state-of-the-art in the field. These systems have been successfully investigated with
computational DFT-based methods. The results are organized in two blocks. The
first block corresponds to photoredox catalysis. First, a formally typical photoredox
process will be discussed where the synthesis of indolines from alkenes and
iodoacetanilide is accomplished by a combination of a Ni(0) catalyst and a
Ru-based photosensitizer. Next, the light-driven catalytic trichloromethylation of
acylpyridines catalyzed by an Ir complex will be discussed. The peculiarity of this
system is that the same metal complex absorbs the light and catalyzes the reaction,
without an external photosensitizer. The second block contains two examples that
cannot be strictly defined as photocatalysis, but rather as photoactivation. They are
the photoinduced oxidative carboxylation of a nonheme Fe(III) complex and the
light-induced insertion of dioxygen into a Pt(II)-methyl bond.
2 Photoredox-Catalyzed Reactions
2.1 Dual Ni–Photoredox Catalysis
Nickel catalysis has been extensively used in the recent years for the development of
novel transformations [43]. Nickel is more abundant and less toxic than other
second- and third-row transition metals. On top of that, Ni exhibits an inherent
ability to change its oxidation state by one unit at a time [44]. This behavior is
complementary to the more traditional two-electron chemistry found in other metals
such as Pd [45, 46]. Thus, the combination of nickel-based catalysts with photoredox
active complexes such as [Ru(bpy) 3 ]
2+ can take advantage of the ability of these
systems for the SET processes [47, 48]. This gives access to intermediates not
reachable by other means, promoting a different reactivity [49].
Here, we will discuss the computational characterization of the full catalytic cycle
for the synthesis of indoline products catalyzed by a symbiotic photoredox Ni system
carried out in our group [50]. This system is remarkable because of the diversity of
nickel oxidation states present in the catalytic cycle. Calculations were carried out
using a B3LYP-D3 functional with a continuum acetone solvent introduced with the
SMD methodology. The SET steps between the photosensitizer and the Ni catalyst
were studied using the Marcus theory. The whole computed mechanism will be
discussed here, with special attention to the steps with participation of the
photosensitizer.
The reaction starts with the oxidative addition of the iodoacetanilide reactant to
the Ni(0) complex; the Gibbs energy profile is shown in Fig. 4. The transition state
TS ox.add for the concerted oxidative addition has a barrier of 19.8 kcal/mol. This is,
however, not the favored path for this step. Ni complexes have been previously
shown to react via halide abstraction to activate aryl halide substrates, and this is the
case also here. Halide abstraction proceeds through the open-shell singlet (OSS)
surface via an inner-sphere single electron transfer event. The activation barrier
found for this mechanism TS 1–2 is 6.1 kcal/mol, 13.7 kcal/mol lower than the
136
A. de Aguirre et al.
computational DFT-based methods. The results are organized in two blocks. The
first block corresponds to photoredox catalysis. First, a formally typical photoredox
process will be discussed where the synthesis of indolines from alkenes and
iodoacetanilide is accomplished by a combination of a Ni(0) catalyst and a
Ru-based photosensitizer. Next, the light-driven catalytic trichloromethylation of
acylpyridines catalyzed by an Ir complex will be discussed. The peculiarity of this
system is that the same metal complex absorbs the light and catalyzes the reaction,
without an external photosensitizer. The second block contains two examples that
cannot be strictly defined as photocatalysis, but rather as photoactivation. They are
the photoinduced oxidative carboxylation of a nonheme Fe(III) complex and the
light-induced insertion of dioxygen into a Pt(II)-methyl bond.
2 Photoredox-Catalyzed Reactions
2.1 Dual Ni–Photoredox Catalysis
Nickel catalysis has been extensively used in the recent years for the development of
novel transformations [43]. Nickel is more abundant and less toxic than other
second- and third-row transition metals. On top of that, Ni exhibits an inherent
ability to change its oxidation state by one unit at a time [44]. This behavior is
complementary to the more traditional two-electron chemistry found in other metals
such as Pd [45, 46]. Thus, the combination of nickel-based catalysts with photoredox
active complexes such as [Ru(bpy) 3 ]
2+ can take advantage of the ability of these
systems for the SET processes [47, 48]. This gives access to intermediates not
reachable by other means, promoting a different reactivity [49].
Here, we will discuss the computational characterization of the full catalytic cycle
for the synthesis of indoline products catalyzed by a symbiotic photoredox Ni system
carried out in our group [50]. This system is remarkable because of the diversity of
nickel oxidation states present in the catalytic cycle. Calculations were carried out
using a B3LYP-D3 functional with a continuum acetone solvent introduced with the
SMD methodology. The SET steps between the photosensitizer and the Ni catalyst
were studied using the Marcus theory. The whole computed mechanism will be
discussed here, with special attention to the steps with participation of the
photosensitizer.
The reaction starts with the oxidative addition of the iodoacetanilide reactant to
the Ni(0) complex; the Gibbs energy profile is shown in Fig. 4. The transition state
TS ox.add for the concerted oxidative addition has a barrier of 19.8 kcal/mol. This is,
however, not the favored path for this step. Ni complexes have been previously
shown to react via halide abstraction to activate aryl halide substrates, and this is the
case also here. Halide abstraction proceeds through the open-shell singlet (OSS)
surface via an inner-sphere single electron transfer event. The activation barrier
found for this mechanism TS 1–2 is 6.1 kcal/mol, 13.7 kcal/mol lower than the
136
A. de Aguirre et al.