Still-Gennari reaction mechanisms

Also obtained were significant amounts of allyl alcohol 49 and its trans-isomer 53, resulting from the reduction of the aldehyde 54. The structures were confirmed by comparison with authentic samples that were prepared from the trans Still-Gennari olefin 51. The mechanism of this double bond isomerization is not clear. It occurs at -78 °C even before the aldol reaction takes place and may be the consequence of an addition/elimination process of chloride or triethylamine induced by boron coordination to the aldehyde oxygen atom, but this is speculative. However, I should note at this point that exposing aldehyde 8 separately to all the reagents used in the process produces no change apart from minimal reduction in the presence of (+)-DIP-Cl. 

The mechanism for the Still-Gennari HWE reaction is shown in above for an aldehyde condensation. The phosphoryl-stabilized carbanion 23 attacks the carbonyl to give oxyanion intermediate 24, which then decomposes via a transient four-centered intermediate 25 to yield olefin 26. The stereochemistry is in favour of erythro isomer, a kinetic adduct. 

Use of diisopinocampheyl boron chloride in place of the triflate affords E(0)-enolates, but the isopinocampheyl ligands were ineffective for anti aldol reactions [48]. Encouraged by the molecular mechanics analysis of the ZfOj-enolate additions, Gennari and Paterson used computational methods to design a new boron ligand for use with E(G)-enolates [97]. The design was cued by Still s comment [98] that cis-2-... 

This reaction has been modified to occur under mild conditions such as those of Masamune and Roush, Still, and Ando. In addition, chiral phosphonates (or phospho-namides) have been used for this reaction. In particular, the method developed by Still and Gennari using [bis(trifiuoroethyl)phosphono] esters, generally known as Gennari-Still phosphonates, is very useful in the preparation of Z-olefins. Illustrated is the HWE reaction using Gennari-Still phosphonates to give c/ -olefins, and its mechanism. 

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