Vinyl ethers isomerization

Unimolecular reactions that take place by way of cyclic transition states typically have negative entropies of activation because of the loss of rotational degrees of freedom associated with the highly ordered transition state. For example, thermal isomerization of allyl vinyl ether to 4-pentenal has AS = —8eu. ... 

Isomerization of the double bond in allylic alcohols may result in aldehydes or ketones (I07a). The reaction can have synthetic value (8bJ3c). If isomerization is desired, palladium is probably the preferred catalyst, operated best under hydrogen-poor conditions (/47fl). Allylic ethers can be converted to alcohols by isomerization with (Ph3P)3RhCl at pH 2 to the vinyl ether, which undergoes hydrolysis (36a). 

The isomerization of allyl ethers and allyl acetals to vinyl ethers or vinyl acetals, respectively, has found many applications in organic synthesis (Equation (17)). Various transition metal catalysts have been reported in the literature for the isomerization of allyl ethers and allyl acetals. 

Ruthenium-carbenoid complex 1 catalyzed the isomerization of /3,7-unsaturated ethers to the corresponding vinyl ethers. This reaction is useful in the deprotection of allyl and homoallyl ethers (Scheme 39).65... 

The stereoselective isomerization of unsymmetrical diallyl ethers to allyl ( )-vinyl ethers was also carried out in the presence of a cationic iridium(l) catalyst. The catalyst prepared in situ by treating [Ir(cod)(PPh2Me)2]PF6 with hydrogen was found to be an excellent catalyst for the selective isomerization of a less substituted allyl group to an ( )-vinyl ether (Scheme 44).72... 

This paper is about a reinterpretation of the cationic polymerizations of hydrocarbons (HC) and of alkyl vinyl ethers (VE) by ionizing radiations in bulk and in solution. It is shown first that for both classes of monomer, M, in bulk ([M] = niB) the propagation is unimolecular and not bimolecular as was believed previously. This view is in accord with the fact that for many systems the conversion, Y, depends rectilinearly on the reaction time up to high Y. The growth reaction is an isomerization of a 7t-complex, P +M, between the growing cation PB+ and the double bond of M. Therefore the polymerizations are of zero order with respect to m, with first-order rate constant k p]. The previously reported second-order rate constants kp+ are related to these by the equation... 

We now turn to the problem of torsional isomerism in the 1-butenyl anion. By following arguments similar to those for methyl vinyl ether, we conclude that the Cs conformation will be lower in energy than the Ts conformation due to an attractive 1,5 nonbonded interaction. 

Ru(H20)6], which is a precursor of ROM polymerization of cyclic dienes has also been found to possess good alkene isomerization activity [1], Among others it catalyzed the isomerization of allylphenyl ether to a vinylphenyl ether (Scheme 9.1) at room temperature. Allyl ethers are stable to acids and bases, while vinyl ethers are easily cleaved in acidic solutions. Therefore this isomerization gives a mild method for removal of protecting allyl groups under exceedingly mild conditions. 

Vinyl ethers are important raw materials in the production of glutaraldehyde, as well as of vinyl polymer materials which contain oxygen and are expected to degrade easily in Nature. The [IrCl(cod)]2 catalyzes an efficient exchange reaction between vinyl acetate 57 and alcohols or phenols 58, leading to the corresponding vinyl ethers 59 (Equation 10.11) [27]. Usually, the acid-catalyzed exchange reaction between alcohols and vinyl acetate results in alkyl acetates 60, and also to vinyl alcohol 61 which is readily isomerized to acetaldehyde 62. 

Enehydroxylamines (102) are invoked as intermediates in the rearrangement of O-vinyl, acyl or aryl oximes (101) (equation 31). Varlamov and coworkers demonstrated that the heterocyclization of ketoximes (103) with acetylene in snper basic medium and in the presence of metal hydroxides proceeds by a [3,3]-sigmatropic rearrangement of the enehydroxylamine 105 of the corresponding oxime vinyl ethers 104 (equation 32). The unreactivity of 3-methyl-2-azabicyclo[3.3.1]nonan-9-one oxime (106) in the same reaction conditions was explained by its inability to isomerize to the corresponding enehydroxylamine. 

N-Sulfinylsulfonimides treated with vinyl ethers give 1,2-thiazetidine 1-oxide (Eq. 35a). Investigation of the stereochemistry of this reaction showed that no isomerization occurs between the forms 172 and 173, which is the rule for the corresponding azetidines. ... 

A series of pyrido[2,3-rf pyrimidine-2,4-diones bearing substituents at C-5 and/or C-6 were synthesized using palladium-catalyzed coupling of uracil derivative 417 with vinyl substrates or allyl ethers to give the regioisomeric mixtures of 418/419 and 420/421, respectively. The ratio of the isomeric structures was dependent on the substituent R. In the case of the reaction with -butyl vinyl ether, only the product 419 was obtained. However, the reactions with acrylonitrile, ethyl acrylate, 2-trifluoromethylstyrene, and 3-nitrostyrene afforded only 418. Also, reaction with allyl phenyl ether gave only 420. The key intermediate 417 was prepared by the reaction of 6-amino-l-methyluracil with DMF-DMA (DMA = dimethylacetamide), followed by N-benzylation with benzyl chloride and vinyl iodination with iV-iodosuccinimide (NIS) (Scheme 15) <2001BML611>. 

The isomerization of allyl ethers to 1-propenyl ethers, which is usually performed with potassium tert-butoxide in dimethyl sulfoxide, can also be carried out under milder conditions using tris(triphen-ylphosphine)rhodium chloride,208 and by an ene reaction with diethyl azodicarboxylate,209,210 which affords a vinyl ether adduct. Removal of an O-allyl group may be achieved by oxidation with selenium dioxide in acetic acid,211 and by treatment with N-bromosuccinimide, followed by an aqueous base.201,212... 

The kinetically controlled product in the reaction of 99a,b with a large excess of ethyl vinyl ether was the thermodynamically less stable diastereo-mer 137 (Ph and OR trans) (Scheme 37). Complex 137 was present in solution as two conformers that rapidly interconverted. At room temperature in CDC13 the complexes 137 isomerized completely within several hours to form the diastereomer 138. In 138 the substituents OEt and Ph occupy mutual cis positions, thus minimizing steric interaction with the bulky W(CO)5 fragment on the heteroatom.250... 

The copper-chromium oxide has two different active sites in a reduced state. The cuprous ions associated with a hydride and two anionic vacancies are the hydrogenation (HYD) sites. The chromium ions in the same environment are the sites where occur the isomerization (I) and the hydrodeoxygenation (HDO) reactions. The use of unsaturated ethers permits to confirm and to precise the nature and the role of the active sites. With the compounds which have the oxygen atom kept away of the catalyst s surface, the HYD activity is very low and the HDO/I ratio too, whereas, in the opposite case, these values increase. With the vinylic ethers, the saturated compound is the main product because the I and the HDO reactions proceed via a concerted mechanism with a common preliminar step and an allylic rearrangement which is impossible with geminate functions. 

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). 

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