Experimental determination of carbon vs. oxygen regioselectivity in reactions of gas‐phase enolate ions

Abstract

A gas-phase experimental investigation of the competition between carbon and oxygen alkylation of a series of cyclic and acyclic enolate ions is described. Perfluoropropylene is shown to react in a characteristic way with oxyanions and carbanions to produce distinctive ionic products. The relative yields of these products formed in reactions with ambident enolate ions provides a measure of their intrinsic carbon vs. oxygen regioselectivity. The results for a series of enolates derived from aldehydes, ketones, esters, amides and related compounds show a wide range of reactivity which is a function of the nature of the central substituent. Most aldehyde and ketone enolates react mainly through oxygen, while enolates with a-acceptor of n-donor type central substituents react mainly through carbon. Ring-size in cyclic ketone enolates also influences C vs. 0 regioselectivity, i.e. small-ring enolates react mainly through carbon, while larger ring enolates (C7-G) react preferentially through oxygen. The enolate reactivity patterns can be generally accounted for by the ketc-enol energy differenc-5 for the parent carbonyl compounds, although some exceptions are evident. The origins of the highly variable, kinetically determined regioselectivities are discussed.

Benzocyclobutenone enolate: an anion with an antiaromatic resonance structure †

“…One explanation involves the energy difference between the keto and enol tautomers (∆H KE ) of the carbonyl compound. 11 A thermodynamic argument works in this case because the kinetically controlled ratios have been shown to be a reflection of the initial reaction step energetics (i.e. carbanion vs. oxy anion attack).…”

Four decades of joy in mass spectrometry

“…Ambident species having two different reaction sites are well known in organic chemistry (Klopman, 1974). Enolate anions are such species that in reactions with pentafluoroanisole (Ingemann et al, 1982) and hexafluoropropene (Brickhouse & Squires, 1989) had been found to react at both the oxygen and carbon center. From the systematic study with hexafluoropropene it was suggested that reaction at the oxygen center would be favored if the difference in heats of formation of the keto and enol forms of the neutral carbonyl compounds would be <60 kJ/mol (Brickhouse & Squires, 1989).…”

“…Enolate anions are such species that in reactions with pentafluoroanisole (Ingemann et al, 1982) and hexafluoropropene (Brickhouse & Squires, 1989) had been found to react at both the oxygen and carbon center. From the systematic study with hexafluoropropene it was suggested that reaction at the oxygen center would be favored if the difference in heats of formation of the keto and enol forms of the neutral carbonyl compounds would be <60 kJ/mol (Brickhouse & Squires, 1989). However, various enolate anions violated this criterion and some of them were observed in my group to react in an opposite way with regard to O/C site addition to the substrates hexafluorobenzene and hexafluoropropene (Freriks, de Koning, & Nibbering, 1991).…”

The Reactivity of Ambident Nucleophiles: Marcus Theory or Hard and Soft Acids and Bases Principle?

“…Many factors affect the ambident reactivity, such as counter metal ions, structures, steric effects, and dielectric factor and hydrogen bonding of solvents, so on and so forth. Computational studies are also convenient to validate reaction mechanisms for ambident reactions .…”