4 FLP-Mediated C–H-Activation
141
The rigid framework reduces the entropy cost associated with the activation of
methane. It was calculated that the activation of methane by these FLPs is thermoneutral, suggesting that such process would be in equilibrium. However, the experimental demonstration would be challenging, since these FLPs represent a significant
synthetic challenge.
Further insight on the possibility of C–H activation by FLPs was
given by Erker and co-workers when they reported zwitterionic species
(Mes) 2 (H)P
+ CH 2 CH 2 B
– (C 6 F 5 ) 2 (Me), which is the formal product of C–H activation of methane by the corresponding FLP[76]. The isolation of this species
supports the idea that the activation product should be possible, but unfortunately the
FLP (Mes) 2 PCH 2 CH 2 B(C 6 F 5 ) 2 (6) did not react with methane. They demonstrated
computationally that the formation of the activation product is endergonic, but that
the transition state for the liberation of methane is 54.0 kcal‧mol
–1 . Based on the
concept of micro reversibility, the corresponding C–H activation product is out of
reach for this specific FLP.
In order to get better insight on the possibility of FLPs to activate the C sp3 –H
group, Fontaine and co-workers examined the reactivity of aminoboranes having an
intramolecular C sp3 –H bond positioned close to the Lewis pair. Looking computationally at the (underlined) C–H bond activation energy of the R group in 1-Me 2 N2-B(H)(R)-C 6 H 4 (R = -CH 2 CH 2 CH 2 CH 3 (X-Bu), o-C 6 H 4 CH 2 CH 3 (X-ArEt), oC 6 H 4 OCH 3 (X-ArOMe), o-C 6 H 4 N(CH 3 ) 2 ), it was found that the G
‡ is high for
an aliphatic chain (33.7 kcal‧mol
–1 ), but relatively accessible for activated species,
especially for the dimethylamine analogue that has a C sp3 –H activation barrier of
25.5 kcal‧mol
–1 . The synthesis of HB(C 6 H 4 -NMe 2 ) 2 was carried out, although it
required the stabilization by a Lewis base (PPh 3 , pyridine) in order to prevent degradation. Upon mild heating, these species cyclized to what is formally the product of
C sp3 –H activation after the release of H 2 , as illustrated in Fig. 4.8 [77]. These results
contrast with the pyrolysis of tris-n-octylborane, which upon heating at 250–350 °C
generates bicycloorganoboranes, presumably via H 2 elimination in a four-membered
transition state similar to TS12 in Fig. 4.8 [78, 79]. Indeed, such a transition state
for the formation of the cyclized product is significantly higher in energy (30.0 kcal
‧mol
–1 ). This latter process supports the idea that the release of H 2 is often a driving
force in the C–H activation products by FLPs.
4.6 Catalytic C–H Borylation by FLPs
Inspired by the work of Repo and co-workers on the FLP cis-hydrogenation of
alkynes[74] and the Pd concerted C–H functionalization systems, first described by
Ryabov and later exploited by Fagnou [16, 19, 20], Fontaine and co-workers investigated the ability of FLPs to functionalize C–H bonds. In a pioneering study, they
used [1-TMP-2-BH 2 -C 6 H 4 ] 2 (16), reported the same year to activate H 2 , [74] for the
C–H activation of heteroarenes [80]. The reaction was possible in the presence of
141
The rigid framework reduces the entropy cost associated with the activation of
methane. It was calculated that the activation of methane by these FLPs is thermoneutral, suggesting that such process would be in equilibrium. However, the experimental demonstration would be challenging, since these FLPs represent a significant
synthetic challenge.
Further insight on the possibility of C–H activation by FLPs was
given by Erker and co-workers when they reported zwitterionic species
(Mes) 2 (H)P
+ CH 2 CH 2 B
– (C 6 F 5 ) 2 (Me), which is the formal product of C–H activation of methane by the corresponding FLP[76]. The isolation of this species
supports the idea that the activation product should be possible, but unfortunately the
FLP (Mes) 2 PCH 2 CH 2 B(C 6 F 5 ) 2 (6) did not react with methane. They demonstrated
computationally that the formation of the activation product is endergonic, but that
the transition state for the liberation of methane is 54.0 kcal‧mol
–1 . Based on the
concept of micro reversibility, the corresponding C–H activation product is out of
reach for this specific FLP.
In order to get better insight on the possibility of FLPs to activate the C sp3 –H
group, Fontaine and co-workers examined the reactivity of aminoboranes having an
intramolecular C sp3 –H bond positioned close to the Lewis pair. Looking computationally at the (underlined) C–H bond activation energy of the R group in 1-Me 2 N2-B(H)(R)-C 6 H 4 (R = -CH 2 CH 2 CH 2 CH 3 (X-Bu), o-C 6 H 4 CH 2 CH 3 (X-ArEt), oC 6 H 4 OCH 3 (X-ArOMe), o-C 6 H 4 N(CH 3 ) 2 ), it was found that the G
‡ is high for
an aliphatic chain (33.7 kcal‧mol
–1 ), but relatively accessible for activated species,
especially for the dimethylamine analogue that has a C sp3 –H activation barrier of
25.5 kcal‧mol
–1 . The synthesis of HB(C 6 H 4 -NMe 2 ) 2 was carried out, although it
required the stabilization by a Lewis base (PPh 3 , pyridine) in order to prevent degradation. Upon mild heating, these species cyclized to what is formally the product of
C sp3 –H activation after the release of H 2 , as illustrated in Fig. 4.8 [77]. These results
contrast with the pyrolysis of tris-n-octylborane, which upon heating at 250–350 °C
generates bicycloorganoboranes, presumably via H 2 elimination in a four-membered
transition state similar to TS12 in Fig. 4.8 [78, 79]. Indeed, such a transition state
for the formation of the cyclized product is significantly higher in energy (30.0 kcal
‧mol
–1 ). This latter process supports the idea that the release of H 2 is often a driving
force in the C–H activation products by FLPs.
4.6 Catalytic C–H Borylation by FLPs
Inspired by the work of Repo and co-workers on the FLP cis-hydrogenation of
alkynes[74] and the Pd concerted C–H functionalization systems, first described by
Ryabov and later exploited by Fagnou [16, 19, 20], Fontaine and co-workers investigated the ability of FLPs to functionalize C–H bonds. In a pioneering study, they
used [1-TMP-2-BH 2 -C 6 H 4 ] 2 (16), reported the same year to activate H 2 , [74] for the
C–H activation of heteroarenes [80]. The reaction was possible in the presence of