1.1 A Brief History of Rh-Catalyzed C–H Functionalization
3
makes the site-selective cleavage and functionalization of C–H bonds to be feasible
[76–78].
In recent decades, the selective formation of carbon–carbon and carbon–
heteroatom bonds through transition metal-catalyzed direct functionalization of
unreactive carbon–hydrogen bonds has attracted considerable attention in chemical research and industry from the perspective of atom and step economy [79–86].
High valence transition metal can obtain electrons from nucleophile, which leads to
the transformation of nucleophile into electrophile. The newly generated electrophile
can couple with other nucleophiles to form a covalent bond, which is named oxidative
coupling reaction. Meanwhile, the reduced transition metal can be oxidized by an
exogenous oxidant for regeneration. Correspondingly, low valence transition metal
can donate electrons to electrophile leading to the transformation of electrophile
into nucleophile, which can react with another electrophile to form a covalent bond.
Accordingly, it is named a reductive coupling reaction. The oxidized transition metal
also can be reduced by exogenous reductant.
Transition metal catalysts, especially Pd, Rh, Ru, Ir, Ni, and Cu catalysts, are
crucial for C–H bond cleavage and further transformations [87–92]. In this area,
extensive efforts have been devoted to the development of Rh catalysts in catalytic
conversion of C–H bonds. It has become an increasingly important strategy for
the construction of complex organic products. A broad range of transformations
could be achieved through rhodium-catalyzed C–H bond functionalization. For
example, Ellman and Bergman have developed a series of efficiently and intermolecularly alkylation of nitrogen heterocycles with a wide range of functionalized
olefins via Rh-catalyzed C–H bond activation [88, 93, 94]. In addition, an array of
Rh(I) and Rh(III)-catalyzed C–H bond functionalization reactions that take advantage of a chelating directing group have also been studied by the same group [95–
102]. The use of chelation control in C–H bond functionalization offers several
advantages with respect to substrate scope and application to total synthesis [103].
Glorius reported a Cp*Rh(III)-catalyzed reaction of N-phenoxyacetamide with 7azabenzonorbornadiene leads to a dearomatized spiro structure with a cyclopropane
unit [104]. Li and co-workers have also reported a series of C–H activation of
arenes using Cp*Rh(III) complexes with high reactivity, stability, and functional
group compatibility [105, 106]. In addition, Chang’s group has made a contribution to the field of development of Rh-catalyzed direct C–H amination reactions
with organic azides [107]. Under their optimized Rh(III)-catalyzed amination conditions, not only sulfonyl azides but also aryl- and alkyl azides could be utilized as
facile amino sources in reaction with various types of C(sp
2 )–H bonds. In order to
realize Rh(III)-catalytic enantioselective transformations, Cramer and co-workers
have introduced two families of chiral cyclopentadienyls ligand. The first generation
consists of a fused cyclohexane unit having pseudo axial methyl groups as chiral
selectors and a rigidifying acetal moiety [108]. The second ligand generation derives
from an atop-chiral biaryl-backbone and which possesses adjustable substituents at
its 3,3’-positions. Rovis also describes a stunning effect on reactivity and selectivity
of Rh-catalyzed C–H bond functionalization reactions by the introduction of achiral
mono-, di-, or pentasubstituted cyclopentadienyl ligands [109–111].
3
makes the site-selective cleavage and functionalization of C–H bonds to be feasible
[76–78].
In recent decades, the selective formation of carbon–carbon and carbon–
heteroatom bonds through transition metal-catalyzed direct functionalization of
unreactive carbon–hydrogen bonds has attracted considerable attention in chemical research and industry from the perspective of atom and step economy [79–86].
High valence transition metal can obtain electrons from nucleophile, which leads to
the transformation of nucleophile into electrophile. The newly generated electrophile
can couple with other nucleophiles to form a covalent bond, which is named oxidative
coupling reaction. Meanwhile, the reduced transition metal can be oxidized by an
exogenous oxidant for regeneration. Correspondingly, low valence transition metal
can donate electrons to electrophile leading to the transformation of electrophile
into nucleophile, which can react with another electrophile to form a covalent bond.
Accordingly, it is named a reductive coupling reaction. The oxidized transition metal
also can be reduced by exogenous reductant.
Transition metal catalysts, especially Pd, Rh, Ru, Ir, Ni, and Cu catalysts, are
crucial for C–H bond cleavage and further transformations [87–92]. In this area,
extensive efforts have been devoted to the development of Rh catalysts in catalytic
conversion of C–H bonds. It has become an increasingly important strategy for
the construction of complex organic products. A broad range of transformations
could be achieved through rhodium-catalyzed C–H bond functionalization. For
example, Ellman and Bergman have developed a series of efficiently and intermolecularly alkylation of nitrogen heterocycles with a wide range of functionalized
olefins via Rh-catalyzed C–H bond activation [88, 93, 94]. In addition, an array of
Rh(I) and Rh(III)-catalyzed C–H bond functionalization reactions that take advantage of a chelating directing group have also been studied by the same group [95–
102]. The use of chelation control in C–H bond functionalization offers several
advantages with respect to substrate scope and application to total synthesis [103].
Glorius reported a Cp*Rh(III)-catalyzed reaction of N-phenoxyacetamide with 7azabenzonorbornadiene leads to a dearomatized spiro structure with a cyclopropane
unit [104]. Li and co-workers have also reported a series of C–H activation of
arenes using Cp*Rh(III) complexes with high reactivity, stability, and functional
group compatibility [105, 106]. In addition, Chang’s group has made a contribution to the field of development of Rh-catalyzed direct C–H amination reactions
with organic azides [107]. Under their optimized Rh(III)-catalyzed amination conditions, not only sulfonyl azides but also aryl- and alkyl azides could be utilized as
facile amino sources in reaction with various types of C(sp
2 )–H bonds. In order to
realize Rh(III)-catalytic enantioselective transformations, Cramer and co-workers
have introduced two families of chiral cyclopentadienyls ligand. The first generation
consists of a fused cyclohexane unit having pseudo axial methyl groups as chiral
selectors and a rigidifying acetal moiety [108]. The second ligand generation derives
from an atop-chiral biaryl-backbone and which possesses adjustable substituents at
its 3,3’-positions. Rovis also describes a stunning effect on reactivity and selectivity
of Rh-catalyzed C–H bond functionalization reactions by the introduction of achiral
mono-, di-, or pentasubstituted cyclopentadienyl ligands [109–111].