Enolate isomerization

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

The effect of HMPA on the reactivity of cyclopentanone enolate has been examined.44 This enolate is primarily a dimer, even in the presence of excess HMPA, but the reactivity increases by a factor of 7500 for a tenfold excess of HMPA at -50° C. The kinetics of the reaction with CH3I are consistent with the dimer being the active nucleophile. It should be kept in mind that the reactivity of regio- and stereoisomeric enolates may be different and the alkylation product ratio may not reflect the enolate composition. This issue was studied with 2-heptanone.45 Although kinetic deprotonation in THF favors the 1-enolate, a nearly equal mixture of C(l) and C(3) alkylation was observed. The inclusion of HMPA improved the C(l) selectivity to 11 1 and also markedly accelerated the rate of the reaction. These results are presumably due to increased reactivity and less competition from enolate isomerization in the presence of HMPA. 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

In the late 1960s, methods were developed for the synthesis of alkylated ketones, esters, and amides via the reaction of trialkyl-boranes with a-diazocarbonyl compounds (50,51), halogen-substituted enolates (52), and sulfur ylids (53) (eqs. [33]-[35]). Only one study has addressed the stereochemical aspects of these reactions in detail. Masamune (54) reported that diazoketones 56 (Ri = CH3, CH2Ph, Ph), upon reaction with tributylborane, afford almost exclusively the ( )-enolate, in qualitative agreement with an earlier report by Pasto (55). It was also found that E) - (Z)-enolate isomerization could be accomplished with a catalytic amount of lithium phenoxide (CgHg, 16 hr, 22°C) (54). 

B. Keto-Enol Isomerization on Acidic Zeolite HZSM-5 Evidenced by H/D Exchange... 

This deprotonation may reform the ketone enolate that was the intermediate en route to the Michael adduct. However, the regioisomeric ketone enolate also can be formed. Figures 13.71-13.74 show such enolate isomerizations B — D, which proceed via the intermediacy of a neutral Michael adduct C. This neutral adduct is a 1,5-diketone in Figure 13.71, a 5-ketoaldehyde in Figure 13.72, and a 5-ketoester in Figure 13.73. 

The different enolate anions that may be obtained from octalone (XII) are shown in Fig. 8. It has been shown that the homoannular enolate corresponding to compound XX is the initial product formed on reaction of A4 -3-ketosteroids with a strong base and the only product formed on reaction with a weak base (28). This enolate isomerizes to the heteroannular species, such as compound XXI, on further treatment with strong base. Similar results have also been observed on reaction of octalone (XII) with strong base (29). This later work as well as a study of enamine formation (29) from XII indicated that the other enolate (XXII) is not present to any large extent. The homoannular enolate (XX) can be adsorbed in the cis and trans arrangement (Fig. 8), but the cis-adsorbed species is the less hindered. The heteroannular enolate (XXI) is almost planar with cis and trans adsorption occurring with almost equal facility. 

Stereospecific cis addition of dialkylcuprates (in excess) to propynoic acids or propynoates can be effected in ether at a very low temperature (61, 175, 260). The intermediate is configurationally unstable above — 78°C and isomerizes, presumably via the enolate. Isomerization is retarded by THF as the medium (61) or by the presence of pyrrolidine (260) or TMEDA (61) as ligands. As a copper enolate is thermally stable at room temperature for long periods but addition of methyl-... 

The time scale of the classical temperatine-jnmp experiment ( 1 qs) as originally pioneered by Eigen has been shortened to nanoseconds and very recently to approximately 5 ps using lasers. The classical temperatnre-jump experiment has found only limited application to biological systems, in spite of its great success in determining, for example, proton transfer rates or keto-enol isomerizations. An important reason for its limited apphcation to enzyme research, apart from experimental difficulties such as optical artifacts as a result of the temperature-jump, is the relatively small deviation from equihbrium AG = AH —... 

MIB)PPh4, where MIB stands for the methyUsobutyrate carbanion 43, which is the unimeric model of the MMA propagating species, was synthesized and characterized by UV and NMR . The aforementioned equilibrium was confirmed, in which the dormant ylide is the major species . Whenever the polymerization of MMA is initiated by (MIB)PPh4 in THF at 20 °C, the initiation efficiency is low (20%) and the molecular weight distribution is broad (Mw/M = 2.1). This observation was accounted for by the ylide-to-enolate isomerization, which is slower than the chain propagation . 

STEPS 9-10 Hydrolysis and isomerization. Hydrolysis of the phosphate group at Cl of fructose 1,6-bisphosphate, followed by keto-enol isomerization of the carbonyl group from C2 to Cl, then completes gluconeogenesis. The isomerization is the exact reverse of step 2 in glycolysis (Figure 29.4). 

A large volume of work has been reported on rapid devolatilization of coal (heating rates approximating process conditions (21,22). Recently, the effects of coal minerals on the rapid pyrolysis of a bituminous coal were reported by Franklin, et al ( 23). They found that only the calcium minerals affected the pyrolysis products. Addition of CaCO3 reduced the tar, hydrocarbon gas and liquid yields by 20-30%. The calcium minerals also altered the oxygen release mechanism from the coal. Franklin, et al. attribute these effects to CaCOj reduction to CaO, which acts as a solid base catalyst for a keto-enol isomerization reaction that produces the observed CO and H2O. 

The process by which enols are converted to aldehydes or ketones is called keto-enol isomerism (or keto-enol tautomerism) and proceeds by the sequence of proton transfers shown in Figure 9.6. Proton transfer to the donble bond of an enol occurs readily because the carbocation that is produced is a very stable one. The positive charge on carbon is stabilized by electron release from oxygen and may be represented in resonance terms as shown on the following page. 

KETO-ENOL ISOMERISM ON TRANSITION METAL SURFACES, A DENSITY FUNCTIONAL THEORY STUDY... 

Keto-enol isomerism on transition metal surfaces... 

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