The retro-Diels-Alder reaction has shown great utility in thermally-responsive materials, and it likewise has great potential for targeted delivery of molecular cargo bound as diene or dienophile units. Just as substitution has been established to profoundly effect the reactivity and regioselectivity of 1,3-dienes in the Diels-Alder reaction, we have found that alkyl substitution on 1,3-cyclohexadienes significantly impacts thermal stability toward cycloreversion. To study this effect, a model system was developed by synthesis of Diels-Alder adducts via the in situ formation of a nitrosocarbamide dienophile by the oxidation of <i>N</i>-4-nitrophenyl-<i>N</i>′-hydroxyurea in the presence of various 1,3-cyclohexadienes. The substitution patterns studied include the parent 1,3-cyclohexadiene, α-phellandrene, 2,3-dimethyl-1,3-cyclohexadiene, 1-methanol-1,3--cyclohexadiene, ergosterol, and 7-dehydrocholesterol. By simultaneously observing the concentration of both the adduct and hydrolyzed product (4-nitrophenylaniline) concentrations through UV-visible spectroscopy, we were able to apply an apparent first-order kinetic model with steady state approximation to determine the rate constants and half-lives associated with each diene. By measuring the rate constants at different temperatures (in the range of 20-60 °C), the Arrhenius activation energy for each diene can be determined.

Probing the effects of 1,3-cyclohexadiene substitution on the rates of retro-nitroso-Diels-Alder reactions

253rd National Meeting (2017)

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