Substitution reactions allyl acetates, resolution

Catalysts lacking phosphorus ligands have also been used as catalysts for allylic substitutions. [lr(COD)Cl]2 itself, which contains a 7i-accepting diolefin ligand, catalyzes the alkylation of allylic acetates, but the formation of branched products was only favored when the substitution reaction was performed with branched allylic esters. Takemoto and coworkers later reported the etherification of branched allylic acetates and carbonates with oximes catalyzed by [lr(COD)Cl]2 without added ligand [47]. Finally, as discussed in Sect. 6, Carreira reported kinetic resolutions of branched allylic carbonates from reactions of phenol catalyzed by the combination of [lr(COE)2Cl]2 and a chiral diene ligand [48]. 

Allyl acetates 1/ent-l that possess identical R groups undergo aUyUc substitution via an achiral intermediate 2. Both enantiomers of starting material proceed via the same intermediate. In the absence of any controlling influence, approach of the nucleophile via pathways a and b is equally likely, and a racemic product 3/ent-3 will be formed (Scheme 1). However, the opportunity for an asymmetric catalytic reaction exists if the reaction can be channeled through one pathway selectively. Overall, the process represents a dynamic resolution, since a racemic starting material is converted into an enan-tiomericaUy emiched product. 

When a racemic allylic substrate is employed in an enantioselective substitution reaction, one of the two substrate enantiomers may react more quickly than the other. This effect is a kinetic resolution and has been noted reasonably often in enantioselective allylic substitution reactions. Several studies on kinetic resolution have been reported, - and a few highlight reactions are noted in Scheme 45. These include recovery of unreacted cy-clohexenyl acetate 92, as well as the tetraacetate 225. Kinetic resolution has also been observed in allylic snbstitution using a snlfinate nucleophile (Li02STlu) with allyl acetate... 

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