Allylic double inversion process

Treatment of the enantiomerically enriched acyclic allylic carbonate (S)-l (97% ee) under the standard reaction conditions furnished the allylic alkylation product (S)-14 (95% ee) in 86% yield, with net retention of absolute configuration (Eq. 3). This result implies that the displacement occurs via a classical double inversion process, albeit through a configurationally stable distorted u-allyl or enyl ff+n) organorhodium intermediate. This is supported by the fact that the achiral ff-spedes iii would undoubtedly have afforded the racemate of 14 (Scheme 10.3). Additionally, the enyl (a+n) organo-metallic intermediate provides a model for the regio- and enantiospedfidty observed in the reaction. 

In combination with the inversion step in the oxidative addition, allyl—X ir-allyl precursors show overall retention of configuration relative to the C—X bond via a double inversion process (equation 152). 

Similarly, lithium enolates of ketones add to allyl acetates via Pd° catalysis by a double inversion process,110 and potassium ketone enolates have been shown to add to preformed rr-allyl complexes with inversion (equations 156 and 157)."1... 

The Mitsunobu reaction is one of the staple reactions for clean nucleophilic substitution with inversion of configuration. It came as a surprise, therefore, to find that a Mitsunobu reaction on allylic alcohol 143.1 [Scheme 8.143] using rerr-butyl 2-(trimethylsilyl)ethylsulfonylcarbamate (143.2) as the nucleophile occurred with retention of configuration. o unusual stereochemistry was explained by a double inversion process in which neighbouring group participation first leads to the intermediate 1433. A subsequent second nucleophilic substitution by 1433 then gave the product 143.4 in 86% yield. 

In instances where the two termini of the allyl moiety possess nonidentical groups, the issues of both regiocontrol and enantiocontrol can become important (Scheme 21). Typically, the nucleophile approaches from the less sterically hindered terminus, and the mechanism proceeds via a double inversion (overall retention) of stereochemistry process. Consequently, the use of a racemic substrate is unlikely to lead to a single regio-isomeric product with high enantioselectivity. 

Treatment of the fraw.Mrart.s-germacranolide, epitulipanolide (477) and two of its derivatives with selenium dioxide and t-butyl hydroperoxide gives the melampolide-type alcohol (478).218 A mechanism for this allylic oxidation with inversion of configuration at the A1-10 double bond is proposed. It should be noted, however, that this is not the first report of such a process (cf. Vol. 11, p. 55). 

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