Nickel catalysis cyclotrimerization

Reinhard, S., Soba, P., Rominger, F., and Bluemel, J. (2003) New silica-immobilized nickel catalysts for cyclotrimerizations of acetylenes. Advanced Synthesis and Catalysis, 345, 589-602. 

Related co-cyclotrimerizations of two alkyne molecules with limited isocyanates have also been achieved using cobalt and nickel catalysts. With respect to intramolecular versions, two examples of the cobalt(I)-catalyzed cycloaddition of a,m-diynes with isocyanates have been reported to afford bicyclic pyri-dones only in low yields, although 2,3-dihydro-5(lff)-indolizinones were successfully obtained from isocyanatoalkynes and several silylalkynes with the same cobalt catalysis [19]. On the other hand, the ruthenium catalysis using Cp RuCl(cod) as a precatalyst effectively catalyzed the cycloaddition of 1,6-diynes 21 with 4 equiv. of isocyanates in refluxing 1,2-dichloroethane to afford bicyclic pyridones 25 in 58-93% yield (Eq. 12) [20]. In this case,both aryl and aliphatic isocyanates can be widely employed. 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

Alkynes can also serve as substrates in [3 -I- 2] cycloadditions to MCP, as exemplified by the reaction of but-2-yne under nickel/phosphite catalysis to provide l,2-dimethyl-4-methylene-cyclopent-l-ene (2). 2 However, alkyne oligomerization, specifically cyclotrimerization, cannot be avoided with alkyl-substituted alkynes. When alkynes with electron-withdrawing substituents are employed, cyclotrimerization becomes the exclusive reaction. 

Since Reppe s discovery of the cyclotrimerization of acetylene to benzene in the presence of nickel carbonyl-phosphine complexes, the use of nickel catalysts in many organic transformations has become popular. Transition metal complex catalysis provides many elegant entries to carbon-carbon bond-forming reactions in organic synthesis. One notable example is carbocyclic ring expansion mediated by nickel(O) complexes. ... 

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