Tetrabutylammonium salts dissociation

The exciplexes formed between arenes and good acceptors such as dicyanobenzene can dissociate to radical ion pairs if the solvent used is sufficiently polar. The radical cation of the arene is then susceptible to attack by nucleophiles and this can lead to products of addition or substitution of the arene. A preliminary account of how salts such as tetrabutylammonium tetrafluoroborate can serve to mediate the charge separation in less polar solvents was reported last year. A second paper from the same group has now been published which describes the formation of the amino substituted dihydrophenanthrene (86) during the irradiation of a solution of phenanthrene, dicyanobenzene and propylamine in relatively non-polar solvents such as THF in the presence of tetrabutylammonium tetrafluoroborate. In the absence of the salt no product is formed. 

Admittedly, the above is a considerable simplification of what is a complex, multi-variable process, but some frame of reference seems required. The use of a quaternary ammonium salt instead of an alkali metal cation allows for the dissolution of ammonium enolate ion pairs in nonpolar media [54]. Whereas these ion pairs are relatively dissociated in such solvents as DMSO and give appreciable amounts of 0-alkylation, in benzene or toluene, the C/0 ratio is quite high. Brandstrom and Junggren found that isopropylation of tetrabutylammonium acetylacetonate lead to a C/0 ratio of 0.72 in DMSO and 13.8 in toluene solution (see Eq. 15.17) [54]. 

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