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

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 σ‐acceptor of π‐donor type central substituents react mainly thorugh carbon. Ring‐size in cyclic ketone enolates also influences C vs. O regioselectivity, i.e. small‐ring enolates react mainly through carbon, while larger ring enolates (C 7 –C 9 ) react preferentially through oxygen. The enolate reactivity patterns can be generally accounted for by the keto–enol energy differences for the parent carbonyl compounds, although some exceptions are evident. The origins of the highly variable, kinetically determined regioselectivities are discussed.