3.6 Rh-Catalyzed C–H Bond Annulation
71
Fig. 3.67 Free energy profiles for Rh(III)-catalyzed phosphoryl-directed C–H bond activation and annulation of alkynes with arylphosphonic acid monoesters. The values are the relative energies given in kcal/mol calculated at the B3-LYP-D3/6-311++G(d,p)/SDD//B3-LYP/631G(d)/LANL2DZ level of theory in 2-methyl-2-propanol
hydrogen transfer from phosphonic acid to acetate occurs via transition state 3-353ts
to generate the O–Rh bond intermediate 3-354. The covalent directed C–H bond
cleavage proceeds via a CMD-type transition state 3-355ts to form the five-membered
rhodacycle 3-356 with a free energy barrier of 20.9 kcal/mol. The following migratory insertion of the coordinated alkyne into the C(aryl)-Rh bond takes place via
3-359ts with a barrier of 17.5 kcal/mol to form extended rhodacycle 3-360. The
C(alkenyl)-O reductive elimination would give the annulation product coordinated
Rh(I) complex 3-362. The Rh(I) would be oxidized by Ag 2 CO 3 to Rh(III) to complete
the catalytic cycle. The C(alkenyl)-O reductive elimination was considered to be the
rate-determining step in the catalytic cycle with an overall activation free energy of
30.7 kcal/mol.
3.6.3 Directing Group as Internal Oxidants
In lots of experimental cases, Cp*Rh(III) can be used as competent catalyst for the C–
H bond activation under oxidative conditions, which allows easily accessible starting
materials [160–162]. The strategy typically makes use of the directing group to realize
cyclometalation at the C–H bond. Then, the insertion of unsaturated bonds can be
followed by reductive elimination to yield the desired heterocycle. The Cp*Rh(III)catalyzed oxidative coupling of arene C–H bonds with olefins, alkynes, or aldehydes
generally require external oxidants like copper or silver salts to regenerate the catalyst
and complete the catalytic cycle [60, 65, 163, 164]. When directing groups can act
as internal oxidants, external oxidant was unnecessary to turn over the high-valence
Rh catalyst.
71
Fig. 3.67 Free energy profiles for Rh(III)-catalyzed phosphoryl-directed C–H bond activation and annulation of alkynes with arylphosphonic acid monoesters. The values are the relative energies given in kcal/mol calculated at the B3-LYP-D3/6-311++G(d,p)/SDD//B3-LYP/631G(d)/LANL2DZ level of theory in 2-methyl-2-propanol
hydrogen transfer from phosphonic acid to acetate occurs via transition state 3-353ts
to generate the O–Rh bond intermediate 3-354. The covalent directed C–H bond
cleavage proceeds via a CMD-type transition state 3-355ts to form the five-membered
rhodacycle 3-356 with a free energy barrier of 20.9 kcal/mol. The following migratory insertion of the coordinated alkyne into the C(aryl)-Rh bond takes place via
3-359ts with a barrier of 17.5 kcal/mol to form extended rhodacycle 3-360. The
C(alkenyl)-O reductive elimination would give the annulation product coordinated
Rh(I) complex 3-362. The Rh(I) would be oxidized by Ag 2 CO 3 to Rh(III) to complete
the catalytic cycle. The C(alkenyl)-O reductive elimination was considered to be the
rate-determining step in the catalytic cycle with an overall activation free energy of
30.7 kcal/mol.
3.6.3 Directing Group as Internal Oxidants
In lots of experimental cases, Cp*Rh(III) can be used as competent catalyst for the C–
H bond activation under oxidative conditions, which allows easily accessible starting
materials [160–162]. The strategy typically makes use of the directing group to realize
cyclometalation at the C–H bond. Then, the insertion of unsaturated bonds can be
followed by reductive elimination to yield the desired heterocycle. The Cp*Rh(III)catalyzed oxidative coupling of arene C–H bonds with olefins, alkynes, or aldehydes
generally require external oxidants like copper or silver salts to regenerate the catalyst
and complete the catalytic cycle [60, 65, 163, 164]. When directing groups can act
as internal oxidants, external oxidant was unnecessary to turn over the high-valence
Rh catalyst.