Mesoionics

Copper-Catalyzed Aza-Iminosydnone-Alkyne Cycloaddition Reaction Discovered by Screening

“…Mesoionics are an old, intriguing class of dipoles often considered as exotic structures, even within the heterocyclic community. As a consequence, their chemistry and reactivity have been only sporadically explored; a rapid survey of literature indicates that less than 5% of the possible mesoionic structures have been described . These compounds were originally defined as dipolar five-membered heterocycles with multiple aromatic resonances that cannot be satisfactorily represented without charge separation.…”

Click and Bio-Orthogonal Reactions with Mesoionic Compounds

“…The synthesis of mesoionics has been the topic of numerous reviews. ,− Here, we will develop only the most representative mesoionics that have found applications in click and bio-orthogonal reactions. Historically, four main strategies were used to construct the scaffolds of mesoionics.…”

“…Finally, one-pot strategies were developed, avoiding the isolation of the toxic nitroso-amine intermediate. , In 2019, an alternative method using mechano-chemistry was developed by the group of Lamaty . Interestingly, the sydnone core offers various postfunctionalization possibilities, which have been the topic of several works, including reviews (Scheme b). ,− Indeed, the proton in position C 4 possesses an acidic character and can be abstracted in the presence of different organometallic species. The intermediate sydnone anion can further react with a large variety of electrophiles or participate in metal-catalyzed cross-coupling reaction via transmetalation. ,− Among the possible functionalization, halogenation (typically achieved in the presence of N -halosuccinimides) is of particular interest, as 4-halo-sydnones are good substrates for cross-coupling reactions. − More recently, Moran discovered that direct arylation, alkenylation, and alkynylation could be performed with 4- H -sydnones. , Following this work, Suzuki couplings and oxidative Heck reactions were developed. , …”

“…Those key intermediates can be further functionalized in the presence of a large variety of nucleophiles. , The other possibilities to further derivatize iminosydnones are relatively similar to those used for sydnones, since the C 4 position is nucleophilic and acidic, as well. However, lithiated iminosydnones are less stable and less nucleophilic than their sydnone counterparts. , Halogenation of the C 4 position is also possible but might be capricious …”

Strain‐Promoted 1,3‐Dithiolium‐4‐olates–Alkyne Cycloaddition

“…Mesoionics are an exotic class of dipoles holding much promise in cycloaddition reactions.A lthough their synthesis and reactivity have been reported since the 60s, their potential is far from being fully explored. [1] Mesoionic compounds represent al arge family of five-membered heterocycles that cannot be represented by Lewis structures not involving charge separation. Over the last decade,t he popularity of this exotic family of heterocycles has suddenly increased and caught much attention.…”