Halogenated arenes and alkenes are of prime importance in many areas of science, especially in the pharmaceutical, agrochemical, and chemical industries. While the simplest ones are commercially available, some of them are still hardly accessible depending on their substitution patterns and the nature of the halogen atom. Reactions enabling the selective and efficient replacement of the halogen atom of an aryl or alkenyl halide by another one, lighter, or heavier, are therefore of major importance since they can be used for example to turn a less reactive aryl/alkenyl chloride into the more reactive iodinated derivatives or, in a reversed sense, to block an undesired reactivity, for late-stage modifications or for the introduction of a radionuclide. If some halogen exchange reactions are possible with activated substrates, they usually require catalysis with metal complexes. Remarkably efficient processes have been developed for metal-mediated halogen exchange in aryl and vinyl halides: they are overviewed, in a comprehensive manner, in this review article.
Does size matter? The bulkiest, yet still highly flexible, N-heterocyclic carbene to date is reported. Its imidazolium salt proved to be easily synthesized in only four steps on a 3.6 g scale. This new ligand was fully characterized and engaged in the formation of complexes with silver(I), copper(I), rhodium(I), and palladium(II).
Disubstituted malonic acid derivatives are smoothly converted into diketones and ketoesters in good to excellent yield (68% to 91%) under electrochemical conditions. The scope can be extended to transform trisubstituted bis-malonic acids into tetraketones in 77% to 86% yield. The new method was applied to the total synthesis of cis-jasmone.
A plethora of drug
molecules and agrochemicals contain the sulfonamide
functional group. However, sulfonamides are seldom viewed as synthetically
useful functional groups. To confront this limitation, a late-stage
functionalization strategy is described, which allows sulfonamides
to be converted to pivotal sulfonyl radical intermediates. This methodology
exploits a metal-free photocatalytic approach to access radical chemistry,
which is harnessed by combining pharmaceutically relevant sulfonamides
with an assortment of alkene fragments. Additionally, the sulfinate
anion can be readily obtained, further broadening the options for
sulfonamide functionalization. Mechanistic studies suggest that energy-transfer
catalysis (EnT) is in operation.
Methods for establishing the absolute configuration of sulfur-stereogenic aza-sulfur derivatives are scarce, often relying on cumbersome protocols and a limited pool of enantioenriched starting materials. We have addressed this by exploiting, for the first time, a feature of sulfonimidamides in which it is possible for tautomeric structures to also be enantiomeric. Such sulfonimidamides can readily generate prochiral ions, which we have exploited in an enantioselective alkylation process. Selectivity is achieved using a readily prepared bis-quaternized phase-transfer catalyst. The overall process establishes the capability of configurationally labile aza-sulfur species to be used in asymmetric catalysis. Scheme 1. a) Examples of aza-sulfur compounds, including sulfonimidamides. b) Proposed enantioselective synthesis of aza-sulfur compounds by exploiting configurational lability.
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