2-Bromo[6]helicene as a Key Intermediate for [6]Helicene Functionalization

Article abstract of DOI:10.1021/acs.joc.7b03234

The synthesis of 2-bromo[6]helicene was revised and improved up to 51% yield. Its reactivity was thoroughly investigated, and a library of 17 different carbon, boron, nitrogen, phosphorus, oxygen and sulfur substituted derivatives was prepared. The racemization barrier for 2-bromo[6]helicene was determined, and the usage of enantiomers in the synthesis of optically pure helicenes was rationalized. The three most energy-demanding reactions using enantiomerically pure 2-bromo[6]helicene were tested in order to confirm the predicted enantiomeric excess.

Full text of DOI:10.1021/acs.joc.7b03234

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2-Bromo[6]helicene as a key intermediate for [6]helicene functionalization
Martin Jakubec, Tomáš Beránek, Pavel Jakubík, Jan
Sykora, Jaroslav Žádný, Vladimir Cirkva, and Jan Storch
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b03234 • Publication Date (Web): 13 Mar 2018
Downloaded from http://pubs.acs.org on March 13, 2018
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2-Bromo[6]helicene as a key intermediate for [6]helicene functionalization
Martin Jakubec, Tomáš Beránek, Pavel Jakubík, Jan Sýkora*, Jaroslav Žádný,
Vladimír Církva and Jan Storch
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Institute of Chemical Process Fundamentals of the CAS, v. v. i., Rozvojová 2/135, Prague 6, 165 02
Email: sykora@icpf.cas.cz
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Abstract
The synthesis of 2-bromo[6]helicene was revised and improved up to 51% yield. Its reactivity was
thoroughly investigated and a libraryof 17 different carbon, boron, nitrogen, phosphorus, oxygen and
sulfur substituted derivatives was prepared. The racemization barrier for 2-bromo[6]helicene was
determined and the usage of enantiomers in the synthesis of optically pure helicenes was rationalized.
The three most energy-demanding reactions using enantiomerically pure 2-bromo[6]helicene were
tested in order to confirm the predicted enantiomeric excess.
Introduction
Helicenes are a class of helically chiral compounds which possess a fully conjugated aromatic
system1. Their highly delocalized π-electron system along with the inherent chirality predicts unique
optical and electronic properties. Helicenes exhibit high values of specific rotation2, strong circular
dichroism3, electroluminescence4,5, circularly polarized luminescence6,7, and non-linear optical
properties8–11. Furthermore, helicenes haverevealedsemiconductivebehavior12,13.All theseproperties
predetermine their utilization in a wide range of applications in organic electronic devices14–16. These
molecules have proved to be useful in asymmetric catalysis, both as ligands or as organocatalysts17–
19. Helicenes can be also used as building blocks in functional polymers20.
Generally, two methods can be employed for the synthesis of helicene-based molecules: a suitable
substituent can beintroduced to themoleculeofaheliceneprecursorbeforethefinal cyclizationstep21,
or the appropriate helicene-based compound can be functionalized after cyclization16,22. Despite the
fact that 2-bromo[6]helicene 1, first described in 197223, offers plenty of possibilities for similar
derivatization owing to the versatility of the bromine atom, it was scarcely used for further reactions
in the past. There were only few exceptions, mainly focused on reverse preparation of unsubstituted
[6]helicene in order to retain some specific properties of the original material. For example, this
approach was employed in the crucial attribution of the dextrorotation to the (P)-helicene
enantiomer23. Other synthetic utilizations of 2-bromo[6]helicene have not been found. Since the
reactivity of 1 is somewhat different from other bromo-substituted aromatic compounds and the
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availability of this compound is limited, the potential of this molecule in helicene chemistry remains
unexploited.
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In this article, we present a revised synthesis of this compound, featuring shorter and more efficient
synthetic pathway when compared to the original synthesis23. Similarly to our previous study on
9-bromo[7]helicene24, we present a coherent exploration of its reactivity, providing robust procedures
for preparation of 17 different compounds. The emphasis is on high-yielding metal catalyzed cross-
coupling reactions and microwave-assisted synthesis. Furthermore, the racemization barrier was
determined and the synthesis of optically pure derivatives from 2-bromo[6]helicene enantiomers was
rationalized.
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Results and discussion
The synthetic procedure for preparation of 2-bromo[6]helicene 1 was first published in 197223 and is
shown in Scheme 1. The key reactions in the 6-step sequence are Wittig reactions, providing two
stilbene-like precursors 3 and 7, and the subsequent photocyclization taking place under irradiation
by a medium-pressure mercury vapor lamp. However, there are two low-yielding reactions, which
significantly decrease the overall yield of the synthesis to only 28 %. The first problematic step is a
radical bromination of 4, which provides the benzylic bromide 5 in 69 % yield, and the other is the
second Wittig reaction of 6 with 73 % yield.
Scheme 1: Original synthesis of 1.
In order to avoid the low-yielding reaction steps, the original synthesis was revisited and several
improvements were implemented (Scheme 2). The key benzo[c]phenanthrene precursor 5 containing
a bromomethylene group was replaced by precursor 10 possessing a carbonyl group. This measure in
fact reduced the number of reaction steps needed for 2-bromo[6]helicene 1 preparation by one.
Furthermore, the Horner-Wadsworth-Emmons reaction employing easily prepared diethyl(4-
bromobenzyl)phosphonate was utilized for both stilbene-forming steps instead of the Wittig reaction.
Both photocyclization reactions were performed in a flow reactor, which greatly reduced the reaction
time and solvent consumption of the process. All above-mentioned improvements increased the
overall yield of the reaction sequence from the original 28 % up to 51 %.
Scheme 2: Improved synthesis of 1.
The optimization of 2-bromo[6]helicene 1 synthesis was followed by a rigorous screening of its
reactivity, since the bromine atom unlocks the possibility of introducing a plethora of substituents
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into position 2 of the helicene skeleton. The formation of C-C bonds via cross-coupling reactions
represents one of the main areas of interest and therefore Sonogashira, Suzuki-Miyaura and Heck
coupling reactions were tested (Scheme 3). The reaction of 1 with (trimethylsilyl)acetylene provided
product 11a in conversions only up to 20 %, while using phenylacetylene under the same conditions
of Sonogashira coupling provided full conversion and product 11b was isolated in 87 % yield. The
low conversion in the first case can be attributed to the higher steric demands of trimethylsilyl in the
proximity of a helicene moiety in comparison with a phenyl substituent. 2-Phenyl[6]helicene 12a was
formed in 76 % yield in Suzuki-Miyaura coupling in the presence of Pd2dba3, XPhos, K3PO4 and
phenylboronic acid. A similar reaction conducted with heteroarylboronic acids can be used to
introduce different heteroatoms to the molecule, e.g. reaction with 3-thienylboronic acid provides 2-
(thien-3-yl)[6]helicene 12b in 63 % yield. Moreover, 2-styryl[6]helicene 13 was prepared in 62 %
yield when trans-bis(acetato)bis[o-(di-o-tolylphosphino)benzyl]dipalladium(II) known as Hermann I
precatalyst in the presence of DABCO and styrene were used. The latter is particularly intriguing, as
the stilbene-like product potentially allows for the extension of the helicene system via
photocyclization as proposed by Martin22.
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Scheme 3: C-C couplings of 1
Rosemund-von Braun cyanation (Scheme 4) was chosen as another example of metal promoted
process providing 2-cyano[6]helicene 14 (79 % yield), another readilytransformable intermediate for
further derivatization. This reaction was also performed in a microwave reactor, therefore the full
conversion is achieved in less than 3 hours.
Scheme 4: Rosemund-von Braun reaction of 1.
The cyano compound 14 can be used for preparation of various carbonyl group-containing
derivatives, however it is more convenient to prepare these directly by the reaction of carbon
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electrophiles with helicene nucleophile generated by alkyllithium. Reaction of DMF or ethyl
chloroformate and acidic workup gives [6]helicene-2-yl carbaldehyde 15 or carboxylic acid ester 16,
respectively (Scheme 5). Yields of the reaction are highly dependent on the stability of the lithiated
intermediate. When the lithiation is conducted at -78 °C for 30 minutes, carbaldehyde 15 is isolated
in 54 % yield. Lowering the temperature to -98 °C in a N2(l)/MeOH cooling bath leads to a slight
increase in isolated yield (63 %). A significant improvement is achieved when the lithiation time is
reduced to only1 minute before the addition of DMF. This adjustment provided 89 % yield of product
15. The remaining material can be recovered as unsubstituted [6]helicene.
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Scheme 5: Lithiation of 1 and reaction with carbon electrophiles.
Similarly, lithiated helicene can react with heteroatom electrophiles. Using the above-mentioned
conditions for lithiation, helicene-2-yl pinacol boronate 17 can be prepared in 83 % yield (Scheme 6).
The same compound can also be prepared by Pd-catalyzed borylation with bis(pinacolato)diboron,
however only in 39% yield. This derivative can be used as a transmetallating agent in a Suzuki-
Miyaura reaction.
Scheme 6: Lithiation of 1 and reaction with boron electrophile.
Hydroxyl groups are another in-demand functionalities. They can be obtained by ipso-oxidation of
boronates using urea-hydrogen peroxide adduct (UHP)from precursor 1725. Upon overnight stirring
of the reaction mixture, it is possible to obtain 2-hydroxy[6]helicene 18 in 62 % yield (Scheme 7).
Scheme 7: 2-Hydroxy [6]helicene 18 synthesis via ipso-oxidation of boronate 17.
Buchwald-Hartwig amination is one of the most effective ways for preparation of aryl amines26. In
our case, this palladium catalyzed process was used in reaction of 1 with benzylamine and
benzophenone imine. The reactants accompanied by palladium acetate, BINAP and sodium tert-
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butoxide afforded products in very good yields of 75 % (19) and 86 % (20) respectively (Scheme 8).
Microwave irradiation reduced the time needed for full conversion to less than 2 hours. 2-
Amino[6]helicene 21 was obtained after acidic hydrolysis of imine precursor 20 in 93 % yield.
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Scheme 8: Buchwald-Hartwig amination of 1.
Other nitrogen-containing compounds can be prepared by different oxidation techniques.
Aminohelicene 21 can be oxidized by excess of 3-chloroperbenzoic acid to a corresponding nitro
compound 22 in 49 % yield after recrystallization. When the same substrate is treated with 20
equivalents of potassium peroxymonosulphate in a two-phase solvent system, 2-nitroso[6]helicene
23 can be isolated in 37 % yield (Scheme 9). Vigorous stirring of the reaction mixture is necessary to
achieve a sufficient reaction rate and to avoid possible side reactions.
Scheme 9: Oxidation of 21.
Sulfur-containing compounds are of great importance, especially in material and surface
chemistry27,28. Similarly to our previously published protocol for this reaction on a different
substrate24, 2-(methylsulfanyl)[6]helicene 24 was synthesized in 63 % yield in the presence of
Pd(PPh3)4 and sodium methanethiolate (Scheme 10).
Scheme 10: Introduction of sulfanyl group.
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With regard to the possible use of helicenes in asymmetric catalysis, phosphino-helicene 25 was
prepared accordingtothepreviouslypublished methodologyinspired byKappe29.Acoupling reaction
of diphenylphosphine with 2-bromo[6]helicene 1 in a microwave reactor gave product 25 in 61%
yield after borane protection. Protection by the formation of borane complex had to be used, due to
the rapid oxidation of free phosphine 26 to the corresponding phosphine oxide 27. Complex 25 is
stable and can be easily deprotected to phosphine 26 by reaction with tetrafluoroboric acid30.
Phosphine oxide 27 can be prepared quantitativelyfrom phosphine complex 25 after deprotection and
subsequent oxidation reaction with hydrogen peroxide in quantitative yield (Scheme 11).
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Scheme 11: Preparation of helicenes 25 – 27 bearing different phosphorus functionalities.
All of the above-mentioned reactions were optimized for racemic starting material 1. Regarding the
reaction conditions for derivatization of 2-bromo[6]helicene 1, some of these reactions can be easily
applied also to individual 2-bromo[6]helicene enantiomers resulting in an optically pure functional
molecule without significant loss of enantiomeric excess (ee). On the contrary, reactions performed
above approximately 150°C are generally considered inappropriate for preparation of optically pure
helicenes due to their tendency to racemize at elevated temperatures. Three of the described reactions
were conducted at significantlyelevated temperatures; namelythe preparation of phosphanylhelicene
25 (160°C), the Buchwald-Hartwig amination providing aryl amine 19 (170°C), and the Rosemund-
von Braun cyanation affording 2-cyano[6]helicene 14 (210°C). In order to judge the suitability of
certain conditions for the enantioselective reaction and to predict the ee of the products reliably, the
racemization barrier of 1 became a matter of interest.
The enantiomers (M)-1 and (P)-1 were isolated using a chiral HPLC and the racemization process
was monitored at 185, 192, 202 and 212 °C, subsequently. The determined Gibbs free energyat 465 K
(36.5 kcal/mol) was close to that reported for unsubstituted [6]helicene31,32. Based on rate constants
obtained at different temperatures, a simple prediction of enantiomeric excess for any reaction
conditions can be made. The solvent effect, racemization of products and other possible influences
were omitted in the first approximation. The prediction was then confronted with the ee values
obtained by enantioselective reactions. Phosphanylhelicene (P)-25 was prepared with 99 %ee after
60 minutes at 160 °C compared to 99 %ee. 2-Benzylamino[6]helicene (P)-19 was obtained with 96
%ee after 120 minutes at 170 °C compared to the calculated 96 %ee, which is in excellent agreement
with the prediction. Contrary to the predicted 24 %ee, the preparation of 2-cyano[6]helicene (M)-14
provided only 1 %ee after 60 minutes at 210 °C. The collected data clearly indicate that a reaction
conducted for several hours at a temperature up to 160 °C can be used for derivatization of (P)-1 or
(M)-1 without a significant decrease in helicene enantiopurity (see SI for details).
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Conclusion
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2-bromo[6]helicene 1 was shown to be a suitable starting material for synthesis of functional
molecules based on a [6]helicene skeleton. The method of 2-bromo[6]helicene 1 preparation was
revised and improved, bringing the overall yield up to 51 % over 5 steps. Its reactivity was
investigated and a library of 17 different carbon, boron, nitrogen, phosphorus, oxygen and sulfur
substituted derivatives bearing a [6]helicene-2-yl moiety was synthesized including 13 previously
unknown [6]helicene derivatives. Thermodynamic data for the racemization barrier of
2-bromo[6]helicene 1 were determined. The effect of reaction conditions on racemization in reactions
using pure enantiomer was rationalized. Three of the most energetically demanding reactions were
performed with pure enantiomer 1 providing enantiomeric excesses of the products close to the
predicted values. Therefore, the individual enantiomers of 2-bromo[6]helicene 1 can also be utilized
in the synthesis of optically pure [6]helicene derivatives opening further applications of these
molecules in various fields of chemistry and material science.
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Experimental Section
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1H, 13C{1H}, 19F, 31P and 31P{1H}, 11B and 11B{1H} spectra were recorded using Bruker Avance 400
and Varian Inova 500 MHz instruments. Chemical shifts (δ) are reported in parts per million (ppm)
relative to TMS, CFCl3 and ext. H3PO4 and 15 % BF3.OEt2 or referenced to residuals of CDCl3 (δ =
7.26 ppm and δ = 77.00 ppm, respectively); CD2Cl2 (δ = 5.30 ppm and 54.00 ppm respectively) and
CD3CN (δ = 1.94 ppm and 118.69 ppm respectively). The coupling constants J are given in Hz. For
the correct assignment of both the 1H and 13C NMR spectra of key compounds, COSY, HSQC, and
HMBC experiments were performed. Electron impact (EI) mass spectra were measured at an ionizing
voltage of 70 eV in a positive mode and the m/z values are given along with their relative intensities
(%). For exact mass measurement, the spectra were internally calibrated using Na-formate or APCI-
TOF tuning mix. ESI and APCI high-resolution mass spectra were measured in positive mode using
a micrOTOF QIII mass spectrometer (Bruker) and were determined by software Compass Data
Analysis. Infrared spectroscopy spectra were determined on a Nicolet 6700 in chloroform. TLC was
performed on Silica gel 60 F254- or 60 RP-18 F254 aluminum sheets and compounds were visualized
by UV light (254 and 366 nm) or PMA solution. Column chromatography was performed on an
HPFC Biotage SP1 or Isolera One system with pre-packed flash silica gel or C18 columns. The
standard Schlenk technique was used for all reactions; cannula filtration technique was used for
filtrations under inert atmosphere. Microwave experiments were performed on an Anton Paar
Monowave 300 equipped with simultaneous temperature measurement with IR and a fiber optic
sensor. Commercially available reagent-grade materials were used as received. Diethyl 4-
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bromobenzylphosphonate33 and Herrmann
I
precatalyst (trans-bis(acetato)bis[o-(di-o-
tolylphosphino)benzyl]dipalladium(II))34 were prepared according to published procedures. Solvents
were degassed by five freeze-pump-thaw cycles. Tetrahydrofuran and toluene were freshly distilled
from sodium/benzophenone under an atmosphere of argon. The ee values were determined by
integration of UV traces (255 nm) of HPLC chromatograms (at Varian ProStar 230 SDA + ProStar
330 PDAdetector (200 – 400 nm). The HPLC analyses were performed on Kromasil Cellucoat (Akzo
Nobel) column (250 x 4.6 mm, 10 μm) using n-heptane/2-propanol (99.65:0.35 for 1, 99.25:0.75 for
14, 98.75:1.25 for 19 and 99.5:0.5 for 25) as mobile phase at a flow rate of 2 mL/min (approx. 102
atm at 28 °C). Specific rotations were measured at 589 nm in chloroform at 20 °C on JASCO P-2000
Polarimeter and are given in deg cm3 g-1 dm-1 as an average value from 50 measurements.All melting
points are uncorrected and were taken on Kofler melting point apparatus with microscope.
2-(4-Bromostyryl)naphtalene (8). A round bottom flask was charged with sodium hydride (60% in
mineral oil, 0.27 g, 6.7 mmol, 1.05 eq) underAr and dryTHF (50 mL) was added. Into the suspension,
diethyl 4-bromobenzylphosphonate (2.06 g, 6.7 mmol, 1.05 eq) was added dropwise and stirred for
10 minutes. 2-Napthaldehyde (1 g, 6.4 mmol, 1 eq) was dissolved in dry THF (30 mL) and added
dropwise. The mixture was heated up to 50 °C and stirred overnight. Reaction was quenched with
water (100 mL) and THF was evaporated. The suspension was filtered, collected precipitate was
dissolved in dichloromethane, dried over magnesium sulfate and evaporated. The residue was then
filtered through a pad of silicagel, eluting with petroleum ether. Volatiles were removed to yield 2-(4-
bromostyryl)naphthalene 8 (1.65 g, 83%) in the form of white powder. 1H NMR (400 MHz, CDCl3):
δ 7.89–7.79 (m, 4H), 7.73 (dd, J1 = 8.7 Hz, J2 = 1.7 Hz, 1H), 7.53–7.40 (m, 6H), 7.27 (d, J = 16.3 Hz,
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1H), 7.16 (d, J = 16.3 Hz, 1H). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C18H14Br 309.0273;
found 309.0278. Spectra are in accordance with known literature35.
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2-Bromobenzo[c]phenantrene (9). 2-(4-Bromostyryl)napthalene 8(4 g, 13 mmol, 1 eq) was dissolved
in 4 Lof toluene and iodine (300 mg, 1.18 mmol, 15 mol%) was added into the mixture. The mixture
was pumped through flow photoreactor (300 m of capillary(ID = 2 mm), 6 x 400 Wmedium pressure
mercury discharge lamps, 50 mL/min). After the reaction, the mixture was washed through a column
with a layer of sodium thiosulfate (50 g) and a layer of silicagel (30 g). The solvents were evaporated
and crude product recrystallized from methanol, yielding 2-bromobenzo[c]phenantrene 9 as a white
solid (2.43 g, 61%). The rest of the reaction mixture was purified by column chromatography with
30:1 hexane/ethylacetate, yielding an additional 1.07 g (27%) of white powder. The combined yield
of the reaction was 3.50 g of 2-bromobenzo[c]phenantrene 9 (88%). 1H NMR (400 MHz, CDCl3): δ
9.29 (d, J = 1.8 Hz, 1 H), 9.05 (d, J = 8.5 Hz, 1H), 8.03 (dd, J1 = 7.9 Hz, J2 = 1.4 Hz, 1H), 7.93 (d, J
= 8.5 Hz, 1H), 7.89 (d, J = 8.6 Hz, 1H), 7.86–7.85 (m, 2H), 7.82 (d, J = 8.5 Hz, 1H), 7.74 (ddd, J1 =
6.9 Hz, J2 = 6.9 Hz, J3 = 1.5 Hz, 1H), 7.71 (dd, J1 = 6.7 Hz, J2 = 1.9 Hz, 1H), 7.66 (ddd, J1 = 6.9 Hz,
J2 = 6.9 Hz, J3 = 1.2 Hz, 1H). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C18H12Br 307.0116; found
307.0115. Spectra are in accordance with known literature36.
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Benzo[c]phenantrene-2-carbaldehyde (10). 2-Bromobenzo[c]phenantrene 9 (6.26 g, 20.4 mmol, 1
eq) was dissolved in dry tetrahydrofurane (150 mL) underAr atmosphere and cooled down to -78 °C.
nBuLi (2.5M, 10.6 mL, 26.5 mmol, 1.3 eq) was added dropwise and the mixture was stirred for an
hour. Afterwards, N,N-dimethylformamide (4 mL, 51.7 mmol, 2.53 eq) was added and the reaction
was stirred for additional 1 hour. ExcessivenBuLi was quenched byMeOH (10 mL), reaction mixture
was extracted with dichloromethane (3x100 mL), and combined organic layers were dried over
magnesium sulfate, filtered and solvents evaporated. Crystallization from DCM/MeOH gave 5.07 g
(97 %) of white solid. 1H NMR (400 MHz, CDCl3): δ 10.29 (s, 1H), 9.60 (s, 1H), 9.08 (d, J = 8.4 Hz,
1H), 8.16–8.09 (m, 2H), 8.08 (dd, J1 = 7.9 Hz, J2 = 1.4 Hz, 1H), 8.02–7.94 (m, 3H), 7.87 (d, J = 8.5
Hz, 1H), 7.78 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.5 Hz, 1H), 7.70 (ddd, J1 = 7.0 Hz, J2 = 7.0 Hz, J3
= 1.2 Hz, 1H). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C19H13O 257.0961; found 257.0959.
Spectra are in accordance with known literature37.
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2-(4-Bromostyryl)benzo[c]phenantrene (7). A Round bottom flask was charged with sodium hydride
(40%, 0.71 g, 17.8 mmol, 1.01 eq) under Ar atmosphere and dry THF (100 mL) was added. Into the
suspension, diethyl (4-bromobenzyl)phosphonate (5.4 g, 17.6 mmol) was added dropwise and stirred
for 10 minutes. Benzo[c]phenantrene-2-carbaldehyde 10 (3.5 g, 13.7 mmol) was dissolved in dry
THF (70 mL) and added dropwise. The mixture was heated up to 50 °C and stirred overnight. The
reaction was quenched with water (100 mL) and THF was evaporated. The mixture was filtered;
precipitate was dissolved in dichloromethane, dried over MgSO4 and filtered through the silicapad.
DCM was evaporated and residual volatiles removed by high vacuum, yielding
2-(4-bromostyryl)benzo[c]phenantrene 7 (5.25 g, 94 %) as a yellow powder. 1H NMR (400 MHz,
CDCl3): δ 9.14 (d, J = 8.2 Hz, 2H), 8.05 (dd, J1 = 7.9 Hz, J2 = 1.3 Hz, 1H), 8.00 (d, J = 8.4 Hz, 1H),
7.94–7.79 (m, 5H), 7.77–7.71 (m, 1H), 7.69–7.63 (m, 1H), 7.54–7.46 (m, 4H), 7.37 (d, J = 16.3 Hz,
1H), 7.22 (d, J = 16.3 Hz, 1H). 13C NMR {1H} (100 MHz, CDCl3): δ 136.3, 134.8, 133.5, 133.2,
131.8 (2C), 131.3, 130.6, 130.3, 130.0, 129.0, 128.6, 128.0 (2C), 127.8, 127.7, 127.6, 127.33,
127.32, 127.10, 127.04, 126.9, 126.3, 125.9, 123.1, 121.4. IR (CHCl3, ν cm-1): 1627, 1326, 985,
962, 827, 607, 411. EI MS: 408 (100 %, M+), 328 (46 %), 313 (22 %), 300 (17 %), 226 (20 %), 164
(39 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C26H18Br 409.0586; found 409.0580.
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2-Bromo[6]helicene (1). 2-(4-Bromostyryl)benzo[c]phenantrene 7 (2 g, 4.9 mmol, 1 eq) was
dissolved in 4 L of toluene and iodine (200 mg, 1.58 mmol) was added. The mixture was pumped
through flow photoreactor (300 m of capillary (ID = 2 mm), 6 x 400 W medium pressure mercury
discharge lamps, 50 mL/min). After the reaction, the mixture was washed trough the column with a
layer of sodium thiosulfate (50 g) and a layer of silicagel (30 g). Solvents were evaporated and crude
product recrystallized from DCM/MeOH to yield 1.51 g (76 %) of yellow powder. Mp = 253–254
°C. 1H NMR (400 MHz, CDCl3): δ 8.06–7.94 (m, 7H), 7.90–7.85 (m, 2H), 7.74 (d, J = 1.7 Hz,
1H), 7.68 (d, J = 8.5 Hz, 1 H), 7.56 (d, J1 = 8.6 Hz, 1H), 7.34–7.28 (m, 2 H), 6.74 (ddd, J1 =
6.9 Hz, J2 = 6.9 Hz, J3 = 1.3 Hz, 1H). 13C NMR {1H} (100 MHz, CDCl3): δ 133.1, 132.2, 131.6,
131.5, 131.0, 130.5, 130.2, 129.6, 128.9, 128.5, 128.1, 127.8, 127.6, 127.5, 127.5, 127.3,
127.08, 127.07, 126.9, 126.8, 126.7, 126.2, 125.9, 124.7, 123.9, 119.1. IR (CHCl3, ν cm-1): 3050,
1603, 1519, 1509, 1472, 1437, 1375, 1192, 1143, 1079, 1043, 1035, 961, 909, 867, 848, 824, 686,
621, 571, 480, 427. EI MS: 408 (87 %, M.+), 406 (82 %), 326 (94 %), 300 (100 %), 162 (43 %).
HRMS (ESI/QTOF) m/z [M ]+ Calcd for C26H15Br 409.0413; found 409.0417. Chiral separation: tR =
6.17 min ((-)-1), 13.27 min ((+)-1). Specific rotation of dextrorotatory (+)-1 and levorotatory (-)-1
[ ]20
[ ]20
were measured in CHCl3 α
= +3494 ± 4° (c 0.0534) and α = −3456 ± 6° (c 0.0622). Mp =
D
D
282–284 °C ((+)-1), Mp = 281–283 °C ((-)-1).
2-(2-trimethylsilylethyn-1-yl)[6]helicene (11a). Compound 11a was prepared according to the
procedure described in 11b. Due to the low conversion (GC-MS), product was not isolated. EI MS:
424 (100 %, M+.), 409 (13 %), 351 (42%), 337 (26 %), 324 (14 %), 300 (15 %), 207 (19 %).
2-(Phenylethynyl)[6]helicene (11b). 2-Bromo[6]helicene 1 (100 mg, 0.25 mmol, 1 eq)
tetrakis(triphenyl)palladium (17 mg, 0.02 mmol) and copper (I) iodide (4 mg, 0.02 mmol) were
dissolved in dimethylformamide (2 mL) and trimethylamine (2 mL) under argon atmosphere.
Phenylacetylene (82 µL, 0.75 mmol, 3 eq) was added into the reaction. The reaction mixture was
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stirred underAr atmosphere at 70°C overnight. After the reaction an aqueous solution of ammonium
chloride (20 mL, 25% V/V aqueous solution) was added and the product was extracted with ethyl
acetate (20 mL). The organic layer was washed with HCl (20 mL, 10% V/V aqueous solution) and
brine (20 mL, saturated solution). Then, the organic layer was dried over magnesium sulfate, filtered,
and the solvent was evaporated at the reduced pressure. Flash-chromatography (petroleum ether /
ethyl acetate 30:1) gave 2-(phenylethynyl)[6]helicene 11b (91 mg, 87%) as a yellow powder. Mp =
65–66 °C. 1H NMR (500 MHz, CDCl3): δ 8.07–7.93 (m, 7H), 7.89 (d, J = 8.5 Hz, 1H), 7.85 (d, J =
8.0 Hz, 1H), 7.81–7.76 (m, 2H), 7.59 (d, J = 8.5 Hz, 1H), 7.35–7.29 (m, 6H), 7.28–7.25 (m, 1H), 6.74
(ddd, J1 = 6.8 Hz, J2 = 6.8 Hz, J3 = 1.0 Hz, 1H). 13C NMR {1H} (125 MHz, CDCl3): δ 133.2, 132.13,
132.11, 131.5, 131.42, 131.41 (2C), 131.2, 129.7, 129.3, 128.2 (2C), 127.91, 127.90, 127.8, 127.7,
127.6, 127.5, 127.42, 127.40 (2C), 127.34, 127.30, 127.13, 127.12, 126.9, 126.2, 125.9, 124.7, 124.0,
123.5, 119.4, 89.4, 88.5. IR (CHCl3, ν cm-1): 3086, 2205, 1598, 1571, 1490, 1443, 1156, 1070, 1037,
691, 623, 524. EI MS: 428 (100 %, M+.), 427 (85 %), 425 (15 %), 424 (37 %), 411 (24 %), 337 (34
%), 206 (38 %). HRMS (APCI/QTOF) m/z [M]+ Calcd for C34H20 428.1559; found 428.1554.
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2-Phenyl[6]helicene (12a). 2-Bromo[6]helicene 1 (50 mg, 0.123 mmol, 1 eq), Pd2dba3.CHCl3 (2.6
mg, 2.5 µmol, 2 mol%), XPhos (4.7 mg, 9.8 µmol, 8 mol%), phenyl boronic acid (37.3 mg, 0.246
mmol, 2 eq) and K3PO4 (52 mg, 0.246 mmol, 2 eq) were added to a pre-dried Schlenk flask, which
was subsequently secured. Dry toluene (5 mL) was then added and the reaction mixture was heated
at 90 °C for3 hunderAr atmosphereandthen cooled to room temperature. The crudereaction mixture
was filtered through a short pad of silica gel, eluting with toluene (100 mL). Crude product was
recrystallized from a DCM/MeOH mixture to give 2-phenyl[6]helicene 12a (37.7 mg, 76%) as a pale
yellow powder in satisfactory purity. Mp = 206–208 °C. 1H NMR (500 MHz, CDCl3): δ 8.04–7.93
(m, 9H), 7.93–7.85 (m, 2H), 7.77 (d, J = 8.5 Hz, 1H), 7.49 (dd, J1 = 8.2 Hz, J2 = 1.8 Hz, 1H), 7.24
(ddd, J1 = 7.0 Hz, J2 = 7.0 Hz, J3 = 1.0 Hz, 1H), 7.22–7.14 (m, 3H), 6.80 - 6.72 (m, 3H). 13C NMR
{1H} (125 MHz, CDCl3): δ 141.0, 137.9, 133.2, 132.4, 131.6, 131.4, 130.9, 130.1, 129.8, 128.2,
128.12, 128.08 (3C), 127.6, 127.58, 127.56, 127.47, 127.34, 127.31, 127.23 (2C), 127.03, 126.99,
126.73, 126.68 (2C), 126.3, 126.1, 124.92, 124.86, 124.1. IR (CHCl3, ν cm-1): 3083 1602, 1575, 1493,
1140, 1333, 1192, 1075, 1035, 990, 699, 624. EI MS: 404 (100 %, M+), 387 (10 %) 374 (9 %), 327
(23 %), 313 (9 %), 300 (37 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C32H21 405.1638; found
405.1632.
3-([6]Helicen-2-yl)thiophene (12b). ASchlenk flask was charged with 2-bromo[6]helicene 1 (50 mg,
0,123 mmol, 1 eq), Pd2(dba)3.CHCl3 (2.5 mg, 2.5 µmol, 2 mol%), XPhos (dicyclohexyl(2',4',6'-
triisopropyl-[1,1'-biphenyl]-2-yl)phosphine) (2.3 mg, 4.9µmol, 4 mol%), (thiophen-3-yl)boronic acid
(47.1 mg, 0.368 mmol, 3 eq) and K3PO4 (313 mg, 1.473 mmol, 12 eq). The mixture was heated at 90
°C for 3 h in dry toluene (10 mL) under Ar atmosphere and then cooled to room temperature. The
crude reaction mixture was filtered through a short pad of silica gel, eluting with toluene (100 mL).
Crude product was recrystallized from DCM/MeOH mixture to give 3-([6]Helicen-2-yl)thiophene
12b (31.5 mg, 63%) as a pale yellow powder in a satisfactory purity. Mp = 206–207 °C. 1H NMR
(500 MHz, CDCl3): δ 8.04–7.97 (m, 6H), 7.95 (d, J = 1.5 Hz, 1H), 7.92 (s, 2H), 7.86–7.81 (m, 2H),
7.74 (d, J = 8.6 Hz, 1H), 7.46 (dd, J1 = 8.2, J2 = 1.7 Hz, 1H), 7.21 (ddd, J1 = 7.0 Hz, J2 = 7.0 Hz, J3 =
1.0 Hz, 1H), 7.14 (dd, J1 = 4.9 Hz, J2 = 3.0 Hz, 1H), 6.75 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.3 Hz,
1H), 6.68 (dd, J1 = 2.9 Hz, J2 = 1.3 Hz, 1H), 6.61 (dd, J1 = 5.0 Hz, J2 = 1.3 Hz, 1H). 13C NMR {1H}
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(126MHz, CDCl3): δ 142.2, 133.2, 132.4, 132.1, 131.6, 131.4, 130.8, 130.1, 129.8, 128.07, 128.06,
128.05, 127.60, 127.58, 127.56, 127.42, 127.35, 127.25, 127.0 (2C), 126.7, 126.3, 126.10, 126.08,
125.8, 125.4, 124.9, 124.4, 124.1, 120.0 ppm. IR (CHCl3, ν cm-1): 3050, 1540, 1405, 1250, 1085,
1033, 848, 830, 684, 657. EI MS: 410 (100 %, M+), 350 (12 %), 324 (14 %), 313 (10 %), 300 (33 %).
HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C30H19S 411.1202; found 411.1205.
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2-Styryl[6]helicene (13). Apre-dried microwave vial was charged with 2-bromo[6]helicene 1 (50 mg,
0.1228 mmol, 1 eq), Herrmann I catalyst (11.5 mg, 0.0123 mmol, 10 mol%), DABCO (27.5 mg,
0.246 mmol, 2 eq), styrene (25.5 mg, 0.246 mmol, 2 eq) and DMF (4 mL). The reaction mixture was
bubbled through with argon and reacted in a microwave reactor for 90 minutes at 150 °C. After the
conversion was complete, solvent was removed from the reaction and the residue was dissolved in
dichloromethane (20 mL) and washed with brine (3 x 25 mL) and water (25 mL). The organic phase
was dried overMgSO4, evaporated and consequentlypurified byflash chromatographyusing reverse-
phase silica gel and acetonitrile as a mobile phase to give 2-styryl[6]helicene 13 as a pale yellow
amorphous solid (32.6 mg, 62 % yield). The cis-13 : trans-13 ratio was estimated from 1H NMR
spectrum (approx. 1:5). trans-13: 1H NMR (500 MHz, CDCl3): δ 8.02 (m, 4H), 8.02 (d, J = 8.1 Hz,
1H), 7.98 (d, J = 8.1 Hz, 1H), 7.91 (d, J = 8.5 Hz, 1H), 7.89 (s, 1H), 7.88 (d, J = 8.5 Hz, 1H), 7.82 (m,
1H), 7.93 (d, J = 8.2 Hz, 1H), 7.71 (d, J = 8.5 Hz, 1H), 7.33 (m, 5H), 7.23 (m, 1H), 7.21 (ddd, J1 =
6.8 Hz, J2 = 6.8 Hz, J3 = 1.1 Hz, 1H), 6.74 (ddd, J1 = 6.8 Hz, J2 = 6.8 Hz, J3 = 1.2 Hz, 1H), 6.55 (d, J
= 16.0 Hz, 1H), 6.24 (d, J = 16.0 Hz, 1H). trans-13: 13C NMR {1H} (125 MHz, CDCl3): δ 137.4,
134.0, 133.2, 132.1, 131.6, 131.3, 131.3, 129.80, 129.75, 128.5 (2C), 128.21, 128.20, 128.0, 127.9,
127.8, 127.6, 127.54, 127.47, 127.38, 127.33, 127.31, 127.2, 127.01, 127.00, 126.5, 126.23 (2C),
126.20, 126.0, 125.6, 124.7, 124.5, 124.1. cis-13: 1H NMR (500 MHz, CDCl3): δ 8.02 (m, 4H), 7.99
(m, 1H), 7.94 (s, 2H), 7.83 (m, 2H), 7.67 (d, J = 8.5 Hz, 1H), 7.55 (d, J = 8.3 Hz, 1H), 7.53 (d, J = 0.8
Hz, 1H), 7.28 (ddd, J1 = 7.0 Hz, J2 = 7.0 Hz, J3 = 1.3 Hz, 1H), 7.14 (m, 3H), 7.04 (dd, J1 = 8.2 Hz, J2
= 1.6 Hz, 1H), 6.95 (m, 2H), 6.81 (ddd, J1 = 6.9 Hz, J2 = 4.9 Hz, J3 = 1.4 Hz, 1H), 6.29 (d, J = 12.1
Hz, 1H), 5.84 (d, J = 12.1 Hz, 1H). cis-13: 13C NMR {1H} (125 MHz, CDCl3): δ 137.1, 134.0, 133.1,
131.9, 131.4, 131.3, 130.8, 130.03, 129.97, 129.87, 129.7, 129.0, 128.9 (2C), 128.5, 127.89 (2C),
127.88, 127.81, 127.78, 127.6, 127.54, 127.51, 127.22, 127.19, 126.89, 126.87, 126.85, 126.81, 126.2
(2C), 125.8, 124.5, 124.0. IR (CHCl3, ν cm-1): 3058, 1621, 1600, 1571, 1489, 1447, 1412 (cis), 1290,
1177, 1157,1001, 960 (trans), 842, 719 (trans), 701, 637, 438. EI MS: 430 (100 %), 350 (6 %), 337
(29 %), 326 (21 %), 313 (9 %), 300 (29 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C34H23
431.1794; found 431.1792.
2-Cyano[6]helicene (14). 2-Bromo[6]helicene 1 (50 mg, 0.12 mmol, 1 eq) and copper (I) cyanide (55
mg, 0.62 mmol, 5 eq) were dissolved in N-methyl-2-pyrrolidone (2 mL) under Ar atmosphere. The
reaction was heated in a microwave reactor to 210 °C for 3 hours. After the reaction an aqueous
solution of ammonium chloride (20 mL, 25% V/V aqueous solution) was added and the product was
extracted with ethyl acetate (2 x 20 mL). Combined organic layers were washed with brine (20 mL,
saturated solution), dried over MgSO4, filtered, and the solvent was evaporated at the reduced
pressure. A recrystallization from DCM/MeOH (1:1) mixture gave 2-cyano[6]helicene 14 (34 mg,
79%) as a light brown powder. The same protocol for synthesis of (M)-14 was used. The reaction
((M)-1, 10 mg, 0.025 mmol, 1 eq) was conducted at 210 °C for one hour. Crude reaction mixture was
analyzed on chiral HPLC (1 %ee of (M)-14). Mp = 269 °C. 1H NMR (500 MHz, CDCl3): δ 8.10 (d,
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J = 1.9 Hz, 1H), 8.08 (d, J = 2.3 Hz, 1H), 8.04 (d, J = 1.1 Hz, 2H), 8.01–7.90 (m, 6H), 7.88 (d, J = 8.2
Hz, 1H), 7.44 (d, J = 8.6 Hz, 1H), 7.36 (dd, J1 = 8.2 Hz, J2 = 1.5 Hz, 1H), 7.28 (ddd, J1 = 6.9 Hz, J2 =
6.9 Hz, J3 = 1.1 Hz, 1H), 6.69 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.4 Hz, 1H). 13C NMR {1H} (125
MHz, CDCl3): δ133.41, 133.40, 133.4, 132.2, 131.8, 131.6, 129.7, 129.2, 129.0, 128.6, 128.5, 127.24,
128.22, 128.1, 127.24, 127.22, 127.19, 127.02, 126.98, 126.96, 126.5, 126.2, 126.0, 124.8, 123.7,
118.8, 107.7. IR (CHCl3, ν cm-1): 2229, 555. EI MS: 353 (100 %, M+.), 349 (15 %), 325 (26 %), 300
(22 %), 176 (12 %), 161 (13 %), 207 (95 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C27H16N
354.1277; found 354.1272. Chiral separation: tR = 9.14 min ((-)-14), 23.91 min ((+)-14).
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[6]helicene-2-yl carbaldehyde (15). 2-Bromo[6]helicene 1 (100 mg, 0.246 mmol, 1 eq) was placed
in a pre-dried flask and secured. Anhydrous THF (10 mL) was added and the reaction mixture was
cooled to -78 °C in a dry ice/ethanol bath. n-Butyl lithium (2.5M in THF, 128 µl, 0.320 mmol, 1.3 eq)
was added dropwise and the reaction mixture was stirred for 1 min at the same temperature, before
the addition of anhydrous DMF (23.3 mg, 0.3194 mmol, 1.3 eq). The reaction was stirred for 30 min
and then removed from the bath and allowed to warm to room temperature. The reaction mixture was
then quenched with diluted HCl (0.5 mL, 10% V/V aqueous solution) and evaporated to dryness.
Residue was dissolved in DCM (20 mL) and washed with brine (2 x 20 mL, saturated solution) and
water (20 mL). Combined organic phases were then dried over MgSO4, evaporated, and the residue
purified using column chromatography with petroleum ether / ethyl acetate 8:1. [6]Helicene-2-yl
carbaldehyde 15 was obtained as a beige amorphous solid (77.8 mg, 89 % yield). Mp = 238–240 °C.
1H NMR (400 MHz, CDCl3): δ 9.08 (d, J = 0.7 Hz, 1H), 8.12–7.92 (m, 9H), 7.90 (d, J = 8.3 Hz, 1H),
7.81 (dd, J1 = 8.1 Hz, J2 = 1.0 Hz, 1H), 7.70 (dd, J1 = 8.2 Hz, J2 = 1.6 Hz, 1H), 7.52–7.48 (m, 1H),
7.17 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.2 Hz, 1H), 6.65 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.4 Hz,
1H). 13C NMR {1H} (101 MHz, CDCl3): δ 191.9, 135.2, 135.0, 133.4, 132.7, 131.9, 131.6, 131.5,
129.7, 129.3, 129.2, 128.6, 128.2, 128.1, 127.84, 127.79, 127.75, 127.69, 127.5, 127.4, 127.2, 127.1,
126.3, 125.8, 124.8, 123.8, 122.1. IR (CHCl3, ν cm-1): 2813, 2762, 2726, 1690, 1398. EI MS: 356
(100 %, M+), 337 (23 %), 327 (50 %), 313 (10 %), 300 (66 %), 162 (28 %), 150 (13 %). HRMS
(ESI/QTOF) m/z [M]+ Calcd for C27H17O 356.1197; found 356.1192.
Ethyl 2-[6]helicenoate (16). 2-Bromo[6]helicene 1 (100 mg, 0.256 mmol, 1 eq) was dissolved in
tetrahydrofuran (10 mL) underAr atmosphere. The reaction was cooled to -78°C and n-BuLi (2.5 M
in THF, 128 µL, 0.32 mmol, 1.3 eq) was added to the reaction. The reaction mixture was stirred at the
same temperature for 1 minute and ethyl chloroformate (38 µL, 0,40 mmol, 1.6 eq) in THF (2 mL)
was added to the reaction. The reaction mixture was warmed to room temperature and ammonium
chloride (1 mL, 25% V/V aqueous solution) was added, and the product was extracted with ethyl
acetate (3 x 15 mL). The combined organic layers were washed with brine (2 x 20 mL, saturated
solution) and water (20 mL). The organic layer was dried over MgSO4, filtered, and the solvent was
evaporated at the reduced pressure. Flash-chromatography (petroleum ether/ethyl acetate 20:1) and
recrystallization from DCM/MeOH 1:1 gave ethyl 2-[6]helicenoate 16 (50 mg, 51%) as a pale brown
powder. Mp = 160–163°C. 1H NMR (500 MHz, CDCl3): δ 8.39 (s, 1H), 8.08–7.91 (m, 8H), 7.88–
7.81 (m, 2H), 7.78 (d, J = 7.9 Hz, 1H), 7.54 (d, J = 8.5 Hz, 1H), 7.19 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz,
J3 = 1.1 Hz, 1H), 6.66 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.3 Hz, 1H), 4.15–4.07 (m, 1H), 4.01–3.92
(m, 1H), 1.18 (t, J = 7.1 Hz, 3H). 13C NMR {1H} (125 MHz, CDCl3): δ 166.3, 134.2, 133.3, 132.3,
131.5, 131.4, 130.4, 129.4, 129.0, 128.7, 128.4, 128.1, 127.9, 127.7, 127.6 (2C), 127.5, 127.4, 127.30,
127.1, 126.9, 126.2, 126.1, 125.7, 125.3, 124.6, 124.0, 60.3, 14.1. IR (CHCl3, ν cm-1): 2979, 2929,
2875, 2855, 1708, 1476, 1465, 1405, 1303, 1277, 1260, 1121, 1109, 1046, 1022, 960. EI MS: 400
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(100 %, M+.), 353 (7 %), 327 (63 %), 300 (51 %), 162 (16 %), 150 (9 %). HRMS (ESI/QTOF) m/z
[M + H]+ Calcd for C29H21O2 401.1536; found 401.1530.
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4,4,5,5-tetramethyl-2-([6]helicene-2-yl)-1,3,2-dioxaborolane (17). Method A: 2-Bromo[6]helicene 1
(100 mg, 0.256 mmol, 1 eq) was dissolved in tetrahydrofuran (10 mL) under Ar atmosphere. The
reaction was cooled to -78°C and n-BuLi (128 µL, 0.32 mmol, 1.3 eq) was added into the reaction.
The reaction mixture was stirred at the same temperature for 1 minute and iso-propoxyboronate
pinacol ester (65.1 µL, 0.3194 mmol, 1.3 eq) was added. The reaction was slowly allowed to warm
up to room temperature and stirred for an additional 60 min. The reaction was quenched with diluted
HCl (0.5 mL, 10% V/V aqueous solution) and evaporated to dryness. The residuum was dissolved in
dichloromethane (30 mL) and washed with water (3 x 30 mL). The organic phase was dried over
MgSO4, evaporated, and the crude product was further purified by means of column chromatography
on silica gel with petroleum ether/ethyl acetate 8:1 as a mobile phase. Boronate 17 was obtained as a
pale yellow amorphous solid (92.6 mg, 83 %). Method B: 2-Bromo[6]helicene 1 (50 mg, 0.1228
mmol, 1 eq), PdCl2 (2.2 mg, 0,0123 mmol, 10 mol%), 1,1′-ferrocenediyl-bis(diphenylphosphine) (6.8
mg, 0.01228 mmol, 10 mol%), bis(pinacolato)diboron (46.8 mg, 0.1843 mmol, 1.5 eq), and sodium
acetate (20.1 mg, 0.2457 mmol, 2 eq) were charged in a pre-dried microwave vial before DMF (3
mL) was added. The vial was reacted in a microwave reactor at 160 °C for 2 hours. Afterwards, the
solvent was removed in vacuo and the residue chromatographed on silica gel with petroleum
ether/ethylacetate 8:1 as a mobile phase. Boronate 17 was obtained as a pale yellow amorphous solid
(21.5 mg, 39 %). Mp = 232–233 °C. 1H NMR (400 MHz, CDCl3): δ 8.12 (s, 1H), 8.03–7.87 (m, 8H),
7.80 (d, J = 7.9 Hz, 1H), 7.75 (dd, J1 = 8.0 Hz, J2 = 1.5 Hz, 1H), 7.57 (dd, J1 = 7.9 Hz, J2 = 1.1 Hz,
1H), 7.53 (d, J = 8.5 Hz, 1H), 7.17 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.2 Hz, 1H), 6.62 (ddd, J1 =
6.9 Hz, J2 = 6.9 Hz, J3 = 1.4 Hz, 1H), 1.19 (s, 6H), 1.18 (s, 6H). 13C NMR {1H} (101 MHz, CDCl3):
δ 135.9, 133.5, 133.1, 132.1, 131.3, 131.1, 130.1, 129.7, 128.8, 128.21, 128.19, 127.69, 127.66,
127.58 (2C), 127.4, 127.3, 127.0, 126.8, 126.7, 126.5, 126.2, 125.2, 124.4, 124.1, 83.3 (2C),
24.9(2C), 24.3 (2C). The carbon directly attached to the boron atom was not detected, likely due to
quadrupolar relaxation. IR (CHCl3, ν cm-1): 1481, 1406, 1395, 1380, 1369, 1330, 1167, 1143 1097,
963, 851. EI MS: 454 (100 %, M+), 353 (21 %), 339 (8 %), 326 (40 %), 313 (11 %), 300 (27 %).
HRMS (ESI/QTOF) m/z [M]+ Calcd for C32H27BO2 454.2104; found 454.2146.
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2-Hydroxy[6]helicene (18). 4,4,5,5-tetramethyl-2-([6]helicene-2-yl)-1,3,2-dioxaborolane 17 (100
mg, 0.2202 mmol, 1 eq) was dissolved in 25 mL of methanol before urea-hydrogen peroxide adduct
(UHP) (103.5 mg, 1.101, 5 eq) was added in two parts over 5 minutes. The reaction was stirred until
full conversion (indicated by TLC analysis). The solvent was then evaporated, the residue dissolved
in ethylacetate (40 mL) and washed with brine (2 x 30 mL) and water (30 mL). After drying over
MgSO4 and evaporation of the organicphase, the crude product was recrystallized from DCM/MeOH
to give 18 as a pale yellow powder (46.9 mg, 62 %). Mp = 238–240 °C. 1H NMR (500 MHz, CDCl3):
δ 8.05–7.77 (m, 9H), 7.75 (d, J = 8.5 Hz, 1H), 7.69 (d, J = 8.5 Hz, 1H), 7.29 (ddd, J1 = 6.9 Hz, J2 =
6.9 Hz, J3 = 1.1 Hz, 1H), 6.93 (d, J = 2.6 Hz, 1H), 6.84 (dd, J1 = 8.5 Hz, J2 = 2.5 Hz, 1H), 6.76 (ddd,
J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.4 Hz, 1H), 4.38 (bs, 1H). 13C NMR {1H} (125 MHz, CDCl3):δ 153.1,
132.9, 131.8, 131.5, 131.3, 131.2, 130.0, 129.4, 127.64, 127.61, 127.5, 127.37, 127.34, 127.2, 127.00,
126.98, 126.96, 126.95, 126.8, 126.4, 125.9, 124.8, 124.2, 123.8, 116.0, 111.5. IR (CHCl3, ν cm-1):
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3591, 1172. EI MS: 344 (100 %, M+), 327 (33 %), 313 (24 %), 287 (11 %), 207 (14 %), 162 (23 %),
150 (16 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C26H17O 345.1274; found 345.1278.
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2-(Benzylamino)[6]helicene (19). A pre-dried microwave vial was charged with 2-bromo[6]helicene
1 (200 mg, 0.4914 mmol, 1 eq), Pd(OAc)2 (11.0 mg, 0.0491 mmol, 10 mol%), BINAP (30.5 mg,
0.0491 mmol, 10 mol%), tBuOK (82.5 mg, 0.7371 mmol, 1.5 eq) and benzylamine (80.4 µL, 0.7371
mmol, 1.5 eq). Dry toluene (18 mL) was added, argon was bubbled through the reaction mixture for
10 min before the vial was sealed. The reaction was performed in a microwave reactor at 170 °C for
2 h. After the reaction was complete, the solvent was removed and the residue was chromatographed
on silica gel using petroleum ether/ethyl acetate 5:1 as a mobile phase. Product 19 was obtained as a
yellow solid (159 mg, 75 %). The same protocol for synthesis of (M)-19 was used. The reaction ((M)-
1, 10 mg, 0.025 mmol, 1 eq) was conducted at 170 °C for 2 hours. Crude reaction mixture was
analyzed on chiral HPLC (96 %ee of (M)-19). Mp = 152–154 °C. 1H NMR (500 MHz, CDCl3): δ
8.03–7.90 (m, 7H), 7.87–7.80 (m, 2H), 7.73 (d, J = 8.4 Hz, 1H), 7.66 (d, J = 8.6 Hz, 1H), 7.38 (ddd,
J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.1 Hz, 1H), 7.31–7.20 (m, 3H), 7.07–7.02 (m, 2H), 6.85 (ddd, J1 = 6.8
Hz, J2 = 6.8 Hz, J3 = 1.4 Hz, 1H), 6.80 (d, J = 1.9 Hz, 1H), 6.64 (dd, J1 = 8.6 Hz, J2 = 2.4 Hz, 1H),
3.44–3.33 (m, 2H). 13C NMR {1H} (125 MHz, CDCl3): δ 145.4, 138.9, 132.9, 132.1, 131.5, 131.4,
131.1, 130.5, 128.6, 128.3 (2C), 127.7 (2C), 127.6 (3C), 127.50, 127.3, 127.11, 127.05, 126.95,
126.89, 126.59, 126.57 (2C), 125.7, 125.2, 124.6, 124.4, 122.1, 115.8, 107.6, 47.5. IR (CHCl3, ν cm-
1): 3448, 1622, 1602, 1581, 1530, 1495, 1454, 1298, 1188, 1144, 1071, 1029, 984, 895, 831 700, 624,
469. EI MS: 433 (100 %, M+), 340 (23 %), 326 (33 %), 313 (13 %), 300 (34 %), 207 (11 %), 162 (11
%). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C33H24N 434.1903; found 434.1901. Chiral
separation: tR = 18.22 min ((-)-19), 31.92 min ((+)-19).
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N-(helicene-2-yl)-1,1-diphenylmethanimine (20). A pre-dried microwave vial was charged with 2-
bromo[6]helicene1(500mg, 1.228mmol, 1 eq), Pd(OAc)2 (27.5 mg, 0.123mmol, 10mol%), BINAP
(76.4 mg, 0.1228 mmol, 10 mol%), tBuOK (206 mg, 1.842 mmol, 1.5 eq) and benzophenone imine
(308 µL, 1.842 mmol, 1.5 eq). Dry toluene (18 mL) was added, argon was bubbled through the
reaction mixture for 10 min before the vial was sealed. The reaction was performed in a microwave
reactor at 170 °C for 2 h. After the reaction was complete, the cooled reaction mixture was filtered
through a pad of silica gel, eluting with toluene. Solvent was evaporated in vacuo and the crude
product was then recrystallized from DCM/MeOH mixture to give 20 as yellow powder (535 mg, 86
%). Mp = 273–275 °C. 1H NMR (500 MHz, CDCl3): δ 7.98–7.74 (m, 10H), 7.53–7.46 (m, 3H), 7.45–
7.38 (m, 2H), 7.36–7.30 (m, 3H), 7.21-7.08 (m, 3H), 6.91 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.4,
1H), 6.58–6.56 (m, 2H), 6.42 (dd, J1 = 8.4 Hz, J2 = 1.9 Hz, 1H).13C NMR {1H} (125 MHz, CDCl3):
δ 167.1, 147.8, 139.9, 135.8, 132.9, 131.9, 131.5, 131.4, 131.0, 130.4, 130.06, 129.96 (2C), 129.3
(2C), 128.40, 128.37, 128.04, 127.96 (2C), 127.91, 127.86, 127.77, 127.76, 127.75, 127.70 (2C),
127.6, 127.14, 127.06, 126.74, 126.72, 126.3, 125.6, 125.0, 124.7, 124.1, 121.5, 120.2. IR (CHCl3, ν
cm-1): 1664, 1595, 1495, 1448, 1333, 1291, 1191, 1177, 1156, 1029, 981, 846, 624, 462, 406. EI
MS: 507 (100 %, M+), 415 (10 %), 341 (10 %), 326 (24 %), 300 (34 %), 281 (21 %), 207 (59 %), 73
(16 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C39H26N 508.2059; found 508.2051.
2-Amino[6]helicene (21). Iminohelicene 20 (429 mg, 0.848 mmol, 1 eq) was dissolved in dry THF
(30 mL), water (45.7 µL, 2.543 mmol, 3 eq) and HCl (423 µL, 3 eq, 6M solution in isopropanol) was
then added. The color of the reaction mixture turned red immediately after the addition of the HCl
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only to fade to yellow in a few minutes while grey precipitate started to form. After the completion of
the reaction (monitored by TLC), the solvent was removed under reduced pressure and the crude
product was triturated in Et2O (30 mL) for 30 min. The solids were filtered off, suspended in EtOAc
(200 mL) and washed with saturated Na2CO3 (3 x 100 mL). The combined organic phases were dried
over MgSO4, evaporated, and dried in a vacuum to afford 2-amino[6]helicene 21 as a dark yellow
amorphous solid (270.4 mg, 93 %). Mp = 211–214 °C. 1H NMR (500 MHz, CDCl3): δ 8.03–7.87 (m,
6H), 7.85–7.78 (m, 2H), 7.75–7.69 (m, 2H), 7.65 (d, J = 8.4 Hz, 1H), 7.31 (ddd, J1 = 7.0 Hz, J2 = 7.0
Hz, J3 = 1.1, 1H), 6.82 (d, J = 2.1 Hz, 1H), 6.78 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.4, 1H), 6.69 (dd,
J1 = 8.4 Hz, J2 = 2.3 Hz, 1H). 13C NMR {1H} (125 MHz, CDCl3): δ 143.8, 132.8, 132.0, 131.6, 131.3,
131.1, 130.2, 128.8, 127.8, 127.7, 127.5, 127.4, 127.2, 127.1, 127.0, 126.8, 126.70, 126.67, 126.3,
125.78, 125.76, 124.6, 124.4, 122.6, 116.2, 111.7. IR (CHCl3, ν cm-1): 3485, 3367, 1627. EI MS: 343
(100 %, M+), 326 (19 %), 313 (10 %), 300 (33 %), 281 (29 %), 207 (73 %). HRMS (ESI/QTOF) m/z
[M + H]+ Calcd for C26H18N 344.1441; found 344.1439.
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2-Nitro[6]helicene (22). 2-Amino[6]helicene 21 (50 mg, 0.145 mmol, 1 eq) and 3-chloroperbenozoic
acid (72 mg, 0.436 mmol, 3 eq) was dissolved in dichloromethane (15 mL) and stirred for an hour. A
complete consumption of starting material was observed by TLC. The reaction mixture was then
washed withsaturated brine(2x15mL) and water(15 mL). Theorganicphasewas dried overMgSO4
and solvents were evaporated. The crude product was purified by column chromatography with
petroleum ether : ethylacetate 8 : 1 as a mobile phase. The crude product was recrystallized from
DCM/MeOH to give 2-nitro[6]helicene 22 (27 mg, 49%) as a dark yellow powder. Mp = 239–241
°C. 1H NMR (400 MHz, CDCl3): δ 8.59 (d, J = 1.8 Hz, 1H), 8.16–8.10 (m, 2H), 8.09–7.95 (m, 7H),
7.91 (d, J = 8.7 Hz, 1H), 7.83 (d, J = 7.9 Hz, 1H), 7.50 (d, J = 8.5 Hz, 1H), 7.23–7.19 (m, 1H), 6.70–
6.66 (m, 1H). 13C NMR {1H} (125 MHz, CDCl3): δ 144.3, 134.7, 133.4, 132.6, 132.0, 131.7, 130.5,
129.2, 128.8, 128.67, 128.61, 128.4 (2C), 128.3, 128.2, 127.3, 127.2, 126.97, 126.96, 126.8, 126.2,
125.8, 124.7, 124.4, 123.8, 119.0. IR (CHCl3, ν cm-1): 1515, 1508, 1339. EI MS: 373 (18 %), 357
(100 %), 340 (44 %), 326 (61 %), 313 (21 %), 300 (12 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd
for C26H16NO2 374.1176; found 374.1178.
2-Nitroso[6]helicene (23). 2-amino[6]helicene 21 (50 mg, 0.14 mmol, 1 eq) was dissolved in a
minimum amount of dichloromethane (3 mL) and potassium peroxymonosulfate (443 mg, 2.91
mmol, 20 eq) in water (3 mL) was added. The two-phase mixture was stirred vigorously until the
complete consumption of starting material was observed by TLC. The reaction mixture was then
diluted with dichloromethane (20 mL) and washed with water (3 x 20 mL). The organic phase was
then dried with MgSO4 and consequently evaporated. Crude product was purified by column
chromatography with petroleum ether : ethylacetate 8:1. After evaporation, 2-nitroso[6]helicene 23
was obtained (19 mg, 37 %) as a yellow amorphous solid. Mp = 215–216 °C. 1H NMR (400 MHz,
CDCl3): δ 9.14 (d, J = 1.5 Hz, 1H), 8.21–7.90 (m, 8 H), 7.79–7.72(m, 2H), 7.60 (dm, J = 8.5
Hz, 1H), 7.08 (ddd, J1 = 7.1 Hz, J2 = 7.1 Hz, J3 = 1.2, 1H), 6.83 (dd, J1 = 8.6 Hz, J2 = 1.8 Hz, 1H),
6.64 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.4, 1H). 13C NMR {1H} (100 MHz, CDCl3): δ 163.2, 135.9,
135.3, 133.6, 132.3, 132.0, 131.8, 131.3, 130.2, 129.4, 129.2, 128.7, 128.5, 128.3, 128.2, 128.1, 127.8,
127.6, 127.10, 127.06, 127.01, 126.2, 125.7, 124.7, 124.0, 108.3. IR (CHCl3, ν cm-1): 1453. EI MS:
358 (22 %, M+), 341 (100 %), 326 (63 %), 314 (16 %). HRMS (ESI/QTOF) m/z [M + H]+ Calcd for
C26H16NO 358.1226; found 358.1223.
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2-(Methylsulphanyl)[6]helicene (24). A pre-dried microwave vial was charged with 2-
bromo[6]helicene 1 (100 mg, 0.246 mmol, 1 eq), sodium methanethiolate (86.0 mg, 1.228 mmol, 5
eq) and Pd(PPh3)4 (29.6 mg, 0.027 mmol, 10 mol%) in 5 mLof dryDMF.Argon was bubbled through
the reaction mixture for 10 min, after which the vial was placed in a microwave initiator and reacted
at 170 °C for 30 min. Solvent was then removed and the residue was dissolved in DCM (20 mL) and
washed with brine (2 x 20 mL, saturated aqueous solution) and water (20 mL). The organic phase was
dried over MgSO4, drying agent was filtered off and the solvent was removed under reduced pressure.
Crude product was chromatographed on reverse-phase silica gel using MeCN/H2O 95:5 as a mobile
phase to yield 2-(methylsulphanyl)[6]helicene 24 as a pale yellow amorphous solid (57.9 mg, 63 %).
1H NMR (400 MHz, CDCl3): δ 8.05–7.82 (m, 9H), 7.71 (d, J = 8.4 Hz, 1H), 7.65 (d, J = 8.5 Hz, 1H),
7.46 (s, 1H), 7.31–7.26 (m, 1H), 7.13 (dd, J1 = 8.4 Hz, J2 = 1.8 Hz, 1H), 6.77–6.73 (m, 1H), 1.65 (s,
3H). 13C NMR {1H} (125 MHz, CDCl3): δ 135.4, 133.2, 132.0, 131.7, 131.3, 130.4, 129.8, 129.5,
127.9, 127.8, 127.7, 127.57, 127.52, 127.46, 127.3, 127.2, 127.14, 127.08, 127.0, 126.5, 126.0, 125.5,
125.3, 124.9, 124.0, 123.8, 14.8. IR (CHCl3, ν cm-1): 2855, 1438, 1318. EI MS: 374 (100 %, M+),
326 (54 %), 300, (34 %), 162 (30 %). HRMS (ESI/QTOF) m/z [M]+ Calcd for C27H18S 374.1219;
found 374.1232.
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Borane 2-(diphenylphosphanyl)[6]helicene complex (25). 2-Bromo[6]helicene 1 (50 mg, 0.123
mmol, 1 eq), diphenylphosphine (47 µL, 0.246 mmol, 2 eq), sodium acetate (21 mg, 0.246 mmol, 2
eq) and Herrmann I (5 mg, 6.1 µmol, 5 mol%) were dissolved in THF (2 mL) underAr atmosphere.
The reaction was heated in a microwave reactor to 160 °C for 30 minutes. The reaction mixture was
cooled to room temperature and 1M borane-tetrahydrofuran complex solution in THF (1.4 mL, 1.4
mmol, 12 eq) was added dropwise. The reaction mixture was stirred under Ar atmosphere at room
temperature overnight. The solvent was removed at reduced pressure and the crude product was
dissolved in dichloromethane and filtered through a short silica gel column and eluted with
dichloromethane (50 mL). Borane 2-(diphenylphosphanyl)[6]helicene complex 25 (39 mg, 61%) was
obtained after recrystallization from dichloromethane/methanol as a yellow powder. The same
protocol for synthesis of (M)-25 was used. The reaction ((M)-1, 10 mg, 0.025 mmol, 1 eq) was
conducted at 160 °C for one hour. Crude reaction mixture was analyzed on chiral HPLC 99 %ee of
(M)-25). Mp = 222–223 °C. 1H NMR {31P} (500 MHz, CDCl3): δ 8.08 (bd, J = 13.3 Hz, 1H), 8.04
(d, J = 8.5 Hz, 1H), 8.00 (bd, J = 8.1 Hz, 1H), 7.97–7.87 (m, 5H), 7.84 (bd, J = 8.6 Hz, 1H), 7.79 (d,
J = 8.6 Hz, 1H), 7.62 (d, J = 7.9 Hz, 1H), 7.54 (d, J = 8.5 Hz 1H), 7.44–7.37 (m, 3H), 7.32–7.25 (m,
4H), 7.17 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 = 1.0, 1H), 7.11–7.04 (m, 4H), 6.73 (ddd, J1 = 6.9 Hz, J2
= 6.9 Hz, J3 = 1.3, 1H), 1.10–0.40 (bs, 3H). 11B {1H} NMR (160 MHz, CDCl3): δ -38.35 (s). 13C
NMR {1H} (125 MHz, CDCl3): δ 133.6 (d, JP-C = 11.3 Hz), 133.3, 133.1 (d, JP-C = 9.7 Hz, 2C), 132.9
(d, JP-C = 9.6 Hz, 2C), 132.6 (d, JP-C = 2.3 Hz), 132.3, 131.4, 131.3 (d, JP-C = 0.6 Hz), 130.7 (d, JP-C
=
2.5 Hz), 130.6 (d, JP-C = 2.5 Hz), 130.1 (d, JP-C = 11.7 Hz), 129.3 (d, JP-C = 57.8 Hz), 129.0, 128.7,
128.6 (d, JP-C = 9.8 Hz), 128.57 (d, JP-C = 58.1 Hz), 128.55 (d, JP-C = 1.7 Hz, 2C), 128.50 (d, JP-C = 0.5
Hz), 128.47 (d, JP-C = 1.7 Hz, 2C), 128.2 (d, JP-C = 10.4 Hz) 128.05, 128.02, 127.96, 127.6, 127.5,
127.4, 127.2 (d, JP-C = 0.8 Hz), 126.8, 126.5, 126.2, 125.6, 125.3 (d, JP-C = 58.8 Hz) , 124.5, 123.7.
31P {1H} NMR (121 MHz, CDCl3): δ 20.43 (bs). IR (CHCl3, ν cm-1): 2387, 2346, 1587, 1486, 1438,
1106, 1060, 1029, 1000, 914, 826, 693, 578. HRMS (ESI/QTOF) m/z [M + H]+ Calcd for C38H26P
513.1766; found 513.1767. Chiral separation: tR = 23.21 min ((-)-23), 28.99 min ((+)-23).
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(2-[6]helicenyl)diphenylphosphine oxide (27). Borane 2-(diphenylphosphanyl)[6]helicene complex
25 (50 mg, 0.097 mmol, 1 eq) was dissolved in dichloromethane (2 mL). Then diethyl ether-
tetrafluoroboric acid complex (78 µL, 0.582 mmol, 6 eq) was added dropwise. The reaction mixture
was stirred for 15 min at room temperature and then potassium carbonate (263 mg, 1.94 mmol, 20
eq) was added and the reaction mixture was stirred at the same temperature for another 15 min.
Inorganic salts were filtered off and aqueous solution of hydrogen peroxide (1 mL, 30 % wt.) was
added into the reaction. Reaction mixture was stirred overnight. After the reaction the crude mixture
was extracted with dichloromethane (20 mL). The organic layer was washed with water (2 x 20 mL)
and was dried over MgSO4, filtered, and the solvent was evaporated at the reduced pressure. The
reaction mixture gave (2-[6]helicenyl)diphenylphosphineoxide 27 (47 mg, 94%) as a yellow powder.
Mp = 285–287 °C. 1H NMR (500 MHz, CDCl3): δ 8.05 (d, J = 8.6 Hz, 1H), 8.01–7.89 (m, 7H), 7.85
(d, J = 8.6 Hz, 1H), 7.80 (d, J = 8.6 Hz, 1H), 7.70 (dd, J1 = 8.0 Hz, J2 = 1.4 Hz, 1H), 7.61 (ddd, J1 =
8.2 Hz, J2 = 8.2 Hz, J3 = 1.5, 1H), 7.57 (dm, J = 8.6 Hz, 1H), 7.51–7.46 (m, 1H), 7.41–7.37 (m, 1H),
7.37–7.31 (m, 2H), 7.29–7.23 (m, 3H), 7.13–7.04 (m, 4H), 6.77 (ddd, J1 = 6.9 Hz, J2 = 6.9 Hz, J3 =
1.4, 1H). 13C NMR {1H} (125 MHz, CDCl3): δ 133.3, 133.2 (d, JP-C = 2.6 Hz), 132.58 (d, JP-C = 104.1
Hz), 132.55 (d, JP-C = 11.8 Hz), 132.21, 132.0 (d, JP-C = 10.1 Hz, 2C), 131.74 (d, JP-C = 9.9 Hz, 2C),
131.74 (d, JP-C = 104.2 Hz), 131.46, 131.46 (d, JP-C = 2.8 Hz), 131.36 (d, JP-C = 1.0 Hz), 131.32 (d,
JP-C = 2.7 Hz), 129.6 (d, JP-C = 13.7 Hz), 129.1, 128.8 (d, JP-C = 105.4 Hz), 128.6, 128.42, 128.29 (d,
JP-C = 12.2 Hz, 2C), 128.28 (d, JP-C = 12.1 Hz), 128.23 (d, JP-C = 12.2 Hz, 2C), 128.20, 128.18, 128.07,
127.9, 127.7, 127.5 (d, JP-C = 7.5 Hz), 127.4 (d, JP-C = 9.6 Hz), 127.3 (d, JP-C = 1.1 Hz), 126.9, 126.6,
126.3, 125.7, 124.8, 123.7. 31P {1H} NMR (121 MHz, CDCl3): δ 29.39 (s) ppm. IR (CHCl3, ν cm-1):
1589, 1482, 1438, 1250, 1172, 1028, 1001, 694. EI MS: 528 (8 %, M+.), 527 (17 %), 281 (17 %), 280
(30 %), 253 (10 %), 252 (10 %), 209 (15 %), 208 (29 %), 207 (96 %), 206 (100 %). HRMS
(ESI/QTOF) m/z [M + H]+ Calcd for C38H26PO 529.1715; found 529.1712.
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Racemization of 2-Bromo[6]helicene 1. Enantiomerically pure (M)-1 (2.0 mg) was dissolved in
hexadecane (2 mL) in a capped (PP-cap and rubber/PTFE septa) vial equipped with a stirring bar. The
vial was wrapped into aluminum foil and heated to a constant temperature (458, 465, 475 or 487 K)
using a hot plate magnetic stirrer with Pt1000 probe and stirred Wood’s metal bath. The temperature
was cross-checked using an external mercury-in-glass thermometer with 0.5 °C accuracy. The
increasing concentration of (P)-1 was monitored by chiral HPLC (Kromasil Cellucoat® column) in
the course of time (for details see SI).
Supporting Information
Thermodynamic data, 1H and 13C NMR spectra, HPLC chromatograms.
Author Information
CorrespondingAuthor
*E-mail: sykora@icpf.cas.cz
ORCID ID
Martin Jakubec: 0000-0001-7377-9359
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Tomáš Beránek: 0000-0002-2393-2084
Pavel Jakubík: 0000-0001-5079-0555
Jan Sýkora: 0000-0002-2930-8880
Jaroslav Žádný: 0000-0002-3737-3519
Vladimír Církva: 0000-0001-5351-4003
Jan Storch: 0000-0002-1970-8485
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Note
The authors declare no competing financial interest.
Acknowledgment
This work was supported by The Czech Science Foundation (grant No. 15-12719S).
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