Allylic Oxidation
Allylic oxidation can be carried out using a peroxide together with a copper (I) complex.20 Enantioselective processes have now been developed using tertiary-butylperbenzoate and either copper (I) or copper (II) complexes in acetonitrile (Figure 11.11).2223
This work has been developed further through the use of a copper(I) bipyridyl complex catalyst, incorporating a chiral and sterically demanding ligand, and
Me3C
CMe,
Cu catalyst
PhC020CMe3 MeCN
Catalysts
Figure 11.11 Enantioselective allylic hydroxylation.
Cu(OTf)2L PhNHNH,
PhC020CMe3
A^Ph
Figure 11.12 Enantioselective allylic hydroxylation.
Figure 11.12 Enantioselective allylic hydroxylation.
reducing copper(II) to copper(I) in situ with phenylhydrazine (Figure 11.12). The yield is excellent, but the enantiomeric excess needs further improvement.24'25
11.7 HYDROXYLATION OF ß-KETOESTERS
The tartrate (or TADDOL) derived approach to catalyst design has also been applied to the enantioselective a-hydroxylation of ß-ketoesters. In this case, an enantiospecific titanium(IV) complex combines with a sulfonyloxaziridine as the
(V^ Ticata|yst
Ti catalyst
Ti catalyst
Figure 11.13 Enantioselective hydroxylation of p-ketoesters.
oxidant to give high yields, and also high enantiomeric excess for tertiary-butyl esters (Figure 11.13).26
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