Kinetics and mechanisms of homogeneous catalytic reactions. Part 12. Hydroalcoxycarbonylation of 1-hexene using palladium/triphenylphosphine systems as catalyst precursors

Article abstract of DOI:10.1007/s10562-014-1335-0

Systems prepared in situ by addition of n equivalents of triphenylphosphine to palladium dichloride in the presence of m equivalents of para-toluenesulfonic acid (TSA), PdCl₂/nPPh₃/mTSA (n and m varying between 2 and 10), were used as precatalysts for the olefin carbonylation (1-hexene, cyclohexene and styrene) with alcohols (MeOH, EtOH, n-PrOH and i-PrOH) to generate the corresponding esters (hydroalcoxycarbonylation), under mild reaction conditions. For 1-hexene carbonylation in presence of methanol (1-hexene hydromethoxycarbonylation), the most active system was PdCl₂/6PPh₃/5TSA at P(CO) = 50 atm and T = 125°C, which was also active for the hydromethoxycarbonylation of other olefins (1-hexene > styrene > cyclohexene). This system was regioselective towards the linear product for 1-hexene and towards the branched product for styrene. A kinetic study of 1-hexene hydromethoxycarbonylation catalyzed by PdCl₂/6PPh₃/5TSA showed that the initial reaction rate (r_o) was first order on Pd and MeOH concentrations and fractional order with respect to CO concentration; for olefin concentration was found a saturation curve. These kinetic results, together with coordination chemistry and computational DFT studies, allow us to propose a catalytic cycle involving species of the type [Pd(H)(L)(PPh₃)₂]⁺ⁿ (L = Cl, n = 0; L = CO, MeOH, olefin and PPh₃, n = 1) as the catalytically active species and three sequential reactions: (1) olefin insertion into the Pd-H bond to yield Pd-alkyl species, (2) CO insertion into the Pd-C bond to generate Pd-acyl intermediates, and (3) the methanolysis of Pd-acyl species to produce the corresponding methyl esters, regenerate the active species and restart the cycle; the last reaction is considered the rate-determining step (rds) of the mechanism.

Full text of DOI:10.1007/s10562-014-1335-0

Catal Lett (2014) 144:1717–1727
DOI 10.1007/s10562-014-1335-0
Kinetics and Mechanisms of Homogeneous Catalytic Reactions.
Part 12. Hydroalcoxycarbonylation of 1-Hexene Using Palladium/
Triphenylphosphine Systems as Catalyst Precursors
Merl ´ı n Rosales
•
In e´ s Pacheco
•
Jairo Medina
•
•
•
Angel Gonz a´ lez Rodolfo Izquierdo
Pablo J. Baricelli
´
•
Juan Fern a´ ndez
Luis G. Melean
•
Received: 15 March 2014 / Accepted: 4 August 2014 / Published online: 23 August 2014
Ó Springer Science+Business Media New York 2014
Abstract Systems prepared in situ by addition of n
equivalents of triphenylphosphine to palladium dichloride
in the presence of m equivalents of para-toluenesulfonic acid
found a saturation curve. These kinetic results, together with
coordination chemistry and computational DFT studies,
allow us to propose a catalytic cycle involving species of the
?
n
(
TSA), PdCl /nPPh /mTSA (n and m varying between 2 and
2
type [Pd(H)(L)(PPh3)2]
(L = Cl, n = 0; L = CO,
3
1
0), were used as precatalysts for the olefin carbonylation
MeOH, olefin and PPh , n = 1) as the catalytically active
3
(
1-hexene, cyclohexene and styrene) with alcohols (MeOH,
species and three sequential reactions: (1) olefin insertion
into the Pd–H bond to yield Pd–alkyl species, (2) CO
insertion into the Pd–C bond to generate Pd–acyl interme-
diates, and (3) the methanolysis of Pd–acyl species to pro-
duce the corresponding methyl esters, regenerate the active
species and restart the cycle; the last reaction is considered
the rate-determining step (rds) of the mechanism.
EtOH, n-PrOH and i-PrOH) to generate the corresponding
esters (hydroalcoxycarbonylation), under mild reaction
conditions. For 1-hexene carbonylation in presence of
methanol (1-hexene hydromethoxycarbonylation), the most
active system was PdCl /6PPh /5TSA at P(CO) = 50 atm
2
3
and T = 125 °C, which was also active for the hydro-
methoxycarbonylation of other olefins (1-hexene [ sty-
rene [ cyclohexene). This system was regioselective
towards the linear product for 1-hexene and towards the
branched product for styrene. A kinetic study of 1-hexene
hydromethoxycarbonylation catalyzed by PdCl /6PPh /
Keywords Hydromethoxycarbonylation ꢀ Palladium ꢀ
Phosphine ꢀ Kinetics ꢀ Mechanisms
2
3
5
TSA showed that the initial reaction rate (r ) was first order
o
1 Introduction
on Pd and MeOH concentrations and fractional order with
respect to CO concentration; for olefin concentration was
Carbonylation of unsaturated substrates in presence of
alcohols (hydroalcoxycarbonylation), one of the Reppe
reactions, is a promising clean method for esters synthe-
sis, which are valuable products as solvents and for fine
chemistry, mainly as flavorings and perfumes; when
methanol is used as the nucleophile, the reaction is called
hydromethoxycarbonylation [1]. Among the precatalysts
employed for this reaction, palladium/triphenylphosphine
systems promoted with strong acids are of special interest;
the protonic acid stabilizes the catalyst and besides,
accelerates the reaction by forming the active metal-
hydride species [2]. Kiss [3], Toniolo and coauthors [4]
and Beller et al. [5] have published comprehensive
reviews dealing with recent advances in palladium-cata-
lyzed olefin hydroalcoxycarbonylation, including some
aspect of the reaction kinetics. In other studies, the use of
M. Rosales (&) ꢀ I. Pacheco ꢀ J. Medina ꢀ J. Fern a´ ndez ꢀ
´
A. Gonz a´ lez
Laboratorio de Qu ´ı mica Inorg a´ nica (LQI), Departamento de
Qu ´ı mica, Facultad Experimental de Ciencias, Universidad del
Zulia (L.U.Z.), Apdo. 526, Maracaibo, Venezuela
e-mail: merlinrosalesaiz@gmail.com; merlin2002@cantv.net
R. Izquierdo
Laboratorio de Qu ´ı mica Te o´ rica y Computacional (LQTC),
Departamento de Qu ´ı mica, Facultad Experimental de Ciencias,
Universidad del Zulia (L.U.Z.), Apdo. 526, Maracaibo,
Venezuela
L. G. Melean ꢀ P. J. Baricelli
Centro de Investigaciones Qu ´ı micas, Facultad de Ingenier ´ı a,
Universidad de Carabobo, Valencia, Venezuela
123

1
718
M. Rosales et al.
alternative sources for the formation of metal-hydride
complexes (water, hydrogen and acids) [6, 7], the effect
of the oxidation state of the palladium center on the
catalyst activity [8], as well as on the kinetic and mech-
anistic studies of these reactions [9–16] have been per-
formed. Most of research works initially reported have
involved the influence of mono- and bidentatephosphines
on the activity and regioselectivity of palladium systems
of this reaction [17–21], including modified precatalysts
containing phosphorous ligands with the metallocene
fragment [22–29]; the effect of the nature of diphosphine
on the formation of either esters or polyketones in the
palladium-ethylene carbonylation have been also reported
well as the characterization, reactivity and roles of some
intermediates in the catalytic cycle.
In spite of the importance of the olefin hydroalcoxy-
carbonylation, only a few studies on the kinetics of these
reactions have been reported [3, 7, 9–15], from which only
two are related to hydromethoxycarbonylation reactions [7,
14]. In the present work, we report an exhaustive study of
the olefin hydroalcoxycarbonylation (olefin = 1-hexene,
cyclohexene and styrene; alcohol = MeOH, EtOH,
n-PrOH, i-PrOH) catalyzed by palladium systems con-
taining triphenylphosphine, including detailed kinetic,
coordination chemistry and computational DFT studies on
the 1-hexene hydrometoxycarbonylation.
[
4, 30, 31].
Recently reported papers involve the use of new mono-
and bidentate phosphines for the hydroalcoxycarbonylation
reactions. Aguirre et al. [32] reported the use of palladium
precatalysts with P,N ligands [2-(diphenylphosphinoamino)-
2 Experimental
All manipulations and reactions were conducted with rig-
orous exclusion of air. TSA was purified by recrystalliza-
tion and dried under vacuo, whereas 1-hexene, styrene,
cyclohexene and solvents were dried over the corre-
sponding reagents and distilled at reduced pressure prior to
use [41]. The IR spectra of the complexes (in KBr disks)
pyridine (Ph PNHpy), 2-(diphenylphosphinomethyl)pyri-
2
dine (Ph PCH py) and 2-(diphenylphosphino)quinoline], in
2
2
the presence of para-toluenesulfonic acid (TSA) for the
hydromethoxycarbonylation of cyclohexene, 1-hexene
and styrene; the complex [PdCl(PPh )(Ph NHpy)]Cl and
3
2
1
the equimolar mixture of PdCl (Ph NHpy) and PPh
3
were recorded on a Shimadzu 8300 FT-IR instrument. H-,
3
2
2
1
1
31
1
C{ H}- and P{ H}-NMR spectra were recorded on a
demonstrated to be the most active systems for these
reactions. Some of these authors also reported that palla-
dium complexes containing the 1,1-bis(diphenylphos-
phino)ferrocene (dppf) ligand showed higher catalytic
activity than those having the naphthyridine ligand in its
structure for the olefin hydromethoxycarbonylation; again,
Bruker AM-300 spectrometer; chemical shift are expressed
in ppm upfield from Me Si and H PO , respectively.
4
3
4
2.1 Catalytic Reactions and Kinetic Study
the addition of PPh to these systems increased the activity
3
The catalytic reactions including those corresponding to
the kinetic study were carried out in a high pressure reactor
(125 mL), supplied by Parr Instruments (model 4561),
which was provided with arrangements for sampling of
liquid contents, automatic temperature and pressure control
as well as variable stirring speed. In a typical run, a solu-
and selectivity of these precatalysts [33]. Subsequently,
Klahn and coauthors [34] used
a
diphenylpho-
sphinocyrhetrene palladium complex in the styrene
5
hydromethoxycarbonylation;
trans-[(g -C H PPh )-
5
4
2
Re(CO) ]PdCl (NCMe) in the presence of PPh3
3
2
(
ratio = 1:2) and HCl as acidic promoter, showed good
tion of PdCl , the corresponding equivalents of triphenyl-
2
catalytic activity and excellent regioselectivity towards the
branched products.
phosphine and TSA, the substrate, the alcohol, isooctane
(7.5 mmol) as the internal standard and toluene were added
to the reactor. The solution was carefully deoxygenated
with argon, and the reactor charged with CO (5–50 atm)
and heated to a pre-set temperature. At the end of the
reaction, the catalytic mixtures were analyzed by gas
chromatography, using a 3300 Series VARIAN instrument
fitted with a flame ionization detector (FID) and a 2 m
20 % SP-2100 on a 0.1 % carbowax 100/120 Supelcoport
Claver and coauthors [35] reported the use of palladium
complexes containing novel cis-1,2-bis(di-t-butyl-phos-
phinomethyl) carbocyclic ligands as catalytic precursors
for the ethylene hydromethoxycarbonylation; some aspect
of catalytic cycle have been elucidated from HP NMR
spectroscopic study.
On the other hand, Pringle and coauthors [36, 37] reportedthe
efficient and chemoselective ethylene hydromethoxycarbonyla-
tion by using of a series of Pd-complexes containing heterodi-
column, using N as carrier gas. Products were quantified
2
with a microcomputer coupled to Measurement Computing
interphase using the Cromat 1.2 software; the organic
products were additionally identified by GC/MS HP
5890/5971 coupled system using a Quadrex PONA 5 %
phenyl methyl silicone, 25 m, 320 lm column. Each
reaction was repeated at least twice in order to assure there
0
phosphines of the type o-Ph(CH PR )(CH PR ); the complexes
2
2
2
2
containing the bulkiest ligands showed the highest activity.
Finally, Toniolo and coauthors [38–40] have recently pub-
lished a series of new triphenylphosphine modified palladium
precatalysts for the ethylene hydromethoxycarbonylation as
1
23

Kinetics and Mechanisms of Homogeneous Catalytic Reactions
1719
producibility of the results. Statistical studies were per-
formed using analysis of variance (ANOVA) to test the
difference between means. Significant differences were
considered when P \ 0.05.
2.3.3 Interaction of PdCl
(PPh ) with the Other
2 3 2
Components of the Catalytic Reactions
In an NMR tube, complex PdCl
(PPh
)
2
(10 mg) was
2
3
dissolved in d -benzene (0.5 mL) and the reaction mixture
6
was heated at 100 °C for 30 min; NMR spectra were
acquired. In a series of different experiments, to this
solution were added the components of the catalytic reac-
tion one by one and consecutively (TSA, substrate and
methanol) in different order of addition each time. After
the addition of each component, the tube was heated at
100 °C by 30 min and NMR spectra were acquired before
the addition of the following component.
2
.2 Kinetic Calculations
The percentage of conversion was restricted to 5–10 % in
order to calculate the initial rate (r ) of the reaction [42].
o
The data were plotted as molar concentration of the pro-
ducts versus time yielding straight lines, which were fitted
2
by conventional linear regression programs (r [ 0.95); r
values were obtained from the corresponding slopes. The
dissolved CO concentrations in the reaction medium were
calculated from solubility data reported elsewhere [43].
o
2.4 Computational Details
The optimized energy calculations for all palladium inter-
mediates involved in olefins hydromethoxycarbonylation
2
.3 Coordination Chemistry Related with Olefin
Hydromethoxycarbonylation
process were carried out using simplified PH analogues
3
and ethylene as the substrate in order to reduce the com-
putational cost. These calculations were performed with the
Gaussian 2003 (G03) computational package [45]. The
unconstrained geometry optimizations and frequency cal-
culations were performed at the hybrid functional B3LYP
The coordination chemistry studies related with the olefin
hydromethoxycarbonylation reactions were performed by
the reaction of PdCl /6PPh or a palladium triphenyl-
2
3
phosphine complex with one or several of the reaction
components (TSA, olefin, methanol and/or carbon mon-
oxide), either in a high pressure reactor or in an NMR tube.
[
46–48] combined with the effective core potential (ECP)
LanL2DZ for the Pd, P and Cl atoms, and the extended
1
31
1
The reaction was monitored by H, P{ H} and/or
3
basis set 6-31?G(d,p) for the C, H and O atoms [49–52].
1
1
C{ H} NMR.
2
.3.1 Interaction of PdCl /6PPh with Carbon Monoxide
2
3 Results and Discussion
3
In a reactor, PdCl (89 mg, 0.5 mmol) and an excess of
2
The carbonylation of olefins with alcohols (hydroalcoxy-
carbonylation) was studied using palladium systems con-
taining triphenylphosphine as precatalysts (olefin = 1-
hexene, styrene and cyclohexene; alcohol = MeOH,
EtOH, n-PrOH and i-PrOH); linear and branched esters
were obtained in the case of 1-hexene and styrene (Eq. 1,
R = n-Bu, Ph), whereas in the cyclohexene carbonylation
only one product was obtained (Eq. 2). The catalytic sys-
tems were prepared in situ by addition of n equivalents of
triphenylphosphine to palladium dichloride in the presence
of m equivalents of para-TSA, PdCl /nPPh /mTSA, n and
triphenylphosphine (0.8 g, 3 mmol) were placed in toluene
(
10 mL) under a carbon monoxide pressure of 10 atm; the
reaction mixture was heated at 125 °C. After 4 h, the
solution was transferred to a Schlenk tube and the solvent
was evaporated at reduced pressure. The obtained yellow
solid was thereafter characterized as PdCl (PPh ) [44].
2
3 2
1
Yield = 50 %. H NMR (CDCl , 300 MHz, ppm): 7.7 and
3
3
.4 (series of m); P{ H} NMR (CDCl , 121 MHz, ppm),
1
1
7
3
2
4.5 (s).
2
3
m varying in the range of 2–10.
2
.3.2 Interaction of PdCl /6PPh /5TSA with Olefins
2
3
in Presence of Carbon monoxide
O
O
CO/R'OH
cat
R
These reactions were carried out through the procedure
described in the prior section adding five equivalents of
TSA (2.5 mmol) and either 1-hexene or cyclohexene
R
+
R
OR'
OR'
R = n-Bu, R' = Me, Et, n-Pr, i-Pr
R = Ph, R' = Me
(
1 mL) obtaining yellow oils, which were characterized by
ð1Þ
NMR.
123

1
720
M. Rosales et al.
100
Table 1 Hydromethoxycarbonylation of 1-hexene catalyzed by pal-
ladium systems PdCl /nPPh /mTSA
2
3
Entry Acid
T (°C) [Pd]:[TSA] [Pd]:[PPh
3
]
TON l/b
8
6
4
0
0
0
1
2
3
4
5
6
7
8
9
0
TSA
TSA
TSA
TSA
TSA
TSA
TSA
TSA
125
125
125
125
125
125
125
100
125
125
1:2
1:5
1:10
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:6
1:6
1:6
1:2
1:4
1:8
1:10
1:10
1:6
1:6
24
180
87
1.1
1.0
0.8
1.0
1.0
1.1
1.2
1.3
1.0
0.9
1-hexene
Heptanoic acid methyl ester
89
2
-methylhexanoic acid methyl ester
135
90
48
20
0
39
2
H SO
4
111
120
1
HCl
0
2000 4000 6000 8000 10000 12000 14000 16000
t (s)
[
Pd] = 1.67 mM,
[1-hexene] = 0.50 M,
[MeOH] = 1.25 M,
p(CO) = 50 atm, solvent: toluene (total volume 30 mL), t = 4 h.
Catalytic activity expressed in turnover number (TON) and regiose-
lectivity in linear to branched ratio (l/b)
Fig. 1 Typical reaction of the 1-hexene hydromethoxycarbonylation
catalyzed by the PdCl /6PPh /5TSA system
2
3
and 10). The order of activity was H SO \ HCl \ TSA,
2
4
which is consistent with the strength of the acid; the sul-
furic acid has the highest acidity and the system shows the
lowest activity whereas the TSA is the weakest acid and
therefore the catalytic system presents the best activity; in
all cases, the l/b ratio was close to the unit. The higher
O
CO/R'OH
cat
OR'
ð2Þ
activity of TSA with regard to HCl and H SO may be also
2
4
explained due to the formation in the reaction medium of
different palladium complexes, such as Pd(TS) (PPh ) ,
R' = Me
2
3 2
Pd(SO )(PPh ) or PdCl (PPh ) , and to the presence of an
4
3
.1 Hydromethoxycarbonylation of Olefins
3 2
2
3 2
ion-pair with TSA [6, 7, 14, 39].
For comparison, the hydromethoxycarbonylation of
other olefins (styrene and cyclohexene) were also studied
using the PdCl /6PPh /5TSA system as precatalyst,
Preliminary tests of the carbonylation reactions were carried
out using 1-hexene and methanol (1-hexene hydro-
methoxycarbonylation). As PPh and TSA concentrations
3
2
3
although this ratio may depend on the substrate nature;
these carbonylations proceeded even under lower reaction
conditions: [Pd] = 1.67 mM; [MeOH] = 1.25 M; [sub-
strate]:[catalyst] ratio = 300:1; P(CO) = 8 atm. This pal-
ladium system was an efficient precatalyst for the
hydromethoxycarbonylation of all substrates; the linear
olefin (1-hexene) was carbonylated three times faster
are important factors in the catalytic processes, reactions
were carried out at several [Pd]:[PPh ] and [Pd]:[TSA]
3
ratios. All of these systems were efficient and regioselective
precatalysts for the 1-hexene hydromethoxycarbonylation
0
(
(
(
Eq. 1, R = C H , R = Me) to give methyl heptanoate
4
9
heptanoic acid methyl ester) and methyl 2-methylhexanoate
2-methylhexanoic acid methyl ester); results are shown in
-
1
-1
(TOF = 143 h ) than styrene (TOF = 44 h ), whereas
Table 1.
the cyclic olefin (cyclohexene) showed the lowest activity
The increase of either acid (entries 1–3) or PPh con-
3
-
TOF = 10 h ). Under these conditions, the regioselec-
1
(
centrations (entries 2 and 4–7) had a positive effect on the
tivity of this system was higher towards the linear product
for 1-hexene (l/b = 2.4) and towards the branched product
for styrene (l/b = 0.4).
activity up to a threshold value above which the inhibition
of the reaction was observed; the best [Pd]:[TSA] and
[
Pd]:[PPh ] ratios were 1:5 and 1:6, respectively (entry 2).
3
Regarding the effect of temperature, the activity of the
system PdCl /6PPh /5TSA was four times lower at 100 °C
entry 8) than at 125 °C (entry 2), whereas at temperatures
3.2 Kinetics Investigation of the 1-Hexene
Hydromethoxycarbonylation
2
3
(
above 125 °C, this system was not catalytically active
possibly due to the formation of palladium metal. We
further examined the effect of acid nature on the activity
and selectivity of PdCl /6PPh /5HX systems (entries 2, 9
The
kinetic
study of
the
1-hexene
hydro-
methoxycarbonylation catalyzed by PdCl /6PPh /5TSA,
2
3
the most active system, was performed using toluene as the
2
3
1
23

Kinetics and Mechanisms of Homogeneous Catalytic Reactions
1721
Table 2 Kinetic data for the hydromethoxycarbonylation of 1-hex-
ene catalyzed by Pd/6PPh /5TsOH
(0.5) on catalyst concentration for cyclohexene hydro-
3
methoxycarbonylation catalyzed by a Pd/PPh system;
3
3
Entry 10 [Pd] [olefin] 10 [CO] [MeOH] 10 r
2
5
however, these authors did not give any explanation
about this finding.
i
l/b
-
(M s )
1
(M)
(M)
(M)
(M)
2
.
The initial carbonylation reaction varies according to a
saturation curve with increasing 1-hexene concentra-
tion. When the 1-hexene concentration is lower than
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
0.75
1.25
1.76
2.11
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.68
1.68
1.68
1.68
1.25
0.38
0.38
0.38
0.38
0.28
0.30
0.46
0.54
0.60
0.38
0.38
0.38
0.38
0.38
0.38
0.38
0.38
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
1.9
7.1
11.8
2.8
2.8
2.8
2.8
2.8
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.49
0.61
1.23
1.54
0.93
3.91
5.56
8.95
9.63
3.87
4.10
5.78
6.14
6.13
3.90
2.2
2.6
2.8
3.6
2.7
2.8
3.1
2.7
2.6
3.1
1.9
1.5
3.1
2.8
2.7
2.9
2.8
0
.38 (entries 2, 5 and 6), the reaction is close to first
order, whereas the reaction rate does not depend on the
-hexene at concentrations higher than 0.54 (entries 9
1
and 10). This result is consistent with the substrate
involved in a pre-equilibrium prior to the rate-deter-
mining step (rds) of the cycle, which is displaced to the
right at high substrate concentrations. Benedek et al.
1
1
1
1
1
1
1
1
[
53] reported that the alkyl intermediate formed in the
11.2
14.2
2.28
deuteroalkoxycarbonylation of 1-hexene catalyzed by
PdCl (PPh ) undergo reversible b-elimination even at
2
3 2
low reaction temperature.
4.02
6.24
7.10
2.02
3.
The dependence of the initial rate with respect to the
dissolved carbon monoxide concentration (entries 2 and
10–12) was of fractional order; the graph of log r versus
o
a
log [CO] yields a straight line, whose slope was 0.7. This
order indicates that CO is also involved in a reversible
reaction prior to the rds of the mechanism.
Solvent: toluene (total volume 40 mL). T = 398 K. Regioselectivity
expressed in linear to branched ratio (l/b)
a
CD
3
OD
4. The initial reaction rate shows a direct dependence as a
function of the methanol concentration (entries 2 and
14–17), thus indicating a first order dependence with
solvent at 398 K. A typical reaction profile is presented in
Fig. 1, which shows the decrease of the substrate (1-hex-
ene) concentration and the increase of the corresponding
products (methyl heptanoate and methyl 2-methylhexano-
ate) as a function of time. In all of reactions, there is an
induction period (20–30 min) which was necessary for the
formation of catalytically active species.
respect to this parameter. Consistent with other reports
[7–14], this finding may be explained if methanol is
directly involved in the rds of the catalytic cycle.
5. A decreasing in reaction rate was observed when the
reaction was carried out in d -MeOH (1-hexene
4
deuteromethoxycarbonylation) instead of MeOH
(entry 17); a primary kinetic isotope effect (KIE) of
2.9 was found, thus confirming that methanol is
involved in the rds of the reaction.
In order to study the effect of each component on the
reaction rate, four experiment sets were carried out varying
the concentration of only one of the component within each
set while keeping constant the concentration of the other
components at 398 K. The concentration ranges studied
were: [Pd] = 0.75–2.11 mM, [1-hexene] = 0.28–0.60 M,
6. The linear to branched ratio (l/b) of the esters formed
by the use of palladium precursor varied from 1.5 to
3.6. A negative effect on l/b ratios was observed with
the increment of dissolved CO and methanol concen-
trations and a positive effect with the increase of
catalyst concentration; no substantial changes in reg-
ioselectivity with the variation of substrate concentra-
tion were observed. Similar results have been reported
previously for the hydroformylation of olefins with Rh/
PPh [54] and Rh/AsPh [55] systems.
-
2
[
MeOH] = 0.49–1.54 M; [CO] = 1.9 9 10 –0.118 M.
The results are summarized in Table 2; the standard con-
ditions are depicted in entry 2. Plots of log r versus the
o
logs of the concentration of each component are shown in
Fig. 2. The kinetic results indicate that:
3
3
1
.
The dependence of the reaction rate on the concentra-
tion of the catalyst (entries 1–4) is close to one as the
In view of the previous observations, the 1-hexene
hydromethoxycarbonylation catalyzed by PdCl /6PPh /
graph of the log r versus log [Pd] yields a straight line
o
2
3
with a slope of 0.92. This finding is consistent with a
mononuclear complex as the catalytically active spe-
cies. Aver’yanov et al. [14] reported a fractional order
5
TSA system proceeds according to the rate law:
a
r0 ¼
½Pdꢁ½olefinꢁ½COꢁ½MeOHꢁ ð3Þ
b þ c½olefinꢁ þ d½CO]
123

1
722
M. Rosales et al.
-
-
5
5
6,5x10
6,0x10
5,5x10
5,0x10
4,5x10
4,0x10
3,5x10
(a)
(b)
-
-
-
-
-
4,0
4,1
4,2
4,3
4,4
-5
-
-
-
-
5
5
5
5
0
,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65
-3,2
-3,1
-3,0
-2,9
-2,8
-2,7
-2,6
[1-hexene]
log [Pd]
-
-
-
-
-
-
-
-
3,8
3,9
4,0
4,1
4,2
4,3
4,4
4,5
-4,0
(c)
(d)
-
4,1
4,2
4,3
4,4
4,5
4,6
4,7
4,8
-
-
-
-
-
-
-
-
1,8
-1,6
-1,4
-1,2
-1,0
-0,8
-0,3
-0,2
-0,1
0,0
0,1
0,2
Log [CO]
log [MeOH]
Fig. 2 Plots of log r
o
versus the log of the concentration of each component: catalyst (a), 1-hexene (b), carbon monoxide (c) and methanol (d)
in each experimental kinetic study. The simplicity of our
Where a, b, c and d are constants, rate constants, equilib-
rium constants and/or their products.
findings may stem from the performance of the kinetic
studies under the better [PPh ]:[Pd] and [TSA]:[Pd] ratios
3
Our findings considerably differ from kinetic studies
performed by other authors. In his review, Kiss [3] described
that kinetics of palladium-catalyzed olefin hydrocarboxyla-
tion, a reaction related to hydroalcoxycarbonylation, is of
first order on palladium catalyst, varies between first and zero
order on substrate, while for water (the nucleophile) and CO,
the order is greater than one. On the other hand, for the
cyclohexene hydromethoxycarbonylation, Toniolo and
coauthors [7] reported for the Pd(TS) (PPh ) -catalyzed
and the use of toluene as a non-coordinating solvent.
3.3 Hydroalcoxycarbonylation of 1-Hexene
The carbonylation of 1-hexene was carried out using other
alcohols (EtOH, n-PrOH and i-PrOH) as nucleophiles (1-
hexene hydroalcoxycarbonylation); the results are listed in
Table 3. As is depicted (entries 1–3, Table 3), the values of
TOF (turnover frequency, moles of products per mole of
2
3 2
reaction (TS is the anion para-toluenesulfonate), that the
reaction rate is of first order on catalyst, substrate and
methanol and passes through a maximum with increasing of
the CO pressure, whereas Aver’yanov et al. [14] reported for
the PdCl (PPh ) /PPh /TSA-catalyzed reaction that it is of
catalyst in 1 h of reaction) and r (initial reaction rate)
o
increased with the increment of carbon length of linear
aliphatic alcohols such as methanol, ethanol and n-propa-
nol, thus reflecting an electronic effect (inductive effect).
Hence, the reaction with n-propanol was the fastest, fol-
lowed closely by ethanol while methanol was the slowest
one. On the other hand, increasing the bulkiness of the
alcohol slightly retarded the reaction: the reaction rate with
2
3 2
3
fractional order on catalyst (0.5), first order on cyclohexene
and passes through maxima for CO and methanol. All of
these differences may be explained by the variety of reaction
conditions employed as well as the nature of the solvent used
1
23

Kinetics and Mechanisms of Homogeneous Catalytic Reactions
1723
Table 3 Data for the hydroalcoxycarbonylation of 1-hexene cata-
lyzed by Pd/6PPh /5TsOH
NMR spectrum showed only two multiplets at 7.7 and
3
7
.3 ppm corresponding to phenyl groups of the triphenyl-
3
1
1
-
1
)
5
10 r
-1
phosphine, whereas the P{ H} NMR showed a singlet at
4.5 ppm. These spectroscopic data are consistent with the
formula PdCl (PPh ) (1); this complex was also prepared
Entry
Alcohol
TOF (h
o
(M s
)
l/b
2
1
2
3
4
Methanol
Ethanol
123
131
137
108
5.56
10.90
13.00
9.64
2.6
1.9
2.0
2.1
2
3 2
by reaction of PdCl with concentrated HCl to generate
2
Propanol
Isopropanol
H [PdCl ], thereafter followed by the addition of PPh
3
2
4
[
44]. On the other hand, this reaction carried out in the
Conditions: [Pd] = 1.25 mM; T: 125 °C; P(CO) = 8 atm;
ROH] = 0.93 M. Solvent: toluene (total volume 40 mL)
presence of Zn, or the reaction of complex 1 with an excess
of four equivalents of PPh in presence of Zn, yielded a
[
3
mixture of Pd(PPh ) (2), complex 1, triphenylphosphine
3
4
3
oxide and free PPh , as corroborated by the P{ H} NMR
1
1
3
spectrum. The reaction of 1 or 2 with excess of TSA did
not yield the corresponding metal-hydride complexes,
which was indicative of hydride species too unstable to be
isolated from the reaction medium.
When the reaction of PdCl /6PPh /5TSA (or 1/4PPh /
3
2
3
5
TSA) with CO was carried out adding an excess of olefin
1-hexene or cyclohexene), hard-to-handle bright yellow
oils were obtained, which could not be isolated as solids.
(
1
31
1
13
1
H, P{ H} and C{ H} NMR spectra of these oils
indicated the presence of several species, thus precluding
1
3
1
their full elucidation. However, the C{ H} NMR spectra
showed two signals at 179 and 182 ppm, which can be
possibly attributed to the presence of two acyl type species,
[Pd–C(O)(CH ) CH and Pd–C(O)CH(CH )(CH ) CH ]
2 5 3 3 2 3 3
Fig. 3 Coordination chemistry related to 1-hexene hydromethoxy-
carbonylation catalyzed by the PdCl /6PPh /5TSA system
for 1-hexene, and one signal at 179 for the reaction with
cyclohexene [Pd–C(O)(c-C H )]. Similar types of acyl
2
3
6
11
complexes have been reported to be isolated from the
palladium-catalyzed hydroalcoxycarbonylation of olefins.
Toniolo and coauthors [56] have reported the isolation of
complex trans-PdCl(C(O)(CH ) CH )(PPh ) in the hy-
isopropanol (entry 4) was lower than with n-propanol
entry 3). These results are, together with the first order
2
5
3
3 2
(
droalcoxycarbonylation of 1-hexene, as well as the reac-
tivity studies of this type of Pd–acyl complexes, which is
relevant to the catalytic cycle [57]; more recently, some of
them reported the isolation and characterization of trans-
PdCl(C(O)CH CH )(PPh ) [39].
kinetics on MeOH concentration and the values of KIE, in
concordance with the alcoholysis as the rds of the hydro-
alcoxycarbonylation reaction.
2
3
3 2
3
.4 Coordination Chemistry Related with Olefin
Hydromethoxycarbonylation
The reactions of complex 1 with each component of the
catalytic reaction were also performed in an NMR tube
using d -toluene as the solvent at 100 °C. The only reaction
8
In order to gain further understanding on the mechanism of
the olefin hydrometoxycarbonylation catalyzed by the
PdCl /6PPh /5TSA system, particularly to isolate and/or
that yielded a new species was the reaction of 1 with
MeOH in the presence of 4 equivalents of PPh . After 20 h,
3
approximately 50 % of complex 1 was transformed into
complex 2; in this reaction, formaldehyde was also detec-
ted by GC. This finding may be explained through the
Eq. 4 as proposed by Toniolo and coauthors [6, 7] and
Aver’yanov et al. [14]. Formaldehyde in the presence of
methanol yields hemiacetal or acetal but small amounts of
water may hydrolyze these products back to the original
2
3
detect some intermediates of the reaction, the interaction of
PdCl with each component of the catalytic mixture was
2
carried out separately (triphenylphosphine, TSA, olefin,
methanol and carbon monoxide); results are displayed in
Fig. 3.
The reaction of PdCl with six equivalents of triphen-
2
ylphosphine in toluene at 125 °C under 10 atm of CO
yielded a yellow precipitate. The FT-IR spectrum of this
compounds (CH O and MeOH). When isopropanol is used
2
instead of MeOH, small amounts of acetone were also
detected [59].
1
complex showed no bands corresponding to m(CO). The H
123

1
724
M. Rosales et al.
3.5 Proposed Mechanism for Olefin
Hydroalcoxycarbonylation
On the basis of our experimental findings (kinetic and
coordination chemistry studies), of some studies using HP
NMR spectroscopy performed for palladium complexes
containing triphenylphosphine [38–40] and diphosphine
ligands [35], and the present knowledge on palladium-
catalyzed olefin hydroalcoxycarbonylation [3–5], we pro-
pose a catalytic cycle depicted in Fig. 4 for the linear ester
(branched ester excluded for clarity); experimental evi-
dence is more in favor of the so-called ‘‘hydride mecha-
?
n
nism’’. Complexes of the type [Pd(H)(L)(PPh ) ] (A),
3
2
L = Cl, n = 0; L = CO, MeOH, olefin, n = 1, in a cis
?
configuration or [Pd(H)(PPh ) ] are considered the pos-
3
3
sible catalytically active species (although trans species are
also possible, vide infra). Zudin et al. [60] elucidated that
?
Pd(H)(PPh ) ] was the key palladium intermediate for
[
3
3
Fig. 4 Catalytic cycle for the 1-hexene hydromethoxycarbonylation
catalyzed by the PdCl /6PPh /5TSA system
the hydrocarbonylation of ethylene to diethyl ketone, a
reaction also related to the hydroalcoxycarbonylation of
olefins.
2
3
The first step of the cycle is the reversible insertion of
ðnꢂ2Þ PPh3
n ¼ 2ꢂ4
the olefin in the Pd–H bond of A (K ) to yield the Pd–alkyl
1
PdCl2ðPPh3Þ þ CH3OH ꢂ! PdðPPh3Þn
2
?n
species [Pd(C H )(L)(PPh ) ] (B). The following step is
6
13
3 2
þ CH2O þ 2 HCL
Based on this coordination chemistry results, we think
ð4Þ
the CO insertion into the Pd–C bond of B to generate the
n
?
Pd–acyl
intermediate
[Pd(C(O)C H )(L)(PPh ) ]
6 13 3 2
(
C) according to the equilibrium K ; intermediates B and
2
that under the reaction conditions, complex 1 is trans-
a
C are possibly generated through pentacoordinated inter-
mediates containing olefin and CO ligands, respectively.
The cycle is completed by the reaction of Pd–acyl species
C with methanol (methanolysis) to yield the methyl esters,
regenerate the active species and restart the cycle; this
reaction is considered the rds of the mechanism and could
occur through an intra- or intermolecular attack of meth-
anol. van Leeuwen et al. [61] and Liu et al. [62] established
that the alcoholysis of acetyl intermediates in diphosphine
palladium systems is thought to be an intramolecular
nucleophilic attack of alcohol at the acyl group; however,
Donald et al. [63], using theoretical DFT calculations,
proposed that the reaction is intermolecular for diphos-
phine palladium systems. We believe that the reaction
catalyzed by the triphenylphosphine palladium system
could proceed through either a pentacoordinated species
with the alcohol bonded to the palladium metal center and
then an hexacoordinated intermediate resulting from the
oxidative addition of the methanol (intramolecular path-
way) or an external nucleophilic attack of alcohol on the
acyl carbon atom (intermolecular pathway).
formed into
palladium(0) species of the type
Pd(L)(PPh ) , which contains some other neutral ligand
3
2
(
L = methanol, olefin, CO or PPh ), which are protonated
3
by TSA to form the corresponding cationic hydrido-palla-
?
dium species of the type [Pd(H)(L)(PPh ) ] . Neutral
3
2
complexes of the type Pd(L)(PPh ) with L = CO, PR
3
3
2
[
17] or olefin [17, 58] and the corresponding cationic hy-
drido species containing MeOH [35, 59] or a monophos-
phine ligand [17, 40, 60] have been isolated. This notion
was further supported by the fact that when the catalysis
was performed in the presence of Zn (conditions as in entry
2
1
of Table 2), the reaction took place faster (7.85 9
-
6
-1
M s ) than in its absence (5.56 9 10 M s ),
-6
-1
0
although the TOF values were rather similar (123 and
-
33 h , respectively). Although, we suggest that the
1
1
active species were cationic hydride complexes, the neutral
species PdHCl(PPh3)2, also generated from complex 1,
could not be discarded. Another possibility is that under the
catalytic conditions, the palladium hydride intermediate
could also obtain directly from PdCl (PPh ) and MeOH
2
3 2
via b-hydride elimination from a molecule of alcohol
coordinate to the palladium center as reported by Clegg
et al. [59]; in this case, TSA could act as a stabilizer of the
Pd–H species against deprotonation to generate Pd(0)
species.
According to this mechanism, the rate law for 1-hexene
hydromethoxycarbonylation catalyzed by Pd/triphenyl-
phosphine system may be derived by applying the Equi-
librium Approximation. As methanolysis of Pd–acyl
1
23

Kinetics and Mechanisms of Homogeneous Catalytic Reactions
1725
species is considered the rds, the reaction rate may be
expressed as:
Table 4 Comparison between calculated and experimental structural
and PdCl
parameters for PdCl
2
(PH
)
3 2
2
(PPh
3 2
)
Parameter
PdCl
2
(PPh
)
3 2
PdCl
2 3
(PH )
2
% error
r ¼ k ½Cꢁ½MeOHꢁ
ð5Þ
Considering the equilibria K ([B] = K [A][olefin]) and
o
3
´
˚
Pd–P (A)
2.33
2.29
2.34
2.37
0.04
3.4
1
1
´
˚
Pd–Cl (A)
K2 ([C] = K [B][CO]), the concentration of C ([C] =
2
K K [A][olefin][CO]) can be substituted in Eq. 4 and the
1
P–Pd–Cl (°)
92.0
92.1
0.1
2
rate expression may be rewritten as:
ro ¼ K1K2k3½Aꢁ½olefinꢁ½COꢁ½MeOHꢁ
Taking into consideration the mass balance of palladium
ð6Þ
In order to reinforce the mechanisms of the reaction
catalyzed by the PdCl /6PPh /5TsOH system which was
2
3
(
[Pd] = A ? B ? C), Eq. 6 may be transformed in:
o
proposed on the basis of our experimental findings, the
Gibbs free energy (DG°) for the three possible elementary
steps (olefin insertion, CO insertion and methanolysis)
were calculated via DFT. We use ethylene as a model for
K1K2k3
r0 ¼ 1
þ K1½olefinꢁ þ K1K2½olefinꢁ½COꢁ
½
Pdꢁ ½olefinꢁ½COꢁ½MeOHꢁ
ð7Þ
This equation is similar to the rate law obtained in the
0
the substrate and the simplified PH model, in which the
3
phenyl groups of the original ligand were replaced by
hydrogen. In order to test our model-methodology, we
calculated the structural parameters of the optimized
structure of PdCl (PH ) and compared it with experi-
kinetic study, where a = K K k , b = 1, c = K and
2 3
1
1
d = K K [olefin], and explains the fractional order
1
2
obtained on CO concentration and the saturation curve
observed for [olefin].
2
3 2
mental studies (Fergunson) of X-ray structure for the
complex PdCl (PPh ) [64]. The obtained bond distances
At low substrate concentration (K [olefin] ? K K [ole-
1
1
2
2
3 2
fin][CO] ꢃ 1), Eq. 7 is transformed to Eq. 6 (first order on
and angles for PdCl (PH ) were in good agreement with
2 3 2
each component), whereas at high [olefin], K [ole-
1
the experimental findings (Table 4). The agreement
between the calculated structural parameters and the
experimental values (less than 5 % error) showed that the
hybrid method used in this work can provide reliable
geometries for the species involved along the entire cata-
lytic cycle.
fin] ? K K [olefin][CO] ꢄ 1, therefore, Eq. 6 can be
1
2
rewritten as:
K2k3
þ K2½COꢁ
r0 ¼
½Pdꢁ ½COꢁ½MeOHꢁ
ð8Þ
0
1
which explains the zero order with respect to olefin when
its concentration was higher than 0.54 M; under these
conditions the order on CO concentration is fractional and
it is first order on catalyst and MeOH concentrations.
Similar mechanisms should operate when the reaction of
The calculation were performed starting from five dif-
?
ferent active species, [Pd(H)(L)(PH ) ] (L = Cl, n = 0;
n
3
2
L = CO, MeOH or olefin, n = 1) both in a cis or trans
?
configuration and [Pd(H)(PH ) ] . Calculations were car-
3
3
ried out at 398 K and 8 atm in order to simulate the
experimental conditions; the results are listed in Table 5.
As may be observed, the three elementary step of the
cycle are thermodynamically favored (DG is negative)
1
-hexene carbonylation is performed in presence of other
alcohols (hydroalcoxycarbonylation), such as EtOH,
n-propanol and i-PrOH, instead of methanol.
?
for[Pd(H)(PH ) ] and for both configurations (cis and
3
3
?
n
3
.6 DFT-Calculations of the Elemental Steps
of the Olefin Hydromethoxycarbonylation
Mechanisms
trans) of [Pd(H)(L)(PH ) ] (L = Cl, n = 0; L = CO,
3 2
MeOH or olefin, n = 1), except the step of CO insertion on
the Pd–alkyl species containing ethylene ligand in a trans
configuration, for which a positive DG was obtained. In
general, the elementary steps involving species in cis
configuration were somewhat more spontaneous than those
involving trans ones. The obtained theoretical results
indicate that the catalytic cycle may be initiated by any of
In spite of its industrial importance, the olefin hydro-
methoxycarbonylation had not received much attention
from the molecular modeling point of view. Donald et al.
[
63] investigated the methanolysis of a Pd–acyl species, an
?
n
elementary reaction which is considered the rds in our
mechanism, through density functional theory; calculations
suggest that intermolecular attack of methanol may be
important in the methanolysis of simple Pd–acyl systems
and that the energetics of this process are strongly depen-
dent on the metal coordination environment.
the [Pd(H)(L)(PH ) ] species; however, we believe that
3 2
?
the species [Pd(H)(MeOH)(PPh ) ] and [Pd(H)(PPh ) ]
?
3
2
3 3
may be the most probably catalytically active species
participating in the catalytic cycle since [Pd(H)(MeOH)
?
(diphosphine)] [59] and [Pd(H)(PPh ) ] [60] have been
?
3
3
isolated in prior works.
123

1
726
M. Rosales et al.
Table 5 Gibbs free energy
-
Elementary step in the cycle
L
1
(
DG°, K cal mol ) of the
possible elementary steps of the
Pd-catalyzed
Cl
CO
MeOH
CH
2
=CH
2
PPh
3
trans cis
trans cis
trans cis
trans
cis
hydromethoxycarbonilaci o´ n of
olefins calculated from the DFT
study at 398 K and 8 atm
1. Olefin insertion
2. CO insertion
3. Methanolysis
-2.5 -5.5 -3.2 -6.4 -2.8 -6.8
-6.3 -8.1 -6.2 -5.6 -8.1 -5.2
-4.5 -3.9 -2.7
16.5 -5.8 -5.6
-8.0 -3.2 -7.4 -4.7 -5.7 -4.3 -30.8 -7.0 -8.5
More detailed DFT theoretical calculations are in pro-
gress in order to explore the potential energy surfaces
10. Kron TE, Terekhova MI, Noskov YG, Petrov ES (2001) Kinet
Catal 42:204
1
1
1. Noskov YG, Petrov ES (2001) Russ Chem Bull (Int Ed) 50:1839
2. Kron TE, Terekhova YG, Petrov ES (2004) Kinet Catal 45:551
(
PES) that connect to these step sequences of the catalytic
process (transition states), aimed at better understanding
each step of the catalytic cycle.
13. Aver’yanov VA, Sevost’yanova NT, Batashev SA, Nosova NM
(2006) Kinet Catal 46:405
1
1
1
1
4. Aver’yanov VA, Batashev SA, Sevost’yanova NT, Nosova NM
2006) Kinet Catal 47:375
5. Averyanov VA, Nosova NM, Astashina EV, Sevostyanova NT
(2007) Kinet Catal 47:167
6. Seayad A, Jayasree S, Damodaran K, Toniolo L, Chaudhari RV
(
4
Conclusions
(
2000) J Organomet Chem 601:100
7. P e´ rez PJ, Calabrese JC, Brunel EE (2001) Organometallics
0:337
The PdCl /6PPh /5TSA system (125 °C, 8–50 bar of CO)
2
3
was an efficient and regioselective precatalyst for the
hydromethoxycarbonylation of olefins (1-hexene, cyclo-
hexene and styrene) to generate the corresponding methyl
esters, under mild reaction conditions; the order of indi-
vidual activities was 1-hexene [ styrene [ cyclohexene. A
kinetic study of the 1-hexene hydromethoxycarbonylation
catalyzed by PdCl /6PPh /5TSA together with coordina-
2
18. Del Rio I, Ruiz N, Claver C, van der Veen LA, van Leeuwen
PWNM (2000) J Mol Catal A 161:39
1
9. Mu n˜ oz B, Marinetti A, Ruiz A, Castillon S, Claver C (2005)
Inorg Chem Commun 8:1113
2
0. Guiu E, Caporali M, Mu n˜ oz B, M u¨ ller C, Lutz M, Spek AL,
Claver C, van Leeuwen PWNM (2006) Organometallics 25:3102
21. Mu n˜ oz BK, Santos Garcı
A, Claver C (2008) Eur J Inorg Chem 2008:4625
´
a E, Godard C, Zangrando E, Bo C, Ruiz
2
3
tion chemistry and computational DFT studies allowed us
2
2. Gusev OV, Kalsin AM, Peterleitner MG, Akhmedov NG, Bian-
chini C, Meli A, Oberhauser W (2002) Organometallics 21:3637
to propose a catalytic cycle involving species of the type
?
n
[
Pd(H)(L)(PPh ) ]
3
(L = Cl, n = 0; L = CO, MeOH,
23. Gusev OV, Kalsin AM, Petrovskii PV, Lyssenko KA, Oprunenko YF,
Bianchini C, Meli A, Oberhauser W (2003) Organometallics 22:913
2
olefin or PPh , n = 1) as the catalytically active species
3
2
4. Bianchini C, Meli A, Oberhauser W, van Leeuwen PWNM,
Zuideveld MA, Freixa Z, Kamer PCJ, Spek AL, Gusev OV,
Kalsin AM (2003) Organometallics 22:2409
and three sequential reactions: olefin insertion in the Pd–H
bond, CO insertion into the Pd–C bond and the methan-
olysis of Pd–acyl species to yield the products, regenerate
the active species and restart the cycle, being the last
reaction the rds of the mechanisms.
25. Bianchini C, Meli A, Oberhauser W, Parisel S, Gusev OV, Kalsin
AM, Vologdin NV, Dolgushin FM (2004) J Mol Catal A 224:35
2
6. Bianchini C, Meli A, Oberhauser W, Parisel S, Passaglia E,
Ciardelli FW, Gusev OV, Kalsin AM, Vologdin NV (2005)
Organometallics 24:1018
Acknowledgments Financial supports from ONCTI (Proyecto
2
2
7. Kalsin AM, Vologdin NV, Peganova TA, Petrovskii PV, Lys-
senko KA, Dogulshin FM, Gusev OV (2006) J Organomet Chem
011001188) and CONDES-LUZ are gratefully acknowledged.
6
91:921
2
2
8. Godard C, Ruiz A, Claver C (2006) Helv Chim Acta 89:1610
9. Diab L, Gouygou M, Manoury E, Kalck P, Urrutigo ¨ı ty M (2008)
Tetrahedron Lett 49:5186
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