Quinone methide stabilization

Using the same reaction, Cole and Wan later photogenerated mcta-quinone methides 110-112 from the appropriate m-hydroxy-a-phenylstyrenes.78 The three were characterized by LFP, with 110 and 111 showing strong and sharp absorptions at 430 and 450 nm, respectively, similar to those observed for 104 and 105. The dimethoxy-susbstituted 112 showed a much broader absorption centered at 420 nm. Addition of the electron-donating methoxy groups stabilized the quinone methides the lifetimes of 110-112 in 1 1 aqueous acetonitrile were 5-200 times longer than that of 104. 

Awad, H. M. Boersma, M. G. Boeren, S. van Bladeren, P. J. Vervoort, J. Rietjens, I. M. C. M. Quenching of quercetin quinone/quinone methides by different thiolate scavengers stability and reversibility of conjugate formation. Chem. Res. Toxicol. 2003,16, 822-831. 

Weinert, E. E. Dondi, R. Colloredo-Melz, S. Frankenheld, K. N. Mitchell, C. H. Freccero, M. Rokita, S. E. Substituents on quinone methides strongly modulate formation and stability of their nucleophilic adducts. J. Am. Chem. Soc. 2006, 128, 11940-11947. 

Amouri, H. Vaissermann, J. Rager, M. N. Grotjahn, D. B. Rhodium-stabilized o-quinone methides synthesis, structure, and comparative study with their iridium congeners. Organometallics 2000, 19, 5143-5148. 

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... 

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... 

Stabilization of quinone methides by coordination to transition metals might have various applications, beyond the important structural and spectroscopic studies of these coordinated compounds. For example, it is possible to manipulate their structure while coordinated to the metal center and affect their controlled release and trapping, leading to new synthetic procedures. 

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. 

Rabin, O. Vigalok, A. Milstein, D. Metal-mediated generation, stabilization, and controlled release of a biologically relevant, simple para-quinone methide BHT-QM. 

Thus, in complex 17, the stabilized ortho-quinone methide 3 was evidently not present in its traditional quinoid form, but in the form of a zwitterionic, aromatic... 

FIGURE 6.16 ortho-Quinone methide 3 stabilization of the zwitterionic rotamer in a complex with /V-methyImorpholine /V-oxide (17). The zwitterionic, aromatic precursor 3a affords the common quinoid form of the o-QM 3 by in-plane rotation of the exocyclic methylene group. 

FIGURE 6.17 Oxidation of a-tocopherol (1) conventionally leads to its spiro dimer (9) via ortho-quinone methide 3 (path A). The zwitterionic o-QM precursor 3a is stabilized by NMMO in complex 17, which upon rapid heating produces small amounts of new dioxocine dimer 18 (path B). Acid treatment of 18 causes quantitative conversion into spiro dimer 9, via o-QM 3 (path C). 

It was shown that complexes 19 of the zwitterionic precursors of ortho-quinone methides and a bis(sulfonium ylide) derived from 2,5-di hydroxyl 1,4 benzoquinone46 were even more stable than those with amine N-oxides. The bis(sulfonium ylide) complexes were formed in a strict 2 1 ratio (o-QM/ylide) and were unaltered at —78 °C for 10 h and stable at room temperature under inert conditions for as long as 15—30 min (Fig. 6.18).47 The o-QM precursor was produced from a-tocopherol (1), its truncated model compound (la), or a respective ortho-methylphenol in general by Ag20 oxidation in a solution containing 0.50-0.55 equivalents of bis(sulfonium ylide) at —78 °C. Although the species interacting with the ylide was actually the zwitterionic oxidation intermediate 3a and not the o-QM itself, the term stabilized o-QM was introduced for the complexes, since these reacted similar to the o-QMs themselves but in a well defined way without dimerization reactions. 

FIGURE 6.18 Oxidation of ortAo-methylphenols to the corresponding ortho-quinone methide via transient zwitterionic intermediates that are stabilized by forming a complex 19 with the 2,5-dihydroxy[l,4]benzoquinone-derived bis(sulfonium ylide). 

Selected Substituent-Stabilized Tocopherols and Conjugatively Stabilized Ortho-Quinone Methides... 

FIGURE 6.46 Oxidation chemistry of 5-(4-methylphenyl)- y-tocopherol (76), establishing a reaction system phenylogous to a-tocopherol (1), with quinone methide 77 and benzyl bromide 78 being the conjugatively stabilized, phenylogous counterparts of o-QM 3 and 5a-bromo-a-tocopherol (46), respectively. 

Patel, A. Netscher, T. Rosenau, T. Stabilization of ortho-quinone methides by a bis (sulfonium ylide) derived from 2,5-dihydroxy-[l,4]benzoquinone. Tetrahedron Lett. 2008, 49, 2442-2445. 

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