Flowing afterglow studies of gas phase enolates

Mark David Brickhouse, Purdue University

Abstract

The reactivity of gas phase enolates was studied using the flowing afterglow instrument, and later, the flowing afterglow triple quadrupole instrument. A review of previous studies of gas phase enolates is provided, as well as a description of the instrumentation and experimental protocols developed for this study. An extensive study was performed on the competitive Bronsted and Lewis acid/base reactions of 6,6-dimethylfulvene (DMFU) with gas phase anions. Localized anions were found to react primarily as bases, while delocalized anions produced both proton transfer and nucleophilic addition. The reactivity of enolate ions varied markedly, the enolates of compounds having high keto-enol energy differences (i.e., esters and amides) produced both proton transfer and nucleophilic addition while the enolates of aldehydes and ketons, compounds with low keto-enol energy differences, produced only proton transfer. This second class of enolates is believed to abstract protons from DMFU at both their carbon and oxygen termini, enhancing their relative basicity. A general kinetic differences of the parent compounds correlate scheme and simplified reaction coordinate is also proposed. Enolate regioselectivity was investigated by studying the reaction of gas phase enolates with perfluoropropylene (C$\sb3$F$\sb6$). C$\sb3$F$\sb6$ is found to produce distinctive ionic products upon reaction with oxyanions and carbanions and a mixture of these products upon reaction with enolates. As with DMFU, product distributions varied greatly, the ester and amide enolates undergoing mainly C-alkylation while ketone and aldehyde enolates produced largely O-alkylation. With a few notable exceptions, keto-enol energy differences of the parent compounds correlate well with enolate regioselectivity. Using the flowing afterglow triple quadrupole instrument, methods were developed for differentiating isomeric enolates, and this technique was used to study the kinetic vs. thermodynamic deprotonation of 2-butanone (CH$\sb3$COCH$\sb2$CH$\sb3$). The equilibrium constant (K$\sb{\rm eq}$) for the reaction CH$\sb3$CH$\sb2$COCH$\sb2\sp-$ + MeOH $\sbsp{\gets}{\to}$ CH$\sb3$COCHCH$\sb3\sp-$ + MeOH was determined by measuring the forward and reverse rate constants. It was found that K$\sb{\rm eq}$ = 5.0 $\pm$ 2.5, so that $\delta\Delta$G = $-$0.95 $\pm$ 0.24 kcal/mole. Preliminary results indicate that for methylisporopyl ketone and ethylisopropyl ketone, the less substituted tautomers are both kinetically and thermodynamically favored. The general concepts of kinetic and thermodynamic control, in both condensed phase and gas phase, are discussed.

Degree

Ph.D.

Advisors

Squires, Purdue University.

Subject Area

Organic chemistry

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