1
Introduction
C–H functionalization has emerged as one of the most efficient and straightforward
approaches to construct carbon–heteroatom bonds directly from C–H bonds.[1] This strategy is advantageous especially from an atom economy and sustainability
point of view as it does not require pre-functionalization of the substrates. Although
it may offer more direct access to target functionality, selective activation of potentially
reactive C–H bonds has remained a great challenge.[2] Along with great advances in organometallic chemistry, the modern application of
transition-metal complexes in the catalytic C–H functionalizations has ushered in
a new era in synthetic chemistry by providing precision to control selectivity and
high reaction efficiency in a wide variety of chemical transformations.[3]
Among a range of transition-metal-based catalytic systems, Cp*M(III) complexes (Cp*
= pentamethyl-cyclopentadienyl) centered on iridium and rhodium, in particular, have
found pervasive utilities as an effective catalyst for various C–H functionalization
reactions.[4] However, the environmental and economic concerns associated with the use of expensive
and nongreen precious metals prompted the search for milder alternatives. In this
context, the pioneering work of Matsunaga and Kanai on the use of [Cp*Co(III)] catalyst
for the direct C(sp2)–H alkylation (Scheme [1]) provided a promising direction for sustainable and efficient catalysis.[5]
Scheme 1 Pioneering discovery of Cp*Co(III) catalytic system in the C–H activation catalysis
The Cp*Co(III) system is not only favorable from an economical or sustainable standpoint,
but the unique properties of the cobalt metal core also lead to better efficiencies
for certain types of functionalization reactions when even compared to the Ir or Rh
counterparts.[6] For example, Glorius and co-workers reported a comparative study on the catalytic
activity of group 9 Cp*M(III) toward the C–H amidation and cyclization cascade of
aryl imidates, whereby the efficiency of Cp*Co had been proven superior in comparison
to the Ir or Rh congeners (Scheme [2a]).[7] The authors attributed such an observation to the relatively stronger Lewis acidity
of cobalt that may facilitate the cyclization reactivity, as well as to its smaller
ionic radius that allows for the circumvention of the unwanted over-amidation pathway
presumably via steric perturbations.
Scheme 2 Unique reactivity of cobalt complexes in the catalytic C–H functionalization reactions
In addition, the inherent physicochemical property of cobalt metal arising from the
d-electronic structure gave rise to new reactivity not seen in conventional catalytic
reactions. Using a cobalt-porphyrin catalytic system, the Zhang group highlighted
the involvement of cobalt-based metalloradical catalysis for the direct amination
of electron-poor C–H bonds, which are difficult to achieve with the widely studied
Rh2 or other closed-shell systems (Scheme [2b]).[8]
In 2014, the versatile application of the Cp*Co(III) catalytic system in various C–H
functionalizations was recognized by Glorius and colleagues through their research
work.[9] In this study, C–H cyanation, halogenation, and allylation reactions were realized
for the first time by Cp*Co(III)-catalyzed formal SN-type reactions (Scheme [3]). This catalytic procedure was successfully applied to various types of sp2 carbons, including arenes, heteroarenes, and alkenes, thereby resulting in value-added
organo nitriles, halides, as well as allylated indoles.
Scheme 3 Versatile utility of Cp*Co(III) catalysts in the C–H cyanation, halogenation, and
allylation reactions
The cobalt(III) metal with Cp-type ligands has also served as a productive platform
for the catalytic enantioselective C–H functionalization reactions.[10] The first successful application of Cp*Co(III) complexes for asymmetric C–H functionalization
was proposed by Ackermann in 2018 (Scheme [4a]). In this work, the cooperative catalytic systems between Cp*Co(III) and chiral
carboxylic acid were employed to promote the enantioselective C–H alkylation on indoles
in a high position- and regioselective manner.[11] Cramer and co-workers revealed that the Cp* variant of the cobalt(III) complex
also enabled asymmetric C–H functionalization (Scheme [4b]).[12] The Co(III) complex equipped with a trisubstituted chiral cyclopentadienyl ligand
was found to be effective to provide high enantioselectivity as well as regioselectivity
in the synthesis of dihydroisoquinolones from N-chlorobenzamides and alkenes, outperforming the best rhodium(III)-based methods for
this type of reaction.
Scheme 4 Utilization of cyclopentadienyl Co(III) catalysts in the asymmetric C–H functionalization
reactions
As a consequence of the synthetic advantages and distinctive activity, the Cp*Co catalytic
systems have exploded in use for a wide range of other C–H functionalization reactions[13] as well, which have been comprehensively summarized in recent review articles by
Yu,[14] Ackermann,[15] and Matsunaga.[16] Our group joined the historical evolution of this research area at a relatively
early stage, particularly in the field of C–H amidation. In this Account, we present
our own efforts on the recent development of Cp*Co-catalyzed C–N bond formation, which
are discussed in two separate sections based on the two distinct mechanistic pathways:
inner- and outer-sphere C–H functionalization (Scheme [5]).
Scheme 5 Two distinct mechanistic pathways of Cp*Co-catalyzed C–H amidation
The inner-sphere mechanism, also denoted as ‘C–H activation catalysis’ (depicted in
red), is postulated to take place via C–H metalation to initially construct a cobaltacycle
II. The corresponding cobalt complex in turn oxidatively activates an amino source to
result in a cobalt-nitrenoid III, which then inserts into the internal Co–C bond to forge a new C–N bond. In this
reaction pathway, the cobalt metal interconnects the key C–H activation and C–N bond-forming
processes, so is referred as the inner-sphere pathway. In the outer-sphere pathway
(illustrated in blue), in sharp contrast, an initially generated cobalt-nitrenoid
species V directly interacts with the C–H bonds of substrate for the C–N bond formation. This
mechanistic manifold involves ‘C–H insertion catalysis’ in that the C–H bond functionalization
occurs at the nitrenoid moiety.
2
Cp*Co-Catalyzed Inner-Sphere C–H Amidation
In 2014, the viability of Cp*Co catalyst for C–H amination was disclosed for the first
time by Matsunaga and Kanai (Scheme [6]).[17] They demonstrated that a readily available cobalt complex, Cp*Co(CO)I2, was efficient for the C2-selective directed C–H amidation of indoles using sulfonyl
azides as an amino source. Despite the novelty and high efficiency of this transformation,
the limited substrate scope and harsh reaction conditions (high temperatures, 100
°C) implied that there would be room for improvement in Co-catalyzed C–H amination.
In this regard, we became interested in devising competent amino sources that can
confer direct C–N bond formation from various C–H bonds under mild Cp*Co catalytic
conditions.
Scheme 6 First example of Cp*Co(III)-catalyzed C–H amination
Scheme 7 Scope of the Cp*Co(III)-catalyzed inner-sphere C–H amidation using O-acylcarbamates as an amino source
In 2015, our group reported the utilization of O-acylcarbamates as a convenient amidating source in a Cp*Co(III)-catalyzed C–H amidation
of arenes via chelation-assisted regiocontrol (Scheme [7]).[18] Under mild and external oxidant-free conditions, Co(III) catalysts exhibited high
amidation efficiency for the construction of synthetically versatile N-aryl carbamate products, with excellent compatibility for a wide range of arenes
possessing various substituents and directing groups such as pyridine, isoquinoline,
pyrimidine, and purines.
A catalytic cycle of this amidation reaction was proposed to start via the generation
of a cationic cobalt species VI, which can facilitate the C–H bond activation to form a 5-membered cobaltacycle VII (Scheme [8]). Coordination of an amidating reagent to the metallacycle followed by oxidative
activation and insertion of the resulting nitrene species IX into the arene gives a Co(III)-amido species X. Subsequent protonolysis of X provides the final product with the regeneration of the active cobalt species VI.
Scheme 8 Plausible mechanism of Cp*Co-catalyzed inner-sphere C–H amidation
The success in the Co-catalyzed inner-sphere C–H amidation prompted us to further
expand the Cp*Co system using alternative nitrogen sources. When dioxazolones[19] were used as an amino reagent, low catalyst loading (1 mol%) of Cp*Co(III) was sufficient
for the effective C–H amidation of anilides, relatively challenging arene substrates
that were previously absent in the Cp*Co-catalyzed C–H functionalization (Scheme [9]).[20] A comparative study on the catalytic activity with Cp*Rh(III) and Cp*Ir(III) complexes
disclosed that the efficiency of the cobalt system was superior in the desired C–H
amidation of anilides when compared to that of the other group 9 analogues. The cobalt
catalyst system could also be readily applied to a wide range of alternative substrate
classes, including benzamides, phenylpyridines, phenyl pyrazoles, thiophenylpyridines,
and benzoquinonlines to provide the desired amide products (Scheme [10]).
Scheme 9 Catalytic activity of group 9 [Cp*MCl2]2 complexes in the amidation of anilides
Scheme 10 Scope of the Cp*Co(III)-catalyzed inner-sphere C–H amidation using dioxazolones as
an amino source
Utilizing robust dioxazolones as an amidating agent, Dixon, Seayad, and co-workers
demonstrated the first example of Cp*Co(III)-catalyzed C(sp3)–H amidation in the presence of a thioamide as a directing group (Scheme [11a]).[21] This method was later further expanded to an asymmetric transformation using a hybrid
catalytic system consisted of achiral cyclopentadienyl and chiral carboxylic acid
by Yoshino and Matsunaga (Scheme [11b]).[22]
3
Cp*Co-Catalyzed Outer-Sphere C–H Amidation
Since the seminal work of Breslow in 1982 on the catalytic transfer of nitrene species
for the amidation of cyclohexane,[23] transition-metal-catalyzed group transfer of metal-bound nitrene has emerged as
one of the most powerful and efficient strategies for the construction of carbon–nitrogen
bonds in organic synthesis.[24] In 2018, diverting our efforts from the inner-sphere C–H amidation reactions[25] to achieve an alternative strategy with group 9 Cp*M complexes, we established a
novel outer-sphere C–H amidation process leveraging the intermediacy of Ir-acylnitrenoid
species (Scheme [12]).[26] The use of Cp*Ir(III) bearing a strong σ-donating bidentate (L,X-type) ligand allowed
for the desired intramolecular C–H amidation with a dioxazolone motif as the nitrene
precursor, thereby leading to the efficient synthesis of γ-lactams while precluding
the unwanted Curtius-type rearrangement pathway. We rationalized that the electron-donating
ligand plays a pivotal role in effectively suppressing the Curtius decomposition pathway
as it is closely related to the elevation of energy barrier of Curtius rearrangement,
which is sensitive to changes in the partial charge of the metal center.
Scheme 11 Thioamide-directed Cp*Co(III)-catalyzed inner-sphere C(sp3)–H amidation
Scheme 12 Outer-sphere C–H amidation utilizing putative metal-nitrenoid species
This milestone outer-sphere strategy has spurred the development of group-transfer
protocols based on the piano-stool metal-nitrenoid intermediates for a wide range
of C–H amidation reactions.[27] For instance, this mechanistic platform has proven successful for the chiral γ‑lactam
synthesis,[28] unconventional C(sp
2)–N bond formation via spirocyclization,[29] oxyamidation of olefin,[30] haloamidation of alkynes,[31] migratory amidation of alkenyl alcohols,[32] chemodivergent C–H amidation,[33] benzylic selective C–H amidation reactions,[34] and selective α-amidation of esters.[35] Driven by the success of outer-sphere amidation approaches using iridium or ruthenium,
we envisioned cobalt could also be implemented in the piano-stool catalytic system
for effective group transfer of the putative nitrenoid intermediacy.
The Cp*Co(III)(κ2-N,O chelate) complexes turned out to be readily accessible by mixing a solution of
dimeric precursor [Cp*CoCl2]2 with 2 equivalents of the corresponding N,O-type ligand in the presence of base at
ambient temperatures (Scheme [13]).[36] The resulting cobalt complexes were found to be air-stable, thus their preparations
could conveniently be operated under atmospheric conditions and avoid the need for
glovebox. Given that the catalytic activity is closely related to the facile formation
and delivery of electrophilic metal-nitrenoid species,[24] electronic properties of κ2-N,O chelating ligands were finely tuned by changing substituents in N,O-ligands.
We found that 8-hydroxylquinoline ligands with electron-withdrawing groups were highly
facile in the desired C–H amidation. Among those, Co1 bearing 5,7-dichloro quinolinol ligand turned out to be the most efficient in the
catalytic outer-sphere C–N bond-forming reactions investigated below (vide infra).
Scheme 13 Preparation of the Cp*Co(III)(N,O) complexes, exemplified by the synthesis of Co1
3.1
C(sp2)–N Bond Formation
Cyclic carbamates are a key structural motif not only present in numerous medicinally
active compounds,[37] but also utilized as a useful functional handle in chemical synthesis (e.g., as
a chiral auxiliary).[38] Despite their importance, metal-nitrenoid-based C(sp2)–H amidation of arenes was rarely applied to the synthesis of benzo-fused cyclic
carbamates, mainly due to the difficulty in controlling the regioselectivity.[39] In this context, we explored the utility of Cp*Co(LX) catalyst (Co1) in the C(sp2)–H amidation of phenyl azidoformates, harnessing the azide motif as a nitrene precursor
(Scheme [14]).[36] The customized catalyst Co1 indeed led to the desired intramolecular C–H amidation of various types of aryl azidoformates
to give five-membered cyclic carbamates, indicating that the Cp*Co(N,O)-type system
is suitable for outer-sphere C–H amidation pathway. When benzyl azidoformate was subjected,
the spirocyclization and subsequent skeletal rearrangement (C–C migration) took place
to afford six-membered cyclic carbamate.
Scheme 14 Cp*Co(III)-catalyzed intramolecular C(sp2)–H amidation.
a 40 °C. b 60 °C.
With the optimized Co1 system, high regioselectivity (C6/C2 amidation) was achieved in the C–H amidation
of meta-substituted phenyl azidoformates (Scheme [15a]). It was of our special interest to observe that the regioselectivity was modulated
according to the central metal within the same ligand system. For instance, an iridium
analogue (Ir1) containing an identical ligand as that of Co1 was also able to promote the desired C–H amidation with similar efficiency but with
reduced C6/C2 selectivity. To understand the contrasting regioselectivity, we briefly
compared the structural features of the two metal complexes by X-ray crystallographic
analysis of the solid structures of Co1 and Ir1 (Scheme [15b]). The distances between the Co metal center and the coordinating ligands (center
of the Cp* plane, N, and O atoms) in Co1 were noticeably shorter than those of Ir1. This is presumably due to the smaller ionic metal radius of cobalt than iridium.
Considering the previous report that the smaller ionic metal radius of cobalt rendered
Cp*Co more susceptible to steric perturbations,[7] we postulated that the distinct structural features of Co1 lead to favoring the insertion of key metal-nitrenoid intermediates at the less-hindered
C6 position.
The invention of the Cp*Co(N,O) system for outer-sphere C(sp2)–H amidation inspired us to explore further utility of cobalt-nitrenoid transfer
into more specialized aromatic systems. Given that arenium species bearing a quaternary
carbon center were known to stimulate alkyl migration,[40] we speculated that the insertion of Co-nitrenoid into alkyl-substituted aromatic
carbon would generate the arenium species and trigger C–C rearrangement. When 2,6-dialkyl-substiuted
phenyl azidoformate was employed, the Cp*Co catalyst (Co1) exhibited unconventional amidation reactivity accompanying an intriguing [1,2]-relocation
of the isopropyl or benzyl group (Scheme [16]).[41] This cascade strategy represents the first example of alkyl migration reactions
mediated by Co-nitrenoid insertion into aromatic systems. When the cobalt system was
applied to the linear ethyl-substituted substrate, [1,2]-ethyl rearrangement, as well
as [1,4]-ethyl shift took place to give the regioisomeric amidation products.
Scheme 15 Origin of high regioselectivity in Cp*Co-catalyzed C–H amidation of meta-substituted phenyl azidoformates
Scheme 16 Cp*Co(III)-catalyzed amidative alkyl migration
Mulliken spin density analysis suggested that the key cationic arenium species A is indeed generated after the cobalt-nitrenoid insertion to the alkyl-substituted
ortho-carbon, considering that the spin is localized at the cobalt metal center (Scheme
[17a]). For the first time, an ‘alkyl-walking mechanism was proposed as a mechanistic
mode for the [1,4]-carbon relocation, wherein the key complex A experiences the migration in sequence by traversing TS1, TS2, and TS3 with reasonable barriers (Scheme [17b]). The mechanistic scenario suggested by the computational simulation was strongly
supported by an experimental observation of formal cyclohexyl migration (Scheme [17c]). The formal cyclohexyl relocation accompanying with [1,3]-ethyl shifts can best
be rationalized by the intervention of the alkyl-walking and subsequent spirocyclization
rearrangements,[42] thereby this finding provides a strong evidence for the novel mechanism.
Scheme 17 Mechanistic investigation on amidative alkyl migration
While nonafluoro-tert-butanol (NFTB) solvent was effective for the above-mentioned amidative alkyl migration,
the use of hexafluoro-isopropanol (HFIP) as solvent completely changed the chemical
reactivity (Scheme [18]).[43] In HFIP solvent system, subjecting 2,6-dimethyl phenyl azidoformate to the identical
Co1 catalytic conditions afforded HFIP-incorporated bisamidated endo-cyclodimers resulting from Diels–Alder dimerization. Interestingly, additional alkyl
alcohols could also be introduced into the dimeric Diels–Alder scaffold when they
were used in a co-solvent system containing NFTB. In this manner, a range of alcohols
such as methyl, allyl, or benzyl moieties could be embedded to furnish the corresponding
Meoc-, Alloc-, and Cbz-equipped dimeric products.
Scheme 18 Cp*Co(III)-nitrenoid insertion mediated alcohol-incorporating Diels–Alder dimerization
Scheme 19 Computational investigation on the solvent effects in amidative Diels–Alder dimerization
Next, DFT calculations were conducted to elucidate the underlying mechanism of Diels–Alder
dimerization with the choice of the specific alcoholic solvents (Scheme [19]). Quantum-chemical calculations indicated that the ring opening of arenium carbamate
E is efficient to generate the key ortho-quinamine scaffold F. The o-quinamine F is then considered to undergo alcohol incorporation for subsequent Diels–Alder dimerization.
The insertion of HFIP to the structure F (12.4 kcal/mol) was calculated to be energetically more feasible than the competing
alkyl migration path (15.5 kcal/mol), thus proceeding with following Diels–Alder dimerization
to give dimeric products. On the other hand, the corresponding NFTB incorporation
(19.3 kcal/mol) is energetically much more demanding, thus the alkyl migration becomes
more favored in NFTB solvent system.
Additionally, this intriguing amidative Diels–Alder dimerization reaction was coupled
with light irradiation to provoke further structural diversity in the bisamidated
endo-cyclodimer (Scheme [20]). In the presence of UV light (365 nm), a ring-fused cage compound was obtained
in high yield from [2+2] cycloaddition of the dimeric product. It should be highlighted
that this simple cobalt-based catalytic system enabled the transformation of aromatic
scaffold into a dearomatized and fully saturated cage structure loaded with bisamides.
Scheme 20 Further structural diversification
3.2
C(sp3)–N Bond Formation
As a natural extension, we wondered whether the present Co-nitrenoid catalytic system
could also be applied to C(sp3)–H amidation reactions. This premise was implemented in a model reaction with phenethyl
azidoformate under the same catalytic system with Co1 (Scheme [21]).[36] The cyclization was highly selective for the sp
3-benzylic site, and a competing C(sp2)–H amidation was not observed. The cobalt catalyst Co1 was also effective in amidation of aliphatic substrates containing tertiary, secondary,
and primary C–H bonds, providing the corresponding oxazolidinones. In terms of nitrenoid
insertion to the C–H bond α to the heteroatom, the Co1 system was much more efficient than the Ir1 system, demonstrating another interesting feature that can be achieved by altering
the central metal within the same ligand environment.
Scheme 21 Cp*Co(III)-catalyzed intramolecular C(sp3)–H amidation
Next, we turned our attention to extending the application of the Cp*Co-nitrenoid
catalytic platform from an intramolecular manner to an intermolecular version, particularly
to unactivated alkanes containing inert C–H bonds. To tackle the challenging alkane
C–H bonds, the electronic property of the (N,O)-type ligands of cobalt catalysts was
modulated by introducing a range of substituents (Scheme [22]).[44] In a model reaction of n-hexane and 2,2,2-trichloroethoxycarbonyl azide (TrocN3) as a nitrene precursor, cobalt catalysts with ligands containing electron-withdrawing
groups, especially a 5-nitro group (Co2), turned out to be optimal for catalytic intermolecular C–H amidation.
Scheme 22 Qualitative analysis of amidation yields and β-LUMO+2 energy of cobalt-nitrenoids
Frontier molecular orbital analysis of this reaction suggested that β-LUMO+2 of Co-nitrenoid
plays a crucial role in abstracting a hydrogen atom from alkanes and thus is closely
related to the catalytic amidation efficiency. In this respect, the computations provided
a corroborative trend that the stronger the electron-withdrawing property present
in the (N,O)-ligand of cobalt catalyst, the lower the β-LUMO+2 energy of the cobalt
nitrenoid, indicating that electronic perturbation at the Co-nitrenoid indeed alter
the amidation reactivity.
The proposed mechanism of the outer-sphere C–H amidation of the unactivated alkanes
is outlined in Scheme [23]. Upon TrocN3 coordination to cobalt, the resultant adduct XI undergoes nitrogen extrusion to engender the key triplet cobalt-nitrenoid XII. Subsequently, the radical-type hydrogen atom abstraction/radical rebound pathway
was suggested to operate on the basis the EPR and radical clock experimental results
along with the consistent computational results.
Scheme 23 Plausible mechanism of Cp*Co(III)-catalyzed outer-sphere C(sp3)–H amidation
With the newly optimized cobalt catalyst Co2, we evaluated the reaction efficiency and site selectivity in the intermolecular
C(sp3)–H amidation of a range of readily available unactivated hydrocarbon feedstock (Table
[1]). The cobalt catalytic system was found to be highly facile for the intermolecular
amidation of light alkanes, such as n-butane, propane, and ethane in a standard pressure reactor. More complex alkanes
bearing multiple carbon chains were also successfully amidated under the Cp*Co catalytic
system to provide isomeric mixtures of amidation products in moderate yield.
Table 1 Cp*Co(III)(κ2-LX)-Catalyzed Intermolecular C(sp3)–H Amidation of Unactivated Alkanes
 
|
|
In terms of site selectivity, Co2 catalytic system offered a high degree of secondary C–H bond selectivity in the presence
of primary or tertiary C–H bonds. Given that the C–H functionalization involved in
the radical-type hydrogen atom abstraction (HAA) mechanism tends to favor tertiary
C–H bonds over secondary C–H bonds due to lower bond-dissociation energies, the secondary
C–H bond preference can be attributed to the catalyst-mediated site selectivity, reversing
the intrinsic tertiary C–H selectivity. We rationalized that the collective two-point
steric interaction of two ligands (Cp* and N,O-ligand)[33] appears to impose difficulties accessing the sterically hindered tertiary C–H bonds.
4
Conclusion and Outlook
In this Account, we have summarized our research efforts toward utilization of Cp*Co(III)
catalysts in a broad range of C–N bond-forming reactions, featuring high reaction
efficiency and site selectivity. This cobalt catalytic system can adopt two distinct
mechanistic reaction pathways to forge C–N bonds: the inner-sphere and the outer-sphere
pathway. The inner-sphere C–H amidation of arenes was enabled via chelation-assisted
regiocontrol with the utilization of O-acylcarbamates and dioxazolones as convenient amidating sources. To enable outer-sphere
C–H amidation, we developed for the first time a Cp*Co(III)(κ2-N,O chelate) complex. This catalytic system allowed an efficient transfer of putative
cobalt-nitrenoids into a wide range of C(sp
2)–H and C(sp
3)–H bonds to produce various types of five- or six-membered cyclic carbamates.
Despite the advances of Cp*Co(III)-catalyzed C–H amidation reactions, several issues
still remain for future research directions. One of the major challenges is the successful
implementation of the Cp*Co catalytic system in the late-stage functionalization of
complex molecules, which requires the further endeavors to develop site- and/or chemoselective
C–H functionalization protocols. Next, although Cp*Co(III)-catalyzed outer-sphere
amidation have been investigated with both activated and unactivated C(sp3)–H bonds, the reactivity of these systems have been mainly limited to racemic systems.
Thus, the development of enantioselective outer-sphere C(sp3)–H amination reaction is highly desirable. The integration of Cp*Co(III) catalysts
with photocatalysis or electrocatalysis to establish novel oxidative C–H functionalization
would be also a promising research subject, especially from a sustainable perspective.
In addition, efforts should be made to employ alternative nitrene precursors in addition
to azidoformates in the outer-sphere C–N bond-forming reactions to produce more diverse
types of amino functional groups. The application of carbene precursors will also
be an interesting research focus for the efficient C–C bond formation utilizing the
current Cp*Co(III) catalytic system.
We believe that Cp*Co(III) complexes, characterized by inexpensive and abundant 3d-metal-based
catalyst with the readily tunable ligand system, will serve as an attractive catalytic
platform to achieve further exciting breakthroughs in the relevant C–H functionalization
reactions. We hope this Account will provide a concise overview of Cp*Co(III) catalytic
system for synthetic elaboration entailed to reach other methodological advances.
