Hexafluorobenzene reaction with carbenes

One approach, which has proved revealing in the case of 9-carbena-fiuorene and which may be generally applicable, is to examine the infiuence of additives which can deactivate electronically excited carbenes without undergoing reaction themselves (e.g. hexafluorobenzene) or which react preferentially with carbenes of a particular spin state (e.g. oxygen and butadiene, which are both thought to react preferentially with triplet carbenes). The technique is analogous to well-... 

The extremely electrophilic nature of bis(trifluoromethyl)carbene has already been illustrated by the formation of addition products, even with hexafluorobenzene [83] (Section lllB), and the high reactivity results in more side-reactions, such as insertion into C—H bonds, than with difluorocarbene (Figure 6.62). 

Because of the extraordinary strength of the carbon-fluorine bond, transition metal-mediated activation of fluoroalkanes and arenes is not easy to achieve. Nevertheless, activation of the C-F bond in highly electron-deficient compounds such as 2,4,6-trifluoropyrimidine, pentafluoropyridine, or hexafluorobenzene is possible with stoichiometric amounts of bis(triethylphosphano) nickel(O) [101] (Scheme 2.45). More recently Herrmann and coworkers [102] have described a variant of the Kumada-Corriu cross-coupling reaction [103] between fluorobenzene and aryl Grignard compounds which uses catalytic amounts of nickel carbene complexes. Hammett analysis of the relative kinetic rate constants indicated that the reaction proceeds via initial oxidative addition of the fluoroaromatic reactant to the nickel(O) species. 

Laser flash photolysis has, as usual, illuminated the problem. Jones and Rettig photodecomposed 9-diazofluorene (188) in hexafluorobenzene and cw-4-methyl-2-pentene mixtures, and showed that the degree of stereoselectivity in the cyclopropane products depended on the concentration of the alkene. Laser flash photolysis showed that the first detectable intermediate in the photolysis reaction is the triplet carbene, and suggests that the product studies are consistent with initial formation of a singlet fluorenylidene which has an extremely short life (less than 5 ns) before forming the triplet. The singlet can be trapped only by high alkene concentrations while the more stable triplet is easily trapped. 

The carbene complex [Ni2(Pr 2G3N2H2)4(GOD)] activates the G-F bond of hexafluorobenzene to give [NiF(G6F5)(Pp2G3N2H2)2] within 1 h. Reaction of [NiEt2(bpy)] (bpy = 2,2 -bypyridine) with hexafluorobenzene leads to the formation of [Ni(G6Fs)2(bpy)]. ... 

---