Radical reactions catalysed by ruthenium(II) complexes with anionic carborane phosphine ligands: Kharasch addition to olefins and controlled polymerisation

Article abstract of DOI:10.1016/S0040-4039(00)00840-6

Ruthenium-carborane complexes 1-3 mediated the Kharasch addition of carbon tetrachloride across olefins with high yields which markedly depended on the catalyst and the substrate used. In addition, the atom transfer radical polymerisation of methyl methacrylate could be carried out in a highly controlled way. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00840-6

Tetrahedron Letters 41 (2000) 5347±5351
Radical reactions catalysed by ruthenium(II) complexes with
anionic carborane phosphine ligands: Kharasch addition to
ole®ns and controlled polymerisation
Francois Simal,a Sophie Sebille,a Albert Demonceau,a,* Alfred F. Noels,a
Rosario Nunez,b Mar Abad,b Francesc Teixidorb and Clara Vinasb
a
Á
Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liege, Sart-Tilman (B.6a),
Á
B-4000 Liege, Belgium
Institut de Ciencia de Materials de Barcelona, CSIC, Campus de Bellaterra, Cerdanyola, 08193 Barcelona, Spain
b
Á
Received 6 April 2000; accepted 23 May 2000
Abstract
Ruthenium-carborane complexes 1±3 mediated the Kharasch addition of carbon tetrachloride across
ole®ns with high yields which markedly depended on the catalyst and the substrate used. In addition, the
atom transfer radical polymerisation of methyl methacrylate could be carried out in a highly controlled
way. # 2000 Elsevier Science Ltd All rights reserved.
Keywords: carboranes; catalysis; ole®ns; polymerisation; radicals and radical reactions; ruthenium and compounds.
The beginnings of the Kharasch reaction date back to 1945 when was ®rst reported the direct
addition of carbon tetrachloride to ole®nic double bonds catalysed by peroxides as radical
initiators. A major breakthrough occurred by the mid-1950s with the discovery that a number of
transition metal complexes were able to catalyse the reaction.1 After two decades of moderate
interest, the Kharasch addition has known an incredible renaissance with Sawamoto's2 and
Matyjaszewski's3 independent ®ndings that the Kharasch reaction could be successfully extended
to ole®n polymerisation. In the presence of an excess of ole®n (monomer) relative to the halo-
genated compound (initiator), highly controlled polymers in terms of molecular weights (Mn) and
molecular weight distributions (Mw/Mn) could be synthesised provided favourable equilibria
between the active species (±Pꢀ ) and the dormant species (±P±X) were established (Scheme 1).4
This is the so-called `atom transfer radical polymerisation' (ATRP) which was coined from `atom
transfer radical addition' (ATRA)Ðthe Kharasch addition.
Ruthenium has played a prominent role in Kharasch chemistry with RuCl2(PPh3)3 displaying
some of the highest eciency and versatility for halocarbon activation and addition to ole®ns.5
* Corresponding author. Tel: +32-4-366 34 95; fax: +32-4-366 34 97; e-mail: A.Demonceau@ulg.ac.be
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd All rights reserved.
PII: S0040-4039(00)00840-6

5348
Scheme 1.
Formation of unsaturated 14-electron ruthenium species (through phosphine dissociation) is
assumed to take place prior to halocarbon activation.6 In a search for potential ruthenium catalysts
in Kharasch chemistry, we recently introduced [RuCl2(arene)(PR3)]7 and [RuCl2(ˆCHPh)(PR3)2]8
complexes for which the lability of the arene9 and phosphine10 ligands has been clearly demon-
strated in ole®n metathesis. Since ligand release depends on several factors including the ancillary
ligands and the temperature, we anticipated that stable, well-de®ned 14-electron complexes
should give a direct access to the catalytic species. Coordinatively unsaturated ruthenium com-
plexes have often been obtained with bulky tertiary phosphines (PR3) as ligands that provide
steric protection to the empty coordination sites at the metal.11 The steric crowding forces the R
group of the ligand in close proximity to the metal centre to induce an agostic interaction between
a C±H bond in the R group and the metal atom, thus preventing dimerisation or solvent coor-
dination. Similar features have been met with ruthenium±carborane complexes in which B±H!Ru
agostic bonds are favoured.12
With this in mind, we investigated the catalytic activity of three 14-electron ruthenium(II)
complexes (1, 2, and 3) with one or two nido-diphosphinocarborane anions (L=7,8-(PPh2)2-7,8-
C2B9H10)^) for the Kharasch addition of carbon tetrachloride to four representative ole®ns, using
standardised reaction conditions (Table 1, Scheme 2). The outcome of the reaction dramatically
depended both on the substrate and the complex used. Thus, almost, 100% conversion of
(meth)acrylates was observed with all three complexes, and the bis(diphosphinocarborane) com-
pound 1 proved to be more ecient than the mono(diphosphinocarborane) derivatives 2 and 3.
Performing the reactions with styrene and 1-decene led to a striking di€erentiation between the
former and the latter complexes. Compound 1 catalysed the reactions with a conversion lower
than 10%, whereas 2 and 3 showed conversions higher than 50%, and Kharasch addition yields
ranging from 40 to 80%. Examination of data gathered in Table 1 also shows that the ruthenium±
hydride complex 2 is more ecient than its chloride analogue 3. In terms of activity, catalyst

5349
Table 1
Kharasch addition of carbon tetrachloride and chloroform to representative ole®ns catalysed by ruthenium
complexes 1±3a
Scheme 2.
loading and reaction conditions, complexes 1±3 compare favourably with RuCl2(PPh3)3 which is
so far the most active ruthenium catalyst in Kharasch chemistry.5,13 Interestingly, with the
hydride complex 2, addition of carbon tetrachloride to styrene reached 78%. Addition of
chloroform to ole®ns is a much more dicult task in Kharasch chemistry. As already observed
with RuCl2(PPh3)3, complexes 1±3 were inecient for the addition of CHCl3 to styrene at 60ꢁC.
In most instances, whatever the halogenated compound used, low molecular weight polymers
were obtained upon precipitation of the reaction mixture in methanol or heptane. It is therefore
inferred that the di€erence between substrate conversion and Kharasch addition yield likely stems
from the formation of short oligomers or telomers. Furthermore, it is worth noting that no
metathesis occurred under these reaction conditions.

5350
When the ole®n was in excess relative to carbon tetrachloride, ruthenium complexes 1±3 success-
fully extended the Kharasch addition to the polymerisation of methyl methacrylate and styrene
(Table 2). Most of the polymerisations were not controlled. However, the hydride complex 2
polymerised methyl methacrylate at 85ꢁC in high yield and, very interestingly, in a highly con-
trolled way since the molecular weight distribution was as narrow as 1.25. Also noteworthy is the
observation that complex 2 is highly ecient without cocatalyst activation, whereas RuCl2(PPh3)3
(the standard ruthenium complex in ATRP) requires Lewis acid addition for being active. From
the preparative viewpoint, complex 2 constitutes therefore a valuable tool for ATRP of meth-
acrylates. A series of halogenated compounds were employed successfully as initiators in order to
examine the e€ects of the initiator-halogen on the polymerisation of MMA. Furthermore, suitable
Lewis acid cocatalysts such as aluminium derivatives also exerted a pronounced in¯uence on the
polymerisation control. A detailed report thereon will be published elsewhere.14
Table 2
Polymerisation of methyl methacrylate and styrene catalysed by ruthenium complexes 1±3a
In conclusion, unsaturated 14-electron ruthenium complexes with two B±H!Ru agostic
interactions are highly ecient catalysts for promoting radical reactions, and they appear to be
attractive for manifold catalytic applications.15
Acknowledgements
This work was supported by DGICYT (Project PB94-0226) and CICYT (MAT98-0921),
Spain. We thank the Fonds National de la Recherche Scienti®que (F.N.R.S.), Belgium, for the
purchase of major instrumentation.
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