Cyclocotrimerizations of Alkynes with Other Unsaturated Molecules and Related Reactions

Cyclocotrimerizations of Alkynes with Other Unsaturated Molecules and Related Reactions 

The transition metal-catalyzed [2 -i- 2 -i- 2] cyclocotrimerization of two molecules of an alkyne with an alkene has studied to a lesser degree compared to the parent alkyne cyclotrimerization [9], although the resultant cyclohexadiene is a valuable synthetic intermediate (e.g., a diene component for the Diels-Alder reaction). This is because a 2 1 coupling of an alkyne and an alkene is generally difficult to compete with the more facile alkyne cyclotrimerization. The success of the selective coupling depends on the electronic balance between the employed alkyne and alkene components the combinations of an electron-deficient alkene with a neutral alkyne [35] or an electron-deficient alkyne with a neutral alkene [36] were successful in the previous 

The transition metal-mediated [2 -i- 2 -i- 2] cyclocotrimerization of two alkynes and a nitrile is a powerful and straightforward route to substituted pyridines [9]. In particular, catalytic cyclocotrimerization is undoubtedly desirable as a metal-atom economically and environmentally benign process. Effective catalysis, however, has been confined to cobalt [40], although a variety of transition metals (Ti [41], Zr/Ni [42], Ta [43], Co [44], and Rh [45]) have been found to mediate the stoichiometric cyclocotrimerization. With respect to ruthenium, pyridine formation from acetoni- 

The recent DFT calculations on model reactions support the regiodiemistry difference observed for the above cydocotrimerizations of heterocumulenes [33]. 

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