1.2 Using Computational Tool to Study the Mechanism …
11
1.2.2 Mechanistic Study of Rh-Catalyzed C–H
Functionalization by Theoretical Methods
Recently, the computational study of Rh-catalyzed C–H functionalization has
achieved significant accomplishments [133, 136, 155]. The advances in computational methods and computing power make theoretical calculation a practical and
powerful tool for mechanistic study. Both the detailed reaction pathway and origin of
selectivity in Rh-catalyzed C–H functionalization can be elucidated through computational study [134, 135, 137]. For instance, Houk and Wu [139] reported density
functional theory investigations of the mechanism of the rhodium (III)-catalyzed
redox coupling reaction of N-phenoxyacetamides with alkynes. The computational
results suggest that the Rh
III -Rh
V -Rh
III mechanism is much more favorable than the
Rh
III -Rh
I -Rh
III mechanism. Natural bond orbital analysis confirms the identity of
the Rh
V intermediate in the catalytic cycle. Chang [107] and co-workers reported a
mechanistic study of the rhodium-catalyzed direct C–H amination reaction. Experimental data and DFT calculations reveal that a stepwise pathway involving a key
Rh(V)-nitrenoid species that subsequently undergoes amido insertion is favored over
a concerted C–N bond formation pathway. DFT calculations and kinetic studies also
suggest that the rate-limiting step in the C–H amination reaction is more closely
related to the formation of a Rh-nitrene intermediate than the presupposed C–H activation process. Breit [156] and co-workers reported a thorough mechanistic investigation of the rhodium-catalyzed propargylic C–H activation reaction by various
spectroscopic and spectrometric methods in combination with DFT calculations. The
experimental data and DFT results show that in contrast to the originally proposed
mechanism, the catalytic cycle involves intramolecular protonation and not oxidative insertion of rhodium into the O–H bond of the carboxylic acid. In addition,
Lan [157] and co-workers reported rhodium/copper-cocatalyzed trans-selective 1,2diheteroarylation of alkynes with azoles in cooperation with You. The calculated
results show that the catalytic cycle involves C–H bond activation, alkyne insertion,
transmetallation with aryl-Cu, formation of a second C–C bond via an unpredictable
trans-nucleophilic addition, and two single electron transfer steps.
Although the tremendous progress in theoretical studies has increased the understanding of the mechanism of rhodium-catalyzed C–H bond functionalization, these
studies are often case by case discussions. There have been few reviews of the computational advances in this area, which leads chemists to not having a systematic knowledge of the mechanism. Therefore, there is an urgent need for a new review detailing
the recent computational progress in rhodium-catalyzed C–H bond functionalization.
Here, we choose to focus on the theoretical aspects of rhodium-catalyzed C–H bond
functionalization. Our review will provide the first comprehensive and systematical
summary of the theoretical advances in rhodium-catalyzed C–H functionalization
in the past decade. In this context, arylation, alkylation, vinylation, alkynylation,
carbonylation, hydroacylation, and cyclization catalyzed by Rh are discussed. In
each part, the order of the elementary reactions, change in the oxidation state of Rh,
11
1.2.2 Mechanistic Study of Rh-Catalyzed C–H
Functionalization by Theoretical Methods
Recently, the computational study of Rh-catalyzed C–H functionalization has
achieved significant accomplishments [133, 136, 155]. The advances in computational methods and computing power make theoretical calculation a practical and
powerful tool for mechanistic study. Both the detailed reaction pathway and origin of
selectivity in Rh-catalyzed C–H functionalization can be elucidated through computational study [134, 135, 137]. For instance, Houk and Wu [139] reported density
functional theory investigations of the mechanism of the rhodium (III)-catalyzed
redox coupling reaction of N-phenoxyacetamides with alkynes. The computational
results suggest that the Rh
III -Rh
V -Rh
III mechanism is much more favorable than the
Rh
III -Rh
I -Rh
III mechanism. Natural bond orbital analysis confirms the identity of
the Rh
V intermediate in the catalytic cycle. Chang [107] and co-workers reported a
mechanistic study of the rhodium-catalyzed direct C–H amination reaction. Experimental data and DFT calculations reveal that a stepwise pathway involving a key
Rh(V)-nitrenoid species that subsequently undergoes amido insertion is favored over
a concerted C–N bond formation pathway. DFT calculations and kinetic studies also
suggest that the rate-limiting step in the C–H amination reaction is more closely
related to the formation of a Rh-nitrene intermediate than the presupposed C–H activation process. Breit [156] and co-workers reported a thorough mechanistic investigation of the rhodium-catalyzed propargylic C–H activation reaction by various
spectroscopic and spectrometric methods in combination with DFT calculations. The
experimental data and DFT results show that in contrast to the originally proposed
mechanism, the catalytic cycle involves intramolecular protonation and not oxidative insertion of rhodium into the O–H bond of the carboxylic acid. In addition,
Lan [157] and co-workers reported rhodium/copper-cocatalyzed trans-selective 1,2diheteroarylation of alkynes with azoles in cooperation with You. The calculated
results show that the catalytic cycle involves C–H bond activation, alkyne insertion,
transmetallation with aryl-Cu, formation of a second C–C bond via an unpredictable
trans-nucleophilic addition, and two single electron transfer steps.
Although the tremendous progress in theoretical studies has increased the understanding of the mechanism of rhodium-catalyzed C–H bond functionalization, these
studies are often case by case discussions. There have been few reviews of the computational advances in this area, which leads chemists to not having a systematic knowledge of the mechanism. Therefore, there is an urgent need for a new review detailing
the recent computational progress in rhodium-catalyzed C–H bond functionalization.
Here, we choose to focus on the theoretical aspects of rhodium-catalyzed C–H bond
functionalization. Our review will provide the first comprehensive and systematical
summary of the theoretical advances in rhodium-catalyzed C–H functionalization
in the past decade. In this context, arylation, alkylation, vinylation, alkynylation,
carbonylation, hydroacylation, and cyclization catalyzed by Rh are discussed. In
each part, the order of the elementary reactions, change in the oxidation state of Rh,