Unprecedented One-Pot Reaction towards Chiral, Non-Racemic Copper(I) Complexes of [2.2]Paracyclophane-Based P,N-Ligands

Article abstract of DOI:10.1002/chem.201704115

Herein, we report a simple one-pot route to enantiopure copper(I) complexes featuring a unique [2.2]paracyclophane-based P,N-ligand system. Phosphine and pyridine moieties can be varied allowing the modular synthesis of these rigid and stable [2.2]paracyclophane-based P,N-ligands. These P,N-ligands are a new ligand class for different transition-metal complexes, which is shown exemplarily for palladium(II).

Full text of DOI:10.1002/chem.201704115

A Journal of
Accepted Article
Title: Unprecedented one-pot reaction to chiral, non-racemic copper(I)
complexes of [2.2]paracyclophane-based P,N-ligands
Authors: Carolin Braun, Martin Nieger, and Stefan Bräse
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To be cited as: Chem. Eur. J. 10.1002/chem.201704115
Link to VoR: http://dx.doi.org/10.1002/chem.201704115
Supported by

Chemistry - A European Journal
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COMMUNICATION
Unprecedented one-pot reaction to chiral, non-racemic copper(I)
complexes of [2.2]paracyclophane-based P,N-ligands
[
a]
[b]
[a,c]
Carolin Braun, Martin Nieger, and Stefan Bräse*
Abstract: Herein, we report a simple one-pot reaction to enantiopure
copper(I) complexes featuring a unique [2.2]paracyclophane-based
P,N-ligand system. Phosphine and pyridine moiety can be varied
allowing the modular synthesis of these rigid and stable
[
2.2]paracyclophane-based P,N-ligands. These P,N-ligands are a
new ligand class for different transition metal complexes, which is
shown exemplarily for palladium(II).
Chiral transition metal complexes are cornerstones in applications
ranging from catalysis to material sciences. A unique motif is
represented by [2.2]paracyclophanes which are thermally stable
and feature a rigid backbone. Among the various disubstituted
[
2.2]paracyclophane derivatives, only those with an ortho-,
pseudo-ortho- or pseudo-geminal-substitution pattern can form
stable bidentate complexes with transition metals. The pseudo-
ortho-disubstitution is easiest accessible via the respective
dibromide. One of the earliest, and most successful, pseudo-
Figure 1: Known pseudo-ortho-disubstituted [2.2]paracyclophane-based
ortho-disubstituted
[2.2]paracyclophane
derivatives
was
bidentate P,P- (1 and 2), P,N- (3 and 4) and N,N-ligands (5).
[
1]
PhanePhos 1 (Figure 1). First reported by Pye and Rossen in
[
1a]
1997,
this ligand has successfully been used in asymmetric
Interestingly, we obtained the coupling product 6[6] only in 4%
yield and the mono coupling product only in traces. This is
significantly lower than for the regioisomeric pseudo-para-
bispyridyl[2.2]paracyclophane, that was isolated in 29% yield
using the pseudo-para-dibromide under the same reaction
conditions.[ Instead, we observed the formation of the copper(I)
complex 10-Cl with an unsymmetrically substituted, mixed P,N-
ligand (Scheme 1). In this reaction, the origin of the diphenyl
phosphine substituent is attributed to the triphenyl phosphine
substituents of the palladium(0) catalyst that was used for the
cross-coupling reaction in catalytic amounts. Consequently, we
could improve the yield of 10-Cl to 46% by addition of
hydrogenation reactions. The general P,P-ligand framework has
been thoroughly studied and extensively modified and combined
with other transition metals for many other asymmetric catalytic
[2]
transformations. Furthermore, mixed P,N-ligands like oxazoline
[
3]
or pyridine 4 as well as N,N-ligands like bisoxazoline 5[5]
[4]
3
7]
have been synthesized. However, the synthesis of many of these
ligands requires the introduction of space moieties that
significantly alter bite angle and mode of coordination.
Nevertheless, apart from PhanePhos there are only few examples
for bidentate, pseudo-ortho-disubstituted [2.2]paracyclophane-
based ligands. Therefore we aimed to synthesize the pseudo-
[
6]
ortho-bispyridyl[2.2]paracyclophane
6
as alternative for a
bidentate N,N-ligand via our elaborated Stille cross-coupling
protocol for pyridyl-substituted [2.2]paracyclophanes from the
3
stoichiometric amounts of PPh .
[
7]
pseudo-ortho-dibromide 8.
[a]
C. Braun, Prof. Dr. S. Bräse
Institute of Organic Chemistry
Karlsruhe Institute of Technology (KIT)
Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
E-mail: braese@kit.edu
[b]
Dr. M. Nieger
Department of Chemistry, University of Helsinki
P.O. Box 55, 00014 University of Helsinki, Finland
Prof. Dr. S. Bräse
[c]
Institute of Toxicology and Genetics
Karlsruhe Institute of Technology (KIT)
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany
Supporting information for this article is given via a link at the end of
the document.
Scheme 1: Unexpected phosphination reaction of 8 to the copper(I) complex
0-Cl that is chelated by a mixed P,N-ligand.
1
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The successful cross-coupling of stable triphenyl phosphines is
[
8]
[9]
reported by Chan et al. , but also by other groups . Traditional
methods for phosphination reactions follow two strategies, using
either the reaction between Grignard or organolithium compounds
with chlorophosphines or transition metal-catalyzed reactions with
[
10]
diphenyl phosphine derivatives. Thus, the formation of 10-Cl is
somewhat unexpected, because a preferred phosphination
reaction over a “regular” cross-coupling reaction with an organo-
metallic coupling partner is rather rare. Nevertheless, none of
these reports mention C–P bond formation by the activation of
triphenylphosphine in the presence of an organometallic coupling
partner, such as stannane 9a. The copper(I) additive is key to the
Stille reaction but its role in the phosphination is unclear; to the
best of our knowledge there are no examples of phosphination
competing with Stille cross-couplings, although ligands exchange
between palladium and phosphorus under mild conditions is
[
11]
known. The mechanism of this exchange is often discussed via
reductive elimination of a phosphonium salt followed by oxidative
[
12]
addition of another P-C-bond.
A similar mechanism is also
[
8a, 8e-g]
assumed for the palladium-catalyzed phosphination.
As traces of both the dipyridyl paracyclophane 8 and the mono
pyridyl bromide were observed it is reasonable to suppose that
the first Stille coupling proceeds as expected. In our experiments,
no other substitution pattern leads to the phosphination reaction
Figure 2: Molecular structure of 10-Cl (hydrogen atoms and solvent omitted for
clarity, displacement parameters are drawn at 50% probability level).
Characteristic bond lengths: Cu–N 2.031(3) Å; Cu1–P1 2.1677(9) Å; Cu1–Cl1
(
see ESI for details). This is not unexpected for the pseudo-para-
2
1
.2248(10) Å, N22–P1 3.618(3) Å. Characteristic bond angles: N22–Cu1–P1
18.97(10)°, N22–Cu1–Cl1 107.55(10)°, P1–Cu1–Cl1 132.53(4)°.
and para-isomers, but especially surprising for the related
pseudo-geminal-substituted bispyridyl[2.2]paracyclophane 12.
However, 12 is formed without any evidence for a competing
phosphination (Scheme 2) despite its steric shielding and its
molecular structure could be confirmed by X-ray crystallography
Single crystals of 10-Cl suitable for X-ray crystallography were
obtained. Interestingly, from another sample we obtained crystals
of the bromide 10-Br, so an unselective reaction for the halide has
to be assumed. The molecular structure, which is shown in Figure
(
see ESI for details).
2
possesses a slightly distorted trigonal-planar coordination
geometry. For such a monomeric coordination geometry of
copper(I) halide complexes, only a handful examples are reported
[
13]
in literature,
whereas usually dinuclear copper(I) complexes
with two bridging halide ligands are observed.[
14]
Regarding
complex 10-Cl, there are two reasons for the preferred monomeric
structure that are attributed to the rigid structure of the
[
2.2]paracyclophane. That causes the large bite angle of almost
Scheme 2: Stille cross-coupling of bromide 11 to obtain 12 as product of the
regular" reactivity.
"
120° and steric congestion around the copper(I) centre, favouring
a trigonal-planar coordination geometry.
Therefore it is then believed that, because of the specific spatial
geometry of the pseudo-ortho-isomer, the copper coordinates to
the pyridine to direct the phosphination in an intramolecular
fashion together with the palladium, enabling the competitiveness
of this reaction pathway. This is further supported by the fact that
the palladium complex and the free ligand is never isolated and
that the regioisomers are not allowing this special orientation
between palladium and coordinated copper. Ongoing
experiments and in silico calculations have to shed light on the
mechanism of this one-pot reaction.
Subsequently we altered the pyridyl- and phosphinyl-substituents.
The variation of the pyridyl substituent is rather straightforward
and is achievable using another stannane as cross-coupling
partner. Therefore, the copper(I) complex 13-Cl with a methyl-
substituted pyridyl group was synthesized by using stannane 9b
(Scheme 3). From 13-Cl single crystals were obtained that were
suitable for X-ray crystallography. The molecular structure of 13-
Cl is very similar to that of 10-Cl with the main difference that a
mixed chloride/bromide structure with a ratio of 83/17 was
observed (see ESI for details).
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The applicability for complexation of other transition metals was
shown exemplarily for palladium(II). The respective palladium(II)
complex 15 was synthesized in almost quantitative yield through
addition of Pd(COD)Cl to a solution of the [2.2]paracyclophane-
2
based P,N-ligand 7 in dichloromethane (Scheme 5).
Scheme 5: Synthesis of the palladium(II) complex 15 from the free [2.2]para-
cyclophane-based P,N-ligand 7.
Scheme 3: Modular approach towards copper(I) complexes of [2.2]paracyclo-
phane-based P,N-ligands.
From the palladium(II) complex 15 we were able to obtain single
crystals that were suitable for X-ray crystallography. The
molecular structure of 15 confirms the suggested mononuclear
palladium(II) complex with a slightly distorted square-planar coor-
dination geometry around the palladium centre that possesses
To introduce alternate phosphinyl substituents requires a catalyst
[
8a, 8b, 8e, 8g]
that does not contain a phosphine ligand.
screened a number of phosphine sources like Pd(OAc)
Pd dba in combination with PPh and obtained 10-Cl in 17% and
1% yield, respectively. This result shows the superiority of
Pd(PPh as catalyst system, but enables the variation of the
Therefore, we
2
and
2
3
3
cis-configuration. This cis-configuration was also observed for the
1
[1b]
respective
PhanePhosPdCl
2
complex,
although
the
3 4
)
PhanePhos ligand shows with 103.69° the distinctly larger bite
phosphine substituent. As shown in Scheme 3, we confirmed our
modular approach also for the phosphinyl substituent with
angle compared to 95.40° for the P,N-ligand in 15. The bond
lengths are comparable to other reported dichloropalladium(II)
Pd(OAc)
Cl with reasonable yield.
2
and P(p-tolyl)
3
and obtained the copper(I) complex 14-
[15]
complexes of P,N-ligands
and show as well a longer Pd–Cl
[2e]
bond trans to phosphorus. In contrast to the PhanePhos ligand,
no alternative formation of a dinuclear, halogenide-bridged
palladium(II) complex was observed.
The synthesis of these enantiopure copper(I) complexes from the
pseudo-ortho-dibromo[2.2]paracyclophane is simple; the
C
2
symmetry of the starting material means the initial Stille coupling
always gives a single configuration of the product. Thus, by using
enantiopure bromide 8, the enantiopure complexes 10-Cl and 13-
Cl were accessible.
Scheme 4: Complex dissociation of the copper(I) complex 10-Cl to obtain the
free P,N-ligand 7.
Moreover, we were interested in the complex dissociation to
isolate the free P,N-ligand 7, which enables the synthesis of
various other transition metal complexes through reaction with the
respective transition metal precursors. The decomplexation is
achievable very easily by addition of a commercially available
metal scavenger to a solution of 10-Cl in dichloromethane
Figure 3: Molecular structure of 15 (hydrogen atoms and solvent omitted for
clarity, displacement parameters are drawn at 50% probability level).
Characteristic bond lengths: Pd1–N22 2.0888(15) Å; Pd1–P1 2.2673(5) Å; Pd1–
Cl2 2.2769(4) Å; Pd1–Cl1 2.3579(4) Å, N22–P1 3.224(2) Å. Characteristic bond
angles: N22–Pd1–P1 95.40(4)°, N22–Pd1–Cl1 84.08(4)°, N22–Pd1–Cl2
(
Scheme 4). Using this procedure, the free ligand 7 was obtained
173.27(4)°, P1–Pd1–Cl2 91.327(16)°, P1–Pd1–Cl1 178.308(16)°, Cl1–Pd1–Cl2
in 94% yield. In contrast to PhanePhos 1, the free P,N-ligand 7 is
stable against oxidation to the respective phosphine oxide for
weeks.
89.261(16)°.
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Chemistry - A European Journal
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COMMUNICATION
In summary, we developed a flexible one-pot reaction that allows
the modular synthesis of copper(I) complexes with P,N-ligands,
where the phosphine and the pyridine moieties can be varied.
Subsequent complex dissociation allows an efficient and
convenient access to [2.2]paracyclophane-based P,N-ligands.
Ongoing research in our laboratories will elaborate their potential
for the complexation of other transition metals and the properties
of the resulting complexes.
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PPh (1.00 equiv.), Pd(PPh
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We thank the SFB/TR 88 for financial support. C. B. gratefully
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Keywords: Cross-coupling • Cyclophanes • Copper •
Phosphanes • Chirality
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