coordinated by both Ru centres in a μ
2
Àη
1 :η
2 -bonding mode which may stabilise
these intermediate structures. Interestingly, upon prolonged reaction of 12 with
excess phenylacetylene, the methylene unit is eventually lost through insertion into
the alkyne C–H bond to yield 1-phenylpropyne (PhC CMe) and complex 15.
Complex 15, which contains a phenylacetylene unit bridging the two Ru centres, is
entirely inactive for the dimerisation reaction. Fortunately the researchers were able
to characterise a number of related compounds that resemble the intermediate
structures 13 and 14 [32, 33]. Complex 16, which contains a μ–CO ligand in
place of the bridging methylene group found in 12, was found to react with
phenylacetylene or 1-hexyne to yield the acetylide–hydride and acetylide–alkenyl
complexes 17 and 18, respectively (Scheme 7b). Compound 16 however was not
found to be an effective catalyst for the dimerisation of alkynes, highlighting the
crucial role the bridging methylene unit plays in activating catalyst 12.
2.3 Nucleophilic Substitution of Propargylic Alcohols
The OH group of propargylic alcohols (HC CCHROH) is readily displaced by a
range of nucleophiles, and this enables the facile attachment of an alkyne group to
an organic molecule (Scheme 8a) [34]. The alkyne moiety is a versatile entity for
further chemical transformations and is itself an important subunit in many fine
chemicals and natural products. The bimetallic ruthenium complex 7, which is an
effective catalyst for the dimerisation of alkynes (Scheme 5), also catalyses the
nucleophilic substitution of propargylic alcohols using a range of nucleophiles such
as alcohols, amines, thiols, ketones and alkenes, among others (Scheme 8a) [35–
39]. Unlike the alkyne dimerisation mechanism described above, the propargylic
substitution reaction requires only one Ru centre to activate the alkyne, yielding the
allenylidene intermediate 19 (Scheme 8b). Complex 19 (where R ¼ Me, CHR
0
¼ C
(tolyl) 2 ) was in fact isolated from the reaction of 7 with NH 4 BF 4 and HC CC
(tolyl) 2 OH and was then shown to liberate the propargylic substitution product
HC CC(Tol) 2 OEt upon treatment with EtOH. Despite only one Ru centre
interacting with the substrate in the proposed mechanism, conventional mononuclear ruthenium complexes were not effective catalysts for the propargylic substitution reaction indicating that the diruthenium core was essential for catalysis.
Computational investigations show that the π-back-donating ability of the reactive
Ru centre is reduced by its bonding to the second Ru centre [40]. This destabilises
the vinylidene and allenylidene intermediates and lowers the energy barrier
between them. It also labilises the η
2 -coordinated alkyne product facilitating its
displacement by a second equivalent of substrate in the final turnover step.
112
M.J. Page et al.
2
Àη
1 :η
2 -bonding mode which may stabilise
these intermediate structures. Interestingly, upon prolonged reaction of 12 with
excess phenylacetylene, the methylene unit is eventually lost through insertion into
the alkyne C–H bond to yield 1-phenylpropyne (PhC CMe) and complex 15.
Complex 15, which contains a phenylacetylene unit bridging the two Ru centres, is
entirely inactive for the dimerisation reaction. Fortunately the researchers were able
to characterise a number of related compounds that resemble the intermediate
structures 13 and 14 [32, 33]. Complex 16, which contains a μ–CO ligand in
place of the bridging methylene group found in 12, was found to react with
phenylacetylene or 1-hexyne to yield the acetylide–hydride and acetylide–alkenyl
complexes 17 and 18, respectively (Scheme 7b). Compound 16 however was not
found to be an effective catalyst for the dimerisation of alkynes, highlighting the
crucial role the bridging methylene unit plays in activating catalyst 12.
2.3 Nucleophilic Substitution of Propargylic Alcohols
The OH group of propargylic alcohols (HC CCHROH) is readily displaced by a
range of nucleophiles, and this enables the facile attachment of an alkyne group to
an organic molecule (Scheme 8a) [34]. The alkyne moiety is a versatile entity for
further chemical transformations and is itself an important subunit in many fine
chemicals and natural products. The bimetallic ruthenium complex 7, which is an
effective catalyst for the dimerisation of alkynes (Scheme 5), also catalyses the
nucleophilic substitution of propargylic alcohols using a range of nucleophiles such
as alcohols, amines, thiols, ketones and alkenes, among others (Scheme 8a) [35–
39]. Unlike the alkyne dimerisation mechanism described above, the propargylic
substitution reaction requires only one Ru centre to activate the alkyne, yielding the
allenylidene intermediate 19 (Scheme 8b). Complex 19 (where R ¼ Me, CHR
0
¼ C
(tolyl) 2 ) was in fact isolated from the reaction of 7 with NH 4 BF 4 and HC CC
(tolyl) 2 OH and was then shown to liberate the propargylic substitution product
HC CC(Tol) 2 OEt upon treatment with EtOH. Despite only one Ru centre
interacting with the substrate in the proposed mechanism, conventional mononuclear ruthenium complexes were not effective catalysts for the propargylic substitution reaction indicating that the diruthenium core was essential for catalysis.
Computational investigations show that the π-back-donating ability of the reactive
Ru centre is reduced by its bonding to the second Ru centre [40]. This destabilises
the vinylidene and allenylidene intermediates and lowers the energy barrier
between them. It also labilises the η
2 -coordinated alkyne product facilitating its
displacement by a second equivalent of substrate in the final turnover step.
112
M.J. Page et al.