Quinone methides ligand

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

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. 

Colloredo-Melz, S. Dorr, R. T. Verga, D. Freccero, M. Photogenerated quinone methides as useful intermediates in the synthesis of chiral BINOL ligands. J. Org. Chem. 2006, 71, 3889-3895. 

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... 

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],... 

An enantiosdective version of this reaction was presented shortly thereafter, which uses the oxazohne ligand (Eq. (4.14)) [27]. The reaction results in good to excellent enantioinduction for a series of 2 -monosubstituted alkenes, while a dimethyl substitution at the terminal alkene carbon leads to 0% ee. This was interpreted by an overall mechanism for which the enantioselection is derived from the first step of alkoxypalladation followed by faster palladium loss from the corresponding quinone methide intermediate than nucleophilic attack by the second alkoxide. 

The role of 7i-alkene, a-alkenyl and 7i-allyl complexes in the Pd-catalysed oxidative esterification of alkenes. Changes in selectivity were affected by solvent and chirality changes in the ancillary ligand sets. " The stabilisation of p-quinone methides was achieved by coordination to the [Pd(dppe)] fragment to yield compopund 39 which was structurally characterised. ... 

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