Quinone methide stabilization metal complexation

An alternative route for stabilization of quinone methides by metal coordination involves deprotonation of a ri5-coordinated oxo-dienyl ligand. This approach was introduced by Amouri and coworkers, who showed that treatment of the [Cp Ir(oxo-ri5-dienyl)]+ B1, 22 with a base (i-BuOK was the most effective) resulted in formation of stable Cp Ir(r 4-o-QM) complexes 23 (Scheme 3.14).25 Using the same approach, a series of r 4-o-QM complexes of rhodium was prepared (Scheme 3.14)26 Structural data of these complexes and a comparison of their reactivity indicated that the o-QM ligand is more stabilized by iridium than by rhodium. 

Moreover, the stability of the QM pincer complexes allows selective modifications of both the metal center and the carbonyl part of the quinone methide moiety, still with... 

An additional notable mode of the reactivity of these quinone methide complexes is formation of metal stabilized p- and o-xylylenes.10... 

These reactions clearly indicate that the exocyclic carbon of the complexed QM in these systems is nucleophilic in character, in contrast to its electrophilic nature in free o-quinone methides. The Cp Ir metal center stabilizes the mesomeric form in which the exocyclic carbon experiences high electron density (Scheme 3.18).29... 

Thus, research in the relatively new field of quinone methide complexes is attractive from both fundamental and applied point of views. Advantages of metal complexation include stabilization of the reactive QM moiety, the ability to affect its controlled... 

Vigalok, A. Shimon, L. J. W. Milstein, D. Methylene arenium cations via quinone methides and xylylenes stabilized by metal complexation. J. Am. Chem. Soc. 1998, 120, 477 183. 

Quinone derivatives such as quinone methides (the monomethylene analogues of quinones) have also been extensively studied ([183] and references cited therein) because they possess biological activity, particularly as antitumor agents. The first thermally stable quinone methide having no substituents in the methylene group ( simple quinone methide ) was crystallographically characterized [183a], It was shown that stabilization of the quinone methide can be achieved by complexation to a transition metal center (5.25) ... 

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