Kharasch addition catalysed by half-sandwich ruthenium complexes. Enhanced activity of ruthenacarboranes

Article abstract of DOI:10.1016/j.tetlet.2003.09.100

Ruthenium complexes of the type [RuH(η⁵-CB)(PPh ₃)₂] {CB is a monoanionic charge-compensated carborane ligand such as [9-SR₂-7,8-C₂B₉H ₁₀]^- and [9-SR₂-7-CH₃-7,8-C ₂B₉H₉]^-} efficiently catalyse the Kharasch addition of CCl₄ across olefins and, with maximum total turnover numbers of 9000 and initial turnover frequencies of 1900 h ^-1 at 40°C, highly surpass their Ru-Cp^# analogues in these reactions.

Full text of DOI:10.1016/j.tetlet.2003.09.100

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8421–8425
Kharasch addition catalysed by half-sandwich ruthenium
complexes. Enhanced activity of ruthenacarboranes
Oscar Tutusaus,a,† Se´bastien Delfosse,b Albert Demonceau,b,* Alfred F. Noels,b Clara Vin˜asa,* and
Francesc Teixidora
aInstitut de Cie`ncia de Materials de Barcelona, CSIC, Campus de la U.A.B., 08193 Bellaterra, Spain
bLaboratory of Macromolecular Chemistry and Organic Catalysis, University of Lie`ge, Sart-Tilman (B.6a), B-4000 Lie`ge,
Belgium
Received 5 August 2003; revised 30 August 2003; accepted 12 September 2003
Abstract—Ruthenium complexes of the type [RuH(h5-CB)(PPh3)2] {CB is a monoanionic charge-compensated carborane ligand
such as [9-SR2-7,8-C2B9H10]− and [9-SR2-7-CH3-7,8-C2B9H9]−} efficiently catalyse the Kharasch addition of CCl4 across olefins
and, with maximum total turnover numbers of 9000 and initial turnover frequencies of 1900 h−1 at 40°C, highly surpass their
Ru–Cpc analogues in these reactions.
© 2003 Elsevier Ltd. All rights reserved.
Cyclopentadienyl and derivatives are ubiquitous ligands
in organometallic chemistry. In the period between the
discovery of ferrocene in 19511 and the development of
metallocene catalysts for the polymerisation of olefins,2
a wide variety of useful transition–metal complexes has
been discovered. Besides the classic cyclopentadienyl
[(h5-C5H5)−, Cp−] and pentamethylcyclopentadienyl
[(h5-C5Me5)−, Cp*−] ligands, a great number of suitably
tailored indenyl and fluorenyl derivatives have been
synthesised with the aim of designing more efficient
catalyst systems.
To this end, ruthenium–carborane complexes 1–6 (see
Scheme 3) were synthesised, in which the uninegative
carborane ligands, [9-SR2-7,8-C2B9H10]−, were obtained
upon substitution of one terminal open face hydrogen
of the dicarbollide dianion [(C2B9H11)2−] by an SR2
group.9 Contrary to the dicarbollide dianions, the
monoanionic carborane ligands (also known as charge-
compensated carborane ligands) allow for proper com-
parisons with their (C5H5–nRn)− analogues in terms of
steric and electronic effects, when they are investigated
in organometallic chemistry and in homogeneous
catalysis.
In the last 30 years, Hawthorne has developed the
coordination chemistry of dicarbollide ligands [(7,8-
C2B9H11)2− and (7,9-C2B9H11)2−], which are isolobal to
cyclopentadienyl and derivatives (Cpc−).3 However,
apart from olefin hydrogenation,4 metallacarboranes
have scarcely been employed as catalysts. In our ongo-
ing efforts to investigate the potential of metallacarbo-
ranes in homogeneous catalysis,4,5 we turned our
attention to the addition of alkyl halides to olefins (the
so-called Kharasch reaction or atom transfer radical
addition (ATRA), Scheme 1),6 which has seen an
incredible renaissance in the last few years as the result
of the development of atom transfer radical polymerisa-
tion (ATRP).7,8
Scheme 1. Kharasch addition of polyhalogenated compounds
to olefins.
Keywords: catalysis; olefins; radicals and radical reactions; ruthenium
and compounds.
* Corresponding authors. Fax: +32/(0)4366 3497 (A.D.); fax: +34/
(0)93 580 5729 (C.V.); e-mail: A.Demonceau@ulg.ac.be;
clara@icmab.es
† O. Tutusaus is enrolled in the U.A.B. PhD Program.
Scheme 2. Structure of [RuCl(Cpc)(PPh3)2] complexes.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.100

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O. Tutusaus et al. / Tetrahedron Letters 44 (2003) 8421–8425
Herein, we wish to report that complexes 1–6 display
remarkable activity in Kharasch chemistry,10 and sur-
pass [RuCl(Cpc)(PPh3)2] (Cpc=Cp* and indenyl,
Scheme 2) which have so far set the standard in the
field.11
First, we explored the addition of carbon tetrachloride
to methyl methacrylate (MMA) at 40°C, under stan-
dardised conditions,8b,11 and found that ruthenacarbo-
ranes 1–4 were equipotent catalysts. MMA conversion
was complete and adduct formation was almost quanti-
tative within a few hours (Table 1). However, analysis
of the kinetics of the reactions revealed that complexes
1 and 2 were more active than their derivatives, 5 and
6, in which the carborane moiety possesses a methyl
group bonded exo-cluster to a carbon atom (Fig. 1).
Under the same conditions, a quantitative conversion
of styrene into (1,3,3,3-tetrachloropropyl)benzene was
also obtained in the presence of complexes 1–6 (Table
1).
Figure 1. Methyl methacrylate (ꢀ, 2, ꢁ, ꢂ) and adduct
(ꢃ, ", ꢄ, ꢅ) versus time for the Kharasch addition of
carbon tetrachloride to MMA catalysed by complexes 1 (ꢀ,
ꢃ), 2 (2, "), 5 (ꢁ, ꢄ), and 6 (ꢂ, ꢅ), at 40°C. Reaction
conditions are the same as in Table 1.
n-Butyl acrylate, on the other hand, was less prone to
undergo Kharasch addition of carbon tetrachloride
than methyl methacrylate and styrene (Table 1).
Indeed, a 70–75% conversion and a 25–60% yield
(depending on the catalyst used) of the Kharasch
adduct were obtained after 16 h at 40°C. In all cases,
higher oligomers also formed and accounted for the
mass balance. As expected, 1-decene was the least reac-
tive substrate among those under study (Fig. 2), and
only moderate yields (35–60%) were obtained after 16
h. Prolonged reaction times (up to 80–100 h), however,
gave around 75–80% conversion and 70–75% yield.
With 1-decene, complex 3 showed the highest catalytic
activity, giving the desired adduct (1,1,1,3-tetra-
chloroundecane) in 58% yield after 16 h. Notably, all
reactions catalysed by complexes 1 and 2 were faster
than those catalysed by their methyl-substituted deriva-
tives, 5 and 6.
Scheme 3. Complexes 1–6.
In light of these results, it became of interest to com-
pare the activity of complexes 1–6 with those of
[RuCl(Cpc)(PPh3)2] (Cpc=Cp* and indenyl), the most
efficient ATRA ruthenium-based catalysts reported so
Table 1. Kharasch addition of carbon tetrachloride to
representative olefins catalysed by ruthenacarboranes 1–6a
Complex
Substrate conversion (%)/Kharasch addition (%)b
n-Butyl
Methyl
Styrene
1-Decene
acrylate
methacrylate
1
2
3
4
5
6
70/47
70/46
72/58
76/47
75/25
75/23
99/95
99/95
99/99
99/99
97/89
96/89
100/99
100/99
100/99
100/99
100/99
100/99
55/52
52/48
66/58
57/51
50/43
41/37
a Reaction conditions. Prior to use, the reagents, the solvent (toluene)
and the internal standard (dodecane) were dried using well estab-
lished procedures, distilled and kept under nitrogen at −20°C. The
catalyst (0.03 mmol) was dissolved in toluene (1 mL) and subse-
quently added through a septum to the solution of alkene (9 mmol),
CCl4 (13 mmol), dodecane (0.25 mL) in toluene (3 mL). Tempera-
ture, 40°C. Reaction time, 16 h.
Figure 2. Substrate (ꢀ, 2, ꢁ, ꢂ) and adduct (ꢃ, ", ꢄ, ꢅ)
versus time for the Kharasch addition of carbon tetrachloride
to n-butyl acrylate (ꢀ, ꢃ), methyl methacrylate (2, "),
styrene (ꢁ, ꢄ), and 1-decene (ꢂ, ꢅ), catalysed by complex
2 at 40°C. Reaction conditions are the same as in Table 1.
b Conversion and yield are based on the alkene, and determined by
GC using dodecane as internal standard.

O. Tutusaus et al. / Tetrahedron Letters 44 (2003) 8421–8425
8423
far (Fig. 3).11 Interestingly, 1–6 exhibited reaction rates
that were much faster than the rates determined in our
earlier investigation using [RuCl(Cpc)(PPh3)2] as the
catalysts.11 With these encouraging results in hand, we
then aimed to decrease the catalyst loading in order to
determine the limitations of catalysts 1–6. To this end,
complex 3 was selected from among those investigated.
The rate profiles corresponding to the addition of CCl4
to methyl methacrylate for representative catalyst load-
ings are shown in Figure 4. They revealed that the
Kharasch addition remained quantitative for MMA/
catalyst molar ratios up to 3000. Beyond this value, the
addition was no longer quantitative, and oligomerisa-
tion became competitive with monoaddition. The
kinetic data also allowed us to estimate the maximum
(unoptimised) turnovers. Thus, with methyl methacryl-
ate, the maximum total turnover number (TTN) and
initial turnover frequency (TOF) reached 4200 and
1900 h−1, respectively. Likewise, with styrene, a maxi-
mum TTN of 9000 and TOF of 1500 h−1 were mea-
sured, as opposed to a maximum TTN of 1600–1700
and TOF of 400 h−1 obtained with [RuCl(Cpc)(PPh3)2]
(Cpc=Cp* or indenyl).11 In addition, the TTN for 3
was also higher than that obtained with the pincer
N,C,N-chelating aryldiaminonickel complex, the most
efficient ATRA catalyst hitherto reported, with a TTN
of 1730 and a TOF of 400 h−1 for MMA.6a,12
Figure 3. Methyl methacrylate (ꢀ, 2, ꢁ, ꢂ) and adduct
(ꢃ, ", ꢄ, ꢅ) versus time for the Kharasch addition of
carbon tetrachloride to MMA catalysed by complexes 3 (ꢀ,
ꢃ),
4
(2, "), [RuCl(Cp*)(PPh3)2] (ꢁ, ꢄ), and
[RuCl(Ind)(PPh3)2] (ꢂ, ꢅ), at 40°C. Reaction conditions are
the same as in Table 1.
Encouraged by these findings, we then investigated the
reaction of chloroform, a much less reactive halocarbon
than carbon tetrachloride, with methyl methacrylate
and styrene. Preliminary results of these experiments
are outlined in Table 2. Unfortunately, the Kharasch
addition of CHCl3 to MMA at 80°C was unsuccessful,
whereas with styrene moderate yields were obtained.
Interestingly enough, with styrene, complex 3 was once
again the most efficient catalyst among those investi-
gated, since most of the styrene consumed during the
reaction (35%) was converted into the Kharasch addi-
tion product (33%).
Figure 4. Methyl methacrylate (ꢀ, 2, ꢁ, ꢂ) and adduct
(ꢃ, ", ꢄ, ꢅ) versus time for the Kharasch addition at 40°C
of carbon tetrachloride to MMA catalysed by complex 3.
Influence of catalyst loading: MMA/3=300 (ꢀ, ꢃ), 4500
(2, "), 6000 (ꢁ, ꢄ), and 9000 (ꢂ, ꢅ). Reaction conditions
are the same as in Table 1.
Table 2. Kharasch addition of chloroform to methyl
methacrylate and styrene catalysed by ruthenacarboranes
1–6a
Substrate conversion (%)/Kharasch addition (%)b
Ruthenium complexes 1–6 are 18-electron species, and
it is generally agreed that the catalytic activity of half-
sandwich ruthenium complexes of the general structure
[RuX(Cpc)(PPh3)2] depends on the relative facility of
dissociation of one phosphine ligand.11,13 It is therefore
reasonable to assume that, in the present case, the
initial stage of the catalytic process implies the release
of one triphenylphosphine ligand from complexes 1–6
Complex
Methyl methacrylate
Styrene
1
2
3
4
5
6
44/3
40/4
42/7
40/4
46/2
48/2
46/17
38/21
35/33
36/10
39/20
36/18
to
yield
the
corresponding
16-electron
[RuH(CB)(PPh3)] species. In order to test this hypothe-
sis, Kharasch reactions were performed in the presence
of free PPh3, reasoning that addition of phosphine
would decrease the rate significantly. Figure 5 does
conform to our expectations. Addition of free PPh3
slowed down the Kharasch reaction rate, and the
effects were more pronounced as the amount of PPh3
increased.
a Reaction conditions are the same as in Table 1, except the tempera-
ture (80°C) and the reaction time (24 h).
b Conversion and yield are based on the alkene, and determined by
GC using dodecane as internal standard.
(halide assisted) from a carbonꢀchlorine bond of CCl4
could then occur, yielding both the corresponding 17-
electron ruthenium(III) species, [RuHCl(CB)(PPh3)],
’
Having thus generated a coordinatively unsaturated
16-electron ruthenium centre, the electron-transfer
and the CCl3 radical. The latter could then add to the
olefin, therefore initiating the catalytic cycle (Scheme 4),

8424
O. Tutusaus et al. / Tetrahedron Letters 44 (2003) 8421–8425
or react with the RuIII species just formed. On this
’
point, two pathways seem viable: either CCl3 recom-
bines with the halogen according to the equilibrium
indicated in Scheme 4, or abstracts the hydrogen atom
from the RuIII species to yield chloroform. Studies
along this line are underway. On the other hand, the
radical nature of the mechanism has been confirmed by
Figure 5. Methyl methacrylate (ꢀ, 2, ꢁ, ꢂ) and adduct
(ꢃ, ", ꢄ, ꢅ) versus time for the Kharasch addition of
carbon tetrachloride to MMA catalysed by complex 2, with-
out added PPh3 (ꢀ, ꢃ) and in the presence of PPh3, 1.02
equiv. (2, "), 1.88 equiv. (ꢁ, ꢄ), and 3.24 equiv. (ꢂ, ꢅ),
at 40°C. Reaction conditions are the same as in Table 1.
Figure 6. Methyl methacrylate (ꢀ, 2, ꢁ, ꢂ) and adduct
(ꢃ, ", ꢄ, ꢅ) versus time for the Kharasch addition of
carbon tetrachloride to MMA catalysed by complex 1, with-
out galvinoxyl (ꢀ, ꢃ) and in the presence of galvinoxyl, 1.05
equiv. (2, "), 2.85 equiv. (ꢁ, ꢄ), and 5.35 equiv. (ꢂ, ꢅ),
at 40°C. Reaction conditions are the same as in Table 1.
adding galvinoxyl (a radical scavenger) to the reaction
medium. Indeed, the Kharasch addition of CCl4 to
methyl methacrylate was retarded in the presence of
one equivalent of galvinoxyl, and almost completely
inhibited when 5 equivalents of galvinoxyl were added
to complex 1 (Fig. 6).11
In conclusion, ruthenium complexes 1–6 bearing
charge-compensated carborane ligands have proven to
be highly efficient catalyst precursors for the Kharasch
addition of carbon tetrachloride to methyl methacrylate
and styrene under mild reaction conditions (40°C). In
addition, they surpass the most efficient ATRA cata-
lysts reported so far. However, the precise role of the
charge-compensated carborane ligand, of the phosphi-
nes, and of the hydride as well, makes detailed mecha-
nistic analysis a challenging endeavour at this time.
Further studies of this chemistry are in progress.
Acknowledgements
This work was financially supported by a grant-in-aid
(O.T.) joined to the project MAT2001-1575 from
CICYT (Spain) and a Scientific Cooperation Project
between Spain and Belgium (2000BE0003). Part of this
work has also been carried out in the framework of the
COST D17/0006/00 and TMR-HPRN CT 2000-10
‘Polycat’ programmes. The Belgian team is also grateful
to the ‘Fonds National de la Recherche Scientifique’
(F.N.R.S.), Brussels, for the purchase of major
instrumentation.
Scheme 4. Tentative mechanism for the Kharasch addition
catalysed by ruthenacarboranes 1–6.

O. Tutusaus et al. / Tetrahedron Letters 44 (2003) 8421–8425
8425
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