1-Hexyne reactions with carbon dioxide

Already in 1977 Inoue discovered the reaction of hexynes with carbon dioxide catalyzed by nickel complexes [86]. 1-Hexyne reacted with CO2 in the presence of a nickel/dppe-catalyst to give the butyl-substituted pyrone in 50 % yield (Figure 31). As by-products the oligomers of 1-hexyne were formed. Similar reactions proved to be possible with alkynes containing internal double bonds, such as 3-hexyne and 4-octyne [87,88], giving tetrasubstituted pyrones. Yields up to 60 % could be obtained. 

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

Carbon dioxide reacts with 1-hexyne to give 4,6-dibutyl-2-pyrone in addition to tributylbenzenes " [equation (13.239)]. A stoichiometric reaction of dienes with CO2... 

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