Quantitative formation of the intermediate of alkene insertion in the copolymerization of p-methylstyrene and carbon monoxide catalyzed by [(PriDAB)Pd(Me)(NCMe)]+BAr4-

Article abstract of DOI:10.1039/a801158c

The intermediate after the first CO and p-methylstyrene insertion in the chain growth process of the copolymerization of these monomers has been quantitatively obtained from [(PrⁱDAB)Pd(Me)(NCMe)⁺(3,5(CF₃) ₂C

Full text of DOI:10.1039/a801158c

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Quantitative formation of the intermediate of alkene insertion in the
copolymerization of p-methylstyrene and carbon monoxide catalyzed by
2
[(PriDAB)Pd(Me)(NCMe)]+BAr4
Carla Carfagna,*† Mauro Formica, Giuseppe Gatti, Alfredo Musco and Annarita Pierleoni
Universita` degli Studi di Urbino, Istituto di Scienze Chimiche, Piazza Rinascimento, 6-61029 Urbino (PS), Italy
The intermediate after the first CO and p-methylstyrene
insertion in the chain growth process of the copoly-
merization of these monomers has been quantitatively
N
N
Me
N
N
Me
Me
Me
Me
Ar4B–
N
N
Pd
=
CH
CH
NCMe
1
obtained
from
[(PriDAB)Pd(Me)(NCMe)]+[{3,5-
PRECATALYST
(CF3)2C6H3}4B]2 in CHCl3 at 20 °C.
CO
In recent years there has been increasing interest in carbon
monoxide olefin copolymerization.1 We and others have found
that ionic Pd complexes with non-coordinating anions and
chiral nitrogen ligands, such as bioxazolines, are active
precatalysts for the copolymerization of styrene and carbon
monoxide, to yield isotactic optically active polymers.2,3 The
use of achiral nitrogen ligands, however, leads to syndiotactic
copolymers.4
Me
Me
Me
Pd
Me
Me
Me
Pd
Me
CH
N
O
CH
N
C
Me
Me
CO
Ar4B–
Ar4B–
–CO
+CO
N
CO
N
CH
CH
Me
From our recent study, the objective of which was to test the
catalytic properties of various nitrogen ligands with different
2
3
p-acceptor
capacity,5
the
complex
[PriDABPd-
CDCl3
–10→20 °C
p-methylstyrene
(Me)(NCMe)]+[{3,5-(CF3)2C6H3}4B]2 1 (PriDAB = 1,4-di-
isopropyl-1,4-diaza-buta-1,3-diene) proved to be a very active
precatalyst in the copolymerization of styrene and 4-methylstyr-
ene with carbon monoxide, giving alternating syndiotactic
polyketones under very mild conditions (1 bar CO and 0 °C). In
addition, the complex 1 is a convenient starting point for
modelling the first steps of the polymer chain growth, as
described here.
Me
Me
Me
Me
Me
Me
Me
CHd
CH
N
O
Hx
Ar4B–
Ar4B–
Ha
Hb
Hx
Ha
N
Pd
By exposure of the precatalyst solution 1 to a CO atmosphere,
under conditions used for the copolymerization reaction, the
corresponding acetyl carbonyl complex 2‡ was obtained, which
reversibly releases one CO group; bubbling nitrogen through a
CH2Cl2 solution at 215 °C resulted in the formation of 3, and
exposure of the solution of 3 to CO atmosphere led to the
regeneration of 21j (Scheme 1). The structure of 3§ was
confirmed by the presence, in the 13C NMR spectrum, of the
Pd–CO resonance at d 174.8 and by the absence of the acetyl
signals.
We then focused our attention on the next step, the olefin
insertion, to see whether our catalytic system would allow
quantitative insertion and isolation of the reaction product. We
performed the reaction using directly the more reactive
compound 3 which was isolated as a white powder from 2. By
adding an equimolecular amount of p-methylstyrene at 210 °C
to a slightly yellow CHCl3 solution of 3 and heating gently to
20 °C, a colour change to dark orange occurred. The new
compound formed in 100% yield corresponds to the 5-mem-
bered palladacycle 4, in agreement with the NMR evidence.¶ In
particular, in the carbonyl region of the 13C NMR spectrum, the
resonance of a typical acetyl group at d 224.9 is present, while
the Pd–CO signal has disappeared. This result is confirmed by
the IR spectrum, which shows an absorption at 1721 cm21 for
the acetyl CO coordinated to Pd.1j,6f Moreover, the CH2 and CH
signals are detected in the DEPT 13C NMR spectrum. The CH–
CH2 fragment is confirmed by the corresponding three-spin
pattern observed in the 1H NMR spectrum, where the relatively
high value of the geminal coupling Jab = 219.7 Hz is typical of
a CH2 linked to a CO group, while the values of the two vicinal
couplings, J = 8.3 and 3.5 Hz, are representative of two
CO
CDCl3 –10 °C
Pd
Hb
Me
O
N
O
Me
N
CH
CHe
Me
Me
Me
5
100%
4
Scheme 1
substantially different dihedral angles. Evidence for two non-
equivalent isopropyl groups is given by the two different CH
chemical shifts observed in both the 1H and 13C NMR spectra.
Moreover, the two methyl groups, within each isopropyl group,
are non-equivalent due to the dissymmetry introduced by the
styrene monomeric unity. The signals of the two meta aromatic
hydrogens are assigned on the basis of NOEs with the adjacent
methyl, while the remaining ortho aromatic hydrogens show
NOEs with CHx. The ortho hydrogens also show NOEs with
CHd, thus allowing the assignment of the latter. Moreover the
NOE observed between the ortho hydrogens and CHa but not
CHb is in agreement with an orientation of the phenyl group
closer to CHa. Therefore the conformation across the CH–CH2
bond is as depicted in the Newmann projection in Fig. 1, in
agreement also with the values of Jax = 8.3 and Jbx = 3.5 Hz,
H
a C6H4Me
Pd
MeOC
Hb
Hx
Fig. 1
Chem. Commun., 1998
1113

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¶ Selected data for 4: dH(CDCl3, 23 °C) 7.80 and 7.78 (s, 1 H each, CHNN),
7.71 (s, 8 H, Ar-Ho), 7.54 (s, 4 H, Ar-Hp), 7.20 and 7.00 (d, 2 H each, Ph-Ho,
Ph-Hm), 3.88 [sept, 1 H, CHd(CH3)2], 3.68 [sept, 1 H, CHe(CH3)2], 3.76 (dd,
due to nearly anti and gauche relationships, respectively. This
suggests that the conformation of the pentatomic ring is slightly
folded.
JHH 8.3 and 3.5, 1 H, CHx–CH2), 3.04 (dd, JHH 19.7 and 8.3, 1 H, CH–CHa),
Insertions of different alkenes into metal–acyl bonds have
already been reported6,1j but to the best of our knowledge there
are in the literature only two examples of reaction of styrene
with a palladium–acyl complex. After insertion, the reaction
products have been described either as a mixture of allylic and
chelate structures in very rapid equilibrium1c or as an allylic
intermediate.7 Therefore it appears that the quantitative forma-
tion of 5-membered palladacyle 4 as the only observable species
is unprecedented. Compound 4 can be isolated as a relatively
stable dark red oil by removing the solvent under reduced
pressure; in CHCl3 solution it proved to be stable up to
50 °C.
In the next step, CO was bubbled through a solution of 4 in
CHCl3 at 210 °C for 15 min, yielding the six-membered chelate
complex 5 in solution.∑ Evidence for the insertion of CO comes
from the doubling of the CO signals in the 13C NMR spectrum
and in the IR spectrum. The deshielding observed for both the
proton and carbon resonances of the CH group of the CH–CH2
fragment shows that the CO has inserted into the Pd–CH bond,
as expected.1j
In conclusion, the results of this work show that by using the
proper ligand it has been possible for the first time to
quantitatively obtain and determine the conformation of the first
intermediate in the syndiotactic copolymerization of CO and
styrene derivatives, i.e. the 5-membered palladacyle 4; it results
from the insertion of p-methylstyrene into the Pd–acyl bond
formed in 3 as a result of a-methyl migration. Furthermore, it
has been possible to obtain the new complex 5 which represents
the second step of the copolymerization reaction.
2.70 (dd, JHH 19.4 and 3.5, 1 H, CH–CHb), 2.39 (s, 3 H, CO–CH3), 2.19
(Ph–CH3), 1.29, 1.17 and 1.09 [d, JHH 6.4, 12 H, CH(CH3)2]; dC(CDCl3,
220 °C) 224.9 (COCH3), 161.7 (q, JCB 49.2, Ar-Ci), 161.2 and 157.1
(CNN), 139.9 (Ph-Ci), 134.8 (Ar-Co), 132.1 (Ph-Co), 130.8 (Ph-Cp), 128.8
(q, 2JCF 31.5, Ar-Cm), 124.5 (q, 1JCF 272.4, CF3), 119.9 (Ph-Cm), 117.6 (Ar-
Cp), 62.1 and 59.5 [CH(Me)2], 52.0 (CH–CH2), 47.0 (CH–CH2), 29.2
(COCH3), 22.8, 22.0, 21.4 and 21.3 [CH(CH3)2], 21.9 (Ph-CH3);
nmax(film)/cm21 1721 (CO), 1610 (CNN).
1
∑ Selected data for 5: dH(CDCl3, 220 °C) 7.80 (s, 2 H, CHNN), 7.73 (s, 8 H,
Ar-Ho), 7.58 (s, 4 H, Ar–Hp), 7.22 (s, 4 H, Ph-Ho, Ph-Hm), 4.65 (dd, JHH
10.6 and 2.0, 1 H, CH–CH2), 3.79 [br s, 2 H, CH(CH3)2], 3.60 (dd, JHH 18.7
and 11.3, 1 H, CH–CH2), 2.80 (d, JHH 18.7, 1 H, CH–CH2), 2.33 (s, 3 H, Ph–
CH3), 2.29 (s, 3 H, CO–CH3), 1.21 and 1.10 [d, JHH 6.1, 12 H, CH(CH3)2];
1
dC(CDCl3, 220 °C) 211.7 and 207.1 (CO–CH3, CO–CH), 161.7 (q, JCB
49.6, Ar-Ci), 140.4 (Ph-Ci), 134.8 (Ar-Co), 130.9 (Ph–Co), 129.5 (Ph-Cm),
128.8 (q, 2JCF 30.5, Ar-Cm), 124.5 (q, 1JCF 272.7, CF3), 117.8 (Ar-Cp), 61.9
(CH–CH2), 46.2 (CH–CH2), 30.1 (CO–CH3), 22.5 [CH(CH3)2], 21.2 (Ph-
CH3); nmax(film)/cm21 1722 (br, CO), 1610 (CNN).
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We are currently exploring the use of the isolable complex 4,
as a preformed catalyst, for the synthesis of syndiotactic
copolymers.
We thank Professor C. J. Elsevier (University of Amsterdam)
for helpful discussions. This work was supported by Ministero
dell’Universita` e della Ricerca Scientifica e Tecnologica and
Consiglio Nazionale delle Ricerche.
Notes and References
† E-mail: schim@fis.uniurb.it
‡ Selected data for 2: dH(CD2Cl2, 275 °C) 7.98 and 7.92 (s, 1 H each,
CH = N), 7.65 (s, 8 H, Ar-Ho), 7.47 (s, 4 H, Ar-Hp), 3.72 [m, 2 H,
CH(CH3)2], 2.67 (s, 3 H, COCH3), 1.10 [d, JHH 6.1, 12 H, CH(CH3)2];
dC(CD2Cl2, 275 °C) 214.5 (COCH3), 172.9 (Pd–CO), 164.1 and 160.5
(CNN), 162.1 (q, 1JCB 49.4, Ar-Ci), 134.9 (Ar-Co), 128.9 (q, 2JCF 31.4, Ar-
Cm), 124.7 (q, 1JCF 272.6, CF3), 117.9 (Ar-Cp), 63.8 and 59.5 [CH(CH3)2],
41.6 (COCH3), 23.2 and 22.0 [CH(CH3)2]; nmax(Nujol)/cm21 2137 (Pd–
CO), 1750 (COCH3), 1611 (CNN). Compound 2 can also be synthesized
5 Experimental details and full spectroscopic characterization will be given
in a forthcoming publication.
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directly by reaction of PriDABPdMeCl and Na+BAr4 in the presence of
2
CO.
§ Selected data for 3: dH(CDCl3, 220 °C) 7.96 and 7.92 (s, 1 H each,
CHNN), 7.69 (s, 8 H, Ar-Ho), 7.54 (s, 4 H, Ar-Hp), 4.13 and 3.77 [sept, 1 H
each, CH(CH3)2], 1.36 (s, 3 H, Pd–CH3), 1.25 [d, JHH 6.7, 12 H, CH(CH3)2];
1
dC(CD2Cl2, 0 °C) 174.8 (Pd–CO), 165.8 and 160.1 (CNN), 161.5 (q, JCB
2
1
49.6, Ar-Ci), 134.6 (Ar-Co), 128.7 (q, JCF 30.0, Ar-Cm), 124.3 (q, JCF
272.5, CF3), 117.5 (Ar-Cp), 63.4 and 56.3 [CH(Me)2], 22.7 and 21.8
[CH(CH3)2], 5.7 (Pd–CH3); nmax(Nujol)/cm21 2130 (Pd–CO), 1610 (CNN).
Compound 3 was isolated as a white powder.
7 G. P. C. M. Dekker, C. J. Elsevier, K. Vrieze, P. W. N. M. van Leeuwen
and C. F. Roobeek, J. Organomet. Chem., 1992, 430, 357.
Received in Liverpool, UK, 9th February 1998; 8/01158C
1114
Chem. Commun., 1998