Copper complexes allylic oxidation

Scheme 10. Results of proline-copper catalyzed allylic oxidation, prom (105).] X-ray crystal structure of (R.Sf-complex 146. [Adapted from (106).]...

Copper-catalyzed allylic oxidation allows the functionalization of unactivated alkenes into chiral allylic carboxylates.1347 The use of oxazoline-containing ligands give good enantioselectivities, but the reaction is extremely slow.1348-1351 Chiral bipyridine complexes, in turn, are much more active and give products in good yields and enantioselectivities up to 70% when applied in benzoyloxylation of cycloalkenes with rm-butyl perbenzoate.1352,1353... 

Copper-catalyzed allylic oxidation with complexes of peresters (oxazole derivatives as ligands) 02T845. 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

In 1965, Denney et al. (98) reported the reaction of a number of alkenes with ferf-butyl hydroperoxide (TBHP) and cupric salts of chiral acids. The use of ethyl camphorate copper complex 144 in the allylic oxidation of cyclopentene provides, upon reduction of the camphorate ester, the allylic alcohol in low yield and low selectivity, Eq. 82. The initial publication only provided the observed rotation of cyclopentenol, but comparison to subsequent literature values (99) reveals that this reaction proceeds in 12% ee and 43% yield (based on the metal complex). 

Allylic oxidation can be carried out using a peroxide together with a copper (I) complex. Enantioselective processes have now been developed using tertiary-butylperbenzoate and either copper (I) or copper (II) complexes in acetonitrile (Figure ll.ll). ... 

Other examples of this type of reaction include those conducted by Andrus and co-workers using the copper(I) complex of ligand 224 in the allylic oxidation of cyclohexene.As shown in Figure 9.65, this reaction afforded the oxidation product, (15)-2-cyclohexen-l-yl 4-nitrobenzoate 225 in 76% yield and 73% ee. Clark and co-workers also experimented with the allylic oxidation of cyclohexene using inda-box ent-9h to afford the oxidation product, (15)-2-cyclohexen-l-yl benzoate, 223b in 76% yield (71% ee). ... 

Feringa reported an enantioselective allylic oxidation of cyclohexene to optically active 2-cyclohexenyl propionate 25 by using a chiral copper complex prepared from Cu(OAc)2 and (S)-proline, as chiral catalyst (Scheme 9.14) [32], In the absence of additives, a negative NLE was observed, whereas in the presence of a catalytic amount of anthraquinone, a positive NLE (asymmetric amplification) was observed. Moreover, higher enantioselectiv-ity was attained when enantiopure (S)-proline was used. However, the role of the additive remains elusive. 

Allylic oxidation of a variety of cyclic alkenes with copper complexes of different pybox ligands (8) and with various peresters shows high enantioselectivity (80-96% ee). Use of phenylhydrazine as an additive and acetone as solvent accelerates the reaction. It has been suggested that the phenylhydrazone is responsible for the observed acceleration. Using EPR spectra, it has been shown that the Cu(II) species is reduced to Cu(I) by phenylhydrazine and phenylhydrazone. It has been found that the presence of a gem-diphenyl group at C(5) and a secondary or tertiary alkyl substituent at the chiral centre at C(4) of the oxazoline rings is crucial for high enantioselectivity. 

From Cu(OTf)2 or Cu(OTf) and the chiral Cj-symmetric tris(oxazoline) ligand 9, copper complexes are obtained that are capable of catalyzing the allylic oxidation of cyclopentene by /er/-butyl perbenzoate in up to 84 % ee [12]. Even today, for most oxidations with chiral or achiral ligand systems, the structures of the real active metal catalysts are unknown. Because of this it is difficult to give a scientific rationale for the selectivities and inductions observed. 

Oxidations. A widely used method for allylic oxidation is the Kharash-Sosnovsky reaction using a peroxide and a copper(I) salt system. Enantioselective allylic oxidations of cycloalkenes such as cyclopentene, cyclohexene and cycloheptene with tert-butyl peibenzoate were investigated with a variety of catalysts derived from bis(oxazoline) ligands and copper(I) triflate complexes (eq 18). The ligand-copper(I) complexes from the /-Bu-... 

Kochi s study of the copper salt-catalyzed oxidation of butenes by peresters (17) is interesting because the role of a ligand in controlling stereoselectivity was clearly demonstrated. This reaction, which involves the oxidation of allylic radicals by cupric ion, results in the formation of high yields of allylic ester in which the double bond is terminal, and it is described as occurring within a metal complex. When phenanthroline... 

Asymmetric allylic oxidation of alkenes using peresters is possible when the ligand L of the Cu(III) intermediate is chiral. Copper complexes of chiral bis(pyri-dine)- and bis(oxazoline)-type ligands have been used with fert-butyl perbenzoate to obtain optically active allylic benzoates. 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use 1s made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant,8 our catalyst combination is completely regioselective for allcyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

In 1995, three different chiral oxazoline derivatives were introduced as chiral ligands which remarkably improved enantioselectivity in allylic oxidation [19]. Pfaltz et al. reported that copper(I)-bis(oxazoline) (15) complex show good to high enantioselectivity (up to 84% ee at -20 °C) in the oxidation of cyclopentene (Scheme 11) [19a], Enantioselectivity is dependent on the solvent used, and acetonitrile gives... 

Allylic oxidations. Allylic esters are obtained in the 50-75% ee range using peroxide, peracid, or perester oxidants in the presence of copper complexes of proline or bis(oxazolinyl)methanes." ... 

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