Oxidation of allenes

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-... 

The palladium(II)-catalyzed oxidation of allenes with chloride was studied by Hege-dus et al. [3], In this reaction the dimeric products 4 and 6 as shown in Scheme 17.4 were obtained. The (allene)palladium(II) complex formed can react with chloride ions in two different ways (Scheme 17.4) [4]. Attack at the terminal carbon gives a vinylpalladium intermediate 2 whereas attack at the middle carbon produces a 2-chloro(jt-allyl)palladium complex 3. The former complex is the kinetic intermediate (k2 > kj) and is in equilibrium with the (allene)palladium complex. The 2-chloro(jt-allyl)palladium complex is formed more slowly but is more stable and has been isolated [2]. The vinyl complex can undergo further reaction with excess allene to give a new (jt-allyl)palladium complex, which undergoes attack with chloride to give the observed dimer 6 [3]. The dichloride from attack on the 2-chloro-(jT-allyl)palladium complex 3 was not observed. 

In 1997, Laux and Krause showed that ruthenium(III) chloride can catalyze the oxidation of allenes 48 to a,a -dihydroxy ketones 49 [17]. They used sodium periodate as a stoichiometric oxidant and obtained the products in moderate to good yields, probably owing to overoxidation (Scheme 17.16). The dihydroxy compound was formed with high diastereomeric excess in one example when a chiral allene was oxidized. 

The oxidation of allenes gives a,a -dihydroxy ketones (Eq. 3.48) [80]. Various het-eroatom-containing compounds undergo oxidation of methylene groups at the a-position. Ethers are converted into esters and lactones [81]. The efficiency of the... 

The 1,2- and 1,4-addition reactions have been observed also for alkyl-substituted aromatics, but the yields are often low, in particular for benzene derivatives, owing to competing side-chain substitution reactions. Examples include the addition of MeOH to methyl-substituted benzenes [38-42], naphthalenes [43], and anthracenes [43,44]. In a similar fashion, the anodic oxidation of 1,3-dienes in MeOH [45,46] results in mixtures of 1,2- and 1,4-addition products accompanied by substitution products and methoxy-containing dimers and trimers [46]. Styrenes are oxidized in MeOH to the 1,2-addition product together with products formed by dimerization-addition [47,48]. Oxidation of allenes results in most cases in complicated product mixtures resulting from single and double addition reactions [49-52]. 

Oxidation of allenes and alkynes. The oxidation promoted by cetylpyri-dinium peroxotungstophosphate in an alcoholic solvent produces a-alkoxy ketones from allenes and a,/3-epoxy ketones from alkynes (along with a,/3-unsaturated ketones). Diphenylacetylene gives benzil (93% yield at 45% conversion). 

Oxidation of aOenic alcohols. French chemists have reported an interesting preparation of 3-oxacyclanones (equations I and II) by oxidation of allenic alcohols with this peracid. Yields are low when the alcohols arc primary, around 60% when the alcohols are secondary, and almost quantitative when the alcohols... 

A two-electron oxidation of allenes (1) (A = S, NMe) has been found to yield carbenes (2) (Scheme 1) dimerization and reaction with (2,2,6,6-tetraamethyl-piperidin-l-yl)oxyl (TEMPO) support a triplet ground state as predicted by computations. Triplet diphenylcarbenes bearing bulky substituents at the para positions have been generated and studied in rigid matrixes at low temperatures by electron spin resonance (ESR) and UV-Vis spectroscopy as well as by laser flash photolysis (LFP) in solution at room temperature. Their reactivity upon LFP was shown to be dominated by dimerization unless triplet carbene quenchers (such as oxygen and cyclohexadiene) were present. 

Pig. 6. Oxidation of allenes to dihydroxy ketones using RuCl2(PPh3)3 as catalyst. 

In analogy with the Pd(II)-catalyzed oxidation of dienes,the Pd-catalyzed oxidation of allenes with CuCla as the oxidant was found to give the dimerization product (Scheme 24). 

Oxidations of Allenes. The intramolecular addition of a tosylamine to a terminal allene affords a piperidine derivative... 

Where 0.1 [Pg.176]

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