Carbon=oxygen bond cleavage

Apart from the carbon-halogen bond, the carbon-oxygen one is rather active toward the reductive cleavage due to its polarity, so different types of compounds bearing a carbon-oxygen bond are able to undergo this reaction. 

As was noted in Scheme 12, distonic radical cations obtained from cyclopropane bond cleavages add oxygen rapidly, producing products with two CO bonds. So do some alkene radical cations. Addition of O2 to an alkene radical cation is formally a nucleophilic attack by the single alkene n electron on O2, and oxidizes both carbon atoms (an alkene radical cations has formally two -f carbons, and the adduct a 1+ and an oxygen-bound carbon). The oxygenation of the radical cation of bia-damantylidine (96) leads to dioxetanes such as 98 in chain reactions (see Scheme 21) [110]. The reactions may be initiated electrochemically or photochemically, but tris(o,p-dibromophenyl)amine hexafluoroantimonate, 97, is a superior catalyst for the dark reaction of certain tetraalkylalkenes, with turnovers up to ca. 800 at... 

Primary alcohols do not dehydrate as readily as secondary or tertiary alcohols and their dehydration does not involve a primary carbocation A proton is lost from the (3 carbon m the same step m which carbon-oxygen bond cleavage occurs The mechanism is E2... 

In the acid catalyzed dehydration of 2 methyl 1 propanol what carbocation would be formed if a hydride shift accompanied cleavage of the carbon-oxygen bond in the alkyloxonium lon" What ion would be formed as a result of a methyl shift" Which pathway do you think will predominate a hydnde shift or a methyl shift" ... 

The transition state involves the carbonyl oxygen of one carboxyl group—the one that stays behind—acting as a proton acceptor toward the hydroxyl group of the carboxyl that IS lost Carbon-carbon bond cleavage leads to the enol form of acetic acid along with a molecule of carbon dioxide... 

The cleavage of dialkyl ethers by hydrogen halides was discussed in Section 16 8 where It was noted that the same pair of alkyl halides results irrespective of the order in which the carbon-oxygen bonds of the ether are broken... 

Thermal decomposition of dihydroperoxides results in initial homolysis of an oxygen—oxygen bond foUowed by carbon—oxygen and carbon—carbon bond cleavages to yield mixtures of carbonyl compounds (ketones, aldehydes), esters, carboxyHc acids, hydrocarbons, and hydrogen peroxide. 

Hydrogenolysis is analogous to hydrolysis and ammonolysis, which involve the cleavage of a bond induced by the action of water and ammonia, respectively. Chemical bonds that are broken by hydrogenolysis reactions include carbon—carbon, carbon—oxygen, carbon—sulfur, and carbon—nitrogen bonds. An example of hydrogenolysis is the hydrodealkylation of toluene to form benzene and methane ... 

The amyl alcohols undergo the typical reactions of alcohols which are characterized by cleavage at either the oxygen—hydrogen or carbon—oxygen bonds. 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

In contrast to phosphorus esters, sulfur esters are usually cleaved at the carbon-oxygen bond with carbon-fluorine bond formation Cleavage of esteri nf methanesulfonic acid, p-toluenesidfonic acid, and especially trifluoromethane-sulfonic acid (tnflic acid) by fluoride ion is the most widely used method for the conversion of hydroxy compounds to fluoro derivatives Potassium fluoride, triethylamine trihydrofluoride, and tetrabutylammonium fluoride are common sources of the fluoride ion For the cleavage of a variety of alkyl mesylates and tosylates with potassium fluoride, polyethylene glycol 400 is a solvent of choice, the yields are limited by solvolysis of the leaving group by the solvent, but this phenomenon is controlled by bulky substituents, either in the sulfonic acid part or in the alcohol part of the ester [42] (equation 29)... 

The carbon-oxygen bond is strong, like the normal carbon-fluorine bond, it is difficult to reduce However, some structural features facilitate its reductive cleavage Aryl esters of perfluoroalkanesulfomc acids can be cleaved in good yield by... 

Examine the geometry of the most stable radical. Is the bonding in the aromatic ring fuUy delocalized (compare to model alpha-tocopherol), or is it localized Also, examine the spin density surface of the most stable radical. Is the unpaired electron localized on the carbon (oxygen) where bond cleavage occurred, or is it delocalized Draw all of the resonance contributors necessary for a full description of the radical s geometry and electronic structure. 

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