4.2 C–N Bond Formation
105
Similar to Rh-carbene, the Rh-nitrene also can react with a C–H bond by an
outer-sphere nitrene insertion into C–H bond to construct C–N bond. In 2017,
Itoh and co-workers developed the first strategy of Rh(III)-catalyzed intermolecular C(sp
3 )–H amination through intramolecular ligand-to-nitrene single electron
transfer. The active Rh(III) catalyst 4-55 can successfully achieve the C–H amination by employing xanthene as substrate and tosyl azide (TsN 3 ) as nitrene source.
KIE experiment (k H /k D = 5.0) indicated that the C(sp
3 )–H bond cleavage is involved
in the rate-determining step (Scheme 4.12).
The DFT calculation indicated that, in the open-shell singlet diradical state of
Rh(III)-nitrene complex 4-56 and corresponding triplet diradical state, one single
electron locates at nitrene moiety and another one delocalizes over the redox-active
tetradentate ligand. Those two diradicals are close to each other in energy but energetically more stable than the corresponding closed-shell singlet state. The calculated
free energy profile for the stepwise pathway of the C(sp
3 )–H amination of xanthene
is shown in Fig. 4.13. The intermolecular hydrogen abstraction from xanthene by the
nitrene radical in complex 4-56 takes place via transition state 4-58ts with a barrier
of 10.8 kcal/mol to give amino-Rh(III) complex 4-60 and xanthene radical 4-59 with
20.1 kcal/mol exergonic. Then a radical rebound process occurs to yield the productcoordinated Rh(III) complex 4-62 via the singlet transition state 4-61ts with a low
barrier of only 1.1 kcal/mol. The calculated results revealed that the rate-determining
step in the catalytic cycle is the C(sp
3 )–H bond activation, which is consistent with
KIE experiments.
10 mol % Rh
III L 676
10.0 eq. Boc 2 O
O
+
S
N 3
O
O
O
S
HN
O
O
Ax = H 2 N
toluene, 100 ºC, 24 h
91 % yield
t Bu
t Bu
N
O
N
NO 2
Rh
III
O
t Bu
t Bu
A
x
A
x
4-55
Scheme 4.12 Rh(III)-catalyzed intermolecular C(sp 3 )–H activation and amination reaction of
xanthene with tosyl azide
105
Similar to Rh-carbene, the Rh-nitrene also can react with a C–H bond by an
outer-sphere nitrene insertion into C–H bond to construct C–N bond. In 2017,
Itoh and co-workers developed the first strategy of Rh(III)-catalyzed intermolecular C(sp
3 )–H amination through intramolecular ligand-to-nitrene single electron
transfer. The active Rh(III) catalyst 4-55 can successfully achieve the C–H amination by employing xanthene as substrate and tosyl azide (TsN 3 ) as nitrene source.
KIE experiment (k H /k D = 5.0) indicated that the C(sp
3 )–H bond cleavage is involved
in the rate-determining step (Scheme 4.12).
The DFT calculation indicated that, in the open-shell singlet diradical state of
Rh(III)-nitrene complex 4-56 and corresponding triplet diradical state, one single
electron locates at nitrene moiety and another one delocalizes over the redox-active
tetradentate ligand. Those two diradicals are close to each other in energy but energetically more stable than the corresponding closed-shell singlet state. The calculated
free energy profile for the stepwise pathway of the C(sp
3 )–H amination of xanthene
is shown in Fig. 4.13. The intermolecular hydrogen abstraction from xanthene by the
nitrene radical in complex 4-56 takes place via transition state 4-58ts with a barrier
of 10.8 kcal/mol to give amino-Rh(III) complex 4-60 and xanthene radical 4-59 with
20.1 kcal/mol exergonic. Then a radical rebound process occurs to yield the productcoordinated Rh(III) complex 4-62 via the singlet transition state 4-61ts with a low
barrier of only 1.1 kcal/mol. The calculated results revealed that the rate-determining
step in the catalytic cycle is the C(sp
3 )–H bond activation, which is consistent with
KIE experiments.
10 mol % Rh
III L 676
10.0 eq. Boc 2 O
O
+
S
N 3
O
O
O
S
HN
O
O
Ax = H 2 N
toluene, 100 ºC, 24 h
91 % yield
t Bu
t Bu
N
O
N
NO 2
Rh
III
O
t Bu
t Bu
A
x
A
x
4-55
Scheme 4.12 Rh(III)-catalyzed intermolecular C(sp 3 )–H activation and amination reaction of
xanthene with tosyl azide