j, S > 200;
(R)-2k, S = 193; (R)-2l, S > 300; (R)-2n, S > 300]. In most
cases, the unreacted secondary sulfones were recovered in
high to excellent enantiopurity. Moreover, extensive efforts
were also spent on the reaction of sulfones with an aliphatic
chain. Only the primary-alkyl-substituted substrates can be
resolved with good selectivity [(R)-2p, S > 200; (R)-2q,
S = 48].
Entry
Catalyst
Solvent
Conv. [%][b]
ee [%][c]
S[d]
18
42
6
n.d.
n.d.
n.d.
5
1
2
3
4
5
6
7
8
9
(R)-A
(R)-B
(R)-C
(R)-D
(R)-E
–
(R)-B
(R)-B
(R)-B
(R)-B
(R)-B
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
CH2Cl2
toluene
furan
THF
1,4-dioxane
49
51
48
11
26
26
45
52
57
45
50
75
90
52
n.d.
<5
0
43
54
62
56
5
5
9
10
11[e]
97
>200
[a] Reaction conditions: 2a (0.1 mmol), KF (0.12 mmol), catalyst
(0.01 mmol) in solvent (0.75 mL) at room temperature for 48 h, unless
otherwise specified. [b] Determined by 1H NMR analysis. [c] The ee value
was determined by HPLC analysis (see the Supporting Information). The
absolute configuration of the unreacted (R)-2a was assigned by
comparison of the measured optical rotation with the reported value.
[d] Selectivity values were calculated by the methods of Fiaud:
S=ln[(1ÀConv.)(1Àee1)]/ln [(1ÀConv.)(1+ee1)]. [e] 1.5 equiv KF,
1.0 mL solvent was used. n.d.=not determined, THF=tetrahydrofuran.
In addition to a wide range of substitutions on R1 and R2,
different functional groups (R3) on the sulfone were also
tolerated under our catalytic system (Table 3). Substrates
bearing both electron-donating and electron-withdrawing
groups underwent b-elimination smoothly to give the
unreacted sulfones in high enantioselectivity [(R)-2r–x; S
value from 11 to 30]. In addition, a heteroaryl sulfone
substrate was also well-tolerated [(R)-2y, S = 12].
polyether chain proved to play a crucial role in the catalytic
performance in our reaction (entries 1–3). The catalyst
bearing three ether units [(R)-B; S = 42] proved to be
a much more effective catalyst than the catalysts bearing
longer [four ether units; (R)-C; S = 6] or shorter [two ether
units; (R)-A; S = 18] ether units. The 3,3’-diiodo-substituted
catalyst (R)-B showed the highest activity and enantioselec-
tivity, and can be explained by the polarizability of the iodine
atoms and strong coordination of the iodine atoms to the
potassium cation. The large radius of the iodine atom might
also influence the chiral environment of the transition state,
thus resulting in the observed enhancement of enantioselec-
tivity. More catalysts, such as (R)-D, derived from a chiral
backbone, were also examined and demonstrated a reduced
catalytic performance in terms of both conversion and
enantioselectivity (entry 4). When the reaction was carried
out using a catalyst in which the 2,2’-hydroxy groups were
methylated, low conversion and negligible enantioselectivity
were observed (entry 5). Meanwhile, the background reaction
was also examined, and the same level of conversion and
enantioselectivity compared to the result shown in entry 5
were observed under the same reaction conditions. This result
indicated that the di-O-methyl protected (R)-E was com-
The mechanism of b-elimination of sulfones to form the
olefins have been the subject of a large number of inves-
tigations.[16] In our catalytic system, all the experimental
results (see the Supporting Information for more details)
strongly support a stepwise carbanion elimination mecha-
nism. The proposed catalytic cycle is illustrated in Scheme 3.
The catalyst is believed to chelate the potassium ion from KF
to form a chiral ion pair, which then engages the sulfone
substrate to afford the complex I. Although it was difficult to
isolate I in an analytically pure form, it could be observed by
HR-MS. Deprotonation of the a-proton by fluoride then
leads to the formation of the complex II. Finally, the
elimination of the PhSO2 group from the b-carbon atom
yields the olefinic product, the potassium salt, and regener-
ates the catalyst. In this catalytic cycle, the formation of I
serves as the enantiodetermining step, and the chiral environ-
ment of the R-configured catalyst only permits the S-
configured sulfonyl ketone to enter the catalytic cycle. It is
proposed that both the sulfone and the ketone oxygen atom
on S-configured sulfonyl ketone, both of which are hydrogen-
bond acceptors and electron-deficient, are essential for
332
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 331 –335

Angewandte
Communications
Chemie
Table 2: Scope of kinetic resolution with respect to the sulfones.[a]
Table 3: Scope of kinetic resolution with respect to the sulfones.[a]
[a] Unless otherwise indicated, the reactions were carried out with 2
(0.1 mmol), 2.0 equiv of potassium fluoride, and catalyst (10 mol%) in
1,4-dioxane at 258C for 48 hours.
naphthalene moiety on the catalyst may be involved, as well
to further lock the conformation of I during the elimination.
To demonstrate the potential utility of this method,
several transformations of the sulfone (R)-2 f were explored
(Scheme 4). Under the Beckmann rearrangement reaction
conditions, (R)-2 f could be transformed into the amide
product 4 in 71% yield with 96% ee. Moreover, the ketone
group on (R)-2 f could be readily converted into the
hydrazone 5 or alcohol 6 in good yield under mild reaction
conditions. The highly diastereoselective formation of 6 by
LiAlH4 reduction could be attributed by the stereocontrol of
the chiral sulfone moiety on the substrate (R)-2 f. These
examples exhibit a diversified application of our products in
the preparation of various precursors of biologically active
compounds.[17]
[a] Unless otherwise indicated, the reactions were carried out with 2
(0.1 mmol), 1.5 equiv of potassium fluoride, and catalyst (10 mol%) in
1,4-dioxane at 258C for 48 hours. [b] The reaction time was 60 hours.
[c] The reaction time was 20 hours. [d] The reaction time was 28 hours,
20% mmol catalyst, 2.0 equiv of KF.
A unique character of our b-elimination strategy is that
the achiral enone product can be easily converted back into
the racemic substrate by the addition of sulfinic acid. In this
way, the inherent limitation of 50% yield of the kinetic
forming I from the chiral ion pair through hydrogen-bonding
interactions and Lewis acidic potassium. Moreover, the p,p-
stacking between the aryl ring of the substrate and the
Angew. Chem. Int. Ed. 2016, 55, 331 –335
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
333

Angewandte
Communications
Chemie
catalytic enantioselective elimination. After three cycles of
resolution, (R)-2a was obtained in 85% yield with 97% ee.
In conclusion, we have developed the first asymmetric b-
elimination of sulfones catalyzed by a chiral cation-binding
polyether and using potassium fluoride as the base. This
catalytic procedure provides a practical preparation of
various chiral secondary sulfones which are useful building
blocks in organic synthesis. Current efforts in our laboratory
are focused on the detailed mechanistic studies to further
elucidate the origin of the asymmetric induction as well as
extending the synthetic utility of the system to other func-
tional molecules.
Experimental Section
The secondary sulfone (2a–y; 0.1 mmol), potassium fluoride (8.7 mg,
0.15 mmol), and the catalyst (11.9 mg, 0.01 mmol) were added to
a 10 mL flame-dried Schlenk tube containing a stirring bar. 1,4-
Dioxane (1 mL) was injected into the tube at room temperature.
After stirring for 48 hours, the mixture was purified by silica gel
chromatography (petroleum ether/actone 7:1) to afford the products
(R)-2a–y as white solids.
Scheme 3. Proposed reaction mechanism.
Acknowledgements
This work was supported by the Fundamental Research
Funds for the Central Universities (Grant No:
0236015205004) and the 100 Young Plan by Chongqing
University (0236011104404) and NSFC (21402016).
Keywords: asymmetric catalysis · ion pairs · kinetic resolution ·
organocatalysis · sulfones
How to cite: Angew. Chem. Int. Ed. 2016, 55, 331–335
Angew. Chem. 2016, 128, 339–343
[1] a) K. R. Brower, J. S. Chen, J. Am. Chem. Soc. 1965, 87, 3396;
b) B. M. Trost, Acc. Chem. Res. 1978, 11, 453; c) C. J. M. Stirling,
Acc. Chem. Res. 1979, 12, 198; d) N. Ono, R. Tamura, T.
Nakatsuka, J. Hayami, A. Kaji, Bull. Chem. Soc. Jpn. 1980, 53,
3295; e) J. M. Baskin, Z. Wang, Tetrahedron Lett. 2002, 43, 8479;
f) F. Ye, A. Orita, A. Doumoto, J. Otera, Tetrahedron 2003, 59,
5635.
[2] a) P. R. Blakemore, J. Chem. Soc. Perkin Trans. 1 2002, 2563;
b) R. J. K. Taylor, G. Casy, Org. React. 2003, 62, 357; c) J. C.
Carretero, R. G. Arrayµs, J. Adrio, in Organosulfur Chemistry in
Asymmetric Synthesis, Wiley-VCH, Weinheim, 2009, pp. 291 –
320.
[3] a) T. J. Wallace, J. E. Hofmann, A. Schriesheim, J. Am. Chem.
Soc. 1963, 85, 2739; b) P. J. Kocienski, B. Lythgoe, S. Ruston, J.
Chem. Soc. Perkin Trans. 1 1978, 829; c) C. R. Johnson, R. A.
Kirchhoff, J. Am. Chem. Soc. 1979, 101, 3602; d) T. Iwama, T.
Kataoka, O. Muraoka, G. Tanabe, Tetrahedron 1998, 54, 5507;
e) L. Gupta, A. Ramírez, D. B. Collum, J. Org. Chem. 2010, 75,
8392.
Scheme 4. Representative product transformations.
resolution process can be overcome by recycling the enone
product. As an example, the substrate (R)-2a was tested for
a multicycle experiment on a gram scale. As shown in
Scheme 5, after the first enantioselective elimination reac-
tion, the generated enone product 1a could be easily
separated and quantitatively converted into the racemic
sulfone 2a. The regenerated 2a then underwent another
[4] a) G. Zhong, D. Shabat, B. List, J. Anderson, S. C. Sinha, R. A.
Lerner, C. F. Barbas III, Angew. Chem. Int. Ed. 1998, 37, 2481;
Angew. Chem. 1998, 110, 2609; b) A. Gayet, S. Bertilsson, P. G.
Andersson, Org. Lett. 2002, 4, 3777.
[5] For selected reviews on kinetic resolution, see: a) H. B. Kagan,
J. C. Fiaud, Top. Stereochem. 1988, 18, 249; b) J. M. Keith, J. F.
Larrow, E. N. Jacobsen, Adv. Synth. Catal. 2001, 343, 5; c) E.
Scheme 5. Recycling experiment with 2a.
334
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 331 –335

Angewandte
Communications
Chemie
Vedejs, M. Jure, Angew. Chem. Int. Ed. 2005, 44, 3974; Angew.
f) X.-T. Ma, R.-H. Dai, J. Zhang, Y. Gu, S.-K. Tian, Adv. Synth.
Chem. 2005, 117, 4040.
Catal. 2014, 356, 2984.
[10] a) I. Shimizu, Y. Ohashi, J. Tsuji, Tetrahedron Lett. 1984, 25,
5183; b) R. Grigg, Tetrahedron: Asymmetry 1995, 6, 2475; c) T.
Llamas, R. G. Arrayµs, J. C. Carretero, Org. Lett. 2006, 8, 1795.
[11] a) M. Kennedy, M. A. McKervey, A. R. Maguire, G. H. P. Roos,
J. Chem. Soc. Chem. Commun. 1990, 361; b) M. Honma, M.
Nakada, Tetrahedron Lett. 2003, 44, 9007; c) M. Honma, T.
Sawada, Y. Fujisawa, M. Utsugi, H. Watanabe, A. Umino, T.
Matsumura, T. Hagihara, M. Takano, M. Nakada, J. Am. Chem.
Soc. 2003, 125, 2860; d) M. Takano, A. Umino, M. Nakada, Org.
Lett. 2004, 6, 4897; e) T. Sawada, M. Nakada, Adv. Synth. Catal.
2005, 347, 1527; f) H. Takeda, H. Watanabe, M. Nakada,
Tetrahedron 2006, 62, 8054.
[6] For a review on asymmetric ion-pairing catalysis, see: a) J.
Lacour, D. Moraleda, Chem. Commun. 2009, 7073; b) J.-F.
Bri›re, S. Oudeyer, V. Dalla, V. Levacher, Chem. Soc. Rev. 2012,
41, 1696; c) K. Brak, E. N. Jacobsen, Angew. Chem. Int. Ed. 2013,
52, 534; Angew. Chem. 2013, 125, 558.
[7] a) S. Doswald, H. Estermann, E. Kupfer, H. Stadler, W. Walther,
T. Weisbrod, B. Wirz, W. Wostl, Bioorg. Med. Chem. 1994, 2, 403;
´
´
b) J. Skarz˙ewski, R. Siedlecka, E. Wojaczynska, M. Zielinska-
Blajet, Tetrahedron: Asymmetry 2002, 13, 2105; c) M. Teall, P.
Oakley, T. Harrison, D. Shaw, E. Kay, J. Elliott, U. Gerhard, J. L.
Castro, M. Shearman, R. G. Ball, N. N. Tsou, Bioorg. Med.
Chem. Lett. 2005, 15, 2685; d) F. Reck, F. Zhou, M. Girardot, G.
Kern, C. J. Eyermann, N. J. Hales, R. R. Ramsay, M. B. Grave-
stock, J. Med. Chem. 2005, 48, 499; e) J. P. Scott, S. F. Oliver,
K. M. J. Brands, S. E. Brewer, A. J. Davies, A. D. Gibb, D.
Hands, S. P. Keen, F. J. Sheen, R. A. Reamer, R. D. Wilson, U.-
H. Dolling, J. Org. Chem. 2006, 71, 3086; f) J. P. Scott, D. R.
Lieberman, O. M. Beureux, K. M. J. Brands, A. J. Davies, A. W.
Gibson, D. C. Hammond, C. J. McWilliams, G. W. Stewart, R. D.
Wilson, U.-H. Dolling, J. Org. Chem. 2007, 72, 4149; g) N.
Neamati, G. W. Kabalka, B. Venkataiah, R. Dayam, WO Patent
081966, 2007; h) M. Nielsen, C. B. Jacobsen, N. Holub, M. W.
Paix¼o, K. A. Jørgensen, Angew. Chem. Int. Ed. 2010, 49, 2668;
Angew. Chem. 2010, 122, 2726.
[12] M. C. Bernabeu, R. Chinchilla, L. R. Falvello, C. Nµjera,
Tetrahedron: Asymmetry 2001, 12, 1811.
[13] a) S. MossØ, A. Alexakis, Org. Lett. 2005, 7, 4361; b) H. Li, J.
Song, X. Liu, L. Deng, J. Am. Chem. Soc. 2005, 127, 8948; c) T.-
Y. Liu, J. Long, B.-J. Li, L. Jiang, R. Li, Y. Wu, L.-S. Ding, Y.-C.
Chen, Org. Biomol. Chem. 2006, 4, 2097; d) Z. Jin, J. Xu, S. Yang,
B.-A. Song, Y. R. Chi, Angew. Chem. Int. Ed. 2013, 52, 12354;
Angew. Chem. 2013, 125, 12580.
[14] a) J. W. Lee, H. Yan, H. B. Jang, H. K. Kim, S.-W. Park, S. Lee,
D. Y. Chi, C. E. Song, Angew. Chem. Int. Ed. 2009, 48, 7683;
Angew. Chem. 2009, 121, 7819; b) H. Yan, H. B. Jang, J.-W. Lee,
H. K. Kim, S. W. Lee, J. W. Yang, C. E. Song, Angew. Chem. Int.
Ed. 2010, 49, 8915; Angew. Chem. 2010, 122, 9099; c) H. Yan, J.-
S. Oh, C. E. Song, Org. Biomol. Chem. 2011, 9, 8119; d) H. Yan,
J. S. Oh, J.-W. Lee, C. E. Song, Nat. Commun. 2012, 3, DOI:
10.1038/ncomms2216; e) S. Y. Park, J.-W. Lee, C. E. Song, Nat.
Commun. 2015, 6, DOI: 10.1038/ncomms8512.
[8] a) P. Bertus, P. Phansavath, V. Ratovelomanana-Vidal, J.-P.
GenÞt, A. R. Touati, T. Homri, B. Ben Hassine, Tetrahedron
Lett. 1999, 40, 3175; b) P. Mauleón, J. C. Carretero, Chem.
Commun. 2005, 4961; c) H.-L. Zhang, X.-L. Hou, L.-X. Dai, Z.-
B. Luo, Tetrahedron: Asymmetry 2007, 18, 224; d) T. Llamas,
R. G. Arrayµs, J. C. Carretero, Angew. Chem. Int. Ed. 2007, 46,
3329; Angew. Chem. 2007, 119, 3393; e) Z. Ding, J. Yang, T.
Wang, Z. Shen, Y. Zhang, Chem. Commun. 2009, 571; f) T. Zhou,
B. Peters, M. F. Maldonado, T. Govender, P. G. Andersson, J.
Am. Chem. Soc. 2012, 134, 13592; g) B. K. Peters, T. Zhou, J.
Rujirawanich, A. Cadu, T. Singh, W. Rabten, S. Kerdphon, P. G.
Andersson, J. Am. Chem. Soc. 2014, 136, 16557.
[15] See the Supporting Information for details.
[16] a) J. E. Hofmann, T. J. Wallace, A. Schriesheim, J. Am. Chem.
Soc. 1964, 86, 1561; b) H. Kwart, T. Takeshita, J. L. Nyce, J. Am.
Chem. Soc. 1964, 86, 2606; c) M. A. Aleskerov, S. S. Yufit, V. F.
Kucherov, Russian Chem. Rev. 1978, 47, 134; d) J. W. Cubbage,
B. W. Vos, W. S. Jenks, J. Am. Chem. Soc. 2000, 122, 4968.
´
[17] a) J. Skarz˙ewski, M. Zielinska-Błajet, I. Turowska-Tyrk, Tetra-
hedron: Asymmetry 2001, 12, 1923; b) M. Zielinska-Błajet, R.
Kowalczyk, J. Skarz˙ewski, Tetrahedron 2005, 61, 5235.
[9] a) K. Hiroi, K. Makino, Chem. Lett. 1986, 617; b) B. M. Trost,
M. G. Organ, G. A. OꢀDoherty, J. Am. Chem. Soc. 1995, 117,
9662; c) H.-J. Gais, T. Jagusch, N. Spalthoff, F. Gerhards, M.
Frank, G. Raabe, Chem. Eur. J. 2003, 9, 4202; d) M. Ueda, J. F.
Hartwig, Org. Lett. 2010, 12, 92; e) X.-S. Wu, Y. Chen, M.-B. Li,
M.-G. Zhou, S.-K. Tian, J. Am. Chem. Soc. 2012, 134, 14694;
Received: August 30, 2015
Revised: September 14, 2015
Published online: November 18, 2015
Angew. Chem. Int. Ed. 2016, 55, 331 –335
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
335