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Structures and Reactivities of Sodiated Evans Enolates: Role of Solvation and Mixed Aggregation on the Stereochemistry and Mechanism of Alkylations

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journal contribution
posted on 21.11.2018, 00:00 by Zirong Zhang, David B. Collum
Oxazolidinone-based sodiated enolates (Evans enolates) were generated using sodium diisopropylamide (NaDA) or sodium hexamethyldisilazide (NaHMDS) in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA), (R,R)-trans-N,N,N′,N′-tetramethylcyclohexanediamine [(R,R)-TMCDA], or (S,S)-TMCDA. 13C NMR spectroscopic analysis in conjunction with the method of continuous variations (MCV), x-ray crystallography, and density functional theory (DFT) computations revealed the enolates to be octahedral bis-diamine-chelated monomers. Rate and computational studies of an alkylation with allyl bromide implicate a bis-diamine-chelated-monomer-based transition structure. The sodiated Evans enolates form mixed dimers with NaHMDS, NaDA, or sodium 2,6-di-tert-butylphenolate, the reactivities of which are examined. Stereoselective quaternizations, aldol additions, and azaaldol additions are described.

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