A σ4, λ5-phosphinine palladium complex: A new type of phosphorus ligand and catalyst. Application to the Pd-catalyzed formation of arylboronic esters

Article abstract of DOI:10.1039/b203550b

A palladium(II) complex of a S-P-S pincer ligand featuring a metallated ylidic P-atom efficiently catalyzes the coupling between pinacolborane and various iodoaryls to yield the corresponding arylboronic esters (TON between 5100 and 76500).

Full text of DOI:10.1039/b203550b

A s4, l5-phosphinine palladium complex: a new type of phosphorus
ligand and catalyst. Application to the Pd-catalyzed formation of
arylboronic esters†
Marjolaine Doux, Nicolas Mézailles, Mohand Melaimi, Louis Ricard and Pascal Le Floch*
Laboratoire ‘Hétéroéléments et Coordination’ UMR CNRS 7653, Ecole Polytechnique, 91128 Palaiseau
Cedex, France. E-mail: lefloch@poly.polytechnique.fr
Received (in Cambridge, UK) 11th April 2002, Accepted 7th June 2002
First published as an Advance Article on the web 20th June 2002
A palladium(II) complex of a S–P–S pincer ligand featuring
a metallated ylidic P-atom efficiently catalyzes the coupling
between pinacolborane and various iodoaryls to yield the
corresponding arylboronic esters (TON between 5100 and
76500).
some comments. As expected for a d8 center the overall
geometry around palladium is square planar. Both intracyclic
P–C bond lengths appear to be rather short, attesting the ylidic
character of the ligand. The most significant feature concerns
the external P–C bond distances at 1.758(8) and 1.760(7) which
are relatively short for single P–C bonds. Conversely, the PNS
bonds at 2.031(4) and 2.037(3) are found to be slightly
lengthened suggesting a substantial electronic delocalisation in
each C–P–S moity.
The catalytic activity of 4 was first tested in the classical
Heck reaction in the coupling of methyl–acrylate with iodo-
benzene. Heating a mixture of the compounds at 100 °C in N-
methylpyrolidone for 5 days with 0.01% of catalyst yielded the
methyl trans-cinnamate in quantitative yield. A conversion
yield of 31% was obtained after 4 days with 1 3 1024 mol% of
catalyst under the same experimental conditions. In view of this
promising result and taking into account the importance of
arylboronic esters in organic synthesis, we then turned our
attention on the Myaura cross-coupling process which still
requires quite substantial loading of catalyst even when
iodoaryls are used as substrates.9 Last year, we reported on a
very efficient catalyst which allows this coupling reaction, in
the case of iodoaryls, with a TON up to 3000.10 The catalytic
activity of 4 was investigated in the coupling of pinacolborane
with some iodoaryls (Scheme 2).11 As can be seen in Table 1,
though the reaction proceeds quite slowly, only a low-loading of
catalyst is needed. In the case of iodobenzene, which yielded the
Tertiary phosphines play a central role as ligands in homoge-
neous catalysis where they are essentially used as two-electron
donor ligand via coordination of the phosphorus atom lone
pair.1 Though the use of metal phosphide-based complexes is
well-documented in heterogeneous catalysis,2 complexes in-
3
4
volving a sigma bonding between the metal and a s or s -
coordinated phosphorus atom are still rare.3 This void results
from the high reactivity of the P–metal bond in most metal–
3
phosphide complexes (s compounds) and from the preference
for C–metal bonding in ylidic type structures.4 We recently
found that a possible way to circumvent this limitation is to
incorporate the phosphorus atom in a ylidic delocalized type-
5
structure such as l -phosphinines. Indeed, recent calculations
have showed that these heterocycles possess a mixed ylidic–
aromatic character depending on the substitution scheme of the
5
P-atom.5 Metal complexes of l -phosphinines are very rare and
only one example, partially characterized, was reported by
Dimroth and coworkers in the case of a cyclopentadienyl iron
complex.6
As part of a program aimed at studying mixed phosphinine-
based S–P–S pincer ligands, we recently found that the reaction
of butyllithium with the easily available sulfide 2 (made by
sulfurization of 1)7 resulted in the formation of the correspond-
ing 1-n-butylcyclophosphahexadienylanion 3. Subsequent reac-
tion of 3 with one equivalent of [Pd(COD)Cl2] cleanly affords
the palladium complex 4 which was structurally characterized
(Scheme 1).8 An ORTEP view of one molecule of 4 is presented
in Fig. 1.‡ Complex 4, which was recovered as an orange
powder, is highly robust and can be stored without special
precautions. Thus, no apparent decomposition was observed
after heating 4 with water for hours. The structure of 4 deserves
Fig. 1 Molecular structure of complex 4 (hydrogen atoms have been omitted
for clarity). Selected bond lengths (Å) and angles (°): P(1)–Pd(1) 2.180(2),
Pd(1)–Cl(1) 2.377(3), Pd(1)–S(1) 2.321(2), Pd(1)–S(2) 2.317(2), P(3)–S(2)
2.031(4), P(2)–S(1) 2.037(3), P(1)–C(1) 1.762(7), P(1)–C(5) 1.761(7),
C(1)–P(2) 1.760(7), C(5)–P(3) 1.758(8); P(1)–Pd(1)–S(1) 87.32(8), P(1)–
Pd(1)–S(2) 88.09(8), Cl1–Pd(1)–S(1) 93.84(8).
1
Scheme 1 Reagents and conditions: i, ⁄4 S8, toluene reflux, 12 h; ii, BuLi (1
equiv.), thf, 278 to 25 °C, 10 min; iii, [Pd(COD)Cl2] (1 equiv.), thf, 278
to 25 °C, 15 min.
† Electronic supplementary information (RSI) available: experimental
procedures for a detailed preparation of complex 4 and cross-coupling
experiments. See http://www.rsc.org/suppdata/cc/b2/b203550b/
Scheme 2 Coupling reaction of iodoaryls with pinacolborane.
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This journal is © The Royal Society of Chemistry 2002

Table 1 Myaura cross-coupling reaction of iodoaryls with pinacolborane
using complex 4 as catalyst
4 (a) W. C. Kaska, Coord. Chem. Rev., 1983, 48, 58; (b) H. Schmidbaur,
Angew. Chem., Int. Ed. Engl., 1983, 22, 907.
5 Z.-X. Wang and P. v. Ragué Schleyer, Helv. Chim. Acta, 2001, 84,
Entry Substrate
t
GCa (%) Yieldb (%) TON
1578.
6 T. Dave, S. Berger, E. Bilger, E. Kaletsch, J. Pebler, J. Knecht and K.
Dimroth, Organometallics, 1985, 4, 1565.
7 (a) N. Avarvari, P. Le Floch and F. Mathey, J. Am. Chem. Soc., 1996,
118, 11978; (b) N. Avarvari, L. Ricard, P. Le Floch and F. Mathey,
Organometallics, 1997, 16, 4089.
1
2
3
4
5
6
7
Iodobenzene
4-Iodotoluene
2-Iodotoluene
2-Iodothiophene
2-Iodonaphthalene
15 h
48 h
5 d
5 d
5 d
100
100
50
73
90
95
96
45
68
87
62
45
10000
10000
5000
7300
9000
6600
5100
8 Synthesis of 2 and 4: phosphinine 1 (4.0 g, 6.5 mmol) was heated in
toluene with elemental sulfur (0.415 g, 13 mmol) under reflux for 12 h.
After cooling to room temperature, sulfide 2 was collected by filtration
and washed with toluene: Yield: 4.23 g (96%); 31P NMR (121.5 MHz,
CD2Cl2, 298 K), d 43.4 (AB2, m, 2JPP 115.0 Hz, PB), 253.1 (AB2, PA).
A solution of n-butyllithium (1.6 M in hexane) (3.4 mL, 4.4 mmol) was
added at 280 °C to a solution of sulfide 2 (3 g, 4.4 mmol) in THF (100
mL). After 10 min stirring, the solution was warmed to room
temperature and stirred for an additional 10 min. The complete
formation of intermediate 3 was checked by 31P NMR (121.5 MHz,
THF, 298 K), d 45.8 (d, JPP 156.0 Hz) and –66.2 (t). After cooling to
278 °C, [Pd(COD)Cl2] (1.25 g) was added and the solution was
warmed to room temperature. After evaporation of the solvent and
washings with hexane (3 3 10 mL), complex 4 was recovered as a very
stable orange solid; Yield: 3.29 g (85%); 31P NMR (121.5 MHz,
CD2Cl2, 298K), d 49.17 (AB2, m, 2JPP 87.0 Hz, PB), 55.38 (AB2, m, 2JPP
87.0 Hz, PA). 1H NMR (300 MHz, CD2Cl2, 298 K, TMS), d 1.06 (t, 3JHH
7.4 Hz, 3H, CH3), 1.60 (qt, 3JHH 7.4 Hz, CH2), 2.00 (m, 2H, CH2), 2.26
(m, 2H, CH2), 5.50 (A2X, t, 4JHP 4.7 Hz, 1H, H4), 6.60–7.80 (m, 30H,
CH of C6H5). 13C NMR (75.5 MHz, CD2Cl2, 298 K), d 13.9 (s, CH3),
24.2 (s, CH2), 24.6 (d, 2JCP 2.6 Hz, CH2), 38.0 (m, CH2), 72.5 (ABB’X,
m, C2), 118.7 (AB2X, m, C4H), 127.7–133.3 (m, CH and Cq of C6H5),
139.9 (m, C3), 159.4 (s, Cq of C6H5).
9 (a) See for example: A. Fürstner and G. Seidel, Org. Lett., 2002, 4, 541;
(b) T. Ishiyama, K. Ishida and N. Miyaura, Tetrahedron, 2001, 57, 9813;
(c) O. Baudoin, D. Guénard and F. Guéritte, J. Org. Chem., 2000, 65,
9268; (d) M. Murata, T. Oyama, S. Watanabe and Y. Masuda, J. Org.
Chem., 2000, 65, 164.
10 M. Melaimi, F. Mathey and P. Le Floch, J. Organomet. Chem., 2001,
640, 197.
11 Procedure for coupling reactions: a solution of catalyst 4 (1.0 mg
mL21) in CH2Cl2 was prepared. 100 ml (1 3 1024 mmol) of this solution
were placed in a Schlenk tube and the solvent was evaporated. The
iodoaryl (1.0 mmol), pinacolborane (1 M in THF, 1.5 mmol),
triethylamine (3.0 mmol) in dioxane (4 mL) were then successively
added. The resulting solution was then heated at 80 °C and the progress
of the reaction was monitored by GC. All arylboronic esters were
purified by chromatographic separation on silicagel (pretreated with
Et3N).
4-Bromoiodobenzene 5 d
4-Iodoanisole 5 d
66
51
a Based on GC analysis with external standards. b Isolated yields by column
chromatography; products fully characterized by NMR and MS by
comparison with literature data.
best result, a TON of 76500 was obtained after 10 days using 1
3 1023 mol% of catalyst.
In conclusion, we have developed a straightforward access to
a new type of phosphorus based catalyst involving a sigma
bonding between the phosphorus and the metal. The high
activity of such systems very likely results from the rigidity
provided by the pincer type backbone but also from the
conjugated ring structure which stabilizes the ylide form. Note
that the particular electronic structure of these ligands, which
combines a strong donor binding site (P atom) with two pendant
acceptor ligands (S atoms), may also play an important role.
Studies aimed at elucidating the nature of the active species,
as well as expanding this procedure to arylbromides and
chlorides by modifying the nature of the substituents at
phosphorus or by using a more reactive borane precursor, are
currently in progress.12,13 The use of these new ligands in
coordination chemistry and in other catalytic processes will also
constitute a priority.
Notes and references
‡ Crystal
data
for
[C4Ph2P(PPh2S)2]PdCl·CH2Cl2
(4):
C
45H40ClP3PdS2·CH2Cl2, M = 964.62. orthorhombic, space group Pna21,
a = 19.482(5), b = 24.081(5), c = 9.337(5) Å, U = 4380(3) Å3, T =
150.0(10), Z = 4, Dc = 1.463 g cm23, m = 0.844 cm21, KappaCCD
diffractometer, l(Mo-Ka) = 0.71069 Å, 5292 independent reflections,
wR(F2) = 0.1106, R1 = 0.0475 for 4122 reflections used with Fo > 2s(I)
and 497 parameters. Flack parameter = 0.01(4). CCDC reference number
185527. See http://www.rsc.org/suppdata/cc/b203550b/ for crystallo-
graphic data in CIF or other electronic format.
12 A. Albrecht and G. Van Koten, Angew. Chem., Int. Ed., 2001, 40,
3750.
1 Applied Homogeneous Catalysis with Organometallic Compounds, ed.
B. Cornils and W. A. Herrmann, VCH, Weinheim, 1996.
2 R. Iwamoto and J. Grimblot, J. Adv. Catal., 1999, 44, 417.
3 See for example:K. Kunz, G. Erker, S. Döring, R. Fröhlich and G. Kehr,
J. Am. Chem. Soc., 2001, 123, 6181.
13 Pd P–C–P pincer-based catalysts have been successfully used in the
Suzuki cross-coupling reaction (formation of C–C bonds) but not in the
Myaura cross-coupling process (formation of C–B bonds). For a
reference, see: R. B. Bedford, S. M. Draper, P. Noelle Scully and S. L.
Welch, New. J. Chem., 2000, 24, 745.
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