Highly efficient Kharasch addition catalysed by RuCl(Cp*)(PPh3)2

Article abstract of DOI:10.1016/S0040-4039(00)01048-0

Air-stable and readily available RuCl(Cp*)(PPh₃)₂ is so far the best ruthenium-based catalyst precursor for promoting the addition of CCl₄ and CHCl₃ across olefins at a temperature as low as 40°C. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01048-0

Tetrahedron Letters 41 (2000) 6071±6074
Highly ecient Kharasch addition catalysed by
RuCl(Cp*)(PPh3)2
Francois Simal, Lise Wlodarczak, Albert Demonceau* and Alfred F. Noels
Á
Laboratory of Macromolecular Chemistry and Organic Catalysis, CERM, University of Liege,
Á
Sart-Tilman (B.6a), B-4000 Liege, Belgium
Received 25 May 2000; accepted 21 June 2000
Abstract
Air-stable and readily available RuCl(Cp*)(PPh3)2 is so far the best ruthenium-based catalyst precursor
for promoting the addition of CCl4 and CHCl3 across ole®ns at a temperature as low as 40ꢀC. # 2000
Elsevier Science Ltd. All rights reserved.
Keywords: catalysis; ole®ns; radicals and radical reactions; ruthenium and compounds.
The Kharasch reaction and related processes have become increasingly important in synthetic
organic chemistry because the resulting adducts may have an array of multifunctional groups
which can be subjected to numerous transformations.1 Furthermore, the extension of the
Kharasch addition to the controlled radical polymerisation of vinyl monomers (ATRP, atom
transfer radical polymerisation)2 has recently revitalised the research in this area as well. This
carbon±carbon and carbon±halogen bond-formation is now typically catalysed by transition
metal complexes (Scheme 1).
Scheme 1.
The major drawbacks of the Kharasch addition are shown by its limited scope of (poly)-
halogenated substrates and by the rather harsh reaction conditions (high reaction temperatures
and long reaction times) that are generally required. To overcome its shortcomings, much e€ort
* 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)01048-0

6072
has been made on using new transition metal complexes or adding Lewis acid cocatalysts to
activate the carbon±halogen bond in ATRP. Among various transition metals, ruthenium has
been successfully utilised to promote the Kharasch reaction, and RuCl2(PPh3)3 is so far the
classical ruthenium complex.3 During our study on the Kharasch process, we found that ATRP
could proceed smoothly in the presence of RuCl2(arene)(PR3) complexes with no need for a
Lewis acid.4,5 Concurrently, we also found that both the Kharasch addition and ATRP could be
controlled by using RuCl2(ˆCHPh)(PR3)2,4,6 the Grubbs' ruthenium benzylidene complex which
is now exceedingly popular in ole®n metathesis.7 These and other results8 clearly indicate that
some of the shortcomings of the Kharasch addition can beÐpartlyÐmitigated through ligand
modi®cation at the metal centre.2a,b,9 In this paper, we report our preliminary results on the
outstanding eciency of RuCl(Cp*)(PPh3)2, an air-stable and easily available complex,10 for the
Kharasch addition to ole®ns (Scheme 2).
Scheme 2.
In the ®rst set of experiments, the Kharasch addition was realised under standard conditions,6b
by reacting carbon tetrachloride with four representative ole®ns at 60±85ꢀC, in the presence of a
catalytic amount of RuCl(Cp*)(PPh3)2 (Table 1). Only a few hours were found necessary for the
complete consumption of methyl methacrylate, n-butyl acrylate and styrene, and for the synthesis
of the resulting monoadduct with yields ranging from 85 to 100%. 1-Decene, a non-activated
substrate, was found to be much less reactive, since only 46% conversion and 27% addition were
reached after 24 h at 60ꢀC. These encouraging results prompted us to perform the same
experiments under milder conditions. At a temperature as low as 40ꢀC (Table 1) and even at
room temperature, the results remained quite good. Total turnover numbers of 1600±1700, and
initial turnover frequencies of around 400 h^1 were observed, so that we can ascertain that
Table 1
Kharasch addition of carbon tetrachloride to representative ole®ns catalysed by [RuCl(Cp*)(PAr3)2]
complexes 1±3a

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RuCl(Cp*)(PPh3)2 greatly surpasses the best ruthenium systems reported so far.3,6b Its
outstanding eciency was further con®rmed when we reacted chloroform (a substrate reluctant
to undergo Kharasch addition) to methyl methacrylate and styrene (Table 2). Again,
RuCl(Cp*)(PPh3)2 proved to be highly active at moderate temperatures, especially with styrene,
whereas RuCl2(PPh3)3 showed a similar reactivity only at temperatures around 120ꢀC.3
Table 2
Kharasch addition of chloroform to methyl methacrylate and styrene catalysed by [RuCl(Cp*)(PAr3)2]
complexes 1±3a
Since it is generally agreed that the key step in the Kharasch reaction is the pseudo-oxidative
addition of the haloalkane onto the metal complex (Mn), we anticipated that the oxidative
process and, hence, the ecacy of the catalyst, should be ®ne-tuned through ligand modi®cation
(Scheme 3).
Scheme 3.
First, we used RuCl(Cp)(PPh3)2 whose cyclopentadienyl ligand is less electron-donating than
Cp*, and we observed that it was much less ecient that RuCl(Cp*)(PPh3)2, except with methyl
methacrylate. Second, we replaced the PPh3 ligands in 1 by p-substituted triarylphosphines so as
to modify the electronic properties of the phosphine while maintaining the cone angle constant at
145ꢀ. Thus, two phosphines were selected on the basis of their electron-donating ability:
P(p-CF3C6H4)3<PPh3<P(p-CH3OC6H4)3.11 An examination of the results collected in Tables 1
and 2 reveals that the corresponding ruthenium±phosphine complexes 2 and 3 were less ecient
than the parent complex 1. In all cases, the reactivity order was 3<2<1, at variance with the
electron-donating ability of the phosphines (2<1<3). These results suggest that the stabilisation
of the RuIIICl intermediate species by the phosphine ligand is not predominant. Instead, the
reactivity order correlates very well with the ruthenium±phosphine bond energy, as indicated by
the enthalpies of formation of RuCl(Cp*)(PAr3)2 from the reaction of RuCl(Cp*)(COD)
(COD=1,5-cyclooctadiene) with P(p-XC6H4)3: ±ÁH=18.1 (1, PPh3) <20.7 (2, P(p-CF3C6H4)3)
<21.8 (3, P(p-CH3OC6H4)3) kcal/mol.12 These results suggest therefore that the release of a
phosphine ligand from RuCl(Cp*)(PAr3)2 is of utmost importance and most likely occurs prior to
the activation of the halogenated compound by the unsaturated ruthenium centre.
In conclusion, RuCl(Cp*)(PPh3)2 is so far the best ruthenium-based catalyst precursor for
promoting the Kharasch addition of CCl4 and CHCl3 across ole®ns under mild reaction
conditions. A better understanding of the exact nature of ligand e€ects should help our synthetic
and catalytic research e€orts. Further investigations are presently underway.

6074
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