Mechanisms ether cleavage

Selective ether cleavage comes about during the substitution step, which obeys an Sn2 mechanism. Therefore, selective cleavage requires selective attack by Y on one of the electrophilic carbons in the protonated ether. Determine if selective attack is likely by examining the shape of the lowest-unoccupied molecular orbital (LUMO) in protonated ethyl propyl ether. Is this orbital larger near one carbon than the other If so, what product combination will result What other atom(s) contribute to the LUMO What would happen if 1 attacked this atom(s) ... 

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

The use of iodotrimethylsilane for this purpose provides an effective alternative to known methods. Thus the reaction of primary and secondary methyl ethers with iodotrimethylsilane in chloroform or acetonitrile at 25—60° for 2—64 hours affords the corresponding trimethylsilyl ethers in high yield. The alcohols may be liberated from the trimethylsilyl ethers by methanolysis. The mechanism of the ether cleavage is presumed to involve initial formation of a trimethylsilyl oxonium ion which is converted to the silyl ether by nucleophilic attack of iodide at the methyl group. tert-Butyl, trityl, and benzyl ethers of primary and secondary alcohols are rapidly converted to trimethylsilyl ethers by the action of iodotrimethylsilane, probably via heterolysis of silyl oxonium ion intermediates. The cleavage of aryl methyl ethers to aryl trimethylsilyl ethers may also be effected more slowly by reaction with iodotrimethylsilane at 25—50° in chloroform or sulfolane for 12-125 hours, with iodotrimethylsilane at 100—110° in the absence of solvent, " and with iodotrimethylsilane generated in situ from iodine and trimcthylphenylsilane at 100°. ... 

The quinone methide can also be generated in situ, at least in aqueous NaOH, directly from the peracetate, as hydrolysis of the phenolic acetate is faster than the benzylic acetate (see an example in Section 12.5.3). This method was used to demonstrate the addition of anthrahydroquinone (AHQ) and anthranol to (actual polymeric) lignin quinone methides in studies elucidating the anthraquinone (AQ)-catalyzed 8-0-4-aryl ether cleavage mechanisms in alkaline pulping.64-66... 

Figure 3.15 Mechanism of aryl ether cleavage during alkaline pulping.

Figure 10. A proposed depolymerization mechanism of lignin by laccase. (1) Side chain cleavage (2) fi-O-4 ether cleavage.

Mechanism forthe cleavage of an ether by a hydrohalic acid. 

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

Scheme 2.5 Mechanism of cleavage of />-methoxybenzyl ethers with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Note SET=single-electron transfer.

The rate law of this El reaction in the form of Equation 4.19 reveals unimolecularity once more. However, it hides what the more detailed form of Equation 4.18 discloses namely, that the elimination rate increases with increasing CF3C02H concentration, which means that this is a bimolecular El elimination. From Equation 4.18 we can also see that the El rate increases at a given concentration of the acid when a more acidic acid is used. In the end Equation 4.18 implies that the rate of ether cleavage following the El mechanism increases with the basicity of the substrate. 

On the other hand, alkaline cleavage of the etherified P-aryl ether unit (47) proceeds by an intramolecular displacement mechanism (D). Under kraft cooking conditions, the p-aryl ether cleavage of phenolic type could be 12-50 times faster than that of the etherified type depending on the hydroxide and sulfide ion concentrations [331]. 

Etherified units. Consistent with the generally accepted mechanism (D, Fig. 14), the erythro dimer of etherified P-aryl ether was about four times more reactive than the threo isomer [350]. The ether cleavage, besides being enhanced by increasing alkalinity, was facilitated in the presence of monoetha-nolamine [351] or in a DMSO-potassium-tertiary butoxide solution [330]. 

In the case of the tricyclic epoxides la-c the outcome of the reaction depends on the ring size.55 The formation of 2 and 3 was explained by the following mechanism.55 Cleavage of the epoxide ring with boron trifluoride diethyl ether complex gives the zwitterionic intermediate 4. Subsequent transfer of fluoride anion to the cationic center C2 leads to fluoroborate 5 (path a), which is hydrolyzed to yield 2. Spiro ketone 3 is obtained by a 1,2-alkyl shift in 4 (path b). 

Some fairly hindered catalysts, such as the rosindones [97] and metal porphyrins [98] are as efficient as AQ at low concentrations in delignifying wood and/or fragmenting models. This observation is contrary to what one would expect with an adduct mechanism metal ion complexes have been proposed to fragment lignin models by SET pathways. Moreover, carbohydrates have been shown to facilitate p-ether cleavage in lignin models [99] and can also transfer electrons to QMs [87]. 

TJ Fullerton, AL Wilkins. The Mechanism of Cleavage of 3-ether Bonds in Lignin Model Compounds by Reducing Sugars. JWood Chem Technol 5 189-201, 1985. 

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