Experimental and Theoretical Studies of Selective Thiol-Ene and Thiol-Yne Click Reactions Involving N-Substituted Maleimides

A combination of experimental and computational methods has been used to understand the reactivity and selectivity of orthogonal thiol-ene and thiol-yne click reactions involving N-allyl maleimide (1) and N-propargyl maleimide (2). Representative thiols methyl-3-mercaptopropionate and beta-mercaptoethanol are shown to add exclusively and quantitatively to the electron poor maleimide alkene of 1 and 2 under base (Et3N) initiated thiol-Michael conditions. Subsequent radical-mediated thiol-ene or thiol-yne reactions can be carried out to further functionalize the remaining allyl or propargyl moieties in near quantitative yields (>95%). Selectivity, however, can only be achieved when base-initiated thiol-Michael reactions are carried out first, as radical-mediated reactions between equimolar amounts of thiol and N-substituted maleimides give complex mixtures of products. CBS-QB3 calculations have been used to investigate the energetics and kinetics of reactions between a representative thiol (methyl mercaptan) with N-allyl and N-propargyl maleimide under both base-initiated and radical-mediated conditions. Calculations help elucidate the factors that underlie the selective base-initiated and nonselective radical-mediated thiol-ene/yne reactions. The results provide additional insights into how to design selective radical-mediated thiol-ene/yne reactions.