New bicyclic phosphorous ligands: Synthesis, structure and catalytic applications in ionic liquids

Article abstract of DOI:10.1016/j.tet.2010.11.023

New azadioxaphosphabicyclo[3.3.0]octane ligands showing a trans arrangement with regard to the two five-membered heterocycles, were obtained as a mixture of three conformers, in agreement with molecular modelling studies. The stability of oxaphosphane ligands was studied under basic catalytic conditions, monitored by NMR spectroscopy. Palladium catalytic systems containing these ligands were active in Suzuki C-C cross-coupling reactions between phenylboronic acid and aryl halides (bromide and chloride derivatives) bearing electron-donor or electron-withdrawing substituents, in both organic and ionic liquid solvents. The catalytic systems showed a high stability even under the most severe reaction conditions used in this work. The ionic liquid catalytic phase could be recycled up to ten times without significant activity loss. Copyright

Full text of DOI:10.1016/j.tet.2010.11.023

Tetrahedron 67 (2011) 421e428
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
New bicyclic phosphorous ligands: synthesis, structure and catalytic applications
in ionic liquids
a
,
b
a
,
a,
ꢀ b
ꢀ
*
*
ꢀ
Martha V. Escarcega-Bobadilla , Emmanuelle Teuma , Anna M. Masdeu-Bulto , Montserrat Gomez
a
ꢀ ꢀ
ꢀ
ꢀ
Laboratoire Heterochimie Fondamentale et Appliquee, UMR CNRS 5069, Universite Paul Sabatier, 118 route de Narbonne, 31120 Toulouse cedex 9, France
b
ꢁ
Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Marcel.lí Domingo s/n, 43007 Tarragona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
New azadioxaphosphabicyclo[3.3.0]octane ligands showing a trans arrangement with regard to the two
five-membered heterocycles, were obtained as a mixture of three conformers, in agreement with mo-
lecular modelling studies. The stability of oxaphosphane ligands was studied under basic catalytic
conditions, monitored by NMR spectroscopy. Palladium catalytic systems containing these ligands were
active in Suzuki CeC cross-coupling reactions between phenylboronic acid and aryl halides (bromide and
chloride derivatives) bearing electron-donor or electron-withdrawing substituents, in both organic and
ionic liquid solvents. The catalytic systems showed a high stability even under the most severe reaction
conditions used in this work. The ionic liquid catalytic phase could be recycled up to ten times without
significant activity loss.
Received 7 September 2010
Received in revised form 2 November 2010
Accepted 4 November 2010
Available online 13 November 2010
Keywords:
Bicyclophosphanes
Palladium
Ionic liquids
Suzuki CeC cross-coupling
Allylic alkylation
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
In addition to saturated heterocyclic phospholanes, other five-
membered P-cyclic ligands, such as phospholes,7 2- and 3- phos-
In the field of enantioselective catalysis, a large variety of chiral
phosphorous-based ligands has been developed since the pioneer-
ing works of Knowles,1 Horner2 and Kagan.3 Looking for high
asymmetric inductions, rigid and well-defined coordination envi-
ronments around the metallic centres besides electronic effects,
appeared beneficial to achieve high enantiomeric excesses.4
Therefore, phosphorous atom placed on a heterocycle can play
a crucial role in the asymmetric induction.5 The noteworthy con-
tribution of Burk and co-workers developing the efficient phos-
pholane DuPhos (Chart 1), which gave excellent enantioselectivities
especially in asymmetric hydrogenation,6 stimulated the catalytic
applications of five-membered phosphacycle-based chiral ligands.
pholenes8 and phosphaferrocenes9 (Fig. 1) have been also applied
in metal-catalyzed organic processes, including bicyclic backbones,
such as phosphanorbornadiene and phosphanorbornene structures
described by Mathey and co-workers.10
P
Fe
P
R
P
R
P
R
phospholes 2-phospholenes 3-phospholenes phosphaferrocenes
Fig. 1. Skeletons of phosphorous-containing unsaturated five-membered heterocycles.
However, P-donor ligands based on fused saturated five-mem-
bered bicycle structures have been less commonly applied in ca-
talysis (Fig. 2). The first work was reported by Vedejs and Daugulis
preparing P,C-sterogenic phosphabicyclo[3.3.0]octanes used in
enantioselective acylations (A in Fig. 2).11 Some years later, Pie-
trusiewicz and co-workers developed related phospholane and also
phospholene (B in Fig. 2) ligands to be employed in Pd-catalyzed
CeC bond formation reactions.12 RajanBabu and Yan described the
applications in Pd-catalyzed asymmetric allylic alkylations of
P
P
DuPhos
Chart 1.
* Corresponding authors. Tel.: þ34 977558779; fax: þ34 977559563 (A.M.M.-B);
tel.: þ33 561557738; fax: þ33 561558204; (M.G); e-mail addresses: annamaria.
mono- and bis-phospholanes synthesized from
D-mannitol (C in
Fig. 2).13 In addition, the work of Buono and co-workers described
ꢀ
ꢀ
masdeu@urv.cat (A.M. Masdeu-Bulto), gomez@chimie.ups-tlse.fr (M. Gomez).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.11.023

ꢀ
M.V. Escarcega-Bobadilla et al. / Tetrahedron 67 (2011) 421e428
422
O
O
R
PhPCl2 / Et3N / THF / 18 h / rt
R
O
PhP
P
P
N
Bn
PR
O
HO
HO
1
80 %
N
Bn
A
B
C
O
4
1. PCl3 / Et3N / THF / 18 h / rt
2. PhOH / Et3N / THF / 18 h / rt
PhOP
N
Bn
NR'
O
N
N
P
P
O
OR
OR
2
88 %
D
E
Scheme 1. Synthesis of oxyphosphanes 1 and 2.
Fig. 2. P-heterocyclic ligands placed on fused saturated five-membered bicycle
structures.
a
diamidophosphite derived from 1,3-diaza-2-phosphabicyclo
[3.3.0]octane and its further catalytic uses (Cu-catalyzed enantio-
selective conjugate addition of diethylzinc to enones and Pd-cata-
lyzed allylic substitutions) (D in Fig. 2).14 Other groups have
employed this kind of bicyclic diamidophosphites in asymmetric
catalytic processes.15 Finally, chiral bicyclic phosphoramidites (E in
Fig. 2) gave high enantiomeric excesses in Rh-catalyzed olefin hy-
drogenation reactions.16
In this context, azadioxaphosphabicyclo[3.3.0]octane com-
pounds (1 and 2 in Fig. 3) represent a new class of ligands con-
taining two fused five-membered rings derived from tartaric acid.
Few reports are described in the literature concerning the synthesis
of trans fused five-membered heterocycles17,13 and for the best of
our knowledge, only one crystal structure has been described.18
Herein, we report the synthesis of phosphonite 1 and phosphite
2 derived from natural tartaric acid and their applications in Pd-
catalyzed CeC bond formation processes in organic and ionic liquid
solvents. Ionic liquids present different advantages in front of
conventional solvents, such as the catalyst recovery for futher
recycling, especially interesting in asymmetric catalysis.19 Bis-
(phosphinite) 320 was also studied in order to compare the effect of
the rigidity in bicyclic ligands. The robustness of these ligands in
[BMI][PF6] (BMI¼1-butyl-3-methyl imidazolium) under catalytic
conditions has been proved, in contrast to that observed in toluene.
In order to evidence the coordination ability of these ligands, two
complexes, Pd1 and Pd2, have been synthesized and fully charac-
terized. A conformational study of the new azadioxaphosphane
bicyclo[3.3.0]octane ligands 1 and 2 has been carried out.
Fig. 4. 1H (500.13 MHz) and 31P{1H} (202.5 MHz) NMR spectra for phosphonite 1 (a, b)
and phosphite 2 (c, d) in CD2Cl2 and CDCl3, respectively, at 298 K. For 1H NMR spectra,
only the methylene and methinic protons region is exhibited.
studied temperature range (298e193 K). The three isomers could
be only distinguished by the signals of the methylene of the benzyl
group. The 2D-HSQC experiment confirmed the presence of these
three isomers (Fig. S1 in Supplementary data). In order to corrob-
orate this structural behaviour, the new related ligand 2 was pre-
pared following the two-step synthetic methodology for
phosphites described elsewhere (Scheme 1).23 An analogous iso-
meric behaviour in solution was found by 1H NMR (isomeric ratio 1/
0.9/0.6), but in this case the 31P{1H} NMR spectrum showed three
singlets (Fig. 4c and d). For both compounds, NOESY experiments at
323 K and 298 K did not reveal exchange signals.
With the aim to understand the nature of these isomers,
a modelling study was carried out. Concerning the two five-
membered fused rings, a trans arrangement is imposed due to the
controlled stereochemistry of the two stereogenic carbon atoms.
Because of the substituents on P (phenyl or phenoxide for 1 and 2,
respectively) and N (benzyl for both ligands) atoms, two confor-
mations could be possible, syn and anti, if both groups point to the
same or opposite direction, respectively (Fig. 5).
O
O
O
O
Ph2PO
Ph2PO
PhP
PhOP
N
Bn
N Bn
N
Bn
1
2
3
Fig. 3. Azadioxaphosphabicylo[3.3.0]octane compounds
phinite 3.
1 and 2, and the diphos-
2. Results and discussion
2.1. Synthesis and characterization of
azadioxaphosphabicyclo[3.3.0]octane compounds
Ligand 1 has been prepared following the methodology repor-
ted for related phosphonite compounds,21 starting from the opti-
cally pure (S,S)-4 diol22 by reaction with phenyl dichlorophosphine,
giving a white solid in a good yield (80%) after filtration through
anhydrous basic alumina to remove the acidic by-products derived
from the PhPCl2 (Scheme 1).
Taking into account that the three isomers are only differenti-
ated by the methylene group of the benzyl substituent on the ni-
trogen, they might arise from the different relative spatial
disposition of this group in the ligand structure (R0 in Fig. 5). This
31P{1H} NMR spectrum of 1 exhibited two singlets pointing to
a mixture of isomers (Fig. 4b). In fact, the 1H NMR spectrum (Fig. 4a)
showed three species with an invariable ratio (1/0.63/0.46) in the

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423
In order to study the stability of ligands 1e3 under catalytic
conditions, a 31P{1H} NMR monitoring study was carried out in
toluene and ionic liquids under basic conditions. It is well known
that PeO bonds in oxyphosphane ligands can be easily hydrolised
to form the corresponding phosphonic acid derivatives.24 Ligands
1e3 dissolved in [BMI][PF6] in the presence of an aqueous solution
of sodium carbonate at 60 ꢀC (conditions used for the catalytic
reactions, see below), proved to be stable, without showing any
R'
R'
R
H
N
P
N
P
R
anti
syn
H
Ph
R' =
R = Ph, OPh
H
Ph
H
,
,
Ph
H
H
sign of degradation (Fig. 7; for full spectra in the range
d
þ200 to
Fig. 5. Syn and anti conformations for trans fused five-membered heterocycles.
0 ppm, see Fig. S6 in Supplementary data). On the contrary, the
three ligands quickly decomposed using toluene or [EMI][HPO(O)-
OMe] (EMI¼1-ethyl-3-methyl imidazolium) as solvent (Figs. S7 and
S8 in Supplementary data).
fact generates three possible isomers for each syn and anti con-
formation. The structures of the six conformations were modelled
at semi-empirical PM3 level and their energies calculated by den-
sity functional theory (DFT B3LYP) using 6-31G* polarization basis
set and pseudopotentials (Figs. S2 and S3 in Supplementary data).
For each compound, two conformations exhibited high energy
(more than 2.2 kcal/mol in relation to the most stable conforma-
tion) and were not considered. The other four conformations of
close energy (between 0.01 and 0.36 kcal/mol in relation to the
most stable conformation) led to a Boltzmann distribution at 298 K
of ca. 1/0.75/0.6/0.5 and 1/1/0.9/0.6 for 1 and 2, respectively (Fig. 6).
Conformers anti-1a and syn-1b (Fig. 6) cannot probably be dis-
criminated by NMR. In this case, the three conformers (anti-1a or
syn-1b/anti-1c/syn-1d) would be in a relative ratio of 1/0.74/0.53. In
the case of 2, conformers with anti arrangement could not be
distinguished by NMR, leading to a ratio 1/0.88/0.6 (anti-2/syn-2c/
syn-2d). These data are in agreement with the experimental ob-
servations (see above). It is important to note that the rotation
around the NeBn bond leads to conformations of different energy,
as demonstrated by the corresponding energy calculations (see
Figs. S4 and S5 in Supplementary data for ligand 1 and 2, re-
spectively), in agreement with the interchange lack stated by NMR
between the different conformers at rt (see above).
2.2. Preparation of palladium complexes
With the aim to prove the coordination ability of mono- and
bidentated oxaphosphane ligands, palladium complexes [PdCl2(k1
-
P-1)2] and [PdCl2(k
2-P,P-3)] were prepared from [PdCl2(COD)] in
toluene at 60 ꢀC in the presence of the appropriate ligand 1 and 3,
respectively (Scheme 2).
In the case of Pd1 containing two monodentate phosphonites 1,
one singlet at
d
þ130 ppm was observed in the 31P{1H} NMR
spectrum in agreement with Pd related complexes.25 1H and 13C
NMR spectra showed signals corresponding to only one isomer in
the studied temperature range (298e173 K) (Fig. S9 in
Supplementary data), pointing to the different ligand conforma-
tions are undistinguished upon coordination, in contrast to the free
ligand, which exhibits three isomers in solution (see above).
The 31P{1H} NMR spectrum of Pd3, containing a seven-mem-
bered metallacycle, showed one singlet at
d
þ122 ppm, character-
istic for Pd complexes coordinated to bidentated phosphinites.26
1
The H NMR spectrum of Pd3 was similar to that observed for the
free ligand, exhibiting slight changes in the chemical shifts (Fig. S10
in Supplementary data).
2.3. Catalytic results
2.3.1. Suzuki CeC cross-couplings. Reaction between 4-R-bromo-
benzene derivatives (R¼CF3, OCH3) and phenylboronic acid under
basic biphasic conditions, solventeH2O,27 was carried out using
palladium catalysts containing oxyphosphane ligands 1e3. Water
was necessary to favour the solubility of sodium carbonate. Cata-
lytic precursors were generated in situ from [PdCl2(COD)] and the
appropriate ligand. Toluene and ionic liquids ([BMI][PF6] and [EMI]
[HPO(O)OMe]) were used as a reaction medium. The catalytic re-
sults are summarized in Table 1.
The three catalytic systems, Pd/1, Pd/2 and Pd/3, were active in
[BMI][PF6] and in toluene for both substrates 1-bromo-4-tri-
fluoromethylbenzene and 1-bromo-4-methoxybenzene. Higher
conversions were obtained for 1-bromo-4-trifluoromethylbenzene
(entries 1, 3, 5, 6 and 8 in Table 1) than for 1-bromo-4-methoxy-
benzene (entries 2, 4, 7 and 9 in Table 1) as expected by the induced
activation when electron-withdrawing substituents are involved.28
In both catalytic reactions, an excellent chemoselectivity was ob-
served, only giving the expected cross-coupling product. Catalytic
system Pd/3 was more active than Pd/1 in both toluene and [BMI]
[PF6] (entries 1e4 vs 6e9). Catalytic system Pd/2 gave similar
conversion than Pd/3 in [BMI][PF6] (entry 5, Table 1).
In order to check the positive influence of the P-donor ligands,
we evaluated the catalytic behaviour of the Pd system with diol 4.
This compound is one of the plausible by-products formed by hy-
drolysis of PeO bonds in oxaphosphane ligands. For the most active
substrate, 1-bromo-4-trifluoromethylbenzene, in [BMI][PF6], Pd/4
system provided ca. 80% conversion (entry 10 in Table 1) lower than
for Pd/3 (entry 6 in Table 1) but close to that obtained using Pd/1
Fig. 6. Modelled structures (PM3(tm)) for the four more stable conformations for: (a)
phosphonite 1 and (b) phosphite 2. In brackets, relative energies calculated by DFT.

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424
Fig. 7. 31P{1H} NMR (121.5 MHz, 298 K, CDCl3) spectra of ligands 1 (a, b, c), 2 (d, e, f) and 3 (g, h, i): in [BMI][PF6] (a, d, g); in [BMI][PF6] in the presence of Na2CO3(aq) at rt (b, e, h) and
after heating at 60 ꢀC for 1 h (c, f, i).
O
reaction, favoured using heterogeneous catalysts.29 Similar result
PhP
2
N
N
Bn
Bn
1
[PdCl2(k -P-1)2]
Pd1
was obtained in the absence of any ligand (entry 11 in Table 1). In
toluene, only 12% substrate conversion was afforded in the absence
of any ligand, mainly giving the corresponding homo-coupling
product coming from the substrate (4-40-CF3-1,10-biphenyl).
With the aim to avoid the addition of a base, we used [EMI][HPO-
(O)OMe] as solvent, which contains a basic anion, following the
encouraging results previously obtained for Heck coupling re-
actions.30 In this solvent, Pd/3 catalytic system using 1-bromo-
4-trifluoromethylbenzene as substrate was inactive even in the
presence of water (entry 13 in Table 1) and poorly active under
aqueous basic conditions (entries 12 and 14 in Table 1).
O
1
59%
[PdCl2(COD)] / toluene / 60 °C / 3 h
Ph2PO
Ph2PO
2
[PdCl2(k -P,P-3)]
Pd3
77%
3
Scheme 2. Synthesis of complexes Pd1 and Pd3.
Table 1
After these promising results where activation of 1-bromo-4-
methoxybenzene was achieved in both toluene and [BMI][PF6], we
evaluated the catalytic behaviour of Pd/L systems using less re-
active chloroaryl substrates (Table 2).31
Suzuki CeC couplings between 4-R-bromobenzene derivatives (R¼CF3, OCH3) and
phenylboronic acid catalyzed by Pd/L systems (L¼1e3)a
Table 2
Entry
Ligand
Solvent
R
t [h]
Conv. (%)
Suzuki CeC couplings between 4-R-chlorobenzene derivatives (R¼CF3, OCH3, NO2,
NH2) and phenylboronic acid catalyzed by Pd/L systems (L¼1e3)a
1
2
3
4
5
6
7
8
1
1
1
1
2
3
3
3
3
4
d
3
3
3
[BMI][PF6]
[BMI][PF6]
Toluene
Toluene
[BMI][PF6]
[BMI][PF6]
[BMI][PF6]
Toluene
Toluene
[BMI][PF6]
[BMI][PF6]
[EMI][HPO(O)OMe]
[EMI][HPO(O)OMe]
[EMI][HPO(O)OMe]
CF3
OMe
CF3
OMe
CF3
CF3
OMe
CF3
OMe
CF3
CF3
CF3
1
15
1
15
1
1
15
1
15
1
1
1
80b
24c
66b
20c
98b
91b
32c
94b
65c
79b,d
77b,e
36b
5b,f
Solvent / [Pd/L]
+
R
Cl
R
B(OH)2
Na2CO3 aq
R= CF3, OMe, NO2, NH2
Entry
L
Solvent
R
t [h]
Conv. (%)
9
1
2
3
4
5
6
7
8
1
1
1
1
2
2
3
3
3
3
1
1
3
3
[BMI][PF6]
[BMI][PF6]
Toluene
CF3
OMe
CF3
OMe
CF3
OMe
CF3
15
99c
91d
82c
85d
90c
26d
96c
31d
18c
55d
41d
29d
35d
26d
10
11
12
13
14
48b
15
Toluene
48b
15
CF3
CF3
1
1
[BMI][PF6]
[BMI][PF6]
[BMI][PF6]
[BMI][PF6]
Toluene
0b,g
48b
15
a
Reaction
conditions:
Pd:L:arylbromide:phenylboronic
acid:sodium
48b
15
carbonate¼1:1.2:100:120:250 mmol in 1 cm3 of [BMI][PF6] or toluene or [EMI][HPO-
OMe
CF3
(O)OMe] and 2 cm3 of water at 60 ꢀC; all reactions by duplicate.
9
48b
15
b
Determined by 19F NMR and GCeMS.
10
11
12
13
14
Toluene
OMe
NO2
NH2
NO2
NH2
c
Determined by 1H NMR and GCeMS.
[BMI][PF6]
[BMI][PF6]
[BMI][PF6]
[BMI][PF6]
48b
15
d
Cross-coupling product:dehalogenated product¼73:26 (in [BMI][PF6]).
e
Cross-coupling product:dehalogenated product¼85:15.
48b
f
Without base.
g
Without base and water.
a
Reaction conditions: Pd:L:arylchloride:phenylboronic acid:sodium carbo-
nate¼1: 1.2:100:120: 250 mmol in 1 cm3 of [BMI][PF6] or toluene and 2 cm3 of
water at 60 ꢀC; all reactions by duplicate.
catalyst (entry 1 in Table 1). However for Pd/4, ca. 23% of the
product corresponded to the dehalogenated substrate, tri-
fluoromethylbenzene. Furthermore, the catalytic mixture became
black after time in contrast to yellow solutions obtained using P-
donor ligands. TEM analysis of the post-catalytic ionic liquid sus-
pension for Pd/4 system, showed the presence of palladium
nanoparticles (PdNP) (Fig. S11 in Supplementary data). Their for-
mation was probably due to the absence of good donor ligands
(such as 1e3), which favour the formation of more active and se-
lective catalytic molecular species. The surface reactivity of the
PdNP generated in situ could be responsible of the dehalogenation
b
T¼100 ꢀC.
c
Determined by 19F NMR and GCeMS.
d
Determined by 1H NMR and GCeMS.
As expected, in relation to the analogous bromo derivatives, the
chloroaryl substrates required longer reaction times to achieve high
conversions
(more
than
90%
for
1-chloro-4-tri-
fluoromethylbenzene after 15 h of reaction at 60 ꢀC entries 1, 5 and
7 in Table 2). For 1-bromo-4-methoxybenzene, from moderate
(31e55% conversion, entries 8 and 10 in Table 2) to excellent con-
versions (up to 91%, entries 2 and 4 in Table 2) at 100 ꢀC and after

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425
two days of reaction, were obtained. In general for both catalysts,
Pd/1 and Pd/3, higher activities were achieved in [BMI][PF6] than in
toluene, in particular for Pd/1 (for Pd/1, see entries 1e2 vs 3e4; for
Pd/3, see entries 7 vs 9 in Table 2). This behaviour could be due to
the ligand degradation in toluene under basic conditions (see
above), especially noteworthy at long times and high temperatures.
Surprisingly for chloro-substrates, Pd/1 catalytic system exhibited
higher activity than Pd/3 (entries 1e4 vs 7e10 in Table 2), in con-
trast to that observed for bromo-substrates (entries 1e8 in Table 1).
Pd/2 gave similar results than Pd/3 in [BMI][PF6] (entries 5 and 6,
Table 2).
Similar catalytic behaviour was observed using Pd/1 and Pd/3
systems in [BMI][PF6] for activated 1-chloro-4-nitrobenzene (en-
tries 11 and 13, Table 2) and for deactivated 4-chloroaniline (entries
12 and 14, Table 2).
Taking into account the stability of Pd/3 catalytic system under
biphasic [BMI][PF6]/water conditions, we evaluated the recycla-
bility for the cross-coupling reaction between 1-bromo-4-tri-
fluoromethylbenzene and phenylboronic acid. The ionic liquid
catalytic phase could be reused up to ten times without loss of
conversion (95e99%), and preserving the chemoselectivity towards
the cross-coupling product (Fig. 8).
conversion was low (entry 2 in Table 3). In [BMI][PF6], only the
dehalogenated product was obtained (entry 1, Table 3). Pd/3 cata-
lytic system mainly gave the dehalogenated and homo-coupling
product in [BMI][PF6] and toluene, respectively (entries 3 and 4,
Table 3). At lower temperatures, the catalytic systems were
inactive.
2.3.3. Asymmetric allylic alkylation. Palladium/L systems were
tested in the benchmark allylic alkylation starting from the racemic
substrate 3-acetoxy-1,3-diphenyl-1-propene and dimethylmalo-
nate as nucleophile, under basic Trost conditions.32 The catalytic
results are summarized in Table 4. The catalytic precursor was
generated in situ form [PdCl(C3H5)]2 and the appropriate ligand
(1e3). Pd/1 was the most active catalytic system in organic solvent,
giving full conversion towards the expected product after 1h of
reaction (entry 1 vs 2 and 3 in Table 4). This system led to the best
asymmetric induction, however only 26% of enantiomeric excess
was achieved. Pd/1 was also used in [BMI][PF6]. The system was
slower than in dichloromethane (entry 4 vs 1 in Table 4), giving also
a lower enantioselectivity but towards the opposite enantiomer
than that obtained in dichloromethane. The reversal of product
configuration depending on the solvent nature (ionic liquid vs
dichloromethane) has been also previously reported for copper
catalyzed DielseAlder reactions.33 This effect can be due to the
influence of the anion nature in the catalytic intermediates.
Table 4
Asymmetric allylic alkylation catalyzed by Pd/L systems (L¼1e3)a
Entry
Ligand
Solvent
t (h)
Conv.b(%)
eec%
1
2
3
4
1
2
3
1
CH2Cl2
CH2Cl2
CH2Cl2
[BMI][PF6]
1
4
4
24
100
100
100
74
26 (S)
4 (R)
10 (R)
11 (R)
Fig. 8. Histogram representing 1-bromo-4-trifluoromethylbenzene conversion to 4-
(trifluoromethyl)-1,10-biphenyl after each run of recycling using Pd/3 catalytic system.
a
Reaction conditions: Pd:1,2¼2.5, 3¼1.25; Pd:substrate¼50 in 4 cm3 of CH2Cl2 or
1 cm3 [BMI][PF6] at rt results obtained by duplicate experiments.
2.3.2. Asymmetric Suzuki CeC cross-coupling. In an attempt of
performing the asymmetric version of the reaction, several re-
actants (ortho and diortho substituted iodides and bromides as
well as ortho and diortho substituted boronic acids) under different
reaction conditions, were tested. Unfortunately, in all cases only the
deboronated or dehalogenated products were mainly obtained. The
most relevant results are collected in Table 3.
b
Conversion determined by 1H NMR.
Enantiomeric excess determined by HPLC on a Chiralcel OJ-H column.
c
3. Conclusions
New azadioxaphosphane bicyclo[3.3.0]octane ligands 1 and 2
derived from natural tartaric acid were prepared and characterized.
NMR studies revealed that at least three conformers are present in
solution due to the relative spatial disposition of benzyl and
phosphorus substituents in both syn and anti arrangements.
Modelling studies are in good agreement with the isomeric ratios
observed by NMR spectroscopy. Nevertheless upon palladium co-
In the case of the catalytic system Pd/1 in toluene, the desired
cross-coupling compound was the main product, although the
Table 3
Asymmetric Suzuki CeC couplings between arylhalide and phenylboronic acid
derivatives catalyzed by Pd/L systems (L¼1, 3)a
ordination to give [PdCl2(
distinguished.
k
1-P-1)2], the conformers are not thus far
The catalytic systems Pd/1e3 were highly active and selective in
Suzuki CeC cross-couplings with substrates bearing both electron-
donor and electron-withdrawing groups for 4-R-bromobenzene
and 4-R-chlorobenzene derivatives. Using [BMI][PF6] as solvent, the
systems worked without formation of by-products allowing the
activation of CeCl bonds for substrates containing electron-donor
substituents.
Entry
L
Solvent
Conv.b(%)
Selectivity I:II:IIIc(%)
1
2
3
4
1
1
3
3
[BMI][PF6]
Toluene
[BMI][PF6]
Toluene
100
28
100
100
0:0:100
82:8:18
0:0:100
3:97:0
It is noteworthy that ligands 1e3 showed higher stability under
catalytic basic biphasic conditions in [BMI][PF6] than that observed
in toluene and [EMI][HPO(O)OMe]. This behaviour allowed the
recycling of the catalytic phase in [BMI][PF6] up to ten cycles
without loss of activity.
a
Reaction
conditions:
Pd:L:arylhalide:arylboronic
acid:sodium
carbonate¼1:1.2:100:120:250 mmol in 1 cm3 of [BMI][PF6] or toluene and 2 cm3 of
water at 100 ꢀC for 24 h; all reactions by duplicate.
b
Conversion determined by GCeMS.
Determined by GCeMS.
c

ꢀ
M.V. Escarcega-Bobadilla et al. / Tetrahedron 67 (2011) 421e428
426
Unfortunately, asymmetric Suzuki reactions were not selective
CHar). 13C NMR (125.5 MHz, CD2Cl2, rt):
d
¼58.0 (CH2CH,
towards the desired cross-coupling product, only giving the
expected product in a very low yield and selectivity (substrate
conversion less than 30% using Pd/1 catalytic system in toluene).
The catalytic evaluation of these chiral ligands for the formation of
new stereogenic centre in the Pd-catalyzed asymmetric allylic al-
kylation model reaction gave low enantioselectivities.
JCP¼2.5 Hz), 59.8 (CH2CH), 60.3 (CH2Bn), 78.1 (CH, JCP¼11.3 Hz), 84.7
(CH, JCP¼16.3 Hz), 127.1 (CHar), 128.2 (CHar), 128.3 (CHarP,
JCP¼8.8 Hz), 128.6 (CHarP), 128.8 (CHar), 130 (CHarP, JCP¼21.3 Hz),
130.3 (CHar), 138.3 (CipsoBn), 140.5 (CipsoP, JCP¼23.8 Hz). 31P NMR
(121.4 MHz, CD2Cl2, rt):
d
¼150.69.
Isomer ‘C’ (22%): 1H NMR (500.13 MHz, CD2Cl2, rt):
d¼2.06 (m,
2H, CH2CH), 2.17 (m, 2H, CH2CH), 3.37 (d, 1H, CHHBn), 3.22 (d, 1H,
CHHBn), 4.39 (m, 1H, CH), 4.97 (m, 1H, CH), 7.13e7.65 (m, 10 CHar).
4. Experimental section
4.1. General data
13C NMR (125.5 MHz, CD2Cl2, rt):
d
¼58.0 (CH2CH, JCP¼2.5 Hz), 59.5
(CH2Bn), 59.8 (CH2CH), 78.1 (CH, JCP¼11.3 Hz), 84.7 (CH,
JCP¼16.3 Hz), 126.9 (CHar), 128.2 (CHar), 128.3 (CHarP, JCP¼8.8 Hz),
128.6 (CHar), 128.8 (CHar), 130 (CHarP, JCP¼21.3 Hz), 130.3 (CHar),
138.2 (CipsoBn), 140.5 (CipsoP, JCP¼23.8). 31P NMR (121.4 MHz,
Syntheses were performed using standard Schlenk techniques
under nitrogen or argon atmosphere. Organic solvents were dried
following the procedures described in literature.34 rac-3-acethoxy-
1,3-diphenyl-1-propene was synthesized following the methodol-
ogy described in the literature.35 PhPCl2, PCl3, Ph2PCl, substrates,
boronic acids, dimethylmalonate, potassium acetate, N,O-bis(tri-
CD2Cl2, rt):
d¼150.69.
4.3. (3aS,6aS)-5-Benzyl-2-phenoxytetrahydro-3aH-[1,3,2]
dioxaphospholo[4,5-c]pyrrole, 2
methylsilyl)acetamide
(BSA),
[Pd(
m-Cl)(
h
3-C3H5)]2
and
[PdCl2(COD)] were purchased from SigmaeAldrich, Acros and
Strem chemicals and used without further purification. [BMI][PF6]
and [EMI][HPO(O)OMe] (99.5%) were purchased from Solvionic and
treated under reduced pressure at 60 ꢀC for 48 h prior to use. 1H, 13C
and 31P NMR spectra were recorded on a Bruker AVe300 spec-
trometer (300.13 MHz for 1H) or a Brucker AVe400 (400.16 MHz for
1H) or a Brucker AVe500 (500.13 MHz for 1H). Chemical shifts are
expressed in parts per million upfield from SiMe4. NOESY, 1H and
13C correlation spectra were obtained using standard procedures.
TEM experiments were performed at the ‘Service Commun de
PCl3 0.023 cm3 (0.26 mmol) was dropwise added to a solution of
50 mg (0.26 mmol) of 4 and 0.072 cm3 of Et3N in 20 cm3 of THF at
ꢁ110 ꢀC and stirred at 70 ꢀC for 5 h. Then the solvent was removed
under reduced pressure and 20 cm3 of THF were added and mixture
cooled at ꢁ110 ꢀC, after 24 mg of phenol in 20 cm3 of THF and
0.036 cm3 of Et3N were dropwise added over a period of 15 min.
The reaction mixture was stirred 3 h affording a white suspension.
The suspension was filtered through anhydrous basic alumina un-
der argon atmosphere and solvent was then evaporated, obtaining
the monophosphite as a white powder (73 mg, 88%). [
a
]
25 þ51.5 (c
D
Microscopie Electronique de l’Universite Paul Sabatier’ on a Philips
1.1 in CH2Cl2) nmax (IR, KBr, pellet)/cmꢁ1 1099 (PeOeCar, st, w), 838
(PeO, st, s), 744 (PeOeC, st, w). HRMS (CIeCH4) found m/z:
316.1102 ([M]þH) C17H19NO3P requires 316.1103.
ꢀ
CM12 electron microscope operating at 120 kV with resolution of
ꢂ
4.5 A. The images of particles dispersed in [BMI][PF6] were obtained
from a transmission electron microscope running at 120 kV. A drop
of solution was deposited on a holey carbon grid and the excess of
[BMI][PF6] was removed in order to obtain a film as thin as possible.
Images were recorded on the film of IL lying on the holes of the grid.
IR spectra were recorded on an FTIR Nicolet Impact 400 spec-
trometer. Optical rotations were measured in a PerkineElmer
241MC polarimeter.
Isomer ‘A’ (40%): 1H NMR (500.13 MHz, CDCl3, rt):
1H, CHHCH), 2.73 (m, 1H, CHHCH), 2.95 (m, 1H, CHHCH), 3.06 (m,
1H, CHHCH), 3.63 (d, 1H, CHHBn), 3.59 (d, 1H, CHHBn), 5.28 (m, 1H,
CH), 4.71 (m, 1H, CH), 7.01e7.36 (m, 10 CHar). 13C NMR (125.5 MHz,
d
¼2.58 (m,
CDCl3):
d
¼58.3 (CH2CH, JCP¼5 Hz), 59.6 (CH2CH), 59.9 (CH2Bn), 78.4
(CH, JCP¼2.5 Hz), 78.6 (CH, JCP¼20.1 Hz), 119.6e129.7 (CHar), 133.9
(CipsoBn), 152.4 (CipsoP, JCP¼8.8 Hz). 31P NMR (202.5 MHz, CDCl3):
d
¼128.7.
4.2. (3aS,6aS)-5-Benzyl-2-phenyltetrahydro-3aH-[1,3,2]
dioxaphospholo[4,5-c]pyrrole, 1
Isomer ‘B’ (36%): 1H NMR (500.13 MHz, CDCl3, rt):
d¼2.55 (m,
1H, CHHCH), 2.75 (m, 1H, CHHCH), 2.95 (m, 1H, CHHCH), 3.058 (m,
1H, CHHCH), 3.77 (d, 1H, CHHBn ), 3.67 (d, 1H, CHHBn), 4.78 (m, 1H,
CH), 5.16 (m, 1H, CH), 7.01e7.36 (m, 10 CHar). 13C NMR (125.5 MHz,
A solution of 100 mg (0.5 mmol) of 4 and 0.72 cm3 of Et3N in
6.6 cm3 of THF were added dropwise to a solution of 0.07 cm3 of
PhPCl2 (0.5 mmol) in 1 cm3 of toluene at rt. The reaction mixture was
stirred overnight affording a white suspension. The suspension was
filtered through anhydrous basic alumina under argon atmosphere
and solvent was then evaporated, obtaining the monophosphonite
CDCl3, rt):
d
¼58.2 (CH2CH, JCP¼2.5 Hz), 59.7 (CH2CH), 60.1 (CH2Bn),
76.5 (CH, JCP¼6.3 Hz), 77.4, 119.5e129.7 (CHar), 138.2 (CipsoBn), 152
(CipsoP, JCP¼6.3 Hz). 31P NMR (202.5 MHz, CDCl3, rt):
d
¼128.64.
Isomer ‘C’ (24%): 1H NMR (500 MHz, CDCl3, rt):
d
¼2.81 (m, 2H,
CH2CH), 3.14 (m, 2H, CH2CH), 3.60 (d, 1H, CHHBn), 3.64 (d, 1H,
as a white powder (120 mg, 80%). [
a
]
25 þ46.7 (c 0.97 in CH2Cl2) nmax
CHHBn), 5.36 (m, 2H, CH), 7.01e7.36 (m, 10 CHar). 13C NMR
D
(IR, KBr, pellet)/cmꢁ1 1104 (PeOeC, st, s), 746 (PeOeC, st, w), 828
(PeC, st, w) HRMS (CIeCH4) found m/z: 299.1081 [M]þ. C17H18NO2P
requires 299.1075.
(125.5 MHz, CDCl3, rt):
d
¼59.4 (CH2CH, JCP¼2.5 Hz), 60.2 (CH2Bn),
75.5 (CH, JCP¼6.3 Hz), 119.6e129.7 (CHar), 137.7 (CipsoBn), 152
(CipsoP, JCP¼5). 31P NMR (202.5 MHz, CDCl3, rt):
¼127.69.
d
Isomer ‘A’ (48%): 1H NMR (500.13 MHz, CD2Cl2, rt):
1H, CH2CH), 2.40 (m, 1H, CH2CH), 2.57 (m, 1H, CH2CH), 2.98 (m, 1H
CH2CH), 3.53 (d, 1H, CHHBn ), 3.45 (d, 1H, CHHBn), 4.59 (m, 1H, CH),
4.77 (m, 1H, CH), 7.13e7.65 (m, 10 CHar). 13C NMR (125.5 MHz,
d
¼2.30 (m,
4.4. Palladium(II) dichloro-k
1-P-bis-{(3aS,6aS)-5-benzyl-2-
phenyltetrahydro-3aH-[1,3,2]dioxaphospholo[4,5-c]pyrrole]},
Pd1
CD2Cl2, rt):
d
¼58.5 (CH2CH, JCP¼3.8 Hz), 58.9 (CH2CH, JCP¼3.8 Hz),
59.8 (CH2Bn), 79.3 (CH, JCP¼11.3 Hz), 84.1 (CH, JCP¼8.8 Hz), 127
(CHar), 128.1 (CHar), 128.3 (CHarP, JCP¼8.8 Hz), 128.3 (CHar), 128.5
(CHar), 128.6 (CHar), 130 (CHarP, JCP¼21.3 Hz), 130.2 (CHar), 138.2
(CipsoBn), 140.1 (CipsoP, JCP¼23.8 Hz). 31P NMR (121.4 MHz, CD2Cl2,
To a solution of 0.046 g (0.154 mmol) of ligand 1 in 20 cm3 of
anhydrous and deoxygenated toluene it was added 0.027 g
(0.077 mmol) of the palladium precursor [PdCl2(COD)]. The re-
action mixture was then heated at 50 ꢀC and stirred during 2 h.
Then the solvent was evaporated under reduced pressure and the
precipitate washed with ethyl ether. The complex was obtained as
a yellowish powder (0.070 g, 59%). dH 1H NMR (400.16 MHz, CD2Cl2,
rt):
d
¼149.66.
Isomer ‘B’ (30%): 1H NMR (500.13 MHz, CD2Cl2, rt):
d
¼2.83 (m,
2H, CH2CH), 3.18 (m, 2H, CH2CH), 3.74 (d, 1H, CHHBn), 3.68 (d, 1H,
CHHBn), 4.39 (m, 1H, CH), 4.97 (m, 1H, CH), 7.13e7.65 (m, 10H
213 K)
d 3.05 (m, 4H, CH2CH), 3.16 (m, 4H, CH2CH), 3.70 (d, 2H,

ꢀ
M.V. Escarcega-Bobadilla et al. / Tetrahedron 67 (2011) 421e428
427
CH2Bn), 3.78 (d, 2H, CH2Bn), 5.04 (br s, 2H, CH), 6.14 (m, 2H, CH),
7.27e7.63 (m, 20H, ar). 13C NMR (125.47 MHz, CD2Cl2, rt):
4.8. General procedure for catalytic asymmetric allylic
alkylation in ionic liquid
d¼57.5
(pt, JCP¼5 Hz, CH2CH), 58.3 (pt, JCP¼5 Hz, CH2CH), 59.3 CH2Bn, 83
(pt, JCP¼2.5 Hz, CH), 83.1 (pt, JCP¼2.5 Hz, CH), 127.5 CHar, 127.6 (pt,
JCP¼7 Hz, CHarP), 128.5 CHar, 128.7 CHar, 130.8 (pt, JCP¼5.7 Hz,
CHarP), 132.2 CHar, 134.0 (d, JCP¼96 Hz, CipsoP), 137.5CipsoBn. 31P
[Pd(m-Cl)(h
3-C3H3)]2 3.7 mg (0.02 mmol) and 0.1 mmol of ligand
were disolved in 1 cm3 of [BMI][PF6] and stirred under vacuum for
30 min then 252 mg (1 mmol) of substrate (rac-3-acethoxy-1,3-
diphenyl-1-propene), 396 mg (1 mmol) of dimethylmalonate,
610 mg and 3 mg of potassium acetate were added at rt.
After the reaction, the products were extracted with diethyl
ether and then extracted with NH4Cl 10% and water. The organic
phase was dried over MgSO4, filtered and solvent was removed
under vacuum.
NMR (121.50 MHz, CD2Cl2, rt):
d
¼130.44. nmax (IR, KBr, pellet)/cmꢁ1
749 (PeC, st, w), 1022 (PeOeCal, st), 1437 (CeN, st, w), 2961 (C]C,
st, w). EA C 52.69, H 4.72, N 3.57. C34H36Cl2N2O4P2Pd requires C
52.63, H 4.68, N 3.61. HRMS (ESI) found m/z: 737.0886 [M]þꢁCl.
C34H36N2O4P2ClPd requires 737.0879.
4.5. Palladium(II) dichloro-k
2-P,P-(3S,4S)-1-benzyl-3,4-bis
(diphenylphosphinooxy)pyrrolidine], Pd3
4.9. General procedure for catalytic asymmetric allylic
alkylation in organic solvent
To a solution of 0.043 g (0.077 mmol) of ligand 3 in 20 cm3 of
anhydrous and deoxygenated toluene it was added 0.027 g
(0.077 mmol) of the palladium precursor [PdCl2(COD)]. The re-
action mixture was then heated at 50 ꢀC and stirred during 2 h.
Then the solvent was evaporated under reduced pressure and the
precipitate washed with ethyl ether. The complex was obtained as
a yellowish powder. (44 mg, 77%). 1H NMR (300.13 MHz, CD2Cl2, rt):
[Pd(m-Cl)(h
3-C3H3)]2 3.7 mg (0.02 mmol) and 0.05 or 0.1 mmol of
the corresponding ligand were dissolved in 1 cm3 of CH2Cl2 and
stirred for 30 min then 252 mg (1 mmol) of substrate (rac-3-ace-
thoxy-1,3-diphenyl-1-propene) dissolved in 1 cm3 of CH2Cl2,
396 mg (1 mmol) of dimethylmalonate in 1 cm3 of CH2Cl2, 610 mg of
BSAdissolved in 1 cm3 of CH2Cl2 and 3 mg of potassium acetate were
added. After the reaction, 10 cm3 of diethyl ether were added and
then extracted with NH4Cl 10% and water. The organic phase was
dried over MgSO4, filtered and solvent was removed under vacuum.
d
¼2.59 (m, 2H, CH2CH), 2.85 (m, 2H, CH2CH), 3.40 (d, 1H, CH2Bn),
3.64 (d, 1H, CH2Bn), 4.81 (2H, m, 2CH), 7.055e8.1 (25H, m, ar). 13
NMR (75.47 MHz, CD2Cl2, rt):
C
d
¼57.3 (pt, JCP¼3.5, 3.5 Hz, CH2), 59.8
(CH2), 77.2 (CH2Bn), 80.6 (2CH), 127.5 (CHar), 127.9 (pt, JCP¼6 Hz,
CHarP), 128.4 (CHar), 128.7 (CHar), 128.8 (pt, JCP¼6 Hz, CHarP),
130.8 (dd, JCP¼27.8, 27 Hz, CipsoP), 131.2 (CHar), 132.1 (pt, JCP¼6 Hz,
CHarP), 133.1 (CHar), 134.9 (pt, JCP¼6.8 Hz, CHarP), 135.1 (dd,
Acknowledgements
We gratefully acknowledge the financial support of the Centre
31
JCP¼135.1, 93.8 Hz, CipsoP), 136.8CipsoBn. P NMR (121.4 MHz,
ꢀ
National de la Recherche Scientifique, the UniversitePaul Sabatier,
CD2Cl2, rt):
d
¼121.79; nmax (IR, KBr, pellet)/cmꢁ11047 (PeOeC, st, s),
ꢀ
the Ministerio de Ciencia e Innovacionn (CTQ2007-63510PPQ and
745 (PeOeC, st, w), 717 (OeC, st, w), HRMS (CIeCH4) found m/z:
702.0712 [M]þꢁCl. C35H33NO2P2ClPd requires 702.0710.
CSD2006-0003) and Generalitat de Catalunya (CTP 2007ITT-
ꢀ
00007). M.V. Escarcega-Bobadilla thanks Generalitat de Catalunya
and European Social Fund for a pre-doctoral fellowship (FI and
BE2008).
4.6. General procedure for catalytic Suzuki CeC coupling in
ionic liquids
Supplementary data
Dichloro(cycloocta-1,5-diene)-palladium(II) (2.8 mg, 0.01 mmol)
and the corresponding ligand (0.012 or 0.024 mmol) were dissolved
in 1 cm3 of the corresponding ionic liquid and stirred for 30 min
under vacuum at rt, giving a colourless or yellowish solution. After
this time, the substrate (1 mmol), the boronic acid (1.5 mmol) and
Na2CO3 (265.0 mg, 2.5 mmol) dissolved in 2 cm3 of deoxygenated
water were consecutively added, forming a biphasic system. The
mixturewas then heated at 60 or 100 ꢀC during 1, 24 or 48 h and then
cooled at rt. The catalytic mixture was extracted with ethyl ether
(5ꢂ2 cm3) then the organic phase washed with 1 cm3 of NaOH 1 M,
1 cm3 of water and dried with anhydrous Na2SO4, filtered and con-
centrated under reduced pressure, yielding the product either as
a solid or as yellow oil.
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2010.11.023.
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