Publication
Due to the high ring strain and unique bonding, cyclopropenes represent important 3-carbon synthons in organic synthesis. Intrigued by the unique reactivity of cyclopropenes, we were interested in further exploration of their chemical properties. Trying to expand the borders of current knowledge, we focused our attention on the underdeveloped reactivities of cyclopropenes: reactions with radicals, electrophiles, and the chemistry of cyclopropenium cations. In the first project, radical azidation of cyclopropenes for the synthesis of quinolines was developed. Multisubstituted quinoline products were obtained in 34â 81% yield. The reaction was most efficient for 3-trifluoromethylcyclopropenes, affording valuable 4-trifluoromethylquinolines. The transformation probably proceeds through the cyclization of an iminyl radical formed by the addition of the azide radical on the cyclopropene double bond, followed by ring-opening and fragmentation. In the second project, we developed electrophile-induced semipinacol rearrangement of cyclopropenylcarbinols. This transformation gives access to various polyfunctionalized cyclopropanes under mild metal-free conditions. The scope of the reaction includes iodine, sulfur and selenium electrophiles, aryl and strained ring migrating groups, and diverse substitution patterns on the cyclopropene. The reaction is particularly efficient for the synthesis of small ring-containing spirocycles, which are important rigid three-dimensional building blocks for medicinal chemistry. During initial trials to achieve semipinacol rearrangement using platinum catalysis, the unexpected formation of 1,3-dienes was observed. This led to the development of side project, that was completed with the help of three Master students: Antonin Homassel, Leander Choudhury, and Percy Rossignol. Diverse dienylated alcohols were obtained in 42-98% yield. The synthetic potential of the products was demonstrated by their use in Diels-Alder cycloadditions with various dienophiles. Isotope labeling studies provide strong support for a mechanism involving pericyclic [1,5]-Ï -bond rearrangement of a vinyl platinum carbene intermediate. Finally, we wanted to address the limitations in the synthesis of cyclopropenes. Therefore, we developed photocatalytic decarboxylative functionalization of cyclopropenes via cyclopropenium cation intermediates. Starting from cyclopropenyl redox-active esters, easily accessible through the rhodium- or silver-catalyzed cyclopropenation of alkynes, we could access a wide range of diverse cyclopropenes under mild photoredox conditions. The transformation displayed a broad scope and a remarkable functional group tolerance. Mechanistic studies supported aromatic cyclopropenium cations as reaction intermediates.