DOI: 10.1002/ejoc.201901434 Full Paper
Weakly Coordinating Anions
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(C
6
F
5
)
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 counter-
Introduction
Boron- and aluminum-based weakly coordinating anions
(WCAs) have found widespread application in molecular chem-
istry.
[1]
This is particularly true for borates containing highly
Figure 1. Chiral congeners of [B(C
6
F
5
)
4
]
–
where one of the fluorine atoms at the aryl groups has been replaced by chiral moieties; [Tr]
+
= triphenylmethylium,
[
Me
Tr]
+
= diphenyl(4-tolyl)methylium.
[a] Institut für Chemie, Technische Universität Berlin,
Straße des 17. Juni 115, 10623 Berlin, Germany
E-mail: mar[email protected]
http://www.organometallics.tu-berlin.de
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201901434.
© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial License, which permits use, distribution and re-
production in any medium, provided the original work is properly cited and
is not used for commercial purposes.
Eur. J. Org. Chem. 2019, 7240–7246 © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7240
anion-directed Diels–Alder reactions and Mukaiyama aldol ad-
ditions 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.
fluorinated aryl groups such as tetrakis[3,5-bis(trifluoro-
methyl)phenyl]borate ([BAr
F4
]
–
)
[2]
and tetrakis(pentafluoro-
phenyl)borate {[B(C
6
F
5
)
4
]
–
}.
[3]
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(C
6
F
5
)
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been replaced by 1,1′-binaphthalene-2-yl groups (Figure 1,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 we
did not observe 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 as in [2]
–
,aMu-
kaiyama aldol reaction afforded 16 % ee as proof of concept.
[6]
Full Paper
Despite these modest prospects, we decided to further pur-
sue the development of chiral, partially fluorinated tetraaryl-
borates. Our initial goal had been to design chiral counter-
anions for silylium ions and silylium-ion-like Lewis acids to drive
our silylium-ion-catalyzed Diels–Alder reactions of cyclohexa-
1,3-diene enantioselectively.
[5,6]
Counteranion [1]
–
with its π-do-
nating naphthyl groups is not chemically resistant against those
strong electrophiles. We, therefore, considered more robust ali-
phatic rather than aromatic chiral units for the modification of
the [B(C
6
F
5
)
4
]
–
platform, and we report here the synthesis and
characterization of the myrtanyl-substituted borates [3]
–
and
[4]
–
with various countercations (Figure 1, right).
Results and Discussion
To replace the fluorine atom in the para position of the C
6
F
5
group by myrtanyl groups, we targeted intermediate 6. Its syn-
thesis began with literature-known myrtanal (5) derived from
(–)-β-pinene
[7]
in two steps (Scheme 1, top left).
[8]
The alcohol
6was obtained by the addition
[8b]
of the Grignard reagent pre-
pared from 1-bromo-2,3,5,6-tetrafluorobenzene (5→6). The
hydroxy group in 6can be seen as a useful handle for further
derivatization in the benzylic position. Defunctionalization was
achieved by the Barton–McCombie deoxygenation subsequent
to xanthate formation (6→7→8); alternative palladium-cata-
lyzed methods using H
2
or Et
3
SiH 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-disubsti-
tuted alkene employing Wilkinson's catalyst proceeded quanti-
tatively with good diastereoselectivity (10 →11). We did not
succeed improving the d.r. = 87:13 further. For example, iridium-
catalyzed enantioselective hydrogenation
[10]
of 10 did not over-
ride the substrate control, and yields were consistently lower
(see the Supporting Information for details). The assignment of
Scheme 1. Preparation of the borate precursors 8and 11.
Eur. J. Org. Chem. 2019, 7240–7246 www.eurjoc.org © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7241
the relative configuration by nOe measurements was not con-
clusive. Attempts to transform ketone 9into gem-dimethyl-
substituted 12 by geminal dimethylation
[11]
resulted in decom-
position of the starting material. The detour involving cyclo-
propanation followed by hydrogenolysis was not feasible due
to low conversion of the Simmons–Smith reaction under vari-
ous reaction conditions.
[12]
The lithium borate [Li]
+
[3]
–
was accessible by chemoselective
deprotonation of 8using n-butyllithium followed by the reac-
tion with BCl
3
(Scheme 2, top). We used a salt metathesis reac-
tion with an excess of NaCl ([Li]
+
[3]
–
→[Na]
+
[3]
–
) to ensure
complete removal of the formed LiCl prior to the next step. The
absence of LiCl was verified by
7
Li NMR spectroscopy. The so-
dium borate [Na]
+
[3]
–
was then reacted with trityl chloride
([Na]
+
[3]
–
→[Tr]
+
[3]
–
). However, the steady formation of tri-
phenylmethane was observed but we were unable to deter-
mine 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 coordinat-
ing solvents from the purification process, we started the salt
metathesis with an excess of Cs
2
CO
3
which allowed isolation of
solvent- and LiCl-free [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 the formation of
the desired trityl borate [Tr]
+
[4]
–
but again, the formation of
triphenylmethane was observed. Hydride abstraction from the
benzylic position was excluded by
2
H-labeling of 11 (for the
characterization of 11-d
2
and the corresponding borates [4-d
2
]
–
,
see the Supporting Information). For [Na]
+
[4-d
2
]
–
to [Tr]
+
[4-d
2
]
–
,
the formation of non-deuterated triphenylmethane persisted,
and deuterated triphenylmethane was not detected. As a con-
sequence, we turned towards reducing the hydride affinity of
the trityl cation by moving from TrCl to diphenyl(4-tolyl)methyl
chloride (
Me
TrCl). Despite the reduced hydride affinity of
Full Paper
Scheme 2. Formation of the chiral borates [3]
–
and [4]
–
with various countercations.
[
Me
Tr]
+
[4]
–
,
[13]
the formation of diphenyl(4-tolyl)methane was
not fully prevented.
With the modified trityl salt [
Me
Tr]
+
[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 [
Me
Tr]
+
[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 enantioselec-
tive 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 de-
sired 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 conver-
sion.
[15c]
Scheme 3. Representative trityl-cation-catalyzed Diels–Alder reactions of
cyclohexa-1,3-diene (14) with different enophiles.
Eur. J. Org. Chem. 2019, 7240–7246 www.eurjoc.org © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7242
Enantioselective Mukaiyama aldol reactions either promoted
by chiral carbocations
[16]
or performed in the presence of chiral
counteranions
[6,17]
are known. We applied [
Me
Tr]
+
[4]
–
in the
model aldol reaction of 18 with benzaldehyde (19). Although
our trityl salt [
Me
Tr]
+
[4]
–
is potent enough to catalyze the reac-
tion, we could only isolate the adduct 20 as a racemic mixture
(Scheme 4, top).
[18]
Examination of [Na]
+
[4]
–
in List's model re-
action of silylketene acetal 21 and 2-naphthaldehyde (22)
[6]
gave only racemic aldol adduct 23 (Scheme 4, bottom).
Scheme 4. Representative Mukaiyama aldol reactions.
To assess the stability of borate [4]
–
towards silylium ions,
we treated Et
3
SiH with [
Me
Tr]
+
[4]
–
in ClC
6
D
5
to achieve the es-
tablished silicon-to-carbon hydride transfer.
[19]
The formation of
the chlorobenzene-stabilized silylium ion [Et
3
Si(ClC
6
D
5
)]
+
[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 chlorobenzene, [Et
3
Si(13)]
+
[4]
–
did form as major product
with a chemical shift of
29
Si NMR = 46.0 ppm; this was con-
Full Paper
firmed by comparison with [Et
3
Si(13)]
+
[B(C
6
F
5
)
4
]
–
prepared by a
literature-known procedure.
[20]
However, the formation of
hexamethyldisiloxane as a sideproduct was also observed in
small amounts (
29
Si NMR = 8.6 ppm). Cyclohexa-1,3-diene (14)
was then added to verify the reactivity of [Et
3
Si(13)]
+
[4]
–
and
the Diels–Alder adduct 15 was isolated in good yield but with-
out enantiomeric excess (Scheme 5).
Scheme 5. Preparation of chalcone-stabilized silicon cation [Et
3
Si(13)]
+
[4]
–
by
Corey's hydride abstraction with subsequent Diels–Alder reaction,
29
Si NMR
resonance signal determined by
1
H/
29
Si HMQC (500/99 MHz, ClC
6
D
5
).
Conclusion
In summary, a new class of para-myrtanyl-substituted chiral
borates based on the ubiquitous [B(C
6
F
5
)
4
]
–
anion has been in-
troduced. Their synthesis hinges on the easily accessible 2,3,5,6-
tetrafluorophenyl-substituted benzyl alcohol 6[three steps
from (–)-β-pinene]. To turn the derived chiral borates into coun-
teranions suitable for strong Lewis acids such as trityl or silicon
cations, a series of salt metathesis reactions had to be per-
formed 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 pre-
cursor (to form [
Me
Tr]
+
[4]
–
). Representative Diels–Alder and
Mukaiyama aldol reactions were feasible but with no enantio-
induction. The generation of a silicon cation from Et
3
SiH with
[4]
–
as 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 spec-
troscopic data for literature-known compounds see the Supporting
Information.
[(1S,2R,5S)-6,6-Dimethylbicyclo[3.1.1]heptan-2-yl](2,3,5,6-tetra-
fluorophenyl)methanol (6): Based on a literature-known proce-
dure
[8b]
1,2-dibromoethane (3 drops) was added to a suspension of
magnesium turnings (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-tetrafluoro-
benzene (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
Eur. J. Org. Chem. 2019, 7240–7246 www.eurjoc.org © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7243
at room temperature, the reaction was quenched by slow addition
of EtOH (10 mL). The brown suspension was extracted with tert-
butylmethyl ether (3 × 100 mL), the combined organic phases
washed with H
2
O (100 mL) and dried with Na
2
SO
4
. 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
1
H NMR analysis by integration of the baseline-
separated signals at δ= 4.91 ppm and δ= 4.96 ppm. HRMS (APCI)
for C
16
H
17
F
4+
[M – OH]
+
: calculated 285.1261, found 285.1258. Mi-
nor diastereomer:
1
H NMR (500 MHz, C
6
D
6
): 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 (m
c
, 1H).
13
C{
1
H} NMR
(126 MHz, C
6
D
6
): 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).
19
F{
1
H} NMR (471 MHz, C
6
D
6
): δ/ppm = –143.5 (dd, J=
22 Hz, J= 13 Hz, 2F), –139.3 (dd, J=23Hz,J= 13 Hz, 2F). Major
diastereomer:
1
H NMR (500 MHz, C
6
D
6
): 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 (m
c
, 1H), 1.79 (m
c
, 1H), 2.29–2.36 (m, 1H),
2.39–2.44 (m, 1H), 2.52 (m
c
, 1H), 4.96 (d, J= 10.8 Hz, 1H), 6.23 (m
c
,
1H).
13
C{
1
H} NMR (126 MHz, C
6
D
6
): 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).
19
F{
1
H} NMR (471 MHz, C
6
D
6
):
δ/ppm = –143.5 (dd, J=23Hz,J= 13 Hz, 2F), –139.4 (dd, J=23Hz,
J= 13 Hz, 2F).
O-{[(1S,2R,5S)-6,6-Dimethylbicyclo[3.1.1]heptan-2-yl](2,3,5,6-
tetrafluorophenyl)-methyl} S-Methyl Carbonodithioate (7): 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 temperature before CS
2
(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 NH
4
Cl solution
(5.0 mL) at 0 °C. The phases were separated, the aqueous phase
was extracted with tert-butylmethyl ether (3 × 5.0 mL), and the
combined organic phases were dried with Na
2
SO
4
.Afterremovalof
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 brown oil.
HRMS (APCI) for C
18
H
21
F
4
OS
2+
[M+H]
+
: calculated 393.0964, found
393.0966.
1
H NMR (500 MHz, C
6
D
6
): 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 (m
c
, 1H), 6.16 (m
c
, 1H), 7.17 (d, J= 8.4 Hz, 1H).
13
C{
1
H} NMR
(126 MHz, C
6
D
6
): 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.
19
F{
1
H} NMR
(471 MHz, C
6
D
6
): δ/ppm = –141.0–[–140.6] (br m, 2F), –138.6 (dd,
J=23Hz,J= 13 Hz, 2F).
(1S,2S,5S)-6,6-Dimethyl-2-(2,3,5,6-tetrafluorobenzyl)bicyclo-
[3.1.1]heptane (8): A solution of nBu
3
SnH (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 × ) 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
Full Paper
as eluent gave the title compound 8(0.24 g, 49 %) as colorless oil.
HRMS (APCI) for C
16
H
17
F
4+
[M – H]
+
: calculated 285.1261, found
285.1265.
1
H NMR (500 MHz, C
6
D
6
): 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),
1.77–1.88 (2H), 2.16–2.25 (m, 2H), 2.53–2.64 (m, 2H), 6.21 (m
c
, 1H).
13
C{
1
H} NMR (126 MHz, C
6
D
6
): 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]
D
20
= +9.08 (c= 1.00,
CHCl
3
).
[(1S,2R,5S)-6,6-Dimethylbicyclo[3.1.1]heptan-2-yl](2,3,5,6-tetra-
fluorophenyl)methanone (9): A solution of the alcohol 6(1.7 g,
5.6 mmol, 1.0 equiv.) in CH
2
Cl
2
(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 H
2
O (100 mL). The pha-
ses were separated, the organic phase was washed with H
2
O(5×
100 mL) and dried with MgSO
4
. 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 diastereo-
meric ratio was determined by GLC analysis. HRMS (APCI) for
C
16
H
15
F
4
O
+
[M – H]
+
: calculated 299.1054, found 299.1051.
1
H NMR
(700 MHz, C
6
D
6
): 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 (m
c
,
1H), 6.08 (m
c
,1H).
13
C{
1
H} NMR (176 MHz, C
6
D
6
): 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.
19
F NMR (659 MHz, C
6
D
6
):
δ/ppm = –142.4 (m
c
, 2F), –137.6 (m
c
, 2F).
(1S,2R,5S)-6,6-Dimethyl-2-[1-(2,3,5,6-tetrafluorophenyl)-
vinyl]bicyclo[3.1.1]heptane (10): Dimethyltitanocene (0.43
M
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 reac-
tion was cooled to room temperature, quenched by addition of H
2
O
(5.0 mL) and extracted with tert-butylmethyl ether (2 × 10 mL). The
combined organic phases were dried with MgSO
4
. After removal of
all volatiles, the residue was purified by flash column chromatogra-
phy on silica gel using n-pentane as eluent to afford the 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 C
17
H
17
F
4+
[M – H]
+
: calculated 297.1261, found 297.1265.
1
H NMR (500 MHz,
C
6
D
6
): 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 (m
c
, 1H), 2.27 (m
c
,1H),
3.07 (m
c
, 1H), 4.95 (d, J= 2.0 Hz, 1H), 5.21 (d, J= 2.2 Hz, 1H), 6.24
(m
c
,1H).
13
C{
1
H} NMR (126 MHz, CDCl
3
): 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-andmeta carbon atoms of the aromatic ring could not
be detected.
19
F{
1
H} NMR (471 MHz, C
6
D
6
): δ/ppm = –142.3 (dd, J=
13 Hz, J= 24 Hz, 2F), –139.4 (dd, J=13Hz,J= 23 Hz, 2F).
(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 (Ph
3
P)
3
RhCl (6.9 mg, 7.5 μmol, 5.0 mol-
%) were placed under a nitrogen atmosphere and dissolved in de-
gassed benzene (2.0 mL). The reaction vessel was transferred to an
autoclave pressurized with H
2
(30 bar) and stirred for 18 h at 30 °C.
The vial was then removed from the autoclave and the crude mate-
rial 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 by
1
H NMR analysis by integration of the baseline-separated signals at
δ= 3.21 ppm and δ= 3.34 ppm and by GLC analysis. Major dia-
Eur. J. Org. Chem. 2019, 7240–7246 www.eurjoc.org © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7244
stereomer:
1
H NMR (500 MHz, C
6
D
6
): 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 (m
c
, 1H), 1.68–1.90 (m, 4H), 2.17 (m
c
, 1H), 2.34–2.46 (m,
1H), 3.21 (m
c
,1H),6.18(m
c
, 1H).
13
C{
1
H} NMR (126 MHz, C
6
D
6
):
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=17Hz).Theortho-andmeta
carbon atoms of the aromatic ring could not be detected.
19
F NMR
(471 MHz, C
6
D
6
): δ/ppm = –144.6–[–141.0] (br m, 2F), –140–[–139.2]
(brm,2F).Minordiastereomer(selectedsignals):
1
HNMR(500MHz,C
6
D
6
):
δ/ppm = 0.77 (d, J= 9.7 Hz, 1H), 2.10 (m
c
, 1H), 2.30 (m
c
,1H),3.34
(m
c
, 1H), 6.24 (m
c
, 1H). Optical rotation: [α]
D
20
= +2.5 (c= 1.4, CHCl
3
).
Lithium Tetrakis(4-{[(1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]-
heptan-2-yl]methyl}-2,3,5,6-tetrafluorophenyl)borate ([Li]
+
[3]
–
):
To a solution of alkane 8(0.40 g, 1.4 mmol, 5.5 equiv.) in Et
2
O
(20 mL) was added dropwise nBuLi (2.7
M
in hexane, 0.48 mL,
1.3 mmol, 5.0 equiv.) at –78 °C, and the resulting mixture stirred for
3h.AfterwardsBCl
3
(1
M
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 ad-
dition of H
2
O (20 mL) and extracted with tert-butylmethyl ether (3 ×
10 mL). After removal of all volatiles, the residue was purified by
flash column chromatography on silica gel using subsequent cyclo-
hexane (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 C
64
H
68
BF
16–
[M]
–
: calculated 1151.5164, found 1151.5165.
1
H NMR (500 MHz, (CD
3
)
2
CO): 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),
11
B{
1
H} NMR
(160 MHz, (CD
3
)
2
CO): δ/ppm = –16.3.
13
C{
1
H} NMR (126 MHz,
(CD
3
)
2
CO): δ/ppm = 22.5, 23.3, 27.0, 28.5, 30.3 (determined by
1
H/
13
C 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.
19
F NMR (471 MHz,
(CD
3
)
2
CO): δ/ppm = –150.5 (m
c
, 8F), –133.4 (br s, 8F).
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 Et
2
O (60 mL) was added dropwise nBuLi (2.7
M
in hexane, 1.4 mL,
3.7 mmol, 4.4 equiv.) at –78 °C and the resulting mixture stirred for
3h.AfterwardsBCl
3
(1
M
in heptane, 0.84 mL, 0.84 mmol, 1.0 equiv.)
was added dropwise and the solution was warmed up to room
temperature overnight slowly. The reaction was quenched by addi-
tion of H
2
O (10 mL) and extracted with n-pentane (3 × 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 solid. HRMS (APCI) for
C
68
H
76
BF
16–
[M]
–
: calculated 1207.5790, found 1207.5754.
1
H NMR
(500 MHz, C
6
D
6
): 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).
7
Li NMR (194 MHz, C
6
D
6
):
δ/ppm = 0.0.
11
B{
1
H} NMR (160 MHz, C
6
D
6
): δ/ppm = –15.8.
13
C{
1
H}
NMR (126 MHz, C
6
D
6
): 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.
19
F NMR (471 MHz, C
6
D
6
): δ/ppm =
–150.5–[–143.7] (br m, 8F), –138.2–[–131.7] (br m, 8F).
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
Full Paper
(1.0 mL) was added a saturated aqueous solution of Cs
2
CO
3
(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 × 5.0 mL), and the combined organic phases were
washed with H
2
O (5.0 mL). The volatiles were removed under re-
duced 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 purification.
1
H NMR
(500 MHz, C
6
D
6
): 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).
11
B{
1
H} NMR (160 MHz,
C
6
D
6
): δ/ppm = –15.8.
13
C{
1
H} NMR (126 MHz, C
6
D
6
): 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.
19
F NMR (471 MHz, C
6
D
6
):
δ/ppm = –150.2–[–142.2] (br m, 8F), –132.2 (br s, 8F).
Sodium Tetrakis(4-{[(1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]hep-
tan-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
CH
2
Cl
2
(2.0 mL) was added a saturated aqueous solution of NaCl
(2.0 mL), and the two-phase mixture was vigorously stirred over-
night at room temperature. The phases were then separated, the
organic phase was dried with Na
2
SO
4
, and all volatiles were re-
moved under reduced pressure. 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.
1
H NMR (500 MHz, C
6
D
6
):
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 (m
c
,8H).
11
B{
1
H} NMR (160 MHz, C
6
D
6
):
δ/ppm = –15.5.
13
C{
1
H} NMR (126 MHz, C
6
D
6
): 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
1
H/
13
C HMBC NMR), 145.1 (determined by
1
H/
13
C 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.
19
F{
1
H} NMR
(471 MHz, C
6
D
6
): δ/ppm = –147.7 (s, 8F), –134.7 (s, 8F). Optical rota-
tion: [α]
D
20
= +23.4 (c= 1.07, CHCl
3
).
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 with Na
2
SO
4
, and all volatiles were removed under re-
duced pressure. The resulting residue was transferred to a glove
box, resuspended in benzene (3 mL), and the solution stirred over-
night 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 borate [Na]
+
[4]
–
(78 mg,
75 %) as a white solid. HRMS (APCI) for C
68
H
76
BF
16–
[M]
–
: calculated
1207.5790, found 1207.5797.
1
H NMR (500 MHz, C
6
D
6
): 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).
11
B{
1
H} NMR (160 MHz, C
6
D
6
): δ/ppm = –15.8.
13
C{
1
H} NMR (126 MHz, C
6
D
6
): 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.
19
F NMR (471 MHz, C
6
D
6
): δ/ppm =
–151.5–[–143.6] (br m, 8F), –139.6–[–131.1] (br m, 8F). Optical rota-
tion: [α]
D
20
= +18.1 (c= 1.22, CHCl
3
).
Triphenylmethylium Tetrakis(4-{[(1S,2S,5S)-6,6-dimethylbicy-
clo[3.1.1]heptan-2-yl]methyl}-2,3,5,6-tetrafluorophenyl)borate
Eur. J. Org. Chem. 2019, 7240–7246 www.eurjoc.org © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7245
([Tr]
+
[3]
–
): 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 sus-
pended in n-hexane (6.0 mL) and stirred overnight at room temper-
ature. The suspension was filtered under nitrogen atmosphere, and
the remaining solid was washed with n-hexane (2 × 3.0 mL). The
red orange residue was redissolved in CH
2
Cl
2
(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 by-product. The
amount of triphenylmethane was determined by
1
H NMR analysis
by integration of the baseline-separated signals at δ=7.58–
7.70 ppm and δ= 7.13 ppm. HRMS (APCI) for C
64
H
68
BF
16–
[M]
–
:
calculated 1151.5164, found 1151.5144. HRMS (APCI) for C
19
H
15+
[M]
+
: calculated 243.1168, found 243.1163.
1
H NMR (500 MHz,
CD
2
Cl
2
): 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).
11
B{
1
H} NMR (161 MHz, CD
2
Cl
2
): δ/ppm = –16.4.
13
C{
1
H} NMR (176 MHz, CD
2
Cl
2
): 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
1
H/
13
C 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.
19
F NMR (471 MHz,
CD
2
Cl
2
): δ/ppm = –149.8 (m
c
, 8F), –134.1 (br s, 8F).
Triphenylmethylium Tetrakis(4-{1-[(1S,2S,5S)-6,6-dimethylbicy-
clo[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 sus-
pended in n-hexane (6.0 mL) and stirred overnight at room temper-
ature. The suspension was filtered under nitrogen atmosphere, and
the remaining solid was washed with n-hexane (6 × 3.0 mL). The
red orange residue was redissolved in CH
2
Cl
2
(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 by-product. The
amount of triphenylmethane was determined by
1
H NMR analysis
by integration of the baseline-separated signals at δ= 7.64 ppm
and δ= 7.13 ppm.
1
H NMR (400 MHz, CD
2
Cl
2
): 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).
11
B{
1
H} NMR (161 MHz, CD
2
Cl
2
): δ/ppm = –16.5.
13
C{
1
H} NMR
(101 MHz, CD
2
Cl
2
): 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.
19
F NMR (471 MHz,
CD
2
Cl
2
): δ/ppm = –151.8–[–145.7] (br m, 8F), –134.1 (br s, 8F).
Diphenyl(4-tolyl)methylium Tetrakis(4-{1-[(1S,2S,5S)-6,6-di-
methylbicyclo[3.1.1]heptan-2-yl]ethyl}-2,3,5,6-tetrafluoro-
phenyl)borate ([
Me
Tr]
+
[4]
–
): The borate [Na]
+
[4]
–
(0.10 g,
0.081 mmol, 1.0 equiv.) and diphenyl(4-tolyl)methyl chloride
(25 mg, 85 μmol, 1.1 equiv.) were dissolved in CH
2
Cl
2
(2.5 mL) and
stirred overnight at room temperature. The supernatant was trans-
ferred into a Schlenk tube, and the remaining solid was washed
with CH
2
Cl
2
(2 × 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 [
Me
Tr]
+
[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 by-product. The amount of diphenyl(4-tolyl)methane was
determined by
1
H NMR analysis by integration of the baseline-sepa-
rated signals at δ= 7.67 ppm and δ= 7.01 ppm. HRMS (APCI)
Full Paper
for C
68
H
76
BF
16–
[M]
–
: calculated 1207.5790, found 1207.5792. HRMS
(APCI) for C
20
H
17+
[M]
+
: calculated 257.1325, found 257.1329.
1
H
NMR (500 MHz, CD
2
Cl
2
): 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).
11
B{
1
H} NMR (161 MHz, CD
2
Cl
2
): δ/ppm = –16.5.
13
C{
1
H}
NMR (126 MHz, CD
2
Cl
2
): 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.
19
F NMR (471 MHz,
CD
2
Cl
2
): δ/ppm = –151.5–[–145.1] (br m, 8F), –134.1 (br s, 8F).
Acknowledgments
M. O. is indebted to the Einstein Foundation Berlin for an en-
dowed professorship.
Keywords: Cations · Chirality · Diels–Alder reaction · Lewis
acids · Trityl group
[1] For leading reviews, see: a) I. Krossing, I. Raabe, Angew. Chem. Int. Ed.
2004,43, 2066–2090; Angew. Chem. 2004,116, 2116–2142; b) S. H.
Strauss, Chem. Rev. 1993,93, 927–942; for representatives examples, see:
c) T. J. Barbarich, S. T. Handy, S. M. Miller, O. P. Anderson, P. A. Grieco,
S. H. Strauss, Organometallics 1996,15, 3776–3778; d) S. Moss, B. T. King,
A. de Meijere, S. I. Kozhushkov, P. E. Eaton, J. Michl, Org. Lett. 2001,3,
2375–2377; e) L. L. Anderson, J. Arnold, R. G. Bergman, J. Am. Chem. Soc.
2005,127, 14542–14543; f) Y. Li, B. Diebl, A. Raith, F. E. Kühn, Tetrahedron
Lett. 2008,49, 5954–5956; g) T.-T.-T. Nguyen, D. Türp, M. Wagner, K. Mül-
len, Angew. Chem. Int. Ed. 2013,52, 669–673; Angew. Chem. 2013,125,
697–701; h) S. Fischer, J. Schmidt, P. Strauch, A. Thomas, Angew. Chem.
Int. Ed. 2013,52, 12174–12178; Angew. Chem. 2013,125, 12396–12400.
[2] A. G. Massey, A. J. Park, J. Organomet. Chem. 1964,2, 245–250.
[3] H. Kobayashi, T. Sonoda, H. Iwamoto, Chem. Lett. 1981,10, 579–580.
[4] For an authoritative review, see: a) T. James, M. van Gemmeren, B. List,
Chem. Rev. 2015,115, 9388–9409; b) M. Mahlau, B. List, Angew. Chem.
Int. Ed. 2013,52, 518–533; Angew. Chem. 2013,125, 540–556; c) R. J.
Phipps, G. L. Hamilton, F. D. Toste, Nat. Chem. 2012,4, 603–614.
[5] a) P. Pommerening, J. Mohr, J. Friebel, M. Oestreich, Eur. J. Org. Chem.
2017, 2312–2316; for an application of our sodium borate [Na]
+
[1]
–
as a
chiral solvating agent, see: b) E. Tayama, T. Sugawara, Eur. J. Org. Chem.
2019, 803–811.
[6] C. K. De, R. Mitra, B. List, Synlett 2017,28, 2435–2438.
[7] Commercially available (1S)-(–)-β-pinene was used without further en-
richment of enantiomers (92–96 % ee). For an enrichment method of
pinene, see: H. C. Brown, N. N. Joshi, J. Org. Chem. 1988,53, 4059–4062.
[8] For the synthesis of myrtanol, see: a) M. F. Sainz, J. A. Souto, D. Regent-
ova, M. K. G. Johansson, S. T. Timhagen, D. J. Irvine, P. Buijsen, C. E.
Eur. J. Org. Chem. 2019, 7240–7246 www.eurjoc.org © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7246
Koning, R. A. Stockman, S. M. Howdle, Polym. Chem. 2016,7, 2882–2887;
for the oxidation of myrtanol to myrtanal (5), see: b) P. E. Peterson, G.
Grant, J. Org. Chem. 1991,56, 16–20.
[9] Attempts for the oxidation of 6or olefination of 9under basic conditions
led to enolate formation with epimerization in α-position to the carbonyl
group. For confirmation of the configuration by nOe experiments, see
the Supporting Information.
[10] For iridium catalyzed asymmetric hydrogenation of di- and trisubstituted
alkenes, see: a) A. Lightfoot, P. Schnider, A. Pfaltz, Angew. Chem. Int. Ed.
1998,37, 2897–2899; Angew. Chem. 1998,110, 3047–3050; b) J. Mazuela,
J. J. Verendel, M. Coll, B. Schäffner, A. Börner, P. G. Andersson, O. Pàmies,
M. Diéguez, J. Am. Chem. Soc. 2009,131, 12344–12353.
[11] For direct geminal dimethylation, see: a) M. T. Reetz, J. Westermann, R.
Steinbach, Angew. Chem. Int. Ed. Engl. 1980,19, 900–901; Angew. Chem.
1980,92, 931–933; b) M. T. Reetz, J. Westermann, R. Steinbach, J. Chem.
Soc., Chem. Commun. 1981, 237–239; c) M. T. Reetz, J. Westermann,
S.-H. Kyung, Chem. Ber. 1985,118, 1050–1057.
[12] For an overview of the Simmons–Smith reaction, see: a) A. B. Charette,
A. Beauchemin, Org. React. 2001,58, 1–415; for the original work, see:
b) H. E. Simmons, R. D. Smith, J. Am. Chem. Soc. 1958,80, 5323–5324,
for more reactive versions of the original Simmons–Smith procedure,
see: c) J. Furukawa, N. Kawabata, J. Nishimura, Tetrahedron Lett. 1966,7,
3353–3354; d) J. Furukawa, N. Kawabata, J. Nishimura, Tetrahedron 1968,
24, 53–58; e) S. E. Denmark, J. P. Edwards, J. Org. Chem. 1991,56, 6974–
6981.
[13] M. Horn, H. Mayr, J. Phys. Org. Chem. 2012,25, 979–988.
[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. 2014,20,
1066–1072; b) J. Bah, V. R. Naidu, J. Teske, J. Franzén, Adv. Synth. Catal.
2015,357, 148–158.
[15] For trityl-cation-catalyzed enantioselective Diels–Alder and hetero-Diels–
Alder reactions, see: a) J. Lv, Q. Zhang, X. Zhong, S. Luo, J. Am. Chem.
Soc. 2015,137, 15576–15583 (with chiral phosphate counteranion); b) S.
Ni, V. R. Naidu, J. Franzén, Eur. J. Org. Chem. 2016, 1708–1713 [with chiral
bis(sulfuryl)amide counteranion]; c) Q. Zhang, J. Lv, S. Luo, Beilstein J.
Org. Chem. 2019,15, 1304–1312 [with chiral iron(III)-based bisphosphate
counteranion].
[16] C.-T. Chen, S.-D. Chao, K.-C. Yen, C.-H. Chen, I.-C. Chou, S.-W. Hon, J. Am.
Chem. Soc. 1997,119, 11341–11342.
[17] a) P. García-García, F. Lay, P. García-García, C. Rabalakos, B. List, Angew.
Chem. Int. Ed. 2009,48, 4363–4366; Angew. Chem. 2009,121, 4427–4430;
b) M. Sai, H. Yamamoto, J. Am. Chem. Soc. 2015,137, 7091–7094.
[18] A related Hosomi–Sakurai allylation of benzaldehyde (18) using meth-
allyltrimethylsilane resulted in a complex mixture.
[19] J. Y. Corey, J. Am. Chem. Soc. 1975,97, 3237–3238.
[20] For [Et
3
Si(benzene)]
+
[B(C
6
F
5
)
4
]
–
, see: J. B. Lambert, S. Zhang, C. L. Stern,
J. C. Huffman, Science 1993,260, 1917–1918.
Received: September 30, 2019
