Nucleophilic substitution enantioselective elimination

TT-Allylpalladium intermediates described in this section can undergo either a /3-H elimination or a nucleophilic substitution reaction. Enantioselective nucleophilic substitution can afford the corresponding chiral compounds with high enantiomeric excesses. 

Allylic substitution using hard nucleophiles proceeds through a different mechanism. Instead of attacking the allyl group of the 71 allyl-metal complex, hard nucleophiles attack the metal first and the product is subsequently formed by reductive elimination. Nickel(O) complexes have often been used for this purpose. Reports of good enantioselectivities in this type of reaction are limited. 

The desymmetrization of meso-e poxides such as cyclohexene epoxide (55, Scheme 13.27) has been achieved both by enantioselective isomerization, e.g. to allylic alcohols (56, path A, Scheme 13.27) or by enantiotopos-differentiating opening by nucleophiles, affording trans-/ -substituted alcohols and derivatives (57, path B, Scheme 13.27). As indicated in Scheme 13.27, the allylic alcohols 56 can also be prepared from the ring-opening products 57 by subsequent elimination of the nucleophile. 

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