62
3 Theoretical Study of Rh-Catalyzed …
Scheme 3.53 The
classification of directing
groups in Rh-catalyzed C–H
bond annulations
DGs
Electrophile
Nuclephile
Internal
oxidant
because it inherently enables the efficient construction of organic building blocks
to prepare a wide range of functional compounds [2, 17, 129–131]. However, the
directing group itself in most such reactions remains intact during the reaction and
would be removed after the C–H functionalization process, which leads to the lower
atom and step economy. One possible way to circumvent this drawback is developing
certain effective C–H annulation processes, in which the directing group could be
utilized as a reactive moiety to be incorporated into the desired product [132–135].
Through this strategy, Rh-catalyzed C–H bond activation and subsequent cyclization
with alkynes and alkenes have evolved into one of the most powerful methods in the
construction of various heterocycles [60, 136–139]. Mechanistically, directing group
plays an important role, which can be utilized as a directing group to participate in
the following transformation in Rh-catalyzed C–H bond activation and annulation
reactions (Scheme 3.53).
3.6.1 Directing Group as Electrophile
The unsaturated groups often can play as π-acid ligand, which would be employed as
directing group in Rh-mediated C–H activation [20, 140–143]. After C–H activation,
the unsaturated bond can undergo a nucleophilic attack to further functionalize in
annulations. In this type of reaction, both redox and non-redox mechanisms are
reasonable depending on the C–H activation step. As shown in Scheme 3.54a, the
redox mechanism involves oxidative addition of C(aryl)-H bond onto Rh(I) species,
migratory insertion of the unsaturated directing group, and C(aryl)-C(alkyl) reductive
elimination. In redox-involved mechanism (Scheme 3.54b), the Rh(III)-catalyzed C–
H bond activation occurs via a CMD type mechanism. The following olefin insertion,
unsaturated directing group insertion, and protonation would give the annulation
product.
3 Theoretical Study of Rh-Catalyzed …
Scheme 3.53 The
classification of directing
groups in Rh-catalyzed C–H
bond annulations
DGs
Electrophile
Nuclephile
Internal
oxidant
because it inherently enables the efficient construction of organic building blocks
to prepare a wide range of functional compounds [2, 17, 129–131]. However, the
directing group itself in most such reactions remains intact during the reaction and
would be removed after the C–H functionalization process, which leads to the lower
atom and step economy. One possible way to circumvent this drawback is developing
certain effective C–H annulation processes, in which the directing group could be
utilized as a reactive moiety to be incorporated into the desired product [132–135].
Through this strategy, Rh-catalyzed C–H bond activation and subsequent cyclization
with alkynes and alkenes have evolved into one of the most powerful methods in the
construction of various heterocycles [60, 136–139]. Mechanistically, directing group
plays an important role, which can be utilized as a directing group to participate in
the following transformation in Rh-catalyzed C–H bond activation and annulation
reactions (Scheme 3.53).
3.6.1 Directing Group as Electrophile
The unsaturated groups often can play as π-acid ligand, which would be employed as
directing group in Rh-mediated C–H activation [20, 140–143]. After C–H activation,
the unsaturated bond can undergo a nucleophilic attack to further functionalize in
annulations. In this type of reaction, both redox and non-redox mechanisms are
reasonable depending on the C–H activation step. As shown in Scheme 3.54a, the
redox mechanism involves oxidative addition of C(aryl)-H bond onto Rh(I) species,
migratory insertion of the unsaturated directing group, and C(aryl)-C(alkyl) reductive
elimination. In redox-involved mechanism (Scheme 3.54b), the Rh(III)-catalyzed C–
H bond activation occurs via a CMD type mechanism. The following olefin insertion,
unsaturated directing group insertion, and protonation would give the annulation
product.