Radical Addition to Iminium Ions and Cationic Heterocycles

Abstract

Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated cationic substrates with various types of C-radicals, including alkyl, alkoxyalkyl, trifluoromethyl, aryl, acyl, carbamoyl, and alkoxycarbonyl species. Despite its high reactivity, the strong interaction of the radical's SOMO with the LUMO of the cation frequently results in a high regioselectivity. Intra-and intermolecular processes such as the Minisci reaction, the Porta reaction, and the Knabe rearrangement will be discussed along with transition metal and photoredox catalysis or electrochemical methods to generate the odd-electron species.

Philicity of Acetonyl and Benzoyl Radicals: A Comparative Experimental and Computational Study

“…After the successful acid‐catalyzed addition reactions of ketone‐derived radicals such as 2 to olefins, we wished to utilize them for the functionalization of arenes. First, we employed isoquinoline ( 6 ), a common substrate in Minisci reactions that shows a strong preference for being attacked at the 1‐position . For the in situ generation of alkenyl peroxides from acetone and t BuOOH, we used excess p ‐toluenesulfonic acid ( p TsOH) to maintain acidic conditions in the presence of the basic 6 .…”

Photocatalysis with TiO2 Applied to Organic Synthesis

“…[73][74][75] In particular, the addition of carbon-centred radicals is interesting for the synthesis of biologically active heterocyclic compounds. [74][75][76] The Minisci reaction has also been performed under different photochemical conditions. [77] Heterogeneous photocatalysis with TiO 2 was thus applied.…”

“…A hole in the valence band of the electronically excited semiconductor particle is quenched by electron transfer from the tertiary N-methylpyrrolidine 117. 75 After deprotonation, the corresponding a-aminoalkyl radical 122 is generated. Addition of 122 to the unsaturated lactone 118 leads to the oxoallyl radical 123.…”

Neue Entwicklungen auf dem Gebiet der Minisci‐Reaktion

“…[26] Die Radikaladdition kann dabei in Abhängigkeit des jeweils gewählten Substrats sowohl reversibel als auch irreversibel stattfinden. Üblicherweise wird eine Säure als stçchiometrisches Additiv zugesetzt, da Protonierung des basischen Heteroarens dessen LUMO-Energie absenkt und so die Radikaladdition begünstigt.…”