phenylacetylene simultaneously to yield a bis-phenylethenylidene complex
(structure A, Scheme 19). The authors therefore proposed that the selectivity of
the bimetallic catalysed reaction is determined by the small size of the cavity
between the two Ru complex fragments (RuÁ Á ÁRu ¼ 9.2 Å). This constrains the
phenylethenylidene units to orient in such a manner that the phenyl groups point out
of the cavity directing attack by the carboxylic acid towards the trans face of the
alkylidene to yield the E-isomer of the product (structure B). Notably, the stereoselectivity of 52 was lost if phenylacetylene was substituted for 1-octyne, which has
considerably less steric bulk, or if the steric bulk of the carboxylic acid was
increased (R ¼ Ph).
5.4 Hydroamination of Alkynes, Allenes and Alkenes
The hydroamination of alkynes is a highly atom-efficient approach to the synthesis
of enamines and imines, as well as to the synthesis of N-heterocyclic compounds
such as indoles and pyrroles, which are widely occurring functional groups in
biologically active molecules. Also included in this section is the hydroamination
of allenes and alkenes, as the reaction of these substrates with chiral bimetallic
catalysts has been shown to yield the chiral amine products with high
enantioselectivity.
A number of reports have demonstrated the utility of monometallic Ir(III)
catalysts for the hydroamination of 2-alkynylanilines to yield indole products
[98, 99]. In an effort to enhance the efficiency of such catalysts, the bimetallic
Ir(III) complex 54 was prepared and investigated as a catalyst for the
hydroamination of 2-alkynylanilines (Scheme 20) [100]. During the catalytic
cycle it was anticipated that the bridging chlorides of 54 would dissociate to free
a vacant coordination site on each metal, through which the alkyne substrate could
coordinate. It was hoped that the compact naphthalene scaffold would enforce a
close alignment of the two metal fragments and thereby lead to a cooperative
enhancement of the reaction rate. While complex 54 was found to be an effective
catalyst for this reaction, comparison with an analogous monometallic catalyst
(generated in situ by reaction of complex 55 with one equiv. of AgPF 6 ) revealed
that no cooperative enhancement of the reaction rate was obtained by the bimetallic
system. It is possible that the conformational freedom available to the two Ir
fragments upon cleavage of the Ir–μ–Cl bonds prevents a favourable alignment of
the metals during catalysis.
Using the bimetallic ruthenium complex 56 as catalyst for the intermolecular
hydroamination of phenylacetylene demonstrated that the relative positions of the
two Ru fragments must be restricted to achieve effective promotion of the reaction
(Scheme 21) [101]. Complex 56 promotes the hydroamination reaction in three
steps. Initial protonation of the bridging phenylacetylene unit was shown to yield
the isolable vinyl complex 57, where the vinyl ligand is stabilised by a bimetallic
126
M.J. Page et al.
(structure A, Scheme 19). The authors therefore proposed that the selectivity of
the bimetallic catalysed reaction is determined by the small size of the cavity
between the two Ru complex fragments (RuÁ Á ÁRu ¼ 9.2 Å). This constrains the
phenylethenylidene units to orient in such a manner that the phenyl groups point out
of the cavity directing attack by the carboxylic acid towards the trans face of the
alkylidene to yield the E-isomer of the product (structure B). Notably, the stereoselectivity of 52 was lost if phenylacetylene was substituted for 1-octyne, which has
considerably less steric bulk, or if the steric bulk of the carboxylic acid was
increased (R ¼ Ph).
5.4 Hydroamination of Alkynes, Allenes and Alkenes
The hydroamination of alkynes is a highly atom-efficient approach to the synthesis
of enamines and imines, as well as to the synthesis of N-heterocyclic compounds
such as indoles and pyrroles, which are widely occurring functional groups in
biologically active molecules. Also included in this section is the hydroamination
of allenes and alkenes, as the reaction of these substrates with chiral bimetallic
catalysts has been shown to yield the chiral amine products with high
enantioselectivity.
A number of reports have demonstrated the utility of monometallic Ir(III)
catalysts for the hydroamination of 2-alkynylanilines to yield indole products
[98, 99]. In an effort to enhance the efficiency of such catalysts, the bimetallic
Ir(III) complex 54 was prepared and investigated as a catalyst for the
hydroamination of 2-alkynylanilines (Scheme 20) [100]. During the catalytic
cycle it was anticipated that the bridging chlorides of 54 would dissociate to free
a vacant coordination site on each metal, through which the alkyne substrate could
coordinate. It was hoped that the compact naphthalene scaffold would enforce a
close alignment of the two metal fragments and thereby lead to a cooperative
enhancement of the reaction rate. While complex 54 was found to be an effective
catalyst for this reaction, comparison with an analogous monometallic catalyst
(generated in situ by reaction of complex 55 with one equiv. of AgPF 6 ) revealed
that no cooperative enhancement of the reaction rate was obtained by the bimetallic
system. It is possible that the conformational freedom available to the two Ir
fragments upon cleavage of the Ir–μ–Cl bonds prevents a favourable alignment of
the metals during catalysis.
Using the bimetallic ruthenium complex 56 as catalyst for the intermolecular
hydroamination of phenylacetylene demonstrated that the relative positions of the
two Ru fragments must be restricted to achieve effective promotion of the reaction
(Scheme 21) [101]. Complex 56 promotes the hydroamination reaction in three
steps. Initial protonation of the bridging phenylacetylene unit was shown to yield
the isolable vinyl complex 57, where the vinyl ligand is stabilised by a bimetallic
126
M.J. Page et al.