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Stereoselective enolization is needed for stereoselective aldols

المؤلف:  Jonathan Clayden , Nick Greeves , Stuart Warren

المصدر:  ORGANIC CHEMISTRY

الجزء والصفحة:  ص870-871

2025-07-21

587

Stereoselective enolization is needed for stereoselective aldols

The cyclic transition state explains how enolate geometry controls the stereochemical outcome of the aldol reaction. But what controls the geometry of the enolate? For lithium enolates of ketones the most important factor is the size of the group that is not enolized. Large groups force the enolate to adopt the cis geometry; small groups allow the trans enolate to form. Because we can’t separate the lithium enolates, we just have to accept that the reactions of ketones with small R will be less diastereoselective.

With boron enolates, we don’t have to rely on the structure of the substrate—we choose the groups on boron—and we can get either cis or trans depending on which groups these are. Boron enolates are made by treating the ketone with an amine base (often Et3N or i-PrNEt₂) and R2B–X, where X− is a good leaving group such as chloride or triflate (CF₃SO₃⁻). With bulky groups on boron, such as two cyclohexyl groups, a trans enolate forms from most ketones. The boron enolate reacts reliably with aldehydes to give ant aldol products through the same six membered transition state that you saw for lithium enolates.

With smaller B substituents, the cis enolate forms selectively. Here, the boron is part of a bicyclic structure known as 9-BBN (9-borabicyclononane). The bicyclic part may look large but, as far as the rest of the molecule is concerned, it’s ‘tied back’ behind the boron, and the methyl group can easily lie cis to oxygen. The cis enolate then gives syn aldol products. Di-n butyl boron triflate (Bu₂BOTf) also gives cis enolates.