New pyridine - Imidazoline ligands for palladium-catalyzed copolymerization of carbon monoxide and styrene

Article abstract of DOI:10.1002/1099-0682(200112)2001:12<3009::AID-EJIC3009>3.0.CO;2-8

C₁-symmetrical pyridine - imidazoline ligands have been synthesised and used in the preparation of Pd^II complexes of general formula [PdMe(NCMe)(N-N′)][BAr′₄] [Ar′ = 3,5-(CF₃)₂C₆H₃]. The introduction of electronically different substituents in the imidazoline moiety determines the coordination position of the methyl group in the cationic complexes, which are catalyst precursors for the copolymerization of carbon monoxide and 4-tert-butylstyrene (TBS). Interestingly, the use of a precatalyst with a different stereochemistry provides copolymers with a different stereoregularity.

Full text of DOI:10.1002/1099-0682(200112)2001:12<3009::AID-EJIC3009>3.0.CO;2-8

SHORT COMMUNICATION
New Pyridine؊Imidazoline Ligands for Palladium-Catalyzed Copolymerization
of Carbon Monoxide and Styrene
[a]
[a]
[a]
[b]
´
Amaia Bastero, Aurora Ruiz, Carmen Claver,* and Sergio Castillon
Keywords: N ligands / Palladium / Copolymerization / Stereochemistry / Nitrogen heterocycles
C1-symmetrical pyridine−imidazoline ligands have been syn-
thesised and used in the preparation of PdII complexes of
general formula [PdMe(NCMe)(N-NЈ)][BArЈ4] [ArЈ = 3,5-
(CF3)2C6H3]. The introduction of electronically different sub-
tion position of the methyl group in the cationic complexes,
which are catalyst precursors for the copolymerization of car-
bon monoxide and 4-tert-butylstyrene (TBS). Interestingly,
the use of a precatalyst with a different stereochemistry pro-
stituents in the imidazoline moiety determines the coordina- vides copolymers with a different stereoregularity.
Introduction
In this paper we show that the stereochemistry of the
[PdMe(NCMe)(N-NЈ)][BArЈ4] complexes can be modified
by changing the electronic properties of the C1-symmetrical
pyridineϪimidazoline ligands. This fact influences the ster-
eoregularity of the CO/TBS copolymers.
Chelating nitrogen ligands have been reported to be the
most efficient for the Pd-catalyzed alternating copolymeri-
zation of carbon monoxide and styrene.[1Ϫ6] Catalysts con-
taining planar, achiral ligands give syndiotactic copolymers
with high stereocontrol.[3,7,8] This stereoregularity has been
attributed to a chain-end control due to the interaction of
the growing chain with the incoming styrene unit, which
inserts exclusively in a 2,1-fashion.[3] Enantiomerically pure
C2-symmetrical N-N ligands yield highly isotactic copoly-
mers due to the strict enantiomorphic site control provided
by the chiral ligand.[9Ϫ11]
To the best of our knowledge there is only one example
of a C1-symmetrical bisnitrogen ligand. Surprisingly, the
use of (S,S)-4,5-dihydro-4-methoxymethyl-3-phenyl-2-(2-
pyridyl)oxazole provides a syndiotactic copolymer. It has
been proposed that, due to a pronounced site-selective co-
ordination of the olefin, the influence of the chirality of the
growing chain is more efficient than the enantiosite con-
trol.[9,12,13] The related C1-symmetrical phosphaneϪ
oxazoline ligands provide isotactic poly(styrene-alt-CO). It
seems that with these ligands the enantioface discrimination
is determined by the site-selective coordination of the olefin
(trans to the P atom) and a secondary insertion of the in-
coming styrene units.[12]
Results and Discussion
The pyridineϪimidazoline ligand 1a was prepared sim-
ilarly to the oxazolines by reaction of 2-cyanopyridine with
meso-1,2-diphenylethylenediamine but using YbOTf3 as
catalyst.[15,16] Further reaction of 1a with BnBr, TsCl or
Tf2O, provided the 1-substituted-4,5-dihydro-4,5-diphenyl-
2-(2-pyridyl)-imidazoles 1bϪd, respectively, in a racemic
manner (Figure 1).[16] The neutral [PdClMe(N-NЈ)] com-
plexes 2aϪd were isolated from the stoichiometric reaction
of [PdClMe(cod)] (cod ϭ 1,5-cyclooctadiene) and the li-
gands 1aϪd in anhydrous toluene.[17,18] The cationic palla-
dium complexes [PdMe(NCMe)(N-NЈ)][BAr4Ј] (3aϪd) were
prepared following reported procedures.[19]
The stereochemistry of the neutral complexes 2aϪd was
assigned by means of NOE experiments at room temper-
ature, showing that all the complexes have the methyl group
coordinated trans to the pyridine ring. This is also con-
1
firmed by the downfield shift in the H NMR spectra of
the proton H6, next to the N-pyridine atom, which is indic-
ative of the presence of the chloride ligand in a cis posi-
tion.[17] However, the cationic compounds [Pd(Me)(NC-
Me)(N-NЈ)][BArЈ4] (3aϪd) show a different stereochemistry
Recently it has been reported that electronic effects in
palladium catalysts modified with chiral ferrocenyldiphos-
phanes may increase the differentiation of the two coordi-
nating P atoms and change the catalytic activity in the en-
antioselective copolymerization of carbon monoxide and
propene dramatically.[14]
1
depending on the ligand. The H NMR spectra of 3cϪd at
[a]
´
´
`
Departament de Quımica Fısica i Inorganica, Universitat
Rovira i Virgili,
`
Pl. Imperial Tarraco 1, 43005 Tarragona, Spain
Fax: (internat.) ϩ34-977/559-563
E-mail: claver@quimica.urv.es
[b]
´
´
´ `
i Quımica Organica,
Departament de Quımica Analıtica
Universitat Rovira i Virgili,
`
Pl. Imperial Tarraco 1, 43005 Tarragona, Spain
Figure 1. Synthesis of the pyridine-imidazoline ligands
Eur. J. Inorg. Chem. 2001, 3009Ϫ3011
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/1212Ϫ3009 $ 17.50ϩ.50/0
3009

´
A. Bastero, A. Ruiz, C. Claver, S. Castillon
SHORT COMMUNICATION
Figure 2. NOE difference experiments show the stereochemistry of
the complexes
room temperature show a downfield shift of the PdϪMe
signal to around δ ϭ 1.[19] NOE difference experiments at
room temperature allowed the determination of the stereo-
chemistry of these complexes and showed that in the com-
plexes 3aϪb the methyl group is trans to the pyridine moi-
ety whereas it is cis in the complexes 3cϪd (Figure 2).
Figure 3. Comparative 13C NMR spectrum of the copolymers in
the region of the methylene carbon atom using the epimerized co-
polymer as reference
Therefore the use of different substituents (R) with elec- merized copolymer as reference (Figure 3). The molecular
tron-donor or electron-withdrawing character results in an weights were high and similar in all the cases. In general,
effective electronic determination in the methyl coordina- we did not find any relation between the nature of the R
tion. The methyl group is always coordinated trans to the substituent and the activity of the catalytic systems.
less basic nitrogen of the ligand.
In conclusion, we have prepared pyridineϪimidazoline
ligands with several substituents in the imidazoline moiety
and their related [Pd(Me)(NCMe)(N-NЈ)][BAr4Ј] com-
plexes. Interestingly, the nature of the substituents deter-
mines the stereochemistry of the complexes. The PdII com-
plexes are active as catalysts in the CO/TBS copolymeri-
zation reaction and show that the electronic properties of
the ligands strongly influence the stereoregularity of the co-
polymers. We are currently studying how this electronic dis-
crimination affects the different steps of the catalytic reac-
tion.
Under copolymerization conditions the coordination of
carbon monoxide and the migratory insertion of the methyl
group take place to form an acyl species; the styrene coor-
dinates in the vacant position. The electronic effects of these
ligands could therefore affect the steroregularity of the co-
polymer obtained with the catalytic precursors. The new
PdII complexes with imidazoline-containing ligands were
tested as catalysts for the alternating CO/TBS copolymeri-
zation. In a typical experiment 3aϪd were placed in chloro-
benzene under 1 bar CO and TBS was added (Table 1).
Acknowledgments
We thank the Spanish Ministerio de Educacion y Cultura for the
financial support (DGES PB-97-0407-C05-01, DGES PB-98-0315
Table 1. Alternating CO/TBS copolymerization catalysed by
´
[PdMe(NCMe)(N-NЈ)][BArЈ4][a]
´
and 2FD97-1565) and the Ministerio de Ciencia y Tecnologıa for
awarding a research grant (to A. B.).
Entry Precursor Productivity[b] Stereoregularity MN
[g(gPd ϫ h)Ϫ1
]
(l diads) [%]
(MW/MN)
[1]
1
2
3
4
3a
3b
3c
3d
2
8.9
7
65
52
15
18
42200 (1.1)
49750 (1.5)
59250 (1.2)
39700 (1.5)
P. Corradini, C. De Rosa, A. Panunzi, G. Petrucci, P. Pino,
Chimia 1990, 44, 52Ϫ54.
M. Barsacchi, G. Consiglio, L. Medici, G. Petrucci, U. W.
Suter, Angew. Chem. Int. Ed. Engl. 1991, 30, 989Ϫ991.
M. Brookhart, F. C. Rix, J. M. DeSimone, J. C. Barborak, J.
[2]
12.8
[3]
[a]
Reaction conditions: 0.0125 mmol catalyst, [TBS]/[cat] ϭ 620, 5
Am. Chem. Soc. 1992, 114, 5894Ϫ5895.
[b]
[4]
mL chlorobenzene, p(CO) ϭ 1 bar, 24 h at room temperature.
Productivity calculated from the isolated copolymer.
E. Drent, P. H. M. Budzelaar, Chem. Rev. 1996, 96, 663Ϫ681.
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248Ϫ253.
[6]
B. Milani, G. Mestroni, Comments Inorg. Chem. 1999, 20,
301Ϫ326.
As Table 1 shows, introducing electron-withdrawing
groups (entries 3 and 4) leads to a greater proportion of u
diads giving highly syndiotactic copolymers. With the sub-
stituents H and Bn (entries 1 and 2 respectively) such a
clear effect is not observed, probably due to the smaller
electronic differentiation of the two rings (pyridine and im-
idazoline), although there is a bigger proportion of l diads.
The degree of stereoregularity was evaluated from the 13C
NMR spectrum of the copolymers by integrating the signals
in the region of the methylene carbon atom using the epi-
[7]
A. Bastero, A. Ruiz, J. A. Reina, C. Claver, A. M. Guerrero, F.
´
A. Jalon, B. R. Manzano, J. Organomet. Chem. 2001, 619,
287Ϫ292.
[8]
[9]
C. Carfagna, G. Gatti, D. Martini, C. Pettinari, Organometal-
lics 2001, 20, 2175Ϫ2182.
M. Brookhart, M. I. Wagner, G. G. A. Balavoine, H. A. Had-
dou, J. Am. Chem. Soc. 1994, 116, 3641Ϫ3642.
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mun. 1995, 16, 9Ϫ14.
[10]
[11]
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2000, 122, 996Ϫ997.
3010
Eur. J. Inorg. Chem. 2001, 3009Ϫ3011

New PyridineϪImidazoline Ligands for Palladium-Catalyzed Copolymerization
SHORT COMMUNICATION
[12]
3
4
5
A. Aeby, G. Consiglio, Inorg. Chim. Acta 1999, 296, 45Ϫ51.
A. Aeby, A. Gsponer, M. Sperrle, G. Consiglio, J. Organomet.
Chem. 2000, 603, 122Ϫ127.
C. Gambs, S. Chaloupka, G. Consiglio and A. Togni, Angew.
Chem. Int. Ed. 2000, 39, 2486Ϫ2488.
J. V. Allen, S. J. Coote, G. J. Dawson, C. G. Frost, C. J. Martin,
J. M. J. Williams, J. Chem. Soc., Perkin Trans. 1994, 1,
2065Ϫ2072.
CDCl3, room temp.): δ ϭ 9.27 (ddd, J ϭ 5.1, J ϭ 1.7, J ϭ
0.8 Hz, 1 H, H6), 8.32 (ddd, 3J ϭ 7.9, J ϭ 1.2, J ϭ 0.8 Hz, 1
4
5
[13]
[14]
[15]
3
4
H, H3), 8.15 (td, J ϭ 7.9, J ϭ 1.7 Hz, 1 H, H4), 7.83 (ddd,
3J ϭ 7.9, J ϭ 5.1, 4J ϭ 1.2 Hz, 1 H, H5), 7.24Ϫ6.69 (m, 10
3
H, Harom), 6.11 (s, 2 H, CHN), 0.45 (s, 3 H, PdϪCH3);
C22H19ClF3N3O2PdS (588.32): calcd. C 45.0, H 3.3, N 7.1;
found C 45.4, H 3.8, N 6.5.
[20]
The standard synthesis is described in ref. 7. As representative
examples 3b and 3d are chosen. Selected data for 3b: 1H NMR
[16]
2-Cyanopyridine and meso-1,2-diphenylethylenediamine were
refluxed in chlorobenzene during 72 h using YbOTf3 as cata-
lyst. 1H NMR (400 MHz, CDCl3, room temp.): δ ϭ 8.63 (ddd,
3
4
(300 MHz, CDCl3, room temp.): δ ϭ 8.44 (dd, J ϭ 5.2, J ϭ
1.8 Hz, 1 H, H6), 7.98 (dd, 3J ϭ 8.1, 4J ϭ 1.0 Hz, 1 H, H3),
3
4
7.86 (td, J ϭ 8.1, J ϭ 1.8 Hz, 1 H, H4), 7.72 (s, 8 H, Harom),
4
5
3
3J ϭ 5.0, J ϭ 1.8, J ϭ 1.1 Hz, 1 H, H6), 8.36 (dt, J ϭ 7.7,
7.53 (s, 4 H, Harom), 7.43 (ddd, 3J ϭ 8.1, 4J ϭ 5.2, 5J ϭ 1.0 Hz,
4J ϭ 1.1, 5J ϭ 1.1 Hz, 1 H, H3), 7.81 (td, 3J ϭ 7.7, 4J ϭ 1.8 Hz,
3
1 H, H5), 7.36Ϫ6.77 (m, 15 H, Harom), 5.54 (d, J ϭ 11.7 Hz,
1 H, CHN), 5.43 (d, J ϭ 11.7 Hz, 1 H, CHN), 5.11 (d, J ϭ
17.6 Hz, 1 H, CH2), 4.45 (d, J ϭ 17.6 Hz, 1 H, CH2), 2.27 (s,
1 H, H4), 7.40 (ddd, 3J ϭ 7.7, J ϭ 5.0, J ϭ 1.1 Hz, 1 H, H5),
7.02Ϫ6.94 (m, 10 H, Harom.), 5.51 (s, 2 H, CHN); C20H17N3
(299.37): calcd. C 80.2, H 5.4, N 14; found C 79.3, H 5.0, N
13.8.
3
4
3
2
2
3 H, CH3CN), 0.56 (s, 3 H, PdϪCH3); C62H41BF24N4Pd
(1415.2): calcd. C 52.6, H 2.9, N 3.9; found C 50.8, H 2.9, N
3.4. Selected data for 3d: 1H NMR (400 MHz, CDCl3, room
temp.): δ ϭ 8.51 (dt, 3J ϭ 5.6, 4J ϭ 0.8 Hz, 1 H, H6), 8.41
[17]
Selected data for 1d as an example: 1H NMR (300 MHz,
CDCl3, room temp.): δ ϭ 8.78 (ddd, J ϭ 4.8, J ϭ 1.8, J ϭ
1.0 Hz, 1 H, H6), 7.96 (dt, J ϭ 7.7, J ϭ 1.0, J ϭ 1.0 Hz, 1
3
4
5
3
4
5
(ddd, J ϭ 7.8, J ϭ 1.4, J ϭ 0.8 Hz, 1 H, H3), 8.24 (td, 3J ϭ
3
4
5
3
4
7.8, 4J ϭ 1.5 Hz, 1 H, H4), 7.76 (ddd, 3J ϭ 7.8, J ϭ 5.6, J ϭ
3
4
H, H3), 7.87 (td, J ϭ 7.7, J ϭ 1.8 Hz, 1 H, H4), 7.49 (ddd,
3
4
3J ϭ 7.7, J ϭ 4.8, J ϭ 1.0 Hz, 1 H, H5), 7.11Ϫ6.99 (m, 10
H, Harom.), 5.98 (d, 3J ϭ 8.7 Hz, 1 H, CHN), 5.92 (d, 3J ϭ
8.7 Hz, 1 H, CHN); C21H16F3N3O2S (431.43): calcd. C 58.46,
H 3.74, N 9.74, S 7.43; found C 58.85, H 3.82, N 9.42, S 7.28.
1.4 Hz, 1 H, H5), 7.72 (s, 8 H, Harom), 7.53 (s, 4 H, Harom), 7.2-
3
6.64 (m, 10 H, Harom), 6.09 (d, J ϭ 8.0 Hz, 1 H, CHN), 6.01
3
(d, J ϭ 8.0 Hz, 1 H, CHN), 1.5 (s, 3 H, CH3CN), 1.06 (s, 3
H, PdϪCH3); C56H34BF27N4O2PdS: calcd. C 46.2, H 2.4, N
3.8, S 2.2; found C 48.3, H 2.4, N 3.7, S 2.3.
Received August 7, 2001
[18]
[19]
R. E Rülke, J. M Ernsting, A. L Spek, C. J. Elsevier, P. W. N.
M. van Leeuwen, K. Vrieze, Inorg. Chem. 1993, 32, 5769Ϫ5778.
Selected data for 2d as an example: 1H NMR (400 MHz,
[I01300]
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