Chemistry Reference
In-Depth Information
OH
CHO
TEMPO (cat.), NaOCl, KBr (cat.)
NaHCO
3
, CH
2
Cl
2
OH
OH
(100 %)
Scheme 12.3
NaOCl (1.1 e
quiv.)
(CH
2
)
7
CHO
(CH
2
)
7
OH
MeO-TEMPO (cat.), KBr (cat.)
CH
2
Cl
2
, H
2
O, pH 9.5
(68 %)
OH
OH
(CH
2
)
7
CHO
NaOCl (2.2 equiv.)
(69 %)
O
Scheme 12.4
primary alcohols react much faster than secondary alcohols,
6,45,65
and complete chemoselectivity can be
observed in many reactions (see, for example, Schemes 12.4, 12.21, 12.22, 12.29, 12.30, 12.34, 12.35,
12.37 - 12.39, and 12.51). A prototypical example of a selective oxidation of a primary alcohol in the
presence of a secondary alcohol is shown in Scheme 12.3. Optically active 4-methyl-5-hexene-1,3-diol is
oxidized to the corresponding hydroxy aldehyde in quantitative yield.
66
The typical reaction conditions are
also mild enough to not cause any racemization of chiral carbons in the molecule (see also, for example,
Schemes 12.31, 12.34, 12.37 - 12.39, 12.41, and 12.51).
Stoichiometric control can be exerted to oxidize the secondary alcohol subsequently to the primary
alcohol. Thus, oxidation of 1,9-undecanediol results, depending on the stoichiometric ratio of substrate to
secondary oxidant, in the formation of either 9-hydroxyundecanal or 9-oxoundecanal (Scheme 12.4).
67
12.5 Chemoselectivity: oxidation of primary vs benzylic alcohols
Benzylic alcohol oxidations are facile (see, e.g., Scheme 12.36).
68
Primary benzylic alcohols are oxidized
faster than secondary benzylic alcohols, even if the secondary alcohol is located at a double-benzylic
position.
69
Under certain conditions, benzylic alcohols can be oxidized with high selectivity (albeit in
low yield) in the presence of primary alcohols, as the example shown in Scheme 12.5 demonstrates.
70
However, the absence of multiple examples of this reaction leaves open the question of the generality of
this particular selectivity.
OH
OH
OH
O
O
O
TEMPO (cat.), laccase (cat.), O
2
,
H
2
O, MeCN, citrate buffer, pH 5, rt, 24 h
OMe
OMe
MeO
MeO
OMe
OMe
(10 %)
Scheme 12.5
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