Pd-catalysed asymmetric SuzukieMiyaura reactions using chiral mono- and bidentate phosphorus ligands

Article abstract of DOI:10.1016/j.jorganchem.2013.06.022

A series of monodentate and bidentate chiral phosphorus based ligands including diphosphites and diphosphonites derived from carbohydrates were tested in the asymmetric Suzuki coupling of aryl halides with 2-substituted 1-naphthylboronic acids. Good activ

Full text of DOI:10.1016/j.jorganchem.2013.06.022

Journal of Organometallic Chemistry 743 (2013) 31e36
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Pd-catalysed asymmetric SuzukieMiyaura reactions using chiral
mono- and bidentate phosphorus ligands
Angelica Balanta Castillo a, Bernabé F. Perandones a, Ennio Zangrando b, Serafino Gladiali c,
,
a,
*
Cyril Godard a , Carmen Claver
*
a Departament de Química Física i Inorgánica, Universitat Rovira i Virgili, C/ Marcel$li Domingo s/n, 43007 Tarragona, Spain
b Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, via Licio Giorgieri 1, 34127 Trieste, Italy
c Dipartimento di Chimica, University of Sassari, via Vienna 2, 07100 Sassari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of monodentate and bidentate chiral phosphorus based ligands including diphosphites and
diphosphonites derived from carbohydrates were tested in the asymmetric Suzuki coupling of aryl ha-
lides with 2-substituted 1-naphthylboronic acids. Good activities and selectivities were achieved,
although modest ee’s were obtained (up to 37%). The X-ray structure of PtCl2(12) bearing the C2-sym-
metry diphosphite ligand 12 derived from carbohydrate, is also reported.
Received 12 March 2013
Received in revised form
14 June 2013
Accepted 19 June 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Palladium
Asymmetric
SuzukieMiyaura
Monodentate
Bidentate
Phosphorus ligands
1. Introduction
independently in 2000 by Buchwald [14] and Cammidge [15]. Later,
Johannsen and Jensen reported the Pd-catalysed asymmetric
The SuzukieMiyaura reaction is a powerful process for the
synthesis of biaryl compounds [1e4], with many inherent advan-
tages over other methods such as the tolerance to a broad range of
functional groups, non-toxic by-products, and the stability of the
organoboronic partners that are inert to air and water [5,6]. Axially
chiral biaryls are important structural motifs in biologically active
molecules [7] and ligands for catalysis [8e10]. Two examples are
shown in Fig. 1: the vancomycin, which is a clinically used glyco-
peptide antibiotic [11] and the steganacin, which is a cytotoxic
tubulin-binding dibenzocyclooctadiene lignin [12]. The chirality of
these compounds arises from the restricted rotation around the
arylearyl bond [13]. Binaphthalenes are among the most repre-
sentative compounds exhibiting axial chirality and over the last
years, the synthesis of optically active 1,1-binaphthalenes through
asymmetric SuzukieMiyaura coupling has received much attention
[14e23]. The first reports on the synthesis of axially chiral biaryls
by enantioselective SuzukieMiyaura reaction were published
coupling reaction using Pd catalysts with the dialkyl arylferroce-
nylphosphines ligands with moderate ee’s (up to 54%) [16]. Mikami
et al. reported that excellent conversion and enantioselectivity
could be achieved in short reaction times using cationic Pd/BINAP
complexes [17]. Espinet and co-workers also improved the reaction
times required to achieve high conversions and ee’s in this asym-
metric process using microwave technology [18].
In terms of ligands, phosphines are the most used ligands in Pd-
catalysed asymmetric SuzukieMiyaura coupling (Fig. 2) [19]. For
instance, Buchwald and co-workers reported the enantioselective
synthesis of axially chiral biaryls for a large substrate scope
employing the catalytic system Pd(OAc)2/KenPhos (B, Fig. 2) with
ee’s in the range 88e94% ee [20]. However, recently, other types of
ligands were successfully employed in this asymmetric process.
Phosphonite ligands (D, Fig. 2) were shown to be efficient in the
asymmetric cross coupling of aryl chlorides using low catalyst
loading and short reaction times (ee up to 78%) [21]. Pd/phos-
phoramiditeeoxazoline systems (ligand E, Fig. 2) were used by
Guiry in the asymmetric SuzukieMiyaura coupling between 2-
methylnaphthylboronic acid and 1-bromonaphthalene, leading to
up to 46% ee at room temperature [22]. Recently, Song and Gong
reported the use of chiral PCN pincer Pd complexes (I, Fig. 2) in the
* Corresponding authors. Tel.: þ34 977 559575; fax: þ34 977 559563.
E-mail addresses: ezangrando@units.it (E. Zangrando), gladiali@uniss.it
(S. Gladiali), cyril.godard@urv.cat (C. Godard), carmen.claver@urv.cat (C. Claver).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.022

32
A.B. Castillo et al. / Journal of Organometallic Chemistry 743 (2013) 31e36
Fig. 1. Examples of natural products containing a chiral biaryl unit.
asymmetric SuzukieMiyaura reaction with ee’s up to 26% [23].
Furthermore, ligands such as chiral dienes (H, Fig. 2) were shown to
be efficient in Pd-catalysed asymmetric SuzukieMiyaura coupling
reaction with ee’s up to 90% [24]. Lassaletta and co-workers re-
ported the use of C2-symmetric nitrogen donor ligands (F, Fig. 2)
with excellent ee’s (up to 98%) [25]. Later, the same authors re-
ported the use of phosphinohydrazone ligands (G, Fig. 2) in this
reaction with high activity and good ee’s [26].
ligands including the commercial phosphine 1 and other ligands
bearing a chiral naphthyl moiety and bidentate ligands such as
chiral diphosphites and diphosphinites based on carbohydrate [28].
2. Results and discussion
2.1. Pd-catalysed asymmetric SuzukieMiyaura coupling using Pd
catalysts bearing monodentate chiral ligands 1e5
A recyclable catalytic system was also reported by Uozumi and
co-workers, who developed a polymer-supported ligand (K, Fig. 2)
for the Pd-catalysed asymmetric SuzukieMiyaura coupling of
several naphthyl halide derivatives with excellent ee’s [27].
Here, we present our results obtained in the coupling of napthyl
bromides and iodides using a series of monodentate P-donor
First, the catalytic conditions were optimised using the com-
mercial ligand 1 (Scheme 1) in the Pd-catalysed asymmetric
SuzukieMiyaura coupling of 2-ethoxy-1-naphthylboronic acid and
1-bromonaphthalene in the presence of various Pd/L ratios, palla-
dium precursors, bases and solvents (see Supplementary
C
B
A
F
E
D
G
H
I
J
K
Fig. 2. Selected ligands reported in asymmetric Suzuki-coupling reactions.

A.B. Castillo et al. / Journal of Organometallic Chemistry 743 (2013) 31e36
33
Scheme 1. Chiral monodentate ligands used in this study.
information). The best results were obtained [PdCl2(NCPh)2] as Pd
source, CsF as a base at 70 ꢀC in THF during 4 h.
obtained (entry 13). With 2-benzyloxynaphthalene-1-boronic acid,
50% conversion was achieved but without enantioselectivity (Entry
14).
The catalytic systems with the ligands 1e5 were then tested
under these conditions in the Pd-catalysed asymmetric Suzukie
Miyaura reaction of bromonaphthalene with naphthylboronic acids
bearing methoxy-, ethoxy- and benzyloxy substituents in 2-
position (Table 1). In all cases, excellent selectivities to the cross
coupling products were obtained (>95%).
The system Pd/1 afforded 76, 56 and 80% of conversion but only
10, 14 and 1% of ee respectively (Entries 1e3). Similar results were
obtained with the catalytic system bearing the ligand 2 with 76 and
49% of conversion and 10 and 9% ee, respectively (entries 4e5).
However, using the phosphine ligand 3, lower conversions and up
to 24% ee were achieved (entry 6e8).
The catalytic system containing the ligand 4 afforded low con-
version and ee up to 17% (entries 9e11). Finally, using the phos-
phoramidite ligand 5, the reaction between 1-bromonaphthalene
and 2-methoxynaphthalene-1-boronic acid afforded 23% of con-
version and 10% of ee (entry 12). When 2-ethoxynaphthalene-1-
boronic acid was used 33% of conversion and 12% of ee was
To summarise, the monodentate ligands 1e5 were used in the
Pd-catalysed asymmetric SuzukieMiyaura coupling reactions of
bromonaphthalene and 2-substituted naphthyl boronic acids and
afforded conversions up to 80% but with only moderate to low ee’s.
Next, these catalytic systems were evaluated in the Pd-catalysed
asymmetric SuzukieMiyaura reactions of iodonaphthalene with
the same 2-alkoxy naphthylboronic acids and in the presence of CsF
as a base at 70 ꢀC during 4 h (Table 2). As expected, higher con-
versions were achieved with this substrate, but low enantiose-
lectivity was again obtained in all cases.
Other substrates were also tested using the Pd/1 catalytic sys-
tem (Table 3). The reactions of 2-methoxy-1-bromonaphthalene
with 2-OEt and 2-Me-1-naphthylboronic acids (Entries 1e2) led
to fair conversions but selectivity and ee were moderate to poor.
Similar results were obtained when 2-Me-1-bromonaphthalene
was the substrate, independently of the naphthyl boronic acid
used (Entries 3e5). No significant change was observed when 2-
bromotoluene was the substrate (Entry 6).
Table 1
Table 2
Pd-catalysed asymmetric SuzukieMiyaura coupling of 1-bromonaphthalene with 2-
substituted 1-naphthylboronic acids using ligands 1e5.a
Pd-catalysed asymmetric SuzukieMiyaura coupling of 1-iodonaphthalene with
2-substituted 1-naphthylboronic acids using ligands 1e5.a
Entry
L
R
Convb (%)
Selb (%)
eec (%)
Entry
L
R
Conv (%)b
Sel (%)b
ee (%)c
1
2
3
4
5
6
7
8
1
eOMe
eOEt
eOBn
eOMe
eOEt
eOMe
eOEt
eOBn
eOMe
eOEt
76
56
80
76
49
15
21
11
19
15
3
98
98
99
99
99
94
98
99
98
99
e
10
14
<1
10
9
24
9
<1
7
17
<1
10
12
<1
1
2
3
4
5
6
7
8
1
1
1
2
2
3
3
3
4
4
4
5
5
5
eOMe
eOEt
eOBn
eOMe
eOEt
eOMe
eOEt
eOBn
eOMe
eOEt
80
66
83
73
60
63
17
73
3
67
73
64
54
40
98
99
99
93
96
98
99
99
e
22
14
1
15
7
12
9
4
2
3
9
4
5
9
e
10
11
12
13
14
10
11
12
13
14
98
99
98
99
99
14
3
16
7
eOBn
eOMe
eOEt
eOBn
eOMe
eOEt
23
33
50
97
98
99
eOBn
eOBn
2
a
a
Reaction conditions: 1.0 mmol bromonaphthalene, 2.0 mmol naphthylboronic
acid, 5 mmol CsF, 1 mol % Pd precursor, 1.5 mol% ligand, 3 mL of THF, T ¼ 70 ꢀC,
t ¼ 4 h.
Reaction conditions: 1.0 mmol iodonaphthalene, 2.0 mmol naphthylboronic
acid, 5 mmol base, 1 mol % Pd precursor, 1.5 mol% ligand, 3 mL of THF, T ¼ 70 ꢀC,
t ¼ 4 h.
b
b
Determined by GC.
Determined by GC.
c
c
Determined by HPLC.
Determined by HPLC.

34
A.B. Castillo et al. / Journal of Organometallic Chemistry 743 (2013) 31e36
Table 3
ligand 12, which is very efficient in allylic alkylation and amination
reactions [30].
Pd-catalysed asymmetric SuzukieMiyaura coupling of various substrates using Pd
catalyst bearing monodentate chiral ligand 1.a
Although crystals of Pd complexes bearing the ligand 12 could not
be obtained, suitable colourless crystals for X-ray diffraction were
obtained for the platinum derivative [PtCl2(12)] by slow diffusion of
Et2O into a CH2Cl2 solution of the complex. The X-ray analysis shows
thepresence of large voids (20.5%) inthe unitcell, butonlyone diethyl
ether molecule per complex was successfully refined. The molecular
structure of the complex is shown in Fig. 4 and the corresponding
selected bond distances and angles are summarised in Table 4. The
complex is located on a crystallographic two-fold axis that, passing
through the metal and the tetrahydrofuranyl oxygen O4, relates the
two chlorine atoms and the phosphite moieties.
Entry
1
Aryl halide
Boronic acid
Conv (%)b Sel (%)b,c ee (%)d
The metal presents a square planar geometry with coplanar
ꢀ
57
65
89
50
35
12
donor atoms. The PteP bond lengths (2.201(4) A) are slightly
ꢀ
shorter than the PteCl ones (2.335(4) A). It is worth noting the
difference between the Cl(1)ePteCl(10) and the chelating P(10)e
PteP(1) bond angle (89.9(2) vs 101.91(18)ꢀ), likely induced by steric
constraints and by the geometry of the chelating ligand. The rings
of the biphenyl moiety are tilted by ca. 50ꢀ.
2
After optimisation of the reaction conditions using ligand 12, the
optimum Pd/L ratio was determined to be 1/1.5 (see Supplementary
information). The catalytic asymmetric Pd-catalysed coupling of
iodonaphthalene with substituted naphthyl boronic acids were
carried out using the ligands 6e13. All the catalysts showed high
chemoselectivity towards the desired cross coupling products
(>97%). The results are summarised in Table 5.
3
4
53
48
89
80
24
28
With the diphosphine ligands 6e7, moderate conversions (ca.
30e50%) with very low ee’s (up to 17%) were achieved (Entries 1e4).
With the C2-symmetry diphosphinite ligand 8, similar conversions
and ee were obtained (Entries 5e6). Slightly higher ee’s (ca. 30%) but
large difference in conversions were observed for both boronic acids
when the C1-symmetry diphosphinite 9 was used (Entries 7e8). The
diphosphite 10, featuring the same chiral backbone than 9, gave an
excellent conversion and a slight increase in ee to 37% (Entry 9).
Finally, with the diphosphite ligands 11e13, moderate conversion
(ca. 50%) and low ee’s were obtained (Entries 10e15). To the best of
our knowledge, this is the first time that chiral diphosphite ligands
are applied in the asymmetric SuzukieMiyaura reaction.
5
67
99
26
6
65
87
10
a
Reaction conditions: 1.0 mmol bromonaphthalene, 2.0 mmol naphthylboronic
acid, 5 mmol CsF, 1 mol % [PdCl2(PhCN)2], 1.5 mol% ligand 1, 3 mL of THF, t ¼ 24 h,
T ¼ 70 ꢀC.
These results thus indicate that diphosphite ligands are efficient
ligands in this asymmetric process, especially in terms of activity
and selectivity, although structural modification of the ligands are
required for the enantioselectivity of the process to be improved.
b
Determined by GC.
Selectivity to the cross-coupling product.
Determined by HPLC.
c
d
3. Conclusions
When the reaction described in entry 1 (Table 3) was repeated at
a larger scale (5 mmol of substrate), the conversion, selectivity and
ee were slightly lower (50, 75 and 20%, respectively). The enan-
tiomeric mixture was isolated in 25% yield.
To summarise, the Pd system bearing the ligand 1 catalyses the
asymmetric SuzukieMiyaura coupling reaction of several bromide
substrates and substituted 2-naphthylboronic acids with moderate
conversions (up to 67%) and low to moderate ee’s (up to 35%).
In view of the poor enantioinduction achieved with mono-
dentate ligands, we decided to screen in the asymmetric Suzukie
Miyaura coupling a range of bidentate ligands including chiral
diphosphines, diphosphinites and diphosphites.
A range of Pd catalytic systems with mono- and bidentate li-
gands, including diphosphonites and diphosphites derived from
carbohydrates, were screened for the first time in the asymmetric
SuzukieMiyaura coupling of iodo- and bromonaphthalenes and
substituted naphthylboronic acids. As to monodentate ligands, the
neomenthyl phosphine ligand 1 provided the highest activity with
an excellent selectivity to the cross coupling product (up to >99%)
and moderate enantioselectivity (up to ca. 35%). The results ob-
tained with the bidentate ligands 6e13 indicate that, even if the
ee’s obtained in this work are far from being exciting,
gands such as diphosphites hold the promise to be efficient in the
p-acidic li-
Pd-catalysed asymmetric Suzuki coupling of these substrates.
2.2. Pd-catalysed asymmetric SuzukieMiyaura coupling using Pd
catalysts bearing bidentate chiral ligands
4. Experimental part
The bidentate ligands used in this study are presented in Fig. 3.
The diphosphinite ligand 8, which was reported in the enan-
tioselective Rh-and Ir-catalysed asymmetric hydrogenation of un-
saturated substrates [29], features the same backbone than the
4.1. General methods
All air- or water-sensitive reactions were performed using
standard Schlenk techniques under
a nitrogen atmosphere.

A.B. Castillo et al. / Journal of Organometallic Chemistry 743 (2013) 31e36
35
Fig. 3. Chiral bidentate ligands used in Pd-catalysed asymmetric SuzukieMiyaura coupling reactions.
Chemicals were purchased from Aldrich Chemical Co and Fluka. All
4.2. General procedure for asymmetric SuzukieMiyaura coupling
reaction
solvents were distilled over drying reagents and were deoxygen-
ated before use. The precursors [PdCl2(COD)] [31] (COD ¼ 1,5-
cyclooctadiene) and [PdCl2(PhCN)2] [32] were prepared following
previously described methods. The boronic acids used in this study
were either purchased from Aldrich Chemical or synthesised ac-
cording to literature procedures [33]. The syntheses of the ligands
2e5 [34] and 8e13 were performed according to previously re-
ported procedures [28b,35].
A Schlenk tube was charged with 0.25 mmol of naphthyl halide,
0.0025 mmol of Pd precursor, 0.00375 mmol of the appropriate
chiral ligand, 0.5 mmol of boronic acid, and 1.25 mmol of base.
Anhydrous solvent was added, the flask was sealed and the mixture
was stirred and heated at the corresponding temperature. The re-
action mixture was treated with 10 mL of distilled water, extracted
with 3 ꢁ 10 mL of CH2Cl2, dried over MgSO4, and purified by flash
chromatography to obtain the corresponding products. The con-
version and selectivity was monitored by gas chromatography. The
ee values were determined by High Performance Liquid
Chromatography.
The deuterated solvents for NMR measurements were dried
over molecular sieves prior to use. NMR spectra were acquired on a
Varian Mercury 400 MHz spectrometer.
Merck silica gel 60 (0.040e0.063 mm) was employed for flash
chromatography. The conversion of the reaction was measured by
GC on a HewlettePackard HP 5890 A instrument (split/splitless
injector, J&W scientific, HP5, 25 m column, internal diameter
0.25 mm, film thickness 0.33 mm, carrier gas: 150 kPa He, F.I.D.
detector) equipped with a HewlettePackard eHP3396 series II
integrator. The ee values were determined by High Performance
Liquid Chromatography using a Daicel AD-H column.
4.3. Crystallographic measurements
Data collection was carried out on a Cu rotating anode equipped
ꢀ
with a Bruker CCD2000 detector (
l
¼ 1.54178 A). Cell refinement,
indexing and scaling of the data set were performed using pro-
grams Denzo and Scalepack [36]. The structure was solved by direct
methods, subsequent Fourier analyses [37] and refined by the full-
matrix least-squares method based on F2 with all observed re-
flections [37]. The asymmetric unit of the compound contains half
crystallographic independent OEt2 molecule. Since the crystal
presents a large void of 1137.7 A correspondent to 20.5% of the unit
cell, the program Squeeze [38] was applied to the data set indi-
cating 130 electrons per unit cell. The calculations were performed
using the WinGX System, Ver 1.80.05 [39].
3
ꢀ
Crystal data: C94H126Cl2O9P2PtSi2.C4H10O, M ¼ 1858.18, mono-
clinic, space group C2,ꢀa ¼ 21.970(2), b ¼ 19.724(2), c ¼ 13.19
3
ꢀ
3
ꢀ
80(4) A,
b
¼ 104.120(11) , V ¼ 5546.4(10) A , Z ¼ 2, Dc ¼ 1.113 g/cm ,
Table 4
ꢀ
ꢀ
Selected bond lengths (A) and angles ( ) for complex [PtCl2(12)].
Bond angles (ꢀ)
ꢀ
Bond lengths (A)
PteP(1)
PteCl(1)
P(1)eO(1)
P(1)eO(2)
P(1)eO(3)
2.201(4)
2.335(4)
1.577(8)
1.576(7)
1.580(7)
Cl(1)ePteCl(10)
P(1)ePteP(10)
P(10)ePteCl(1)
P(1)ePteCl(1)
C(13)eO(3)eP(1)
89.9(2)
101.91(18)
174.00(17)
84.09(11)
125.9(5)
Fig. 4. Molecular structure of the complex [PtCl2(12)] (methyl groups of tBu not shown
for clarity).
Primed atoms at ꢂx þ 1, y, ꢂz þ 1.

36
A.B. Castillo et al. / Journal of Organometallic Chemistry 743 (2013) 31e36
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eB(OH)2
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ee (%)c
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2
3
4
5
6
7
8
6
6
7
7
8
8
9
9
10
11
11
12
12
13
13
eOMe
eOEt
eOMe
eOEt
eOMe
eOEt
eOMe
eOEt
eOEt
eOMe
eOEt
eOMe
eOEt
eOMe
eOEt
32
40
50
31
36
56
14
73
96
50
49
65
56
58
43
99
99
97
98
99
97
98
99
99
98
99
98
98
99
98
3
4
17
3
22
17
30
21
37
4
9
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12
a
Reaction conditions: 1.0 mmol iodonaphthalene, 2.0 mmol naphthylboronic
acid, 5 mmol CsF, 1.0 mol% Pd precursor, 1.0 mol% ligand, 3 mL of THF, t ¼ 4 h,
T ¼ 70 ꢀC.
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Determined by GC.
Determined by HPLC.
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m
(Cu-K
a
) ¼ 3.661 mmꢂ1, F(000) ¼ 1948,
q
range ¼ 3.05e46.16ꢀ.
Final R1 ¼ 0.0454, wR2 ¼ 0.1130, S ¼ 1.072 for 521 parameters,
11,510 reflections, 4217 unique (R(int) ¼ 0.0440), residuals in
DF
ꢀꢂ3
map 0.954, ꢂ0.505 e. A
.
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Chem. 77 (2012) 4740.
Acknowledgements
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The authors are grateful to the Spanish Ministerio de Economía
y Competitividad (CTQ2010-14938/BQU, Juan de la Cierva fellow-
ship to B. F. Perandones and Ramon y Cajal fellowship to C. Godard)
and the Generalitat de Catalunya (for financial support 2009SGR116
and for awarding a research grant to Angelica Balanta (FI program/
2009FI)).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.06.022.
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