134
K. Melis et al. / Journal of Organometallic Chemistry 671 (2003) 131Á136
/
aliphatic alkynes proceeds in quantitative yield (run 3, 5
and 8). Only the transformation of phenylacetylene
reaches merely 44.8% and the reaction products consist
of 53% of enol esters and 47% of dimeric products (run
1
(
). Also, the intramolecular addition proceeds smoothly
run 7). The vinylation of most terminal alkynes
proceeds selectively by the attack on the external C1
carbon atom of the terminal alkyne and thus production
of Markovnikov adducts. The reaction between 3,3-
dimethyl-1-butyne and acetic acid exhibits a reversed
regioselectivity for addition on the internal C carbon
2
atom of the triple bond (run 3). The formed Anti-
Markovnikov adducts mainly consist of the cis isomers
Fig. 1. Influence of nature of carboxylic acid on the nucleophilic
addition of the carboxylic acid on phenylacetylene. (^) No acid; (k)
formic acid; (I) acetic acid; (ꢀ) isovaleric acid; (\), trichloroacetic
/
(
67%).
A second catalysed carbonÁ
acid. Reaction conditions: reactions were carried out by using 3.58
mmol of carboxylic acid, 2.94 mmol of phenylacetylene and 0.032
mmol of catalyst 4 in toluene (3 ml) under nitrogen. The reaction
mixture was stirred at 110 8C. The reaction is monitored by Raman
/
carbon bond formation
reaction is the dimerisation or homo-coupling of 1-
alkynes. A solution of 4 (0.032 mmol) in 3 ml toluene
was heated at 110 8C. 1-Alkyne (3.2 mmol) was added
and the reaction mixture was stirred at 110 8C for 150
min. The activity of catalyst 4 for the dimerisation of the
spectroscopy by following the diminishing intensity of the CÅC of
/
phenylacetylene using a calibration curve. Selectivity as determined by
1
H-NMR.
1
-alkynes shows a decreasing order from 1-octyneꢀ
/
1,7-
vinylation occurs preferentially by attack on the internal
C carbon atom of the triple bond and thus production
of Markovnikov adducts. The intermolecular addition
octadiyneꢀphenylacetyleneꢀ3,3 dimethyl-1-butyne.
/
/
The head-to-tail/tail-to-tail ratio is strongly dependent
on the nature of the terminal alkyne. The aromatic
substituent affords a very good regioselectivity with a
head-to-tail/tail-to-tail ratio of 98:2 (run 2). For the di-
alkyne, the ratio dramatically decreases to 60:40 (run 9).
In the presence of the steric tert-butyl group, the
selectivity reverses to production of the tail-to-tail
adduct as the major adduct (24:76) (run 4). No
dimerisation of 1-octyne occurs, although after 4 h
2
of two 1-alkynes proceeds mainly by addition on the C2
atom and formation of head-to-tail enynes. Only formic
acid shows a regioselectivity for the tail-to-tail addition.
Although detailed mechanism of the catalytic reaction
and nature of the inorganic species after thermal
treatment of Cl (PCy )(triazol-5-ylidene)RuÄ
/
CHPh (4)
have not been elucidated, the outcome of the thermal
treatment of Cl (PCy )(triazol-5-ylidene)RuÄCHPh (4)
2
3
7
7% of the 1-octyne is consumed (run 6). Investigation
/
2
3
of the reaction mixture reveals that the reaction
products consist of oligomers and polymers.
The influence of the substituent on the COOH group
on the reactivity of catalyst 4 for the vinylation of
phenylacetylene is depicted in Fig. 1. The overall yield
increases at decreasing pKa [16]. The intermolecular
addition of trichloroacetic acid (pK ꢁ0.66) on pheny-
lacetylene proceeds smoothly. After 1 h, 91.2% of the
phenylacetylene is consumed. The addition of formic
and the reactivity of 4 with phenylacetylene combined
with the catalytic performance of 4 suggest some
mechanistic insights (Scheme 3). Decoordination of a
PCy ligand takes place on the metal center. This creates
3
a vacant site for the, respectively, incoming carboxylic
acid (cycle 1) or 1-alkyne (cycle 2).
The nucleophilic addition of the carboxylic acid on
the alkyne or vinylation (cycle 1) occurs preferably by
the regioselective intermolecular attack of the acid on
/
a
acid (pK ꢁ
/
3.75) reaches a total yield of 77.7% after 4 h
of reaction. Acetic and isovaleric acid, which possess a
similar acidity (pK ꢁ4.76 and 4.78, respectively), only
a
the internal C carbon atom of the terminal alkyne and
2
thus production of Markovnikov adducts (8 b). Gen-
erally, Ru-based systems afford enol esters bearing exo-
olefins. The ring closing of the intramolecular addition
/
a
reach 44.8 and 47.2% conversion of the triple bond. In
the absence of carboxylic acid, 32.8% of the triple bond
is converted into the dimeric enyne products. Next to
enol esters, the reaction products consist of dimeric
products for acetic, formic and isovaleric acid. The
vinylation/dimerisation ratio decreases from 60/40 for
formic acid, 52/48 for acetic acid and 35/65 for isovaleric
acid. Only trichloroacetic acid exclusively yields enol
esters.
proceeds by an attack on the C carbon atom of the
2
triple bond and formation of the Markovnikov 5-exo
compound (8 b). Only the addition of acetic acid on 3,3-
dimethyl-1-butyne proceeds by the attack on the ex-
ternal C1 carbon atom of the terminal alkyne and
formation of Anti-Markovnikov adducts (8 a).
In the absence of carboxylic acids, only dimerisation
(cycle 2) occurs for all terminal alkynes. For arylacety-
lene derivatives, the intermolecular attack proceeds
The preference for the dimerisation of terminal
alkynes rises at increasing pK and increasing sterical
hindrance of the substituent on the COOH group. The
a
preferentially on the C of the terminal alkyne (10 b).
1