protected hydroxyl as well as carbonyl, ester, and halide groups underwent selective
isomerization (Scheme 6).
Analogous pincer complexes possessing substituents not capable of agostic
interactions were reported nearly inactive or not selective under the described
reaction conditions; therefore, it is suggested that the reaction is operated by a
mechanism in which the appended sidearm on the backbone acts as a shuttle.
Aiming at more tunable catalysis, Miller et al. chose another strategy to adjust the
properties of the pincer catalysts by means of an additional crown ether-based
hemilabile sidearm. Over the years, much information about crown ether-cation
interactions has been gained; therefore, a catalyst crafted with an appended crown
ether function would offer a rare possibility to tune the hemilability of the donor via
allosteric interactions. Moreover, the controlled hemilability may also directly
control the primary coordination sphere, thus fine-tuning a balance between stability
and reactivity.
The first NCOP pincer ligand bearing an aza-15-crown-5 ether macrocycle
as the amine donor (22) was prepared in ca. 50% yield by amination of
3-bromomethylphenol (21) with aza-15-crown-5 with subsequent phosphination
with chlorodiisopropylphosphine [69]; advantageously, aza-crown ethers of different ring sizes as well as phosphine derivatives having various electronic and steric
properties can be installed using only slightly modified protocols (Scheme 7) [70].
Since it is an aromatic PCN pincer ligand, 22 can accommodate a variety of late
transition metals via oxidative or electrophilic C(sp
2 )-H activation (Scheme 8).
Depending on the reaction conditions, the aza-crown-based pincer ligand can
adopt tridentate, tetradentate, or pentadentate coordination modes to satisfy the
ligancy of the metal. The interconversion between the modes was found to be highly
reversible owing to weak dative Ir-O bonds that were theoretically estimated (the
Scheme 6 Representative isomerization of alkenes catalyzed by 20
Scheme 7 Synthesis of the pincer ligand bearing appended aza-15-crown-5 functionality
102
A. Singh et al.
isomerization (Scheme 6).
Analogous pincer complexes possessing substituents not capable of agostic
interactions were reported nearly inactive or not selective under the described
reaction conditions; therefore, it is suggested that the reaction is operated by a
mechanism in which the appended sidearm on the backbone acts as a shuttle.
Aiming at more tunable catalysis, Miller et al. chose another strategy to adjust the
properties of the pincer catalysts by means of an additional crown ether-based
hemilabile sidearm. Over the years, much information about crown ether-cation
interactions has been gained; therefore, a catalyst crafted with an appended crown
ether function would offer a rare possibility to tune the hemilability of the donor via
allosteric interactions. Moreover, the controlled hemilability may also directly
control the primary coordination sphere, thus fine-tuning a balance between stability
and reactivity.
The first NCOP pincer ligand bearing an aza-15-crown-5 ether macrocycle
as the amine donor (22) was prepared in ca. 50% yield by amination of
3-bromomethylphenol (21) with aza-15-crown-5 with subsequent phosphination
with chlorodiisopropylphosphine [69]; advantageously, aza-crown ethers of different ring sizes as well as phosphine derivatives having various electronic and steric
properties can be installed using only slightly modified protocols (Scheme 7) [70].
Since it is an aromatic PCN pincer ligand, 22 can accommodate a variety of late
transition metals via oxidative or electrophilic C(sp
2 )-H activation (Scheme 8).
Depending on the reaction conditions, the aza-crown-based pincer ligand can
adopt tridentate, tetradentate, or pentadentate coordination modes to satisfy the
ligancy of the metal. The interconversion between the modes was found to be highly
reversible owing to weak dative Ir-O bonds that were theoretically estimated (the
Scheme 6 Representative isomerization of alkenes catalyzed by 20
Scheme 7 Synthesis of the pincer ligand bearing appended aza-15-crown-5 functionality
102
A. Singh et al.