380
F. Holtrop et al.
(E red = –0.56 V vs Fc
+ /Fc) and [Ph 3 C][BF 4 ] (E ox –0.11 V vs Fc/Fc
+ ) indicate this
(E = 10.4 kcal/mol) [55, 56]. In the case of the germylene/B(C 6 F 5 ) 3 system 25,
redox potentials of both components are known (25: E red = –0.36 V vs Fc
+ /Fc and
B(C 6 F 5 ) 3 : E ox –1.52 V vs Fc/Fc
+ ) and show that the energy required for SET is higher
than thermally feasible (43.4 kcal/mol), indicating that this process is likely photoinduced. In addition, this could mean that the observed absorption band at 544 nm is
actually the charge-transfer band corresponding to the photoinduced SET.
10.4.3 Applications in Synthesis
So far, this chapter has detailed SET events within FLP-type systems and analysed the resulting radical species. Recently, such reactivity has been successfully
harnessed to drive catalytic C–C bond formation [25, 26]. Ooi et al. reported the
photoinduced reaction of dimethylaniline 32 and methylvinylketone catalysed by
B(C 6 F 5 ) 3 , yielding species 33 [26]. They found that the dimethylaniline donor
forms an EDA-complex with B(C 6 F 5 ) 3 , which upon irradiation leads to formation
of the corresponding radical ion pair: ArMe 2 N
•+ and B(C 6 F 5 ) 3
•– . This led to the
proposal that the reaction proceeds via photoinduced formation of ArMe 2 N
•+ and
B(C 6 F 5 ) 3
•– ,after which the former reacts with methylvinylketone affording 33 + . This
species is then reduced by B(C 6 F 5 ) 3
•– to yield the product 33. However, due to the
known rapid degradation of B(C 6 F 5 ) 3
•– , it seems unlikely that the reaction would
proceed efficiently if this species is a necessary intermediate. Analysis of the reactivity of pseudo-FLP species Cp* 2 Fe/B(C 6 F 5 ) 3 with substrates featuring oxygen
moieties (e.g. quinones, N 2 O) showed these proceed via initial Lewis acid coordination at the oxygen, after which SET leads to product formation (see Sect. 10.3.2),
thus completely avoiding the formation of the unstable B(C 6 F 5 ) 3
•– as a free species.
This mechanism was also shown for the reaction of P/B-type FLP systems with
tetrachloro-p-quinone (see Sect. 10.4.1). As methylvinylketone, the substrate utilized
by Ooi et al. features a Lewis basic carbonyl moiety, the SET is likely to operate
via this mechanism. In addition, Slootweg et al. demonstrated computationally that
Lewis acid coordination of B(C 6 F 5 ) 3 to methylvinylketone is feasible and leads to an
increase in electron affinity [42]. Subsequent EDA-complex formation between the
methylvinylketone-B(C 6 F 5 ) 3 adduct and dimethylphenylaniline 32 and irradiation
would lead to the formation of radical species ArMe 2 N
•+ and 34. Radical coupling
and release of B(C 6 F 5 ) 3 would then generate the products and regenerate the catalyst
(Scheme 10.27).
Melen et al. also utilized FLP-based radical formation to drive C–C bond formation [25]. They utilized PMes 3 /B(C 6 F 5 ) 3 to couple 35 and styrenes to form 37
(Scheme 10.28). They proposed, though cautiously, that the reaction proceeds via
SET from the Lewis base to the Lewis acid, forming the Mes 3 P
+ /B(C 6 F 5 ) 3
•– radical
ion pair. The borane radical anion is then proposed to react with ester species 35,
leading to expulsion of diarylmethyl radical 40. Radical addition to the olefin and
subsequent radical abstraction by PMes 3
•+ then lead to formation of the product
F. Holtrop et al.
(E red = –0.56 V vs Fc
+ /Fc) and [Ph 3 C][BF 4 ] (E ox –0.11 V vs Fc/Fc
+ ) indicate this
(E = 10.4 kcal/mol) [55, 56]. In the case of the germylene/B(C 6 F 5 ) 3 system 25,
redox potentials of both components are known (25: E red = –0.36 V vs Fc
+ /Fc and
B(C 6 F 5 ) 3 : E ox –1.52 V vs Fc/Fc
+ ) and show that the energy required for SET is higher
than thermally feasible (43.4 kcal/mol), indicating that this process is likely photoinduced. In addition, this could mean that the observed absorption band at 544 nm is
actually the charge-transfer band corresponding to the photoinduced SET.
10.4.3 Applications in Synthesis
So far, this chapter has detailed SET events within FLP-type systems and analysed the resulting radical species. Recently, such reactivity has been successfully
harnessed to drive catalytic C–C bond formation [25, 26]. Ooi et al. reported the
photoinduced reaction of dimethylaniline 32 and methylvinylketone catalysed by
B(C 6 F 5 ) 3 , yielding species 33 [26]. They found that the dimethylaniline donor
forms an EDA-complex with B(C 6 F 5 ) 3 , which upon irradiation leads to formation
of the corresponding radical ion pair: ArMe 2 N
•+ and B(C 6 F 5 ) 3
•– . This led to the
proposal that the reaction proceeds via photoinduced formation of ArMe 2 N
•+ and
B(C 6 F 5 ) 3
•– ,after which the former reacts with methylvinylketone affording 33 + . This
species is then reduced by B(C 6 F 5 ) 3
•– to yield the product 33. However, due to the
known rapid degradation of B(C 6 F 5 ) 3
•– , it seems unlikely that the reaction would
proceed efficiently if this species is a necessary intermediate. Analysis of the reactivity of pseudo-FLP species Cp* 2 Fe/B(C 6 F 5 ) 3 with substrates featuring oxygen
moieties (e.g. quinones, N 2 O) showed these proceed via initial Lewis acid coordination at the oxygen, after which SET leads to product formation (see Sect. 10.3.2),
thus completely avoiding the formation of the unstable B(C 6 F 5 ) 3
•– as a free species.
This mechanism was also shown for the reaction of P/B-type FLP systems with
tetrachloro-p-quinone (see Sect. 10.4.1). As methylvinylketone, the substrate utilized
by Ooi et al. features a Lewis basic carbonyl moiety, the SET is likely to operate
via this mechanism. In addition, Slootweg et al. demonstrated computationally that
Lewis acid coordination of B(C 6 F 5 ) 3 to methylvinylketone is feasible and leads to an
increase in electron affinity [42]. Subsequent EDA-complex formation between the
methylvinylketone-B(C 6 F 5 ) 3 adduct and dimethylphenylaniline 32 and irradiation
would lead to the formation of radical species ArMe 2 N
•+ and 34. Radical coupling
and release of B(C 6 F 5 ) 3 would then generate the products and regenerate the catalyst
(Scheme 10.27).
Melen et al. also utilized FLP-based radical formation to drive C–C bond formation [25]. They utilized PMes 3 /B(C 6 F 5 ) 3 to couple 35 and styrenes to form 37
(Scheme 10.28). They proposed, though cautiously, that the reaction proceeds via
SET from the Lewis base to the Lewis acid, forming the Mes 3 P
+ /B(C 6 F 5 ) 3
•– radical
ion pair. The borane radical anion is then proposed to react with ester species 35,
leading to expulsion of diarylmethyl radical 40. Radical addition to the olefin and
subsequent radical abstraction by PMes 3
•+ then lead to formation of the product