8 research outputs found
N–O Bond as External Oxidant in Group 9 Cp*M(III)-Catalyzed Oxidative C–H Coupling Reactions
Group 9 Cp*MÂ(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*MÂ(III) complexes. By using Cp*RhÂ(III)-catalyzed
isocoumarin synthesis as a model reaction, experimental and theoretical
mechanistic studies were conducted. The results concluded that the
RhÂ(III)–RhÂ(I)–RhÂ(III) rather than the RhÂ(III)–RhÂ(V)–RhÂ(III)
pathway is more likely involved in the mechanism, and both the C–H
activation and oxidation of the Cp*RhÂ(I) species were involved in
the turnover-limiting step
Mild Mn(OAc)<sub>3</sub>‑Mediated Aerobic Oxidative Decarboxylative Coupling of Arylboronic Acids and Arylpropiolic Acids: Direct Access to Diaryl 1,2-Diketones
A simple and efficient
method for the synthesis of diaryl 1,2-diketones
has been developed. The reaction represents the first example of diaryl
1,2-diketones that are synthesized directly from arylboronic acids
and arylpropiolic acids by a radical pathway in moderate to good yields.
This reaction proceeds under mild reaction conditions and with good
tolerance of a variety of functional groups. Preliminary mechanistic
studies were conducted
Cp*Co(III)-Catalyzed Annulations of 2‑Alkenylphenols with CO: Mild Access to Coumarin Derivatives
Cp*CoÂ(III)-catalyzed annulations
of 2-alkenylphenols with CO for
the synthesis of coumarin derivatives have been developed. The reaction
features mild reaction conditions, broad substrate scope, and good
functional group tolerance. Preliminary mechanistic studies were conducted,
suggesting that C–H activation is the turnover limiting step.
Furthermore, the efficiency of this reaction was demonstrated by the
rapid total synthesis of three natural products herniarin, xanthyletin,
and seselin
Regioselective Synthesis of 5‑Aminooxazoles via Cp*Co(III)-Catalyzed Formal [3 + 2] Cycloaddition of <i>N</i>‑(Pivaloyloxy)amides with Ynamides
A simple
and efficient protocol for the regioselective synthesis
of 5-aminooxazoles is disclosed. The reaction, catalyzed by a cheap
Cp*CoÂ(III) catalyst, starts from easily accessible <i>N</i>-(pivaloyloxy)Âamides and ynamides. Mild reaction conditions, a broad
substrate scope, good functional group tolerance, and good to excellent
yields were observed
From Indoles to Carbazoles: Tandem Cp*Rh(III)-Catalyzed C–H Activation/Brønsted Acid-Catalyzed Cyclization Reactions
A tandem Cp*RhÂ(III)-catalyzed C–H
activation/Brønsted
acid-catalyzed intramolecular cyclization allows a facile synthesis
of carbazoles from readily available indoles. The reaction proceeds
under rather mild reaction conditions with the generation of water
and N<sub>2</sub> as the only byproducts. Broad substrate scope, excellent
functional group tolerance, and high yields were observed. The benzannulation
of pyroles for the synthesis of indoles is also feasible using the
same protocol
Three-Component Catalytic Carboxygenation of Activated Alkenes Enabled by Bimetallic Rh(III)/Cu(II) Catalysis
A novel cascade Cp*RhÂ(III)-catalyzed
C–H alkylation/CuÂ(II)-promoted
α-oxygenation which enabled a three-component carboxygenation
of activated alkene is reported. Mild reaction conditions, broad substrate
scope, and good functional group tolerance were observed. The synthetic
utility of the protocol was showcased by the facile transformations
of the product to a variety of structurally diverse molecules. Preliminary
mechanistic studies were conducted
Three-Component Catalytic Carboxygenation of Activated Alkenes Enabled by Bimetallic Rh(III)/Cu(II) Catalysis
A novel cascade Cp*RhÂ(III)-catalyzed
C–H alkylation/CuÂ(II)-promoted
α-oxygenation which enabled a three-component carboxygenation
of activated alkene is reported. Mild reaction conditions, broad substrate
scope, and good functional group tolerance were observed. The synthetic
utility of the protocol was showcased by the facile transformations
of the product to a variety of structurally diverse molecules. Preliminary
mechanistic studies were conducted
Experimental and Theoretical Studies on Rhodium-Catalyzed Coupling of Benzamides with 2,2-Difluorovinyl Tosylate: Diverse Synthesis of Fluorinated Heterocycles
Fluorinated heterocycles play an
important role in pharmaceutical
and agrochemical industries. Herein, we report on the synthesis of
four types of fluorinated heterocycles via rhodiumÂ(III)-catalyzed
Cî—¸H activation of arenes/alkenes and versatile coupling with
2,2-difluorovinyl tosylate. With <i>N</i>-OMe benzamide
being a directing group (DG), the reaction delivered a monofluorinated
alkene with the retention of the tosylate functionality. Subsequent
one-pot acid treatment allowed the efficient synthesis of 4-fluoroisoquinolin-1Â(2<i>H</i>)-ones and 5-fluoropyridin-2Â(1<i>H</i>)-ones.
When <i>N</i>î—¸OPiv benzamides were used, however,
[4 + 2] cyclization occurred to provide <i>gem</i>-difluorinated dihydroisoquinolin-1Â(2<i>H</i>)-ones. Synthetic applications have been demonstrated and
the ready availability of both the arene and the coupling partner
highlighted the synthetic potentials of these protocols. Mechanistically,
these two processes share a common process involving Nî—¸H deprotonation,
Cî—¸H activation, and olefin insertion to form a 7-membered rhodacycle.
Thereafter, different reaction pathways featuring β-F elimination
and Cî—¸N bond formation are followed on the basis of density
functional theory (DFT) studies. These two pathways are DG-dependent
and led to the open chain and cyclization products, respectively.
The mechanistic rationale was supported by detailed DFT studies. In
particular, the origins of the intriguing selectivity in the competing
β-F elimination versus CN bond formation were elucidated.
It was found that β-F elimination is a facile event and proceeds
via a <i>syn</i>-coplanar transition state with a low energy
barrier. The Cî—¸N bond formation proceeds via a facile migratory
insertion of the Rhî—¸CÂ(alkyl) into the RhÂ(V) amido species.
In both reactions, the migratory insertion of the alkene is turnover-limiting,
which stays in good agreement with the experimental studies