Two Alkynes, and CO or Isocyanides

The rhodium-entrapped cage compound which is formed using a stoichiometric amount of [RhCl(CO)2]2 is a notable paradigm of the rhodium-catalyzed [2-I-2-I-1] al-kyne-alkyne and CO coupling [35]. Heating 57 in acetone at 50 °C for 8 h or irradiation by a tungsten or mercury lamp provided the cage compound in 50% yield based on NMR spectroscopy. However, due to mechanical losses it was isolated in only 16% yield from the reaction mixture, by crystallization as the hexafluorophosphate salt 58 (Eq. 13). 

The Co2(CO)g- and [Ir(COD)Cl]2-catalyzed processes may be carried out in the presence of the appropriate ligand under a CO atmosphere. These conditions, as well as the direct use of Vaska s complex [IrCl(CO)(PPh3)2], are applicable to alkynes bearing bulky substituents, such as 59, for synthesis of the parent products 60. However, it should be noted that, in many cases, the thermodynamically more stable products 60  

Alternatively, rhodium catalysts have been revealed to be effective for the coupling of two alkynes with an isocyanide to afford the iminocyclopentadienes 62 and 62 in high yield (Tab. 11.9) [36bj. The coordinating solvent dibutyl ether, in combination with por-tionwise addition of the isocyanides, is key to the success of this transformation. 

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