Cotrimerizations of Acetylenic Compounds

Abstract

The cyclotrimerization of acetylenic compounds is a useful method for the construction of three new bonds in a one‐step process. Although symmetry‐allowed, there is a paucity of examples of purely thermal [2+2+2] cycloadditions. The first example reported was in 1866, which was the cyclization of acetylene to benzene. High temperatures are required for this reaction and a mixture of products is formed. In 1948, Reppe discovered the first transition‐metal catalyzed version of this reaction in which nickel was used and substituted benzenes were formed. Since then catalysts based on transition metals, lanthanides, actinide, and aluminum have been developed for the cycloaddition of substituted alkynes to benzene derivatives. Many compounds can take part in the cyclization reactions to give four‐, five‐,six, or eight membered rings. Many of these reactions have found many applications in organic synthesis This chapter is devoted to early to late transition metal‐mediated cotrimerization of acetylenic compounds directed towards organic synthesis. It addresses the scope and generality of the reaction, as well as the current state of the art, with regard to region‐and stereoselectivities. In the context of selectivity, a mechanistic overview is provided. Inter‐and intramolecular versions of the cyclizations are presented with an emphasis on applications in synthesis. The literature up to mid‐2004 is covered.

A Diversity‐Oriented Approach to Diphenylalkanes by Strategic Utilization of [2+2+2] Cyclotrimerization, Cross‐Enyne Metathesis and Diels–Alder Reaction

“…[9] In particular, rhodium-catalyzed [2+2+2] cyclotrimerization reactions [13][14][15] are widely used to design complex molecular arrays. Also, we have demonstrated a tactical utilization of Wilkinson's catalyst, RhCl(PPh 3 ) 3 to deliver highly functionalized compounds.…”

“…Alkyne building blocks have been used as starting materials in many processes such as Sonogashira reaction, [20] Glaser coupling, [21] Pauson-Khand reaction, [22] enyne metathesis, [23] and [2+2+2] cyclotrimerization [9] reactions. This has resulted in the emergence of various methods to generate terminal acetylenic compounds.…”

“…[8] Motivated by these findings, the two acetylenic ends of un-wise functionalization of acetylenic termini of α,ω-diyne scaffold to build functionalized aromatic units. tethered α,ω-diyne scaffolds (for instance 5) have been used for the assembly of aromatic units and a new approach to polysubstituted diphenylalkane derivatives via a strategic utilization of [2+2+2] cyclotrimerization reaction, [9] crossenyne metathesis (CEM) [10] and Diels-Alder (DA) reaction [11] as key steps has been conceived. The methodology proposed here constitutes a diversity-oriented approach [12] as there are several points of diversity where structural variations can be incorporated in the target molecule.…”

Unexpected Solvent Effects in the Isomerization of iPrPCPIr(η2‐PhC≡CPh) to a 1‐Iridaindene

“…Transition metal mediated transformations of alkynes have a rich history of study and find use in a number of reactions, [1,2] including trimerization to form aromatic compounds, [3,4] and the Pauson-Khand reaction [5] which generates cyclopentenones. The formation of a metallacyclopentadiene complex is implicated in a number of metal-alkyne reactions, arising from the metal assisted 2 + 2 cycloaddition of two alkyne molecules.…”

Catalytic Cyclooligomerization of Enones with Three Methylene Equivalents

“…The most prominent class of cyclooligomerization reactions involves the use of alkynes as substrates. 2 Cyclotrimers are favored under most transition-metal-catalyzed conditions because of the high thermodynamic stability of benzenes relative to cyclobutadienes, cyclooctatetraenes, and higher-order annulenes. Cyclooligomerization reactions using other π-components, such as 1,3-dienes 1a,3 and allenes, 4 have also been studied but generally exhibit poor selectivity and narrow substrate scopes.…”