Asymmetric Oxidation of Alcohols and Phenol Derivatives with Air as Oxidant - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 238

9.3.1). 17a The mechanistic study of this coupling reaction revealed that the
dimeric iron(salan) complex 6 is dissociated in the presence of 2-naphthol to
the monomeric iron(salan)(2-naphthoxo) complex 7, which transfers an
electron to dioxygen and affords radical cation species 8. 17b,c
The radical cation in 8 should be distributed over the naphthoxy
oxygen and a similar radical cation structure has been proposed as the active
site for alcohol oxidation by galactose oxidase. 18 The participation of a
similar radical cation intermediate has also been proposed for alcohol oxi-
dation using a ruthenium-salen complex as the catalyst (Section 9.2.2.,
Scheme 9.14). 19 Hence it was expected that the iron(salan) complex 6 would
also catalyze asymmetric alcohol oxidation in the presence of a naphthol.
However, if the additive is 2-naphthol, the coupling of 2-naphthol should
compete with alcohol oxidation. Therefore, several naphthols were screened
and 1-naphthol was found to be the additive of choice. Indeed, complex 6
catalyzed OKR of secondary alcohols using air as oxidant in the presence of
catalytic amount of 1-naphthol at 50-60 1C. 20 This method was successfully
applied not only to activated secondary alcohols such as benzylic and
allylic alcohols, irrespective of the presence or absence of either electron-
donating or -withdrawing groups, but also to aliphatic alcohols such as
4-phenylbutan-2-ol and 1-cyclopentylethan-1-ol (Scheme 9.11).

d n 4 r 4 n g | 1

9.2.2 Asymmetric Oxidative Desymmetrization of meso-Diols
Some catalysts for OKR described in Section 9.2.1 have been applied to the
aerobic oxidative desymmetrization of meso-diols (Scheme 9.2). For example,
the desymmetrization of meso-diols with the palladium-sparteine system,
which is an ecient catalyst system for OKR, successfully provides chiral
acyclic andcyclichydroxy ketonesunderO 2 . 21 Katsuki andco-workers reported
the first example of desymmetrization using ambient air as oxidant. 19,23 They
discovered that achiral ruthenium(salen) complex 9 bearing quaternary car-
bons at its ethylenediamine moiety catalyzes the selective oxidation of a pri-
mary alcohol in the presence of a secondary alcohol (Scheme 9.12) and,
moreover, the oxidation of 1,n-diols to give lactols or n-hydroxyaldehydes. 22
Based on these results, they further developed asymmetric oxidative desym-
metrization of meso-diols using ruthenium(salen) complexes as catalyst in air
at room temperature under photoirradiation (Scheme 9.13). 19,23
Complexes 10 that bear chiral quaternary carbons at the diamine moiety
or a bulky aryl substituent at C2 00 oxidize meso-diols enantioselectively to give
the optically active lactols, the enantiomeric excesses (ee) of which were
determined after pyridinium dichromate (PDC) oxidation to the corres-
ponding lactones. During this study, it was found that the enantioselectivity
of the reaction is affected by the nature of the apical ligand of the catalyst.
Hence the scope of this desymmetrization can be broadened by an appro-
priate combination of the apical ligand and the C2 00 -aryl group.
Investigations of reaction mechanisms were conducted through kinetic,
kinetic isotope effect (KIE) and spectroscopic studies and it was found that

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