Enolate cations

The order of enolate reactivity also depends on the metal cation which is present. The general order is BrMg < Li < Na < K. This order, too, is in the order of greater dissociation of the enolate-cation ion pairs and ion aggregates. Carbon-13 chemical shift data provide an indication of electron density at the nucleophilic caibon in enolates. These shifts have been found to be both cation-dependent and solvent-dependent. Apparent electron density increases in the order > Na > Li and THF/HMPA > DME > THF >ether. There is a good correlation with observed reactivity under the corresponding conditions. 

It is well known that neutral simple ketones 163 in general are more stable than their isomeric enol form 164b0 6l). However, for the corresponding cation radicals 165 and 166 it has been shown both experimentally62,63) and computationally64,65 that, depending on the substituent R, the ionized ketones 165 are significantly less stable than their isomeric enol cation radicals 166 (41). Moreover, CA spectroscopy... 

Acid catalysis was observed in the formation of 171 from 168, and the enol cation 170 was proposed as an (invisible) intermediate.830... 

The difference in conjugation between neutral molecules and their ion-radicals can also be traced for keto-enol tautomerism. As a rule, enols are usually less stable than ketones. Under the equilibrium conditions, enols exist only at a very low concentration. However, the situation becomes different in the corresponding cation-radicals, where gas-phase experiments have shown that enol cation-radicals are usually more stable than their keto tautomers. This is because enol cation-radicals profit from allylic resonance stabilization that is not available to ketones (Bednarek et al. 2001, references therein). 

As revealed, cation radicals of ferrocenyl ethylenes do not undergo the cis-to-trans isomerization. Calculations show that the cation radical s center is located exclusively at the iron atom, with no participation of the ethylene bond (Todres et al. 1992). Hence, one-electron oxidation of this ferrocenyl ethylene occurs at the iron atom exclusively anything else would be extremely unusual in this case. Thus, the recently described stable enol linked to a ferrocene redox center gives the cation radical upon one-electron oxidation. This species is better characterized as a ferricenium salt than as an enol cation radical (Schmit-tel Langels 1998). 

The reactions of enol ester radical cations formed in anodic oxidations were pioneered by Shono [220-225] almost two decades ago. A reaction mode was identified that formally corresponds to that of enol cation radicals. Depending on the electrolysis conditions enol acetates were either converted to a-acetoxy ketones (high concentration of acetate) or to enone products (absence of acetate). Similarly, a-methoxy ketones were obtained through electrolysis in methanol-Et4NOTs. Yields for additional reactions not listed here varied between 29% and 90% [222,223]. 

Fe(n) forms the enolate cation [FeOC(Me)=CH2]+ in the gas phase in the reaction of diverse FeX+ (X = H, Me, All, NH2, OH, F) ions with acetone. Other products are also observed. The relative reactivities of the FeX+ cations relate to electronic and thermodynamic properties of the displaced substituent X. 

Table 1 Dependence of the diastereofacial selectivity on the nature of the enolate cation and the oxygen protecting group...

It appears that the diastereofacial selectivity may be correlated with the nature of the enolate cation and the 0-protecting group. The sodium cation, known to be a weaker Lewis acid, cannot efflciently coordinate the silyloxy group of imine especially if it is large, so that facial selectivity follows, at least in part, a Felkin-Ahn model leading to preponderant formation of the anti isomer. However, the more basic imine nitrogen atom still coordinates the sodium cation so that the simple distereoselectivity still proceeds via a closed boat-like Zimmerman-Traxel transition state leading exclusively to the formation of (raru-azetidinones (Scheme IS). 

After ionization of 6 in Ar, the UVA is spectrum showed the characteristic bands of the enol cation at 600-700 nm, but the band at 400 - 500 nm looked different than those seen in other o-quinoid enol cations (solid line in Figure 4.7a). CASPT2 excited state calculations, which were performed for the first time at this stage of the project, predicted a weak band at about 650 nm, a medium sized one at 490 nm, and an intense one at 425 nm (see vertical bars in Figure 4.7a), which were in excellent accord with the observed spectrum, except that the second band looked more intense than predicted. 

Mariott WR, Chen EYX. Stereospecific, coordination polymerization of acrylamides by chiral dKSd-metaUocenium alkyl and ester enolate cations. Macromolecules. 2004 37 4741-4743. 

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