another equivalent of NaOCH 3 and produces tetradentate 84 by intramolecular
coordination to a deprotonated hydroxyl function stabilized by a weak hydrogen
bond between the hydroxypyridine and pyridonate arms (Scheme 35).
As was demonstrated in previous studies on bifunctional complexes possessing
appended hydroxyl groups, an interplay between alkoxide and alkoxyl coordination
can facilitate bond activation/formation processes [99–101]. Therefore, the catalytic
potential of the coordination switching to 84 was evaluated by exposing it to
H-BPin, which resulted in hydrogen evolution and the formation of a new compound
(85) bearing two pinacolyl boronate groups (Scheme 36) [98]. The solid-state and
NMR data indicate the presence of a weak B-H interaction, suggesting that 85 may
represent a plausible intermediate state of B-H heterolysis and Lewis acidity of the
boron centers.
Based on this observation, compound 84 was subjected to catalytic nitrile
hydroboration with HBPin, and it was found to be a prominent catalyst after being
activated by deprotonation. The active species 86 exhibited full conversion of a wide
range of benzonitriles into the corresponding benzylamines with TOF as high as
1.19 Â 10
À2 s
À1 (Scheme 37).
The plausible mechanism for the nitrile hydroboration reaction shown in Scheme
38 suggests that the real catalytic species is the bifunctional pincer complex 87
possessing a Lewis acidic secondary coordination sphere [98]. The catalytic cycle
starts with the activation of the B-H bond that takes place in the reaction of 86 with
HBPin with the loss of phosphine (87), as described in Scheme 38. The H and BPin
transfer across the nitrile substrate proceeds following outer-sphere activation by the
pendant Lewis acid groups (89 and 91).
After demonstrating the critical role of the in situ-formed secondary coordination
in cooperative substrate activation, a structurally well-defined catalyst was designed
Scheme 35 Coordination chemistry of the ligand 81
N
N
N
N
N
Ru
O
O
PPh 3
PPh 3
H
84
N
N
N
N
N
Ru
O
O
B
PPh 3
H
O
O
B
O
O
85
HBPin
-H 2 , -PPh 3
Scheme 36 Plausible intermediate state of B-H heterolysis by 84
Cooperative Reactivity by Pincer-Type Complexes Possessing Secondary. . .
117
coordination to a deprotonated hydroxyl function stabilized by a weak hydrogen
bond between the hydroxypyridine and pyridonate arms (Scheme 35).
As was demonstrated in previous studies on bifunctional complexes possessing
appended hydroxyl groups, an interplay between alkoxide and alkoxyl coordination
can facilitate bond activation/formation processes [99–101]. Therefore, the catalytic
potential of the coordination switching to 84 was evaluated by exposing it to
H-BPin, which resulted in hydrogen evolution and the formation of a new compound
(85) bearing two pinacolyl boronate groups (Scheme 36) [98]. The solid-state and
NMR data indicate the presence of a weak B-H interaction, suggesting that 85 may
represent a plausible intermediate state of B-H heterolysis and Lewis acidity of the
boron centers.
Based on this observation, compound 84 was subjected to catalytic nitrile
hydroboration with HBPin, and it was found to be a prominent catalyst after being
activated by deprotonation. The active species 86 exhibited full conversion of a wide
range of benzonitriles into the corresponding benzylamines with TOF as high as
1.19 Â 10
À2 s
À1 (Scheme 37).
The plausible mechanism for the nitrile hydroboration reaction shown in Scheme
38 suggests that the real catalytic species is the bifunctional pincer complex 87
possessing a Lewis acidic secondary coordination sphere [98]. The catalytic cycle
starts with the activation of the B-H bond that takes place in the reaction of 86 with
HBPin with the loss of phosphine (87), as described in Scheme 38. The H and BPin
transfer across the nitrile substrate proceeds following outer-sphere activation by the
pendant Lewis acid groups (89 and 91).
After demonstrating the critical role of the in situ-formed secondary coordination
in cooperative substrate activation, a structurally well-defined catalyst was designed
Scheme 35 Coordination chemistry of the ligand 81
N
N
N
N
N
Ru
O
O
PPh 3
PPh 3
H
84
N
N
N
N
N
Ru
O
O
B
PPh 3
H
O
O
B
O
O
85
HBPin
-H 2 , -PPh 3
Scheme 36 Plausible intermediate state of B-H heterolysis by 84
Cooperative Reactivity by Pincer-Type Complexes Possessing Secondary. . .
117