Precise control of the formation of a covalent and an ionic bond in carbocation-carbanion combination reactions

The electronic effect on the selectivity of covalent or ionic bond formation was examined for the reaction of Kuhn's anion 1- (C67H39-; tris(7H-dibenzo[c,g]fluorenylidenemethyl)methide ion) and 1-aryl-2,3-dicyclopropylcyclopropenylium ions. The carbocation stability was progressively changed by varying the substituent on the phenyl ring, while the steric effect was kept essentially unchanged. The cations having the p-chlorophenyl (2a+), phenyl (2b+), m-methylphenyl (2c+), or m,m'-dimethylphenyl (2d+) group gave a covalent product, whereas a carbocation-carbanion salt was obtained from the cations having the p-methylphenyl (2e+) or p-methoxyphenyl (2f+) group. The reduction potentials E(red) of the cations, as determined by cyclic voltammetry, showed that the formation of the covalent or ionic product is switched by a small difference in stability (less-than-or-equal-to 0.4 kcal/mol) between 2d+ and 2e+. In chloroform, the salts 1-2e+ and 1-2f+ were transformed into covalent forms 1-2e and 1-2f, which can exist only in solution. When 1-(2a-d) and 1-2e,f+ were dissolved in DMSO, equilibrium between a covalent compound and ions was established. A plot of the free energy of heterolysis DELTAG(het)-degrees for 1-(2a-f) against the E(red) of the corresponding cations 2a-f+ showed that DELTAG(het)-degrees decreases as the cation is more stabilized. The heterolysis in DMSO was shown to be enhanced by ca. 13 kcal/mol both by the steric congestion in the covalent molecules and the stabilization of the cyclopropenylium ions by solvation.