Diastereoselective Synthesis of Planar Chiral Phosphoramidites with a Ferrocenophane Scaffold

Article abstract of DOI:10.1021/acs.organomet.7b00700

A new family of planar chiral phosphoramidites with a [3]ferrocenophane structure was synthesized. The synthetic strategy involved diastereoselective formation of the chiral ferrocene units from suitable substituted bis-cyclopentadienyl derivatives. Preliminary coordination studies of these ligands were undertaken with the synthesis of palladium and platinum complexes.

Full text of DOI:10.1021/acs.organomet.7b00700

Article
Cite This: Organometallics XXXX, XXX, XXX-XXX
pubs.acs.org/Organometallics
Diastereoselective Synthesis of Planar Chiral Phosphoramidites with
a Ferrocenophane Scaffold
Mathilde Neel, Pascal Retailleau, Arnaud Voituriez,* and Angela Marinetti*
Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Universite
1198 Gif-sur-Yvette, France
́ ́
Paris-Sud, Universite Paris-Saclay, 1, av. de la Terrasse,
9
*
S Supporting Information
ABSTRACT: A new family of planar chiral phosphoramidites with a [3]ferrocenophane structure was synthesized. The
synthetic strategy involved diastereoselective formation of the chiral ferrocene units from suitable substituted bis-
cyclopentadienyl derivatives. Preliminary coordination studies of these ligands were undertaken with the synthesis of palladium
and platinum complexes.
INTRODUCTION
■
Chiral phosphoramidites represent one of the major classes of
1
phosphorus ligands for asymmetric organometallic catalysis.2
They stand out especially among monodentate chiral ligands
for their efficiency in reactions such as rhodium-promoted
3
hydrogenation of olefins, copper-promoted conjugate 1,4-
4
5
additions, iridium-promoted allylic substitutions, and gold-
6
catalyzed cycloisomerizations. Chiral phosphoramidites usually
display cyclic structures derived from C -symmetric chiral diols,
2
mainly axially chiral biaryl diols, e.g. BINOL I, spiranic diols II,
or diols with central chirality, e.g. TADDOL III (Figure 1). On
the other hand, it may be noted that chiral phosphoramidites
based on planar chiral scaffolds have been rather neglected so
7
far. The main achievement in this field has been the recent
disclosure of unsymmetric phosphoramidites based on a
Figure 1. Representative phosphoramidites with axial, central, and
planar chiral backbones.
[
2.2]paracyclophane scaffold and their successful use in
7b
copper-catalyzed conjugate additions. In this context, inspired
by the high potential of planar chirality in other series o8f
phosphorus ligands, including ferrocene-derived phosphines,
we have started recently to develop various synthetic
of aryl substituents on both of the arylidene units, ortho to the
O−P−O tether. Unlike phosphoramidites I−III, the diol
precursor of IV is an achiral species and planar chirality is
generated at the cyclization step which involves the achiral diol
approaches to phosphoramidites displaying C -symmetric
2
planar chiral scaffolds. Thus, in 2016 we have disclosed the
synthesis of phosphoramidites displaying the [3.4]-
paracyclophane scaffold IV and their use in gold-promoted
9
Special Issue: Organometallic Chemistry in Europe
Received: September 13, 2017
cycloisomerizations. The macrocyclic paracyclophane structure
IV is made of two arylidene units tethered by O−P−O and
biphenylene units. Planar chirality is generated by the presence
©
XXXX American Chemical Society
A
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and a suitable dichlorophosphoramidite. In the synthesis of IV
this step displayed moderate diastereoselectivity, as it afforded a
complex mixture of the d,l and meso isomers.
Thus, we have adapted the same strategy to O−Si−O tethered
cyclopentadienyls, in order to access the corresponding
ferrocenophanes, as shown in Scheme 1. The key feature of
In parallel studies, wishing to access a more diverse range of
planar chiral phosphoramidites, we have investigated the
unprecedented use of 1,3-dioxa-2-phospha[3]ferrocenophanes
as structural units. The targeted structures V are shown in
Figure 1. This paper reports on our first achievements in this
field.
Scheme 1. Diastereoselective Synthesis of Aryl-Substituted
O−Si−O Tethered Ferrocenes
RESULTS AND DISCUSSION
■
1
,3-Dioxa-2-phospha[3]ferrocenophanes with various phospho-
rus functions have been known since the late 1980s
phosphonates, phosphorochloridates, phosphonites, phos-
phites, phosphates, phosphorochloridites
10
11
(
11b,12
); however,
phosphoramidites have not been described so far. Moreover,
only one of the known ferrocenophanes above display
substituted ferrocene units, as required for planar chiral
1
3
derivatives. Recently, a 2,2′-dimethyl-substituted ferroceno-
phane phosphate has been isolated as an inseparable mixture of
meso and d,l isomers from lithiation/methylation of the
12b
unsubstituted 1,3-dioxa-2-phosphaferrocenophane.
Thus, the rather poor development of this field encouraged
us to investigate the new class of planar chiral phosphor-
amidites V. In principle, the desired phosphoramidites V should
be available from the corresponding 2,2′-diaryl-1,1′-ferrocene-
diols that, as far as we know, have not been described so far
the targeted bis(cyclopentadienyloxy)silanes 3 are the aryl
substituents on their 2-positions. A variety of aryl groups can be
entered in this position by palladium-promoted Suzuki coupling
of 2-iodocyclopentenone with arylboronic acids. The method is
illustrated in Scheme 1. Starting from cyclopentenone 1, a
regioselective iodation was accomplished using a literature
(Figure 2).
17
procedure (I , 4-dimethylaminopyridine (DMAP), K CO in
2
2
3
a H O/THF 1/1 mixture). The 2-iodocyclopent-2-enone was
2
obtained in 74% yield on a 10 g scale. Subsequently, Suzuki
couplings between this iodide and four different boronic acids
were accomplished in the presence of PdCl (PPh ) as the
2
3 2
catalyst and sodium carbonate as the base. The desired 2-
phenyl-, 2-(1-naphthyl)-, 2-(2-naphthyl)-, and 2-(9-
phenanthryl)cyclopent-2-enones 2a−d were obtained in 69−
8
0% isolated yields. This easy approach to the aryl-substituted
enones 2 is crucial for the development of the new family of
phosphoramidites, as it ensures a modular and flexible access to
the corresponding ferrocenes, as shown hereafter. Next, our
strategy involves formation of the bis(cyclopentadienyl) silyl
Figure 2. Retrosynthetic pathway leading to phosphoramidites V.
18
The surprisingly scarce development of this class of ferrocene
derivatives might be related to their high air sensitivity, as
ethers 3 as the key intermediates. This step has been
accomplished through reaction of cyclopentenones 2 with di-
tert-butylsilyl bis-trifluoromethanesulfonate, in the presence of
triethylamine. It generates the desired bis(5-arylcyclopenta-1,4-
dienyloxy)silanes 3a−d in 40−77% yields as mixtures of
isomers, since the basic conditions of these reactions enable
migration of the double bonds to various positions of the
cyclopentadiene rings. For clarity reasons, Scheme 1 displays
only one of these isomeric species.
11a,14
reported for the parent 1,1′-ferrocenediol.
Thus, the first
step of our work has been the search for a suitable access to
these diols, with the additional challenge of controlling the
diastereoselectivity of their synthesis, since only the d,l isomer
is relevant to our purposes. With this in mind, we have
envisioned to assemble the substituted ferrocene directly from
FeCl and bis-cyclopentadienyl derivatives tethered by an O−
2
15
Si−O chain (Figure 2). This strategy relies on the reasonable
hypothesis that steric constraints at the cyclization step will
induce some diastereoselectivity in the assembly of the
ferrocene unit. Removal of the silicon group will then generate
in situ the desired diol.
Starting from 3a−d, deprotonation of the cyclopentadiene
units with nBuLi at −78 °C followed by treatment of the
19
resulting anions with FeCl2 afforded the desired O−Si−O
tethered ferrocenes 4a−d in 50−91% yields. Most notably,
these reactions take place with excellent stereocontrol, leading
to ferrocenes 4 as single diastereomers. These diastereomers
were demonstrated to be the chiral d,l forms through further
experiments and X-ray studies disclosed hereafter. The d,l
isomers are expected to be largely favored by steric effects, since
Previous work of Plenio et al. has shown that ferrocenyl 1,1′-
bis-silyl ethers can be obtained easily from symmetrically
substituted cyclopentadienyl silyl ethers and FeCl and that
2
16
they can serve as synthetic equivalents of hydroxyferrocenes.
B
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the two aryl substituents will be located on opposite faces of the
O−Si−O bridges. The observed high stereocontrol means that
the synthetic method largely benefits from the template effect
of the tethered cyclopentadienes, as anticipated in our initial
design.
Scheme 3. Resolution of the Phosphoramidite 5a via Chiral
Palladacyclic Complexes
The last step of the reaction sequence leading to the
ferrocenophane-type phosphoramidites 5 is typified in Scheme
2
. It involves replacement of the O−Si(tBu) −O linker by a
2
Scheme 2. One-Pot Conversion of the 1,3-Dioxa-2-
sila[3]ferrocenophanes 4 into the Corresponding
Phosphoramidites 5
O−P(NMe )−O linker following a one-pot desilylation/
2
phosphorylation procedure, in order to avoid isolation of the
air-sensitive intermediate 1,1′-dihydroxyferrocenes.
Cleavage of the silicon bridge has been carried out with
16b
tetrabutylammonium fluoride (TBAF)
in THF at room
temperature. After a workup, the crude mixture was diluted in
THF, and then triethylamine and Me NPCl were added and
2
2
the reaction mixture was stirred for 4 h at room temperature.
The desired phosphoramidites 5a−d were purified by column
chromatography on silica gel and isolated in 30−53% yields
over two steps, as single isomers. Thus, the reaction conditions
of the one-pot procedure above allow total retention of the
stereochemistry of the planar chiral ferrocene scaffold. We have
also considered alternative methods for the cyclization step,
such as the use of P(NMe) as the phosphorus reagent, but
3
they did not afford improved yields. The new phosphoramidites
5
a−d have been fully characterized by spectroscopic methods,
but unfortunately, we could not get crystals suitable for X-ray
diffraction studies.
Figure 3. ORTEP view of complex (R,R,R)-7b. Selected bond
distances (Å) and angles (deg): P−O(1) = 1.639(5), P−O(2) =
Overall, these preliminary investigations have highlighted a
suitable method for the diastereoselective synthesis of
unprecedented phosphoramidites displaying 2-phospha[3]-
ferrocenophane scaffolds. Reactions in Scheme 2 show that
the ferrocene substituents can be modulated easily. The size of
the aryl substituents does not affect the cyclization step
markedly, which looks promising for further extension of the
method.
1
.629(6), P−N, 1.618(8), P−Pd = 2.190(2), O(1)−C(1) = 1.38(1);
O(1)−P-O(2) = 104.1(3), P−O(1)-C(1) = 129.5(5); P−O(2)-C(12)
= 126.3(5); O(1)−C(1)−C(2) = 123.4(7); O(1)−C(1)−C(5) =
129.1(7); O(2)−C(12)−C(13) = 126.7(7); O(2)−C(12)−C(16) =
1
21.9(7).
Measurements reveal bond angle and distance values in the
normal range for the phosphoramidite unit. However, they
highlight a significant bending of the ferrocene unit due to ring
strain: the oxygen atoms remain coplanar with the respective
cyclopentadienyl rings, while the two cyclopentadienyl rings are
tilted toward each other, forming an angle of 11.9°. Thus, the
tilt of the flexible ferrocene unit balances the strain generated
by the short O−P−O tether.
The following step of this study (Scheme 3) allowed us to
both isolate pure enantiomers of the representative phosphor-
amidite 5a and to validate the structural assignment of 5a by X-
ray diffraction of a palladium complex. For the optical
resolution of racemic 5a we have applied the standard
procedure involving separation of diastereomeric palladium(II)
2
0
complexes. The chiral metallacyclic complexes 7a,b have been
formed by reacting the cyclopalladated (R)-N,N-dimethyl-1-
Finally, the enantiomerically pure phosphoramidites (S,S)-5a
and (R,R)-5a have been removed from their palladium
complexes by stirring with 1 equiv of 1,2-bis-
(diphenylphosphino)ethane (dppe) at room temperature in
CH Cl . The same resolution procedure has been applied to
21
naphthylethylamine complex (R)-6 with (±)-5a at room
temperature in acetone. They have been separated easily by
flash chromatography on silica gel.
2
2
After isolation of the pure diastereomers, we could grow
crystals of 7b suitable for X-ray diffraction studies. An ORTEP
drawing is shown in Figure 3. The phosphorus ligand occupies
complexes 5c,d (see the Supporting Information).
As a preliminary investigation of the coordinating behavior of
these ligands, we have considered the synthesis of platinum(II)
metallacyclic complexes of the general formula 9 (Scheme 4).
2
2
the trans-to-N position in the palladacyclic complex.
C
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Article
Scheme 4. Synthesis of the Platinum(II) Complex 10
bulk of the ferrocene unit being increased further by the phenyl
substituents, ortho to the O−P−O bridge.
The platinacyclic complex 10 displays an additional axial
chirality, since its unsymmetrical NHC ligand is oriented
25
“
orthogonally” to the square-planar platinum complex and
two different ligands are located ortho to the NHC (P and C in
Figure 5). In principle, this axially chiral core combined with
the planar chiral phosphoramidite should generate a mixture of
24
diastereomeric complexes 10.
These complexes display six-membered platinacyclic units
containing NHC− and σ-aryl−platinum bonds. Complexes of
23
this class had been investigated extensively by our group. The
platinacycles were generated from
a
NHC−
Figure 5. Schematic view of complex 10, with stereochemical
descriptors.
0
Pt (divinyltetramethyldisiloxane) complex, such as 8, by
intramolecular oxidative addition of the aryl iodide and then
combined in situ with a monodentate phosphorus ligand so as
to complete the coordination sphere of platinum(II). We had
shown notably that a variety of monodentate phosphoramidites
coordinate platinum(II) easily to generate such complexes. In
most of the square-planar complexes 9 the phosphorus ligand L
The reaction leading to 10 (Scheme 4) proved to be totally
diastereoselective, as only the R,R,R diastereomer has been
isolated. From the X-ray structure, it can be seen that the R,R,R
relative configuration minimizes the steric hindrance between
the phenyl substituent of the phosphoramidite and the
platinacyclic unit. This might account for the diastereoselectiv-
ity observed in this reaction.
24
(
L = phosphine, phosphoramidite) enters trans to the carbene
ligand, but cis coordination is also possible. Thus, as a
complement to these previous studies, we have envisioned the
synthesis of analogous complexes by reacting phosphoramidite
CONCLUSION
0
■
(
R,R)-5a with the (NHC)Pt (dvtms) complex 8. The reaction
31
In summary, we have disclosed here a suitable synthetic
approach to planar chiral phosphoramidites with [3]-
ferrocenophane structures. As the key step, the method
involves diastereoselective building of the ferrocene units
from suitable bis-cyclopentadienyl derivatives displaying an
O−Si−O tether. The phosphoramidites have been obtained in
enantiomerically pure form by resolution of their racemic
mixtures. In spite of their high steric bulk, these new
phosphoramidites proved to be suitable ligands for palladium
and platinum square-planar complexes. Investigations of the
catalytic properties of these phosphoramidites as chiral ligands
are currently ongoing.
afforded the expected platinum complex, as a single isomer ( P
NMR δ 145.8 (JP−Pt = 3008 Hz)).
The crystal structure of 10 (Figure 4) revealed that
phosphoramidite 5a coordinates cis to the NHC ligand. This
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications Web site. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.7b00700.
General information, experimental procedures, X-ray
1
crystal structure determinations, and NMR spectra ( H,
C, P) (PDF)
Figure 4. ORTEP view of complex 10. Selected bond distances (Å)
and angles (deg): Pt−P = 2.266(4), Pt−C(32) = 1.96(2), Pt−C(23) =
13
31
2
.09(2), Pt−I = 2.666(1), P−N = 1.64(1), P−O(1) = 1.655(9), P−
Accession Codes
O(2) = 1.63(1); P−Pt−C(32) = 95.5(5), P−Pt−I = 90.5(1), I−Pt−
C(23) = 90.1(4), P−Pt−C(23) = 172.8(4).
CCDC 1543585 and 1543727 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
coordination mode had been encountered previously with a
24c
TADDOL-derived phosphoramidite. The preference for cis
coordination had been tentatively assigned to the increased
steric hindrance of the TADDOL phosphoramidite, with
respect to Monophos and other phosphoramidites. Formation
of the cis complex 10 might corroborate this hypothesis, as
phosphoramidite 8 is a significantly bulky species, the steric
AUTHOR INFORMATION
Corresponding Authors
*E-mail for A.V.: arnaud.voituriez@cnrs.fr.
■
D
DOI: 10.1021/acs.organomet.7b00700
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Organometallics
E-mail for A.M.: angela.marinetti@cnrs.fr.
Article
*
(14) (a) Nesmejanow, A. N.; Ssazonowa, W. A.; Drosd, V. N. Chem.
Ber. 1960, 93, 2717. (b) Korb, M.; Schaarschmidt, D.; Lang, H.
Organometallics 2014, 33, 2099.
(15) The attempted use of bis-cyclopentadienyl derivatives with O−
P(Y)−O tethering chains failed to give the desired ferrocenes in
reasonable yields. This strategy had been applied before to the
ORCID
Arnaud Voituriez: 0000-0002-7330-0819
Notes
The authors declare no competing financial interest.
̈
synthesis of a 2-phospha[3]ferrocenophane: Hocher, T.; Cinquantini,
A.; Zanello, P.; Hey-Hawkins, E. Polyhedron 2005, 24, 1340.
ACKNOWLEDGMENTS
(16) (a) Plenio, H.; Aberle, C. Chem. Commun. 1996, 2123.
b) Plenio, H.; Aberle, C. Organometallics 1997, 16, 5950.
■
(
The authors acknowledge the support of the Institut de Chimie
des Substances Naturelles (ICSN) and the Centre National de
la Recherche Scientifique (CNRS).
(17) Krafft, M. E.; Cran, J. W. Synlett 2005, 2005, 1263.
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bis(cyclopentadienyloxy)silanes, see: (a) Rochin, C.; Babot, O.;
Moulines, F.; Duboudin, F. J. Organomet. Chem. 1984, 273, C7−
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(
13) Planar chiral 2-phospha[3]ferrocenophane units can be found in
trialkyl- and aryldialkylphosphines of the FerroPHANE series:
a) Fleury-Bregeot, N.; Panossian, A.; Chiaroni, A.; Marinetti, A.
Eur. J. Inorg. Chem. 2007, 2007, 3853. (b) Voituriez, A.; Panossian, A.;
Fleury-Bregeot, N.; Retailleau, P.; Marinetti, A. J. Am. Chem. Soc. 2008,
30, 14030. (c) Voituriez, A.; Panossian, A.; Fleury-Bregeot, N.;
Retailleau, P.; Marinetti, A. Adv. Synth. Catal. 2009, 351, 1968.
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DOI: 10.1021/acs.organomet.7b00700
Organometallics XXXX, XXX, XXX−XXX