Stereochemical "memory effect

Keywords Alkene radical cations Ion pairs Kinetics Stereochemical memory effects Tandem reactions... 

In an extensive investigation of the stereochemical memory effect, a series of six diastereomeric pairs of substrates was prepared to probe the effect of single, then multiple substituents on the 5-exo cyclization of amines onto alkene radical cations [144,145]. Overall, these cyclizations were highly dia-stereoselective and were accounted for by a transition-state model employing a chairlike transition state with attack of the nucleophilic amine on the opposite face of the alkene radical to the one shielded by the phosphate anion in the initial contact ion pair (Scheme 34), as exemplified in Schemes 35 and 36. 

Starting from enantioenriched branched substrates, a strong stereochemical memory effect is observed. With sNus, the substitution reaction occurs with net retention of configuration at the reacting center. If hNus are used, the attack at the aUyl system proceeds according to an inner-sphere mechanism resulting in a net inversion (Scheme 12.88). 

Scheme 12.88 Stereochemical memory effect in Rh-catalyzed allylic alkylation.

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