Radical reactions catalysed by homobimetallic ruthenium(II) complexes bearing Schiff base ligands: Atom transfer radical addition and controlled polymerisation

Article abstract of DOI:10.1016/S0040-4039(02)00834-1

The homobimetallic ruthenium Schiff base complexes Ia-f mediated the atom transfer radical addition (ATRA) of carbon tetrachloride across olefins in excellent yields which markedly depended on the catalyst and the substrate used. The best catalytic system Ic is able to compete with the highest performing ruthenium catalysts reported so far for ATRA reactions. In addition these systems are also capable of performing atom transfer radical polymerisation (ATRP) reactions with styrene in high yields and with good control over molecular weight and molecular weight distribution.

Full text of DOI:10.1016/S0040-4039(02)00834-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4687–4690
Radical reactions catalysed by homobimetallic ruthenium(II)
complexes bearing Schiff base ligands: atom transfer radical
addition and controlled polymerisation
Bob De Clercq and Francis Verpoort*
Ghent University, Department of Inorganic and Physical Chemistry, Krijgslaan 281 (S-3), 9000 Ghent, Belgium
Received 29 March 2002; accepted 30 April 2002
Abstract—The homobimetallic ruthenium Schiff base complexes Ia–f mediated the atom transfer radical addition (ATRA) of
carbon tetrachloride across olefins in excellent yields which markedly depended on the catalyst and the substrate used. The best
catalytic system Ic is able to compete with the highest performing ruthenium catalysts reported so far for ATRA reactions. In
addition these systems are also capable of performing atom transfer radical polymerisation (ATRP) reactions with styrene in high
yields and with good control over molecular weight and molecular weight distribution. © 2002 Elsevier Science Ltd. All rights
reserved.
In the 60 years since the discovery of M. S. Kharasch
that peroxides could promote the anti-Markovnikov
addition of CCl4 to olefins,1 there has been extensive
research on this groundbreaking reaction.2 The discov-
ery in the mid-1950s that this carbonꢀcarbon and car-
bonꢀhalogen forming reaction could be catalysed by
transition metal complexes, lead to a repression of
oligomerisation and telomerisation side processes and
the awareness of both industrial and academic synthetic
organic chemists in the possibilities of this reaction.2 In
the 40 years thereafter, organometallic catalytic com-
plexes based on Cu, Fe, Ni, Pd, Rh and Ru were
developed in order to improve selectivities, to extend
the range of possible substrates and to perform the
ATRA reaction under milder conditions.3
In the mid-1990s, the research in atom transfer radical
addition (ATRA) got a powerful injection because of
the independent discovery of Sawamoto4 and
Matyjaszewski5 that ATRA could be extended to atom
Scheme 1. Mechanism of atom transfer radical addition and extension to atom transfer radical polymerisation.
Keywords: atom transfer radical addition; atom transfer radical polymerisation; homogeneous catalysis; ruthenium and compounds; olefins.
* Corresponding author. Tel.: +32-9-264-44-36; fax: +32-9-264-49-83; e-mail: francis.verpoort@rug.ac.be
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00834-1

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B. De Clercq, F. Verpoort / Tetrahedron Letters 43 (2002) 4687–4690
tuted aromatic Schiff base Ic reaches systematically
higher yields than the isopropyl containing Schiff base
Ie. Examination of data from Table 1 further shows
that the electronic properties of these ligands also exert
a big influence on the catalytic activity of these cata-
lysts. This is best illustrated by comparing the yields of
Ia with Ib, Ic with Id and Ie with If. In each series, the
compound containing the electron withdrawing nitro
group has the lowest catalytic activity. The catalytic
system Ic clearly has the best combination of steric
crowding and electronic balance, as this complex
exhibits the best catalytic performance of our systems
and this for all olefins. For example, when the reaction
mixture is heated for 8 h at 65°C, Ic converts the best
olefins methyl methacrylate (MMA), n-butylacrylate
(BA) and isobutyl methacrylate (IBMA) into the
monoadduct in 86, 93 and 95 yield, respectively. Also
methyl acrylate (MA) and styrene (Styr.) are converted
smoothly under these reaction conditions, reaching 61
and 78% yield, respectively. When the reaction is per-
formed for 8 h at 85°C instead of 8 h at 65°C, MMA,
IBMA, BA and Styr. are converted in nearly quantita-
tive yields. When working under these reaction condi-
tions, MA and acrylonitrile (AN) are also converted
smoothly as the ATRA products are obtained in 73 and
66% yield, respectively. To the best of our knowledge,
no ruthenium complex reported so far succeeds in
performing ATRA of CCl4 to AN in such good yields.
Moreover, system Ic (i) easily surpasses the catalytic
performance of the most commonly used ruthenium
carbene systems today for ATRA of CCl4 to MMA,
BA and Styr. and (ii) competes with the best ruthenium
system reported so far for ATRA of CCl4 to these
olefins. In what follows we will support these state-
ments by facts. Fig. 2 depicts some ruthenium systems
for which ATRA of CCl4 to MMA, BA and Styr. was
reported in literature. In 1999 Sawamoto et al. reported
on the ATRP activity of systems 1 and 3.7 Soon there-
after Simal et al. tested these systems for their ATRA
activity.8 In 2000 Simal et al. reported on the ATRA
activity of the analogous Cp* containing complexes
2a–c.9 To the best of our knowledge, 2a is the best
ruthenium system reported so far to mediate ATRA.
Matsumoto reported in 1973 on the ATRA activity of
system 4.10 Before the report on the exceptional activity
of 2a, system 4 was considered as the most efficient and
versatile ruthenium catalyst for ATRA reactions. In
transfer radical polymerisation (ATRP). Indeed, as can
be seen in Scheme 1, the mechanism of ATRP rests on
the iteration of the ATRA reaction. Recently, we suc-
ceeded in synthesising and characterising a new class of
homobimetallic complexes Ia–f (Fig. 1).6 We now
report on the excellent activity of these readily available
and robust complexes Ia–f for ATRA of carbon tetra-
chloride across various olefins. Moreover, we also
checked the catalytic performance of these systems in
ATRP reactions.
In a first set of experiments, we checked the ability of
ruthenium complexes Ia–f to catalyse ATRA of CCl4 to
six representative olefins under standardised reaction
conditions (Table 1).
The results gathered in Table 1 reveal that the outcome
of the reaction depended very much on the olefin and
catalytic system used. It is clear that the Schiff base
ligand should introduce enough steric bulkiness in the
catalytic structure in order to have some reasonable
catalytic activity. Indeed, the catalytic performance of
systems Ia–b is much lower compared to compounds
Ic–f that contain a more bulky Schiff base, and this is
irrespective of the olefin used. However, the steric
influence of the Schiff base is rather subtle as is demon-
strated by the fact that too bulky ligands lead to a
decrease in catalytic activity. Indeed, the methyl substi-
Figure 1.
Table 1. ATRA of carbon tetrachloride to representative olefins catalysed by ruthenium complexes Ia–fa
Iab
Ibb
Icb
Idb
Ieb
Ifb
Icc
Methyl methacrylate
Isobutyl methacrylate
Methyl acrylate
n-Butyl acrylate
Styrene
26
19
17
12
22
7
11
B5
7
B5
8
86
95
61
93
78
49
62
75
47
69
56
22
79
86
54
84
72
41
54
59
31
55
43
17
94
98
73
96
91
66
Acrylonitrile
B5
a Yields (%) based on GLC using dodecane as internal standard.
b Reaction conditions. Prior to use, the reagents, the solvent (toluene) and the internal standard (dodecane) were dried using well established
procedures, distilled and kept under nitrogen at −20°C. The catalyst (0.006 mmol) was dissolved in toluene (1 ml) and subsequently added
through a septum to the solution of alkene (9 mmol), CCl4 (13 mmol), dodecane (0.25 ml) in toluene (3 ml). The catalyst loading was thus 0.066
mol%. The reaction mixture was heated at 65°C for 8 h.
b
c Same as but here the reaction mixture was heated at 85°C for 8 h.

B. De Clercq, F. Verpoort / Tetrahedron Letters 43 (2002) 4687–4690
4689
Figure 2. Depiction of some ruthenium systems for which the ATRA reaction of CCl4 to MMA, BA and Styr. was reported in
literature.
1999, the groups of Snapper et al.11 and Simal et
al.3,12,13 showed that catalytic complexes 5 and 6, which
are known to be highly active olefin metathesis cata-
lysts, were also highly active for promoting ATRA
reactions of CCl4 to electron poor olefins. In this class
of ruthenium carbene catalysts, complex 5a is by far the
most active system reported so far. Catalytic compound
7, developed in the laboratory of Noels et al. and being
the highest performing catalytic ruthenium system
reported so far for ATRP of vinyl monomers,14 was
tested in 1999 by Simal et al. in ATRA reactions.12
Table 2 summarises the yields (%) of these ruthenium
catalysts for the above-mentioned ATRA reactions.
When comparing the results from Table 1 and Table 2,
the first thing one observes is the superior activity of
our homobimetallic ruthenium carbene complex Ic in
comparison with the class of ruthenium carbene com-
plexes 5a–c and 6.
active ruthenium system reported so far) with our com-
plex Ic reveals that they have comparable turnover
numbers. However, the turnover frequencies of system
2a are considerably higher than those obtained with Ic.
For example, the turnover numbers/turnover frequen-
cies for MMA, BA and Styr. with catalyst Ic (tested at
85°C for 8 h) varies around 1400 and 175 h−1, respec-
tively, whereas for 2a values of around 1600–1700 and
400 h−1, respectively, were reported. A major drawback
of system 2a, however, is the very difficult, tedious and
time-consuming synthesis of the complex requiring the
use of stringent reaction conditions and rigorously
dried and purified solvents and reactants.15 In contra-
diction to this, the synthesis of our complex is very easy
and moreover most of the reaction intermediates are air
stable.6 So despite the lower turnover frequencies, it is
reasonable to state that our complex is able to compete
with system 2a because of their comparable turnover
numbers and the much faster, easier, cheaper and more
efficient synthesis of our complex.
For these types of ruthenium complexes, our related
system Ic exhibits by far the best catalytic activity in
ATRA reactions reported so far. Our complex is also
much more performing than system 4, which set the
standards in the field of ruthenium catalysed ATRA
reactions from 1973 until 1999. When comparing the
catalytic performance of systems 1, 2b–c and 3 with our
highest performing system Ic, one has to bear in mind
that (i) despite the fact that these catalysts reach high
yields in considerable less time than our system, at the
time the yields were determined, for system 3 almost
quantitative total conversions (this includes the conver-
sion of the substrates in ATRA products, telomers and
oligomers) were obtained and (ii) the difference in
catalyst loading with a factor of 5. This leads to the
conclusion that our system again performs better. Com-
paring the catalytic performance of system 2a (the most
In a second set of experiments we checked the catalytic
activity of our complexes Ia–f in ATRP reactions with
MMA and IBMA (initiator: ethyl 2-methyl-2-bromo-
propionate), MA and BA (initiator: methyl 2-bromo-
propionate) and Styr. (initiator: (1-bromoethyl)-
benzene). None of our catalysts was able to convert
MA, BA, MMA, IBMA or AN. Although the relation-
ship between ATRP and ATRA is beyond dispute,
not all catalysts that are efficient in ATRA display
the same control/activity in ATRP, and vice versa.
For example, the outstanding ATRP catalyst 7 ex-
hibits a very poor ATRA activity. However, when
styrene is used as a monomer our systems display a
totally different behaviour. The yields (%) and charac-
teristics of the formed polystyrene with Ia–c are
Table 2. Yields (%) of some ruthenium catalysts for which the ATRA reaction of CCl4 to MMA, BA and Styr. was
reported in literaturea
1
2ab/2b/2c
3
4c
5a/5b/5cc
6c
7c
MMA (2 h, 85°C)
n-BuA (4 h, 85°C)
Styr. (5 h, 60°C)
97
14
36
97/68/54
85/25/13
95/61/38
98
62
81
60
45
88
73/32/11
–
57/45/24
4
4
29
B2
B2
B2
a Yields (%) based on GLC using dodecane as internal standard. The catalyst loading was in all cases 0.33 mol%.
b Here the authors reported that with the substrates indicated, turnover numbers up to 1600–1700 and turnover frequencies of around 400 h−1
could be reached.
c Here the reaction mixture was allowed to react for 24 h at the indicated temperature.

4690
B. De Clercq, F. Verpoort / Tetrahedron Letters 43 (2002) 4687–4690
Table 3. Yield (%) for the ATRP of styrene catalysed by
Ia–f and properties of the formed polymers with systems
Ia–f
Monomer=styrene
Ia
Ib
Ic
Id
Ie
If
Yield (%)a
11
–
–
–
–
–
–
–
–
–
71
34
46
1.35
0.87
44
25
38
1.51
0.73
63
31
44
1.42
0.85
36
27
40
1.49
0.56
b
Mn (×103)
b
Mw (×103)
(Mw/Mn)b
fic
a Reaction conditions: [monomer]0: [initiator]0: [Ru]0=800:2:1, initia-
tor: (1-bromoethyl)benzene, temperature: 110°C, reaction time: 17 h.
b Mn, Mw and the PDIs are determined by size-exclusion chromato-
graphy (SEC) with polystyrene calibration.
Figure 4. Time dependence of ln([M0]/[Mt]) for the ATRP of
styrene and using catalytic system Ic. [M0] and [Mt] are the
monomer concentrations at times 0 and t (y=0.074x; r2=
0.9948).
c fi=Initiation
([monomer]0/[initiator]0)×MW(monomer)×conversion.
efficiency=Mn,theor./Mn,exp.
with
Mn,theor.=
Acknowledgements
depicted in Table 3. Again catalyst Ic is the best
performing. Besides a good yield of 71%, the good
initiation efficiency of 0.87 and the polydispersity (Mw/
Mn) of 1.35 indicate that the polymerisation proceeds in
a controlled fashion. For Ic we also followed the
monomer conversion and the number average molecu-
lar weight (Mn) in function of time. The dependence of
molecular weight and polydispersity on monomer con-
version are illustrated in Fig. 3. The linear dependence
observed for Mn is in agreement with a controlled
process with a constant number of growing chains. In
addition, the significant decrease of the polydispersity
with polymerisation time indicates that the radicals are
long-lived. Moreover, also the first order kinetic plot is
linear, so that one can conclude that termination reac-
tions are almost completely excluded (Fig. 4).
B.D.C. is indebted to the IWT (Vlaams instituut voor
de bevordering van het wetenschappelijk-technologisch
onderzoek in de industrie) for a research grant. F.V. is
indebted to the FWO-Flanders (Fonds voor weten-
schappelijk onderzoek-Vlaanderen) for financial sup-
port and to Research funds of Ghent University.
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In conclusion, the catalytic system Ic is the most active
ruthenium carbene catalyst reported so far for the
ATRA reaction of CCl4 across olefins. Moreover, our
homobimetallic catalytic complex Ic can compete with
2a, which is the highest performing system of all types
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activity and control in ATRP reactions with styrene.
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Figure 3. Dependence of the molecular weight Mn and Mw/
Mn on monomer conversion for styrene and using catalytic
system Ic.