Synthesis of a Chiral Borate Counteranion, Its Trityl Salt, and Application Thereof in Lewis-Acid Catalysis

Article abstract of DOI:10.1002/ejoc.201700239

The preparation of a chiral derivative of [B(C₆F₅)₄]^– in which the fluorine atom in the para position of each of the C₆F₅ groups is replaced by a 1,1′-binaphthalen-2-yl group is described. The new counteranion was isolated as its lithium, sodium, and trityl salts. The chiral trityl salt was then used as a catalyst in selected counteranion-directed Diels–Alder reactions and a Mukaiyama aldol addition, but no asymmetric induction was achieved. Application of the chiral trityl salt to the generation of silicon cations by silicon-to-carbon hydride transfer from hydrosilanes failed, presumably as a result of the incompatibility of the relatively electron-rich naphthyl groups in the borate and the cationic silicon electrophiles.

Full text of DOI:10.1002/ejoc.201700239

A Journal of
Accepted Article
Title: Synthesis of a Chiral Borate Counteranion, its Trityl Salt, and
Application Thereof in Lewis-Acid Catalysis
Authors: Phillip Pommerening, Jens Mohr, Jonas Friebel, and Martin
Oestreich
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10.1002/ejoc.201700239
European Journal of Organic Chemistry
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Synthesis of a Chiral Borate Counteranion, its Trityl Salt, and
Application Thereof in Lewis-Acid Catalysis
Phillip Pommerening, Jens Mohr, Jonas Friebel, and Martin Oestreich*[a]
Abstract: The preparation of a chiral derivative of [B(C6F5)4]– where
the fluorine atom in the para position of each of the C6F5 groups is
replaced by a 1,1’-binaphthalen-2-yl group is described. The new
counteranion is isolated as its lithium, sodium, and trityl salt. The
chiral trityl salt is then used as a catalyst in selected counteranion-
directed Diels–Alder reactions and a Mukaiyama aldol addition but
no asymmetric induction is achieved. Application of the chiral trityl
salt to the generation of silicon cations by silicon-to-carbon hydride
transfer from hydrosilanes failed, presumably as a result of the
incompatibility of the relatively electron-rich naphthyl groups in the
borate and cationic silicon electrophiles.
[4]–. We therefore also considered testing the corresponding
trityl salt [Ph3C]+·[4]– as catalyst in asymmetric Diels–Alder
reactions.[16–18] Franzén and co-workers had recently shown that
trityl cations promote challenging Diels–Alder reactions of
cyclohexa-1,3-diene,[17a,b] and the same group accomplished a
related enantioselective Diels–Alder reaction of 2,3-
dimethylbuta-1,3-diene directed by a chiral counteranion.[16a]
We report here the preparation of the chiral counteranion [4]–
as its lithium and sodium salts [Li]+·[4]– and [Na]+·[4]– and the
transformation of the latter into [Ph3C]+·[4]–. The chiral trityl salt
was then applied as a catalyst in representative Diels–Alder
reactions and a Mukaiyama aldol addition while the generation
of cognate silicon cations by Corey’s hydride abstraction[19] from
hydrosilanes had failed.
Introduction
Weakly coordinating anions (WCAs) have become an enabling
technology,[1] being widely used in fundamental inorganic and
organic chemistry[2] as well as finding broad application in
polymer science.[3] Various types of these counteranions with
different elements as the central atom, e.g., boron and aluminum,
have been reported.[1] Boron-based systems with electron-
deficient aryl groups are particularly popular, and the fluorinated
borates tetrakis[3,5-bis(trifluoromethyl)phenyl] borate ([1]–;
[BArF ]–)[4] and tetrakis(pentafluorophenyl) borate ([2]–;
4
[B(C6F5)4]–)[5] are the typical congeners of this class of
compounds (Figure 1). Asymmetric catalyses with and without
transition metals where a chiral counteranion is the source of
chirality have recently witnessed tremendous growth[6] but none
make use of a chiral tetraaryl-substituted borate.[7] The majority
of chiral borates are derived from catechols and binols[7,8] while
just a few borates without boron–oxygen linkages are known[9,10]
(e.g., [3]–; Figure 1). We are not aware of an application of [3]– or
related compounds in asymmetric counteranion-directed
catalysis.[6] With our interest in silicon-cation-catalyzed
enantioselective Diels–Alder reactions of cyclohexa-1,3-
diene,[11–14] we entertained the idea of replacing weakly
coordinating [B(C6F5)4]– ([2]–) by the chiral, partially fluorinated
counteranion [4]– (Figure 1).[15] The oxophilicity of silicon cations
precludes the use of the prevalent oxygen-containing borates,
and the enormous electrophilicity of these Lewis acids could be
incompatible with the relatively electron-rich naphthyl groups in
Figure 1. Typical weakly coordinating borates [1]– and [2]– by Park and
Kobayashi, respectively, as well as known and targeted chiral borates [3]– and
[4]–.
Results and Discussion
We decided to replace the four para fluorine atoms in [B(C6F5)4]–
([2]–) by 1,1’-binaphthalen-2-yl groups. The required precursor
(R)-7 was prepared from binol-derived (R)-5[20] (Scheme 1, top).
The aryl boronic ester (S)-6 was made from (R)-5 by palladium-
catalyzed coupling with pinacolborane[21] [(R)-5 → (S)-6].[22]
Desired (R)-7 was then obtained by subsequent Suzuki–Miyaura
coupling of (S)-6 with 1-bromo-2,3,5,6-tetrafluorobenzene [(S)-6
[a]
P. Pommerening, Dr. J. Mohr, J. Friebel, Prof. Dr. M. Oestreich
Institut für Chemie, Technische Universität Berlin
Straße des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
Homepage: http://www.organometallics.tu-berlin.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201700239
European Journal of Organic Chemistry
FULL PAPER
→ (R)-7]. High catalyst loading, excess of the aryl bromide, strict
exclusion of oxygen, and long reaction times were mandatory to
furnish (R)-7 in decent 67% yield. HPLC analysis of (R)-7 (98%
ee) on a chiral stationary phase shows no thermally induced
racemization. Also, crystals suitable for X-ray diffraction were
obtained from a solution of (R)-7 in CH2Cl2 by slow evaporation
of the solvent at room temperature and ambient atmosphere
(see the Supporting Information). Attempts to directly arrive at
(R)-7 from (R)-5 were unsuccessful.
Scheme 1. Preparation of precursor (R)-7 and its transformation into various borate salts of [4]–.
We had initially planned to install one chiral unit in
[B(C6F5)4]–-type borates starting from B(C6F5)3 and, e.g., [1,1'-
binaphthalen]-2-yllithium but the targeted heteroleptic borate
was formed along with [Li]+·[2]–, indicating transfer of a C6F5
group.[23] Therefore, we turned directly toward the preparation of
homoleptic [4]– (Scheme 1, bottom). Deprotonation of (R)-7 with
sec-butyllithium[24] followed by the reaction with BCl3 led to the
air- and water-stable lithium borate [Li]+·[4]– in near-quantitative
yield {(R)-7 → [Li]+·[4]–}. The direct conversion of [Li]+·[4]– into
[Ph3C]+·[4]– with trityl chloride was feasible but resulted in the
difficult separation of [Ph3C]+·[4]– from LiCl due to its solubility in
organic solvents. Preceding salt metathesis with excess NaCl
([Li]+·[4]– → [Na]+·[4]–) allowed for straightforward filtration of
difficultly soluble NaCl after the reaction of [Na]+·[4]– with trityl
chloride ([Na]+·[4]– → [Ph3C]+·[4]–). This procedure provided
access to lithium-free [Ph3C]+·[4]–.[25]
enantioselective counteranion-directed Diels‒Alder reaction with
methacrolein (11) with no success (traces of the Diels‒Alder
adduct).[16a] However, the same catalyst promoted the
cycloaddition of another diene/dienophile combination, 12/11,
with 53% ee at low conversion (cf. Scheme 2, middle).[12b] For
comparison, we also investigated the reactions of 2,3-
dimethylbuta-1,3-diene (12) as well as cyclohexa-1,3-diene (9)
with methacrolein (11, Scheme 2, middle and bottom). In both
cases, we isolated the cycloadducts 13 and 14 in moderate yield
without enantiomeric excess.
As mentioned above, Franzén and co-workers had recently
shown that trityl cations catalyze Diels‒Alder reactions, even
those involving cyclohexa-1,3-diene as enophile.[17a,b] With the
LiCl-free trityl salt [Ph3C]+·[4]‒ in hand, we tested the difficult
Diels‒Alder reaction of cyclohexa-1,3-diene (9) and chalcone (8)
(Scheme 2, top).[12b] Cycloadduct 10 was isolated in moderate
yield. The trityl cation with [4]‒ as counteranion is promoting this
Diels‒Alder reaction at lower rate than parent [Ph3C]+·[B(C6F5)4]‒;
the yield obtained with [Ph3C]+·[B(C6F5)4]‒ is 84%. We note here
that Franzén and co-workers had also tested
9 in the
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10.1002/ejoc.201700239
European Journal of Organic Chemistry
FULL PAPER
[14b,32]
the corresponding hydrosilane in 1,2-Cl2C6D4
was also
unsuccessful. Decomposition of both the ferrocene backbone
and the borate [4]– was assumed based on several resonance
signals in the 1H and 11B NMR spectra.
Conclusions
In summary, we presented a reliable synthesis of an air- and
water-stable (potentially weakly coordinating) chiral borate [4]–
with various countercations, i.e., [Li]+·[4]–, [Na]+·[4]–, and
[Ph3C]+·[4]–. To demonstrate its usefulness, we tested
[Ph3C]+·[4]– as a catalyst in representative trityl-cation-catalyzed
Diels–Alder reactions and a Mukaiyama aldol addition but there
was no enantiomeric excess. We also verified whether
[Ph3C]+·[4]– could be used in Corey’s silicon-to-carbon hydride
transfer for the generation of silicon cations from hydrosilanes.
However, unlike commonly employed [Ph3C]+·[B(C6F5)4]–, the
highly electrophilic silicon cations were not compatible with the
chiral counteranion [4]– wrapped in relatively electron-rich
naphthyl groups.
Scheme 2. Representative trityl-cation-catalyzed Diels–Alder reactions of
cyclohexa-1,3-diene (top/bottom) and 2,3-dimethylbuta-1,3-diene (middle).
Experimental Section
We also applied [Ph3C]+·[4]– to the typical Mukaiyama aldol
reaction of 15 and 16 (Scheme 3).[26,27] Examples of
enantioselective Mukaiyama aldol additions have been reported
with chiral triarylcarbenium ions as catalysts[28] or in the
presence of chiral counteranions.[29] Our catalyst yielded adduct
17 with moderate efficiency in racemic form. It is worth
mentioning that we also observed rapid decoloration of the
reaction mixture,[28] an indication of competing trityl-cation and
silicon-cation catalysis.[27]
For general remarks as well as experimental procedures and
spectroscopic data for literature-known compounds see the Supporting
Information.
(S)-2-([1,1'-Binaphthalen]-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane [(S)-6]
According to a modified procedure by Masuda,[21a] a mixture of (R)-(1,1’-
binaphthalen)-2-yl trifluoromethanesulfonate [(R)-5, 0.87 g, 2.2 mmol, 1.0
equiv] and triethylamine (0.90 mL, 0.65 g, 6.5 mmol, 3.0 equiv) in 1,4-
dioxane (6.0 mL) was added to
a
solution of [1,1′-
bis(diphenylphosphino)ferrocene]dichloropalladium(II) (48 mg, 65 µmol,
3.0 mol %) in 1,4-dioxane (3.0 mL). Pinacolborane (1.4 mL, 1.3 g, 9.8
mmol, 4.5 equiv) was added, and the resulting mixture was maintained at
120 °C for 19 h. The reaction was quenched by the addition of water (30
mL), and the phases were separated. The aqueous phase was extracted
with CH2Cl2 (2 × 30 mL), and the combined organic phases were dried
over Na2SO4. After evaporation of the volatiles under reduced pressure,
the residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate = 30/1 as eluent to afford the title
compound (S)-6 (0.7 g, 85%) as a white solid. M.p. = 114 °C. Rf = 0.42
(cyclohexane/ethyl acetate = 20/1). GLC (SE-54): tR = 29.0 min. IR
(ATR): nu(tilde) = 1471, 1378, 1354, 1301, 1264, 1134, 1109, 964, 850,
[Ph3C] ·[4]
(10 mol %)
O
O
OTBDMS
OTBDMS
H
EtO
CH2Cl2
78 °C
4.5 h
EtO
15
16
(1.0 equiv)
17: 0% ee
55%
(1.1 equiv)
Scheme 3. Representative trityl-cation-catalyzed Mukaiyama aldol addition.
We also probed [Ph3C]+·[4]– in Corey’s silicon-to-carbon
hydride transfer.[19] Treatment of Et3SiH with [Ph3C]+·[4]‒ in C6D6
to form [Et3Si(benzene)]+·[4]– failed.[30] Although we detected the
diagnostic resonance signal of the methine proton of
818, 800, 774, 745 cm–1. HRMS (APCI) for C26H26BO2 [(M+H)]:

calculated 381.2020, found 381.2023. 1H NMR (500 MHz, CDCl3): δ/ppm
= 0.80 (s, 6H), 0.93 (s, 6H), 7.24–7.35 (m, 3H), 7.41–7.53 (m, 4H), 7.56–
7.61 (m, 1H), 7.87–7.99 (m, 5H). 13C{1H} NMR (126 MHz, CDCl3): δ/ppm
= 24.3, 24.4, 83.3, 125.1, 125.5, 125.8, 125.9, 126.6, 126.7, 126.9,
127.37, 127.4, 127.9, 128.0, 128.2, 130.0, 132.9, 133.6, 134.1, 134.7,
139.1, 144.9. The Cq–Bpin signal could not be detected. The
spectroscopic data are in accordance with those previously reported.[21b]
triphenylmethane
in
the
1H
NMR
spectrum,
the
[Et3Si(benzene)]+·[4]– was not be observed.[31] Several
resonance signals in the 29Si NMR spectrum hinted
decomposition of [Et3Si(benzene)]+·[4]–, probably due to
interactions of the strong silicon electrophile and the relatively
electron-rich naphthyl groups in counteranion [4]–. The
generation of our intramolecularly stabilized, ferrocenyl-
substituted silicon cation [FcSi(tBu)Me]+ (Fc = ferrocenyl) from
(R)-2-(2,3,5,6-Tetrafluorophenyl)-1,1’-binaphthalene [(R)-7]
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10.1002/ejoc.201700239
European Journal of Organic Chemistry
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A
mixture of (S)-2-([1,1'-binaphthalen]-2-yl)-4,4,5,5-tetramethyl-1,3,2-
According to a modified procedure by Schulz,[33] sodium tetrakis(2-
[2,3,5,6-tetrafluorophenyl]-1,1’-binaphthalene)borate [Na]+·[4]– (0.10 g, 60
µmol 1.0 equiv) and triphenylmethyl chloride (0.86 mg, 0.31 mmol 5.0
equiv) were suspended in n-hexane (6 mL) and stirred overnight at room
temperature. The solution was filtered under nitrogen atmosphere and
the resulting precipitate washed with n-hexane (2 × 5 mL). The brown
solid was dissolved in CH2Cl2 (10 mL), and the resulting solution was
concentrated under reduced pressure to approximately one third of its
volume. n-Pentane (20 mL) was added rapidly, and the supernatant was
removed via syringe. This procedure was repeated three times. The
resulting precipitate was dried under high vacuum to afford the title
compound [Ph3C]·[4]– (95 mg, 85%) as an orange to brown solid. M.p. =
189 °C. IR (ATR): nu(tilde) = 3046, 1579, 1506, 1481, 1440, 1356, 1277,
dioxaborolane [(S)-6, 1.50 g, 3.94 mmol, 1.00 equiv],
tetrakis(triphenylphospine)palladium(0) (456 mg, 0.395 mmol, 10.0
mol %), bromo-2,3,5,6-tetrafluorobenzene (1.87 mL, 3.52 g, 15.4 mmol,
3.90 equiv), and Cs2CO3 (3.16 g, 9.70 mmol, 2.46 equiv) in 1,4-
dioxane/water (10/1, 22 mL) was heated at 95 °C for 4 d. The reaction
mixture was filtered through Celite®, and the volatiles were removed
under reduced pressure. Purification of the residue by flash column
chromatography on silica gel using cyclohexane/toluene = 10/1 as eluent
afforded the title compound (R)-7 (1.07 g, 67%, 98% ee) as a white solid.
M.p. = 158 °C. Rf = 0.30 (cyclohexane). GLC (SE-54): tR = 29.0 min. IR
(ATR): nu(tilde) = 1491, 1387, 1365, 1281, 1255, 1170, 968, 934, 871,
849, 821, 803, 781, 755, 711 cm–1. HRMS (APCI) for C26H14F4 [M]:
calculated 402.1026, found 402.1033. 1H NMR (500 MHz, CDCl3): δ/ppm
= 6.77 (mc, 1H), 7.27–7.61 (m, 9H), 7.83–7.89 (m, 2H), 8.03 (mc, 1H),
8.10 (mc, 1H). 13C{1H} NMR (126 MHz, CDCl3): δ/ppm = 105.0 (mc),
121.9 (mc), 125.1, 125.7 (mc), 126.0, 126.1, 126.2, 126.2, 126.8, 127.0,
127.2, 127.3, 128.1, 128.1, 128.2, 128.4, 132.5, 133.3, 133.4, 133.8,
135.5, 139.1, 142.4–143.0 (m), 144.3–144.9 (m, 2C), 146.3–146.6 (m).
19F{1H} NMR (471 MHz, CDCl3): δ/ppm = –140.0–(–139.8) (m, 2F), –
1182, 964, 824, 760, 703 cm–1. HRMS (APCI) for C104H52BF16 [M–]:
–
calculated 1615.3912, found 1615.3849. HRMS (APCI) for C19H15 [M]:

calculated 243.1168, found 243.1174. 11B NMR (161 MHz, CD2Cl2):
δ/ppm = –17.3. Optical rotation: [α]20D = +49 (c = 1.0, CHCl3).
Acknowledgments
139.5–(–139.3) (m, 1F), –138.8–(–138.6) (m, 1F). Optical rotation: [α]20
D
= –1128 (c = 1.05, CHCl3). The enantiomeric excess was determined by
HPLC analysis on a chiral stationary phase (Daicel Chiralcel OJ-RH
column, 20 °C, acetonitrile/H2O = 60/40, flow rate 0.40 mL/min, λ = 254
nm): tR = 52.0 min for (S)-7, tR = 63.6 min for (R)-7. The crystallographic
data is available online in the CCDC database under number CCDC
1521041.
M.O. is indebted to the Einstein Foundation (Berlin) for an
endowed professorship. We thank Dr. Elisabeth Irran (TU Berlin)
for the X-ray analysis.
Keywords: Cations • Chirality • Diels–Alder reaction • Lewis
acids • Trityl group
Lithium
Tetrakis(2-[2,3,5,6-tetrafluorophenyl]-1,1’-binaphthalene)-
borate ([Li]+·[4]–)
[1]
[2]
For leading reviews, see: a) I. Raabe, I. Krossing, Angew. Chem. Int.
Ed. 2004, 43, 2066–2090; b) S. H. Strauss, Chem. Rev. 1993, 93, 927–
942.
To a solution of (R)-2-(2,3,5,6-tetrafluorophenyl)-1,1’-binaphthalene [(R)-
7, 0.50 g, 1.2 mmol, 5.5 equiv] in diethyl ether (20 mL), sec-butyllithium
(1.3 M in cyclohexane, 0.90 mL, 1.1 mmol, 5.0 equiv) was added
dropwise at –78 °C, and the resulting mixture was stirred for 3 h. BCl3
(1.0 M in n-heptane, 0.20 mL, 0.23 mmol, 1.0 equiv) was added dropwise,
and the resulting mixture was stirred overnight by allowing the solution to
slowly warm to room temperature. The reaction was quenched by the
addition of water (20 mL), and the volatiles were removed under reduced
pressure to give [Li]·[4]– (0.35 g, 96%) as a colorless solid. M.p. =
219 °C. IR (ATR): nu(tilde) = 3650, 3045, 1611, 1505, 1437, 1365, 1276,
For representatives examples, see: a) T. J. Barbarich, S. T. Handy, S.
M. Miller, O. P. Anderson, P. A. Grieco, S. H. Strauss, Organometallics
1996, 15, 3776–3778; b) S. Moss, B. T. King, A. de Meijere, S. I.
Kozhushkov, P. E. Eaton, J. Michl, Org. Lett. 2001, 3, 2375–2377; c) L.
L. Anderson, J. Arnold, R. G. Bergman, J. Am. Chem. Soc. 2005, 127,
14542–14543; d) Y. Li, B. Diebl, A. Raith, F. E. Kühn, Tetrahedron Lett.
2008, 49, 5954–5956; e) T.-T.-T. Nguyen, D. Türp, M. Wagner, K.
Müllen, Angew. Chem. Int. Ed. 2013, 54, 669–673; f) S. Fischer, J.
Schmidt, P. Strauch, A. Thomas, Angew. Chem. Int. Ed. 2013, 52,
12174–12178.
1174, 962, 822, 758, 704 cm–1. HRMS (APCI) for C104H52BF16 [M]:

calculated 1615.3912, found 1615.3850. 7Li NMR (194 MHz, CDCl3):
δ/ppm = –1.94. 11B NMR (160 MHz, CDCl3): δ/ppm = –17.1.
[3]
a) J. A. S. Roberts, M.-C. Chen, A. M. Seyam, L. Li, C. Zuccaccia, N. G.
Stahl, T. J. Marks, J. Am. Chem. Soc. 2007, 129, 12713–12733; b) M.-
C. Chen, J. A. S. Roberts, A. M. Seyam, L. Li, C. Zuccaccia, N. G. Stahl,
T. J. Marks, Organometallics 2006, 25, 2833–2850; c) M. Mager, S.
Becke, H. Windisch, U. Denninger, Angew. Chem. Int. Ed. 2001, 40,
1898–1902.
Sodium
Tetrakis(2-[2,3,5,6-tetrafluorophenyl]-1,1’-binaphthalene)-
borate ([Na]+ [4]–)
To freshly prepared brine (5 mL), a solution of lithium tetrakis(2-[2,3,5,6-
tetrafluorophenyl]-1,1’-binaphthalene)borate [Li]·[4]– (377 mg, 0.232
mmol) in CH2Cl2 (5 mL) was added and stirred overnight. The reaction
mixture was diluted with water (10 mL), the phases were separated, and
the aqueous phase extracted with CH2Cl2 (2 × 10 mL). The volatiles were
removed under reduced pressure, and the residue was purified by flash
column chromatography on silica gel using ethyl acetate as eluent to
afford the title compound [Na]·[4]– (202 mg, 53%) as a gray solid. M.p. >
220 °C. Rf = 0.07 (ethyl acetate). IR (ATR): nu(tilde) = 3651, 3048, 1611,
1506, 1438, 1365, 1277, 1176, 962, 823, 759, 705 cm–1. HRMS (APCI)
[4]
[5]
[6]
A. G. Massey, A. J. Park, J. Organomet. Chem. 1964, 2, 245–250.
H. Kobayashi, T. Sonoda, H. Iwamoto, Chem. Lett. 1981, 579–580.
For authoritative reviews, see: a) M. Mahlau, B. List, Angew. Chem. Int.
Ed. 2013, 52, 518–533; b) R. J. Phipps, G. L. Hamilton, F. D. Toste, Nat.
Chem. 2012, 4, 603–614.
[7]
[8]
For a review, see: J. Lacour, V. Hebbe-Viton, Chem. Soc. Rev. 2003,
32, 373–382.
For an example of a binol-derived borate as a counteranion in a
copper-catalyzed reaction, see: D. B. Llewellyn, D. Adamson, B. A.
Arndtsen, Org. Lett. 2000, 2, 4165–4168.

for C104H52BF16 [M]: calculated 1615.3912, found 1615.3878. 11B NMR
[9]
K. Torssell, Acta Chem. Scand. 1962, 16, 87–93.
(160 MHz, CDCl3): δ/ppm = –17.3. Optical rotation: [α]20D = –14 (c = 0.98,
CHCl3).
[10] D. J. Owen, D. VanDerveer G. B. Schuster, J. Am. Chem. Soc. 1998,
120, 1705–1717.
Triphenylcarbenium
Tetrakis(2-[2,3,5,6-tetrafluorophenyl]-1,1’-
binaphthalene)borate ([Ph3C]+·[4]–)
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10.1002/ejoc.201700239
European Journal of Organic Chemistry
FULL PAPER
[11] For planar chiral, ferrocene-stabilized silicon cations as catalysts, see:
R. K. Schmidt, H. F. T. Klare, R. Fröhlich, M. Oestreich, Chem. Eur. J.
2016, 22, 5376–5383.
[33] M. Lehmann, A. Schulz, A. Villinger, Angew. Chem. Int. Ed. 2009, 48,
7444–7447.
[12] For axial chiral, sulfur-stabilized silicon cations as catalysts, see: a) V.
H. G. Rohde, M. F. Müller, M. Oestreich, Organometallics 2015, 34,
3358–3373; b) P. Shaykhutdinova, M. Oestreich, Organometallics 2016,
35, 2768–2771.
[13] For an axial chiral, acetonitrile-stabilized silicon cation as catalyst, see:
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This article is protected by copyright. All rights reserved.

10.1002/ejoc.201700239
European Journal of Organic Chemistry
FULL PAPER
Suggestion for the Entry for the Table of Contents
FULL PAPER
Chiral Counteranion. Chiral
modification of the popular
P. Pommerening, J. Mohr, J. Friebel, M.
Oestreich*
perfluorinated borate counteranion
[B(C6F5)4]– resulted in an air- and
water-stable, potentially weakly
coordinating borate with various
countercations. Its utility is shown in
trityl-cation-catalyzed Diels–Alder
reactions and a Mukaiyama aldol
addition.
Page No. – Page No.
Synthesis of a Chiral Borate
Counteranion, its Trityl Salt, and
Application Thereof in Lewis-Acid
Catalysis
This article is protected by copyright. All rights reserved.