Mechanism of Ruthenium-Catalyzed Cycloadditions

A myriad of metallacyclopentadiene complexes relevant to cyclotrimerization have been isolated representativeruthenacyclopentadienes are depicted in Table 3.16. Except for 91, these complexes are not only reactive intermediates but also competent catalysts for relevant [2 -i- 2 -i- 2] cycloadditions. For example, carbonyl complexes 1 and 90 catalyze the cyclotrimerization of electron-deficient alkynes [8a,20], and 

99 from the reaction of isolated 98 with acetylene. Naphthoquinone-fiised complex 

100 was also prepared in a good yield from 6 and l,2-bis(phenylpropiolyl)benzene. In contrast to the other examples, 100 gradually isomerized to cyclobutadiene complex 

On the basis of DPT calculations, mechanisms similar to that of alkyne cyclotrimer-ization were proposed for the cyclocotrimerizations of acetylene either with electron-deficient nitriles [57,85] or heterocumulenes [86]. Aza- or thia-ruthenabicycle intermediates were postulated for these cyclocotrimerizations. 

The mechanistic considerations above imply that the substitution pattern of cyclopentadienyl ligands influences the regiochemical outcome of the cycloaddition of unsymmetrical diynes using catalyst 6. The stereoelectronic effect of the 

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