4.1 C–O Bond Formation
99
12.5 kcal/mol endothermic. In this catalytic cycle, the oxidative state of Rh remains
at +3 and reveals a non-redox process.
4.2 C–N Bond Formation
Nitrogen-containing compounds are widely present in natural products and synthetic
intermediates. The development of efficient and selective C–N bond formation reactions has been a highly important research topic in chemical synthesis [8, 16–18]. As
a result, much effort has been devoted to the conversion of an unactivated C–H bond
to a C–N bond through Rh-mediated C–H bond activation in the past years [19–22].
The C–H activation can provide nucleophilic aryl-Rh or alkyl-Rh species, which can
react with nitrene precursors to construct C–N bond. Alternatively, azide also can be
used as a nucleophilic nitrogen source in Rh-mediated oxidative couplings.
4.2.1 Rh-Mediated C–H Amination by Using Nitrene
Precursor
The Rh-nitrene complexes play important roles in C–N bond formation reactions after
Rh-catalyzed C–H bond activation [23]. In this chemistry, a series of nitrene precursors can be used to construct Rh-nitrene complex, including azides [24], dioxazolones
[25–27], anthranils [28–31], and N-phenoxyacetamides [32, 33]. The varied leaving
group in those nitrene precursors leads to a series of different mechanisms for the
generation of Rh-nitrene complexes as well as their transformations. Based on the
previous experimental and theoretical studies, a common approach has been proposed
in Scheme 4.3. The catalytic cycle is based on Rh-mediated C–H bond activation
to form an aryl-Rh(III) intermediate 4-10. The coordination of nitrene precursor
leads to the dissociation of leaving group and afford Rh-nitrene complex 4-11. Then
nitrene inserts into C(aryl)–Rh(III) bond through an inner-sphere process to form
C–N bond. Rh-catalyst can be regenerated by the following protonolysis. Generally,
the oxidative state of Rh-nitrene complexes was deemed to +5, which is regarded as
a redox process.
The azides are frequently used nitrene precursors in Rh-catalyzed arene aminations. Chang and co-workers [21, 34, 35] have made a significant contribution in
Rh(III)-catalyzed intermolecular arene C–H bond aminations using organic azides
as amino source. These reactions do not require external oxidants and release only
nitrogen as a byproduct. Serious directing groups are effective for this reaction to
furnish desired ortho-aminated products in good yields with excellent selectivity
(Scheme 4.4).
They [36] also performed DFT studies to investigate the detailed mechanism of
nitrene insertion, which leads to C–N bond formation. The five-membered rhodacycle
99
12.5 kcal/mol endothermic. In this catalytic cycle, the oxidative state of Rh remains
at +3 and reveals a non-redox process.
4.2 C–N Bond Formation
Nitrogen-containing compounds are widely present in natural products and synthetic
intermediates. The development of efficient and selective C–N bond formation reactions has been a highly important research topic in chemical synthesis [8, 16–18]. As
a result, much effort has been devoted to the conversion of an unactivated C–H bond
to a C–N bond through Rh-mediated C–H bond activation in the past years [19–22].
The C–H activation can provide nucleophilic aryl-Rh or alkyl-Rh species, which can
react with nitrene precursors to construct C–N bond. Alternatively, azide also can be
used as a nucleophilic nitrogen source in Rh-mediated oxidative couplings.
4.2.1 Rh-Mediated C–H Amination by Using Nitrene
Precursor
The Rh-nitrene complexes play important roles in C–N bond formation reactions after
Rh-catalyzed C–H bond activation [23]. In this chemistry, a series of nitrene precursors can be used to construct Rh-nitrene complex, including azides [24], dioxazolones
[25–27], anthranils [28–31], and N-phenoxyacetamides [32, 33]. The varied leaving
group in those nitrene precursors leads to a series of different mechanisms for the
generation of Rh-nitrene complexes as well as their transformations. Based on the
previous experimental and theoretical studies, a common approach has been proposed
in Scheme 4.3. The catalytic cycle is based on Rh-mediated C–H bond activation
to form an aryl-Rh(III) intermediate 4-10. The coordination of nitrene precursor
leads to the dissociation of leaving group and afford Rh-nitrene complex 4-11. Then
nitrene inserts into C(aryl)–Rh(III) bond through an inner-sphere process to form
C–N bond. Rh-catalyst can be regenerated by the following protonolysis. Generally,
the oxidative state of Rh-nitrene complexes was deemed to +5, which is regarded as
a redox process.
The azides are frequently used nitrene precursors in Rh-catalyzed arene aminations. Chang and co-workers [21, 34, 35] have made a significant contribution in
Rh(III)-catalyzed intermolecular arene C–H bond aminations using organic azides
as amino source. These reactions do not require external oxidants and release only
nitrogen as a byproduct. Serious directing groups are effective for this reaction to
furnish desired ortho-aminated products in good yields with excellent selectivity
(Scheme 4.4).
They [36] also performed DFT studies to investigate the detailed mechanism of
nitrene insertion, which leads to C–N bond formation. The five-membered rhodacycle