Dicationic palladium(II)-spiro bis(isoxazoline) complex for highly enantioselective isotactic copolymerization of CO with styrene derivatives

Article abstract of DOI:10.1055/s-0028-1087667

The enantioseletive alternating copolymerization of CO with styrene derivatives catalyzed by a dicationic palladium(II) complex of tetra isopropyl-substituted spiro bis(isoxazoline) (i-Pr-SPRIX) is developed. This polymerization proceeded efficiently to p

Full text of DOI:10.1055/s-0028-1087667

310
LETTER
Dicationic Palladium(II)–Spiro bis(isoxazoline) Complex for Highly
Enantioselective Isotactic Copolymerization of CO with Styrene Derivatives
E
nantioselective Is
a
otactic Cop
n
olymerization of
C
B
O with
S
tyrene
D
.
erivatives Bajracharya, Priti S. Koranne, Tetsuya Tsujihara, Shinobu Takizawa, Kiyotaka Onitsuka, Hiroaki Sasai*
The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka, Ibaraki-shi, Osaka 567-0047, Japan
Fax +81(6)68798469; E-mail: sasai@sanken.osaka-u.ac.jp
Received 27 September 2008
Although several catalytic systems have been developed
Abstract: The enantioseletive alternating copolymerization of CO
for the copolymerization of CO with styrene, polyketones
were produced in poor chemical yields. Thus, the devel-
opment of an efficient catalytic system to produce
with styrene derivatives catalyzed by a dicationic palladium(II)
complex of tetra isopropyl-substituted spiro bis(isoxazoline) (i-Pr-
SPRIX) is developed. This polymerization proceeded efficiently to
produce isotactic polyketones with high molecular weights and high poly(styrene-alt-CO) with both high yield and high pro-
molar optical rotation values.
ductivity is a challenge. Furthermore, the formation of ful-
ly isotactic poly(styrene-alt-CO) usually results in low
molecular weight. We herein report a highly efficient di-
cationic palladium(II) complex ligated with tetraisopro-
pyl-substituted spiro bis(isoxazoline) (i-Pr-SPRIX, 1) for
the alternating copolymerization of CO with styrene de-
rivatives to produce isotactic polyketones with high mo-
lecular weight (Scheme 1).
Key words: copolymerization, isotactic polyketones, spiro
bis(isoxazoline) ligands (SPRIX), dicationic palladium(II) complex
Alternating copolymerization of CO with olefins promot-
ed by chiral transition-metal complexes is one of the most
powerful tools to obtain copolymers (CP) with consider-
able stereoregularity.1 The efficiency of the process main-
ly relies on the nature of the chiral ligands that control the
stereocenters in the main polymer chain. With styrenes as
olefins, the use of cationic palladium(II) complexes with
weakly coordinating counteranions and C2-symmetric bi-
dentate nitrogen donor ligands such as bisoxazoline,2
bioxazoline,3 azabis(oxazoline),4 or phosphorus-based
ligand BINAPHOS5 and phosphino-oxazoline6 afford
more or less isotactic copolymers.7
Recently, we reported unusual reactivity and selectivity in
Wacker-type reactions through activation of olefinic
bonds using SPRIX in palladium catalysis.9 In the Pd(II)–
SPRIX-catalyzed enantioselective intramolecular amino-
carbonylation of alkenes, we disclosed the insertion of CO
in the alkyl-palladium intermediate to afford the nitrogen
containing heterocycles.9c This result prompted us to test
the feasibility of SPRIX in copolymerization of styrene
with CO in the presence of Pd(i-Pr-SPRIX)(Me)(Cl) com-
plex 3, which was readily prepared as shown in
Scheme 2.10 Mixing of (M*,S*,S*)-1 with one equivalent
of Pd(MeCN)2Cl2 followed by treatment with tetrameth-
yltin produced 3 in 88% yield.11 Monocationic precata-
lysts 4a and 4b (1 mol%), prepared in situ by
dehalogenation of 3 using AgBF4 and NaBArF
{BArF = [3,5-(CF3)2C6H3]4B}, respectively, were used in
the copolymerization of CO with styrene (ambient pres-
sure) in CH2Cl2 (4.8 M) at 25 °C. Although very high
yields of the polyketones were resulted (71% and 99% af-
ter 48 h using precatalysts 4a and 4b, respectively), the
molecular weight was relatively low with a range of
1400–2000.
Most of the reported procedures make use of a methyl-Pd
complex as the precatalyst due to the facile formation of
an intermediary acyl-palladium species, styrene insertion
then propagates the copolymerization process. Complete
2,1-insertion of styrene in the alternating copolymeriza-
tion with CO is observed when the reaction is promoted
by palladium(II) complexes of bidentate sp2 nitrogen
ligands.8 In contrast, predominant 1,2-insertion of styrene
is observed when phosphorus ligands are employed.5d,6c
O
[Pd(MeCN)4](BF4)2-i-Pr-SPRIX
(0.01–1 mol%)
Next we used a dicationic palladium(II) complex for the
copolymerization.6c,12 Gratifyingly, 1 mol% of dicationic
complex [Pd(i-Pr-SPRIX)2](BF4)2 (5), prepared in situ by
mixing (M,S,S)-i-Pr-SPRIX (1) and [Pd(MeCN)4](BF4)2
(L/Pd = 2:1) in CH2Cl2, produced poly(styrene-alt-CO) in
22% chemical yield (after 48 h) with a Mw of 26700 and a
narrow polydispersity (Mw/Mn = 1.2). The 13C NMR spec-
trum showed single peaks for the carbonyl carbon and the
ipso carbon, indicating formation of an isotactic copoly-
mer.10 The stereoregularity of the copolymer was further
confirmed by a molar optical rotation [a] value of –366
p-benzoquinone (1 equiv to Pd)
CH2Cl2, 25 °C
CO/Ar (1:1) (PCO/Ar 0.1 MPa)
n
+ n CO
M
R
R
n
H
H
S
S
up to Mw = 72600
R = H, Me, t-Bu, OMe
i-Pr
i-Pr
i-Pr
i-Pr
N
N
O
O
i-Pr-SPRIX
Scheme 1
SYNLETT 2009, No. 2, pp 0310–0314
x
x.
x
x.
2
0
0
9
Advanced online publication: 15.01.2009
DOI: 10.1055/s-0028-1087667; Art ID: U10408ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Enantioselective Isotactic Copolymerization of CO with Styrene Derivatives
311
H
H
i-Pr
i-Pr
i-Pr
i-Pr
Pd(MeCN)2Cl2
CH2Cl2, r.t., 2 h
SnMe4
(M*,S*,S*)-1
N
N
CH2Cl2
40 °C, 2 h
O
O
Pd
Cl
Cl
quant
88%
2
X–
+
H
H
H
H
AgBF4 or NaBArF
(1 equiv)
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
N
N
i-Pr
N
N
CH2Cl2/MeCN (1:1)
r.t.
O
O
O
O
Pd
Pd
Me Cl
Me MeCN
3
4a: X = BF4
4b: X = BArF
Scheme 2 Preparation of monocationic precatalysts
and the configuration of the asymmetric carbon was deter- active palladium(II). To check the effect of partial CO
mined to be R.5d,9c
pressure, argon was mixed with CO in different ratios and
used in the CO/styrene copolymerization. The optimum
benefit was obtained using a 1:1 ratio of CO and argon,
producing poly(styrene-alt-CO) in 25% yield with a Mw of
35000 (Table 1, entry 3).
X-ray crystallographic analysis of a single yellow crystal
of racemic 5 revealed that complex 5 has a slightly distort-
ed square-planar structure (Figure 1); the dihedral angles
between the planes that consist of Pd and two N atoms of
i-Pr-SPRIX are approximately 14°.13 This phenomenon is The product yield as well as the Mw gradually increased
likely due to the intramolecular steric repulsion between with time, indicating a continuous growth of the polymer
the two i-Pr-SPRIX ligands, because the geometry of chain, and at the same time inhibition of the formation of
Pd(i-Pr-SPRIX)(CF3CO2)2 is close to an ideal square-pla- the palladium black was noticed (entries 1–3). Decreasing
nar structure.9b The six-membered diazametallacycles the catalyst loading to 0.5 mol% produced the polyketone
formed by chelate coordination of 1 nearly lie on the same with Mw of 35500 and showed the highest isotacticity with
plane, which is in sharp contrast to the structure of free a molar optical rotation value of –641° (entry 4). When
i-Pr-SPRIX (1) with a dihedral angle between the imino the copolymerization was conducted in the presence of 1
planes of 28.60(8)°.9 Thus, the imino groups in 5 are dis- mol% of p-benzoquinone ([BQ]/[Pd] = 1), a productivity
torted; most of the torsion angles at the trans position of 62 g CP/g Pd was achieved with 49% chemical yield of
around the imino groups are in the range of 157–165°, the copolymer (entry 5). Increase in the amounts of p-ben-
whereas those at the cis position are less than 7°. These zoquinone led to a steady drop in the molecular weight of
structural features have also been observed in Pd(i-Pr- poly(styrene-alt-CO), while the productivity was not af-
SPRIX)(CF3CO2)2.
fected (entries 6 and 7).6e,15 For further improvement of
the productivity, a decrease in the catalyst loading was the
obvious remedy. Indeed, the productivity increased to 97
by employing 0.25 mol% catalyst with [BQ]/[Pd] = 1 (en-
try 8). Despite a decreased chemical yield and somewhat
wider polydispersity, fully isotactic copolymer was ob-
tained with the highest Mw of 72600. It is important to
mention that in all cases using a 1:1 mixture of CO and Ar,
either in the presence or absence of p-benzoquinone, ste-
reoregular isotactic polyketones were produced.
We were encouraged by this result, since even in the ab-
sence of an oxidant, copolymerization proceeded smooth-
ly producing isotactic polyketone.14 Deposition of
palladium black was observed during the progress of the
process, indicating the complex was less stable under CO
atmosphere. We sought to increase the lifetime of palladi-
um complex 5 by: (a) decreasing CO pressure, and (b) ad-
dition of p-benzoquinone as an oxidant to regenerate
The efficiency of the catalytic system in the copolymer-
ization was tested by employing various styrene deriva-
tives (Table 2). Besides unsubstituted styrene, both p-
methylstyrene and p-tert-butylstyrene smoothly under-
went copolymerization in the presence of in situ generated
precatalyst 5 producing the corresponding polyketones
with high molecular weights and optical activities in good
yields (entries 1–3). However, p-methoxystyrene pro-
duced an atactic polyketone (entry 4), while a p-chloro
substituent halted the copolymerization process (entry 5).
We further investigated the compatibility of the copoly-
merization in the scale-up process with a reduction in cat-
alyst loading. When a tenfold excess of styrene (0.55 mL,
Figure 1 ORTEP drawing with the atom labelling of complex 5;
counteranions and hydrogen atoms are omitted for clarity
Synlett 2009, No. 2, 310–314 © Thieme Stuttgart · New York

312
G. B. Bajracharya et al.
LETTER
Table 1 Reaction Optimization for the Copolymerization of CO with Styrenea
Entry
Cat. Pd (mol%) [BQ]/[Pd]
Time (h)
18
Yield (%)b
g CP/g Pd
Mw (Mw/Mn)c
10700 (1.1)
20200 (1.2)
35000 (1.3)
35500 (1.4)
35700 (1.2)
17000 (1.3)
12900 (1.5)
72600 (1.6)
[a]58925 (°)d
1
2
3
4
5
6
7
8e
1
0
0
9
22
25
14
49
50
50
19
11
28
32
35
62
63
63
97
–207
–360
–411
–641
–511
–467
–432
–538
1
36
1
0
48
0.5
1
0
48
1
48
1
5
48
1
10
1
48
0.25
48
a Precatalyst was prepared by mixing (M,S,S)-i-Pr-SPRIX (1) and [Pd(MeCN)4](BF4)2 (L/Pd = 2:1) in CH2Cl2 (2.4 M) at 25 °C for 2 h prior to
addition of p-benzoquinone (when added), styrene (0.48 mmol), and CO/Ar (1:1) under balloon pressure (PCO/Ar = 0.1 MPa).
b Starting material was recovered.
c Determined by GPC using polystyrene standards.
d Concentration 0.5 g/L in CHCl3.
e A 1.92 mmol amount of styrene was used.
Table 2 Copolymerization of CO with Styrene Derivatives under Mixed CO/Ar (1:1) Atmosphere (PCO/Ar = 0.1 MPa)a
Entry
styrene derivatives
styrene
Yield (%)b
g CP/g Pd
Mw (Mw/Mn)c
35700 (1.2)
62000 (1.6)
34000 (1.6)
8600 (3.7)
ND
[a]58925 (°)d
–511
1
2
3
4
5
49
62
28
74
90
ND
p-Me-styrene
p-tert-Bu-styrene
p-MeO-styrene
p-Cl-styrene
20
–404
41
–376
58
ND
trace
ND
a Precatalyst was prepared by mixing (M,S,S)-i-Pr-SPRIX (1) and [Pd(MeCN)4](BF4)2 (L/Pd = 2:1) in CH2Cl2.
Reaction conditions: nPd = BQ = 4.8·10–3 mmol (1 mol%), styrene derivative (0.48 mmol), CH2Cl2 V = 0.2·10–3 L (2.4 M), 25 °C, 48 h.
b Starting material was recovered.
c Determined by GPC using polystyrene standards.
d Concentration = 0.5 g/L in CHCl3. ND = not determined.
4.8 mmol) was copolymerized in the presence of 0.2 species. Alternate insertion of styrene and CO produces 7
mol% of in situ generated precatalyst 5 in CH2Cl2 (9.6 M), as the propagating species. Termination is expected to be
1
the poly(styrene-alt-CO) was obtained in 16% yield with through a b-H elimination process, since the H NMR
a productivity of 99 g CP/g Pd.10 This copolymer dis- spectrum of the polyketones exhibited signals due to inter-
played Mw and [a] values of 39800 (Mw/Mn = 1.3) and – nal olefinic protons with poor intensity around d = 6.6–6.8
488, respectively. Upon further reduction of catalyst load- ppm.10
ing to 0.01 mol% and using 5.5 mL of styrene (48 mmol),
When styrene was treated with
1
mol% of
the polyketone was obtained in an excellent combination
of high productivity (445 g CP/g Pd), Mw value of 61600
(Mw/Mn = 1.7) and stereoregularity {[a] –505}.10 These
results clearly indicate a higher catalytic activity of the di-
cationic palladium(II) complex of i-Pr-SPRIX in compar-
ison to previously known catalytic systems in the
enantioselective synthesis of poly(styrene-alt-CO).
Although further study is necessary to get insight into the is known in alcoholic medium,16b thus compound 8 may
identification of active initiator in the polymerization pro- be generated form an intermediate of type 7 (X = OMe).
cess, we speculate that the initiation step could be through This experiment suggests 2,1-insertion of styrene into the
formation of a palladium-hydride species of type 6, gen- i-Pr-SPRIX-Pd(II) complex in the present copolymeriza-
erated via a water-gas shift reaction (Scheme 3).16,17 Inser- tion.
[Pd(MeCN)4](BF4)2 and 2 mol% of (M*,S*,S*)-i-Pr-
SPRIX (1) in the presence of hydroquinone in MeOH un-
der CO, methyl cinnamate (8) was obtained in 20% yield
together with methyl-3-methoxy-3-phenylpropanoate (9,
16% yield) and dimethyl 2-phenylbutandioate (10, 18%
yield) as a double CO insertion product (Scheme 4).3b,10,18
Conversion of palladium-hydride into palladium-alkoxide
tion of styrene to 6 probably generates an alkyl-palladium
Synlett 2009, No. 2, 310–314 © Thieme Stuttgart · New York

LETTER
Enantioselective Isotactic Copolymerization of CO with Styrene Derivatives
313
2+
+
N
N
L
CO, styrene
N
N
N
N
CO, H2O
Pd
Pd
*
*
*
*
H
5
6
+
M
X
O
N
N
N
N
chain
growth
H
H
S
S
Pd
2
=
*
1
i-Pr
i-Pr
i-Pr
N
i-Pr
N
O
O
Ph
7
1 (i-Pr-SPRIX)
X = growing copolymer
Scheme 3 Suggested mechanism
(2) (a) Brookhart, M.; Wagner, M. I.; Balavoine, G. G. A.;
[Pd(MeCN)4](BF4)2 (1 mol%)
(rac)-i-Pr-SPRIX (1) (2 mol%)
Haddou, H. A. J. Am. Chem. Soc. 1994, 116, 3641.
CO
MeOH
+
+
(b) Brookhart, M.; Wagner, M. I. J. Am. Chem. Soc. 1996,
118, 7219.
p-benzoquinone (1.2 equiv)
hydroquinone (0.6 equiv)
25 °C, 24 h
(1 atm) (as solvent)
(3) (a) Bartolini, S.; Carfagna, C.; Musco, A. Macromol. Rapid
Commun. 1995, 16, 9. (b) Scarel, A.; Durand, J.; Franchi,
D.; Zangrando, E.; Mestroni, G.; Carfagna, C.; Mosca, L.;
Seraglia, R.; Consiglio, G.; Milani, B. Chem. Eur. J. 2005,
11, 6014.
(4) Schätz, A.; Scarel, A.; Zangrando, E.; Mosca, L.; Carfagna,
C.; Gissibl, A.; Milani, B.; Reiser, O. Organometallics 2006,
25, 4065.
O
OMe
+
O
OMe
+
O
OMe
MeO
MeO2C
(5) (a) Nozaki, K.; Sato, N.; Takaya, H. J. Am. Chem. Soc. 1995,
117, 9911. (b) Nozaki, K.; Sato, N.; Tonomura, Y.;
Yasutomi, M.; Takaya, H.; Hiyama, T.; Matsubara, T.;
Koga, N. J. Am. Chem. Soc. 1997, 119, 12779. (c) Nozaki,
K.; Kawashima, Y.; Nakamoto, K.; Hiyama, T. Polym. J.
1999, 31, 1057. (d) Nozaki, K.; Komaki, H.; Kawashima,
Y.; Hiyama, T.; Matsubara, T. J. Am. Chem. Soc. 2001, 123,
534.
(6) (a) Sperrle, M.; Aeby, A.; Consiglio, G.; Pfaltz, A. Helv.
Chim. Acta 1996, 79, 1387. (b) Aeby, A.; Bangerter, F.;
Consiglio, G. Helv. Chim. Acta 1998, 81, 764. (c)Aeby,A.;
Gsponer, A.; Consiglio, G. J. Am. Chem. Soc. 1998, 120,
11000. (d) Aeby, A.; Consiglio, G. Inorg. Chim. Acta 1999,
296, 45. (e) Gsponer, A.; Schmid, T. M.; Consiglio, G. Helv.
Chim. Acta 2001, 84, 2986.
(7) For reviews, see: (a) Sen, A. Acc. Chem. Res. 1993, 26, 303.
(b) Okamoto, Y.; Nakano, T. Chem. Rev. 1994, 94, 349.
(c) Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663.
(d) Bianchini, C.; Meli, A. Coord. Chem. Rev. 2002, 225,
35. (e) Nakano, K.; Kosaka, N.; Hiyama, T.; Nozaki, K.
Dalton Trans. 2003, 4039. (f) Nozaki, K. J. Polym. Sci.,
Part A: Polym. Chem. 2004, 42, 215. (g) Durand, J.; Milani,
B. Coord. Chem. Rev. 2006, 250, 542.
8 (20%)
9 (16%)
10 (18%)
Scheme 4
In conclusion, we reported an efficient dicationic palladi-
um complex 5 for the enantioselective alternating copoly-
merization of CO with styrene derivatives. Compared to
the previously known ligands, chiral spiro ligand 1 pro-
duced the polyketones in good productivity, molecular
weight as well as stereoregularity under ambient temper-
ature and pressure.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
G.B.B. thanks the Japan Society for the Promotion of Science
(JSPS) for a postdoctoral fellowship. This work was supported by
Grants-in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology (MEXT), Japan. We
thank the technical staff in the Materials Analysis Center of ISIR,
Osaka University.
(8) Binotti, B.; Carfagna, C.; Gatti, G.; Martini, D.; Mosca, L.;
Pettinari, C. Organometallics 2003, 22, 1115; and references
cited therein.
(9) (a) Arai, M. A.; Arai, T.; Sasai, H. Org. Lett. 1999, 1, 1795.
(b) Arai, M. A.; Kuraishi, M.; Arai, T.; Sasai, H. J. Am.
Chem. Soc. 2001, 123, 2907. (c) Shinohara, T.; Arai, M. A.;
Wakita, K.; Arai, T.; Sasai, H. Tetrahedron Lett. 2003, 44,
711. (d) Muthiah, C.; Arai, M. A.; Shinohara, T.; Arai, T.;
Takizawa, S.; Sasai, H. Tetrehedron Lett. 2003, 44, 5201.
References and Notes
(1) (a) Okamoto, Y.; Yashima, E.; Yamamoto, C. In Materials-
Chirality; Green, A. M.; Nolte, R. J. M.; Meijer, E. W., Eds.;
Wiley-Interscience: New York, 2003, 157. (b) Consiglio,
G.; Milani, B. In Catalytic Synthesis of Carbon Monoxide/
Alkenes Copolymers and Cooligomers; Sen, A., Ed.; Kluwer
Academic: Dordrecht, 2003, 189. (c) Nozaki, K. In
Catalytic Synthesis of Carbon Monoxide/Alkenes
(10) See the supporting information for this article.
(11) Because of the low coordination ability of i-Pr-SPRIX(1), it
could not replace COD as ligand in [Pd(cod)(Me)(Cl)], thus
an alternative route was developed to synthesize complex 3.
For a general literature procedure to synthesize a methyl–Pd
complex, see: Rülke, R. E.; Ernsting, J. M.; Spek, A. L.;
Copolymers and Cooligomers; Sen, A., Ed.; Kluwer
Academic: Dordrecht, 2003, 217.
Synlett 2009, No. 2, 310–314 © Thieme Stuttgart · New York

314
G. B. Bajracharya et al.
LETTER
Elsevier, C. J.; van Leeuwen, P. W. N. M.; Vrieze, K. Inorg.
Chem. 1993, 32, 5769.
(15) A similar effect of p-benzoquinone has been reported. When
the amount of the oxidant is increased, the selectivity of the
copolymerization is shifted towards formation of copolymer
with a low Mw. See: Barsacchi, M.; Consiglio, G.; Medici,
L.; Petrucci, G.; Suter, U. W. Angew. Chem., Int. Ed. Engl.
1991, 30, 989.
(16) (a) Ford, P. C. Acc. Chem. Res. 1981, 14, 31. (b) Jiang, Z.;
Dahlen, G. M.; Houseknecht, K.; Sen, A. Macromolecules
1992, 25, 2999.
(17) A small amount of water in the reaction media might assist
in the conversion of 5 to 6. The reactions in CH2Cl2/D2O
(500:1, 100:1) were found to yield shorter Mn of polymer
than that of Mn when using anhyd CH2Cl2. However, the 2H
NMR spectrum of copolymer exhibited no deuterium-
labeled initiating group.
(12) For dicationic Pd(II) complexes used in CO/styrene
copolymerization, see: (a) Jiang, Z.; Adams, S. E.; Sen, A.
Macromolecules 1994, 27, 2694. (b) Aeby, A.; Gsponer, A.;
Sperrle, M.; Consiglio, G. J. Organomet. Chem. 2000, 603,
122. (c) Milani, B.; Scarel, A.; Durand, J.; Mestroni, G.;
Seraglia, R.; Carfagna, C.; Binotti, B. Macromolecules
2003, 36, 6295. (d) Bergman, S. D.; Goldberg, I.; Carfagna,
C.; Mosca, L.; Kol, M.; Milani, B. Organometallics 2006,
25, 6014; and references cited therein.
(13) Crystal Data
C46H76B2F8N4O4Pd, monoclinic; space group C2/c (#15);
a = 21.325 (9) Å, b = 15.977 (4) Å, c = 61.30 (3) Å;
b = 96.74 (3)°; V = 20742 (14) Å3; Z = 16; R = 0.123,
Rw = 0.142; Dc = 1.330 g/cm3. The unit cell contained two
crystallographically independent molecules with similar
structures.
(18) The use of hydroquinone assists in polymer termination, see:
Pisano, C.; Nefkens, S. C. A.; Consiglio, G.
Organometallics 1992, 11, 1975.
(14) The Pd(II)-promoted copolymerization is reported to be
suppressed in the absence of an oxidant, see ref. 7c and 4.
Synlett 2009, No. 2, 310–314 © Thieme Stuttgart · New York

Copyright of Synlett is the property of Georg Thieme Verlag Stuttgart and its content may not
be copied or emailed to multiple sites or posted to a listserv without the copyright holder's
express written permission. However, users may print, download, or email articles for
individual use.