52
3 Theoretical Study of Rh-Catalyzed …
process of formal alkyne insertion into an aryl C–H bond. The non-redox mechanism
can be summarized in Scheme 3.38a, where Rh(III)-catalyzed C–H bond activation
starts with a CMD type C(aryl)–H bond cleavage. The following alkyne insertion and
protonation would result in the final alkenylation product. In an alternative redoxinvolved mechanism (Scheme 3.38b), the oxidative addition of C(aryl)–H bond onto
Rh(I) species provide an aryl hydride-Rh(III) intermediate. After acetylene insertion
into Rh–H bond, C(aryl)-C(vinyl) reductive elimination also could yield alkenylation
product and revive Rh(I) species.
In 2014, Wang and co-workers [96] reported a Rh(III)-catalyzed alkenylation
reactions of 8-methylquinolines with alkynes undergoing a C(sp
3 )–H bond activation.
The KIE was found to be k H /k D = 4.0 indicating that the C–H bond cleavage was
involved in the rate-determining step. The deuterium-labeling experiments revealed
that the C(sp
3 )–H bond activation is a reversible process. The deuterium-labeling
experiments also indicated that protonolysis may be involved in the catalytic cycle
(Scheme 3.39).
In 2015, Morokuma and co-workers [97] performed a detailed theoretical study to
investigate the mechanism of the above reaction. As shown in Fig. 3.40, the coordination of 8-methylquinoline in complex 3-196 leads to the activation of benzylic C–H
bond, which can form a hydrogen bond with the coming copper acetate complex.
The C–H activation undergoes intermolecular deprotonation with copper acetate
complex as a base via transition state 3-197ts with an overall activation free energy
of 26.4 kcal/mol to give alkyl-Rh(III) intermediate 3-199. The following acetylene
Scheme 3.38 General
mechanism for the
Rh-catalyzed C–H bond
alkenylation by using
alkynes
Protonation
(a)
(b)
3-190
[Rh(III)]Y 2
3-187
Ar H
DG
HY
C-H bond
cleavage
Ar
DG
Rh(III)]Y
3-188
R
DG
Rh(III)]Y
Ar
R
3-189
Alkyne
insertion
DG
Ar
R
HY
Reductive
elimination
3-194
[Rh(I)]
3-191
Ar H
C-H bond
cleavage
Ar Rh(III)]
3-192
R
3-193
Alkyne
insertion
Ar
R
Ar Rh(III)]
H
R
3 Theoretical Study of Rh-Catalyzed …
process of formal alkyne insertion into an aryl C–H bond. The non-redox mechanism
can be summarized in Scheme 3.38a, where Rh(III)-catalyzed C–H bond activation
starts with a CMD type C(aryl)–H bond cleavage. The following alkyne insertion and
protonation would result in the final alkenylation product. In an alternative redoxinvolved mechanism (Scheme 3.38b), the oxidative addition of C(aryl)–H bond onto
Rh(I) species provide an aryl hydride-Rh(III) intermediate. After acetylene insertion
into Rh–H bond, C(aryl)-C(vinyl) reductive elimination also could yield alkenylation
product and revive Rh(I) species.
In 2014, Wang and co-workers [96] reported a Rh(III)-catalyzed alkenylation
reactions of 8-methylquinolines with alkynes undergoing a C(sp
3 )–H bond activation.
The KIE was found to be k H /k D = 4.0 indicating that the C–H bond cleavage was
involved in the rate-determining step. The deuterium-labeling experiments revealed
that the C(sp
3 )–H bond activation is a reversible process. The deuterium-labeling
experiments also indicated that protonolysis may be involved in the catalytic cycle
(Scheme 3.39).
In 2015, Morokuma and co-workers [97] performed a detailed theoretical study to
investigate the mechanism of the above reaction. As shown in Fig. 3.40, the coordination of 8-methylquinoline in complex 3-196 leads to the activation of benzylic C–H
bond, which can form a hydrogen bond with the coming copper acetate complex.
The C–H activation undergoes intermolecular deprotonation with copper acetate
complex as a base via transition state 3-197ts with an overall activation free energy
of 26.4 kcal/mol to give alkyl-Rh(III) intermediate 3-199. The following acetylene
Scheme 3.38 General
mechanism for the
Rh-catalyzed C–H bond
alkenylation by using
alkynes
Protonation
(a)
(b)
3-190
[Rh(III)]Y 2
3-187
Ar H
DG
HY
C-H bond
cleavage
Ar
DG
Rh(III)]Y
3-188
R
DG
Rh(III)]Y
Ar
R
3-189
Alkyne
insertion
DG
Ar
R
HY
Reductive
elimination
3-194
[Rh(I)]
3-191
Ar H
C-H bond
cleavage
Ar Rh(III)]
3-192
R
3-193
Alkyne
insertion
Ar
R
Ar Rh(III)]
H
R