158
Y. Soltani and F.-G. Fontaine
4.8 Frustrated Lewis Radicals
Boron containing radicals have been of synthetic utility in C–H functionalization
protocols for several decades. Notably, the BEt 3 /O 2 mixture [92] which generates a
[BEt 3
· ]
– radical, has been used in industry as a cheap and non-toxic radical initiator.
It is therefore not surprising that recent advancement in the field of FLP chemistry
has included radicals generated through FLP cooperation.
In 2013, Stephan and co-workers reported that the combination of tBu 3 P,
Al(C 6 F 5 ) 3 and N 2 O can lead to the formation of a radical pair, which activated
the C–H bond of one of the tBu groups of the phosphine and generated ion pair 28
(Scheme 4.51) [93]. The same report demonstrated that tris(aryl)phosphine reacted
in the same fashion to generate radicals that can activate the C–H bond of bromobenzene and toluene (Scheme 4.52). This single-electron transfer in FLP transformation
has been shown to be a common reactivity pattern, especially with trisarylphosphines
[94].
Recently, Melen and co-workers reported a strategy using stoichiometric amounts
of radical FLPs to promote Heck-type coupling reactions between esters and styrenes
[95] (Scheme 4.53). The mechanism proposed for this reactivity (Fig. 4.16) is similar
to other radical Heck-type reactions promoted by transition metals [96]. The radical
anion [
· B(C 6 F 5 ) 3 ]
– interacts with the ester to generate the carboxylate borate anion
and an organoradical, which can react with styrene to generate the benzyl radical.
The phosphorous base radical can abstract H
. , which will lead to the corresponding
Heck product. In these transformations, the E-isomer is favourably generated.
P
N N
O Al(C 6 F 5 ) 3
tBu 3
N 2 O
Al(C 6 F 5 ) 3
tBu 3 P
P
tBu tBu
Me
Me
HO
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
P
N N
O Al(C 6 F 5 ) 3
tBu 3
Al(C 6 F 5 ) 3
- N 2
tBu 3 P
O
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
C-H
activation
28
Scheme 4.51 C–H activation of tBu using frustrated Lewis radicals
Y. Soltani and F.-G. Fontaine
4.8 Frustrated Lewis Radicals
Boron containing radicals have been of synthetic utility in C–H functionalization
protocols for several decades. Notably, the BEt 3 /O 2 mixture [92] which generates a
[BEt 3
· ]
– radical, has been used in industry as a cheap and non-toxic radical initiator.
It is therefore not surprising that recent advancement in the field of FLP chemistry
has included radicals generated through FLP cooperation.
In 2013, Stephan and co-workers reported that the combination of tBu 3 P,
Al(C 6 F 5 ) 3 and N 2 O can lead to the formation of a radical pair, which activated
the C–H bond of one of the tBu groups of the phosphine and generated ion pair 28
(Scheme 4.51) [93]. The same report demonstrated that tris(aryl)phosphine reacted
in the same fashion to generate radicals that can activate the C–H bond of bromobenzene and toluene (Scheme 4.52). This single-electron transfer in FLP transformation
has been shown to be a common reactivity pattern, especially with trisarylphosphines
[94].
Recently, Melen and co-workers reported a strategy using stoichiometric amounts
of radical FLPs to promote Heck-type coupling reactions between esters and styrenes
[95] (Scheme 4.53). The mechanism proposed for this reactivity (Fig. 4.16) is similar
to other radical Heck-type reactions promoted by transition metals [96]. The radical
anion [
· B(C 6 F 5 ) 3 ]
– interacts with the ester to generate the carboxylate borate anion
and an organoradical, which can react with styrene to generate the benzyl radical.
The phosphorous base radical can abstract H
. , which will lead to the corresponding
Heck product. In these transformations, the E-isomer is favourably generated.
P
N N
O Al(C 6 F 5 ) 3
tBu 3
N 2 O
Al(C 6 F 5 ) 3
tBu 3 P
P
tBu tBu
Me
Me
HO
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
P
N N
O Al(C 6 F 5 ) 3
tBu 3
Al(C 6 F 5 ) 3
- N 2
tBu 3 P
O
Al(C 6 F 5 ) 3
Al(C 6 F 5 ) 3
C-H
activation
28
Scheme 4.51 C–H activation of tBu using frustrated Lewis radicals