Bisxantphos: Stereoselective synthesis and coordination behaviour of a new class of cyclic double bridged diphosphines

Article abstract of DOI:10.1016/j.jorganchem.2011.06.019

A new class of cyclic double bridged diphosphine ligands was developed and the coordination properties are described. The reaction of 4,5-dilithioxanthene with dichlorophenylphosphine gave a cyclic double bridged diphosphine ligand based on two xanthene b

Full text of DOI:10.1016/j.jorganchem.2011.06.019

Journal of Organometallic Chemistry 696 (2011) 3113e3120
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Bisxantphos: Stereoselective synthesis and coordination behaviour
of a new class of cyclic double bridged diphosphines
,
c
René den Heeten a, Erik Zuidema a, Martin Lutz b, Anthony L. Spek b, Piet W.N.M. van Leeuwen a
,
,
Paul C.J. Kamer d
*
a Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
b Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
c Institute of Chemical Research of Catalonia (ICIQ), Avenida Països Catalans 16, 43007 Tarragona, Spain
d EaStCHEM, School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of cyclic double bridged diphosphine ligands was developed and the coordination properties
are described. The reaction of 4,5-dilithioxanthene with dichlorophenylphosphine gave a cyclic double
bridged diphosphine ligand based on two xanthene backbones (4) as a single stereoisomer. X-ray crystal
structure determination revealed that the groups bridging the two phosphorus atoms are arranged in
a syn-disposition. This new cyclic bisxantphos structure was modified with bulky residues at the third
substituent of the phosphorus atoms, thus forming cavity-shaped ligands.
Received 12 April 2011
Received in revised form
8 June 2011
Accepted 15 June 2011
Keywords:
Ó 2011 Elsevier B.V. All rights reserved.
Bis-Xantphos
Metallo-cavitands
Cyclic diphosphine
Ligand design
Bite angle
1. Introduction
cavities with catalytic centres was shown to lead to shape-selective
catalysts. Also the use of cyclodextrins as synthetic cavitands
As part of our continuous effort in search of new ligand systems
with interesting properties in catalysis [1], we here report the
synthesis of cyclic double bridged diphosphines based on two
xanthene backbones. Implementing features of biological systems
in transition metal catalysts [2], the ligand design aims to create
metallo-cavitands. Enzymatic reactions typically impose selectivity
by control of the geometry in the enzymeesubstrate complex. In
homogeneous catalysis sophisticated modular ligand systems have
been designed using bioinspired supramolecular interactions [3] or
by introducing bulky organic scaffolds, like cyclodextrins or calix-
arenes, for substrate encapsulation [4ꢀ6]. The environment of
a bound substrate can be closely controlled by confinement of the
metal centre, in particular enabling a catalytic reaction to be driven
towards the product that best adapts to the steric restraints
imposed by the confining structure.
received much attention in recent years and there are numerous
reports on their application in catalysis [6].
Cyclic double bridged diphosphines in which the two phosphine
lone pairs are in syn orientation form another class of ligands in
which the metal embracement is particularly effective. Each
phosphorus atom has one substituent R forced to reside at the front
side of the metal centre, and therefore, structural modulations are
possible around the metal centre by varying R groups (Fig. 1).
Substituents that provide large steric bulk can have a pronounced
effect on substrate coordination to the metal centre. Synthetic
routes to these cyclic diphosphines are relatively undeveloped and
the compounds are usually obtained as mixtures of stereoisomers.
Stereoselective synthetic methods have been developed for 1,n-
diphosphacycloalkanes by Alder et al. [7], for 1,5-diaza-3,7-
disphosphacyclooctane derivatives by Karasik et al. [8] and for
phosphorus-bridged [1.1]ferrocenophanes by Mizuta and
coworkers [9].
Phosphorus-based calix[4]arenes [4] and calix[6]arenes [5] have
been employed as scaffolds for the development of structurally
well-defined pocket-shaped ligands. The combination of calixarene
In this paper, we report the stereoselective synthesis of a new
class of cyclic double bridged diphosphines based on two xanthene
backbones, as well as its coordination behaviour to transition
metals. The rigid ligand structure should not only determine the
* Corresponding author. Tel.: þ44 1334 467285; fax: þ44 1334 463808.
E-mail address: pcjk@st-andrews.ac.uk (P.C.J. Kamer).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.06.019

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R. den Heeten et al. / Journal of Organometallic Chemistry 696 (2011) 3113e3120
t
t
Bu
Bu
t
t
Bu
Bu
Bu
P
P
O
R
R
M
O
O
R
R
1: R = Br
2: R = Li
t
P
P
BuLi
Fig. 1. Schematic representation of a cyclic double bridged diphosphine.
t
t
Bu
PCl2
electronic properties of the metal, but also introduce geometrical
effects that may determine the way the substrate interacts with the
catalyst. In addition, cyclic bisxantphos ligands that are modified
with residues that are sterically demanding were developed in
order to create cavity-shaped ligands.
9
4
Scheme 2. Synthetic route to bisxantphos 4.
dilithio intermediate), resulted in the formation of merely traces of
bisxantphos 4-Me. Alternatively, 4,5-bis(chlorophenylphosphino)-
2,7-di-tert-butyl-9,9-dimethylxanthene (6) was synthesized as a
mixture of diastereomers by reacting compound 2 (Scheme 2) with
two equivalents of PPh(NEt2)Cl, followed by the exchange of the NEt2
moieties bychlorides using PCl3. This compound was highly insoluble,
however, hampering the final ring closure with a second equivalent of
dilithio compound 2.
2. Results and discussion
As the preparation of target compound 4 (Scheme 2) inevitably
involved a troublesome double cyclization, we studied stepwise
synthesis procedures (Scheme 1). The reaction of PhPCl2 with two
equivalents of the monolithiated analogue of 4,5-dibromo-2,7,9,9-
tetramethylxanthene (1-Me) gave the expected bisxantphos
monophosphine derivative 3.
Bromide-lithium exchange of the two remaining bromide moieties
using four equivalents of t-BuLiandsubsequentreactionwithasecond
equivalent of PhPCl2 under high dilution (due to low solubility of the
We also explored the possibility of a templated synthesis of
compound 4. The transition metal template bis(dichlorophenyl-
phosphino)tetracarbonyl-molybdenum and the PeN templates
N,N-bis(dichlorophosphino)aniline,
N,N-bis(dichlorophosphino)
methylamine, 1,2-bis(dichlorophosphino)-1,2-dimethylhydrazine
and 1,4-bis(dichlorophosphino)piperazine were reacted with dili-
thio compound 2 under high dilution, but in neither of these
reactions a cyclization product was observed.
Finally, the successful synthetic route to cyclic bisxantphos is
shown in Scheme 2. The commercially available 4,5-dibromo-2,7-
di-tert-butyl-9,9-dimethylxanthene (1) was again chosen as a
starting material. Lithiation of 1 with four equivalents of t-BuLi
provided compound 2. Slow simultaneous addition of this
compound and dichlorophenylphosphine (9) to a stirred ether
solution using a double syringe pump gave cyclic diphosphine 4
in 10% yield. In addition to the cyclic bisxantphos 4, the
monophosphine derivative and oligomers were formed. The
isolation of compound 4 from the reaction mixture was achieved
by centrifugally accelerated, radial, thin-layer chromatography.
The yield of merely 10% is low and optimization attempts did not
lead to yields exceeding 20% so far. It should be noted that such low
yields are not uncommon in the synthesis of challenging structures
such as cyclic double bridged disphosphines. Zabel et al. reported
for the analogues reaction of 1,10-dilithioferrocene with
O
n
1. BuLi
Br
t
1. BuLi
Ph
P
Br
4-Me
1-Me
X
X
O
2. Cl2PPh
2. Cl2PPh
3
t
t
Bu
Bu
n
1. BuLi
2
1
4
O
2. ClPPh(NEt2)
Ph
P
R
P
R
Ph
5: R = NEt2
6: R = Cl
PCl3
(ꢀ)-dichloromenthylphosphine to form
a
trans-phosphorus-
bridged [1.1]ferrocenophane a yield of only 2% [10]. In addition, the
overall yield of the laborious multistep synthesis of 1,n-diphos-
phacycloalkanes and syn-phosphorus-bridged [1.1]ferrocenophanes
by, respectively, Karasik and Mizuta did not exceed 30% [8,9].
In the 31P {1H} NMR spectrum of 4, one sharp signal was observed
t
t
Bu
Bu
Bu
O
t
1. BuLi
1
P
P
at
acyclic 2,7-di-tert-butyl-9,9-dimethyl-4,5-bis(diphenylphosphino)
xanthene (
d
¼ ꢀ37.9 ppm, which is an upfield shift compared to the related
4
X
2.
Cl2P
PCl2
O
d
¼ ꢀ16.2 ppm) [11]. The upfield shift can be attributed to
7
t
t
Bu
a flattening of the phosphorus atoms, caused by the conformational
constraints imposed by the cyclic structure [12]. The presence of
only one phosphorus signal suggested that a single stereoisomer
was formed [13]. From the 1H NMR spectrum it was evident that
compound 4 is highly symmetrical. The tert-butyl protons are
observed as one sharp singlet and two distinct resonances corre-
8
CO
Mo
CO
Ph
N
P
P
CO
CO
N
N
P
N
N
P
P
P
=
,
,
,
P
P
P
P
sponding to the methyl groups were found at
Crystals suitable for X-ray diffraction were obtained by slow
diffusion of methanol into dichloromethane solution of
d
¼ 1.73 and 1.52 ppm.
a
Scheme 1. Stepwise synthesis procedures towards bisxantphos 4.

R. den Heeten et al. / Journal of Organometallic Chemistry 696 (2011) 3113e3120
3115
Table 1
compound 4. The structure is shown in Fig. 2 and selected angles
Selected bond lengths and angles for compound 4.
and bond lengths are given in Table 1.
bond angle (ꢁ)
The crystal structure shows that the two bridging xanthene
groups are arranged in a syn-disposition. The structure has CS
symmetry, with an exact, crystallographic mirrorplane passing
through the two phosphorus atoms and the attached phenyl rings.
There is no additional symmetry present in the molecule. P1 and P2
and their environments are thus independent. The potential C2v
symmetry of the molecule is broken by the different orientations of
tert-butyl groups in the crystal structure. In solution, these groups
can freely rotate and therefore the tert-butyl groups and the
phosphorus atoms are equivalent according to the NMR observa-
tions. Here, the compound shows a time averaged C2v symmetry.
The P/P distance is 4.2890(13) Å, significantly longer than the
4.080 Å observed for the Xantphos ligand [14]. In metal complexes
of ligands of the Xantphos family, the P/P distance is even shorter
[15], except in complexes where the ligand displays trans-coordi-
nation, in which case the distance is around 4.5 Å [16]. The
increased rigidity of ligand 4 makes prediction of its coordination
behaviour based on previous results with the Xantphos family
difficult. On the one hand, the longer P/P distance results in
a significantly larger bite-angle, and this disfavours cis-coordina-
tion of this ligand in square planar or octahedral complexes. On the
bond length (Å)
C21eP2
P2eC61
C21eC22
C21eC26
C11eP1
P1eC51
C11eC12
C11eC16
P1/P2
1.843 (3)
1.846 (4)
1.401 (4)
1.408 (4)
1.847 (2)
1.837 (3)
1.399 (3)
1.386 (3)
4.2890 (13)
C21eP2eC21i
C21eP2eC61
P2eC21eC22
P2eC21eC26
C11eP1eC11i
C11eP1eC51
P1eC11eC12
P1eC11eC16
99.04 (16)
102.52 (13)
118.71 (19)
123.4 (2)
99.59 (15)
102.67 (10)
118.36 (18)
123.67 (18)
other hand, the reduced flexibility of 4 could also hamper trans-
coordination. Therefore, the formation of several transition metal
complexes of bisxantphos 4 was examined.
When [(PhCN)2PtCl2] was reacted with one equivalent of
diphosphine 4 in methylene chloride, the resulting 31P {1H} NMR
spectrum displayed one sharp signal at
d
¼ ꢀ23.6 ppm with a 1J(Pt,P)
coupling constant of 1781 Hz. The value of the 1J(Pt,P) can be directly
correlated to the coordination mode of the diphosphine. Values
below 3000 Hz (usually around 2500 Hz) are encountered for trans-
diphosphines, and higher values (around 3600 Hz) for complexes in
which the phosphines occupy cis-positions. The low value of
1781 Hz is probably due to a distorted trans geometry [17]. The
large deviation from the usual value around 2500 Hz for most trans-
complexes is probably due to the strained structure of the double
bridged diphosphine, which also causes an unusual chemical shift
in the 31P NMR of
d
¼ ꢀ37 ppm for the free ligand. The formation of
trans-[(4)PtCl2] is interesting since the development of only trans-
coordinating diphosphine ligands is the subject of ongoing research
[18]. We performed MM2 and PM3 analysis on [(4)PtCl2] and the
calculated bond lengths and angles are depicted in Tables 2 and 3.
The P1ePteP2 bond angle is approximately 140ꢁ. Despite this small
bite angle, a trans-complex can not be excluded. In trans-spanning
XantphosePd complexes, P1ePdeP2 angles of 150ꢁ have previ-
ously been observed [16a,b]. A calculated optimized structure of
[(4)PtCl2] shows the bisxantphos ligand coordinated in a trans-
fashion (Fig. 3).
The reaction of ligand 4 with [Pd(cod)Cl2] in methylene chloride
at room temperature gave [(4)PdCl2] as a yellow powder. A 31P {1H}
NMR spectrum of this complex showed one sharp signal at
d
¼ ꢀ14.5 ppm. We also studied the formation of palladiumeallyl
complexes with bisxantphos 4, but these were only analyzed in situ,
because isolation of the complexes invariably led to product
decomposition. When [Pd(h
3-C3H5)Cl]2 was mixed with two
equivalents of ligand 4 in CD2Cl2, the resulting 31P NMR spectrum
displayed one singlet at
less clear. The terminal protons syn and anti to the central proton of
d
¼ ꢀ21.22 ppm. The 1H NMR spectrum was
the allyl group in complex [(4)Pd(
on the NMR time scale and consequently two signals were
h
3-C3H5)]Cl interconverted slowly
observed in the 1H NMR spectrum at
d
¼ 3.23 and 2.90 ppm. This
syneanti isomerization takes place via an h3
e
h1
e
h
3 rearrangment.
Table 2
Calculated bond lengths and angles (MM2) for [(4)PtCl2].
bond length (Å)
bond angle (ꢁ)
C21eP2
P2eC61
C21eC22
C21eC26
P1/P2
PteCl
1.86
1.84
1.41
1.40
4.44
2.29
C21eP2eC21i
C21eP2eC61
P2eC21eC22
P2eC21eC26
P1ePteP2
106.1
117.3
121.8
118.6
142.3
165.2
Fig. 2. Molecular structure of 4, determined by X-ray crystallography. Displacement
ellipsoids are drawn at the 50% probability level. Symmetry operation i: 2-x, y, z.
Hydrogen atoms and solvent molecules are omitted for clarity. Top: view along the
crystallographic [0,0,1] direction. Bottom: view along the crystallographic [-1,0,0]
direction.
ClePteCl

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R. den Heeten et al. / Journal of Organometallic Chemistry 696 (2011) 3113e3120
Table 3
n
1. BuLi
Calculated bond lengths and angles (PM3) for [(4)PtCl2].
Br
Br
Br
PR2
2. ClP(NEt2)2
bond length (Å)
bond angle (ꢁ)
10
11: R = NEt2
12: R = Cl
C21eP2eC21i
C21eP2eC61
P2eC21eC22
P2eC21eC26
P1ePteP2
99.2
106.9
121.5
121.8
139.3
167.7
n
1. BuLi
2. PCl3
C21eP2
P2eC61
C21eC22
C21eC26
P1/P2
PteCl
1.88
1.82
1.41
1.39
4.23
2.32
PCl3
Scheme 3. Synthetic routes to compound 12.
ClePteCl
possible by varying the aryldichlorophosphine species in the
cyclization step. We chose to make use of arylbromides as third
subsituent on the phosphorus atoms. After the cyclization step, the
arylbromides were converted to benzaldehyde moieties to allow
the elegant and mild introduction of e.g. bulky amino acid frag-
ments via reductive amination.
There were no signals observed of an olefinic CH2-unit (typically
two double doublets around
¼ 5.0 ppm). This excludes the
presence of an
1-allyl complex and we can conclude that ligand 4
forms an
3-allyl complex.
The palladium complex [(4)Pd(
situ by addition of diphosphine 4 to [Pd(
two equivalents of silver triflate. One singlet at
d
h
h
h
3-C3H5)]OTf was prepared in
3-C3H5)Cl]2, followed by
Therefore, the synthesis of aryldichlorophosphine 12 was
explored. Arylbromide 12 has previously been synthesized using
a FriedeleCrafts reaction between PCl3 and bromobenzene, but the
desired para substituted product could not be separated from its
ortho and meta isomers [21]. A different approach starts with
monolithiation of dibromobenzene 10, followed by the addition of
bis(diethylamino)chlorophosphine to yield the arylphosphorus
diamide 11 (Scheme 3). Diethylamino phosphane 11 can be con-
verted to dichloride 12 by treatment with PCl3. However, the
formed Et2NPCl2 reacted with 12 during work-up to generate
ArPCl(NEt2) and this was difficult to separate from the desired
product. In a shorter albeit less controllable route, compound 12
was obtained by reacting the monolithiated analogue of 1,4-
dibromobenzene with PCl3. The principal side-product encoun-
tered was Ar2PCl and compound 12 was obtained by fractional
distillation.
h
d
¼ ꢀ21.34 ppm was
observed in the 31P NMR spectrum. Interestingly, the dynamics of
the methyl and tert-butyl groups of the backbone varied with the
counterion. A single resonance corresponding to the tert-butyl
groups and two resonances for the backbone methyl groups were
observed in [(4)Pd(
tert-butyl groups and four distinct resonances for the methyl
groups were observed in complex [(4)Pd(
3-C3H5)]OTf. Similar
behaviour was previously observed for [(Xantphos)Pd(
3-C3H5)]X
h
3-C3H5)]Cl, whereas two resonances for the
h
h
complexes for which the interconvertion of the backbone substit-
uents was ascribed to reversible dissociation of one-half of the
bidentate ligand or pseudorotation via a pentacoordinate palla-
dium complex formed by coordination of the counterion [19]. In the
pentacoordinated [(4)Pd(
h
3-C3H5)]Cl complex the chloro anion
3 (or
) rearrangment of the allyl
facilitates both the h3 h1
e
e
h
pesep
Following the same procedure as used for the synthesis of
double bridged diphosphine 4, aryldichlorophosphine 12 was
employed to obtain cyclic bisxantphos 13 as an air-stable powder.
Lithiation of 13 with n-BuLi/TMEDA in diethyl ether at ꢀ78 ꢁC
generated the dilithio derivative that was quenched with anhy-
drous DMF to afford the bisaldehyde 14 in high yield after acidic
work-up (Scheme 4). The addition of TMEDA (N,N,N’,N’-tetrame-
thylethane-1,2-diamine) was required to prevent decomposition of
the dilithiated intermediate. The formyl groups formed could be
clearly identified by their NMR resonances (1H NMR:
moiety [20] and the synesyn, antieanti exchange (apparent rota-
tion of the allyl group). The latter is a faster exchange process than
the syneanti isomerization. As a result an apparent high symmetry
(C2v) is obtained as indicated by one average resonance for the tert-
butyl groups and two resonances for the methyl groups. In [(4)
Pd(h
3-C3H5)]OTf, with the non-coordinating OTf anion, both
processes are slower than in the Cl-complex. If these processes are
slow on the NMR time scale, the symmetry of the complex is C1,
resulting in four methyl signals and two tert-butyl signals in the 1H
NMR spectrum of [(4)Pd(h
3-C3H5)]OTf.
d
¼ 10.05 ppm, 13C {1H} NMR:
d
¼ 192.2 ppm).
Finally, as an example of sterically demanding residues, L-
phenylalanine methyl ester moieties were introduced via reductive
amination of bisaldehyde 14 using sodium triacetoxyborohydride
giving bisxantphos 15. Unfortunately we were unable to grow
crystals suitable for X-ray diffraction, but the characterization of 15
was achieved by multinuclear NMR spectroscopy.
2.1. Synthesis of functionalized bisxantphos derivatives
Next, sterically demanding residues were introduced in the
cyclic bisxantphos structure in order to create a pocket around the
transition metal. Structural modulations of bisxantphos 4 are
2.2. Catalysis
In the rhodium catalyzed hydroformylation of 1-octene ligand 4
gave very low activity and selectivity (conditions: 20 bar CO/H2,
80 ꢁC). The systems was 15 times less active than Xantphos based
t
t
Bu
Bu
13 R = Br
14 R = CHO
15: R =
O
O
R
P
P
R
Ph
HN
t
t
Bu
Bu
CO2Me
Fig. 3. Calculated optimized structure of [(4)PtCl2] using PM3.
Scheme 4. Bisxantphos with various substituents.

R. den Heeten et al. / Journal of Organometallic Chemistry 696 (2011) 3113e3120
3117
catalysts and gave a l/b ratio of only 3. Isomerization to internal
alkenes took place (30%). Therefore no further hydroformylation
reactions were pursued.
phase was dried over MgSO4 and concentrated under reduced
pressure. The residue was dissolved in CH2Cl2 and purified by silica
gel circular column chromatography (20% CH2Cl2 in hexanes).
Finally, recrystallization from CH2Cl2/MeOH yielded 4 as a white
3. Conclusions
powder (0.22 g, 10%). 1H NMR (300 MHz, CD2Cl2, 293 K):
d
¼ 1.19 (s,
36H, C(CH3)3), 1.52 (s, 6H, CH3), 1.73 (s, 6H, CH3), 7.27e7.32 (m, 10H,
In summary, we have described a stereoselective route to cyclic
double bridged diphosphine ligands based on two xanthene
backbones. The reaction of dilithio-xanthene with dichlor-
ophenylphosphine gave cyclic bisxantphos (4) as a single stereo-
isomer. X-ray crystal structure determination revealed that the
phosphorus bridging groups are arranged in a syn-disposition.
Double bridged cyclic diphosphines in which the two phosphorus
lone pairs are cis oriented are useful chelates in transition metal
catalysts. The coordination behaviour of the new ligand to several
transition metals was investigated. The route is flexible in the
choice of aryldichlorophosphine and should permit a range of
substituents to be introduced on the phosphorus atoms. The new
bisxantphos structure was modified with sterically demanding
residues on the third substituent of the phosphorus atoms which
opens the way to create a shape-selective pocket upon coordination
of the ligands to a metal centre.
H-arom), 7.46e7.50 (m, 8H, H-arom); 13C {1H} NMR (75 MHz,
CD2Cl2, 293 K):
d
¼ 30.8, 34.0, 34.2, 34.9, 123.1, 125.3 (t, J ¼ 12.8 Hz),
126.3, 127.3, 127.8, 129.5 (t, J ¼ 4.0 Hz), 132.0 (t, J ¼ 15.7 Hz), 138,6 (t,
J ¼ 12.1 Hz), 145.3, 151.7 (t, J ¼ 23.3 Hz); 31P {1H} NMR (121 MHz,
CD2Cl2, 293 K):
d
¼ ꢀ37.90; HRMS (FABþ): m/z calcd. for
C58H67O2P2 (MþHþ): 857.4616; found: 857.4622.
4.2.2. X-ray crystal structure determination of 4
Crystals of 4 suitable for X-ray diffraction were grown by slow
diffusion of methanol into a solution of compound 4 in methylene
chloride. C58H66O2P2$CH2Cl2 þ disordered solvent, Fw ¼ 941.97[*]
,
colourless block, 0.50 ꢂ 0.50 ꢂ 0.30 mm3, orthorhombic, Cmc21
(no. 36), a ¼ 19.4478(2), b ¼ 18.7047(1), c ¼ 16.9358(1) Å,
V ¼ 6160.66(8) Å3, Z ¼ 4, Dx ¼ 1.016 g/cm3[ ]
,
m
¼ 0.19 mmꢀ1[ ]
.
*
*
57324 Reflections were measured on a Nonius Kappa CCD diffrac-
tometer with rotating anode (graphite monochromator,
l
¼ 0.71073 Å) up to a resolution of (sin
q
/
l
)
¼ 0.65 Åꢀ1 at
max
4. Experimental section
a temperature of 150(2) K. Intensity integration was performed
with EvalCCD [23]. The SADABS [24] program was used for
absorption correction and scaling based on multiple measured
reflections (0.83e0.94 transmission range). After merging the
Friedel pairs, 3768 Reflections were unique (Rint ¼ 0.0365), of which
4.1. General remarks
Unless stated otherwise, reactions were carried out under an
atmosphere of argon using standard Schlenk techniques. THF and
diethyl ether were distilled from sodium/benzophenone. Tertiary
amines, CH2Cl2 and methanol were distilled from CaH2 and toluene
was distilled from sodium. Deuterated solvents were distilled from
the appropriate drying agents. Unless stated otherwise, all chem-
icals were obtained from commercial suppliers and used as
received. Bis(diethylamino)chlorophosphine was synthesized
according a published procedure [22]. Optical rotations were
measured on a PerkineElmer 241 polarimeter. NMR spectra were
recorded on a Varian Mercury 300, a Varian Inova 500 or a Bruker
Avance DRX-300 spectrometer. Chemical shifts are reported in ppm
and are given relative to tetramethylsilane (1H, 13C) and 85% H3PO4
3240 were observed [I > 2s(I)]. The structure was solved with
Direct Methods using the program SHELXS-97 [25]. The structure
was refined with SHELXL-97 [25] against F2 of all reflections. The
crystal structure contained solvent accessible voids (1269 Å3/unit
cell) filled with disordered solvent molecules. Their contribution to
the structure factors was secured by back-Fourier transformation
using the SQUEEZE routine of the program PLATON [26] resulting in
411 eꢀ/unit cell. Non hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were introduced in
calculated positions and refined with a riding model. 315 Parame-
ters were refined with one restraint. R1/wR2 [I > 2s(I)]: 0.0420/
0.1133. R1/wR2 [all refl.]: 0.0509/0.1174. S ¼ 1.047. Residual electron
density between ꢀ0.24 and 0.40 e/Å3. Geometry calculations and
checking for higher symmetry was performed with the PLATON
program [26]. [*] derived quantities do not contain the contribution
of the disordered solvent.
(
31P). High Resolution Mass Spectra were recorded at the Depart-
ment of Mass Spectrometry at the University of Amsterdam using
Fast Atom Bombardment (FAB) ionization on a JOEL JMS SX/SX102A
four-sector mass spectrometer, coupled to a JEOL MS-MP9021D/
UPD system program. Samples were loaded in a matrix solution
(3-nitrobenzyl alcohol) on to a stainless steel probe and bombarded
with xenon atoms with an energy of 3 KeV. Elemental analyses
were carried out by Kolbe Mikroanalytisch Labor, Mülheim an der
Ruhr (Germany).
4.2.3. Preparation of [(4)PdCl2]
Ligand 4 (10.0 mg, 11.7
mmol) and [Pd(cod)Cl2] (3.7 mg,
11.7 mol) were placed in a Schlenk flask. Methylene chloride
m
(2 mL) was added and the reaction mixture was stirred overnight at
room temperature. The solvent was removed under reduced pres-
sure. The resulting yellow powder was washed with diethyl ether
and dried in vacuo (11.6 mg, 96%). 1H NMR (500 MHz, CD2Cl2,
4.2. Synthesis and coordination behaviour of 4
4.2.1. Cyclic bisxantphos 4
293 K):
7.32 (m, 4H, H-arom), 7.48e7.52 (m, 10H, H-arom), 7.85 (m, 4H, H-
arom); 13C {1H} NMR (125 MHz, CD2Cl2, 293 K):
d
¼ 1.19 (s, 36H, C(CH3)3), 1.45 (s, 6H, CH3), 1.75 (s, 6H, CH3),
To
a solution of 4,5-dibromo-2,7-di-tert-butyl-9,9-dimeth
ylxanthene (2.50 g, 5.2 mmol) in diethyl ether (36.4 mL) was
added dropwise t-BuLi (1.5 M in pentane, 13.6 mL, 20.8 mmol)
at ꢀ78 ꢁC. The reaction mixture was allowed to reach room
temperature overnight. The resulting yellow solution was trans-
ferred into a 50 mL gastight glass syringe. A second gastight syringe
was filled with a solution of dichlorophenylphosphine (0.93 g,
5.2 mmol) in diethyl ether (50 mL). Both solutions were added
simultaneously to a flask charged with diethyl ether (50 mL) using
a syringe pump at a rate of 0.2 mL minꢀ1. After the addition was
completed, the reaction mixture was allowed to stir for another 2 h.
The mixture was washed with water (100 mL) and the organic
d
¼ 26.7, 27.3, 28.2,
31.1, 31.2, 35.0, 37.9, 124.4, 127.2 (t, J ¼ 11.3 Hz), 128.0, 129.9, 135.0 (t,
J ¼ 9.3 Hz), 135,8 (t, J ¼ 5.0 Hz), 147.2 (t, J ¼ 5.7 Hz), 154.0 (t,
J ¼ 7.7 Hz); 31P {1H} NMR (202 MHz, CD2Cl2, 293 K):
d
¼ ꢀ14.48;
HRMS (FABþ): m/z calcd. for C58H66O2P2Pd (MeCl2): 962.3593;
found: 962.3599.
4.2.4. Preparation of [(4)PtCl2]
Ligand 4 (10.0 mg, 11.7
11.7 mol) were placed in a Schlenk flask. Acetonitrile (2 mL) was
added and the reaction mixture was stirred overnight at room
mmol) and [(PhCN)2PtCl2] (5.5 mg,
m

3118
R. den Heeten et al. / Journal of Organometallic Chemistry 696 (2011) 3113e3120
temperature. The solvent was removed under reduced pressure.
butyllithium (2.4 M in hexanes, 2.2 mL, 5.3 mmol) was added
slowly at ꢀ78 ꢁC. The solution was stirred for 2 h. Then, a solution of
chloro(diethylamino)phenylphosphine (1.21 g. 5.6 mmol) in
pentane (20 mL) was slowly added to the solution. The resulting
mixture was stirred at ꢀ78 ꢁC for 1 h and was subsequently allowed
to warm to room temperature overnight. The mixture was filtered
over basic alumina under argon atmosphere and the filter material
was rinsed twice using methylene chloride (20 mL). The combined
solvents were removed in vacuo, yielding 5 as an off-white solid
(3.2 g, 89%, as a 2:1 mixture of rac- and mesodiastereomers). The air
and moisture sensitive product was used without further purifi-
The resulting white solid was washed with hexanes and dried in
vacuo (12.5 mg, 95%). 1H NMR (500 MHz, CD2Cl2, 293 K):
d
¼ 1.19 (s,
36H, C(CH3)3), 1.46 (s, 6H, CH3), 1.75 (s, 6H, CH3), 7.33 (m, 4H, H-
arom), 7.50 (m, 10H, H-arom), 7.62 (m,4H, H-arom); 13C {1H} NMR
(125 MHz, CD2Cl2, 293 K):
d
¼ 28.1, 31.3, 34.6, 34.8, 35.5, 123.7, 125.9
(dd, J ¼ 12.7 Hz, J ¼ 9.3 Hz), 126.9, 127.8, 129.8, 130.2 (t, J ¼ 4.0 Hz),
132.6 (t, J ¼ 16.1 Hz), 139,3 (t, J ¼ 12.2 Hz), 145.9, 152.3 (t,
J ¼ 24.3 Hz); 31P {1H} NMR (202 MHz, CD2Cl2, 293 K):
d
¼ ꢀ23.56
(1JPteP ¼ 1781 Hz); HRMS (FABþ): m/z calcd. for C58H66O2P2Pt
(MeCl2): 1051.4191; found: 1051.4216.
cation. Major rac-isomer: 1H NMR (300 MHz, C6D6, 293 K):
d
¼ 0.89
4.2.5. Computational details
(t, J ¼ 9.3 Hz, 12H, CH3), 1.38 (s, 18H, C(CH3)3), 1.73 (s, 6H, CH3), 3.07
Molecular mechanics calculations were performed using the
Cache WorkSystem (Fujitsu Ltd.) Pro Version 7.5.0.85, using the
MM2 and PM3 programs without changing parameters.
(m, 8H, CH2), 7.12e7.30 (m, 6H, H-arom), 7.35e7.60 (m, 8H, H-
arom); 31P {1H} NMR (121 MHz, C6D6, 293 K):
d
¼ 55.98; Minor
meso-isomer: 1H NMR (300 MHz, C6D6, 293 K):
d
¼ 0.98 (t,
J ¼ 9.8 Hz, 12H, CH3), 1.35 (s, 18H, C(CH3)3), 1.66 (s, 3H, CH3), 1.75 (s,
4.2.6. In situ preparation of [(4)Pd(
Ligand 4 (10.0 mg, 11.7 mol) and [Pd(
mol) were placed in a Schlenk flask. CD2Cl2 (1 mL) was added
and the mixture was stirred for 1 h before analyzed by NMR
spectroscopy. 1H NMR (300 MHz, CD2Cl2, 293 K):
h
3-C3H5)]Cl
3H, CH3), 3.25 (m, 8H, CH2), 7.0e7.12 (m, 6H, H-arom), 7.35e7.60
m
h
3-C3H5)Cl]2 (2.2 mg,
(m, 8H, H-arom); 31P {1H} NMR (121 MHz, C6D6, 293 K):
d
¼ 53.64;
5.9
m
The 13C {1H} NMR spectrum was highly complex, and the signals
could not be assigned to the individual isomers. Therefore all
observed resonances are listed. 13C {1H} NMR (75 MHz, C6D6,
d
¼ 1.17 (s, 36H,
C(CH3)3), 1.57 (s, 6H, CH3), 1.66 (s, 6H, CH3), 2.90 (m, 2H), 3.23 (m,
293 K):
d
¼ 14.74, 31.78, 32.28, 32.52, 34.70, 34.92, 44.77, 122.48,
2H), 5.56 (m, 1H, Hmeso), 7.18e7.22 (m, 8H, H-arom), 7.49e7.56 (m,
127.59, 127.86, 128.09, 128.87, 128.29, 129.19, 129.53, 131.86, 131.99,
132.13, 132.52, 132.67, 132.81, 140.53, 141.07, 145.11, 149.90, 150.73.
10H, H-arom); 31P {1H} NMR (121 MHz, CD2Cl2, 293 K):
d
¼ ꢀ21.11.
4.2.7. In situ preparation of [(4)Pd(
Ligand 4 (10.0 mg, 11.7 mol) and [Pd(
mol) were placed in a Schlenk flask. CD2Cl2 (1 mL) was added
and the reaction mixture was stirred for 1 h. Next AgOTf (2.8 mg,
11 mol) was added and the mixture was filtered before analyzed
by NMR spectroscopy. 1H NMR (300 MHz, CD2Cl2, 293 K):
h
3-C3H5)]OTf
3-C3H5)Cl]2 (2.2 mg,
4.3.3. 4,5-Bis(chlorophenylphosphino)-2,7-di-tert-butyl-9,9-
dimethylxanthene (6)
m
h
5.9
m
1.0 g of 5 (1.47 mmol) was dissolved at 0 ꢁC in 10 mL of freshly
distilled PCl3. The mixture was heated to 50 ꢁC and stirred at that
temperature for 2 h. After cooling the mixture to room tempera-
ture, the solvents were removed using high vacuum and the
remaining solid was washed using hexanes (2 ꢂ 10 mL). This yiel-
ded 0.89 g of 6 as a 1:1 mixture of diastereomers (1.44 mmol, 98%).
This air and moisture sensitive mixture was used without further
m
d
¼ 1.18 (s,
18H, C(CH3)3), 1.19 (s, 18H, C(CH3)3), 1.54 (s, 3H, CH3), 1.64 (s, 3H,
CH3), 1.67 (s, 3H, CH3), 1.69 (s, 3H, CH3), 2.90 (m, 2H), 3.23 (m, 2H),
5.51 (m, 1H, Hmeso), 7.17e7.33 (m, 8H, H-arom), 7.50e7.58 (m, 10H,
H-arom); 31P {1H} NMR (121 MHz, CD2Cl2, 293 K):
d
¼ ꢀ21.34.
purification. 1H NMR (300 MHz, C6D6, 293 K):
d
¼ 1,26 (s, 18H,
C(CH3)3), 1.40 (s, 3H, CH3), 1.42 (s, 3H, CH3), 1.43 (s, 6H, CH3), 6,94
(m, 3H, H-arom), 7.0e7.18 (m, 4H, H-arom), 7.47 (dd, J ¼ 7.1 Hz,
J ¼ 3.2 Hz, 2H, H-arom), 7.55e7.70 (m, 2H, H-arom), 7.85 (bs, 1H, H-
arom), 7.91 (bs, 2H, H-arom); 31P {1H} NMR (121 MHz, C6D6, 293 K):
4.3. Stepwise synthesis procedures towards 4
4.3.1. Bis-(5-bromo-2,7,9,9-tetramethyl-9H-xanthen-4-yl)-
phenylphosphine (3)
d
¼ 76.9, 77.1.
To
a
solution of 4,5-dibromo-2,7-dimethyl-9,9-dimeth
ylxanthene (2.0 g, 5.0 mmol) in THF (50 mL), n-butyllithium (2.4 M
in hexanes, 2.0 mL, 4.9 mmol) was added dropwise at ꢀ78 ꢁC. The
solution was stirred for 1.5 h. Then, a solution of dichlor-
ophenylphosphine (0.35 mL, 2.6 mmol) in pentane (10 mL) was
slowly added to the solution. The resulting mixture was stirred
at ꢀ78 ꢁC for 2 h and was subsequently allowed to warm to room
temperature overnight. The mixture was hydrolyzed using a 1.0 M
solution of HCl in water and extracted with methylene chloride
(3 ꢂ 20 mL). The combined organic layers were dried over MgSO4.
Removal of the solvent in vacuo yielded a light yellow solid. The
product was recrystallized from methylene chloride/methanol,
yielding 1.4 g of colourless needles (74%). 1H NMR (300 MHz, CDCl3,
4.4. Synthesis of functionalized bisxantphos derivatives
4.4.1. 4-Bromophenyl-bis(diethylamino)phosphine (11)
To a solution of 1,4-dibromobenzene 10 (11.8 g, 50.0 mmol) in
diethyl ether (125 mL) was added dropwise n-BuLi (2.5 M in
hexanes, 20.0 mL, 50.0 mmol) at ꢀ78 ꢁC. After stirring for 1 h at that
temperature, bis(diethylamino)chlorophosphine
A
(10.5 g,
50.0 mmol) was added and the solution was allowed to warm to
room temperature over a 16 h period. The reaction mixture was
filtered and the filtrate was concentrated in vacuo. Flash column
chromatography (neutral Al2O3, light petroleum) afforded 11
(11.7 g, 71%) as a white solid. 1H NMR (500 MHz, C6D6, 293 K):
293 K):
d
¼ 1.66 (s, 6H, CH3),1.69 (s, 6H, CH3), 2.20 (s, 6H, CH3), 2.27 (s,
d
¼ 0.97 (t, J ¼ 7.1 Hz, 12H, CH3), 2.90e2.97 (m, 8H, CH2), 7.24e7.27
6H, CH3), 6.61 (bs, 2H, H-arom), 7.20 (m, 5H, H-arom), 7.36 (bs, 4H, H-
(m, 2H, H-arom), 7.35e7.37 (m, 2H, H-arom); 13C {1H} NMR
arom), 7.63 (bs, 2H, H-arom); 13C {1H} NMR (75 MHz, CDCl3, 293 K):
(125 MHz, C6D6, 293 K):
d
¼ 15.1 (d, J ¼ 3.5 Hz), 43.4 (d, J ¼ 17.2 Hz),
d
¼ 21.0, 21.5, 32.0, 32.4, 35.2, 111.0, 125.3, 127.0, 128.6, 129.1, 129.3,
131.8, 132.4, 132.7, 133.2, 135.3 (d, J ¼ 22 Hz), 135.8 (d, J ¼ 12 Hz),
145.8,150.4,150.6; 31P {1H} NMR (121 MHz, CDCl3, 293 K):
¼ ꢀ24.7.
121.5, 122.4, 132.0 (d, J ¼ 2.9 Hz), 133.5 (d, J ¼ 16.5 Hz); 31P {1H}
NMR (121 MHz, C6D6, 293 K):
d
¼ 96.59; HRMS (FABþ): m/z calcd.
d
for C14H25N2PBr (MþHþ): 331.0939; found: 331.0945.
4.3.2. 4,5-Bis-(N,N-diethylaminophenylphosphino)-2,7-di-tert-
4.4.2. (4-Bromophenyl)dichlorophosphine (12)
butyl-9,9-dimethylxanthene (5)
A solution of 1,4-dibromobenzene (14.2 g, 60.0 mmol) in 200 mL
of THF was cooled to ꢀ78 ꢁC n-BuLi (2.5 M in hexanes, 24.0 mL,
60.0 mmol) was added dropwise and the reaction mixture was
To
a
solution of 4,5-dibromo-2,7-di-tert-butyl-9,9-dimet
hylxanthene (2.5 g, 5.2 mmol) in diethyl ether (80 mL), n-

R. den Heeten et al. / Journal of Organometallic Chemistry 696 (2011) 3113e3120
3119
stirred for 1 h keeping the temperature at ꢀ78 ꢁC. The resulting
white, cloudy solution was added via a cannula to a solution of PCl3
(32.9 g, 240 mmol) in THF (50 mL) at ꢀ78 ꢁC. The reaction mixture
was allowed to warm to room temperature and stirred for 16 h. The
precipitation was filtered off and solvent and excess PCl3 were
removed under reduced pressure. Purification by fractional distil-
lation (135e140 ꢁC, 14 mbar) yielded 12 (5.83 g, 37%) as a colourless
CH2), 7.14e7.35 (m, 18H, H-arom), 7.43e7.48 (m, 8H, H-arom); 13C
{1H} NMR (75 MHz, CDCl3, 293 K):
d
¼ 31.6, 32.5, 34.8, 35.1, 38.8,
51.6, 52.5, 65.5, 123.4, 124.3 (t, J ¼ 18.1 Hz),126.3, 126.4, 126.5, 128.9,
129.2, 129.6, 130.0, 134.2 (t, J ¼ 20.3 Hz), 137.7, 138.0, 139.1 (t,
J ¼ 13.5 Hz), 145.6, 150.7 (t, J ¼ 19.5 Hz), 175.6; 31P {1H} NMR
(121 MHz, CDCl3, 293 K):
d
¼ ꢀ36.95; HRMS (FABþ): m/z calcd. for
C80H93N2O6P2 (MþHþ): 1239.6509; found: 1239.6434.
liquid. 1H NMR (500 MHz, CDCl3, 293 K):
arom), 7.71e7.76 (m, 2H, H-arom); 13C {1H} NMR (125 MHz, C6D6,
d
¼ 7.61e7.64 (m, 2H, H-
Acknowledgements
293 K):
d
¼ 121.5, 132.0 (d, J ¼ 32.1 Hz), 132.5 (d, J ¼ 7.9 Hz), 133.6,
139.7 (d, J ¼ 53.5 Hz); 31P {1H} NMR (121 MHz, CDCl3, 293 K):
This work was supported by the Netherlands Organization for
Scientific Research (NWO/CW) and the NRSCC.
d
¼ 158.59.
4.4.3. Cyclic bisxantphos 13
Appendix. Supplementary data
Following the procedure as described for the synthesis of
compound 4, bisxantphos 13 was obtained starting from xanthene
1 (2.50 g, 5.2 mmol) and dichlorophosphine 12 (1.34 g, 5.2 mmol) in
10% yield (0.26 g) as a white solid. 1H NMR (500 MHz, CDCl3, 293 K):
CCDC 789421 (4) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
d
¼ 1.21 (s, 36H, C(CH3)3), 1.53 (s, 6H, CH3), 1.74 (s, 6H, CH3),
7.33e7.35 (m, 8H, H-arom), 7.43e7.47 (m, 8H, H-arom); 13C {1H}
NMR (125 MHz, CDCl3, 293 K):
d
¼ 31.7, 34.8, 34.9, 35.6, 121.2, 123.7,
Appendix. Supplementary material
125.6 (t, J ¼ 19.6 Hz), 129.7, 130.3, 130.8, 134.5 (t, J ¼ 16.1 Hz), 138.6
(dd, J ¼ 7.5 Hz, J ¼ 5.0 Hz), 146.0, 152.6 (t, J ¼ 24.8 Hz); 31P {1H} NMR
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2011.06.019.
(121 MHz, CDCl3, 293 K):
d
¼ ꢀ37.15; HRMS (FABþ): m/z calcd. for
C59H65Br2O2P2 (MþHþ): 1015.2815; found: 1015.2813. Anal. calcd.
for C58H64Br2O2P2: C, 68.64; H, 6.36; Found: C, 68.39; H, 6.45.
References
4.4.4. Cyclic bisxantphos 14
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At ꢀ78 ꢁC, n-butyllithium (2.5 M in hexanes, 0.8 mL, 2.0 mmol)
was added dropwise to a stirred solution of 13 (0.23 g, 0.2 mmol)
and TMEDA (0.3 mL, 2.0 mmol) in diethyl ether (10 mL). The
reaction mixture was stirred for 30 min at ꢀ78 ꢁC. Then, DMF
(0.15 mL, 2.0 mmol) was added and the reaction mixture was stir-
red for another 30 min allowing to warm to room temperature. The
solution was washed with 2 M aqueous HCl (10 mL). The organic
layer was dried over MgSO4 and concentrated under reduced
pressure. Purification by silica gel column chromatography (10%
ethyl acetate in light petroleum) yielded 14 (0.17 g, 95%) as a white
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1113e1122;
powder. 1H NMR (500 MHz, CD2Cl2, 293 K):
C(CH3)3), 1.58 (s, 6H, CH3), 1.76 (s, 6H, CH3), 7.44e7.45 (m, 4H, H-
arom), 7.48e7.49 (m, 4H, H-arom), 7.69e7.71 (m, 4H, H-arom),
7.85e7.86 (m, 4H, H-arom), 10.05 (s, 2H, CHO); 13C {1H} NMR
d
¼ 1.24 (s, 36H,
(125 MHz, CD2Cl2, 293 K):
d
¼ 31.3, 34.8, 34.9, 35.6, 124.3, 124.9 (t,
J ¼ 18.9 Hz), 128.7, 129.7, 130.3, 132.9 (t, J ¼ 16.2 Hz), 135.2, 146.5,
149.1 (t, J ¼ 15.0 Hz), 152.2 (t, J ¼ 24.1 Hz), 192.2; 31P {1H} NMR
(121 MHz, CDCl3, 293 K):
d
¼ ꢀ34.71; HRMS (FABþ): m/z calcd. for
C60H67O4P2 (MþHþ): 913.4515; found: 913.4490. Anal. calc. for
C60H66O4P2: C, 78.92; H, 7.29; Found: C, 78.62; H, 7.61.
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4.4.5. Cyclic bisxantphos 15
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methylene chloride (2 mL) was treated with sodium acetate
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reaction mixture was cooled to 0 ꢁC and sodium triacetoxybor-
ohydride (17.0 mg, 0.08 mmol) was added. The suspension was
warmed to room temperature and stirred for another 3 h. Brine
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(2 ꢂ 3 mL). The combined organic layer was dried over Na2SO4 and
concentrated under reduced pressure. Purification by silica gel
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NMR (300 MHz, CDCl3, 293 K):
CH3), 1.72 (s, 6H, CH3), 2.99 (m, 4H, H
J ¼ 14.0 Hz, 2H, CH2), 3.67 (s, 6H, OCH3), 3.85 (d, J ¼ 15.0 Hz, 2H,
d
¼ 1.20 (s, 36H, C(CH3)3), 1.52 (s, 6H,
b
), 3.57 (m, 2H, H ), 3.66 (d,
a

3120
R. den Heeten et al. / Journal of Organometallic Chemistry 696 (2011) 3113e3120
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