Allylic substitution leaving group effect

The effect of the leaving group was briefly examined, but cinnamyl bromide gave a substantially lower ee (38%). Cinnamyl dimethyl phosphonate, or acetate, gave very poor results. The cyclohexyl-substituted allylic acetate 21, on the other hand, afforded a completely y-selective reaction, but the product turned out to be racemic. Changing the Grignard reagent halide from bromide to either chloride or iodide resulted in very low ees. 

The effect of the nature of ion pairs as nucleophiles in a metal-catalysed substitution reaction has been investigated by determining product ratios for the Pd-catalysed allylic alkylations of substrates (9)-(ll) under various conditions, particularly with respect to catalyst ligands, nucleophiles, and counterions. Each dienyl acetate ionizes to form initially the vinyl (7r-allyl)-Pd intermediate corresponding most closely to the leaving group, i.e. (12) from (9), (13) from (11), but (12) and (13) from (10). The initial intermediate can then either be trapped by the nucleophile or it can begin to equilibrate to some mixture of vinyl 7r-allyl intermediates. If nucleophilic addition occurs before full equilibration, the product ratio is different for each substrate if... 

The allyl cation thus formed may stabilize itself either by readdition of the leaving group — leading to a 2,3-dihalopropene — or by the addition of a nucleophile. The influences of steric and electronic effects on the stereochemistry and on the solvolysis rates of various alkyl-substituted chlorocyclopropanes have been investigated by Parham and co-workers [165, 166], who could show for example that os-2,3-dipropyl-l,l-dichlorocyclopropane solvolyzes 24 times faster than its trans-isomer, in accordance with predictions based on orbital symmetry arguments. When one propyl substituent of the trans-isomer is replaced by an ethoxy group the rate of solvolysis increases 200 fold. 

Couplings WS Allyl Intermediates- Excellent reviews cover selectivities and mechanism in palladium catalysed allyl reactions. General examples of NaCH(CX)2R)2 additions to allyl acetates, carbonates, and chlorides have appeared. Trifluotoacetale is the preferred leaving group for retention of (E)/( stereochemistry in the case of substituted allyls. Ketomalonates are susceptible to diallylation under PdL2 catalysis especially in the presence of diacetates whidi prove effective routes to bicyclic systems (Equation 6). ... 

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