Quinone Methide Metal Complexes

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

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... 

The synthesized complex does not react with air, carbon monoxide, or trimethylpho-sphine. It is noted [183a] that the rhodium center is very strongly bound to the quinonoid ligand. Recently, Vaissermann et al. have synthesized and fully characterized (even by x-ray diffraction) the first o-quinone methide complex, with Ir as the metal center [183b]. This complex is highly stable at room temperature, a notable difference when it is compared with the simplest known o-quinone methide that is unstable above — 100°C [183c],... 

Phenol complexes of [Os] display pronounced reactivity toward Michael acceptors under very mild conditions. The reactivity is due, in part, to the acidity of the hydroxyl proton, which can be easily removed to generate an extended enolate. Reactions of [Os]-phenol complexes are therefore typically catalyzed using amine bases rather than Lewis acids. The regio-chemistry of addition to C4-substituted phenol complexes is dependent upon the reaction conditions. Reactions that proceed under kinetic control typically lead to addition of the electrophile at C4. In reactions that are under thermodynamic control, the electrophile is added at C2. These C2-selective reactions have, in some cases, allowed the isolation of o-quinone methide complexes. As with other [Os] systems, electrophilic additions to phenol complexes occur anti to the face involved in metal coordination. 

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 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. 

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... 

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 methides have been generated by reaction of their transition metal complexes.127 The T)2-methylene-coordinated complex 77 forms stable solutions in water and methanol. NMR spectroscopy showed that 77 and dibenzy-lideneacetone (DBA) in methanol undergo rapid conversion to 79 (Scheme 37). This is consistent with the reaction of DBA with the palladium ligand at 77 to... 

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) ... 

---