Ch_322a_8.011

Ch_322a_8.011 - Oxidations of Alkenes Syn Hydroxylation The...

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Unformatted text preview: Oxidations of Alkenes Syn Hydroxylation The stereospecific formation of 1,2-diols (or glycols) from alkenes may be carried out in two ways: C=C KMnO 4 , HO- cold H 2 O C C OH HO C=C (i) OsO 4 , pyridine (ii) Na 2 SO 3 /H 2 O or NaHSO 3 /H 2 O C C OH HO The Mechanisms of the Hydroxylation Reactions The mechanisms of the two hydroxylation reactions are very similar. The initial step is oxidative addition to the alkene to produce cyclic ester-type intermediates. C=C Mn = = = O : : : O : : O : : O : :- permanganate ion +7 C C Mn = O : : O : : O : O : : +5 :- : manganate ester two-electron redox Os = = = O : : O : : O : : O : : +8 = C=C osmium tetroxide C C Os = O : : O : : O : O : : +6 : = osmate ester two-electron redox Conversion to 1,2-Diols The syn stereochemistry of the hydroxylation reactions is set with formation of the cyclic intermediates. The cyclic esters are converted to the diols as follows: C C Mn = O O O- manganate ester alkaline hydrolysis HO- H 2 O C C OH HO + MnO 2 O Hydrolysis of the manganate ester occurs...
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Ch_322a_8.011 - Oxidations of Alkenes Syn Hydroxylation The...

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