Chelates substitution reactions

The addition of vinylmagnesium bromide to methyl (S)-3-benzyloxy-4-oxobutanoate (5) in tetrahydrofuran proceeded with a slight preference for the nonchelation-controlled reaction product (40 60)5°. A reversal of the diastereoselectivity (80 20) could be observed when the Grignard reagent, as a solution in tetrahydrofuran, was added to a dichloromethane solution of the aldehyde which had been precomplexed with one equivalent of magnesium bromide. The almost exclusive formation of the chelation-controlled reaction product 6 was achieved when tetrahydrofuran was completely substituted by dichloromethane the presence of tetrahydrofuran interferes with the formation of the chelate complex, which is a prerequisite for high chelation-controlled diastereoselection. 

Reactions through chelated TS Reactions of a- or (3-oxy-substituted aldehydes often show chelation-controlled stereoselectivity with Lewis acids that can accommodate five or six ligands. Chelation with substituents in the allylic reactant can also occur. The overall stereoselectivity depends on steric and stereoelectronic effects in the chelated TS. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

Except for one recent example, all iridium-catalyzed allylic substitution reactions have been performed under an inert atmosphere with dry solvent and reagents. The iridium metalacycle is sensitive to protonation, which opens the metalacycle and results in the formation of a less-active complex containing a K -phosphoramidite ligand. A paper by Helmchen et al. addressed this issue [107]. Nearly all iridium catalysts used for allylic substitution consist of an iridium fragment chelated by COD. In the presence of a catalyst containing dibenzo[a,c]cyclooctatetraene (dbcot) in place of COD, allylic substimtion reactions occur in air with results that are comparable to those of reactions performed under an inert atmosphere (Scheme 35). 

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