Synthesis, structural analysis, and catalytic properties of tetrakis(binaphthyl or octahydrobinaphthyl phosphate) dirhodium(II,II) complexes

Article abstract of DOI:10.1021/om300935u

The X-ray structural analyses of homoleptic Rh(II) complexes made of enantiopure (R)-1,1′-binaphthyl and (R)-(5,5′,6,6′,7,7′, 8,8′-octahydro)binaphthyl phosphate ligands are for the first time presented. The possibility to introduce halogen atoms at the 3,3′-positions is also reported. The isolated dirhodium complexes were further tested as catalysts (1 mol %) in enantioselective cyclopropanations and Si-H insertion reactions, affording chiral cyclopropanes and silanes in good yield but moderate enantioselectivity (ee max 63%).

Full text of DOI:10.1021/om300935u

Article
pubs.acs.org/Organometallics
Synthesis, Structural Analysis, and Catalytic Properties of
Tetrakis(binaphthyl or octahydrobinaphthyl phosphate)
Dirhodium(II,II) Complexes
Radim Hrdina,† Laure Guenee,‡ Delphine Moraleda,† and Jerome Lacour*,†
́
́
́
̂
†Department of Organic Chemistry, University of Geneva, quai Ernest-Ansermet 30, 1211 Geneva 4, Switzerland
‡Laboratory of Crystallography, University of Geneva, quai Ernest-Ansermet 24, 1211 Geneva 4, Switzerland
S
* Supporting Information
ABSTRACT: The X-ray structural analyses of homoleptic
Rh(II) complexes made of enantiopure (R)-1,1′-binaphthyl
and (R)-(5,5′,6,6′,7,7′,8,8′-octahydro)binaphthyl phosphate
ligands are for the first time presented. The possibility to
introduce halogen atoms at the 3,3′-positions is also reported.
The isolated dirhodium complexes were further tested as
catalysts (1 mol %) in enantioselective cyclopropanations and Si−H insertion reactions, affording chiral cyclopropanes and
silanes in good yield but moderate enantioselectivity (ee max 63%).
in, we describe the first X-ray structural analyses of dirhodium
phosphate complexes. We also report that halogen atoms are
not easily accommodated as 3,3′-substituents unless a partial
hydrogenation of the binaphthyl backbone is performed. New
dirhodium complexes of type 2 were formed and characterized
(Figure 2). Results on the use of these complexes as catalysts
for the enantioselective cyclopropanation of styrene and the
Si−H insertion of aryldiazoacetates are reported. Unfortunately,
erosion in yields and enantioselectivity is observed upon the
addition of the 3,3′-substituents.
INTRODUCTION
■
Decomposition of diazo compounds in the presence of chiral
dirhodium complexes constitutes an important field of
enantioselective catalysis.1 Such complexes are usually prepared
by surrounding the central Rh−Rh core with four enantiopure
bridging ligands, carboxylates and imidates in particular. The
reactivity of these homoleptic complexes has been extensively
examined.2 Dirhodium complexes made of chiral biaryl
phosphate ligands can also be prepared (Figure 1). In 1992,
Pirrung and Zhang demonstrated that the treatment of
Rh2(OAc)4 with an excess of 1,1′-binaphthyl-2,2′-diyl hydrogen
phosphate, BNP-H, formed the homoleptic Rh2(BNP)4
complex 1a.3 In the same year, McKervey et al. prepared a
heteroleptic Rh(II) complex in which two homochiral BNP
anions and two hydrogen carbonates surrounded the Rh−Rh
backbone.4 Later, a series of homoleptic 3,3′-, 4,4′-, or 7,7′-
substituted binaphthyl phosphate dirhodium complexes were
RESULTS AND DISCUSSION
■
At the start of this study, there was a lack of precise structural
information for complexes 1a and 1b, these complexes being
essentially characterized through NMR spectroscopy. Care was
thus taken to repeat the synthesis of Rh2(BNP)4 1a and
attempt to determine its structure through X-ray analysis.
Compound 1a was prepared according to the original protocol
by refluxing eight equivalents of enantiopure (R)-1,1′-
binaphthyl-2,2′-diyl hydrogen phosphate 3a with Rh2(OAc)4
in chlorobenzene for 18 h (Scheme 1).3 The desired complex
was separated from the excess of acid 3a by column
chromatography (SiO2, CH2Cl2) and isolated in 79% yield.
Taking advantage of its moderate solubility in 1,4-dioxane, X-
ray quality crystals of 1a were obtained and subjected to
structural analysis (Figure 3, Table S1 in the Supporting
Information).
developed by Muller and Hodgson.5−7 These phosphate
̈
dirhodium complexes were used as chiral catalysts by these
authors and the community.8−25
Remarkably, despite the important synthetic activity in this
field, only one dirhodium complex was reported with
substituents at the 3,3′-positions of the binol moiety (Figure
1, R1 = methyl, 1b).26 In view of the actual importance of such
a substitution pattern in modern chiral phosphoric acid
catalysis,27−31 it seemed important to introduce atoms or
groups other than methyl at the R1 positions of Rh2(BNP)4
complexes. However, it was a challenge to predict whether the
formation of such complexes would be possible since the
structural determination of complex 1a and derivatives had
never been reported. Estimating which substituents R1 would fit
within the crowded environment around the dirhodium
backbone was thus difficult, and assessing their influence on
subsequent enantioselective transformations impossible. Here-
The X-ray analysis revealed a pseudo-C4-symmetrical
distribution of four bridging phosphate ligands around a Rh−
Rh core (Rh−Rh bond distance 2.521(1) Å) and a
coordination of the remaining Lewis acidic sites of the rhodium
Received: October 4, 2012
Published: January 7, 2013
© 2013 American Chemical Society
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Figure 1. Known chiral homoleptic phosphate dirhodium complexes.
Neighboring O−Rh−Rh planes are essentially perpendicular to
each other (angle 90.2(1)°). Phosphorus atoms lie out of this
plane, forming a five-membered (P−O−Rh−Rh−O) envelope-
like conformation. The angle between planes (Rh−Rh−O) and
(O−P−O) is 32.7(1)°, and the distance of P from the Rh−
Rh−O plane is 0.413(1) Å (Figure 4). Within each biaryl
ligand, the average dihedral angle between the two naphthyl
planes is 54.5(1)° (see Table S4).
Encumbrance around the hydrogen atoms at the 3- and 3′-
positions was found to be moderate. The closest neighbors to
these H atoms are the oxygen atoms connected to the
binaphthyl moiety of adjacent BNP ligands (minimal distance
H3···O 2.77(1) Å, Table S10, Supporting Information). Purely
on the basis of sterics, a substitution of these hydrogens by
atoms or groups with van der Waals radii smaller than 2.77 Å
seemed therefore feasible, and fluorine, chlorine, and bromine
atoms in particular.32,33 The formation and the study of the
stability of complexes 1c, 1d, and 1e containing these
electronegative atoms in proximity to the lone pairs of the
adjacent oxygens was attempted.
Figure 2. Targeted dirhodium complexes.
Scheme 1. Synthesis of Pirrung’s Complex 1a Starting from
Enantiopure (R)-BNP-H
Bromo-substituted binaphthyl phosphoric acid 3c (R1 = Br)
was first prepared, according to the literature procedure.34 The
synthesis of the corresponding complex was attempted using
the experimental conditions used for 1a (chlorobenzene, 155
°C, 18 h). Differences were immediately noticed, as the
exchange of the acetate ligands was incomplete after 18 h. The
crude contained an inseparable mixture of heteroleptic and
atoms by one molecule of water and one molecule of 1,4-
dioxane. The four bridging oxygen atoms of opposite
phosphate ligands are quite perfectly aligned in a plane
(maximum atomic deviation <0.1 Å and torsion angles O−
Rh−Rh−O ∼3.6°; see Tables S4−S5, Supporting Information).
Figure 3. ORTEP views along the Rh−Rh axis (left) and on the side (right) of the crystal structure of (R)-1a. Thermal ellipsoids are drawn at 50%
probability. All hydrogen atoms, except those at 3,3′-positions, and solvent molecules are omitted for clarity.
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Figure 4. Rh−Rh core of 1a with defined planes, selected distances, and angles (Table S2, Supporting Information).
Scheme 2. Synthesis of 3,3′-Difluoro 1,1′-Binapthyl Phosphoric Acid 3e
homoleptic dirhodium complexes with phosphate and acetate
ligands around the metals. Prolongation of the reaction time to
24 h or the use of a 16-fold excess of acid 3c did not lead to a
complete exchange of the acetates.
complicated crude mixture was obtained, and the major
component 4 was isolated in 78% yield. This complex, made
of two phosphate and two trifluoroacetate ligands around the
Rh−Rh bond, was analyzed by NMR spectroscopy. The cis- or
trans-disposition of the bridging ligands could not, however, be
ascertained (Scheme 3).
3,3′-Dichloro and 3,3′-difluoro-1,1′-binaphthyl hydrogen
phosphates, 3d and 3e, respectively, were then tested, hoping
that the formally smaller chlorine and fluorine atoms would
enable the exchange of the ligands. These binol-derived acids
have, to our knowledge, never been reported in the literature.
Their preparation was however straightforward starting from
MOM-protected (R)-binol.35 As described in Scheme 2 (fluoro
series), a selective deprotonation with n-BuLi (3.5 equiv) at 0
°C (THF, 60 min) followed by treatment with an electrophilic
source of chlorine (C2Cl6, 4 equiv)36 or fluorine (NFSI, 6
equiv) and acidic hydrolysis in 1,4-dioxane afforded the 3,3′-
dichloro and 3,3′-difluoro binaphthol in 34% and 24%
combined yield (X-ray data in Table S11 for the fluoro binol,
Supporting Information). Then, reaction with phosphorus
oxychloride and hydrolysis in the presence of pyridine gave
acids 3d and 3e in 50% and 58% yields, respectively.
Scheme 3. Ligand Exchange Reactions of 3c, 3d, and 3e with
Rh2(OAc)4 and Rh2(TFA)4
Unfortunately, the exchange of acetate by hydrogen
phosphate ligands 3d and 3e was also incomplete. In the case
of dichloro 3d, it was possible to isolate, in low 3% yield,
homoleptic complex 1d, made of four phosphates, while with
difluoro 3e, heteroleptic complex 6, made of three phosphate
and one acetate ligand, was characterized with a 5% yield. In
view of these results, explained by the relatively poor leaving
group ability of acetate ligands, dirhodium precursor
Rh2(TFA)4 (TFA = trifluoroacetate) was used instead of
Rh2(OAc)4. The exchange reaction was first tested with 3,3′-
dibromo-1,1′-binaphthyl hydrogen phosphate 3c. A less
Trying to find a solution for making the homoleptic
derivatives, we reflected on the synthesis of complex 1b,
which is effective despite the presence of rather large R1 methyl
substituents.26 We considered that its success is the result of the
electron-rich nature of ligands of 1b, this ligand being able to
substitute the acetates of Rh2(OAc)4 more easily than ligands of
1c or 1d.37 Then, modifications of the binaphthyl skeleton of
these acids that would enable (i) a transformation into an
electron-rich moiety and (ii) the possibility to still introduce
halogen atoms at the desired 3,3′-positions would be favorable
for complete substitution. With this guideline, a partial
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Figure 5. Crystal structure of pseudo-C4-symmetrical tetrakis[(R)-1,1′-(5,5′,6,6′,7,7′,8,8′-octahydrobinaphthylphosphate]dirhodium, Rh2[(R)-H8-
BNP]4, 2a.
Figure 6. Rh−Rh core of 2a with defined plane and selected distances and angles (Tables S7−S9).
saturation of the naphtho rings was considered as a possible
solution.38 (R)-(5,5′,6,6′,7,7′,8,8′-Octahydro)binaphthyl phos-
phoric acid 7a39 and its 3,3′-dibrominated analogue 7b were
prepared according to literature procedures.40
Refluxing eight equivalents of acid 7a with Rh2(OAc)4 in
chlorobenzene for 18 h resulted in the expected formation of
the desired complex 2a (82%).41 Monocrystals were obtained
from the mixture of acetonitrile and dichloromethane, and a
structural X-ray analysis was performed (Figure 5, Table S6 in
the Supporting Information). The resulting complex presented
again a pseudo-C4-symmetrical geometry and two molecules of
acetonitrile as apical ligands on the rhodium atoms. The Rh−
Rh bond distance is 2.513(1) Å, very close to that in complex
1a (Table S7, Supporting Information).
S8), 5° more than for compound 1a. This larger value for the
torsion angle was expected on the basis of previous
observations.42 It might be the reason for the slightly increased
average O−P−O angle value observed within the chelating ring
(118.9° and 118.2° for 2a and 1a, respectively), which in turn
might be the reason for the twisted envelope-like (Rh−Rh−O−
P−O) conformation (Figure 6, Table S7).
Measurement on each half of the complexes of the distances
between the four H3 and H3′ atoms revealed an interesting
aspect. For compound 2a, these hydrogen atoms are distant on
average by 7.12 and 7.20 Å on each half, forming comparable
quadrilaterals of rather rectangular shape.
The chiral pockets created by the ligands around the
rhodium atoms are thus virtually identical on each side for 2a,
and their size is intermediate to those measured for 1a. In fact,
the analogous analysis performed on 1a revealed a contrasted
situation. The average distances between H3 and H3′ atoms are
5.95 and 7.53 Å on each side respectively, indicating the
For each ligand, a twist of the chelating ring can be noticed
with a mean torsion angle O−Rh−Rh−O of 16.4(1)° (Figure
6, Table S9, Supporting Information). The average dihedral
angle between the two phenyl planes is around 59.9(1)° (Table
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Figure 7. Comparison of complexes 1a (left) and 2a (right) considering the free volume of the apical electrophilic sides of rhodium atoms.
existence of a tight ligand pocket around one rhodium atom
and a much larger one around the other (Table S10,
Supporting Information). The quadrilaterals are drawn in
Figure 7 (left) and compared to that of 2a (right). Interestingly,
the tight pocket surrounds the bound 1,4-dioxane molecule and
not the small water ligand. The ligand exchange reaction was
then performed with 3,3′-dibromo-5,5′,6,6′,7,7′,8,8′-octahydro-
binaphthyl hydrogen phosphate 7b. In this case, in contrast to
3c, the process went to completion and the homoleptic
complex 2b was isolated in 30% yield (Scheme 4). This positive
With complexes 2a and 2b in hand, their catalytic efficiency
was evaluated using first the cyclopropanation of styrene with
methyl phenyldiazoaceate 8a, a reaction known to occur with
the classic BNP catalyst 1a (Scheme 5).25 While both 2a and
2b performed well in terms of reactivity (80% and 74% yields),
a lower selectivity was achieved. Cyclopropane 9 was obtained
as a single diastereomer in 38% and 23% ee, respectively; all
three catalysts afforded the product in the same predominant
enantiomeric form.
In view of these rather negative results, another attempt was
performed using the Si−H insertion reactions of methyl
phenyldiazoacetates 8 (Scheme 5). Reactions of this type had
been reported only in dirhodium catalysis with chiral
carboxylate-derived complexes.43−57 For instance, Moody and
co-workers showed that Rh2(MEPY)4 affords the highest
selectivity (55% ee) of a series of dirhodium carboxylates and
carboxamidates.51 Davies et al. reported the application of
Rh2(DOSP)4 in this reaction to afford products of type 10 with
85% ee,45 and the group of Hashimoto developed a
Rh2(PTPA)4 complex that provided the insertion products
with enantioselectivity values up to 72%.49
Scheme 4. Synthesis of Complexes 2a and 2b
Scheme 6. Dirhodium-Catalyzed Si−H Insertions of
Phenyldiazoacetates
result seems to validate the hypothesis of easier exchange
reactions with partially saturated biaryl ligands. Crystals of 2b
were obtained from a 1,4-dioxane solution. The quality was
sufficient to determine a C4-symmetrical arrangement of the
ligands, but insufficient for a more precise X-ray analysis.
Introducing even larger substituents would formally be
interesting, but X-ray structures of 2a and 2b indicate that
formation of such complexes is highly unlikely for steric
reasons.
Scheme 5. Dirhodium-Catalyzed Cyclopropanation of Styrene
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Table 1. Dirhodium-Catalyzed Si−H Insertions of Phenyldiazoacetates
entry
catalyst
diazo (R =)
silane
solvent
CH2Cl2
T [°C]
yield [%]
ee [%]
1
2
3
4
5
6
1a
1a
1a
1a
1a
1a
1a
1a
1a
2a
2b
H (8a)
H (8a)
H (8a)
H (8a)
H (8a)
H (8a)
H (8a)
OMe (8d)
NO2 (8e)
H (8a)
H (8a)
Et3SiH
Me2PhSiH
Ph3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
25
25
83 (10a)
28 (10b)
16 (10c)
60 (10a)
90 (10a)
42 (10a)
66 (10a)
81 (10d)
46 (10e)
72 (10a)
48 (10a)
47; (−)-10a
43; 10b
CH2Cl2
CH2Cl2
CHCl3
25
44; 10c
25
49; (−)-10a
34; (−)-10a
38; (−)-10a
63; (−)-10a
62; (−)-10d
44; (−)-10e
44 ; (−)-10a
20; (+)-10a
toluene
cyclohexane
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
25
25
a
7
−78
25
8
9
25
10
11
25
25
a
12 h reaction time at −78 °C.
Methyl-2-diazophenylacetate 8a (R = H) and Rh2(BNP)4 1a
free of charge via the Internet at http://pubs.acs.org. CCDC-
904048 to CCDC-904050 contain the supplementary crystallo-
graphic data for this paper (compounds 1a, 2a, and (R)-3,3′-
difluoro-1,1′-bi-2-naphthol). These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
were first chosen as model reactant and catalyst (1 mol %),
respectively. The silanes Et3SiH, PhMe2SiH, and Ph3SiH were
tested, and the results are reported in Table 1 (entries 1 to 3).
Et3SiH provided the corresponding product 10a in good yield
(83% yield) and with the best enantiomeric excess (47% ee) of
the three silanes. Et3SiH was thus used for further optimization.
A rapid evaluation of common solvents (toluene, cyclohexane,
CHCl3, CH2Cl2) was performed, and it indicated that CH2Cl2
was better as far as yield and enantiomeric purity of 10a are
concerned. Low temperature (−78 °C) improved the selectivity
of the reaction (63% ee) at the expense of the reactivity
however (10a: 66% after 12 h). Selectivity could also be
improved using more reactive diazo compound 8d (R = p-
MeO), as resulting product 10d was obtained in 62% ee.
Substitution of the diazo reactant with an electron-withdrawing
group (8e: R = p-NO2) had the reverse effect on yield and
enantiomeric excess.
Partially saturated catalysts 2a and 2b were then tested. In
the case of catalyst 2a and substrate 8a, a slight decrease of
reactivity and selectivity was observed at room temperature
(72% yield, 44% ee), the major enantiomer having the same
absolute configuration as that obtained predominantly with 1a.
For the reaction with complex 2b, adduct 10a was afforded in
only 48% yield and 20% enantiomeric excess, the product being
of opposite (dextrorotatory) configuration of the previous
examples. Obviously, unlike what had been desired initially,
lower reactivity and enantioselectivity were obtained with 3,3′-
disubstituted complex 2b, an effect of the bromine atoms being
clear from the reversal of the absolute sense of stereoinduction.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jerome.lacour@unige.ch.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support of this work by the COST
Action CM0802, Swiss National Science Foundation, and the
State Secretariat for Education and Science. We also acknowl-
edge the contributions of the Sciences Mass Spectrometry
(SMS) platform at the Faculty of Sciences, University of
Geneva, and the help of Dr. S. Michalet in particular.
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■
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ASSOCIATED CONTENT
* Supporting Information
Spectroscopic data for novel compounds and procedures
describing the synthesis of stated compounds are available
■
S
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