Carbon-Oxygen Bonds in Ethers and Esters

Cleavage of Carbon-Oxygen Bonds in Ethers and Esters 

The cleavage of carbon-oxygen bonds in ethers or esters by nucleophilic substitution is frequently a useful synthetic transformation. 

Good yields are generally observed, especially for methyl ethers. The combination of boron tribromide with dimethyl sulfide has been found to be particularly effective for cleaving aryl methyl ethers.91 

The boron trifluoride-alkyl thiol reagent combination also operates on the basis of nucleophilic attack on an oxonium ion generated by reaction of the ether with boron trifluoride.90 

Trimethylsilyl iodide (TMSI) cleaves methyl ethers in a period of a few hours at room temperature.89 Benzyl and f-butyl systems are cleaved very rapidly, whereas secondary systems require longer times. The reaction presumably proceeds via an initially formed silyl oxonium ion. 

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SECTION 3.3. NUCLEOPHILIC CLEAVAGE OF CARBON-OXYGEN BONDS IN ETHERS AND ESTERS... 

Reductive eliminations from nickel(ll) complexes to form carbon-heteroatom bonds in amines and ethers have also been reported. Like the mechanisms for oxidative additions to Ni(0) and Pd(0) that cleave carbon-heteroatom bonds, the mechanisms for reductive elimination from nickel(II) and palladium(II) complexes to form caibon-heteroatom bonds are different from each other. Most reductive eliminations from Ni(II) to form carbon-nitrogen bonds occur after oxidation of the Ni(II) to Ni(III) with ferro-ceruum, oxygen, or iodine (Equations 8.53 and 8.54). Reductive eliminations from Ni(II) to form carbon-oxygen bonds in ethers also requires oxidation of ttie Ni(II) to Ni(III) (Equation 8.55). In contrast, reductive eliminations from Ni(II) to form the ester group of a lactone occurred after a proposed insertion of CO into the nickel-carbon bond of an oxametallacycle without oxidation. Reductive eliminations from isolated arylnickd complexes to form amines and ethers have not been reported. 

FUNCTIONAL GROUPS THAT CONTAIN OXYGEN Carbon-Oxygen Single Bonds in Alcohols and Ethers Carbon-Oxygen Double Bonds in Aldehydes and Ketones Carbon-Oxygen Bonds in Carboxylic Acids and Esters... 

Williams83 has recently considered the possibility of ion molecule reactions in alcohols, ketones, ethers, and esters. He postulates that the primary reactions of the parent ion are the inter- or intra-molecular abstraction of a hydrogen atom and the formation of a x-bond between oxygen and the adjacent carbon atom after homolytic scission of a bond to that adjacent carbon atom. After examining some of the liquid phase data on the radiolysis of these oxygen compounds it was concluded that such ion molecule reactions may be of importance in these systems also. 

There are many classes of organic compounds that contain carbon-oxygen bonds, including alcohols, carboxylic acids, ethers, peroxides, aldehydes, ketones, esters, and acid anhydrides. Not all classes of compounds are included in the following discussion, so the interested smdent is advised to look at the more detailed references already mentioned for additional information. 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

We ll first consider the carbon-oxygen bond breaking which occurs upon protonation of t-butyl esters or ether. The Yates r treatment gives clearly unreasonable orders" in water of -8.9 and -3.15. The slope parameter, instead, is negative in both... 

BXj-Promoted Carbon-Oxygen Bond Cleavage in Ethers, Acetals, and Esters. 

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