T. Opstal, F. Verpoort / Tetrahedron Letters 43 (2002) 9259–9263
9261
chloroform are significantly lower compared to CCl4.
The reason for this can be explained by the preference
to form oligomers with this halide which was confirmed
when the mixed ligand systems containing one N-hete-
rocyclic carbene are used. For instance, the addition of
benzoic acid to phenylacetylene results in the formation
of the (Z)-alk-1-en-1-yl ester (M/anti-M=0.21) with
system 1, while catalyst 2 yielded almost exclusively the
alk-1-en-2-yl ester (88%). Transforming complexes 1
and 2 into their cationic counterparts has a pronounced
effect on the catalytic activity in vinylation reactions.
Complexes 3 and 4 give access to the formation of
2-styryl benzoate in quantitative yields in a three times
shorter period compared with systems 1 and 2. The
addition of formic acid and acetic acid to phenyl-
acetylene were followed in function of time and an
analogous accelerating effect was observed as with ben-
zoic acid (Figs. 2 and 3). A possible explanation is
given in Fig. 4. It is known from the literature that the
first step involves the dissociation of a phosphine ligand
4
,13,14
by others.
Abstracting a chloride entity in com-
plexes 1 and 2 does not improve their performance
since the total yields for all the substrates are lower
with both systems. This is quite surprising because an
17
opposite tendency was observed for ATRP reactions.
It must be said that an eventually solvent effect on the
outcome of the reaction has not yet been investigated.
If we compare our results in terms of average turnover
frequency (TOF) with the traditional alkylidene system
1
2,13
RuCl (ꢀCHPh)(PR ) (R=Ph, 5, R=Cy, 6),
the
2
3
obtained turnover frequencies in this work are consider-
ably higher for comparable reaction conditions. For
−
1
example, for 1-octene the TOF for 5 is 0.9 h and for
−
1
6
a value of 3.25 h is reached, while the vinylidene
−1
18
complexes 1 and 2 easily get 2.25 and 7 h . Next to the
higher activity, the vinylidene complexes have the
advantage that they are also easily accessible via com-
before the acid coordinates. This phosphine entity can
compete every moment in the reaction cycle with the
incoming acid. When a chloride is abstracted, the acid
can directly enter the coordination sphere of the Ru-
center and no equilibrium with a competitive phosphine
ligand is established.
15
mercial available and air-stable compounds.
Complexes 1–4 can be implemented in the formation of
enol-esters from the reaction of several terminal alkynes
with carboxylic acids (Table 2). With phenylacetylene
as the substrate nearly quantitative yields are obtained
For the vinylation of the more steric t-butylacetylene
with acetic acid, globally lower conversions are
obtained (55–77%), but again a preference for the
Markovnikov product is noticed (M/anti-M:3). The
fraction of the alk-1-en1-yl esters consist almost exclu-
with
a
preference for the formation of the
Markovnikov adduct. It is also seen that the catalytic
activity and the regioselectivity (M/anti-M >3) increases
a
Table 2. Vinylation reaction catalyzed by complexes 1–4
% Yieldb
% Mc % anti-M (Z)
c
2
% anti-M (E)
c
2
Dimerizationc
Alkyne
Acid
Cat.
Time (h)
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
t-Butylacetylene
t-Butylacetylene
t-Butylacetylene
HCOOH
1
1
1
2
2
2
3
3
3
4
4
4
1
2
3
4
1
2
3
4
1
2
3
4
6
6
6
6
6
6
2
4
2
2
2
2
6
6
6
6
6
6
6
6
2
2
1
1
80
93
97
85
96
98
82
94
95
85
96
100
55
60
68
77
72
78
66
70
17
75
76
88
89
87
100
77
89
99
67
70
74
76
44
48
34
8
80
25
24
–
11
10
–
10
5
1
2
–
–
3
–
4
–
–
–
–
–
–
–
3
–
22
5
–
–
12
–
–
–
–
4
–
2
5
–
–
–
–
–
–
–
–
–
–
CH COOH
3
C H COOH
6
5
HCOOH
CH COOH
3
C H COOH
6
5
HCOOH
CH COOH
3
C H COOH
–
3
6
5
HCOOH
CH COOH
2
3
C H COOH
–
6
5
CH COOH
29
25
26
21
14
10
17
15
–
3
CH COOH
3
CH COOH
3
t-Butylacetylene
CH COOH
3
d
1,7-Octadiyn
1,7-Octadiyn
1,7-Octadiyn
1,7-Octadiyn
CH COOH
3
d
d
d
CH COOH
3
CH COOH
80
83
100
100
100
100
52
46
100
100
100
100
–
–
–
–
–
–
3
CH COOH
3
4-Pentynoic acid
4-Pentynoic acid
4-Pentynoic acid
4-Pentynoic acid
–
–
–
a
b
c
General conditions: The catalyst (0.04 mmol) was dissolved in toluene (1 ml) and subsequently added through the septum of a glass vessel to
a mixture of alkyne (4 mmol), carboxylic acid (4.4 mmol), dodecane (0.25 ml) and toluene (2 ml). Reaction temperature 110°C.
The total yield was determined with Raman spectroscopy by following the diminishing intensity of the wCꢂC of the alkyne in combination with
appropriate calibration curves.
Selectivities were determined by GC–MS making use of the different fragmentations of the isomers. GC–MS measurements excluded the
formation of other products than those reported here. Dimerization products exclusively consist off the (E)-enyn.
The rest of the products consist of mono-substituted product (Markovnikov addition).
d