Cyclotrimerization of alkynes with nitriles

Selective cyclotrimerization of alkynes with nitriles produced pentasubstituted pyridines (1) with little formation of benzenoid products (Scheme 1) <94CB(127)2535>. 

Numerous transition metal-mediated [2 + 2+2] cycloadditions have been utilized in the synthesis of pyridines . Selective cyclotrimerization of alkynes with nitriles leads to pentasubstituted pyridines 310 with minimal formation of benzenoid byproducts (Scheme 157) <20000L3131>. Different alkynes can be utilized in the same strategy if a sequential approach is used (Scheme 158) <2000JA4994>. 

Among transition-metal complex catalyzed reactions of alkynes with carbon-heteroatom unsaturated compounds the most studied is co-cyclotrimer-ization of alkynes with nitriles to pyridines. For this process the same complexes can be used as for the cyclotrimerization of alkynes. The first report of a cyclopentadienylcobalt complex catalyzed co-cyclotrimerization of alkynes with nitriles appeared in 1973 [92] and was soon followed by other papers [93]. Co-cyclotrimerization of alkynes and nitriles with all its aspects has been recently reviewed [94] and because of that we will focus only on recent developments in this area. In this regard, advances have been made in simple co-cy-clotrimerization of ethyne with various nitriles [95], combinatorial synthesis of substituted pyridines [96], and co-cyclotrimerization of hydroxyalkynes with nitriles in aqueous media catalyzed by cobalt complex with hydrophobic chain attached to the cyclopentadienyl ring [97]. 

---