Chiral Modification of the Tetrakis(pentafluorophenyl)borate Anion with Myrtanyl Groups

Article abstract of DOI:10.1002/ejoc.201901434

The synthesis and characterization of chiral [B(C₆F₅)₄]^– derivatives bearing a myrtanyl group instead of a fluoro substituent in the para position are described. These new chiral borates were isolated as their bench-stable lithium, sodium, and cesium salts. The corresponding trityl salts were prepared and tested as catalysts in representative counteranion-directed Diels–Alder reactions and Mukaiyama aldol additions but no enantioselectivity was obtained. Preformation of a chalcone-derived silylcarboxonium ion with the chiral borate as counteranion did not lead to any asymmetric induction in a reaction with cyclohexa-1,3-diene.

Full text of DOI:10.1002/ejoc.201901434

A Journal of
Accepted Article
Title: Chiral Modification of the Tetrakis(pentafluorophenyl)borate
Anion with Myrtanyl Groups
Authors: Phillip Pommerening and Martin Oestreich
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901434
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10.1002/ejoc.201901434
European Journal of Organic Chemistry
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Chiral Modification of the Tetrakis(pentafluorophenyl)borate
Anion with Myrtanyl Groups
Phillip Pommerening[a] and Martin Oestreich*[a]
Abstract: The synthesis and characterization of chiral [B(C6F5)4]–
derivatives bearing a myrtanyl group instead of a fluoro substituent
in the para position are described. These new chiral borates were
isolated as their bench-stable lithium, sodium, and cesium salts. The
corresponding trityl salts were prepared and tested as catalysts in
representative counteranion-directed Diels–Alder reactions and
Mukaiyama aldol additions but no enantioselectivity was obtained.
Preformation of a chalcone-derived silylcarboxonium ion with the
chiral borate as counteranion did not lead to any asymmetic
induction in a reaction with cyclohexa-1,3-diene.
This is particularly true for borates containing highly fluorinated
aryl groups such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
([BArF ]–)[2] and tetrakis(pentafluorophenyl)borate ([B(C6F5)4]–).[3]
4
Chiral congeners of these anions are essentially unknown but
their use as chiral counteranions in asymmetric catalysis[4] is
attractive. Recently, we[5] and List and co-workers[6]
independently introduced chiral versions of [B(C6F5)4]– where the
fluorine atoms in the para positions have been replaced by 1,1’-
binaphthalene-2-yl groups (Figure 1, left).[5,6] We showed that
the trityl salt of [1]– promotes Diels–Alder reactions as well as a
Mukaiyama aldol addition but did not obtained any
enantioselectivity.[5] Similar observations were made by List and
co-workers; however, when shifting the chiral unit from the para
to the meta position in [2]–, a Mukaiyama aldol reaction afforded
16% ee as proof of concept.[6]
Introduction
Boron- and aluminum-based weakly coordinating anions (WCAs)
have found widespread application in molecular chemistry.[1]
Figure 1. Chiral congeners of [B(C6F5)4]– where one of the fluorine atoms at the aryl groups has been replaced by chiral moieties; [Tr]+ = triphenylmethylium,
[
MeTr]+ = diphenyl(4-tolyl)methylium.
Despite these modest prospects, we decided to further
cyclohexa-1,3-diene enantioselectively.[5,6] Counteranion [1]–
with its π-donating naphthyl groups is not chemically resistant
against those strong electrophiles. We therefore considered
more robust aliphatic rather than aromatic chiral units for the
modification of the [B(C6F5)4]– platform, and we report here the
synthesis and characterization of the myrtanyl-substituted
borates [3]– and [4]– with various countercations (Figure 1, right).
pursue the development of chiral, partially fluorinated
tetraarylborates. Our initial goal had been to design chiral
counteranions for silylium ions and silylium-ion-like Lewis acids
to drive our silylium-ion-catalyzed Diels–Alder reactions of
[a]
Institut für Chemie, Technische Universität Berlin,
Straße des 17. Juni 115, 10623 Berlin, Germany
E-mail: martin.oestreich@tu-berlin.de
Results and Discussion
http://www.organometallics.tu-berlin.de
To replace the fluorine atom in the para position of the C6F5
group by myrtanyl groups, we targeted intermediate 6. Its
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201901434
European Journal of Organic Chemistry
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synthesis began with literature-known myrtanal (5) derived from
(–)-β-pinene[7] in two steps (Scheme 1, left).[8] The alcohol 6 was
obtained by the addition[8b] of the Grignard reagent prepared
from 1-bromo-2,3,5,6-tetrafluorobenzene (5 → 6). The hydroxy
group in 6 can be seen as a useful handle for further
derivatization in the benzylic position. Defunctionalization was
achieved by the Barton–McCombie deoxygenation subequent to
xanthate formation (6 → 7 → 8); alternative palladium-catalyzed
methods using H2 or Et3SiH as reducing agents gave no
conversion. Another building block with a methyl group at the
benzylic center was obtained by Dess–Martin oxidation (6 → 9)
followed by methenylation using the Petasis reagent[9] (9 → 10).
Substrate-controlled hydrogenation of the 1,1-disubstituted
alkene employing Wilkinson’s catalyst proceeded quantitatively
with good diatereoselectivity (10 → 11). We did not succeed
improving the d.r.
= 87:13 further. For example, iridium-
catalyzed enantioselective hydrogenation[10] of 10 did not
override the substrate control, and yields were consistently lower
(see the Supporting Information for details). The assignment of
the relative configuration by nOe measurements was not
conclusive. Attempts to transform ketone 9 into gem-dimethyl-
substituted 12 by geminal dimethylation[11] resulted in
decomposition of the starting material. The detour involving
cyclopropanation followed by hydrogenolysis was not feasible
due to low conversion of the Simmons–Smith reaction under
various reaction conditions.[12]
Scheme 1. Preparation of the borate precursors 8 and 11.
The lithium borate [Li]+[3]– was accessible by chemoselective
deprotonation of 8 using n-butyllithium followed by the reaction
with BCl3 (Scheme 2, top). We used a salt metathesis reaction
with excess of NaCl ([Li]+[3]– → [Na]+[3]–) to ensure complete
removal of the formed LiCl prior to the next step. The absence of
[Cs]+[4]–. However, an exchange from cesium to sodium as
countercation is crucial for the formation of trityl borates
([Cs]+[4]– → [Na]+[4]–). Treatment of the sodium salt [Na]+[4]–
with trityl chloride resulted in formation of the desired trityl borate
[Tr]+[4]– but again, the formation of triphenylmethane was
observed. Hydride abstraction from the benzylic position was
7
LiCl was verified by Li NMR spectroscopy. The sodium borate
[Na]+[3]– was then reacted with trityl chloride ([Na]+[3]–
→
excluded by H-labeling of 11 (for the characterization of 11-d2
2
[Tr]+[3]–). However, the steady formation of triphenylmethane
was observed but we were unable to determine the origin of the
hydride. For comparison, we subjected 11 with a more sterically
hindered benzylic C–H to a similar reaction sequence (Scheme
2, bottom). The lithium borate [Li]+[4]– was obtained in high yield.
To fully remove coordinating solvents from the purification
process, we started the salt metathesis with an excess of
Cs2CO3 which allowed isolation of solvent- and LiCl-free
and the corresponding borates [4-d2]–, see the Supporting
Information). For [Na]+[4-d2]– to [Tr]+[4-d2]–, the formation of non-
deuterated triphenylmethane persisted, and deuterated
triphenylmethane was not detected. As a consequence, we
turned towards reducing the hydride affinity of the trityl cation by
moving from TrCl to diphenyl(4-tolyl)methyl chloride (MeTrCl).
Despite the reduced hydride affinity of
formation of diphenyl(4-tolyl)methane was not fully prevented.
[
MeTr]+[4]–,[13] the
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10.1002/ejoc.201901434
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Scheme 2. Formation of the chiral borates [3]– and [4]– with various countercations.
With the modified trityl salt [MeTr]+[4]– in hand, we tested its
catalytic activity in two representative Diels–Alder reactions[15]
and two Mukaiyama aldol additions.[6] Franzén and co-workers
demonstrated that trityl cations are able to catalyze difficult
Diels–Alder reactions involving cyclohexa-1,3-diene (14) as
enophile in good yields.[14] We applied trityl salt [MeTr]+[4]– to the
cycloaddition of chalcone (13) with diene 14 (Scheme 3, top).
The cycloadduct 15 was isolated in good yield but without
enantiomeric excess. Franzén had also tested an
enantioselective counteranion-directed Diels–Alder reaction of
14 with methacrolein (16) but could only observe cycloadduct 17
in trace amounts.[15b] Even though our catalyst enabled the
desired reaction of 16 and 14 in moderate yield, there was no
enantioinduction (Scheme 3, bottom). Diels–Alder reactions with
anthracene as the enophile did not show any conversion.[15c]
Enantioselective Mukaiyama aldol reactions either promoted
by chiral carbocations[16] or performed in the presence of chiral
counteranions[6,17] are known. We applied [MeTr]+[4]– in the model
aldol reaction of 18 with benzaldehyde (19). Although our trityl
salt [MeTr]+[4]– is potent enough to catalyze the reaction, we
could only isolate the adduct 20 as a racemic mixture (Scheme 4,
top).[18] Examination of [Na]+[4]– in List’s model reaction of
silylketene acetal 21 and 2-naphthaldehyde (22)[6] gave only
racemic aldol adduct 23.
[
MeTr] [4]
O
O
OTBDMS
OTBDMS
(10 mol%)
H
EtO
CH2Cl2
78 °C
4.5 h
EtO
18
19
(1.0 equiv.)
20: 0% ee
49%
(1.2 equiv.)
[
MeTr] [4]
O
[Na] [4]
(10 mol%)
TMSCl
(5.0 mol%)
Ph
Ph
C6H6
r.t.
17 h
OTMS
O
O
OH
(0.50 equiv.)
Ph(O)C
15: 0% ee
Ph
MeO
H
Ar
MeO
Ar
13
14
toluene
80 °C
80%
(1.0 equiv.)
(2.0 equiv.)
21
22
(1.0 equiv.)
23: 0% ee
Ar = naphth-2-yl
22 h
85% conv.
[
MeTr] [4]
(1.3 equiv.)
(5.0 mol%)
O
Me
Me
Scheme 4. Representative Mukaiyama aldol reactions.
CH2Cl2
20 °C
23 h
H
H
O
16
14
17: 0% ee
To assess the stability of borate [4]– towards silylium ions,
we treated Et3SiH with [MeTr]+[4]– in ClC6D5 to achieve the
established silicon-to-carbon hydride transfer.[19] The formation
of the chlorobenzene-stabilized silylium ion [Et3Si(ClC6D5)]+∙[4]–
was not observed. However, the same reaction in the presence
of a carbonyl group as a Lewis base did result in the formation of
the corresponding silylcarboxonium ion. With chalcone (13) in
46%
(1.0 equiv.)
(1.0 equiv.)
Scheme 3. Representative trityl-cation-catalyzed Diels–Alder reactions of
cyclohexa-1,3-diene (14) with different enophiles.
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10.1002/ejoc.201901434
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chlorobenzene, [Et3Si(13)]+[4]– did form as major product with a
chemical shift of 29Si NMR = 46.0 ppm; this was confirmed by
comparison with [Et3Si(13)]+[B(C6F5)4]– prepared by a literature-
temperature, the reaction was quenched by slow addition of EtOH (10
mL). The brown suspension was extracted with tert-butylmethyl ether (3 x
100 mL), the combined organic phases washed with H2O (100 mL) and
dried over Na2SO4. After removal of all volatiles under reduced pressure,
the residue was purified by flash column chromatography on silica gel
using cyclohexane/tert-butylmethyl ether = 10/1 as eluent to afford the
title compound 6 (d.r. = 70:30, 4.0 g, 51%) as a brown oil. The
diastereomeric ratio was determined in 1H NMR analysis by integration of
the baseline-separated signals at δ 4.91 ppm and δ 4.96 ppm. HRMS
known
procedure.[20]
However,
the
formation
of
hexamethyldisiloxane as a sideproduct was also observed in
small amounts (29Si NMR = 8.6 ppm). Cyclohexa-1,3-diene (14)
was then added to verify the catalytic activity of [Et3Si(13)]+[4]–
and the Diels–Alder adduct 15 was isolated in good yield but
without enantiomeric excess (Scheme 5).

(APCI) for C16H17F4 [M–OH]: calculated 285.1261, found 285.1258.
Minor diastereomer: 1H NMR (500 MHz, C6D6): δ/ppm = 0.72 (d, J = 9.7
Hz, 1H), 1.01 (s, 3H), 1.02 (s, 3H), 1.37–1.41 (m, 1H), 1.66 (d, J = 7.2 Hz,
1H), 1.72–1.98 (m, 5H), 2.10–2.17 (m, 1H), 2.51–2.60 (m, 1H), 4.91 (dd,
J = 7.3 Hz, J = 11.4 Hz, 1H), 6.17 (mc, 1H). 13C{1H} NMR (126 MHz,
C6D6): δ/ppm = 19.9, 23.0, 26.4, 27.9, 33.6, 38.6, 41.4, 43.8, 46.8, 71.0,
104.9 (t, J = 23 Hz), 123.7 (t, J = 15 Hz), 144.8 (dm), 146.3 (dm). 19F{1H}
NMR (471 MHz, C6D6): δ/ppm = –143.5 (dd, J = 22 Hz, J = 13 Hz, 2F), –
139.3 (dd, J = 23 Hz, J = 13 Hz, 2F). Major diastereomer: 1H NMR (500
MHz, C6D6): δ/ppm = 0.75 (d, J = 9.7 Hz, 1H), 1.01 (s, 3H), 1.03–1.11 (m,
1H),1.11–1.22 (m, 1H), 1.18 (s, 3H), 1.53–1.64 (m, 2H), 1.73 (mc, 1H),
1.79 (mc, 1H), 2.29–2.36 (m, 1H), 2.39–2.44 (m, 1H), 2.52 (mc, 1H), 4.96
(d, J = 10.8 Hz, 1H), 6.23 (mc, 1H). 13C{1H} NMR (126 MHz, C6D6):
δ/ppm = 18.2, 22.9, 26.3, 27.1, 27.2, 28.2, 33.4, 38.7, 41.5, 42.4, 46.6,
69.4, 104.9 (t, J = 22 Hz), 123.5, (t, J = 15 Hz), 144.4 (dm), 146.1 (dm).
19F{1H} NMR (471 MHz, C6D6): δ/ppm = –143.5 (dd, J = 23 Hz, J = 13 Hz,
2F), –139.4 (dd, J = 23 Hz, J = 13 Hz, 2F).
Scheme 5. Preparation of chalcone-stabilized silicon cation [Et3Si(13)]+[4]– by
Corey’s hydride abstraction with subsequent Diels–Alder reaction, 29Si NMR
resonance signal determined by 1H/29Si HMQC (500/99 MHz, ClC6D5).
O-(((1S,2R,5S)-6,6-Dimethylbicyclo[3.1.1]heptan-2-yl)(2,3,5,6-
tetrafluorophenyl)-methyl) S-methyl carbonodithioate (7)
Conclusion
In summary, a new class of para-myrtanyl-substituted chiral
borates based on the ubiquitous [B(C6F5)4]– anion has been
introduced. Their synthesis hinges on the easily accessible
To a suspension of NaH (60% in mineral oil, 47 mg, 1.2 mmol, 2.5 equiv.)
and imidazole (1.6 mg, 24 μmol, 5.0 mol%) in THF (4.0 mL) was added a
solution of the alcohol 6 (0.14 g, 0.47 mmol, 1.0 equiv.) in THF (3.0 mL)
at 0°C. The resulting suspension was stirred for 0.5
h at room
2,3,5,6-tetrafluorophenyl-substituted benzyl alcohol
6 (three
temperature before CS2 (70 μL, 90 mg, 1.2 mmol, 2.5 equiv.) was added
dropwise. The mixture was stirred for an additional 0.5 h, then MeI (0.17
g, 1.2 mmol, 2.5 equiv.) was added dropwise, and the solution was
stirred 1.5 h at room temperature. The reaction was quenched by
addition of saturated aqueous NH4Cl solution (5.0 mL) at 0°C. The
phases were separated, the aqueous phase extracted with tert-
butylmethyl ether (3 x 5.0 mL), and the combined organic phases dried
over Na2SO4. After removal of all volatiles under reduced pressure, the
residue was purified by flash column chromatography on silica gel using
cyclohexane as eluent to afford the title compound 7 (0.12 g, 65%) as a
steps from (–)-β-pinene). To turn the derived chiral borates into
counteranions suitable for strong Lewis acids such as trityl or
silicon cations, a series of salt metathesis reactions had to be
performed to obtain LiCl-free material ([Li]+ to [Na]+ or ([Li]+ to
[Cs]+ to [Na]+). To increase the chemical stability of the borate
anion, the hydride affinity of the trityl salt was attenuated with the
use of diphenyl(4-tolyl)methyl chloride as carbocation precursor
(to form [MeTr]+[4]–). Representative Diels–Alder and Mukaiyama-
aldol reactions were feasible but with no enantioinduction. The

brown oil. HRMS (APCI) for C18H21F4OS2 [M+H]: calculated 393.0964,
generation of
a
silicon cation from Et3SiH with [4]– as
found 393.0966. 1H NMR (500 MHz, C6D6): δ/ppm = 0.74 (d, J = 9.9 Hz,
1H), 1.11 (s, 3H), 1.13–1.21 (m, 2H), 1.18 (s, 3H), 1.53–1.63 (m, 1H),
1.70–1.80 (m, 2H), 2.03 (s, 3H), 2.25–2.32 (m, 1H), 2.32–2.38 (m, 1H),
3.07 (mc, 1H), 6.16 (mc, 1H), 7.17 (d, J = 8.4 Hz, 1H). 13C{1H} NMR (126
MHz, C6D6): δ/ppm = 17.4, 18.9, 22.8, 26.1, 27.9, 33.1, 38.5, 41.3, 42.5,
43.3, 78.7, 106.3 (t, J = 23 Hz), 118.5 (t, J = 15 Hz), 145.3 (dm, J = 249
Hz), 146.1 (dm, J = 248 Hz), 215.9. 19F{1H} NMR (471 MHz, C6D6):
δ/ppm = –141.0–[–140.6] (br m, 2F), –138.6 (dd, J = 23 Hz, J = 13 Hz,
2F).
counteranion was successful as its chalcone adduct. Its
subsequent reaction with cyclohexa-1,3-diene (14) gave the
cycloaddition product as a racemic mixture.
Experimental Section
For general remarks as well as experimental procedures and
spectroscopic data for literature-known compounds see the Supporting
Information.
(1S,2S,5S)-6,6-Dimethyl-2-(2,3,5,6-
tetrafluorobenzyl)bicyclo[3.1.1]heptane (8)
((1S,2R,5S)-6,6-Dimethylbicyclo[3.1.1]heptan-2-yl)(2,3,5,6-
tetrafluorophenyl)methanol (6)
A solution of nBu3SnH (2.2 mL, 8.4 mmol, 5.0 equiv.), DBPO (49 mg,
0.20 mmol, 0.12 equiv.) and the xanthate 7(0.66 g, 1.7 mmol, 1.0 equiv.)
in toluene (22 mL) was degassed (3 x) and maintained at 105°C for 15 h.
The reaction was then cooled to room temperature, diluted with
cyclohexane (5.0 mL), and all volatiles were removed under reduced
pressure. Purification of the residue by flash column chromatography
using cyclohexane as eluent gave the title compound 8 (0.24 g, 49%) as
Based on a literature-known procedure[8b] 1,2-dibromoethane (3 drops)
was added to a suspension of magnesium turning (1.0 g, 41 mmol, 1.6
equiv.) in THF (7.0 mL). After stirring for 5 min a solution of 1-bromo-
2,3,5,6-tetrafluorobenzene (4.7 mL, 39 mmol, 1.5 equiv.) in THF (35 mL)
was added slowly. The resulting dark brown solution was stirred for 1.5 h
at room temperature and then 1 h at 60°C. The mixture was cooled to
room temperature, and a solution of aldehyde 5 (3.9 g, 26 mmol, 1.0
equiv.) in THF (8.0 mL) was added quickly. After stirring for 5 h at room

colorless oil. HRMS (APCI) for C16H17F4 [M–H]: calculated 285.1261,
found 285.1265. 1H NMR (500 MHz, C6D6): δ/ppm = 0.69 (d, J = 9.7 Hz,
1H), 1.07 (s, 3H), 1.14 (s, 3H), 1.34–1.46 (m, 1H), 1.60–1.73 (m, 3H),
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10.1002/ejoc.201901434
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1.77–1.88 (2H), 2.16–2.25 (m, 2H), 2.53–2.64 (m, 2H), 6.21 (mc, 1H).
13C{1H} NMR (126 MHz, C6D6): δ/ppm = 22.0, 22.9, 26.6, 28.2, 30.0, 33.9,
38.9, 41.4, 41.6, 45.3, 103.6 (t, J = 23 Hz), 121.1 (t, J = 19 Hz), 145.3
(dm), 146.0 (dm). Optical rotation: [α]20D = +9.08 (c = 1.00, CHCl3).
= 17 Hz). The ortho- and meta carbon atoms of the aromatic ring could
not be detected. 19F NMR (471 MHz, C6D6): δ/ppm = –144.6–[–141.0] (br
m, 2F), –140–[–139.2] (br m, 2F). Minor diastereomer (selected signals):
1H NMR (500 MHz, C6D6): δ/ppm = 0.77 (d, J = 9.7 Hz, 1H), 2.10 (mc,
1H), 2.30 (mc, 1H), 3.34 (mc, 1H), 6.24 (mc, 1H). Optical rotation: [α]20
=
D
+2.5 (c = 1.4, CHCl3).
((1S,2R,5S)-6,6-Dimethylbicyclo[3.1.1]heptan-2-yl)(2,3,5,6-
tetrafluorophenyl)methanone (9)
Lithium
tetrakis(4-(((1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]heptan-2-
yl)methyl)-2,3,5,6-tetrafluorophenyl)borate ([Li]+[3]–)
A solution of the alcohol 6 (1.7 g, 5.6 mmol, 1.0 equiv.) in CH2Cl2 (60 mL)
was cooled to 0°C. Dess–Martin periodinane (3.6 g, 8.4 mmol, 1.5 equiv.)
was added in one portion, and the resulting mixture stirred 4 h at room
temperature. The reaction was quenched by addition of H2O (100 mL).
The phases were separated, the organic phase washed with H2O (5 x
100 mL) and dried over MgSO4. After removal of all volatiles, the
resulting white solid was removed by filtration through a pad of cotton to
afford the ketone 9 (d.r. = 95:5, 1.4 g, 84%) as an orange brown oil; it
was used without further purification. The diastereomeric ratio was
determined by GLC analysis. HRMS (APCI) for C16H15F4O [M–H]:
calculated 299.1054, found 299.1051. 1H NMR (700 MHz, C6D6): δ/ppm
= 0.85 (d, J = 9.9 Hz, 1H), 0.88 (s, 3H), 1.06 (s, 3H), 1.55–1.65 (m, 2H),
1.67–1.71 (m, 1H), 1.76–1.83 (m, 1H), 2.12–2.19 (m, 1H), 2.25–2.31 (m,
1H), 2.34–2.39 (m, 1H), 3.13–3.20 (mc, 1H), 6.08 (mc, 1H). 13C{1H} NMR
(176 MHz, C6D6): δ/ppm = 14.4, 22.7, 25.1, 27.1, 30.8, 39.0, 40.8, 43.0,
54.3, 107.2 (t, J = 23 Hz), 121.3 (t, J = 21 Hz), 142.9 (dm), 146.0 (dm),
196.9. 19F NMR (659 MHz, C6D6): δ/ppm = –142.4 (mc, 2F), –137.6 (mc,
2F).
To a solution of alkane 8 (0.40 g, 1.4 mmol, 5.5 equiv.) in Et2O (20 mL)
was added dropwise nBuLi (2.7M in hexane, 0.48 mL, 1.3 mmol, 5.0
equiv.) at –78°C, and the resulting mixture stirred for 3 h. Afterwards BCl3
(1M in heptane, 0.26 mL, 0.26 mmol, 1.0 equiv.) was added dropwise,
and the solution was allowed to slowly warm to room temperature
overnight. The reaction was quenched by addition of H2O (20 mL) and
extracted with tert-butylmethyl ether (3 x 10 mL). After removal of all
volatiles, the residue was purified by flash column chromatography on
silica gel using subsequent cyclohexane (200 mL) and ethyl acetate (800
mL) as eluent. The lithium borate [Li]+[3]– (0.25 g, 96%) was obtained as
–
a white solid. HRMS (APCI) for C64H68BF16 [M]–: calculated 1151.5164,
found 1151.5165. 1H NMR (500 MHz, (CD3)2CO): δ/ppm = 0.86 (d, J =
9.5 Hz, 4H), 1.14 (s, 12H), 1.19 (s, 12H), 1.57–1.67 (m, 4H), 1.80–1.94
(m, 16H), 1.95–2.02 (m, 4H), 2.24–2.37 (m, 8H), 2.65–2.76 (m, 8H),
11B{1H} NMR (160 MHz, (CD3)2CO): δ/ppm = –16.3. 13C{1H} NMR (126
MHz, (CD3)2CO): δ/ppm = 22.5, 23.3, 27.0, 28.5, 30.3 (determined by
1H/13C HSQC NMR experiment), 34.3, 39.4, 42.2, 42.3, 46.0, 114.7 (t, J =
19 Hz), 144.9 (dm, J = 240 Hz), 149.2 (dm, J = 243 Hz). The carbon
atoms of the C–B bonds could not be detected. 19F NMR (471 MHz,
(CD3)2CO): δ/ppm = –150.5 (mc, 8F), –133.4 (br s, 8F).
(1S,2R,5S)-6,6-Dimethyl-2-(1-(2,3,5,6-
tetrafluorophenyl)vinyl)bicyclo[3.1.1]heptane (10)
Dimethyltitanocene (0.43M in THF, 5.9 mL, 2.5 mmol, 1.5 equiv.) was
added to a solution of the ketone 9 (0.50 g, 1.7 mmol, 1.0 equiv.) in THF
(10 mL) and heated at 65°C until full conversion monitored by GLC (18–
48 h). The reaction was cooled to room temperature, quenched by
addition of H2O (5.0 mL) and extracted with tert-butylmethyl ether (2 x 10
mL). The combined organic phases were dried over MgSO4. After
removal of all volatiles, the residue was purified by flash column
chromatography on silica gel using n-pentane as eluent to afford the
Lithium tetrakis(4-(1-((1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]heptan-2-
yl)ethyl)-2,3,5,6-tetrafluorophenyl)borate ([Li]+[4]–)
To a solution of alkane 11 (d.r. = 87:13, 1.1 g, 3.8 mmol, 4.5 equiv.) in
Et2O (60 mL) was added dropwise nBuLi (2.7M in hexane, 1.4 mL, 3.7
mmol, 4.4 equiv.) at –78°C and the resulting mixture stirred for 3 h.
Afterwards BCl3 (1M in heptane, 0.84 mL, 0.84 mmol, 1.0 equiv.) was
added dropwise and the solution was allowed to warm up to room
temperature overnight slowly. The reaction was quenched by addition of
H2O (10 mL) and extracted with n-pentane (3 x 30 mL). After removal of
all volatiles the residue was purified by flash column chromatography on
neutral aluminum oxide using subsequent n-pentane (500 mL), tert-
butylmethyl ether (500 mL), n-pentane (500 mL) and acetonitrile (2 L) as
eluent. The lithium borate [Li]+[4]– (0.94 g, 92%) was obtained as a white
alkene 10 (d.r.
= 96:4, 0.28 g, 57%) as a colorless liquid. The
diastereomeric ratio was determined by GLC analysis. HRMS (APCI) for

C17H17F4 [M–H]: calculated 297.1261, found 297.1265. 1H NMR (500
MHz, C6D6): δ/ppm = 0.81 (d, J = 9.8 Hz, 1H), 1.00 (s, 3H), 1.15 (s, 3H),
1.48–1.70 (m, 3H), 1.75–1.88 (m, 2H), 2.18 (mc, 1H), 2.27 (mc, 1H), 3.07
(mc, 1H), 4.95 (d, J = 2.0 Hz, 1H), 5.21 (d, J = 2.2 Hz, 1H), 6.24 (mc, 1H).
13C{1H} NMR (126 MHz, CDCl3): δ/ppm = 19.6, 23.6, 26.2, 28.1, 33.5,
38.7, 41.6, 44.0, 44.6, 104.5 (t, J = 23 Hz), 117.2, 141.7. The ortho- and
meta carbon atoms of the aromatic ring could not be detected. 19F{1H}
NMR (471 MHz, C6D6): δ/ppm = –142.3 (dd, J = 13 Hz, J = 24 Hz, 2F), –
139.4 (dd, J = 13 Hz, J = 23 Hz, 2F).
–
solid. HRMS (APCI) for C68H76BF16 [M]–: calculated 1207.5790, found
1207.5754. 1H NMR (500 MHz, C6D6): δ/ppm = 0.72–0.82 (br m, 4H),
1.05–1.14 (br m, 24H), 1.14–1.30 (br m, 12H), 1.44–1.55 (br m, 4H),
1.61–1.75 (br m, 4H), 1.76–1.88 (br m, 8H), 1.88–1.99 (br m, 8H), 2.13–
2.26 (br m, 4H), 2.41–2.57 (br m, 4H), 3.22–3.35 (br m, 4H). 7Li NMR
(194 MHz, C6D6): δ/ppm = 0.0. 11B{1H} NMR (160 MHz, C6D6): δ/ppm = –
15.8. 13C{1H} NMR (126 MHz, C6D6): δ/ppm = 18.0, 22.2, 23.0, 27.0, 28.3,
34.2, 37.2, 38.6, 41.5, 45.2, 45.5, 120.1, (t, J = 17 Hz), 144.7 (dm, J =
242 Hz), 149.4 (dm, J = 238 Hz). The carbon atoms of the C–B bonds
could not be detected. 19F NMR (471 MHz, C6D6): δ/ppm = –150.5–[–
143.7] (br m, 8F), –138.2–[–131.7] (br m, 8F).
(1S,2S,5S)-6,6-Dimethyl-2-(1-(2,3,5,6-
tetrafluorophenyl)ethyl)bicyclo[3.1.1]heptane (11)
In a glass vial, the alkene 10 (45 mg, 0.15 mmol, 1.0 equiv.) and
(Ph3P)3RhCl (6.9 mg, 7.5 μmol, 5.0 mol%) were placed under a nitrogen
atmosphere and dissolved in degassed benzene (2.0 mL). The reaction
vessel was transferred to an autoclave pressurized with H2 (30 bar) and
stirred for 18 h at 30°C. The vial was then removed from the autoclave
and the crude material filtered through a plug of silica. Removal of all
volatiles under reduced pressure gave the alkane 11 (d.r. = 87:13, 44 mg,
quant.) as a colorless liquid. The diastereomeric ratio was determined in
1H NMR analysis by integration of the baseline-separated signals at δ
3.21 ppm and δ 3.34 ppm and by GLC analysis. Major diastereomer: 1H
NMR (500 MHz, C6D6): δ/ppm = 0.72 (d, J = 9.7 Hz, 1H), 1.01 (s, 3H),
1.03 (s, 3H), 1.13 (d, J = 7.0 Hz, 3H), 1.33–1.44 (m, 1H), 1.51 (mc, 1H),
1.68–1.90 (m, 4H), 2.17 (mc, 1H), 2.34–2.46 (m, 1H), 3.21 (mc, 1H), 6.18
(mc, 1H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm = 17.4, 21.9, 22.9, 26.8,
28.3, 34.2, 37.3, 38.6, 41.4, 45.06, 45.11, 103.6 (t, J = 23 Hz), 125.9 (t, J
Cesium tetrakis(4-(1-((1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]heptan-2-
yl)ethyl)-2,3,5,6-tetrafluorophenyl)borate ([Cs]+[4]–)
To a solution of borate [Li]+[4]– (0.11 g, 0.091 mmol) in benzene (1.0 mL)
was added a saturated aqueous solution of Cs2CO3 (1.0 mL), and the
two-phase mixture was vigorously stirred for 5 h at room temperature.
The phases were then separated, extracted with benzene (2 x 5.0 mL),
and the combined organic phases were washed with H2O (5.0 mL). The
volatiles were removed under reduced pressure, and the resulting
residue dried under high vacuum (130°C/10–3 mbar) giving the cesium
borate [Cs]+[4]– (0.11 g, 92%) as a white solid; it was used without further
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10.1002/ejoc.201901434
European Journal of Organic Chemistry
FULL PAPER
purification. 1H NMR (500 MHz, C6D6): δ/ppm = 0.71–0.84 (br m, 4H),
1.03–1.16 (br m, 24H), 1.22–1.37 (br m, 12H), 1.45–1.58 (br m, 4H),
1.64–1.75 (br m, 4H), 1.75–1.88 (br m, 8H), 1.88–2.01 (br m, 8H), 2.11–
2.28 (br m, 4H), 2.46–2.64 (br m, 4H), 3.26–3.41 (br m, 4H). 11B{1H}
NMR (160 MHz, C6D6): δ/ppm = –15.8. 13C{1H} NMR (126 MHz, C6D6):
δ/ppm = 18.2, 22.2, 23.1, 27.0, 28.3, 34.3, 37.3, 38.6, 41.5, 45.3, 45.6.
The aromatic carbon atoms could not be detected. 19F NMR (471 MHz,
C6D6): δ/ppm = –150.2–[–142.2] (br m, 8F), –132.2 (br s, 8F).
analysis by integration of the baseline-separated signals at δ 7.58–7.70
–
ppm and δ 7.13 ppm. HRMS (APCI) for C64H68BF16 [M]–: calculated
+
1151.5164, found 1151.5144. HRMS (APCI) for C19H15 [M]+: calculated
243.1168, found 243.1163. 1H NMR (500 MHz, CD2Cl2): δ/ppm = 0.81 (d,
J = 9.4 Hz, 4H), 1.11 (s, 12H), 1.18 (s, 12H), 1.51–1.62 (m, 4H), 1.76–
1.91 (m, 16H), 1.91–2.00 (m, 4H), 2.22–2.34 (m, 8H), 2.60–2.71 (m, 8H).
7.58–7.70 (m, 6H), 7.84 (t, J = 7.5 Hz, 6H), 8.18–8.29 (m, 3H). 11B{1H}
NMR (161 MHz, CD2Cl2): δ/ppm = –16.4. 13C{1H} NMR (176 MHz,
CD2Cl2): δ/ppm = 22.3, 23.1, 26.8, 28.3, 30.2, 34.1, 39.1, 41.8, 41.9, 45.7,
114.5, 131.0, 140.3, 143.1, 144.0, 211.1 (determined by 1H/13C HMBC
NMR experiment). The ortho- and meta carbon atoms of the aromatic
rings as well as the carbon atoms of the C–B bonds could not be
detected. 19F NMR (471 MHz, CD2Cl2): δ/ppm = –149.8 (mc, 8F), –134.1
(br s, 8F).
Sodium
tetrakis(4-(((1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]heptan-2-
yl)methyl)-2,3,5,6-tetrafluorophenyl)borate ([Na]+[3]–)
To a solution of the lithium borate [Li]+[3]– (0.30 g, 0.26 mmol) in CH2Cl2
(2.0 mL) was added a saturated aqueous solution of NaCl (2.0 mL), and
the two-phase mixture was vigorously stirred overnight at room
temperature. The phases were then separated, the organic phase dried
over Na2SO4, all volatiles removed under reduced pressure, and the
residue was dried under high vacuum (130°C/10–3 mbar) for 10 h giving
the sodium borate [Na]+[3]– (0.24 mg, 77%) as a white solid. 1H NMR
(500 MHz, C6D6): δ/ppm = 0.63 (d, J = 9.7 Hz, 4H), 1.05 (s, 12H), 1.09 (s,
12H), 1.38–1.51 (m, 4H), 1.56–1.67 (m, 8H), 1.73–1.84 (m, 12H), 2.08–
2.15 (m, 4H), 2.18–2.28 (m, 4H), 2.59 (mc, 8H). 11B{1H} NMR (160 MHz,
C6D6): δ/ppm = –15.5. 13C{1H} NMR (126 MHz, C6D6): δ/ppm = 21.8, 23.1,
26.7, 28.2, 30.2, 34.0, 38.8, 41.5, 41.6, 46.0, 115.6 (determined by
1H/13C HMBC NMR), 145.1 (determined by 1H/13C HMBC NMR). The
meta carbon atoms of the aromatic rings as well as the carbon atoms of
the C–B bonds could not be detected. 19F{1H} NMR (471 MHz, C6D6):
δ/ppm = –147.7 (s, 8F), –134.7 (s, 8F). Optical rotation: [α]20D = +23.4 (c
= 1.07, CHCl3).
Triphenylmethylium
tetrakis(4-(1-((1S,2S,5S)-6,6-
dimethylbicyclo[3.1.1]heptan-2-yl)ethyl)-2,3,5,6-
tetrafluorophenyl)borate ([Tr]+[4]–)
The borate [Na]+[4]– (0.10 g, 0.081 mmol, 1.0 equiv.) and triphenylmethyl
chloride (0.11 g, 0.41 mmol, 5.0 equiv.) were suspended in n-hexane (6.0
mL) and stirred overnight at room temperature. The suspension was
filtered under nitrogen atmosphere, and the remaining solid was washed
with n-hexane (6 x 3.0 mL). The red orange residue was redissolved in
CH2Cl2 (2.0 mL) and then dried under high vacuum (50°C/10–3 mbar).
The trityl salt [Tr]+[4] (86 mg, 0.059 mmol, 73%) was obtained as an
orange solid with triphenylmethane (2.0 mg, 0.010 mmol, 12%) as
byproduct. The amount of triphenylmethane was determined by 1H NMR
analysis by integration of the baseline-separated signals at δ 7.64 ppm
and δ 7.13 ppm. 1H NMR (400 MHz, CD2Cl2): δ/ppm = 0.77 (br d, J = 9.2
Hz, 4H), 1.02 (br s, 24H), 1.21 (br s, 12H), 1.40–1.55 (br m, 4H), 1.55–
1.70 (br m, 4H), 1.76–1.92 (br m, 8H), 1.92–2.11 (br m, 8H), 2.16–2.28
(br m, 4H), 2.28–2.42 (br m, 4H), 3.06–3.19 (br m, 4H), 7.64 (d, J = 7.9
Hz, 6H), 7.85 (t, J = 7.6 Hz, 6H), 8.25 (t, J = 7.6 Hz, 3H). 11B{1H} NMR
(161 MHz, CD2Cl2): δ/ppm = –16.5. 13C{1H} NMR (101 MHz, CD2Cl2):
δ/ppm = 18.0, 22.2, 22.9, 27.1, 28.3, 34.3, 36.8, 38.7, 41.7, 45.1, 45.5,
131.0, 140.3, 143.0, 144.0, 211.1. The ortho- and meta carbon atoms of
the aromatic rings as well as the carbon atoms of the C–B bonds could
not be detected. 19F NMR (471 MHz, CD2Cl2): δ/ppm = –151.8–[–145.7]
(br m, 8F), –134.1 (br s, 8F).
Sodium tetrakis(4-(1-((1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]heptan-2-
yl)ethyl)-2,3,5,6-tetrafluorophenyl)borate ([Na]+[4]–)
To a solution of the cesium borate [Cs]+[4]– (0.11 g, 0.084 mmol) in
benzene (1.5 mL) was added a saturated aqueous solution of NaCl (1.5
mL), and the two-phase mixture was vigorously stirred for 3 h at room
temperature. The phases were then separated, the organic phase dried
over Na2SO4, and all volatiles were removed under reduced pressure.
The resulting residue was transferred to a glove box, resuspended in
benzene (3 mL), and the solution stirred overnight over molecular sieves
(4 Å). The molecular sieves was filtered off, and the resulting solution
dried under high vacuum (130°C/10–3 mbar) for 10 h to afford the sodium
Diphenyl(4-tolyl)methylium
tetrakis(4-(1-((1S,2S,5S)-6,6-
borate [Na]+[4]– (78 mg, 75%) as a white solid. HRMS (APCI) for
dimethylbicyclo[3.1.1]heptan-2-yl)ethyl)-2,3,5,6-
tetrafluorophenyl)borate ([MeTr]+[4]–)
–
C
68H76BF16 [M]–: calculated 1207.5790, found 1207.5797. 1H NMR (500
MHz, C6D6): δ/ppm = 0.72–0.82 (br m, 4H), 1.05–1.14 (br m, 24H), 1.14–
1.30 (br m, 12H), 1.44–1.55 (br m, 4H), 1.61–1.75 (br m, 4H), 1.76–1.88
(br m, 8H), 1.88–1.99 (br m, 8H), 2.13–2.26 (br m, 4H), 2.41–2.57 (br m,
4H), 3.22–3.35 (br m, 4H). 11B{1H} NMR (160 MHz, C6D6): δ/ppm = –15.8.
13C{1H} NMR (126 MHz, C6D6): δ/ppm = 18.0, 22.2, 23.0, 27.0, 28.3, 34.2,
37.2, 38.6, 41.5, 45.2, 45.5, 120.3, (t, J = 17 Hz), 144.8 (dm, J = 248 Hz),
149.3 (dm, J = 236 Hz). The carbon atoms of the C–B bonds could not
be detected. 19F NMR (471 MHz, C6D6): δ/ppm = –151.5–[–143.6] (br m,
8F), –139.6–[–131.1] (br m, 8F). Optical rotation: [α]20D = +18.1 (c = 1.22,
CHCl3).
The borate [Na]+[4]– (0.10 g, 0.081 mmol, 1.0 equiv.) and diphenyl(4-
tolyl)methyl chloride (25 mg, 85 mol, 1.1 equiv.) were dissolved in
CH2Cl2 (2.5 mL) and stirred overnight at room temperature. The
supernatant was transferred into a Schlenk tube, and the remaining solid
was washed with CH2Cl2 (2 x 2.0 mL). The red orange solution was
transferred out of the glovebox and connected to a vacuum-nitrogen
manifold to remove all volatiles under high vacuum (50°C/10–3 mbar).
The trityl salt [MeTr]+[4] (83 mg, 0.056 mmol, 70%) was obtained as an
orange solid with diphenyl(4-tolyl)methane (1.0 mg, 0.006 mmol, 7%) as
byproduct. The amount of diphenyl(4-tolyl)methane was determined by
1H NMR analysis by integration of the baseline-separated signals at δ
7.67 ppm and δ 7.01 ppm. HRMS (APCI) for C68H76BF16– [M]–: calculated
Triphenylmethylium
tetrakis(4-(((1S,2S,5S)-6,6-
dimethylbicyclo[3.1.1]heptan-2-yl)methyl)-2,3,5,6-
tetrafluorophenyl)borate ([Tr]+[3]–)
+
1207.5790, found 1207.5792. HRMS (APCI) for C20H17 [M]+: calculated
257.1325, found 257.1329. 1H NMR (500 MHz, CD2Cl2): δ/ppm = 0.77 (br
d, J = 9.3 Hz, 4H), 1.03 (br s, 24H), 1.22 (br s, 12H), 1.39–1.56 (br m,
4H), 1.56–1.68 (br m, 4H), 1.78–1.92 (br m, 8H), 1.92–2.09 (br m, 8H),
2.15–2.27 (br m, 4H), 2.28–2.41 (br m, 4H), 2.70 (br s, 3H), 3.08–3.18 (br
m, 4H), 7.54–7.62 (m, 6H), 7.67 (d, J = 7.9 Hz, 2H), 7.81 (t, J = 7.7 Hz,
4H), 8.18 (t, J = 7.5 Hz, 2H). 11B{1H} NMR (161 MHz, CD2Cl2): δ/ppm = –
16.5. 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm = 18.1, 22.3, 22.9, 23.7,
27.1, 28.4, 34.4, 36.9, 38.7, 41.7, 45.1, 45.5, 119.0 (t, J = 17 Hz), 130.7,
132.5, 138.2, 140.1, 142.0, 142.7, 143.7, 160.8, 208.4. 19F NMR (471
MHz, CD2Cl2): δ/ppm = –151.5–[–145.1] (br m, 8F), –134.1 (br s, 8F).
The borate [Na]+[3]– (0.10 g, 0.085 mmol, 1.0 equiv.) and triphenylmethyl
chloride (0.12 g, 0.43 mmol, 5.0 equiv.) were suspended in n-hexane (6.0
mL) and stirred overnight at room temperature. The suspension was
filtered under nitrogen atmosphere, and the remaining solid was washed
with n-hexane (2 x 3.0 mL). The red orange residue was redissolved in
CH2Cl2 (2.0 mL) and then dried under high vacuum (50°C/10–3 mbar).
The trityl salt [Tr]+[3]– (87 mg, 0.063 mmol, 75%) was obtained as an
orange solid with triphenylmethane (2.7 mg, 0.011 mmol, 13%) as
byproduct. The amount of triphenylmethane was determined by 1H NMR
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10.1002/ejoc.201901434
European Journal of Organic Chemistry
FULL PAPER
[14] For trityl-cation-catalyzed Diels–Alder and hetero-Diels–Alder reactions
in the racemic series, see: a) J. Bah, J. Franzén, Chem. Eur. J. 2014,
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Acknowledgments
M.O. is indebted to the Einstein Foundation (Berlin) for an
endowed professorship.
[15] For trityl-cation-catalyzed enantioselective Diels–Alder and hetero-
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(with chiral iron(III)-based bisphosphate counteranion).
Keywords: Cations • Chirality • Diels–Alder reaction • Lewis
acids • Trityl group
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10.1002/ejoc.201901434
European Journal of Organic Chemistry
FULL PAPER
Suggestion for the Entry for the Table of Contents
FULL PAPER
New chiral versions of the ubiquitous
[B(C6F5)4]‒ anion decorated with
myrtanyl backbones and paired with
various countercations are presented
(Tr = Ph3C). Their use as catalysts in
Diels–Alder reactions and Mukaiyama
aldol additions is demonstrated, and
their stability against highly
P. Pommerening, M. Oestreich*
Page No. – Page No.
Chiral Modification of the
Tetrakis(pentafluorophenyl)borate
Anion with Myrtanyl Groups
electrophilic silylium-ion-like species is
investigated.
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