3-heptyn-1-ol with a monometallic Au complex in the presence of a proton sponge
[86]. The researchers also investigated several bimetallic complexes for this reaction with the expectation that the formation of gem-diaurated species may be
favoured. Surprisingly a range of coordination modes were characterised depending
on the nature of the bridging ligand scaffold. For example, while the ferrocenyl
digold complex 34 was shown to yield the expected gem-digold intermediate A, an
analogous structure could not be characterised with the binaphthyl bridged digold
complex 35. The NMR spectra of the reaction containing 35 suggested multiple
oligomeric diaurated species of irregular structure might be present. Presumably the
helical twist of the binaphthyl unit precludes a simple bimetallic interaction with
the substrate in this case. While it has yet to be reported how the different
monometallic and bimetallic structures in Scheme 15b affect their efficiency as
catalysts for the hydroalkoxylation of alkynes, chiral biaryl catalysts analogous to
35 have been used for the enantioselective hydroalkoxylation of allene substrates
[87]. For example, the bimetallic Au catalyst 36 was shown to catalyse the
intramolecular hydroalkoxylation of allene alcohols with exceptional enantioselectivity (>93% ee) (Scheme 15c).
Moving from Au to other metal centres for promoting C–O bond formation,
Messerle and coworkers demonstrated that monometallic Rh(I) and Ir(I) complexes
containing bidentate N-donor ligands such as bis(1-pyrazolyl)methane (bpm) are
excellent catalysts for the intramolecular dihydroalkoxylation of alkynediols to
yield spiroketals, in a one-pot tandem reaction [88]. Recently it was demonstrated
that the efficiency of these catalysts could be enhanced by linking two bpm complex
fragments by a bridging organic scaffolds to yield a series of bimetallic catalysts
(Scheme 16) [89, 90]. An initial investigation showed that the bimetallic complexes
38, 39 and 40 were all superior catalysts compared to the monometallic analogue 37
for the dihydroalkoxylation of a variety of alkynediol substrates. The reaction rates
achieved by catalysts 38–40 were also found to increase as the apparent separation
between the metal fragments decreased, such that the efficiency of the catalysts
could be ordered 38 < 39 < 40. It was also found that increasing the flexibility of
the scaffold, such as in the complexes 41, 42 and 43 (linked by hexyl, heptyl and
ferrocenyl groups, respectively), considerably diminished the bimetallic synergism
for these catalysts. The bimetallic complexes 44 and 45 containing a xanthene and
dibenzofuran scaffold, respectively, did not achieve catalytic rates as high as the
structurally very similar anthracene containing complex 40. Despite a superficial
similarity in structure of the complexes 40, 44 and 45, DFT modelling of their
structures revealed that the intermetallic distance in their lowest energy conformation differed significantly, which may contribute to their dissimilar reactivity. The
weakly basic oxygen heterocycle present in the scaffold of 44 and 45 may also
interact with the substrate to alter the reactivity of these catalysts.
To better understand the structure–activity relationship with the bimetallic
catalysts 38–40, 44 and 45, computational modelling of their structural confirmations was performed [90]. A high degree of conformational flexibility existed
within the structures due to free rotation of the bpm–arene bond and flipping of
the bpm–Rh metallocycle between two possible boat conformations (Fig. 3).
Alkyne Activation Using Bimetallic Catalysts
121
[86]. The researchers also investigated several bimetallic complexes for this reaction with the expectation that the formation of gem-diaurated species may be
favoured. Surprisingly a range of coordination modes were characterised depending
on the nature of the bridging ligand scaffold. For example, while the ferrocenyl
digold complex 34 was shown to yield the expected gem-digold intermediate A, an
analogous structure could not be characterised with the binaphthyl bridged digold
complex 35. The NMR spectra of the reaction containing 35 suggested multiple
oligomeric diaurated species of irregular structure might be present. Presumably the
helical twist of the binaphthyl unit precludes a simple bimetallic interaction with
the substrate in this case. While it has yet to be reported how the different
monometallic and bimetallic structures in Scheme 15b affect their efficiency as
catalysts for the hydroalkoxylation of alkynes, chiral biaryl catalysts analogous to
35 have been used for the enantioselective hydroalkoxylation of allene substrates
[87]. For example, the bimetallic Au catalyst 36 was shown to catalyse the
intramolecular hydroalkoxylation of allene alcohols with exceptional enantioselectivity (>93% ee) (Scheme 15c).
Moving from Au to other metal centres for promoting C–O bond formation,
Messerle and coworkers demonstrated that monometallic Rh(I) and Ir(I) complexes
containing bidentate N-donor ligands such as bis(1-pyrazolyl)methane (bpm) are
excellent catalysts for the intramolecular dihydroalkoxylation of alkynediols to
yield spiroketals, in a one-pot tandem reaction [88]. Recently it was demonstrated
that the efficiency of these catalysts could be enhanced by linking two bpm complex
fragments by a bridging organic scaffolds to yield a series of bimetallic catalysts
(Scheme 16) [89, 90]. An initial investigation showed that the bimetallic complexes
38, 39 and 40 were all superior catalysts compared to the monometallic analogue 37
for the dihydroalkoxylation of a variety of alkynediol substrates. The reaction rates
achieved by catalysts 38–40 were also found to increase as the apparent separation
between the metal fragments decreased, such that the efficiency of the catalysts
could be ordered 38 < 39 < 40. It was also found that increasing the flexibility of
the scaffold, such as in the complexes 41, 42 and 43 (linked by hexyl, heptyl and
ferrocenyl groups, respectively), considerably diminished the bimetallic synergism
for these catalysts. The bimetallic complexes 44 and 45 containing a xanthene and
dibenzofuran scaffold, respectively, did not achieve catalytic rates as high as the
structurally very similar anthracene containing complex 40. Despite a superficial
similarity in structure of the complexes 40, 44 and 45, DFT modelling of their
structures revealed that the intermetallic distance in their lowest energy conformation differed significantly, which may contribute to their dissimilar reactivity. The
weakly basic oxygen heterocycle present in the scaffold of 44 and 45 may also
interact with the substrate to alter the reactivity of these catalysts.
To better understand the structure–activity relationship with the bimetallic
catalysts 38–40, 44 and 45, computational modelling of their structural confirmations was performed [90]. A high degree of conformational flexibility existed
within the structures due to free rotation of the bpm–arene bond and flipping of
the bpm–Rh metallocycle between two possible boat conformations (Fig. 3).
Alkyne Activation Using Bimetallic Catalysts
121