Steric and electronic factors in the promoting activity of diphosphine ligands in cyclohexene hydrocarbomethoxylation catalyzed by palladium acetate

Article abstract of DOI:10.1134/S0023158412040076

Cyclohexene hydrocarbomethoxylation catalyzed by Pd(OAc) ₂-p-Toluenesulfonic acid-diphos-phine systems has been investigated for a wide range of diphosphine structures and concentrations. The fac-Tors controlling the activity of the palladium-containing catalysts include the hydrocarbon moiety of the ligand and the mutual arrangement of the phosphine groups. A comparison between the promoting effects of monophosphine and diphosphine ligands has demonstrated that bridged trans-diphosphines are more effi-cient in kinetic and concentration terms (TOF and P/Pd ratio, respectively). In particular, the promoting activity of diphosphines is one order of magnitude higher than that of triphenylphosphine, and this effect is attained at 8-65 times lower P/Pd ratios. It is discussed how the catalytic properties of the systems depend on the chelate effect and on the geometric compatibility between the diphosphine structure and the arrange-ment of vacant s and d orbitals of the palladium center. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0023158412040076

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2012, Vol. 53, No. 4, pp. 462–469. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © I.E. Nifant’ev, S.A. Batashev, S.A. Toloraya, A.N. Tavtorkin, N.T. Sevost’yanova, A.A. Vorob’ev, V.V. Bagrov, V.A. Aver’yanov, 2012, published in Kinetika
i Kataliz, 2012, Vol. 53, No. 4, pp. 483–490.
Steric and Electronic Factors in the Promoting Activity
of Diphosphine Ligands in Cyclohexene Hydrocarbomethoxylation
Catalyzed by Palladium Acetate
I. E. Nifant’eva, b, *, S. A. Batashevc, S. A. Tolorayad, A. N. Tavtorkinb, N. T. Sevost’yanovac,
A. A. Vorob’evc, V. V. Bagrova, and V. A. Aver’yanovc
a Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia
b Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 119991 Russia
c Department of Organic and Biological Chemistry, Tula State Pedagogical University, Tula, 300026 Russia
d Moscow State Pedagogical University, Moscow, 119991 Russia
* eꢀmail: inif@org.chem.msu.ru
Received October 31, 2011
Abstract—Cyclohexene hydrocarbomethoxylation catalyzed by Pd(OAc)2–pꢀtoluenesulfonic acid–diphosꢀ
phine systems has been investigated for a wide range of diphosphine structures and concentrations. The facꢀ
tors controlling the activity of the palladiumꢀcontaining catalysts include the hydrocarbon moiety of the
ligand and the mutual arrangement of the phosphine groups. A comparison between the promoting effects of
monophosphine and diphosphine ligands has demonstrated that bridged transꢀdiphosphines are more effiꢀ
cient in kinetic and concentration terms (TOF and P/Pd ratio, respectively). In particular, the promoting
activity of diphosphines is one order of magnitude higher than that of triphenylphosphine, and this effect is
attained at 8–65 times lower P/Pd ratios. It is discussed how the catalytic properties of the systems depend
on the chelate effect and on the geometric compatibility between the diphosphine structure and the arrangeꢀ
ment of vacant s and d orbitals of the palladium center.
DOI: 10.1134/S0023158412040076
Olefin carbonylation catalyzed by transition metal
complexes is a promising way of producing various
organic compounds, including pharmaceuticals and
agrochemicals [1]. An important carbonylation reacꢀ
tion is alkene hydrocarboalkoxylation in the presence
of a palladium complex. This reaction provides means
to obtain various esters from readily available alkenes
in one step [2]. In 2008, Lucite company commercialꢀ
ized the ethylene hydrocarbomethoxylation process
[3]:
Although it is obviously essential to understand
how the efficiency of palladium complexes in alkene
hydrocarboalkoxylation depends on the structure of
the diphosphine ligand, there have been no systematic
studies in this area. Because of this, we studied the
effect of the structure of various diphosphines on the
kinetic parameters of cyclohexene hydrocarꢀ
bomethoxylation as a model reaction. This reaction
was chosen for the reason that all reaction sites in
cyclohexene are chemically equivalent and the only
hydrocarbomethoxylation product is methyl cycloꢀ
hexanecarboxylate. In addition, we did not expect any
considerable extent of copolymerization between
cyclohexane and CO, so the run of the reaction was
anticipated to characterize the efficiency of the cataꢀ
lytic system as such.
R1
R3
R2
COOR
R4
R1
R3
R2
R4
+ ROH + CO[Pd]/phosphine
H
.
ꢀ
H
Among the catalytic systems employed in alkene
hydrocarboalkoxylation, of particular interest are palꢀ
ladium derivatives promoted with strong protonic
acids and free phosphines [4–7]. Among the latter,
diphosphines are attracting increasing interest, for
they act as a chelating agent on the palladium center of
the catalyst and form more stable complexes than
ordinary phosphines do. This produces a significant
effect on the kinetics and regioselectivity of alkene
hydroalkoxycarbonylation [3, 8–10].
The cyclohexene hydrocarbomethoxylation proꢀ
moters examined in this study were bis(diphenylphosꢀ
phino)alkanes
three, and fourꢀmethylene bridge, respectively, and
bisdiphenylphosphines IV XVI, which have a fourꢀ
I–III, whose molecules have a twoꢀ,
–
atom bridge and can form palladium complexes with a
bite angle suitable for efficient catalysis of hydrocarꢀ
bomethoxylation [11]. The structures of synthesized
diphosphines are presented below:
462

STERIC AND ELECTRONIC FACTORS IN THE PROMOTING ACTIVITY
463
Ph2P (CH2)2
Ph2P (CH2)3
Ph2P (CH2)4
PPh2
PPh2
PPh2
I
II
III
PPh2
PPh2
PPh2
PPh2
PPh2
PPh2
IV
V
VI
PPh2
PPh2
PPh2
PPh2
PPh2
VII
PPh2
VIII
PPh2
IX
PPh2
PPh2
P(C(CH3)3)2
P(C(CH3)3)2
PPh2
X
XI
XII
F
CH3
S
CH3
S
F
P
P
P
S
P
F
S
CH3
H3C
F
XIII
XIV
CH3
O
H3C
H3C
CH3
O
P
P
CH3
P
CH3
P
H3C
O
XV
XVI
H3C
O
EXPERIMENTAL
thyl)norbornene was obtained in the same way as its
trans isomer, using the corresponding ditosylate as the
starting compound. Aryltetramethylenediphosphines
XIII and XIV were synthesized from the correspondꢀ
ing 1,4ꢀtetrachlorodiphosphinebutane using our origꢀ
inal, recently reported procedure [19].
Diphosphine Synthesis
Some of the diphosphines used in this work were
synthesized via standard procedures. These are comꢀ
pounds
[16], VIII [17], IX [18], XI [9], and XII [9]. Hitherto
unreported
I–III [12], IV [13], V [14, 15], VI [14, 15], VII
Synthesis of diphosphine VI. Finely divided Li (1 g,
cisꢀ2,3ꢀbis(diphenylphosphinemeꢀ 143 mmol) was added to a solution of triphenylphosꢀ
KINETICS AND CATALYSIS Vol. 53
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2012

464
NIFANT’EV et al.
phine (12.45 g, 47.5 mmol) in 70 mL of absolute tetꢀ Na2SO4. The solvent was removed in vacuo. The resiꢀ
due was recrystallized from ethanol. Yield: 5 g (42%).
rahydrofuran (THF). The reaction mixture was stirred
at room temperature overnight, and a solution of cis
31P NMR (CDCl3), ppm: –17.14.
ꢀ
1H NMR (CDCl3), ppm: 7.54 (m, 10H), 7.38 (m,
10H), 6.14 (s, 2H), 3.06 (s, 2H), 2.41 (s, 4H), 2.28 (s,
2H), 2.20 (m, 2H), 1.43 (d, 1H), 1.17(d, 1H).
13C NMR (CDCl3), ppm: 139.6 (d), 138.4 (d),
135.5 (s), 133.0 (d), 132.6 (d), 128.5 (d), 128.38 (s),
128.34 (d), 48.6 (s), 47.3 (d), 39.54 (d), 39.4(d),
2,3ꢀtosyloxymethylnorbornene (12.2 g, 26.4 mmol)
obtained via a procedure described by Bricklebank
et al. [12], in 30 mL of absolute THF, was then added.
The mixture was stirred at room temperature for
20 min and was treated with an aqueous NH4Cl soluꢀ
tion, 10% Н2SO4, and a saturated NaCl solution. The
organic layer was separated and was dried over 29.1 (d).
F
F
F
1. BuLi
2.
Cl2P
PCl2
P
Et O
2
P
F
F
Br
Synthesis of diphosphine XIII. A solution of BuLi treated with water (20 mL). The organic layer was sepꢀ
(38.5 mmol) in hexane (1.6 N, 24 mL) was added dropꢀ
wise to 4ꢀfluorobromobenzene (4.2 mL, 38.5 mmol) in
40 mL of absolute diethyl ether under cooling
arated and was dried over MgSO4, and the solvent was
removed in vacuo. The residue was dissolved in hot
ethanol. The precipitated substance was filtered out
and dissolved in benzene, and the solution was filtered
through a silica gel bed. Yield: 1.8 g (47%).
(⎯40°С). The reaction mixture was warmed to room
temperature and was stirred for 4 h. Thereafter, a soluꢀ
tion of 1,4ꢀtetrachlorodiphosphinebutane (2 g,
7.7 mmol) in 2 mL of absolute diethyl ether was added
dropwise to the reaction mixture under cooling with a
water–ice mixture. The reaction mixture was stirred at
31P NMR (CDCl3), ppm: –18.53.
1H NMR (CDCl3), ppm: 7.34 (m, 8H), 7.04 (t,
room temperature overnight. On the next day, it was 8H), 1.97 (t, 4H), 1.52 (m, 4H).
CH3
S
1. BuLi
2.
CH3
Cl2P
H3C
S
S
PCl2
P
Et O
2
P
S
S
CH3
H3C
Synthesis of diphosphine XIV. A solution of BuLi with water (20 mL). The organic layer was separated
and was dried over MgSO4, and the solvent was
removed in vacuo. The residue was dissolved in hot
ethanol. The precipitated substance was collected on a
filter and vacuumꢀdried. Yield: 2.8 g (72%).
(38.5 mmol) in hexane (1.6 N, 24 mL) was added
dropwise to 2ꢀmethylthiophene (3.9 mL, 40.4 mmol)
in 40 mL of absolute diethyl ether under cooling
(⎯40°С). The reaction mixture was warmed to room
31P NMR (CDCl3), ppm: –41.21.
1H NMR (CDCl3), ppm: 7.15 (dd, 4H), 6.72 (m,
4H), 2.52 (s, 12H), 2.09 (t, 4H), 1.58 (m, 4H).
temperature and was stirred for 4 h. Thereafter, a solution
of 1,4ꢀtetrachlorodiphosphinebutane (2 g, 7.7 mmol) in
2 mL of absolute diethyl ether was added dropwise to
the reaction mixture under cooling with a water–ice
mixture. The reaction mixture was stirred at room
13C NMR (CDCl3), ppm: 145.99, 137.40, 135.24,
temperature overnight. On the next day, it was treated 126.1, 31.76, 27.37, 15.60.
KINETICS AND CATALYSIS Vol. 53
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2012

STERIC AND ELECTRONIC FACTORS IN THE PROMOTING ACTIVITY
465
Li
H3C
P
H3C
Cl2P
CH3
PCl2
P
Et O
2
CH3
CH3
Synthesis of diphosphine XV [20]. A solution of oꢀbroꢀ perature overnight. On the next day, it was treated with
motoluene (4.6 mL, 38.5 mmol) in 15 mL of absolute water (20 mL). The precipitate was collected on a filter,
diethyl ether was added dropwise to Li (0.53 g, 77 mmol) washed with ethanol (20 mL, two times) and with benzene
in 2 mL of absolute diethyl ether. The reaction mixture was (20 mL, two times), and vacuumꢀdried. Yield: 1.5 g (41%).
boiled for a few hours, and a solution of 1,4ꢀtetrachloꢀ
rodiphosphinebutane (2 g, 7.7 mmol) in 2 mL of absolute
diethyl ether was then added under cooling with a water–
31P NMR (CDCl3), pp: –38.18.
1H NMR (CDCl3), ppm: 7.16 (m, 16 H), 2.40 (s,
ice mixture. The reaction mixture was stirred at room temꢀ 12H), 1.97 (t, 4H), 1.60 (m, 4H).
CH3
O
CH3
H3C
O
O
1. BuLi
2.
Cl2P
P
PCl2
P
Et O
2
H3C
O
Br
O
H3C
Synthesis of diphosphine XVI [21]. A solution of temperature and CO pressure. The constant temperaꢀ
BuLi (38.5 mmol) in hexane (1.6 N, 24 mL) was added
dropwise to ꢀbromoanisole (7.2 mL, 38.5 mmol) in
40 mL of absolute diethyl ether under cooling
40 ). Next, the reaction mixture was warmed to
ture was maintained by circulating a hot organic
medium through the reactor jacket. All experiments
p
were performed at 105
and the following concentrations (mol/L): [C6H10] =
0.1, [CH3OH] = 0.45, [(СН3СОО)2Pd] = 1.0
10–3,
[TsOH] = 1.2
10–2. In each series of experiments, we
°С, a CO pressure of 2.1 ×
106 Pa,
(
⎯
°С
room temperature and was stirred for 4 h. Thereafter,
a solution of 1,4ꢀtetrachlorodiphosphinebutane (2 g,
7.7 mmol) in 2 mL of absolute diethyl ether was added
dropwise to the reaction mixture under cooling with a
water–ice mixture. The reaction mixture was stirred at
room temperature overnight. On the next day, it was treated
with water (20 mL). The organic layer was separated and
was dried over MgSO4, and the solvent was removed in
vacuo. The residue was chromatographed in a SiO2ꢀpacked
column (initially in the benzene : petroleum ether = 1 : 1
system and then in benzene). Yield: 1.8 g (43%).
×
×
varied the concentration of one of the ligands. At cerꢀ
tain intervals, the reaction mass was sampled from the
reactor, and the samples were analyzed by GLC on a
Tsvetꢀ163 chromatograph (flameꢀionization detector,
3000
× 3 mm glass column, Chromosorb W (80/100
mesh) as the support, OVꢀ275 (3%) as the stationary
phase, argon as the carrier gas (30 mL/min), injection
port temperature of 225
out in the temperatureꢀprogrammed mode in the temꢀ
perature range from 75 to 205 at a heating rate of
°С). The analyses were carried
31P NMR (CDCl3), ppm: –27.85.
°С
1H NMR (CDCl3), ppm: 7.10 (m, 16H), 3.84 (s,
6H), 3.74 (s, 6H), 2.00 (m, 4H), 1.78 (m, 4H).
8 K/min. Chromatographic calculations were perꢀ
formed using the MultiKhrom program. The reactants
and products were identified as their retention times,
and the component concentrations were determined
by comparing the observed chromatograms with the
Kinetic Experiments
Cyclohexene hydrocarbomethoxylation was carꢀ
ried out in toluene in a batch reactor [22] at a constant chromatograms of artificial mixtures with known
KINETICS AND CATALYSIS Vol. 53
No. 4
2012

466
Concentration, mol/L
NIFANT’EV et al.
w0
×
104, mol/L–1 min–1
2
0.10
0.08
0.06
0.04
0.02
4
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
1
а
3
0
100
200
300
400
500
Time, min
0
0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
[PPh3], mol/L
Fig. 1. Methyl cyclohexanecarboxylate accumulation
–3
kinetics at a diphosphine concentration of 3.0
) ligand IV, ( ) ligand , ( ) ligand VI, and (
×
10 mol/L:
Fig. 2. Effect of the triphenylphosphine concentration on
the initial cyclohexene hydrocarbomethoxylation rate.
(1
2
V
3
4
) ligand VIII
.
component concentrations. The internal standard was sponds occurs at a much larger P/Pd ratio (~65) than
ꢀxylene.
in the case of the systems involving the diphosphine
ligands (III IX). For diphosphines III and VIII, the
maximum rate is observed at a P/Pd ratio close to 2;
for ligands IV VII and IX, at much larger P/Pd values
of . The maximum initial rate w0 for ligands III IX
is higher than for PPh3 (0.150
103 mol L–1 min–1). It
o
–
RESULTS
In order to study the effects of the diphosphine
–
≈
8
–
ligand structure and concentration on the cyclohexene
hydrocarbomethoxylation rate, we carried out 12
series of experiments in which the catalyst compoꢀ
×
is significant that transꢀdiphosphine VII is a 3 times
more effective promoter than its cis isomer VI. Note
that the other diphosphine ligands with the cis
arrangement of phosphine groups (X–XII) practically
do not accelerate cyclohexene hydrocarbomethoxylaꢀ
tion. It is noteworthy that the presence of a multiple
bond has an effect on the efficiency of the diphosꢀ
phines. For example, unsaturated diphosphine VII is a
nearly 2 times less effective promoter than its hydrogeꢀ
nents were (CH3COO)2Pd,
pꢀTsOH, and one of
diphosphines XII. For comparing the behaviors of
I–
the diphosphine and monophosphine ligands, we perꢀ
formed an additional series of experiments in which
triphenylphosphine was used as the ligand.
Figure 1 plots typical dependences of the concenꢀ
tration of the resulting methyl cyclohexanecarboxylate
on the reaction time for different catalytic systems.
Each concentration versus time curve has an autocatꢀ
alytic segment indicating the formation of catalytically
active complexes during the reaction. With the system
nated analogue V.
In order to study the effect of the substituents at the
phosphorus atoms on the promoting properties of the
diphosphine ligands in cyclohexene hydrocarꢀ
bomethoxylation, we carried out four series of experiꢀ
involving ligand
V, there is almost no induction
period. Initial reaction rates were derived from the
slope of the initial portions of the curves after the end
of the autocatalytic period. The kinetic data obtained
in this way are presented as plots of the initial hydroꢀ
carboalkoxylation rate versus the phosphine ligand
concentration (Figs. 2, 3). The curves obtained for
ments on diphosphines XIII–XVI, varying their conꢀ
centrations, with the other conditions fixed (Fig. 4).
The increase in electronꢀdonating power in the order
XIII < XV < XVI is accompanied, on the qualitative
level, by a decreases in the catalytic activity of the sysꢀ
tem. Unsubstituted diphosphine III and methylthꢀ
iopheneꢀsubstituted ligand XIV are outliers from this
order. This is indicated by the turnover frequency
ligands III
involving ligands
tive throughout the ligand concentration range examꢀ
–
IX show an extremum. The systems
I,
II and XII are practically inacꢀ
,
X–
ined. It is interesting that the reaction rate maximum (TOF) data referring to the maximum reaction rate for
for the system involving triphenylphosphine correꢀ the TOF catalytic systems:
I
,
II
,
X
–
XII PPh3
III
IV
V
VI
VII
VIII
IX
XIII XIV
3.0 3.5
XV
XVI
Ligand
TOF, h–1
0
9.0
21.0 42.0 150.0 15.0 54.0 63.0 57.0
1.5
0.9
KINETICS AND CATALYSIS Vol. 53
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2012

STERIC AND ELECTRONIC FACTORS IN THE PROMOTING ACTIVITY
467
×
104, mol L–1 min–1
4
–1
–1
w0
w
×
10 , mol L min
0
4
2.5
2.0
1.5
1.0
0.5
0
1
3
2
1
2
6
7
3
5
3
2
4
1
4
5
0
0.01
0.02
0.03
0.04
0
0.005
0.010
0.015
0.020
Ligand concentration, mol/L
Ligand concentration, mol/L
Fig. 3. Effect of the diphosphine ligand concentration on
the rate of cyclohexene hydrocarbomethoxylation in the
presence of the catalytic systems with the following
Fig. 4. Effect of the diphosphine ligand concentration on
the cyclohexene hydrocarbomethoxylation rate for the folꢀ
ligands: (
1
) Ph P–(CH ) –PPh (III), (
2
) Ph P–CH –
lowing ligands with electronꢀwithdrawing substituents: (
Ph P–(CH ) –PPh III), (H C–C H S) P–
(CH ) –P(SC H –CH ) (FC H ) P–
6 4 2
1)
2
2 4
2
2
2
C H –CH –PPh
(
IV), (
3)
trans
ꢀ
I
),
2
– Ph P–CH –
(
(2)
4
6
2
2
2
2
2
2
2
2 4
2
3
)
4
2
2
C H –CH –PPh (IV),
3
–
трансꢀС Н (СН PPh )
(
V
),
trans
VIII),
(
XIV),
XIII),
XV), and
C H ) P–(CH ) –P(C H –O–CH ) (XVI).
(
3
4
6
7
10
2
2 2
2 4
2 4
2 4
4 2
4
2
3 2
(4
)
cisꢀC H (CH PPh )
(
VI),
(
5
)
ꢀ
(CH ) –P(C H F)
(
(
4
)
(H C–C H ) P–
(5) (H C–O–
3
7
8
2
2 2
6
6
4
4
2
3 6 4 2
C H (CH PPh )
(
VII), (
6
) C H (CH PPh )
(
(CH ) –P(C H –CH )
(
4
7
8
2
2 2
8
12
2
2 2
3 2
and (7) (C H CHCHCH PPh ) (IX).
6 4 2 2 2
6
2 4
6
3 2
DISCUSSION
suggests that the concentration of active complexes is
determined by pure kinetic factors. Apparently, these
complexes form under conditions of a nearly irreversꢀ
ible reaction. This is the reason why the ascending
portions of the curves plotted in Fig. 3 are highly slopꢀ
ing at low diphosphine concentrations. The fairly
sharp decrease in the reaction rate after its passage
through the maximum is evidence that the diphosꢀ
phines are involved in rapid ligand exchange yielding
less active complexes.
The catalytic activity versus ligand concentration
curve for the Pd(OAc)2–PPh3 system (Fig. 2) differs
noticeably in shape from the same curves for the
Pd(OAc)2–diphosphine ligand systems, which have an
extremum (Fig. 3). The initial segment of the
curve presented in Fig. 2 (up to point а) indicates a
sharp increase in catalytic activity. This is seemingly
due to the fact that, in this region, the catalyst compoꢀ
nent ratio is [PPh3]/[Pd(OAc)2] < 2, well below
the stoichiometric ratio. Under these conditions,
the concentration of the resulting active phosphine–
The high promoting activity of the diphosphine
ligands with trans phosphine groups at bridging strucꢀ
tures might seem paradoxical. All of the transꢀdiphosꢀ
palladium
complexes—Pd(PPh3)2(TsO)2
and
phines (IV
ing power to the cis isomers (VI
diphosphines. This is apparently due to the fact that
coordination between palladium and a rigid trans
,
V,
VII
–
IX) are indeed superior in promotꢀ
Pd(PPh3)2(OAc)2 —is limited by the amount of triphꢀ
enylphosphine introduced into the system. However,
as the PPh3 concentration is further raised, the resultꢀ
ing active complexes are increasingly involved in fast,
reversible, ligandꢀexchange reactions yielding less
active complexes. Thus, because of the existence of a
complicated equilibrium involving the formation of
active complexes and their conversion into lowꢀactivꢀ
ity complexes via ligand exchange, the concentration
of active complexes as a function of [PPh3] passes
through an extremum and the hydrocarbomethoxylaꢀ
tion rate as a function of [PPh3] has a kink. Even at
small [PPh3]/[Pd(OAc)2] ratios, the catalytic system
remains homogeneous owing to the stabilizing effect
,
X) and to the nonrigid
ꢀ
diphosphine with a suitable geometry (specifically, a
suitable length of the bridge between the phosphorus
atoms) allows generation of species displaying the
highest catalytic efficiency. This reasoning underlies
the concept of a bite angle formulated by van Leenwen
et al. [11].
The low activity of ligand VII, which has a double
bond, as compared to the activity of its hydrogenated
analogue V, indicates that VII possibly forms oligoꢀ
mers whose coordination to the palladium center
makes it sterically less accessible to the reactants.
of
pꢀtoluenesulfonic acid and the alkene on pallaꢀ
dium [6].
We believe that the almost complete inertness of
The existence of an extremum in the depeLndence ligands
of the reaction rate on the diphosphine concentration and the noticeable promoting effect of ligand III can
I and II in cyclohexene carbomethoxylation
KINETICS AND CATALYSIS Vol. 53
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2012

468
NIFANT’EV et al.
be explained in terms of the chelate effect, which favorable [24, 25]. Accordingly, for diphosphines I and
makes the formation of sterically unstrained metallaꢀ II, a more likely process is the competitive formation
cycles via coordination between a linear bidentate of inactive polynuclear complexes whose metal atoms
ligand and a complexꢀforming metal energetically are diphosphineꢀbridged:
(CH2)n
PPh2
(CH2)n
PPh2
~
~
PPh2 (CH2)n
PPh2 Pd PPh2(CH2)nPPh2
(CH2)n
Pd
PPh2
PPh2
(CH2)n
(CH2)n
Our finding that the promoting power of substiꢀ
tuted 1,4ꢀbisdiphenylphosphines increases in the bridge structure and the mutual arrangement of the
order XIII XV XVI is in qualitative agreement with phosphine groups in diphosphine ligands, it is possible
Thus, we have demonstrated that, by varying the
<
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the activating effect of electronꢀwithdrawing substituꢀ to control the activity of the palladium–phosphine
ents in diphosphines on PdCl2ꢀcatalyzed styrene catalysts in cyclohexene hydrocarbomethoxylation.
hydrocarboalkoxylation [25]. This can be explained in The transꢀdiphosphines are much stronger promoters
terms of the strengthening of back donation in the forꢀ than the cisꢀdiphosphines, and the most effective one
mation of Pd–P bonds in the palladium–diphosphine is V.
complexes. The higher activity of the base ligand III
compared to its substituted analogues XIII, XV, and
A comparison between the kinetic behaviors of the
monophosphine and diphosphine ligands in cyclohexꢀ
ene hydrocarbomethoxylation demonstrated that the
bridged transꢀdiphosphines are more effective and are
superior to the other ligands in both kinetic and conꢀ
centration terms: they afford a higher reaction rate and
XVI is likely due to the absence of steric hindrance to
palladium chelation with III. The unexpectedly strong
promoting effect of the methylthiophene substituents
in XIV suggests that this ligand forms more stable comꢀ
plexes owing to its higher denticity. The decrease in
TOF on passing to the catalytic systems with ligands
XIII–XVI is due to the sharp decrease in the reaction
rate and to the shift of the rate peak to higher diphosꢀ
phine concentrations.
operate at smaller P/Pd ratios. In particular, the trans
ꢀ
diphosphines show a one order of magnitude higher
promoting activity than triphenylphosphine and allow
the P/Pd ratio to be 8–65 times smaller than in the
case of PPh3
.
On the whole, our data point out the necessity of
systematically investigating the electronic and steric
effects of substituents in diphosphine ligands on the
rate and regioselectivity of the reaction.
The role of the chelate effect and of the geometric
compatibility between the ligand structure and the
positions of vacant
has been explained.
s and d
orbitals in the Pd2+ cation
The marked difference between the P/Pd values
maximizing the reaction rate for the diphosphineꢀ and
triphenylphosphineꢀcontaining systems deserves speꢀ
cial attention. We believe that this difference stems
from the fact that the chelates are more stable than the
complexes with monodentate ligands. This higher staꢀ
bility is due to the synergism of the entropic and enerꢀ
getic factors in chelate formation [24]. As a conseꢀ
quence, a substantially lower concentration of phosꢀ
phine groups is sufficient for the formation of chelateꢀ
type reactive intermediates than for the formation of
active complexes with monophosphine ligands [25].
The palladium chelates with transꢀdiphosphine
ligands have been demonstrated to be promising carꢀ
bonylation catalysts.
ACKNOWLEDGMENTS
This work was supported by the Federal Target Proꢀ
gram “Scientific and Scientific–Pedagogical Personꢀ
nel of Innovative Russia” for 2009–2012 (state conꢀ
tract no. 02.740.11.0266) and by the Russian Foundaꢀ
tion for Basic Research (grant no. 09ꢀ08ꢀ00890).
Use of palladium chelates with transꢀdiphosphine
ligands as carbonylation catalysts provides opportuꢀ
nity to design highꢀperformance and economically
efficient processes for the synthesis of esters and other
valuable oxygenꢀcontaining products. Owing to their
stability, the palladium chelates are anticipated to be
multiply reusable in carbonylation processes.
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