Novel Cs-symmetric 1,4-diphosphine ligands in the copolymerization of propene and carbon monoxide: High regio- and stereocontrol in the catalytic performance

Article abstract of DOI:10.1002/hlca.200590002

New C_s-symmetric aryl 1,4-diphosphine ligands were synthesized and tested in the copolymerization of carbon monoxide and propene. The electronic properties of the two different P-atoms did not affect the high enantioselectivity of the catalyst

Full text of DOI:10.1002/hlca.200590002

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Helvetica Chimica Acta ± Vol. 88 (2005)
Novel Cs-Symmetric 1,4-Diphosphine Ligands in the Copolymerization of
Propene and Carbon Monoxide: High Regio- and Stereocontrol in the
Catalytic Performance
by Antonella Leone and Giambattista Consiglio*
Eidgenössische Technische Hochschule, Departement Chemie und Angewandte Biowissenschaften, ETH
Hönggerberg, CH-8093 Zurich
(e-mail: consiglio@chem.ethz.ch)
New Cs-symmetric aryl 1,4-diphosphine ligands were synthesized and tested in the copolymerization of
carbon monoxide and propene. The electronic properties of the two different P-atoms did not affect the high
enantioselectivity of the catalyst precursors, thus resulting in high ꢀregioꢁ- and ꢀstereoregularꢁ copolymers.
Introduction. ± Cationic Pd complexes modified with diphosphine ligands are very
active catalysts for the carbonylation of alkenes, a field of considerable industrial and
academic interest [1][2]. This reaction makes a wide range of products accessible,
starting with high-molecular-weight polyketones [3][4] to methyl propanoate [5], a key
intermediate in the synthesis of methyl methacrylate (ˆ methyl 2-methylprop-2-
enoate). A detailed mechanistic study of the intermediates involved in the commer-
cially important Pd-catalyzed methoxycarbonylation of ethene was reported [6]. The
complex [Pd(dppb)(dba)]1) was used to generate in situ the species involved in the
catalytic cycle. The same type of ligand was employed in the Pd-catalyzed carbon-
ylation of butadiene and of methyl pent-3-enoate [7 ± 9].
We tested the performance of Pd complexes modified with 1,2-bis[di(aryl)phos-
phinomethyl]benzene ligands in the copolymerization of carbon monoxide (CO) and
propene, investigating the relationship between the electronic characteristics of the
catalytic system and the microstructure of the resulting copolymer [10]. In particular,
we were interested in the factors influencing the insertion mode of propene, as this step
of the catalytic process gives rise to different chain configurations. For ligands of the
type 1a, electronic factors strongly influence the regio- and enantioselectivity in the
efficient and largely isotactic copolymerization of CO and propene. Oligomerization
experiments [10] have shown that, under certain conditions, a highly regioselective
secondary insertion of propene occurs. The catalytic systems [Pd(P^P)(H2O)2)]-
2
(CF3SO3)2 ) modified with the ligands 1b and 1c show opposite regioselectivity in the
insertion of propene, but similar isotacticity with regard to the copolymerization
reaction. For these C2v-symmetric ligands, the discrimination of the enantiotopic faces
during the copolymerization process should be chain-end controlled. As chain-end
1
)
Abbreviations: dbbp ˆ ꢀ1,2-bis[di(tert-butyl)phosphinomethyl]benzeneꢁ; dba ˆ ꢀdibenzylideneacetoneꢁ
(ˆ(E,E)-1,5-diphenylpenta-1,4-dien-3-one).
)
The term (P^P) refers to a chelating diphoshine ligand.
2
ꢂ 2005 Verlag Helvetica Chimica Acta AG, Zürich

Helvetica Chimica Acta ± Vol. 88 (2005)
211
control and secondary insertion both lead to syndiotactic copolymers in the case of
styrene [11], this may have enabled a syndiotactic structure to form in the
copolymerization of propene and CO.
Results and Discussion. ± Keeping the same C backbone of the above ligands,
different Cs-symmetric diphosphine ligands were synthesized to introduce two different
phosphino moieties. From the results obtained with corresponding C2v-symmetric
diphosphines, the investigation of this new system appeared to be particularly
interesting. We were interested in the question whether the regiochemistry of this
reaction would be altered with such new ligands, possibly resulting in new copolymer
structures.
The Cs-symmetric 1,4-diphosphine ligands 2 and 3 were synthesized according to a
procedure reported in the literature [12]. The starting material was the cyclic sulfate 4
(Scheme), which was obtained upon treatment of 1,2-phenylenedimethanol with
SOCl2, followed by oxidation with NaIO4 [13]. Cleavage of the CÀO bond and opening
of the sulfate occurred rapidly and quantitatively after adding a solution of LiPAr2 at
À 788. After reaching room temperature, the solution was cooled again to À 788.
Addition of the second lithium phosphide, LiPAr'2, introduced the nonequivalent
moiety with substitution of the sulfate group. The yield of the chelate ligands 5 ranged
from 55 to 75%.
Scheme
The catalyst-precursor complexes [Pd(P^P')(OH2)2](OTf)2 were prepared accord-
ing to the methods reported in the literature [14], and tested according to the protocol
published elsewhere [15] (Table 1). Moreover, a series of experiments with in situ

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Table 1. Copolymerization of Carbon Monoxide and Propene with Preformed Catalyst Precursors
[Pd(P^P')(OH2)2](CF3SO3)22). Reaction conditions: 0.03 mmol catalyst precursor, 75 ml THF and 4.5 ml
MeOH, 1.73 mmol 1,4-naphtoquinone, 45 mmol propene, 80bar CO, 44 8.
Ligand
Reaction time [h]
Productivity
[g (g(Pd) h)À1
Mn
ꢀRegioregularityꢁa)
ꢀStereoregularityꢁb)
[%]
]
hh
ht
tt
2
16
20
25
25
58
126
123
15
3400
0
1
1
1
1
0
67
82
71
76
3a
3b
3c
5400
15100
1700
0.02
0.04
0.03
0.03
0.05
0.04
a) Relative intensity of 12C-NMR signals centered at d ca. 223, 219, and 214 ppm, resp. b) Intensity [%] of the
most-intense band in the ht range of the carbonyl signals in the 13C-NMR spectra; h and t refer to ꢀheadꢁ and
ꢀtailꢁ, resp.
formation of the catalyst precursors were carried out, generating the catalyst from a
suspension of Pd(AcO)2 and the diphosphine ligand in CH2Cl2 (Table 2). The general
features of the copolymers were found to be very similar for both the preformed Pd
complexes and the in situ systems Pd(AcO)2/P^P'/BF3 ´ OEt2.
The productivities of the preformed catalysts were similar to those reported for the
C2v-symmetric analogs previously tested [16]. However, there was a general improve-
ment in the activity, especially in the case of 2 compared to the analog bearing only
cyclohexyl groups at the two P-atoms. The productivity was higher for the catalyst
precursors generated in situ (Table 2). This can be attributed either to the different
solvent mixture or to a counter-ion effect. However, these factors do not affect the
enantioselectivity of the cationic complex toward the insertion of the olefin, leading to
copolymers with similar isotacticities.
Table 2. Copolymerization of Carbon Monoxide and Propene with in situ Catalyst Precursors from Pd(AcO)2,
Ligand, and BF3 ´ OEt2. Reaction conditions: 0.03 mmol Pd(AcO)2; 0.036 mmol P^P' ligand; 0.79 mmol BF3 ´
OEt2; 75 ml CH2Cl2 and 4.5 ml MeOH; 1.73 mmol 1,4-naphtoquinone; 45 mmol propene; 80bar CO; 44 8.
Ligand
Reaction time [h]
Productivity
[g (g(Pd) h)À1
Mn
ꢀRegioregularityꢁa)
ꢀStereoregularityꢁb)
[%]
]
hh
ht
tt
2
23
27
23
43
205
163
239
167
3800
4800
4100
6900
0.01
1
1
1
1
0
0
0
0
77
72
79
74
3a
3b
3c
0
0
0
a) Relative intensity of 13C-NMR signals centered at d ca. 223, 219, and 214 ppm, resp. b) Intensity [%] of the
most intense band in the ht range of the carbonyl signals in the 13C-NMR spectra; h and t refer to ꢀheadꢁ and ꢀtailꢁ
resp.
From a 13C-NMR analysis (Fig.) of the produced copolymers, only one stereo-
chemical arrangement, corresponding to an isotactic structure, was found. In contrast
to what could have been expected, an essentially regular enchainment must, thus, have
occurred. The ꢀstereoregularityꢁ ranged from 67 to 82% for the isotactic pentad

Helvetica Chimica Acta ± Vol. 88 (2005)
213
Figure. 13C-NMR ((CF3)2C(D)OD, 125 MHz) of the poly(propene-alt-CO) copolymers prepared with the
catalyst precursors [Pd(P^P')(OH2)2](CF3SO3)2 (see Table 1). The chelating phosphine ligands P^P correspond
to 2, 3a, 3b, and 3c (from top to bottom), resp.
(corresponding to a minimum content of 92 and 96% l-diads, resp. [17]), which is
comparable to the values reported for the catalyst precursors modified with the
corresponding C2v-symmetrical 1,4-diphosphine ligands [16]. This shows that the
insertion mode of the propene unit is not affected by a variation of the electronic
character and basicity at the P-atom. The insertion mode of the olefin generates the
same configuration along the polymer chain as observed for the C2v-symmetric
diphosphine ligands. The ligand 2 showed a remarkable improvement in ꢀstereo-
regularityꢁ, when compared with the C2v-symmetric diphosphine ligand bearing only
cyclohexyl substituents. The ꢀregioregularityꢁ showed the predominance of ht (head-to-
tail) enchainment, as expected for a basic ligand [18].
To elucidate the geometry of the species responsible for the insertion, a carbon-
ylation study of the model complex [PdMe(P^P')MeCN](OTf), where P^P' is 3c, was
carried out. [PdMe(P^P')Cl], with a stereoisomer ratio of 6.5 :1, was transformed with
AgOTf into a monocationic complex for which the ratio became 2 :1. After bubbling
CO into the reaction mixture, the acyl complex [PdC(O)Me(P^P')MeCN](OTf) was
formed as a mixture of stereoisomers in a ratio of 3.3 :1. In all the cases, the major
isomer has the Me group trans to the stronger electron-withdrawing substituents.
Even though two isomers may have been present, the copolymerization proceeded
under high regio- and stereocontrol. This suggests that isomerization of the
intermediates takes place very easily by exchanging the coordination site of the olefin
and the growing polymer chain, thus implying site-selective coordination of the olefin.
Ã
1
In fact, the HMQC H, 31P{H} correlation spectrum of the complex [PdMe(P^P')-
MeCN](OTf) displayed considerable fluctuation at room temperature.

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Experimental Part
General. All reactions were carried out under Ar gas using standard Schlenk techniques. All solvents were
purified according to standard procedures and deoxygenated prior to use. The precursor sulfate 4 was prepared
according to the method of Gao and Sharpless [13]. The synthetic procedure for preparing the ligands was an
adaptation of that described by Fries et al. [12]. The diaqua palladium complexes were prepared according to
published methods [19].
1H- and 31P-NMR spectra were recorded on a Bruker AMX-500 apparatus; d in ppm rel. to Me4Si (1H or
13C) or 85% H3PO4 (31P), J in Hz. The copolymerization reactions were carried out in a 250-ml stainless-steel
autoclave, which was placed in an oil bath equipped with a thermostat.
1,5-Dihydro-2,4,3-benzodioxathiepin 3,3-Dioxide (4). SOCl2 (4 ml, 0.055 mol) was slowly added to
benzene-1,2-dimethanol (6.1696 g, 0.0446 mol) in CCl4 (30ml) by means of a syringe. The resulting brownish
soln. was refluxed for 1.5 h. After cooling to r.t., the mixture was concentrated on a rotary evaporator to give a
brown oil. The latter was dissolved in a mixture of ice-cold CCl4 (30ml), MeCN (30ml), and H 2O (45 ml),
Then, RuCl3 ´ n H2O (82 mg, 0.308 mmol) and NaIO4 (19 g, 0.089 mol) were added while stirring the mixture
vigorously. The mixture was stirred at 258 for 1 h, diluted with H2O (200 ml), and extracted with Et2O (4 Â
50ml). The org. phase was dried (Na 2SO4), filtered through Celite (to remove the Ru salts), and evaporated to
dryness. Recrystallization from THFand hexane afforded 6.43 g (72%) of 4 as a pure, colorless crystalline solid,
which was stored at À 208 to avoid thermal decomposition. 1H-NMR (500 MHz, CDCl3, 258): 5.43 (s, 2 CH2);
7.35 ± 7.37 (m, 2 arom. H); 7.44 ± 7.46 (m, 2 arom. H). 13C-NMR (125.7 MHz, CDCl3, 258): 73.61 (CH2); 129.51,
130.05, 134.07 (arom. C). Anal. calc. for C8H8O4S: C 47.99, H 4.03, O 31.69, S 16.08; found: C 48.03, H 4.24, O
31.75, S 15.98.
Dicyclohexyl ({2-[(diphenylphosphino)methyl]phenyl}methyl)phosphine (2).
A 1.6m soln. of BuLi
(8.40ml, 13.44 mmol) in hexane was added to a soln. of dicyclohexylphosphine (2.21 g, 11.14 mmol) in THF
(25 ml) at À 788 in the presence of hexamethylphosphoramide (HMPA; 5 ml). The mixture was allowed to
warm to 258, stirred for 30min at this temp., cooled to À 788, and then added dropwise to a soln. of 4 (2.22 g,
11.09 mmol) in THF (45 ml) precooled to À 788. The mixture was stirred for 30min at this temp., stirred for
30min at r.t., cooled again to À 788, and treated with a soln. of Ph2Li (prepared from Ph2PH (2.05 g,
11.04 mmol) in THF (25 ml) and 8.30 ml (13.28 mmol) of a 1.6m soln. of BuLi in hexane). The mixture was
allowed to reach r.t., and then heated at 608 for 3 h. The solvent was removed under vacuum, and the residue
was redissolved in Et2O (50ml). The soln. was quenched with H 2O (20ml). The aq. phase was extracted with
Et2O (3 Â 10ml). The org. phase was separated, dried (Na 2SO4), filtered, and concentrated under vacuum. The
residue was recrystallized from degassed MeOH to afford 2 in 65% yield. White solid. 1H-NMR (500 MHz,
C6D6, 258): 1.07 ± 1.34 (br., 11 H, cyclohexyl); 1.49 ± 1.87 (br., 11 H, cyclohexyl); 3.05 (br., CH2P(cyclohexyl)2);
3.89 (d, J(P,H) ˆ 2.31, CH2PPh2); 6.79 (d, 1 arom. H, C6H4); 6.87 (m, 1 arom. H, C6H4); 6.99 ± 7.09 (m, 7 arom. H,
C6H4 and Ph); 7.24 (m, 1 arom. H, Ph); 7.42 ± 7.48 (m, 3 arom. H, Ph). 31P{1H}-NMR (202.5 MHz, C6D6, 258):
À 3.89 (br., P(cyclohexyl)2); À 13.8 (d, J(P,P) ˆ 1.6, PPh2). Anal. calc. for C32H40P2: C 78.98, H 8.28; found: C
78.38, H 8.28.
Compounds 3a ± c were prepared as described for 2 and isolated as white air-sensitive solids in 56% (3a),
75% (3b), and 47% (3c) yield, resp.
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Received November 26, 2004