Mechanisms ether formation

Lauder, T., and J. H. Green Mechanism of Formation of Ether, Using... 

The mechanism for ether formation is quite simple given the recent work of Chini (19) that shows that rhodium clusters can react to give strong acids ... 

Misonidazole [27 l-methoxy-3-(2-nitroimidazol-l-yl)-2-propanol] and the model compound l-methyl-2-nitroimidazole have been used as radiosensitizers in the treatment of certain types of human tumors. One important property of these compounds is that they are more toxic to hypoxic cells than to aerobic cells, indicating that reductive metabolism of the drug is involved in the toxicity. Results of a number of studies suggest that intracellular thiols play a significant role in the hypoxic cell toxicity, and it was found that reduction products formed stable thio ethers with GSH (for literature see References 181-183). The reaction mechanism of thio ether formation has not been fully established. It has been suggested that the 4-electron reduction product was involved in thio ether formation181,184,185, and that the hydroxylamine rather than the nitroso derivative was the reactant. On the other hand, an intermediate nitroso derivative is expected to give a sulfenamide cation (see Scheme 1) which easily allows thio ether formation. 

The mechanism for formation of the cycloheptenone is exactly the same as discussed in the book. After a Diels-Alder reaction, the enol ether is hydrolyzed to the ketone by an acid-catalyzed mechanism. 

The positive slope for ether formation found on alumina (series 47), which contrasts with the negative one for alkene formation, has been interpreted by Knozinger as evidence of different mechanisms for these two, often in parallel proceeding transformations of alcohols. It has been suggested that the first step of the dehydration to an ether is the formation of a surface alkoxide, which is then attacked by a weakly bonded alcohol molecule. 

Dithioacetals derived from heteropine 177 smoothly react with methylene iodide in the presence of a zinc-copper couple in refluxing ether to give the corresponding fused thiophenes 178. The suggested mechanism involves formation of an ylide which undergoes intramolecular aldol-type condensation assisted by coordination of zinc with a carbonyl followed by demethylation of the S-methylthiophenium species (Scheme 35 (1989TL3093)). 

Jones and Barteau [275] have studied the cyclization and related reactions of iodoethanol on Ag(llO). The mechanism of diethyl ether formation on Ag(llO) and its dependence on the coadsorbed oxygen species has been studied by Jones etal. [276]. 

However, the investigations of the mechanism of olefin and ether formation from alcohol (Sect. 2.2) revealed the importance of basic sites. It is feasible that, for esterification also, pairs of acidic and basic sites might be necessary. 

More recent investigations into the mechanism of formation of pentaerythritol have been made both by Wawzonek and Rees [3] and by Barth, Snow and Wood [4]. Both groups tried to explain the process in terms of the formation of jjw-pen-taerythritol ether (dipentaerythritol), and other ethers, e.g. the methyl ether. The latter is produced whenever the formaldehyde used for the reaction contains methyl alcohol ... 

The coupling reaction by which the aromatic group from one residue of mono- or diiodotyrosine is joined in ether linkage with a second residue is also catalyzed readily by peroxidases. One dehydroalanine residue is formed for each molecule of hormone released.108 A possible mechanism involves formation of an electron-deficient radical, which can undergo (3 elimination to produce a dehydroalanine residue and an aromatic radical. The latter could couple with a second radical to form triiodothyronine or thyroxine. However, as depicted in Eq. 25-6, the radical coupling may occur prior to chain cleavage. While P elimination (pathway... 

At lower temperatures the oxonium salt or the alkyl hydrogen sulfate may react by an SN displacement mechanism with excess alcohol in the reaction mixture, thereby forming a dialkyl ether. Although each step in the reaction is reversible, ether formation can be enhanced by distilling away the ether as fast as it forms. Diethyl ether is made commercially by this process ... 

Cationic methylplatinum(II)-nitrile complexes of the type m is-PtMeL2(NCR)+X have been isolated by the reaction of //vms-PtMeClL2, where L = dimethylphenylphosphine or trimethyl-arsine, with an aryl nitrile and AgX, where X = BF4, PF. Use of pentafluorobenzonitrile and 2,3,5,6-tetrafluorotetraphthalonitrile in alcohol has led to the synthesis of a series of imino ether complexes. A mechanism for imino ether formation, involving nucleophilic attack by an alcohol at a coordinated nitrile, is suggested and the course of the reaction is shown to be dependent not only on the alcohol but also on the size of the anion used.110... 

Scheme 7.23. Potential mechanism for formation of silylmethyl allylic silanes from gem-disubstituted homoallylic ethers.

Br bond dipoles are oriented antiparallel and thus able to avoid repulsion). If this chair conformer eliminates HBr, the dihedral angle between the C-H and the SC-Br that are to be broken on the way to 3-bromocyclohexene measures a favorable 180°. the dihedral angle between the H and Br atoms to be eliminated to afford 1 -bromocyclohexene is an unfavorable 60°. As long as the reaction temperature is not too low and/or the potassium alkoxide not too undemanding sterically so that ether formation by the SN2 mechanism becomes competitive, the 3-bromocyclohexene eliminates a second equivalent of HBr in another 1,2-elimination, forming 1,3-cyclohexadiene. 

It is obvious (see Figure 9.19) that the catalyst is stabilized by water addition to the feed. It is well known that the mechanism of formation of diethyl ether is of the Eley-Rydeal type [148] (see Section 9.6), and the mechanism of formation of ethylene is of the Langmuir-Hinshelwood type [145], Figure 9.20 shows the fitting of the experimental data obtained during the kinetic study... 

The synthesis of 4,5-dicyano-l,2,3-trithiole 2-oxide (172) starts from sodium cyanide and carbon disulfide which via (170) gave the disodium salt of 2,3-mercaptomaleonitrile (171 M = Na). Treatment of the corresponding silver salt (171 M = Ag) with thionyl chloride yielded (172) <66HC(2l-l)l). Phenylsulfine (174), prepared in situ by dehy-drohalogenation of phenylmethanesulfinyl chloride (173), slowly decomposed in ether solution at room temperature to give cis- and trans-stilbenes, mms-4,5-diphenyl-l,2,3-trithiolane 1,1-dioxide (36) and a 5,6-diphenyl-l,2,3,4-tetrathiane dioxide (68JCS(C)1612). The mechanisms of formation of these heterocycles are obscure. 

The review starts with a discussion of the mechanism of keto-enol tautomerisation and with kinetic data. Included in this section are results on stereochemical aspects of enolisation (or enolate formation) and on regioselec-tivity when two enolisation sites are in competition. The next section is devoted to thermodynamic data (keto-enol equilibrium constants and acidity constants of the two tautomeric forms) which have greatly improved in quality over the last decade. The last two sections concern two processes closely related to enolisation, namely the formation of enol ethers in alcohols and that of enamines in the presence of primary and secondary amines. Indeed, over the last fifteen years, data have shown that enol-ether formation and enamine formation are two competitive and often more favourable routes for reactions which usually occur via enol or enolate. 

Sn2 reactions can occur at elements other than carbon. Common examples in organic chemistry are silicon, phosphorus, sulfur, and the halogens. The formation of the tosylate above by attack of the alcohol on TsCl is an example of an Sn2 reaction at sulfur. Later in this chapter you will see that alcohols attack phosphorus very easily and that we use the reaction between ROH and PBra to make alkyl bromides. Alcohols also react rapidly with Si-CI compounds such as MegSiCI to give silyl ethers by an Sn2 reaction at silicon. You have already seen several examples of silyl ether formation (p. 240, for example), though up to this point we have not discussed the mechanism. Here it is B represents a base such as triethyl amine. 

Brunow G, Sipila J, Makela T (1989) On the mechanism of formation of non-cyclic benzyl ethers during lignin biosynthesis Holzforschung 43 55-59 Ede RM, Brunow G, Simola LK, Lemmetyinen J (1990) Two-dimensional H- H chemical shift correlation and J-resolved NMR studies on isolated and synthetic lignins Holzforschung 44 95-101... 

There are a number of claims4 40 51,69,150,247,268-274 for the isolation of bimolecular ethers from various other heterocyclic cations although the structures of these products have rarely been unambiguously established. The reaction mechanism outlined for the formation of 114 probably does occur in other heterocyclic systems, particularly in those cases in which alkoxide ion formation from the pseudobase readily occurs. Solubility considerations may dictate the precipitation of the bimolecular ether rather than the pseudobase from basic aqueous solutions containing relatively high concentrations of the heterocycle. However, such bimolecular ether formation will usually be in direct competition with the pseudobase disproportionation reaction (Section V,D) which shows the same pH dependence. 

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