Group 4 metal benzyl catalysts mechanism

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

Further evidence for the proposed mechanism in which both the alcohol and ketone are co-ordinated to the same Lewis-acid metal site came from the liquid-phase reaction of prochiral phenylacetone with (5)-2-butanol [33]. Both the aluminum- and titanium-containing zeolite Beta catalysts gave the corresponding alcohol in an enantiomeric excess (e. e.) of 34%. The positive rotation of the reaction mixture after removal of the excess alcohol proved the preferred formation of the (5)-(+)-l-phenyl-2-propanol enantiomer. The formation of the (S) enantiomer can be explained by the preferential orientation of the bulky benzyl group of phenylacetone relative to the spatially smaller methyl group of (5)-2-butanol in the transition state (Scheme 6). 

Following the work on metal-phosphine coordination polymers, the group started investigating mechanical dissociation of silver(I)-coordination complexes with A-heterocyclic carbene (NHC) functionalised polymers [81]. It has been shown that polymers with an Ag(NHC)2 coordination complex in the pTHF main chain have significantly lower values. pTHF has an M around 40 kg moP whereas the for Ag(NHC-pTHF)2PF6 is lower than 13 kg mol [75]. Thus external force selectively breaks Ag-NHC bonds and yields fi-ee NHC, which was used to catalyse the transesterificatiOTi of benzyl alcohol and vinyl acetate under sonication [82, 83] (Fig. 15). The complex form of the carbene displayed no activity, proving the latency of the catalyst. Control experiments confirmed that the catalyst was activated mechanically. 

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