Stereochemical memory

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. 

A stereoselective intramolecular aldol reaction of thiazolidinecarboxylate (39) proceeds with retention of configuration to give fused heterocycles (40a,b separable) and (41), the product of a retroaldol-acylation reaction. The selectivity is suggested to be directed by self-induced axial chirality, in which the enolate generated in the reaction has a stereochemical memory, being generated in an axially chiral form (42). The retroaldol step also exemplifies a stereoretentive protonation of an enolate. 

The loss of stereochemical memory in the non-vertical excited state implies that stereospecific, concerted reactions of an alkene singlet state may take place from the vertical state. Of particular importance is that the change from cis or trans geometry to something in between opens up a route for converting one geometrical isomer of an alkene to another, and this is a photoisomerization reaction that will be described in the next section. 

Porter NA, Mills KA, Caldwell SE, Dubay GR (1994) The mechanism of the [3,2] allylperoxyl rearrangement a radical-dioxygen pair reaction that proceeds with stereochemical memory. J Am Chem Soc 116 6697-6705... 

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.

Castilla AM, Ousaka N, Bilbeisi RA, Valeri E, Ronson TK, Nitschke JR (2013) High-fidelity stereochemical memory in a FeIl4L4 tetrahedral capsule. J Am Chem Soc 135(47) 17999-18006... 

The conformational change from the chair form in the radical precursors to the boat form in the anomeric glycosyl radicals is so rapid that stereochemical memory is lost upon the generation of the radical. Thus, the reduction of P- and a-haloglycosides 1 and 7 by tributyltin deuteride afforded a-deuterated product 8 regardless of the stereochemistry of the starting materials (Scheme 4). ... 

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