The Stereochemistry of Allenic Enol Tautomerism – Independent Generation and Reactivity of the Enolates

Abstract

A priori, allenic enolates as reaction intermediates may be protonated to afford (Z)‐ or (E)‐α,β‐unsaturated carbonyl products. The allenic enolates are tautomeric with α‐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 α,β‐unsaturated carbonyl products. Under kinetic conditions, these highly reactive species are protonated in the α,β‐π plane with preference (E) to the larger β 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Heteropoly Compound Catalyzed Synthesis of Both Z‐ and E‐α,β‐Unsaturated Carbonyl Compounds

“…The tendency of the transformation into Z isomers was found to depend on the electronic nature of the aryl substituent at the propargyl position ( Table 2, On the basis of several experiments mentioned below, the unprecedented Z selectivity can be accounted for by the characteristic bulkiness and acidity of the heteropoly compounds. First, monitoring the reaction of 1 a with Ag 3 [PMo 12 O 40 ]·n H 2 O by 1 H NMR spectroscopy revealed that (Z)-2 a formed about ten times more quickly than (E)-2 a (Figure 1). Although a large amount of the dimer 3 was generated at the beginning of the reaction, 3 was found to react with Ag 3 [PMo 12 O 40 ]·n H 2 O under the same conditions to preferentially afford (Z)-2 a [(Z)-2 a/(E)-2 a % 10:1].…”

“…First, monitoring the reaction of 1 a with Ag 3 [PMo 12 O 40 ]·n H 2 O by 1 H NMR spectroscopy revealed that (Z)-2 a formed about ten times more quickly than (E)-2 a (Figure 1). Although a large amount of the dimer 3 was generated at the beginning of the reaction, 3 was found to react with Ag 3 [PMo 12 O 40 ]·n H 2 O under the same conditions to preferentially afford (Z)-2 a [(Z)-2 a/(E)-2 a % 10:1]. [10] We also observed a similar selective formation of (Z)-2 a with Na 3 [PMo 12 O 40 ]·n H 2 O as the catalyst.…”

“…Although a large amount of the dimer 3 was generated at the beginning of the reaction, 3 was found to react with Ag 3 [PMo 12 O 40 ]·n H 2 O under the same conditions to preferentially afford (Z)-2 a [(Z)-2 a/(E)-2 a % 10:1]. [10] We also observed a similar selective formation of (Z)-2 a with Na 3 [PMo 12 O 40 ]·n H 2 O as the catalyst. It is worth noting that even H 3 [PMo 12 O 40 ]·n H 2 O preferentially generated (Z)-2 a at an early stage of the reaction.…”

Heteropoly Compound Catalyzed Synthesis of Both Z‐ and E‐α,β‐Unsaturated Carbonyl Compounds

“…Among the various cations, a silver salt exhibited higher reactivity and Z selectivity (entries 2, and 5-7). The reaction of 1 a with Ag 3 [PMo 12 O 40 ]·n H 2 O in EtOAc gave (Z)-2 a in 66 % yield (NMR spectroscopy); furthermore, the use of the same silver salt in acetone dramatically improved both the yield and the stereoselectivity (entries 7-9). [9] Under the optimal reaction conditions, we subsequently examined the reactions of a series of secondary propargyl alcohols 1 b-g ( Table 2).…”

“…[7] One of the most common heteropoly acids is the Keggin-type compounds with the general formula H n [XM 12 O 40 ] in which X is the central heteroatom and M is the addenda atom. They possess a strong acidity and redox properties, both of which can be tuned by simply changing the cationic moiety and the polyanion chemical composition.…”