Allyl ethers elimination reactions

Longer reaction times are usually required for propoxylation reactions due to the preference for lower reaction temperatures necessary to maintain product color and avoid troublesome side reactions such as elimination of a terminal alcohol to generate allyl ethers. Propoxylation reactions can be selectively accelerated at lower temperatures with the use of a double metal cyanide catalyst. The catalyst is extremely sensitive and easily deactivated by moisture and conventional groups I and II catalysts, and will not effectively catalyze ethoxylation reactions. 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

Vinyl ethers can also be generated by thermal elimination reactions. For example, base-catalyzed conjugate addition of allyl alcohols to phenyl vinyl sulfone generates 2-(phenylsulfinyl)ethyl ethers that can undergo elimination at 200° C.223 The sigmatropic... 

Since nucleophilic addition to a metal-coordinated alkene generates a cr-metal species bonded to an -hybridized carbon, facile 3-H elimination may then ensue. An important example of pertinence to this mechanism is the Wacker reaction, in which alkenes are converted into carbonyl compounds by the oxidative addition of water (Equation (108)), typically in the presence of a Pd(n) catalyst and a stoichiometric reoxidant.399 When an alcohol is employed as the nucleophile instead, the reaction produces a vinyl or allylic ether as the product, thus accomplishing an etherification process. 

The reaction proceeds through ligand exchange and a subsequent P-elimination akin to the oxidative addition of Cp2Zr to allylic ethers [58], In this way, allyltitanium compounds can be obtained from readily available allylic alcohol derivatives and inexpensive Ti(OiPr)4. The method allows the preparation of functionalized allyltitaniums bearing functional groups such as ester or halide (Scheme 13.28). 

In view of the proposed mechanism for this cleavage reaction, it was reasonable to expect that certain allylic ethers for which an elimination pathway existed (Scheme II) would also be susceptible to cleavage under thermolysis, acidolysis or a combination of both. 

Phenylselenoetherification (8, 26-28). This cyclization has been described in detail.6 The 16 examples reported indicate that the reaction is applicable to unsaturated primary, secondary, and tertiary alcohols as well as to phenols. The most important use is for synthesis of allylic ethers by syn-selenoxide elimination, which proceeds selectively away from the oxygen. The value of this methodology for synthesis of natural products is illustrated by a synthesis of a muscarine analog (1), outlined in equation (I). 

Finally, upon treatment with acid, both diene and allyl ether complexes of [Os(NH3)5]2+ yield Os(IV) i73-allyl species, the latter reaction proceeding by the elimination of alcohol (180). 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

Ene-type products are obtained by Co- and Fe-catalysed reaction of dienynes. Cocatalysed cyclization of substrate 111 proceeds smoothly with respect to the diene, acetylene and allylic ether moiety to afford 114. In this cyclization, the 7i-allyl complex 112 is formed by insertion of the diene to Co—H, followed by domino insertions of the triple and double bonds to give 113. The final step is the elimination of the /J-alkoxidc group from 113 to form 114 [47], The six-membered ene-type products 117 and 118 are obtained from the reaction of 115 catalysed by an Fe bipyridyl complex. The reaction seems to involve oxidative cyclization to form 116. Subsequent -elimination and reductive elimination provide 117 and 118. As another possibility, insertion of the diene to Fe—H gives a 7i-allyl complex. Then double bond insertion and -elimination should give 117 and 118 [48],... 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

The synthesis of a variety of dihydrobenzofuranes can be accomplished via the Aryl SN1 mechanism. The acetone-sensitized irradiation of various o-chlorophenyl allyl ethers in polar solvents affords the aryl cation, which adds onto the tethered double bond following the 5-exo-trig mode leading to (dihydro)benzofurans. The reaction is solvent-dependent, and byproducts are also generally obtained. The best results are found when ethyl acetate is used. Depending on the substrate employed, the cation intermediate can yield products as a result of proton elimination or trapping by chloride ions (Scheme 10.54) [70],... 

The reaction of oxetane with carbenes follows two major pathways carbonhydrogen insertion or the formation of an oxygen ylide by reaction of the carbene and the oxetane oxygen. The oxygen ylide can produce a tetrahydrofuran by a Wittig rearrangement or generate an allyl ether by an intermolecular -elimination process (Scheme 61). 

Elimination reactions are typically found in transition metal chemistry. The conversion of allylic ethers into allylzirconocenes [18] is a well-known zirconium-mediated process of this type. In contrast, the y-elimination reactions are less frequently encountered, and only a few examples of y-elimination involving zirconocenes have been reported. When studying isonitrile insertion into zirconacycles, Whitby and coworkers observed an interesting reaction leading to a cyclopropane derivative [19]. The overall transformation is depicted in Scheme 12. Treatment of the diene 18 with preformed butene-zirconocene gave... 

Therefore, the isomerization reaction was also performed starting from the a-substituted enol ether 119 (Scheme 52). By using the tandem allylic C-H activation-elimination reactions, Z-120c is initially formed and by a transmetalation reaction into organocopper with a stoichiometric amount of CuCI/ 2LiCl, followed by heating at +50 °C for 1 h and reaction with allyl chloride, the resulting ( , )-diene 122 is obtained with an isomeric ratio of 90 10 but in a low 40% yield as described in Scheme 52. 

Ally) ethers are selectively cleaved with titanium(lV) isopropoxide and commercially available Grignard reagents like /i-butyl- or cyclohexylmagnesium chloride [Scheme 4.229].432 Neither benzylidene acetals nor more highly substituted allylic ethers suffer under the reaction conditions. A mechanism for the reaction implicates formation of the titanacyclopropane intermediate 229.1 as the first step. Ligand exchange with an unsubstituted allyl ether affords intermediate 229.2. -Elimination to the allyltitanium(lV) alkoxide 2293 followed by hydrolysis returns the deprotected alcohol. The reaction closely resembles an earlier method based on zirconium.433... 

As in the selenium case (Scheme 17) the oxidation of alkyl phenyl telluride with excess MCPBA in the presence of alcohols results in a facile substitution of a PhTe moiety by an alkoxy group. The reaction is assumed to proceed via a similar tellurone-MCPBA adduct intermediate. Oxidation of cycloalkyl telluride (61) was accompanied by ring contraction to produce an acetal (62), < while the bromination-hydrolysis method affords the allylic ether by telluroxide elimination (Scheme 22). ... 

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