Abstract
The publications covering new, transition metal-free cross-coupling reactions of pyrroles with electrophilic haloacetylenes in solid medium of metal oxides and salts to regioselectively afford 2-ethynylpyrroles are discussed. The reactions proceed at room temperature without catalyst and base under solvent-free conditions. These ethynylation reactions seem to be particularly important, since the common Sonogashira coupling does not allow ethynylpyrroles with strong electron-withdrawing substituents at the acetylenic fragments to be synthesized. The results on the behavior of furans, thiophenes, and pyrazoles under the conditions of these reactions are also provided. The reactivity and structural peculiarities of nucleophilic addition to the activated acetylene moiety of the novel C-ethynylpyrroles are considered.
Highlights
Functionalized five-membered aromatic heterocycles represent a frequent structural motif of bioactive natural products and pharmaceuticals [1,2,3,4,5,6,7,8,9,10,11,12]
The combination of the electron-rich, five-membered aromatic heterocyclic nucleus with highly reactive carbon-carbon triple bond in one molecule allows using these compounds for the targeted synthesis of various complex heterocyclic systems
It was shown that some other metal oxides (MgO, CaO, BaO) [33] and salts (K2 CO3 ) [35] can be beneficially applied instead of Al2 O3
Summary
Functionalized five-membered aromatic heterocycles represent a frequent structural motif of bioactive natural products and pharmaceuticals [1,2,3,4,5,6,7,8,9,10,11,12]. As far as the relationship between the reactivity of the heterocycle and the solid salt used is concerned, a broad screening of various metal oxides and salts as mediators for the cross-coupling has Molecules 2020, 25, 2490; doi:10.3390/molecules25112490 www.mdpi.com/journal/molecules. A selection of specific metal oxides (Al2 O3 or K2 CO3 ) for a particular reaction is determined experimentally because the results significantly depend on the structure of both the pyrrole and haloacetylene employed. This methodology was already partially documented in recent reviews [44,45,46,47,48,49,50,51,52]. This survey covers the recent publications (since 2014) concerning this reaction and the related chemistry which have not been yet summarized in a review
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