2444-101011 - 1 Oxidative Cleavage Ozonolysis...

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Oxidative Cleavage: Ozonolysis 1 • 2,3-dimethyl-2-butene reacts with ozone (O 3 , 61 ) by to give a 1,2,3-trioxolane, 62 . 62 is unstable and rearranges to a 1,2,4-trioxolane (an ozonide, 63 ), which is the final product. • Ozonide 63 is not isolated but rather treated with hydrogen peroxide, or another reagent to generate two carbonyl compounds, in this example acetone. • Ozonolysis of an alkene leads to cleavage of the C=C unit and formation of an aldehyde or a ketone, and sometimes a carboxylic acid. • The ! -bond is cleaved in this reaction via the dipolar addition, and then the " -bond is cleaved via the rearrangement step. • The net result is cleavage of both bonds of the C=C unit, and transformation of each carbon of the C=C unit to a C=O unit, a carbonyl compound (aldehyde, ketone, or carboxylic acid). • The conversion of an alkene to an ozonide is known as ozonolysis and is an example of an oxidative cleavage reaction . O O O O O O O O O O O –78°C H 2 O 2 61 62 63
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Oxidative Cleavage: Ozonolysis 2 • When the alkene has a hydrogen atom attached to the C=C unit, as in 2,4-dimethyl- 3-hextene ( 64 ), reaction with ozone gives the expected 1,2,3-trioxolane product 65 , and rearrangement to ozonide 66 . • Subsequent reaction of 66 with dimethyl sulfide, a reducing agent in this case, gives 2-butanone and 2-methylpropanal. • When 66 is treated with hydrogen peroxide, a good oxidizing agent, the cleavage products are 2-butanone and 2-methylpropanoic acid. • With H 2 O 2 , the initially formed aldehyde is oxidized to the acid. When When there is a hydrogen there is a hydrogen atom on the C=C unit, ozonolysis and oxidation leads to a atom on the C=C unit, ozonolysis and oxidation leads to a carboxylic acid, but reduction of the ozonide leads to an aldehyde. carboxylic acid, but reduction of the ozonide leads to an aldehyde. O O O O O O H H H H O O OH O O S Me Me O 3 H 2 O 2 65 66 64
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Oxidative Cleavage: Ozonolysis 3 Hydrogen peroxide may be used as the oxidizing agent and either dimethyl sulfide or zinc and acetic acid may be used as the reducing dimethyl sulfide or zinc and acetic acid may be used as the reducing agent. agent. • Ozonolysis of a cyclic alkene leads to oxidative cleavage to a diketone, a dialdehyde, a keto-aldehyde, a keto acid or a dicarboxylic acid. • When 1,3-dimethylcyclopentene ( 67 ) is treated with ozone and then with zinc and acetic acid, oxidative cleavage leads to keto-aldehyde 68 (2-ethyl-5-oxooctanal). H O 1. O 3 , –78°C 2. Zn , CH 3 CO 2 H 68 O 67 O CHO
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Oxidative Cleavage: Diols 4 • Vicinal diols are also subject to oxidative cleavage and the products are aldehydes or ketones. • Two common reagents used for this purpose are periodic acid (HIO 4 ) and lead tetraacetate [Pb(OAc) 4 ]. • When hexane-2,3-diol ( 69 ) is treated with HIO 4 (periodic acid), a cyclic product is formed ( 70 ), which decomposes under the reaction conditions to generate butanal and ethanal (acetaldehyde).
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