Peroxy esters allylic oxidation

Several catalytic systems based on copper can also achieve allylic oxidation. These reactions involve induced decomposition of peroxy esters (see Part A, Section 11.1.4). When chiral copper ligands are used, enantioselectivity can be achieved. Table 12.1 shows some results for the oxidation of cyclohexene under these conditions. 

Copper-catalyzed687-689 or photochemical690 reaction of alkenes with peroxy-esters, usually with tert-butyl peracetate (or rm-BuOOH in acetic acid), may be used to carry out acyloxylation or the synthesis of the corresponding allylic esters in good yields. In contrast to the oxidation with Se02, preferential formation without rearrangement of the 3-substituted esters takes place from terminal alkenes 691... 

Oxidation of C—bonds by copper ion catalyzed reaction with an organic peroxy ester (the Kha-rasch-Sosnovsky reaction) was at one time very popular for allylic oxidation and has been thoroughly reviewed. The reaction is usually carried out by dropwise addition of peroxy ester (conunonly r-butyl peracetate or r-butyl perbenzoate) to a stirred mixture of substrate and copper salt (0.1 mol % commonly copper(I) chloride or bromide) in an inert solvent at mildly elevated temperature (60-120 C). The mechanism involves three steps (i) generation of an alkoxy radical (ii) hyttogen atom abstractitm and (iii) radical oxidation and reaction with carboxylate anion (Scheme 11). 

The Kharasch-Sosnovsky reaction may be carried out in the presence of carboxylic acids to introduce the acyloxy moiety of the acid used, and may also be conducted photochemically at room temperature using UV irradiation. Peioxy acids,diacyl peroxides, and peroxyphosphates and peroxyphospho-nates are alternative oxidants. /-Butyl hydroperoxide may also be used in place of peroxy esters with broadly similar results, although formations of mixed peroxides and /-butyl ethers can then compete with allyl ester production. 

An example of this reaction is the reaction of cyclohexene with t-butyl perbenzoate, which is mediated by Cu(I)." The initial step is the reductive cleavage of the peroxy ester. The t-butoxy radical then abstracts hydrogen from cyclohexene to give an allylic radical. The radical is oxidized by Cu(II) to the carbocation, which captures benzoate ion. 

Allylic hydrogens are replaced by acyloxy groups by reaction of peroxy esters in the presence of catalytic amounts of copper salts, including Cu(OAc)2- The reaction probably proceeds via the formation of an aUylic radical, which reacts quickly with Cu to form a Cu intermediate that generates the most substituted aUcene, probably via a pericyclic transition state (eq 6)T Allylic oxidation can be enantioselective when performed in AcOH and pivalic acid in the presence of Cu(OAc)2 and an L-amino acid. ... 

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