Continuous-flow synthesis of 3,5-disubstituted pyrazoles via sequential alkyne homocoupling and Cope-type hydroamination

Catalytic Hydrofunctionalization Reactions of 1,3-Diynes

Metal-catalyzed hydrofunctionalization reactions of alkynes, i.e., the addition of Y–H units (Y = heteroatom or carbon) across the carbon–carbon triple bond, have attracted enormous attention for decades since they allow the straightforward and atom-economic access to a wide variety of functionalized olefins and, in its intramolecular version, to relevant heterocyclic and carbocyclic compounds. Despite conjugated 1,3-diynes being considered key building blocks in synthetic organic chemistry, this particular class of alkynes has been much less employed in hydrofunctionalization reactions when compared to terminal or internal monoynes. The presence of two C≡C bonds in conjugated 1,3-diynes adds to the classical regio- and stereocontrol issues associated with the alkyne hydrofunctionalization processes’ other problems, such as the possibility to undergo 1,2-, 3,4-, or 1,4-monoadditions as well as double addition reactions, thus increasing the number of potential products that can be formed. In this review article, metal-catalyzed hydrofunctionalization reactions of these challenging substrates are comprehensively discussed.

Hydroelementation of diynes

This review highlights the hydroelementation reactions of conjugated and separated diynes, which depending on the process conditions, catalytic system, as well as the type of reagents, leads to the formation of various products: enynes, dienes, allenes, polymers, or cyclic compounds. The presence of two triple bonds in the diyne structure makes these compounds important reagents but selective product formation is often difficult owing to problems associated with maintaining appropriate reaction regio- and stereoselectivity. Herein we review this topic to gain knowledge on the reactivity of diynes and to systematise the range of information relating to their use in hydroelementation reactions. The review is divided according to the addition of the E-H (E = Mg, B, Al, Si, Ge, Sn, N, P, O, S, Se, Te) bond to the triple bond(s) in the diyne, as well as to the type of the reagent used, and the product formed. Not only are the hydroelementation reactions comprehensively discussed, but the synthetic potential of the obtained products is also presented. The majority of published research is included within this review, illustrating the potential as well as limitations of these processes, with the intent to showcase the power of these transformations and the obtained products in synthesis and materials chemistry.

Recent advances in pyrazole synthesis employing diazo compounds and synthetic analogues

This review summarizes the recent developments in the construction of pyrazoles using diazo compounds, nitrile imines and their synthetic equivalents.

Sequential [3+2] annulation reaction of prop-2-ynylsulfonium salts and hydrazonyl chlorides: synthesis of pyrazoles containing functional motifs

A novel sequential [3+2] annulation reaction has been developed using prop-2-ynylsulfonium salts and hydrazonyl chlorides, affording a series of pyrazoles with functional motifs that can be post modified in the preparation of various drugs or drug candidates. Further transformation and gram-scale operations could also be achieved efficiently.

Denitrative imino-diaza-Nazarov cyclization: synthesis of pyrazoles

An iodine-catalyzed denitrative imino-diaza-Nazarov cyclization (DIDAN) methodology has been developed for the synthesis of pyrazoles with high to excellent yields by using α-nitroacetophenone derivatives and in situ generated hydrazones. The key transformation of this oxidative 4π-electrocyclization proceeds through an enamine-iminium ion intermediate. This rapid one-pot DIDAN protocol results in the selective generation of C-C and C-N bonds and cleavage of a C-N bond.