Transition metal-mediated cycloadditions

Cobalt, as its CpCo(CO)2 complex, has proven to be especially suited to catalyze [2 + 2 + 2] cycloadditions of two alkyne units with an alkyne or alkene. These cobalt-mediated [2 + 2 + 2] cycloaddition reactions have been studied in great detail by Vollhardt337. The generally accepted mechanism for these cobalt mediated cycloadditions, and similar transition metal mediated cycloadditions in general, has been depicted in equation 166. Consecutive co-ordination of two triple bonds to CpCo(CO)2 with concomitant extrusion of two molecules of carbon monoxide leads to intermediates 578 and 579 via monoalkyne complex 577. These react with another multiple bond to form intermediate 580. The conversion of 578 to 580 is said to be kinetically favored over that of 579 to 580. Because intermediates like 580 have never been isolated, it is still unclear whether the next step is a Diels-Alder reaction to form the final product or an insertion to form 581. The exact circumstances might determine which pathway is followed. 

Scheme 13.6 Representative transition metal-mediated cycloadditions for seven-membered ring synthesis.

The chemistry discussed in this section is described in detail by D. M. T. Chan in Volume 5, Part 3, Chapter 3.2 of this series Transition Metal-Mediated Cycloadditions . The elegant and related work of P. Binger and coworkers is also discussed there. 

Schore, N. E. Transition metal-mediated cycloaddition reactions of alkynes in organic synthesis. Chem. Rev. 1988, 88, 1081-1119. 

---