The reactions of metal-ion complexes with hydrocarbons. Part 5. Some catalytic reactions of palladium acetate and its derivatives: the oxidations of cyclohexene, cyclo-octene, 3,3-dimethylbut-1-ene, and n-hexenes and the isomerisation of linear alkene derivatives

Abstract

The oxidation of cyclohexene, (28), cyclo-octene, (32), 3,3-dimethylbut-1-ene, (1), hex-1-ene, (15), and cis-hex-2-ene, (16) by dioxygen catalysed by ‘palladium acetate’ has been studied in acetic acid solution at 25 °C and ca. 1 atm pressure. Trimeric palladium acetate or a dissociation product is catalytically active but is converted by olefins into complexes of the type [Pd₃(η³-allyl)₂(µ-O₂CMe)₄] which are also catalytically active probably by virtue of the presence of one palladium atom not complexed by an allyl group. Compounds (1) and (15) are oxidised by addition of Pd(O₂CMe) to the double bond and then by elimination of PdH to give 3,3-dimethylbut-1-en-2-yl acetate, (2), and hex-1-en-2-yl acetate, (19), respectively and very little of the corresponding primary esters. The palladium hydride generated by (15) leads to extensive isomerisation of the hexenyl ester products and to selective formation of cis- and trans-hex-2-ene. In the case of (1), a complication is the oxidative coupling of the olefin to di(t-butyl) butadienes and di(t-butyl)cyclobutene which remain complexed to the catalyst as addition compounds of η³-allylic structure. Some 1,3,5-tri(t-butyl)benzene is also formed. Studies of the continuous catalytic rate with (1) and (28) and of the initial rate with (15) show that the reaction of olefins with the catalytic species (displacement) is usually rate determining, although under some circumstances decomposition of a complex or reaction with dioxygen (‘regeneration’) can be rate determining. In the case of (15), the addition of sodium acetate to the acetic acid promoted the formation of isomerised ester products such as hex-4-en-2-yl actate. Perchloric acid promotes the rapid formation of stable η³-allyl complexes that are catalytically inactive. Oxidation of (16) by the addition–elimination route is very slow and, in the presence of dioxygen, a free-radical process yielding allylic esters such as hex-1-en-3-yl acetate and unsaturated alcohols and ketones becomes dominant.