Synthetic methods 2 + 2 + 2 cyclotrimerization

Recently, the Yamamoto group and the Guitian group as pioneers in this field have developed efficient Pd-catalyzed reactions of arynes, particularly cyclotrimerization and cocyclization with alkynes, offering useful synthetic methods for polycyclic aromatic compounds [78]. The Spanish group reported that the cyclotrimerization of benzyne, generated from 251, occurred in situ at room temperature in the presence of Pd(PPh3)4 in MeCN, and obtained triphenylene (253) in 83 % yield [79]. 

In 2002, Wang and Yu reported that tris(2-pyridyl)-phosphine complex [P(2-py)3 W(C0)(N0)2] (Bp4)2 was found to be an efficient Lewis acid catalyst for the tandem reactions of Friedel-Crafts/aldehyde cyclotrimerization of (151) and (26a) that lead to the formation of a series of hyperbranched star polymers (153) (Scheme 16.40) [47]. These tandem reactions are carried out in neat substrates at ambient temperature with a low catalyst loading (0.05-1 mol%) to afford quantitative insoluble polymers almost instantaneously. The hyperbranched star structure and high contents of heterocycles are difficult to generate through conventional synthetic methods, and they are believed to be important elements for high-temperature polymers. 

Arene complexes are usually prepared by the following methods (i) metal vapor synthesis (see Metal Vapor Synthesis of Transition Metal Compounds).(ii) AEAIX3 reduction of a metal halide in the presence of the arene and (hi) alkyne cyclotrimerization (see Cyclodimerization -tri-merization Reactions). Synthetic procedmes to obtain r] -arene derivatives have been reviewed by Pampaloni and Calderazzo. Over the past ten years, studies in this... 

A common limitation of most classes of cyclotrimerizations is the difficulty associated with controlling the chemoselectivity of the process. In most instances involving inter-molecular couplings, multiple incorporation of the same aUcyne imit is imavoidable. However, with systems in which two aUcynes, or one alkene and one aUcyne are tethered, synthetically useful processes become possible. The first example of a selective addition of two equivalents of an allcyne and one alkene was reported by ChalkTsuda has applied this process to applications in polymer chemistry,and Ikeda has developed a similar method employing cyclic enones (Scheme 47).P l... 

The cyclotrimerization of alkynes catalyzed by transition metals is a general method for building substituted benzenes from aliphatic precursors. Multiple bonds are formed in these reactions in a single operation. Although the reaction of thermal trimerization relates to allowed electro-cyclic processes, it is catalyzed by several transition metals, such as Co, Ni, Rh, Pd, Rh, and Ru [38]. Most recent publications show promise for the participation of transition metal complexes in [2+2+2] cycloaddition reactions based on zirconium, titanium, and indium [9]. This reaction has synthetic potential for using metallocyclopentadienes as intermediates in the cyclotrimerization of alkynes. The reaction mechanism is shown in Scheme 2.1 [3, 38]. Two alkyne molecules coordinated to the metal, that is, complex 2.1, couple to form cyclopentadiene 2.2. Next there is either addition of the alkyne to the metallocycle 2.3 to form the metallocycle... 

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