N-O Bond as External Oxidant in Group 9 Cp?M(III)-Catalyzed Oxidative C-H Coupling Reactions

Article abstract of DOI:10.1021/acscatal.7b00677

Group 9 Cp?M(III) (M = Co, Rh, Ir) complexes have been extensively investigated as catalysts in a variety of C-H activation reactions. Typically, late metal-based silver or copper salt was used (while needed) as oxidant in these catalytic systems. Herein, we report our discovery of a potentially general type of N-O bond-containing oxidants, which allowed the mild and efficient syntheses of isocoumarins, isoquinolines, isoquinolinone, and styrenes via C-H activation catalyzed by group 9 Cp?M(III) complexes. By using Cp?Rh(III)-catalyzed isocoumarin synthesis as a model reaction, experimental and theoretical mechanistic studies were conducted. The results concluded that the Rh(III)-Rh(I)-Rh(III) rather than the Rh(III)-Rh(V)-Rh(III) pathway is more likely involved in the mechanism, and both the C-H activation and oxidation of the Cp?Rh(I) species were involved in the turnover-limiting step.

Full text of DOI:10.1021/acscatal.7b00677

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Article
N-O Bond as External Oxidant in Group 9 Cp*M(III)-
Catalyzed Oxidative C-H Coupling Reactions
Xu-Ge Liu, Hui Gao, Shang-Shi Zhang, Qingjiang Li, and Honggen Wang
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017
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NꢀO Bond as External Oxidant in Group 9
Cp*M(III)ꢀCatalyzed Oxidative C−H Coupling
Reactions
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XuꢀGe Liu,†a Hui Gao,†a ShangꢀShi Zhang,a Qingjiang Li, a and Honggen Wang* a.
aSchool of Pharmaceutical Sciences, Sun Yatꢀsen University, Guangzhou, 510006, China
ABSTRACT: Group 9 Cp*M(III) (M = Co, Rh, Ir) complexes have been extensively
investigated as catalysts in a variety of C−H activation reactions. Typically, late metalꢀbased
silver or copper salt was used (while needed) as oxidant in these catalytic systems. Herein, we
report our discovery of a potentially general type of NꢀO bondꢀcontaining oxidants, which
allowed the mild and efficient syntheses of isocoumarins, isoquinolines, isoquinolinone and
styrenes via CꢀH activation catalyzed by group 9 Cp*M(III) complexes. By using Cp*Rh(III)ꢀ
catalyzed isocoumarin synthesis as a model reaction, experimental and theoretical mechanistic
studies were conducted. The results concluded that the Rh(III)ꢀRh(I)ꢀRh(III) rather than Rh(III)ꢀ
Rh(V)ꢀRh(III) pathway is more likely involved in the mechanism, and both the CꢀH activation
and oxidation of the Cp*Rh(I) species were involved in the turnꢀover limiting step.
KEYWORDS : CꢀH activation· Cp*M(III)·heterocycle·NꢀO bond·oxidation
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INTRODUCTION
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The past decades have witnessed a great success on transitionꢀmetalꢀcatalyzed direct
functionalization of inert C−H bonds, which provides a streamlined synthetic strategy to access
structurally diverse molecules while avoiding lengthy preparation of preꢀactivated starting
materials and thereby, minimizing cost and waste production.1 Among the numerous C−H
activation reactions, the oxidative couplings of C−H bonds with different nucleophiles (with
regard to redoxꢀneutral C−H couplings with electrophiles) provide ample opportunities for C−C
and C−heteroatom bond formations largely due to the ready availability of diverse carbon and
heteroatomꢀcentered nucleophiles. Mechanistically, the bond polarity of these arene substrate
and the coupling reactions requires the employment of (super)stoichiometric amount of oxidant
for catalyst turnover.
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Recently, there is an increasing interest in exploring group 9 Cp*M(III) (M = Co, Rh, Ir)
complexes as catalysts in C−H activation reactions.2 Interestingly, with very few exceptions,3 a
silver or copper salt was commonly used as an oxidant in these catalytic systems.4 Given their
high costs, undesirable metal residues and potential side reactions (caused by the diverse
reactivities of Ag or Cu), synthetic utility of these reactions might be significantly compromised.
From sustainability point of view, and the demand for discovery of new catalytic reactivity, there
is an intensive need for new and greener oxidants. In this regards, it was occasionally found that
O2, alone5 or with the aid of copper6 or photoredox catalyst,7 could serve as the terminal oxidant
in several oxidative coupling reactions. An alternative while fruitful strategy is to use
multifunctional oxidizing directing group, which acts as the chelating group as well as an
internal oxidant.8 In this respect, on the basis of the seminal work by Fagnou9 and Glorius,10 a
variety of N−O,11 N−N,12 O−O,13 or N−S14 bondꢀcontaining directing groups have been designed
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and applied to effect intriguing reactions (Figure 1a). On one hand, the use of these oxidizing
directing groups renders improved reactivity, milder reaction conditions, and more selective
transformations. On the other hand, however, the installation of an oxidizing directing group is
not always trivial and may pose synthetic challenges. Mechanistic controversies exist on how the
internal oxidant oxidizes the metal catalyst (Figure 1b). More specifically, the reductive
elimination may take place from a M(III) complex followed by the oxidation (regeneration) of
the Mn(I), namely the M(III)ꢀM(I)ꢀM(III) pathway.15 Alternatively, the M(III) complex could be
oxidized to M(V) prior to the reductive elimination, namely the M(III)ꢀM(V)ꢀM(III) pathway.16
The wide application of N−O based oxidizing directing group inspired us to question the
feasibility of a N−O bondꢀbased external oxidant for group 9 Cp*M(III) catalysis. To this end,
several challenges may arise: 1) the capability of bimolecular oxidation of the lowꢀvalent metal
to its high oxidation state with a N−O bond; 2) the strong coordination property of the N−O
oxidant or its reduced residues (amine or carboxylate) may poison the catalyst, thus leading to
poor effciency; 3) the undesired C−H amination or carboxylation could be the potential side
reaction.17 Herein, we present our findings that the N−O bond could serve as a potentially
general external oxidant in group 9 Cp*M(III)ꢀcatalyzed oxidative C−H coupling reactions,18 as
demonstrated by its effectiveness in the mild synthesis of several classes of heterocycles and in
oxidative Heck reactions (Figure 1c). Experimental and theoretical studies pointed out that the
M(III)ꢀM(I)ꢀM(III) mechanistic pathway is likely operational.
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Figures 1. Group 9 Cp*M(III)ꢀcatalyzed oxidative coupling reactions.
RESULTS AND DISCUSSION
Initial Discovery. We were interested in exploration of oxidizing directing group in Cp*Ir(III)ꢀ
catalyzed C−H annulation with alkynes for the synthesis of isocoumarins,13,19 which are among
the most important structural motifs in bioactive compounds.20 Initial optimization studies
revealed that the reaction of OꢀbenzoylꢀN,Nꢀdiisopropylhydroxylamine19i (1a, 0.4 mmol) with
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1,2ꢀdiphenylethyne (2a, 0.2 mmol) took place smoothly to give isocoumarin 3a under the
catalysis of Cp*Ir(III) with benzoic acid (4a, 0.2 mmol) as an additive (eq 1). Surprisingly,
however, the switch of 1a to another substrate 1b under the otherwise identical conditions led to
the formation of desired 3b, together with relatively higher amount of 3a, which was derived
from the reaction of the benzoic acid 4a (eq 2). These results gave a strong hint that the N−O
containing 1b at least serve as an external oxidant for this transformation. This intriguing
finding, along with the numerous applications of N−O containing molecules as internal oxidant
promoted us to question whether N−O bond can be a general oxidant in group 9 Cp*M(III)
catalysis. If this would be the case, the advantage of using organic NꢀO containing external
oxidant is apparent in terms of cost, solubility, metalꢀresidue, environmental influence and ease
of product purification.
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Optimization Studies. The Cp*Ir(III)ꢀcatalyzed coupling reaction of benzoic acid (4a, 0.2
mmol) with 1,2ꢀdiphenylethyne (2a, 0.2 mmol) was chosen as a model reaction (Table 1).
Previous studies have showed that this reaction was effective while using AgOAc (2.0 equiv) as
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an oxidant at 60 C.19h To uncover a new oxidant, a series of Oꢀacyl hydroxylamines (OX-
1−OX-7) and peresters (OX-8−OX-12), containing labile NꢀO and OꢀO bond, respectively, were
prepared and evaluated. As shown in Table 1, Oꢀacyl hydroxylamines OX-3, OX-5, and OX-7
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bearing bulky isopropyl group exhibited prospective reactivity, delivering the desired product
3aa in moderate yields (entries 1ꢀ7). Peresters (OX-8−OX-12) were much less effective (entries
8ꢀ12). OX-7 was then chosen for further screening. Out of our expectation, the reaction
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temperature could be lowered to 30 C, with a good isolated yield of 70% by prolonging the
reaction time to 24 h (entry 13). The increase of concentration of OX-7 to 0.3 M (1.5 equivalents
with respect to 4a) was beneficial for the yield (entry 14), and the best yield of 95% was
obtained while doubling the loading of benzoic acid 4a (entry 15). The omission of OX-7 gave
negligible amount of product, confirming that OX-7 was the effecting oxidant (entry 17).
Notably, no C−H amination or carboxylation products were observed in these reactions.
Table 1. Reaction Optimization for Cp*Ir(III)ꢀCatalyzed Oxidative Coupling of 4a with 2a
Using NꢀO or OꢀO Containing Oxidanta
entry
1
[oxidant] (M)
temp. (oC)
80
time (h)
10
yield (%)b
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OX-1 (0.2 M)
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OX-10 (0.2 M)
OX-11 (0.2 M)
OX-12 (0.2 M)
OX-7 (0.2 M)
OX-7 (0.3 M)
OX-7 (0.3 M)
OX-7 (0.3 M)
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trace
aReaction conditions: 4a (0.2 mmol), 2a (0.2 mmol), [Cp*IrCl2]2 (5 mol%), OX, TFE (1.0 mL),
under air, temp.. bIsolated yields. c4a (0.4 mmol). d2a (0.4 mmol).
The above success encouraged us to further investigate the effectiveness of Cp*Rh(III) and
Cp*Co(III) in this reaction. Gratifyingly, preliminary screening quickly identified that OX-6 was
an optimal oxidant for the Cp*Rh(III) version of the reaction, giving 90% yield of 3aa under the
conditions of [Cp*Rh(CH3CN)3](SbF6)2 (5 mol %), OX-6 (0.3 M, 1.5 equivalent respect to 2a),
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Cs2CO3 (20 mol %), and TFE (1 mL), N2, at 60 C (eq 3). It should be mentioned although
Cu(OAc)2 and Cu(OAc)2/air have been reported to be effective oxidants for this reaction, harsh
conditions (> 120 C and/or under microwave irradiation) were employed.19a,b,e Very recently,
o
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the use of electronꢀdeficient CpERh(III) complex allowed an efficient coupling under ambient
conditions.19k The Cp*Co(III)ꢀcatalyzed oxidative coupling of benzoic acids with acetylenes was
known only very recently.19m In our hand, a maximum yield of 30% was obtained while using
Cp*Co(III) as catalyst in combination with OX-2 as oxidant.
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Scope on the Synthesis of Isocoumarins. The scope of the Cp*Ir(III) and Cp*Rh(III)ꢀcatalyzed
synthesis of isocoumarins using a novel NꢀO containing external oxidant was then investigated
and found to be quite broad (Table 2). A variety of substituted benzoic acids tested were all
competent substrates. In accordance with previous observations, electronꢀdeficient benzoic acid
exhibited lower reactivity.19 In the Cp*Ir(III)ꢀcatalyzed system, increase of the temperature to 50
oC was beneficial to challenging substrates such as electronꢀdeficient or sterically congested
benzoic acids. Heteroaryl carboxylic acid 4h was also a viable substrate, delivering valuable
indole derivatives 3ha. The paraꢀmethoxyl substituted diphenylacetylene were problematic in
previous studies.19h However, using our protocols, good yields were obtained (3ab). Not only
diarylacetylenes, but also dialkylacetylenes were compatible with the reaction conditions.
Table 2. Group 9 Cp*M(III)ꢀCatalyzed Isocoumarin Synthesis Using NꢀO as an External
Oxidanta
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aReaction conditions: Cp*Ir: 4 (0.4 mmol), 2 (0.2 mmol), [Cp*IrCl2]2 (5 mol%), OX-7 (0.3 M),
TFE (1.0 mL), 30ꢀ50 oC, air, 24ꢀ48 h. Cp*Rh: 4 (0.24 mmol), 2 (0.2 mmol),
[Cp*Rh(CH3CN)3](SbF6)2 (5 mol%), OX-6 (0.3 M), Cs2CO3(20 mol %), TFE (1.0 mL), 60 C,
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24 h, N2. Cp*Co: 4 (0.4 mmol), 2 (0.2 mmol), [Cp*Co(CH3CN)3](SbF6)2 (10 mol%), OX-2 (0.4
M), TFE (1 mL), 60 oC, 24 h, N2.
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Scope on the synthesis of isoquinolines and isoquinolinone
To further demonstrate the generality of NꢀO bond as external oxidant in group 9 Cp*M(III)
catalysis, the oxidative annulation of aryl imines with alkynes toward the synthesis of
isoquinolines has been attempted.3g,21
It was found a 37% yield of isoquinoline 6aa could be obtained under the catalysis of Cp*Ir(III)
in combination with OX-7 as oxidant (Table 3). Though still less efficient, this reaction
represents the first example of Cp*Ir(III)ꢀcatalyzed isoquinoline synthesis via CꢀH activation.
Cp*Rh(III) complexes have been used as catalysts for isoquinolines synthesis via external
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oxidant (Cu(OAc)2, 2.1 equiv, > 80 C)21a,b,d or internal oxidant strategy. In the presence of
[Cp*RhCl2]2 (5 mol %) and OX-2 (0.4 M), the reaction of diphenylmethanimine (5a, 0.5 mmol)
and 1,2ꢀdiphenylethyne (2a, 1.0 mmol) proceeded smoothly in TFE (2.5 mL) at 80 oC, affording
6aa in 89% isolated yield (Table 3). The protocol could be applied to the reaction of a variety of
substituted diphenylmethanimines and alkynes, in moderate to excellent yields.
Very recently, several groups reported the use of internal oxidant strategy for the synthesis of
multiꢀsubstituted isoquinolines via Cp*Co(III)ꢀcatalyzed [4+2] annulation of oximes and
alkynes,21kꢀn wherein the reaction temperature ranged from 80 to 120 C. We have also
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discovered that Ce(SO4)2 was an effective external oxidant in Cp*Co(III)ꢀcatalyzed CꢀH
annulation of NꢀH imines with alkynes, leading to an efficient synthesis of isoquinolines at 100
oC.3g With OX-6 as oxidant, we were pleased to find that annulation of imine 5 with alkynes
proceeded smoothly under the catalysis of Cp*Co(III) at 80 oC (Table 3).
Table 3. Group 9 Cp*M(III)ꢀCatalyzed Isoquinolines Synthesis Using NꢀO as External Oxidanta
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R1
R1
group 9 [Cp*MIII]
OX, TFE, 80 oC
N
R2
R2
+
R
NH
R
R2
7
8
9
R2
5
2
6
known procedure:
[Cp*IrIII]: no precedent
[Cp*RhCl2]2 (1 mol %), Cu(OAc)•2H2O (2 equiv), DMF, 80 oC, N2.
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ref 21b
[Cp*RhCl2]2 (4 mol %), Cu(OAc)•2H2O (2.1 equiv), THF, 85 oC, N2.
ref 21d
Cp*Co(CO)I2 (10 mol %), AgOTf (20 mol %), Ce(SO4)2 (2.0 equiv), KOAc (40 mol %), 3 Å MS,
CH3NO2, N2, 100 oC.ref 3g
N
N
N
N
OMOM
OMe
Br
OMOM
OMe
Br
6aa
6ab
6af
6ae
Ir:
37%
Rh:
Co:
81%
63%
70%
73%
51%
42%
89%
85%
OMe
N
N
N
N
MeO
6ag
6ad
6ba
6ca
30%
Rh:
Co:
78%
83%
72%
70%
75%
90%
61%
N
6ah
Rh:
Co:
40%
80%
aReaction conditions: Cp*Ir: 5 (0.2 mmol), 2 (0.4 mmol), [Cp*IrCl2]2 (5 mol%), OX-7 (0.4 M),
TFE (1.0 mL), 80 oC, 12 h, N2. Cp*Rh: 5 (0.5 mmol), 2 (1.0 mmol), [Cp*RhCl2]2 (5 mol%), OX-
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2 (0.4 M), TFE (2.5 mL), 80 C, 12 h, N2. Cp*Co: 5 (0.2 mmol), 2 (0.4 mmol),
[Cp*Co(CH3CN)3](SbF6)2 (10 mol%), OX-6 (0.4 M), TFE (1 mL), 80 oC, 24 h, N2.
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7
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9
Initial attempts demonstrated that Nꢀmethylbenzamide 7 was also a viable substrate for
cyclization. Thus, in the presence of [Cp*RhCl2]2 (5 mol %) and OX-2 (0.6 M, 3.0 equivalents
with respect to 7), the corresponding isoquinolinone 8 could be constructed in 40% yield (eq
4).22
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Oxidative Heck reaction
In addition to the oxidative annulation reactions, the external NꢀO oxidant was also examined in
Cp*Rh(III)ꢀcatalyzed oxidative Heck reaction (Table 4).5c, 23 In this case, the use of OX-1 (0.6 M,
3.0 equivalents with respect to 9) allowed the generally efficient coupling of 2ꢀphenylpyridines 9
o
with styrenes 10 at 80 C. Heterocyclic and electronꢀdeficient substrates were found to be
relatively less efficient. The use of unsubstituted 2ꢀphenylpyridine and 2ꢀphenylpyrimidine gave
a mixture of monoꢀ and difunctionalized products in high yields, indicating the high reactivity of
the catalytic system.
Table 4. Cp*Rh(III)ꢀCatalyzed Oxidative Heck Reaction Using NꢀO as External Oxidanta
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aReaction conditions: 9 (0.5 mmol), 10 (2.0 mmol), [RhCp*(CH3CN)3](SbF6)2 (5 mol %), OX-
1 (0.6 M), TFE (2.5 mL), N2, 80 oC, 24 h.
Experimental Mechanistic Analysis. The high efficiency and general behavior of the NꢀO
bondꢀcontaining oxidant in group 9 Cp*M(III) catalysis prompted us to probe the mechanism in
detail, specifically on: (1) What is the turnoverꢀlimiting step? (2) Is the break of the NꢀO bond a
facile process and how is it cleaved? (3) Is any Cp*M(V) intermediate involved in the catalytic
cycle? The answer to these questions should aid in the discovery of new reactions using this
catalytic system and may provide implications, to some extent, to the action mode of internal
oxidant.
Kinetic Isotope Effect. The Cp*Rh(III)ꢀcatalyzed isocoumarin synthesis was chosen for
mechanistic studies. Pronounced kinetic isotope effect (KIE) values and PH/PD values, regardless
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of the substitution patterns of alkyne, were determined from parallel experiments and
intermolecular competition experiments, respectively. These results supported that CꢀH bond
cleavage is involved in the turnoverꢀlimiting step.
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Rateꢀdependence on Oxidant.
The rate dependence on oxidant was conducted during the first 10% conversation of the substrate
with a wider range of oxidant concentrations (Figure 2). Interestingly, at low concentrations, the
increase of oxidant (from 0.1 M to 0.2 M, then to 0.4 M) led to similar reaction rates. However,
when the oxidant concentration was further increased, the initial reaction rate dramatically
dropped. These results might suggest that: 1) the oxidation of the metal species is involved, at
least partially, in the turnoverꢀlimiting step; and 2) an inhibitory effect of the oxidant at a higher
concentration probably due to the competing ligation of the oxidant or its decomposed residues
(carboxylates or amines) to the metal.
6.0E-06
5.0E-06
4.0E-06
3.0E-06
2.0E-06
1.0E-06
0.0E+00
0
0.2
0.4
0.6
0.8
1
1.2
1.4
[OX-6] (M)
Figure 2. Reaction profiles of Cp*Rh(III)ꢀcatalyzed isocoumarin synthesis with different oxidant
concentrations
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Computational studies
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To further cast light on details of the mechanism, theoretical calculations were performed by at
the density functional theory level (M06).24 The Cp*Rh(III)ꢀcatalyzed oxidative coupling of
benzoic acids (4a) with 2ꢀbutyne was chosen as a model reaction for this study. The proposed
mechanism was broken into 3 key steps: CꢀH activation, alkyne insertion, reductive elimination
and catalyst regeneration.
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CꢀH activation and alkyne insertion. Figure 3 shows the computational results for the formation
of the sevenꢀmembered rhodacycle intermediate INT-6. Complex Cp*Rh(HCO3)2 was set to a
relative zero value in the whole free energy profiles. The dissociative ligand exchange between
Cp*Rh(HCO3)2 and benzoic acid affords INT-2. Subsequently, the CMD type CꢀH activation
takes place via TS-1 associated with an overall activation free energy of 25.2 kcal/mol, leading
to the formation of INT-3. Thereafter, substitution of the ligated H2CO3 with 2ꢀbutyne gives
alkyne complex INT-5 which is exergonic by 9.2 kcal/mol. The alkyne insertion occurs via
transition state TS-2 with an activation energy of 15.3 kcal/mol to produce the sevenꢀmembered
rhodacyclic intermediate INT-6.
Figure 3. Computed pathways for CꢀH activation and alkyne insertion
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Reductive elimination and catalyst regeneration. As mentioned before, two mechanistic
possibilities exist for the formation of the final product and regeneration of catalyst, namely
M(III)ꢀM(I)ꢀM(III) pathway15 and M(III)ꢀM(V)ꢀM(III)16 pathway. To better understand the
mechanism, especially the working model of the external NꢀO oxidant, both possible pathways
have been evaluated by DFT calculations.
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The Rh(III)−Rh(I)−Rh(III) pathway. As shown in Figure 4, the direct reductive elimination from
intermediate INT-6 gives a product ligated Rh(I) intermediate INT-7 (red lines). The barrier for
this step is found to be 22.4 kcal/mol. The release of the product is followed by the association of
NꢀO oxidant to form INT-8, which is endergonic by 22.6 kcal/mol. The oxidative addition of
Rh(I) to the NꢀO bond proceeds via transition state TS-4, which is accompanied by an overall
activation energy of 24.5 kcal/mol (0.7 kcal/mol lower than activation energy of CꢀH
activation).25 The resulting Rh(III) complex then undergoes ligand exchange to regenerate the
active Rh(III) catalyst.
The Rh(III)−Rh(V)−Rh(III) pathway. The direct oxidation of the Rh(III) complex INT-6 with Nꢀ
O oxidant via transition state TS-5 is found to be highly endergonic by 36.8 kcal/mol (Figure 4,
blue lines). Reductive elimination from the thus formed Rh(V) intermediate (via TSꢀ6) delivers
the final product and INT-9. This step requires an overall energy of 41.6 kcal/mol.
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41.6
TS-6
MeO
Gsolv
kcal/mol
MeO
N
O
O
N
*Cp
O
Rh
*Cp
33.6
O
Rh
MeO
O
5
6
MeO
O
TS-5
O
O
28.9
*Cp
Rh
INT-11
O
O
MeO
7
8
9
O
N
19.2
*Cp
RhV
Cp*
Rh
OMe
O
TS-3
Cp*
Rh
OMe
OMe
MeO
O
O
OMe O
13.1
INT-10
MeO
O
O
O
N
O
N
O
N
OMe
O
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OMe
O
4.9
O
N
3.0
INT-8
TS-4
OMe O
OMe
*Cp
O
Rh
N
O
O
-3.2
O
O
INT-6
OMe
O
Cp*
Rh
O
-19.6
Cp*
RhI
O
INT-7
2 H2CO3 -53.5
INT-9
Cp*
Rh
-53.5
HOAc
Cp*
Rh
Et2NH
-68.0
Cat.
OMe
INT-9
O
O
O
OMe
O
N
-68.0
Cat.
O
2 H2CO3
N
HOAc
Et2NH
O
OMe
OMe
Rh(III)-Rh(I)-Rh(III) Pathway
Rh(III)-Rh(V)-Rh(III) Pathway
Figure 4. Reductive elimination and catalyst regeneration. Red lines: Rh(III)−Rh(I)−Rh(III)
pathway; Blue lines: Rh(III)−Rh(V)−Rh(III) pathway.
Taken together, the above results suggest the Rh(III)−Rh(I)−Rh(III) pathway is more likely to be
operative in our external oxidative coupling reactions. The mechanistic scenario is in good
agreement with the observations of a pronounced kinetic isotope effect and similar reaction rates
at low concentrations, indicating that both the CꢀH activation and oxidation of Cp*Rh(I) species
were involved in the turnꢀover limiting step.
CONCLUSIONS
In summary, a new type of NꢀO bondꢀcontaining oxidants were identified as efficient and
potentially general oxidants in group 9 Cp*M(III)ꢀcatalyzed oxidative CꢀH coupling reactions.
The use of these oxidants, in lieu of the commonly used silver or copper salts, allowed for the
mild and efficient syntheses of isocoumarins, isoquinolines, isoquinolinone, and styrenes with
different Cp*M(III) catalysts. Experimental and theoretical mechanistic studies using
Cp*Rh(III)ꢀcatalyzed isocoumarin syntheses as a model reaction were conducted. The results
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suggested the Rh(III)ꢀRh(I)ꢀRh(III) pathway is more likely involved in the mechanism. Given
the great power of group 9 Cp*M(III) complexes in catalysis, we anticipate that our findings
should aid in the discovery of new and more efficient catalytic reactions.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
Experimental procedure and compound characterization (PDF)
AUTHOR INFORMATION
Corresponding Author
* Email: wanghg3@mail.sysu.edu.cn.
Author Contributions
†
These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support by the National Natural Science Foundation of China (Nos. 21472250,
81402794 and 21502242), the Key Project of Chinese National Programs for Fundamental
Research and Development (2016YFA0602900), the "1000ꢀYouth Talents Plan", and the Startꢀ
up Grant from Sun Yatꢀsen University is gratefully acknowledged.
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The coupling reaction of 4a with 2a with a Rh(I) catalyst [RhCp*(C6Me6)] in the presence of
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this reaction was much slower than that with Cp*Rh(III) catalyst. For a full discussion, see SI.
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