Insight into the Activation of In Situ Generated Chiral RhI Catalysts and Their Application in Cyclotrimerizations

Article abstract of DOI:10.1002/chem.201703198

We report a detailed study concerning the efficient generation of highly active chiral rhodium complexes of the general structure [Rh(diphosphine)(solvent)₂]⁺ as well as their exemplary successful utilization as catalysts for cyclotrimerizations. Such solvent complexes could likewise be prepared from novel ammonia complexes of the type [Rh(diphosphine)(NH₃)₂]⁺. A valuable, feasible approach to generate novel chiral Rh^I complexes was found by in situ generation from Wilkinson's catalyst [RhCl(PPh₃)₃] with chiral P,N ligands. The generated catalysts led to moderate to good enantioselectivities and excellent yields in the cyclotrimerizations of triynes, showcasing their usefulness in the synthesis of axially chiral benzene derivatives.

Full text of DOI:10.1002/chem.201703198

A Journal of
Accepted Article
Title: Insight into the Activation of In Situ-Generated Chiral Rh(I)-
Catalysts and their Application in Cyclotrimerizations
Authors: Indre Thiel, Moritz Horstmann, Phillip Jungk, Sonja Keller,
Fabian Fischer, Hans-Joachim Drexler, Detlef Heller, and
Marko Hapke
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To be cited as: Chem. Eur. J. 10.1002/chem.201703198
Link to VoR: http://dx.doi.org/10.1002/chem.201703198
Supported by

Chemistry - A European Journal
10.1002/chem.201703198
FULL PAPER
Insight into the Activation of In Situ-Generated Chiral Rh(I)-
Catalysts and their Application in Cyclotrimerizations
[
a]
[a]
[a]
[a]
[a]
Indre Thiel, Moritz Horstmann, Phillip Jungk, Sonja Keller, Fabian Fischer, Hans-Joachim
[a]
[a]
[a,b]
Drexler, Detlef Heller* and Marko Hapke*
Abstract: We herein report a detailed study concerning the efficient
generation of highly active chiral rhodium complexes of the general
activated (pre)catalysts
+
structure [Rh(diphosphine)(solvent)] as well as their exemplary
High ∆ or hν
or both required
throughout the reaction
successful utilization as catalysts for cyclotrimerizations. Such
Co
solvent complexes could likewise be prepared from novel ammonia
Ind'CoL2
3
(L = diolefin or P(OR) )
"[Ind'Co(solvent)]"
+
complexes of the type [Rh(diphosphine)(NH
3
)
2
] . A valuable feasible
approach to generate novel chiral Rh(I)-complexes was found by in
situ generation from Wilkinson’s catalyst RhCl(PPh with chiral P,N
Treatment with H
solvent: CH Cl or
ClCH CH Cl
2
)
3 3
[Rh(cod) ](BF )
P
2
2
2
4
2
2
or
1/2 "[Rh(diphosphine)]2+"
ligands. The generated catalysts led to moderate to good
enantioselectivities and excellent yields in cyclotrimerizations of
triynes, showcasing their usefulness in the synthesis of axially chiral
benzene derivatives.
Rh
Ir
+
+
∗
∗
[RhCl(cod)]2/
(and other species)
P
P
AgBF4
in many cases ∆ required
throughout the reaction
[IrCl(cod)]2
"[IrCl(diphosphine)]"
P
Scheme 1. General but simplified methods to obtain active chiral group 9
metal catalysts for asymmetric cyclotrimerizations (Ind’ = chiral indenyl, cod =
Introduction
1,5-cyclooctadiene).
In recent years various new catalytic systems for
enantioselective [2+2+2] cycloaddition reactions have been
described, significantly broadening their scope of application.
The first chiral cobalt systems were derived from chiral
cyclopentadienyl (Cp) or indenyl (Ind) frameworks and activation
occurred by irradiation (Scheme 1, top). Hapke and Heller et al.
[1]
Nevertheless, enantioselective [2+2+2] cycloadditions are still a
challenging transformation, stimulating the search for novel
chiral catalysts and concepts for asymmetric induction. Out of
the many metals which in general catalyze this transformation,
the group 9 transition metals are clearly dominating the field of
the asymmetric cyclotrimerizations. However, within this group,
the properties of the catalyst precursor as well as the type and
efficiency of the active catalyst promoting achiral reactions may
vary significantly, as earlier investigations could already
[3]
reported
on
enantioselective
Co(I)-catalyzed
[2+2+2]
cycloaddition reactions under photochemical conditions, where a
chiral menthyl-derived indenyl-based Co(I)-complex was shown
to yield enantiomerically enriched atropisomers of axially chiral
[ 4 ]
1-aryl-5,6,7,8-tetrahydroisoquinolines and biarylphosphines.
The implementation of phosphite ligands in this chiral indenyl-
Co(I) complex led to an active catalyst without the imperative
need of irradiation, which was exemplified in the synthesis of
[
2 ]
demonstrate.
An overview of the general methods and
[5]
chiral pyridines from diynes/nitriles and triaryls from triynes.
conditions applied to generate active group 9 metal catalysts
used in enantioselective [2+2+2] cycloadditions is presented in
Scheme 1.
Only very recently we reported on the first in situ-generated
chiral Co(I)-catalyst based on P,N ligands, which was able to
promote asymmetric cyclizations of triynes to chiral triaryls under
[6]
mild conditions.
Shibata et al. were the first to explore the iridium-catalyzed
enantioselective [2+2+2] cycloaddition reaction in more depth.
They reported on various intramolecular cyclization reactions
such as e. g. the synthesis of chiral cyclohexa-1,3-dienes from
[
7 ]
enediynes,
atropisomeric chiral ortho-diarylbenzenes from
[
8]
triynes
hexaynes.
or axially chiral biaryl systems from tetraynes and
[a]
Dr. I. Thiel, S. Keller, Dr. P. Jungk, M. Horstmann, F. Fischer, Dr. H.-
J. Drexler, Prof. Dr. D. Heller, Prof. Dr. M. Hapke
Leibniz-Institut für Katalyse e.V. an der Universität Rostock (LIKAT
Rostock)
[
9 ]
Intermolecular cyclization reactions allowed the
construction of axially chiral tetraaryl compounds from diynes
[10]
and alkynes. The active catalyst was generally assembled by
combining a simple Ir(I)-precursor such as [IrCl(cod)] and a
2
chiral diphosphine ligand, achieving the cyclizations in good
yields and high enantioselectivities (Scheme 1, bottom).
However, in most cases higher reaction temperatures are
required as well as structural requirements to ensure
Albert-Einstein-Straße 29a, D-18059 Rostock (Germany)
E-mail: detlef.heller@catalysis.de; marko.hapke@catalysis.de
Johannes Kepler Universität Linz
Institut für Katalyse (INCA)
Altenberger Straße 69, A-4040 Linz (Austria)
E-mail: marko.hapke@jku.at
[b]
Supporting Information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201703198
FULL PAPER
configurational stability of the chiral products against
epimerization.
Rh(I)-catalyzed enantioselective [2+2+2] cycloaddition reactions
are the most popular and were explored in depth especially by
[
11]
Tanaka et al.
Like in the case of the Ir(I)-systems, Rh(I)-
](BF ) in combination with chiral
precursors of the type [Rh(cod)
2
4
Scheme 2. Solvent-dependent (pre)catalyst generation.
ligands such as BINAP and Segphos were applied. The
generated catalysts promoted intra- as well as intermolecular
[
2+2+2] cycloaddition reactions affording axially chiral
[
12
]
[ 13 ]
The prehydrogenation activation procedure was widely adopted
phthalides
and biaryls
in high yields and high
[
23]
for cyclization reactions with BINAP also by other groups, and
enantioselectivities. While the application of tetraynes for the
synthesis of axially chiral biaryls, bipyridines and bipyridones
resulted in moderate ee values and yields,
enantioselective cycloaddition between diynes and isocyanates
yielding 2-pyridones was accomplished with high enantiomeric
excesses.
enantioselective
carbaparacyclophanes using [Rh(cod)
diphosphine ligand in good yields. Gandon and Aubert et al.
reported on the enantioselective [2+2+2] cycloaddition of diynes
and sulfonimines yielding the respective 1,2-dihydropyridines
using partially the same catalytic system, although a substantial
part of reactions in the study were carried out using [RhCl(1,5-
[
24]
extended to a series of other achiral and chiral diphosphines.
The cyclization itself was usually carried out in chlorinated
alkanes (CH Cl , 1,2-dichloroethane), but examples in other
solvents, e.g. chlorobenzene, were also reported.
[
14
]
the
2
2
[
25]
[
15
]
The herein described investigations aim to provide detailed
information on the activation process of rhodium precatalysts
and to disclose a novel synthetic route not requiring an
activation step (prehydrogenation). The latter will be achieved by
using the Wilkinson catalyst as a novel Rh(I)-precatalyst. Such
chiral catalysts generated by both routes will be applied in
asymmetric cyclotrimerization reactions. Furthermore, the
optimization of the catalyst generation can minimize the amount
of precious metal and ligands involved, which is a criterion for
sustainable transition metal catalysis.
Recently, Tanaka et al. described the
synthesis
of
](BF
planar-chiral
and chiral
2
4
)
a
[16]
[
17
]
hexadiene)
2
]
as metal source.
Further examples
demonstrated the broadness of applicability of chiral in situ-
generated rhodium catalyst systems.
Depending on the metal, different protocols are applied to
[18]
generate the active chiral catalyst. The efficiency of such Results and Discussion
protocols though can vary significantly, thus affecting the
amount of metal effectively available for the catalytic reaction.
Unique is the case of Rh(I)-catalysts prepared in situ from a
diolefin precursor (Scheme 1, middle) and a diphosphine:
generation of the catalytic active species requires displacement
of 1,5-cyclooctadiene from the metal coordination sphere and
this is achieved through its hydrogenation into cyclooctene or
even cyclooctane. Although widely applied, this procedure may
not be selective and the type of Rh(I)-species it affords can be
strongly affected by the delicate role of the solvent used for the
prehydrogenation reaction. In initial work on Rh-catalyzed
Synthesis of solvent complexes and studies of their
catalytic activity and selectivity
Catalyst prehydrogenation and subsequent transformations like
the [2+2+2] cycloadditions are generally and conveniently
performed in chlorinated reaction solvents (CH Cl , ClH CCH Cl).
2
2
2
2
The latter however can react with the active catalyst leading to
undesired side reactions. Although typical of low-valent late
transition metal complexes, oxidative addition reactions with
[
26]
[
[
2+2+2] cycloadditions by Tanaka et al. the precursor
Rh(cod) ](BF ) was reacted with the chosen diphosphine for a
2
Cl ,
chlorinated alkanes and arenes
can also occur with Rh(I)-
2
4
complexes. In addition also the potential formation of two- and
trinuclear Rh(I)-complexes in such media can in principle not be
short period of time (usually 5 min) under argon in CH
2
[
27]
before being hydrogenated for 30 min. After removal of the
solvent, the substrates for the cyclotrimerization reaction are
excluded.
This can be illustrated by analyzing the mixture
obtained from precatalyst hydrogenation in CH Cl . Depending
2
2
added, usually also in
CH Cl .
2 2
a
non-coordinating solvent like
A closer view on the catalyst generation process
reveals details of the transformation. Combination of
diphosphine like BINAP and [Rh(cod)] (BF ) in a 1 : 1 ratio in
CH Cl resulted in the formation of [Rh(cod)(BINAP)](BF
When the latter is (pre-)hydrogenated, the arene-bridged dimer
on the reaction conditions (especially the reaction time before
hydrogenation and reaction temperature) other species, beside
the desired arene-bridged dimers [Rh(BINAP)] (BF ) , can be
[12,19,20]
a
2
4 2
2
4
2 4
detected in significant amounts, among which [Rh(BINAP) ](BF )
[
20,21]
[28]
2
2
4
).
should be cited. These problems do not occur in coordinating
solvents like methanol. Here, the formation and subsequent
splitting of the diphosphine-bridged dimeric Rh-species can be
avoided entirely.
[
22]
[
Rh(BINAP)]
solvents like MeOH, THF or acetone the corresponding solvent
complexes [Rh(BINAP)(solvent) ](BF are formed instead
Scheme 2).
2
(BF
4
)
2
is obtained quantitatively.
In coordinating
2
4
)
Systematic investigations into the hydrogenation of cationic
(
Rh(I)-complexes of the type [Rh(I)(P
proved that the prehydrogenation time is strongly affected by
͡
P)(diolefin)](anion) clearly
[
29]
the type of diphosphine.
More specifically, we have focused
on complexes relevant to asymmetric [2+2+2] cycloaddition
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Chemistry - A European Journal
10.1002/chem.201703198
FULL PAPER
reactions in order to evaluate the time required to fully
22.3% theoretically expected value after 20 min hydrogenation.
Beside the aforementioned complexes no further organometallic
Rh(I)-species could be detected, giving evidence for a very
clean conversion. This was also experimentally verified for the
[
30 ]
hydrogenate diolefin ligands, specifically cod,
and thus
generate the corresponding catalytically active solvent
complexes (Table 1).
[
33]
8
H -BINAP ligand.
arene-bridged dimer
Table 1. (Pseudo) first order rate constants and required times for complete
prehydrogenation (7 half-lives correspond to 99.2% cod conversion) of
[Rh(BINAP)] (BF )
2 4 2
81 %
[Rh(BINAP)(cod)](BF
19 %
4
)
͡
͡
[
[
Rh(PP)(diolefin)](BF
4
)
to
the
corresponding
solvent
complex
[
31]
Rh(PP)(MeOH) ](BF ) in MeOH at 25.0 °C.
2 4
[
a]
#
Diphosphine
Hydrogenation
rate constant
Time [min] for
99.2% of cod
conversion
Ref.
[
b]
[1/min]
5
5
50
45
40
35
30
25
20
ppm
-
-
-
-
-
-
-
1
1
1
2
1
2
1
1
2
3
4
5
6
7
8
9
BINAP
-BINAP
Segphos
2.3*10
8.5*10
4.7*10
6.3*10
1.6*10
2.8*10
6.8*10
21.1
5.7
[21]
31
1
Figure 1. P{ H} NMR spectrum after 20 min of hydrogenation at 25.0 °C
reaction of the complex [Rh(BINAP)(cod)](BF ) in CH Cl
H
8
this work
this work
this work
this work
[32]
4
2
2
.
10.3
77.0
30.3
173.3
7.1
DTBM-Segphos
Difluorphos
Et-Duphos
These results hinted at a discrepancy between the common
practice reported in the literature (30 min prehydrogenation time)
and the observed timeframe required for a full conversion (Table
1). The reported 30 min are either too short or too long,
regardless of the solvent used. In the first case only a portion of
the applied Rh precatalyst is activated and can be used in the
catalytic reaction, while in the latter case the prehydrogenation
time is too long, possibly allowing the formation of undesired di-
Duanphos
this work
[34b]
-
1
Tangphos
3.75*10
12.9
24.3
-
1
Et-Ferrotane
2.0*10
this work
[34,35]
and trinuclear hydrides.
Having evaluated the optimal prehydrogenation times required
for each catalyst to obtain the desired solvent complexes in
methanol, we set out to apply such complexes in [2+2+2]
cycloaddition reactions of alkynes. Cycloaddition of phenyl
acetylene (1) using the conventionally generated Rh-DTBM-
Segphos complex as catalyst (Scheme 3) was performed as
[
a]
An overview on the investigated ligands is given in the Supporting
[
b]
Information.
The determination of the rate constants was performed under
the following conditions: 0.01 mmol precatalyst, ca. 1.0 mmol cod and 15.0 mL
anhydrous MeOH, normal pressure, 25.0 °C. Detailed information is given in
ref. [29].
In MeOH large differences in the prehydrogenation times were
observed depending on the ligand. Even diphosphines with large
similarities in their structural features, e.g. BINAP vs. H
(
1
2 2
initial test reaction. The cyclization in CH Cl at 30 °C for 16 h
gave a conversion of 93% for 1 and the 1,2,4- and 1,3,5-
substituted phenyl isomers 2 and 3 were obtained in a 83:17
8
-BINAP
Table 1, entry 1 and 2) or Segphos vs. DTBM-Segphos (Table
, entry 3 and 4) required significantly different reaction times.
[
36]
ratio (Scheme 3, Condition A).
[
29]
This behaviour can be observed also for other solvents
including dichloromethane.
A more detailed discussion allows the comparison of measured
pseudo) first order rate constants for the cod hydrogenation in
(
CH
and H
2
Cl
2
at 25.0 °C for the ligands BINAP (k‘cod = 0.0751 1/min)
-BINAP (k‘cod = 0.275 1/min). Emanating from these rate
8
constants the required prehydrogenation times for BINAP and
-BINAP are 65 min and 18 min, respectively. To further
corroborate these findings experimentally, the stoichiometric
hydrogenation of [Rh(BINAP)(cod)](BF ) was stopped after 20
min reaction time, hydrogen removed by repeated freeze-thaw
H
8
4
3
1
cycles in vacuo and the P NMR of the solution measured
Figure 1). Because the allowed prehydrogenation time is
shorter than the 7-fold half-life time, the unreacted starting
complex [Rh(BINAP)(cod)](BF ) should be detectable. Indeed,
beside the arene-bridged dimer [Rh(BINAP)] (BF , the NMR
spectrum does show the presence of [Rh(BINAP)(cod)](BF ) in
9% relative amount, which is in very good agreement with the
(
Scheme 3. Rh-catalyzed [2+2+2] cycloaddition test reaction of
phenylacetylene under standard conditions for catalyst generation (Condition
4
A, applying 30 min pretreatment of complex solution in CH
2
Cl
and applying the complex “[Rh(DTBM-
)”; Condition B, solution of [Rh(DTBM-Segphos)(cod)](BF ) in
MeOH, 77 min pretreatment with H , addition of substrate).
2 2
with H
2
4
)
2
according to Tanaka et al.
Segphos)](BF
[19]
4
4
4
1
2
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Chemistry - A European Journal
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FULL PAPER
By carrying out the reaction in MeOH and raising the
prehydrogenation time to 77 min to fully convert the precatalyst
into the solvent complex [Rh(DTBM-Segphos)(MeOH) ](BF ) the
2 4
cycloaddition was significantly accelerated requiring only 45 min
at 25 °C to achieve the same conversion with lower catalyst
loading (Scheme 3, Condition B). In the latter case the
catalyst/substare ration can even be raised without detrimental
effects for the reaction, therefore requiring even less catalyst for
the reaction to occur.
7
(Sc,
(Sc,
R
R
p
)-Duanphos
)-Duanphos
MeOH/THF
THF/Aceton
30.9
18.6
24.0
20.6
47.8
94 (27.7)
7a
p
(2:1)
8
(S,S,R,R)-Tangphos
(S,S,R,R)-Tangphos
(R,R)-Et-Ferrotane
MeOH/THF
MeOH/THF
8a
(50°C)
9
MeOH/THF
69 (50.1)
[
a]
Experimental details and results for all investigated ligands are given in the
[
b]
experimental section.
without isolation. Isolated yields and selectivities for selected examples.
Selectivity screening from crude reaction mixtures
[
c]
Application of solvent complexes in asymmetric [2+2+2]
cycloaddition reactions
8
Interestingly, the commonly applied ligands BINAP and H -
BINAP (Table 2, entry
1 and 2) only gave insignificant
The observed high reactivity of chiral Rh-diphosphine solvent
complexes lead us to investigate their performance in
asymmetric cyclotrimerization reactions of suitable alkyne
substrates. The evaluation of their reactivity and selectivity in
comparison to the conveniently applied catalytic systems would
allow interesting insights into the influence of the catalyst
generation step. For this purpose we chose triyne 4a as
standard cyclization substrate for the evaluation of different
phosphine ligands and their solvent complexes under mild
conditions. The results for selected systems are displayed in
Table 2.
selectivities in the cyclization of 4a. The best result with a
selectivity of up to 50% ee was accomplished by Et-Ferrotane as
ligand (Table 2, entry 9). Comparable selectivities were
observed with Duanphos and Difluorphos (Table 2, entry 5 and
7). Raising the reaction temperature to 50 °C did not improve the
selectivity, as selected experiments showed (Table 2, entry 8
and 8a). Using solvent systems other than methanol/THF lead to
decreased selectivities as exemplified with Difluorphos and
Duanphos (Table 2, compare entries 5, 5a-c and 7,7a). Isolated
yields were obtained for several examples and are in general
excellent (Table 2, entries 2, 5, 7 and 9).
͡
2 4
Having established that [Rh(PP)(solvent) ](BF ) complexes are
competent for cycloaddition reactions, we wondered whether
catalysts other than those containing diolefins or solvent
molecules could be used to generate active species.
Replacement of diolefins would allow to skip the
prehydrogenation step, making the overall procedure
significantly more convenient. On the other hand, solvent
complexes are so reactive they can only be generated in situ.
Table 2. Evaluation of [Rh(diphosphine)(MeOH)
intramolecular cyclotrimerization reactions.
2 4
](BF )
complexes in
[
a]
͡
Rh(I)-diphosphine complexes of the type [Rh(PP)(NH
3 2 4
) ](BF )
Isolation
Yield [%]
may at first sight look unsuitable as catalyst precursors because
Selectivity
Screening
{Selectivity
[
37,38]
of the stable coordination of ammonia to rhodium.
by adding a stoichiometric amount of an acid such as HBF
However,
Reaction
solvent
#
Ligand
(Selectivity
-
4
(BF
4
[c]
[b]
[%ee])
[
%ee]}
is the anion in the complex anyway), ammonia should be easily
displaced generating the solvent complexes, thus circumventing
the need for a prehydrogenation step. In order to prove such a
1
2
3
4
5
(R
a
)-BINAP
MeOH/THF
MeOH/THF
MeOH/THF
MeOH/THF
MeOH/THF
2
2
concept, the complexes [Rh(Duanphos)(NH
3
)
2
](BF
) were synthesized and characterized. The
structures of complexes [Rh((Sc, )-
](BF ) and [Rh((R,R)-Et-Ferrotane)(NH ](BF
4
) and [Rh(Et-
a
(R )-H
8
-BINAP
>98 (2)
3 2 4
Ferrotane)(NH ) ](BF
molecular
R
p
(R)-Segphos
(R)-DTBM-Segphos
(R)-Difluorphos
6.8
27
Duanphos)(NH
3
)
2
4
3
)
2
4
)
were determined by X-ray structure analysis of suitable crystals
and the molecular structure of the first mentioned complex is
43.4
>98 (16.9)
[
38]
displayed in Figure 2.
2,2,2-
5
a
(R)-Difluorphos
(R)-Difluorphos
Trifluoro-
ethanol
10.4
13.5
Propylene
carbonate
5
b
c
5
(R)-Difluorphos
(S,S)-Et-Duphos
THF
31.9
39.0
6
MeOH/THF
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Chemistry - A European Journal
10.1002/chem.201703198
FULL PAPER
p 3 2 4
Figure 2. Molecular structure of [Rh((Sc,R )-Duanphos)(NH ) ](BF ); ORTEP
plot, 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Having these complexes in hand, the solvent complexes
[
Rh(Duanphos)(MeOH)
2
](BF
4
)
and
[Rh(Et-
Ferrotane)(MeOH) ](BF
2
4
) as well as the corresponding ammonia
complexes were investigated regarding their activity towards
triynes, which have been used in earlier Co(I)-catalyzed
[
41c]
cyclizations (Scheme 4).
The results showed that the
Duanphos ligand performed superior compared to the Et-
Ferrotane ligand in terms of selectivity (Scheme 4). In the
cyclizations of symmetrical triynes 4c and 4e excellent
enantioselectivities for the products 5c and 5e approaching 90%
ee were obtained. For selected examples the products were
isolated with yields ranging between 35 and >98%.
For the activation of the ammonia complexes (Scheme 4, entries
e and f) the addition of 2 equiv. HBF
4
immediately leads to the
quantitative formation of the corresponding solvent complexes,
3
1
[39]
as could be corroborated by P NMR analysis.
This novel
approach to the generation of catalytic active species was tested
in the cyclization of triynes 4a and 4e. The cyclizetion of 4e
using [Rh(Duanphos)(NH ) ](BF ) under the above mentioned
3 2 4
conditions gave 5e in comparably high selectivity compared to
the independently generated solvent complex (Scheme 4,
compare entries d and e). Cyclization of the test substrate 4a
Scheme 4. [2+2+2] Cycloadditions of selected triynes catalyzed by Rh(I)-
diphosphine solvent complexes and isolated Rh-diphosphine ammonia
complexes as precatalysts (6 mol% loading). Identification and selectivities of
crude product were determined by comparison with the known racemate.
promoted
by
[Rh(Et-Ferrotane)(NH
3
)
2
4
](BF )
furnished
3 3
Wilkinson catalyst RhCl(PPh ) as metal source for chiral
selectivities of 44.8% ee, which matches the result obtained with
the pure solvent complex (Scheme 4, entry f). The cyclization of
the structurally related malonate-bridged triynes 4c and 4d
utilizing solvent complexes of Et-Ferrotane and Duanphos gave
superior results for the latter with 71% ee for 5c (Scheme 4,
entries b and c), while the result for the Et-Ferrotane ligand is
comparable to the solvent complex (Table 2, entry 9).
catalysts with P,N ligands in cyclotrimerizations
The avoidance of metal-olefin complexes as precursors by using
other convenient Rh(I)-precursor could open a bypass to obtain
the catalyst without the hydrogenation as prerequisite step, as
exemplified already by the use of ammonia complexes. Classical
3 3
metal(I)-precursors for group 9 metal catalysis are MCl(PPh )
(
M = Co, Rh) complexes. While the catalytic reactivity of
[40]
3 3
CoCl(PPh ) has only been explored in a few cases and even
[
41]
less for [2+2+2] cycloaddition reactions,
homolog is the commercially available and widely applied
Wilkinson complex RhCl(PPh which was quite often
investigated as precatalyst in cyclotrimerization reactions.
Consequentially, there are a number of reports on its application
the heavier group
3 3
) ,
[
42 ]
[
43]
in [2+2+2] cycloadditions,
taking place under mild conditions
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Chemistry - A European Journal
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and at temperatures as low as 20 °C, enabling the synthesis
influence on the reaction outcome (Table 3, entry 2 and 3).
These systems represent the first chiral Rh(I)-based catalysts
[
44]
with sensitive substrates even in natural product synthesis.
Yet none of these reactions was carried out enantioselectively.
Therefore our goal was to assess the potential of RhCl(PPh
3 3
generated in situ from RhCl(PPh ) , avoiding the prerequisite
[
11]
3
)
3
hydrogen treatment. We also tested the successfully applied
as Rh(I)-source in combination with a chiral ligand as in situ
O-PINAP ligand in related iridium-catalyzed cyclotrimerizations
catalyst in enantioselective [2+2+2] cycloaddition reactions.
2 a
with triyne 4a. However, applying [IrCl(cod)] and (R,S )-O-
PINAP as catalytic system at 95 °C, only 27% yield and low
The development of general reaction conditions and screening
of in situ generated chiral catalysts in the cyclization of test
substrate 4f led to our delight to moderate to high yields and
good ee values between 42-87% for the triaryl 5f. While the use
selectivity of 4.7% ee were obtained after 72 h.
3 3 a
Having identified RhCl(PPh ) and (R,S )-O-PINAP as the
most promising in situ catalytic system, we ventured to
of P,P ligands such as (R
selectivities (Table 3, entry 1), selected chiral P,N ligands such
as (R )-QUINAP or (R,R )-O-PINAP afforded moderate ee
a
)-BINAP resulted in no significant
investigate the substrate scope. Therefore various triynes we
[
41c]
have synthesized previously
have been cyclized yielding the
a
a
respective product in moderate to good yields (Table 4).
values of 49% and 61%, respectively (Table 3, entries 2 and 7).
NMR investigations corroborated the assumption of facile
exchange of two PPh for the P,N ligand at the Rh center.
3
For the firstly investigated substrates 4e,g and h low yields
of product with no or rather low selectivity were obtained (Table
4, entries 1 to 3). Symmetrically substituted triynes 4i-k were
cyclized in moderate to quantitative yields with high ee values of
[
45]
7
5-85% for 5i and 5j respectively (Table 4, entries 4 and 5) and
an only moderate ee for the quinoline product 5k (Table 5, entry
). Surprisingly, the isoquinoline-substituted triyne analogue of
6
substrate 4k (namely, triyne 4b, Scheme 4) was not converted,
even at elevated temperatures of up to 80 °C. The
unsymmetrically substituted triynes 4l-n gave altogether
moderate yields of the corresponding benzene derivatives (5l-n),
however, in very low enantioselectivities (Table 4, entries 8-10).
We have also investigated different malonate-bridged triynes
under the aforementioned reactions conditions. Higher reaction
temperatures were required compared to ether-bridged triynes
and basically no enantioselectivities were observed (as an
3 3
Table 3. Catalyst screening of RhCl(PPh ) using various chiral ligands.
[
a]
[b]
#
1
2
3
4
5
6
7
8
9
Chiral Ligand
Yield [%]
83
d/l:meso
ee [%]
(R
(R
(R
a
a
a
)-BINAP
1 : 2.4
1 : 1.4
1 : 1.5
1 : 2.3
1.3 : 1
1.7 : 1
1.2 : 1
1.4 : 1
1 : 1.2
6
)-QUINAP
)-QUINAP
)-QUINAP
41
49
42
[
46]
[
c]
example see conversion of triyne 4c, Table 4, entry 11). This
trend is similar to that noticed with asymmetric cobalt-catalyzed
cyclizations of triynes, where in only few cases the selective
formation of arene products from malonate-bridged triynes was
37
(S
a
48
(-)47
27
(R,R
a
)-N-PINAP
88
[
6]
detected.
(R,S
a
)-N-PINAP
62
(-)87
61
(R,R
a
)-O-PINAP
85
(R,S
a
)-O-PINAP
94
(-)85
(-)45
(R)-PHOX
78
The reaction screening was performed on a 1 mmol triyne substrate scale. [a]
1
Ratio determined by integration of peak areas in the H NMR. [b] The ee
values were determined by chiral HPLC. [c] 2 equiv. ligand were used.
Only (R,R
a
)-N-PINAP led to mediocre 27% enantiomeric excess
Table 3, entry 5). Accordingly, the (S )-derivatives – namely
)-QUINAP, (R,S )-N-PINAP and (R,S )-O-PINAP – led to
(
(
a
S
a
a
a
similar or even better ee values of 47%, 87% and 85%
respectively, while favoring the other enantiomer (Table 3,
entries 4,
6
and 8). By turning to
a
different,
phosphinooxazoline-based P,N ligand class (PHOX), which
does not contain the chiral information in the backbone structure
but in a group close to the nitrogen moiety, high yields and
moderate ee values of up to 45% could be achieved (Table 3,
entry 9). Application of 2 equiv. of ligand has no significant
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8
5
1
0
1
4l
5l (65 °C, 18 h)
9
Table 4. Substrate screening with ether- and malonate-bridged triynes.
39
#
Substrate
Product
Yield [%]
(
react. temp., °C;
react. time, h)
(d/l:meso)
Sel. [%ee]
7
a)
4m
5m (60 °C, 24 h)
1
28
10
(
1 : 5.2)
Rac
[
b]
67
F1: (-)7
F2: rac
4e (X=O)
5e (25 °C, 6 h)
2
5n (65 °C, 4 d)
4n
11
7
(
1.1 : 1)
37
57
(1.5:1)
rac
5g (40 °C, 27 h)
4g (X=O)
4c
3
5c (90 °C, 3 d)
[
a] Scale of the reaction was 0.125-0.5 mmol triyne substrate. The ee values
3
1 : 1.2)
-)6
9
were determined by chiral HPLC. Detailed reaction conditions and meso:d/l
ratios are listed in the Supporting Information. [b] Total yield of both pair of
enantiomers, which were isolated separately after column chromatography.
(
(
5h (40 °C, 27 h)
4h
We have considered the simple synthesis and isolation of a
a
dinuclear chlorido-bridged Rh(I)-complex with the (R,S )-O-
4
5
PINAP ligand as catalyst precursor, which would circumvent the
in situ generation from two components. Furthermore, it would
possibly allow to analyze whether there is any retarding effect
from the free PPh released from the Wilkinson catalyst during
3
complexation. The Rh(I)-ethylene complex was used as
7
5
(
1 : 2.7)
-)85
(
4i (X=O)
5i (25 °C, 22 h)
precursor, which easily reacted with the O-PINAP upon mixing in
CH
2 2
Cl , furnishing the dinuclear complex 6 in excellent yield
3
6
(
Scheme 5).
(
11.8 : 1)
84
4j (X=O)
5j (55 °C, 24 h)
6
7
>
95
1.3 : 1)
-)31
(
(
4k (X=O)
5k (65 °C, 24 h)
Scheme 5. Synthesis of the dinuclear chiral Rh(I)-O-PINAP complex 6.
40
(-)2
With isolated complex 6 we investigated the cyclization of triyne
4a as test substrate in the presence of AgNTf
2
as described
4a
5a (50 °C, 23 h)
above (see Table 4 for conditions). The results indicated that
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under identical conditions (50 °C, 72 h reaction time; Table 4,
entry 7) only 10% yield of product 5a with 6.6% ee was obtained,
which is less than obtained with in situ catalyst. The best yield
and slightly higher selectivity were obtained after heating to
solution of ammonia in THF (2 mL) for 1 h at room temperature.
Complete conversion was confirmed by P NMR. The bright yellow
solution was layered with 6 ml heptane. Yellow crystals suitable for X-ray
31
1
analysis could be obtained after one day. H NMR (benzene-d
6
, 400
t
3
MHz): δ = 0.78 (s, Bu, 16H), 2.47 (s, NH , 6H), 3.14 (2H), 3.30 (2H), 3.68
, 162 MHz): δ = 120.8 ( JRh-P
77.6 Hz) ppm; N NMR (40.6 MHz benzene-d , reference is MeNO ,
50 °C for 4 h and 100 °C for 20 h, giving biaryl 5a with 57% yield
31
1
(
1
2H), 7.05 (8H) ppm; P NMR (benzene-d
6
=
and 12% ee. Heating to 95 °C for 23.5 h resulted in identical
selectivity and 34% yield. These experiments proved that
cyclotrimerizations with complex 6 required even higher reaction
temperatures to proceed, under conditions otherwise identical to
those used with the catalyst generated in situ from O-PINAP
15
6
2
1
(40.6 MHz, C D , MeNO )): δ = -398 ( J = 67.9 Hz) ppm.
6
6
2
H-N
[Rh((R,R)-Et-Ferrotane)(NH ) ](BF ): The complex [Rh((R,R)-Et-
3
2
4
4
Ferrotane)(cod)](BF ) (23.8 mg, 0.033 mmol) was stirred in a saturated
solution of ammonia in THF (2 ml) for 1 h at room temperature. Complete
conversion was confirmed by P NMR. The bright yellow solution was
3 3
ligand and RhCl(PPh ) .
31
layered with 6 mL heptane. Yellow crystals suitable for X-ray analysis
In conclusion, we have presented a detailed investigation into
1
could be obtained after one day. H NMR (THF-d
8
, 400 MHz): δ = 0.78 (t,
, 4H), 1.46 (t, J = 7.1 Hz, CH , 6 H),
.80 (m, 2H), 2.04-2.49 (m, 6H), 2.5 (m, 2H), 2.61 (s, NH , 6H), 4.26 (bs,
Cp-H, 2H), 4.41 (bs, Cp-H, 2H), 4.50 (bs, Cp-H, 2H), 4.53 (bs, Cp-H, 2H)
the quantitative formation of highly reactive chiral
J = 7.1 Hz, CH
3
, 6H), 1.11 (m, CH
2
3
+
[
Rh(diphosphine)(solvent)
2
]
complexes from the corresponding
1
3
cod complexes by applying appropriate prehydrogenation
conditions. Furthermore we have shown that isolated ammonia
31
1
15
ppm; P NMR (THF-d , 162 MHz): δ = 71.6 ( J
8
Rh-P
= 179.9 Hz) ppm;
N
1
complexes of the type [Rh(diphosphine)(NH
catalysts prepared from the Wilkinson complex RhCl(PPh
3
)
2
](BF
4
)
and
and
6 6 2
NMR (40.6 MHz, C D , MeNO ): δ = -396 ( JH-N= 67.5 Hz) ppm.
3
)
3
suitable P,N ligands like O-PINAP are competent catalyst
precursors for [2+2+2] cycloaddition reactions. The following
points deserve attention: a) It was shown that the time required
to fully hydrogenate and thus displace the diolefin from Rh(I)-
olefin complexes is strongly affected by the type of diphosphine.
Therefore the prehydrogenation step required to ensure
complete conversion of the starting complex into the active
precatalyst should be assessed for each different catalytic
system; b) catalyst generation in chlorinated solvents, although
widely applied, may afford metal species other than those
obtained in non-chlorinated solvents, potentially leading to
undesired side products; c) Rh(I)-ammonia complexes can
afford catalytic active species when treated with stoichiometric
amounts of acid, thus circumventing the prehydrogenation step
required by Rh(I)-olefin complexes; d) application of the
[RhCl{(R,S
R,S )-O-PINAP (184.0 mg, 0.328 mmol) were weighted into a 25 mL
Schlenk tube in the glove-box and 2.8 mL CH Cl added as solvent,
a 2 2 4 2
)-O-PINAP}] (6): [RhCl(C H )] ) (63.0 mg, 0.162 mmol) and
(
a
2
2
resulting in the formation of a deep red solution. The solution was stirred
at 20 °C for 3 h, concentrated to 1 mL in vacuo and diethyl ether (16 mL)
was added. An olive green precipitate formed immediately. Solids
attached to the glass wall were removed by freezing and thawing of the
solution and treatment in an ultrasound bath at 30 °C for 30 min. The
suspension was cooled to 0 °C and the solvent removed by a filter tube.
The remaining solid was again suspended in diethyl ether (9 mL), stirred
strongly and cooled to 0 °C and the solvent filtered off. This was repeated
a third time with 5 mL of diethyl ether. After removal of the solvent by
filtration the green powder was dried in high vacuo, yielding 201 mg
1
(
96%) product with minimal solvent residues. H NMR (CD
2 2
Cl , 300
3
MHz): δ = 0.93 (d, CH , J = 6.2 Hz, 6H), 5.52 (q, CH, J = 6.2 Hz, 2H),
6.75-7.00 (m, 13H), 7.05-7.15 (m, 6H), 7.23-7.35 (m, 13H), 7.35-7.45 (m,
4
H), 7.48-7.58 (m, 4H), 7.63-7.73 (m, 4H), 7.94-8.00 (m, 2H), 8.00-8.11
31
1
Wilkinson complex RhCl(PPh
3
)
3
as convenient Rh(I)-source
(m, 4H) ppm; P NMR (benzene-d
Hz) ppm; P NMR (CD
6
, 162 MHz): δ = 120.8 ( JRh-P = 177.6
31
1
avoids the necessity of prehydrogenation; here P,N ligands
proved to be better ligands in cyclotrimerization reactions
compared to P,P ligands.
2 2
Cl
, 121 MHz): δ = 63.3 ppm ( JRh-P = 174 Hz).
General Procedure for the preparation of the solvent complexes
͡
͡
[
Rh(PP)(solvent) ](BF ): The precatalyst [Rh(PP)(cod)](BF ) (0.01 mmol)
2
4
4
While the cyclotrimerization of alkynes and triynes was
investigated as a showcase in the presented study, the details of
catalyst generation are likewise highly interesting for a range of
asymmetric transformations mediated by Rh(I)-complexes,
which until now applied Rh(I)-olefin complexes that had to be
activated via hydrogenation. The significantly higher efficiency
achieved with the catalytic systems we have presented here
represents a step forward towards more sustainable syntheses.
was dissolved in anhydrous MeOH (15.0 mL) and treated with hydrogen
under normal pressure according to the time given in Table 1 at 25.0 °C.
Afterwards the hydrogen was removed by five freeze-thaw-cycles.
Catalytic cyclotrimerization experiments:
Procedure for the cyclization of phenylacetylen: The precursor
[
4
Rh(DTBM-Segphos)(cod)](BF ) (31 mg, 0.02 mmol) was dissolved in
methanol (5 mL). The resulting solution was exposed to hydrogen for 77
min and then the hydrogen removed by five freeze-thaw-cycles.
Subsequently phenylacetylene (0.2 mL, 2 mmol) was added via syringe
under vigorous stirring. After 45 min the reaction mixture was quenched
by exposition to air. To obtain the product composition, the samples have
been analyzed by GC/MS and the individual products been identified by
their fragmentation pattern.
Experimental Section
All reactions have been performed under argon using Schlenk technique.
All solvents were distilled over appropriate drying reagents in an inert
atmosphere or taken from a solvent purification system.
General procedure for the cyclization of triynes with
Synthesis of Rh(I)-complexes:
͡
[
Rh(PP)(solvent)
2
](BF
(0.01 mmol) was dissolved in 1 mL MeOH and
stirred under hydrogen for respective time (see Table 1 and Supporting
4
) complexes (Table 2): The precursor complex
͡
[
Rh(PP)(cod)](BF )
4
[Rh((Sc,
R
p
)-Duanphos)(NH
3
)
2
4
](BF ):
The
complex
p
[Rh((Sc,R )-
4
Duanphos)(cod)](BF ) (21.8 mg, 0.033 mmol) was stirred in a saturated
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Chemistry - A European Journal
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Information) to obtain practically complete conversion into solvent
Acknowledgements
͡
complex [Rh(PP)(MeOH)
dissolved in 0.5 mL THF was added and the reaction mixture stirred for
6 h. In case of other solvent systems than MeOH, the MeOH was
2 4
](BF ). Afterwards the substrate (0.165 mmol)
We are indebted to Prof. Uwe Rosenthal for his enduring
support, helpful discussions and comments. M.H. thanks the
DFG (HA-3511/3-1) for financial support and the Fonds der
Chemischen Industrie for a Kekulé Fellowship for I. T. We thank
Kirill Faust for the synthesis of [RhCl{(R,S )-O-PINAP}] . Our
a 2
appreciation also goes to Dr. Elisabetta Alberico for helpful
comments and suggestions.
1
removed after hydrogenation and the remaining solvent complex and
substrate were dissolved in the respective solvent system. Reactions in
solvent mixtures like MeOH/THF result from the addition of substrate
solution in THF due to better solubility to the solution of the precatalyst in
MeOH. The complete screening table with all screened ligands is given in
the Supporting Information.
General procedure for the cyclization of triynes with
Keywords: [2+2+2] cycloaddition • rhodium • asymmetric
͡
[
Rh(PP)(solvent)
2
](BF
complex [Rh(PP)(diolefine)](BF
MeOH and stirred under hydrogen atmosphere for respective time (Table
and Supporting Information) to obtain 99.2% conversion into the
4
)
complexes (Scheme 4): The appropriate
͡
catalysis • triynes • diphosphine ligands
4
) (0.01 mmol) was dissolved in 1 mL
1
͡
solvent complex [Rh(PP)(MeOH)
(
2 4
](BF ). Afterwards the triyne substrate
[
1]
a) K. Tanaka, T. Shibata in Transition-Metal-Mediated Aromatic Ring
Construction, John Wiley & Sons, 2013, pp. 255-280; b) M. Amatore, C.
Aubert, Eur. J. Org. Chem. 2015, 265-285.
0.165 mmol) dissolved in 0.5 mL THF was added and the reaction
mixture stirred for 16 h at room temperature. The complete screening
table with all screened ligands is given in the Supporting Information. In
case of ammonia complexes, the solvent complexes were generated by
[
2]
3]
N. Weding, R. Jackstell, H. Jiao, A. Spannenberg, M. Hapke, Adv.
Synth. Catal. 2011, 353, 3423-3433.
4
addition of 2 equiv. HBF .
[
a) R. L. Halterman, Chem. Rev. 1992, 92, 965-994; b) Recent
overview: C. G. Newton, D. Kossler, N. Cramer, J. Am. Chem. Soc.
General procedure for the chiral ligands screening with Wilkinson
catalyst RhCl(PPh in the cyclotrimerization of triyne 4f (Table 3):
Triyne 4f (1 mmol), RhCl(PPh (5 mol% in regard to the triyne), the
respective chiral ligand (5 mol% in regard to the triyne) and AgNTf (5
2
016, 138, 3935-3941.
3 3
)
[4]
a) A. Gutnov, B. Heller, C. Fischer, H.-J. Drexler, A. Spannenberg, B.
3 3
)
Sundermann, C. Sundermann, Angew. Chem. Int. Ed. 2004, 43, 3795-
2
3797; b) B. Heller, A. Gutnov, C. Fischer, H.-J. Drexler, A.
mol% in regard to the triyne) were dissolved in THF (3 mL) and the
solution was stirred at room temperature for 4 h. At the end of the
reaction, the solvent was removed under reduced pressure and the
residue purified by column chromatography (n-hexane/ethyl acetate 1:1,
v/v) to yield the benzene derivative 5f. The ee values were determined by
chiral HPLC analysis (Cellulose 2, n-heptane/isopropanol 95:5, v/v, 1
mL/min).
Spannenberg, D. Redkin, C. Sundermann, B. Sundermann, Chem. Eur.
J. 2007, 13, 1117-1128; c) M. Hapke, K. Kral, C. Fischer, A.
Spannenberg, A. Gutnov, D. Redkin, B. Heller, J. Org. Chem. 2010, 75,
3993-4003.
[5]
[6]
[7]
P. Jungk, T. Täufer, I. Thiel, M. Hapke, Synthesis 2016, 48, 2026-2035.
P. Jungk, F. Fischer, M. Hapke, ACS Catal. 2016, 6, 3025-3029.
T. Shibata, H. Kurokawa, K. Kanda, J. Org. Chem. 2007, 72, 6521-
6525.
General procedure for the cyclization with Wilkinson catalyst
[8]
[9]
T. Shibata, K. Tsuchikama, M. Otsuka, Tetrahedron: Asymmetry 2006,
17, 614-619.
RhCl(PPh
Table 4): The triyne (0.125-0.5 mmol), RhCl(PPh
to the triyne), (R,S )-O-PINAP (5 mol% with regard to the triyne) and
AgNTf (5 mol% with regard to the triyne) were dissolved in THF/toluene
1 mL) and the solution was stirred at 25-95 °C for a specified time. At
3
)
3
and chiral ligands, exemplified for (R,S
a
)-O-PINAP
(
3
)
3
(5 mol% with regard
a) T. Shibata, K. Tsuchikama, Org. Biomol. Chem. 2008, 6, 1317-1323;
b) T. Shibata, S. Yoshida, Y. Arai, M. Otsuka, K. Endo, Tetrahedron
2008, 64, 821-830.
a
2
(
[10] a) T. Shibata, T. Fujimoto, K. Yokota, K. Takagi, J. Am. Chem. Soc.
2004, 126, 8382-8383; b) T. Shibata, Y. Arai, K. Takami, K.
Tsuchikama, T. Fujimoto, S. Takebayashi, K. Takagi, Adv. Synth. Catal.
2006, 348, 2475-2483.
the end of the reaction, the solvent was removed under reduced pressure
and the residue purified by column chromatography to yield the benzene
derivative. The ee values were determined by HPLC analyses on chiral
phases.
[11] For reviews about enantioselective Rh-catalyzed [2+2+2] cycloadditions,
see: a) K. Tanaka, Synlett 2007, 1977-1993; b) K. Tanaka, Chem.
Asian J. 2009, 4, 508-518; c) Y. Shibata, K. Tanaka, Synthesis 2012,
44, 323-350; d) K. Tanaka, Heterocycles 2012, 85, 1017-1043. e) K.
Tanaka, Y. Kimura, K. Murayama, Bull. Chem. Soc. Jpn. 2015, 88, 375-
385.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited at the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC-
1
[
531752
Rh(Duanphos)(NH
4
Ferrotane)(NH ](BF ). Copies of the data can be obtained free of
for
[Rh(BINAP)
2
](BF
4
)
2
,
CCDC-1531753
for
[Rh(Et-
3
)
2
](BF and
4
)
CCDC-1531754
for
[12] K. Tanaka, G. Nishida, A. Wada, K. Noguchi, Angew. Chem. Int. Ed.
2004, 43, 6510-6512.
3
)
2
charge on application to CCDC, 12 Union Road, Cambridge, CB21EZ,
UK (fax: int. code + (1223) 336-033; e-mail: deposit@ccdc.cam.ac.uk).
[13] K. Tanaka, G. Nishida, M. Ogino, M. Hirano, K. Noguchi, Org. Lett.
2005, 7, 3119-3121.
[14] G. Nishida, N. Suzuki, K. Noguchi, K. Tanaka, Org. Lett. 2006, 8, 3489-
3492.
[15] K. Tanaka, A. Wada, K. Noguchi, Org. Lett. 2005, 7, 4737-4739.
[16] T. Araki, K. Noguchi, K. Tanaka, Angew. Chem. Int. Ed. 2013, 52,
5617-5621.
[17] M. Amatore, D. Lebœuf, M. Malacria, V. Gandon, C. Aubert, J. Am.
Chem. Soc. 2013, 135, 4576-4579.
[18] Recent examples: a) S. Yoshizaki, Y. Nakamura, K. Masutomi, T.
Yoshida, K. Noguchi, Y. Shibata, K. Tanaka, Org. Lett. 2016, 18, 388-
391; b) R. Shintani, R. Takano, K. Nozaki, Chem. Sci. 2016, 7, 1205-
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
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FULL PAPER
3
1
1
211; c) M. Fernández, M. Parera, T. Parella, A. Lledó, J. Le Bras, J.
Muzart, A. Pla-Quintana, A. Roglans, Adv. Synth. Catal. 2016, 358,
848-1853; d) Y. Aida, H. Sugiyama, H. Uekusa, Y. Shibata, K. Tanaka,
[31]
P NMR data of the cod as well as the MeOH solvent complexes and
an overview on the structures of ligands investigated in this work are
compiled in the Supporting Information.
1
Org. Lett. 2016, 18, 2672-2675; e) K. Masutomi, H. Sugiyama, H.
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31
yl)ethane) as ligand. As shown in the Supporting Information, the
P
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8
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31
[
39] See Supporting Information for P NMR spectra of this experiment.
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[
28] Following the general procedure described by Tanaka et al.,
(
[Rh(cod)
argon, followed by treatment with H
of resulting complexes like [Rh(BINAP)
structure) can be found in the Supporting Information.
2
](BF
4
) and BINAP were reacted in CH
for 30 min. Details on the isolation
](BF (including X-ray
2 2
Cl for 5 min under
2
2
4
)
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differences between the complexes is dictated mostly by the ring size
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31
[45] For details and
P
NMR spectra of the experiments
see the Supporting Information.
[46] For details on the experimental conditions and results, see the
Supporting Information, section 9.
[16].
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201703198
FULL PAPER
Abstract:
FULL PAPER
Indre Thiel, Moritz Horstmann, Phillip
Jungk, Sonja Keller, Fabian Fischer,
Hans-Joachim Drexler, Detlef Heller*
and Marko Hapke*
Page No. – Page No.
Insight into the Activation of In Situ-
Generated Chiral Rh(I)-Catalysts and
their Application in Cyclotrimeri-
zations
All roads lead to Rome, but the most frequented routes might not be the most
efficient and sustainable. The detailed investigation on the generation of chiral
Rh(I)-complexes from different precursors and their use exemplified in
cyclotrimerizations of triynes allowed insights into the efficient generation of chiral
Rh(I)-catalysts.
This article is protected by copyright. All rights reserved.