Sulfide, dimethyl alkenes, ozone

The second method of preparation that we studied is the oxidative cleavage of carbon-carbon double bonds—ozonolysis (Section 12-12). Exposure to ozone followed by treatment with a mild reducing agent, such as zinc metal or dimethyl sulfide, cleaves alkenes to give aldehydes and ketones. 

Aldehydes are easily oxidized to carboxylic acids under conditions of ozonide hydroly SIS When one wishes to isolate the aldehyde itself a reducing agent such as zinc is included during the hydrolysis step Zinc reduces the ozonide and reacts with any oxi dants present (excess ozone and hydrogen peroxide) to prevent them from oxidizing any aldehyde formed An alternative more modem technique follows ozone treatment of the alkene m methanol with reduction by dimethyl sulfide (CH3SCH3)... 

An elegant one-pot reduction of ozonides consisting of the treatment of a crude product of ozonization of an alkene in methanol with dimethyl sulfide (36% molar excess) gives 62-97% yields of very pure aldehydes [244. ... 

The next step is to react the unknown alkene with ozone, followed by treatment with dimethyl sulfide. The products of this reaction are isolated and identified (by other... 

Addition of ozone to the 7t bond of the alkene forms an unstable intermediate called a molozon-ide, which then rearranges to an ozonide by a stepwise process. The unstable ozonide is then reduced without isolation to afford carbonyl compounds. Zn (in H2O) or dimethyl sulfide (CH3SCH3) are two common reagents used to convert the ozonide to carbonyl compounds. 

In the preparation of aldehydes by ozonolysis of alkenes, it is important to add the correct amount of ozone to the solution because an excess of O3 can lead to side reactions. Ozonolysis in alcoholic solvents traps the carbonyl oxide as a hydroperoxide. Dimethyl sulfide reduces hydroperoxides under very mild conditions and generates the corresponding aldehydes in excellent yields. This workup procedure is recommended when the aldehyde is the desired reaction product. 

Ozone is a 1,3 dipole and adds to alkenes to give a molozonide by the 6e path, then breaks by the reverse process, and adds together to form the ozonide by the 6e path. The ozonide is cleaved by dimethyl sulfide to give two carbonyls. Show the mechanism. 

Ozonolysis consists of bubbling ozone into a very cold (—78°C) solution of the alkene in CH2CI2, followed by treatment of the solution with dimethyl sulfide (or zinc and acetic acid). The overall result is as follows ... 

Write the structures of the alkenes that would yield the following carbonyl compounds when treated with ozone and then with dimethyl sulfide. 

Reaction of an alkene with ozone leads to a 1,2,3-trioxolane, which rearranges to a 1,2,4-trioxolane (an ozonide). Subsequent treatment with hydrogen peroxide or with dimethyl sulfide leads to an aldehyde, ketone, or carboxylic acid product. When an ozonide contains a C-H unit, oxidation with hydrogen peroxide leads to a carboxylic acid, but reaction with dimethyl sulfide leads to an aldehyde 51, 52, 53, 54, 78,82,83,117. 

For any alkene, hydrogen peroxide may be used as the oxidizing agent and either dimethyl sulfide or zinc and acetic acid may be used as the reducing agent (see Chapter 18, Section 18.4.3). 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). 

Draw the structural formula of the alkene that reacts with ozone followed by dimethyl sulfide to give each product or set of products. 

Although in some cases the ozonide can be isolated, these compounds are generally fragile and decompose explosively. Usually, the ozonides are reductively decomposed immediately after formation with zinc (Zn) in acetic add (ethanoic acid, CH3CO2H) or dimethyl sulfide [( 113)28] to give two equivalents of aldehyde, two equivalents of ketone, or one equivalent of each (depending upon the structure of the alkene). The overall process of ozonization coupled with reductive hydrolysis is known as ozonolysis. 

In order to optimize the synthesis of alcohol III-50, the corresponding aldehyde III-49 was isolated by treatment of alkene III-48 with ozone followed by addition of dimethyl sulfide (Scheme 4.27) [129]. After an extensive screening of reducing agents (NaBHU, DIBALH, Ha/PtOa) [130, 131], solvents, and conditions, we obtained alcohol ni-50 using zinc borohydride in ether[132,133] with 84 % yield without purification and 50 % after column chromatography. 

Ozone, O3, is produced in the laboratory in an instrument called an ozonator, in which an arc discharge generates 3 % ozone in a dry oxygen stream. The gas mixture is passed through a solution of the alkene in methanol or dichloromethane. The first isolable intermediate is a species called an ozonide, which is reduced directly in a subsequent step by exposure to zinc in acetic acid or by reaction with dimethyl sulfide. The net result of the ozonolysis-reduction sequence is the cleavage of the molecule at the carbon-carbon double bond oxygen becomes attached to each of the carbons that had originally been doubly bonded. 

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