Asymmetric Oxidation of Alcohols and Phenol Derivatives with Air as Oxidant - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 244

R

6 (4 mol%)
air

R

OH
OH

2

60 ºC

d n 4 r 4 n g | 1

OH

R

( R )

Ph Ph

H H

R = Me, 89%, 77% ee
R = Ph, 94%, 93% ee
R = 4PhC 6 H 4 , 91%, 92% ee
R = 2-n a phthyl, 92%, 94% ee
R = 91%, 96% ee
R = 69%, 97% ee
R = Cl, 82%, 94% ee
R = Br, 86%, 94% ee
R = I, 77%, 96% ee

N N

Fe

O
Ph

O

C CPh,

Ph

CCTMS,

( R )

HO

2

6

Scheme 9.18 Iron(salan)-catalyzed AOC of C3-substituted 2-naphthols.

possess a non-coordinating group at their C3 position, whereas the corres-
ponding iron(salen) complex does not catalyze the coupling, mainly owing to
its high oxidation potential and inflexibility. 17b,32
Based on mechanistic studies including kinetics, correlation between
ee values of the catalyst and product and X-ray analysis, it was
proposed that the dimeric complex 6 dissociates into monomeric species
7 in the presence of 2-naphthol, which is oxidized to give a radical cation
species 8 and undergoes coupling via a radical anion mechanism
(Scheme 9.19). 17b,c
The SET oxidation is the rate-determining step and the monomeric
species bearing a more electron-rich naphthoxo ligand is oxidized in
preference to that bearing a less electron-rich naphthoxo group. In agree-
ment with this mechanism, the monomeric species can be isolated at lower
temperature and participates in the asymmetric catalysis of 2-naphthol
coupling under the coupling conditions, whereas the complexes that
cannot dissociate into a monomeric species do not show catalysis
of oxidative coupling. Such a catalytically inactive complex 14 gives a
m-naphthoxo-m-oxo-iron(salan) complex 15 upon treatment with 3-bromo-
2-naphthol 16 (Scheme 9.20). 17b
Understanding of the mechanism of the coupling gave a clue to the
highly enantio- and cross-selective coupling of electronically different
2-naphthols. Previously reported cross-coupling reactions gave a mixture of
two homo- and one cross-coupling products. According to the proposed
mechanism, it is anticipated that a 2-naphthol derivative bearing a strong
electron-withdrawing group such as an ester or ketone cannot be oxidized
and the homo-coupling of the derivative does not proceed. However,

.