Chiral ditopic cyclophosphazane (CycloP) ligands: Synthesis, coordination chemistry, and application in asymmetric catalysis

Article abstract of DOI:10.1002/chem.201302327

A series of dichlorocyclophosphazanes [{ClP(μ-NR)}₂] containing chiral and achiral R groups was obtained from simple commercially available amines and PCl₃. Their condensation reactions with axially chiral biaryl diols yielded ansa-bridged chiral cyclophosphazane (CycloP) ligands. This highly modular methodology allows extensive elaboration of the ligand set, in which the chirality can be introduced at the diol bridge and/or the amido R group. This provides the possibility to observe match and mismatch effects in catalysis. A series of twenty CycloP ligands was synthesized and characterized by multinuclear NMR spectroscopy, HRMS, elemental analysis, and in selected cases, single-crystal X-ray diffraction. These studies show that all of the ditopic CycloP ligands are C₂ symmetric, rendering their metal coordination sites symmetry equivalent. Two well-established enantioselective reactions were explored by using late-transition metal CycloP complexes as catalysts; the gold-catalyzed hydroamination of γ-allenyl sulfonamides and the asymmetric nickel-catalyzed three-component coupling of a diene and an aldehyde. The steric demands of the CycloP ligands have a subtle influence on the reactivity and selectivity observed in both reactions. Good enantiomeric ratios (e.r.) as high as 89:11 in the gold-catalyzed reaction and 92:8 in the nickel-catalyzed bis-homoallylation reaction were observed. One ligand - two chiral binding sites: A library of new, chiral, bridged cyclophosphazane ligands, in which chirality can be introduced at two positions (at the bridge and/or at the phosphorus bridging amido groups), gives rise to good enantiomeric ratios in Au^I- and Ni⁰-catalyzed enentioselective reactions (see scheme, Ts=tosyl). Copyright

Full text of DOI:10.1002/chem.201302327

FULL PAPER
DOI: 10.1002/chem.201302327
Chiral Ditopic Cyclophosphazane (CycloP) Ligands: Synthesis, Coordination
Chemistry, and Application in Asymmetric Catalysis
Torsten Roth,[a] Hubert Wadepohl,[a] Dominic S. Wright,*[b] and Lutz H. Gade*[a]
Dedicated to Professor Brian F. G. Johnson on the occasion of his 75th birthday
Abstract: A series of dichlorocyclo-
phosphazanes [{ClP(m-NR)}2] contain-
gands was synthesized and character-
ized by multinuclear NMR spectrosco-
py, HRMS, elemental analysis, and in
selected cases, single-crystal X-ray dif-
fraction. These studies show that all of
the ditopic CycloP ligands are C2 sym-
metric, rendering their metal coordina-
tion sites symmetry equivalent. Two
well-established enantioselective reac-
tions were explored by using late-tran-
sition metal CycloP complexes as cata-
lysts; the gold-catalyzed hydroamina-
tion of g-allenyl sulfonamides and the
asymmetric nickel-catalyzed three-
component coupling of a diene and an
aldehyde. The steric demands of the
CycloP ligands have a subtle influence
on the reactivity and selectivity ob-
served in both reactions. Good enantio-
meric ratios (e.r.) as high as 89:11 in
the gold-catalyzed reaction and 92:8 in
the nickel-catalyzed bis-homoallylation
reaction were observed.
AHCTUNGTRENNUNG
ing chiral and achiral R groups was ob-
tained from simple commercially avail-
able amines and PCl3. Their condensa-
tion reactions with axially chiral biaryl
diols yielded ansa-bridged chiral cyclo-
phosphazane (CycloP) ligands. This
highly modular methodology allows ex-
tensive elaboration of the ligand set, in
which the chirality can be introduced
at the diol bridge and/or the amido R
group. This provides the possibility to
observe match and mismatch effects in
catalysis. A series of twenty CycloP li-
Keywords: asymmetric catalysis
·
chiral ligands · phosphazanes · syn-
thetic methods · X-ray diffraction
Introduction
Chiral phosphorus compounds occupy a pivotal role as li-
gands and precursors for a variety of transformations and
are ubiquitous in a broad range of industrial and academic
asymmetric catalytic reactions.[1] Early examples of chiral
phosphorus ligands introduced by Knowles et al.[2] and
Horner et al.[3] were monodentate, however, these were rap-
idly superseded by C2-symmetric bidentate ligands intro-
duced by Kagan et al.[4] More than two decades later Alexa-
kis et al.,[5] Feringa et al.,[6] Reetz et al.,[7] and Pringle et al.[8]
successfully challenged the conventional wisdom of the su-
perior enantioselectivity of bidentate chiral catalysts by in-
troducing monodentate phosphordiamide, phosphoramidite,
phosphite, and phosphonite ligands (Figure 1), which gave
comparable enantioselectivities to the established C2-sym-
Figure 1. Monodentate phosphoramidite, phosphite, and phosphonite li-
gands.
metric bidentate counterparts. The main advantages of these
ligand scaffolds compared to previously existing arrange-
ments of chiral aryl and alkyl phosphines is their robustness
as well as the ease by which they can be prepared and their
steric and electronic demands elaborated.[9]
The modular construction of ligands of this type enables
chirality to be introduced either through the chelating com-
ponent, the amine substituent, or both. Modification of the
chirality of both components provides the potential for dia-
stereomeric ligand pairs. A conspicuous feature of the very
successful phosphoramidite and phosphite ligands is their re-
liance on their axially-chiral, rigid diol fragments as the
source of chirality and conformational constraint. Besides
the ubiquitous binol unit, taddol, vanol, and spinol have also
proven to be “privileged” groups in numerous transforma-
tions in recent years.[10]
[a] Dipl.-Chem. T. Roth, Prof. Dr. H. Wadepohl, Prof. Dr. L. H. Gade
Anorganisch Chemisches Institut, Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax : (+49)6221-54-5609
E-mail: lutz.gade@uni-hd.de
[b] Prof. Dr. D. S. Wright
Chemistry Department, Cambridge University
Lensfield Rd, Cambridge CB2 1EW (UK)
Fax : (+44)1223-336362
E-mail: dsw1000@cam.ac.uk
In 1977, Thompson and Keat reported that the reaction of
dichlorophosphazanes with diols and diamines gives transan-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302327.
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Following our recent work on the investigation of the in-
fluence of multiple metal centers on the enantioselectivity
of gold-catalyzed transformations,[20] we became interested
in the cyclophosphazane moiety, its dimetallic complexes,
and its potential for cooperative interactions in catalytic re-
actions. In the current paper we show that 1) a very large
number of CycloP ligands of the type II can be easily ac-
cessed, which are chiral at the transannular bridge or (for
the first time) at the amido group or both, 2) these ligands
coordinate catalytically active late-transition metals such as
rhodium, copper, iridium, gold, and nickel, and 3) the metal
complexes of the CycloP ligands can be employed as cata-
lysts for asymmetric transformations with good enantioselec-
tivity.
Scheme 1. Synthesis of diol-bridged cyclophosphazanes by Thompson and
Keat.[11]
nular bicyclic phosphazanes in which the normal tendency
for cis–trans-isomerization and dissociation of the P2N2 ring
unit is suppressed (see Scheme 1).[11]
More recently, by using the same methodology, Swamy
et al. prepared several biaryl-bridged representatives of this
class of cyclophosphazanes[12] (Figure 2, I) and the first
Results and Discussion
Synthesis and characterization of chiral cyclophosphazane
(CycloP) ligands
Synthesis and characterization of dichlorocyclophosphazane
precursors: The first step in the systematic elaboration of
chiral cyclophosphazanes was to synthesize a range of new
Figure 2. Achiral and chiral bridged cyclophosphazanes I and II as syn-
thesized by Swamy et al. and dianionic bisACHTNUTGRNEUNG(amido)cyclodiphosphazanes
III.
dichlorodimers of the type [{ClPACHTNUGTRENUNG(m-NR)}2]. Previously, only
a limited number of dimers of this type had been structural-
ly characterized.[21] One of the primary aims of the initial
synthetic studies was to broaden the range of the available
dimers, which could be used as precursors for the synthesis
of the CycloP ligands described in the next section. A fur-
ther aim was to synthesize the first dichlorophosphazane
chiral representatives by employing
a binol backbone
(Figure 2, II).[13] However, despite being structurally and
electronically closely related to the phosphoramidites, the
bidentate cyclophosphACTHUNGTRENNG(U III)azanes have so far found only
limited use in catalysis and there are no examples of asym-
metric catalysis involving this type of ligand framework.[14]
A noteworthy exception are the dianionic chelating bis-
À
dimers containing chiral groups within the R N substituent.
This was thought to be a key element in the formation of
new CycloP scaffolds having chirality both at the bridge and
(m-NR)}2]2À (Figure 2,
ACHTUNGTRENNUNG(amido)cyclodiphosphazanes, [{(RN)PACHTUNGTRENNUGN
III) that were reported by Stahl and Chivers et al.,[15] which
have recently garnered attention as ligands for Group 4-
based olefin polymerization catalysts.[16]
the R N group, hence the potential for match and mismatch
À
ligand effects.
Initially, a range of synthetic approaches for the formation
of dichlorocyclophosphazanedimers was evaluated, based on
known literature procedures. It quickly became apparent
that the best approach to these species using a broad range
of organic substituents was the condensation of PCl3 with
primary amines in the presence of excess triethylamine as
the Brønsted base (Scheme 2).
Chiral cyclophosphazane (CycloP) ligands like II in
Figure 2 have several unique and important characteristics,
which are in stark contrast to more conventional ligands
used in catalysis. In particular they have a convex ligand
topology and an exodentate spatial arrangement of the
phosphorus lone pairs making them potentially ditopic li-
gands. Importantly, with regard to their application as
stereo-directing ligands in catalysis, the cyclophosphazane
framework should be remarkably easily elaborated by a
series of modular reactions both at the bridging amido
groups within the P2N2 units and at the transannular bridge.
Similar to the ligand systems shown in Figure 1, there is also
the possibility of introducing chirality at either of these key
positions, that is, introducing the potential for matched and
mismatched pairs. In view of the bis-monodentate character
of CycloP ligands, that is, coordination of two metal centers
rather than it acting as a chelate ligand[17] there is also a
unique potential for cooperative activation by using both
metal centers,[18] as commonly utilized in enzyme biocataly-
sis.[19]
Scheme 2. General synthesis of dichlorocyclophosphazanes.
The organic substituents (R) were selected based on their
steric demands. The use of bulky substituents ensured the
retention of the P2N2 ring units because it is known from
previous studies that small substituents (such as R=Me, Et)
could result in the formation of higher cyclic oligomers or
cages.[22]
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Chiral Ditopic Cyclophosphazane Ligands
FULL PAPER
Figure 4. Molecular structure of compound 6·Cl2, thermal ellipsoids set at
the 50% probability level. Hydrogen atoms are omitted for clarity. Se-
À
À
lected bond lengths [ꢂ] and angles [8]: P Cl 2.1276(4), P’ N 1.6983(10),
À
P N 1.7137(9); P’-N-P 99.09(5), N’-P-N 80.90(5), P-N-C(1)-C(6) 133.3(1).
Chiral cyclophosphazane ligands: The next step was to
obtain the CycloP ligands by incorporating a chiral back-
bone. These ligands were prepared by using the condensa-
tion reactions of the dichlorphosphazanes 1·Cl2–11·Cl2 with
the bridging diols binol, vanol, and vapol in the presence of
NEt3 as the Brønsted base (Scheme 3).
Figure 3. Numbering scheme for the N-alkylated and N-arylated cyclo-
phosphazanes. Numbers correspond to the P2(NR)2 fragment.
Initially, we investigated the behavior of the bis(mentho-
late) phosphazane dimer A prepared by the reaction shown
in Scheme 4. Its structural chemistry illustrates the level of
The previously reported compounds 1·Cl2–4·Cl2 (R=tBu
(1), cyclohexyl (Cy, 2), Ph (3), 2,6-diisopropylphenyl (dipp,
4)) were prepared according to literature procedures, where-
as the new P2N2 dimers 5·Cl2–11·Cl2 were obtained in mod-
erate yields by the condensation reaction shown in
Scheme 2 (see Figure 3 for the numbering scheme used in
the text). Importantly, the products could be prepared on a
multigram scale by this procedure. In addition to 31P and
1H NMR spectroscopy, all of the new compounds 5·Cl2–
11·Cl2 were characterized by a combination of LIFDI(+)
mass spectrometry, elemental analysis, and, in the case of
6·Cl2, by single-crystal X-ray diffraction. For all of the com-
pounds the LIFDI(+) mass spectra in toluene gave a highest
molecular ion peak corresponding to [M]+, thus indicating,
that the dimeric structures were maintained in the gas
phase. Particularly diagnostic is the 31P NMR spectroscopic
shift of dimers 1·Cl2–11·Cl2, which was found in the range
d=200 to 225 ppm. This chemical shift range is typical of di-
meric cis-P2N2 units and can be compared to dimers in
which the chloride substituents are trans to the P2N2 plane
(shifted by d=80–90 ppm to lower field compared to the cis
dimer).[23] It is known that for relatively sterically unencum-
bered substituents R, which have low electronegativity, the
cis structure is marginally more stable in solution and in the
gas phase.[23] The dichlorocyclophosphazane 9·Cl2 is note-
worthy in being a rare example of a P2N2 dimer containing
donor-functionalized R groups that can potentially coordi-
nate metal centers.[24]
Scheme 3. General synthesis of bridged cyclophosphazane ligands.
Scheme 4. Synthesis of the bis(mentholato) cyclophosphazane A.
complexity to be encountered when not using diols that give
rise to the well-defined bicyclic structures discussed in detail
below.
The molecular structure of A determined by X-ray dif-
fraction shows a cis arrangement of the menthol groups with
respect to the P2N2 ring. In addition, the menthol rings
themselves adopt exo and endo orientations with respect to
the P2N2 ring unit (Figure 5). Such exo/endo orientation of
À
The solid-state molecular structure of dimer 6·Cl2, which
was a key precursor in later studies is shown in Figure 4.
The plane of the N-bonded aryl substituent is skewed by ap-
proximately 508 out of the plane of the P2N2 ring.
N( H)- and O-bonded substituents has been seen previously
in the solid-state structures of cyclophosphazane macrocy-
cles.[25] As for these macrocycles a variable-temperature
(VT) 31P NMR study revealed that rapid exo/endo inversion
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D. S. Wright, L. H. Gade et al.
Figure 5. Molecular structure of compound A. The menthol moiety on
O(1) adopts an endo and the menthol group on O(2) an exo orient-
ACHTUNGTRENNUNG
ation.[25a] Thermal ellipsoids set at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths [ꢂ] and angles [8]:
Figure 6. Numbering scheme for chiral diol bridging groups. Letters cor-
respond to the diaryloxy fragment.
À
À
À
P(1) O(1) 1.626(4), P(2) O(2) 1.622(5), P N 1.713(5)…1.737(5); P(1)-
N(2)-P(2) 96.7(2), N(1)-P(1)-N(2) 82.4(2), O(1)-P(1)-P(2)-O(2) 2.2(3).
Scheme 5. Fluxional behavior of bis(mentholato) cyclophosphazane A in
solution.
Scheme 6. Observed fragmentation–condensation reaction of dichlorocy-
clophosphazane 4·Cl2.
of the oxygen atoms occurs at room temperature
(Scheme 5).
sponding phosphoramidite as suggested by NMR spectro-
scopy (Scheme 6). In addition, the reaction of sterically de-
manding vapol i with 1·Cl2 only yielded an intractable mix-
ture of products.
The corresponding free energy of activation (DG°) for
this process of 31 kJmolÀ1 was found to be similar to that
previously reported for oxygen-bridged phosphazane macro-
cycles.[25b]
We therefore decided to focus on the binol framework in
all further studies. Whereas almost all of the binols a–e are
commercially available, binols f and g are novel derivatives
and were conveniently prepared by Suzuki coupling reac-
tions (see the Supporting Information). Our thinking in
regard to these ligand frameworks was that steric and elec-
tronic modification at the 3,3’-positions would have the
greatest impact on the metal-coordinating phosphorus
donor site. Most importantly 3,3’-substitution of the binol
core provided the maximum possible transmission of chirali-
ty to the phosphorus site. This “long-arm effect” had recent-
ly been exploited in binol and spinol phosphoramidites.[26]
Figure 7 lists all of the new CycloP ligands, which were
prepared in this study. All products obtained are colorless
solids and remarkably air stable, for example, air exposure
of 1a for several days resulted in only partial decomposition,
typically giving a yellow discoloration. The new CycloP li-
gands were characterized by a combination of HRMS
(FAB(+)), multinuclear (1H, 31P, 13C, 29Si, 19F) NMR spectro-
scopy, and elemental analysis., and in selected cases by
single-crystal X-ray diffraction (compounds 1b, 1c, 2a, 5a,
6a, and 7c). All of the products listed in Figure 7 were char-
acterized by a single singlet 31P{1H} NMR shift (generally
The significance of the fluxionality of A is that ligand sys-
tems involving separate and disconnected chiral groups
bonded to phosphorus are unlikely to be efficient ligands
for asymmetric metal catalysis because the steric domain of
the coordination environment on either side of the ligand
does not remain static. In contrast, ansa-bridged chiral
CycloP ligands should not suffer from this problem. Follow-
ing the general procedure depicted in Scheme 3, we pre-
pared the binol derivative 1a[12a] (72% yield) and the vanol-
bridged ligand 1h (59% yield) (see Figures 3 and 6 for the
numbering scheme used throughout this work).
The retention of the P2N2 ring unit and the formation of
the desired chiral bridged cyclophosphazane proved to be
critically dependent on the steric bulk of the amido group
À
R N and of the bridging framework. In general, it is nota-
ble, that the sterically less demanding combinations of diol
and dichlorophosphazane resulted in cleaner product forma-
tion, whereas the bulkier derivatives had to be further puri-
fied by column chromatography. An extreme case of this
was observed in the reaction of the sterically encumbered
compound 4·Cl2 with unsubstituted binol a, which gives rise
to disruption of the P2N2 ring and formation of the corre-
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Chiral Ditopic Cyclophosphazane Ligands
FULL PAPER
Figure 7. Complete list of the chiral cyclophosphazane ligands synthesized in this work. One amido R group is omitted for clarity.
between d=164 and 185 ppm),
confirming, that the molecular
C2-symmetric axis is maintained
in each case (Figure 8).
The main features of the new
CycloP ligands were thus either
chirality at the bridge only or
Figure 8. Illustration of the C2
rotational axis in cyclophos-
both at the bridgehead and
the amide group R. The latter
allowed access to diastereomer-
ic pairs and the probing of
phazane ligands containing
chirality in the binol backbone
and the amido substituent.
match/mismatch
phenomena.
These features are illustrated most clearly in the structurally
characterized complexes
1b (chiral at the bridge alone, Figure 9), and the diastereom-
ers (Rbinol,Ramine)-10a (Figure 10) and (Sbinol,Ramine)-10a
(Figure 11).
Figure 9. Molecular structure of compound (R)-1b, thermal ellipsoids set
at the 50% probability level. Hydrogen atoms are omitted for clarity. Se-
À
À
lected bond lengths [ꢂ] and angles [8]: P(1) N(1) 1.710(11), P(1) O(1)
1.685(8); P(1)-N(1)-P(2) 97.6(6), N(1)-P(1)-N(2) 82.1(5).
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D. S. Wright, L. H. Gade et al.
there were no previous reports of the coordination behavior
of chiral phosphazane counterparts.
Initially, we investigated the reaction of ligand 1a with
the common gold(I) precursor [(Me2S)AuCl] (1:2 equiva-
lents) and found the transformation to the digold(I) com-
plex was complete within 30 min at room temperature
(Scheme 7). In situ 31P{1H} NMR spectroscopy showed that
Scheme 7. General procedure for the synthesis of chiral digold(I)cyclo-
phosphazane complexes.
Figure 10. Molecular structure of compound (Rbinol,Ramine)-10a, thermal
ellipsoids set at the 50% probability level. One molecule of toluene and
hydrogen atoms are omitted for clarity. Selected bond lengths [ꢂ] and
the bis-coordination to two gold centers resulted in a de-
crease of the chemical shifts of the singlet resonance of ap-
À
À
angles [8]: P(1) N(1) 1.717(2), P(1) O(1) 1.664(2); P(1)-N(1)-P(2)
98.5(1), N(1)-P(1)-N(2) 81.4(1).
1
proximately d=50 ppm. Significantly, the H and 31P NMR
spectra of [1a·ACHTNUTRGNEUNG(AuCl)2] confirmed that the ligand remained
intact in solution and that there was no rearrangement or
oligomerization occurring upon metal coordination.
This was also confirmed by a single-crystal X-ray structure
analysis of [1a·ACTHNUTRGNEUNG(AuCl)2] (Figure 12). The structure reveals a
Figure 11. Molecular structure of compound (Sbinol,Ramine)-dia-10a, ther-
mal ellipsoids set at the 50% probability level. Hydrogen atoms are omit-
À
ted for clarity. Selected bond lengths [ꢂ] and angles [8]: P(1) N(1)
À
1.704(2), P(1) O(1) 1.667(2); N(1)-P(1)-N(2) 81.2(1), P(1)-N(1)-P(2)
98.6(1).
Figure 12. Molecular structure of compound (R)-[1a·ACTHNURGTNEUNG(AuCl)2], thermal el-
lipsoids set at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected distances [ꢂ], bond lengths [ꢂ], and angles [8]:
Coordination of chiral cyclophosphazanes to metals: In the
next step of the study we investigated the complexation be-
havior of the two phosphorus lone pairs of the CycloP li-
gands to several late transition metals, in particular to assess
the rigidity of the chiral ligands upon metal coordination.
This was an important issue because previous studies sug-
gested that ansa cyclophosphazanes of this type could poten-
tially oligomerize giving dimeric macrocyclic species,[25b] and
the presence of Lewis acidic metal centers could also result
in ring-expansion.[27]
Moreover, the spatial arrangement around the catalytical-
ly active sites and any dynamic behavior at the metal centers
were of interest. The coordination chemistry of achiral di-
meric phosphazanes (related to the chiral system we report
here) had been elaborated in great detail by several groups,
resulting in a plethora of interesting mono- and oligo-metal-
lic structures.[28] However, to the best of our knowledge,
À
À
À
Au(1)···Au(2) 5.7321(2), P(1) Au(1) 2.199(1), P(1) N(1) 1.687(4), P(1)
O(1) 1.600(3); P(1)-Au(1)-Cl(1) 174.39(5), P(1)-N(1)-P(2) 94.8(2), N(1)-
P(1)-N(2) 84.9(2).
very long intramolecular AuI ··AuI distance of 5.7321(2) ꢂ,
which is well outside the range of aurophilic interactions.[29]
This indicated that cooperativity between the two metal cen-
ters was unlikely in catalytic reactions.
A series of such gold(I) complexes containing CycloP li-
gands was prepared and fully characterized in the current
work. The NMR spectroscopic characteristics of all of these
complexes were similar to those discussed above for [1a·-
AHCTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
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Chiral Ditopic Cyclophosphazane Ligands
FULL PAPER
Figure 14. Molecular structure of dimeric compound [9a·ACTHUNRGTNEUNG(AuCl)2], ther-
mal ellipsoids set at the 50% probability level. The second set of the dis-
ordered OBn group and hydrogen atoms are omitted for clarity. Selected
À
À
bond lengths [ꢂ] and angles [8]: P(1) Au(1) 2.205(2), P(2) Au(2)
Figure 13. Molecular structure of compound [1h·ACTHNUTRGENUG(N AuCl)2], thermal ellip-
soids set at the 50% probability level. Only one of the two independent
molecules is shown. Hydrogen atoms are omitted for clarity. Selected
À
À
À
2.200(2), Au(1) Au(51) 3.093(1), Au(1) Cl(1) 2.282(2), Au(2) Cl(2)
2.278(3); P(1)-Au(1)-Cl(1) 171.50(9), P(2)-Au(2)-Cl(2) 175.76(10), P(1)-
Au(1)-Au(51)-P(51) 138.28(9).
À
À
bond lengths [ꢂ] and angles [8]:P(51) O(51) 1.601(5), P(51) N(51)
À
À
1.678(6), P(51) Au(51) 2.186(2), Au(51) Cl(51) 2.257(2); P(51)-Au(51)-
Cl(51) 176.89(9).
the AuI···AuI distances in the series of CycloP complexes
[1a·
ACHTUNGTRENNUNG(AuCl)2], [1b·ACHTUNGTRENNUNG(AuCl)2], [1c·ACHTUNEGRTG(NNUN AuCl)2], [1e·ACHTNUGRTENN(UGN AuCl)2], and
[1g·(AuCl)2], in which there is increasing steric bulk of the
ACHTUNGTRENNUNG
3,3’ substituents within the binol bridge, revealed that there
is no resulting reduction in the metal···metal distance
(ꢀ5.71–5.98 ꢂ). Interestingly, the 3,3’-aryl-substituted com-
Scheme 8. Reaction of 1a with the dimeric precursors [{RhACHTUNGTRENNUNG(nbd)Cl}2]
(left) and [{Pd(2-Me-allyl)Cl}2] (right).
plexes [1e·ACHTUNGTRENNUNG(AuCl)2] and [1g·HCATUNGTRENN(UGN AuCl)2] feature relatively short
intramolecular paryl···AuI distances of approximately 3.65 ꢂ.
Such short p-cation distances may exert significant stabiliz-
ing effects on catalytically active metal centers, an advant-
age exploited in the Buchwald ligand family.[1d,30]
A comprehensive discussion of all of the structurally char-
acterized complexes is not provided here, however, the two
is closely related to a number of rhodium(I) compounds
containing achiral cyclophosphazane ligands.[31]
1H and VT 31P NMR studies of complex [1a·{Rh
ACTHNGUTERU(NNG nbd)Cl}2]
are consistent with the presence of rotamers with respect to
[32]
À
complexes [1h·
terest. Complex [1h·
terized complex in the current work containing a vanol-de-
rived ligand (Figure 13), whereas compound [9a·(AuCl)2] is
A
R
the phosphorus rhodium bonds (Scheme 9). Thus, in con-
1
N
trast to the H NMR spectrum of the uncoordinated ligand,
which shows the expected six individual sharp resonances in
the aromatic region at room temperature, the 3,3’-protons of
A
the only complex containing a CycloP ligand combining a
the binol ligand in [1a·{RhACTHUNGTERNU(NG nbd)Cl}2] are extremely broad-
chiral R N substituent and chiral bridge (Figure 14). The
ened (Figure 16). The room-temperature 31P{1H} NMR spec-
trum shows two broad apparent doublets. As the tempera-
ture is lowered to approximately À138C these broad reso-
nances are resolved into three distinct multiplets, a doublet
of doublets and a doublet of doublet of doublets are ob-
À
solid-state structure of this complex also shows intermolecu-
lar aurophilic interactions.
To demonstrate the generality of the coordination behav-
ior of the chiral cyclophosphazane ligands, the reactions of
1a with a range of other transition-metal fragments, which
are of interest in catalysis, were also explored. The metal
complexes [1a·{RhACHTUNGTRENNUNG(nbd)Cl}2] (nbd=norbornadiene) and
[1a·{Pd(2-Me-allyl)Cl}2] were prepared by the reactions of
1a shown in Scheme 8.
Both complexes were obtained as analytically pure solids
after column chromatography and were characterized by
multinuclear NMR spectroscopy, MS, and elemental analy-
sis, and in the case of the rhodium complex, by single-crystal
X-ray diffraction. The solid-state molecular structure is C2
symmetric with each rhodium(I) atom adopting a slightly
distorted square-planar geometry (Figure 15). This complex
Scheme 9. Rotational isomers of 1a·[RhACTHNUTRGNEN(UG nbd)Cl]2 in solution.
Chem. Eur. J. 2013, 19, 13823 – 13837
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13829

D. S. Wright, L. H. Gade et al.
Figure 17. 31P{1H} NMR spectrum of [1a·{Rh
233 K. The black diamonds mark the signals corresponding to the asym-
ACHTUNGRTENUN(GN nbd)Cl}2] in CD2Cl2 at
1
2
metric b isomer (P1: d=138.0 ppm, dd, J
d=133.1 ppm, ddd, 1J(P,Rh)=272 Hz, 2J
3.5 Hz); the black square denotes the C2-symmetric rotational a isomer
G
ACHTUNGTRENNUNG
E
N
ACHTUNGTRENNUNG
Figure 15. Molecular structure of compound [1a·{RhACTHNURTGNENG(U nbd)Cl}2], thermal
ellipsoids set at the 50% probability level. Hydrogen atoms are omitted
1
(d=136.7 ppm, m, JACTHNUTRGNEUGN(P,Rh)=276 Hz).
À
for clarity. Selected bond lengths [ꢂ] and angles [8]: P(1) Rh 2.233(1),
À
À
Rh Cl 2.347(1), P(1) N(1) 1.694(4); P(1’)-N(1)-P(1) 96.1(2), N(1’)-P(1)-
N(1) 83.9(2).
Figure 18. Illustration of the four isomeric forms of dimetallic CycloP
allyl palladium complexes in solution.
barrier allows the observation of four distinct conformers in
solution at room temperature (Figure 18).[33] These are also
apparent in the 1H NMR spectrum of [1a·{Pd(2-Me-al-
lyl)Cl}2] at 358C, showing the presence of four distinct dou-
blet resonances for the 3,3’-protons of each isomer
(Figure 19). In the room-temperature 13C DEPT NMR
AHCTUNGTREGsNNUN pectrum the six resonances for the carbon atoms of the
binol bridge are each split into four singlet resonances
Figure 16. 1H NMR spectrum of [1a·{Rh
ACTHNUTRGEN(UNG nbd)Cl}2] in CD2Cl2 at 295 K.
For the atom labeling see Scheme 9.
served for the phosphorus atoms of rotamer b and a
(second-order) double multiplet is found for the phosphorus
atoms of rotamer a (Scheme 9 and Figure 17). The ratio of
these two rotational isomers changes with the temperature,
so, for example, at À138C the ratio of a/b is 1.2:1, whereas
at À808C the ratio is 8:1 (K=a/b, DH=(À7Æ2) kJmolÀ1,
DS=(À24Æ7) JmolÀ1). The bias towards the conformation-
ally more stable a rotamer at low temperature (the ob-
served solid-state structure of [1a·{RhACHTUNGERTN(NUNG nbd)Cl}2], Figure 15)
is as expected.
A similar dynamic situation is apparent in the palladium
complex [1a·{Pd(2-Me-allyl)Cl}2], this time, however, with a
much higher rotational energy barrier. This higher energy
Figure 19. 1H NMR spectrum of [1a·{Pd(2-Me-allyl)Cl}2] at 310 K in
CD2Cl2.
13830
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Chem. Eur. J. 2013, 19, 13823 – 13837

Chiral Ditopic Cyclophosphazane Ligands
FULL PAPER
Table 1. Examination of the catalytic properties in the intramolecular
allene hydroamination reaction.[a]
Entry
[Ligand·
E
T
[8C]
t
Conv[b]
[%]
e.r.
[h]
R/S[c]
1
2
3
4
5
6
7
8
(R)-1a
(R)-1a
(S)-1b
(R)-1c
(R)-1e
(R)-1 f
(R)-1g
(R)-1h
25
50
25
25
25
25
25
25
72
19
93
120
130
144
144
68
98
95
96
28
83
45
86
92
39:61
42:58
39:61
36:64
16:84
49:51
49:51
51:49
[a] All reactions were carried out with 5 mol% of AuI in toluene (1 mL).
[b] Conversion was monitored by 1H NMR spectroscopy. [c] Enantiose-
lectivities were determined by chiral HPLC.
Figure 20. Aromatic section of the 13C DEPT NMR spectrum of
[1a·{Pd(2-Me-allyl)Cl}2] at 295 K in CD2Cl2.
but meagre enantiomeric ratios (e.r.s) of 39:61 and 42:58
(Table 1, entries 1 and 2). We found, that gradually increas-
ing the steric bulk of the 3,3’-positions led to a higher enan-
tioselectivity and a concomitant retardation of the reaction
rate (Table 1, entries 3–5). However, both, a further increase
of the steric demand and replacement of the binol by the
vanol backbone gave essentially racemic mixtures of the
product in the catalytic transformation (Table 1, entries 6–
8).
Because the variation of the binol scaffold did not give
rise to an increased selectivity, we next explored the effect
of derivatization of the amine substituent, reasoning that its
spatial proximity to the coordinating phosphorus atoms
might have a greater impact on the selectivity. Thus, we set
out to explore the influence of chirality on the amine com-
ponent. By using the digold complexes of the CycloP ligands
7a, 8a, and 9a in the hydroamination reaction, the selectivi-
ty was considerably increased to an e.r. of 88:12, 15:85, and
19:81, respectively (Table 2, entries 1–3).
(Figure 20).[34] These isomers interconvert at approximately
1
658C in CD2Cl2 according to H and 31P NMR studies.[35]
Evaluation in model catalysis
Gold(I)-catalyzed cyclohydroamination of a g-allenyl sulfo-
namide: Cationic gold(I) compounds are known to be ex-
ceptional catalysts for the activation of carbon–carbon mul-
tiple bonds toward nucleophilic attack and have thus been
À
À
À
À
used for the formation of C C, C O, C N, and C S
bonds.[36] For the purpose of assessing the activity and selec-
tivity of the gold(I) complexes of the CycloP ligands descri-
bed above, we chose the cyclohydroamination of g-allenyl
sulfonamides, which is a well understood reference reaction
in the domain of gold catalysis (Scheme 10).[37] We were es-
Table 2. Examination of the catalytic properties in the intramolecular
allene hydroamination reaction.[a]
Entry
[Ligand·
E
t
Conv[b]
[%]
e.r.
[h]
R/S[c]
Scheme 10. Cyclohydroamination of g-allenyl sulfonamide s1 (Ts=tosyl).
1
2
3
4
5
6
7
(Sbinol,Ramine)-7a
(Rbinol,Samine)-8a
24
37
16
17
25
24
45
100
99
93
87
87
88:12
15:85
19:81
12:88
28:72
45:55
23:77
A
,
binol A
pecially interested in the behavior of the new digold(I) spe-
cies in this reaction because a wide range of di- and trinu-
clear gold catalysts had previously been shown to give excel-
lent enantioselectivities in hydroamination reactions.[20,38]
Initially, we were interested in establishing to what extent
the variation of the steric demands in the 3,3’-positions of
the CycloP gold complexes might influence the reactivity
and selectivity. By using literature conditions (room temper-
ature, toluene, 5 mol% Au, 5 mol% AgOBz (Bz=benzyl))
(Rbinol,Ramine)-7a
(R)-5a
(R)-6a
96
100
(R)-11a
[a] All reactions were carried out with 5 mol% of AuI in toluene (1 mL)
at 258C. [b] Conversion was monitored by 1H NMR spectroscopy.
[c] Enantioselectivities were determined by chiral HPLC.
Furthermore, to investigate possible match/mismatch ef-
fects within the CycloP scaffold the diastereomeric counter-
part to [7a·ACHTUNGTERNNU(G AuCl)2] (dia-[7a·ACHTUGNRTEN(UNGN AuCl)2]) was prepared by in-
verting the chirality of the binol component. Surprisingly,
this diastereomer achieved an enantiomeric ratio of 12:88
(Table 2, entry 4). In other words, both diastereomeric di-
the series of the digold(I) complexes [1a·
(AuCl)2], [1c·(AuCl)2], [1e·(AuCl)2], [1 f·(AuCl)2], [1g·-
(AuCl)2], and [1h·(AuCl)2] was employed in the cyclohy-
droamination reference reaction (the results are shown in
Table 1). The unsubstituted complex [1a·(AuCl)2] gave full
conversion after 72 h at room temperature and 19 h at 508C
ACHTUNGTERN(NUNG AuCl)2], [1b·-
A
R
G
U
A
U
AHCTUNGTRENNUNG
AHCTUNGTREGgNNNU old(I) complexes of ligand 7a achieved similar enantiose-
Chem. Eur. J. 2013, 19, 13823 – 13837
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13831

D. S. Wright, L. H. Gade et al.
Table 3. Examination of the catalytic properties of CycloP ligands in the
Mori–Tamaru three component coupling.[a]
lectivities and the absolute configuration of the enantioen-
riched product obtained depends solely on the chirality of
the binol employed. Because the absolute configuration of
the amine is not crucial to the enantioselectivity in this
transformation, the achiral amine cyclophosphazane digol-
d(I) derivatives of 5a, 6a, and 11a were next employed
(Table 2, entries 5–7). However, the enantioselectivities ob-
tained in the reference reaction were considerably lowered
(28:72, 45:55, and 23:77, respectively). Thus, we conclude
Entry
Ligand
t
Conv
[%][b]
anti/syn[b]
e.r.
(+)/À
[c]
[h]
1
2[d]
3[e]
4
5
6
7
8
9
10
11
12
13
14
15
16
17
(Sbinol,Ramine)-7a
(Sbinol,Ramine)-7a
(Sbinol,Ramine)-7a
(R)-5a
18
21
18
20
15
20
21
20
18
26
26
20
21
20
13
18
13
97
100
97
98
78
99
100
92
81
55
95
100
81
99
26
11:1
15:1
13:1
10:1
12:1
12:1
11:1
15:1
14:1
14:1
14:1
12:1
11:1
15:1
8:1
14:86
15:85
24:76
91:9
70:30
74:26
53:47
85:15
64:36
40:60
9:91
92:8
88:12
9:91
50:50
87:13
50:50
(R)-6a
(Rbinol,Ramine)-dia-7a
(Rbinol,Ramine)-7c
(Rbinol,Samine)-8a
À
that although the chirality of the bridging N R group does
not appear to affect the enantioselectivity there is clearly a
steric influence of these groups.
A
,
binol A
(Rbinol,Ramine)-10a
(Sbinol,Ramine)-dia-10a
(R)-11a
(R)-1e
(R)-1 f
(R)-5e
(R)-5h
(R)-7e
Nickel(0)-catalyzed enantio- and diastereoselective coupling
of a 1,4-diphenylbuta-1,3-diene and benzaldehyde: Having
explored the key factors involved in the optimization of the
CycloP ligand system in the gold-catalyzed cyclohydroami-
nation reaction we decided to assess the potential of the cy-
clophosphazane ligands in a different late-transition-metal-
catalyzed reaction. The Mori–Tamaru nickel-catalyzed cou-
pling of 1,3-dienes and aldehydes (Scheme 11)[39] was chosen
98
33
11:1
8:1
[a] All reactions were carried out with 0.003 mmol CycloP ligand and
0.005 mmol of the nickel precursor at room temperature in toluene
(1 mL). [b] Conversion and anti/syn ratio were determined by 1H NMR
spectroscopy. [c] Enantioselectivities for the major diastereomer were de-
termined by chiral HPLC. [d] [Ni
cyclooctadiene) was used.
ACHTUNGTNERNUG(acac)2] was used. [e] [NiACHTUNGTERN(NUGN cod)2] (cod=
ture with an enantiomeric ratio of 14:86 and an anti/syn
ratio of approximately 11:1 (Table 3). A further screening of
solvents showed, that other aromatic solvents such as ben-
zene, mesitylene, and fluorobenzene gave very similar enan-
tioselectivities, whereas more polar solvents, for example, di-
ethylether, tetrahydrofuran, dioxane, pyridine, and acetoni-
trile, gave reduced enantio- and/or diastereoselectivities (see
the Supporting Information). Next, the nickel precursors
were varied. However, whereas nickel(II)acetylacetonate
gave comparable selectivities, the enantiomeric ratio drop-
ped to 24:76 when using the nickel(0) compound bis(cy-
clooctadiene)nickel.
By screening the CycloP ligands 5a, 6a, dia-7a, 7c, 8a,
9a, 10a, dia-10a, and 11a, we found that the reactivity and
selectivity of the nickel-catalyzed reactions depended on the
cyclophosphazane scaffold used (Table 3, entries 4–12). No-
tably, the achiral amine-substituted binol cyclophosphazanes
5a and 11a gave selectivities of 9:91 and 8:92 e.r., respec-
tively, with acceptable anti/syn ratios (Table 3, entries 4 and
12), whereas the chiral amine-derived ligands gave lower or
similar selectivities (Table 3, entries 7–11). It is also note-
worthy, that although both ligands 7a and 11a contain the
same (R)-binol backbone but different amines, the opposite
enantiomers are preferentially formed in each case. In addi-
tion, it is apparent that ligand 7a represents the matched
case, whereas the diastereomeric ligand dia-7a is the mis-
matched system (Table 3, entry 6) (cf. the gold-catalyzed re-
action in which there is no distinct influence of the chirality
of the amine). Due to these unexpected trends, which are
markedly different or even the opposite of the gold-cata-
lyzed reactions, the activity of the bulky binol-derived li-
gands 1e and 1 f were evaluated in the nickel reference
Scheme 11. Nickel(0)-catalyzed bis-homoallylation of benzaldehyde with
1,4-diphenylbutadiene and diethylzinc.
because this three-component coupling offers several dis-
tinct challenges with respect to the ligand system that were
not examined in the gold(I)-catalyzed reaction; 1) unlike the
gold(I)-catalyzed cyclohydroamination reaction, the coordi-
nated nickel atom undergoes a change in the oxidation state
in this transformation, 2) this transformation gives rise to
two stereocenters and thus is potentially dia- and enantiose-
lective, 3) the nickel-catalyzed reaction involves more ag-
gressive reagents than the previously explored gold(I) reac-
tion, and 4) this transformation is known to be catalyzed by
complexes containing monodentate phosphorus ligands.[40]
The asymmetric intermolecular version of the nickel-cata-
lyzed coupling reaction was first reported by Zhou et al. in
2007 by using substituted spirobiindane phosphoramidite li-
gands.[41–43] The reaction proceeds through an oxidative cy-
clometallation, forming an allylalkoxynickel(II) intermedi-
ate, which is subsequently re-reduced to the catalytically
active nickel(0) species. Since then, several groups have
broadened the scope of this reaction by using other reducing
agents, which introduce alkyl, aryl, boronates, or silyl groups
stereoselectively at the allylic position.[44]
Initially, the chiral ligand 7a was employed, which gave
the best enantioselectivities in the previous gold(I)-catalyzed
reaction. Addition of benzaldehyde and diethylzinc to a sol-
ution of ligand 7a, the nickel precursor [NiBr2ACTHNUTRGNE(NUG dme)]
(dme=dimethoxyethane), and 1,4-diphenylbutadiene in tol-
uene gave the desired product after 18 h at room tempera-
13832
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Chem. Eur. J. 2013, 19, 13823 – 13837

Chiral Ditopic Cyclophosphazane Ligands
FULL PAPER
chiral Daicel Chiracel AD-H column. High-resolution mass spectra were
acquired on Bruker ApexQe hybrid 9.4 T FT-ICR (ESI) and JEOL JMS-
700 magnetic sector (FAB, EI, LIFDI) spectrometers at the mass spec-
trometry facility of the Institute of Organic Chemistry of the University
of Heidelberg. Elemental analyses were carried out in the Microanalysis
Laboratory of the Heidelberg Chemistry Department. Compounds
1·Cl2,[47] 2·Cl2,[48] 3·Cl2,[49] 4·Cl2,[50] b,[51] c,[52] 1a,[13a] and s1[53] were synthe-
sized according to reported procedures. For compound numbering see
the Supporting Information. All chemicals were obtained from commer-
cial suppliers and were used without further purification.
system (Table 3, entries 13 and 14). Enantiomeric ratios of
12:88 and 9:91 were found, respectively.
Based on these results, the ligands 5e, 5h, and 7e were in-
vestigated (Table 3, entries 15–17), containing bulky binol
and amido groups. However, the additional steric bulk in 5e
and 7e resulted in racemic mixtures. In contrast, the vanol
derivative 5h gave a selectivity of 87:13 e.r. These results
show that the enantioselectivity is highly dependent on the
steric demands of both the bridging scaffold and the amido
group. This situation is similar to the gold system, which
however, appears to have a lower tolerance with respect to
steric congestion of the ligand.
General procedure for the synthesis of dichlorocyclophosphazanes
(GP1): A solution of amine (1 equiv) and triethylamine (10 equiv) in
THF was added dropwise to
a solution of phosphorus trichloride
(1 equiv) in THF at À788C. The mixture was slowly warmed to room
temperature overnight (18 h). The solvent was evaporated in vacuo and
the residue was extracted with toluene. The combined extracts were con-
centrated to a minimum amount. At this point the workup was varied de-
pending on the compound:
Conclusion
Workup a) compounds 5–7, and 11: The formation of a colorless precipi-
tate was noted during the evaporation of the solvents. In this case, the
solid material was dissolved in a minimum amount of boiling toluene and
then gradually cooled to room temperature. The supernatant was re-
moved by filtration. The residue was washed with a small amount of cold
toluene and n-pentane and dried on high vacuum to yield the dichlorocy-
clophosphazanes as colorless to slightly yellow solids.
The work described in this paper shows that CycloP ligands
are very easily prepared by using commercially available
precursors and can be elaborated extensively, by introducing
chiral groups both at the bridgehead and in the amido posi-
tion. These chiral phosphorus ligands are robust in solution
and in the solid state and can be used to support enantiose-
lective catalytic reactions with moderate to good enantiose-
lectivities being observed. In the gold-catalyzed cyclohy-
droamination reaction the enantioselectivity is imparted by
the chirality of the bridge only, whereas in the nickel-cata-
lyzed three-component coupling reaction the enantioselec-
tivity can be influenced by the chirality of both the bridging
group and the amido groups. In both catalytic reactions ex-
treme steric bulk of the substituent groups lowered the
enantioselectivity and reaction rate. Based on the results so
far it is clear that CycloP ligands are a promising family of
chiral ligands. Further elaboration of these ligand sets is un-
derway in order to improve the enantioselectivity in a
number of key catalytic transformations.
Workup b) compounds 8–10: Removal of the solvent resulted in the for-
mation of an oily residue or foam. In this case, the residue was dissolved
in a small amount of toluene and layered with n-pentane. The mixture
was then stored in the freezer (À30 or À788C) until the formation of a
powdery precipitate was observed. The supernatant was removed by fil-
tration and the residue was washed with n-pentane to yield the crude di-
chlorocyclophosphazane as slightly yellow or brownish solids. The di-
chlorocyclophosphazanes were used in the next steps without any further
purification.
Data for compound 7: Colorless solid; 35% yield; 1H NMR ([D8]THF,
600.13 MHz, 295 K): d=7.89 (s, 2H; H-12), 7.86 (d, J=8.5 Hz, 2H; H-5),
7.85–7.80 (m, 4H; H-7,H-10), 7.58–7.55 (m, 2H; H-4), 7.47–7.43 (m, 4H;
H-8, H-9), 4.81–4.74 (m, 2H; H-2), 1.65 ppm (d, J=6.7 Hz, 6H; H-1);
13C{1H} NMR ([D8]THF, 150.92 MHz, 295 K): d=138.71 (m, CAr), 134.27
(s, CAr), 134.22 (s, CAr), 129.40 (s, C-5), 128.66 (s, CAr-H), 128.26 (s, CAr
-
H), 127.47 (s, C-12), 126.86 (s, CAr-H), 126.84 (s, CAr-H), 125.54 (s, C-4),
55.07 (t, J=6.6 Hz, C-2), 21.74 ppm (t, J=6.0 Hz, C-1); 31P{1H} NMR
([D8]THF, 242.92 MHz, 295 K): d=219.35 ppm (s); MS (LIFDI(+)): m/z
(100%) calcd for C24H22Cl2N2P2: 470.1; found 469.8 [M]+; elemental anal-
ysis calcd (%) for C24H22N2P2Cl2: C 61.16, H 4.71, N 5.94; found: C 61.08,
H 4.84, N 5.95. The spectroscopic data for the dichlorocyclophosphazanes
5, 6, 8, 9, 10, and 11 can be found in the Supporting Information.
Experimental Section
All manipulations, except those indicated, were carried out under exclu-
sion of air and moisture by using standard Schlenk and glovebox techni-
ques. As inert gas, Argon 5.0, purchased from Messer Group GmbH, was
used after drying over Granusic phosphorpentoxide granulate. Solvents
were dried over activated alumina columns by using a solvent purifica-
tion system (M. Braun SPS 800) or according to standard literature-
known methods[45] and stored in glass ampules under an argon atmos-
phere. Diethyl ether and n-pentane were distilled from sodium/potassium
alloy, tetrahydrofuran, benzene, and n-hexane from potassium, methanol
from magnesium, dichloromethane, chloroform and triethylamine from
calcium hydride, and toluene from sodium. The same procedures were
used to dry the deuterated solvents. Degassed solvents were obtained by
three successive freeze–pump–thaw cycles. Phosphorus trichloride was
distilled prior to use and triethylamine was degassed. NMR spectra were
recorded on Bruker Avance (400, 500, and 600 MHz) instruments. Chem-
ical shifts (d) are reported in parts per million [ppm] and are referenced
to residual proton solvent signals or carbon resonances.[46] H3PO4 (31P)
and CCl3F (19F) were used as external standards. The following abbrevia-
tions were used: s=singlet, d=doublet, dd=doublet of doublets, t=trip-
let, m=multiplet, br=broad signal. Enantioselectivities were measured
on a HPLC Agilent Technologies 1200 Series instrument by using a
General procedure for the synthesis of CycloP ligands (GP2): A solution
of the diol (1 equiv) and triethylamine (10 equiv) in toluene was added
to a solution of the dichlorocyclophosphazane (1 equiv) in toluene at
room temperature. The resulting mixture was heated to 1108C overnight
(18 h), cooled to room temperature, and filtered. The obtained solid was
then purified by column chromatography (SiO2, n-pentane/ethyl acetate)
to obtain the pure CycloP ligands as colorless or pale yellow solids.
Data for 1b: Colorless solid; 40% yield; 1H NMR (C6D6, 600.13 MHz,
295 K): d=8.04 (s, 2H; H-4), 7.33 (d, J=8.16 Hz, 2H; H-6), 6.95–6.91
(m, 2H; H-7), 6.79 (d, J=8.65 Hz, 2H; H-9), 6.71–6.67 (m, 2H; H-8),
0.94 ppm (s, 18H; H-12); 13C{1H} NMR (C6D6, 150.92 MHz, 295 K): d=
149.23 (m, CAr), 134.19 (s, CAr), 132.61 (s, C-4), 131.36 (s, CAr), 128.35 (s,
CAr), 127.47 (s, C-8), 127.02 (s, C-6), 126.09 (s, C-9), 125.78 (s, C-7),
119.49 (s, C-3), 52.89 (t, J=12.15 Hz, C-11), 31.01 ppm (t, J=6.60 Hz, C-
12); 31P{1H} NMR (C6D6, 242.92 MHz, 295 K): d=177.31 ppm (s); HRMS
(ESI(+)):m/z (9%) calcd for C28H29Br2N2O2P2: 645.0071; found 645.0058
[M+H]+; elemental analysis calcd (%) for C28H28N2O2P2Br2: C 52.04, H
4.37, N 4.33; found: C 52.07, H 4.41, N 4.36. The spectroscopic data for
the CycloP ligands 1c, 1e, 1 f, 1g, 1h, 2a, 3a, 5a, 5e, 5h, 6a, 7a, 7c, 7e,
8a, 9a, 10a, and 11a can be found in the Supporting Information.
Chem. Eur. J. 2013, 19, 13823 – 13837
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13833

D. S. Wright, L. H. Gade et al.
General procedure for the synthesis of digold(I) CycloP metal complexes
(GP3): Dimethylsulfidegold(I) chloride (2 equiv) was added in one por-
tion to a solution of the CycloP ligand (1 equiv) in dichloromethane. The
clear solution was stirred at room temperature for 30 min, then the sol-
vent was removed and the residue was recrystallized from dichlorome-
thane/n-pentane. The colorless solid was washed with n-pentane and
dried on high vacuum to yield the digold(I) CycloP complex as a color-
less solid.
Daicel Chiracel AD-H column: conditions: n-hexane/isopropanol=95:5,
208C, 1 mLminÀ1, t(S) =13.6, t(R) =14.9 min.
Nickel-catalyzed three-component coupling: This reaction was conducted
similarly as described previously by Zhou et al.[41] Benzaldehyde
(0.2 mmol) was added to a solution of the CycloP ligand (0.003 mmol),
[NiBr2ACTHNUGRTENUNG(dme)] (0.005 mmol) and trans,trans-1,4-diphenyl-1,3-butadiene
(0.1 mmol) in the indicated solvent (1 mL). The solution was stirred at
room temperature for 5 min and then a 1m solution of diethylzinc in
hexane (0.24 mmol) was added dropwise. The reaction mixture was stir-
red for the indicated time at room temperature and then quenched by
the addition of 1m HCl. Extraction with ethyl acetate, washing with satu-
rated aqueous NaHCO3 solution and evaporation of the solvent yielded
the crude coupling product p2. From this crude mixture the diastereo-
meric ratio was determined by 1H NMR spectroscopy, then the solid was
further purified by column chromatography (SiO2, n-pentane/ethyl ace-
tate=4:1). Spectroscopic properties of p2 were in accordance to previous
reports in the literature.[41] Enantioselectivities for product p2 were deter-
mined by chiral HPLC employing a Daicel Chiracel AD-H column: con-
Data for [1a·ACHTUNGTRENNUNG
(AuCl)2]: Colorless solid; 76% yield; 1H NMR (CDCl3,
500.05 MHz, 300 K): d=8.06 (d, J=8.91 Hz, 2H; H-4), 7.91 (d, J=
8.11 Hz, 2H; H-6), 7.48 (d, J=8.94 Hz, 2H; H-3), 7.47–7.42 (m, 2H; H-
7), 7.22–7.18 (m, 2H; H-8), 6.47 (d, J=8.57 Hz, 2H; H-9), 1.24 ppm (s,
18H; H-12); 13C{1H} NMR (CDCl3, 125.74 MHz, 300 K): d=148.83 (m,
CAr), 134.57 (s, CAr), 131.53 (s, CAr), 131.12 (s, C-4), 128.24 (s, C-8), 128.17
(s, C-6), 126.79 (s, C-7), 125.92 (s, C-9), 124.66 (m, CAr), 123.25 (s, C-3),
56.45 (t, J=2.21 Hz, C-11), 31.44 ppm (t, J=5.19 Hz, C-12);
31P{1H} NMR (CDCl3, 242.92 MHz, 295 K): d=119.14 ppm (s); HRMS
(ESI(+)): m/z (4%) calcd for C28H30Au2Cl2N2O2P2Na: 975.0388; found
975.0357 [M+a]+; elemental analysis calcd (%) for C28H30N2O2P2Au2Cl2:
C 35.28, H 3.17, N 2.94; found: C 34.91, H 3.30, N 2.81.
ditions: n-hexane/isopropanol=95:5, 208C, 1 mLminÀ1, t(+) =17.1, t(À)
=
19.7 min.
X-ray crystal structure determinations: Crystal data and details of the
structure determinations are listed in the Supporting Information. Inten-
sity data were collected at low temperature with a Bruker AXS Smart
1000 (MoKa radiation, sealed tube, graphite monochromator), an Agilent
Technologies Supernova-E (MoKa or CuKa radiation, microfocus tube,
multilayer mirror optics), a Nonius Kappa CCD diffractometer (MoKa ra-
diation, sealed tube, graphite monochromator) or a Bruker AXS Smart
X2S (MoKa radiation, sealed microfocus tube, silicon monochromator)
CCD diffractometers. Data were corrected for air and detector absorp-
tion, Lorentz and polarization effects;[54–56] absorption by the crystal was
treated analytically,[55,57] numerically (Gaussian grid)[55] or with a semiem-
pirical multiscan method.[56,58–60] The structures were solved by the charge
flip procedure,[61] by the heavy-atom method combined with structure ex-
pansion by direct methods applied to difference structure factors,[62] by
direct methods with dual-space recycling,[63,64] or by conventional direct
methods.[65–67] Refinement was carried out by full-matrix least-squares
methods based on F2 against all unique reflections.[66,68] All non-hydrogen
atoms were given anisotropic displacement parameters. Hydrogen atoms
were generally input at calculated positions and refined with a riding
model. When justified by the quality of the data the positions of some
hydrogen atoms were taken from difference Fourier syntheses and re-
fined. When found necessary, disordered groups and/or solvent molecules
where subjected to suitable geometry and adp restraints. Due to severe
disorder and fractional occupancy, electron density attributed to solvent
The spectroscopic data for the CycloP complexes [1b·
(AuCl)2], [1e·(AuCl)2], [1 f·(AuCl)2], [1g·(AuCl)2], [1h·(AuCl)2], [5a·-
(AuCl)2], [6a·(AuCl)2], [7a·(AuCl)2], dia-[7a·(AuCl)2], [8a·(AuCl)2],
[9a·(AuCl)2], and [11a·(AuCl)2] can be found in the Supporting Informa-
tion.
Synthesis of the dirhodium(I) CycloP complex [1a·{Rh
chloromethane (15 mL) was added to a Schlenk flask containing the
CycloP ligand 1a (347 mg, 0.753 mmol, 1 equiv) and [Rh(nbd)Cl]2
(368 mg, 0.753 mmol, 1 equiv) and the mixture was stirred at room tem-
perature overnight. The solvent was removed and the residue was puri-
fied by column chromatography (SiO2, n-pentane/ethyl acetate=1:1 to
pure ethyl acetate) to give the dirhodium(I) CycloP complex as an
orange solid (487 mg, 68%). Single crystals suitable for X-ray diffraction
analysis were grown by carefully layering a solution of the complex in di-
chloromethane with n-pentane. 1H NMR (CD2Cl2, 600.13 MHz, 295 K):
d=8.01 (d, J=8.81 Hz, 2H; H-4), 7.88 (d, J=8.15 Hz, 2H; H-6), 7.76–
7.25 (m, 4H; H-3, H-7), 7.13–7.07 (m, 2H; H-8), 6.45 (d, J=8.42 Hz, 2H;
H-9), 5.62–4.92 (m, 4H; H-14, H-15), 4.29–3.62 (m, 8H; H-13, H-16, H-
17, H-18), 1.65–1.46 (m, 4H; H-19), 1.41–1.23 ppm (m, 18H; H-12);
13C{1H} NMR (CD2Cl2, 150.92 MHz, 295 K): d=151.43–150.22 (m, CAr),
134.95 (s, CAr), 131.25 (s, CAr), 129.96 (s, C-4), 128.12 (s, C-6), 127.20 (m,
C-8), 126.18 (s, C-9), 125.67 (m, C-7, CAr), 124.26 (m, C-3), 94.35–87.31
(m, C-14, C-15), 65.18 (m, C-19), 57.22–55.39 (m, Cnbd), 54.66 (m, C-11);
51.54–47.61 (m, Cnbd), 33.30–30.64 ppm (m, C-12); due to significant
broadening and overlapping of the 13C resonances the signals were as-
ACHTUNGTERN(NUNG AuCl)2], [1c·-
G
R
G
R
ACHTUNGTRENNUNG
R
G
G
E
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
AHCTUNGTREG(NNUN nbd)Cl}2]: Dry di-
AHCTUNGTRENNUNG
of crystallization was removed from the structures of 1h·
[AuCl]2 with the BYPASS procedure,[69] as implemented in PLATON
(SQUEEZE).[70] Partial structure factors from the solvent masks were in-
cluded in the refinement as separate contributions to Fobs
CCDC-939645 (A), 939646 ([A·(AuCl)2]), 939647 ([1a·(AuCl)2]), 939648
([1a·{Rh(nbd)Cl}2]), 939649 (1b), 939650 ([1b·(AuCl)2]), 939651 (1c),
939652 ([1c·(AuCl)2]), 939653 ([1e·(AuCl)2]), 939654 ([1g·(AuCl)2]),
939655 ([1h·(AuCl)2]), 939656 (2a), 939657 (5a), 939658 (6·Cl2), 939659
(6a), 939660 (7c), 939661 ([9a·(AuCl)2]), 939662 ((Rbinol,Ramine)-10a), and
ACHTUNGTERN[NUNG AuCl]2 and 9a·-
AHCTUNGTRENNUNG
signed
ACHTUNGTRENNUNG
by
analogy
to
the
previously
reported
[RhCl-
(nbd)(phosphoramidite)] complexes by Albinati and coworkers;[32]
.
31P{1H} NMR (CD2Cl2, 242.92 MHz, 295 K): d=138.82–136.17 (m,
ꢀ1.5P), 134.78–132.23 ppm (m, ꢀ0.5P); for comparison: a low-tempera-
ture 31P{1H} NMR spectrum (T=233 K) comprising of sharp resolved res-
onances is shown in Figure 17; HRMS (FAB(+)):m/z (100%) calcd for
C42H46ClN2O2P2Rh2: 913.0833; found: 913.0892 [MÀCl]+; elemental anal-
ysis calcd (%) for C42H46N2O2P2Rh2Cl2: C 53.13, H 4.88, N 2.95; found C
52.63, H 4.94, N 3.05.
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
939663 ((Sbinol,Ramine)-10a) contain 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.
The spectroscopic data for the CycloP complex [1a·{Pd(2-Me-allyl)Cl}2]
can be found in the Supporting Information.
Gold-catalyzed cyclohydroamination: A mixture of the digold(I) complex
(0.0025 mmol) and the corresponding silver(I) salt (0.005 mmol) in tol-
uene (dry and degassed, 0.5 mL) was stirred at room temperature for
15 min in the dark. Then, a solution of the g-allenyl sulfonamide s1
(0.1 mmol) in toluene (dry and degassed, 0.5 mL) was added and the re-
action mixture was left to stir at room temperature for the indicated
time. The progress of the reaction was monitored by 1H NMR spectrosco-
py. Upon completion, the crude mixture was loaded directly onto a silica
gel column and purified by column chromatography (SiO2, n-pentane/
ethyl acetate=5:1) to yield the cyclized product p1. Spectroscopic prop-
erties of p1 were in accordance to previous reports in the literature.[37b]
Enantioselectivities for p1 were determined by chiral HPLC employing a
Acknowledgements
We gratefully acknowledge the EU (Erasmus-Program (T.R.), ERC-Ad-
vanced Investigator Grant (D.S.W.)] and the award of a Ph.D. grant to
T.R. from the Landesgraduiertenfçrderung (LGF Funding Program of
the state of Baden-Wꢃrttemberg). This work has also been funded by the
DFG (SFB 623, TP B6). We also thank Y. Lv and Dr. J. E. Davies (Cam-
13834
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 13823 – 13837

Chiral Ditopic Cyclophosphazane Ligands
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Received: June 18, 2013
Published online: August 28, 2013
Chem. Eur. J. 2013, 19, 13823 – 13837
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www.chemeurj.org
13837