New Advances in Selected Transition Metal-Catalyzed Annulations

Abstract

This account describes the recent advances on the Pdcatalyzed [4+2]-benzannulation reaction, development of chemoand regiospecific formal [2+2+2]-cyclotrimerization of alkynes, as well as discovery and evolution of the transition metal-catalyzed cycloisomerization of alkynyl ketones and imines into five-membered heteroaromatics.

Recent progress in cobalt-mediated [2 + 2 + 2] cycloaddition reactions

“…A few methods have been proposed to overcome this crucial issue, but despite their impressive efficiency, these strategies remain restricted to specific cases. [8][9][10][11] For instance, a calixarene bound titanium complex allowed the highly selective cyclization of monosubstituted alkynes into 1,2,4-trisubstituted benzenes. 9 Chemoselective assembly of terminal alkynes with DMAD via iridium and ruthenium catalysis was also reported, yet with moderate levels of regioselectivity.…”

“…10 Although limited to 1,2diynes, the palladium-catalyzed homodimerization of terminal alkynes and subsequent [4 + 2] benzannulation gave tetra-and pentasubstituted benzenes from three different alkynes with excellent levels of regio-and chemoselectivity. 11 Stepwise strategies involving stoichiometric amounts of zirconium and titanium complexes were also proposed for the selective formation of benzenes and pyridines. 12 In the cobalt series, examples remain rare and specific.…”

Z-Selective Dimerization of Aromatic Terminal Alkynes Catalyzed by an Iridium(I)-N-Heterocyclic Carbene-Phosphine System

“…Scheme 1 displays the potential enyne isomers obtainable via this dimerization process. In addition to these products, competing cyclotrimerization 10 or butatriene formation 8c,d,11 are also observed reaction outcomes. Nevertheless, the formation of branched enynes, that is, headto-tail coupled products A, has been readily achieved via Au and Pd catalysis.…”

1,3-Butadiene Dicarbofunctionalization Enabled by the Dual Role of Diaryl Ketone in Photo-HAT/Chromium Catalysis

“…Upon irradiation, an n → π* triplet state will be formed, which is commonly depicted as a diradical with significant reactivity toward aliphatic C–H bonds, resulting in the generation of alkyl radicals and the ketyl radical . The merging of diaryl ketone with photoredox nickel and copper catalysis has emerged as a powerful tool for constructing otherwise challenging chemical bonds …”