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.
Highlights
Radicals, representing odd-electron species, possess the peculiarity of combining nucleophilicity and electrophilicity at the same reaction center since their HOMO and LUMO are located in one common molecular orbital, the SOMO
In 1971, Minisci established a new method for the generation of alkyl radicals by silver-catalyzed oxidative decarboxylation of carboxylic acids with peroxydisulfate and demonstrated the positive effect of acidic conditions on the homolytic alkylation of heteroarenes (Scheme 2) [2]
As the cyclopropylmethyl radical is known for its very fast ring-opening rate constant of 1.3 × 108 s−1 [54], the rate of the radical addition to the very electron deficient N-acyliminium ion has to be of similar velocity
Summary
Radicals, representing odd-electron species, possess the peculiarity of combining nucleophilicity and electrophilicity at the same reaction center since their HOMO and LUMO (highest occupied/lowest unoccupied molecular orbital) are located in one common molecular orbital, the SOMO (singly occupied molecular orbital). Depending on the SOMO energy, either the SOMO-HOMO or the SOMO-LUMO interaction may be dominant as the result of a smaller energy difference In the former case, the radical can be classified as electrophilic while nucleophilic properties can be ascribed to it in the latter case (Figure 1) [1]. Even radicals with an estimated electrophilic character are capable of reacting as nucleophiles if a reactant with a low lying LUMO is present. This behavior is an important aspect of the characteristic reactivity of radicals, making them a powerful, yet sometimes less predictable tool for organic synthesis
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