The stereochemistry of allenic enol tautomerism - Independent generation and reactivity of the enolates

A priori, allenic enolates as reaction intermediates may be protonated to afford (Z)- or (E)-alpha,beta-unsaturated carbonyl products. The allenic enolates are tautomeric with alpha-vinylcarbanions. The literature on the behavior of these species on protonation is highly varied both in stereochemical outcome and in mechanistic interpretation. The current study has provided an independent mode of generation of the allenic enolates and has investigated the reaction stereochemistry of protonation to afford the stereoisomeric alpha,beta-unsaturated carbonyl products. Under kinetic conditions, these highly reactive species are protonated in the alpha,beta-pi plane with preference (E) to the larger beta group. Under thermodynamic conditions, addition/elimination equilibrates the two product stereoisomers. The kinetic protonation stereochemistry is a function of solvent, proton donor, and donor concentration. Computations serve to clarify the reaction mechanism. It was found that the stereochemistry of ketonization of allenic enolates follows the reaction course suggested as possible some decades earlier and common to less unique enolates. Additionally, the linear versus the bent enolate structure proves to depend on the countercation. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).