Structures of three lithium ester enolates by x-ray diffraction: derivation of reaction path for cleavage into ketene and alcoholate

Abstract

Crystal structure analyses have been carried out for lithium enolates of the following three esters: tert-butyl propionate ((Z)-1), tert-butyl2-methylpropionate (2), and methyl 3,3-dimethylbutanoate ((Z)-3). Enolates (Z)-1 and 2 are dimeric (with one TMEDA per Li atom), whereas (2)-3 is tetrameric (with one THF per Li atom). The Z configuration of 1 and 3 established by X-ray analysis is in agreement with that assigned by Ireland. From a detailed analysis of the geometry of the ester enolate grouping, the reaction path trajectory for the breakdown of this type of molecule can be derived. The implication of ketenes as intermediates, suggested by this analysis, could be confirmed chemically. In spite of the central importance of ester enolates as reactive intermediates in synthetic organic chemistry, very little is known about the actual structures of the metallated species involved. This lack of information is largely attributable to the low stability of ester enolates in general. In fact, the structures of only two metal derivatives of esters have been established by X-ray analysis: one is the Reformatksy reagent derived from tert-butyl bromoacetate,2 and the other is a lithium derivative of the highly acidic penta- kis(methoxycarbonyl)cyclopentadiene.3 Neverthless, it has been more than 10 years since the Lochmann4 and Rathke5 groups described the isolation of pure lithium derivatives of esters as solid substances. The analysis of these compounds was at that time limited to what could be achieved by IR and NMR spectroscopy; the deduction was that they are to be formulated as enolates. Deprotonation of esters of the type RR'CH-COOR with LDA (lithium diisopropylamide) gives in principle (E)- or (Z)-con- figurated enolates, shown as (E)-1 and (Z)-1 for the example of