The Trityl Cation Embedded into a [7]Helicene-Like Backbone: Preparation and Application as a Lewis Acid Catalyst

Article abstract of DOI:10.1055/a-1404-4966

The synthesis of a helically chiral carbenium ion is reported. The new motif is essentially a trityl cation embedded into a [7]helicene-like framework. The key step in its preparation establishes the π-extended fluorenone system in one step by an unprecedented palladium-catalyzed carbonylative annulation of a 4,4′-biphenanthryl-3,3′-diyl precursor. The racemic form of the new carbon Lewis acid was found to catalyze a representative set of reactions typically promoted by the trityl cation.

Full text of DOI:10.1055/a-1404-4966

SYNTHESIS
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2021, 53, A–E
paper
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B. M. Gross, M. Oestreich
Paper
Synthesis
The Trityl Cation Embedded into a [7]Helicene-Like Backbone:
Preparation and Application as a Lewis Acid Catalyst
Benjamin M. Gross
Martin Oestreich*
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OTf
Institut für Chemie, Technische Universität Berlin,
Strasse des 17. Juni 115, 10623 Berlin, Germany
martin.oestreich@tu-berlin.de
O
–
[BArF
]
4
δ (13C) 194.7 ppm
(CD2Cl2)
MIenMastraititCilunomtrOfraüerrsttpiCnrohe.oniecedmhsintirege,iATceuhct@htnouir-sbcehreliUnn.dieversität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
Received: 31.01.2021
Accepted: 03.03.2021
Published online: 03.03.2021
DOI: 10.1055/a-1404-4966; Art ID: ss-2021-z0041-op
ion unit is embedded into a helically chiral backbone
(Scheme 1, top right). We disclose herein the synthesis and
characterization of the racemic [7]helicene-like cation 2+
and its application as a Lewis acid catalyst.10
Abstract The synthesis of a helically chiral carbenium ion is reported.
The new motif is essentially a trityl cation embedded into a [7]helicene-
like framework. The key step in its preparation establishes the -ex-
tended fluorenone system in one step by an unprecedented palladium-
catalyzed carbonylative annulation of a 4,4′-biphenanthryl-3,3′-diyl
precursor. The racemic form of the new carbon Lewis acid was found to
catalyze a representative set of reactions typically promoted by the tri-
tyl cation.
Helically Chiral Trityl Cation (this work)
[WCA]–
[(P)-2][WCA]
helically chiral trityl cation
WCA = weakly coordinating anion
[WCA]–
[1][WCA]
trityl cation
Key words carbenium ions, carbonylation, chirality, catalysis, Lewis
acids, palladium
Chiral Trityl Cation in Asymmetric Mukaiyama Aldol Reactions (Chen 1997, ref. 6)
Et
Et
[(R,R)-3][ClO4]
(20 mol%)
O
OTBS
OH O
Carbenium ions are ubiquitous Lewis acids, but they are
still underrepresented in non-metal-based catalysis.1 The
long-known triphenylmethylium ion (1+; trityl cation) is an
archetypical congener of these reactive intermediates
(Scheme 1, top left),2 and the parent compound 1+ and de-
rivatives thereof have been employed as catalysts in, for ex-
ample, Mukaiyama aldol and Diels–Alder reactions.1,3–5
Rendering these reactions enantioselective by using chiral
trityl cations has largely been unsuccessful, and asymmet-
ric Lewis acid catalysis with chiral carbenium ions contin-
ues to be a daunting challenge. As a proof of concept, Chen
and co-workers accomplished the Mukaiyama aldol reac-
tion of 4 and 5 with modest yield and promising enantioin-
duction for (R)-6 (Scheme 1, bottom).6 Denmark and Chen’s
dibenzosuberol-derived carbenium ion (R,R)-3+ was used in
this reaction.7 At the same time, Kagan and co-workers in-
troduced -ferrocenyl-substituted carbenium ions with
planar chirality (not shown).8 However, no enantioinduc-
tion was achieved in Diels–Alder reactions.8,9 Despite the
work of Chen and co-workers, the design of chiral trityl cat-
ions remains underexplored. To fill this gap, we present a
new motif for chiral carbenium ions in which the trityl cat-
+
CH2Cl2
–78 °C, 3 h
Ph
H
OEt
Ph
OEt
4
5
(R)-6: 40%
–
ClO4
Ar
[(R,R)-3][ClO4]
(Ar = 4-tBuC6H4)
38% ee
(after aq. HF/MeCN)
Scheme 1 The trityl cation and chiral congeners
Helicenes and their analogs have received considerable
attention in recent years for various applications in materi-
als science, supramolecular chemistry, and organic chemis-
try.11 As such, their synthesis is a point of interest and re-
mains difficult.12 In thinking about the synthesis of the cat-
ionic helical framework of 2+, we envisioned starting from
the -extended fluorenone derivative rac-7 (Scheme 2).
This would provide a carbonyl group as a functional handle
for the installation of different aryl substituents for later
optimization. Compound rac-7 had been synthesized before
by -extension of an existing functionalized fluorenone.13
We decided to access rac-7 from rac-814 with the 4,4′-bi-
phenanthryl-3,3′-diyl framework already established. Our
approach relies on an unprecedented palladium-catalyzed
carbonylative annulation.15 Nozaki and co-workers used a
© 2021. Thieme. All rights reserved. Synthesis 2021, 53, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
B. M. Gross, M. Oestreich
Paper
Synthesis
similar strategy for the preparation of oxygen- and nitro-
gen-containing hetero[7]helicenes through palladium ca-
talysis.14
Table 1 Selected Examples of the Optimization of the Carbonylative
Annulationa
Pd(OAc)2 (10 mol%)
dppp (15 mol%)
HC(O)OPh (1.2 equiv)
Hünig's base (12 equiv)
OTf
OTf
CO2Ph
CO2Ph
OTf
OTf
O
+
O
DMF
[WCA]–
[rac-2][WCA]
120 °C, 12–15 h
rac-7
rac-8
(S)-9
rac-10
(S)-11
Scheme 2 Planned synthesis of the [7]helicene-based trityl cation
Entry
Variation
Yield of
Yield of
We serendipitously noticed that fluorenone rac-10
could be accessed from bis-triflate (S)-9 by employing reac-
tion conditions similar to those reported by Manabe and
co-workers (Table 1).16 Although the original report aimed
to convert (S)-9 into the bis-phenyl ester (S)-11 by using
phenyl formate as a CO source, we observed rac-10 as a by-
product in substantial amounts when conducting the reac-
tion on an 18-mmol scale (entry 1).17 The scale-up was re-
sponsible for the initial discovery because a lower yield was
obtained on a smaller scale of 3.3 mmol (entry 2). However,
with adjustment of the equivalents of phenyl formate, the
yield was optimized to 39% on a small scale (entries 3–5).
The screening of different bidentate phosphine ligands (en-
tries 6–8) and various palladium precatalysts (entries 9–12)
did not bring about any improvement except the use of
Pd(OCOCF3)2 instead of Pd(OAc)2 (54% vs 39%, entries 9 and
5). Lowering of the reaction temperature to 100 °C did not
yield any product (entry 13). The optimized reaction setup
was also applied on a 1.0 mmol scale (see the Supporting
Information).
With the carbonylation protocol in hand, we set out to
apply it to the synthesis of [7]helicene-like rac-7 from the
known bis-triflate rac-814 (Scheme 3). To our delight, the
carbonylative annulation yielded rac-7 in 47% isolated yield.
Several [7]helicene derivatives are configurationally stable
even at high temperatures.18 We therefore prepared enan-
tioenriched (R)-8 (97% ee) according to the procedure of
Nozaki and co-workers (see the Supporting Information).14
The reaction of the enantioenriched material to form -ex-
tended fluorenone (M)-7 proceeded in 45% yield, but with
an eroded enantiomeric excess of less than 10%. Hence, the
racemization barrier of fluorenone 7 is too low for the reac-
tion temperature of 120 °C.
rac-10 (%)d
(S)-11 (%)d
1b
2c
3
HC(O)OPh (8.0 equiv), neat
HC(O)OPh (8.0 equiv), neat
HC(O)OPh (3.0 equiv)
HC(O)OPh (2.0 equiv)
none
20e
< 10
20
12e
n.r.
< 10
trace
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
trace
4
25
5
39
6
dppm (15 mol%)
n.r.
7
dppe (15 mol%)
n.r.
8
dppb (15 mol%)
trace
54
9
Pd(OCOCF3)2 (10 mol%)
PdCl2 (10 mol%)
10
11
12
13
trace
trace
trace
n.r.
[(η3-C3H5)PdCl]2 (5.0 mol%)
Pd(dba)2 (10 mol%)
100 °C instead of 120 °C
a Unless noted otherwise, reactions were performed on a 0.1-mmol scale
with a substrate concentration of 0.2 M. n.r. = no reaction. n.d. = not deter-
mined.
b 18-mmol scale.
c 3.3-mmol scale.
d Determined by 1H NMR spectroscopy with mesitylene as an internal stan-
dard.
e Isolated yield after recrystallization.
The synthesis of the targeted carbenium ion rac-2+ re-
quired three further steps from fluorenone rac-7 (Scheme
4). Nucleophilic arylation of the carbonyl group in rac-7
furnished the tertiary alcohol rac-12 in high yield. Direct
conversion of such alcohols into the corresponding carbeni-
um ions bears the risk of residual water or traces of Brønsted
acid, eventually leading to competing Brønsted acid cataly-
sis.9 We therefore converted tertiary alcohol rac-12 into the
‘trityl’ chloride rac-13 in good yield by treatment with ace-
tyl chloride. Abstraction of chloride from rac-13 with
Pd(OCOCF3)2 (10 mol%)
dppp (15 mol%)
HC(O)OPh (1.2 equiv)
Hünig's base (12 equiv)
Na[BArF ] [ArF = 3,5-bis(trifluoromethyl)phenyl] in CH2Cl2,
4
subsequent filtration, and removal of the solvent afforded
OTf
OTf
O
DMF
120 °C, 16 h
[rac-2][BArF ] as a dark green solid. The salt could be stored
4
at room temperature under an inert atmosphere because
no decomposition was observed, even after several days.
The carbocation carbon atom was detected in the 13C NMR
spectrum at a chemical shift of  = 194.7 ppm in CD2Cl2.
rac-8
rac-7: 47%
Scheme 3 Carbonylative annulation for the assembly of the [7]heli-
cene-like fluorenone
Compared with that of the parent trityl cation [1][BArF ],
4
the resonance signal is less deshielded, likely as a result of
© 2021. Thieme. All rights reserved. Synthesis 2021, 53, A–E

C
B. M. Gross, M. Oestreich
Paper
Synthesis
[rac-2][BArF
(5.0 mol%)
]
]
conformational planarization in the fluorene unit and thus
4
OTBS
O
OTBS
H
O
better delocalization. The Lewis acidity of [rac-2][BArF ]
4
+
+
(1)
Ph
CH2Cl2
–78 °C, 4 h
H
Ph
was estimated to be 88% of that of trityl cation [1][B(C6F5)4]
by a Gutmann–Beckett analysis19 (see the Supporting Infor-
mation for details).
14
4
15: 70%
d.r. = 80:20
[rac-2][BArF
4
Ph
O
O
PhLi
(1.2 equiv)
(5.0 mol%)
(2)
(3)
CO2Me
OH
1,2-Cl2C6H4
r.t., 30 h
Ph
CO2Me
O
THF
16
17
18: 73%
d.r. >95:5
–78 °C, 1 h
(2.1 equiv)
rac-7
rac-12: 81%
[rac-2][BArF
(5.0 mol%)
]
4
O
AcCl
neat
Ar
Ph
+
(excess) 50 °C, 18 h
1,2-Cl2C6H4
r.t., 18 h
O
Ph
Ar
19
20
21: 65%
d.r. >95:5
endo:exo >95:5
Na[BArF
(1.0 equiv)
]
(Ar = 4-PhC6H4)
(2.1 equiv)
4
Cl
r.t.
[rac-2][BArF
(2.0 mol%)
]
4
–
[BArF
]
4
O
CH2Cl2, 10 min
O
[rac-2][BArF4]: quant.
δ (13C) = 194.7 ppm
(CD2Cl2)
rac-13: 78%
Me
+
(4)
Me
N
CH2Cl2
r.t., 24 h
N
Me
Me
22
23
24: quant.
(1.5 equiv)
Scheme 4 Preparation of the [7]helicene-like trityl cation
Scheme 5 Application of the [7]helicene-like trityl cation in Lewis acid
catalysis
As several applications of trityl cations in Lewis acid
catalysis are present in the literature,1,3–10 we conducted
example reactions to assess the reactivity of Lewis acid
CH2Cl2, ethyl acetate, n-pentane, and methanol) were distilled prior to
use. All reactions were performed in flame-dried glassware by using
conventional Schlenk techniques under a static pressure of nitrogen
unless otherwise stated. Liquids and solutions were transferred with
syringes. Analytical thin layer chromatography was performed on sil-
ica gel 60 F254 glass plates from Merck. Flash column chromatogra-
phy was performed on silica gel 60 (40–63 m, 230–400 mesh, ASTM)
from Grace by using the indicated solvents. 1H, 13C, 29Si, and 19F NMR
spectra were recorded in CDCl3, CD2Cl2, or C6D6 on Bruker AV400 and
AV500 instruments. Chemical shifts are reported in ppm and are ref-
erenced to the residual solvent resonance as the internal standard
(CHCl3:  = 7.26 ppm for 1H NMR and CDCl3:  = 77.0 ppm for 13C
NMR; CDHCl2:  = 5.32 ppm for 1H NMR and CD2Cl2:  = 53.8 ppm for
[rac-2][BArF ] by using catalyst loadings of 5.0 mol% or low-
4
er in all cases (Scheme 5). The Mukaiyama aldol reaction of
14 and 4 yielded aldol product 15 in 70% yield (eq. 1).7 Two
representative Diels–Alder reactions, 16 + 17 → 18 recently
reported by Luo and co-workers5 and 19 + 20 → 21,1b,4a,20 as
an example of a difficult dienophile/enophile combination
gave the cycloadducts in good yields (eqs. 2 and 3). The
Friedel–Crafts reaction of indole 22 with methyl vinyl ke-
tone (23) yielded the 3-substituted indole 24 quantitatively
(eq. 4).1b These reaction outcomes compare well with those
in the literature reports.
1
13C NMR; C6D5H:  = 7.16 ppm for H NMR and C6D6:  = 128.0 ppm
for 13C NMR). Other nuclei (19F and 29Si) are referenced in compliance
with the unified scale for NMR chemical shifts as recommended by
the IUPAC by stating the chemical shift relative to BF3·Et2O, CCl3F, and
Me4Si.21 Data are reported as follows: chemical shift, multiplicity (s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet, mc = cen-
trosymmetric multiplet, br = broad signal), coupling constants (Hz),
and integration. Gas–liquid chromatography was performed on an
Agilent Technologies 7820A gas chromatograph equipped with an Ag-
ilent Technologies HP-5 column (30 m × 0.32 mm, 0.25 μm film thick-
ness) by using the following program: N2 carrier gas; injection tem-
perature 250 °C; detector temperature 300 °C; flow rate: 1.7 mL/min;
temperature program: start temperature 40 °C, heating rate 10
°C/min, end temperature 280 °C for 10–30 min. Gas chromatography–
mass spectrometry was performed on an Agilent Technologies 5975C
gas chromatograph equipped with an Agilent Technologies HP-5 col-
umn (30 m × 0.32 mm, 0.25 μm film thickness) by using the following
program: N2 carrier gas; injection temperature 280 °C; detector tem-
perature 280 °C; flow rate: 0.8 mL/min; temperature program: start
temperature 40 °C, heating rate 10 °C/min, end temperature 280 °C
for 10 min. Enantiomeric excesses were determined by analytical
high-performance liquid chromatography analysis on an Agilent
We have disclosed herein a new design for chiral conge-
ners of the trityl cation. Unlike previous systems relying on
central6,7 or planar8 chirality, our system makes use of heli-
cal chirality. The carbenium ion is embedded into a [7]heli-
cene-like backbone. To access this motif, a novel carbonyla-
tive annulation15,17 for the synthesis of -extended fluo-
renones was developed. The helically chiral trityl cation
derivative is a Lewis acid that catalyzes several model reac-
tions equally effectively as the parent compound.1a Future
work will be directed toward the preparation of an enantio-
enriched derivative and its application in asymmetric catal-
ysis.
All chemicals were purchased from commercial suppliers and used as
received unless otherwise stated. All solvents (CH2Cl2, n-hexane, tolu-
ene, Et2O, tert-butyl methyl ether, and THF) were dried and purified
by following standard procedures. Technical grade solvents for ex-
traction or chromatography (tert-butyl methyl ether, cyclohexane,
© 2021. Thieme. All rights reserved. Synthesis 2021, 53, A–E

D
B. M. Gross, M. Oestreich
Paper
Synthesis
Technologies 1290 Infinity instrument with a chiral stationary phase
by using a Daicel Chiralcel OD-H column (n-heptane/isopropanol
mixtures as solvent). Infrared spectra were recorded on an Agilent
Technologies Cary 630 FTIR spectrophotometer equipped with an
ATR unit and are reported as wave numbers (cm–1). Melting points
were determined with a Leica Galen III melting point apparatus (Wag-
ner & Munz). High-resolution mass spectra were obtained from the
Analytical Facility at the Institut für Chemie, Technische Universität
Berlin, on a Thermo Fisher Scientific LTQ Orbitrap XL apparatus by us-
ing APCI/ESI/LIFDI techniques with a linear ion trap analyzer.
HRMS (APCI): m/z [M – OH]+ calcd for C35H21+: 441.1638; found:
441.1635.
1H NMR (500 MHz, CD2Cl2):  = 7.88–7.82 (m, 2 H), 7.69–7.77 (m, 4
H), 7.69–7.60 (m, 4 H), 7.60–7.50 (m, 4 H), 7.30–7.22 (m, 3 H), 7.12
(mc, 2 H), 6.29 (mc, 2 H), 2.90 (s, 1 H).
13C NMR (125 MHz, CD2Cl2):  = 152.2, 150.7, 143.2, 138.1, 137.0,
133.9, 132.2, 132.0, 131.2, 130.9, 129.8, 129.7, 128.7, 128.4, 127.9,
127.7, 127.7, 127.5, 127.4, 127.1, 127.1, 127.1, 127.0, 126.8, 126.7,
126.0, 123.6, 123.5, 122.3, 122.0, 83.5.
rac-9-Chloro-9-phenyl-9H-cyclopenta[1,2-c:4,3-c′]phenanthrene
(rac-13)
rac-9H-Cyclopenta[1,2-c:4,3-c']diphenanthrene-9-one (rac-7)
rac-[4,4′-Biphenanthrene]-3,3′-diylbis(trifluoromethanesulfonate)
(rac-8, 558 mg, 1.00 mmol, 1.00 equiv), palladium(II) trifluoroacetate
(28.5 mg, 85.0 mol, 10.0 mol%), and 1,3-bis(diphenylphosphino)pro-
pane (52 mg, 0.12 mmol, 15.0 mol%) were placed in a flame-dried re-
action vial under an inert atmosphere. The reaction vessel was evacu-
ated and flushed with N2 (3 ×), before Hünig’s base (1.8 mL, 10 mmol,
12 equiv) and DMF (3 mL) were added. Phenyl formate (0.11 mL, 1.0
mmol, 1.2 equiv) was added dropwise, and the reaction mixture was
heated to 120 °C in a heating block. The reaction mixture was stirred
at this temperature for 16 h under an open N2 atmosphere (0.1 bar
over pressure). After letting the mixture cool to room temperature,
EtOAc (20 mL) was added, and the organic phase was washed succes-
sively with a sat. aq. solution of NaHCO3 (5 mL), aq. HCl (2 M, 5 mL),
and brine (5 mL). The organic layer was dried over MgSO4, filtered,
and concentrated under reduced pressure. The residue was dissolved
in EtOAc (5 mL), filtered over a plug of silica, and washed with cyclo-
hexane:EtOAc (1:1, 100 mL). The solvents were then evaporated un-
der reduced pressure. The crude product was recrystallized from cy-
clohexane/CH2Cl2 under slow evaporation of the solvent mixture to
yield rac-9H-cyclopenta[1,2-c:4,3-c′]diphenanthrene-9-one (rac-7,
156 mg, 410 mol, 47%) as dark red crystals; Rf = 0.7 (cyclohex-
ane:EtOAc = 10:1); mp: > 230 °C.
Alcohol rac-12 (42 mg, 91 mol, 1.0 equiv) was dissolved in acetyl
chloride (2 mL), and the reaction mixture was heated to 50 °C in a
heating block and stirred for 18 h. After the reaction mixture had
cooled to room temperature, the solvent was removed under reduced
pressure and the crude product was recrystallized from cyclohex-
ane/dichloromethane under slow evaporation of the solvent mixture.
Chloride rac-13 (34 mg, 0.071 mmol, 78%) was obtained as a yellow
solid; mp: > 230 °C.
IR (ATR, diamond): 3040, 1704, 1594, 1487, 1442, 1262, 1209, 1157,
1029, 948, 828, 739 cm–1
.
HRMS (APCI): m/z [M – Cl]+ calcd for C35H21+: 441.1638; found:
441.1635.
1H NMR (500 MHz, CD2Cl2):  = 7.93–7.89 (m, 2 H), 7.80–7.72 (m, 5
H), 7.69–7.55 (m, 7 H), 7.34–7.29 (m, 3 H), 7.15–7.10 (m, 2 H), 6.33–
6.26 (m, 2 H).
13C NMR (125 MHz, CD2Cl2):  = 150.3, 149.7, 141.0, 137.1, 136.5,
133.9, 133.9, 132.2, 132.1, 131.1, 130.9, 129.9, 129.9, 128.9, 128.5,
128.2, 128.1, 128.0, 127.9, 127.8, 127.7, 127.2, 127.0, 127.0, 126.9,
126.8, 123.7, 123.0, 122.9, 75.7.
rac-9-Phenyl-9H-cyclopenta[1,2-c:4,3-c′]diphenanthren-9-ylium
Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate ([rac-2][BArF4])
IR (ATR, diamond): 3384, 3041, 1698, 1564, 1388, 1303, 947, 855, 740,
694 cm–1
.
In a glovebox, chloride rac-13 (23.8 mg, 5.00 mol, 1.00 equiv) and
[NaBArF4] (44.3 mg, 5.00 mol, 1.00 equiv) were placed in a reaction
vessel, dissolved in dichloromethane (3 mL), and stirred for 10 min.
The dark green reaction mixture was filtered under an inert atmo-
sphere, and the solvent was removed under reduced pressure. Trityl
salt [rac-2][BArF4] (65.2 mg, 5.00 mol, quant.) was obtained as a dark
green solid.
HRMS (APCI): m/z [M + H]+ calcd for C29H17O+: 381.1235; found:
381.1253.
1H NMR (500 MHz, CDCl3):  = 7.91 (d, J = 7.5 Hz, 2 H), 7.76 (d, J = 7.5
Hz, 2 H), 7.65 (d, J = 8.7 Hz, 2 H), 7.61 (d, J = 8.7 Hz, 2 H), 7.56 (d, J = 8.0
Hz, 2 H), 7.53 (d, J = 8.0 Hz, 2 H), 7.11 (mc, 2 H), 6.30 (mc, 2 H).
13C NMR (125 MHz, CDCl3):  = 193.3, 147.2, 138.1, 135.2, 132.0,
130.6, 129.7, 128.5, 128.0, 127.6, 127.0, 126.7, 123.8, 120.8.
+
HRMS (APCI): m/z [M – C32H12BF24
]
calcd for C35H21+: 441.1638;
–
found: 441.1635; m/z [M – C35H21
]
calcd for C32H12BF24–: 863.0654;
rac-9-Phenyl-9H-cyclopenta[1,2-c:4,3-c′]diphenanthrene-9-ol
(rac-12)
found: 863.0693.
1H NMR (500 MHz, CD2Cl2):  = 8.12 (d, J = 8.3 Hz, 2 H), 7.84 (t, J = 7.7
Hz, 1 H), 7.74 (s, 8 H), 7.62 (t, J = 7.7 Hz, 2 H), 7.57 (s, 4 H), 7.47 (d, J =
7.7 Hz, 2 H), 7.02–6.90 (m, 6 H), 6.48 (t, J = 7.6 Hz, 2 H), 6.43 (d, J =
8.3Hz, 2 H), 6.31 (d, J = 7.8 Hz, 2 H), 6.25 (d, J = 7.8 Hz, 2 H).
13C NMR (125 MHz, CD2Cl2):  = 194.7, 162.1 (q, J = 49.3 Hz), 150.4,
145.8, 140.2, 139.5, 138.8, 136.6, 135.1, 133.6, 133.0, 132.1, 131.9,
131.3, 131.0, 130.3, 129.8, 129.0, 126.6, 126.3, 123.6, 120.9, 117.8.
Fluorenone rac-7 (75 mg, 0.19 mmol, 1.0 equiv) was dissolved in THF
(3 mL), and the reaction mixture was cooled to –78 °C. A solution of
phenyllithium (1.9 M in dibutylether, 0.12 mL, 0.24 mmol, 1.2 equiv)
was added dropwise, and the reaction mixture was stirred for 1 h. A
solution of methanol/destilled H2O (5:1, 0.3 mL) was added dropwise,
and the reaction mixture was warmed to room temperature and con-
centrated under reduced pressure. Purification by flash column chro-
matography on silica gel with cyclohexane:EtOAc = 50:1 as the eluent
afforded alcohol rac-12 as a light yellow solid (74 mg, 0.16 mmol,
81%); Rf = 0.4 (cyclohexane:EtOAc = 10:1); mp: > 230°C.
19F NMR (471 MHz, CD2Cl2):  = –62.8 ppm.
11B NMR (160 MHz, CD2Cl2):  = –6.5 ppm.
IR (ATR, diamond): 3524, 3041, 1488, 1445, 1294, 1158, 1019, 828,
Conflict of Interest
741 cm–1
.
The authors declare no conflict of interest.
© 2021. Thieme. All rights reserved. Synthesis 2021, 53, A–E

E
B. M. Gross, M. Oestreich
Paper
Synthesis
Funding Information
137, 15576. (b) Ni, S.; Naidu, V. R.; Franzén, J. Eur. J. Org. Chem.
2016, 1708. See also: (c) Pommerening, P.; Mohr, J.; Friebel, J.;
Oestreich, M. Eur. J. Org. Chem. 2017, 2312. (d) Pommerening, P.;
Oestreich, M. Eur. J. Org. Chem. 2019, 7240.
M.O. is indebted to the Einstein Foundation (Berlin) for an endowed
professorship.
E
i
nsteinStitfu
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g
B
erlin()
(11) (a) Chen, C.-F.; Shen, Y. Helicene Chemistry: From Synthesis to
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Y.; Chen, C.-F. Chem. Rev. 2012, 112, 1463.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/a-1404-4966.
S
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p
ortingI
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formati
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© 2021. Thieme. All rights reserved. Synthesis 2021, 53, A–E