Stabilization of Unstable Intermediates

The examples given in this section are intended to illustrate the use of transition metal complexes in stabilizing compounds that cannot normally be isolated. In principle, the reactive intermediate or structure may then be generated, for reaction in situ or rapid examination, by removal of the scaffolding. Sometimes, useful transformations can be carried out on the complex before removal of the metal. 

Although cyclobutadiene is stable in a noble gas matrix at 8°K (Schmidt, 1973), this is not a convenient form for use in organic synthesis. The tricar-bonyliron complex of cyclobutadiene is a readily prepared, stable, pale yellow liquid [Eq. (156) (Pettit and Henery, 1970)]. Free cyclobutadiene (Schmidt, 

1973) can be generated in situ by oxidation of the complex with ceric ion (Watts and Pettit, 1966 Sanders et al., 1974). This reaction has been used in the synthesis of a variety of compounds such as Dewar benzenes (Watts et, al. 1965 Reeves et al., 1969 Meinwald and Mioduski, 1974), cubanes [Eq. (157) (Barborak and Pettit, 1967)], and caged keto sulfones [Eq. (158) (Paquette and Wise, 1967)]. Intramolecular reactions give higher yields of products if 

In a related reaction leading to a synthesis of homopentaprismanone, the cyclobutadiene complex behaves as the dienophile [Eq. (160) (Ward and Pettit, 1970)]. 

Variously substituted tricarbonyliron-cyclobutadiene complexes are readily prepared (Fitzpatrick et ah, 1965 Roberts et al., 1969 Agar et al., 1974) and cyclobutadiene complexes with other transition metals are known (Maitlis, 1966), but few of these have been used in organic synthesis. 

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Stabilization of Unstable Intermediates. Transition metals can stabilize normally unstable or transient organic intermediates. Cyclobutadiene has never been isolated as a free molecule, but it has been isolated and fully characterized as an iron tricarbonyl complex (138) ... 

Metal TT-complex formation is utilized for such seemingly unrelated applications as the separation or purification for analysis, resolution of racemic mixtures, and the stabilization of unstable intermediates. 

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