Ring-closing metathesis, Kharasch addition and enol ester synthesis catalysed by a novel class of ruthenium(II) complexes

Article abstract of DOI:10.1016/S0040-4039(01)01952-9

Ruthenium Schiff base complexes I-III mediated the Kharasch addition of carbon tetrachloride across olefins with high yields which markedly depended on the catalyst and the substrate used. In addition, ring-closing metathesis of some representative diolefins was carried out. The best catalytic system III is able to form tri- and tetrasubstituted double bond products. Finally, dependent of the catalytic system and the reaction conditions used, these systems can catalyse the stereoselective formation of enol esters or enynes in excellent yields.

Full text of DOI:10.1016/S0040-4039(01)01952-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8959–8963
Ring-closing metathesis, Kharasch addition and enol ester
synthesis catalysed by a novel class of ruthenium(II) complexes
Bob De Clercq and Francis Verpoort*
Ghent University, Department of Inorganic and Physical Chemistry, Krijgslaan 281 (S-3), 9000 Ghent, Belgium
Received 28 August 2001; revised 8 October 2001; accepted 17 October 2001
Abstract—Ruthenium Schiff base complexes I–III mediated the Kharasch addition of carbon tetrachloride across olefins with high
yields which markedly depended on the catalyst and the substrate used. In addition, ring-closing metathesis of some representative
diolefins was carried out. The best catalytic system III is able to form tri- and tetrasubstituted double bond products. Finally,
dependent of the catalytic system and the reaction conditions used, these systems can catalyse the stereoselective formation of enol
esters or enynes in excellent yields. © 2001 Elsevier Science Ltd. All rights reserved.
7
1
Since ruthenium has a 4d 5s electronic configuration, it
In the enol ester synthesis the ruthenium(II) complexes
promote the electrophilic activation of alkynes to give
Markovnikov or anti-Markovnikov (E and Z) addition
has the widest range of oxidation states (from −2 in
Ru(CO)4 to +8 in RuO ) of all the elements and
−
2
4
various coordination geometries in each electronic
configuration, which represents a great potential for the
exploitation in catalytic reactions. However, until the
reactions of carboxylic acids to the triple bond (Scheme
4
3
).
1
980s the useful methods were limited to a few reac-
There exists an extensive chemistry of [(arene)-
RuX PR ] complexes and their activity in ring-closing
tions including proposals for oxidations with RuO4,
2
3
homogeneous hydrogenations for fine chemicals, hydro-
metathesis, Kharasch addition reactions and enol ester
formylations and hydrogen transfer reactions. As the
coordination chemistry of ruthenium complexes has
progressed, characteristic features of ruthenium (e.g.
high electron transferability, low redox potentials, sta-
bility of reactive metallic species, metallacycles, metal
carbenes) have opened for a broad variety of catalytic
5
synthesis. In contrast, analogous half-sandwich Schiff
base complexes are virtually unknown. Therefore, we
investigated the activity of three new Schiff base-arene-
ruthenium(II) complexes (Scheme 4) for the three
above-mentioned reactions.
1
transformations. These include olefin metathesis, Kha-
rasch addition reactions and enol ester synthesis. Olefin
metathesis is a catalytic reaction in which alkenes are
converted into new products via the rupture and refor-
mation of CꢀC double bonds. Depending on the start-
ing material (cyclic or acyclic alkenes) and the reaction
parameters, ring-closing metathesis (RCM), acyclic
diene metathesis (ADMET) or ring-opening metathesis
2
polymerisation (ROMP) proceed (Scheme 1).
The Kharasch reaction consists of the addition of a
polyhalogenated alkane across an olefin through a rad-
3
ical mechanism (Scheme 2).
Scheme 1.
Scheme 2.
Keywords: ring-closing metathesis; Kharasch addition; enol ester syn-
thesis; catalysis; ruthenium and compounds.
Corresponding author. Tel.: +32-9-264-44-36; fax: +32-9-264-49-83;
e-mail: francis.verpoort@rug.ac.be
*
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01952-9

8
960
B. De Clercq, F. Verpoort / Tetrahedron Letters 42 (2001) 8959–8963
Scheme 3.
tion times), the conversion increases dramatically.
Under these forced conditions, the best catalytic system
III) reaches conversions of 71 and 23% for the tri- and
tetrasubstituted malonatederivate, respectively (entries
and 3). Furthermore, also diallylphtalate (entry 7)
(
2
and linalool (entry 8) are converted smoothly. With the
best catalytic system III and when heating the reaction
mixture for 17 h at 85°C conversions of 94 and 76%,
respectively are reached. One important limitation for
RCM reactions catalysed by these complexes, however,
was encountered during this investigation. All attempts
to convert diallylamine (entry 6) failed. Even when
using the more harsh reaction conditions, no ring clos-
ing metathesis occurred.
Scheme 4.
These systems were fully characterised by Raman, IR,
1
13
6
H, C and elemental analysis. To generate the metal
carbene which is necessary in metathesis reactions, a
catalytic amount of trimethylsilyldiazomethane
TMSD) was slowly added to the Ru-species. Studies
The Kharasch addition reactions were performed using
standard conditions. It appeared that the outcome of
the reaction depended very much on the olefin and the
catalytic system used (Table 2). Methylmethacrylate
(
by other teams dealing with metathesis reactions medi-
ated by ruthenium arene complexes have shown that
the release of the arene ligand is crucial and is responsi-
7
and styrene underwent a clean monoaddition of CCl in
ble for the generation of the active catalyst. Therefore,
4
the presence of the best catalytic complex III, and
yielded as much as, respectively, 73 and 88% product
after 17 h at 65°C. Performing the reactions with
diethylallylmalonate led to a striking differentiation
between the different catalytic complexes. Compounds I
and II catalysed the reaction with a conversion round
40%, whereas III showed a conversion of 56%. Exami-
nation of data gathered in Table 2 also shows that with
all three systems methacrylates are more smoothly con-
verted than acrylates. For the best catalytic system III
the reaction was also carried out at 85°C instead of
65°C (reaction time 17 h). In this case all substrates
reached higher conversions. Styrene which is the best
substrate for this type of catalysts, was nearly quantita-
we suggest that the mechanism depicted in Scheme 5 is
responsible for the formation of the catalytically active
species in RCM reactions with I, II and III.
The ring-closing metathesis activities of complexes I, II
and III are summarised in Table 1. Catalytic RCM of
dienes diethyldiallylmalonate (entry 1), 1,7-octadieen
(
entry 4) and diallylether (entry 5) was performed quan-
titatively by all three systems. As can further be seen
from the results in Table 1, the reactivity of the com-
plexes is sufficiently high to allow the preparation of
trisubstituted cycloalkenes at 70°C in moderate yields
(
entry 2). By using more harsh reaction conditions
however (higher reaction temperatures and longer reac-
Scheme 5.

B. De Clercq, F. Verpoort / Tetrahedron Letters 42 (2001) 8959–8963
8961
Table 1. Ring-closing metathesis results of representative diolefins
tively transformed but also methylmethacrylate reached
up to 84% conversion.
however, is very dependent of the type of catalyst and
acid used. Again system III is the most active. Whereas
with formic acid a total yield of 71% is obtained, up to
76% of total conversion is reached for the addition of
acetic acid to octadiyne. To our knowledge, system III
is one of the most active ruthenium catalysts reported
so far for the selective formation of (E)-alk-1-en-yl
esters out of alkadiynes and saturated carboxylic acids.
Besides the formation of the (E)-alk-1-en-yl ester, also
In Table 3 the results of the enol ester synthesis using
systems I, II and III are summarised. When octadiyne is
used as a substrate, the addition of both carboxylic
acids result in the selective formation of (E)-alk-1-en-yl
esters corresponding to a regio- and stereoselective
anti-Markovnikov addition of the acid to the triple
bond. The percentage of (E)-alk-1-en-yl ester is for all
three catalytic systems and both acids more or less the
same and varies in the range of 71–79%. The total yield,
a
small percentage of (Z)-alk-1-en-yl ester,
Markovnikov addition products and disubstituted enol
esters is obtained.

8
962
B. De Clercq, F. Verpoort / Tetrahedron Letters 42 (2001) 8959–8963
Table 2. Kharasch addition of carbon tetrachloride to
representative olefins catalysed by ruthenium complexes I,
plexes to dimerise terminal alkynes into enynes (Scheme
8,9
6
).
a
II, III
b
b
b
c
When phenylacetylene is used as an alkyne source, the
above-mentioned dimerization reaction becomes very
important. With compound II, almost no nucleophilic
addition of the carboxylic acid to the triple bond occurs
and the (E)-enyne is formed with a selectivity of about
I
II
III
III
Methylmethacrylate
Isobutylmethacrylate
Methylacrylate
Butylacrylate
Styrene
Acrylonitrile
62
66
26
22
12
72
73
34
26
12
88
84
52
33
18
91
24
18
9
72
B5
7
5% reaching a total yield of about 80%. Compounds I
and III show a totally different behaviour. When using
phenylacetylene, the outcome of the reaction depends
on the carboxylic acid used. With formic acid, both
systems form selectively the (E)-alk-1-en-yl ester. Per-
forming the reaction with acetic acid leads to the
stereoselective formation of the (E)-enyne. In this case,
system III reaches an excellent total yield of 96% and a
selectivity of 78% for the formation of the (E)-enyne.
B5
42
B5
56
B5
76
Diethylallylmalonate
38
a
Yields based on GLC using dodecane as internal standard.
b
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),
CCl (13 mmol), dodecane (0.25 ml) in toluene (3 ml). The reaction
4
In conclusion, the catalytic systems I, II and III are
highly efficient multifunctional catalysts for promoting
ring-closing metathesis and Kharasch addition reac-
tions. Furthermore, dependent on the catalytic com-
pound and the reaction conditions, they can catalyse
the stereoselective formation of enol esters or enynes in
very good yields.
mixture was heated at 65°C for 17 h.
c
b
Idem as but here the reaction mixture was heated at 85°C for 17 h.
It is known from literature that a limitation of rutheni-
um(II) complexes containing an electron-donating
bidentate nitrogen ligand is the ability of these com-
Acknowledgements
B.D.C. is indebted to the IWT (Vlaams instituut voor
de bevordering van het wetenschappelijk-technologisch
onderzoek in de industrie) for a research grant. F.V. is
indebted to the FWO (Fonds voor wetenschappelijk
onderzoek-Vlaanderen) for financial support.
Scheme 6.
a
Table 3. Synthesis of enol esters from phenylacetylene, octadiyne and ruthenium catalysts I, II and III
c
anti-M. (Z)c
(%)
anti-M. (E)c
(%)
c
c
Total yield
M. (%)
Head-to-tail
enyn (%)
(Z)-Enyn (%) (E)-Enyn (%)
b
c
(
%)
I
ph.ac.
ph.ac.
ph.ac.
f. ac.
ac. ac.
f. ac.
ac. ac.
f. ac.
ac. ac.
16
44
78
80
74
96
19
5
7
7
12
3
17
3
2
2
/
64
10
9
7
79
9
–
2
–
4
–
3
–
9
6
8
4
5
4
71
76
74
5
II
III
2
78
%
Disubstituted
enol ester
I
octad.
octad.
octad.
f. ac.
ac. ac.
f. ac.
ac. ac.
f. ac.
ac. ac.
19
49
25
64
71
76
12
14
12
14
11
13
10
6
9
9
6
72
75
71
73
76
79
6
5
8
4
7
3
II
III
5
a
The reactions were carried out by using 4.4 mmol of carboxylic acid, 4.4 mmol of alkyne and 0.04 mmol of catalyst in 3 ml toluene at 100°C
for 4 h under an inert atmosphere.
b
c
The total yield was determined with Raman spectroscopy by following the diminishing intensity of the wCꢁC of phenylacetylene (ph.ac.) and
octadiyne (octad.) and using a calibration curve (f. ac.=formic acid, ac. ac.=acetic acid). Conformation was obtained by GC–MS.
The selectivities were determined by GC–MS making use of the different fragmentations of the isomers. GC–MS measurements excluded also
the formation of other products than those reported here. (% M=percentage Markovnikov addition product, % anti-M=percentage
anti-Markovnikov addition product)

B. De Clercq, F. Verpoort / Tetrahedron Letters 42 (2001) 8959–8963
8963
References
W.; Saive, E.; Noels, A. F. Macromolecules 1997, 30, 3127;
c) Simal, F.; Demonceau, A.; Noels, A. F. Recent Res.
(
1
. Cornils, B.; Herrmann, W. A.; Schl o¨ gl, R.; Wong, C.-H.
Catalysis from A to Z. A Concise Encyclopedia; Wiley–
VCH: Weinheim, 2000.
Devel. Org. Chem. 1999, 3, 455–464; (d) Hansen, H. D.;
Nelson, J. H. Organometallics 2000, 19, 4740–4755.
6. Submitted for publication in J. Mol. Cat.
7. Karlen, T.; Ludi, A.; M u¨ hlebach, A.; Bernhard, P.; Phar-
isa, C. J. Polym. Sci., Part A: Polym. Chem. 1995, 33,
1665.
8. Howell, J. A. S.; Ashford, N. F.; Dixon, D. T.; Kola, J.
C.; Albright, T. A.; Kang, S. K. Organometallics 1991, 10,
1852.
2
3
4
5
. (a) Randall, M.; Snapper, M. J. Mol. Cat. A: Chem. 1998,
1
33, 29; (b) Ivin, K. J. Mol. Cat. A: Chem. 1998, 133, 1.
. Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem. Rev. 1994, 94,
19–564.
5
. Doucet, H.; Martin-Vaca, B.; Bruneau, C.; Dixneuf, P. H.
J. Org. Chem. 1995, 60, 7247–7255.
. (a) Demonceau, A.; Noels, A. F.; Saive, E.; Hubert, A. J.
9. Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999, 32,
311–323.
J. Mol. Cat. 1992, 76, 123; (b) Demonceau, A.; Stumpf, A.