Synthesis and Reactivity of Chiral, Wide-Bite-Angle, Hybrid Diphosphorus Ligands

Article abstract of DOI:10.1002/ejic.201301255

Effective and modular synthetic approaches toward phosphine-phosphite ligands and phosphine-phosphonite ligands featuring a diphenyl ether backbone have been developed. The phosphine-phosphite ligands are obtained by a two-step protocol from 2-bromo-2′-methoxydiphenyl ether. The phosphine-phosphonite ligands are prepared in a four-step synthetic protocol that involves a novel, unsymmetrical diphenyl ether derived phosphine-phosphorusdiamide as key building block. Structural studies on Pt^II complexes with either phosphine-phosphite or phosphine-phosphonite ligands indicate strict cis coordination for these ligand systems. High-pressure NMR spectroscopy studies of Rh complexes under syngas indicate the presence of two ea isomers for Rh(H)(CO)₂(PP). The existence of this mixture is further supported by high-pressure IR spectroscopy studies. In order to benchmark the activity and selectivity of these novel, wide-bite-angle, mixed-donor ligands, they were screened in Pd-catalyzed asymmetric allylic alkylation as well as Rh-catalyzed hydrogenation and hydroformylation reactions. The ligands give 100-% conversion and low-to-moderate enantioselectivity in the allylic alkylation of 1,3-diphenyl-2-propenyl acetate and cyclohexyl-2-enyl acetate with dimethyl malonate. In the hydroformylation of styrene, good conversion and regioselectivities are achieved but only moderate enantioselectivity. The ligands give good conversions in asymmetric hydrogenation of typical substrates, with good-to-excellent enantioselectivities of up to 97-% depending on the substrate. Copyright

Full text of DOI:10.1002/ejic.201301255

FULL PAPER
DOI:10.1002/ejic.201301255
Synthesis and Reactivity of Chiral, Wide-Bite-Angle,
Hybrid Diphosphorus Ligands
CLUSTER
ISSUE
Christine Fee Czauderna,[a][‡] David B. Cordes,[a]
Alexandra M. Z. Slawin,[a] Christian Müller,[b][‡‡]
Jarl Ivar van der Vlugt,[c] Dieter Vogt,[b][‡‡‡] and Paul C. J. Kamer*[a]
Dedicated to Professor David Cole-Hamilton on the occasion of his retirement
Keywords: P ligands / Synthesis design / Palladium catalysis / Rhodium catalysis
Effective and modular synthetic approaches toward phos-
phine–phosphite ligands and phosphine–phosphonite li-
gands featuring a diphenyl ether backbone have been devel-
oped. The phosphine–phosphite ligands are obtained by a
two-step protocol from 2-bromo-2Ј-methoxydiphenyl ether.
The phosphine–phosphonite ligands are prepared in a four-
step synthetic protocol that involves a novel, unsymmetrical
diphenyl ether derived phosphine–phosphorusdiamide as
key building block. Structural studies on PtII complexes with
either phosphine–phosphite or phosphine–phosphonite li-
gands indicate strict cis coordination for these ligand sys-
supported by high-pressure IR spectroscopy studies. In order
to benchmark the activity and selectivity of these novel,
wide-bite-angle, mixed-donor ligands, they were screened in
Pd-catalyzed asymmetric allylic alkylation as well as Rh-cat-
alyzed hydrogenation and hydroformylation reactions. The
ligands give 100% conversion and low-to-moderate enantio-
selectivity in the allylic alkylation of 1,3-diphenyl-2-propenyl
acetate and cyclohexyl-2-enyl acetate with dimethyl malon-
ate. In the hydroformylation of styrene, good conversion and
regioselectivities are achieved but only moderate enantio-
selectivity. The ligands give good conversions in asymmetric
tems. High-pressure NMR spectroscopy studies of Rh com- hydrogenation of typical substrates, with good-to-excellent
plexes under syngas indicate the presence of two ea isomers
for Rh(H)(CO)2(PP). The existence of this mixture is further
enantioselectivities of up to 97% depending on the substrate.
challenging substrates, to induce higher activity or to im-
prove the enantioselectivity. To achieve high control over
the coordination geometry and to transfer chiral infor-
mation efficiently, bidentate ligands are frequently used.
Initially, diphosphorus ligands bearing two identical phos-
phorus donors, either with a C2-symmetric or a non-C2-
symmetric backbone[2,3] were applied, but more recently hy-
brid diphosphorus ligands,[4] featuring two non-equivalent
P groups were successfully applied in, for example, asym-
metric hydrogenation,[5] allylic substitution[6] and hydrofor-
mylation, resulting in high chemo-, regio- and stereoselecti-
vity.[7–12] Ligands bearing two distinctly different donor
entities, e.g. a σ-donating group (Figure 1, left) and a π-
accepting group (Figure 1, right), can influence the coordi-
nation of the substrate to the metal centre via electronic
(trans-effect) and steric differentiation (Figure 1).[13] For the
Rh-catalyzed hydrogenation, Achiwa and co-workers dis-
covered that in the substrate complex, the Ptrans atom has
mainly an electronic effect on the coordinated substrate and
the interaction between the Pcis atom and the coordinated
substrate is a steric effect.[14] In catalytic reactions, the coor-
dination behaviour of the two donor atoms can control the
reactivity and stereoselectivity.
Introduction
Over the last years, transition-metal catalysis has grown
to be a powerful tool in asymmetric organic synthesis of
high-value-added compounds such as flavours, fragrances,
pharmaceuticals and agrochemicals.[1] Many new chiral
phosphorus-based ligands have been developed to convert
[a] EaSTCHEM, School of Chemistry, University of St. Andrews,
St. Andrews, Fife, KY16 9ST, United Kingdom
E-mail: pcjk@st-andrews.ac.uk
http://www.st-andrews.ac.uk/chemistry/contact/academic/#pcjk
[b] Department of Chemistry and Chemical Engineering,
Eindhoven University of Technology,
The Netherlands
[c] Homogeneous & Supramolecular Catalysis,
van ’t Hoff Institute for Molecular Sciences,
University of Amsterdam,
Science Park 904, 1098 XH Amsterdam, The Netherlands
[‡] Current address: Janssen Pharmaceutica, 2340 Beerse, Belgium
[‡‡] Current address: Institut für Anorganische Chemie, Freie
Universität Berlin, Deutschland
[‡‡‡] Current address: EASTCHEM, School of Chemistry,
University of Edinburgh, United Kingdom
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301255.
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bone. We anticipate that the σ-donating phosphine group
and π-accepting phosphonite or phosphite fragment, bear-
ing a chiral auxiliary, will induce a strongly unsymmetrical
environment around the catalytic centre. The use of dif-
ferent, tuneable chiral building blocks in combination with
easily accessible scaffolds provides opportunities to create a
library of ligands and the possibility to screen for active
and selective catalysts (Figure 3).
Figure 1. Interplay between steric and electronic character of two
different P-donor groups. P1: electron-donating, sterically de-
manding. P2: electron-accepting, sterically undemanding.
A number of hybrid diphosphorus ligands, including the
recently described phosphite–phospholane bobphos (1),[8]
phosphine–phosphite BINAPhos (2)[9] and the very related
phosphine–phosphoramidite YanPhos (3),[10] as well as
phosphine–phosphonite ligands with a xanthene backbone
(4)[11] (Figure 2) have allowed for significant breakthroughs
in the asymmetric hydroformylation of various prochiral
alkenes, including heterocyclic olefins.
Figure 3. Schematic scheme of novel, mixed-donor P,PЈ-ligands. X
indicates either a Br (to introduce phosphonite) or an OH group
(to arrive at a phosphite unit).
In order to arrive at these hybrid ligand systems, straight-
forward synthetic routes to two new classes of mixed-donor
P,PЈ-ligands have been developed, i.e. phosphine–phos-
phonite and phosphine–phosphite derivatives. To test the
influence of the different chiral auxiliaries and to determine
whether the wide bite angle in combination with the dif-
ferent donating character has a pronounced effect on the
reactivity of transition-metal complexes, the coordination
behaviour of these ligands was studied and they were tested
in several catalytic reactions.
Results and Discussion
Synthesis of Hybrid Phosphine-Phosphonite Ligands
A straightforward synthetic route was established for
phosphine–phosphonite mixed-donor ligands based on
DPEphos through the selective, sequential introduction of
the two different phosphorus moieties, with aminophos-
phine 7 as key intermediate (Scheme 1). The two methyl
groups in the 4,4Ј-position of the diphenyl ether backbone
Figure 2. Examples of hybrid diphosphorus ligands: phosphite–
phospholane (bobphos 1), phosphine–phosphite (BINAPhos 2),
phosphine–phosphoramidite (YanPhos 3) and phosphine–phos-
phonite (4).
Based on the encouraging results obtained recently with are required to avoid side-reactions during lithiation. Selec-
rigid phosphine–phosphonite ligands (4),[11] a strategy was tive lithiation of only one of the Br groups followed by reac-
devised to combine mixed-donor ligand design with a wide- tion with chlorodiphenylphosphine gave species 6. Gratify-
bite-angle but flexible ligand backbone; the latter inspired ingly, formation of the unwanted cyclic phenoxaphosphine
by the successful application of the related diphosphine li- side-product during the second lithiation step, concomitant
gand DPEphos in several catalytic reactions with good re- with P–Ph cleavage,[18] was completely suppressed by pro-
gioselectivity.[15–17] Ultimately, chiral ligands based on di- tecting phosphine 6 as its borane adduct, intermediate 6-
phenyl ether and featuring two inequivalent phosphorus (BH3) (31P δ = 18.5 ppm, broad). This not only rendered
moieties may be expected to manifest both high stereoselec- the phosphine air-stable, enabling facile purification, but it
tivity and regioselectivity. Formation of such hybrid systems also allowed straightforward introduction of the P(NEt2)2
could be achieved by sequential introduction of, for exam- unit through a lithium/bromide exchange reaction followed
ple, a phosphine and a chiral phosphite or phosphonite by addition of ClP(NEt2)2. To facilitate purification and
group at the two phenyl rings of the diphenyl ether back- prevent hydrolysis, also this intermediate 7 was converted
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Scheme 1. Synthetic route to phosphine–phosphonite ligands.
into the BH3-product 7-(BH3)2. The 31P NMR spectrum of
7-(BH3)2 in CD2Cl2 at room temperature shows an AX spin
system with broad doublets at δ = 17.8 and 88.8 ppm (1JPB
coupling constant of 105 Hz), assigned to the BH3 adducts
of the aminophosphine and diphenylphosphine groups,
respectively.
Compound 7-(BH3)2 is stable towards oxidation and hy-
drolysis and can be easily purified by precipitation from
acetone and methanol in good yields (80%). Crystals of 7-
(BH3)2 suitable for X-ray crystallography were obtained by
recrystallization by slow diffusion of hexane into a diethyl
ether solution (Figure 4). The P–C bond length of the back-
bone to the diphenylphosphine group [1.815(4) Å] is vir-
tually the same as the P–C bond length of the backbone to
the diethylaminophosphine group [1.819(4) Å]. Both P–C
bond lengths are similar to the values found for other wide-
bite-angle ligands such as Xantphos.[16] The intramolecular
distance between the two phosphorus atoms [5.803(2) Å] is
Figure 4. ORTEP representation of compound 7-(BH3)2. Displace-
ment ellipsoids are drawn at the 50% probability level. All hydro-
larger than in Xantphos (4.080 Å), and obviously the flexi- gen atoms, except those bound to boron, are omitted for clarity.
Selected bond lengths (Å) and angles (°): P1–C1 1.819(4); P1–B1
ble backbone has to rearrange to allow chelation of a metal
1.918(6); P2–N21 1.673(3); O1–C2 1.394(4); O1–C22 1.408(4); B1–
atom. The P–B bonds lengths [1.910(4) and 1.918(6) Å] of
P1–C8 115.8(2); C2–O1–C22 114.8(3); B21–P2–N21 101.91(15);
both phosphorus groups are almost indistinguishable. The
B21–P2–N22 112.66(19).
tetrahedral geometry is distorted around both phosphorus
centres, judging from, for example, the B–P–C angles.
The BH3 protecting groups were quantitatively and
smoothly removed by reaction with an equimolar amount
of DABCO in toluene at 60 °C overnight without affecting
the P–NEt2 bond. The 31P NMR spectrum shows one sing-
let at δ = 90.9 ppm (aminophosphine) and one singlet at
–18.0 ppm (phosphine). The chemical shifts of the amino-
phosphine and phosphine groups are in the same range as
previously reported for related species.[11] Ultimately, two
hybrid phosphine–phosphonite ligands were prepared by
reacting (S)-BINOL derivatives with the aminophosphine
moiety in refluxing toluene for one day (for 8a) or two days
(8b). The longer reaction time for 8b was expected as this
quantitative yield after flash chromatography (silica) under
an argon atmosphere. The respective 31P NMR spectra for
these species show two doublets, one for the phosphinite
group at δ = 179.5 and 179.8 ppm for 8a and 8b and one
for the phosphine group at –16.1 and –15.2 ppm, respec-
tively, which is in the same range as for previously reported
related ligands.[3]
Synthesis of Hybrid Phosphine–Phosphite Ligands
The rather air- and moisture-sensitive nature of phos-
reaction can be hampered by steric hindrance when using phine–phosphonite ligands 8a and 8b complicated synthetic
too bulky diols,[19] The products were isolated in almost and workup procedures. The phosphine–phosphite ana-
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Scheme 2. Synthetic route to phosphine–phosphite ligands.
Figure 5. Phosphine–phosphite ligands 12a–12e.
logues could be assembled in a more facile manner by reac- 140 ppm and –17 ppm (7JP,P coupling of around 7 Hz), as-
tion of a DPE-based phenol with a chiral phosphorochlor- signed to the phosphite and diphenylphosphine moieties,
idite in the final step, after introduction of the phosphine respectively.
via selective lithiation. Hence, a straightforward four-step
synthetic procedure led to a series of new chiral phosphine–
phosphite ligands (Scheme 2). In order to allow for selective
monophosphorylation to obtain the key monophosphine–
Coordination Behaviour toward Platinum
monophenol intermediate in good yield, a diphenyl ether
The PtCl2(L) complexes were synthesized by facile reac-
scaffold with one methoxy group on one phenyl ring and a tion of PtCl2(MeCN)2 with an equimolar amount of the
bromine on the other phenyl ring was required. Adapted respective ligand L in CH2Cl2. Due to the inequivalent P-
from related literature procedures, building block 9 was ob- donor groups, two signals are observed in the 31P NMR
tained by the copper catalyzed C–O-coupling of 2-bro- spectra, with additional 195Pt-satellites and with larger P–P
mophenol and 2-bromoanisole in the presence of 2,2,6,6- coupling constants than found for the corresponding free
tetramethylheptane-3,5-dione as ligand.[20]
ligands. Selected NMR spectroscopic data are presented in
Subsequently, selective lithiation of the bromine func- Table 1. In case of PtCl2(12c) and PtCl2(12d) the P–P cou-
tionality with nBuLi followed by phosphorylation by using pling is in the range 19.2–19.7 Hz. The phosphorus signals
chlorodiphenylphosphine gave monophosphine 10. The of PtCl2(8a) and PtCl2(12b) are broad and therefore no P–
methyl ether was cleaved to generate a free phenol function- P coupling is visible. As expected, all of the complexes show
1
ality by using BBr3 in CH2Cl2. 31P NMR spectroscopy re- a coupling constant JPt,P of 3538–3690 Hz for the phos-
1
veals
a
phosphinoborane intermediate 11Ј (1JPB
=
phine moiety and a coupling constant JPt,P of 5045 Hz for
1
146.4 Hz). The P–B bond could be cleaved under basic con- the phosphonite and JPt,P larger than 6000 Hz for the
ditions i.e. by rapid stirring of a biphasic mixture of aque- phosphite moiety. These coupling constants are similar to
ous NaHCO3 and CH2Cl2 solution of 11 overnight. After previously reported PtCl2(PP)complexes with phosphine,[16]
full conversion of 11Ј a singlet is observed at –18.14 ppm phosphite[21] and phosphonite[22] ligands and indicate a mu-
for the desired pure monophosphine–monophenol 11. The tual cis coordination of the ligands in the square-planar Pt
last step in the phosphite formation was the condensation complexes. Unfortunately, despite many attempts no suit-
of the phenol 11 with the appropriate phosphorochloridite able samples could be obtained for X-ray crystallographic
reagent [(RO)2PCl] to generate a small library of these analysis of these complexes, which prohibits the determi-
mixed-donor ligands. The 31P NMR spectrum of ligands nation of true experimental bite angles for these ligand
12a–12e (Figure 5) shows an AX spin system around series.
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Table 1. 31P NMR spectroscopic data, PtCl2(L) complexes with li- spectroscopy. These experiments were carried out with one
gands 8a and 12b–12d.[a]
representative example for each of the two hybrid ligand
classes, i.e. ligands 8b and 12c.
Figure 6. Bis(equatorial) (ee) and equatorial–apical (ea) coordina-
tion modes of bidentate phosphorus ligands in the resting state of
the hydroformylation catalyst.
Complex Ligand
δ (ppm)
2JP,P (Hz) 1JPt,P (Hz)
13
14
15
16
17
12c
12d
12b
8a
83.8
4.6 (–PPh2)
83.3
4.7 (–PPh2)
44.85
0.7 (–PPh2)
111.6
19.7
–
6052
3538
6060
3535
6829
3555
5045
3690
3795
[b]
19.2
[b]
–
[b]
–
High-Pressure NMR Spectroscopy
[b]
–
[b]
–
The Rh(H)(L)(CO)2 complex was formed from
Rh(acac)(CO)2 (20 mm) and P,P-ligand 8b and 12c under
20 bar of CO/H2 pressure in [D8]toluene at room tempera-
ture (Table 2). The proton-coupled 31P NMR spectra of
[Rh(H)(8b)(CO)2] and [Rh(H)(12c)(CO)2] show two charac-
teristic signals attributed to the two non-equivalent phos-
phorus moieties of the ligands 8b and 12c in the Rh-phos-
phine and Rh-phosphite/phosphonite region as expected.
4.1 (–PPh2)
DPEphos[c] (–PPh2)
2.0
[a] Reactions in dichloromethane/MeCN at room temperature for
16 h. [b] Broad signal, unresolved. [c] Literature values for
PtCl2(DPEphos) from ref.[17]
Coordination to Rhodium Complexes
Furthermore, the formation of a Rh(H)(L)(CO)2 species is
1
The coordination of ligand 12c towards rhodium was ini- confirmed by the hydride signal in the H NMR spectrum.
tially investigated by in situ NMR spectroscopy by reaction For the phosphine–phosphite ligand 12c, a doublet-of-dou-
with [Rh(cod)2]BF4 in CHCl3 at room temperature. The 31
P
blets-of-doublets signal is observed between –9.6 and
NMR spectrum of this species 18 shows two slightly broad- –9.9 ppm and for the phosphine–phosphonite 8b, a pseudo-
ened doublets-of-doublets i.e. the phosphonite (δ triplet-of-doublets signal between –8.5 and –8.8 ppm. The
=
173.4 ppm) and the phosphine (δ = 22.5 ppm) for the major rhodium-hydride coupling constants (JH-Rh = 9.5 Hz for the
1
species (≈90%), with JP-Rh (225.4 Hz and 141.2 Hz) and phosphine–phosphite ligand and 8 Hz for the phosphine–
2JP,P (333 Hz) coupling constants, which suggests that the phosphite ligand) indicate a phosphorus atom trans to the
complex can indeed be formulated as [Rh(cod)(12c)]BF4.
The catalyst resting state in the rhodium-catalyzed hy- ordination of the phosphine–phosphite moieties.
droformylation is typically five-coordinate [Rh(H)- Large trans-P–H coupling constants are normally re-
(L)(CO)2] complex, which can exist as a mixture of two ported in [Rh(H)(L)(CO)2] complexes with equatorial–api-
equilibrating trigonal-bipyramidal isomers, i.e. bis- cal coordination of the diphosphine and diphosphite li-
hydride and therefore typically an equatorial–apical (ea) co-
a
a
(equatorial) (ee) and an equatorial-apical (ea) iso- gands (typically 150–180 Hz for phosphites and 90–110 Hz
mer.[20,21,23] In case of mixed-donor diphosphorus ligands, for phosphines),[24] whereas relatively small cis coupling
further distinction between ea A and ea B is necessary (Fig- constants (1–10 Hz) have been observed in [Rh(H)(L)(CO)
ure 6). To obtain more information about the active species 2] with bis(equatorial) coordination of the diphosphine and
in the hydroformylation and to determine if the change in diphosphite ligands.[25] The intermediate phosphorus–hy-
bite angle on going from the phosphine–phosphite to the dride coupling constants (2JP-H) of 85 and 45 Hz for ligand
phosphine–phosphonite ligand has any effect on the chela- 12c and 56 and 51 Hz for ligand 8b suggest that these com-
tion of the ligand structure, the respective catalyst resting plexes exist as mixtures of the two equatorial–apical (ea)
states were studied by high-pressure (HP) NMR and IR isomers A and B that are in dynamic equilibrium[21] (Fig-
1
Table 2. Selected high-pressure NMR spectroscopic data (top: H; bottom: 31P) for Rh(H)(8b)(CO)2 and Rh(H)(12c)(CO)2.[a]
Complex
L
δ (ppm)
2JP-H (Hz)[b]
2JP-H (Hz)[c]
2JH-Rh (Hz)
19
20
8b
12c
9.1 (pseudo-td)
9.8 (ddd)
60.1
84.8
52.2
43.4
10
9.5
Phosph(on)ite
δ(ppm)
Phosphine
δ (ppm)
2JP-H (Hz)
2JP-H (Hz)
2JH-Rh (Hz)
2JP-Rh (Hz)
2JP-Rh (Hz)
19
20
187.1 (ddd)
164.8 (ddd)
53.5
43.4
3.2
33.1
191.5
225.2
23.5 (dd)
21.6 (b)[d]
60.1
114.1
–
[a] Conditions: Rh(acac)(CO)2 as precursor, [Rh] = 20.1 mm, Rh/L = 1:1, p = 20 bar, T = 298 K. [b] Phosphine. [c] Phosph(on)ite. [d]
Broadened signal, unresolved.
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ure 6) although the presence of minor ee species is not ex- Table 3. Selected high-pressure IR data of Rh(H)(L)(CO)2.[a]
cluded. Rapid interconversion of the two isomers, as de-
Complex
Ligand
ee
˜
ea
˜
scribed previously for diphosphite complexes,[24] inter-
changes the phosphorus atoms, and hence intermediate
chemical shifts and coupling constants are observed. The
ν [cm–1]
ν [cm–1]
19
20
8b
12c
DPEphos
2052, 1980
–
2049, 1970
2006, 1959
2021, 1975
1992, 1943
31P NMR spectra of both complexes show significant 21
broadening of the signals for the phosphine moiety at room
[a] Conditions: Rh(acac)(CO)2, [Rh] = 1 mm, Rh/L = 1:2.1, p =
20 bar, T = 323 K.
temperature and indicate a fast equilibrium between the
two isomers A and B. This has also been observed for other
phosphine–phosphite ligand systems,[26,27] whereas systems
with flexible bridges show static behaviour.[28] The signals
of the phosphite and phosphonite moieties are well resolved
as doublet-of doublets-of-doublets. Remarkably, the phos-
phorus-phosphorus coupling constants of [Rh(H)(12c)-
(CO)2] and [Rh(H)(8b)(CO)2] differ significantly (76 vs.
6 Hz). This suggests a more ideal cis coordination for the
phosphine–phosphonite ligand 8b. The phosphorus-hydride
coupling constants for the phosphine are larger than for the
corresponding phosphite and phosphonite moieties. Hence,
the isomer with the phosphine occupying the apical posi-
tion trans to the hydride is preferred. These results were also
observed with other phosphine–phosphite ligands.[26–28] In
contrast to this, the [Rh(H)(BINAPhos)(CO)2] complex ex-
ists dominantly as the ea isomer in which the phosphite
moiety preferably occupies the apical position and the
phosphine coordinates in the equatorial position.[21,29] In
the proton-decoupled 31P{1H} NMR spectrum of the
[Rh(H)(8b)(CO)2] complex, the two signals appear as two
sharp doublets with Rh–P coupling constants of 191 Hz for
the phosphonite and 114 Hz for the phosphine, which
corroborates the very small JP-P indicative of cis coordina-
tion.
hydroformylation product (C=O stretching vibration
around 1800 cm–1) appeared immediately after reapplying
CO/H2 pressure (20 bar).
Hybrid Ligands in Pd-Catalyzed Allylic Substitution
In the asymmetric substitution of allylic substrates, the
combination of high regio- and stereoselectivity is still
problematic. Ligands with two different donor atoms such
as P–N ligands have been successfully applied.[5] However,
hybrid systems with two phosphorus atoms as donor atoms
have been investigated less intensively as ligands for allylic
substitution reactions so far.[12,31] As a benchmark reaction,
the novel ligands were applied in the Pd-catalyzed asym-
metric allylic alkylation of 1,3-diphenyl-2-propenyl acetate
(24) and cyclohexyl-2-enyl acetate (22) with dimethyl ma-
lonate as nucleophile (Scheme 3). The results are summa-
rized in Figure 7.
High-Pressure IR Spectroscopy
The HP-IR spectroscopy experiments were carried out
by in situ formation of the Rh(H)(CO)2(PP) complex from
Rh(acac)(CO)2 and the hybrid ligand under actual catalysis
conditions by using the same rhodium concentration
(1 mm), a ligand-to-metal ratio of 2:1, 20 bar of CO/H2
Scheme 3.
In all cases, the allylic substitutions of 1,3-diphenyl-2-
pressure and 50 °C. The results are summarized in Table 3. propenyl acetate (24) and cyclohexyl-2-enyl acetate (22)
The IR spectrum of Rh(H)(12c)(CO)2 shows two distinct proceeded to full conversion of the starting material within
absorption bands at 1975 (strong) and 2021 cm–1 (medium). 20 h but with low to moderate enantioselectivity. With re-
Similar results were observed for ligand 8b as well as for gard to the enantioselectivity, the individual ligand per-
the related diphosphine ligand DPEphos. The formation of formance was substrate dependent. The phosphine–phos-
the Rh(H)(L)(CO)2 complex was completed within 10 min phite ligand with TADDOL gave moderate enantio-
for ligand 12c, within 20 min for ligand 8b and within selectivity for 1,3-diphenyl-2-propenyl acetate and low
60 min for DPEphos. The major bands can be assigned to enantioselectivity with cyclohexyl-2-enyl acetate. In case of
the ea isomer, whereas the smaller bands are assumed to BINOL as chiral auxiliary, the enantioselectivity of 1,3-di-
belong to the ee isomer.[30] In case of complex 20, only one phenyl-2-propenyl acetate increased with increased steric
minor band is observed for the ea isomer, the second band bulk of the ortho substituents, i.e. H Ͻ Ph Ͻ SiMe3. With
is probably hidden underneath of the large band of the ea cyclohexenyl-2-enyl acetate as substrate, the size of the or-
isomer.
tho substituent has no clear effect on the enantioselectivity.
The palladium-catalyzed allylic alkylation of (E)-1-phen-
After formation of the Rh(H)(L)(CO)2 complexes, the
syngas pressure was released, styrene introduced, and the ylbut-2-en-1-yl acetate (26) with ligand 8a–8b and 12a–12e
mixture was re-pressurized with CO/H2 (20 bar) at 50 °C. was also investigated, in order to study the regioselectivity
The band corresponding to the aldehyde fragment of the of these novel P,PЈ-ligands. The catalytic reactions were car-
Eur. J. Inorg. Chem. 2014, 1797–1810
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Rh-Catalyzed Hydrogenation of C=C Unsaturated
Substrates
The hybrid ligands were also examined in the Rh-cata-
lyzed asymmetric hydrogenation of the benchmark sub-
strates dimethyl itaconate, α-acetamidocinnamic acid and
methyl α-acetamidoacrylate. All reactions resulted in full
conversions of the substrate after 20 h (Figure 8). The hy-
drogenation of α-acetamidocinnamic acid gave moderate to
good enantioselectivities (67–69%) with ligands 12c–12e
from the series of phosphine–phosphite ligands. The S-ste-
reoisomer was formed, albeit in low enantiomeric excess,
when phosphine–phosphonite ligands 8a–8b and phos-
phine–phosphite ligand 12b were used, while the R-stereo-
isomer was obtained with the phosphine–phosphite ligands
12c–12e containing the same chiral binaphthol fragments
as for 8a–8b.
Various levels of enantioselectivity were reached for the
hydrogenation of methyl α-acetamido acrylate. With the
phosphine–phosphite ligand 12b moderate enantio-
selectivity (33%) was obtained. The phosphine–phos-
phonite ligands 8a–8b gave ees of up to 17%. Gratifyingly,
ligands 12c–12e induced relatively good enantioselectivities
(75–78%) for this particular substrate. Notably also in this
case, the opposite enantiomer was formed (S for 8a–8c, R
for 12c–12e), and the ee was found to strongly depend on
the particular ligand employed.
The best results were obtained in the Rh-catalyzed asym-
metric hydrogenation of dimethyl itaconate (Figure 8). The
phosphine–phosphite ligand 12b with (R,R)-TADDOL as
chiral auxiliary only led to a low level of enantioselectivity.
Compared to the previous substrates – methyl α-acetamido
acrylate and α-acetamido cinnamic acid – the phosphine–
phosphonite ligands 8a–8b induced higher but still moder-
ate enantioselectivities in the hydrogenation of dimethyl ita-
conate. Enantioselectivities of up to 97% were achieved by
using the phosphine–phosphite ligands 12c–12e. Similarly
as for the other two substrates, use of ligands 12b and 8a
or 8b results in the (S)-isomer of the product, whereas li-
gands 12c–12e produce the (R)-isomer of the product. For
all substrates tested, the enantioselectivity increased with an
increase in the steric bulk introduced by the ortho substitu-
ents on the chiral diol moiety (SiMe3 Ͼ Ph Ͼ H). To gain
insight into the kinetics of the hydrogenation, the H2 gas
uptake was monitored in time by performing the hydrogen-
ation in an automated set-up employing an AMTEC reac-
tor. At 25 °C and at a hydrogen pressure of 10 bar, the hy-
drogenation of dimethyl itaconate with 12c/[Rh(cod)2]BF4
was quantitative after 20 h and the initial turn-over fre-
quency (TOF) over the first 30% conversion was 44.6 h–1.
Interestingly, phosphine–phosphite ligands 12c–12e
yielded the opposite isomer for the product compared to
phosphine–phosphonite ligands 8a–8b for all substrates,
whilst bearing the same chiral auxiliary [derivates of (S)-
BINOL]. It seems reasonable to assume that the change in
conformation of the hydrogenation product is due to dif-
ferent coordination to the rhodium centre. In case of the
phosphine–phosphite ligands a six-atom ligand bridge ex-
Figure 7. Palladium-catalyzed allylic alkylation of rac-1,3-diphenyl-
2-propenyl acetate (red bars) and cyclohexyl-2-enyl acetate (blue
bars) in CH2C12. Full conversion after 20 h reaction time (GC).
Reaction conditions: L/[Pd(η3-allyl)Cl2]2 = 2:1, [S]/[C] = 100,
0.1 mmol substrate (rac-1,3-diphenyl-2-propenyl acetate and cyclo-
hexyl-2-enyl acetate), 1 mL CH2C12, 20 °C, 0.3 mmol dimethyl
malonate, 0.3 mmol N,O-bis(trimethylsilyl)acetamide (BSA) and a
catalytic amount of KOAc. Enantiomeric excess determined by chi-
ral HPLC for PhPhAc as substrate and chiral GC for CyAc as
substrate.
ried out under the same conditions as before. The results
are summarized in Table 4. All reactions proceeded to com-
plete conversion of the starting material within 20 h, as de-
termined by GC. The regioselectivity of the reaction
changed depending on the ligand employed. In case of the
phosphine–phosphite ligands (12c–12e) with (S)-BINOL as
chiral auxiliary, the regioselectivity slightly decreased with
increasing steric bulk of the ortho substituents, i.e. H Ͼ Ph
Ͼ SiMe3. By using the phosphine–phosphite ligand 12b
with the relatively small chiral diol (R,R)-TADDOL, the
regioselectivity increased further. When the phosphine–
phosphonite ligands 8a–8b were used, the regioisomer ratio
of 27:28 was 9:1. The enantioselectivity increased with steric
bulk of the ortho substituents, but the overall enantio-
selectivity was at most 14%.
Table 4. Palladium-catalyzed allylic alkylation of (E)-1-phenylbut-
2-en-1-yl acetate.[a]
Ligand
27:28
ee [%][b]
12a
12b
12c
12d
12e
8a
90:10
95:5
92:8
90:10
88:12
90:10
90:10
–
1
1
10
14
12
14
8b
[a] Reaction conditions: L/[Pd(η3-allyl)Cl2]2 = 2:1, [S]:[C] = 100,
0.1 mmol substrate (l,3-diphenyl-2-propenyl acetate and cyclo-
hexyl-2-enyl acetate),1 mL CH2C12, 20 °C, 0.3 mmol dimethyl
malonate, 0.3 mmol N,O-bis(trimethylsilyl)acetamide (BSA) and a
catalytic amount of KOAc. [b] ee = enantiomeric excess; deter-
mined by chiral HPLC for PhMeAc as substrate.
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Figure 8. Rhodium-catalyzed hydrogenation of dimethylitaconate {precursor: ([Rh(nbd)2]BF4) (dark green bars) or ([Rh(cod)2]BF4) (light
green bars)} and methyl α-acetamidoacrylate {precursor: ([Rh(nbd)2]BF4) (blue bars) or ([Rh(cod)2]BF4) (red bars) in CH2C12}.
ists between the two phosphorus atoms that probably leads also in a decreased branched-to-linear ratio. These condi-
to a large bite angle as well as a rather large flexibility tions need further optimization. The phosphine–phos-
range. In case of the phosphine–phosphonite species, this phonite ligands 8a–8b gave only low to moderate conver-
ligand-bridge between the two phosphorus atoms is only sion with moderate enantioselectivity.
five atoms, which results in an overall smaller bite angle
Table 5. Rhodium-catalyzed hydroformylation of styrene.[a]
and more rigidity.
Ligand
T [°C]
Conversion [%] ee [%]
b/l
12b
12c
12d
12e
8a
50
50
50
50
50
50
40
40
40
40
40
40
77
81
83
84
54
42
29
25
32
29
16
3
23 (S)
29 (R)
30 (R)
32 (R)
25 (R)
24 (R)
26 (S)
34 (R)
33 (R)
35(R)
3.32
7.77
7.88
8.13
2.85
2.87
3.8
7.21
9.45
10.14
3.14
2.90
Rh Catalyzed Hydroformylation of Styrene
The novel hybrid ligands were also tested in the rhodium-
catalyzed asymmetric hydroformylation of styrene as model
prochiral vinylarene substrate. Because of the particular re- 8b
12b
action set-up, styrene and the internal standard were added
to the Rh(acac)(CO)2/L solution without prior prefor-
mation of the active catalyst. Styrene was hydroformylated
at 40 and 50 °C in toluene. The results are given in Table 5.
Good reactivities and rather good branched-to-linear ratios
were obtained at 50 °C for the phosphine–phosphite ligands
12b–12e. All ligands induced moderate enantioselectivity
for the catalytic system investigated. The highest enantio-
selectivities were obtained by using phosphine–phosphite li-
gands with derivatives of (S)-BINOL, and the enantio-
selectivity increased with increased steric bulk of the ortho
substituents, i.e. SiMe3 Ͼ Ph Ͼ H. As expected, an increase
12c
12d
12e
8a
30 (R)
27 (R)
8b
[a] Reaction conditions: L/Rh(acac)(CO)2 = 2.2:1, [S]:[C] = 500,
0.1 mmol styrene,1 mL toluene, 20 bar CO/H2, [Rh] = 1.0 mm.
Conclusions
By using modular preparative routes, two classes of
in both the enantioselectivity and branched-to-linear (b/l) novel, mixed-donor bidentate ligands based on the diphenyl
ratio was observed by lowering the temperature from 50 ether backbone have been established, i.e. phosphine–phos-
to 40 °C. Interestingly, in case of the phosphine–phosphite phite and phosphine–phosphonite derivatives with different
ligands a change in the diol from (S)-BINOL to (R,R)- chiral auxiliaries. Optimization of the phosphine–phosphite
TADDOL resulted not only in the formation of the oppo- synthesis resulted in air-stable key intermediates that al-
site enantiomer of the hydroformylation product 36, but lowed for easy handling of these compounds. Coordination
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3,3Ј-dibromo-2,2Ј-dimethoxy-1,1Ј-bi-2-naphthalene,[36,37] (S)-3,3Ј-
studies toward PtII with ligands 8a and 12b–12d strongly
suggested a strict mutual cis coordination of the two phos-
phorus donors in the square-planar PtCl2(L) complexes.
High-pressure NMR studies on Rh(H)(CO)2(L), the cata-
lyst resting state during hydroformylation, indicated the
dibromo-1,1Ј-binaphthol,[38]
(S)-3,3Ј-bis(trimethylsilyl)-1,1Ј-bi-
naphthol,[39] (S)-3,3Ј-biphenyl-1,1Ј-bi-2-naphthol,[40] phosphoro-
chloridite compounds[41] and 2,2Ј-dibromo-4,4Ј-dimethyldiphenyl
ether[42] were synthesized according to literature procedures.
existence of two ea isomers. The phosphine fragment pre- Synthesis of 6-BH3: A tBuLi hexane solution (17.66 mL of a 1.6 m
solution, 28.24 mmol, freshly titrated) was added to a solution of
2,2Ј-dibromo-4,4Ј-dimethyldiphenyl ether (5.00 g, 14.13 mmol) in
diethyl ether (100 mL) at –100 °C over the course of 10 min. The
temperature was kept at –100 °C for 30 min before slowly warming
up the mixture to –30 °C. A clear orange solution was obtained
and the lithium–bromine exchange was followed by GC/MS after
quenching a sample with H2O in THF. Upon full conversion the
reaction mixture was cooled to –78 °C, and chlorodiphenylphos-
phine (2.53 mL, 3.13g, 14.2 mmol) was added dropwise to the reac-
tion mixture, which was then warmed up to room temperature and
dominantly occupies the axial position in these stereoiso-
mers. This coordination geometry was also detected in the
corresponding high-pressure IR spectroscopy studies.
In the Pd-catalyzed asymmetric allylic alkylation of sev-
eral allyl acetate substrates, all ligands provided good con-
versions but only low to moderate enantioselectivities, de-
pending on the substrate. Use of the hybrid ligands in the
Rh-catalyzed asymmetric hydroformylation of styrene re-
sulted in good catalyst activities and also in satisfying
branched-to-linear ratios but with moderate enantio- stirred for another 20 h. After quenching with water (2–3 mL), the
solvents were removed in vacuo. The precipitate was azeotropically
dried with toluene (3ϫ 20 mL) and dissolved in THF (100 mL)
before a BH3SMe2 solution in THF (14.12 mL of a 2 m solution,
28.34 mmol) was added. After 1 h, the reaction mixture was con-
centrated and the precipitate was azeotropically dried with toluene
(3ϫ 20 mL) to remove excess BH3SMe2. The precipitate was sus-
pended in DCM and washed with brine (50 mL) and water
(50 mL). The organic layer was dried with MgSO4, and the solvent
was removed in vacuo. The precipitate was washed with hot meth-
anol to obtain a white powder (5.43 g, 11.44 mmol, 81%). 1H
selectivity. The ligands also led to active catalysts for the
Rh-catalyzed hydrogenation of benchmark substrates, di-
methyl itaconate, methyl α-acetamido acrylate and α-acet-
amido cinnamic acid. Depending on the substrate, moder-
ate-to-high enantioselectivities (of up to 97% for dimethyl
itaconate) were obtained. Notably, in the hydroformylation
and hydrogenation reactions, the ee was influenced by the
size of the ortho substituent on the chiral auxiliary group
of the phosphite or phosphonite fragment. Interestingly, use
of the (S)-BINOL-based phosphine–phosphite ligand re- NMR (400 MHz, CD2Cl2, 297 K): δ = 7.74–7.68 (m, 5 H), 7.42–
3
3
7.24 (m, 9 H), 6.88–6.85 (m, 1 H), 6.50 (dd, JHH = 8.4; JHP
=
sulted in the opposite enantiomer in asymmetric hydrogena-
tion relative to that for the phosphine–phosphonite ligands.
3.6 Hz, 1 H), 6.19 (d, 3JHH = 8.4 Hz, 1 H), 2.33 (s, 3 H, CH3), 2.23
(s, 3 H, CH3), 1.8–0.75 (br. q, BH3) ppm. 13C NMR (100 MHz,
CD2Cl2, 297 K): δ = 157.3, 150.1, 136.8 (d, JCP = 13.6 Hz), 136.3,
134.8, 134.0, 133.4, 133.3, 133.2, 131.2, 129.7, 128.9, 128.8, 120.9,
118.3, 117.8, 116.7 (d, JCP = 4.5 Hz), 115.1, 20.8, 20.5 ppm. 31P
Experimental Section
1
NMR (161 MHz, CD2Cl2, 296 K): δ 18.53 (br. d, JPB = 52.5 Hz)
General Methods: All reactions were carried out by using standard
Schlenk techniques under an atmosphere of purified argon. Tolu-
ene was distilled from sodium, THF and Et2O from sodium/benzo-
phenone, hexane from sodium/benzophenone/triglyme, methanol
and ethanol from magnesium and DCM from CaH2 under argon
atmosphere. NMP, stored over molecular sieves, was purchased
from Fluka. Chemicals were purchased from Acros Organics,
Sigma–Aldrich and Alfa Aesar. Diethylamine and triethylamine
were distilled from K2CO3. PPh2Cl and PCl3 were distilled under
argon atmosphere before use. Washing solutions (water, brine) were
degassed by three freeze-pump-thaw cycles and all precursors were
dried azeotropically with toluene. Silica and aluminium oxide were
degassed by heating under vacuum before use. Chromatography
was carried out under argon atmosphere with flame-dried glass-
ware. TLC analysis was executed by using silica F254 TLC plates
ppm. M.p.: 167 °C.
Deprotection of Borane Adduct 6-BH3: DABCO (475 mg,
4.25 mmol) was added to a suspension of 6 (500 mg, 1.05 mmol) in
diethyl ether (40 mL). The reaction mixture turned clear while heat-
ing and was stirred for 20 h at 60 °C. After cooling down to room
temperature, toluene was removed under vacuum. Purification by
filtration through silica (CH2Cl2) yielded a white sticky solid that
was used as such. 31P NMR (161 MHz, CD2Cl2, 296 K): δ = –15.70
(s) ppm.
Synthesis of 7-(BH3)2: A tBuLi hexane solution (4.6 mL of a 1.6 m
solution, 7.27 mmol, freshly titrated) was added to a solution of 6-
BH3 (1.5 g, 3.16 mmol) in THF (250 mL) at –100 °C over 10 min.
The temperature was kept at –100 °C for 30 min before slowly
from VWR. Silica gel 60 (0.063–0.2 mm; Fluka) was used for flash warming up to –78 °C. The reaction mixture was cooled to
chromatography. Melting points were determined on a Gallenkamp
MF-370 melting point apparatus in open capillaries. 1H, 13C and
31P spectra were measured on a Bruker Advance II 400 or a Bruker
Advance 300 NMR spectrometer. CDCl3was distilled from CaH2
and stored over K2CO3 under argon. Other deuterated solvents
were degassed by three freeze-pump-thaw cycles. Mass spectra were
collected by using a Micromass GC mass spectrometer or a
Thermo Scientific DSQ II Single Quadrupole GC/MS spectrome-
ter. FTIR measurements were carried out on a Nicolet 6700 spec-
trometer (Thermo Fisher Scientific) in transmission mode with a
CsI pellet in a N2 filled glovebox. 3,3Ј,5,5Ј-Tetra(tert-butyl)-2,2Ј-
biphenol,[32] dimethyl-2,3-O-isoproylidene-l-tartrate,[33,34] (R,R)-
TADDOL,[35] (S)-2,2Ј-dimethoxy-1,1Ј-bi-2-naphthalene,[36] (S,S)-
–100 °C, and a solution of P(NEt2)2Cl (0.750 mL, 750 mg,
3.56 mmol) in THF (50 mL) at –78 °C was added dropwise to the
reaction mixture over 30 min. The reaction mixture was warmed
up to room temperature. After 20 h a BH3SMe2 solution in THF
(4 mL of a 2 m solution, 8 mmol) was added followed by stirring
the mixture for 2 h. The reaction mixture was concentrated by ap-
plying a vacuum, and the precipitate was azeotropically dried with
toluene (3ϫ 20 mL) to remove excess BH3SMe2. The precipitate
was suspended in DCM and washed with brine (50 mL) and water
(50 mL). The organic layer was dried with MgSO4, and the solvent
was removed in vacuo. Recrystallization from acetone/methanol
yielded the product as a white powder (1.311 g, 2.25 mmol, 71%).
1
M.p. 182 °C. H NMR (400 MHz, CD2Cl2, 295 K): δ = 7.67–7.62
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3
(m, 1 H), 7.53–7.36 (m, 9 H), 7.29–7.23 (m, 1 H), 7.02 (d, JHH
=
of diol A (720 mg, 1.672 mmol) in toluene (10 mL). The reaction
mixture was heated under reflux at 120 °C under an argon atmo-
sphere for 16 h. The reaction was followed by 31P NMR spec-
4
3
4
8.4, JHH = 2.2 Hz, 1 H), 6.97 (d, JHH = 12.2, JHH = 1.8 Hz, 1
3
3
3
H), 6.82 (d, JHH = 8.4, JHP = 4.4 Hz, 1 H), 6.39 (d, JHH = 8.4,
3JHP = 4.4 Hz, 1 H), 3.10 (m, 4 H), 2.70–2.64 (m, 2 H, CH2), troscopy. Upon full conversion, the solvent was removed in vacuo,
2.512.44 (m, 2 H, CH2), 2.28 (s, 3 H, CH3), 2.21 (s, 3 H, CH3), leaving an oily solid. Purification by flash filtration through silica
3
3
1.123 (t, JHH = 7.0 Hz, 6 H, CH3), 1.07 (t, JHH = 7.0 Hz, 6 H,
(hexane/CH2Cl2, 1:1) led to a white, very hydrolysis-sensitive pow-
CH3) ppm. 13C NMR (101 MHz, CDCl3, 297 K): δ = 160.3 (d, JCP der (yield: 991 mg, 1.23 mmol, 71%). M.p. 120 °C. 1H NMR
= 3.4 Hz), 159.8 (d, JCP = 5.6 Hz), 135.5 (d, JCP = 6.8 Hz), 134.6,
133.75–133.55 (m), 133.1, 133.0, 131.4 (d, JCP = 16.9 Hz), 130.4,
129.8, 129.4, 129.1 (d, JCP = 10.8 Hz), 128.8, 125.0, 124.3, 123.6
(400 MHz, CDCl3, 297 K): δ = 8.13 (s, 1 H), 8.04–7.99 (m, 2 H),
3
3
7.63 (d, JHH = 8.7 Hz, 1 H), 7.54 (d, JHH = 8.7 Hz, 1 H), 7.43–
3
4
7.26 (m, 14 H), 7.17–7.15 (m, 1 H), 7.01 (dd, JHH = 8.4, JHH
=
=
4
3
(d, JCP = 5.4 Hz), 122.9 (d, JCP = 5.9 Hz), 120.2, 119.7, 42.2 (d,
2.2 Hz, 1 H),6.90–6.84 (m, 3 H), 6.68 (dd, JHH = 1.96, JHP
2
3
3
2JCP = 4.1 Hz), 40.7 (d, JCP = 3.6 Hz), 20.9, 20.8, 15.0, 14.3 ppm. 4.3 Hz, 1 H), 6.55 (dd, JHH = 8.4, JHP = 4.2 Hz, 1 H), 2.20 (s,
31P NMR (161 MHz, CD2Cl2, 296 K): δ = 17.77 (br. s), 88.78 (br. CH3 DPE), 1.94 (s, CH3 DPE), 0.34 (s, 9 H, SiCH3), 0.30 (s, 9 H,
1
d, JPB = 109.8 Hz) ppm. (FT-MS + p-APCI corona): m/z calcd.
SiCH3) ppm. 13C NMR (100 MHz, CD2Cl2, 297 K): δ = 159.6 (d,
for [C34H48B2N2O1P2 + H]+ 583.3344 [M – H]+, found 583.3356 2JCP = 20.9 Hz), 158.4 (d, JCP = 16.8 Hz), 151.8, 151.0 (d, JCP
=
=
2
2
[M – H]+; calcd. for [C34H48B2N2OP2 – BH2]+ 571.3173 [M –
BH2]+, found 571.3175 [M – BH2]+ (mixture of [M – H]+ and [M –
BH2]+ was observed).
6.3 Hz), 138.3 (m), 138.2 (m), 138.2 (m), 137.9 (m), 137.7 (m),
135.9, 135.8, 135.7, 135.5 (d, JCP = 3.9 Hz), 135.4 (d, JCP
2
2
3.9 Hz), 135.2, 134.6, 134.5, 134.1, 133.7, 132.5, 132.5, 132.1, 131.9,
131.7, 131.6 (m), 131.4 (m), 130.4 (d, 2JCP = 6.6 Hz), 130.2 (d, 2JCP
Synthesis of 7: A solution of DABCO (1.48 g, 13.23 mmol) in tolu-
ene (40 mL) was added to a solution of 7-(BH3)2 (967 mg,
1.65 mmol, azeotropically dried with toluene) in diethyl ether
(100 mL). The reaction mixture was heated for 20 h at 60 °C. After
cooling down to room temperature, toluene was removed in vacuo.
Purification by filtration through neutral alumina (CH2Cl2) re-
sulted in a white sticky solid that was used without further purifica-
2
2
= 6.6 Hz), 129.9, 129.8 (d, JCP = 14.5 Hz), 127.0 (d, JCP
=
2
9.3 Hz), 126.0 (d, JCP = 5.6 Hz), 124.8, 123.6, 123.0, 119.8, 118.7,
20.8 (CH3), 20.5 (CH3), 0 (SiCH3) ppm. 31P NMR (161 MHz,
7
7
CD2Cl2, 296 K): δ = –15.17 (d, JPH = 4.6 Hz), 179.78 (d, JPH
=
4.6 Hz) ppm. TOF-MS EI+: m/z calcd. for [C52H50O3P2Si2 + H]+
839.2695 [M + H]+, found 839.2690 [M + H]+.
1
Synthesis of 1-Bromo-2-(2-methoxyphenoxy)benzene (9): 1-Bromo-
2-(2-methoxyphenoxy)benzene (9) was synthesized in a manner
similar to the literature.[20] Cs2CO3 (34.59 g, 106.03 mmol.) was
added to a solution of 3-methoxyphenol (11.66 mL, 13.16 g,
106.03 mmol) in NMP (200 mL). The flask was evacuated and
backfilled with argon three times. 3-Bromoiodobenzene (11.35 mL,
25 g, 88.36 mmol), 2,2,6,6-tetramethyl-hepta-3,5-dione (1.84 mL,
1.627 g, 8.83 mmol) and CuCl (0.437 g, 4.418 mmol) were added
successively. The flask was evacuated and backfilled with argon
three times before the reaction mixture was heated to 120 °C under
argon; the reaction was monitored by GC/MS and TLC. After full
conversion of the starting material, the solvent was removed under
reduced pressure. The product was purified by Kugelrohr distil-
lation under reduced pressure from the crude reaction mixture. The
product contained 5 to 10% impurities and further purification was
not successful. 1H NMR (400 MHz, CD2Cl2, 295 K): δ = 7.61 (dd,
3JHH = 7.9, 4JHH = 1.3 Hz, 1 H), 7.1.9–7.16 (m, 1 H), 7.04 (d, 3JHH
tion. H NMR (300 MHz, CD2Cl2, 295 K): δ = 7.34–7.25 (m, 10
H), 7.20–7.18 (m, 1 H), 7.05–7.01 (m, 1 H), 6.90–6.86 (m, 1 H),
3
3
6.62–6.57 (m, 1 H), 6.36 (dd, JHH = 8.1, JHP = 4.5 Hz, 1 H),
3
3.02–2.82 (m, 8 H), 2.27, 2.13, 0.99 (t, JHH = 7.1 Hz, 12 H) ppm.
13C NMR (101 MHz, CDCl3, 297 K): δ = 158.7 (d, JCP = 18.4 Hz),
156.9 (d, JCP = 17.6 Hz), 137.6 (d, JCP = 12.4 Hz), 134.5–134.3,
133.3 (d, JCP = 13.1 Hz), 132.9–132.7 (m), 131.3, 129.8, 129.0–
128.7 (m), 128.5 (d, JCP = 14.9 Hz), 119.0, 118.7, 44.0 (d, JCP
=
2
18.6 Hz), 21.1, 20.8, 15.1 (d, JCP = 2.7 Hz) ppm. 31P NMR
(121 MHz, CD2Cl2, 296 K): δ = –18.06 (s), 90.89 (s) ppm.
Synthesis of Phosphine–Phosphonite Ligand 8a: A solution of 7
(310.6 mg, 0.56 mmol) in toluene (15 mL) was added to a solution
of 1,1Ј-bi-2-naphthol (160 mg, 0.56 mmol) in toluene (5 mL). The
reaction mixture was heated under reflux at 120 °C under argon
atmosphere for 16 h. After purification by flash filtration through
silica (hexane/CH2Cl2, 1:1), a white, hydrolysis-sensitive powder
was obtained (yield: 298 mg, 0.43 mmol, 76%). M.p. 120 °C. 1H
3
= 7.9 Hz, 1 H), 6.96–6.91 (m, 2 H), 6.70 (br. d, JHH = 8.2 Hz, 1
3
H), 3.08 (s, 3 H) ppm. 13C NMR (101 MHz, CDCl3, 297 K): δ =
155.2, 151.8, 144.7, 134.0, 129.0, 126.1, 124.2, 121.7 121.6,117.5,
113.7, 112.9, 56.4 ppm. GC-MS (EI): m/z = 277.9.
NMR (300 MHz, CDCl3, 297 K): δ = 8.04 (d, JHH = 8.8 Hz, 1
3
3
H), 7.97 (d, JHH = 8.2 Hz, 1 H), 7.83 (d, JHH = 8.1 Hz, 1 H),
3
3
7.64 (d, JHH = 8.8 Hz, 1 H), 7.55 (br. d, JHH = 8.1 Hz, 1 H),
7.48–7.17 (m, 16 H), 7.02 (d, JHH = 8.4, JHH = 1.8 Hz, 1 H),
6.90–6.82 (m, 3 H), 6.69 (d, JHP = 4.3, JHH = 1.8 Hz, 1 H), 6.58
3
4
Synthesis of 2-Diphenylphosphino-2Ј-Methoxydiphenyl Ether (10): A
nBuLi hexane solution (9.05 mL of a 1.6 m solution, 14.47 mmol,
freshly titrated) was added to a solution of 1-bromo-2-(2-meth-
oxyphenoxy)benzene (9) (4.04 g, 14.47 mmol) in THF (100 mL)
over the course of 10 min whilst maintaining the temperature at
–78 °C. After 60 min the lithium–bromine exchange was checked
by GC-MS after quenching a sample with H2O in THF. Chlorodi-
phenylphosphine (2.60 mL, 14.47 mmol) was added dropwise to
the reaction mixture at –78 °C. The reaction mixture was warmed
up to room temperature and stirred for 20 h at this temperature.
After quenching with water (2–3 mL), the solvents were removed
under high vacuum. The precipitate was suspended in DCM and
washed with brine (50 mL) and water (50 mL). The organic layer
was dried with MgSO4, and the solvent was removed under high
vacuum. A white solid was obtained by recrystallization from
DCM and hexane (4.314 g, 11.28 mmol, 78%). M.p. 146 °C (de-
comp.). 1H NMR (400 MHz, CDCl3, 297 K): δ = 7.47–7.37 (m, 10
3
4
3
3
(d, JHH = 8.4, JHP = 4.1 Hz, 1 H), 2.19 (s, 9 H), 1.91 (s, 9 H)
ppm. 13C NMR (100 MHz, CD2Cl2, 297 K): δ = 157.4 (d, JCP
=
=
2
2
2
21.1 Hz), 156.1 (d, JCP = 16.0 Hz),149.4, 148.5 (d, JCP
6.3 Hz),135.9 (d, JCP = 11.3 Hz),135.6 (d, JCP = 11.3 Hz),133.7,
133.3 (d, JCP = 6.2 Hz), 133.1 (d, JCP = 6.2 Hz), 132.9, 132.5,
132.1, 131.7, 131.5, 130.8, 130.3, 129.4 (d, JCP = 3.1 Hz), 128.4,
128.3, 128.2, 128.0, 127.9 (d, JCP = 7.0 Hz), 127.7, 127.6, 127.5,
127.4, 125.9 (d, JCP = 13.2 Hz), 125.4, 125.1, 124.5, 124.1, 123.9
2
2
2
2
2
2
2
2
2
(d, JCP = 5.6 Hz), 123.8, 122.7 (d, JCP = 2.5 Hz), 121.5, 120.8,
117.5, 116.5,19.8 (CH3), 19.4 (CH3) ppm. 31P NMR (161 MHz,
7
7
CD2Cl2, 296 K): δ = –15.20 (d, JPH = 4.5 Hz), 179.48 (d, JPH
=
4.5 Hz) ppm. TOF-MS ESI+: m/z calcd. for [C46H34O3P2 – H]+
695.1905 [M – H]+, found 695.1905 [M – H]+.
Synthesis of Phosphine–Phosphonite Ligand 8b: A solution of 7
(930 mg, 1.672 mmol) in toluene (30 mL) was added to a solution
Eur. J. Inorg. Chem. 2014, 1797–1810
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FULL PAPER
H), 7.28–7.22 (m, 1 H), 7.14–7.09 (m, 1 H), 7.00–6.95 (m, 1 H),
Synthesis of 12b: The reaction was carried out according to the
6.90–6.78 (m, 3 H), 6.67 (dd, 3JHH = 8.1, 3JHP = 4.5 Hz, 1 H), 3.79 general procedure for phosphine–phosphite ligand formation. A
(s, 3 H, CH3) ppm. 13C NMR (100 MHz, CD2Cl2, 297 K): δ = solution of PCl(TADDOL) (890 mg, 1.8 mmol) in THF was added
160.14 (d, JCP = 16.0 Hz), 151.84, 144.6, 136.8 (d, JCP = 11.03 Hz),
134.2 (d, JCP = 20.3 Hz), 133.9, 130.2, 128.6, 128.4 (d, JCP
7.0 Hz), 126.6 (d, JCP = 14.4 Hz), 125.2, 122.5, 121.9, 121.2, 114.7,
113.4, 56.1 ppm. 31P NMR (161 MHz, CD2Cl2, 296 K): δ = –16.64
ppm. TOF-MS ESI+: m/z calcd. for [C25H21O2P + Na]+ 407.1177
[M + Na]+, found 407.1183 [M + Na]+.
to a solution of 2-diphenylphosphino-2Ј-hydroxydiphenyl ether
(11) (555.1 mg, 1.5 mmol) in diethyl ether. The solution was cooled
to 0 °C, and triethylamine (0.315 mL, 227 mg, 2.25mmol) was
added. The product was obtained after 20 h as a white powder
=
1
(1.05 mg, 1.22 mmol, 89%). H NMR (400 MHz, CDCl3, 297 K):
δ = 7.55–7.53 (m, 2 H), 7.47–7.33 (m, 4 H), 7.35–7.10 (m, 25 H),
3
6.96 (t, JHH = 7.4 Hz, 1 H), 6.89–6.87 (m, 2 H), 6.82–6.79 (m, 1
Synthesis of 2-Diphenylphosphino-2Ј-Hydroxydiphenyl Ether (11): A
solution of BBr3 (1.57 mL, 4.08 g, 16.31 mmol) was added drop-
wise to a solution of 2-diphenylphosphino-2Ј-methoxydiphenyl
ether (10) (2.397 mg, 6.27 mmol) in dry DCM (150 mL) at –78 °C.
The reaction mixture was warmed up to room temperature and was
stirred for 20 h. The reaction was checked by 31P NMR spec-
3
H), 6.64–6.2 (m, 1 H), 6.52–6.49 (m, 2 H), 5.36 (d, JHH = 8.2 Hz,
3
1 H), 5.30 (t, JHH = 8.1 Hz, 1 H),0.79 (s, 3 H, CH3), 0.57 (s, 3 H,
CH3) ppm. 13C NMR (100 MHz, CD2Cl2, 297 K): δ = 160.0 (d,
JCP = 16.7 Hz), 147.3 (d, JCP = 2.7 Hz), 146.0, 145.8, 144.2 (d, JCP
= 2.6 Hz), 141.7 (d, JCP = 3.4 Hz), 141.2, 137.0, 136.9, 134.4 (d,
JCP = 3.4 Hz), 134.1 (d, JCP = 3.4 Hz), 134.0, 130.4, 129.1, 128.7
1
troscopy [δ = –0.5 (s, 8%), –18.1 (q, JPB = 146.4 Hz, 92 %) ppm
(d, JCP = 5.9 Hz), 128.6 (d, JCP = 6.9 Hz), 128.4, 128.2 (d, JCP
=
]. After full conversion of the starting material, excess BBr3 and
DCM were evaporated, and the solid was dried azeotropically with
toluene (3ϫ 2 mL). The mixture was quenched with ice-cold water
(20 mL), and after phase separation, the aqueous phase was ex-
tracted with DCM (3ϫ 20 mL). The combined organic phases were
subsequently washed with water (30 mL) and brine (30 mL), and
dried with MgSO4. After filtration through a short plug of silica
(DCM), 2-diphenylphosphino-2Ј-hydroxydiphenyl ether was used
without further purification (4.314 g, 11.28 mmol, 78%). M.p.
15.9 Hz), 128.0, 127.9, 127.6, 127.5, 127. 5, 127.3, 127.2, 124.3 (d,
JCP = 6.2 Hz), 123.2, 122.3 (d, JCP = 7.8 Hz), 121.3, 117.1, 113.8,
86.9 (d, JCP = 10.9 Hz), 84.8 (d, JCP = 4.7 Hz), 82.2 (d, JCP
=
11.2 Hz), 80.3 (d, JCP = 5.04 Hz), 26.9, 26.4 ppm. 31P NMR
7
(202 MHz, CD2Cl2, 296 K): δ = –17.06 (d, JPH = 4.6 Hz), 131.35
7
(d, JPH = 4.6 Hz) ppm. M.p. 133 °C (decomp.). TOF-MS ESI+:
m/z calcd. for [C55H46O4P2 + H]+ 863.2698 [M + H]+, found
863.2692 [M + H]+.
1
104 °C. H NMR (400 MHz, CDCl3, 297 K): δ = 7.47–7.39 (m, 10
Synthesis of 12c: The reaction was carried out according to the
general procedure for phosphine–phosphite ligand formation. A
solution of PCl(BINOL) (700 mg, 2 mmol) in THF was added to
a solution of 2-diphenylphosphino-2Ј-hydroxydiphenyl ether (11)
(672.9 mg, 1.82 mmol) in diethyl ether. The solution was cooled to
0 °C, and triethylamine (0.305 mL, 220.9 mg, 2.19 mmol) was
added. The product was obtained after 20 h as a white powder
H), 7.35–7.28 (m, 1 H), 7.07–7.68 (m, 7 H), 5.52 (s, 1 H, OH) ppm.
13C NMR (100 MHz, CD2Cl2, 297 K): δ = 157.5 (d, JCP
=
15.3 Hz), 147.0, 141.6, 134.9 (d, JCP = 9.1 Hz), 133.2, 132.9, 129.6,
128.3, 127.9 (d, JCP = 7.3 Hz), 127.0 (d, JCP = 13.7 Hz), 124.5,
122.8, 119.5, 119.1, 115.5, 114.6 ppm. 31P NMR (161 MHz,
CD2Cl2, 296 K): δ = –18.1 ppm.
1
(1.096 mg, 1.6 mmol, 88%). H NMR (400 MHz, CDCl3, 297 K):
Synthesis of 12a: The reaction was carried out according to the
general procedure for phosphine–phosphite ligand formation. A
solution of PCl(A) (818 mg, 1.75 mmol) in THF was added to a
solution of 2-diphenylphosphino-2Ј-hydroxydiphenylether (11)
(555 mg, 1.5 mmol) in diethyl ether. The solution was cooled to
0 °C and triethylamine (0.275 mL, 198 mg, 1.95 mmol) was added.
The product was obtained after 20 h reaction time as a white, hy-
drolysis sensitive powder (1.01 mg, 1.22 mmol, 81%). M.p. 156 °C.
δ = 7.97 (t, 3JHH = 9.1 Hz, 2 H), 7.91 (d, 3JHH = 8.2 Hz, 1 H), 7.82
3
(d, JHH = 8.8 Hz, 1 H), 7.47–7.18 (m, 20 H), 7.07–6.98 (m, 3 H),
6.90–6.86 (m, 1 H), 6.76–6.70 (m, 2 H) ppm. 13C NMR (100 MHz,
2
2
CD2Cl2, 297 K): δ = 159.7 (d, JCP = 16.3 Hz), 148.1 (d, JCP
=
4.1 Hz), 147.5 (d, 2JCP = 2.4 Hz, BINOL), 147.4 (d, 2JCP = 2.9 Hz,
BINOL), 143.8 (d, JCP = 5.4 Hz), 136.9 (d, JCP = 7.7 Hz, BI-
2
2
2
NOL), 136.8 (d, JCP = 7.9 Hz, BINOL), 134.6, 134.5, 134.3 (d,
2JCP = 3.2 Hz), 133.2 (s, BINOL), 132.8 (s, BINOL), 132.1 (s, BI-
NOL), 131.6 (s, BINOL), 130.8, 130.2, 129.2–128.5 (m), 128.3 (d,
1JCP = 15.7 Hz), 127.2, 127.6, 126.7, 126.5, 125.7, 125.6, 125. 3 (d,
4
1H NMR (400 MHz, CDCl3, 297 K): δ = 7.4 (d, JHH = 2.4 Hz, 2
H), 7.33–7.24 (m, 12 H), 7.20 (d, 4JHH = 2.4 Hz, 1 H), 7.0 (td, 3JHH
5
3
= 7.4, JHH = 0.8 Hz, 4 H), 6.90–6.81 (m, 1 H), 6.75 (ddd, JHH
=
1JCP = 14.4 Hz), 124.7 (d, JCP = 5.0 Hz), 123.9, 123.1 (d, JCP
=
1
1
3
5
8.1, JHP = 4.4, JHH = 0.9 Hz, 1 H), 6.64–6.58 (m, 1 H), 1.35 (s,
18 H), 1.34 (s, 18 H) ppm. 13C NMR (100 MHz, CD2Cl2, 297 K):
δ = 159.6 (d, 2JCP = 17.1 Hz), 148.1 (d, 2JCP = 2.3 Hz), 147.3, 145.8
(d, 2JCP = 6.2 Hz), 143.2, 1, 137.0 (d, 2JCP = 12.0 Hz), 134.5, 135.4,
1
2.2 Hz), 122.8 (d, JCP = 6.6 Hz), 122.4, 122.2, 121.6, 116.7 ppm.
31P NMR (121 MHz, CD2Cl2, 296 K): δ = –16.42 (d, JPH
=
7
7
8.7 Hz), 144.90 (d, JPH = 8.7 Hz) ppm. M.p. 123 °C. TOF-MS
ESI+: m/z calcd. for [C44H30O4P2 + H]+ 683.1536 [M + H]+, found
683.1527 [M + H]+.
2
2
134.3, 133.2 (d, JCP = 3.8 Hz), 130.7, 129.3 (d, JCP = 16.2 Hz),
2
129.0, 128.8 (d, JCP = 7.0 Hz), 126.8, 125.1, 124.9, 124.3, 124.0,
123.5 (d, 2JCP = 6 Hz), 120.5, 118.2, 35.7, 35.0, 31.6, 31.2 ppm. 31
P
Synthesis of 12d: The reaction was carried out according to the
general procedure for phosphine–phosphite ligand formation. A
solution of PCl(B) (365 mg, 1 mmol) in THF was added to a solu-
tion of 2-diphenylphosphino-2Ј-hydroxydiphenyl ether (485 mg,
0.96 mmol) in diethyl ether. The solution was cooled to 0 °C, and
triethylamine (0.176 mL, 127 mg, 1.25 mmol) was added. The prod-
uct was obtained after 20 h as a white powder (681 mg, 0.81 mmol,
7
NMR (121 MHz, CD2Cl2, 296 K): δ = –17.0 (d, JPH = 10.6 Hz),
136.05 (d, JPH = 10.7 Hz) ppm. TOF-MS EI+: m/z calcd. for
7
[C52H58O4P2 + H]+ 807.3727 [M + H]+, found 807.3730 [M +
H]+.
General Synthesis of Phosphine–Phosphite Ligands: A THF solu-
tion of PCl(diol) (1.1 molequiv.) was added to a solution of 2-[2-
(diphenylphosphanyl)phenoxy]phenol (11) (1 molequiv.) in diethyl
ether by a teflon cannula. The solution was cooled to 0 °C, and
triethylamine (1.2 molequiv.) was added. The reaction mixture
turned cloudy immediately. The suspension was warmed to room
temperature overnight. After 20 h, the suspension was filtered and
concentrated. The ligand was purified by filtration through silica
(hexane/CH2Cl2, 1:1) to obtain a white powder.
1
84%). M.p. 133 °C. H NMR (300 MHz, CDCl3, 297 K): δ = 8.18
(s, 4JHH = 1.8 Hz, 1 H), 8.14 (s, 4JHH = 1.8 Hz, 1 H), 8.06 (d, 3JHH
3
= 8.8 Hz, 1 H), 7.89 (d, JHH = 8.7 Hz, 1 H), 7.73–7.71 (m, 18 H),
7.59–7.56 (m, 3 H), 7.50–7.20 (m, 3 H), 7.08–6.99 (m, 3 H), 6.89
3
3
4
3
(ddd, JHH = 7.6, JHP = 4.3, JHH = 1.6 Hz, 1 H),6.73 (dd, JHH
= 4.4, JPH = 0.9 Hz, 1 H) ppm. 13C NMR (100 MHz, CD2Cl2,
297 K): δ = 158.6 (d, JCP = 16.6 Hz), 147.0 (d, JCP = 4.4 Hz),
3
2
2
Eur. J. Inorg. Chem. 2014, 1797–1810
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FULL PAPER
146.4 (d, 2JCP = 2.2 Hz, BINOL), 146.2 (d, 2JCP = 3.0 Hz, BINOL), dropwise to the ligand solution. The resulting orange solution was
2
2
142.6 (d, JCP = 5.9 Hz), 139.6 (d, JCP = 6.0 Hz), 137.0, 136.8,
stirred for 5 min and subsequently filled into the autoclave under
a light argon flow. The autoclave was purged three times with
16 bar CO/H2 (1:1) at room temperature, pressurized to circa
18 bar and heated to 50 °C. After adjustment of pressure to 20 bar,
the measurement started. After formation of the Rh(H)(L)(CO)2
complexes, the syn-gas pressure was released, styrene introduced,
and the mixture was repressurized with CO/H2 (20 bar) at 50 °C.
The band corresponding to the aldehyde fragment of the hydrofor-
mylation product (C=O stretching vibration at 1700 to 1750 cm–1)
appeared immediately after the reintroduction of CO/H2 (20 bar).
Data were corrected by subtracting the spectrum of the solvent at
the same temperature and pressure, which yielded the spectrum of
the catalyst complex itself.
135.8, 135.7, 135.6, 133.4, 133.2, 133.1, 131.3, 131.1, 130.8, 129.9,
2
129.6, 129.3, 129.1, 128.25, 128.0–127.6 (m), 127.2 (d, JCP
=
15.4 Hz), 126.8, 126.7, 126.5, 126.4 (d, 2JCP = 4.2 Hz), 125.2, 125.0,
2
124.5, 124.1, 123.4, 122.7, 121.9 (d, JCP = 2.3 Hz), 121.7–121.5
(m), 120.5, 115.6 ppm. 31P NMR (161 MHz, CD2Cl2, 296 K): δ =
–16.62 (d, 7JPH = 8.5 Hz), 145.0 (d, 7JPH = 8.5 Hz) ppm. TOF-MS
ESI+: m/z calcd. for [C56H38O4P2+H]+ 835.2162 [M + H]+, found
835.2150 [M + H]+.
Synthesis of 12e: The reaction was carried out according to the
general procedure for phosphine–phosphite ligand formation. A
solution of PCl(C) (890 mg, 1.8 mmol) in THF was added to a
solution of 2-diphenylphosphino-2Ј-hydroxydiphenyl ether
(555.1 mg, 1.5 mmol) in diethyl ether. The solution was cooled to
0 °C, and triethylamine (0.315 mL, 227 mg, 2.25 mmol) was added.
The product was obtained after 20 h as a white powder (1.01 mg,
1.22 mmol, 81%). M.p. 120 °C. 1H NMR (300 MHz, CDCl3,
Selected data: Rh(H)(12c)(CO)2: 1H NMR (500 MHz, [D8]toluene,
297 K): δ = –9.8 (ddd, JRh-H = 9.5, JH-P1 = 45; JH-P2 = 85 Hz) ppm.
31P NMR (162 MHz, [D8]toluene, 297 K): δ = 164.8 (ddd, JRh-P1
=
227, JP1-P2 = 76, JH-P1 = 45 Hz, phosphite P1), 21.7 (broad, JRh-P2
3
297 K): δ = 8.11 (s, 1 H), 8.07 (s, 1 H), 7.98 (d, JHH = 8.8 Hz, 1
H), 7.81 (d, JHH = 8.7 Hz, 1 H), 7.43–7.17 (m, 18 H), 7.07–6.98
=
114 Hz, phosphine P2) ppm. Rh(H)(8b)(CO)2: 1H NMR
3
(400 MHz, [D8]toluene, 297 K): δ = –8.7 (JRh-H = 8, JH-P1 = 51;
JH-P2 = 56 Hz) ppm. 31P NMR (162 MHz, [D8]toluene, 297 K): δ
= 164.8 (ddd, JP1-P2 = 6, JH-P1 = 51 Hz, phosphite P1), 21.7 (br.
dd, JH-P2 = 56 Hz, phosphine P2) ppm. 31P NMR {1H} (202 MHz,
[D8]toluene, 297 K): δ = 187 (phosphonite, d, JRh-P1 = 191 Hz),
23.5 (phosphine, d, JRh-P2 = 117 Hz) ppm.
3
3
4
(m, 3 H), 6.88 (ddd, JHH = 8.2, JHP = 4.2, JHH = 0.6 Hz, 1 H),
3
3
3
6.75 (dd, JHH = 1.7, JPH = 1.9 Hz, 1 H), 6.71 (ddd, JHH = 8.2,
3JHP = 4.2, JHH = 0.6 Hz, 1 H), 0.33 (s, 9 H), 0.32 (s, 9 H) ppm.
4
13C NMR (100 MHz, CD2Cl2, 297 K): δ = 160.9 (d, JCP
=
2
2
2
16.7 Hz), 149.4 (d, JCP = 4.3 Hz), 148.7 (d, JCP = 2.1 Hz, BI-
NOL), 144.9 (d, 2JCP = 2.9 Hz, BINOL), 138.8, 138.4, 138.1, 138.0,
137.8, 135.8–135.3 (m), 134.4, 134.1, 132.6, 132.2, 131.9,–131.7 (m),
High-Pressure NMR Spectroscopy Studies of Rhodium Complexes:
High-pressure NMR experiments were carried out on a Varian
Unity Inova 500 MHz and 400 MHz spectrometer. [D8]toluene was
used as solvent. The experiments were carried out in 10 mm sap-
phire tubes with a titanium valve head. In a typical experiment,
[Rh(acac)(CO)2] (9.7 mg, 37.6 μmol) and ligand (1.05 equiv.) were
weighted. Both were dissolved in deuterated solvent (1 mL), and
the solutions were stirred for 15 min. Subsequently, the solution of
the rhodium precursor was added dropwise to the stirred solution
of the ligand. The resulting solution was stirred for 5 min and
transferred into the sapphire tube under argon atmosphere. The
tube was flushed three times with 10 bar of synthesis gas and finally
pressurized to 20 bar. The pressurized tube was left overnight at
50 °C, in order to be sure preformation was finished.
2
131.4, 130.2–129.8 (m), 129.4 (d, JCP = 15.9 Hz), 127.2, 127.0,
1
1
126.7, 126.3, 125.6 (d, JCP = 5.2 Hz), 124.8, 124.0 (d, JCP
=
1.7 Hz), 123.8 (d, 1JCP = 6.7 Hz), 123.4, 123.2, 233.7, 117.7, 0 ppm.
31P NMR (125 MHz, CD2Cl2, 296 K): δ = –16.30 (d, JPH
=
7
8.6 Hz), 144.79 (d, 7JPH = 8.6 Hz) ppm. TOF-MS ESI+: m/z calcd.
for [C50H46O4P2+H]+ 827.2326 [M + H]+, found 827.2321 [M +
H]+.
Synthesis of [Pd(η3-allyl)(L)]PF6: In a typical experiment, ligand
(0.05 mmol),
(0.05 mmol) were placed in a Schlenk flask, flushed with argon
and dissolved in DCM (5 mL). The reaction mixture was stirred
overnight at room temperature, then added to water and extracted
with DCM (3ϫ 5 mL). The organic layer was dried with MgSO4,
and the solvent was removed in vacuo.
[Pd(3η-allyl)Cl]2
(0.025 mol)
and
NH4PF6
Pd-Catalyzed Asymmetric Allylic Alkylation: In a Schlenk tube
[Pd(η3-C3H5)Cl]2 (0.005 mmol) and the respective ligand
(0.01 mmol) were dissolved in CH2Cl2 (1 mL) and degassed. (E)-
Synthesis of [Rh(L)(cod)]BF4: In a typical experiment, a solution of
ligand (0.02 mmol) and [Rh(cod)2]BF4 (8.2 mg, 0.02 mmol) were 1,3-diphenylprop-2-ene-1-yl acetate, (E)-pent-3-ene-2-yl acetate or
placed in a Schlenk flask, flushed with argon and dissolved in chlo-
roform (1 mL). The reaction mixture was stirred overnight at room
temperature. The conversion was checked by 31P NMR spec-
troscopy.
cyclohex-2-ene-1-yl acetate (1 mmol) was added as the substrate,
and after 20 min of stirring at room temperature, dimethyl malon-
ate (3 mmol), BSA [N,O-bis(trimethylsilyl)acetamide, 3 mmol] and
a catalytic amount of KOAc (0.024 mmol) were added consecu-
tively. The reaction mixture was degassed again and stirred at the
given temperature. After the appropriate reaction time, the reaction
mixtures were filtered through a plug of silica with DCM/methanol
(99:1). The conversion was determined by GC measurement, and
the enantiomeric excess was measured by chiral GC.
High-Pressure IR Spectroscopy Studies of Rhodium Complexes Un-
der Catalytic Conditions: High-pressure FTIR experiments were re-
corded on a Shimadzu FTIR 8300 spectrometer. The spectra were
recorded in the transmission mode. The IR autoclave was flushed
for 1.5 h with argon before the start of the experiment. For refer-
ence spectra, the autoclave was filled with 2-MeTHF (10 mL) un- Rh-Catalyzed Asymmetric Hydrogenation: The hydrogenation ex-
der a light argon flow. The autoclave was flushed by applying 8 bar
of synthesis gas and releasing the pressure three times. The pressure
was then increased to 18 bar, and the solution stirred for 2 min,
allowing the solution to get saturated with the gas. After heating
to 50 °C, the pressure was adjusted to 20 bar. Simultaneously,
Rh(acac)(CO)2 (4.5 mg, 17 μmol) and ligand (2.0 equiv.) were dis-
solved in 2-MeTHF (8 mL) and stirred for 15 min (2-MeTHF was
chosen as a solvent, because toluene interferes in the carbonyl spec-
tral region). The solution of the rhodium precursor was added
periments were carried out in a stainless steel autoclave (total vol-
ume 150 mL) charged with 10- or 14 reaction vessels including Tef-
lon mini stirrer bars for conducting parallel reactions. In a typical
experiment, a reaction vessel was charged with [Rh(cod)2]BF4
(1.0 μmol), bidentate ligand (1.1 μmol), substrate (0.100 mmol) and
internal standard (0.125 mmol, e.g. dodecane, decane) in solvent
(0.5 mL). Before starting the catalytic reactions, the charged auto-
clave was purged three times with 10 bar of dihydrogen and then
pressurized to 10 bar H2. The reaction mixtures were stirred at
Eur. J. Inorg. Chem. 2014, 1797–1810
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20 °C for 20 h. After the reaction time, the autoclave was cooled in
an ice bath for 15 min before the autoclave was depressurized and
the reaction mixtures were filtered through a plug of silica. The
conversion was determined by GC measurement, and the enantio-
meric excess was measured by chiral GC.
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): NMR spectral characterization of all new ligands, high-pres-
sure NMR and IR spectra are presented.
Rh-Catalyzed Asymmetric Hydrogenation in AMTEC SPR16: In a
typical experiment, the stainless steel autoclaves of an AMTEC
SPR16 parallel reactor were heated to 90 °C and flushed with ar-
gon (15 bar) three times. The reactors were cooled down to room
temperature. [Rh(cod)2]BF4 (4.1 mg, 10 μmol, 1 equiv.) and the li-
gand (20.0 μmol) were dissolved in dichloromethane (5 mL) at
room temperature. The solution was injected into the AMTEC via
syringe filled with a solution of dimethyl itaconate (1.58 g,
10 mmol, 1000 equiv.) in dichloromethane (3 mL). The atmosphere
was exchanged with hydrogen gas followed by the adjustment of
the pressure and temperature to reaction conditions. After the reac-
tion time, the autoclave was depressurized and the reaction mix-
tures were filtered through a plug of silica. The conversion was
determined by GC measurement, and the enantiomeric excess was
measured by chiral GC.
Acknowledgments
This work was supported by an EASTCHEM fellowship, an
IDECAT visiting researcher grant and a EU COST PhoSciNet
(cm08602) short-term exchange grant (all to C. F. C.). We would
like to acknowledge the European Union for funding: Marie Curie
Excellence Grant MEXT-2004-014320; Network of Excellence
IDECAT (IDECAT-CT-2005-011730); COST action CM0802
PhoSciNet. We thank Ton Staring for skilful technical assistance
at TU/e.
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Received: September 25, 2013
Published Online: December 3, 2013
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim