Reverse Julia olefination

The modified Julia olefination of aromatic aldehydes with alkyl benzothiazol-2-yl sul-fones has been investigated experimentally, and by DFT, with a view to identifying the origin of the high F-selectivity. The reversibility of the addition is very variable, so the selectivity of the formation of the sulfinate intermediate also varies. However, the syn- and anri-sulfinates both eliminate in concerted processes syn- and anri-periplanar, respectively), thus leading preferentially to ( )-alkene. 

The mechanism and origin of high E selectivity in the modified Julia olefination of aromatic aldehydes have been computationally and experimentally investigated. Reversibility of the addition and hence selectivity of the formation of sulfinate (7) depends on the nature of the sulfone substrate. However, elimination occurs through a concerted antiperiplanar and synperiplanar mechanism for sulfinates anti-0) and syn- l), respectively. The high experimental E selectivity (up to 90% de) is thus explained by -selective elimination, from both the syn- and the anft -diastereomer. 

This mechanistic hypothesis has since been corroborated by several experiments (Scheme 3.37). For example, reaction of substrate 192, possessing both a j8-hydroxy sulfone and a j8-benzoyloxy sulfone, with Sml2 at —78 °C, affords exclusively the olefin 193 originating from the elimination of the sulfonylbenzoate. An even more remarkable example is provided in reaction 3.30, in which exclusive formation of triene 195 from substrate 194 takes place. Despite the closer proximity of the OH and phenylsulfone substituents, only the elimination involving the benzoate is observed. This reverse elimination appears to be specific to benzoate derivatives and opens new vistas in the development of mild conditions for the Julia olefination of sensitive polyfunctional molecules. 

Scheme 19.24 Effect of the base on the stereoselectivity of the Julia-Kocienski olefination. The conplexity of the stereoselectivity issue arises from the reversibility of the first step of the...

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