Synthesis of enol esters and dimerization of terminal alkynes catalyzed by neutral and cationic vinylidene ruthenium complexes

Article abstract of DOI:10.1055/s-2003-37106

In the current study Ru(II) vinylidene complexes of the general type: Cl₂Ru{=C=C(H)R}(PR′₃)L (R = Ph, SiMe₃, R′ = Ph, Cyclohexyl (Cy) and L = phosphine or N-heterocyclic carbene) are synthesized and tested for the addition of carboxylic acids to terminal alkynes. A careful choice of the catalytic system, substrate and carboxylic acid gives access to alk-1-en-2-yl esters, alk-1-en-1-yl esters or enyne dimerization products. Furthermore, an extension was made to synthesize an analogous 14-electron species by treating one of the complexes with AgBF₄ and its influence on the catalytic activity and selectivity are investigated.

Full text of DOI:10.1055/s-2003-37106

314
LETTER
Synthesis of Enol Esters and Dimerization of Terminal Alkynes Catalyzed by
Neutral and Cationic Vinylidene Ruthenium Complexes
Synthesis of
Enol Esters
and
m
Dimerization of Termina
O
l
Alkynes pstal, Francis Verpoort*
Department of Inorganic and Physical Chemistry, Laboratory of Organometallics and Catalysis, Ghent University, Krijgslaan 281 (S-3),
9000 Ghent, Belgium
Fax +32(9)2644983; E-mail: Francis.Verpoort@rug.ac.be
Received 7 January 2003
nylidene species directly generated from terminal alkynes
Abstract: In the current study Ru(II) vinylidene complexes of the
have been recognized as catalytic intermediates in the
general type: Cl2Ru{=C=C(H)R}(PR′3)L (R = Ph, SiMe3, R′ = Ph,
dimerization of alkynes into enynes6 or butatrienes,7 in the
Cyclohexyl (Cy) and L = phosphine or N-heterocyclic carbene) are
cyclization of dienylalkynes or in the coupling of alkynes
synthesized and tested for the addition of carboxylic acids to termi-
nal alkynes. A careful choice of the catalytic system, substrate and with allylic alcochols to generate unsaturated carbonyl
compounds.8 In this context we found it reasonable to fur-
carboxylic acid gives access to alk-1-en-2-yl esters, alk-1-en-1-yl
esters or enyne dimerization products.
ther explore the catalytic activity of easily accessible ru-
Furthermore, an extension was made to synthesize an analogous 14-
electron species by treating one of the complexes with AgBF4 and
its influence on the catalytic activity and selectivity are investigat-
ed.
thenium vinylidenes for this kind of reactions.
The characteristic features of ruthenium (e.g. high elec-
tron transferability, low redox potentials, stability of reac-
tive metallic species, low oxophilicity) have paved the
way to a broad avenue of catalytic transformations.9
Key words: enol esters, homogeneous catalysis, NHC-ligands, ru-
thenium, vinylidene
A relatively recent development of transition metal medi-
ated catalysis is the application of N-heterocyclic car-
benes (NHC) of the imidazole and triazole type as
ancillary ligands due to their increased Lewis basicity
compared to phosphine ligands in combination with the
numerous opportunities for electronic and steric ligand
tuning and this provides an ideal platform for catalytic en-
gineering.10
A great deal of attention has been devoted to the chemistry
of transition-metal vinylidene complexes [M]=C=CR2
during the past two decades.1 One of the most straightfor-
ward routes to vinylidene metal complexes arises from the
activation of a terminal alkyne to give an η2-coordinated
intermediate followed by either a direct 1,2-hydrogen
migration2a or an oxidative addition of the alkyne to the
metal centre and subsequent 1,3-shift of the hydride to the
alkynyl ligand.2b It is recently shown that not only termi-
nal alkynes but also silylacetylenes3a (R′CCSiR3, R and
R′ = Ph, Me), stannylacetylenes3b (R′CCSnR3, R and
R′ = Ph, Me) and alkylthio or iodoalkynes3c,d can be con-
verted in the coordination sphere of transition-metals to
the corresponding vinylidene complex. It is now well-es-
tablished that the stability and properties of such deriva-
tives are a function of both the metal centre and the
ancillary ligands.1 In particular, electron-rich rutheni-
um(II) complexes such as RuCl(PPh3)n, RuH2Cl2(P-i-
Pr3)2, [RuCl2(p-cymene)]2 and RuCl(η5-C9H7 or Cp)L2
(with L2 two phosphines, one phosphine and one CO,
bitentate phosphine, phospho-enolates) have proven to be
appropriate precursors for the preparation of stable vi-
nylidenes.1
Recently, we have found that the salicylaldiminato Ru vi-
nylidene complexes (RuIICl(PCy3)(OC6H4CH=NR){=C=
CHR′} (R = 4-Br-2,6-Me2C6H2 and R′ = Ph, t-But) and
RuIICl2(PCy3)(L){=C=C(H)-t-But} (L = PCy3, N-hetero-
cyclic carbenes) reveal themselves as versatile catalyst for
the nucleophilic addition of carboxylic acids to terminal
alkynes also referred as vinylation reaction and afford
alk-1-en-2-yl esters (I also called Markovnikov adduct)
or alk-1-en-1-yl esters (II and III anti-Markovnikov
adducts) with very good yields and selectivity
(Scheme 1).11,12
Our experience in this field already showed that in some
particular experiments, not the targeted vinylation occurs
but rather an alkyne coupling reaction is favoured
(Scheme 2).11,12,17,18 Changing the ligand environment of
the metal, using different acids and alkynes or working in
aprotic solvents can dramatically alter the observed prod-
uct distribution.
Ruthenium catalyzed activation of alkynes plays a promi-
nent role in the formation of carbon-hetero-atom or car-
bon-carbon bonds and has become a key step in a lot of
new synthetic methodologies.4 Since the discovery of the
one-step formation of alkenylcarbamates,5 ruthenium vi-
We now report on the study of a variety of ruthenium
vinylidene
derivatives
of
the
general
type
RuCl2(PR3)L(=C=CHR′) (R = Ph, Cy; R′ = Ph, SiMe3;
L = PR3 or N-heterocyclic carbene) as catalysts for the vi-
nylation of carboxylic acids to terminal alkynes
(Figure 1). Furthermore the catalytic potential of the ru-
thenium complex 4 in vinylation reactions was extended
to its cationic 14 electron analogue which is easily gener-
Synlett 2003, No. 3, Print: 19 02 2003.
Art Id.1437-2096,E;2003,0,02,0314,0320,ftx,en;D22103ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

LETTER
Synthesis of Enol Esters and Dimerization of Terminal Alkynes
315
Scheme 1
Scheme 2
ated in situ by abstracting of a chloride with AgBF4 in tol- nylidene systems 1 and 3 (Mark/anti-Mark = 0.23 and
uene.
0.17) and rather alk-1-en-2-yl esters with the other cata-
lysts (Mark/anti-Mark>2). A slight increase in yield is ob-
served when the triphenylphosphine ligand is changed by
a more bulky cyclohexylphosphine, however the effect is
more pronounced with the silylvinylidene system. On the
other hand if the ruthenium centre bears one tricylohexy-
lphosphine and one dihydroimidazol-2-ylidene entity, the
yields decrease significantly. Indeed a total yield of 86%
is reached with catalyst 6 whereas with the bisphosphine
analogue 95% yield was found. From Table 1 it is also
seen that the vinylidene function is necessary to create an
active system as the yields with catalysts 7 are much lower
compared to the vinylidene congener 5 (40% vs 69%).
Ruthenium vinylidene complexes (1 and 3) can be easily
prepared from commercial available terminal alkynes and
ruthenium sources and are well-described in literature.13
The corresponding silyl homologues (2 and 4) are ob-
tained by a procedure described in the literature.14 Com-
plexes 5–7 are prepared in a convenient way by the in situ
deprotonation of the commercial available imidazolium
tetrafluoroborate salt with t-BuOK combined with a
substitution of a phosphine ligand in the vinylidene or
alkylidene complex.15,16
In a first set of experiments a vinylation of phenylacety-
lene with a divergent spectrum of acids was targeted.
Therefore, catalysts 1–7 were exposed to phenylacetylene
and the results are summarized in Table 1. The observed
product distribution strongly depends from the used acid
and catalyst. From these results it is clearly seen that the
bisphosphine systems give mainly access to dimerization
products whereas the systems bearing one N-heterocyclic
carbene accomplish the expected vinylation.
When weaker acids such as acetic acid or isovaleric acid
are used, the contribution of the vinylation reaction is dra-
matically lowered and mainly dimerization products are
found in the reaction mixture. This effect is spectacular
for catalysts 1 where respectively 60% and 91% of the
product distribution for acetic and isovaleric acid consist
of dimerization product [(Z)-enyne]. An analogous ten-
dency is observed for catalyst 2 where respectively 65%
and 86% of the reaction products consist of the (Z)-enyne.
Reaction of formic acid with phenylacetylene afforded
preferentially (Z)-alk-1-en-1-yl esters with the phenylvi-
PR3
Ru
PR3
N
N
N
N
H
Cl
Cl
•
H
Cl
Cl
Cl
Cl
Ru
PCy3
•
Ru
R'
R'
Ph
PCy3
7
1 R = Ph, R' = Ph
5 R = Cy, R' = Ph
6 R = Cy, R' = SiMe3
2 R = Ph, R' = SiMe3
3 R = Cy, R' = Ph
4 R = Cy, R' = SiMe3
Figure 1
Synlett 2003, No. 3, 314–320 ISSN 0936-5214 © Thieme Stuttgart · New York

316
T. Opstal, F. Verpoort
LETTER
Table 1 Vinylation of Phenylacetylene Using Catalysts 1–4a
Cat.
Acid
Yield
(%)b
I
II
III
(Z)-Enyne
(E)-Enyne
Head-to-tail
Enyne (%)b
(%)b
(%)b
(%)b
(%)b
(%)b
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
6
6
6
6
7
7
7
7
HCOOH
89
88
93
58
80
84
89
61
92
95
91
70
95
94
98
97
69
66
70
50
86
85
84
90
40
15
70
50
18
17
0
66
5
10
11
0
6
60
91
78
33
65
86
34
2
0
2
0
5
5
0
4
0
2
2
4
8
0
11
0
0
1
0
4
0
0
0
0
0
0
0
0
0
0
0
CH3COOH
(CH3)2CCOOH
C6H5COOH
HCOOH
0
4
8
0
0
14
6
34
16
4
16
18
5
7
CH3COOH
(CH3)2CCOOH
C6H5COOH
HCOOH
3
8
0
3
9
48
76
6
5
2
10
55
38
26
64
78
5
4
4
CH3COOH
(CH3)2CCOOH
C6H5COOH
HCOOH
3
28
39
23
6
0
7
1
15
5
30
3
5
1
26
7
CH3COOH
(CH3)2CCOOH
C6H5COOH
HCOOH
11
3
0
4
2
80
20
10
4
9
15
56
70
63
31
68
75
77
80
79
64
80
85
57
13
26
20
25
32
24
20
16
20
15
10
6
2
6
15
0
2
CH3COOH
(CH3)2CCOOH
C6H5COOH
HCOOH
0
7
5
5
11
0
23
0
10
0
CH3COOH
(CH3)2CCOOH
C6H5COOH
HCOOH
1
0
0
3
0
0
4
0
0
1
0
0
CH3COOH
(CH3)2CCOOH
C6H5COOH
21
8
0
0
2
0
4
5
0
a The catalyst (0.04 mmol) was dissolved in toluene (1 mL) and subsequently added through a septum to the solution of alkyne (4 mmol), dode-
cane (250 µL, internal standard) and carboxylic acid (4.4 mmol) in toluene (3 mL). The reaction mixture was heated at 110 °C for 5 h. The
reaction was monitored by withdrawing samples at timed intervals from the reaction mixture and analyzed by Raman, NMR and GC-MS.
b Total conversion of the alkyne was determined by quantitative Raman analysis (νC≡C) using calibration curves. The yields of enol esters and
dimerization products are determined with GC-MS and 1H NMR-spectroscopy and these data are confirmed by literature.8,17 The reaction prod-
ucts were identified by comparison of the reaction products with the spectral data of authentic samples. Authentic samples were isolated from
concentrated reaction mixtures by silica gel chromatography.
A rather mixed product distribution is obtained with the adduct (55% and 38%) and (Z)-enyne (28% and 39%).
cyclohexylphosphine systems 3 where a vinylation/ Relatively more vinylation is observed with system 4
dimerization ratio of 1.7 and 0.85 is obtained for acetic (vin./dim. = 8, 78% Markovnikov) for acetic acid while
acid and isovaleric acid with preferential Markovnikov for isovaleric acid almost exclusively dimerization (vin./
Synlett 2003, No. 3, 314–320 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Synthesis of Enol Esters and Dimerization of Terminal Alkynes
317
Table 2 Vinylation of Different Alkynes with Acetic Acida
Cat.
Alkyne
Yield (%)b
I
II
III
(Z)-Enyne
(E)-Enyne
Head-to-tail
Enyne (%)b
(%)b
(%)b
(%)b
(%)b
(%)b
4
90
88
10
2
–
–
–
4
4
4
55
71
90
67
81
9
11
–
4
8
–
20
–
–
–
–
–
–
–
100
–
COOH
6
81
90
8
2
–
–
–
6
6
6
40
48
84
74
86
16
6
3
8
–
7
–
–
–
–
–
–
–
100
–
–
COOH
a Condtions: Identical as Table 1.
b Total conversion of the alkyne was determined by quantitative Raman analysis (νC≡C) using calibration curves. The yields of enol esters and
dimerization products are determined with GC-MS and 1H NMR-spectroscopy and these data are confirmed by literature.8,17 The reaction prod-
ucts were identified by comparison of the reaction products with the spectral data of authentic samples. Authentic samples were isolated from
concentrated reaction mixtures by silica gel chromatography.
dim = 0.1, 80% (Z)-enyne) occurred. With catalysts 5–7 γ-methylene-γ-butyrolactone in excellent yields for both
the enol-ester formation is the main reaction and a prefer- catalysts (90 and 84%).
ence for the Markovnikov products is observed even in
the case with acetic acid and isovaleric acid (e.g. catalyst
6 give 77% and 80% Markovnikov for these two acids).
When the complexes 1–7 are exposed to a solution of phe-
nylacetylene in toluene (100 equiv) one expects that a
dimerization reaction should occur. The results of these
With benzoic acid as acid source the obtained products experiments are depicted in Table 3.
strongly depend on the used catalyst. With catalyst 1 par-
ticularly the (Z)-enyne is obtained (78%) while with the
Table 3 Dimerization of Phenylacetylene Using Catalysts 1–7a
other catalysts the vinylation reaction is favoured with
rather poor selectivities for systems 3 and 4 [(max 57%
(Z)-alk-1-en-1-yl ester for catalyst 4] and quite good se-
lectivities for system 5–7 (max 85% alk-1-en-2-yl ester
with catalyst 7).
Cat.
Yield (%)c (Z)-Enyne (E)-Enyne Head-to-tail
(%)c
(%)c
70
49
50
55
13
0
Enyne (%)c
1
2
3
4
5
6
7
39
40
56
46
37
40
10
20
10
19
23
21
0
32
In another experiment, catalysts 4 and 6 were tested for
the vinylation reaction of acetic acid to different alkynes
and the results are depicted in Table 2. These results clear-
ly indicate that the outcome of a vinylation reaction
strongly depends on the used alkyne. Both catalysts give
access to Markovnikov products with tert-butylacetylene
as alkyne source in high yields (81 and 90%) and selectiv-
ity (Mark/anti-Mark>7). With the terminal alkyne, 1-oc-
tyne, the yields are lower but again a preference for the
alk-1-en-2-yl ester is found. Almost no dimerization prod-
ucts are observed for these two alkynes.
27
24
87
100
80
0
17
3
a The catalyst (0.04 mmol) was dissolved in toluene (1 mL) and sub-
sequently added through a septum to the solution of alkyne (4 mmol)
and dodecane (250 µL, internal standard) in toluene (3 mL).
b The reaction mixture was heated at 110 °C for 5 h.
c Total conversion of the alkyne was determined as previous de-
scribed (Table 1).
From Table 2 it also follows that after reaction of acetic
acid with 1,7-octadiyne, a preference for the (geminal,
geminal)dienol diester (Markovnikov adduct) synthesis is
observed for both catalysts and small percentages (E,E)
and (Z,Z) dienol diesters and no traces dimerization prod-
ucts are detected with GC-MS. The alkynyl acid, 4-pen-
tynoic acid, gave after internal vinylation exclusively the
From Table 3 it follows that all the tested catalysts are
moderate active for the dimerization of phenylacetylene.
In the observed product distribution a preference for the
Synlett 2003, No. 3, 314–320 ISSN 0936-5214 © Thieme Stuttgart · New York

318
T. Opstal, F. Verpoort
LETTER
(E)-enyne is found with the systems 1–4 and a preference nylation products were observed. On the other hand the
for the (Z)-enyne is seen for the systems 5–7.
addition of a phosphine sponge such as CuCl to a mixture
of 4 afforded a quantitative conversion after 3 hours.
Therefore we reasoned that the dissociation of a phos-
phine ligand is crucial in the reaction cycle (Scheme 3).
Since it is known from our previous reported results that
abstracting a chloride in neutral vinylidene complexes
which involves the generation of a cationic species, has an
advantageous effect on the catalytic activity, our best sys- In conclusion catalysts 1–4 are efficient catalysts for the
tem was treated with AgBF4 (4) and tested for the vinyla- addition of carboxylic acids to activated alkynes. A care-
tion and dimerization reaction.12 Before the reaction ful choice of the used acid can preferentially steer the re-
starts, 0.04 mmol catalyst has been stirring with AgBF4 at action into one direction either a vinylation reaction with
room temperature in a glass vessel for 30 minutes. After mainly Markovnikov adduct or a dimerization reaction
this period a precipitation of AgCl is observed and the with the (Z)-enyne as the major product. Complexes 5–7
alkyne and acid are subsequently added. The results are have proven to be efficient catalysts for the vinylation re-
summarized in Table 4.
action of formic acid, acetic acid, isovaleric acid and
benzoic acid to phenylacetylene. Finally a cationic variant
of complex 5 was synthesized and has proven to be a more
active system then the neutral complexes.
During the monitoring of the reaction we saw that the dis-
appearance of the alkyne proceeds much faster than with
the neutral complex. After a reaction time of 2.5 hours al-
most a total conversion was obtained. A preference for the
vinylation reaction was established with vin./dim. ratio’s
ranging from 6.7 (isovaleric acid), 9 (formic acid) to
100% (acetic acid) and corresponding Mark/anti-Mark
ratio’s from 0.5 (isovaleric acid), 5 (formic acid) and 32
(acetic acid). The reaction of formic acid with 1,7-oc-
tadiyne leads almost exclusively to the (geminal, geminal)
dienol diester. The dimerization of phenylacetylene
reaches 65% conversion with the cationic system and an
almost equal amount of (E)- and (Z)-enyne. A probably
explanation for the increased activity is that the cationic
complex is more attractive for a nucleophilic attack of an
carboxylic acid.12
Acknowledgment
T.O. is indebted to the Research founds of Ghent University for a
research grant F.V. is indebted to the FWO-Flanders (Fonds voor
Wetenschappelijk Onderzoek-Vlaanderen) and to the Research
Fund of Ghent University for financial support.
References
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for the reaction of benzoic acid with phenylacetylene in-
crease in the following order: 2<6<4<4+ as is indicated by
the relative trend in initial TOF: 4: 44 h–1, 6: 62 h–1, 4, 72
h–1, 4+: 106 h–1.
Several experiments were performed to apprehend the na-
ture of the active species for the vinylation and dimeriza-
tion reactions.18 Addition of 10 equivalents of
tricyclohexylphosphine to a mixture of 4 with alkyne and
formic acid inhibited the reaction immediately and no vi-
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Table 4 Vinylation of Phenylacetylene Using the Cationic Variant of Catalyst 4a
Acid
Alkyne
Yield (%)c
I
II
III (%)c
(Z)-Enyne
(E)-Enyne Head-to-tail
(%)c
(%)c
(%)c
(%)c
10
0
Enyne (%)c
HCOOH
CH3COOH
(CH3)2CCOOH
C6H5COOH
HCOOH
–
Phenylacetylene
97
90
97
98
90
65
75
97
20
30
88
0
10
2
5
1
5
8
2
0
0
0
0
0
0
0
0
4
15
49
10
0
60
13
0
0
0
1.7-Octadiyne
0
phenylacetylene
40
56
a The catalyst (0.04 mmol) was dissolved in toluene (1 mL) and subsequently added through a septum to the solution of alkyne (4 mmol), dode-
cane (250 µL, internal standard) and carboxylic acid (4.4 mmol) in toluene (3 mL). The reaction mixture was heated at 110 °C for 2.5 h.
b Total conversion of the alkyne was determined as previously described (Table 1).
Synlett 2003, No. 3, 314–320 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Synthesis of Enol Esters and Dimerization of Terminal Alkynes
H
319
L
R'
Cl
Cl
Ru
R'
O
H
R'
R'
OCOH
OCOH
R'
O
L
4
R'
Cl
Cl
Ru
O
H
O
3
H
HCOOH
- PCy3
L
L
- H+
Cl
Cl
Cl
Cl
H
Ru
•
R'
Ru
PCy3
R'
+ H+
PCy3
1
2
Scheme 3
(6) (a) Bianchini, C.; Peruzzini, M.; Frediani, F. J. Am. Chem.
Soc. 1991, 113, 5453. (b) Slugovc, C.; Mereiter, K.; Zobetz,
E.; Schmid, R.; Kirchner, K. Organometallics 1996, 15,
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13C NMR (75.41 MHz, C6D6, 25 °C): δ = 279.33 (dt,
J(RuH) = 57.0 Hz, J(PC) = 15.4 Hz, Ru=C=C), 86.45 (dt,
J
(RuH) = 15.7 Hz, J(PC) = 5.1 Hz, Ru=C=C), 150.16 (s, C1 of
PPh), 129.77, 128.00, 126.87 (s, PPh). 31P NMR {1H}
(7) Wakatsuki, Y.; Yamazaki, H.; Kumegawa, N.; Satoh, T.;
Satoh, J. Y. J. Am. Chem. Soc. 1991, 113, 9604.
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5190.
(121.40 MHz, C6D6, 25 °C, ref. H3PO4): δ = 26.3 (s). IR
(KBr): ν = 1625 (C=C) cm–1. Anal. Calcd for
C41H40Cl2P2SiRu: C, 61.96; H, 5.07. Found: C, 62.30; H,
5.82. Cl2Ru{=C=C(H)SiMe3}(PCy3)2(4): 1H NMR (299.89
MHz, C6D6, 25 °C): δ = 2.68–2.59, 2.13–1.97, 1.89–1.64,
1.26–1.16 (each m, 66 H, PCy3), 0.29 (s, 9 H, SiCH3), 0.023
(dt, J(RuH) = 1.9 Hz, J(PH) = 2.8 Hz, 1 H, =CHSiMe3). 13
NMR (75.41 MHz, C6D6, 25 °C): δ = 274.30 (dt,
C
J
(RuH) = 57.2 Hz, J(PC) = 15 Hz, Ru=C=C), 81.20 (dt,
J(RuH) = 16 Hz, J(PC) = 5 Hz, Ru=C=C), 35.46 (pseudo triplet,
J = 8.7 Hz, C1 of PCy), 30.14 (s, C3,5 of PCy), 27.83 (pseudo
triplet, J = 4.2 Hz, C2,6 of PCy), 26.35 (s, C4 of Pcy). 31
P
NMR {1H} (121.40 MHz, C6D6, 25 °C, ref. H3PO4): δ =
31.50 (s). IR (KBr): ν = 1630 (C=C) cm–1. Anal. Calcd for
C41H76Cl2P2SiRu: C, 59.26; H, 9.58. Found: C, 59.52; H,
10.10.
(14) Synthesis of Complexes 2 and 4: To a suspension of
[RuCl2(p-cymene)]2 (0.72 g, 1.17 mmol) in toluene (45 mL),
the phosphine PCy3 (1.32 g, 4.7 mmol) or PPh3 (1.23 g, 4.7
mmol) was added and stirred at r.t. The mixture instantly
changed into a reddish brown solution. Me3SiCCH (3.34
mL, 23.5 mmol) was added and the solution was stirred for
1 h at r.t. The solution instantly darkened to a dark red
solution. After 1 h the temperature was gradually risen to
60 °C and stirred during one night. The analytical pure
compound was obtained (65% yield) after washing the crude
product with methanol. Cl2Ru{=C=C(H)SiMe3}(PPh3)2(2):
1H NMR (299.89 MHz, C6D6, 25 °C): δ = 7.82–7.64 and
7.00–6.65 (each m, 30 H, PPh3), 0.33 (s, 9 H, SiCH3), 0.01
(dt, J(RuH) = 2.1 Hz, J(PH) = 2.5 Hz, 1 H, =CHSiMe3).
(15) (a) Morgan, J. P.; Grubbs, R. H. Org. Lett. 2000, 2, 3153.
(b) Louie, J.; Grubbs, R. H. Angew. Chem. Int. Ed. 2001, 40,
247.
(16) A Typical Procedure for the Preparation of Complex 6 is
as follows. A solution (0.1 M in THF) of the 4,5-
dihydroimidazol-2-ylideen tetrafluoro-borate salt (300 ml)
(STREM), a magnetic stirring bar and toluene (1 mL) were
added to a glass vessel. t-BuOK was added (30 µL 1 M in
Et2O, Aldrich) to the rapidly stirred suspension at r.t.,
resulting in the immediate dissolution of the salt to form a
light yellow solution. After 5 min, a 0.1 M solution of the
vinylidene (3 or 4, 250 µL) or alkylidene [Cl2(PCy3)2Ru
(= CHPh)] complex in toluene were added via cannula. The
mixture was heated to 70 °C for 1 h and subsequently cooled
Synlett 2003, No. 3, 314–320 ISSN 0936-5214 © Thieme Stuttgart · New York

320
T. Opstal, F. Verpoort
LETTER
to r.t. Complex 6: 1H NMR (299.89 MHz, C6D6, 25 °C):
δ = 7.03–6.96 (br, 2 H, Mes), 6.92–6.86 (br, 2 H, Mes), 3.49
(s, 4 H, imidazolium), 2.44–2.10, 1.99–1.84 (br,18 H, Mes),
2.12 (m, 3 H, C1 PCy3), 1.55–1.52, 1.39, 1.02–0.96 (m, 32 H,
PCy3) 0.05 (s, 9 H, SiCH3), –0.15 (dt, J(RuH) = 1.9 Hz,
J(PH) = 2.8 Hz, 1 H, =CHSiMe3). 13C NMR (75.41 MHz,
C6D6, 25 °C): δ = 267.27 (dt, J(RuH) = 56.8 Hz, J(PC) = 15 Hz,
Ru=C=C), 194.46 (s, J(CP) = 80 Hz, Ru-CNN), 144.82,
141.00, 137.64, 134.95, 131.93 (all s, Mes), 129.01 (d,
J(CH) = 150 Hz, Mes), 128.82 (d, J(CH) = 130Hz, Mes), 71.05
(dt, J(RuH) = 15 Hz, J(PC) = 5.5 Hz, Ru= C=C), 31.30 (pseudo
triplet, J = 9 Hz, C1 of PCy3), 29.96 (s, C3,5 of PCy), 27.87
(pseudo triplet, J = 4 Hz, C2,6 of PCy3), 26.36 (s, C4 of PCy3)
21.96, 21.13, 19.54, 18.61 (all s, Mes). 31P NMR {1H}
(121.40 MHz, C6D6, 25 °C, ref. H3PO4): δ = 28.03 (s). IR
(KBr): ν = 1634 (C = C) cm–1. Anal. Calcd for
C44H69N2Cl2PSiRu: C, 61.66; H, 8.11; N, 3.27. Found: C,
62.98; H, 9.23; N, 4.03.
(17) (a) Baratta, W.; Herrmann, W. A.; Rigu, P.; Schwarz, J. J.
Organomet. Chem. 2000, 593, 489. (b) Yi, C. S.; Liu, N.
Synlett 1999, 3, 281.
(18) Melis, K.; Opstal, T.; Verpoort, F. Eur. J. Org. Chem. 2002,
3779.
Synlett 2003, No. 3, 314–320 ISSN 0936-5214 © Thieme Stuttgart · New York