Copper-catalyzed allylic substitution nucleophiles

AUylic substitution catalyzed by copper is a transformation that is related to ally-lie substitutions catalyzed by other transition metals discussed previously in this chapter, but several features of copper-catalyzed allylations make them worth differentiating. First, copper-catalyzed allylic substitutions are conducted with different types of nucleophiles tiian most allylic substitutions catalyzed by other metals. Second, the regioselectivity of the copper-catalyzed reactions is typically different from that of reactions catalyzed by complexes of other metals, particularly of reactions catalyzed by complexes of palladium. Thus, this last section of tiie chapter describes studies on allylic substitution catalyzed by copper, witii an emphasis on enantioselective examples. 

The scope of enantioselective, copper-catalyzed allylic substitution reactions is not limited to so-called hard carbon nucleophiles and achiral acyclic linear electrophiles. A recent report from Ito, Sawamura, and co-workers showed that a diboron reagent can serve as a pronucleophile for enantioselective, copper-catalyzed boronation of (Z)-aUylic carbonates (Equation 20.85). The corresponding chiral allylboronates were isolated in good yields with high enantioselectivities. 

Allylic substitutions with nonstabilized C-nucleophiles are an important domain of organocopper chemistry [51]. However, on close inspection of the literature, it becomes apparent that regioselectivity in favor of the branched allylic alkylation products is only obtained with alkyl copper compounds, while aryl copper compounds mainly give the linear alkylation products. This observation was an incentive for Alexakis et al. [52] to probe the reactions of aryl zinc hahdes in the Ir-catalyzed allylic substitution (Scheme 9.18). 

Scheme 22 Copper(I)-catalyzed allylic substitution of carbonates with boron nucleophiles by Sawamura...

Independent from the work of Oestreich and co-workers, the group of Shintani and Hayashi presented another example of NHC-copper(I)-catalyzed asymmetric allylic substitution of allylic phosphates by nucleophilic silicon (Scheme 28) [61]. The NHC precursor L23 in combination with CuCl and NaOH as base allowed for the enantioselective preparation of several a-chiral allylic silanes. The choice of base is cmcial as aUcoxides had a negative effect on the regioselectivity (not... 

The chloroacetoxylation reaction is synthetically useful since the chloride can be substituted with either retention [Pd(0)-catalyzed reaction] or inversion (Sjv2 reaction) by a number of nucleophiles. In this way both the cis and trans isomers are accessible and have been prepared from a number of allylic acetates (Schemes 5 and 6). In a subsequent reaction the allylic acetate can be substituted by employing a copper- or palladium-catalyzed reaction. The latter reactions are stereo specific. 

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