Homogeneous Catalysis
FULL PAPER
mation of some 4a or 6a, respectively, which proves that
halide metathesis is a realistic process. Moreover, a transme-
tallation experiment with 2 on 4b and on 6b (followed by
31P NMR spectroscopy) in the presence of LiCl showed a
rate acceleration in comparison with the experiments in the
absence of LiCl, proving that halide metathesis is also a ki-
netically relevant process (Figures S5 and S6 in the Support-
ing Information). Complexes 4b and 6b are thermodynami-
cally more stable, since halide metathesis on 4a and 6a in
the presence of LiI (32 equiv) gave a complete conversion
to 4b and 6b, respectively.
To further support our findings on the role of in situ
halide metathesis in the acceleration of the reaction rate,
the Stille reaction of 1a was studied in the presence of LiI.
A decrease of the initial kobsd was observed (Table 1, com-
pare entries 2 and 6) due to an in situ halide metathesis con-
Figure 1. Cyclic voltammetry study of the reduction of [PdIICl
2ACHTUNGTRENNUNG(PPh3)2] by
2. Reactions performed in THF (containing nBu4NBF4, 0.3m) at 208C.
Peak currents were measured at a steady gold disk electrode (d=1 mm)
verting [PdIICl
ACHTUNGTRENNUNG(2-Py)ACHTUNGTERN(NUGN PPh3)2] (4a) into the less reactive
at the scan rate of 0.5 VsÀ1
.
: Decrease of the reduction peak current of
*
[PdIICl
2ACHTUNRGTNEUNG(PPh3)2] (1.94 mm) with time after addition of 2 (38.8 mm). &:
[PdIII
N
ACHTUNGTRENNUNG
Evolution with time of the reduction peak current of the intermediate
in Supporting Information for the decelerating effect of LiI
on the transmetallation of 4a and 6a by 2). However, within
24 h in the presence of LiI a full conversion was achieved,
while in its absence only 48% was obtained.[18] Additionally,
an increase of the reaction rate was observed when LiCl was
added to the Stille reaction of 2-chloropyridine (1a)
(Table 1, compare entries 2 and 5) and when LiI was added
to the Stille reaction of 2-iodopyridine (1b) (Table 1, com-
pare entries 8 and 10), while no effect on the turnover limit-
ing transmetallation was expected. Initially, the promotion
of the transmetallation by the formation of a hypervalent
stannane (=known as nucleophilic assistance) was consid-
ered to be responsible for the rate acceleration of these re-
complex [PdII(Th)
(PPh3)2]. : Evolution with time of the oxidation peak
~
current of the Pd0 complex formed in situ from [PdII(Th)
2ACHTNUTRGNE(UNG PPh3)2].
reach a maximum value and then decreased (Figure 1; &).
This peak was assigned to [PdII(Th)
2A(PPh3)2] (Th=2-thienyl)
formed by double transmetallation of [PdIICl
2A(PPh3)2] with
CTHUNGTRENNUNG
CTHUNGTRENNUNG
2. When the cyclic voltammetry was performed directly to-
wards oxidation potentials, a tiny oxidation peak was ob-
served at +0.12 V, which characterized a Pd0 complex.[9]
This small peak appeared about 90 min after addition of 2
CHTUNGTRENNUNG
to [PdIICl (PPh3)2] and was generated from [PdII(Th)2-
actions.[7] Transmetallation experiments on [PdIICl
N
species was very low (Figure 1; ~) and the yellow solution
A
G
U
first turned brown and then black. This indicates that the
presence of LiCl and LiI, respectively, showed however no
rate effect (Figures S9 and S10 in the Supporting Informa-
tion). Therefore, the observed accelerations cannot be due
to nucleophilic assistance at tin. The obtained results indi-
cate that in situ halide metathesis does play a role in some
cases, but cannot be the only effect of halides on the reac-
tion rate. We then focused on the in situ reduction of
Pd0 generated by reduction of [PdIICl
2ACTHUNGTERN(NUG PPh3)2] by 2 was
quite unstable, owing to the lack of stabilizing ligands (no
chlorides because quenched as ClSnBu3). The reaction of 2
1
with [PdIICl
C
spectroscopy at room temperature (CDCl3). The 1H NMR
spectrum exhibited the three protons of a thienyl group lo-
cated at higher field than in 2, characteristic for aromatic
groups ligated to a PdII center. No stable Pd0 complex in the
reductive elimination could be characterized by 31P NMR
spectroscopy, in agreement with its instability. An important
modification appeared in the cyclic voltammetry measure-
2ACHTUNGTRENNUNG
[PdIICl (PPh3)2] by 2. This process delivers the actual Pd0
catalyst at the very beginning of the catalytic reaction and a
different concentration of catalyst will immediately influ-
ence the rate of the Stille reaction.[19]
ments when the reaction of [PdIICl
2ACHTUNGTERNU(NG PPh3)2] with 2 was per-
formed in the presence of chloride ions introduced as
[PdIICl
G
niques, taking the advantage of the fact that PdII and Pd0
complexes can be detected and characterized by their reduc-
tion or oxidation potentials, respectively, and quantified
since the reduction or oxidation peak currents are propor-
tional to the concentration of electroactive species.[12]
nBu4NCl (9.3 equiv) before the addition of 2 (20 equiv). The
reduction peak current of [PdIICl
2ACHTNUTRGNE(UNG PPh3)2] again decreased
with time, but the reaction was slower than in the absence
of chlorides (compare Figures 1 and 2; *) because [PdIICl2-
AHCTUNGTRENNUNG
[PdIICl
2A
CHTUNGTRENNUNG
SCE in THF at 208C. The reduction peak slowly disap-
peared with time after addition of 2 (20 equiv), evidencing a
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
reaction of [PdIICl
(Figure 2; &). Cyclic voltammetry performed concomitantly
first towards oxidation potentials revealed an oxidation
peak at +0.12 V, characteristic of a Pd0 complex. This oxida-
tantly, a new reduction peak was observed at À1.67 V, the
reduction peak current of which increased with time to
Chem. Eur. J. 2010, 16, 12831 – 12837
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12833