A new insight into ortho-(dimesitylboryl)diphenylphosphines: Applications in Pd-catalyzed Suzuki-Miyaura couplings and evidence for secondary π-interaction

Article abstract of DOI:10.1039/c1cc12611c

ortho-(Dimesitylboryl)phenylphosphines 1 and 2 were applied in Pd-catalyzed Suzuki-Miyaura C-C couplings. Coordination studies were performed in order to rationalize the relationship between structure and reactivity. Full characterization of a Pd(0) complex derived from 1 has evidenced a new coordination mode for phosphine-arylboranes involving secondary π-interaction between one of the mesityl groups at boron and the metal centre.

Full text of DOI:10.1039/c1cc12611c

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COMMUNICATION
A new insight into ortho-(dimesitylboryl)diphenylphosphines: applications
in Pd-catalyzed Suzuki–Miyaura couplings and evidence for secondary
p-interactionw
a
b
a
a
´
Raluca Malacea, Nathalie Saffon, Montserrat Gomez* and Didier Bourissou*
Received 4th May 2011, Accepted 24th May 2011
DOI: 10.1039/c1cc12611c
ortho-(Dimesitylboryl)phenylphosphines 1 and 2 were applied in
Pd-catalyzed Suzuki–Miyaura C–C couplings. Coordination
studies were performed in order to rationalize the relationship
between structure and reactivity. Full characterization of a
Pd(0) complex derived from 1 has evidenced a new coordination
mode for phosphine-arylboranes involving secondary p-interaction
between one of the mesityl groups at boron and the metal centre.
prompted us to further compare these two classes of ligands,
and we naturally became interested in Pd-catalyzed cross-
couplings, especially in the Suzuki–Miyaura reaction.
Here we report the catalytic evaluation of ligands 1 and 2 in
this type of reaction. The influence of Pd precursor, base,
temperature, reaction time and ligand/palladium ratio was
studied. A catalytically relevant Pd(0) complex derived from
1 was prepared and fully characterized.
We studied the coupling of phenyl boronic acid with the
deactivated substrate 4-bromoanisole as a model reaction.11
The catalytic species was generated in situ mixing the appro-
priate palladium precursor and phosphine ligand, starting with
palladium(II) acetate and 1 (catalytic data collected in Table 1).
The effect of the base, temperature and reaction time were
studied (Table S1, ESIw). Anhydrous potassium phosphate
was used as a base at 80 1C for 2 h using a ligand/Pd ratio of 2
(entry 1). Longer reaction time and higher temperature only
slightly improved the catalytic activity (up to 86.5% of cross-
coupling product, entry 2). When higher phenyl boronic acid/
4-bromoanisole ratio was applied, chemoselectivity dramatically
decreased, giving in particular benzene as by-product (entry 3).
A ligand 1/Pd ratio of 1 led to a more active system than
that observed for 1/Pd = 2 (entry 4 vs. 1), affording up to 87%
of the expected product. This suggests that the more active
catalytic species probably involves only one phosphine ligand
per palladium. Pd(II) organometallic precursors, [PdCl(Z3-C3H5)]2
and [PdCl2(cod)], gave similar reactivity as that observed with
palladium acetate (Table S1w). However, [Pd2(dba)3] led to a
less active system (47% conversion after 20 h, Table S1w),
probably due to the slow formation of the active Pd species.
The presence of an Ar-BMes2 moiety in the ligand raises the
question of side cross-couplings.12 With the aim of charac-
terizing the formation of such undesirable products, the catalyst
content was increased to 25 mol% (see ESIw). However,
Palladium catalysis has become a major tool in organic synthesis,
performing a range of valuable transformations.1 In particular,
the last decade has witnessed spectacular progress in cross-
couplings thanks to the use of biaryl-phosphines.2 These readily
accessible and highly modular ligands proved to be extremely
versatile and were successfully applied to a range of C–C and
C-heteroatom bond-forming processes.3,4 Key features of biaryl-
phosphines are their (i) high resistance towards oxidation5
and (ii) ability to stabilize low-valent Pd(0) intermediates by
secondary interactions between the remote aryl ring and the
metal centre.6
Over the last few years, some of us have been investigating
the coordination properties of phosphine-boranes and related
ambiphilic ligands. Four different coordination modes have
been evidenced, differing in the participation of the Lewis acid
site that can (i) remain pendant, (ii) interact with a co-ligand,
typically a halogen atom, (iii) abstract an X co-ligand to give
a zwitterionic complex or (iv) coordinate to the metal as a
s-acceptor ligand.7,8 Recent studies have pointed out some
unexpected analogies between ortho-(dimesitylboryl)phenyl-
phosphines and biaryl-phosphines (Fig. 1). First, no sign of
oxidation of [R2P(o-C6H4)BMes2] (R = Ph for 1; R = iPr for 2)
was noted upon air bubbling in solution at rt.9 Second, very
similar catalytic behaviour was found for the phosphine-borane 1
and biaryl-phosphine [Ph2P(o-C6H4)Ar] 3 (Ar = 2,4,6-iPr3C6H2) in
Rh-catalyzed hydroformylation of 1-octene.10 These observations
a
´
Universite de Toulouse, UPS, LHFA, 118 route de Narbonne,
F-31062 Toulouse Cedex 9, France and CNRS, LHFA UMR 5069,
F-31062 Toulouse Cedex 9, France. E-mail: gomez@chimie.ups-tlse.fr,
dbouriss@chimie.ups-tlse.fr; Fax: +33 561558204, +33 561558204;
Tel: +33 561557738+33 561556803
´
Institut de Chimie de Toulouse, FR 2599, Universite Paul Sabatier,
118 route de Narbonne, 31062 Toulouse Cedex 9, France
b
w Electronic supplementary information (ESI) available. CCDC 802793.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1cc12611c
Fig. 1 Phosphine-boranes 1, 2 and biaryl-phosphine 3.
Chem. Commun., 2011, 47, 8163–8165 8163
c
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Table 1 Suzuki cross-couplings between 4-bromoanisole and phenyl
boronic acid catalyzed by Pd/L systemsa
in 86% yield after 1 h at 65 1C in THF (Scheme 1). NMR
spectroscopic data and elemental analysis are consistent with
the general formula [Pd(1)(ma)]. At 298 K, two 31P NMR
signals in a 4 : 5 ratio were observed at 31.1 and 26.6 ppm,
respectively. The coexistence of two isomers was corroborated
by the coalescence of the two signals upon heating at 75 1C.
1H and 13C NMR spectroscopy confirmed the presence of two
very similar species and revealed the absence of any symmetry
in both. In particular, two sets of signals were observed for the
ma coligand of each isomer. In addition, mesityl groups give
four sets of signals for each isomer, indicating that the
two Mes rings are inequivalent and rotation around B–Cipso
bonds is hindered. Most noticeably, one of the NMR signals
associated to quaternary carbon atoms of the Mes ring is
shifted upfield to a significant extent (d = 104.66 and 107.67 ppm
for the two isomers), while the other carbon atoms resonate
at d 4 125 ppm. Such a feature is typically observed upon
p-coordination of arene rings to transition metals.16 The
11B NMR signal at 69.4 ppm is close to that of the free ligand
(75.0 ppm),17 suggesting the retention of a tricoordinate
environment around boron.
Entry
Ligand
T/1C
t/h
L/Pd
Yield of Ib (%)
1
2
3
4
5
6
7
8
1
1
1
1
80
100
80
2
20
2
2
2
2
1
2
2
2
1
81 (90)
86.5(94)
58 (77)c
80
2
81 (89) [87 (95)]d
85.6 (94)
67 (72)
96.6 (99.8)
82(91) [72 (85)]d
2
100
100
100
80
20
24
20
20
PPh3
3
1-Pd
a
Results from duplicate experiments. Reaction conditions: 1 mmol
4-bromoanisole; 2 ml toluene; K3PO4/boronic acid/4-bromoanisole =
2.1/1.1/1; 0.01 mmol Pd(OAc)2 or 1-Pd; the corresponding amount of
b
L. Determined by GC analysis; in parentheses, substrate conversion.
c
d
Ph–B(OH)2/substrate = 2. In square brackets, data after 6 h of
reaction.
To get more insight into the precise structure of 1-Pd, an
X-ray diffraction study was carried out (Scheme 1). Complex
1-Pd was found to adopt a monomeric structure with a
tricoordinate Pd centre. The ma co-ligand is Z2-coordinated.
More striking is the bidentate coordination of the phosphine-
borane through the phosphorus atom and one of the mesityl
rings at boron. The Pd centre establishes close contacts with
the Cipso carbon atom [2.292 A] and one of the Cortho carbon
atoms [2.590 A], while the other carbon atoms lie at 42.9 A.
The geometry of the Mes ring is almost unchanged upon
p-coordination: the involved Cipso–Cortho bond is not elongated
and both C atoms remain in planar environments. The two
isomers observed by NMR in solution most likely result from
the relative orientation of the facing ma and BMes2 groups.
Despite the coordination of the Mes ring, the B atom does not
participate in the coordination. The Pd–B distance (2.898 A) is
rather long and the 2p(B) orbital does not point toward the Pd
centre, precluding Pd-B back-donation. Consistently, the B
atom does not deviate from planarity. Overall, the BMes2
group of 1 adopts Z2-C–C coordination to Pd, and thereby
stabilizes the 14-electron [(phosphine)Pd(ma)] fragment. Such
weak Pd–arene coordinations have been occasionally evidenced
with biaryl-phosphines,18 and they are likely to play a critical
role in the unique behaviour of these ligands in Pd-catalyzed
cross-coupling reactions.6 The bonding situation observed in
no trace of the two conceivable cross-coupling products
Ar-(4-OCH3-C6H4) (where Ar = Ph2P(o-C6H4) or Mes) was
detected. Thus, the BMes2 group of 1 does not compete with
Ph-B(OH)2 in the cross-coupling reaction, probably due to
the substantial steric protection induced by the presence of
the PPh2.
The Pd/1 system was sensitive to the steric hindrance of the
reactants. While for the coupling between 4-bromoanisole and
2-methylphenyl boronic acid up to 75% of the corresponding
cross-coupling product was achieved, less than 20% of cross-
coupling product was obtained using 2,4,6-tris(isopropyl)bromo-
benzene. Pd/1 also activated the C–Cl bond of 4-chloroanisole,
but only giving 27% yield of I (Scheme S1w).
Surprinsigly, the basicity of the phosphorus atom of the
phosphine-borane ligand did not induce an important effect on
the catalytic behaviour (entries 2 and 5). The Pd/PPh3 catalytic
system (entry 6) exhibited a notable lower activity and about
similar chemoselectivity relative to Pd/1 and Pd/2. Hence, the
dimesityl boron moiety enhances the catalytic activity, conversely
to what was found in the Rh-catalyzed hydroformylation of
1-octene.10
With the aim to compare the catalytic behaviour of our
phosphine-boranes 1 and 2 with the outstanding biaryl-
monophosphines described by Buchwald’s group, the Pd/3
system was tested. This catalytic system was more active than
Pd/1 and Pd/2, giving ca. 97% of cross-coupling with a nearly
complete substrate conversion, and was more chemoselective
(entry 7 vs. 2 and 5): only ca. 3% of by-products were obtained
with negligible formation of benzene and anisole. This behaviour
points to a higher robustness of Pd/3 than that exhibited by
Pd/1 and Pd/2.13
To gain more insight into the behaviour of the ortho-
(dimesitylboryl)phenylphosphines in these catalytic couplings,
coordination studies were performed with the aim of characterizing
some catalytically relevant Pd(0) species.14 The stoichiometric
reaction of 1 with [Pd(nbd)(ma)]15 (nbd = norborna-1,4-diene,
ma = maleic anhydride) gave complex 1-Pd as an orange powder
Scheme 1 Synthesis and crystal structure of 1-Pd (ellipsoids drawn at
50% probability; hydrogen atoms omitted for clarity). Selected bond
lengths (A): P2–Pd1 2.2901(11), Pd1–C31 2.292(4), Pd1–C32 2.590(4),
C53–C54 1.406(6).
c
8164 Chem. Commun., 2011, 47, 8163–8165
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complex 1-Pd suggests that some structural analogy can be
drawn between the phosphine-borane 1 and biaryl-phosphines.
It also provides evidence for a new coordination mode
of phosphine-arylboranes. Thus far, arene coordination was
found to systematically support M-B interactions (M = Rh,
Pd, Pt, Cu) resulting in multi-centre Z2-BC or Z3-BCC
coordinations.19
Chem. Commun., 2011, 47, 859; (c) H. Braunschweig and
R. D. Dewhurst, Dalton Trans., 2011, 40, 549.
9 S. Porcel, N. Saffon, G. Bouhadir, L. Maron and D. Bourissou,
Angew. Chem., Int. Ed., 2010, 49, 6186.
10 M. W. P. Bebbington, S. Bontemps, G. Bouhadir, M. J. Hanton,
R. P. Tooze, H. van Rensburg and D. Bourissou, New J. Chem.,
2010, 34, 1556.
11 (a) Y. C. Lai, H. Y. Chen, W. C. Hung, C. C. Lin and F. E.
Hong, Tetrahedron, 2005, 61, 9484; (b) A. Jutand and A. Mosleh,
Organometallics, 1995, 14, 1810.
To verify that complex 1-Pd can be involved in the catalytic
process, it was evaluated in the model reaction chosen for
the Suzuki–Miyaura coupling (entry 8). The cross-coupling
product was obtained in 82% yield. This gives support to the
contribution of weak p-arene coordination to stabilize catalytic
species deriving from ortho-(dimesitylboryl)phenylphosphines,
similarly to that envisioned with biaryl-phosphines.
12 N. Wang, Z. M. Hudson and S. Wang, Organometallics, 2010,
29, 4007.
13 This behaviour can be attributed to some catalyst aggregation,
favouring
(a) L. Djakovitch, M. Wagner, C. G. Hartung, M. Beller and
K. Koehler, J. Mol. Catal. A: Chem., 2004, 219, 121; (b) F. Fernandez,
B. Cordero, J. Durand, G. Muller, F. Malbosc, Y. Kihn, E. Teuma
and M. Gomez, Dalton Trans., 2007, 5572, and references therein.
dehalogenation
and
deboronation
reactions:
´
´
In conclusion, ligands 1 and 2 were successfully applied in
Pd-catalyzed Suzuki–Miyaura C–C couplings. The presence of
the BMes2 moiety in ortho position to phosphorus is compatible
with the cross-coupling process and actually improves the
catalytic activity. This activity enhancement compared to the
Pd/PPh3 system is likely attributed to the ability of the mesityl
group linked to boron to engage into a weak p-coordination to
the metal centre, as shown by the X-ray diffraction analysis
of 1-Pd. These results as a whole further substantiate some
analogy between o-(dimesitylboryl)phenylphosphines and biaryl-
monophosphines20 and thereby open interesting perspectives for
phosphine-borane ligands in catalysis.21
14 The Pd(II) complex [Pd(2)Cl(allyl)] has been previously prepared.
The spectroscopic and structural data indicated the pendant
character of the BMes2 group: S. Bontemps, G. Bouhadir, D. C.
Apperley, P. W. Dyer, K. Miqueu and D. Bourissou, Chem.–Asian J.,
2009, 4, 428.
15 [Pd2(dba)3] was also investigated. See ESIw for experimental
comments.
16 Most recent literature precedents include weak Z2-coordination of
the remote ring of binaphthyl-phosphines ((a) P. G. A. Kumar,
P. Dotta, R. Hermatschweiler, P. S. Pregosin, A. Albinati and
S. Rizzato, Organometallics, 2005, 24, 1306) and binaphthyl-
aminocarbenes ((b) J. Vignolle, H. Gornitzka, B. Donnadieu,
D. Bourissou and G. Bertrand, Angew. Chem., Int. Ed., 2008,
47, 2271) as well as Z2-BCC and Z3-BCC coordinations of the
diphenylborane moiety of an ambiphilic PSB ligand ((c) B. E.
Cowie, D. J. H. Emslie, H. A. Jenkins and J. F. Britten, Inorg.
Chem., 2010, 49, 4060). See also references therein.
This work was financially supported by the Centre National
de la Recherche Scientifique (CNRS), the Universite Paul
´
17 T. W. Hudnall, Y.-M. Kim, M. W. P. Bebbington, D. Bourissou
and F. P. Gabbaı, J. Am. Chem. Soc., 2008, 130, 10890.
Sabatier (UPS) and the COST action CM0802 (PhoSciNet).
R. M. thanks the CNRS for a post-doc grant. We are grateful
to Dr A. L. Llamas-Saiz (Universidade de Santiago de
Compostela, Spain) for the X-ray diffraction analysis of 1-Pd.
¨
18 (a) J. Yin, M. P. Rainka, X.-X. Zhang and S. L. Buchwald, J. Am.
Chem. Soc., 2002, 124, 1162; (b) S. M. Reid, R. C. Boyle,
J. T. Mague and M. J. Fink, J. Am. Chem. Soc., 2003, 125, 7816;
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20 For some selected examples of efficient P ligands mimicking
biaryl-monophosphines, see: (a) S. Harkal, F. Rataboul,
A. Zapf, C. Fuhrmann, T. Reirmeier, A. Monsees and M. Beller,
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21 The possibility of using the Lewis acid moiety of phosphine-
boranes to anchor incoming substrates has been investigated
early on: (a) A. Boerner, J. Ward, K. Kortus and H. Kagan,
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c
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