Intra-intermolecular Cycloaddition of Alkynes

11 is coordinated sterically to the iridium-cyclopentadienyl catalyst to form the intermediate 2.14 which through the Diels-Alder mechanism affords the derivative 2.15, and its reductive elimination results in the final product 2.12. The efficiency of the dppe ligand supports the assumption that upon coordination with iridium a five-membered chelate ring stabilizing intermediate 2.15 is formed thus facilitating the Diels-Alder reaction. This involves the formation of the w to-product 

Okamoto [9, 40, 41] and Hilt [9, 42, 43] with coworkers have developed a new and improved strategy for the catalytic trimerization of 

In 2003, Tanaka published data on the regioselective intermolecu-lar [2-b2-b2] homotrimerization of 21 terminal alkynes catalyzed by cationic complexes of Rh(I)-biaryldiphosphine [44]. Using 5 mol% [Rh(cod)2]BF4 and dtbm-segphos as a ligand, the cyclotrimerization of 

While testing two different catalysts, Tanaka found that cationic rhodium in a binary system (cationic Rh(I)/H8-binap) is effective in chemo- and regioselective addition reactions of terminal alkynes with acetylenedicarboxylate to form 1,2,3,4-tetra-substituted benzenes with excellent yield of 99% [9, 44, 45]. It is also important to note that this reaction is tolerant to a large number of functional groups, including alkenes, alkyl halides, and esters. Although cationic iridium complex Ir(I) did not give a positive result in the cycloaddition reactions, the authors showed that the catalytic system with neutral Ir(I) can facilitate cycloaromatization of dimethyl acetylenedicarboxylate and terminal alkynes [45]. 

Intermolecular cyclization of diyne with monoynes or intermolecular cyclization of triynes is a general method for the synthesis of substituted benzenes [51]. But a serious drawback of this reaction is the presence of a secondary process of diyne dimerization or monoyne trimerization. The selectivity and yield of the cyclic products strongly depend on the structure of the reactants, solvent, catalyst, etc. A highly efficient and selective [2-I-2-I-2] cyclization of diynes and monoynes was described. A rhodium catalyst allowed the reaction to proceed at room 

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