Grubbs ligand substitution

More recently, Grubbs et al. obtained a refined mechanistic picture of the initiating step by conducting a 31P NMR spectroscopic study of the phosphine exchange in precatalysts 12-A. These investigations revealed that substitution of the phosphine proceeds via a dissociative-associative mechanism, i.e., a 14-electron species 12-B is involved that coordinates the alkene to give a 16-electron species 12-C (Scheme 12) [26a]. Increased initiation rates are observed if the substituents R and the phosphine ligands PR3 in precatalysts... 

Grubbs and co-workers have recently reported a new class of Ru catalysts (83, Eq 2) [29] that bear a chiral monodentate N-heterocyclic carbene ligand [30]. The reactions illustrated in Eq 2 include the highest ee reported (13-90% ee) asymmetric induction is clearly dependent on the degree of olefin substitution (cf. Schemes 18 and 4 for comparison with the Mo-catalyzed reactions of the same substrates). As is the case with nearly catalytic enantioselective reactions [4], the identity of the optimal catalyst depends on the substrate a number of chiral... 

Scheme 12 Substitution of phosphine ligands in first-generation Grubbs catalyst...

Ccr . atives from three alkyne a I n substituted benzenes are il. - Lontaining Ph P ligands. h. ie Grubbs and Wilkinson... 

ROM has been used to prepare phosphine-containing polymer supports (Scheme 20). Norbornyl-substituted monomer 22 was prepared in two steps from d-bromo-iodobenzene. This was then polymerized with diene 23. It was initially envisioned that it would be necessary to convert the phosphine to the borane adduct in order not to poison the metathesis catalyst. Although protection was needed when using the Grubb s type 1 complex as a catalyst, when employing the more active second-generation complex 24, the free phosphine monomer could be used. This has been attributed to the lower affinity of the active form of the catalyst toward coordination of phosphines due to the presence of the electron-rich heterocyclic carbene ligand. 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

Barbasiewicz et al. [99] reported on the Hoveyda-Grubbs metathesis catalyst bearing a chelating benzylidene ligand assembled on / en -substituted naphthalene 8.68, as shown in Eq. (8.22). In contrast to usual naphthalene-based compounds 8.67, it exhibits a very fast initiation behavior for a ring-closing metathesis reaction (Eq. (8.22)), which is attributed to a distorted molecular structure and reduced i-electron delocalization within a weakly stabilized six-membered chelate ring. 

It is well known that the reaction of the Grubbs complexes with alkyl vinyl ethers readily occurs at room temperature to yield the ruthenium complex with alkoxy-substituted carbene ligand, via a ruthenacyclobutane intermediate (Scheme 11) [28]. 

The Attachment of Grubbs-Type Catalysts via the X-Type Ligand The substitution... 

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