Enantioselective copper-catalyzed allylic nucleophiles

Compared to the enantioselective allylic alkylation using soft nucleophiles, the reaction with Grignard reagents has received much less attention. The first enantioselective copper-catalyzed allylic alkylation with alkylmagnesium reagents was reported in 1995 by the groups of Backvall, van Koten and coworkers (Scheme 13). ... 

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. 

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. 

Several catalytic asymmetric allylation reactions have been developed specifically for iminoester derivatives, but the level of enantioselection remains typically only in the lowto high 80s (Table 1.15). Allylstannanes [99], allylsilanes [100], allyltrimethox ysilanes [101], and allylboronates [102] are suitable nucleophiles in these copper or zinc catalyzed processes. 

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... 

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