Oxidations allenes, palladium® bromide

In 1997, Backvall and Jonasson published a procedure for the 1,2-oxidation of terminal allenes 7 [5]. In this case the reaction conditions were chosen so that the (vinyl)palladium complex equilibrates back to the allene complex. Using bromide instead of chloride as a nucleophile, the 2-bromo-jt-allyl complex 9 is the major intermediate present in the reaction mixture. A catalytic reaction was developed with the use of 5 mol% palladium acetate and p-benzoquinone (BQ) as terminal oxidant (Scheme 17.5). 

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

Backvall et al. reported that palladium-catalyzed 1,2-oxidation of the allenic tosylamides 231 with lithium bromide produced the 2-(l-bromoalkenyl)-iV-tosylpyrrolidines 232 in good yields (Scheme 75).148 This reaction proceeds through the coordination of palladium(II) to the allene moiety of 231, followed by nucleophilic attack of bromine atom, formation of the jr-allylpalladium intermediate 233, and the intramolecular nucleophilic attack of NHTs to the jr-allyl center carbon. 

---