Rate and mechanism of the oxidative addition of vinyl triflates and halides to palladium(0) complexes in DMF

Article abstract of DOI:10.1021/om030298c

In a coordinating solvent such as DMF, the fast oxidative addition of vinyl triflates to Pd⁰(PPh₃)₄ performed under stoichiometric conditions gives the cationic complexes [(η¹-vinyl)- Pd(PPh₃)₂(DMF)]⁺TfO^-, which have been characterized by conductivity measurements, electrospray mass spectrometry, and NMR spectroscopy, before their decomposition to vinyl-phosphonium salts [vinyl-PPh₃]⁺TfO^- and Pd⁰ complexes. [(η¹-vinyl)Pd(PPh₃)₂(DMF)]⁺ TfO^- complexes are less stable than [(aryl)Pd(PPh₃)₂(DMF)]⁺TfO^- formed in the oxidative addition of aryl triflates to Pd⁰(PPh₃)₄. The rate constant of the oxidative addition of vinyl triflates and bromides to Pd⁰(PPh₃)₄ has been determined and compared to that of aryl triflates and halides. The following reactivity orders are established in DMF: vinyl-OTf ? vinyl-Br > PhBr and vinyl-OTf ? PhOTf.

Full text of DOI:10.1021/om030298c

Organometallics 2003, 22, 4229-4237
4229
Ra te a n d Mech a n ism of th e Oxid a tive Ad d ition of Vin yl
Tr ifla tes a n d Ha lid es to P a lla d iu m (0) Com p lexes in DMF
Anny J utand* and Serge Ne´gri
Ecole Normale Supe´rieure, De´partement de Chimie, UMR CNRS 8640,
24 Rue Lhomond, F-75231 Paris Cedex 5. France
Received April 23, 2003
In a coordinating solvent such as DMF, the fast oxidative addition of vinyl triflates to
Pd0(PPh3)4 performed under stoichiometric conditions gives the cationic complexes [(η1-vinyl)-
Pd(PPh3)2(DMF)]+TfO-, which have been characterized by conductivity measurements,
electrospray mass spectrometry, and NMR spectroscopy, before their decomposition to vinyl-
phosphonium salts [vinyl-PPh3]+TfO- and Pd0 complexes. [(η1-vinyl)Pd(PPh3)2(DMF)]+TfO-
complexes are less stable than [(aryl)Pd(PPh3)2(DMF)]+TfO- formed in the oxidative addition
of aryl triflates to Pd0(PPh3)4. The rate constant of the oxidative addition of vinyl triflates
and bromides to Pd0(PPh3)4 has been determined and compared to that of aryl triflates and
halides. The following reactivity orders are established in DMF: vinyl-OTf . vinyl-Br >
PhBr and vinyl-OTf . PhOTf.
In tr od u ction
electrophiles.6 However, chloride ions are always re-
quired in coupling reactions of vinyl triflates with
organostannanes (Stille reactions), when catalyzed by
Pd0 complexes ligated to PPh3.5c-e A mechanistic inves-
tigation of the oxidative addition of vinyl triflates to
Pd0L4 (L ) PPh3), reported by Scott and Stille5d in 1986
and reinvestigated by Farina et al.7 in 1993, revealed
that unstable noncharacterized cationic [(η1-vinyl)-
PdIILn]+ (n ) 2, 3?) complexes were probably formed in
THF, whereas stable, neutral, well-characterized (η1-
vinyl)PdIIClL2 complexes were generated when the
oxidative addition was performed in the presence of
chloride anions in THF.
Stang et al. have investigated the mechanism of the
oxidative addition of vinyl triflates to Pt0 complexes.8
With Pt0(PPh3)2(η2-CH2dCH2) as the starting material,
in the absence of extra phosphine, the reaction proceeds
in two steps: a first complexation of the active species
Pt0L2 to the CdC bond of the vinyl triflate to form (η2-
vinyl-OTf)Pt0L2 complexes (eq 1), followed by a fast
In a previous work, we established that stable cationic
trans-[(aryl)PdIIL2S]+TfO- (S ) solvent) complexes are
formed in the oxidative addition of aryl triflates to Pd0L4
(L ) PPh3) in coordinating solvents such as DMF.1
Kinetic data on the rate of the oxidative addition have
been reported with the following reactivity order: PhI
. PhOTf > PhBr. The oxidative addition is faster in
the presence of chloride ions and leads to neutral trans-
(aryl)PdIIClL2 complexes.1 trans-(aryl)PdII(OAc)L2 com-
plexes are formed when the oxidative addition is per-
formed with the Pd0 complexes generated from Pd(OAc)2
and PPh3.2 When the oxidative addition of aryl triflates
is performed in THF with a Pd0 complex ligated to the
bidentate ligand dppf as in Pd0(dppf)(η2-CH2dCHCO2Me),
the cationic [(aryl)PdII(dppf)]+ complexes are not stable
at room temperature. However, stable neutral (aryl)-
PdIICl(dppf) and (aryl)PdII(OAc)(dppf) complexes are
formed in the presence of chloride or acetate ions,
respectively.3 Recently, Alcazar-Roman and Hartwig
reported kinetic data on the oxidative addition of aryl
triflates to Pd0(BINAP)2 in THF. Stable cationic com-
plexes [(aryl)Pd(BINAP)(NH2R)]+TfO- (R ) alkyl) are
obtained only in the presence of an amine.4
Vinyl triflates are involved in palladium-catalyzed
reactions with nucleophiles5 and, to a lesser extent, with
* To whom correspondence should be addressed. Fax: +33-1-4432-
3325. E-mail: Anny.J utand@ens.fr.
(1) J utand, A.; Mosleh, A. Organometallics 1995, 14, 1810.
(2) J utand, A.; Mosleh, A. J . Org. Chem. 1997, 62, 261.
(3) J utand, A.; Hii, K. K.; Thornton-Pett, M.; Brown, J . M. Organo-
metallics 1999, 18, 5367.
(4) Alcazar-Roman, L. M.; Hartwig, J . F. Organometallics 2002, 21,
491.
intramolecular rearrangement step (in fact, the real
oxidative addition step), which gives neutral complexes
(5) For seminal works see: (a) Cacchi, S.; Morera, E.; Ortar, G.
Tetrahedron Lett. 1984, 25, 2271. (b) Cacchi, S.; Morera, E.; Ortar, G.
Tetrahedron Lett. 1984, 25, 4821. (c) Scott, W. J .; Crisp, G. T.; Stille,
J . K. J . Am. Chem. Soc. 1984, 106, 4630. (d) Scott, W. J .; Stille, J . K.
J . Am. Chem. Soc. 1986, 108, 3033. For reviews see: (e) Stille, J . K.
Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (f) Scott, W. J .; McMurry,
J . E. Acc. Chem. Res. 1988, 21, 47. (g) Ritter, K. Synthesis 1993, 735.
(6) (a) J utand, A.; Ne´gri, S. Synlett 1997, 719. (b) J utand, A.; Ne´gri,
S. Eur. J . Org. Chem. 1998, 1811.
(7) Farina, V.; Krishnan, B.; Marshall, D. R.; Roth, G. P. J . Org.
Chem. 1993, 58, 5434.
(8) (a) Kowalski, M. H.; Stang, P. Organometallics 1986, 5, 2392.
(b) Stang, P. J .; Kowalski, M. H.; Schiavelli, M. D.; Longford, D. J .
Am. Chem. Soc. 1989, 111, 3347.
10.1021/om030298c CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/13/2003

4230 Organometallics, Vol. 22, No. 21, 2003
J utand and Ne´gri
(η1-vinyl)PtII(OTf)L2 in toluene (eq 2). Cationic com-
plexes [(η1-vinyl)PtIIL3]+TfO- have been formed from
Pt0(PPh3)4, i.e., in the presence of extra phosphine (eq
3),8 and characterized by conductivity measurements in
nitromethane. Kinetic data on the oxidative addition of
vinyl triflates to Pt0(PPh3)4 in toluene have also been
reported.8b The intermediate (η2-vinyl-OTf)Pt0(PPh3)2
complexes (eq 1) have not been characterized, in con-
trast to (η2-vinyl-X)Pt0(PPh3)2 complexes (X ) Cl, Br).8b
measurements to detect ionic species.12,13 The reaction
was performed at 10 °C with 1 equiv of 1a . The
conductivity κ of a solution of Pd0(PPh3)4 (2 mM) in DMF
(residual conductivity: κ0 ) 3 µS cm-1) increased after
addition of 1 equiv of 1a and reached the limiting value
k
lim, which was constant within the time scale investi-
gated here (Figure 1a). This establishes that ionic
species were formed in a fast reaction (t1/2 ) 4 s) as
determined in Figure 1b, which exhibits the very first
time of the experiment of Figure 1a. The ionic species
were actually generated in the oxidative addition, as
attested to by the concomitant disappearance of the Pd0
complex, monitored independently by amperometry
(vide infra) (Figure 1b). The ionic species were stable
at 10 °C, beyond the time required for the oxidative
addition, as demonstrated by the constant limiting value
of the conductivity klim at the end of the reaction (Figure
1a).
The mechanism of the oxidative addition of vinyl
halides to Pd0 complexes ligated by a bisphosphine
ligand has been reported by Brown and Cooley.9 In THF,
they have observed, at low temperatures, the reversible
formation of intermediate complexes (η2-vinyl-X)Pd0L2
(L2 ) dppf; X ) Br, Cl) in the oxidative addition of vinyl
halides to (η2-CH2dCH2)Pd0L2 (L2 ) dppf) (eq 4), with
subsequent formation of stable (η1-vinyl)PdXL2 (L2
)
dppf; X ) Cl, Br, I) (eq 5).9 In a qualitative comparison
The cationic complex [(η1-vinyl)Pd(PPh3)2(DMF)]+
(3a +) generated in the oxidative addition of the vinyl
triflate 1a (eq 6) was characterized by electrospray mass
spectrometry with its counteranion TfO-, as was 3b+
generated from 1b. The oxidative additions were carried
of reactivity, Brown and Cooley have established that
the rate of the internal rearrangement (eq 5) is halogen
dependent. Indeed, the stability of the intermediate (η2-
vinyl-X)Pd0(dppf) complex follows the order vinyl chlo-
ride > vinyl bromide > vinyl iodide. In other words, the
facility of the activation of the C-X bond by the Pd0
follows the same order as for aryl halides; i.e., C-I >
C-Br > C-Cl.9
out in DMF and the electrospray analysis performed
immediately after mixing and dilution in methanol.
Molecular ions at m/z 713 and 767 were observed for
the [(η1-vinyl)Pd(PPh3)2]+ cation in complexes 3a + and
3b+, respectively, with the predicted isotope pattern due
to the Pd atom. Only two PPh3 groups were ligated to
the PdII atom.
However, kinetic data on the rate of the oxidative
addition of vinylic electrophiles with Pd0 complexes are
scarce. Kinetic data on the comparative reactivity of
Pd0(PPh3)4 with (Z)- and (E)-â-bromostyrene in DMF are
available.10 The reversibility of the oxidative addition
of (Z)- and (E)-1,2-dichloroethylene to Pd0 complexes has
also been established.11 To our knowledge, nothing is
known on the reactivity of vinyl triflates with Pd0
complexes. We report herein some results on the kinet-
ics of the oxidative addition of vinyl triflates to Pd0(PPh3)4
The cationic complexes 3a + and 3b+ were then
1
characterized by H NMR spectroscopy. The oxidative
addition of 1a (1 equiv) to Pd0(PPh3)4 in DMF-d7 was
monitored in an NMR tube. The reaction led to complex
3a +, for which the vinylic and aliphatic protons were
shifted upfield when compared to the protons of 1a (see
Experimental Section). The counteranion TfO- was also
characterized by 19F NMR spectroscopy by comparison
with an authentic sample of nBu4NOTf. The complex
3a +TfO- was the only detectable complex and was
(which is a catalytic precursor for Stille reactions5c-g
)
1
stable for at least 30 min at 25 °C. After 2 h, the H
and a comparison with that of vinyl bromides in DMF.
The cationic complexes trans-[(η1-vinyl)Pd(PPh3)2-
(DMF)]+TfO- have been characterized.
NMR spectrum exhibited, besides the signals of 3a +, a
new set of signals in which the protons of the butyl
group were located closed to those of the butyl group of
1a , but two vinylic protons were located at lower field
than the two vinylic protons of 1a (see Experimental
Section). This new set of signals which appeared at long
times was assigned to the vinylphosphonium salt14
[CH2dC(nC4H9)-PPh3]+TfO- (5a +TfO-), whose cation
was also characterized by electrospray mass spectrom-
etry (m/e 345). After 17 h, the ratio 3a +:5a + was equal
to 1:0.8, indicating that after 17 h the cationic complex
3a + was still the major complex in solution.
Resu lts a n d Discu ssion
Evid en ce of th e F or m a tion of Ca tion ic [(η1-
vin yl)P d (P P h 3)2(DMF )]+ Com p lexes in th e Oxid a -
tive Ad d ition of Vin yl Tr ifla tes to P d 0(P P h 3)4 in
DMF . Since DMF appears to be a good coordinating
solvent for cationic PdII complexes,1 the oxidative ad-
dition of the vinyl triflate 1a (Table 1) to Pd0(PPh3)4 was
performed in DMF and monitored by conductivity
(12) For the characterization of ionic coordination compounds by
conductivity measurements see: Geary, W. J . Coord. Chem. Rew. 1971,
7, 81.
(13) For the use of conductivity measurements for the characteriza-
tion of cationic organopalladium(II) complexes and the determination
of kinetic data see: J utand, A. Eur. J . Inorg. Chem. 2003, 2017.
(14) For a related vinylphosphonium salt see ref 16.
(9) Brown, J . M.; Cooley, N. A. J . Chem. Soc., Chem. Commun. 1988,
1345.
(10) Fauvarque, J . F.; J utand, A. J . Organomet. Chem. 1981, 209,
109.
(11) Amatore, C.; Azzabi, M.; J utand, A. J . Am. Chem. Soc. 1991,
113, 1670.

Addition of Vinyl Triflates and Halides to Pd(0)
Organometallics, Vol. 22, No. 21, 2003 4231
Ta ble 1. 31P NMR Sh iftsa of (η1-vin yl)P d II Com p lexes F or m ed in th e Oxid a tive Ad d ition of Vin yl Tr ifla tes
or Br om id es to P d 0(P P h 3)4 in DMF (L ) P P h 3)
a
b
At 101.2 MHz in DMF containing 10% of acetone-d6. δ values are referred to H3PO4 as external reference. Vinyl-OTf:Pd0(PPh3)4 )
1:1. c Free PPh3 was also detected at -5.27 ppm with an integration close to that of [(η1-vinyl)Pd(PPh3)2(DMF)]+. Preferably added to
d
Pd0(PPh3)4. Free PPh3:PPh3 of [(η1-vinyl)Pd(PPh3)2(DMF)]+ ) 3. e Vinyl-Br:Pd0(PPh3)4 ) 30:1. f Two tiny doublets were observed at δ
29.83 (d, J PP ) 33 Hz) and 32.90 (d, J PP ) 33 Hz) ppm and assigned to (η2-2c)Pd0(PPh3)2. Free PPh3 was also detected at -5.30 ppm
g
with an integration close to that of (η1-vinyl)PdBr(PPh3)2. Added as nBu4NBr to Pd0(PPh3)4 prior to the addition of 1c or 2c.
h
The oxidative addition of 1a to Pd0(PPh3)4 performed
under stoichiometric conditions was also monitored by
31P NMR spectroscopy in DMF. Fifty minutes after
mixing, the spectrum exhibited one singlet assigned to
the cationic complex 3a +TfO- (22.1 ppm), the singlet of
the vinylphosphonium salt 5a +TfO- (24.7 ppm) with the
ratio 3a +:5a + ) 1:0.3, and the singlet of the free PPh3
(-5.16 ppm) (eq 6). The presence of a singlet indicates
that the two PPh3 ligands of 3a + sit in a trans position
on the Pd atom. The fourth coordination site is occupied
by the DMF. The 31P NMR singlet of the vinylphos-
phonium species 5a + increased with time at the expense
of that of complex 3a +, while the signal of the free PPh3
was no longer detected; instead, a broad signal was
found at 3.2 ppm characteristic of Pd0(PPh3)3 in equi-
librium with Pd0(PPh3)2 and PPh3.15 Therefore, the Pd0
complex disappeared and the free PPh3 appeared at
short times (oxidative addition, eq 6); then at longer
times, the Pd0 complex appeared again and the free
PPh3 disappeared (eq 7).
It is thus established that a fast oxidative addition of
a vinyl triflate to Pd0(PPh3)4 generates the cationic
complex [(η1-vinyl)Pd(PPh3)2(DMF)]+ (eq 6), which slowly
decomposes to a vinylphosphonium salt and a Pd0
(15) Amatore, C.; Khalil, F.; J utand, A.; M’Barki, M. A.; Mottier, L.
Organometallics 1993, 12, 3168.

4232 Organometallics, Vol. 22, No. 21, 2003
J utand and Ne´gri
spectrometry. The cation of the vinylphosphonium salt
5b+TfO- was also detected (m/e 399).
The oxidative addition was then monitored by 1H
NMR in DMF-d7 under stoichiometric conditions. Fif-
teen minutes after mixing, the cationic complex 3b+ was
formed in 26% yield and traces of the vinylphosphonium
species 5b+ were already detected.14 The color of the
solution turned progressively from orange to yellow.
After 1 h, the three compounds 1b, 3b+, and 5b+ (easily
detected by their vinylic protons at 5.92, 5.18, and 6.85,
respectively) were present in equal amounts. After 3 h,
only the vinylphosphonium species 5b+ was detected.
Therefore, the oxidative addition of 1b was slower than
that 1a (vide supra, Table 2) and decomposition of 3b+
to 5b+ occurred within a similar time scale. The oxida-
tive addition was monitored by 31P NMR spectroscopy
in DMF. One hour after mixing, two singlets assigned
to 3b+ (22.66 ppm) and 5b+ (23.2 ppm) were observed.
The free PPh3 was not detected, but a broad signal was
found characteristic of Pd0(PPh3)n complexes15 gener-
ated with the vinylphosphonium salt in a reaction
similar to that reported in eq 7.
F igu r e 1. Kinetics of the oxidative addition of CH2d
C(nBu)-OTf (1a ; 2 mM) to Pd0(PPh3)4 (2 mM) in DMF at
10 °C. (a) Kinetics of the formation of [CH2dC(nBu)Pd-
(PPh3)2(DMF)]+TfO- (3a +TfO-): variation of the conductiv-
ity κ versus time. κ ) κexp - κ0 (κexp ) experimental
conductivity at t; κ0 ) initial residual conductivity of 3 µS
cm-1). (b) Variation of the molar fraction of Pd0(PPh3)3
versus time (-), monitored by amperometry and variation
of the molar fraction of the ionic species 3a +TfO- versus
time (- - -), monitored by conductivity measurements (0 <
t < 60 s in Figure 1a).
The complexes [(η1-vinyl)Pd(PPh3)2(DMF)]+TfO-
formed in the stoichiometric oxidative addition of vinyl
triflates 1a -c to Pd0(PPh3)4 were all characterized in
DMF by a 31P NMR singlet (Table 1, entries 1-3),
attesting to a trans geometry for the cationic PdII
complex (eq 8). The complex 3c+TfO- formed in the
complex (eq 7). The formation of vinylphosphonium salts
from PPh3 and vinyl triflates, catalyzed by Pd0 com-
plexes, has been reported by Stang et al.16 It requires
excess PPh3 and refluxing THF. We have now evidence
that the vinylphosphonium salt is generated from the
cationic complex 3a + together with a Pd0 complex (eq
7), probably by a reductive-elimination step. This Pd0
complex can only be observed when the oxidative
addition is performed under stoichiometric conditions.
Indeed, if the vinyl triflate is in excess, the Pd0 formed
together with the vinylphosphonium salt undergoes a
second fast oxidative addition with the excess vinyl
triflate. As a result, we observe a catalytic transforma-
tion of the vinyl triflate to the vinylphosphonium salt,
limited by the number of PPh3 groups delivered by
Pd0(PPh3)4. Consequently, the stable cationic complex
[(η1-vinyl)Pd(PPh3)2(DMF)]+, which needs to be stabi-
lized by at least two PPh3 groups, has little chance of
being detected and characterized when the vinyl triflate
is in excess, because all the stabilizing PPh3 ligands will
be transformed into the vinylphosphonium salt.
Pd0(PPh3)4 + vinyl-OTf f
trans-(η1-vinyl)Pd(PPh3)2(DMF)+ + TfO- + 2PPh3
(8)
oxidative addition of 1c to Pd0(PPh3)4 seems to be the
more stable of the series. When the oxidative addition
was performed in the presence of 4 equiv of PPh3, the
singlet of 3c+TfO- was still observed at short times
(Table 1, entries 3 and 4) with a ratio of the free PPh3
and the ligated PPh3 equal to 3. This excludes the
formation of the three-phosphine-ligated complex [(η1-
vinyl)Pd(PPh3)3]+ as was the case for analogous plati-
num complexes (eq 3) in toluene.8a
Consequently, in a coordinating solvent such as DMF,
cationic trans-[(η1-vinyl)Pd(PPh3)2(DMF)]+ complexes
are generated with TfO- as the counteranion in the
oxidative addition of vinyl triflates to Pd0(PPh3)4 (eq 8).
However, such complexes are less stable than [(aryl-
PdL2(DMF)]+TfO- complexes.1 They decompose to the
vinylphosphonium salt with formation of Pd0 complexes.
The time scale of this decomposition may be close to that
of the oxidative addition.
For m ation of Neu tr al (η1-vin yl)P dBr (P P h 3)2 Com -
p lexes in th e Oxid a tive Ad d ition of Vin yl Br om id e
to P d 0(P P h 3)4 in DMF . The oxidative addition of the
vinyl bromide 2c with Pd0(PPh3)4 was investigated by
31P NMR spectroscopy in DMF. The neutral trans-(η1-
vinyl)PdBr(PPh3)2 complex 4c was expected (eq 9), as
The molar conductivity of 3a +TfO- was calculated in
DMF at 10 °C: ΛM ) 25 ((2) S cm2 mol-1. This value
is indicative of a 1:1 electrolyte in solution in DMF.12
As mentioned above, the complex 3b+TfO- formed in
the oxidative addition of the vinyl triflate 1b (1 equiv)
to Pd0(PPh3)4 was characterized by electrospray mass
(16) (a) Kowalski, M. H.; Hinkle, R. J .; Stang, P. J . J . Org. Chem.
1989, 54, 2783. (b) Hinkle, R. J .; Stang, P. J .; Kowalski, M. H. J . Org.
Chem. 1990, 55, 5033.

Addition of Vinyl Triflates and Halides to Pd(0)
Organometallics, Vol. 22, No. 21, 2003 4233
Ta ble 2. Kin etics of th e Oxid a tive Ad d ition of Vin yl Tr ifla tes a n d Br om id es to P d 0(P P h 3)4 (2 m M) in DMF
a t 30 °Ca
a
Scheme 1 for X ) Br and Scheme 2 for X ) OTf.
reported for related vinyl bromides in nondissociative
solvents.10,17
differed from the singlet of the cationic complex 3c+
(22.66 ppm) formed in the oxidative addition of the vinyl
triflate 1c to Pd0(PPh3)4 (eq 10, entry 3 in Table 1).
Indeed, besides the signal of the free PPh3, a singlet
was observed at 23.37 ppm (Table 1, entry 6) which
The singlet at 23.37 ppm was not affected by the
addition of 10 equiv of bromide ions (Table 1, entry 6).
It characterizes the neutral complex 4c formed in the
oxidative addition (eq 9) which, at the NMR concentra-
(17) (a) Mann, B. E.; Shaw, B. L.; Tucker, N. I. J . Chem. Soc. A
1971, 2667. (b) Loar, M. K.; Stille, J . K. J . Am. Chem. Soc. 1981, 103,
4174.

4234 Organometallics, Vol. 22, No. 21, 2003
J utand and Ne´gri
Sch em e 1. Mech a n ism of th e Oxid a tive Ad d ition
of Vin yl Br om id es to P d 0(P P h 3)4 in DMF
Pd0(PPh3)4 was investigated by amperometry at 30 °C.
The oxidation peak of Pd0(PPh3)3 at +0.06 V vs SCE
disappeared after addition of a vinyl bromide. The
kinetics of the oxidative addition was then monitored
by amperometry at a rotating gold disk electrode polar-
ized at +0.35 V, on the plateau of the oxidation wave of
Pd0(PPh3)3.1,19 After addition of the vinyl bromide, the
decay of the oxidation current i of Pd0(PPh3)3 (propor-
tional to its concentration) was recorded versus time
(Figure 2a).
F igu r e 2. Kinetics of the oxidative addition of (E)-â-
bromostyrene (2h ; 70 mM) to Pd0(PPh3)4 (C0 ) 2 mM) in
DMF at 30 °C. (a) Variation versus time of the oxidation
current i of Pd0(PPh3)3 at a rotating gold disk electrode (i.d.
2 mm, ω ) 105 rad s-1) polarized at +0.35 V, on the plateau
of the oxidation wave of Pd0(PPh3)3. (b) Plot of 2 ln x - x +
1 versus time (x ) [Pd0L3]/[Pd0L3]0 ) i/i0 with i ) oxidation
current of Pd0L3 at time t, i0 ) initial oxidation current of
Pd0L3, and L ) PPh3). 2 ln x - x + 1 ) -kexpt.
By analogy to related metals and ligands (Pt0/PPh3
8b
and Pd0/dppf9), (η2-vinyl-Br)Pd0(PPh3)2 complexes are
probably formed as intermediate complexes in the
course of the oxidative addition (eq 14). The mechanism
of the oxidative addition is then as proposed in Scheme
1.According to Scheme 1, the kinetic law of the overall
oxidative addition (eq 16) is given by eq 17.20a In the
tion (16 mM), did not dissociate in DMF to the cationic
complex 3c+ (eq 11).18
d[Pd0L3]/dt ) -kapp[Pd0L3][vinyl-Br] )
-kBrK1K2[Pd0L3][vinyl-Br]/[L] (17)
presence of excess vinyl bromide (pseudo-first-order
conditions) and taking into account the fact that PPh3
is continuously released in reaction 13 when the oxida-
tive addition proceeds, the integration of eq 17 gives eq
18 (x ) [Pd0L3]/[Pd0L3]0 ) i/i0 with i ) oxidation current
The oxidative addition of the vinyl bromides 2g,h 10,17b
and 2i to Pd0(PPh3)4 was also monitored by 31P NMR
spectroscopy in DMF at 25 °C. The singlet of the free
phosphine was detected together with the singlet of
trans-(η1-vinyl)PdBr(PPh3)2 (eq 12) with similar inte-
gration (Table 1).
2 ln x - x + 1 ) -kBrK1K2[vinyl-Br]t/C0 (18)
of Pd0L3 at time t and i0 ) initial oxidation current of
Pd0L3). When the oxidative addition was performed from
(E)-â-bromostyrene (2h ), the plot of 2 ln x - x + 1 versus
time was linear (Figure 2b): 2 ln x - x + 1 ) -kexpt.
Plotting kexp versus (E)-â-bromostyrene (2h ) concentra-
tion afforded a straight line (Figure 3), which confirms
that the reaction order in the vinyl bromide is 1 (eq 18).
Consequently, kexp ) kBrK1K2[vinyl-Br]/C0. The slope of
this straight line allows the determination of kBrK1K2
(s-1) (Table 2), which characterizes the reactivity of the
Pd0(PPh3)4 + vinyl-Br f
trans-(η1-vinyl)PdBr(PPh3)2 + 2PPh3 (12)
Ra te of th e Oxid a tive Ad d ition of Vin yl Br o-
m id es to P d 0(P P h 3)4 in DMF . Pd0(PPh3)4 (C0 ) 2 mM)
dissociates in DMF to the major complex Pd0(PPh3)3,
which then affords SPd0(PPh3)2 as the reactive complex
in oxidative additions (eq 13).1,15,19 The kinetics of the
oxidative addition of vinyl bromides 2c and 2g-i with
(20) (a) The expression of the reaction rate is v ) k1k2kX[Pd0L3][vinyl-
X]/(k-1k-2[L] + k-1kX[L] + k2kX[vinyl-X]). k1 and k2 are the forward
rate constants and k-1 and k-2 the backward rate constants of the
equilibrium in eq 13 and 14, respectively, and X ) Br. Taking into
account the fact that the equilibrium in eq 13 is fast and is in favor of
Pd0L3 (k-1[L] . k2[vinyl-X]), the general equation simplifies into v )
k1k2kX[Pd0L3][vinyl-X]/{k-1[L](k-2 + kX)}, which in turn simplifies to
eq 17 (see text) when considering an equilibrium in eq 14 (k-2 . kX).
(b) See ref 20a for the expression of the rate law with X ) OTf.
(18) In the course of the oxidative addition of 2c with Pd0(PPh3)4,
two tiny doublets were also observed at 39.83 and 32.90 ppm (J PP
)
33 Hz) together with the singlet of complex 4c. They may characterize
the intermediate complex (η2-2c)Pd0(PPh3)2 observed in related oxida-
tive addition of vinyl bromides.8b,9
(19) Fauvarque, J . F.; Pflu¨ger, F.; Troupel, M. J . Organomet. Chem.
1981, 209, 109.

Addition of Vinyl Triflates and Halides to Pd(0)
Organometallics, Vol. 22, No. 21, 2003 4235
F igu r e 3. Kinetics of the oxidative addition of (E)-â-
bromostyrene (2h ) to Pd0(PPh3)4 (C0 ) 2 mM) in DMF at
30 °C. Reaction order in (E)-â-bromostyrene: plot of kexp
(determined in Figure 2b) versus (E)-â-bromostyrene con-
centration. kexp ) kBrK1K2[vinyl-Br]/C0 (Scheme 1).
Ch a r t 1. Rela tive Rea ctivity of Vin yl Br om id es in
Th eir Oxid a tive Ad d ition to P d 0(P P h 3)4 in DMF a t
30 °C
F igu r e 4. Kinetics of the oxidative addition of the vinyl
triflate 1d (2 mM) to Pd0(PPh3)4 (C0 ) 2 mM) in the
presence of PPh3 (60 mM) in DMF at 30 °C. (a) Variation
versus time of the oxidation current i of Pd0(PPh3)3 at a
rotating gold disk electrode (i.d. 2 mm, ω ) 105 rad s-1
)
vinyl bromide in the oxidative addition to Pd0(PPh3)4
in DMF. The value of kapp ) kBrK1K2/[L] (M-1 s-1) (eq
16) was also calculated using an average value21 of [L]
) 1.5C0 (Table 2). However, the fact that the experi-
mental data fit the kinetic law in eq 18 does not prove
definitively the existence of the intermediate equilibri-
um in eq 14. Indeed, were the (η1-vinyl)PdBrL2 complex
directly generated by a reaction of Pd0L2 with the vinyl
bromide with a rate contant k′Br, the rate law would be
2 ln x - x + 1 ) - k′BrK1[vinyl-Br]t/C0: i.e., mathemati-
cally identical with that expressed in eq 18.22
An order of reactivity has been established (Chart 1)
for the series of vinyl bromides investigated here (the
highest overall rate constant is taken as a reference),
evidencing that the oxidative addition is sensitive to
steric hindrance: the overall rate constant of the
reaction decreases when the steric hindrance increases
(compare (Z) and (E)-â-bromostyrene) and is affected by
electronic effects (the most electron rich CdC bond in
2c is the less reactive one). This is indicative of the
complexation of the CdC bond of the vinyl bromide to
Pd0(PPh3)2 followed by the intramolecular oxidative
addition (Scheme 1).
Ra te of th e Oxid a tive Ad d ition of Vin yl Tr ifla tes
to P d 0(P P h 3)4 in DMF . The oxidative addition of the
vinyl triflates 1a (1 equiv) with Pd0(PPh3)4 (C0 ) 2 mM)
in DMF was monitored at 10 °C by two analytical
techniques. The disappearance of Pd0(PPh3)4 in the
oxidative addition was monitored by amperometry
(Figure 1b), whereas the kinetics of formation of the
cationic complex 3a +TfO- was monitored by conductiv-
ity measurement (Figure 1b). Comparison of the two
kinetics shows that the time scale of both reactions was
polarized at +0.35 V, on the plateau of the oxidation wave
of Pd0(PPh3)3. (b) Plot of 1/x versus time (x ) [Pd0L3]/
[Pd0L3]0 ) i/i0 with i ) oxidation current of Pd0L3 at time
t, i0 ) initial oxidation current of Pd0L3, and L ) PPh3).
1/x ) kOTfK1K2C0t/[L] + 1 (Scheme 2).
the same. The non-S-shaped form of the kinetic curve
featuring the formation of the ionic species (Figure 1b)
also indicates that no neutral intermediate complexes,
either Pd0 or neutral PdII complexes, accumulated in
significant amounts in the course of the overall oxidative
addition (eq 21). This indicates that, on one hand, the
intermediate Pd0 complex (η2-vinyl-OTf)Pd0(PPh3)2 (eq
19) (if formed) behaved as a transient species which
obeys steady-state kinetics. On the other hand, if the
intermediate neutral PdII complex (η1-vinyl)Pd(OTf)-
(PPh3)2 was formed, its dissociation to (η1-vinyl)Pd-
(PPh3)2(DMF)+ and TfO- would be faster than its rate
of formation.
The oxidative additions of the vinyl triflates 1a -f
investigated here were faster than those performed from
vinyl bromides (vide supra). To compare the reactivity
of vinyl triflates to that of vinyl halides, which was
determined at 30 °C, and to observe reasonable reaction
times at 30 °C, the oxidative addition of vinyl triflates
was investigated in the presence of 30 equiv of PPh3,
so as to decrease the rate of the overall oxidative
addition by shifting the equilibrium in eq 13 toward its
left-hand side, i.e. by decreasing the concentration of
the active complex SPd0(PPh3)2.19 Under such condi-
tions and using only 1 equiv of the vinyl triflate per
Pd0(PPh3)4, reasonable time scales were observed (Fig-
ure 4a for the oxidative addition of 1d ). The reactions
were still fast enough to avoid any interference of the
Pd0 complexes (generated by the decomposition of the
cationic complex trans-[(η1-vinyl)Pd(PPh3)2(DMF)]+ into
the vinylphosphonium salt), which could have under-
(21) [L] varied from the initial C0 to 2C0 at the end of the oxidative
addition.
(22) The problematic complexation of the active Pd0 complex to aryl
halides before the oxidative addition step has been discussed in detail
by Alcazar-Roman and Hartwig.4

4236 Organometallics, Vol. 22, No. 21, 2003
J utand and Ne´gri
complexation of Pd0(PPh3)2 with the CdC bond of the
vinyl triflate compared to the vinyl bromide.22
The vinyl triflates are considerably more reactive than
PhOTf (kapp ) 1.7 × 10-3 M-1 s-1 in DMF at 20 °C)1
and even more reactive than PhI (kapp ) 17 M-1 s-1 in
DMF at 20 °C). The vinyl bromides investigated here
are also more reactive than PhBr (kapp ) 10-3 M-1 s-1
in DMF at 20 °C).1 The following reactivity orders have
been established in DMF:
Sch em e 2. Mech a n ism of th e Oxid a tive Ad d ition
of Vin yl Tr ifla tes to P d 0(P P h 3)4 in DMF
vinyl-OTf . vinyl-Br > PhBr
vinyl-OTf > PhI . PhOTf > PhBr
Con clu sion
As for aryl triflates, the oxidative addition of vinyl
triflates to Pd0(PPh3)4 in a coordinating solvent such as
DMF gives the cationic complexes trans-[(η1-vinyl)Pd-
(PPh3)2(DMF)]+TfO-. The latter were characterized by
conductivity measurements, electrospray mass spec-
trometry, and NMR spectroscopy when the oxidative
addition was performed under stoichiometric conditions.
They are less stable than trans-(aryl)Pd(PPh3)2(DMF)]+-
TfO- (formed in the oxidative addition of aryl triflates
to Pd0(PPh3)4), since they decompose to phosphonium
salt [vinyl-PPh3]+TfO- and Pd0 complexes.
gone a second oxidative addition with the vinyl triflate
and disturbed the kinetics.
According to Scheme 2, the kinetic law of the overall
oxidative addition (eq 21) is given by eq 22.20b In the
d[Pd0L3]/dt ) -kapp[Pd0L3][vinyl-OTf] )
-kOTfK1K2[Pd0L3][vinyl-OTf]/[L] (22)
The rate constants of the oxidative addition of vinyl
triflates and bromides to Pd0(PPh3)4 have been deter-
mined. Vinyl triflates are considerably more reactive
than vinyl bromides. Vinyl triflates are also more
reactive than phenyl triflate. Vinyl bromides are more
reactive than phenyl bromide.
presence of excess PPh3 and 1 equiv of the vinyl triflate,
the disappearance of Pd0(PPh3)4 (C0 ) 2 mM) follows
the kinetic law of eq 23 (x ) [Pd0L3]/[Pd0L3]0 ) i/i0 with
1/x ) kOTfK1K2C0t/[L] + 1
(23)
As already reported, the neutral complexes (η1-vinyl)-
PdCl(PPh3)2 were formed when the oxidative addition
was performed in the presence of chloride ions.1,5d
Whereas an accelerating effect of chloride ions on the
rate of the oxidative addition of the poorly reactive aryl
triflates was observed,1 the accelerating effect on the
highly reactive vinyl triflates is still observed but is very
low. Consequently, the positive effect of chloride ions
in catalytic Stille reactions involving vinyl triflates and
organostannanes5c-g does not originate in the oxidative
addition, which is probably not rate determining, but
rather in bypassing the decomposition of the cationic
complex to vinylphosphonium salts by formation of the
neutral complexes (η1-vinyl)PdCl(PPh3)2, as proposed by
Scott and Stille.5d
i ) oxidation current of Pd0L3 at time t and i0 ) initial
oxidation current of Pd0L3, L ) PPh3). The plot of 1/x
versus time gave a straight line which passed through
unity (Figure 4b) in agreement with eq 23 (reaction
order in vinyl triflate of 1). kOTfK1K2 was calculated from
the slope (Table 2). With Pd0(PPh3)4 (C0 ) 2 mM) as
the starting material in the absence of added PPh3, the
apparent rate constant of the overall reaction 21 is kapp
) kOTfK1K2/[PPh3]. The value of the apparent rate
constant kapp may be then calculated in the absence of
added PPh3 with the average value of [PPh3] ) 1.5C0
(Table 2).21
Cyclic vinyl triflates are less reactive than acyclic ones
(compare 1a with 1b-f in Table 2). In the cyclic series,
conjugated triflates are more reactive than nonconju-
gated ones (compare 1c,d with 1b and 1e,f in Table 2).
We have established in a previous work that the
oxidative addition of aryl triflates to Pd0(PPh3)4 was
faster in the presence of added chloride ions.1 In the
case of vinyl triflates, the accelerating effect was still
observed but was less significant (entries 2-4 in Table
2) than for aryl triflates.
The vinyl triflates investigated here are considerably
more reactive than the vinyl bromides. For 1c and 2c,
which possess the same vinylic structure, the vinyl
triflate 1c is ca. 104 times more reactive than the vinyl
bromide 2c (entries 5 and 9 in Table 2). This is in
contrast with the comparable reactivity of PhOTf (kapp
) 1.7 × 10-3 M-1 s-1 at 20 °C) and PhBr (kapp ) 10-3
M-1 s-1 at 20 °C) with Pd0(PPh3)4 in DMF.1 This
suggests that the higher reactivity of the vinyl triflate
compared to that of vinyl bromide arises from a better
Exp er im en ta l Section
Gen er a l Meth od s. All experiments were performed using
standard Schlenk techniques under an argon atmosphere.
DMF was distilled on CaH2 and kept under argon. PPh3 and
anhydrous nBu4NBr were of commercial grade (Aldrich). The
vinyl bromides 2h ,i were of commercial grade (Aldrich) and
used after filtration on alumina. Pd0(PPh3)4,23 the vinyl tri-
flates 1b-f,24 1a ,25 and the vinyl bromides 2c26 and 2g27 were
synthesized according to published procedures.
(23) Rosevear, D. T.; Stone, F. G. A. J . Chem. Soc. A 1968, 164.
(24) (a) Stang, P. J .; Hanack, M.; Subramanian, L. Synthesis 1982,
85. (b) McMurry, J . E.; Scott, W. J . Tetrahedron Lett. 1983, 24, 979.
(c) Pal, K. Synthesis 1995, 1485.
(25) Summerville, R. H.; Senkler, C. A.; Schleyer, P. v. R.; Dueber,
T. E.; Stang, P. J . J . Am. Chem. Soc. 1974, 96, 1100.
(26) Willems, A. G. M.; Pandit, U. K.; Huisman, H. O. Recl. Trav.
Chim. Pays-Bas 1965, 84, 389.
(27) Cristol, S. J .; Norris, W. P. J . Am. Chem. Soc. 1953, 75, 2645.

Addition of Vinyl Triflates and Halides to Pd(0)
Organometallics, Vol. 22, No. 21, 2003 4237
The 31P NMR spectra were recorded on a Bruker spectrom-
eter (101 MHz) using H3PO4 as an external reference, the H
of the vinyl bromides 2g,h with Pd0(PPh3)4 have been
reported.10,17b
1
Vin yl Tr ifla te 1a .25 1H NMR (250 MHz, DMF-d7): δ 0.91
(t, 7.2 Hz, 3 H, CH3), 1.37 (m, 7.2 Hz, 2 H, CH2CH3), 1.52 (m,
7.2 Hz, 2 H, CH2CH2CH3), 2.46 (t, 7.2 Hz, 2 H, dCCH2CH2),
5.25 (d, 4 Hz, 1 H, vinyl H trans to OTf), 5.32 (d, 4 Hz, 1 vinyl
H cis to OTf). 19F NMR (235 MHz, DMF + acetone-d6 10%): δ
-74.12 (s).
Com p lex 3a +TfO-. 1H NMR (250 MHz, DMF-d7): δ 0.47
(m, 3 H, CH3), 0.66 (m, 4 H, CH2CH2CH3), 1.53 (m, 2 H, d
CCH2CH2), 4.68 (m, 1 H, vinyl H), 4.78 (m, 1 H, vinyl H), 7.21-
7.73 (m, H of PPh3). 31P NMR (101 MHz, DMF + acetone-d6
10%): δ 22.1 (s) ppm. 19F NMR (235 MHz, DMF + acetone-d6
10%): δ -77.61 (s). ES MS (DMF/methanol; C42H41P2Pd): m/z
(%) 713 [M+] (100), 631 [Pd(PPh3)2 + H+], 451 [M+ - PPh3].
P h osp h on iu m Sa lt 5a +TfO-. 1H NMR (250 MHz, DMF-
d7): δ 0.77 (t, 7.3 Hz, 3 H, CH3), 1.26 (m, 7.3 Hz, 2 H, CH2CH3),
1.51 (m, 7.3 Hz, 2 H, CH2CH2CH3), 2.46 (dd, 12 Hz (J PH), 7.3
Hz, 2 H, dCCH2CH2), 6.20 (dd, 20 Hz (J PH), 4 Hz, 1 H, vinyl
H trans to PPh3+), 6.83 (m, 1 H, vinyl H cis to PPh3+), 7.2-7.4
(m, 15 H, aromatic H). 31P NMR (101 MHz, DMF + acetone-
d6): δ 24.7 (s). 19F NMR (235 MHz, DMF + acetone-d6 10%):
δ -77.61 (s). ES MS (DMF/methanol; C24H26P (345)): m/z 345
[M+].
NMR spectra on a Bruker spectrometer (250 MHz), and the
19F NMR spectra on a Bruker spectrometer (235 MHz). Cyclic
voltammetry and amperometry were performed with a home-
made potentiostat and a waveform generator GSTP4 (Radi-
ometer analytical). The current was recorded on a Nicolet 301
oscilloscope. The conductivity was measured with a CDM210
conductivity meter (Radiometer Analytical). The cell constant
was 1 cm-1. The conductivity was recorded versus time using
a computerized homemade program. Electrospray mass spec-
trometry was performed on a T100LC spectrometer (J EOL
AccuTOF J MS).
Gen er a l P r oced u r e for 31P NMR Exp er im en ts. Into 0.75
mL of DMF were introduced 15 mg (13 µmol) of Pd0(PPh3)4
followed by the vinyl triflate 1a (3 mg, 2 µL, 13 µmol) or the
vinyl bromide 2c (75 mg, 0.39 mmol). A 75 µL amount of
acetone-d6 was then added for the lock. The 1H NMR spec-
troscopy was performed versus time. In other experiments,
bromide ions were introduced as nBu4NBr or PPh3 when
required (see Table 1).
Gen er a l P r oced u r e for 1H NMR Exp er im en ts. Into 0.75
mL of DMF-d7 were introduced 15 mg (13 µmol) Pd0(PPh3)4
1
followed by the vinyl triflate 1a (3 mg, 2 µL, 13 µmol). The H
NMR spectroscopy was performed versus time. 19F NMR
experiments were also performed concomitantly.
Vin yl Tr ifla te 1b.24b 1H NMR (250 MHz, DMF-d7): δ 0.89
(s, 9 H, CH3), 1.35 (m, 2 H), 1.98 (m, 2 H), 2.22 (m, 1 H), 2.39
(m, 2 H), 5.92 (t, 3 Hz, 1 H, vinyl H). 19F NMR (235 MHz,
Gen er a l P r oced u r e for th e Kin etics of th e Oxid a tive
Ad d ition Mon itor ed by Am p er om etr y. Experiments were
carried out in a three-electrode thermostated cell connected
to a Schlenk line. The counter electrode was a platinum wire
of ca. 1 cm2 apparent surface area; the reference was a
saturated calomel electrode (Radiometer Analytical) separated
from the solution by a bridge (3 mL) filled with a 0.3 M
nBu4NBF4 solution in DMF. A 12 mL portion of DMF contain-
ing 0.3 M nBu4NBF4 was poured into a cell. A 28 mg amount
(24 µmol, 2 mM) of Pd0(PPh3)4 was then introduced into the
cell. The kinetic measurements were performed at a rotating
gold disk electrode (i.d. 2 mm, inserted into a Teflon holder,
EDI 65109, Radiometer Analytical) with an angular velocity
of 105 rad s-1 (Radiometer Analytical controvit). The rotating
electrode was polarized at +0.35 V on the plateau of the
oxidation wave of Pd0(PPh3)3. The appropriate amount of the
vinyl bromide was then added to the cell and the decay of the
oxidation current recorded versus time up to 100% conversion.
When the reactivity of vinyl triflates was investigated, 180 mg
(0.72 mmol) of PPh3 was introduced before the vinyl triflate.
Gen er a l P r oced u r e for th e Kin etics of th e Oxid a tive
Ad d it ion Mon it or ed b y Con d u ct ivit y Mea su r em en t s.
Experiments were carried out in the same thermostated cell
as that used for the electrochemical experiments (vide supra).
A 28 mg amount (24 µmol, 2 mM) of Pd0(PPh3)4 was introduced
into the cell containing 12 mL of DMF. The residual conduc-
tivity was measured. After addition of 1 equiv of the vinyl
triflate 1a (3.7 µL, 24 µmol, 2 mM), the increase of the
conductivity was recorded versus time until a constant final
value was obtained corresponding to the end of the oxidative
addition.
DMF + acetone-d6
10%): δ -73.95 (s).
Com p lex 3b+TfO-. This species was never generated as a
pure compound because it started to decompose to the vinyl-
phosphonium salt 5b+TfO- before the oxidative addition was
over. 3b+TfO- was formed in 26% yield with unreacted 1b at
the very beginning of the oxidative addition (15 min after
mixing). Due to the complexity of the badly resolved corre-
sponding 1H NMR spectra, only the characteristic signals of
3b+TfO- which strongly differ from that of 1b are presented.
1H NMR (250 MHz, DMF-d7): δ 0.55 (s, 9 H, H of tBu), 5.18
(m, 1 H, vinyl H). 31P NMR (101 MHz, DMF + acetone-d6
10%): δ 22.47 (s). 19F NMR (235 MHz, DMF + acetone-d6
10%): δ -77.61 (s). ES MS (DMF/methanol; C46H47P2Pd): m/z
(%) 767 [M+] (100), 631 [Pd(PPh3)2 + H+], 505 [M+ - PPh3].
P h osp h on iu m Sa lt 5b+TfO- (P u r e Com p ou n d ). 1H
NMR (250 MHz, DMF-d7): δ 0.84 (s, 9 H, H of tBu), 1.42 (m,
2 H), 2.02 (m, 1 H), 2.25 (m, 2 H), 2.4 (m, 2 H), 6.85 (br d, 23
Hz (J PH), 1 H, vinyl H), 7.2-7.6 (m, 15 H, aromatic H). 31P
NMR (101 MHz, DMF + acetone-d6): δ 23.2 (s). 19F NMR
(235 MHz, DMF + acetone-d6 10%): δ -77.61 (s). ES MS
(DMF/methanol; C28H32P): m/z 399 [M+].
Ack n ow led gm en t. This work has been supported
by the Centre National de la Recherche Scientifique
(CNRS, UMR-ENS-UPMC 8640) and the Ministe`re de
la Recherche (Ecole Normale Supe´rieure). We thank
J ohnson Matthey for a generous loan of sodium tetra-
chloropalladate.
tr a n s-(η1-vin yl)P d Br (P P h 3)2. The synthesis and charac-
terization of the complexes generated in the oxidative addition
OM030298C