Enantioface discriminating carbonylations of styrene with cationic palladium(II) catalyst precursors

Article abstract of DOI:10.1016/S0022-328X(00)00128-5

The enantioface discriminating copolymerization of styrene with carbon monoxide using palladium complexes [(L^∧L′)Pd(S)₂](X)₂ of the C₁-symmetric ligands (S,S)-2-[2-(diphenylphosphino)phenyl]-3-phenyl-4-methoxyme

Full text of DOI:10.1016/S0022-328X(00)00128-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 603 (2603) 122–127
Note
Enantioface discriminating carbonylations of styrene with cationic
palladium(II) catalyst precursors
Anton Aeby, Achim Gsponer, Martin Sperrle, Giambattista Consiglio *
Swiss Federal Institute of Technology, Industrial and Engineering Chemistry Laboratory, ETH-Zentrum, CH-8092 Zu¨rich, Switzerland
Received 11 November 1999; received in revised form 1 February 2000
Dedicated to Professor Dr Piero Salvadori, a friend, a colleague and a teacher, on the occasion of his 65th birthday.
Abstract
The enantioface discriminating copolymerization of styrene with carbon monoxide using palladium complexes
‚
[(L L%)Pd(S)2](X)2 of the C1-symmetric ligands (S,S)-2-[2-(diphenylphosphino)phenyl]-3-phenyl-4-methoxymethyl-4,5-dihydro-
oxazole or (S,S)-2-(2-pyridyl)-3-phenyl-4-methoxymethyl-4,5-dihydrooxazole gives highly isotactic or syndiotactic poly(1-oxo-2-
phenyl-1,3-propanediyl), respectively. With the former catalytic system, efficient enantioface discrimination takes place only after
the first insertion step and is only slightly influenced by the growing chain, as shown by the chiroptical properties of terpolymers
with ethene. Similar terpolymerization experiments with the latter catalyst precursor show that the fairly good enantioface
discrimination due to the catalyst is in opposition to the influence of the chirality of the growing chain. The latter effect
predominates resulting in the formation of the overwhelmingly syndiotactic copolymer. © 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Enantioselective catalysis; Carbonylation; Olefins; Palladium complexes; Carbon monoxide copolymers
1. Introduction
formation of u-diads [16], independent of the anionic
ligand [17,18]. Optically active, highly isotactic copoly-
‚
Catalytic enantioselective carbonylation reactions
(such as hydroformylation and hydro-carbalkoxylation)
of olefin substrates have considerable potential for in-
dustrial use and have been studied for a long time
[1–4]. However, the level of enantioface discrimination
reached has seldom been high [5] and was only moder-
ate in the most cases. In contrast, essentially
diastereospecific enantioselective carbonylations to
highly isotactic poly(olefin-alt-CO) have been obtained
rather easily [6–13]. When styrene is the substrate,
mers were obtained with [(N N)Pd(CH3)(S)](X) cata-
‚
lyst precursors (where N N is the C2-symmetric ligand
(S,S) - 2,2% - propanediylbis(4,5 - dihydro - 4 - (1 - methyl-
ethyl)-2-oxazole) [8,10,19]. Similarly high isotacticity
associated with high optical activity of the produced
‚
copolymer was obtained using the [(P N)Pd-
‚
(H2O)2](OTf)2 catalyst precursors [20] (where P N is,
e.g. (S,S)-2-[2-(diphenylphosphino)phenyl]-3-phenyl-4-
methoxymethyl-4,5-dihydrooxazole [21]). Surprisingly,
‚
the [(N N%)Pd(CH3)(S)](X) system modified by the chi-
‚
catalyst precursors of the type [(L L)Pd(S)2](X)2 [14]
ral C1-ligand (S,S)-2-(2-pyridyl)-3-phenyl-4-methoxy-
methyl-4,5-dihydrooxazole gave copolymers with a pre-
dominantly syndiotactic structure [8,21]. The new re-
sults presented here are related to end group analysis
and oligomer formation as well as to the alternating
terpolymerization of ethene and styrene with carbon
monoxide and help clarify the mechanism of enantio-
face selection during the carbonylation reactions.
‚
‚
or [(L L)Pd(CH3)(S)](X) [15] (L L is 1,10-phenan-
throline or 2,2%-bipyridine, S is a solvent molecule and
X is a weakly coordinating anion) produce poly(1-oxo-
2-phenylpropane-1,3-diyl) with the prevailing (ꢀ90%)
* Corresponding author. Tel.: +41-1-6323552; fax: +41-1-
6321162.
E-mail address: consiglio@tech.chem.ethz.ch (G. Consiglio)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00128-5

A. Aeby et al. / Journal of Organometallic Chemistry 603 (2000) 122–127
123
Table 1
Alternating co- and terpolymerization of styrene and ethene with carbon monoxide using 1 and 2 as the catalyst precursors a
Entry
Catalyst
precursor
Ethene (mmol)
CO pressure
(bar)
Productivity
(mmol/(g Pd·h))
Polymer composition
(styrene/ethene)
Dm (ꢀ282 nm)
(l·mol−1·cm−1
)
1
2
3
4
5
6
7
1
1
1
2
2
2
2
0
485
497
0
961
633
187
320
280
260
5
10
10
74.4
20.2
68.3
31
1015
170
100/0
0/100
13/87
100/0
7/93
−10.12
0
−12.65
−1.78
−7.56
−6.18
−3.61
19/81
45/55
10
191
a Styrene concentration 435 mmol in 10 ml CH3OH, except for entry 2 for which no styrene was used.
2. Results and discussion
The polymerization experiments were carried out using
‚
‚
[(P N)Pd(H2O)2](OTf)2 (1) or [(N N%)Pd(H2O)2](OTf)2
(2) as the catalyst precursor. The results are shown in
Table 1 and the experimental conditions are described in
the experimental section. The experiments with the two
catalyst precursors were carried out under different
conditions of carbon monoxide pressure, due to the
inverse kinetic order with respect to CO exhibited by the
two types of ligand systems [15,20,22]. As mentioned
briefly elsewhere [21], 1 gives isotactic poly(1-oxo-2-
phenylpropane-1,3-diyl), whereas 2 gives a prevailingly
syndiotactic copolymer. Fig. 1 shows the 13C-NMR
spectrum in the region of the ipso-carbon atom for both
copolymers. The material obtained with 2 shows the four
broad bands associated with the possible triads (ll, ul or
lu, and uu that are centered at about 136.2, 135.8, 135.45
and 134.95 ppm) [16,23]. The other signals at lower fields
are most probably associated to end groups. In contrast,
the copolymer obtained with 1 shows essentially a single
sharp band in that region, consistent with a highly or-
dered structure; in the spectrum of a mixture of the two
copolymers this sharp band is superimposed to the
aforementioned one at 136.2 ppm. The isotacticity of the
latter material is demonstrated by the very high optical
activity and, correspondingly, by the very high intensity
of the circular dichroism band (Dm) in the region of the
n“p* chromophore (ꢀ282 nm). The low molar elliptic-
ity (Dm) value observed for the former copolymer is con-
sistent with the low concentration of the l-diads (Fig. 1).
1
Fig. 2 shows the H-NMR spectrum in the region of
the methyl group of the 2-phenylpropionyl termination
3 for both copolymers. For the material produced with
1, the two internal doublets (1.375 and 1.355 ppm) with
the highest intensities are assigned to the l and u
terminations 6 (diastereomeric excess ꢀ30%), based on
NMR data of 2,4,7-triphenyl-oct-1-en-3,6-dione 7 and
on H-13C-correlation spectra. The prevailing diastereo-
meric termination is unknown so far, but probably
corresponds to the l relative absolute configuration. As
a matter of fact, this class of P N ligands causes
1
‚
enantioselective carbonylation of styrene to homochiral
methyl 2-phenylpropanoate and dimethyl 2-phenyl-
butanedioate [20,24,25]. The doublet at higher field (1.24
ppm) was analogously attributed to one diastereomer of
the alternative regioisomeric termination (l- or u-
CH3CH(C6H5)COꢀCH(C6H5)CH2COꢀ), whereas the
fourth doublet (1.445 ppm) could not be assigned conclu-
sively. Remarkably, the prevailingly syndiotactic copoly-
mer seems to show the signal of only one diastereomeric
termination 6, which is probably broadened by variations
in the microstructure of the following monomer units.
Furthermore the regioselectivity of the insertion of the
first two styrene units in this case appears to be complete.
End group analysis of the copolymers reveals the pre-
sence of terminal 2-phenylpropionyl groups 3 at a con-
centration close to 50%; alternative end groups are us-
ually the two possible unsaturated terminations 4 and 5.
In the reaction mixture of the copolymerization cata-
lyzed by 1 (R)-dimethyl phenylsuccinate (8) (54% ee),

124
A. Aeby et al. / Journal of Organometallic Chemistry 603 (2000) 122–127
possible to identify similar low molecular weight com-
pounds in the copolymerization mixture catalyzed by 2.
2,4-diphenyl-3-oxo-pent-1-en (9) (38% ee) and E-1,4-
diphenyl-3-oxo-pent-1-en (10) (18% ee) were identified.
Accordingly, it is assumed that the polymer chain
starts with secondary styrene insertion into a palla-
dium–hydride species with low enantioface discrimina-
tion to give termination 3. The second insertion of
styrene usually takes place with secondary regioselectiv-
ity and probably with very high enantioselectivity to
give termination 6. Indeed, the enantiomeric excess of
the two oligomers
9 and 10 is similar to the
diastereomeric excess determined for chain end 6. High
enantioface discrimination in the following insertion
steps leads to the formation of isotactic poly(1-oxo-2-
phenylpropane-1,3-diyl). Unfortunately, it was not yet
Fig. 1. 13C-NMR-spectrum (1:1 (CF3)2COD–CDCl3; 125 MHz) (ipso-carbon atom region) of poly(styrene-alt-CO) prepared with 1 (a) and 2 (b).
The most intense signals in the regions of the four triads (ll, ul or lu, lu or ul, and uu) are marked.

A. Aeby et al. / Journal of Organometallic Chemistry 603 (2000) 122–127
125
Fig. 2. 1H-NMR-spectrum (1:1 (CF3)2CDOD–CDCl3; 500 MHz) of poly(styrene-alt-CO) prepared with 1 (a) and 2 (b).
Terpolymerization reactions of ethene and styrene,
using 1 as the catalyst precursor and an ca. 1:1 molar
ratio of the two olefin co-monomers, show the in-
hibitory effect of ethene that was observed previously
with similar phosphino–oxazoline ligands [26,27]. De-
spite the higher reactivity of the styrene co-monomer
with respect to ethene for copolymerization (Table 1,
entry 1 vs. entry 2), the latter olefin is preferentially
inserted in the terpolymerization reaction (Table 1,
entry 3). Therefore only terpolymers with rather low
styrene concentrations were obtained. The molar ellip-
ticity, normalized to the styrene content, is somewhat
higher in the case of the terpolymer than in that of the
copolymer obtained with the same catalyst precursor. It
seems reasonable [9] to assume that the chiroptical
properties of the carbonyl chromophore of these com-
pounds is not substantially influenced by the macro-
molecular environment [10,28]. Thus, the result shows
that enantioface discrimination by the catalytic system
1 is due essentially to the catalyst enantiomorphic site,
with a very low participation of the growing polymer
chain.
With catalyst precursor 2, terpolymers with a high
styrene content are also accessible (Table 1), as re-
‚
ported for other N N ligands [29]. For these materials,
the molar ellipticity increases strongly with increasing
ethene content. Considering that the highest Dm
value ever observed for the styrene copolymers is
that reported in Table 1 (−12.65), enantioface selec-
tion for styrene insertion in the absence of induction
of chirality by the growing chain seems to be rather
high.
3. Conclusions
In the copolymerization of styrene with carbon
monoxide to poly(1-oxo-2-phenylpropane-1,3-diyl) cat-
alyzed by palladium complexes modified with 1,10-
phenanthroline or 2,2%-bipyridine, the growing chain
induces enantioface selection towards the preferential
formation of u-diads [16]. For chiral C2-symmetric
catalytic systems such as those containing (S,S)-2,2%-
propanediyl-bis(4,5-dihydro-4-(1-methylethyl)-2-oxa-

126
A. Aeby et al. / Journal of Organometallic Chemistry 603 (2000) 122–127
3
zole), the mechanism of selection is modified to cause
the preferential formation of l-diads (enantiomorphic
site control) [19]. Similar highly efficient enantioselec-
tivity by the enantiomorphic catalyst is displayed by
systems containing the chiral phosphino-oxazoline
chelate ligands, despite their C1-symmetry. However, in
this case high enantioface discrimination does not
take place in the first insertion step to give the
1-phenylethyl-palladium intermediate; high enantio-
selectivity is displayed probably already for the second
olefin insertion but surely for the third and successive
insertions. The importance of the growing chain for
efficient enantioface selection has been similarly proven
for the Ziegler–Natta olefin polymerization with metal-
locene [30,31]. For the pyridine–oxazoline modified
catalytic system, the chirality of the catalyst site is not
sufficient to overcome the effect of the chirality of the
growing chain. A prevailingly syndiotactic copolymer,
as for the 2,2%-bipyridine ligand, is thus produced. Site
selective coordination of the olefin at the metal center is
probably involved in the stereocontrol of both catalysts
1 and 2.
17.8 Hz, J(H,H)=3.8 Hz), 2.62 (dd, 1H, CH2 (D2),
2J(H,H)=18.1 Hz, J(H,H)=4.0 Hz), 3.41 (dd, 1H,
3
CH2 (D1), 2J(H,H)=17.8 Hz, 3J(H,H)=10.6 Hz), 3.47
2
3
(dd, 1H, CH2 (D2), J(H,H)=18.1 Hz, J(H,H)=10.0
Hz), 3.68 (q, 1H, CH(CH3) (D1 or D2), J(H,H)=7.1
3
3
Hz), 3.88 (q, 1H, CH(CH3) (D1 or D2), J(H,H)=7.1
Hz), 4.75 (dd, 1H, CH (D2), 3J(H,H)=4.0 Hz,
3J(H,H)=10.0 Hz), 4.77 (dd, 1H, CH (D1), 3J(H,H)=
3.8 Hz, 3J(H,H)=10.6 Hz), 5.78 (d, 1H, H2CꢁC (D1 or
2
D2), J(H,H)=0.7 Hz), 5.80 (d, 1H, H2CꢁC (D1 or
2
D2), J(H,H)=0.7 Hz), 6.15 (d, 1H, H2CꢁC (D1 or
2
D2), J(H,H)=0.7 Hz), 6.16 (d, 1H, H2CꢁC (D1 or
2
D2), J(H,H)=0.7 Hz).
2,4-Diphenyl-pent-1en-3-one (9): 1H-NMR (CDCl3,
3
500 MHz): 1.46 (d, 3H, CH3, J(H,H)=6.9 Hz), 4.35
(q, 1H, CH(CH3), 3J(H,H)=6.9 Hz), 5.75 (d, 1H,
H2CꢁC, 2J(H,H)=0.7 Hz), 6.01 (d, 1H, H2CꢁC,
2J(H,H)=0.7 Hz).
4.2. Copolymerization experiments
4.2.1. Copolymerization reaction of styrene and CO
The copolymerization using the catalytic system 1 is
described in detail as an example (entry 1 in Table 1).
A 250 ml stainless steel autoclave under an atmo-
sphere of N2 was charged with 1,4-benzoquinone (0.216
g; 2.0 mmol). The palladium complex (1, 0.11 mmol)
was dissolved in methanol (10 ml) in a Schlenk tube,
and stirred for 15 min. After adding styrene (50 ml; 435
mmol), the solution was transferred to the pre-evacu-
ated autoclave. After pressurising to 320 bar of CO, the
autoclave was placed in an oil bath and the mixture
stirred and heated to 50°C. After 46 h, the autoclave
was cooled down to room temperature (r.t.) and the
residual gas released. The mixture was poured into
MeOH, and the unsoluble copolymer filtered off,
washed again with methanol and dried. Yield 5.5 g
(productivity 74.4 mmol g−1(Pd) h). Prior to NMR
analysis, some of the recovered copolymer was ex-
tracted with MeOH in a Kumagawa extractor.
4. Experimental
4.1. General
Complexes 1 and 2 were prepared as described previ-
ously by reaction of the ligands with PdCl2(NCPh)2 to
‚
PdCl2(L L%), followed by treatment with silver tri-
fluoromethanesulfonate, which led to the corresponding
‚
[Pd(H2O)2(L L%)](OTf)2 [21,32].
The low molecular weight products were character-
ised by NMR and GC-MS. The enantiomeric excess
was determined by gas chromatography using hep-
takis(6-O-TBDMS-2,3-O-methyl)-b-cyclodextrin as the
stationary phase. The NMR spectra were measured on
a Bruker AMX 400 and on AMX 500 with tetramethyl-
silane as the internal standard. The solvent normally
used for co- and terpolymers was a (CF3)2CDOD–
CDCl3 1:1 mixture. 2D spectra (COSY
well as HC- and HCC-correlation) were employed to
obtain correct assignment.
The characterization of dimethyl 2-phenylbutane-
dioate (8) [33] and of (E)-1,4-diphenylpent-1-en-3-one
(10) [34] was already reported. 2,4,7-Triphenyloct-1-en-
3,6-dione (7) and 2,4-diphenylpent-1-en-3-one (9) were
isolated through chromatography on silica from car-
bonylation mixtures of styrene obtained as previously
described [35].
,
INADEQUATE as
4.2.2. Copolymerization reaction of ethene and CO
A similar procedure under comparable conditions
was used for the copolymerization reaction of ethene
and CO (entry 2 in Table 1). 50 ml of toluene, the
solvent, were added to 10 ml methanol. Ethene (485
mmol) was added with a ‘Bu¨chi press-flow gas con-
troller bpc’. During the copolymerization reaction (41
h), the pressure was kept constant by adding a gas
mixture of 1:1 ethene–CO. Yield 0.56 g (productivity
20.2 mmol g−1(Pd) h).
2,4,7-Triphenyl-oct-1-en-3,6-dion (7) was obtained as
a mixture of the two diastereomers (D1 and D2).
1H-NMR (CDCl3, 500 MHz): 1.39 (d, 3H, CH3 (D1 or
4.2.3. Terpolymerization reaction of styrene, ethene and
CO
For the terpolymerisation of ethene, styrene and CO
a similar procedure to that of the copolymerization of
3
D2), J(H,H)=7.0 Hz), 1.41 (d, 3H, CH3 (D1 or D2),
2
3J(H,H)=7.1 Hz), 2.54 (dd, 1H, CH2 (D1), J(H,H)=

A. Aeby et al. / Journal of Organometallic Chemistry 603 (2000) 122–127
127
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ethene and CO was used except that toluene was re-
placed by styrene (50 ml, 435 mmol). The terpolymer-
ization using the catalytic system 1 is reported as an
example (entry 3 in Table 1). 497 mmol ethene were
added with a ‘Bu¨chi press-flow gas controller bpc’ to
the reaction mixture and during the terpolymerization
reaction (46 h) the pressure was also kept constant by
adding a gas mixture of 1:1 ethene–CO. Yield 2.1 g
(productivity 68.3 mmol g−1(Pd) h).
The co- and terpolymerization reactions using cata-
lyst precursor 2 were similarly carried out using the
same quantities of solvents and catalyst and the pres-
sures given in Table 1. Reaction times for entries 4 to 7
were 18.5, 22, 41 and 41 h, respectively.
In all cases, immediate quantitative gas chromato-
graphic analysis for the reaction mixture after filtration
was carried out on a Hewlett Packard HP1 (50 m)
column using acetophenone as the internal standard.
Response factors were obtained from three point cali-
bration with the pure isolated compounds. Correlation
factors of 0.998–1.0 were generally obtained for the
calibration curves.
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Acknowledgements
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This research was supported in part by the Swiss
National Foundation. We are grateful to F. Bangerter
for the very accurate NMR spectra.
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