Palladium-Catalyzed Domino Cycloisomerization/Double Condensation of Acetylenic Acids with Dinucleophiles

Metal-catalyzed cascade processes based on hydrofunctionalization of alkynes are receiving much more attention because of their potential to provide advantageous approaches to otherwise synthetically challenging compounds. An alternative catalyst system has been found for the domino cycloisomerization/cyclocondensation reaction involving acetylenic acids and heterodinucleophiles. A CNN pincer palladium(II) complex, acting as a homogeneous catalyst, provides the corresponding polyheterocycles with a higher substrate/catalyst ratio. Other palladium sources were also tested and discarded, and a number of mechanistic studies including poisoning assays, kinetic plots, TEM images, XRD spectra and UPLC-MS analysis of reaction intermediates were conducted in order to shed light on the role of this pincer catalyst and the catalytic cycle involved in the cascade reaction. As a result, a more nuanced mechanism is tentatively proposed.

Iron-catalyzed alkyne-carbonyl metathesis for the synthesis of 6,7-dihydro-5H-dibenzo[c,e]azonines

The synthesis of nine-membered rings via alkyne-carbonyl metathesis is reported. The alkyne and acetal units contained in a biaryl substrate undergo the intramolecular alkyne-carbonyl metathesis reaction under FeCl₃-catalysis to furnish the unexplored 6,7-dihydro-5H-dibenzo[c,e]azonines. The method provides an alternative to existing routes to nine-membered rings with cheap, non-toxic iron catalysts and milder conditions.

Addition of benzyl ethers to alkynes: a metal-free synthesis of 1H-isochromenes

Bromotrimethylsilane (TMSBr)-promoted intramolecular cyclization of (o-arylethynyl)benzyl ethers to form 1H-isochromenes at room temperature is reported. Further studies indicated that vinyl carbocations are the reaction intermediates which are stabilized by the conjugated aryl groups. Thus, O-addition of benzyl ethers/tetrahydropyrans to alkynes was achieved under metal-free, acidic conditions. These reaction conditions were compatible with an alkynyl Prins reaction; therefore, 1H-isochromenes were produced directly from alkynyl benzaldehydes and alkynyl alcohols using a one-pot procedure.

Radical Chain Isomerization of N-Sulfonyl Ynamides to Ketenimines and Its Application to Furan Dearomatization

A radical chain isomerization of N-sulfonyl ynamides to isolable ketenimines is developed, featuring mild reaction conditions, a high efficiency, ∼100% atom economy, a broad substrate scope, and column chromatography-free workup in most cases. Meanwhile, an unprecedented dearomatization of furans is achieved by the radical chain isomerization-triggered aza-Claisen rearrangement, providing highly chemo-, regio-, stereo-, and diastereoselective access to functionalized quaternary nitriles.

Base-Promoted Stereoselective Hydrogenation of Ynamides with Sulfonyl Hydrazide to Give Z-Enamides

A base-mediated semihydrogenation of ynamides using p-toluenesulfonyl hydrazide as an inexpensive and easy-to-handle hydrogen donor is reported. This transition-metal-free protocol avoids overhydrogenation and reduction of other functional groups, generating the thermodynamically unfavorable Z-enamides exclusively.

Metal-Free [3+2] Annulation of Ynamides with Anthranils to Construct 2-Aminoindoles

A novel metal-free [3+2] annulation of ynamides with anthranils provides a facile, flexible, environmentally friendly, and atom-economical route to 2-aminoindoles. This synthetic process proceeds with efficiency, excellent regioselectivity, and wide functional group tolerance under mild conditions. Moreover, the obtained 2-aminoindole products represent a multifunctional platform for the construction of various 2-aminoindolyl frameworks.

Synthesis of polysubstituted azetidines via cascade trifluoromethylation/cyclization of N-allyl ynamides

A cascade trifluoromethylation/cyclization of N-allyl sulfonylynamides is developed, providing a direct access to azetidine-fused tricyclic compounds at room temperature.

Nonmetal-catalyzed hydroamination of ynamides with amines

A TfOH-catalyzed hydroamination of ynamides with primary and secondary amines under mild reaction conditions is described for the efficient synthesis of N-arylimines and ethene-1,1-diamines.