Organic Chemistry

Section 20.7 Oxidative Cleavage of 1,2-Diols 859

of bonds to oxygen, magnesium and osmium have oxidations states of and

respectively.) Formation of the cyclic intermediate is a syn addition because both

oxygens are delivered to the same side of the double bond. Therefore, the oxidation

reaction is stereospecific—a cis cycloalkene forms only a cis diol.

The cyclic osmate intermediate is hydrolyzed with hydrogen peroxide that reoxidizes

osmium to osmium tetroxide.

Higher yields of the diol are obtained with osmium tetroxide because the cyclic

osmate intermediate is less likely to undergo side reactions.

PROBLEM 18

Give the products that would be formed from the reaction of each of the following alkenes

with , followed by aqueous

a. b.

PROBLEM 19

What stereoisomers would be formed from the reaction of each of the following alkenes

with followed by?

a.trans-2-butene c.cis-2-pentene

b. cis-2-butene d.trans-2-pentene

20.7 Oxidative Cleavage of 1,2-Diols

1,2-Diols are oxidized to ketones and/or aldehydes by periodic acid Periodic

acid reacts with the diol to form a cyclic intermediate. The reaction takes place be-

cause iodine is in a highly positive oxidation state so it readily accepts electrons.

When the intermediate breaks down, the bond between the two carbons bonded to the

OH groups breaks. If the carbon that is bonded to an OH group is also bonded to two

(+7),

(HIO 4 ).

OsO 4 H 2 O 2

CH 3 C CH 2

CH 3

CHCH 2 CH 3

OsO 4 H 2 O 2 :

H

H

H

H

Os Os + OsO 3

OH

OH

H 2 O 2

cyclohexene

a cyclic osmate

intermediate

cis-1,2-cyclohexanediol

mechanism for cis glycol formation

OO

OO

OO

OO

Mn MnO 2

H

H

Mn +

O O−

O O

O O−

O O

a cyclic manganate

intermediate

H

H

OH

OH

H 2 O

cyclopentene

mechanism for cis glycol formation

cis-1,2-cyclopentanediol

+ 7 +8,

3-D Molecules:

Potassium permanganate

Osmium tetroxide (OsO 4 )

(KMnO 4 );

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