Iridium-Catalyzed Asymmetric Allylic Alkylation

Iridium-Catalyzed Asymmetric Allylic Alkylation with Catalysts Derived from LI... 

Giacomina F, Riat D, Alexakis A (2010) o-Ethylenic allylic substrates as alternatives to cyclic substrates in copper- and iridium-catalyzed asymmetric allylic alkylation. Org Lett 12 1156-1159... 

In 2010, Ueda and Hartwig reported on an iridium-catalyzed asymmetric allylation of sodium sulfinates 345 to branched allylic sulfones 348 with high regioselectivities and enantioselectivities (Scheme 46.40). Notably, the reaction proceeded with a broad range of acyclic allylic carbonates 346 and aryl and alkyl sodium sulfinates 345. Most recently, Zhao et al. developed the catalytic asymmetric allylic alkylations of acyclic allylic carbonates 346 using sodium thiophenoxide and alkyl thiolates 348 to give good-to-excellent selectivities for branched products 350 with excellent enantioselectivities. 

Although Helmchen et al. showed that asymmetric iridium-catalyzed allylic substitution could be achieved, the scope of the reactions catalyzed by iridium complexes of the PHOX ligands was limited. Thus, they evaluated reactions catalyzed by complexes generated from [lr(COD)Cl]2 and the dimethylamine-derived phosphoramidite monophos (Scheme 8) [45,51]. Although selectivity for the branched isomer from addition of malonate nucleophiles to allylic acetates was excellent, the highest enantiomeric excess obtained was 86%. This enantiomeric excess was obtained from a reaction of racemic branched allylic acetate. The enantiomeric excess was lower when linear allylic acetates were used. This system catalyzed addition of the hthium salts of A-benzyl sulfonamides to aUylic acetates, but the product of the reaction between this reagent and an alkyl-substituted linear aUylic acetate was formed with an enantiomeric excess of 13%. 

Palladium remains the most widely recognized transition metal to effect stereoselective allylic alkylation reactions. Consequently, diastereoselective and enantioselective Pd-catalyzed processes are extensively discussed in Sections 14.2 and 14.3. More recent advances in the field of metal-catalyzed al-lylation reactions include the use of chiral iridium complexes, dealt with in Section 14.4 [33, 34]. Section 14.5 describes selected stereoselective copper-catalyzed SN2 -allylation reactions [33, 35-37], while a brief presentation of allylation reactions with other transition metals including Mo and Rh is given in Section 14.6 [8, 13, 33, 38, 39]. The closing Section 14.7 deals with selected methods for asymmetric ring-opening reactions of unsaturated heterocycles [38, 40, 41]. 

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