Aerobic Oxidation of Alcohols Catalyzed by Iron or Vanadium Compounds - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 165

2 mol% VOPO 4 , 2 mol% TEMPO, O 2

Ar CHO

Ar C 2 OH

H 2 O

Scheme 6.21 Aerobic oxidation of benzylic alcohols catalyzed by VOPO 4 -TEMPO.

d n 4 r 4 n g | 5

t -Bu t -Bu

OH O

5 mol% VOSO 4 , 10 mol%

N N

Ar R Ar R

H 2 O, O 2 or air (0.1 MPa), 90 o C

Scheme 6.22 Aerobic oxidation of benzylic alcohols catalyzed by vanadium complex
VOSO 4 combined with 4.4 0 -di-tert-butyl-2,2 0 -bipyridyl.

OH

2 mol% (HQ) 2 V(O)(O i Pr), 10 mol% NEt 3

O

R 1 R 2 air, 40-80 o C

R 1 R 2

O

O
O

N

N V i Pr

OH

O

N

VO(acac) 2

i PrOH

Scheme 6.23 Aerobic oxidation of benzylic, allylic and propargylic alcohols cata-
lyzed by (HQ)2V(O)(OiPr)-NEt 3 .

.

(Scheme 6.23). 30 The activated alcohols produced the desired aldehydes and
ketones in high yields. Conversely, simple aliphatic alcohols underwent little
or no oxidation, even when heated at 100 1C. The catalyst can be easily pre-
pared in air from the reaction of VO(acac) 2 with 8-quinolinol in 2-propanol.

6.2.4 Aerobic Oxidation of Alcohols Including Unactivated
Alcohols
Velusamy and Punniyamurthy found that a variety of secondary alcohols,
including unactivated aliphatic alcohols, were oxidized to the corresponding
ketones under an oxygen atmosphere in the presence of 5 mol% V 2 O 5 in
toluene at 100 1C. In the case of a primary alcohol, the ester was obtained
instead of the corresponding aldehyde (Scheme 6.24). 31 The ester was pro-
duced from the subsequent oxidation of a hemiacetal derived from the al-
dehyde and the unchanged alcohol. The formation of the hemiacetal is due
to the weakly acidic nature of the reaction medium (pH 6) (Scheme 6.25).
Addition of K 2 CO 3 to the reaction with primary alcohols increased the pH
to 9,

inhibited hemiacetal formation and afforded the corresponding