Stable Bromiranium Ions with Weakly-Coordinating Counterions as Efficient Electrophilic Brominating Agents

Article abstract of DOI:10.1002/chem.201701643

Electrophilic halogenating agents are an important class of reagents in chemical synthesis. Herein, we show that sterically demanding bromiranium ions with weakly coordinating counterions are highly reactive electrophilic brominating agents. Despite their

Full text of DOI:10.1002/chem.201701643

A Journal of
Accepted Article
Title: Stable Bromiranium Ions with Weakly-Coordinating Counterions
as Efficient Electrophilic Brominating Agents
Authors: Christoph Ascheberg, Jonathan Bock, Florenz Buß, Christian
Mück-Lichtenfeld, Constantin B. Daniliuc, Klaus Bergander,
Fabian Dielmann, and Ulrich Hennecke
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To be cited as: Chem. Eur. J. 10.1002/chem.201701643
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Stable Bromiranium Ions with Weakly-Coordinating Counterions
as Efficient Electrophilic Brominating Agents
Christoph Ascheberg,[a] Jonathan Bock,[a] Florenz Buß,[b] Christian Mück-Lichtenfeld,[a] Constantin G.
Daniliuc,[a] Klaus Bergander,[a] Fabian Dielmann[b] and Ulrich Hennecke*[a]
Abstract: Electrophilic halogenating agents are an important class of
reagents in chemical synthesis. Herein, we show that sterically
demanding bromiranium ions with weakly coordinating counterions
are highly reactive electrophilic brominating agents. Despite their high
reactivity these reagents are stable, in one case even under ambient
conditions and can be applied in electrophilic halogenations of
alkenes as well as heteroatoms.
been proposed.[11] However, from a synthetic perspective, there
does not seem to be an advantage in using haliranium ions as
halogenating
agents
in
halocycloetherifications
or
halolactonisations. Standard electrophilic halogenating agents
such as elemental halogens or N-haloamides are reactive enough
for this type of transformations and clearly more atom-economic.
Introduction
The electrophilic halogenation of alkenes is one of the
fundamental
Dihalogenation,
reaction
classes
of
organic
and other
chemistry.
halo-
halolactonisation
functionalisations are important synthetic methods and have often
been applied in total synthesis.[1] In general, it is assumed that
these reactions take place via the formation of a cyclic, cationic
intermediate, a haliranium ion, which nicely explains the usually
observed anti-addition towards the alkene.[2,3] The existence of
halonium ions and more specifically haliranium ions in solution
was demonstrated by Olah using NMR spectroscopy.[4] Based on
evidence from NMR spectroscopic investigations and theoretical
calculations the haliranium structure seems to be preferred over
β-halocarbenium ion structures in solution, however, this might
not be true in all cases.[5] The first stable haliranium ion was
obtained by bromination of the sterically highly demanding alkene
adamantylidene adamantane by Wynberg.[6] This bromiranium ion
2-Br3 was formed as a rather insoluble tribromide salt. Definite
prove of the cyclic nature of this ion in the solid state was provided
by Brown through X-ray structure analysis.[7] Later on, the triflate
salt of 2-OTf was prepared and proved to be an useful tool for
mechanistic studies of electrophilic alkene brominations.[8] The
bromiranium ion 2-OTf can directly transfer the electrophilic
bromine atom to another alkene, a process called alkene-to-
alkene transfer of bromiranium ions (Scheme 1).[8,9] This direct
transfer via an associative mechanism enabled the use of 2-OTf
as an electrophilic halogenating agent in mechanistic
investigations of halocyclizations reactions.[10] Furthermore, the
use of chiral bromiranium ions as chiral halogenating agents has
Scheme 1. Bromiranium ions and their alkene-to-alkene transfer.
The situation could be different in reactions, which require highly
reactive halogenating agents such as halogen-induced polyene
cyclizations.[12] N-Haloamides and many other halogenating
agents lack sufficient reactivity to induce high-yielding polyene
cyclizations. The use of elemental bromine leads usually only to
dibromides as the bromide anion is too nucleophilic to allow
further cyclizations after the initial electrophilic bromination.
Barluenga showed that his reagent provides good yields for
monocyclizations, but examples with this reagent for
polycyclizations are missing.[13] The importance of the counterion
in these cyclizations was shown by Snyder, who developed
+
BrS(Et)2 SbCl5Br- (BDSB) as highly reactive halogenating
agent.[14,15] Alternatively, catalytic approaches for the activation of
N-haloimides are known, which allow even enantioselective
halogen-induced polyene cyclizations, however, under those
reaction conditions partially cyclized side products are produced
and a second cyclization step is required to achieve good
cyclization yields.[16]
With these facts in mind, we set out to prepare bromiranium ion
salts with weakly-coordinating counterions as possible highly
reactive brominating agents. It could be demonstrated that such
ions with either tetrakis[3,5-bis(trifluoromethyl)phenyl] borate
(BArF-) or Al2Br7- counterions are easily prepared on gram scale.
Furthermore, those new salts proved to be highly reactive
halogenating agents allowing efficient polyene-type cyclizations
as well as the halogenation of heteroatoms.
[a]
C. Ascheberg, J. Bock, Dr. C. Mück-Lichtenfeld, Dr. C. G. Daniliuc,
Dr. K. Bergander, PD Dr. U. Hennecke*
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität
Corrensstr. 40, 48149 Münster, Germany
E-mail: ulrich.hennecke@uni-muenster.de
F. Buß, Dr. F. Dielmann
Institut für Anorganische Chemie
Westfälische Wilhelms-Universität
[b]
Corrensstr. 28/30, 48149 Münster, Germany
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
weak interaction with the counterion in the structure of 2-BArF.
The shortest inter-ion contact of the bromine atom is observed to
two carbon atoms of one of the aryl rings in the BArF anion with
C···Br distances of 357.0 and 367.4 pm being at the sum of their
van der Waals radii (360 pm).[18] This is quite different to 2-Br3
and 2-OTf, which show rather short Br···Br (309.6 pm) in 2-Br3 or
O···Br (294.2 and 308.1 pm for the two independent ions in the
unit cell) contacts, respectively. This suggests that the reactivity
of 2-BArF is close to an ideal, uncoordinated bromiranium ion.
Synthesis of Bromiranium Ion Salts with Weakly-
Coordinating Anions
Originally, the bromiranium ion of adamantylidene adamantane
(1) was prepared as the tribromide salt 2-Br3.[6] However, due to
the presence of the tribromide, which can release bromine and a
nucleophilic bromide ion, this is not a very useful brominating
agent. Instead, Brown prepared the triflate salt 2-OTf, but also the
triflate interacts with the bromiranium ion.[8] The solid state
structure shows a clear contact between the bromine atom and
one of the oxygen atoms of the triflate ion (O···Br distances of
294.2 or 308.1 pm for the two independent ions in the unit cell,
respectively). Furthermore, the triflate is still nucleophilic and 2-
OTf reacts with cyclohexene to give 2-bromocyclohexyl triflate.[10]
Scheme 2. Synthesis of the stable bromiranium salt 2-BArF.
Figure 1. Solid-state structure of 2-BArF. Thermal ellipsoids are shown with
30% probability. Hydrogen atoms have been omitted for clarity.
To obtain a bromiranium salt with a weakly-coordinating anion we
chose to prepare the salt 2-BArF. This salt has been used
previously as a halogenating agent for an osmium carbonyl
complex, but no procedure for its preparation or data on its
reactivity have been reported.[14] For its preparation we used a salt
metathesis reaction with sodium tetrakis[3,5-bis(trifluoromethyl)-
phenyl] borate (NaBArF). The stoichiometric addition of NaBArF
to 2-Br3 in dichloromethane at room temperature formed 2-BArF
(Scheme 2). The reaction was allowed to stir overnight and
sodium bromide and Br2 as byproducts were removed by filtration
and evaporation, respectively. The crude product could be
purified by crystallization from dichloromethane/hexane at -20 °C.
2-BArF precipitates as pale yellow crystals in 79% yield. The
reagent is stable at room temperature and, at least briefly, under
air. Nevertheless, it is recommended to store 2-BArF protected
from light and air at low temperature to prevent decomposition.
The 13C-NMR spectrum shows five resonances for the adamantyl
moiety, which suggests that rapid exchange of the bromenium,
similar to 2-OTf, takes place, possibly via alkene-to-alkene
transfer.[8] This process is operative even at -78°C (see supporting
information for NMR spectra at -78 °C).
Although 2-BArF is an easy-to-handle brominating agent, the
BArF anion is not ideal if atom-economy is concerned. As an
alternative pure inorganic anions were investigated. Attempts to
brominate 1 with a combination of Br2 and SbCl5 did not provide
a pure product. Instead, we tried a direct route to form the
bromiranium ion by using Br2 and AlBr3. Expecting the formation
-
of the Al2Br7 -anion,[19] a combination of one equivalent of Br2 and
two equivalents of AlBr3 were employed for the halogenation of 1
(Scheme 3). The use of 1,2-dichlorobenzene as solvent was
crucial as dichloromethane proved to be not stable under the
reaction conditions. After the addition of pentane to the reaction
mixture, the product 2-Al2Br7 crystallized at -20 °C as yellow solid
in 66% yield. In contrast to 2-BArF the NMR spectrum of 2-Al2Br7
did not show any dynamic effects and all expected signals could
be identified. Again, in contrast to 2-BArF this bromiranium salt is
not stable under ambient conditions. Under air, we could observe
decomposition with production of white smoke. Considering that
2-BArF is stable, most likely decomposition/hydrolysis of the
Al2Br7- anion is responsible for this behavior.
The crystals obtained of 2-BArF allowed the determination of its
solid state structure using X-ray crystallography (Figure 1).
Overall the solid state structure of the bromiranium ion of 2-BArF
is similar to the previous determined structures in 2-Br3 and 2-OTf.
Small differences can be observed in the length of the C-Br bonds,
which are slightly longer and more symmetrical in 2-BArF
(214.9(4) pm and 213.4(4) pm) compared to 2-Br3 (211.6(6) pm
and 219.4(6) pm) and 2-OTf (211.8(10) pm and 213.6(10) pm or
208.9(10) pm and 211.4(20) pm for the two independent ions in
the unit cell, respectively). The main difference is the only very
Scheme 3. Synthesis of the bromiranium salt 2-Al2Br7.
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The crystals obtained from the bromiranium salt 2-Al2Br7 were
suitable for an X-ray crystal structure analysis (Figure 2). The
C···Br distances in the solid-state structures of the two
independent bromiranium ions found in the unit cell are again
quite similar to the previous structures with symmetrical
bromiranium ions (212.6(13) pm and 215.8(14) pm or
212.9(14) pm and 213.7(15) pm for the two independent ions in
the unit cell, respectively). The Br···Br distances between the
positively polarized bromine atom of the bromiranium ion and the
negatively polarized bromine atoms of the anion (356.3 pm) are
below the sum of their van der Waals radii (380 pm[18]), but
significantly longer than the Br···Br distance in 2-Br3 (309.6 pm),
indicating a much weaker interaction between the anions and the
bromiranium cations.
2-BArF with 3d, a very electron-deficient phosphine, showed
significantly
different
behavior.
The
corresponding
bromophosphonium salt 4d was formed only in about 20% in the
stoichiometric reaction indicating a similar bromenium ion affinity
of 1 and 3d. Interestingly, this study indicates that the reaction of
2-BArF with phosphines can be used as a general protocol for the
direct synthesis of bromophosphonium salts containing a weakly
coordinating anion as long as the phosphine is more electron rich
than phosphine 3d.[21]
Scheme 4. Bromination of phosphines using 2-BArF.
In a second set of experiments, the reactivity of 2-BArF towards
diethyl sulfide (5) was investigated. This was carried out to
compare the reactivity of 2-BArF with BDSB, which is a
bromodiethyl sulfonium salt.[14] However, treatment of 2-BArF in
dichloromethane with one equivalent of diethyl sulfide in a NMR
1
tube provided a surprising result. The H- and 13C-NMR spectra
showed only one set of signals (Figure 3, C), which were neither
identical to the starting materials (Figure 3, A and B) nor to
adamantylidene adamantane, which speaks against the formation
of a free bromodiethyl sulfonium ion.
Figure 2. Solid-state structure of 2-Al2Br7. Thermal ellipsoids are shown with
30% probability. Hydrogen atoms have been omitted for clarity.
Bromiranium ions as Brominating agents for heteroatoms
To explore the potential of 2-BArF as bromenium ion transfer
agent towards heteroatoms, we investigated the reactivity of
2-BArF with phosphines comprising different stereoelectronic
properties. Therefore, PPh3 (3a), P(o-tol)3 (3b), P(NBiPr)Ph2 (3c)
and P(C6F5)3 (3d) were included in our study (Scheme 4). The
decoration of phosphines with strong π-donor groups such as the
benzimidazolin-2-ylidenamino (NBiPr) substituent has been
shown to give particularly electron rich phosphines.[20] The
stoichiometric reaction of 2-BArF with phosphines 3a-d was
investigated via reactions in NMR tubes. The NMR analysis
indicated the clean and quantitative formation of the
bromophosphonium salts [(o-tol)3PBr]+ [BArF]− (4a), [Ph3PBr]+
[BArF]− (4b) and [(NBiPr)Ph2PBr]+ [BArF]− (4c), respectively. The
31P-NMR signals of 4a (52.9 ppm) and 4b (46.4 ppm) are shifted
downfield compared to those of the corresponding phosphines,
while the 31P-NMR signal of 4c (16.8 ppm) appears at higher field
compared to that of P(NBiPr)Ph2 (33.7 ppm). The reaction of
Figure 3. Stacked 1H-NMR spectra of 2-BArF (A), SEt2 (5, B), 2-BArF/5: 1/1
(C), 2-BArF/5: 1/5 (D) and 7 (E).
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All signals underwent significant shifts compared to the starting
materials and some signals such as the 13C-NMR signals of C1 of
the adamantylidene groups and the CH2-groups of diethyl sulfide
broad indicating dynamic behavior (see supporting information for
NMR spectra). However, in these cases the addition of an excess
of sulfide/selenide did not lead to the appearance of more than
became very broad. For example, the CH2-group of diethyl sulfide, one set of NMR signals and the NMR spectra remained dynamic.
which resonates at 2.53 ppm in CH2Cl2 (spectrum A) was shifted
to 2.90 ppm (spectrum C) upon 2-BArF addition. A possible
explanation is the coordination of 5 towards the bromine of the
bromiranium ion without release of a bromo sulfonium ion into the
solution (Scheme 5). Such a coordination of a Lewis base towards
a haloiranium can be found in the solid state structure of the
Attempts to crystallize the reaction products have not been
successful so far, but in the case of the reaction of 2-BArF with
an excess (five equivalents) dibutyl selenide ESI-MS spectra
show a signal with m/z 405.2065 corresponding to a butyl
seleniranium ion (C24H37Se).
iodiranium ion of 1, which contains
a coordinated water
molecule.[8b] DFT calculations suggest that an adduct such as 6
can be stable (see supporting information for details). Attempts to
identify adduct 6 by mass spectrometry were not successful. The
ESI-MS spectrum of the 2-BArF/5 1:1 mixture in CD2Cl2 solution
showed an intensive signal at 329.2315, which corresponds to the
ethyl thiiranium ion of adamantylidene adamantane (7,
Scheme 5), and a less intense signal corresponding to the
bromodiethyl sulfonium ion. Furthermore, based on the broad
NMR signals indicating a dynamic process, it cannot be excluded
that several, rapidly equilibrating compounds are present.
Figure 4. Solid state structure of ethylthiiranium ion 7. Thermal ellipsoids are
shown with 30% probability. Hydrogen atoms have been omitted for clarity.
The mechanism of formation of 7 out of 2-BArF and 5 is currently
not clear. One possibility would be that a bromodiethyl sulfonium
ion is released by the reaction of 2-BArF and 5, which could
subsequently react with excess 5 to give ethylsulfenyl bromide
and a triethylsulfonium ion. Sulfenylation of 1 by ethylsulfenyl
bromide would then provide 7 (Scheme S1 in the supporting
information).
Scheme 5. Reaction of 2-BArF with diethyl sulfide (5).
In further investigations, 2-BArF was treated with an excess of
five (for NMR experiments) or ten equivalents (for crystallization
experiments) of diethyl sulfide. The NMR spectra with excess 5
show signals for three different ethyl groups (Figure 3, D), one at
3.07 ppm, one very intense signal at 2.70 and one at 2.58 ppm.
The most intense signal, which should belong to the excess
diethyl sulfide was still shifted compared to pure diethyl sulfide. A
second ethyl group was identified to belong to the thiiranium ion
7, which was formed in almost quantitative yield under these
conditions. The identity of the third ethyl group signal is not fully
clear, but its chemical shifts are significantly different from ethyl
bromide, while being similar to literature data for triethyl sulfonium
triflate.[22]
On a preparative scale, 7 could be obtained as crystalline solid in
63% yield simply by mixing 2-BArF in dichloromethane with ten
equivalents of 5 followed by vapor diffusion of pentane into the
mixture (Scheme 5). A single crystal was analyzed by an X-ray
diffraction study that confirmed the structure of 7 to be the ethyl
thiiranium ion (Figure 4). Bond length and angles in the solid-state
structure of 7 are very similar to values previously observed for
the corresponding S-phenyl thiiranium ion of adamantylidene
adamantane.[23]
Bromiranium-induced polyene cyclizations
To investigate the potential of 2-BArF as brominating agent in
organic synthesis, several examples of bromiranium-induced
polyene cyclizations were investigated (Table 1). Initial
investigations were conducted on homogeranyl benzene 8a.[12i, 14]
This substrate has been investigated previously, and using
conventional electrophilic halogenating agents such as NBS or
TBCO only low yields of tricyclic products are obtained and main
products are either simple addition products without cyclization or,
especially in methods employing a Lewis basic promoter or
catalyst, monocyclization products.[12,14,16] This makes an
additional, Brønstedt acid-catalyzed cyclization necessary to
reach tricyclic product 9a.[12i, 16] Good yields of 9a can be obtained
directly by using BDSB, however, this requires the use of
nitromethane as solvent.[14]
When homogeranyl benzene 8a was treated with 2-BArF in
dichloromethane without the addition of a base, the desired
cyclization product 9a was only obtained in varying, but generally
low quantities. Instead, the proton-induced cyclization product 10
proved to be the main component of the reaction mixture. Its
formation was attributed to the in situ formation of the super acid
When 2-BArF was treated with one equivalent of
tetrahydrothiophene or one equivalent of dibutylselenide, similar
behavior than with diethyl sulfide was observed. The CH2-groups
next to the sulfur/selenium atom showed very significant shifts in
the proton NMR and some proton and 13C-NMR signals became
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HBArF (or, more likely, its aryl ring adduct) in the last step of the
bromocyclization, a SEAr reaction (see supporting information for
a likely mechanism, Scheme S2).
state according to the Stork-Eschenmoser hypothesis.[24]
Cyclization of homogeranyl benzenes with substituents on the
aromatic ring such as an isopropyl group (8b) or a methoxy group
(8c) provided similar results. Starting material 8b with an
isopropyl-substituent provided tricyclic 9b as the main product,
Table 1. Cyclization of homogeranyl benzenes
brominating agent.a
8
using 2-BArF as
but isolation in high purity required
a lengthy column
chromatography to give 9b in moderate 29% yield (Table 1, entry
2). Crystallization of the product from pentane was possible and
a resulting crystal could be analyzed by X-ray diffraction. The solid
state structure of 9b showed relative stereochemistry identical to
9a. Starting material 8c provided product 9c with about 90% purity
in acceptable yield (49%, Table 1, entry 3). Again, crystallization
from acetone led to pure product in lower overall yield (30%).[25]
Entry
Starting
material
Product
Yield
(%)
1
8a (R=H)
64b (26)c
9a
Scheme 6. 2-BArF-induced cyclization of prenylated anisole 11 and N-tosyl
indole 14.
2
8b (R=iPr)
29d
2-BarF was also used to cyclize alkenyl-substituted aromatics.
Following the standard procedure, alkene 11 was reacted to give
12 as the main product in 34% yield (Scheme 6). The relatively
low yield is caused by the sensitivity of the product 12 to the
elimination of HBr under the reaction conditions. The
corresponding elimination products 13a/b could be observed by
GC-MS, however, isolation in pure form proved to be difficult.
Similarly, indole 14 carrying a prenyl side chain in the 3-position
was cyclized to give cyclopentindole 15. When 1.0 equivalent of
2-BarF was used under standard conditions, about 60%
conversion was observed and the product could be isolated in
33% yield.
9b
3
8c (R=OMe)
50b (30)c
[a] 8 (0.4 mmol) and HMDS (0.8 mmol) in CH2Cl2 (0.13 M) were treated with
2-BArF (0.4 mmol) at 0 °C for 3 h. [b] Yield after column chromatography to
give products of about 90% purity. [c] In brackets: yield after crystallization
from acetone. [d] Isolated yield after column chromatography.
Conclusions
To prevent this proton-induced cyclization, a bulky base was
added to the bromocyclization. 2,6-Di(tbutyl)-4-methyl pyridine or
hexamethyldisilazane (HMDS) worked equally well and enabled
efficient bromocyclization reactions. The main product was now
indeed the fully cyclized 9a. However, it was accompanied by
small amounts of a complex mixture of minor byproducts.
Purification by column chromatography enabled the isolation of
the main cyclization product 9a with about 90% purity in good
yield of 64% (Table 1, entry 1). The isolation of highly pure 9a was
possible by crystallization from acetone, however, this was
The results show that stable bromiranium ions based on
adamantylidene
adamantane
with
weakly-coordinating
counterions can be easily obtained by anion exchange from
known 2-Br3 or by direct synthesis. Bromiranium salt 2-BArF
comprising the tetrakis(3,5-bistrifluoromethylphenyl) borate anion
shows high reactivity as an halogenating agent for the
halogenation of heteroatoms as well as of unsaturated
hydrocarbons. The experiments with 2-BArF as halogenating
agent for heteroatoms show that this type of reagent does not only
act as halogenating agent for alkenes via alkene-to-alkene
transfer, but does directly react with more or less Lewis-basic
heteroatoms. Therefore, this halogenating agent could be of high
accompanied by
a significant loss of yield (26% after
crystallization). The relative configuration of 9a could be
established by X-ray structure analysis. The observed
configuration of 9a could result from a chair-chair-type transition
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1-Methoxy-4-(4-methylpent-3-en-1-yl)benzene
(11):
Magnesium
interest in element organic chemistry, where it could provide a
cationic bromine atom ion combined with a weakly-coordinating
counterion and a sterically very demanding (and therefore not
very reactive) alkene as the only side product.
turnings (0.73 g, 30.0 mmol, 1.50 equiv.) were placed in a Schlenk-flask
and Et2O (15 mL) and THF (15 mL) were added. The suspension was
cooled to 0 °C and 4-methoxybenzyl chloride (4.95 mL, 30.0 mmol, 1.0
equiv.) in a Et2O (15 mL)/THF (15 mL) mixture was added dropwise. After
warming to RT the reaction mixture was stirred for 1 h. The reaction
mixture was now slowly added to a precooled solution of 1-bromo-3-
methyl-2-butene (1.20 mL, 10.0 mmol, 1.0 equiv.) in THF (30 mL) at -78 °C
and the resulting mixture was stirred at -78 °C for 1 h. The mixture was
allowed to warm to RT and stirred overnight. Saturated aqueous NH4Cl-
solution was added and the mixture was stirred for 10 min. The mixture
was extracted three times with CH2Cl2, the combined organic phases were
washed with saturated aqueous NaHCO3-solution and brine and dried over
magnesium sulfate. The crude product was purified by column
chromatography (pentane) followed by warming to 60 °C for 3 h in vacuo
(to remove 3-OMe toluene) to provide the product (0.96 g, 5.05 mmol,
51 %) as a colourless liquid. 1H-NMR (400 MHz, CDCl3, 296 K): δ = 7.25
(m, 1H, Ar-H), 6.87-6.76 (m, 3H), 5.27-5.20 (m, 1H), 3.84 (s, 3H), 2.70-
2.64 (m, 2H), 2.42-2.29 (m, 2H), 1.75 (s, 3H), 1.64 (s, 3H). 13C-NMR (101
MHz, CDCl3, 296 K): δ = 159.7, 144.2, 132.2, 129.3, 123.9, 121.0, 114.3,
111.1, 55.2, 36.3, 30.1, 25.8, 17.8. MS-ESI-EM (with added AgCO2CF3):
m/z = 297.0405 calculated for C13H18OAg+ ([M+Ag]+), found: 297.0405.
As a halogenating agent in organic chemistry, 2-BArF shows how
important a weakly-nucleophilic counterion for polyene-type
cyclizations is. Whereas most other halogenating agents are not
capable to induce double cyclization of homogeranyl benzene,
with 2-BArF the tricyclic product is obtained with good efficiency.
In contrast to BDSB, this is already the case with dichloromethane
as solvent and no polar solvent such as nitromethane is
necessary to increase reactivity or solubility.
Experimental Section
General Methods: All manipulations involving air- or moisture-sensitive
reagents or intermediates were carried out in dried glassware under argon
and performed by using standard Schlenk and drybox techniques. Dry and
oxygen-free solvents were employed for reactions. Solvents for extraction
and flash chromatography were distilled before use. Commercial
compounds were purchased by standard suppliers (ABCR, Acros, Alfa
Aesar, Sigma-Aldrich or TCI) and used without further purification. Flash
chromatography (FC) was carried out on silica gel 60 (40-63 µm) under
argon pressure (approximately 1.1-1.5 bar). IR spectra were recorded with
a Digilab FTS 4000 with an attenuated total reflectance (ATR) unit. The
NMR spectroscopic data were recorded on a Bruker DPX-300, Agilent
DD2 600, Bruker AVANCE I 400, Bruker AVANCE III 400 or Bruker
AVANCE II 200 spectrometer. Chemical shifts (δ) are reported in ppm
relative to SiMe4 (1H, 13C), 85% H3PO4 (31P) and referenced to either the
residual solvent signal for 1H-NMR or the solvent signal for 13C-NMR
(δ = 7.26/77.2 ppm for CDCl3; δ = 5.32/54.0 ppm for CD2Cl2) or internally
by the instrument after locking and shimming to the deuterated solvent (11B,
19F, 31P). NMR multiplicities are abbreviated as follows: s = singlet, d =
doublet, t = triplet, sept = septet, m = multiplet, br = broad signal. Mass
spectra with electrospray ionization (MS-ESI-EM, m/z) were recorded on
N-Tosyl-3-prenylindole (14): 3-Prenylindole[x] (200 mg, 1.08 mmol, 1.0
equiv.) was dissolved in CH2Cl2 and stirred at RT. NBu4Cl (30 mg, 0.11
mmol, 10 mol%), powdered NaOH (86 mg, 2.16 mmol, 2.0 equiv.) and
tosyl chloride (226 mg, 1.19 mmol, 1.2 equiv.) were added subsequently
and the resulting mixture was stirred at RT for 1 h. Hydrochloric acid (2 mL,
1 M) and water (10 mL) were added, the phases were separated and the
aqueous phase was extracted with CH2Cl2. The organic phases were
combined, dried over sodium sulfate and the solvent was removed in
vacuo. The crude product was purified by column chromatography
(pentane:Et2O:CH2Cl2 = 80:15:5) to give 15 as a coluorless oil (319 mg,
0.94 mmol, 87%) IR (film): 2977, 2922, 1597, 1447, 1367, 1187, 1121,
1095, 974, 744, 669. 1H-NMR (400 MHz, CDCl3, 295 K) δ = 8.02-7.96 (m,
1H), 7.78-7.72 (m, 2H), 7.49-7.45 (m, 1H), 7.34-7.28 (m, 2H), 7.27-7.18
(m, 3H), 5.41-5.34 (m, 1H), 3.38-3.32 (m, 2H), 2.33 (s, 3H), 1.78 (s, 3H),
1.73 (s, 3H). 13C-NMR (101 MHz, CDCl3, 295 K): δ = 144.8, 135.6, 135.5,
133.8, 131.1, 129.9, 126.9, 124.7, 123.1, 122.9, 122.8, 120.9, 119.7, 113.8,
25.8, 24.0, 21.7, 18.0. MS-ESI-EM: calculated for C20H21NNaO2S+:
362.1185 ([M+Na+]), found: 362.1193.
a Bruker MicroTof or a Thermo Scientific Orbitrap LTQ XL (Nanospray).
(E)-3,7-Dimethylocta-2,6-diene-1-yldiethylphosphate[20]
(3c), homogeranyl benzene[20] (8a) and 1-methoxy-4-homogeranyl
benzene[12a] (8c) were prepared following literature procedures.
[16a]
, P(NBiPr)Ph2
Preparation of 2-BArF: In a Schlenk-flask 2-Br3 (0.86 g, 1.46 mmol, 1.0
equiv.)[5] and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (1.30 g,
1.46 mmol, 1.0 equiv.) were dissolved in CH2Cl2 (50 mL) and stirred at RT
overnight. The reaction mixture was filtered and the solvent was removed
in vacuo. The crude product was crystallized from CH2Cl2 (50 mL) and
hexane (50 mL) at -20 °C overnight. The precipitate was filtered of and
dried in vacuo to give the desired product as a pale yellow solid (1.40 g,
11.6 mmol, 79%). MP: 129-131 °C. IR (solid): 2940, 2869, 2362, 1610,
1457, 1355, 1278, 1126, 889, 839, 712, 682. 1H-NMR (600 MHz, CD2Cl2,
299 K): δ = 7.76-7.72 (m, 8H), 7.58 (s, 4H), 3.07 (s, 4H), 2.59-2.50 (m, 8H),
2.29-2.17 (m, 12H), 2.04 (s, 4H). 13C-NMR (126 MHz, CD2Cl2, 193 K): δ =
(E)-1-(4’,8’-dimethylnona-3’,7’-diene-1’-yl)-4-isopropylbenzene (8b):
Magnesium turnings (1.09 g, 45.0 mmol, 1.50 equiv.) were placed in a
Schlenk-flask, THF (30 mL) was added and the suspension was cooled to
0 °C. 4-Isopropylbenzyl chloride (4.95 mL, 30.0 mmol, 1.00 equiv.) in THF
(40 mL) was added dropwise. After warming to RT the reaction mixture
was stirred for 1 h. The reaction mixture was now slowly added to a
precooled solution of (E)-3,7-dimethylocta-2,6-diene-1-yldiethylphosphate
(4.35 g, 15.0 mmol, 1.00 equiv.) in THF (40 mL) at -45 °C. The reaction
mixture was allowed to warm up RT and stirred overnight. After addition of
NH4Cl (aq., sat.) and H2O the mixture was stirred for 10 min. The aqueous
phase was extracted with EtOAc:pentane 1:2 (3x75 mL) and the combined
organic phases were washed with NaHCO3 (aq., sat.) and NaCl (aq., sat.)
and dried over MgSO4. Purification of the crude product by column
chromatography (cyclohexane) provided the product as a colorless oil
(2.41 g, 8.91 mmol, 59 %). IR (film): 2960, 2923, 1513, 1449, 1382, 1108,
1056, 984, 822, 631, 549, 532, 511, 509. 1H-NMR (400 MHz, CDCl3,
295 K): δ = 7.17-7.11 (m, 4H), 5.23-5.17 (m, 1H), 5.13-5.07 (m, 1H), 2.88
(sept, 3J = 6.9 Hz, 1H), 2.65-2.57 (m, 2H), 2.34-2.25 (m, 2H), 2.11-2.03 (m,
2H), 2.02-1.95 (m, 2H), 1.69 (d, 3J = 0.9 Hz, 3H), 1.61 (s, 3H), 1.57 (s, 3H),
1.25 (d, 3J = 6.9 Hz, 6H). 13C-NMR (101 MHz, CDCl3, 295 K): δ = 146.3,
139.9, 135.8, 131.5, 128.5, 126.4, 124.5, 123.9, 39.9, 35.9, 33.8, 30.2,
26.9, 25.9, 24.2, 17.8, 16.1. MS-ESI-EM (with added AgCO2CF3): m/z =
377.1393 calculated for C20H30Ag+ ([M+Ag]+), found: 377.1398.
1
2
161.5 (q, JC-B = 49 Hz), 157.0 (s), 134.3 (s), 128.3 (qm, JC-F = 32 Hz),
1
122.0 (q, JC-F = 273 Hz), 117.3 (m), 41.4, 36.8, 35.8, 26.3. 11B-NMR
(160 MHz, CD2Cl2, 193 K): δ = -6.8. 19F-NMR (470 MHz, CD2Cl2, 193 K):
δ = -62.4. MS-ESI-EM: m/z = 347.1369 calculated for C20H28Br+ ([M]+),
found: 347.1372; 863.1 calculated for C32H12F24B- ([M]-), found: 863.2.
Elementary analysis: C: 51.55 H: 3.33 calculated for C52H40BBrF24,
found: C: 51.56 H: 3.41. For crystal structure analysis data of 2-BArF, see
supporting information and CCDC 1540672.
Preparation of 2-Al2Br7: In a Schlenk-flask AlBr3 (750 mg, 2.81 mmol,
2.0 equiv.) and adamantylidene adamantane 1 (377 mg, 1.41 mmol,
1.0 equiv.) were dissolved in 1,2-dichlorobenzene (10 mL) at 0 °C. Br2
(36 µL, 1.41 mmol, 1.0 equiv.) was added und the reaction mixture was
allowed to reach RT. Dry pentane (20 mL) was added to crystallize the
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10.1002/chem.201701643
Chemistry - A European Journal
FULL PAPER
product overnight at -20 °C. The supernatant solvent was removed and the
precipitate was washed with dry pentane to give the desired product as a
yellow solid (890 mg, 0.93 mmol, 66 %). MP: 115-117 °C. 1H-NMR
(400 MHz, CD2Cl2, 299 K): δ = 3.13 (s, 4H, H-2), 2.69-2.58 (m, 4H, H-3’a),
2.57-2.51 (m, 4H, H-3a), 2.34-2.20 (m, 12H, H-3b, H-3’b + H-4, H-4’), 2.07
(s, 4H, H-5). 13C-NMR (101 MHz, CD2Cl2, 299 K): δ = 159.8 (C-1), 44.0
(C-3‘), 41.5 (C-3), 38.5 (C-2), 37.1 (C-5), 27.5 (C-4, C-4‘). MS-ESI-EM:
m/z = 347.1369 calculated for C20H28Br+ ([M]+), found: 347.1370. For
crystal structure analysis data of 2-Al2Br7, see supporting information and
CCDC 1540669.
tetrahydrothiophene, thioanisole and dibutylselenide[x] in place of
diethylsulfide (see supporting information for details and NMR spectra).
Preparation of 7: 2-BArF (125 mg, 0.1 mmol) was placed in a 2 mL glass
vial and dissolved in CH2Cl2 (1 mL). Diethyl sulfide (5) (0.1 mL, 1.0 mmol,
10.0 equiv.) was added, the solution was briefly shaken and the glass vial
was placed in a 10 mL glass vial. The area between the larger and the
smaller vial was filled with pentane (ca. 2 mL) and the larger vial was
closed with a screw cap and left standing at RT. After large crystals had
appeared the larger vial was opened, the small vial was removed and the
remaining solvent in the small vial was removed via a pipette. The
colourless, crystalline solid from the small vial was dried in vacuo to give 7
Bromination of phosphines with 2-BArF
1
(75 mg, 0.63 mmol, 61%). H-NMR (600 MHz, CD2Cl2, 299 K): δ = 7.74-
7.71 (m, 8H), 7.56 (s, 4H), 2.58 (q, 3JH-H = 7.8 Hz, 2H), 2.35-2.05 (m, 22H),
1.97-1.88 (m, 4H), 1.69-164 (m, 1H), 1.61 (t, 3JH-H = 7.8 Hz, 3H). 13C-NMR
(151 MHz, CD2Cl2, 299 K): δ = 162.3 (q, 1JC-B = 50 Hz), 135.3, 129.4 (qm,
2JC-F = 32 Hz), 125.2 (q, 1JC-F = 273 Hz), 118.0 (m), 97.1, 39.4, 38.9, 38.7,
37.6, 36.5, 34.5, 31.1, 27.0, 27.0, 24.7, 13.1. 11B-NMR (160 MHz, CD2Cl2,
299 K): δ = -6.6. 19F-NMR (564 MHz, CD2Cl2, 299 K): δ = -62.9. MS-ESI-
EM: m/z = 329.2298 calculated for C22H33S+ ([M]+), found: 329.2313. For
crystal structure analysis data of 7, see supporting information and CCDC
1540673.
The suitability of 2-BArF for the bromination of phosphines was
investigated by NMR experiments according to the following general
procedure: The phosphine (0.1 mmol) was added to a stirred solution of 2-
BArF (0.1 mmol) in CD2Cl2 (1 mL). After stirring the solution for 5 minutes
at room temperature, NMR analysis was performed. In all cases, the
reaction mixture showed the clean formation of the bromophosphonium
salt. No decomposition of the product was observed upon heating the NMR
tube at 80 °C overnight.
Compound 4a: 1H-NMR (400 MHz, CD2Cl2, 300 K): δ = 7.94-7.88 (m, 3H,
aryl-H), 7.78-7.71 (m, 6H, aryl-H and 8H, BArF-H), 7.70-7.62 (m, 3H, aryl-
H), 7.55 (s, 4H, BArF-H). 13C-NMR (101 MHz, CD2Cl2, 300 K): δ = 162.4
(q, 1JC-B = 50 Hz, BArF-C1), 137.9 (d, JC-P = 3 Hz, aryl-C), 135.4 (s, BArF-
C2,C6), 134.3 (d, JC-P = 13 Hz, aryl-C), 131.4 (d, JC-P = 15 Hz, aryl-C),
129.5 (m, BArF-C3,C5), 125.1 (q, 1JC-F = 274 Hz, BArF-CF3), 119.5 (d, JC-P
= 88 Hz, aryl-C), 118.1 (m, BArF-C4). 31P-NMR (162 MHz, CD2Cl2, 300 K):
δ = 52.9 (s). MS-ESI-EM: m/z = 342.0168 calculated for C18H16BrP+ ([M]+),
found: 342.0167.
General procedure for bromocylization of poylenes: In a Schlenk-flask
starting material 8 (0.40 mmol, 1.00 equiv.) and HMDS (129 mg, 0.8 mmol,
2.00 equiv.) were dissolved in CH2Cl2 (3 mL) and cooled to 0 °C. 2-BArF
(0.20 mmol, 242 mg, 1.00 equiv.) was added and the resulting solution
stirred for 3 h. After addition of NaHCO3 (aq., 10%, 2.5 mL) and Na2SO4
(aq., 10%, 2.5 mL) the aqueous phase was extracted with CH2Cl2 (3x5 mL),
the combined organic phases were dried over MgSO4 and the solvent
removed in vacuo. Purification by column chromatography provided the
product 9.
Compound 4b: 1H-NMR (400 MHz, CD2Cl2, 300 K): δ = 7.83-7.77 (m, 3H,
aryl-H), 7.76-7.71 (m, 8H, BArF), 7.63-7.57 (m, 3H, aryl-H), 7.56 (s, 4H,
BArF), 7.52-7.45 (m, 3H, aryl-H), 7.42-7.32 (m, 3H, aryl-H), 2.40 (s, 3H,
rac-(2S*,4aS*,10aR*)-2-Bromo-1,2,3,4,4a,9,10,10a-octahydro-1,1,4a-
trimethyl-phenantrene (9a): Following the general procedure 8a (91 mg,
0.40 mmol, 1.0 equiv.) was cyclized. Purification by column
chromatography (pentane) provided 9a as a colourless liquid (79 mg,
0.26 mmol, 64%) containing about 10% impurities. Crystallization from
acetone provided 9a as a colourless crystalline solid (32 mg, 0.10 mmol,
1
CH3). 13C-NMR (101 MHz, CD2Cl2, 300 K): δ = 162.4 (q, JC-B = 50 Hz,
BArF-C1), 145.1 (d, JC-P = 10 Hz, aryl-C), 137.9 (d, JC-P = 3 Hz, aryl-C),
135.7 (d, JC-P = 16 Hz, aryl-C), 135.4 (s, BArF-C2,C6), 135.3 (d, JC-P
=
=
2
13 Hz, aryl-C), 129.5 (qm, JC-F = 32 Hz, BArF-C3,C5), 128.7 (d, JC-P
1
1
26%). MP.: 97 °C. H-NMR (300 MHz, CDCl3, 299 K): δ = 7.23-7.19 (m,
15 Hz, aryl-C), 125.2 (q, JC-F = 273 Hz, BArF-CF3), 118.1 (m, BArF-C4),
116.8 (d, JC-P = 83 Hz, aryl-C), 23.2 (d, 3JC-P = 15 Hz, CH3). 31P-NMR (162
MHz, CD2Cl2, 300 K): δ = 46.4 (s). MS-ESI-EM: m/z = 383.0559 calculated
for C21H21BrP+ ([M]+), found: 383.0552.
1H), 7.17-7.04 (m, 3H), 4.11-4.02 (m, 1H), 3.02-2.83 (m, 2H), 2.44-2.22
(m, 3H), 2.03-1.95 (m, 1H), 1.90-1.76 (m, 1H), 1.66-1.55 (m, 1H), 1.53-
1.45 (m, 1H), 1.26 (s, 3H), 1.17 (s, 3H), 1.08 (s, 3H), 13C-NMR (101 MHz,
CDCl3, 299 K): δ = 148.8, 134.8, 129.2, 126.0, 125.7, 124.6, 69.0, 51.3,
40.2, 40.0, 38.0, 31.7, 30.9, 30.7, 25.0, 20.7, 18.4, MS-ESI-EM (with
added AgOTFA): m/z = 413.0029 calculated for C17H23BrAg+ ([M+Ag]+),
found: 413.0029. For crystal structure analysis data of 5a, see supporting
information and CCDC 1540671.
Compound 4c: 1H-NMR (400 MHz, CD2Cl2, 300 K): δ = 7.91-7.82 (m, 4H,
Ph), 7.81-7.72 (m, 10H, Ph, BArF), 7.70-7.63 (m, 4H, Ph), 7.62-7.59 (m,
2H, aryl-H), 7.56 (s, 4H, BArF), 7.43-7.36 (m, 2H, aryl-H), 4.74 (sept, 3JH-
H = 7.0 Hz, 2H, CHMe2), 1.45 (d, 3JH-H = 7.0 Hz, 12H, CH3). 13C-NMR (101
MHz, CD2Cl2, 300 K): δ = 162.4 (q, 1JC-B = 50 Hz, BArF-C1), 137.9 (d, JC-P
= 3 Hz, Ph), 135.4 (s, BArF-C2,C6), 132.0 (d, JC-P = 13 Hz, Ph), 130.6 (d,
JC-P = 15 Hz, Ph), 129.5 (m, BArF-C3,C5), 125.2 (q, 1JC-F = 273 Hz, BArF-
CF3), 125.1 (s, aryl-C), 118.1 (m, BArF-C4), 113.9 (s, aryl-C), 50.2
(CHMe2), 20.8 (CH3). 31P-NMR (162 MHz, CD2Cl2, 300 K): δ = 16.8 (s).
MS-ESI-EM: m/z = 480.1204 calculated for C25H28N3BrP+ ([M]+), found:
480.1201.
rac-(2S*,4aS*,10aR*)-2-Bromo-1,2,3,4,4a,9,10,10a-octahydro-6-iso-
propyl-1,1,4a-trimethylphenantrene (9b): Following the general
procedure 8b (108 mg, 0.4 mmol, 1.0 equiv.) was cyclized. Purification by
column chromatography (pentane) provided 9b as a colourless solid
(39 mg, 0.11 mmol, 29%). MP.: 112-113 °C, IR (film): 2958, 2871, 1499,
1461, 1392, 1378, 1264, 1184, 909, 733, 646, 540, 535, 530, 525, 519,
1H-NMR (400 MHz, CDCl3, 295 K): δ = 7.05 (s, 1H), 7.02-6.98 (m, 2H),
4.05 (dd, 3JH-H = 12.4, 4.4 Hz, 1H), 2.92-2.79 (m, 3H), 2.43-2.23 (m, 3H),
Bromination of diethyl sulfide with 2-BArF
1.99-1.91 (m, 1H), 1.86-1.75 (m, 1H), 1.67-1.58 (m, 1H), 1.47 (dd, 3JH-H
=
12.0, 2.1 Hz, 1H), 1.25 (d, 4JH-H = 0.5 Hz, 3H), 1.22 (d, 3JH-H = 6.9 Hz, 6H),
1.15 (s, 3H), 1.06 (s, 3H), 13C-NMR (75 MHz, CDCl3, 295 K): δ = 148.7,
146.5, 132.3, 129.1, 123.8, 122.7, 69.2, 51.5, 40.2, 40.0, 38.1, 34.2, 31.7,
30.7, 30.6, 25.1, 24.3, 24.3, 20.8, 18.4. MS-ESI-EM (with added AgOTFA):
m/z = 455.0498 calculated for C20H29BrAg ([M+Ag]+), found: 455.0498. For
crystal structure analysis data of 5b, see supporting information and CCDC
1540670.
The suitability of 2-BArF for the bromination of diethyl sulfide (5) was
investigated by NMR experiments according to the following general
procedure: 2-BArF (36 mg, 0.03 mmol) was dissolved in CD2Cl2 (1 mL).
This was followed by the addition of diethyl sulfide (5) in different
quantities: sample A (Figure 3): no addition; sample C (Figure 3): 3.2 µL
(0.03 mmol, 1.0 equiv.); sample D (Figure 3): 16 µL (0.15 mmol, 5.0 equiv.).
The solutions were mixed by shaking and submitted to NMR
measurements. Corresponding experiments were also carried out with
This article is protected by copyright. All rights reserved.

10.1002/chem.201701643
Chemistry - A European Journal
FULL PAPER
rac-(2S*,4aS*,10aR*)-2-Bromo-1,2,3,4,4a,9,10,10a-octahydro-6-meth-
oxy-1,1,4a-trimethyl-phenantrene (9c): Following the general procedure
8c (103 mg, 0.4 mmol, 1.0 equiv.) was cyclized. Purification by column
chromatography (pentane) provided 9c as a colourless liquid (67 mg,
0.20 mmol, 50%) containing about 10% impurities. Crystallization from
acetone provided 9c a colourless crystalline solid (41 mg, 0.12 mmol,
8260-8265; c) V. I. Teberekidis, M. P. Sigalas, Tetrahedron 2002, 58,
6171-6178; d) B. K. Ohta, R. E. Hough, J. W. Schubert, Org. Lett. 2007,
9, 2317-2320; e) B. K. Ohta, T. M. Scupp, T. J. Dudley, J. Org. Chem.
2008, 73, 7052-7059; f) X. S. Bogle, D. A. Singleton, J. Am. Chem. Soc.
2011, 133, 17172-17175.
[6]
[7]
[8]
J. Strating, J. H. Wieringa, H. Wynberg, Chem. Commun. 1969,16, 907-
908.
1
30%). MP.: 110-111 °C, H-NMR (300 MHz, CDCl3, 299 K): δ = 6.98 (d,
3JH-H = 8.4 Hz, 1H), 6.75 (d, 3JH-H = 2.6 Hz, 1H), 6.68 (dd, 3JH-H = 8.4, 2.6
H. Slebocka-Tilk, R. G. Ball, R. S. Brown, J. Am. Chem. Soc. 1985, 107,
4504-4508.
3
Hz, 1H), 4.05 (dd, JH-H = 12.5, 4.1 Hz), 3.77 (s, 3H), 2.96-2.74 (m, 2H),
2.39-2.22 (m, 3H), 2.01-1.90 (m,1H), 1.86-1.75 (m, 1H), 1.66-1.58 (m, 1H),
1.45 (dd, 3JH-H = 12.0,2.1 Hz, 1H), 1.25 (d, 4JH-H = 0.5 Hz, 3H), 1.16 (s, 3H),
1.06 (s, 3H).13C-NMR (75 MHz, CDCl3, 299 K): δ = 157.9, 150.1, 130.0,
127.1, 111.2, 110.3, 69.0, 55.4, 51.4, 40.2, 40.0, 38.2, 31.7, 30.7, 30.1,
24.9, 20.9, 18.4. MS-ESI-EM (with added AgOTFA): m/z = 445.0123,
calculated for C18H25BrOAg+ ([M+Ag]+), found: 445.0122.
a) A. J. Bennet, R. S. Brown, R. E. D. McClung, M. Klobukowski, G. H.
M. Aarts, B. D. Santarsiero, G. Bellucci, R. Bianchini, J. Am. Chem. Soc.
1991, 113, 8532-8534; b) R. S. Brown, R. W. Nagorski, A. J. Benuet, R.
E. D. McClung, G. H. M. Aarts, M. Klobukowski, R. McDonald, B. D.
Santaniero, J. Am. Chem. Soc. 1994, 116, 2448-2456.
R. S. Brown, Acc. Chem. Res. 1997, 30, 131-137.
[9]
[10] a) A. A. Neverov, R. S. Brown, Can. J. Chem. 1994, 72, 2540-2543; b)
A. A. Neverov, R. S. Brown, J. Org. Chem. 1996, 61, 962-968.
[11] a) A. A. Neverov, T. L. Muise, R. S. Brown, Can. J. Chem. 1997, 75,
1844-1850; b) D. Lenoir, N. Hertkorna, C. Chiappe, Tetrahedron Lett.
2004, 45, 3003-3005.
rac-2-bromo-6-methoxy-1,1-dimethyl-1,2,3,4-tetrahydronaphthaline
(12): According to the general procedure 11 (38 mg, 0.20 mmol,
1.0 equiv.) was cyclized. Purification by column chromatography (pentane)
provided 12 as a colourless solid (18 mg, 0.07 mmol, 34%). IR (film): 2973,
2947, 2836, 2364, 2337, 1578, 1469, 1451, 1438, 1261, 1081, 780, 740,
566, 558, 546, 531, 513. 1H-NMR (400 MHz, CDCl3, 296 K): δ = 7.06-7.00
[12] a) E. E. van Tamelen, E. J. Hessler, Chem. Comm. 1966, 13, 411-413;
b) T. Kato, I. Ichinose, A. Kamoshida, Y. Kitahara, J. Chem. Soc. Chem.
Commun. 1976, 518-519; c) L. E. Wolinsky, D. J. Faulkner, J. Org. Chem.
1976, 41, 597-600; d) A. G. González, J. D. Martín, C. Pérez, M. A.
Ramírez, Tetrahedron Lett. 1976, 17, 137-138; e) T. R. Hoye, M. J. Kurth
J. Org. Chem. 1978, 43, 3693-3697; f) H.-M. Shieh, G. D. Pretswich,
Tetrahedron Lett. 1982, 23, 4643-4646; g) T. Kato, M. Mochizuki, T.
Hirano, S. Fujiwara, T. Uyehara, J. Chem. Soc. Chem. Commun. 1984,
1077-1078; h) Y. Yamaguchi, T. Uyehara, T. Kato, Tetrahedron Lett.
1985, 26, 343-346; i) A. Sakakura, A. Ukai, K. Ishihara Nature 2007, 445,
900-903; j) C. Recsei, B. Chan, C. S. P. McErlean, J. Org. Chem. 2014,
79, 880-887.
3
(m, 1H), 6.68-6.64 (m, 1H), 6.64-6.60 (m, 1H), 4.33 (dd, JH-H = 9.3, 3.1
Hz, 1H), 3.74 (s, 3H), 3.04-2.90 (m, 1H), 2.89-2.74 (m, 1H), 2.35-2.13 (m,
2H), 1.48 (s, 6H). 13C-NMR (101 MHz, CDCl3, 296 K): δ = 158.9, 136.4,
131.3, 126.9, 121.8, 109.5, 68.5, 55.2, 39.4, 30.5, 30.1, 27.4, 25.8. MS-
ESI-EM (with added AgOTFA): m/z = 642.9978 calculated for
(C13H17BrO)2Ag+ ([2M+Ag]+), found: 642.9975.
rac-2-bromo-3,3-dimethyl-4-tosyl-1,2,3,4-tetrahydrocyclopenta[b]-
indole (15): According to the general procedure N-Ts-3-prenylindole 14
(140 mg, 0.40 mmol, 1.0 equiv.) was cyclized. Purification by column
chromatography (pentane/CH2Cl2: 1/1) provided 15 as a colourless oil
(55 mg, 0.13 mmol, 33%). IR (film): 2966, 2933, 1597, 1363, 1174, 1132,
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2
3
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H
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Acknowledgements
Financial support by the WWU Münster and the DFG (HE 6020/3-
1) is gratefully acknowledged.
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Keywords: alkenes • cations • cyclization • electrophilic addition
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Unidentified ethyl group: 1H-NMR (400 MHz, CD2Cl2): 3.07 (q, J = 7.1
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Sterically demanding bromiranium
ions with weakly coordinating
Christoph Ascheberg, Jonathan Bock,
Florenz Buß, Christian Mück-Lichtenfeld,
Constantin G. Daniliuc, Klaus
Bergander,[a] Fabian Dielmann[b] and
Ulrich Hennecke*[a] Author(s),
counterions are highly reactive
electrophilic brominating agents.
Despite their high reactivity these
reagents are stable and can be
applied in electrophilic halogenations
of alkenes as well as heteroatoms.
Corresponding Author(s)*
Page No. – Page No.
Stable Bromiranium Ions with
Weakly-Coordinating Counterions as
Efficient Electrophilic Brominating
Agents
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