Organic Syntheses by Means of Noble Metal Compounds. XXVIII. Syntheses of a New Type of π-Allylic Palladium Complexes from α, β- and β, γ-Unsaturated Esters and Their Carbonylation

A general method of synthesizing a new type of π-allylic complex of palladium from compounds with an allylic hydrogen activated by a carbonyl group is described. Ethyl 3-butenoate and ethyl 2-butenoate give the same carbethoxy-substituted π-allyl complex, with hydrogen chloride eliminated, when treated with palladium chloride. Unsaturated diesters, such as diethyl glutaconate and diethyl 3-hexenedioate, also react with palladium chloride to give the corresponding π-allylic complexes. The structures of these complexes are discussed by means of a study of their NMR spectra. The carbonylation of these complexes proceeds smoothly, and carbon monoxide is introduced to form unsaturated esters. 3-Butenoate and then glutaconate are prepared, starting from allyl chloride, by a series of complex formations and carbonylation reactions. The possible synthetic application of this method of step-by-step introduction of carbon monoxide to other olefins is suggested. Diethyl 3-hexenedioate gives the complex, which is then converted into diethyl muconate by the elimination of palladium by the action of a base or carbon monoxide.