Mechanistic study on the coupling reaction of aryl bromides with arylboronic acids catalyzed by (iminophosphine)palladium(0) complexes. Detection of a palladium(II) intermediate with a coordinated boron anion

Article abstract of DOI:10.1039/b514405a

The complexes [Pd(η²-dmfu)(P-N)] [P-N = 2-(PPh ₂)C₆H₄-1-CHNR, R = C₆H ₄OMe-4; CHMe₂; C₆H₃Me ₂-2,6; C₆H₃(CHMe₂)-2,6] react with an excess of BrC₆H₄R¹-4 (R¹ = CF₃; Me) yielding the oxidative addition products [PdBr(C ₆H₄R¹-4)(P-N)] at different rates depending on R [C₆H₄OMe-4 > C₆H₃(CHMe ₂)-2,6 > CHMe₂ ≈ C₆H₃Me ₂-2,6] and R¹ (CF₃ ? Me). In the presence of K₂CO₃ and activated olefins (ol = dmfu, fn), the latter compounds react with an excess of 4-R²C₆H ₄B(OH)₂ (R² = H, Me, OMe, Cl) to give [Pd(η²-ol)(P-N)] and the corresponding biaryl through transmetallation and fast reductive elimination. The transmetallation proceeds via a palladium(ii) intermediate with an O-bonded boron anion, the formation of which is markedly retarded by increasing the bulkiness of R. The intermediate was isolated for R = CHMe₂, R¹ = CF₃ and R ² = H. The boron anion is formulated as a diphenylborinate anion associated with phenylboronic acid and/or as a phenylboronate anion associated with diphenylborinic acid. In general, the oxidative addition proceeds at a lower rate than transmetallation and represents the rate-determining-step in the coupling reaction of aryl bromides with arylboronic acids catalyzed by [Pd(η²-dmfu)(P-N)]. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b514405a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Mechanistic study on the coupling reaction of aryl bromides with arylboronic
acids catalyzed by (iminophosphine)palladium(0) complexes. Detection of a
palladium(II) intermediate with a coordinated boron anion
Bruno Crociani,*a Simonetta Antonaroli,a Alessia Marini,a Ugo Matteolib and Alberto Scrivantib
Received 11th October 2005, Accepted 13th February 2006
First published as an Advance Article on the web 7th March 2006
DOI: 10.1039/b514405a
2
=
The complexes [Pd(g -dmfu)(P–N)] [P–N = 2-(PPh2)C6H4-1-CH NR, R = C6H4OMe-4; CHMe2;
C6H3Me2-2,6; C6H3(CHMe2)-2,6] react with an excess of BrC6H4R1-4 (R1 = CF3; Me) yielding the
oxidative addition products [PdBr(C6H4R1-4)(P–N)] at different rates depending on R [C6H4OMe-4 >
C6H3(CHMe2)-2,6 > CHMe2 ≈ C6H3Me2-2,6] and R1 (CF3 ꢀ Me). In the presence of K2CO3 and
activated olefins (ol = dmfu, fn), the latter compounds react with an excess of 4-R2C6H4B(OH)2 (R2 =
2
H, Me, OMe, Cl) to give [Pd(g -ol)(P–N)] and the corresponding biaryl through transmetallation and
fast reductive elimination. The transmetallation proceeds via a palladium(II) intermediate with an
O-bonded boron anion, the formation of which is markedly retarded by increasing the bulkiness of
R. The intermediate was isolated for R = CHMe2, R1 = CF3 and R2 = H. The boron anion is
formulated as a diphenylborinate anion associated with phenylboronic acid and/or as a
phenylboronate anion associated with diphenylborinic acid. In general, the oxidative addition proceeds
at a lower rate than transmetallation and represents the rate-determining-step in the coupling reaction
2
of aryl bromides with arylboronic acids catalyzed by [Pd(g -dmfu)(P–N)].
Introduction
The palladium-catalyzed cross-coupling between organic elec-
trophiles and organoboron compounds (Suzuki–Miyaura reac-
tion) is a very efficient and widely used method for the formation
of carbon–carbon bonds.1 In particular, when aryl halides and
arylboronic acids are used, the reaction yields biaryls,1,2 which
are important intermediates for the synthesis of more complex
molecules widely used as pharmaceuticals, agrochemicals or
advanced materials.
The commonly accepted mechanism of this reaction involves
an initial oxidative addition of the aryl halide to a palladium(0)
species, followed by transfer of the aryl group from the boronic
acid to the palladium(II) centre (transmetallation) and eventually
by reductive elimination to give the biaryl product and regenerate
the catalytically active palladium(0) species (Scheme 1).3
Scheme 1
An alternative mechanism involves the reaction of the arylboronic
acid with the complex [Pd(OR)(Ar)(L)2] in situ formed by ligand
exchange between [PdX(Ar)(L)2] and the base RO− (alkoxide,
hydroxide or carboxylate anion).3
The key step of the catalytic cycle is represented by the particular
type of transmetallation, step (b), because the oxidative addition,
step (a), and reductive elimination, step (c), are also commonly
found in other catalytic reactions leading to carbon–carbon
bond formation, e.g. the coupling of organic electrophiles with
organostannanes (Stille reaction).4 In spite of its importance, the
mechanism of the transmetallation step is not completely clear.
In aqueous alkaline solutions, the transmetallation was proposed
to proceed through the interaction of the hydroxoboronate anion
Following our studies on the catalytic activity of the zerovalent
2
complexes [Pd(g -dmfu)(P–N)] [dmfu = dimethyl fumarate; P–
5
=
N = 2-(PPh2)C6H4-1-CH NR, R = alkyl or aryl group], we
have recently reported that these complexes are quite efficient
catalysts (or catalyst precursors) in the coupling of arylboronic
acids with aryl bromides (◦turnover numbers of up to 105 h−1 in
anhydrous toluene at 110 C, in the presence of K2CO3).6 With
a suitable choice of the reaction conditions, an almost complete
conversion of the substrates to biaryls can be obtained also when
deactivating groups are present on the aryl bromide. These results
prompted us to carry out a mechanistic investigation of the
fundamental steps of the catalytic cycle of Scheme 1 in order to
achieve a better understanding of (i) the factors which affect the
Ar1B(OH)3 with the oxidative addition product [PdX(Ar)(L)2].3
−
aDipartimento di Scienze e Tecnologie Chimiche, Universita` di Roma
”Tor Vergata”, Via della Ricerca Scientifica 00133, Roma, Italy E-mail:
crociani@stc.uniroma2.it; Fax: +39 06 72594328
bDipartimento di Chimica, Universita` di Venezia, Dorsoduro 2137, 30123
Venezia, Italy
2698 | Dalton Trans., 2006, 2698–2705
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Table 1 Completion times for reaction (2) of Scheme 3a
reaction rates and (ii) the mechanism of the transmetallation step,
−
because the formation of the anion Ar1B(OH)3 or the complex
Complex
Aryl bromide
Product
Completion time
[Pd(OR)(Ar)(L)2] can hardly occur under the catalysis conditions
used.
1a
2a
2a
3a
4a
5a
BrC6H4CF3-4
BrC6H4CF3-4
BrC6H4Me-4
BrC6H4CF3-4
BrC6H4CF3-4
BrC6H4CF3-4
1b
2b
5b
3b
4b
1b
ca. 15 min
ca. 55 min
ca. 7 h
ca. 60 min
ca. 35 min
ca. 20 h
Results and discussion
The model reaction chosen for the mechanistic study is reported
in eqn (1):
a For a molar ratio 1a–5a/aryl bromide of 1:10 in toluene at 90 ◦C.
2
−1
=
the m(C O) band the g -bound dmfu at 1685 cm . For the fn
complex 5a, the decreasing intensity of the m(C≡N) band of the
(1)
g -bound olefin at 2203 cm−1 was monitored. In the 31P NMR
2
spectra, the d(31P) singlet of the starting complexes 1a–4a (in the
range 19.3–23.2 ppm) progressively disappears with concomitant
formation of the d(31P) singlet of the products 1b–5b in the range
20.7–28.0 ppm. Even though the reactions are accompanied by
a slight decomposition to palladium metal, the completion times
2
The zerovalent complexes [Pd(g -ol)(P–N)] [ol = dimethyl
fumarate (dmfu) or fumaronitrile (fn); P–N = iminophosphine]
used as catalysts are shown in Scheme 2. Where possible, the
reactions of the single steps of the cycle in Scheme 1 have been
carried out in dry toluene (or toluene-d8) at 90 ◦C in a N2
atmosphere under pseudo-first-order conditions, generally using
a palladium complex/reactant molar ratio of 1:10 with an initial
palladium complex concentration of 1 × 10−2 mol dm−3. The
progress of the reactions was monitored by IR spectroscopy in
the range 2400–1500 cm−1 and by 31P NMR spectroscopy.
evaluated from IR data are very close to those obtained from 31
NMR spectra.
P
As expected, in the reactions of 2a with BrC6H4R1-4 the rate
decreases considerably on going from R1 = CF3 to R1 = Me
in agreement with a lower electrophilic character of the C–Br
carbon atom. The rate of the oxidative addition of BrC6H4CF3-
4 to complexes 1a–5a depends essentially on the electronic and
steric properties of the imino nitrogen substituent R and on the
p-accepting properties of the olefin as it decreases in the order
1a > 4a > 2a ≈ 3a ꢀ 5a. This trend closely parallels that found
for the analogous oxidative additions of IC6H4CF3-4: 1a > 2a
ꢀ 5a.7 From a kinetic study of the latter reactions,7 this trend
was rationalized in terms of electronic effects which reduce the
electron density on the central metal and increase the strength
of the Pd–olefin and Pd–N bonds, on the basis of the following
Step (a): oxidative addition
The reactions studied are summarized in Scheme 3.
With a ten-fold excess of BrC6H4R1-4, the completion times
of Table 1 were evaluated by IR and 31P NMR spectra at
different times.
For the dmfu complexes 1a–4a, the progress of the reaction was
indicated by the increasing intensity of the m(C O) band of free
dmfu at 1725 cm−1 and the concomitant decreasing intensity of
=
Scheme 2
Scheme 3
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2698–2705 | 2699
©

View Article Online
Table 2 Completion times for reaction (3)a
mechanism:
Complex
R
R1
R2
ol
T/min
1b
2b
2b
2b
2b
2b
3b
4b
5b
C6H4OMe-4
CHMe2
CHMe2
CHMe2
CHMe2
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
Me
H
H
H
Me
OMe
Cl
H
H
H
fn
dmfu
fn
fn
fn
fn
fn
fn
fn
ca. 5
(2)
ca. 20
ca. 20
ca. 7
< 5
CHMe2
> 90
ca. 35
ca. 30
ca. 40
C6H3Me2-2,6
C6H3(CHMe2)2-2,6
CHMe2
where S is a solvent molecule and [Pd(g -dmfu)(S)(j1-P–N)] is
an intermediate containing a P-monodentate iminophosphine.
From the data presented it can be seen that steric effects are
also important, but a clear trend cannot be recognized. As a
matter of fact, for the complexes 1a, 3a and 4a, containing N–aryl
substituents, the rate decreases on going from 1a (R = C6H4OMe-
4) to 3a (R = C6H3Me2-2,6), i.e. with increasing the steric bulk of
the N–aryl group, and this effect can be understood on the basis of
the proposed mechanism which involves a solvent assisted rupture
of the Pd–olefin and Pd–N bonds. By contrast, an increased rate
is observed on going from 3a to 4a [R = C6H3(CHMe2)2-2,6]
where the steric requirements of the N–aryl group are further
increased. In a possible rationalization, this may be attributed to
an intramolecular steric clash between the isopropyl groups and
dmfu which would favour the olefin dissociation and/or the Pd–N
bond breaking. However, this explanation needs to be confirmed
by further kinetic measurements.
2
a For initial mixtures 1b–5b/4-R2C6H4B(OH)2/K2CO3/ol in a 1 : 10 : 20 :
1.5 molar ratio at 90 ◦C in toluene.
goes to completion in ca. 20 min when a 2b/ol molar ratio
of 1 : 1.5 is used with ol = dmfu or fn, and also when the
molar ratio 2b/ol is increased to 1 : 10. For this reason and for
2
the greater thermal stability of the complexes [Pd(g -fn)(P–N)]
towards decomposition, fumaronitrile was used as the olefin in
reaction (3). A comparison of the completion times of Table 2
with those of Table 1 shows that the oxidative addition is generally
the rate-determining step in the catalytic cycle of reaction (1) under
the experimental conditions of this work, the only exception being
the coupling of BrC6H4CF3-4 with 4-ClC6H4B(OH)2 when 2a is
used as catalyst.
As commonly accepted,3 the formation of the biaryl and the
complex [Pd(g -ol)(P–N)] occurs through aryl transfer from the
boronic acid to the palladium(II) centre of 1b–5b (transmetalla-
tion) to give a labile intermediate [Pd(C6H4R1-4)(C6H4R2-4)(P–
N)]. The subsequent reductive elimination of the biaryl generates
a coordinatively unsaturated Pd(P–N) fragment which is stabilized
All the complexes 1b–5b have been isolated and characterized
(see Experimental). These palladium(II) derivatives may exist as
two geometrical isomers depending on the relative position of the
bromide and aryl ligands. The available spectroscopic data indicate
the presence of a major isomer (ca. 96%) and of a minor one for 4b
and of a single isomer for the other complexes. A single isomer is
2
also observed for the chloride analogues [PdCl(C6H4R1-4)(P–N)]
2
1
by g -coordination of the olefin. During our experiments, however,
=
[P–N = 2-(PPh2)C6H4-1-CH NCHMe2; R = CF3 (2c), Me (5c)]
signals stemming from an intermediate of the type [Pd(C6H4R1-
4)(C6H4R2-4)(P–N)] were never detected in the 31P NMR spectra
of the reaction mixtures. This implies that reductive elimination
and olefin coordination are much faster than transmetallation. By
contrast, the 31P NMR spectra shows the initial formation of a
palladium(II) intermediate containing a coordinated boron anion
which undergoes the transmetallation step (see later). Thus, the
overall reaction (3) consists of at least three consecutive steps as
shown in Scheme 4.
which can be prepared by different methods (see Experimental).
According to X-ray structural analyses of the related compounds
8
=
[PdI(Ph)(P–N)] [P–N = 2-(PPh2)C6H4-1-CH NR; R = Me, Et],
a configuration with the aryl ligand trans to the imino nitrogen is
proposed for the predominant isomer in the above complexes of
the type [PdX(Ar)(P–N)] (X = Cl, Br). The assignment is further
supported by the low-frequency values of the m(Pd–Cl) bands of
3c and 5c at 284 and 270 cm−1, respectively, indicating that the
chloride ion is trans to a ligand of high trans influence such as the
phosphorus atom of the iminophosphine.9
The formation of an intermediate of the type 6 is particularly
evident in the reaction of 2b with PhB(OH)2. At 25 ◦C and with a
2b/PhB(OH)2 molar ratio increasing from 1 : 3 to 1 : 10, the 31P
NMR spectra at different times show the progressive decrease of
the d(31P) singlet of 2b at 27.7 ppm and the simultaneous increase
of the d(31P) singlet of 6 at 33.2 ppm [spectrum (a) of Fig. 1].
The rate of step (i) in Scheme 4 increases with increasing
concentration of the arylboronic acid, and is also influenced
by the steric requirements of the imino group R and by the
electronic properties of the para substituent R1 on the aryl
ligand. The latter effect is observed in the spectrum (b) of
Fig. 1. Under the same experimental conditions, in the reaction
5b/PhB(OH)2/K2CO3/dmfu (1 : 5 : 10 : 1.5 molar ratio) the
intermediate of type 6 is formed more slowly (ca. 50% after 30 min
from mixing of the reactants). Thus, the rate of step (i) decreases on
Steps (b) and (c): transmetallation and reductive elimination
In dry toluene, the complexes 1b–5b react with arylboronic acids
in the presence of an activated olefin and anhydrous K2CO3
according to eqn (3):
(3)
Despite the presence of solid K2CO3, the progress of the
reactions can be followed by IR and 31P NMR spectroscopy in
toluene and toluene-d8, respectively.
The completion times (Table 2) are not influenced by the
nature of the olefin nor by its concentration: for 2b, the reaction
1
1
=
going from 2b (R CF3) to 5b (R = Me), i.e. when the electron-
withdrawing CF3 substituent on the aryl ligand is replaced by the
2700 | Dalton Trans., 2006, 2698–2705
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Scheme 4 (i) + K2CO3 − KBr, nucleophilic substitution; (ii) transmetallation; (iii) reductive elimination and olefin coordination; Y = boron anion (see
further).
Fig. 2 (a) 31P NMR spectrum of complex 2b in toluene-d8 at 90 ◦C. (b)
and (c) 31P NMR spectra of the reaction mixture 2b/PhB(OH)2/K2CO3/fn
(1 : 10 : 15 : 1.5 molar ratio) in toluene-d8 at 90 ◦C after 8 and 25 min,
respectively, from the mixing of the reactants: ᭹ signal of the intermediate
Fig.
1
(a) 31P NMR spectrum of the reaction mixture
2b/PhB(OH)2/K2CO3/dmfu (1 : 5 : 10 : 1.5 molar ratio) in toluene-d8
at 25 ◦C after 25 min from mixing of the reactants: ᭹ signal of the
intermediate 6, ꢀsignal of the starting complex 2b. (b) 31P NMR spectrum
of the reaction mixture 5b/PhB(OH)2/K2CO3/dmfu (1:5:10:1.5 molar
ratio) in toluene-d8 at 25 ◦C after 25 min from mixing of the reactants: ᭹
signal of the intermediate 6, ꢀ signal of the starting complex 5b.
2
6, ꢁ signal of the product [Pd(g -fn)(P–N)].
6 is not observed during the course of reaction (3). In the 31P
NMR spectra of the reaction mixture at different times (Fig. 3), the
decreasing d(31P) singlet of the starting complex and the increasing
◦
2
electron-donating Me group. At 90 C and with a complex/aryl
d(31P) singlet of the zerovalent product [Pd(g -fn)(P–N)] are the
boronic acid molar ratio of 1 : 10, the formation of 6 is relatively
fast in the reaction of 1b (R = C6H4OMe-4) with PhB(OH)2 and
in the reactions of 2b (R = CHMe2) with 4-R2C6H4B(OH)2 (R2 =
H, Me, OMe, Cl). As can be seen in the spectra (a) and (b) of
Fig. 2, the starting complex 2b has completely disappeared after
8 min from the mixing of the reactants.
only observed signals throughout.
This can be ascribed to a marked rate decrease of step (i) relative
to the subsequent step (ii) brought about by the greater steric
hindrance towards nucleophilic substitution of the bromide ligand
exerted by the bulkier N–aryl groups. From the present data it
appears that formation of the intermediate 6 [step (i)] is rate-
determining in reaction (3) of 3b or 4b with PhB(OH)2, whereas
transmetallation [step (ii)] is rate-determining in reaction (3) of
In the reactions of 3b (R = C6H3Me2-2,6) and 4b [R =
C6H3(CHMe2)2-2,6] with PhB(OH)2, the intermediate of the type
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2698–2705 | 2701
©

View Article Online
Fig. 4 Proposed structures for the intermediate 6b.
and OH groups with the H atom involved in hydrogen-bonding
(lower frequency band). The integration of the 1H NMR spectrum
(CDCl3) in the range 8.2–6.8 ppm gives a value of 35 in good
=
agreement with the number of the phenyl and N CH protons
(34) resonating in that range. Significant structural information
is afforded by the MS spectrum where the peaks at m/z 105 and
165 are assigned to the ions [PhBOH]+ and [Ph2B]+, respectively,
and the cluster of peaks centered at m/z 688 is assigned to
the ion [Pd(OBPh)(C6H4CF3-4)(P–N)]+ [P–N = 2-(PPh2)C6H4-1-
=
CH NCHMe2]. Furthermore, clusters of peaks centered at m/z
436, 582 and 598 are also observed and assigned to the ions [Pd(P–
N)]+, [Pd(C6H4CF3-4)(P–N)]+ and [Pd(OH)(C6H4CF3-4)(P–N)]+,
respectively.
Fig. 3 (a) 31P NMR spectrum of complex 3b in toluene-d8 at 90 ◦C. (b)
and (c) 31P NMR spectra of the reaction mixture 3b/PhB(OH)2/K2CO3/fn
(1 : 10 : 15 : 1.5 molar ratio) in toluene-d8 at 90 ◦C after 12 and 35 min,
respectively, from the mixing of the reactants: ꢀ signal of the starting
In the temperature range −35 ÷ + 25 ◦C, the 19F and 31P NMR
1
spectra show the presence of a sharp singlet only, while the H
NMR spectra are characterized by a single set of resonances.
Accordingly, the intermediate 6b may exist in solution either
with a single structure or with both structures I and II in a fast
interconversion (on the NMR timescale). Furthermore, in the 1H
NMR spectra the methyl protons of the N–CHMe2 group appear
as a single doublet indicating that the coordination plane around
the palladium centre acts as a time-averaged symmetry plane10
for the whole molecule, in accord with the proposed structures.
The 11B NMR spectrum of 6b in Fig. 5 shows two broad signals
at 31.8 and 7.4 ppm, which become somewhat sharper at higher
temperatures.
2
complex 3b, ꢁ signal of the product [Pd(g -fn)(P–N)].
1b with PhB(OH)2 or of 2b with 4-R2C6H4B(OH)2 (R2 = H, Me,
OMe, Cl). In the latter cases, the completion times of Table 2 give
a measure of the reaction rates for the transmetallation step (ii).
It is interesting to note that under comparable steric requirements
the rate increases with increasing electron-withdrawing properties
of the imino substituent R [1b (R = C6H4OMe-4) > 2b (R =
CHMe2)] and with increasing electron-donating properties of the
4-R2 group on the arylboronic acid.
Characterization of the intermediate 6b
The intermediate 6b was isolated as an off-white microcrystalline
solid from the reaction of 2b with PhB(OH)2 in the presence of
K2CO3 (see Experimental). Unfortunately, the crystals proved
unsuitable for X-ray analysis. Therefore, the caracterization is
essentially based on elemental analysis, IR and multinuclear
(1H, 31P, 11B, 19F) NMR spectra along with FAB mass spectra.
The available data suggest a structure containing an O-bonded
boron anion formed by a diphenylborinate anion associated
with a molecule of phenylboronic acid (structure I of Fig. 4)
or by a phenylboronate anion associated with a molecule of
diphenylborinic acid (structure II of Fig. 4).
The IR spectrum in the solid and in CHCl3 solution show the
presence of different OH groups. In the solid, the m(OH) vibrations
are detected as a rather sharp band at 3627 cm−1 and as a much
broader band at 3430 cm−1, while a sharp band at 3596 cm−1
and a broader band at 3410 cm−1 are observed in solution. This
pattern clearly indicates the presence of OH groups with the H
atom not involved in hydrogen-bonding (higher frequency band)
Fig. 5 11B NMR spectrum of 6b in CDCl3 at −10 ◦C.
2702 | Dalton Trans., 2006, 2698–2705
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
From chemical shift considerations, the signal at 31.8 ppm is
assigned to a three-coordinate 11B centre whereas that at 7.4 ppm
is assigned to a four-coordinate 11B centre.11
If our formulation is correct, the presence of the Ph2BO unit
in the structures of the intermediate 6b implies that a Ph/OH
exchange has occurred between two PhBO2 moieties in the course
of the reaction [step (i) of Scheme 4]. We have therefore studied
the interaction of PhB(OH)2 with anhydrous K2CO3 in toluene-d8
[spectra (a) and (b) of Fig. 6].
The intermediacy of 6b in the catalytic cycle is confirmed by
the reaction (2) of Scheme 5, which occurs in ca. 20 min when
the complex is heated at 90 ◦C in toluene in the presence of
fumaronitrile and PhB(OH)2 with a 6b/PhB(OH)2/fn molar ratio
of 1 : 7 : 1.5 [cf. the completion time of ca. 20 min for the reaction
(3) of 2b, under comparable experimental conditions].
Conclusion
From the present mechanistic study it appears that in the cross-
coupling of arylboronic acids with aryl bromides catalyzed by
2
[Pd(g -ol)(P–N)] the oxidative addition of the aryl bromide is
generally the rate-determining step of the catalytic cycle. Thus, the
greater catalytic activity displayed by the zerovalent complexes
with ol = dmfu and with an imino N–aryl group of low steric
requirements, such as Ph and C6H4OMe-4,6 can be related to
the higher rates of their reactions with ArBr. The oxidative
addition products [PdBr(Ar)(P–N)] react with arylboronic acids
in the presence of K2CO3 and of an activated olefin yielding the
corresponding biaryls and regenerating the starting complexes
2
[Pd(g -ol)(P–N)] by transmetallation followed by fast reductive
2
elimination and g -coordination of the olefin. The transmetal-
lation step proceeds through the intermediacy of a complex
containing an O-bonded boron anion which is formulated as an
arylboronate anion associated with a molecule of diarylborinic
acid and/or a diarylborinate anion associated with a molecule of
arylboronic acid.
Experimental
Fig. 6 11B NMR spectra of PhB(OH)2 at 25 ◦C: (a) in toluene-d8; (b) in
toluene-d8 saturated with anhydrous K2CO3; (c) in CDCl3.
1H, 31P and 19F NMR spectra were recorded on a Bruker
AM400 spectrometer operating at 400.13, 161.98 and 376.50 MHz,
respectively. The 11B NMR spectra were recorded on a Bruker
AMX300 spectrometer operating at 96.25 MHz. Chemical shifts
As can be seen, in the presence of K2CO3 the signal of the
phenylboronic acid at 32.0 ppm considerably broadens and a
new broad resonance appears at 7.4 ppm suggesting that a four-
coordinate boron anion is formed. From the close similarity in the
11B chemical shifts, it is likely that such an anion is also present in
the structure of 6b.
In the intermediate, the boron anion is weakly bound to the
metal centre as it can be completely displaced by chloride anions
[reaction (1) of scheme 5].
1
are reported in ppm downfield from SiMe4 for H, from H3PO4
as external standard for 31P, from CFCl3 as external standard
for 19F and from BF3·Et2O as external standard for 11B. The
spectra were run at 25 ◦C except when noted. IR spectra
were recorded on a Perkin–Elmer 983G spectrophotometer. The
FAB mass spectra in a matrix of 3-nitrobenzyl alcohol were
obtained with a VG Quattro Micromass spectrometer. All the
Scheme 5
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2698–2705 | 2703
©

View Article Online
reactions were carried out under N2. Toluene was distilled from
sodium/benzophenone and methanol was dried over magnesium
turnings.12 The aryl bromides BrC6H4R1-4 (R1 = CF3, Me), the
arylboronic acids 4-R2C6H4B(OH)2 (R2 = H, Me, OMe, Cl), the
olefins dmfu and fn, and the anhydrous potassium carbonate are
commercially available and were used without further purification.
Preparation of the intermediate 6b
A mixture of complex 2b (0.33 g, 0.5 mmol), PhB(OH)2 (0.31 g,
2.5 mmol), anhydrous K2CO3 (0.69 g, 5 mmol) and dmfu (0.07 g,
0.5 mmol) in dry toluene (50 cm3) was stirred at room temperature
for 45 min. The insoluble material was filtered off and the solution
◦
was evaporated to dryness at 20 C under reduced pressure. The
2
The complexes [Pd(g -ol)(P–N)] (1a–5a) were prepared as reported
solid residue was dissolved in anhydrous methanol (ca. 6 cm3).
After stirring for a few minutes an off-white product precipitated.
The product was purified by recrystallization from toluene/n-
hexane (1 : 1.5 v/v) at −20 ◦C. The compound must be stored
at −20 ◦C because it decomposes slowly at room temperature.
Freshly recrystallized samples were used for each measurement.
A certain amount of dmfu in the reaction mixture is required
in order to prevent the formation of metallic palladium from
further reactions of the intermediate. The presence of Br− ions
in the insoluble material resulting from the initial filtration of the
reaction mixture in toluene was ascertained by qualitative analysis
with AgNO3.
in the literature.5c,13
Synthesis of [PdBr(C6H4R1-4)(P–N)] (1b–5b)
2
The complex [Pd(g -dmfu)(P–N)] 1a–4a (0.5 mmol) and the aryl
bromide BrC6H4CF3-4 (1.13 g, 5 mmol) were dissolved in toluene
(50 cm3). The mixture was heated at 110 ◦C (5 min for 1a and 4a,
10 min for 2a and 3a) and then rapidly cooled at room temperature
to minimize decomposition. The solvent was removed at reduced
pressure and the solid residue was extracted with CH2Cl2 (2 ×
20 cm3). After addition of activated charcoal and filtration, the
solution was evaporated to about 3 cm3. Upon addition of Et2O,
the product precipitated as a pale-yellow solid. The complex 5b was
prepared in a similar manner starting from 2a (0.58 g, 0.5 mmol)
and BrC6H4Me-4 (1.71 g, 10 mmol) and heating the mixture at
110 ◦C for 20 min. All the compounds were further purified by
recrystallization from CH2Cl2/Et2O.
Any attempt to isolate intermediates of type 6 from analogous
reactions of 2b with 4-R2C6H4B(OH)2 (R2 = Me, OMe, Cl) or of
5b with PhB(OH)2 failed because of the greater solubility of the
intermediates in methanol.
Intermediate 6b. (0.20 g, 45%) (Found C 63.28, H 4.86, N
Complex 1b. (0.24 g, 66%) (Found C 54.18, H 3.40, N 1.94%;
C33H26BrF3NOPPd requires C 54.53, H 3.61, N 1.93%); mmax/cm−1
1.45%; C47H43B2F3NO3PPd requires C 63.72, H 4.89, N 1.58%);
mmax/cm−1 (O–H) 3627 m, 3430 m (br), (B–O) 1322 vs, (C N) 1637
=
=
=
(C N) 1611 ms (Nujol); dH (CDCl3) 8.21 (1 H, s, N CH), 7.8–7.1
(18 H, m, aryl protons), 6.94–6.90 (2 H, m, m-H of C6H4OMe-4),
6.87–6.83 (2 H, m, m-H of C6H4CF3-4), 3.83 (3 H, s, OCH3), dP
(CDCl3) 28.3 (s).
=
m (Nujol); dH (CDCl3) 8.1–6.8 (35 H, m, aryl protons and N CH),
5.20 (1 H, spt, 3J(HH) = 6.4 Hz, CHMe2), 2.20 (0.8 H, s, OH), 1.17
(6 H, d, 3J(HH) = 6.4 Hz, CH3); dP (toluene-d8) 33.2 (s), (CDCl3)
32.8 (s); dF (toluene-d8) −62.01 (s). Selected MS peaks: m/z 105
(21%), 165 (18), 436 (12), 582 (70), 598 (38), 688 (20).
Complex 2b. (0.26 g, 78%) (Found C 52.74, H 3.71, N 2.20%;
C29H26BrF3NPPd requires C 52.53, H 3.95, N 2.11%); mmax/cm−1
Reaction of 6b with aqueous NaCl
=
=
(C N) 1628 ms (Nujol); dH (CDCl3) 8.15 (1 H, s, N CH), 7.8–7.1
(16 H, m, aryl protons), 6.88–6.84 (2 H, m, m-H of C6H4CF3-4),
5.48 (1 H, spt, 3J(HH) = 6.4 Hz, CHMe), 1.25 (6 H, d, 3J(HH) =
6.4 Hz, CH3); dP (CDCl3) 28.7 (s); dF (toluene-d8) −61.54 (s).
A saturated solution of NaCl in water (10 cm3) was added to
a solution of 6b (60 mg, 0.068 mmol) in CH2Cl2 (10 cm3). The
mixture was vigorously stirred for 30 min at room temperature.
The organic layer was separated, dried over anhydrous Na2SO4
and concentrated to a small volume (ca. 3 cm3) at reduced pressure.
Addition of Et2O caused the precipitation of the off-white product
2c which was reprecipitated from a CH2Cl2/Et2O mixture.
Complex 3b. (0.27 g, 75%) (Found C 56.14, H 3.80, N 1.95%;
C34H28BrF3NPPd requires C 56.33, H 3.89, N 1.93%); mmax/cm−1
=
=
(C N) 1622 m (Nujol); dH (CDCl3) 8.27 (1 H, s, N CH), 7.9–7.0
(19 H, m, aryl protons), 6.87–6.83 (2 H, m, m-H of C6H4CF3-4),
2.37 (6 H, s, CH3); dP (CDCl3) 22.7 (s).
Complex 2c. (34 mg, 82%) (Found C 55.94, H 3.95, N 2.25%;
C29H26ClF3NPPd requires C 56.33, H 4.24, N 2.27%); mmax/cm−1
Complex 4b. (0.22 g, 56%) (Found C 58.31, H 4.39, N 1.80%;
C38H36BrF3NPPd requires C 58.44, H 4.65, N 1.79%); mmax/cm−1
=
(C N) 1627 m, (Pd–Cl) 284 m (Nujol); dH (CDCl3) 8.17 (1 H, s,
=
N CH), 7.8–7.1 (16 H, m, aryl protons), 6.90–6.85 (2 H, m, m-H
of C6H4CF3-4), 5.43 (1 H, spt, J(HH) = 6.4 Hz, CHMe), 1.28
=
(C N) 1609 m (Nujol); dH (CDCl3) major isomer: 8.28 (1 H, s,
3
=
N CH), 7.9–7.0 (19 H, m, aryl protons), 6.83–6.79 (2 H, m, m-H
(6 H, d, 3J(HH) = 6.4 Hz, CH3); dP (CDCl3) 30.1 (s).
of C6H4CF3-4), 3.23 (2 H, spt, 3J(HH) = 6.8 Hz), CHMe2), 1.50
(6 H, d, 3J(HH) = 6.8 Hz, CH3) 0.94 (6 H, d, 3J(HH) = 6.8 Hz,
3
=
Synthesis of [PdCl2(2-(PPh2)C6H4-1-CH NCHMe2)]
=
CH3), minor isomer: 8.12 (s, N CH), 3.10 (spt, J(HH) = 6.8 Hz,
3
3
CHMe2), 1.41 (d, J(HH) = 6.8 Hz, CH3), 0.80 (d, J(HH) =
6.8 Hz, CH3); dP (CDCl3) major isomer: 22.1 (s), minor isomer:
23.9 (s).
The iminophosphine 2-(PPh2)C6H4-1-CH NCHMe25c (0.17 g, 0.5
=
mmol) was added to a stirred solution of [PdCl2(N≡CMe)2]14
(0.13 g, 0.5 mmol) in CH2Cl2 (25 cm3). After 30 min the solution
was evaporated to ca. 3 cm3 and diluted with Et2O to precipitate
the product as a yellow microcrystalline solid. The compound was
reprecipitated from CH2Cl2/Et2O (0.22 g, 86.5%) (Found C 51.75,
H 4.21, N 2.65%; C22H22Cl2NPPd requires C 51.94, H 4.36, N
Complex 5b. (0.21 g, 70%) (Found C 56.81, H 4.60, N 2.33%;
C29H29BrNPPd requires C 57.21, H 4.80, N 2.30%); mmax/cm−1
=
=
(C N) 1627 m (Nujol); dH (CDCl3) 8.13 (1 H, s, N CH), 7.8–7.0
(16 H, m, aryl protons), 6.53–6.49 (2 H, m, m-H of C6H4Me-4),
5.47 (1 H, spt, 3J(HH) = 6.4 Hz, CHMe2), 2.06 (3 H, s, CH3), 1.21
(6 H, d, 3J(HH) = 6.4 Hz, CH3); dP (CDCl3) 26.8 (s).
2.75%); mmax/cm−1 (C N) 1624 m, (Pd–Cl) 346 m, 268 m (Nujol);
=
=
dH (CDCl3) 8.06 (1 H, s, N CH), 7.8–6.9 (14 H, m, aryl protons),
2704 | Dalton Trans., 2006, 2698–2705
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
5.59 (1 H, spt, 3J(HH) = 5.6 Hz, CHMe), 1.14 (6 H, d, 3J(HH) =
5.6 Hz, CH3); dP (CDCl3) 32.5 (s).
coupling Reactions, ed. A. de Meijere and F. Diederich, Wiley-VCH,
Weinheim, 2004.
2 S. Stanforth, Tetrahedron, 1998, 54, 263–303; J. Hassan, M. Se´vignon,
C. Gozzi, E. Schulz and M. Lemaire, Chem. Rev., 2002, 102, 1359–1470;
D. A. Horton, G. T. Bourne and M. L. Smythe, Chem. Rev., 2003, 103,
893–930.
=
Synthesis of [PdCl(C6H4Me-4)(2-(PPh2)C6H4-1-CH NCHMe2)]
(5c)
3 N. Miyaura, J. Organomet. Chem., 2002, 653, 54–57, and references
therein.
=
A mixture of [PdCl2(2-(PPh2)C6H4-1-CH NCHMe2)] (0.51 g, 1
4 V. Farina and B. Krishnan, J. Am. Chem. Soc., 1991, 113, 9585–9595;
V. Farina and G. P. Roth, Adv. Met-Org. Chem., 1996, 5, 1–53; A. L.
Casado and P. Espinet, J. Am. Chem. Soc., 1998, 120, 8978–8985; A. L.
Casado, P. Espinet and A. L. Gallego, J. Am. Chem. Soc., 2000, 122,
11771–11782; A. Ricci, F. Angelucci, M. Bassetti and C. Lo Sterzo,
J. Am. Chem. Soc., 2002, 124, 1060–1071; C. Amatore, A. A. Bahsoun,
A. Jutand, G. Meyer, A. Ndedi and L. Ricard, J. Am. Chem. Soc., 2003,
125, 4212–4222.
5 (a) A. Scrivanti, U. Matteoli, V. Beghetto, S. Antonaroli, R. Scarpelli
and B. Crociani, J. Mol. Catal. A: Chem., 2001, 170, 51–56; (b) A.
Scrivanti, U. Matteoli, V. Beghetto, S. Antonaroli and B. Crociani,
Tetrahedron, 2002, 58, 6881–6886; (c) B. Crociani, S. Antonaroli, V.
Beghetto, U. Matteoli and A. Scrivanti, Dalton Trans., 2003, 2194–
2202.
6 A. Scrivanti, V. Beghetto, U. Matteoli, S. Antonaroli, A. Marini, F.
Mandoj, R. Paolesse and B. Crociani, Tetrahedron Lett., 2004, 45,
5861–5864; A. Scrivanti, V. Beghetto, U. Matteoli, S. Antonaroli, A.
Marini and B. Crociani, Tetrahedron, 2005, 61, 9752–9758.
7 B. Crociani, S. Antonaroli, L. Canovese, P. Uguagliati and F. Visentin,
Eur. J. Inorg. Chem., 2004, 732–742.
mmol), 4-MeC6H4B(OH)2 (0.41 g, 3 mmol), anhydrous K2CO3
(1.38 g, 10 mmol) and dmfu (0.14 g, 1 mmol) in dry toluene
(50 cm3) was stirred at room temperature for 30 min. The insoluble
material was filtered off and the solution was evaporated to
dryness at reduced pressure. The solid residue was dissolved in
CH2Cl2 (20 cm3) and the solution was filtered over Celite. Upon
addition of methanol (30 cm3) the solution was concentrated
to a small volume (ca. 5 cm3) and stored at −20 ◦C for 2 h.
The resulting yellowish precipitate was filtered, washed with cold
methanol and recrystallized from CH2Cl2/Et2O to give the off-
white microcrystalline product 5c.
Complex 5c. (0.21, 37%) (Found C 61.31, H 4.86, N 2.49%;
C29H29ClNPPd requires C 61.72, H 5.18, N 2.48%); mmax/cm−1
=
(C N) 1627 m, (Pd–Cl) 270 m (Nujol); dH (CDCl3) 8.16 (1 H, s,
=
N CH), 7.8–7.0 (16 H, m, aryl protons), 6.54–6.50 (2 H, m, m-H
8 L. Crociani, G. Bandoli, A. Dolmella, M. Basato and B. Corain,
Eur. J. Inorg. Chem., 1998, 1811–1820.
9 T. G. Appleton, H. C. Clark and L. E. Manzer, Coord. Chem. Rev.,
1973, 10, 335–422.
10 B. Crociani, S. Antonaroli, M. L. Di Vona and S. Licoccia,
J. Organomet. Chem., 2001, 631, 117–124.
3
of C6H4Me-4), 5.43 (1 H, spt, J(HH) = 6.4 Hz, CHMe2), 2.06
3
(3 H, s, CH3), 1.23 (6 H, d, J(HH) = 6.4 Hz, CH3); dP (CDCl3)
29.3 (s).
11 (a) J. D. Kennedy, in Multinuclear NMR, ed. J. Mason, Plenum, New
York, 1987, ch. 8, pp. 221–258; (b) S. L. Wiskur, J. J. Lavigne, H. Ait-
Haddou, V. Lynch, Y. H. Chiu, J. W. Canary and E. V. Anslyn, Org.
Lett., 2001, 3, 1311–1314.
Acknowledgements
We thank Dr Sergio Tamburini and Dr Laura Crociani of the
Istituto di Chimica Inorganica e delle Superfici, CNR, Padova,
Italy, for recording the 11B NMR spectra.
12 D. D. Perrin and W. L. Armarego, Purification of Laboratory Chemicals,
Pergamon, Oxford, 3rd edn, 1988.
13 S. Antonaroli and B. Crociani, J. Organomet. Chem., 1998, 560, 137–
146; B. Crociani, S. Antonaroli, L. Canovese, F. Visentin and P.
Uguagliati, Inorg. Chim. Acta, 2001, 315, 172–182.
14 The complex [PdCl2(N≡CMe)2] was prepared from reaction of PdCl2
with boiling acetonitrile in the same way as described for the analogue
[PdCl2(N≡CPh)2], see:F. R. Hartley, Organomet. Chem. Rev., Sect. A,
1970, 6, 119–137.
References
1 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457–2483; A. Suzuki,
J. Organomet. Chem., 1999, 576, 147–168; A. Suzuki, J. Organomet.
Chem., 2002, 653, 83–90; N. Miyaura, in Metal-catalyzed Cross-
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2698–2705 | 2705
©