382
F. Holtrop et al.
10.5 Concluding Remarks
This chapter has analysed the different occurrences of radical formation within FLP
systems that are being increasingly reported in the literature and provided mechanistic
insight into the circumstances when this occurs. The understanding of the different
types of radical formation will be highly beneficial for further development of this
field. The reactivity of FLPs with radical substrates showcases the ability of such
systems to capture radicals as well as form highly reactive radical species capable of
bond activation. Furthermore, use of pseudo-FLP systems, in which the Lewis base is
replaced by a single-electron donor, has been applied to force a single-electron radical
pathway within FLP systems, which has led to valuable insights regarding both the
mechanism and product formation occurring via radical pathways. This has revealed
that, in many cases, reactivity proceeds via Lewis acid coordination to a substrate,
thereby enhancing its electron affinity, which facilitates a subsequent single-electron
transfer event in which radical species are generated. This mechanism circumvents
direct SET from the Lewis base to the Lewis acid, and thus avoids the formation
of highly unstable radical anion species formed after reduction of the Lewis acid.
Recent developments in which radical FLP systems have been used for C–C bond
formation reactions are most likely operating via this type of mechanism.
Direct SET from the Lewis base to the Lewis acid has been investigated and
is feasible under certain conditions. The energy difference between the Lewis
acid/Lewis base pair and the corresponding radical ion pair plays a crucial role
in determining the nature of this SET. In case of a large energy gap, the Lewis acid
and Lewis base can form an EDA-complex, after which irradiation leads to SET and
thus radical formation. However, if the energy gap is small, thermal SET becomes
feasible. This understanding prompts thorough analysis of previously reported and
future SET reactivity in FLP systems. Furthermore, knowing which systems require
light for photochemical formation of the radicals can lead to enhanced catalytic
activity and potential applications in photoredox catalysis.
Although the formation of radical species within FLP systems is only a recent
discovery, understanding has recently developed significantly, which is likely to allow
for further rapid development of this field. In addition, the first reactions utilizing
this type of reactivity for bond formation were reported earlier this year. It will be
interesting to see how this radical chemistry will be further utilized and how this
field will further develop.
References
1. Paradies J (2019) From Structure to Novel Reactivity in Frustrated Lewis Pairs. 170–183.
https://doi.org/10.1016/j.ccr.2018.09.014
2. Welch GC, Juan RRS, Masuda JD, Stephan DW (2006) Reversible, Metal-Free Hydrogen
Activation. Science 314:1124–1126. https://doi.org/10.1126/science.1134230
F. Holtrop et al.
10.5 Concluding Remarks
This chapter has analysed the different occurrences of radical formation within FLP
systems that are being increasingly reported in the literature and provided mechanistic
insight into the circumstances when this occurs. The understanding of the different
types of radical formation will be highly beneficial for further development of this
field. The reactivity of FLPs with radical substrates showcases the ability of such
systems to capture radicals as well as form highly reactive radical species capable of
bond activation. Furthermore, use of pseudo-FLP systems, in which the Lewis base is
replaced by a single-electron donor, has been applied to force a single-electron radical
pathway within FLP systems, which has led to valuable insights regarding both the
mechanism and product formation occurring via radical pathways. This has revealed
that, in many cases, reactivity proceeds via Lewis acid coordination to a substrate,
thereby enhancing its electron affinity, which facilitates a subsequent single-electron
transfer event in which radical species are generated. This mechanism circumvents
direct SET from the Lewis base to the Lewis acid, and thus avoids the formation
of highly unstable radical anion species formed after reduction of the Lewis acid.
Recent developments in which radical FLP systems have been used for C–C bond
formation reactions are most likely operating via this type of mechanism.
Direct SET from the Lewis base to the Lewis acid has been investigated and
is feasible under certain conditions. The energy difference between the Lewis
acid/Lewis base pair and the corresponding radical ion pair plays a crucial role
in determining the nature of this SET. In case of a large energy gap, the Lewis acid
and Lewis base can form an EDA-complex, after which irradiation leads to SET and
thus radical formation. However, if the energy gap is small, thermal SET becomes
feasible. This understanding prompts thorough analysis of previously reported and
future SET reactivity in FLP systems. Furthermore, knowing which systems require
light for photochemical formation of the radicals can lead to enhanced catalytic
activity and potential applications in photoredox catalysis.
Although the formation of radical species within FLP systems is only a recent
discovery, understanding has recently developed significantly, which is likely to allow
for further rapid development of this field. In addition, the first reactions utilizing
this type of reactivity for bond formation were reported earlier this year. It will be
interesting to see how this radical chemistry will be further utilized and how this
field will further develop.
References
1. Paradies J (2019) From Structure to Novel Reactivity in Frustrated Lewis Pairs. 170–183.
https://doi.org/10.1016/j.ccr.2018.09.014
2. Welch GC, Juan RRS, Masuda JD, Stephan DW (2006) Reversible, Metal-Free Hydrogen
Activation. Science 314:1124–1126. https://doi.org/10.1126/science.1134230