4 FLP-Mediated C–H-Activation
145
4.6.1 C–H Activation of Arenes with Aminoborane FLPs
Parallel to the work of Fontaine, Repo and co-workers used stoichiometric amounts
of various 2-aminophenylborane derivatives to activate the C–H bonds of arenes
(Scheme 4.38) [82]. When species 1-NMe 2 -2-B(H)(C 6 F 5 )-C 6 H 4 (17) is reacted with
various arenes (Ar–H), there is formation of 1-NMe 2 -2-B(Ar)(C 6 F 5 )-C 6 H 4 with
concomitant release of H 2 . As described in the catalytic borylation by Fontaine,
the first step consists in a FLP C–H activation of the arene to generate a zwitterionic
intermediate, which releases readily H 2 . The rate-limiting step is the C–H activation,
which was calculated using DFT to have a TS of 26.6 kcal‧mol
–1 for the addition
of benzene (Fig. 4.10; Scheme 4.39).
The conditions required to cleave the C–H bond vary in function of the arene,
showing complete conversion at –15 °C in the case of thiophene, while the reaction
NHMe 2
BH(C 6 F 5 ) 2
80 °C, 20 h
- C 6 F 5 H
NMe 2
BH(C 6 F 5 )
PhH (neat)
80 °C, 20 h
- H-H
NMe 2
BPh(C 6 F 5 )
17
Int43
18
C 6 D 6
Scheme 4.38 Activation of benzene by FLP aminoborane
Fig. 4.10 Energy profile for the stoichiometric activation of benzene by aminoborane species 17.
Energies in kcal mol –1