Electrochemistry as a correlation tool with the catalytic activities in [RuCl2(p-cymene)(PAr3)]-catalysed Kharasch additions

Article abstract of DOI:10.1016/j.tetlet.2006.01.138

[RuCl₂(p-cymene)] complexes containing triarylphosphine ligands with various substituents at the para position were used to catalyse the atom transfer radical addition of carbon tetrachloride to various olefins, and their catalytic activities were nicely correlated with their electrochemical parameters.

Full text of DOI:10.1016/j.tetlet.2006.01.138

Tetrahedron Letters 47 (2006) 2077–2081
Electrochemistry as a correlation tool with the catalytic activities
in [RuCl (p-cymene)(PAr )]-catalysed Kharasch additions
2
3
*
Aurore Richel, Albert Demonceau and Alfred F. Noels
Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Li e` ge, Sart-Tilman (B.6a), B-4000 Li e` ge, Belgium
Received 15 December 2005; revised 24 January 2006; accepted 27 January 2006
Available online 14 February 2006
Abstract—[RuCl (p-cymene)] complexes containing triarylphosphine ligands with various substituents at the para position were
2
used to catalyse the atom transfer radical addition of carbon tetrachloride to various olefins, and their catalytic activities were nicely
correlated with their electrochemical parameters.
Ó 2006 Elsevier Ltd. All rights reserved.
Over the past few years, the transition-metal-catalysed
Kharasch addition (also known as ‘ATRA’, atom trans-
fer radical addition) has experienced a sudden rebirth,
which is unquestionably the result of the development
of atom transfer radical polymerisation (ATRP), discov-
ered independently in the mid-nineties by Sawamoto
precursors) is crucial. [RuCl (arene)(PR )] complexes
2 3
afford these advantages: [RuCl (arene)(PR )] complexes
2
3
are readily available on reaction of 2 equiv of a phos-
9
phine with the [RuCl (arene)] dimer and are air-stable.
2
2
In addition, their stereoelectronic features can be fine-
tuned by using suitable phosphine ligands.
1
and Matyjaszewski. Many varieties of metal-based
2
3
4
catalysts, such as copper, nickel and ruthenium
We have recently found that [RuCl (p-cymene)(PR )]
2
3
complexes, have been proposed for Kharasch reactions.
complexes (p-cymene = 4-isopropyltoluene) possessing
a basic and bulky phosphine (typically tricyclohexyl-
phosphine and triisopropylphosphine) were outstanding
catalyst precursors for promoting ATRP of vinyl mono-
Ruthenium, in particular, has played a prominent role in
5
Kharasch chemistry with RuCl (PPh )
displaying
2
3 3
some of the highest efficiency and versatility for halocar-
bon activation and addition to olefins.
1
0
mers. Their use in Kharasch chemistry was however
7
b
unsuccessful.
Conversely, [RuCl (p-cymene)(PR )]
2 3
We have been interested in the development of new
catalyst systems for Kharasch addition, and a variety
a novel ruthenium-based catalysts have been reported,
complexes with less basic and/or less bulky phosphines
(typically triphenylphosphine) were inefficient in
ATRP. We now report on the efficiency of catalytic
1
0
6
7
including ruthenacarboranes, RuCl (@CHPh)(PR )
systems based on [RuCl (p-cymene)(PAr )] for the
2
3 2
#
2
3
#
(
Grubbs’ complexes) and RuCl(Cp )(PAr ) (Cp =
Kharasch addition of carbon tetrachloride to olefins,
especially to unactivated a-olefins (Scheme 1).
3
2
*
8
Cp and indenyl). Despite their high efficiency (total
turnover numbers (TON) up to 9000 and initial turnover
ꢀ
1
frequencies (TOF) of around 1900 h were observed
Since the Kharasch addition rests on the metal-mediated
activation of a carbon–halogen bond and the concomi-
tant one-electron oxidation of the metal centre (Scheme
2), we anticipated that simple modification to the tri-
phenylphosphine ligand in [RuCl (p-cymene)(PAr )] by
6
b,c
with the former complexes ) and their versatility,
these catalyst systems suffer from their difficult, tedious
and time-consuming multi-step synthesis. Given the
importance of radical reactions in both fine organic
synthesis and macromolecular chemistry, the need for
cheap and easily synthesised catalysts (or catalyst
2
3
Cl
CCl4
[RuCl (p-cymene)(PAr )]
CCl3
Keywords: Catalysis; Electrochemistry; Olefins; Radicals and radical
reactions; Ruthenium and compounds.
R
R
2
3
*
Corresponding author. Tel.: +32 4 366 3495; fax: +32 4 366 3497;
e-mail: A.Demonceau@ulg.ac.be
Scheme 1. Kharasch addition of carbon tetrachloride across olefin.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.01.138

2078
A. Richel et al. / Tetrahedron Letters 47 (2006) 2077–2081
II
[Ru ]
III
[Ru X]
100
80
R
X
+
R•
+
Scheme 2. Activation of an alkyl halide by ruthenium.
6
4
0
0
incorporating electron-donating or electron-withdraw-
ing substituents at the para position of the aryl groups,
could be used to tune the catalytic activity of the ruthe-
nium complexes. Thus, five isosteric [RuCl (p-cym-
2
20
0
ene)(P(p-C H X) )] complexes were synthesised, in
6
4
3
which X = OMe, Me, F, Cl and CF (Scheme 3). The
3
experimental data for the Kharasch addition of CCl4
0
100
200
Time, h
300
400
500
to representative olefins catalysed by [RuCl (p-cym-
2
ene)(PAr )] complexes are summarised in Table 1.
3
Figure 1. Influence of the substrate on the Kharasch addition of
carbon tetrachloride catalysed by [RuCl (p-cymene)(PPh )] at 60 °C.
2
3
When styrene was reacted with CCl under anaerobic
4
Styrene (s) and 1-decene (h) and their Kharasch adducts (d, j).
conditions, the Kharasch adduct (1,1,1,3-tetrachloro-3-
phenylpropane) was obtained and, depending upon the
catalyst used and the reaction time, with styrene being
recovered. Small amounts of oligomers, not yet detected
by gas chromatography, most probably account for the
balance of the reaction. With 1-decene as the substrate,
the expected 1,1,1,3-tetrachloroundecane was the only
product along with the starting material (Table 1 and
Reaction conditions are the same as in Table 1.
CH3
[
Ru]
+
CCl4
E
H C E
H
3
C E H C E
3
3
Fig. 1). On the other hand, reaction of CCl with methyl
4
+
methacrylate yielded the expected monoadduct (A,
Scheme 4), together with the diadduct (B) and higher
oligomers.
Cl
3
C
Cl C
3
Cl
Cl
A
B
E = CO CH3
2
On the basis of the results shown in Table 1, we con-
cluded that 1-decene, which is a difficult substrate for
4
Scheme 4. Addition of CCl to methyl methacrylate.
Kharasch additions,11 is all things considered a sub-
strate better than expected, at least with this class of
ruthenium complexes (see also Fig. 1). This prompted
us to evaluate the substrate scope of this protocol with
other unactivated olefins (Table 2). When reacted with
Ru
Cl
Cl
PAr3
CCl , 1-octene and 1-dodecene were also efficiently
4
Scheme 3. [RuCl
2
(p-cymene)(PAr
3
)] complexes under investigation.
transformed into the corresponding Kharasch adducts
in excellent yields. Of note, however, is the significantly
lower reactivity of 1-hexene, compared to higher a-
olefins.
Table 1. Addition of carbon tetrachloride to styrene and 1-decene
a
catalysed by [RuCl
RuCl (p-cymene)
PAr )]
2 3
(p-cymene)(PAr )] complexes
Rationalisation of electronic effects in transition-metal-
mediated catalytic reactions is critically important for
b
[
(
2
Substrate conversion (%) /Kharasch
b
3
addition (%)
1
2
the design of new ligands. In the present case, the sub-
stituents at the para position of the aromatic rings of the
phosphine ligands affect the catalytic activity to a great
extent (Table 1, Figs. 2 and 3). The best results were
obtained with [RuCl (p-cymene)(P(p-C H OCH ) )],
Styrene
0 °C
1-Decene
60 °C
1-Decene
85 °C
MMA
85 °C
6
Ar
p-C
p-C
6
H
H
4
OCH
3
66/61
64/59
62/58
31/27
25/20
45/43
43/42
34/33
28/28
21/21
20/19
12/11
89/89
79/78
67/66
59/58
45/44
64/63
99/40
96/47
87/43
50/21
39/19
51/13
2
6
4
3 3
6
4
CH
3
which contains the most electron-donating tris(p-meth-
oxyphenyl)phosphine ligand of the series. On the con-
trary, [RuCl (p-cymene)(P(p-C H Cl) )], which contains
C
6
H
5
p-C
p-C
p-C
6
6
6
H
H
H
4
4
4
F
Cl
2
6
4
3
the weaker electron-donating tris(p-chlorophenyl)phos-
phine ligand was much less efficient, revealing a nice
qualitative correlation between electronic features of
the phosphine ligand and catalytic activity of the result-
ing ruthenium complex. For instance, with decene at
CF
3
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),
8
5 °C and 30 h runtime, yields were as follows: OMe
(89%), Me (78%), H (66%), F (58%) and Cl (44%).
4
CCl (13 mmol), dodecane (0.25 mL) in toluene (3 mL). Reaction
The same trend was found with methyl methacrylate.
time, 30 h.
b
Conversion and yields based on GLC using dodecane as internal
standard.
[RuCl (p-cymene)(P(p-C H CF ) )], however, bearing
2
6
4
3 3
the least electron-donating phosphine ligand of the

A. Richel et al. / Tetrahedron Letters 47 (2006) 2077–2081
2079
a
Table 2. Addition of carbon tetrachloride to representative a-olefins catalysed by [RuCl
2
3
(p-cymene)(PAr )] complexes
b
b
[
RuCl (p-cymene)(PAr )]
2
3
Substrate conversion (%) /Kharasch addition (%)
1
-Hexene
1-Octene
1-Decene
1-Dodecene
Ar
p-C
p-C
c
c
c
c
c
c
c
c
c
c
c
6
6
H
4
OCH
CH
3
48/39 (82)
44/31 (71)
40/23 (59)
97/92 (95)
86/77 (89)
82/75 (87)
89/89 (100)
79/78 (100)
67/66 (99)
63/61 (96)
56/55 (99)
55/52 (95)
H
4
3
c
C
6
H
5
a
b
c
Reaction conditions are the same as in Table 1 (temperature, 85 °C; 30 h).
Same as in footnote b (Table 1).
Kharasch addition at complete conversion of the olefin.
1
00
100
80
60
40
20
0
These results in Kharasch chemistry were rationalised
on the basis of the electron-donating ability of the phos-
8
6
4
2
0
0
0
0
0
phine ligands, as indicated by the pK value of the phos-
a
1
3
phonium ion, and by measuring the changes in the
1
4,15
redox potential
ene)(PAr )] complexes.
for the different [RuCl (p-cym-
2
1
6
3
60˚C
85˚C
The cyclic voltammograms of [RuCl (p-cymene)(PAr )]
2
3
complexes in dichloromethane (0.1 M nBu NPF ) dis-
4
6
0
100
200
Time, h
300
400
500
0
100
Time, h
200
played a reversible oxidation couple at ca. 0.6–0.8 V ver-
sus ferrocene. The electrochemical data are summarised
in Table 3 and representative cyclic voltammograms of
Figure 2. Influence of the catalyst system, [RuCl
on the Kharasch addition of carbon tetrachloride to 1-decene at 60 and
5 °C. Catalysts: [RuCl (p-cymene)(P(p-C OCH )] (j), [RuCl
p-cymene)(PPh )] (d) and [RuCl (p-cymene)(P(p-C Cl) )] (m).
Reaction conditions are the same as in Table 1.
2 3
(p-cymene)(PAr )],
[
RuCl (p-cymene)(PAr )] complexes (Ar = p-C H OCH ,
2 3 6 4 3
C H and p-C H Cl) are shown in Figure 4. Examina-
8
(
2
6
H
4
3
)
3
2
-
6
5
6
4
3
2
6
H
4
3
tion of the data indicates that the redox potentials vary
significantly with the nature of the phosphine ligand.
0
Compared to [RuCl (p-cymene)(PPh )] (E = 0.680 V),
2
3
slightly less positive potential values of 0.595 V
in [RuCl (p-cymene)(P(p-C H OCH ) )] and 0.624 V in
2
6
4
3 3
1
00
[RuCl (p-cymene)(P(p-C H CH ) )] were observed,
2 6 4 3 3
in line with the presence of the more electron-rich
8
6
4
2
0
0
0
0
0
P(p-C H OCH ) and P(p-C H CH ) ligands that would
make the Ru(II) centre more easily oxidised than that
6
4
3 3
6
4
3 3
in [RuCl (p-cymene)(PPh )]. Using similar arguments,
2
3
the more positive potentials of [RuCl (p-cymene)(P(p-
2
C H F) )] (0.700 V), [RuCl (p-cymene)(P(p-C H Cl) )]
6
4
3
2
6
4
3
(
(
0.740 V)
and
[RuCl (p-cymene)(P(p-C H CF ) )]
2 6 4 3 3
0.820 V) than [RuCl (p-cymene)(PPh )] (0.680 V) were
2
3
attributed to the electron-withdrawing effects of the para
substituents on the aryl groups. Not surprisingly, the
0
50
100
150
Time, h
Figure 3. Influence of the catalyst system, [RuCl
on the Kharasch addition of carbon tetrachloride to methyl metha-
crylate at 85 °C. Catalysts: [RuCl (p-cymene)(P(p-C OCH )] (h,
j), [RuCl (p-cymene)(PPh )] (s, d) and [RuCl (p-cymene)(P(p-
Cl) )] (n, m); methyl methacrylate (h, s, n) and its Kharasch
2 3
(p-cymene)(PAr )],
a
Table 3. pK
a
of the triarylphosphines, PAr
3
and cyclic voltammetry
H
4
3
)
3
b
2
6
data of the [RuCl
RuCl
Ar
(p-cymene)(PAr )] complexes under investigation
2
3
2
3
2
0
c
d
[
2
(p-cymene)(PAr )] pK
3
a
E (V)
DE (V)
C
6
H
4
3
monoadduct (A, Scheme 4) (j, d, m). Reaction conditions are the
same as in Table 1.
p-C H OCH
5.13
4.46
3.28
1.63
0.87
0.595
0.624
0.680
0.700
0.740
0.820
0.068
0.070
0.075
0.075
0.078
0.078
6
4
3
p-C
6
H
4
CH
3
C
6
H
5
p-C
p-C
p-C
6
6
6
H
H
H
4
4
4
F
Cl
CF
series, displayed an unpredictable reactivity profile,
which depended on the substrate and the temperature.
In most cases, the activity of [RuCl (p-cymene)-
3
ꢀ1.39
2
a
13
pK
a
of phosphonium ion.
(
P(p-C H CF ) )] lay between that of [RuCl (p-cym-
b
6
4
3 3
2
Sample, 2 mM; nBu NPF
under nitrogen at room temperature; scan rate, 50 mV s ; potentials
are reported in volt versus ferrocene as an internal standard.
E = (Epa+Epc)/2; Epa and Epc are the anodic and cathodic peak
4
6
(0.1 M) in dry and degassed CH
2 2
Cl ,
ene)(P(p-C H ) )] and [RuCl (p-cymene)(P(p-C H F) )],
ꢀ1
6
5 3
2
6
4
3
except with decene at 60 °C where [RuCl (p-cym-
17
2
c
0
ene)(P(p-C H CF ) )] was the least active catalyst,
6
4
3 3
presumably due to its low solubility in the reaction
mixture.
potentials, respectively.
d
DE
p
= jEpaꢀEpcj.

2080
A. Richel et al. / Tetrahedron Letters 47 (2006) 2077–2081
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plexes in dichloromethane. [RuCl
(
2
(p-cymene)(PAr
3
)] com-
1
2
(p-cymene)(P(p-C
6 4 3 3
H OCH ) )]
1
), [RuCl
Cl) )] (
2
(p-cymene)(PPh
3
)] (
) and [RuCl (p-cymene)(P(p-
2
1
7
538–540; (b) Simal, F.; Delaude, L.; Jan, D.; Demonceau,
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Matyjaszewski, K., Ed.; ACS Symposium Series 768;
American Chemical Society: Washington, DC, 2000; pp
C
6
H
4
3
).
relative activities of the [RuCl (p-cymene)(PAr )] com-
plexes were supported by electrochemical studies.
2
3
2
23–233; (d) Simal, F.; Sebille, S.; Hallet, L.; Demonceau,
In conclusion, [RuCl (p-cymene)(PAr )] are highly
2
3
A.; Noels, A. F. Macromol. Symp. 2000, 161, 73–85; (e)
Simal, F.; Jan, D.; Delaude, L.; Demonceau, A.; Spirlet,
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535.
active catalyst precursors for the Kharasch addition of
carbon tetrachloride to olefins, especially to unactivated
olefins such as a-olefins. They are however somewhat
#
#
*
less active than RuClCp (PPh ) (Cp = Cp , indenyl
11. (a) Quebatte, L.; Haas, M.; Solari, E.; Scopelliti, R.;
3
2
and carboranyl), which set so far the standard in the
field, but have the simplicity and accessibility, which is
ideally required for practical applications. In addition,
we have shown that the catalytic activities of the
Nguyen, Q. T.; Severin, K. Angew. Chem., Int. Ed. 2005,
4
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[
RuCl (p-cymene)(PAr )] complexes are effectively cor-
2 3
1
2. (a) M e´ ndez, N. Q.; Seyler, J. W.; Arif, A. M.; Gladysz, J.
A. J. Am. Chem. Soc. 1993, 115, 2323–2334; (b) Faller, J.
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Acknowledgements
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This work has been carried out in the framework of the
TMR-HPRN CT 2000-10 ‘Polycat’ programme. We are
also grateful to the ‘Fonds National de la Recherche
Scientifique’ (F.N.R.S.), Brussels, for the purchase of
major instrumentation, and the ‘R e´ gion wallonne’
9
1
1
2
(
FIRST Europe programme) for a fellowship to A.R.
(
b) Na, Y.; Lee, C.; Pak, J. Y.; Lee, K. H.; Chang, S.
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configuration consisting of a 3 mm diameter glassy carbon
working electrode, a platinum auxiliary electrode and an
Ag/AgCl/saturated KCl reference electrode. Tetrabutyl-
ammonium hexafluorophosphate was used as the sup-
porting electrolyte and all the potentials were referenced to
1
7. Cyclic voltammetry experiments were conducted at room
temperature with a Princeton Applied Research EG & G
0
+
ferrocene, E (Fc/Fc ) = 0.0 V, added at the end of the
2
36 A potentiostat, using a conventional three-electrode
experiment.