electron with other species in solution. Most of these nonproductive pathways would
return to the main catalytic cycle. They may reduce the reaction rate, but not modify
the final outcome.
In summary, the full mechanism for the indoline synthesis from 2
0
-iodoacetanilide and alkene catalyzed by a combination of Ni/Ru dual system
could be fully characterized. The barriers for the outer-sphere SET were successfully
calculated applying the Marcus theory.
2.2 Light-Driven Catalytic Trichloromethylation
of Acylpyridines
A number of natural products bear trichloromethyl groups. They are interesting
because of their pharmacological properties [53, 54]. Many efforts have been done to
develop strategies for the insertion of this group into organic molecules. These
methodologies can be stoichiometric [55, 56] or catalytic (mainly with Ru [57, 58]
and Ti [59, 60] catalysts). However, methodologies for an effective enantioselective
addition of trichloromethyl groups have been limited. Most of the strategies to
achieve this goal involve redox-mediated radical addition, presenting thus a perfect
opportunity for the treatment of this reaction with a photocatalytic approach.
Trichloromethylation has been achieved in selected case without need of an
external photosensitizer. Meggers and coworkers [61] developed an iridium system
which is capable to photoinduced single electron transfer to catalyze the
enantioselective trichloromethylation of 2-acylpyridines and 2-acyl imidazoles.
The unique features of this chiral Λ-iridium complex are that it acts as both the
asymmetric catalysts and the photosensitizer [62]. The bidentate ligands in this
octahedral complex are responsible for the chirality of the system. They are arranged
in a left-handed propeller-type coordination [63].
In this section, we will present our computational study of the reaction reported
by Meggers and coworkers. Although the original publication analyzed the origin of
enantioselectivity, here we will only focus on the steps where light plays a direct
role. Calculations were carried out with an ONIOM method, ωB97X-D for the QM
region and UFF for the MM region. The MM region consisted of the methyl
substituents in the tert-butyl groups of the catalyst.
2.2.1 Overall Catalytic Cycle
As is the usual case in photoredox catalysis, the whole reaction mechanism consists
of two linked cycles that we can label as “light” and “dark.” They are shown in
Fig. 8. The two cycles cross in two points. The first of them is the transfer of the CCl 3
radical generated by the interaction of the photogenerated excited state iridium
complex A with the BrCCl 3 substrate. The second connection is in the electron
140
A. de Aguirre et al.
return to the main catalytic cycle. They may reduce the reaction rate, but not modify
the final outcome.
In summary, the full mechanism for the indoline synthesis from 2
0
-iodoacetanilide and alkene catalyzed by a combination of Ni/Ru dual system
could be fully characterized. The barriers for the outer-sphere SET were successfully
calculated applying the Marcus theory.
2.2 Light-Driven Catalytic Trichloromethylation
of Acylpyridines
A number of natural products bear trichloromethyl groups. They are interesting
because of their pharmacological properties [53, 54]. Many efforts have been done to
develop strategies for the insertion of this group into organic molecules. These
methodologies can be stoichiometric [55, 56] or catalytic (mainly with Ru [57, 58]
and Ti [59, 60] catalysts). However, methodologies for an effective enantioselective
addition of trichloromethyl groups have been limited. Most of the strategies to
achieve this goal involve redox-mediated radical addition, presenting thus a perfect
opportunity for the treatment of this reaction with a photocatalytic approach.
Trichloromethylation has been achieved in selected case without need of an
external photosensitizer. Meggers and coworkers [61] developed an iridium system
which is capable to photoinduced single electron transfer to catalyze the
enantioselective trichloromethylation of 2-acylpyridines and 2-acyl imidazoles.
The unique features of this chiral Λ-iridium complex are that it acts as both the
asymmetric catalysts and the photosensitizer [62]. The bidentate ligands in this
octahedral complex are responsible for the chirality of the system. They are arranged
in a left-handed propeller-type coordination [63].
In this section, we will present our computational study of the reaction reported
by Meggers and coworkers. Although the original publication analyzed the origin of
enantioselectivity, here we will only focus on the steps where light plays a direct
role. Calculations were carried out with an ONIOM method, ωB97X-D for the QM
region and UFF for the MM region. The MM region consisted of the methyl
substituents in the tert-butyl groups of the catalyst.
2.2.1 Overall Catalytic Cycle
As is the usual case in photoredox catalysis, the whole reaction mechanism consists
of two linked cycles that we can label as “light” and “dark.” They are shown in
Fig. 8. The two cycles cross in two points. The first of them is the transfer of the CCl 3
radical generated by the interaction of the photogenerated excited state iridium
complex A with the BrCCl 3 substrate. The second connection is in the electron
140
A. de Aguirre et al.