Acylation of aromatic substrates with ketenes. II. Hammett–Brown studies on substituted aromatic compounds

Competition reactions using dimethylketene (DMK) – aluminum chloride complexes on a variety of substituted benzene derivatives gave partial rate factors. The logs of these Pfx values when plotted versus σ+ values gave two line segments of ρ = −6.59 for less active substrates, and ρ = −0.92 for more active substrates. Similar results were found for diphenylketene (ρ = −9.47 and −1.07).These results were rationalized by analogy to vinyl cation alkylations. A pair of π complexes (outer then inner) occur between initial approach of the reagent and final, product-determining π-complex formation. In one of these complexes the C=C bond of the ketene complex may be able to back-bond in a manner similar to that found in inorganic complexes of CO and metals.