Kinetic data on the synergetic role of amines and water in the reduction of phosphine-ligated palladium(II) to palladium(0)

Article abstract of DOI:10.1002/ejoc.201402519

Tertiary amines such as EtN(iPr)₂ do not reduce [PdCl ₂(PPh₃)₂] into a Pd⁰ complex. The latter is formed only after the addition of water (generation of HO ^-), as was reported by Grushin and Alper for NEt₃. The mechanism of the Pd^II/Pd⁰ reduction performed in the presence of an excess amount of PPh₃ is established quantitatively by means of electrochemical techniques that provide kinetic data (determination of the equilibrium and rate constants) for the disappearance of [PdCl ₂(PPh₃)₂] and the formation of [Pd ⁰(PPh₃)₃]. [PdCl(OH)(PPh₃) ₂] is formed, which allows reductive elimination between OH and the ligated PPh₃ (zero-order reaction for PPh₃), and this leads to a Pd⁰ complex. The reducing agent is ligated PPh ₃, which ultimately yields (O)PPh₃. The rate of the overall reduction process is controlled by the amount of water that imposes the concentration of HO^-.

Full text of DOI:10.1002/ejoc.201402519

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402519
Kinetic Data on the Synergetic Role of Amines and Water in the Reduction of
Phosphine-Ligated Palladium(II) to Palladium(0)
[
a]
[b]
[a]
[a]
Christian Amatore,* Laurent El Kaïm, Laurence Grimaud,* Anny Jutand,*
[a]
[a]
Alice Meignié, and Georgy Romanov
Keywords: Reaction mechanisms / Water chemistry / Reduction / Palladium / Amines / Phosphanes
0
Tertiary amines such as EtN(iPr)
2
do not reduce [PdCl
2
-
of [PdCl
[PdCl(OH)(PPh
2
(PPh
3
)
)
2
]
and the formation of [Pd (PPh
3
)
3
].
0
(PPh
3
)
2
] into a Pd complex. The latter is formed only after
3
2
] is formed, which allows reductive elimi-
(zero-order reaction
), and this leads to a Pd0 complex. The reducing
agent is ligated PPh , which ultimately yields (O)PPh . The
rate of the overall reduction process is controlled by the
amount of water that imposes the concentration of HO .
–
the addition of water (generation of HO ), as was reported
nation between OH and the ligated PPh
for PPh
3
II
by Grushin and Alper for NEt
3
. The mechanism of the Pd /
Pd reduction performed in the presence of an excess amount
of PPh is established quantitatively by means of electro-
3
0
3
3
3
–
chemical techniques that provide kinetic data (determination
of the equilibrium and rate constants) for the disappearance
Introduction
(zero-order reaction for PPh
was generated in the presence of water,
, Scheme 1). Phosphine oxide
3
[
3a,3b]
the concentra-
Many reactions performed from aryl halides ArX involve
tion of which did not affect the kinetics of the reduction
II
[
Pd X (PPh ) ] (X = halide, OAc) as precatalysts and a ter-
[3a]
2
3 2
process (see Figure 2 in ref. ) because the hydrolysis takes
[1]
0
tiary amine as a base. A Pd complex must be formed to
activate ArX by oxidative addition. It is often considered
place after the rate-determining reductive elimination step
0
(
rate constant k ) in which Pd is formed (Scheme 1).
re
II
that the Pd precatalyst is reduced in situ by tertiary amines
0
to generate the Pd catalyst active in the catalytic reac-
[
2]
tions. In this respect, amines with a hydrogen in the α
position with regard to the N atom (i.e., in a β position
with regard to the Pd atom once the amine is ligated to the
II
Pd center) were proposed to undergo β-hydride elimi-
0
nation ultimately to lead to a Pd complex via a HPdX
Scheme 1. Mechanism of the reduction of [Pd(OAc)
2
(PPh
3
)
2
] to
[
2]
intermediate. This was supposed to happen in Heck reac-
0 [3a,3b]
Pd .
tions [precatalyst: Pd(OAc ) + nPPh ], until it was estab-
2
3
lished in 1992 by some of us that amines did not play any
reducing role in the presence of a phosphine ligand.
The same reduction process was later on reported by Oz-
∧
Water is however required in the case of bidentate P P
[
3a,3b]
ligands such as 2,2Ј-bis(diphenylphosphino)-1,1Ј-bi-
naphthyl (binap), as reported by Osawa and Hayashi, and
1,3-bis(diphenylphosphino)propane (dppp), as reported by
our group. Indeed, owing to the bidentate character of
P P ligands, the O–P reductive elimination step that takes
place in [Pd(OAc) (P P)] is reversible, and the equilibrium
[4]
awa, Kubo, and Hayashi in the presence of NEt and
3
[
4]
[3b]
water. However, the amine is not required.
Indeed,
[3f]
phosphine is the unique reducing agent that is able to re-
∧
0
duce Pd(OAc) to a Pd complex through a rate-determin-
∧
2
2
ing reductive elimination step occurring in [Pd(OAc)2-
0
needs be shifted towards the Pd complex by hydrolysis of
(
PR ) ] (R = Ar, alkyl) between OAc and the ligated PPh
[3f]
3
2
3
the hemiphosphonium (see Schemes 3 and 4 in ref. ).
Amines do not play any kinetic role in this reduction pro-
cess.
[
a] Ecole Normale Supérieure-PSL Research University,
[3f]
Département de Chimie, Sorbonne Universités – UPMC Univ
Paris 06, CNRS UMR 8640 PASTEUR
Grushin and Alper reported in 1993 that [PdCl (PPh ) ]
2
3 2
2
4 rue Lhomond, 75231 Paris, France
was reduced to “Pd metal” in the presence of hydroxide
E-mail: Anny.Jutand@ens.fr
www.chimie.ens.fr
[5]
ions (NaOH), whereas PPh3 was oxidized to OPPh .
3
0
[b] UMR 7652-Ecole Polytechnique/ENSTA/CNRS, ENSTA-
ParisTech,
These authors also observed that no Pd was formed in the
presence of the dry amine NEt , whereas it was formed af-
3
8
28 Bd. des Maréchaux, 91120 Palaiseau, France
[
5]
–
ter the addition of water owing to the formation of HO .
The mechanism proposed by these authors considers the
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402519.
Eur. J. Org. Chem. 2014, 4709–4713
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4709

C. Amatore, L. Grimaud, A. Jutand et al.
SHORT COMMUNICATION
initial formation of [PdCl(OH)(PPh ) ] leading to dation peak when CV was performed directly towards oxi-
3
2
[
HPdCl(PPh )], the latter of which is responsible for the for- dation potentials. Its oxidation peak current increased with
3
mation of Pd after reductive elimination of HCl. However, time (Figure 1, b). Consequently, a Pd0 complex was
0
in our view, the inner-sphere situation in [PdCl(OH)(PPh3) formed in situ from 1 in the presence of EtN(iPr ) only if
2
2]
is similar to that occurring in [Pd(OAc) (PPh ) ] water was added to [PdCl (PPh ) ] (Scheme 2), as observed
2
3 2
2
3 2
5]
[
0
(Scheme 1). On the basis of this analogy, we expected that by Grushin and Alper for NEt3. Conversely, no Pd was
0
[
PdCl(OH)(PPh ) ] could directly evolve into a Pd moiety formed in the presence of water and absence of amine
3 2
through a single step occurring within the palladium coor- (Scheme 2).
dination shell through inner-sphere oxidation of one phos-
phine by hydroxy transfer. Given that the authors did not
produce any kinetic data to firmly validate their postulated
mechanism, we decided to fully investigate the kinetics of
the reduction of [PdCl (PPh ) ] if the dry tertiary amine
2
3 2
EtN(iPr) and water were simultaneously present. Indeed,
2
the exact mechanism involved for the overall reduction of
the Pd center may have important consequences on the
0
loading of the Pd catalyst and consequently on the rate
and success of the catalytic reaction.
Figure 1. (a) Voltammetry of [PdCl
(15 mL, nBu NBF , 0.3 m) at a gold disk electrode (d = 1 mm) at
a scan rate of 0.5 Vs performed in the presence of PPh
(0.5 mL), and H O (1 mL) as a function of time at room
temperature. (b) Oxidation of [Pd (PPh
PdCl (PPh ] as a function of time in the same reaction as that
in (a).
2 3 2
(PPh ) ] (2 mm) in DMF
Results and Discussion
4
4
–
1
3
(6 mm),
All reactions were performed in the presence of PPh3 EtN(iPr)
3 equiv. with respect to [PdCl (PPh ) ] (1)} to follow the
2
2
0
3 3
) ] generated in situ from
{
2
3 2
[
2
3 2
)
0
[6]
kinetics of the formation of stable complex [Pd (PPh ) ]
3
3
[5]
(instead of palladium metal ). Cyclic voltammetry was
used to monitor the evolution of [PdCl (PPh ) ] (C
=
0
2
3 2
The kinetics of the decay of 1 (2 mm) in the presence of
2
mm) in dry DMF in the presence of dry EtN(iPr) (2)
2
2
1
(0.5 mL, 100 equiv. with respect to 1) and H O (1 mL,
2
and known amounts of H O at room temperature. Indeed,
2
852 equiv. with respect to 1) was monitored by chronoam-
perometry at a rotating disk electrode (RDE) polarized at
complex 1 is well characterized by cyclic voltammetry (CV)
p
by its reduction peak at E
= –0.92 V versus the saturated
red
–
1.2 V on the reduction wave of [PdCl (PPh ) ]. The
2
3 2
calomel electrode (SCE),[ and interestingly, its reduction
7]
decreasing reduction plateau current of [PdCl (PPh ) ],
2
3 2
peak current is proportional to its concentration at any
proportional to its concentration, was recorded with time
[
8]
time. If a reaction took place, the reduction peak current
[9]
just after the addition of water (Figure 2, a).
of 1 should decrease with time and the oxidation peak of
In another experiment performed at the same concentra-
0
resulting [Pd (PPh ) ] (3) should appear concomitantly. The
0
3
3
tions, the kinetics of the formation of the Pd complex was
latter is indeed well characterized by its oxidation peak at
monitored by the same technique, and the RDE was polar-
p
E
= –0.03 V versus SCE with an oxidation peak current
0
ox
ized at +0.2 V on the oxidation wave of [Pd (PPh ) ]. The
3
3
[6]
proportional to its concentration at any time. It was first
checked that no Pd complex was formed after the addition
0
increasing oxidation plateau current of [Pd (PPh ) ] (pro-
3
3
0
portional to its concentration) was recorded with time just
after the addition of water. Figure 2 (b) displays the con-
of an excess amount of PPh to 1, even over long reaction
3
times. Thus, free PPh did not reduce 1 to 3 at room tem-
II
3
comitant evolution of the molar fractions of the Pd and
0
perature. No Pd was formed from [PdCl (PPh ) ] in the
0
2
3 2
Pd complexes.
presence of 2 even in large amounts and at long times
Scheme 2).
The plot of ln x versus time is linear (Figure 2, c) (x =
(
[
1] /[1] = i /i ; i = reduction current of 1 at t, i = initial
t 0 t 0 t 0
II
Pd
reduction current of 1) with ln x = –kobs t. The plot of
ln xЈ versus time is linear {xЈ = ([3]fin – [3] )/[3] = (ifin
–
t
fin
i )/i ; i = oxidation current of 3 at t, i = final oxidation
t
fin
t
fin
0
Pd
current of 3} with ln xЈ = –k
of kobs
t (Figure 2, d). The values
and kobs , determined from the slope of the
obs
II
0
Pd
Pd
straight lines in Figure 2 (c, d) respectively, were almost
identical (Table 1, entries 3 and 4) upon considering the fact
II
0
Pd
Pd
that kobs
and kobs
were independently determined in
Scheme 2.
two different experiments (see above). Thus, [PdCl (PPh ) ]
2
3 2
II
Pd
It was only after addition of water that [PdCl (PPh ) ] disappeared (rate characterized by k
obs
) at the same rate
2
3 2
0
0
Pd
progressively disappeared, as attested by the decay in its as [Pd (PPh ) ] was formed (rate characterized by k
),
3
3
obs
reduction peak current with time (Figure 1, a). Further- which suggests that no intermediate complex accumulated
0
more, [Pd (PPh ) ] was formed and detected by its oxi- during the reduction process. Importantly, the concentra-
3
3
4710
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 4709–4713

Synergy in the Reduction of Phosphine-Ligated PdII to Pd0
0
that the Pd complex is formed by a O–P reductive elimi-
nation between OH and ligated PPh in 4 [Scheme 3, Equa-
3
tion (3)]. The cis coordination of these two groups favors
reductive elimination. Therefore, ligated PPh played the
3
role of reducing agent. Phosphine oxide, (O)PPh , ulti-
3
+
mately formed in
(
a fast reaction from HO–PPh3
Scheme 3), was characterized by P NMR spectroscopy in
31
DMF (121 MHz, H PO ) as a singlet at δ = 25.4 ppm.
3
4
Figure 2. Kinetics of the reduction of [PdCl
2
(PPh
(0.5 mL), water (1 mL),
(6 mm) in DMF (15 mL) at 25 °C. (a) Decay of the re-
duction plateau current of [PdCl (PPh ] at a RDE (d = 2 mm)
polarized at –1.2 V vs. SCE vs. time. (b) Evolution of the molar
3 2
) ] (1, 2 mm) to
0
[Pd (PPh
3
)
3
] in the presence of EtN(iPr)
2
2 3 2
Scheme 3. Mechanism for the reduction of [PdCl (PPh ) ] to
0
and PPh
3
3 3
[Pd (PPh ) ] in the presence of a tertiary amine and water from
2
3
)
2
kinetic data.
0
fraction of [PdCl
the decay of [PdCl
3 3
the formation in situ of [Pd (PPh ) ] 3: plot of ln xЈ vs. time.
2
(PPh
3
)
2
] and [Pd (PPh
3 3
) ] vs. time. (c) Kinetics of
Alper and Grushin analyzed their reaction (NEt as the
3
2
(PPh
3
)
2
]: plot of ln x vs. time. (d) Kinetics of
31
II
amine) by P NMR spectroscopy when the Pd was ligated
by a chiral phosphine. Retention was observed at the P
0
[
5]
center in the isolated chiral phosphine oxide; they con-
II
tion of PPh did not affect the kinetics of the reduction cluded that “the Pd phosphine complexes [...] are reduced
3
0
process: t1/2 = 112 and 109 s in the presence of 3 and to a Pd( ) species [...] not by extra phosphine, but by their
6
equiv. of PPh respectively, for reactions performed in the own coordinated phosphine ligands which are oxidized to
3
presence of 2 (0.5 mL) and H O (1 mL) (Figure S1, Sup- phosphine oxides, with OH playing the role as the specific
2
0
[5]
porting Information). Consequently, the Pd complex was promoter”. The role of the ligated phosphine in the re-
formed in an intramolecular process (zero-order reaction duction process is confirmed from our kinetic data. Most
for PPh ) and the free PPh was not the reductant.
importantly, the present kinetics evidence that the reduction
3
3
is zero-order in phosphine, which disproves the involvement
PdII
Table 1. Observed rate constant kobs
for the disappearance of
[5]
0
of a hydrido [HPdCl(PPh )] intermediate on the way to
3
Pd
0
[PdCl
2
(PPh
3
)
2
] and kobs
for the formation of [Pd (PPh
2 3 2 3 3
(PPh ) ] (2 mm) into [Pd (PPh ) ] in the
3
)
3
] in the
0
0
Pd . Furthermore, this establishes on a sound kinetic basis
that the entire two-electron reduction of the Pd center oc-
in situ reduction of [PdCl
presence of EtN(iPr) and water.
[a]
2
PdII
(kobsPd0) /s–1
curs directly within the [Pd(OH)Cl(PPh ) ] inner-sphere
3 2
Entry
EtN(iPr)
equiv.)
2
/mL O /mL
H
2
k
obs
shell, that is, involving direct transfer of the hydroxy onto
one phosphorus with simultaneous formation of
(
(equiv.)
[
[
b]
[b]
[b]
1
2
3
4
5
6
0.05 (10)
0.5 (100)
0.5
0.5
1.0
0.5 (926)
0.5 (926)
slow
0
–
+
b]
–3
[ClPd (PPh )] and [HO–PPh ] [Scheme 3, Equation (3)] in
3 3
1.4ϫ10
3[c]
1
1
0.5
1
3.1ϫ10–
a perfectly analogous manner to that established previously
by us for the palladium acetate case (Scheme 1). Hence, in
both cases the oxidation state of the Pd center readily
changes from II to 0, whereas that of the P center simulta-
–3 [c]
(3.6ϫ10 )
1.5ϫ10–
3
1.0
2.5ϫ10–3
[
a] All reactions were performed in the presence of PPh
in DMF (15 mL) at 25 °C. [b] Equiv. with respect to [PdCl
PPh ]. [c] Determined in a separate series of experiments (see
text).
3
(3 equiv.)
neously goes from III to V through the formation of a [RO–
2
-
+
PPh ] (R = H or Ac) intermediate.
3
(
3 2
)
The effect of the concentration of H O on the rate of
2
formation of 3 was then investigated by maintaining a con-
The crucial role of water upon using an amine involves stant concentration of amine 2 in the presence of various
the reversible formation of hydroxide ions [Scheme 3, Equa- amounts of H O. From the determination of k (Table 1,
2
obs
tion (1)] with the formation of [PdCl(OH)(PPh ) ] [4; entries 2, 3, 5, and 6; see also Figures S2–S5), it was ob-
3
2
Scheme 3, Equation (2)].[ By analogy with the well-estab- served that the higher the concentration of water, the faster
5]
0
lished evolution of [Pd(OAc) (PPh ) ] toward a Pd com- the reaction (with a first-order reaction for H O, Fig-
2
3 2
2
[
3a,3b]
plex through O–P reductive elimination (Scheme 1),
ure S5), in agreement with a higher concentration of
and the zero-order reaction found for PPh , one concludes hydroxides formed in Equation (1), as this shifts the equilib-
3
Eur. J. Org. Chem. 2014, 4709–4713
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4711

C. Amatore, L. Grimaud, A. Jutand et al.
SHORT COMMUNICATION
rium [Equation (2)] towards complex 4. The effect of the
concentration of amine 2 was then investigated by main-
taining a constant concentration of H O (Table 1, entries 2
2
and 5). The higher the concentration of amine, the faster
the reaction at low H O concentrations, in agreement with a
2
higher concentration of hydroxides formed in Equation (1).
The exact kinetic law that takes into account the fact that
the kinetics were not investigated under pseudo-first-order
–
+
conditions for Cl and EtN(iPr) H was determined:
2
2
O]t/2C 2
ln x – 2x + 0.5x + 1.5 = –KOH
K
Cl
k
re[2][H
2
0
The value of KOHK k = 9.2ϫ10–9 s–1 (DMF, 25 °C)
Cl re
2
was determined from the plot of ln x – 2x + 0.5x + 1.5
versus time (Figure S6).
0
Scheme 4. Mechanism for the formation of [Pd (PPh
3
)
3
] from
No kinetic data was published by Grushin and Alper for
[PdCl
2 3 2
(PPh ) ] in the presence of hydroxide in DMF from kinetic
the reduction of [PdCl (PPh ) ] (1) to “Pd metal” in the
2
3 2
data.
[
5]
presence of pure hydroxide ions (introduced via NaOH).
In the present work, the reactions were performed in the
mates the value of KOH = 3.5ϫ10–6 (DMF, 25 °C;
Scheme 3, Equation (1)].
presence of PPh (3 equiv. vs. 1). The electrochemical tech-
3
nique (i.e., RDE) was used to follow the disappearance of
All previous reactions were performed in dry DMF in
[
[
PdCl (PPh ) ] (C0
=
2 mm) and the formation of
2
3 2
0
the presence of dry EtN(iPr)
formed in nondistilled DMF in the presence of the nondis-
tilled amine. The CV of [PdCl (PPh ] (2 mm) still per-
2
. Some experiments were per-
Pd (PPh ) ] in the presence of NaOH (3.3 equiv., from a
3
3
0
mother solution 1 m in water). Once more, stable [Pd -
(
II
2
)
3 2
PPh ) ] was formed at the same rate as the Pd complex
3 3
formed in the presence of PPh (3 equiv.) but in the absence
3
disappeared (Figure 3), which indicates that the reductive
elimination [Scheme 4, Equation (3Ј)] was rate determining,
controlled by the equilibrium in Equation (2) (Scheme 4).
0
of added water revealed that some [Pd (PPh ) ] was already
3
3
formed in situ, which was detected by its oxidation peak.
Again, addition of water greatly increased the rate of for-
0
mation of the Pd complex. Consequently, in the absence of
purposely added water, [PdCl (PPh ) ] could be trans-
2
3 2
0
formed into a Pd complex in the presence of an amine
via hydroxides generated by residual water. The rate of the
reduction process was slow and was controlled by the low
concentration of residual water. Addition of water ac-
0
celerated the formation of the Pd complex. Whatever the
case, the phosphine ligand remains the reducing agent by
an intramolecular reduction process established by kinetic
data (Scheme 3).
Conclusions
Figure 3. Kinetics of the reduction of [PdCl
2
(PPh
] in the presence of NaOH (6.6 mm, 0.1 mL from a
mother solution of NaOH, 1 m in water) and PPh (6 mm) in DMF
15 mL) at 25 °C. (a) Decay of the molar fraction of [PdCl
3 2
) ] (1, 2 mm) to
In conclusion, tertiary amines do not reduce
0
3 3
[Pd (PPh )
[
PdCl (PPh ) ] to a palladium(0) complex, even those pos-
2 3 2
3
sessing a hydrogen in the α position. It is only in the pres-
(
(
(
2
-
0
0
PPh
3
)
2
] vs. time. (b) Increase in the molar fraction of [Pd (PPh
3
)
3
]
ence of H O that a Pd complex is formed; this confirms
2
3) generated in situ vs. time.
[5]
the previous work by Grushin and Alper. The mechanism
for the reduction process is now quantitatively established
The half-reaction time for the formation [Pd (PPh ) ] with the determination of the equilibrium and rate con-
0
3
3
was t1/2 = 7.8 s (Figure 3). This value can be compared to stants thanks to electrochemical techniques that provided
0
t1/2 = 140 s obtained in the presence of EtN(iPr) (0.5 mL, kinetic data on the concomitant formation of the Pd com-
1
2
II
00 equiv.) and water (1 mL, 1852 equiv.; Figure 2, d). Even plex and the disappearance of the Pd complex during the
in the presence of a small amount of purposely added reduction process. The synergetic role of amine and water
hydroxide ions, the reduction process was much faster be- only involves the formation of hydroxide ions in water to
cause the equilibrium in Equation (1) in Scheme 3 was by- deliver [PdCl(OH)(PPh ) ], which allows the formation of a
3
2
0
passed.
Pd complex through reductive elimination between ligated
[
10]
The kinetic law was determined and allowed the calcu- PPh and the OH ligand. Therefore, the reducing agent is
lation of K k = 2.6ϫ10
mination of KOHK k = 9.2ϫ10
3
–
3 –1
s
(Figure S7). From the deter- ligated PPh , which leads to phosphine oxide. The rate of
3
Cl re
–
9 –1
s
(see above), one esti- the reduction is controlled by the amount of water through
Cl re
4712
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 4709–4713

Synergy in the Reduction of Phosphine-Ligated PdII to Pd0
[
2] a) J. P. Collman, L. Hegedus, J. R. Norton, R. G. Finke, Prin-
ciples and Applications of Organotransition Metal Chemistry,
University Science Books, Mill Valley, CA, 1987, p. 725; b) R.
Mc Crindle, G. Ferguson, G. J. Arsenault, A. J. Mc Alees,
D. K. Stephenson, J. Chem. Res. Synop. 1984, 360–361.
adjusting the concentration of the hydroxides. This is why
such reactions are slower than those performed in the pres-
ence of pure hydroxide ions, whereas the reduction process
is essentially the same. Consequently, in catalytic reactions
performed with the use of tertiary amines and [PdCl2- [3] a) C. Amatore, A. Jutand, M. A. M’Barki, Organometallics
(
PPh ) ] as a precatalyst, it is recommended to add water
1992, 11, 3009–3013; b) C. Amatore, E. Carré, A. Jutand,
M. A. M’Barki, G. Meyer, Organometallics 1995, 14, 1818–
3
2
to facilitate the in situ reduction of [PdCl (PPh ) ] into the
2
3 2
1
826; c) C. Amatore, A. Jutand, M. J. Medeiros, New J. Chem.
0
required Pd complex, the catalytic loading of which is thus
made higher. The catalytic reactions become thus faster in
the presence of water.[
1
996, 20, 1143–1148; d) A. Jutand, F. Lemaître, J.-L. Ricard,
S. Kozuch, S. Shaik, J. Organomet. Chem. 2004, 689, 3728–
3734; e) C. Amatore, A. Jutand, Acc. Chem. Res. 2000, 33,
11]
314–321; f) C. Amatore, A. Jutand, A. Tuilliez, Organometallics
2001, 20, 3241–3249.
Experimental Section
[4] F. Ozawa, A. Kubo, T. Hayashi, Chem. Lett. 1992, 2177–2180.
[
5] V. V. Grushin, H. Alper, Organometallics 1993, 12, 1890–1901.
General Procedure for the Kinetic Measurements of the Reduction
of [PdCl (PPh ) ] in the Presence of Amine and Water: The kinetic
2 3 2
measurements were performed by chronoamperometry at a rotating
0
[6] For the characterization of [Pd (PPh ) ] in DMF, see: C. Ama-
3
3
tore, A. Jutand, F. Khalil, M. A. M’Barki, L. Mottier, Organo-
metallics 1993, 12, 3168–3178.
gold disk electrode (d = 2 mm) with an angular velocity of
[
7] For the electrochemical reduction of [PdCl
2 3 2
(PPh ) ] in DMF,
1
05 rads–1 in a cell (see the Supporting Information) containing
see: A. Jutand, S. Negri, Eur. J. Org. Chem. 1998, 1811–1821.
DMF (15 mL, with nBu
PdCl (PPh (16.8 mg, 0.03 mmol, 2 mm), and EtN(iPr)
0.5 mL, 0.6 mmol). The rotating disk electrode was polarized at
4
NBF
4
0.3 m, as supporting electrolyte),
[8] a) A. J. Bard, L. R. Faulkner, Electrochemical Methods: Funda-
mentals and Applications, 2nd ed. Wiley, New York, 2001; b)
A. Jutand, Chem. Rev. 2008, 108, 2300–2347.
[
2
3
)
2
]
2
(
[
9] A diminution of about 13% in the reduction current (not
shown in Figure 2, a) was observed just after the addition of
water (within less than 5 s) because of the dilution induced by
the addition of water (1 mL, ca. 9%). Part of the current de-
crease was also due to the partial formation of complex 4 (not
reduced at –1.2 V) in a fast reaction controlled by the concen-
tration of hydroxide (see text).
–1.2 V vs. SCE, on the plateau of the reduction current of
[PdCl
2
(PPh ]. The decreasing reduction current was recorded ver-
3 2
)
sus time after the addition of H
conversion (Figure 2, a).
2
O (1 mL, 55 mmol), up to 100%
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and kinetic curves.
[
[
10] The kinetic law that takes into account the fact that the kinetics
was not investigated under pseudo-first-order conditions for
–
–
Acknowledgments
Cl and HO was determined: ln x – 3.3ln(2.3 + x) + 4 =
–
3 –1
–1.15kre
KClt. KClkre = 2.6ϫ10
s
was calculated from the
plot of ln x – 3.3ln(2.3 + x) + 4 vs. time (Figure S7).
This work has been supported in part by the Centre National de
la Recherche Scientifique (CNRS) and Ecole Normale Supérieure
11] The ring opening of cyclopropylbromoarylamine catalyzed by
[
PdCl
worked only after the addition of water (53% yield)
Scheme S1); see: a) L. El Kaïm, L. Grimaud, R. Ramozzi,
2 3 2
(PPh ) ] in the presence of a base did not work. It
(ENS).
(
[
1] For books, see: a) E.-i. Negishi (Ed.), Handbook of Organopal-
ladium Chemistry for Organic Synthesis Wiley, New York, 2002,
vol. I and II; b) M. Oestreich (Ed.), The Mizoroki–Heck Reac-
tion, Wiley, Chichester, UK, 2009.
2013, unpublished results; b) R. Ramozzi, PhD Thesis, ENS
University Lyon1, Paris, 2013.
Received: April 30, 2014
Published Online: June 25, 2014
Eur. J. Org. Chem. 2014, 4709–4713
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4713