Ligand-controlled
rhodium(I)-catalyzed denitrogenative transformations
of a range of 4-vinyl-1,2,3-thiadiazoles possessing electron-donating
substituents at the C5-position of the heterocycle have been demonstrated.
With [Rh(COD)2]BF4, vinylic 1,2,3-thiadiazoles
undergo an intramolecular transannulation reaction to afford substituted
furans. In contrast, the [Rh(COD)DPPF]BF4 catalytic system
inhibits the intramolecular reaction but promotes intermolecular transannulation
with both electron-deficient and electron-rich terminal alkynes, providing
access to densely functionalized thiophenes with unexpected regioselectivity.
Experimental and computational mechanistic studies were performed
to gain insights into the Rh(I)-catalyzed intramolecular transannulation
of vinylic 1,2,3-thiadiazoles, with a focus on understanding the influence
of the C5-substituent on reactivity and the role of the DPPF ligand.
Importantly, our crystallographic data uncovered that the true structure
of the organorhodium intermediate involved in Rh(I)-catalyzed denitrogenative
reactions of 1,2,3-thiadiazoles is likely to be a four-membered cyclometalated
Rh(III) complex.
Despite the growing use of denitrogenative reactions of 1,2,3-thiadiazoles in heterocycle synthesis, the origin of the varied reactivity and divergent regioselectivity observed in their transannulation reactions is not well understood....
Simple and convenient methods for the synthesis of a range of new 1,2,3-thiadiazole derivatives, including ethyl esters, alcohols, and aldehydes, were developed. Moreover, twenty-nine new structurally diverse 4-vinyl-1,2,3-thiadiazoles were successfully synthesized in 18-89 % yield via Knoevenagel condensation. NOESY-experiments were performed for E/Z-assignment of isomeric mixtures. The structure of five of the new vinylic derivatives has been confirmed using X-ray diffraction analysis. Using the example of ethyl (E)-2-cyano-3-(5-(pyrrolidin-1-yl)-1,2,3-thiadiazol-4-yl)acrylate, the possibility of dimerization with the formation of diethyl(1R,2R,3S,4S)-1,3-dicyano-2,4-bis(5-(pyrrolidin-1-yl)-1,2,3-thiadiazol-4-yl)cyclobutane-1,3-dicarboxylate was demonstrated, allowing the first 1,2,3-thiadiazole substituted cyclobutane to be synthesized.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.