β Scribed by Herges, Rainer ;Reif, Wolfgang
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A norbornadiene tetracarboxylic acid was synthesized the p__K__~s~ of which can be changed by irradiation forming the quadricyclane derivative or by catalytical reisomerization to the norbornadiene. The p__K__~s~ difference of both isomers in the second and fourth dissociation step is 0.59 and 0.68 units. Photochemical isomerization proceeds without decomposition at high pH (β₯ 9.0). Reisomerization of the quadricyclane to the norbornadiene with a Rh^I^ complex is also a clean reaction, but rather slow with t~1/2~ = 5 d at room temperature. We attribute the pH change of an aqueous solution during isomerization to the formation or cleavage of intramolecular hydrogen bonds between the adjacent carboxyl groups and accumulation or dispersion of negative charge in the quadricyclane and norbornadiene, respectively.
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