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Allylic substitution reactions offer some of the most practical methods for the synthesis of enantiomerically enriched materials. The products formed by these transformations offer multiple functionalities for further elaboration. In the recent years the field has extended from the original palladium catalysts, which preferentially forms new bonds at the less hindered site of an allylic species, to other transition metals that typically react at the more hindered site. Iridium catalysts have played a predominant role in this revolution, as high regioand enantioselectivities for a great range of transformations are observed. Even though problems still remain: substrates are linear, mono-substituted allylic carbonates, esters or halides, nucleophiles often have to be activated prior to addition, and while iridium catalysts preferentially form the branched products, the selectivity is not perfect. Catalytic systems that allow for the use of easily accessible branched unactivated allylic substrates and can directly employ benchstabil nucleophiles in highly regioand enantioselective allylic substitution reactions would be highly desirable. Building on earlier reports from the group, we studied the allylic amination of branched allylic alcohols with sulfamic acid in a highly enantiospecific manner. Easily accessible allylic alcohols of high optical purity were converted under overall retention of configuration to primary allylic amines with enantiospecificity (Scheme I). A novel (P,alkene)-Ir complex was identified as the catalyst. Furthermore an enantioselective protocol for the allylic amination was developed in the group. Scheme I : (P,Alkene)-Ir catalyzed Enantiospecific Allylic Amination Informed by preliminary mechanistic investigations from the allylic amination the (P,alkene)Ir catalyzed allylic substitution protocol was extended to alcohol nucleophiles. Here an enantioselective allylic etherification of racemic allylic alcohols formed products in high yield and enantioselectivity (Scheme II). The identification of another (P,alkene)-Ir complex was followed by the observation of a vast rate difference between the substrate enantiomers. Scheme II : (P,Alkene)-Ir catalyzed Enantioselective Allylic Etherification Finally, the direct use of thiols as nucleophiles for the (P,alkene)-Ir catalyzed enantioselective allylic substitution was accomplished, based on a deeper understanding of the mechanism governing the enantioselective transformation of racemic substrates to single enantiomer products. The use of phosphoric acid as promoter enabled the highly enantioselective allylic thioetherification to occur (Scheme III). Extensive mechanistic studies unveiled an enantioconvergent mechanism in which the substrate enantiomers are converted via two different mechanistic pathways to the same product enantiomers. Scheme III : (P,Alkene)-Ir catalyzed Enantioselective Allylic Etherification The distinguished features of the (P,alkene)-Ir catalyzed allylic substitution reactions are the direct use of branched allylic alcohols and non-ionic hetero-nucleophiles, the high regioselectivity and excellent enantioselectivity of the transformation, and the novel concept of deracemization in allylic substitution.

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Iridium-catalyzed allylic substitution: Unique reactivity of a (P,Alkene) Ir-Complex in the direct, enantioselective substitution of racemic allylic alcohols