Oxidation
reactions of alkenes give cyclic ethers in which both carbons of a double bond
become bonded to the same oxygen atom.
These products are called epoxides or oxiranes.
An important method for preparing epoxides is by reaction
with peracids, RCO3H.
The
epoxidation reaction occurs in a single step with a transition state.
Consequently, epoxidations by peracids always have syn-stereoselectivity
Epoxides
may be cleaved by aqueous acid to give glycols that are often diastereomeric
with those prepared by the syn-hydroxylation reaction described above.
Proton
transfer from the acid catalyst generates the conjugate acid of
the epoxide,
which is attacked by nucleophiles such as water .
The
result is anti-hydroxylation of the double bond .
In the following equation this procedure is
illustrated for a cis-
disubstituted epoxide, which, of course, could be
prepared from the
corresponding cis-alkene. This hydration of an epoxide does
not change
the oxidation state of any atoms or groups.
No comments:
Post a Comment