Cycloruthenated complexes as homogeneous catalysts for atom-transfer radical additions

Article abstract of DOI:10.1016/j.tetlet.2009.12.011

Various cycloruthenated complexes were used as homogeneous catalysts for the atom-transfer radical addition of polyhalogenated compounds to several olefinic substrates. Yields obtained through conventional or microwave heating could reach high values (up to 98% with CBrCl₃ and 88% with CCl₄).

Full text of DOI:10.1016/j.tetlet.2009.12.011

Tetrahedron Letters 51 (2010) 822–825
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Cycloruthenated complexes as homogeneous catalysts for atom-transfer
radical additions
*
Ksenia Parkhomenko, Laurent Barloy , Jean-Baptiste Sortais, Jean-Pierre Djukic, Michel Pfeffer
Institut de Chimie, CNRS-Université de Strasbourg, UMR 7177, Laboratoire de Synthèses Métallo-Induites, 4 rue Blaise Pascal, 67000 Strasbourg, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Various cycloruthenated complexes were used as homogeneous catalysts for the atom-transfer radical
addition of polyhalogenated compounds to several olefinic substrates. Yields obtained through conven-
tional or microwave heating could reach high values (up to 98% with CBrCl and 88% with CCl ).
3 4
Received 4 November 2009
Revised 30 November 2009
Accepted 2 December 2009
Available online 5 December 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Homogeneous catalysis
Cyclometallated complexes
Ruthenium and compounds
Radicals and radical reactions
Olefins
ATRA
The Kharasch reaction is an efficient method for the formation of
C–C bonds. It has encountered renewed interest within the last dec-
after a 6 h reaction at 80 °C. We used cycloruthenated complexes
belonging to the library shown in Scheme 2 as ATRA catalysts; their
synthesis has been reported previously (see Supplementary data).
Overall we observed that the yield and conversion for a given cata-
lyst were nearly the same, the difference between both figures being
less than 5% except for catalyst 2c (entry 7). We can therefore con-
1
ade because the high reactivity of radical intermediates can be
tamed by transition metal catalysts; such metal-catalysed additions
of radicals (e.g., originated from polyhalogenated compounds) onto
olefins are known as atom-transfer radical addition (ATRA) reac-
2
8
tions. Among these ATRA catalysts, ruthenium complexes are now
clude that side reactions such as radical polymerisation of styrene
occur very little.
3
widely used, including many half-sandwich complexes.
Besides, cycloruthenated complexes are easy to synthesise in
good yield, and they have proved to be markedly robust, the me-
tal-carbon bond being stabilized by chelation.4 We have been
All the catalysts except 5b were active, with yields ranging from
42% to 98%. The effect of the bridging ligand X in the 5 series is
remarkable: the presence of a bridging hydroxo ligand (5a, entry
14) instead of a chloro (5b, entry 15) makes the complex catalyti-
5
interested lately in their properties as hydrogen transfer catalysts.
However, examples of applications of cycloruthenated complexes
in other domains of homogeneous catalysis remain relatively
scarce. In the field of radical processes, Alexandrova et al. have re-
cally active. The other l-hydroxo, l-chloro ruthenacyclic dimer 6
exhibits an activity similar to that of 5a.
4
Within the subfamily of mononuclear ruthenium complexes,
6
cently reported the successful use of cycloruthenated complexes in
the best candidates for ATRA were the
g
-arene neutral complexes
atom-transfer radical polymerisation catalysis.6
derived from N,N-dimethylbenzylamines (DMBAs) 1a and 1d (en-
We will report hereafter some unprecedented properties of
cycloruthenated complexes as catalyst precursors in the field of ATRA
reactions. The reaction consisted in the addition of bromotrichlorom-
ethane or carbon tetrachloride with various olefins (Scheme 1).
R
Cl3C
X
Ru catalyst
CXCl3
+
R
3
A first set of catalytictests were run using reactive CBrCl as a rad-
R'
R'
ical addition reagent and styrene as an olefinic substrate (Table 1);
the experimental conditions (molar ratios) reported by Kamigaito
et al. were followed. The yields and conversions were determined
X = Br, Cl
styrene: R = Ph, R' = H
MA: R = COOMe, R' = H
MMA: R = COOMe, R' = Me
7
1
-hexene: R = n-C H , R' = H
4 9
*
Corresponding author. Tel.: +33 368851269; fax: +33 368851526.
Scheme 1. Ruthenium-catalysed Kharasch addition of polyhalogenated compounds
E-mail address: barloy@unistra.fr (L. Barloy).
to olefins.
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.12.011

K. Parkhomenko et al. / Tetrahedron Letters 51 (2010) 822–825
823
Table 1
100
Addition of bromotrichloromethane to styrene catalysed by cycloruthenated
complexesa
Entry
Catalyst
Conversionb (%)
Yieldb (%)
80
60
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
2a
2b
2c
2d
2e
3
4a
4b
5a
5b
6
99
56
43
98
67
49
76
60
88
83
59
72
70
98
56
42
98
67
49
66
59
88
83
58
70
70
4
2
0
0
0
10
12
13
14
15
16
c
c
—
—
0
1
2
3
4
5
6
7
8
60
56
Time, h
a
Reaction conditions: styrene (1.4 mmol), CBrCl
3
(4.2 mmol), catalyst
Figure 1. Styrene (s,h) and adduct (d,j) versus time for the addition of
bromotrichloromethane to styrene catalysed by complexes 1a (d,s) and 2e (j,h)
at 80 °C (conventional heating). The reaction conditions were as listed in Table 1,
except the reaction mixture was evenly charged into nine glass tubes (see
Supplementary data). Determination of conversions and yields: see Table 1.
(
0.01 mmol), standard (0.14 mmol), 80 °C, 6 h (see experimental details in Sup-
plementary data); yield of uncatalysed reaction (blank) is 5%.
b
Conversions and yields were based on the olefin, and were determined by 1
H
NMR using tetraline as an internal standard.
c
Traces.
between neutral and cationic cyclometallated complexes only as
a general positive effect of increased electron density on the metal,
because conversely the electron-donating methoxy substituents on
the metallated phenyl group of complex 2b diminished the yield
with regard to 2a (entries 5 and 6). By contrast, Demonceau et al.
had observed a positive influence of electron-donating groups on
tries 1 and 4), which led almost quantitatively to the 1:1 adduct.
Another kind of catalysts that led to high chemical yields (>80%)
6
were cationic
g
-benzene complexes resulting from cycloruthena-
tion of primary benzylic amines, viz. 2e and 3 (entries 9 and 10). By
contrast with these cyclometallated tertiary and primary amines,
the cycloruthenated secondary amine 2c (NHMe analogue of 2e)
afforded a lower yield.
1
1
phosphine ligands in Ru-catalysed ATRA. In accordance with
our own findings, no correlation was found between the reducing
power of cycloruthenated complexes and their ATRP catalytic
Switching from 1a to the cationic analogue 2a afforded a lower
yield (67%, entry 5); according to the literature on ATRA ruthenium
catalysts, sometimes the catalytic performance decreased similarly
6
activity.
9
We noticed that the bipy-coordinated complexes 4a and 4b
were less efficient than their half-sandwich analogues, respec-
upon abstraction of a chloro ligand, but the reverse effect was also
1
0
reported.
In our case, we cannot interpret the comparison
+
-
+
-
PF6
PF6
R1
R2
Ru
Ru
Ru
Cl
NMe2
NCMe
NCMe
NH2
4
5
NR R
R1
R3
R3
Me
R2
1
2
3
1
1
1
1
a: R1 = R2 = R3 = H
2a: R1 = R2 = R3 = H, R = R5 = Me
4
b: R = R = H, R3 = Me
1
2
2b: R3 = H, R1 = R2 = OMe, R = R5 = Me
4
c: R = R = Me, R2 = iPr
1
3
2c: R1 = R2 = R4 = H, R = R5 = Me
3
d: R = Me, R2 = R3 = H
1
2d: R1 = R = R = R = R5 = H
2
3
4
1
2
4
5
2
e: R = R = R = R = H, R3 = Me
+
-
PF6
OC CO
OC CO
Ru
R1
N(R2)
2
Ru
Cl
Cl
Ru(bipy)(MeCN)2
N
N
N
N
X
O
Ru
Ru
H
CO
CO
CO
CO
4
5
6
4
4
a: R1 = H, R2 = Me
b: R1 = Me, R2 = H
5a: X = µ-OH
5b: X = µ-Cl
bipy = 2,2'-bipyridine
Scheme 2. Cycloruthenated ATRA catalysts.

8
24
K. Parkhomenko et al. / Tetrahedron Letters 51 (2010) 822–825
Table 2
the experimental procedures (absence of stirring in the conditions
shown in Fig. 1).
The ability of cycloruthenated complexes to catalyse Kharasch
3
additions of CBrCl on other substrates, viz. methyl acrylate
(MA), methyl methacrylate (MMA) and 1-hexene, was then evalu-
ated (Table 2). The DMBA-catalysts 1a and 1d, and also the cationic
cycloruthenated primary amines 2e and 3 were selected for that
purpose. Reactions at 80 °C led to yields ranging from 30% to
99%; there was often a gap between conversion and yield, indicat-
ing substantial oligomerisation or polymerisation.
As expected, yields were higher overall with substrates MA and
MMA than with 1-hexene. We noticed that catalyst 1a, which led
to an almost quantitative conversion of MMA into the CBrCl -adduct
3
within 3 h only at 80 °C (entry 3), was still active when the temper-
ature was lowered to 60 °C (63% yield in 6 h, entry 2). It is worth not-
ing that using MA as the substrate, the yield at 80 °C reached 94%
with 1a, whereas it was only 51% with 1d (entries 10 and 12),
although both catalysts differ slightly; furthermore, decreasing the
temperature to 60 °C was not very detrimental to the yield with 1a
Addition of bromotrichloromethane to various alkenes catalysed by cycloruthenated
complexesa
Entry
Substrate
Catalyst
T (°C)
Conversion (%)
Yield (%)
b
1
2
3
4
5
6
7
8
9
MMA
MMA
MMA
MMA
MMA
MMA
MMA
MMA
MA
MA
MA
MA
MA
1a
1a
1a
1a
1d
1d
1d
3
1a
1a
1d
1d
2e
3
45
60
80
1
1
b
99
63
b
c
c
99
99
d
135
100
99
94
100
100
99
96
58
69
88
99
56
63
98
81
89
94
18
51
30
80
44
43
30
b
60
80
b
135d
d
135
b
60
80
60
80
80
80
80
80
b
10
11
12
13
14
15
16
17
b
b
b
b
MA
99
44
45
30
b
1-Hexene
1-Hexene
1-Hexene
1a
1d
3
b
b
80
a
Reaction conditions: alkene (1.4 mmol), CBrCl
3
(4.2 mmol), catalyst
(
89%, entry 9), but it was so with 1d (18%, entry 11).
(
0.01 mmol), standard (0.14 mmol); see experimental details in Supplementary
Demonceau et al. have reported the use of microwave irradia-
data. Determination of conversions and yields: see Table 1.
b
Conventional heating, t = 6 h unless otherwise stated; we found that no reaction
4
tion as the heating source in ATRA reactions with CCl using half-
1
2
took place without catalyst under any of those conditions.
sandwich ruthenium catalysts. This device proved to be a conve-
nient way to provide fast heating of the reaction medium. Thus, we
have also used it to reach higher reaction temperatures easily. In-
c
Conversion and yield after t = 3 h.
Microwave reactor, t = 30 min; yield of uncatalysed reaction (blank) is 10%.
d
3
deed the reaction of MMA with CBrCl proceeded with excellent
yields (81%–99%) within a short time (30 min, entries 4, 7 and 8).
We then tried this method with the less reactive polyhalogenat-
ed species CCl (Table 3). Initial attempts of addition of CCl to var-
tively, 2a and 2e (entries 12 and 13 vs 5 and 9). However, from
these observations in particular and from all data listed in Table
4
4
1
in general, it is difficult to rationalise the influence on the cata-
ious olefins by conventional heating to 80 °C had not been very
lytic efficiency of various factors (charge of the complex, nature
of ancillary ligands, nature of substituents on the cyclometallated
molecule, etc.). Some contradictory trends were even observed;
for instance, comparison between 1a and 1b showed that a methyl
substituent in benzylic position was detrimental to the yield in the
DMBA series, but reversibly it had a positive effect on the yield in
the cationic cycloruthenated benzylamine series (2d vs 2e).
We have represented in Figure 1 the kinetic profiles of the Khar-
successful, as long as five days heating was necessary to attain rea-
1
2
sonable yields and conversions. Easily polymerisable substrates
such as styrene (catalyst 3, entry 1) and MMA (catalyst 1d, entry
6) led to high conversions (92% and 82%) but negligible yields.
The yield of addition of CCl on styrene reached much higher val-
4
ues (23–75%) within shorter times (0.5–1 h) when the reaction
medium was submitted to a temperature of 135 °C in a microwave
oven (entries 2–5). At first sight it seemed surprising that the yield
could increase to 75% at t = 1 h whereas the conversion was already
100% at t = 0.5 h (entries 2 vs 3); this was probably due to the fact
that in the oven, the reaction mixture was not frozen immediately
after the reaction time was elapsed, and the substrate may have
undergone side reactions during the period of time when temper-
ature decreased. A similar phenomenon happening at the begin-
ning of the microwave-assisted ATRA reaction has been reported
3
asch addition of CBrCl on styrene catalysed by 1a and 2e at 80 °C.
They confirm that the conversions and yields were practically
equal over time with both catalysts. The reaction catalysed by 1a
was over within 3 h, whereas 2e was slower; turn-over frequencies
À1
À1
(
2
TOF) of, respectively, 54 h and 21 h were estimated for 1a and
e from the curves shown in Figure 1. The final yield after 6 h reac-
tion with 2e (75%) was slightly lower than the yield (88%) men-
tioned in Table 1; this may be due to minor differences between
1
2
previously.
Table 3
a
Addition of carbon tetrachloride to various alkenes catalysed by cycloruthenated complexes
Entry
Substrate
Catalyst
T (°C)
Reaction time (h)
Conversionb (%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
Styrene
Styrene
Styrene
Styrene
Styrene
MMA
MMA
MMA
1-Hexene
1-Hexene
1-Hexene
1-Hexene
3
3
3
80b
135
135
135
120
0.5
1
0.5
1
92
100
100
68
83
82
98
48
73
46
2
51
75
23
32
0
88
2
73
46
59
21
c
c
c
1d
1d
1d
1d
3
1d
2e
1d
3
c
135
b
80
120
0.5
0.5
120
120
c
135
c
135
b
80
80
b
1
1
1
0
1
2
c
d
135
135
0.5
59
21
c
d
0.5
a
4
Reaction conditions: alkene (1.4 mmol), CCl (4.2 mmol), catalyst (0.01 mmol), standard (0.14 mmol); see experimental details in Supplementary data. We found that no
1
:1 ATRA adduct was formed without catalyst under any of those conditions, unless otherwise stated. Determination of conversions and yields: see Table 1.
b
Conventional heating.
Microwave reactor.
c
d
Yield of uncatalysed reaction under those conditions was 5%.

K. Parkhomenko et al. / Tetrahedron Letters 51 (2010) 822–825
825
It is striking to note that using MMA as the substrate, micro-
wave heating led to a very good yield (88%, entry 7) using catalyst
0086). We also thank Dr. Ernest Graf for experimental assistance
with reactions under microwave heating.
1
d, but it led to a yield of 2% only using catalyst 3 (entry 8); by con-
trast, 3 was superior to 1d with styrene as the ATRA substrate.
Overall, these figures showed that under microwave conditions,
unwanted chemical processes were avoided and the ATRA reaction
was often favoured.
With the non-activated substrate 1-hexene, the final yield in
Kharasch product was always equal to the conversion, indicating
that no polymerisation occurred (which is often expected with this
kind of substrate). Under conventional heating conditions, a good
yield (73%) could be reached after 5 days heating at 80 °C (entry
Supplementary data
Supplementary data (catalysts synthesis, experimental proce-
dures) associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.tetlet.2009.12.011.
References and notes
9
). The reaction time was reduced to 0.5 h under microwave condi-
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(
b) Kharasch, M. S.; Jensen, E. V.; Urry, W. H. Science 1945, 102, 128.
tions with fair yields (21 and 59%, respectively, entries 11 and 12).
Although we have implemented no mechanistic investigations
on ruthenacycle-catalysed Kharasch reactions so far, it is likely that
the reaction starts with the abstraction of X from CXCl
Ru –X transient species and a reactive CCl
2.
(a) Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem. Rev. 1994, 94, 519–564; (b) Gossage,
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1
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in the field of ATRA reactions, even with a moderately reactive
substrate such as 1-hexene. They represent a new advancement
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8
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3 4
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4
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Acknowledgements
1
The authors thank the Agence National de la Recherche for
financing a Young Investigator Program (project ANR-06-JCJC-