Chiral metal complexes electrophilic allylation

Many transition metal complexes catalyse the reaction but palladium systems are the most widely used. Allylic substitution can be used to create C-C as well as C-X (X = heteroatom) bonds under very mild conditions, which are compatible with many functional groups. The allylic substitution reaction is unique in the sense that there are many mechanisms that can be responsible for asymmetric induction and because chiral elements can be placed at the nucleophile, the electrophile or both. 

The Simmons-Smith reaction is an efficient and powerful method for synthesizing cyclopropanes from alkenes [43]. Allylic alcohols are reactive and widely used as substrates, whereas a,j8-unsaturated carbonyl compounds are unreactive. In 1988, Ambler and Davies [44] reported the electrophilic addition of methylene to a,/3-unsaturated acyl ligands attached to the chiral-at-metal iron complex. The reaction of the racemic iron complex 60 with diethylzinc and diiodomethane in the presence of ZnCl2 afforded the c/s-cyclopropane derivatives 61a and 61b in 93 % yield in 24 1 ratio (Sch. 24). 

The Sharpless asymmetric epoxidation (sec. 3.4.D.i) exploits this chelation effect because its selectivity arises from coordination of the allylic alcohol to a titanium complex in the presence of a chiral agent. The most effective additive was a tartaric acid ester (tartrate), and its presence led to high enantioselectivity in the epoxidation.23 An example is the conversion of allylic alcohol 40 to epoxy-alcohol 41, in Miyashita s synthesis of the Cg-Ci5 segment of (-t-)-discodermolide.24 in this reaction, the tartrate, the alkenyl alcohol, and the peroxide bind to titanium and provide facial selectivity for the transfer of oxygen from the peroxide to the alkene. Binding of the allylic alcohol to the metal is important for delivery of the electrophilic oxygen and... 

Many diastereoselective allylations form a new stereocenter at one of the allylic carbons and at the nucleophilic carbon. For example, an iridium complex containing a phosphite ligand catalyzes enantioselective and diastereoselective formation of products containing two stereocenters, one at the original nucleophile and one at the original allyl electrophile (Equation 20.58). In another example shown in Equation 20.59, Trost s palladium catalyst leads to the reaction of allylic esters with chiral azlactone pronucleophiles with high diastereomeric and enantiomeric excess, as does the related molybdenum catalyst. In these cases, the metal appears to control the new stereocenter at the allyl group, as well as the relationship between this stereocenter and the new stereocenter formed at the nucleophile. 

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