Chemistry Reference
In-Depth Information
11.6. ASYMMETRIC DIHYDROXYLATION
The Sharpless asymmetric dihydroxylation of olefi ns is an important chemical process,
providing access to synthetically valuable 1,2-diols with two contiguous stereogenic
centers [133]. Osmium tetroxide, together with cinchona alkaloid-derived ligands, is
employed as a catalyst, and NMO and potassium ferricyanide/potassium carbonate are
common oxidants for the reaction. Several types of chiral cinchona alkaloid-derived
ligands have been introduced in accordance with the substitution pattern of olefi ns, and
high enantioselectivity is obtained in most cases. There are two potential catalytic cycles
for the osmium-catalyzed dihydroxylation of olefi ns using NMO as oxidant (Scheme
11.73). The left cycle, in which the chiral alkaloid ligand binds to the osmium, is called
the fi rst cycle, and the reaction proceeds with high enantioselectivity. On the other hand,
the second cycle does not involve the chiral ligand but 1,2-diol ligand. In general, the
participation of the second cycle leads to reduced enantioselectivity. Thus, rapid hydro-
lysis of Os(VIII) trioxoglycolate is essential for achieving high enantioselectivity.
Recently, however, Fokin and Sharpless discovered a unique utilization of the second
cycle for enantioselective dihydroxylation and aminohydroxylation [134], which is the
aza-analogue of the dihydroxylation and a powerful approach to amino alcohols. In the
course of further investigations of the osmium-catalyzed oxidations, the authors discov-
ered that certain classes of olefi ns undergo rapid dihydroxylation or aminohydroxylation
in the absence of the alkaloid ligand, even with very low catalyst loadings [135]. On the
basis of the observation, the authors set out the research to account for the exceptional
phenomenon and found that chiral
N
- sulfonyl - 1,2 - hydroxyamines serve as effective
ligands for the asymmetric reactions via the second cycle (Scheme 11.74) [136].
R'
H
2
O
R
OH
OH
O
R
R'
O
O
O
R'
Os
O
R
L*
L*
L*
NMM
O
O
R
First cycle
Second cycle
Os
R'
O
O
O
O
Os
O
O
O
R
NMO
R
L*
R
L*
O
R'
H
2
O
O
Os
O
O
L*
R'
OH
R
OH
R
R'
Scheme 11.73.
